Controls on High Sedimentary Accumulation of Organic Matter During Cretaceous Oceanic Anoxic Event 2, Southeast Newfoundland Ridge, North Atlantic Ocean

Deoxygenation and low-oxygen zones are occurring in modern oceans as a consequence of excessive nutrients in runoff. Solving this issue necessitates understanding the relationships between dissolved oxygen, ocean chemistry, inputs of atmospheric CO2, and climate change. As an analog, this study looks back 94 Ma to Oceanic Anoxic Event 2 (OAE2), a global geologic event triggered by greenhouse conditions from increased volcanism. The global carbon cycle was disrupted by OAE2, increasing burial of organic matter on the seafloor. Environmental changes were studied below, within, and above OAE2 sediments from deep-sea sediment cores drilled at Integrated Ocean Drilling Program Site U1407, Southeast Newfoundland Ridge. By analyzing the geochemistry and microscopic fossils, an interval of time within OAE2 was identified when excess nutrients led to oxygen levels so low that microorganisms could not survive on the seafloor and toxic sulfides were present in pore spaces of sediments. After OAE2, nutrient inputs declined, the ocean reoxygenated, and microorganism abundance and diversity recovered. The abnormal OAE2 environmental conditions impacted the sizes and community assemblages of benthic foraminifera, common protists in oceans over the past 500 Myr. To analyze foraminifera, correct sieve sizes for preparing them must be selected so that shell size does not bias results. Abundances recovered from sieves with standard >63 μm and 38-63 μm openings were compared to assess whether key environmental taxa or community trends were lost using larger-sized openings. General trends in both sizes were relatively similar, and no key taxa were lost, so counting large abundances of 38-63 μm shells was not proven time-effective. For >63 μm foraminifera, significant differences in their assemblages were observed at the Cenomanian–Turonian boundary hiatus at the base of the organic-carbon-rich layer deposited during OAE2. Many taxa presumed extinct after the Cenomanian were discovered further up-core in the Turonian, indicating they survived elsewhere and re-populated the area once conditions improved. Future organismal responses to ongoing climate changes that affect local environments will occur under multiple and cumulative stressors, both natural and anthropogenic. Therefore, studying analogs in the geological record provides empirical evidence for predictions of future climate change and deep-sea organismal responses.

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