Oxygen Variability on the Central Chile Margin Across the Last Glacial Cycle

Ocean deoxygenation is a global phenomenon with potentially significant ecological and economic impacts. Paleo-proxy data can reduce model uncertainties for future projections, by providing insight into the mechanism of ocean oxygen change, and its relationship to climate forcing. However, important gaps remain in our picture of ocean oxygen change during the most recent glacial cycle. In particular, while data compilation work has assembled a global database of proxy records of ocean (de)oxygenation since the last glacial maximum (Jaccard and Galbraith 2012), few records cover the earlier parts of the last glacial cycle. To help fill this gap we have extended a record of redox-sensitive trace metals from the central Chile margin through to the inception of the most recent glacial cycle.

At Ocean Drilling Program Site 1234 (36^◦S, 1015 m water depth), redox proxies xsRe and xsMn are consistent with a decrease in bottom water oxygenation during the deglaciation, previously attributed to decreased ventilation by Antarctic Intermediate Water (Muratli et al. 2010). These same proxies suggest Stages 2 and 3 were characterized by well-oxygenated conditions, with no evidence for millennial-scale variability associated with rapid climate events. The last time xsRe was as high as during the Holocene - interpreted as low bottom water oxygen - was during Marine Isotope Stage 5, when there were three distinct peaks. In contrast to the low oxygen conditions present during the Holocene, which are associated with low productivity and high 𝛿¹⁵N, during Marine Isotope Stage 5, low oxygen conditions are associated with high productivity and low sedimentary 𝛿¹⁵N. Although Low 𝛿¹⁵N may reflect a decrease in denitrification within the northern Peru Chile Undercurrent, perhaps in response to better thermocline ventilation (De Pol-Holz et al. 2007), the coincidence of xsRe peaks and 𝛿¹⁵N minima on the central Chile margin with opal flux peaks in the Antarctic zone (Studer et al. 2015) during Marine Isotope Stage 5 suggests both may have been driven by increased nitrate supply from the Southern Ocean. These results highlight the complexity of processes driving ocean oxygen, and the utility of a multi-proxy approach for identifying mechanisms responsible for past changes in ocean oxygen.