The Early Jurassic Toarcian Oceanic Anoxic Event (T-OAE, ~183 Ma) represents a major deep-time hyperthermal in the Phanerozoic, characterized by the rapid injection of isotopically light carbon into the ocean-atmosphere system and a prominent negative carbon-isotope excursion (NCIE), accompanied by global warming and widespread marine anoxia. Traditionally, enhanced silicate weathering and massive marine organic carbon burial during hyperthermals are considered the two primary pathways for drawing down atmospheric CO2 and driving the Earth system toward a cooling recovery. Following this logic, massive carbon burial should have resulted in significant cooling. However, geological records indicate that global high temperatures persisted even after the T-OAE carbon burial phase. As potent greenhouse gases with global warming potentials far exceeding that of CO2, methane (CH4) and nitrous oxide (N2O) emitted from terrestrial lacustrine systems play a pivotal role in perturbing the carbon cycle and exacerbating climate warming. Yet, the emission fluxes of lacustrine N2O and their feedback mechanisms on global climate during deep-time hyperthermals remain poorly constrained.
Recently, a collaborative research team led by Dr. HAN Zhong (Nanjing Institute of Geology and Palaeontology, CAS), Dr. GE Yuzhu (Chengdu University of Technology), and Prof. HU Xiumian (Nanjing University), in collaboration with scholars from multiple domestic and international research institutions, conducted high-resolution sedimentary and geochemical analyses on lacustrine records from the Da'anzhai Member in the Sichuan Basin (Fig. 1). By integrating these data with a nitrogen-cycle mass-balance model, the study unveils a climate feedback mechanism wherein a giant terrestrial freshwater lake sustained global high temperatures by emitting massive amounts of N2O during the hyperthermal. These findings were recently published online in the top-tier geoscience journal Geology.
The research team focused on the expansive Early Jurassic Sichuan mega-lake, which covered an area of over 230,000 square kilometers (roughly equivalent to 10% of the total surface area of modern global lakes). Analyses of drill-core samples revealed a significant increase in total organic carbon (TOC) within the lacustrine sediments during the T-OAE NCIE interval, accompanied by severe bottom-water anoxia. Crucially, the strata record a pronounced positive excursion in bulk nitrogen isotopes (δ15Nbulk), reaching up to +6.2‰ (Fig. 2). This isotopic signature indicates a substantial intensification of water-column denitrification under anoxic conditions.
To decipher why enhanced denitrification led to greenhouse gas emissions, the team delved into the role of trace elements. During denitrification, the reduction of N2O to harmless dinitrogen gas (N2) relies on copper (Cu) as a vital enzymatic catalyst. However, the severe water-column anoxia and organic-rich sedimentation in the Sichuan mega-lake during the T-OAE promoted the extensive complexation of bioavailable dissolved Cu with sulfides and organic matter. Consequently, Cu was firmly sequestered in the sediments, leaving the water column severely depleted in bioavailable Cu for bacterial utilization. This "Cu limitation" effect effectively blocked the final step of the denitrification pathway, forcing the lacustrine system to emit massive amounts of nitrogen into the atmosphere in the form of the greenhouse gas N2O.
By constructing an isotopic mass-balance model for the nitrogen cycle, the team conservatively estimated that the Sichuan mega-lake alone released approximately 0.9 to 1.8 Gt (gigatonnes) of N2O into the atmosphere during the T-OAE. Given that the global warming potential of N₂O is 265 to 298 times greater than that of CO2, the greenhouse effect generated by these N2O emissions was sufficient to completely offset the potential cooling effect driven by the massive burial of organic carbon (~460 Gt) within the lake.
This study is the first to reveal the positive feedback role of giant terrestrial lakes during deep-time hyperthermals from the unique perspective of "trace-element limitation on lacustrine nitrogen cycling." These findings not only provide a novel mechanistic framework explaining why global warming persisted during and after the T-OAE NCIE, but also offer critical deep-time geological insights into the ecological and environmental risks of increasing greenhouse gas emissions from modern lakes under the current backdrop of global warming, eutrophication, and deoxygenation.
This research was jointly supported by the National Key R&D Program of China, the National Natural Science Foundation of China, the Strategic Priority Research Program (B) of the Chinese Academy of Sciences, the National Science and Technology Major Project, and the International Geoscience Programme (IGCP 739).
Reference: Ge, Y., Han, Z., Uveges, B.T., Jenkyns, H.C., Kemp, D.B., Cui, Y., Yang, T., Zhang, S., Tian, H., Hu, X., 2026. Intensified lacustrine nitrous oxide emissions linked to global climate warming during the Toarcian Oceanic Anoxic Event. Geology, https://doi.org/10.1130/G54763.1.
Fig.1 Early Toarcian paleogeography of the Sichuan Basin and integrated stratigraphy of the LA core.
Fig.2 Integrated geochemical, paleontological, and paleotemperature records across the Toarcian hyperthermal. The correlation illustrates the coupling among water-column anoxia, copper (Cu) depletion, and the positive bulk nitrogen-isotope (δ15Nbulk) excursion in the Sichuan megalake, set against the backdrop of global warming and Large Igneous Province (LIP) during the T-OAE NCIE.
Fig.3 Mechanistic model and simulation results of hyperthermal-driven lacustrine greenhouse gas (N2O) emissions. (Left) Conceptual model illustrating the physicochemical mechanism wherein water-column anoxia triggers a Cu limitation effect, which inhibits complete denitrification and leads to massive emissions of the potent greenhouse gas N2O. (Right) Nitrogen-cycle mass-balance model quantitatively confirming the marked surge in lacustrine denitrification fluxes during the hyperthermal.
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