Increased glacial-age ventilation of the Chilean margin by Antarctic Intermediate Water

Antarctic Intermediate Water is, at present, a water mass that brings oxygen to intermediate depths throughout the Southern Hemisphere oceans. Models have suggested that intermediate waters had higher concentrations of oxygen during the last glacial period1,2, consistent with globally reduced denitrification3 and increased production of Antarctic Intermediate Water4. However, some palaeoceanographic reconstructions5,6 have indicated that production decreased in the southeast Pacific Ocean at this time. Here we analyse the concentrations of Re and Mn, the sedimentary concentrations of which are controlled by the amount of dissolved oxygen at the sea floor, from three sediment cores located along the Chilean margin for the past 30,000 years. Our results from the cores, which bracket the present-day water-column extent of Antarctic Intermediate Water, show that the depth range of well-oxygenated Antarctic Intermediate Water increased off Chile during the Last Glacial Maximum. Dissolved oxygen content began to decrease approximately 17,000 years ago, coincident with rapid Antarctic warming and a poleward shift of the southern westerly winds7. Our estimates of productivity from accumulation rates of organic carbon and opal do not co-vary with the seafloor oxygen variations, ruling out local control of seafloor oxygenation. We conclude that the data are best explained by a combination of increased oxygenation and increased flux of Antarctic Intermediate Water during the Last Glacial Maximum.