Subantarctic Eastern South Pacific surface Ocean Dynamics since the Late Miocene (IODP Expedition 383)

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The Antarctic Circumpolar Current (ACC), the world’s strongest zonal current system, connects all three major ocean basins of the global ocean and therefore integrates and responds to global climate variability. The Pacific sector of the ACC clearly lacks information on its Cenozoic paleoceanography from deep-sea drilling records. To advance our understanding of Miocene to Holocene atmosphere- ocean-cryosphere dynamics in the Pacific and their implications for regional and global climate and atmospheric CO2, we aim to investigate surface ocean changes in the northern ACC system. We will provide a unique reference record of sea surface temperature (SST) changes at orbital to suborbital time-scales to (i) evaluate the development of the large scale SST gradients across the subantarctic South Pacific and meridionally along the eastern South Pacific into the tropics, to (ii) capture long-term changes in the ACC water transported through the Drake Passage and entering the Atlantic meridional overturning circulation, and to (iii) assess the links between the thermal history of the eastern South Pacific and the dynamics of the West Antarctic and Patagonian Ice sheets. Our SST reconstructions will be based on biomarker analyses on samples from IODP Site U1543 located at ~3850 m water depth west of southern Chile at the entrance of the Drake Passage. Pleistocene SST records showed that paleo-SSTs based on UK37 paleothermometry is a highly reliable method in this subantarctic region of the Pacific. The ship-board age model for this site suggests continuous sedimentation (~5 cm/ka) throughout the past ~8 Ma. Precise age control is provided through a combination of numerous biostratigraphic markers, an exceptional paleomagnetic record, and a promising stratigraphic tuning of physical property and XRF data back to the uppermost part of the Miocene, which is to our knowledge unprecedented for the Southern Ocean. Except for subantarctic Atlantic ODP Site 1090 and ODP Sites off New Zealand (e.g. Sites 593 and 594), no continuous SST record presently extends beyond 3.5 Ma and the long-term thermal history of the Pacific as the largest Southern Ocean sector is only poorly constrained. We will document surface ocean conditions during the relatively stable warm periods of the late Miocene and Pliocene with atmospheric CO2 similar to today and will analyze how orbital-scale SST variations in the subantarctic ACC responded to different modes of orbital forcing under warmer- than-present climates. Moreover, we will learn how SSTs in the eastern South Pacific relate to the general Plio-Pleistocene cooling trend and the onset of major northern hemisphere glaciation and how orbital-scale variability changed across major Pleistocene climate transitions such as the Mid-Pleistocene Revolution. Finally, we will identify the relevance of eastern South Pacific SSTs for the development and the dynamics of the West Antarctic and Patagonian Ice Sheets.