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Abstract:
Storing carbon in the deep ocean is a key-feedback mechanism that allows astronomical forcing to drive the late Pleistocene glacial/interglacial variations. As carbon storage is intrinsically linked to oxygenation, proxies for sediment oxygenation have been used to quantify changes in carbon storage during the late Pleistocene. However, evidence for astronomically-paced changes in carbon storage beyond the late Pleistocene is limited, hindering our understanding of the stability of this feedback mechanisms.
Here we used molecular fossils (biomarkers) in marine sediment cores that span the last ~3.5 million years to assess the long-term impact of astronomical forcing on deep ocean oxygenation, and hence carbon storage, and explore the stability of this deep ocean feedback mechanism. Using high-resolution records from three independent cores from the North Atlantic, we find that the concentration of biomarkers from anaerobic bacteria is eccentricity paced during the middle and late Pleistocene with high abundances during glacials and absence during interglacials. We interpret this data to reflect a decrease in oxygenation and hence increase in carbon storage during the most recent glacials. Across the MPT this pacing changes to obliquity forcing and we show that this forcing is persistent into the late Pliocene, highlighting the stability of this feedback mechanism. However, prior to 2.7 Myr we find no biomarkers of anaerobic bacteria across the North Atlantic, suggesting reduced carbon storage prior to the intensification of the glaciation of the Northern Hemisphere. Our findings indicate that the lowering of atmospheric CO2 by the sequestration of carbon in the deep ocean in response to astronomical forcing persisted throughout the Quaternary and was essential for the development of Plio/Pleistocene ice ages, but this feedback mechanisms did not persist into the warm Pliocene.
