New research combining the expertise of scientists from the University of California, Davis, the Chinese Academy of Sciences, and Texas A&M University has unveiled a striking pattern in Earth’s ancient environmental history. Approximately 300 million years ago, natural pulses of carbon dioxide emissions—termed “burps”—triggered significant and sustained decreases in oceanic oxygen levels. This groundbreaking discovery sheds new light on the interplay between atmospheric carbon dioxide concentrations and marine oxygen depletion during the late Paleozoic era, providing a deep-time analogue for contemporary climate challenges driven by rising anthropogenic carbon emissions.
The research, recently published in the prestigious journal Proceedings of the National Academy of Sciences, draws from meticulous geochemical analysis of sedimentary cores and sophisticated climate modeling. By examining the uranium isotope composition in carbonate sediments derived from the Naqing geological succession in South China, the investigators reconstructed a detailed record of oceanic oxygen fluctuations correlated to spikes in atmospheric carbon dioxide. Crucially, this multi-proxy approach enabled the identification of five discrete intervals, each lasting roughly 100,000 to 200,000 years, during which ocean oxygen content globally decreased by 4% to 12%.
These phenomena of marine anoxia, characterized by severe oxygen deficiency—or near absence—pose dire consequences for aquatic ecosystems. Oxygen-deprived environments directly impact biodiversity and productivity, leading to interruptions in the evolutionary trajectory of marine species. Although the observed events did not coincide with mass extinctions per se, the timing aligns with documented pauses in biodiversity growth in the fossil record, strongly implicating oceanic oxygen levels as a key ecological driver during these intervals.
The atmospheric context of these ancient events contrasts vividly with present-day conditions, particularly with respect to the oxygen content of the atmosphere itself. Around 300 million years ago, atmospheric oxygen concentrations were estimated to be 40% to 50% higher than modern levels—a fundamental difference in planetary respiration that nonetheless did not preclude episodes of widespread oceanic anoxia. This paradoxical coexistence underscores the enormous impact that elevated carbon dioxide levels had on ocean chemistry and circulation patterns in Earth’s deep past, despite the ostensibly favorable conditions afforded by high atmospheric oxygen.
Senior author Isabel P. Montañez, a distinguished professor at UC Davis, emphasized the contemporary relevance of these findings. She highlighted that the natural bursts of carbon dioxide recorded from ancient sediments offer the only direct analogues for understanding the dramatic increases in atmospheric CO₂ we observe today. However, whereas these ancient carbon pulses were driven by volcanic activity and other geologic phenomena, the current rise is overwhelmingly attributable to human industrial activity, occurring at rates two to three orders of magnitude faster than any natural event in the paleo-record.
To derive these insights, the research team applied cutting-edge climate models tailored specifically for paleoclimate reconstructions. This involved inputting detailed geochemical proxy data into a sophisticated mathematical framework running on supercomputers, allowing simulations to span a wide array of scenarios and uncertainties. Such high-resolution modeling confirmed the timing and magnitude of ocean oxygen depletion aligned precisely with carbon dioxide perturbations inferred from uranium isotope spikes, providing a robust mechanistic link between atmospheric composition and oceanic redox states during the late Carboniferous to early Permian periods.
The use of uranium isotopes as a proxy for ocean oxygenation represents a state-of-the-art approach in paleoceanography. Uranium isotopic ratios in carbonate sediments serve as sensitive indicators of global marine redox conditions, reflecting the extent of anoxic waters. The congruence of carbon dioxide “burps” with dramatic shifts in uranium isotope values in the sedimentary record affirms the cyclical nature of these oxygen-depleting episodes and offers a quantifiable measure of their environmental severity.
While ocean anoxia is often associated with catastrophic biotic crises or mass extinctions in Earth’s past, the new study portrays an intermediate scenario—periodic but sustained drops in oxygen that imposed ecological stress without inducing wholesale faunal turnover. Montañez and colleagues observed that these oxygen minima coincided with paleo-biodiversity stalls, hypothesizing a disproportionate impact on coastal ecosystems where oxygen demand is naturally higher and turnover rates of biomass more sensitive to environmental perturbations.
The research carries profound implications for understanding the limits and resilience of ocean systems under rapid carbon forcing. The authors caution that while the Earth’s ancient atmosphere featured greater oxygen abundance, the oceans still succumbed to anoxic episodes driven by CO₂ increases similar in scale to those experienced today. This finding serves as a sobering warning: the modern ocean, buffered by lower oxygen levels and facing anthropogenic carbon emissions at unprecedented rates, may be equally or more vulnerable to loss of oxygenation, threatening marine biodiversity and the livelihoods dependent on healthy fisheries.
The sediment core analyses, combined with geochemical proxies and high-complexity climate simulations, represent a major leap forward in disentangling the coupled carbon-oxygen dynamics of Earth’s past. This integrative methodology enables a nuanced appreciation of how atmospheric perturbations modulate marine oxygen reservoirs, shaping ecological and evolutionary outcomes across geologic timescales. Importantly, it underscores that oxygen levels in marine environments are tightly coupled to atmospheric carbon dioxide variations, both in deep time and in the present anthropocene epoch.
Looking to the future, the study urges the scientific community and policymakers alike to heed these deep-time lessons. The rapidity and scale of contemporary CO₂ emissions may induce oceanic anoxia similar in magnitude to those ancient “burps,” but occurring over mere centuries rather than hundreds of millennia. Coastal zones, already hotspots for fisheries and ecological diversity, could bear the brunt of hypoxic conditions, undermining ecosystem services and food security. Understanding the underlying processes documented through paleoenvironmental reconstructions is thus critical to forecasting and mitigating the trajectory of ocean deoxygenation under ongoing climate change.
In sum, this multi-disciplinary investigation merges geochemistry, paleoclimate modeling, and ecological interpretation to chart a compelling narrative of how massive natural carbon releases historically drove oxygen declines in the oceans. It raises urgent questions about the potential recurrence of marine anoxia in an era of accelerating anthropogenic emissions—a scenario that could imperil marine ecosystems in ways not previously appreciated. As humanity navigates the climatic challenges of the 21st century, these revelations from Earth’s distant past illuminate both the vulnerabilities and the resilience of the planet’s life-sustaining systems.
Subject of Research: Not applicable
Article Title: Repeated occurrences of marine anoxia under high atmospheric O2 and icehouse conditions
News Publication Date: 23-Jun-2025
Web References: https://doi.org/10.1073/pnas.2420505122
References: Proceedings of the National Academy of Sciences, 2025
Keywords: Paleoclimatology, Climate change, Earth climate, Geologic history, Oceanography
