Pleistocene Open Ocean Antarctic Intermediate Water Variability

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As part of the global thermohaline circulation Antarctic Intermediate Water (AAIW) not only redistributes heat and salt but also ventilates intermediate depths and transports nutrients from the Southern Ocean (SO) to the nutrient deprived tropics. AAIW is therefore of global importance to marine biogeochemistry and carbon cycling. AAIW formation has been intrinsically tied to the upwelling of Circumpolar Deep Water (CDW) in the SO and has been modulated by westerly winds and a seasonal freshening related to sea ice export and melt. Although it is clear that AAIW transport and composition played an important role for the oceanographic and climatic changes during the last deglaciation there are very few longer records of AAIW variability. Although still debated, a consensus on a persistent, if slightly weaker AAIW flow during the last glacial maximum has started to emerge from proxy data for the Atlantic. Neodymium isotope records that suggested greater and more rapid variability have been shown to be biased by sedimentary overprinting, which is a problem inherent to the continental shelf settings where most AAIW depth cores were recovered. To avoid such overprinting and to develop an understanding of longer term AAIW variability we propose a new study utilising intermediate water depth drill cores from open ocean locations in the South Atlantic (DSDP Site 516), the SE Pacific (ODP Site 1236) and the Tasman Sea (DSDP Site 592 and IODP Site U1510). Although the sedimentation rates at these locations are low, first preliminary results indicate that the full amplitude of glacial/interglacial benthic oxygen isotope variability expected for intermediate waters has been recorded and thus also other parameters will be valid without significant effects of bioturbation. Rare earth element patterns obtained from these oxic sediments will complement benthic foraminiferal stable C & O isotopes and trace metal proxies for temperature (Mg/Ca, Li/Mg) and nutrient content (Cd/Ca, Ba/Ca), as well as mixed foraminiferal coating Nd isotopes as proxy for intermediate depth water mass mixing. These tools will allow the reconstruction of the water mass sources contributing to AAIW, its nutrient content and its temperature evolution, spanning approximately the last 1.5 million years and covering the important mid Pleistocene transition (MPT). After the establishment of a high resolution benthic isotope stratigraphy at each site, samples spanning key glacial and interglacial intervals from before, during and after the MTP will be analysed for all proxies. Taken together, these records from different basins will provide important new insights into the potential role of AAIW in shaping the evolution of glacial terminations given that AAIW directly connects the Southern Ocean with the low latitude ocean basins. New insights will also be gained into how ocean circulation responded to orbital and increasing ice sheet forcing during the Pleistocene.