The Earth’s carbon cycle is a finely tuned system, essential for maintaining life and climate stability across geological timescales. Carbon constantly moves between the atmosphere, oceans, living organisms, and geological reservoirs such as rocks and sediments. This delicate balance ensures that carbon, in its many chemical forms, is recycled and stored in various locations, helping regulate global climate and ocean chemistry. However, perturbations to this cycle, particularly during periods of extreme volcanic activity, can send shockwaves through the Earth’s systems, causing severe environmental consequences such as global warming and ocean acidification.
One of the most pressing questions in Earth science is how our planet recovers from massive disturbances in the carbon cycle, such as those triggered by extensive volcanic eruptions that release vast quantities of carbon dioxide (CO2) into the atmosphere. These episodes, which happened periodically in Earth’s deep past, have the potential to drastically alter atmospheric chemistry and marine environments. Yet, geological records indicate that the Earth system eventually stabilizes and recovers, but the mechanisms underlying this resilience have remained elusive until now.
A recent breakthrough study led by Mojtaba Fakhraee of the University of Connecticut, published in Nature Geoscience, sheds light on an overlooked geochemical feedback capable of stabilizing the climate following these catastrophic events. Through sophisticated computational simulations of coupled carbon and sulfur cycles over hundreds of millions of years, the research team uncovered a crucial role played by pyrite (iron sulfide) formation and burial during episodes of ocean anoxia—conditions of severely depleted oxygen in the marine environment.
At the heart of this natural buffering process is the response of ocean chemistry to oxygen loss brought on by the sudden influx of volcanic CO2. When oxygen levels in the ocean plummet, anaerobic respiration pathways generate sulfur species that react with iron to form pyrite, often called “fool’s gold” for its metallic sheen. This pyrite formation acts as an alkalinity source, helping to neutralize acidity and maintain oceanic pH balance. The resulting chemical reaction effectively serves as a long-term stabilizer against rapid ocean acidification.
Fakhraee explains that under typical conditions, oceanic carbon and atmospheric CO2 exist in a quasi-equilibrium, where dissolved inorganic carbon in seawater balances the carbon concentration in the air. However, during extraordinary volcanic episodes, the system is pushed out of this equilibrium due to the sudden surge of carbon dioxide. This creates a state far from balance, leading to oxygen depletion in the ocean interior and the subsequent activation of sulfur cycling pathways. The net effect is a substantial increase in pyrite burial, which enhances alkalinity and draws down acidity to buffer the marine environment.
By incorporating the complex interplay between carbon and sulfur cycles into a global geochemical model, the study recreates several prominent ocean anoxic events (OAEs) documented in the geological record. These OAEs are periods characterized by widespread oxygen deficiency in the world’s oceans, historically associated with mass extinctions and ecological upheavals. The simulations reveal that as volcanic CO2 emissions spike and ocean deoxygenation intensifies, pyrite formation amplifies, contributing to sustained alkalinity increases that counteract the acidifying influence of elevated atmospheric CO2.
This feedback process does not operate instantaneously. Rather, it unfolds over geological timescales—spanning thousands to millions of years—making it ineffective as a short-term solution to the contemporary climate crisis. Fakhraee stresses that while this buffering mechanism helped the Earth system recover after past catastrophes, it is not an escape hatch for humanity’s current CO2 emissions. The rapid pace of anthropogenic climate change outstrips the slow geochemical feedbacks, meaning ecosystems and human societies face immediate risks before these natural stabilizations could take effect.
Interestingly, pyrite formation does occur in localized, oxygen-poor marine settings today, such as certain sedimentary basins and isolated anoxic zones. However, the overall scale of this process under current ocean oxygen levels is minimal, thus exerting negligible influence on global carbon sequestration or ocean pH stabilization. For this buffering feedback to become globally significant, the ocean would have to endure widespread and prolonged deoxygenation—conditions likely associated with dire consequences for marine ecosystems and biodiversity.
The study underscores the intricacy and resilience built into Earth’s biogeochemical systems. Despite the immense challenges brought about by volcanic CO2 injections and oceanic oxygen loss, the coupling between the carbon and sulfur cycles provides a fundamental mechanism through which the planet’s climate and marine chemistry can gradually regain stability. This emphasizes both the vulnerability and the remarkable adaptive capacity of the Earth system over deep time.
Fakhraee’s findings also illuminate how even seemingly detrimental environmental states—such as ocean anoxia—can, paradoxically, play a part in planetary recovery. The geological record reveals numerous instances where mass extinctions and ecological collapses were followed by rebounds facilitated in part by these geochemical feedback loops. This nuanced understanding challenges simplistic assumptions that anoxia is unambiguously harmful, highlighting instead its dual role as both a crisis and a catalyst for stabilization.
Looking into the future, the research invites reflection on the thresholds for ocean deoxygenation and the long-term trajectories of Earth’s carbon cycle. If anthropogenic CO2 emissions continue unabated, areas of the ocean could experience expanding oxygen minimum zones that might activate stronger sulfur cycling and pyrite burial, potentially providing a long-term buffer against acidification. Yet, such outcomes come with profound ecological costs, and humanity’s ability to mitigate emissions remains paramount.
Ultimately, this research reiterates the urgent need to integrate knowledge of Earth’s deep-time feedbacks into climate models and policy discourse. While the planet harbors intrinsic mechanisms to eventually restore equilibrium, the timescales involved are out of reach for human timescales and welfare. The intertwined fate of human civilization and the Earth system hinges on global commitment to reducing greenhouse gas emissions and protecting marine oxygen levels to safeguard both the biosphere and the resilience of planetary cycles.
This new insight into the ancient interplay between volcanic outgassing, ocean anoxia, sulfur cycling, and pyrite burial provides a compelling example of how cutting-edge computational modeling can unlock secrets buried in Earth’s geological past. It is a vivid reminder that the planet’s survival story stretches back hundreds of millions of years, shaped by complex chemical dialogues beneath the ocean surface that continue to influence the trajectory of life today.
Subject of Research: Not applicable
Article Title: Climate stabilization by alkalinity production from pyrite burial during oceanic anoxia
News Publication Date: 21-May-2025
Web References: 10.1038/s41561-025-01698-0
References: Fakhraee, M., et al. Climate stabilization by alkalinity production from pyrite burial during oceanic anoxia. Nature Geoscience (2025).
Keywords: Carbon cycle, Earth systems science, Biogeochemical feedback, Earth sciences, Climatology
