Paleoproterozoic cap carbonates provide vital records of post-glacial environmental and biogeochemical transitions, offering crucial insights into early Earth’s climatic, ocean–atmosphere evolution, and the planet’s habitability[1]. This study reports, for the first time, well-preserved evidence of such cap carbonates from the Aravalli Supergroup, India, identified within calc-silicate horizons embedded in the metavolcanics of the Delwara Formation. Comprehensive geochemical and isotopic analyses confirm their primary depositional signatures and effectively rule out major diagenetic or metamorphic overprinting. The systematically collected samples exhibit negative δ13CV-PDB values, characteristic of global cap-carbonate sequences that formed immediately after the Paleoproterozoic glaciation. These strata are subsequently overlain by dolomites displaying the pronounced positive δ13CV-PDB excursion associated with the Lomagundi–Jatuli Event (LJE). Unlike the Sausar Group of India, which records cap carbonates without evidence of the LJE, the Aravalli Supergroup uniquely preserves both features within its Paleoproteozoic succession[2]. This integrated record establishes the Aravalli Basin as a key site for understanding the temporal link between deglaciation, large-scale carbon-cycle shifts, and atmospheric oxygenation. Furthermore, the coexistence of post-glacial and LJE signatures enables refined global chemostratigraphic correlations with other Paleoproterozoic basins across continents such as South Africa, Canada, and Australia[3]. These findings highlight the Aravalli Basin’s pivotal role in tracing the aftermath of Paleoproterozoic glaciations and provide new perspectives on how early Earth’s surface environments evolved during one of the most transformative intervals in the planet’s history.
References
[1] Bekker et al. [2005]. Precamb Res. 137(3-4), 167-206.
[2] Goswami et al. [2023]. Precamb Res. 399.
[3] Maheshwari et al. [2010]. Gondwana Res. 417, 195-209.
How to cite: Goswami, A., Jang, Y., and Kwon, S.: When Ice Met Oxygen: Unveiling the Oldest Clues of Earth’s Climate Shift from the Aravalli Supergroup, India. , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2953, https://doi.org/10.5194/egusphere-egu26-2953, 2026.
Neoproterozoic “snowball Earth” refers to extreme glaciations when sea ice extended from the poles to the tropics and perhaps to the equator. Despite decades of study, the mechanisms that triggered global glaciation are still debated, although many mechanisms link their onset to reductions in atmospheric CO2 concentration. We use a coupled general circulation model and two geologically constrained paleogeographic reconstructions to re-examine the CO2 threshold for the initiation of the Sturtian snowball Earth (~717 Ma). With modern landmasses, a hard-Snowball transition occurs at 95±5 ppm CO2, consistent with prior estimates. In contrast, one 720 Ma reconstruction, resists global glaciation down to 6±1 ppm CO2 – a threshold so low that initiation via CO2 drawdown might be challenging – while maintaining an "oasis climate" with a small, zonally asymmetric region of open tropical ocean. A second 720 Ma reconstruction glaciates at 110±10 ppm, similar to modern. We show that the oasis climate is possible because the former continental configuration inhibits ocean heat transport out of a small, tropical ocean basin, allowing it to maintain above-freezing sea surface temperatures. While the "oasis climate" lacks the hysteresis expected for snowball glaciations in our climate model, hysteresis might be supplied by land ice sheets. The apparent sensitivity of Earth's snowball glaciation behavior to subtle changes in continental geometry points to a need for better-constrained paleogeographic reconstructions for understanding snowball Earth events and highlight potential challenges to CO2 drawdown mechanisms for snowball initiation.
How to cite: Fu, M., Graham, R., and Abbot, D.: Paleogeography strongly influences CO2 threshold for Sturtian Snowball Earth initiation, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5987, https://doi.org/10.5194/egusphere-egu26-5987, 2026.
During the Neoproterozoic, Earth experienced at least two extreme glaciations with ice extending to tropical latitudes. While the Snowball Earth hypothesis proposes a fully ice-covered planet, geological evidence and the persistence of life suggest that parts of the ocean may have remained ice-free. This has motivated the concept of Waterbelt states: alternative climate equilibria featuring open equatorial oceans that could act as refugia for early life and expand the range of habitable climates relevant to Earth-like exoplanets. Despite their appeal, Waterbelt states remain disputed due to uncertainties in the mechanisms required to halt the ice–albedo feedback at low latitudes, including the role of bare sea-ice albedo and cloud radiative effects.
Here, we investigate whether Waterbelt states are robust solutions of the coupled climate system and identify the processes controlling the stability of low-latitude ice margins. Using a hierarchy of models, this work combines mechanistic insights from a Budyko–Sellers energy balance model with a large ensemble of global climate simulations. In particular, we present results from a coordinated model intercomparison that includes three versions of the ICON model and five versions of the CAM model, all run in the same aquaplanet slab-ocean setup. The simulations are analyzed with respect to three key factors that have been proposed to influence Waterbelt stability: the area of exposed bare sea ice, cloud masking of the ice–albedo feedback, and shortwave cloud radiative feedbacks.
We demonstrate that stable Waterbelt states can be found in a wide variety of models. While ICON Waterbelt states depend on cloud tuning, all CAM models readily simulate stable Waterbelt states over a substantial range of CO2 radiative forcing. These differences are primarily due to cloud radiative effects: the CAM models exhibit stabilizing shortwave cloud feedbacks and stronger cloud masking than ICON. Overall, this suggests that clouds do not present a fundamental obstacle to Waterbelt climates, but instead play a modulatory role that varies across models. This implies that Waterbelt states may be more physically plausible than studies based on a single model have suggested, while at the same time emphasizing the importance of clouds for deep-time climate and exoplanet habitability.
How to cite: Voigt, A. and Hörner, J.: Waterbelt solutions to avoid a hard Snowball Earth, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8384, https://doi.org/10.5194/egusphere-egu26-8384, 2026.
Deep-time glacial intervals provide critical benchmarks for assessing Earth System Model (ESM) performance under past climate states. However, most paleo-simulations lack dynamic icesheets, leaving this key component poorly constrained. Here, we introduce a machine learning approach for reconstructing global glacial extent across the Phanerozoic, integrating paleoclimate simulations, paleo-topography, and a global stratigraphic database of glacial deposits. This framework generates spatially explicit, probabilistic reconstructions that enable quantitative comparison between geological archives and climate model ensembles, highlighting regions of agreement and mismatch.
The Late Palaeozoic Ice Age (LPIA), a >100-million-year glaciation variously attributed to declining atmospheric CO₂, palaeographic changes, and tectonic activity, provides an ideal case-study considered here. A persistent enigma concerning the LPIA is its hemispheric asymmetry, whereby preserved glacial deposits are abundant in the Southern Hemisphere but sparse in the Northern Hemisphere. Whether this bipolarity reflects genuine climate asymmetry or preservation bias remains unresolved. We address this by modelling the distribution of land-ice using environmental predictors such as temperature, precipitation, transpiration, and topography, derived from HadCM3L simulations that do not include dynamic icesheets. This analysis yields time-slice specific probabilistic reconstructions that can be directly compared with the preserved sedimentary record. We calibrate our framework against modern glaciers and LPIA glacial deposits, and subsequently applying it to other Phanerozoic ice ages, producing a consistent reference dataset for model-data comparison. While our approach does not replace fully coupled ice-climate simulations, it highlights some key discrepancies between models and geological evidence and allows climate asymmetry to be distinguished from preservation bias. By quantitatively bridging paleo-archives and climate models, our framework provides a new means of evaluating ESM performance across diverse climate states, strengthening constraints on ice-climate feedback relevant to future projections.
How to cite: Beura, S., Gernon, T., Stockey, R., and Lunt, D.: Reconstructing Late Palaeozoic Land-Ice Distributions: A Machine Learning Framework for Model-Data Comparison, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-19938, https://doi.org/10.5194/egusphere-egu26-19938, 2026.
The Hirnantian glacial maximum was a brief but intense glacial event that occurred during the latest Ordovician (~445-443 million years ago). It was characterized by global cooling, major ice-sheet expansion over Gondwana, and substantial perturbations to the carbon cycle. Previous studies have combined Earth system models with proxy records to investigate the magnitude of the cooling and to explore the mechanisms linking ocean deoxygenation to the Late Ordovician mass extinction. However, the results of these reconstructions exhibit considerable discrepancies, primarily due to the increasing uncertainty of proxy data with geological age and the difficulty of constraining boundary conditions required by models in deep time. Here we introduce state-dependent climate sensitivity, in which the radiative forcing of atmospheric CO2 increases with its concentration, to improve the Earth system modelling. We then perform a series of simulations with varying levels of greenhouse gases and nutrients to identify the climate-productivity conditions that plausibly drove the cooling during the Hirnantian glacial maximum. Applying rigorously screened Late Ordovician sea-surface temperature estimates derived from oxygen isotope studies as constraints, alongside a semi-quantitative constraint based on a new compilation of local redox proxies, we identify a plausible scenario of Hirnantian climate and redox changes. Our results show that deep-ocean deoxygenation during the Hirnantian was driven by a combination of cooling and changes in ocean nutrient inventory, and that temperature-driven microbial respiration can reconcile the spatial distribution of seafloor anoxia as reconstructed, providing new insights into the decoupling of redox conditions between the surface and deep waters. In addition, our simulations suggest that Late Ordovician atmospheric CO2 levels before cooling may have been substantially overestimated (up to 6,720 ppm according to previous studies), likely due to a fixed climate sensitivity assumed in previous modelling studies. This overestimation may not be limited to this event, but could also affect climate simulations of other periods.
How to cite: Zhang, Q. and Fan, J.: Late Ordovician Climate Reconstruction Based on State-Dependent Climate Sensitivity, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16908, https://doi.org/10.5194/egusphere-egu26-16908, 2026.
Organic carbon (OC) burial is a critical process regulating the global carbon cycle and climate system. However, compared to well-studied marine systems, the role and mechanisms of lacustrine OC burial in deep time remain poorly constrained. Despite covering an area only 1/80th that of the oceans, modern lakes contribute 10–50% of the global OC burial, highlighting their exceptional sequestration efficiency. This review synthesizes OC burial records from typical deep-time lacustrine shales, revealing that the geological-scale transition in OC burial capacity was driven by the evolution of lake ecosystems from "dead" and "starved" lakes to "ecologically primary" and "prosperous" ones. Based on the "productivity, preservation, and dilution" ternary equilibrium theory, we evaluate the multi-factor composite controls on the OC burial process, including tectonics, climate, hydro-ecological conditions, volcanic–hydrothermal activities, and marine transgressions. Our findings show that efficient OC burial results from the synergistic coupling of tectonic–climatic–ecological systems. Notably, nutrients from volcanic and hydrothermal activities were crucial for overcoming adverse climatic or ecological conditions—particularly during the "ecologically primary lakes" stage before the Late Paleozoic—thereby enabling effective OC sequestration. Finally, we propose five primary mechanisms for large-scale lacustrine OC burial: (1) volcanic–hydrothermal driven, (2) climate–volcanic activities coupling, (3) climate–basin scale coupling, (4) climate–transgressions coupling, and (5) tectonic–climate coupling. This synthesis not only offers a new perspective from lake records for understanding deep-time Earth's sphere interactions and carbon cycling but also establishes a geological-historical framework for predicting the response of lacustrine carbon reservoirs to future climate change.
How to cite: Liang, C., Chen, A., and Cao, Y.: Lacustrine organic carbon burial in deep time: Perspectives from major geologic events and tectonic-climatic-ecological coupling, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3702, https://doi.org/10.5194/egusphere-egu26-3702, 2026.
Understanding deep-time climatic feedbacks relies on quantifying the initial drivers of Earth system perturbations. Earth system perturbations are highly sensitive to, among other parameters, the timing and duration of volcanic degassing. Currently, these input parameters are coarsely constrained, with volatile estimates coming from melt inclusion data and radiometric dating1. However, recent work has highlighted the power of combining sedimentary mercury (Hg), a volcanic tracer, and simple Hg cycle box models to estimate the tempo and volume of volcanic degassing in deep time2.
Here, we present a quantitative high-resolution degassing history through the Sinemurian - Pliensbachian Boundary Event (SPBE). This protracted negative “U-shaped” carbon isotope excursion lasted for over 3 million years in the Early Jurassic (ca. 190 Ma). We utilise over 1600 samples collected from the recently drilled core at Prees, Cheshire Basin, U.K., as part of the International Continental Scientific Drilling Program JET project, to create this history.
The SPBE is broadly coeval with increased rifting and the associated opening of the Hispanic Seaway, and potentially a late pulse of volcanic activity from the Central Atlantic Magmatic Province3–5, all of which may have contributed to its shape and duration. We quantify the tempo and volume of volcanic degassing during the SPBE using a novel geochemical machine-learning framework to isolate volcanically sourced Hg, followed by identification of the best-fit degassing scenarios using a global Hg box model.
The results of our method have implications regarding the sensitivity and feedbacks of the carbon cycle in deep time. Specifically, we quantify the evolution of emissions during this enigmatic excursion. This will directly aid in understanding climate sensitivity during this period, where the protracted “U-shaped” change in carbon isotopes must now be reconciled with our evidence for distinct pulses of volcanic emissions throughout.
This work helps bridge the gap between the palaeoclimate modelling and proxy communities. By quantitatively linking Hg concentrations to volcanic degassing, we can provide volcanic inputs with a precision of a few thousand years to modellers aiming to simulate deep-time climate change.
References:
1. Hernandez Nava, A. et al. Reconciling early Deccan Traps CO2 outgassing and pre-KPB global climate. Proceedings of the National Academy of Sciences 118, e2007797118 (2021).
2. Fendley, I. M. et al. Early Jurassic large igneous province carbon emissions constrained by sedimentary mercury. Nat. Geosci. 17, 241–248 (2024).
3. Franceschi, M. et al. Early Pliensbachian (Early Jurassic) C-isotope perturbation and the diffusion of the Lithiotis Fauna: Insights from the western Tethys. Palaeogeography, Palaeoclimatology, Palaeoecology 410, 255–263 (2014).
4. Ruhl, M. et al. Astronomical constraints on the duration of the Early Jurassic Pliensbachian Stage and global climatic fluctuations. Earth and Planetary Science Letters 455, 149–165 (2016).
5. Jiang, H. et al. Large-scale volcanogenic Hg enrichment coincided with the Sinemurian-Pliensbachian boundary event (Early Jurassic). Geological Society of America Bulletin https://doi.org/10.1130/B37640.1 (2025)
How to cite: Neilson, O., Fendley, I., Frieling, J., Mather, T., Hesselbo, S., Jenkyns, H., and Ullmann, C.: A novel approach for quantifying the timing and volume of volcanic degassing in deep time: A case study from the Sinemurian – Pliensbachian Boundary Event, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14764, https://doi.org/10.5194/egusphere-egu26-14764, 2026.
Large Igneous Province (LIP) volcanism is widely invoked as a primary driver of major carbon-cycle perturbations and climate extremes in Earth history, yet its short-term eruptive tempo and terrestrial environmental impacts remain poorly constrained. Most existing models assume temporally smoothed volcanic carbon release, largely due to the limited temporal resolution of marine sedimentary archives. Here we present a sub-millennial-resolution lacustrine sedimentary record spanning Oceanic Anoxic Event 1a (OAE1a) from the Aptian Jiufotang Formation in the Kazuo Basin, northeastern China, providing a rare terrestrial perspective on high-frequency LIP activity. A total of 199 samples were collected from a ~130 kyr interval (covering the transition from high to low 187Os/188Os) of organic-rich lacustrine black shales, achieving a temporal resolution of ~0.3–1.0 kyr per sample—comparable to Quaternary paleoclimate studies but applied to a deep-time volcanic event. High-resolution stratigraphic profiles of carbon isotopes reveal repeated, abrupt excursions, indicating episodic volatile release associated with super-eruptive volcanism. These geochemical signals are stratigraphically coupled with sedimentological features, including volcanic ash layers, sulfide laminae, and storm-induced deposits, demonstrating that individual eruptive pulses are not only geochemically resolvable but also sedimentologically expressed. Additional Pb isotope constraints further support an Ontong Java Plateau mantle source. Importantly, the magnitude and frequency of lacustrine carbon isotope excursions exceed those typically observed in coeval marine records, implying strong terrestrial amplification through enhanced organic carbon burial, primary productivity blooms, and potentially intensified methanogenesis. These results challenge conventional time-averaged carbon-cycle models and highlight that the climatic and ecological impacts of LIP volcanism are governed by short-lived, threshold-crossing forcing events. Lacustrine systems thus provide a uniquely sensitive archive for resolving the true temporal structure of deep-time volcanic perturbations and their consequences for Earth’s surface environments.
How to cite: Sun, M.-D., Matsumoto, H., and Xu, Y.-G.: A sub-millennial-resolution lacustrine record of Large Igneous Province volcanism during Early Cretaceous OAE1a, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8621, https://doi.org/10.5194/egusphere-egu26-8621, 2026.
Reconstructing carbon release fluxes during extreme climatic events in Earth history—particularly quantifying the magnitude and climatic impacts of biogenic greenhouse-gas emissions—is crucial for building high-confidence “past–future” climate analog frameworks. In paleoclimate research, the Toarcian Oceanic Anoxic Event (T-OAE; ~183 Ma), one of the most prominent global warming episodes of the Mesozoic, still features key knowledge gaps regarding the coupled mechanisms linking its carbon-isotope excursions (CIEs) to greenhouse-gas release. Here we integrate multi-proxy constraints to develop a global coupled biogeochemical model that explicitly represents methane cycling across the sediment–ocean–atmosphere system, and we apply a Markov chain Monte Carlo (MCMC) Bayesian inversion to systematically quantify methane emission fluxes during the T-OAE for the first time. Model simulations indicate that reproducing the pulsed negative CIEs, the rise in atmospheric pCO2, and the 4–6 °C global warming inferred from paleotemperature proxies requires at least ~4700 Gt (CO2-equivalent) of sustained biogenic methane input to the Earth’s surface system. Notably, the inferred carbon-isotopic composition of the methane (δ¹³C = −50‰ to −70‰) closely matches the characteristic fractionation associated with methanogenic archaeal metabolisms. The model further suggests that methane release may have amplified methanogenesis and increased organic-matter input, while sulfate-depleted ocean conditions reduced methane oxidation, together establishing a positive feedback of “enhanced methane production–suppressed oxidation efficiency.” Sensitivity experiments show that methane emissions of this magnitude could drive an atmospheric pCH₄ increase of >5 ppm, producing additional radiative forcing sufficient to yield ≥2 °C extra surface warming. Moreover, oceanic methane release promotes a millennial-scale decline in dissolved oxygen, triggering systemic collapse of benthic habitats. This nonlinear coupling between biogeochemical cycling and ecosystem responses may have been a key driver of widespread marine biotic losses during the T-OAE.
How to cite: Qiu, R.: Pulsed biogenic methane emissions and episodic carbon cycle perturbations during the Toarcian Oceanic Anoxic Event, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2050, https://doi.org/10.5194/egusphere-egu26-2050, 2026.
Understanding of different types of El Niño events, notably Eastern Pacific (EP) and Central Pacific (CP) El Niño, is hindered by the limited length of observations. Using climate simulations, we investigated the evolution of El Niño flavor from 250 million years ago (Ma) to present. Results show that El Niño has been persistent throughout the entire period the simulation spans, but was dominated by CP El Niño at 250 Ma - 80 Ma. With the emergence of the Atlantic Ocean, which modulated the state of the Pacific Ocean through atmospheric circulation, EP El Niño became the predominant El Nino state (70 Ma - 10 Ma). After the closure of the Central American Seaway (0 Ma), EP and CP El Niño occurred with similar frequencies. Our findings highlight that El Niño types are controlled by geography over tectonic timescales.
How to cite: Hu, Y., Wu, S., and Liu, Y.: Eastern Pacific El Niño activated by the Atlantic Ocean, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2419, https://doi.org/10.5194/egusphere-egu26-2419, 2026.
Silicate weathering regulates Earth’s surface climate over geological timescales by removing atmospheric CO2. Understanding changes in weathering dynamics and rates is key to predicting climate response time scales. We investigated the reactivity of the North American source-to-sink system and the chemical weathering regime during the Paleocene–Eocene Thermal Maximum (PETM). We measured the detrital lithium isotope composition (δ7Li) in a deep-marine sediment core from the Gulf of Mexico, tracking changes in the formation of clay minerals, alongside neodymium isotopes (εNd), to constrain sediment provenance.
We find a buffered negative δ7Li excursion during the PETM body, likely reflecting the mixing of newly formed and reworked clays from continental floodplains, followed by a pronounced negative δ7Li excursion during the recovery phase. This pattern is consistent with the continental Bighorn Basin (Wyoming, USA) δ7Li record (Ramos et al., 2022), indicating a rapid propagation of enhanced weathering and erosion fluxes in response to the PETM, which would have contributed to efficient CO2 drawdown (Jaimes-Gutierrez et al., 2025).
To fully understand weathering–climate feedbacks during the PETM, future work will target the radiometric dating of clay minerals exported to the ocean during this climatic perturbation. Constraining the timing of clay formation and residence on continental floodplains will allow us to distinguish between newly formed and reworked clays. Such age constraints would provide critical insights into the response timescales of continental weathering processes and thereby improve our understanding of carbon budgets during the PETM.
References:
Jaimes-Gutierrez, R., Vimpere, L., Wilson, D.J., Blaser, P., Adatte, T., Sahoo, S., and Castelltort, S., 2025, Lithium isotopes reveal enhanced weathering fluxes in North America during the Paleocene–Eocene Thermal Maximum: Geology, doi:https://doi.org/10.1130/G53708.1.
Ramos, E.J. et al., 2022, Swift Weathering Response on Floodplains During the Paleocene‐Eocene Thermal Maximum: Geophysical Research Letters, v. 49, doi:10.1029/2021GL097436.
How to cite: Jaimes-Gutierrez, R., Vimpere, L., Castelltort, S., Wilson, D. J., Blaser, P., Pogge von Strandmann, P. A. E., Adatte, T., Sahoo, S., and King, G. E.: Lithium isotopes reveal enhanced weathering fluxes in North America during the Paleocene–Eocene Thermal Maximum: Perspectives on clay chronology, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10197, https://doi.org/10.5194/egusphere-egu26-10197, 2026.
The dissolution of calcium carbonate (CaCO3) is a key regulator of long-term changes in oceanic CO2 uptake, through the generation of alkalinity. Geological records from past climatic and carbon-cycle perturbation events contain abundant evidence for ocean acidification and seafloor CaCO3 dissolution. Such events can thus serve as natural laboratories to assess the role of carbonate compensation in mitigating extreme carbon release and stabilizing the Earth system. In this study, we aim to evaluate the magnitude, rate, and climatic significance of carbonate dissolution for the Paleocene-Eocene Thermal Maximum (PETM, ~56 Ma), the most dramatic of the early Cenozoic hyperthermals. Specifically, we use a new high-resolution record from IODP Site U1514 from the Mentelle Basin in the SE Indian Ocean (paleolatitude: ∼60°S at 50 Ma) to quantify the dissolution of seafloor CaCO3 deposited prior to the PETM (‘burndown’), in the earliest phases of the event. Our site is ideally positioned to document this process due to its location in the deep-sea, relatively high sedimentation rates, expanded upper Paleocene record and sensitivity to changes in carbonate saturation. We use precession-scale cyclostratigraphy to create an age model for the late Paleocene and early Eocene at U1514, anchored within 405-kyr astrochronozones and subsequently tied to the established astrochronology of ODP Site 690 in the Weddell Sea, allowing for refined interbasinal stratigraphic alignment across the Southern Ocean. The age model forms the basis for our analysis of ‘burndown’ dissolution and alkalinity generation at our site and across the PETM seafloor. Our work is an important step forward in our ability to quantify alkalinity fluxes from seafloor dissolution and their impact relative to terrestrial weathering, on millenial to orbital timescales.
How to cite: Papadomanolaki, N. M., Jones, H. L., Hanson, E. M., Edgar, K. M., Bialik, O. M., Batenburg, S. J., and De Vleeschouwer, D.: Quantifying PETM Carbonate Burndown and Alkalinity Feedbacks through Cyclostratigraphy , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2001, https://doi.org/10.5194/egusphere-egu26-2001, 2026.
The Earth's climate shifted swiftly from a "greenhouse" state to an "icehouse" state ~34 million years ago (Ma). This climatic transition is characterized by abrupt atmospheric pCO2 drawdown, the initiation of Antarctic glaciation, and perturbations of marine carbon cycling. While previous studies have suggested heterogeneous changes across ocean basins in primary productivity, but a global unchanged state of fish production. The net effect of the marine biological pump on sequestrating atmospheric pCO2 is still an enigma. Marine barite (BaSO4) is a reliable proxy of export productivity owing to its biologically induced formation and refractory nature. Here, we present global records of marine barite accumulation rates from multiple sediment cores representing different oceanographic regions from the late Eocene to the early Oligocene. We reconstruct the temporal and spatial evolution of export productivity between 41 and 28 Ma, and investigate its contribution to the global carbon budget before and after the Eocene–Oligocene Transition. Additionally, we use the Earth System Model IPSL-CM5A2 and biogeochemical model PISCESv2, and compare proxy data with model results of the 40 Ma and 30 Ma simulations. Together, they can help to explore the role of tectonic-driven reorganization of ocean circulation in export productivity. These findings offer implications for understanding feedbacks between tectonic, climate, and carbon cycling at the onset of the early Cenozoic icehouse world.
How to cite: Zhang, R., Pineau, E., Donnadieu, Y., and Yao, W.: Global reconstruction of ocean export productivity from the late Eocene to the early Oligocene, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9562, https://doi.org/10.5194/egusphere-egu26-9562, 2026.
The earliest Cenozoic Antarctic bryozoan fossil records (late Early Eocene) are well documented from the shallow-marine–estuarine clastic succession of the lower part (Telm1-2) of the La Meseta Formation of Seymour Island. In the 800-meters thick stratigraphical profile of the LMF in the basal facies of the (Telm1), the earliest – late Early Eocene bryozoans are represented by the internal moulds of the loosely encrusting, unizooidal, flexible articulated or rooted colonies belonging to cheilostome buguloids and catenicelloideans, which are taxonomically and morphologically different from the overlying fauna. At present, representatives of (Beanidae, Catenicellidae, Savignyellidae and Calwelliidae widely occur in the tropical-warm temperate latitudes in the shallow-marine settings (Hara, 2015). Higher in Telm1 the most common are spectacular in size, massive multilamellar colonies, showing a great variety of shapes dominated by cheilostome celleporiforms and cyclostome cerioporids (Hara, 2001). The stable isotopic δ18O analyses of the bryozoan skeletons from the lower part of the LMF show the temperature range from 13.4 to 14.6°C (Hara, 2022), what is consistent with the isotopic data of other marine macrofaunal fossil records (Ivany et al., 2008).
The distinct free-living lunulitiforms bryozoans, for the first time reported from Antarctica from the middle part of the LMF (Telm4-6, Cucullaea I-II; Ypresian/Lutetian) are represented by the disc-shaped colonies - characteristic for the temperate warm, shallow-shelf environment, with the bottom temperature, which are never lower than 10 to 12°C. The skeletons of Lunulites, Otionellina, and Uharella are formed by the intermediate-Mg calcite (IMC) with the 4.5 mol% MgCO3. Their bimineralic zoaria (with the traces of aragonite, calcite and strontium apatite) are indicative for the sandy, temperate shelf environment (Hara et al., 2018).
Contrary to occurrence of the rich bryozoans of the (Telm1–2), the Late Eocene bryozoans from the upper part of the LMF (Telm6–7), are represented by the scarce lepraliomorphs accompanied by the crustaceans, brachiopods and gadiform fish remains. The bryozoan-bearing horizon is composed of the single taxon tentatively assignated to Goodonia terminating the occurrence of the bryozoans, showing a sharp decline in their biodiversity between the lower and upper part of the formation (Hara, 2001), what is consistent with the overall pattern of Eocene cooling up to around 10,5°C in Telm6 and 7.
References
Hara U. 2001 – Bryozoa from the Eocene of Seymour Island, Antarctic Peninsula. Palaeontologia Polonica. III, 60: 33–156.
Hara U. 2015. Bryozoan internal moulds from the La Meseta Formation (Eocene) of Seymour Island, Antarctic Peninsula. PPR, 36, 25-49.
Hara U., 2022 – Geochemistry of the fossil and Recent bryozoan faunas in the natural diagenetic environments and their significance for the reconstruction of biota and climatic regimes in Cenozoic. Archive PGI-NRI, nr. 5210/2022.
Hara U., Mors T., Hagstrom J., Reguero M.A., 2018 – Eocene bryozoans assemblages from the La Meseta Formation of Seymour Island, Antarctica. Geol. Quar., 62: 705–728.
Ivany L.C., Lohmann K.C., Hasiuk F., Blake D.B., Glass A., Aronson R.B., Moody R.M., 2008 – Eocene climate record of the high southern latitude continental shelf: Seymour Island, Antarctica. Geol. Soc. Amer. Bull., 120, 5–6: 659–678.
How to cite: Hara, U.: Palaeoenvironmental and climatic events (EECO-EOT) in the bryozoan fossil records of the Early Cenozoic of Antarctica, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21840, https://doi.org/10.5194/egusphere-egu26-21840, 2026.
The Namib Desert in Southern Africa is likely the world’s oldest desert, experiencing arid to hyperarid conditions for most of the Cenozoic. The desert is inhabited by a unique flora and fauna, some of which has adapted to obtain water from fog, which develops along the Namibian coastline. However, our knowledge of the climatic history of this desert is fragmentary, based on evidence from lithology and geochemistry. Temporal constraints are often provided by biostratigraphy based on fossilised ratite eggshells, which only gives an approximate sequence of events.
In order to test different scenarios for the development of the Namib Desert, we employ FastScape, a landscape evolution model, combined with a model of orographic rainfall. We use this framework to reconstruct Southern African landscape evolution based on different hypotheses arising from geological data, and infer consequential climatic histories, over the last 100 million years. Modern-day remote sensing and weather station data are used to tune and test the fit of the final model timeslice. This allows us to determine which landscape evolution scenarios are most likely, providing novel insights into the onset and evolution of aridity in the Namib Desert.
How to cite: Allen, B., Braun, J., Acevedo-Trejos, E., Böhm, C., and Feulner, G.: How old is the world’s oldest desert? Investigating the coevolution of landscape and climate in the development of the Namib Desert, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-19878, https://doi.org/10.5194/egusphere-egu26-19878, 2026.
The Lothidok Range west of Lake Turkana, Kenya contains a rich paleontological record, including multiple well-preserved Miocene fossil ape taxa. Our work, as part of the West Turkana Miocene Project, seeks to integrate new paleontological surveys with modern tools in geologic mapping, stratigraphic analysis, geochronology, and proxy-based climatic and environmental reconstructions. The Early Miocene Moruorot and Kalodirr localities are well known for fossils of the ape taxa Afropithecus, Turkanapithecus, and Simiolus. Our work at Moruorot demonstrates that these ape taxa were coeval and are preserved in humid alluvial fan complexes. Paleovegetation proxies based on stable carbon isotope ratios in paleosol organic matter (δ13Com = -28 to -31 ‰) and pedogenic carbonates (δ13Cpc = -9 to -12 ‰) are consistent with C3 plants thriving in a forested ecosystem. This interpretation is bolstered by the presence of calcified branches and fruits in lahar deposits. We also use a paleosol bulk geochemical proxy for mean annual precipitation (MAP), which yields values of 1700-1900 mm, which requires intense seasonality of rainfall for pedogenic carbonate stability. In contrast to the Early Miocene paleoenvironments, nearby Middle Miocene deposits at Esha that contain at least one newly discovered fossil ape taxon preserve floodplain paleosols that suggest seasonal woodland conditions (δ13Com = -19 to - 27‰, δ13Cpc = -6.5 to -12 ‰) with a minor fraction of C4 plants in a C3-dominated biome. The paleosol bulk geochemical proxy yields MAP estimates of 500-1000 mm, notably drier than the Early Miocene paleosols. This multi-proxy investigation demonstrates that the West Turkana region experienced drying from the Early to Middle Miocene, and that both time intervals were much wetter than modern conditions. Our ongoing work is focused on refining the stratigraphy and geochronology at both known and newly discovered Early and Middle Miocene sites, and placing systematically collected fossils within a well resolved geological and paleoenvironmental framework across the southern Lothidok Range.
How to cite: Lukens, W., Peppe, D., Cote, S., Rossie, J., Deino, A., Herold, J., Venters, A., Munyaka, V., and Muchemi, F.: Paleoclimate and Paleoenvironments of Early to Middle Miocene strata in West Turkana, Kenya: proxy records of forests, woodlands, and hydroclimate change, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13842, https://doi.org/10.5194/egusphere-egu26-13842, 2026.
The Oligocene–Miocene Transition (OMT) includes a pronounced ~1‰ positive excursion in benthic oxygen isotope records (δ18O), reflecting Antarctic ice sheet expansion and/or deep ocean cooling, commonly referred to as the Mi-1 glaciation. At present, limited reconstructions of sea surface temperature (SST) evolution across the OMT have been published, leaving the magnitude of global cooling during Mi-1 uncertain. Here we present high-resolution (~10 kyr) SST reconstructions from IODP Site U1406 on the Newfoundland Margin (North Atlantic) using the lipid biomarker TEX86 proxy, based on isoGDGT distributions. Our record shows TEX86 values ranging from 0.64 to 0.76, with a ~0.04 decrease during the Mi-1 event. To assess potential non-thermal overprints on the TEX86 data, we calculated GDGT-based indices, including the Branched-to-Isoprenoid Tetraether (BIT) index. BIT values are relatively high (0.4–0.8), suggesting significant input of terrestrial GDGTs that could bias TEX86. However, TEX86 and BIT show weak correlation (R2 = 0.124), indicating limited terrestrial overprint on the TEX86 signal. Furthermore, a ternary plot of brGDGT compositions shows that the Newfoundland samples differ from modern soils and peats, suggesting marine production of brGDGTs as the source of the high BIT values. These findings suggest that the Newfoundland Margin was not influenced by substantial terrestrial organic matter input across the OMT, and that TEX86 provides a reliable record of SST. Translating TEX86 into temperature, our record indicates warm SSTs ranging from 25 to 31 °C, with a cooling of ~2 °C during the Mi-1 event, consistent with published low-resolution alkenone-derived (UK’37) estimates (Guitián et al., 2019). Future work will focus on determining whether the observed SST cooling at Site U1406 reflects a global climate signal or is driven by latitudinal shifts in the North Atlantic SST gradient. This could be addressed using seawater oxygen isotope (δ18Osw) reconstructions based on the combination of SST proxies and planktic foraminiferal δ18O to infer changes in surface ocean circulation, alongside comparisons with Earth System Model simulations.
How to cite: Agterhuis, T., Stoll, H., Tanner, T., Hollingsworth, E., Foster, G., Wade, B., and Inglis, G.: North Atlantic sea surface temperature evolution across the Oligocene–Miocene Transition from TEX86 paleothermometry, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15171, https://doi.org/10.5194/egusphere-egu26-15171, 2026.
During the Cenozoic Era, the ocean's meridional overturning circulation (MOC) has alternated between North Pacific and North Atlantic sinking modes. The Miocene Climatic Optimum (17.0–14.7 Ma) is a key interval for reconstructing this history because there is partial and inconclusive evidence for both MOC modes during this period. Here we investigate the MOC during the Miocene Climatic Optimum using two different climate models, GFDL CM2.1 and ACCESS-ESM1.5. Simulations are forced with atmospheric CO2 levels of pre-industrial concentration (286 ppm), double (572 ppm) and triple (858 ppm) CO2- the latter two falling within proxy-based estimates for this period.
In the GFDL CM2.1 model, we find either North Pacific overturning or North Atlantic overturning modes at all three CO2 levels, depending on the details of the paleogeography. Arctic-Atlantic gateways are especially important in controlling the freshwater balance, and hence surface density, in the North Atlantic sinking regions. By contrast, in the ACCESS-ESM1.5 model, we find that North Atlantic overturning consistently occurs at pre-industrial CO2 only. At double or triple CO2, the model becomes increasingly stratified, leading to a weakening or collapse of the global overturning circulation. The more stratified regimes are linked to a significantly higher climate sensitivity in ACCESS-ESM1.5, with intensified surface buoyancy changes. These markedly different overturning regimes have major implications for deep ocean oxygenation, with the stratified cases becoming largely hypoxic in the deep ocean, while cases with active overturning remain well oxygenated.
How to cite: Hutchinson, D., Meissner, K., Menviel, L., Wright, N., Berg, J., Acosta, P., and Anthonisz, B.: Pacific and Atlantic Modes of Overturning in the Miocene Climatic Optimum, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18315, https://doi.org/10.5194/egusphere-egu26-18315, 2026.
The Antarctic Ice Sheet (AIS) expansion and global cooling during the Mid-Miocene Climate Transition (MMCT) is thought to be closely linked to marine carbon cycle changes. However, how the marine carbon cycle interacted with the rest of the climate system during this period remains elusive. Here, we reconstruct surface-water CO2 and intermediate-depth seawater carbonate chemistry from the Southern Ocean during the MMCT. We show that a marked surface-water CO2 rise in the Southern Ocean, accompanied by carbon loss from the intermediate depths, coincided with AIS retreat and surface Southern Ocean warming within the MMCT. The release of CO2 from the intermediate depths to the surface ocean was likely caused by the northward shift of the Southern Ocean fronts and possibly strengthening of the Southern Ocean overturning circulation. Southern Ocean circulation reorganization, triggered by AIS expansion and global cooling, was able to transiently interrupt the transition of the Earth’s climate into a cooler state during the MMCT.
How to cite: Dai, Y., Hutchinson, D., Yu, J., Bland, S., and Ellwood, M.: Southern Ocean circulation reorganization led to abrupt CO2 outgassing during the Mid-Miocene Climate Transition, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8390, https://doi.org/10.5194/egusphere-egu26-8390, 2026.
Investigating changes in ocean oxygenation during past warm climates advances our process understanding of biogeochemical and physical dynamics in the ocean and may inform our predictions of future changes. The Miocene Climatic Optimum (MCO) was a warm climate episode ~15, million years ago (Ma), with high atmospheric CO2 concentrations that are comparable to end-of-century predictions for mid-range future emission scenarios. Proxy records suggest that the Oxygen Minimum Zone (OMZ) in the Eastern Tropical Pacific (ETP) was small or non-existent during the high-CO2 MCO, and only expanded when CO2 declined after 15 Ma. In contrast, the OMZ in the Eastern Pacific was already extensive in the recent preindustrial era, and is currently expanding further with increasing CO2, due to ocean warming and stratification. Despite the importance of understanding the controls on Pacific OMZ extent under warm conditions, there are no existing model investigations of these opposing OMZ dynamics. Here, we use a climate model with an offline biogeochemical framework to investigate ocean oxygen concentrations during the Miocene for a range of CO2 concentrations and two different topographic configurations. We compare results to available physical and biogeochemical proxies and assess which combination of boundary conditions best replicates recorded proxy trends. We find that for higher CO2 concentrations, oxygen declines globally and OMZs expand, particularly in the Atlantic Ocean. However, for one of the topographic configurations, OMZs in the ETP contract under higher CO2 concentrations. This contraction can be attributed to regionally reduced export production and remineralization rates, which are caused by weaker upwelling due to a southward shifted Hadley cell and correspondingly weaker southern hemisphere trade winds. This atmospheric response is driven by hemispheric asymmetries in warming due to changes in large scale ocean circulation. These results emphasize the complexity and spatial heterogeneity of the marine oxygen response to climate change.
How to cite: Berg, J., Hutchinson, D., Meissner, K., Pasquier, B., Holzer, M., and Auderset, A.: Simulated Ocean Oxygen under Miocene Boundary Conditions, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5952, https://doi.org/10.5194/egusphere-egu26-5952, 2026.
The mid and late Pleistocene are marked by large-amplitude fluctuations in global ice volume and pronounced climatic variability. Around ~1 Ma, Earth’s climate system underwent a fundamental reorganization, as glacial–interglacial variability shifted from predominantly 41-kyr cycles to higher-amplitude, quasi-100-kyr oscillations. This transition was accompanied by enhanced atmospheric CO2 drawdown during glacial periods. However, how the global carbon cycle adjusted to this shift, and which reservoirs account for the lowered glacial atmospheric CO2 concentrations, remains not fully quantitatively constrained. In this context, marine carbon burial, particularly on continental shelves, represents a potentially important yet underexplored long-term sink for atmospheric CO2.
Here, we quantify variability in organic and carbonate carbon burial on the West Australian shelf and evaluate its potential contribution to Pleistocene atmospheric CO2 drawdown. We measured δ¹³C and calculated relative burial fractions and mass accumulation rates for organic and carbonate carbon in sediments recovered from IODP Expedition 356 Site U1460 (27°22′S, 112°55′E), spanning the last ~210 kyr (MIS 7–MIS 1). The site was drilled at ~214 m water depth in the northern Perth Basin and is situated in a dynamic oceanographic setting influenced by the interaction between the warm, oligotrophic Leeuwin Current (LC) and the cooler, nutrient-rich West Australian Current (WAC).
Our results reveal two pronounced maxima in organic carbon burial relative to carbonate during glacial MIS 6 (~168 ka) and MIS 2 (~26 ka), as well as a more moderate increase at ~109 ka across the MIS 5a–d to MIS 5e transition. These patterns are consistent with previous suggestions of enhanced shelf organic carbon burial during glacial periods (Auer et al., 2021). Variations in organic-to-carbonate burial ratios are paced by eccentricity-modulated glacial–interglacial sea-level changes and Milankovic-driven shifts in seasonality, both of which influence the strength of the LC and its interaction with the WAC. High sea level and enhanced seasonality strengthen the LC, restricting nutrient supply to the West Australian shelf. Conversely, low sea level and reduced seasonality weaken the LC, allowing the nutrient-rich WAC to dominate, thereby enhancing primary productivity and organic carbon burial.
Finally, we use organic carbon mass accumulation rates to place first-order constraints on the potential for carbon storage on the West Australian shelf during Late Pleistocene glacials. Although organic carbon burial increased during glacial intervals, limited accommodation space on the shelf likely restricted total organic carbon accumulation, preventing it from exerting a major influence on global glacial–interglacial atmospheric CO₂ variability.
How to cite: V. Del Gaudio, A., M. Bialik, O., Auer, G., and De Vleeschouwer, D.: Variability and controls of organic and carbonate carbon burial on the West Australian shelf during the Late Pleistocene, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-19757, https://doi.org/10.5194/egusphere-egu26-19757, 2026.
Methane seeping from the submarine has long been recognized as a driver of climate warming, owing to its oxidation that emits carbon dioxide to the atmosphere. Yet, the biogeochemical processes that transfer methane to organic carbon (OC), serving as a negative feedback on warming, remain largely under constrained. Here, we measured concentration and stable and radiocarbon isotopes of the dissolved and sedimentary OC, as well as foraminifera, across contemporary and past methane seepage settings. Our findings reveal that methane undergoes transformation into OC, promoting its long-term burial in sediments and mitigating climate change. At active methane seeps in the South China Sea, methane contributes up to 23% of dissolved OC in the contemporary bottom water. And our results suggest that methane may be emitted to the water column ~700 m above the seafloor during the Last Glacial Maximum, and subsequently undergoes transformation into OC buried in sediments. It accounts for up to 11% of methane-derived OC burial during the Last Glacial Maximum with active methane seepage events, and reduces the radiative forcing caused by methane emission over glacial cycles. Our discovery of the enhanced methane carbon burial calls for reconsideration of methane’s impact on climate warming.
How to cite: Dong, L., Chu, M., and Bao, R.: The transformation and burial of methane-derived organic carbon in the South China Sea, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4416, https://doi.org/10.5194/egusphere-egu26-4416, 2026.
The last glacial cycle is a key period in the environmental and cultural history of the Australian continent, yet the climate of this time period remains poorly understood. Conflicting evidence from spatially disparate lacustrine records and discontinuous fluvial archives have hindered consensus on environmental change during this period. Here, we present two new, highly resolved organic sedimentary records from the Thirlmere Lakes (NSW) and Minjerribah (North Stradbroke Island, QLD) regions of eastern Australia that provide new constraints on long-term climate and environmental variability through the last glacial cycle.
Australian aquatic systems often deviate from biogeochemical frameworks developed largely from Northern Hemisphere environments. The prevalence of low-nutrient conditions results in unusual carbon isotope signatures, complicating the identification of organic carbon sources and their transport between terrestrial and aquatic reservoirs. Through characterisation of modern aquatic carbon isotopes, we develop alternative threshold values for distinguishing organic matter sources and, in turn, demonstrate the utility of sedimentary stable carbon isotopes as robust tracers of environmental and climatic change in southern mid-latitude systems.
Applying these newly developed isotope thresholds, we reconstruct millennial-scale climate variability in eastern Australia from Marine Isotope Stage 5 to the present. The resulting records reveal strong coupling between regional carbon cycling and Southern Hemisphere high-latitude climate, with limited evidence for Northern Hemisphere forcing. These findings highlight the importance of regionally calibrated carbon isotope frameworks and demonstrate the value of stable carbon isotopes for reconstructing past Earth system change in under-represented Southern Hemisphere environments.
How to cite: Cadd, H., Tibby, J., Tyler, J., Barr, C., Forbes, M., Leng, M., Mariani, M., Moss, P., Cohen, T., Li, B., Marx, S., Mazumder, D., Kobayashi, T., and Boesl, F.: Long-term climate dynamics and carbon cycling in eastern Australian from MIS5 to present, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15355, https://doi.org/10.5194/egusphere-egu26-15355, 2026.
Semi-arid and arid regions have traditionally been regarded as peripheral to the global carbon cycle because of their presumed low silicate weathering rates, resulting in their systematic omission from long-term carbon budget assessments. Direct quantification on CO₂ consumption by silicate weathering (CO₂(SIW)) in eolian-dominated drylands, however, remains scarce. Here we reconstruct both silicate weathering rate (RCO₂) and annual CO₂ consumption (CO₂(SIW)) flux using red clay and loess–paleosol sequences from the Chinese Loess Plateau (CLP). We demonstrate that variability in eolian mass accumulation rate (MAR), rather than intrinsic silicate weathering intensity (RCO₂), exerted the primary control on CO₂(SIW), reflecting persistently low to moderate chemical weathering across the CLP. Our results further reveal a rise in CO₂(SIW) from ~3.3 Tg C yr⁻¹ to ~12.3Tg C yr⁻¹ between 4.0 and 1.0 Ma, followed by a subsequent decline to ~9.0 Tg C yr⁻¹, broadly coincident with the late Pliocene decrease in atmospheric CO₂... These findings provide the first long-term quantitative budget of silicate weathering–mediated CO₂ drawdown in drylands and highlight the previously underrecognized role of semi-arid and arid eolian systems as negative feedback on atmospheric CO₂ over both million-year and orbital timescales.
How to cite: Zhang, C., Wu, H., Qiao, Y., and Guo, Z.: Quantification of silicate weathering CO2 consumption in semi-arid and arid eolian-dominated regions since the late Pliocene, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10497, https://doi.org/10.5194/egusphere-egu26-10497, 2026.
Loess deposits are silt sediments that can contain up to 30% carbon. As they are transported into the ocean, whether by wind or by rivers, they can increase the ocean's alkalinity. Recent studies have reported that accounting for the carbon weathering of loess under glacial conditions could increase the global alkalinity flux by more than 50% compared with previous estimates. This, in turn, could lower atmospheric CO2 concentrations by increasing the ocean's buffering capacity. To test this hypothesis in a transient Earth System Modeling framework and quantify the role of loess weathering in orbital-scale global carbon reorganizations, we employed the cGENIE model, nudged the ocean circulation state to a previously conducted transient 3 Ma CESM1.2 simulation, and applied various loess weathering scenarios. Our results suggest that plausible estimates of loess-derived carbon fluxes can explain a considerable fraction of interglacial/glacial CO2 variability during the last 1 Ma.
How to cite: Ishizu, M., Timmermann, A., and Yun, K.-S.: Loess weathering as an important contributor to the glacial atmospheric pCO2 drawdown, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6267, https://doi.org/10.5194/egusphere-egu26-6267, 2026.
During glacial periods, atmospheric CO2 concentrations are known to have been about 90 ppmv lower than during interglacial. However, climate models have not been able to fully reproduce this decrease, partly due to large uncertainties in changes in ocean physical fields. In this study, we evaluate the impact of uncertainties in ocean physical fields on atmospheric pCO2 during the Last Glacial Maximum (LGM) using a single offline ocean biogeochemical model forced by 12 ocean physical states derived from PMIP.
The simulated glacial atmospheric pCO2 reduction is 40.3 ± 7.8 ppmv on average, with a large inter-model spread. This reduction mainly comes from the SST-dependent solubility effect (−30.1 ± 5.6 ppmv) and the enhanced efficiency of the organic matter pump (−21.6 ± 6.6 ppmv), cancelled somewhat by the response of the gas-exchange pump (+6.2 ± 9.4 ppmv). Our analysis suggests that the enhanced efficiency of the organic matter pump is associated with the older deep-water age in the glacial ocean and the response of the gas-exchange pump appears controlled by the SST contrast between the North Atlantic and the Southern Ocean.
We find that models with older radiocarbon deep-water ages exhibit more efficient sequestration of carbon transported by the organic matter pump into the deep ocean, leading to a larger glacial reduction in atmospheric pCO2. However, all models used in this study underestimate the deep-water radiocarbon ages suggested by Δ14C paleoclimate records. In addition, both the global mean SST and the global mean ocean temperature are tend to be underestimated in the model compared to paleoclimate proxy reconstructions, leading to the smaller contribution of the SST-dependent solubility effect to the pCO2 reduction. If such model biases (i.e. underestimation of deep-water ages and the SST cooling) are corrected, we estimate that the corrected model estimate of the glacial pCO2 reduction becomes up to ~65ppmv which is still not enough for 90 ppmv reduction obtained from ice core record. Our results imply that the improvement in the reproducibility of the glacial ocean physical field alone are insufficient to fully account for the glacial atmospheric CO2 reduction and further improvements in the representation of ocean biogeochemical processes are also required under constraints including carbon isotope records.
How to cite: Nishida, M., Oka, A., and Kobayashi, H.: Impact of Ocean Physical Conditions on Ocean Carbon Pumps and Atmospheric CO2, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16243, https://doi.org/10.5194/egusphere-egu26-16243, 2026.
Coccolithophores, calcifying marine phytoplankton, play a dual role in the oceanic
carbon cycle by contributing to carbon fixation through photosynthesis and to carbon
release via calcification (uptake of bicarbonate and release of CO2). To evaluate the net
effect of coccolithophore long-term evolutionary and productivity dynamics on the car-
bon cycle, we analyzed two sediment cores, MD96-2060 (Mozambique Channel) and MD97-
2125 (Coral Sea), spanning the past 1 Myr. Using automated light microscopy and im-
age recognition, we quantified coccolithophore assemblages, morphology, and calcite mass.
These data were complemented by stable isotope analyses (δ13C and δ18O) of coccolith-
dominated the fine fraction (< 30 µm,) sediment samples. Our results reveal pronounced
coccolithophore bloom phases, characterized by high abundances of Gephyrocapsa caribbean-
ica and Emiliania huxleyi, and sharp increases in total Noelaerhabdaceae mass accumu-
lation rate. The Morphological Divergence Index, a proxy for evolutionary divergence,
exhibits similar long-term trends at both sites, in phase with orbital eccentricity cycles.
Fine-fraction δ13C records display long-term patterns that are absent in benthic and plank-
tonic foraminiferal δ13C records, indicating a persistent coccolithophore-driven isotopic
signal. We interpret this signal as the result of species-specific vital effects in dominant
blooming taxa, particularly during periods of low eccentricity, when reduced ecological
niche partitioning may have favored the proliferation of smaller more cosmopolitan species.
This, in turn, may have led to a significant depletion in δ13C values of the fine fraction
during low eccentricity phases, thereby influencing the marine carbon cycle on orbital
timescales.
How to cite: Dauvier, J., Beaufort, L., Sonzogni, C., Bolton, C., Mazur, J. C., Kazuyo, T., Rapuc, W., Thouveny, N., Lichterfeld, Y., and Vidal, L.: Isotopic Imprints of Coccolithophore Blooms Overthe Past Million Years, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8129, https://doi.org/10.5194/egusphere-egu26-8129, 2026.
We use benthic isotope data from 491 sediment cores compiled in the World Atlas of late Quaternary Foraminiferal Oxygen and Carbon Isotope Ratios (Mulitza et al., 2022) to evaluate transient simulations across the last 25 kyr performed with BICYCLE-SE, the solid Earth version of the Box model of the Isotopic Carbon cYCLE (Köhler & Mulitza, 2024), which have been updated by data-based constraints on the deglacial release of ~250 PgC from land via permafrost thaw (Winterfeld et al., 2018), extensive petrogenic organic carbon oxidation (Wu et al., 2022) and biomass burning (Riddell-Young et al., 2025). These additional land carbon fluxes reduce mean ocean δ13C by 0.1‰ since the Last Glacial Maximum (LGM). The increase in mean ocean δ13C is 0.45‰ since the LGM in both data and model, but the rise started only after Heinrich Stadial 1 in the data, but earlier in the simulations. Abrupt reductions in Atlantic Meridional Overturning Circulation during Greenland stadials as suggested from 14C (Köhler et al., 2024) lead to simulated anomalies in δ13C in most ocean boxes, that are not confirmed by the δ13C data. Further model-data offsets suggest that the so far applied assumptions on changes in the Southern Ocean physical and biological carbon pumps during the deglaciation in BICYCLE-SE might need to be revised – or point to the limitations of this simple box model approach.
References:
Köhler, P. and Mulitza, S.: No detectable influence of the carbonate ion effect on changes in stable carbon isotope ratios (δ13C) of shallow dwelling planktic foraminifera over the past 160kyr, Clim. Past, 20, 991–1015, https://doi.org/10.5194/cp- 20-991-2024, 2024.
Köhler, P., Skinner, L. C., and Adolphi, F.: Radiocarbon cycle revisited by considering the bipolar seesaw and benthic 14C data, Earth Planet. Sc. Lett., 640, 118801, https://doi.org/10.1016/j.epsl.2024.118801, 2024.
Mulitza, S., Bickert, T., Bostock, H. C., Chiessi, C. M., Donner, B., Govin, A., Harada, N., Huang, E., Johnstone, H., Kuhnert, H., Langner, M., Lamy, F., Lembke-Jene, L., Lisiecki, L., Lynch- Stieglitz, J., Max, L., Mohtadi, M., Mollenhauer, G., Muglia, J., Nürnberg, D., Paul, A., Rühlemann, C., Repschläger, J., Saraswat, R., Schmittner, A., Sikes, E. L., Spielhagen, R. F., and Tiedemann, R.: World Atlas of late Quaternary Foraminiferal Oxygen and Carbon Isotope Ratios, Earth Syst. Sci. Data, 14, 2553–2611, https://doi.org/10.5194/essd-14-2553-2022, 2022.
Riddell-Young, B., Lee, J. E., Brook, E. J., Schmitt, J., Fischer, H., Bauska, T. K., Menking, J. A., Iseli, R., and Clark, J. R.: Abrupt changes in biomass burning during the last glacial period, Nature, 637, 91–96, https://doi.org/10.1038/s41586-024-08363-3, 2025.
Winterfeld, M., Mollenhauer, G., Dummann, W., Köhler, P., Lembke-Jene, L., Meyer, V. D., Hefter, J., McIntyre, C., Wacker, L., Kokfelt, U., and Tiedemann, R.: Deglacial mobilization of pre-aged terrestrial carbon from degrading permafrost, Nature Communications, 9, 3666, https://doi.org/10.1038/s41467-018-06080-w, 2018.
Wu, J., Mollenhauer, G., Stein, R., Köhler, P., Hefter, J., Fahl, K., Grotheer, H., Wei, B., and Nam, S.-I.: Deglacial release of petrogenic and permafrost carbon from the Canadian Arctic impacting the carbon cycle, Nature Communications, 13, 7172, https://doi.org/10.1038/s41467-022-34725-4, 2022.
How to cite: Köhler, P. and Mulitza, S.: Data-model comparison of marine 13C across Termination I , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3078, https://doi.org/10.5194/egusphere-egu26-3078, 2026.
The efficiency of the ocean to store or release gases, such as carbon, is mainly governed by overturning circulation and air-sea gas exchanges, thereby it regulates the carbon dioxide (CO2) sequestration in the ocean interior and its subsequent outgassing. Changes in the ocean circulation are considered as one of the primary drivers of atmospheric CO2 fluctuations during the last glacial-interglacial cycle. Although Indian Ocean role plays an important role in the global ocean circulation, its role in carbon cycle during the last glacial termination remains scantily studied. In this study, the ventilation records from the northern Indian Ocean over the last 25 kyr has been compiled and examined, where the ventilation ages are calculated as the difference between the radiocarbon ages of coexisting benthic and planktic foraminifera. The most notable feature from our result is the stratification between the intermediate and deep water of the northern Indian Ocean during the Last Glacial Maximum (LGM). During this period, the water mass at a depth of ~2000 m below was poorly ventilated, characterized by low-14C, enrich in CO2 and high ventilation ages exceeding 2000 14C years. In contrast, the reported ventilation ages of water mass above ~2000 m depth were low (~1400 14C years) indicating relatively better ventilated water. This strong vertical stratification between the water masses implies a reduced renewal of deep water in the northern Indian Ocean during the LGM, suggesting that the northern Indian Ocean basin was a part of the glacial ocean aged carbon pool. The condition changed to better-ventilated water during the deglaciation, probably due to increased contribution of the northern sourced deep water to the northern Indian Ocean and outgassing the glacially stored CO2.
How to cite: Kumari, N. and Naik, S.: Radiocarbon evidence for the last glacial-interglacial ventilation changes in the northern Indian Ocean, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8941, https://doi.org/10.5194/egusphere-egu26-8941, 2026.
Reconstructions of atmospheric radiocarbon during the Laschamps geomagnetic excursion show a pronounced increase in Δ14C. The amplitude of this increase remains poorly reproduced by current carbon cycle models driven by independent 14C-production rates derived from 10Be ice-core records or geomagnetic field intensity reconstructions. This mismatch has commonly been attributed to uncertainties in cosmogenic 14C production rates, potentially arising from the underestimation of global production-rate changes in polar ice-core 10Be records during periods of strongly reduced geomagnetic field intensity.
Here we present a new global compilation of 10Be records from ice cores and marine sediments spanning the Laschamps event, providing an improved, globally integrated estimate of cosmogenic nuclide production for the period from 30,000 to 60,000 years BP. This compilation overcomes previous limitations of polar-only ice-core records, is more representative of global production, and is consistent with latest geomagnetic field intensity reconstructions. However, while the revised production rate implies larger 14C production-rate changes than previous estimates, it remains insufficient to reproduce the full amplitude of the observed Δ14C increase when implemented in carbon cycle models under conservative parameterization.
Using transient tuning of a simple carbon cycle model, we show that the remaining model–data mismatch is closely linked to signals observed in independent climate proxies, in particular ice-core δ18O records. This similarity suggests that the interactions between climate changes and carbon cycle dynamics during the glacial period are not adequately represented in current models.
Our results indicate that uncertainties in cosmogenic production alone cannot explain the radiocarbon anomaly associated with the Laschamps event. Instead, they point to a need for improved representations of climate–carbon cycle interactions under glacial conditions. This finding highlights the importance of revisiting carbon cycle dynamics, including carbon reservoir sizes, exchange rates, and circulation changes, in glacial climates, and demonstrates the value of globally integrated cosmogenic isotope records for disentangling production and carbon cycle effects in past radiocarbon variations.
How to cite: Wall, V., Lamy, F., Lembke-Jene, L., Lachner, J., Winkler, S., and Adolphi, F.: Revisiting radiocarbon production and the glacial carbon cycle during the Laschamps geomagnetic excursion, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13087, https://doi.org/10.5194/egusphere-egu26-13087, 2026.
Magnetotactic bacteria produces biogenic magnetite in marine environments with low oxygen (O2) concentrations. These conditions are typical of past global warming events, which has led to generation of biogenic magnetite records that have been interpreted as proxies for O2 variability. However, biogenic magnetite is still poorly studied and there are no records of this mineral in land-based sections. Here, we present a new biogenic magnetite record for the Palaeocene-Eocene Thermal Maximum interval of the land-based Contessa Road section (Gubbio, Italy). We quantified biogenic magnetite in the marine sedimentary rocks of Contessa Road with new geochemical, rock magnetic and electron microscopy data, which indicate that biogenic magnetite contents increase during the PETM body phase and reduce in coincidence with the PETM recovery. These patterns are similar to those of the stable carbon/oxygen isotopes, and reveal warming-induced deoxygenation in the Contessa Road setting in the PETM peak phase, and gradual marine reoxygenation during the PETM interval of carbon uptake. Our results are compared to a new model that confirms strong coupling between the carbon and oxygen cycles during the PETM.
How to cite: Piedrahita, V., Roberts, A., Rohling, E., Heslop, D., Galeotti, S., Florindo, F., Yan, L., and Li, J.: Biogenic magnetite reveals marine deoxygenation during the Paleocene-Eocene Thermal Maximum, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-22682, https://doi.org/10.5194/egusphere-egu26-22682, 2026.
The terrestrial carbon cycle has long been discussed under a framework that focuses on inorganic carbon (i.e. the balance between solid Earth degassing and silicate weathering). Therefore, the role of organic carbon has remained poorly constrained in both the present and past. A recent study highlighted the importance of rock-derived “petrogenic” organic carbon (OCpetro), suggesting that the amount of CO2 released during the exhumation and mobilisation of OCpetro may be comparable to that from volcanism. To determine the response of OCpetro to future climate change, warming events in the geologic record can be investigated. For example, there are biomarker-based evidence for up to an order-of-magnitude increase in the burial of OCpetro in shallow-marine sediments dated to the Paleocene-Eocene thermal maximum (PETM; ∼56 Ma). However, estimates of the proportion of OCpetro lost via oxidation are unavailable due to the lack of suitable techniques.
Raman spectroscopy assesses differences in the crystallinity of OCpetro, allowing the distinction between graphitised and disordered carbon. Modern river systems have shown a shift towards a dominance of graphite downstream, as disordered carbon are more susceptible to oxidation. Here, we explore whether Raman spectroscopy can be used to reconstruct OCpetro oxidation in the past. During the PETM, there is an increase of graphite in the mid-Atlantic Coastal Plain, indicating enhanced OCpetro oxidation. This is consistent with signs of intensified physical erosion and enhanced OCpetro delivery. On the other hand, the distribution of graphitised carbon vs. disordered carbon (and biomarkers) do not change in the Arctic Ocean, implying spatial variability. This study demonstrates, for the first time, the utility of Raman spectroscopy as a novel tool to evaluate OCpetro oxidation in a geological context. Applying this approach to quantify oxidation rates require further ground truthing in settings with different degrees of weathering.
How to cite: Hollingsworth, E., Sparkes, R., Self-Trail, J., Foster, G., and Inglis, G.: The oxidation of petrogenic organic carbon: a source of CO2 during transient warming events?, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21846, https://doi.org/10.5194/egusphere-egu26-21846, 2026.
Globally distributed data from the Last Glacial Maximum (LGM) indicate a significant depletion of radiocarbon in the ocean, equivalent to ~800 14Cyrs. Some interpretations of these data have emphasized a slow-down of the North Atlantic overturning, as well as a reduction or even ‘reversal’ of overturning in the North Pacific. While many model simulations have been able to produce a shoaled and weakened circulation in the Atlantic under glacial conditions, many others (and many of the same) produce a stronger overturning overall and in the Pacific. If the glacial ocean circulation was indeed stronger, despite reduced radiocarbon ventilation, it would constrain the balance of contributions from marine ‘respired’ and ‘disequilibrium’ carbon pools to glacial atmospheric CO2 drawdown. Here we show that global marine radiocarbon fields from the LGM and deglaciation are not consistent with the modern transport when taking into account past air-sea equilibration changes at the sea surface. Rather, they imply a reduced and/or shoaled transport in the North Atlantic (consistent with most interpretations to date), and an enhanced transport throughout the Pacific. Although the latter conflicts with some previous interpretations of LGM North Pacific radiocarbon data, it coheres with several key model simulations in suggesting an overall ‘faster’ glacial mass turnover despite weaker exchange of CO2 between the ocean and atmosphere. This would emphasize the role of the disequilibrium carbon pool (and therefore ocean-atmosphere gas-exchange, influenced by upper ocean mixing, sea ice etc.) in determining the overall ocean’s overall sequestered carbon inventory during the last glacial period.
How to cite: Skinner, L. and Primeau, F.: Enhanced ocean transport despite reduced radiocarbon ventilation at the Last Glacial Maximum: were the models right all along?, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21691, https://doi.org/10.5194/egusphere-egu26-21691, 2026.
Ocean productivity is highly sensitive to climate change, but its future trend remains largely unknown, complicating projections for marine ecosystems and fisheries. Past warm climate events offer valuable analogs for understanding the long-term effects of anthropogenic warming on ocean productivity. The Paleocene–Eocene Thermal Maximum (PETM, about 56 million years ago) is one of the most pronounced global warming events in the Cenozoic era, triggered by massive and rapid injections of isotopically light carbon into the ocean-atmosphere system. While previous studies have evaluated ocean productivity during the PETM, proxy records and model results remain contradictory, and the response of fish productivity is also poorly constrained. Here we present global records of ichthyolith accumulation rates (IAR) from deep-sea sediment cores across the PETM. Our new data show the temporal and spatial evolution of pelagic fish productivity as well as the resilience in fish communities. These IAR data are then compared with export productivity estimates derived from marine barite accumulation rates (BAR) from the same or proximal sites to explore their correlation. Using the Earth system model cGENIE, we further conduct sensitive simulations to investigate the roles of elevated atmospheric pCO2, changes in nutrient supply (internal and external), and ocean circulation in driving carbon export during the PETM. Through combining multi–proxy and model–informed analyses, this study provides an integrated perspective on how ocean productivity and fish communities reacted to abrupt warming, offering a critical long-term context for understanding the future of ocean ecosystems.
How to cite: Zhou, X., Zhang, R., Tsang, M.-Y., and Yao, W.: Reconstructing pelagic fish productivity and export productivity during the Paleocene-Eocene Thermal Maximum, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4725, https://doi.org/10.5194/egusphere-egu26-4725, 2026.
Tides are the main energy source for diapycnal mixing in the ocean interior. However, energy-constrained tidal mixing parameterizations are not routinely included in ocean models applied to the deep-time past of the Earth. Instead, diapycnal mixing is usually parameterised by a constant vertical diffusivity or a prescribed vertical profile of vertical diffusivity.
Here, by leveraging outputs from the DeepMIP project, we compute the power effectively consumed by parameterized diapycnal mixing in each DeepMIP model and for different CO2 concentrations. We show that this power slightly increases with increasing CO2 in simulations integrated to quasi-equilibrium but skyrockets in warming, out-of-equilibrium, simulations. This reflects the increased stratification in a warming ocean, even though in principle the same amount of tidal energy is available for mixing. We find no evident relationships between the intensity of the overturning circulation and the power consumed by diapycnal mixing across the DeepMIP models. Finally, we use coupled climate-biogeochemistry simulations performed with the IPSL-CM5A2 model to show that the marine biogeochemistry is largely impacted by the vertical mixing scheme employed, even if the total power consumed by diapycnal mixing remains similar.
How to cite: Ladant, J.-B., de Lavergne, C., Chan, W.-L., Hutchinson, D., Lunt, D., and Zhu, J.: Diapycnal mixing in the Early Eocene: insights from the DeepMIP intercomparison project phase 1, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18913, https://doi.org/10.5194/egusphere-egu26-18913, 2026.
Proxy records from the Miocene epoch (∼23‐5 Ma) indicate a warmer climate than today with a reduced meridional temperature gradient. These characteristics have been partly attributed to atmospheric CO2 changes and differences in the tectonic setting. In this contribution we present climate simulations using the complex coupled earth system model AWI-ESM2 for Miocene boundary conditions to investigate the impact of different atmospheric CO2 concentrations and paleogeographic configurations. Besides investigating their individual contribution, we will also examine the combination of both forcing factors and differences that arise from different orographic and bathymetric reconstructions. We will discuss implications for global and meridional temperature responses, as well as sea ice changes and high latitude ocean ventilation.
How to cite: Knorr, G. and Lohmann, G.: The impact of paleogeography and atmospheric CO2 concentrations on Miocene warmth in AWI-ESM, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5416, https://doi.org/10.5194/egusphere-egu26-5416, 2026.
Changes in the geological carbon cycle and associated surface input and output CO2 fluxes drive long-term Cenozoic climate trends mainly through magmatic emissions and weathering of silicate minerals. Proxy records, which indirectly reconstruct past climate conditions, demonstrate a steady decline in both surface CO2 and temperature since ˜50 million years ago (Ma), punctuated by shorter periods of climatic optima and hyperthermals such as the PETM, EECO, MECO and MMCO. However, lack of constraints in terms of input and output CO2 fluxes prevents the assessment of responsible processes for these trends. Here, we use a newly developed technique based on a reversible-jump Markov chain Monte Carlo algorithm (rj-McMC) to invert the temporal CO2 changes from the Proxy Integration Project (CENCO2PIP) (Hönisch et al., 2023) and obtain estimates of the surface input and output CO2 fluxes throughout the Cenozoic. We base the inversion on a general formulation of the geological carbon cycle that includes a degassing source and a temperature-dependent sink term, with the temperature time history (Hansen et al., 2023) used as an additional constraint. Reconstructed fluxes reveal that perturbations of the carbon cycle are stronger during the early Cenozoic (i.e., ˜66 – 34 Ma), while these reduce since˜34 Ma. We hypothesise that stronger degassing from the solid-Earth during the EECO and MECO prevent an earlier onset of the Antarctic ice cap during the Eocene. We discuss that the higher carbon emissions during these periods can partially link to the evolution of the Neo-Tethyan magmatic margin, which extinction occurs ˜34 Ma. Results show that carbon flux stabilization since the Oligocene could be due to temperature dependent processes like albedo increase and enhanced silicate weathering in the context of Tibetan Plateau uplift. Finally, we estimate that the net amount of CO2 removed since ˜34 Ma is four times greater than that of the first half of the Cenozoic.
References
Hansen, J. E., Sato, M., Simons, L., Nazarenko, L. S., Sangha, I., Kharecha, P., Zachos, J. C., von Schuckmann, K., Loeb, N. G., Osman, M. B., Jin, Q., Tselioudis, G., Jeong, E., Lacis, A., Ruedy, R., Russell, G., Cao, J., & Li, J. (2023). Global warming in the pipeline. Oxford Open Climate Change, 3(1). https://doi.org/10.1093/oxfclm/kgad008.
Hönisch, B., Royer, D. L., Breecker, D. O., Polissar, P. J., Bowen, G. J., Henehan, M. J., Cui, Y., Steinthorsdottir, M., McElwain, J. C., Kohn, M. J., Pearson, A., Phelps, S. R., Uno, K. T., Ridgwell, A., Anagnostou, E., Austermann, J., Badger, M. P. S., Barclay, R. S., Bijl, P. K., … Zhang, L. (2023). Toward a Cenozoic history of atmospheric CO2. Science, 382(6675). DOI: 10.1126/science.adi517.
How to cite: Castrogiovanni, L., Pasquero, C., Piana Agostinetti, N., Vaes, B., Longman, J., and Sternai, P.: Input and output fluxes of surface CO2 throughout the Cenozoic, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5914, https://doi.org/10.5194/egusphere-egu26-5914, 2026.
Radiocarbon (Δ14C) and stable carbon isotope (δ13C) proxy records provide important constraints on how carbon is redistributed among Earth’s surface reservoirs during major climate transitions. Previous work by Kobayashi et al. (2024, Climate of the Past) showed that transient simulations with the MIROC 4m climate model reproduce the timing of deglacial atmospheric pCO2 changes but underestimate their magnitude. Here, we extend this analysis by using carbon isotope proxies to better diagnose ocean carbon cycle processes during the last deglaciation (21 to 11 ka BP).
We combine three-dimensional transient model output with marine sediment core and ice core records of Δ14C and δ13C to examine how changes in ocean ventilation, biological carbon export efficiency, and alkalinity cycling are reflected in carbon isotope budgets. Particular attention is given to Heinrich Stadial 1 (HS1), the Bølling–Allerød, and the Younger Dryas, periods characterized by abrupt changes in the Atlantic Meridional Overturning Circulation (AMOC) and pronounced interhemispheric climate asymmetry.
We analyze three-dimensional transient model output and compare the results with existing marine sediment core and ice core records of Δ14C and δ13C. This comparison is used to examine how changes in ocean circulation and biological carbon export and remineralization are expressed in carbon isotope budgets. We focus on Heinrich Stadial 1 (HS1), the Bolling-Allerod, and the Younger Dryas, periods associated with abrupt changes in the Atlantic Meridional Overturning Circulation (AMOC) and strong interhemispheric climate asymmetry.
The model reproduces the sequence of atmospheric pCO2 variations across these events, but comparisons with proxy data reveal a systematic underestimation of enhanced deep-ocean ventilation during HS1, particularly in the Southern Ocean and North Pacific, as indicated by marine Δ14C records. Stable carbon isotope data further suggest that reductions in biological carbon export efficiency during HS1 are weaker in the model than implied by benthic and planktonic δ13C records. During the Younger Dryas, proxy records indicate a continued increase in deep-ocean δ13C, whereas the model simulates an opposite trend, pointing to potential biases in simulated AMOC changes, ecosystem responses, or terrestrial carbon exchange.
Overall, radiocarbon and stable carbon isotope comparisons indicate that the redistribution of carbon within the ocean is underestimated in the model. To further investigate these discrepancies, we additionally report sensitivity experiments that revisit the initialization of the Last Glacial Maximum state and assess the respective roles of ocean circulation and the biological pump in shaping deglacial carbon isotope and atmospheric pCO2 evolution.
How to cite: Kobayashi, H., Oka, A., Obase, T., Nishida, M., and Abe-Ouchi, A.: Diagnosing deglacial ocean carbon cycle change through radiocarbon and stable carbon isotopes, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8803, https://doi.org/10.5194/egusphere-egu26-8803, 2026.
Antarctic ice core evidence indicates that atmospheric CO2 levels increased during Heinrich Stadial (HS) 1 and the Younger Dryas (YD) during the last deglaciation. A substantial fraction of this carbon is believed to have stemmed from the ocean interior, released, in part, through enhanced wind-driven upwelling and air-sea CO2 exchange in the Southern Ocean. This was highlighted by two deglacial opal flux peaks identified in sediment core TN057-13-PC4 (53.17 °S, 5.13 °E, 2818 m water depth) from the Atlantic Southern Ocean south of the Polar Front, proximal to the Antarctic Divergence Zone (Anderson et al., 2009). However, there is limited information on changes in deep-ocean 14C ventilation and surface ocean hydrography in the Atlantic Antarctic Divergence, and their role in atmospheric CO2 variations during these two periods of deglacial CO2 rise. Here, we provide a new set of 12 mixed-benthic and 63 planktonic foraminiferal (i.e., Neogloboquadrina pachyderma) 14C ages obtained with a MIni-CArbon-DAting-System (MICADAS) in sediment core TN057-13-PC4, along with high-resolution multi-proxy (sub-)sea surface temperature reconstructions for the same site (N. pachyderma Mg/Ca ratios, TEX86, diatom assemblages). Our data help better constrain the nature, timing, and impacts of deep-ocean upwelling on surface ocean hydrography and on atmospheric CO2 exchange near the Antarctic Divergence of the Southern Ocean. Our data show strong (sub-)surface warming in the Antarctic Divergence during HS1 and YD that is accompanied by a rapid decline in benthic-minus-planktic 14C ages towards mean Holocene values at the onset of the deglaciation. We also observe millennial-scale increases in seawater d18O (paired N. pachyderma Mg/Ca-d18O analyses), hence local surface salinity and marked variations in 14C surface ocean reservoir ages that parallel changes in Antarctic sea ice extent. This corroborates previous evidence indicating increased upwelling of Circumpolar Deep Water in the Atlantic Antarctic Divergence during HS1 and YD, yet suggests an onset of strong Southern Ocean ventilation earlier than what is expected from increases in opal fluxes alone. Our data support a fundamental role of upwelling and CO2 outgassing in the Antarctic Divergence of the Southern Ocean in the two-step atmospheric CO2 rise during the last deglaciation, and further suggest that possible variations in CO2 solubility and sea-ice retreat amplified the effects of physical circulation changes on Southern Ocean air-sea CO2 exchange.
References: Anderson, R.F., Ali, S., Bradtmiller, L.I., Nielsen, S.H.H., Fleisher, M.Q., Anderson, B., Burckle, L.H., 2009. Wind-driven upwelling in the Southern Ocean and the deglacial rise in atmospheric CO2. Science 323, 1443–1448. doi: 10.1126/science.1167441
How to cite: Gottschalk, J., Bartels, C., Anderson, R. F., Crosta, X., Elling, F. J., Esper, O., Frick, D. A., Hartmann, J., Hodell, D. A., Jaccard, S. L., Rosenthal, Y., Skinner, L. C., Szidat, S., and Wacker, L.: Radiocarbon evidence for early deglacial changes in deep ocean upwelling near the Antarctic Divergence Zone in the Atlantic sector of the Southern Ocean, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12349, https://doi.org/10.5194/egusphere-egu26-12349, 2026.
Methane is a powerful greenhouse gas that plays an important role in Earth’s climate. However, its long-term evolution over deep-time remains poorly constrained. Consequently, methane is rarely treated as an explicit, dynamically evolving component in Earth system models, and its potential contribution to long-term climate variability has not been systematically explored.
Here we present a modelling framework coupled to the Earth System Model HadCM3, designed to investigate methane–climate interactions over multi-million-year timescales. The model represents major methane sources, with a particular focus on wetland emissions, and simulates methane sinks through an explicit atmospheric chemistry scheme, enabling a process-based calculation of atmospheric methane concentrations. Methane radiative forcing is subsequently derived from the simulated concentrations to evaluate its long-term climatic impact.
Our preliminary simulations indicate that methane variations exhibit nonlinear and systematic dependencies on background climate state and carbon cycle conditions. The persistent co-variation between CO₂ forcing and global temperature over the Phanerozoic, despite the gradual increase in solar luminosity, implies the presence of additional compensating forcings or feedback mechanisms. Our results indicate that methane radiative forcing alone is insufficient to provide this compensating influence, pointing to the involvement of additional long-term climate factors that are not yet fully understood.
How to cite: Xie, Y., Valdes, P., Hopcroft, P., and Lunt, D.: Methane–Climate Interactions over Phanerozoic Timescales in an Earth System Modelling Framework, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14111, https://doi.org/10.5194/egusphere-egu26-14111, 2026.
Discovery of abundant lake ice-rafted debris (L‑IRD) coeval with dinosaurs in continental strata of the Late Triassic to middle Jurassic of northwestern China (Junggar Basin) led to reevaluation of paleolatitude for that region (1). The basin was inferred to lie north of the Arctic Circle during the Late Triassic/Early Jurassic, along with much of Northeast Asia, consistent with paleomagnetic reference frame data (2–4). Similarities in facies transitions through the Triassic and Jurassic in both the North and South China blocks, together with recent paleomagnetic interpretations, suggest amalgamation with the Siberian plate by the Late Triassic (5, 6), implying a giant Early Mesozoic Arctic continent dwarfing present-day Antarctica.
The L‑IRD shows that the southern margin of the Arctic had freezing winters despite high pCO₂, consistent with climate models (7), and the outsized Arctic continent would have had an enhanced continental climate with even colder winters. With lowlands freezing in winter in the southern Arctic, there were presumably significant mountain glaciers, perhaps even a small ice cap, as a background condition, consistent with glacioeustatic Triassic–Jurassic sea-level fluctuations (8).
The end-Triassic sea-level drop stands out in particular: a ∼10⁵‑year event on a multimillion-year rise, broadly coincident with the end-Triassic mass extinction (ETE) (9). This sea-level drop is coincident with the onset of the Central Atlantic Magmatic Province (CAMP), but modeling suggests that CAMP-related uplift would have had relatively local effects (10). An increase in glacial ice triggered by CAMP volcanic winters provides a possible mechanism (11). Perhaps enhanced via ice–albedo feedback and a consequent increase in Earth System sensitivity to polar orbital forcing, ice-sheet growth may have triggered a recently identified ~400 kyr switch in tropical orbital pacing from expected precession dominance to obliquity dominance and back (12), a temporary transition resembling the onset of the “40 kyr world” at the mid-Miocene transition, plausibly caused by growth of the Antarctic Ice Sheet to near-modern size (13).
This giant Arctic continent may have primed the Earth System to switch from a hothouse to a transient icehouse world during CAMP volcanic winters, causing an abrupt sea-level drop. The same cold perturbations may also have driven the extinction of all large non-insulated land animals, paving the way for dinosaur ecological dominance, as these insulated reptiles were already living in the freezing Arctic beforehand.
1) Olsen et al. 2022. Sci. Adv. 8, eabo6342; 2) Marcilly et al. 2021. http://www.earthdynamics.org/climate/exposed_land.zip; 3) van Hinsbergen et al. 2014. paleolatitude.org; 4) Leonard et al. 2025. Commun. Earth Environ. 6, 508. 5) Yi et al. 2023. Earth Planet. Sci. Lett. 118143; 6) Olsen et al. 2024. Geol. Soc. Lond. Spec. Publ. 538, SP538–2023–2089; 7) Landwehrs et al. 2022. Proc. Natl. Acad. Sci. 119, e2203818119; 8) Wang et al. 2022. Glob. Planet. Change 208, 103706; 9) Fox et al. 2020. Proc. Natl. Acad. Sci.; 10) Austermann et al. 2015. EGU Gen. Assem. Abstr. 3073; 11) Schoene. 2010. Geology 38, 387–390; 12) Olsen et al. 2024. AGU24, Abstr. V22A-05; 13) Westerhold et al. 2020. Science 369, 1383.
How to cite: Olsen, P.: A Giant Arctic Continent During the Early Mesozoic: its Climatic, Eustatic, and Biotic Implications, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15949, https://doi.org/10.5194/egusphere-egu26-15949, 2026.
3D biogeochemical ocean models such as cGENIE can explicitly model depth-dependent carbon cycle processes, such as remineralisation of organic carbon. This potential advantage of 3D models (in comparison to box ocean models) can, however, be limited by coarse spatial resolutions. In particular, continental shelves may be underresolved in 3D model bathymetric grids. Such grids are created by downsampling (palaeo)-digital elevation models (DEMs).
We develop an algorithm (here termed ‘DEM-based upsampling’) to project 3D ocean model output onto its associated DEM. This resolves depth-dependent quantities and fluxes at the seafloor at degree-scale and better captures shallow seafloor, including continental shelves. This is critical for modelling organic carbon cycling, as continental shelves receive more than half of the global flux of organic carbon to the seafloor. We validate the DEM-based upsampling algorithm using area-weighted errors between a modern-Earth model run and observational data (World Ocean Atlas 2023). Upsampling yields statistically significant reductions in error in modelled temperature, salinity, oxygen concentration, and phosphate concentration across bootstrap confidence intervals and paired non-parametric tests.
We then derive the first spatially-resolved model record of ocean organic carbon burial from 25 Ma – present using the PhanerO3D framework, driving cGENIE with SCION biogeochemistry and HadCM3L atmospheric physics. We obtain organic carbon burial flux by upsampling cGENIE’s organic carbon export flux and applying a simple burial scheme. We find the global burial rate peaks in the early Miocene, then declines over the remaining Neogene. This trend agrees well with geochemical records until the latest Miocene – Pliocene. We find global variability to be largely driven by regional changes; notably declining North Atlantic margin burial over the Miocene, and rising West Pacific burial in the Pliocene.
These results highlight the advantages of DEM-based upsampling as a tool in palaeoclimate modelling: better constraining depth-dependent ocean processes, facilitating deeper investigation of spatiotemporal patterns, and potentially facilitating more spatially precise proxy-model comparison.
How to cite: Sartin, A., Stockey, R. G., Vervoort, P., Rohling, E. J., and Gernon, T. M.: Quantifying spatiotemporal variability in Neogene organic carbon burial: a case for ocean model upsampling, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16785, https://doi.org/10.5194/egusphere-egu26-16785, 2026.
Equilibrium climate sensitivity (ECS), defined as the response of the global mean surface temperature response to a sustained doubling of atmospheric CO2 at equilibrium, is a key metric for quantifying the Earth’s climate sensitivity to greenhouse gas emissions. Accurate ECS estimates are therefore fundamental for reliable simulations of the long-term carbon cycle. cGENIE, as an Earth system model of intermediate complexity that integrates ocean circulation, atmospheric energy balance, and global biogeochemical cycling, is widely used to investigate cross-sphere carbon cycle evolution and long-term climate feedback mechanisms. However, previous cGENIE studies have assumed a fixed climate sensitivity (with a default radiative forcing of 4 W m-2 per CO2 doubling), which often led to inaccurate surface temperature estimates compared with proxy reconstructions, limiting the model’s ability to capture state-dependent climate feedbacks. Here we use fully coupled models (e.g., HadCM3 and CESM) to derive the relationship between atmospheric CO2 concentrations and ECS throughout the Phanerozoic. These simulations are considered to closely match proxy reconstructions of temperatures. We then incorporate state-dependent climate sensitivity into cGENIE to enhance its representation of climate feedbacks across varying CO2 levels. Our results show that temperature simulations using the unmodified cGENIE model exhibit substantial discrepancies for periods of rapid cooling and warming, such as the Late Ordovician and the PETM. However, incorporating state-dependent climate sensitivity substantially reduces the discrepancy between simulated and proxy-reconstructed surface temperatures. These findings highlight the importance of accounting for state-dependent climate sensitivity in Earth system models, both for accurately reconstructing past climate extremes and for improving projections of future climate change.
How to cite: Zhang, Q. and Fan, J.: Refining Phanerozoic Extreme Climate Simulations with Equilibrium Climate Sensitivity (ECS) in cGENIE, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18128, https://doi.org/10.5194/egusphere-egu26-18128, 2026.
The Late Cretaceous greenhouse climate experienced a pronounced cooling trend during the Campanian–Maastrichtian, potentially driven by declining atmospheric CO2 and ocean-gateway reorganization. Yet, low-latitude high-resolution reconstructions remain limited, hampering mechanistic interpretations of surface-ocean dynamics. Here, we present a new high-resolution planktonic Mg/Ca-derived sea-surface temperature (SST) record from Ocean Drilling Program (ODP) Sites 1209 and 1210 (Shatsky Rise, western tropical Pacific), spanning ~2.5 Myr (67.0–69.4 Ma). Reconstructed SSTs range between ~32 and 34 °C, consistently exceeding modern tropical surface-ocean temperatures. SSTs rise toward ~68.1 Ma before cooling in the youngest part of the record. While absolute Mg/Ca temperatures are higher than published TEX86 and planktonic δ18O-based SSTs, the major trends agree across proxies. To place these SST changes into a broader paleoceanographic framework, we integrate our record with new high-resolution planktonic δ13C and δ18O data from the same sites. The combined dataset enables evaluation of carbon-cycle perturbations, surface-water salinity variability (δ18Osw), and productivity-related vertical δ13C gradients, as well as their pacing on orbital timescales. Together, these results refine Maastrichtian low-latitude climate variability and highlight a trend toward increased meridional temperature gradients.
How to cite: Fischer, A., Westerhold, T., Röhl, U., Bahr, A., Voigt, S., and Friedrich, O.: Multi-proxy reconstruction of late Maastrichtian surface-ocean dynamics in the tropical Pacific, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16949, https://doi.org/10.5194/egusphere-egu26-16949, 2026.
Geological records suggest that marine phytoplankton might have arisen in the Proterozoic while zooplankton remained absent, and marine productivity was not excessively low. However, quantitative estimates of phytoplankton biomass and net primary productivity (NPP) remain elusive. Here, we use the Earth system model CESM1.2.2, modifyingbiological module and boundary conditions, to simulate marine biogeochemical cycles in the Proterozoic. The simulations demonstrate that, within the expected range of nutrient levels, phytoplankton at sea surface was >2 times denser than present, sustaining a greener ocean due to the absence of predators. Heavier surface chlorophyll in the Proterozoic would block sunlight from penetrating subsurface layers. This so-called self-shielding effect would decrease subsurface NPP significantly. Simulations show that, through the combined influence of low nitrate level under a low-oxygen environment, the absence of diatoms, and self-shielding, the Proterozoic NPP was only ~60% and 30% of the present level in warm (almost ice-free) and cold (sea-ice reaches ~30°N/S) periods, respectively.
How to cite: Liu, Y. and Liu, P.: A Greener but Less Productive Proterozoic Ocean, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10809, https://doi.org/10.5194/egusphere-egu26-10809, 2026.
Oceanic Anoxic Event 1b (OAE1b) occurred near the Aptian–Albian boundary during the mid-Cretaceous and represents a unique long-lasting global perturbation of the carbon cycle, characterized by multiple black shale intervals and four distinct negative carbon isotope excursions (Jacob/113, Kilian, Urbino/Paquier, and Leenhardt events). Compared with other OAEs, OAE1b is notable for its prolonged duration (~4 Myr) and its subdivision into multiple sub-events. Despite extensive marine studies, its triggering mechanisms remain controversial, with proposed drivers including volcanism related to the Southern Kerguelen Plateau, enhanced ocean stratification, intensified monsoonal circulation, and methane hydrate dissociation. However, terrestrial environmental responses to OAE1b remain poorly constrained.
Here we present a high-resolution terrestrial record of OAE1b from the Songliao Basin, northeastern China, based on the ICDP SK-2 borehole. Integrated analyses of organic carbon isotopes (δ¹³Corg), mercury concentrations, mercury isotopes (Δ¹⁹⁹Hg), and major and trace elements, combined with an established astrochronological framework, allow identification of three OAE1b sub-events (Jacob, Kilian, and Paquier) in terrestrial deposits. For the first time, mercury isotope evidence reveals three episodes of globally significant volcanic activity occurring prior to the Jacob event, prior to the Kilian event, and following the Kilian event. These volcanic signals correlate well with records from other basins worldwide, indicating a global volcanic influence.
Notably, the temporal decoupling between volcanic pulses and OAE1b sub-events suggests that volcanism was unlikely the direct trigger of OAE1b. Instead, relatively weak and predominantly subaerial volcanism of the Southern Kerguelen Plateau may have exerted a longer-term climatic influence, promoting a transition from transient cooling to greenhouse conditions and enhancing continental weathering. This long-term forcing, superimposed on orbital-scale monsoon intensification and increased wildfire activity, likely enhanced primary productivity and organic carbon burial, ultimately contributing to the development of OAE1b.
How to cite: Yang, L., Gao, Y., and Wu, Z.: Mercury Isotopic Evidence that global carbon cycle disturbance decoupled from volcanism during the Oceanic Anoxic Event 1b, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10145, https://doi.org/10.5194/egusphere-egu26-10145, 2026.
Climate-induced changes in salinity and hydrological restriction can reshape ecological communities and biogeochemical cycles in anoxic water bodies, thereby altering the productivity–preservation balance and influencing organic carbon burial. This study distinguishes two anoxic depositional modes and employs lipid biomarkers, trace element indices, and C–N stable isotopes to elucidate their ecological and biogeochemical implications. During arid intervals (Mode A), characterized by hypersaline, restricted conditions and high TOC, elevated δ¹⁵N values (~6‰) indicate enhanced denitrification. Although cyanobacterial abundance is relatively high, low Mo/TOC ratios suggest limited Mo availability, which constrains nitrogen fixation. In humid periods (Mode B), corresponding to low‑salinity, open‑system settings with low TOC, δ¹⁵N values decrease (~4.5‰). Increased Mo/TOC ratios point to improved Mo availability that promotes nitrogen fixation, superimposing a nitrogen‑fixation signal on the δ¹⁵N record and causing a slight negative shift even under anoxic conditions. Differences in δ¹³C between the two modes further indicate that higher productivity during arid phases enriches the dissolved inorganic carbon pool in heavier carbon, whereas humid periods are marked by reduced productivity and greater input of terrestrially derived light carbon. Overall, the sensitivity of the nitrogen cycle to environmental perturbation is primarily governed by the supply of Mo—a key cofactor for nitrogenase—rather than cyanobacterial abundance. Meanwhile, aridity‑driven nutrient concentration combined with brief oxidative decomposition under a shallow halocline jointly enhances both organic matter input and preservation, ultimately promoting organic carbon burial. This framework highlights the coupling among climate, nutrient dynamics, trace‑metal limitation, and biological communities, offering an ecological‑process perspective for interpreting nitrogen‑cycle perturbations and carbon‑sink formation in anoxic systems.
How to cite: Chen, A. and Liang, C.: Climate fluctuations drive periodic shifts in anoxic depositional environments: Mo availability regulates nitrogen cycling and organic carbon burial, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3098, https://doi.org/10.5194/egusphere-egu26-3098, 2026.
Long paleoclimate time series combine strong persistence, multiple orbital cycles, and regime shifts, which complicates the analysis of dynamical coupling and predictability. We analyze Cenozoic variability using the Cenozoic global reference benthic foraminiferal carbon and oxygen isotope dataset, in a regime based time series framework that integrates deterministic decomposition, cyclical fractional cointegration, and regime aware forecasting. We divide the record into segments in line with the major Cenozoic climate states. Within each segment, deterministic components are estimated and removed, including linear trends, orbital forcing variables, and harmonic cycles identified via a GARMA based filtering procedure. We then apply cyclical fractional cointegration tests at shared orbital frequencies to assess whether common spectral peaks reflect a stable frequency specific linkage (cointegration) between the proxies and orbital variables. The results reveal pronounced regime dependence. The long eccentricity cycle (405 kyr) shows recurrent evidence of cointegration with both proxies across different climate states. For obliquity, an indication of frequency specific linkage is primarily found after the middle Miocene Climate Transition. Finally, we fit regime specific VAR(2) models to the residuals and report in-sample forecasts, and we generate a 100 kyr out-of-sample projection based on the Icehouse specific dynamics. Forecast behaviour varies across climate states, highlighting that non-stationarity and regime specific dynamics place strong constraints on predictability in long paleoclimate records.
How to cite: Özer, Y., del Barrio Castro, T., Escribano, Á., and Sibbertsen, P.: Modeling Long Memory Cyclical Trends in the Cenozoic, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7593, https://doi.org/10.5194/egusphere-egu26-7593, 2026.
Among the five great extinction events of the Phanerozoic, the Late Ordovician stands out as it is
concomitant with a massive glacial event under high atmospheric pCO2. This apparent climate
paradox was addressed in numerous climate modeling studies. In particular, [1] showed that under
the specific palaeogeographical conditions of the Hirnantian (445 Ma), with an ocean-dominated
Northern Hemisphere, the climate system may undergo a “tipping point” where a small pCO2
variation leads to either glacial or ice-free warm equilibrium state.
Those results were obtained with the intermediate complexity Fast Ocean Atmosphere Model
(FOAM). We have conducted new simulations using the state-of-the-art coupled IPSL-CM5A2-LR
Earth System Model [2], spanning a wide range of pCO2 for the Hirnantian. We find that the climate
tipping point is entirely absent, and that the equilibrium climate sensitivity is strikingly linear in this
set of simulations.
We conducted a detailed model intercomparison and we have identified major differences between
the models in the representation of the radiative transfer, cloud cycle and oceanic eddy dynamics
which contribute to the qualitatively different model behaviors, enhanced under high atmospheric
pCO2 content. Specifically, the FOAM tipping point corresponds to an abrupt transition from a sharp
Northern latitudinal temperature gradient at low pCO2 (cold state) to a flattened gradient with warm
polar latitudes (ice-free warm state). In contrast, the IPSL-CM5A2 temperature gradient is relatively
constant across pCO2, with year-long sea ice confined in the Northern latitudes even under 15X
preindustrial pCO2 level (4200 ppm).
We propose a physical mechanism to link the warm FOAM flattened latitudinal temperature gradient
to the dramatic sea-ice albedo feedback sensitivity via the increased stratification of the superficial
ocean. Since this mechanism is independent of the physical parameterizations and relative
complexity of the models, and comparing our results with other scarce published climate simulations
of the Hirnantian [3,4], we propose that the latitudinal temperature gradient, seen as a model-
dependent emerging feature, may be the main driver of the previously unveiled sea-ice albedo
climate tipping point.
References:
[1] Pohl et al. (2014), Climate of the Past, 10, 6
[2] Sepulchre et al. (2020), Geoscientific Model Development, 13,7
[3] Pohl et al. (2017), Paleoceanography, 32, 4
[4] Valdes et al. (2021), Climate of the Past, 17, 4
How to cite: Naar, J., Donnadieu, Y., Le Hir, G., Pohl, A., and Ladant, J.-B.: Model-dependent latitudinal temperature gradient drives Late Ordovician climate stability, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18645, https://doi.org/10.5194/egusphere-egu26-18645, 2026.
The ~9‰ increase in seawater lithium isotope composition (δ7Li) during the Cenozoic is widely interpreted as evidence for uplift-driven intensification of continental silicate weathering, particularly associated with major orogenic systems such as the Tibetan Plateau. However, this interpretation remains largely untested due to the lack of long-term riverine δ7Li records from tectonically active regions. Here we present the first Neogene paleowater δ7Li records spanning the past ~15 Myr from both the southern and northern Tibetan Plateau, a region that today contributes ~18% of the global riverine Li flux. Our dataset is derived from a 3500-m-thick fluvial sequence (15-5 Ma) in the Siwalik foreland basin (southern, monsoon-dominated Plateau) and a 1700-m drill core (7.3-0.1 Ma) from the Qaidam Basin (northern, arid Plateau). These two archives capture contrasting climatic, lithological and denudation regimes associated with Neogene uplift and cooling. Reconstructed paleowater δ7Li values reveal persistently low values in the southern Plateau and a long-term decrease in the northern Plateau, indicating reduced silicate weathering intensity under conditions of climatic cooling and rapid exhumation. These trends contrast with the coeval rise in seawater δ7Li, challenging the view that enhanced silicate weathering from uplifted mountain belts directly drives the marine lithium isotope record. By integrating our δ7Li reconstructions and reconstructed Li fluxes from the entire Tibetan Plateau into a global lithium cycle model, we show that continental silicate weathering from tectonically active mountains alone is unlikely to account for the observed Neogene increase in seawater δ7Li. Our results highlight the need for direct continental records from major orogenic systems to robustly constrain the links between tectonics, weathering, and the long-term carbon cycle.
How to cite: Liu, Y., Yang, Y., Pogge von Strandmann, P. A. E., Jin, Z., and Fang, X.: Decoupling of Neogene Seawater Lithium Isotopes from Uplift-driven Weathering, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14466, https://doi.org/10.5194/egusphere-egu26-14466, 2026.
