Global Warming Caused Widespread Ocean Anoxia 93 Million Years Ago, Research Suggests

Arthur T Knackerbracket has processed the following story:

Marine anoxia is characterized by the oceans being severely depleted in dissolved oxygen, making them toxic and thus having devastating impacts on the organisms inhabiting them. One such event, known as Oceanic Anoxic Event 2 (OAE2), occurred ~93.5 million years ago across the Cenomanian-Turonian boundary of the Upper Cretaceous and lasted for up to 700,000 years.

During such scenarios, organic matter is buried at an elevated rate, producing distinctive layers of black shale in the geological record, which are depleted in the isotopically-heavier carbon-13, therefore generating a positive carbon isotope excursion of ~6 for this study period.

The specific factors triggering OAE2 are still debated, but the most widely supported is volcanism from the Caribbean Large Igneous Province and High Arctic Large Igneous Province, increasing atmospheric carbon dioxide and therefore warming the planet.

Among the plethora of impacts from a warmer planet is increased weathering of land, with fluvial processes transporting this material to the oceans, providing key nutrients to primary producers in the surface ocean. Enhanced primary productivity produces more oxygen, but trophic food chains ultimately use up more of this oxygen in their metabolic processes.

Compounded by decreased solubility of oxygen in warmer oceans, this results in widespread deoxygenation of Earth's marine realm, the focus of new research published in Climates of the Past.

[...] Revealing the significance of the work, Dr. Abraham said, "Our research delves into the ancient oceans' secrets, specifically a period 93.5 million years ago when much of the ocean was devoid of oxygen. By studying natural chemical fingerprints preserved in marine sediments, we uncover how volcanic activities and climate warming in the past led to drastic ocean deoxygenation. Understanding this in deep time is crucial, as they mirror the challenges we face today with the ongoing climate crisis, helping us predict and mitigate future consequences."

Taking samples of organic matter from the drilled cores, the research team isolated compounds of biological origin that are stable over geological time periods of millions of years, known as biomarkers. Dr. Abraham explains that biomarkers are known as "molecular fossils," adding, "Biomarkers are chemical compounds found in sedimentary rocks that originated from living organisms millions of years ago. Think of them as molecular fossils that, unlike bones or shells, are not easily visible to the naked eye. These compounds, once part of living organisms, have remained chemically stable over vast geological timescales.

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