Dynamics of Surface to Intermediate-Mode Water in the Pleistocene Subantarctic Pacific (DYNAPACC Expedition 383)

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Changes in the subantarctic Southern Ocean and their interaction with the atmosphere are regarded as key components for understanding climate change on orbital to millennial timescales. Glacial to interglacial variations in sea-ice cover, ocean stratification, biological nutrient utilization and ventilation of intermediate and deep water play a key role in natural variations of Pleistocene atmospheric CO2 concentrations. We plan to reconstruct the history of Southern Ocean Intermediate Water (SOIW) for the last ca. 1.5 Ma with regard to (1) (sub)seasurface to deep thermocline stratification, temperature and salinity variations (2) ventilation and carbonate chemistry in relation to Circumpolar Deepwater (CDW), and resulting carbon cycle changes, (2) potential connections to lower latitudes via ocean tunneling. We use a planktic foraminiferal multi-species approach, combining stable isotope (18O, 13C, 11B) and elemental geochemistry (Mg/Ca, B/Ca). Using both shallow-dwelling and species living deeper in thermocline and intermediate waters, a reconstruction of the upper ca. 500 m water column shall be established based on two IODP Sites drilled during recent Expedition 383, U1541 from the pelagic East Pacific Rise and U1542 from the Chilean continental margin. Previous works have indicated differential developments between surface and thermocline patterns, pointing to variations in glacial-interglacial formation of SOIW, or lateral advection within the SE Pacific Gyre, potentially connected to changes of the Southern Westerly Wind Belt. Carbon isotopes shall be used to reconstruct the paleo-chemical history between mid-depth and upper ocean waters, while foraminiferal ∂11B measurements shall provide additional insight into directly associated carbonate chemistry changes. To attain both a zonal reconstruction and a high-resolution view into sub-millennial SOIW dynamics, hemipelagic IODP Site U1542 shall provide information on the physical conditioning and biogeochemical characteristics of SOIW. SOIW was potentially provisioning mid- to lower Pacific latitudes via the ocean tunnel mechanism with nutrients, critically needed to enhance low-latitude biological primary productivity. This southern-sourced nutrient leakage has been recently challenged intermediate water reconstructions from the North Pacific and is a matter of ongoing debate. The outlined foraminiferal analyses are supplemented high-resolution XRF core scanning data at (sub-)millennial-scale resolution on site U1542, and lower (sub)orbital-scale resolution on Sites U1541 to reconstruct productivity and changes in the flow regime of the SE Pacific Gyre and Antarctic Circumpolar Current.