Oceanic Anoxic Event Dynamics in Geological Records

Oceanic Anoxic Events (OAEs) represent intervals in Earth’s history when large expanses of the ocean became depleted in dissolved oxygen, leading to widespread deposition of organic-rich sediments. These episodes are recorded in marine strata by distinct geochemical signatures—positive carbon-isotope excursions, enrichments of redox-sensitive elements and shifts in palaeontological assemblages. Geological records indicate that OAEs were often paced by pulses of volcanic activity from Large Igneous Provinces, which injected greenhouse gases into the atmosphere, drove global warming, enhanced weathering and nutrient delivery to the oceans, and ultimately triggered widespread anoxia. Changes in ocean circulation and continental configuration modulated the distribution and intensity of anoxic conditions, while subsequent burial of organic carbon helped restore redox balance. Proxies such as TEX₈₆ palaeothermometry, osmium and calcium isotope stratigraphy, neodymium isotopes and sedimentary mercury concentrations have been instrumental in reconstructing temperature trends, water-mass sources, carbonate chemistry and volcanic triggers across different OAEs. Together, these lines of evidence reveal a dynamic interplay between tectonics, climate, ocean circulation and biogeochemical cycling. Understanding the tempo and drivers of past anoxic events has direct relevance for assessing modern deoxygenation trends and for predicting future carbon-cycle feedbacks under ongoing climate change.

Research from Nature Portfolio

Recent studies have used mercury and radiogenic isotope proxies to pinpoint the timing and source of volcanic pulses that triggered Oceanic Anoxic Event 2 around 94 Ma. Elevated mercury concentrations in Mentelle Basin sediments, coupled with shifts in neodymium and strontium isotopes, point to Kerguelen Plateau volcanism as the principal driver of greenhouse gas release, climatic warming and enhanced carbon burial during the mid-Cenomanian. A complementary multiproxy investigation of the Yezo Group on the northwestern Pacific margin has identified seven discrete volcanic pulses, five of which coincide with elevated humidity, catchment weathering and nutrient runoff. These processes appear to have synchronously driven a depletion of oxygen in the Pacific and contributed to the global anoxic episode. Foundational coupled ocean-atmosphere modelling has further demonstrated that Late Cretaceous rearrangements of continental masses and seaways altered deep-water formation sites, intensifying circulation changes that either exacerbated or mitigated ocean deoxygenation at key intervals.

Oceanic Anoxic Event Dynamics in Geological Records publication trend

The graph below shows the total number of articles in oceanic anoxic event dynamics in geological records across all publications each year (not limited to Nature Index journals).

Technical terms

Oceanic Anoxic Event (OAE): A prolonged interval when large portions of the ocean were depleted in oxygen, preserving organic-rich sediments.

Large Igneous Province (LIP): A region of massive volcanic basalt emplacement linked to rapid greenhouse gas injections and environmental change.

TEX₈₆ palaeothermometer: An organic geochemical proxy based on membrane lipid distributions of marine archaea, used to estimate past sea-surface temperatures.

Carbon isotope excursion: A global positive or negative shift in the ratio of ¹³C to ¹²C in marine carbonates or organic matter, indicative of perturbations in the carbon cycle.

Isotope stratigraphy: The use of sequential variations in isotope ratios (e.g., Os, Ca, Nd) in sedimentary layers to reconstruct the timing and drivers of environmental events.

References

1. Oceanic Anoxic Event 2 triggered by Kerguelen volcanism. Nature Communications (2024).
2. Large igneous province activity drives oceanic anoxic event 2 environmental change across eastern Asia. Communications Earth & Environment (2024).
3. Cretaceous sea-surface temperature evolution: Constraints from TEX86 and planktonic foraminiferal oxygen isotopes. Earth-Science Reviews (2017).
4. Marine 187Os/188Os isotope stratigraphy reveals the interaction of volcanism and ocean circulation during Oceanic Anoxic Event 2. Earth and Planetary Science Letters (2014).
5. A better-ventilated ocean triggered by Late Cretaceous changes in continental configuration. Nature Communications (2016).
6. Ca isotope stratigraphy across the Cenomanian–Turonian OAE 2: Links between volcanism, seawater geochemistry, and the carbonate fractionation factor. Earth and Planetary Science Letters (2015).