Orbital- to Millennial-scale Changes in Pacific Circumpolar Deepwater Circulation and Ventilation During the Pleistocene

Southern Ocean deep water mass dynamics, their interaction with the upper ocean and atmosphere in the Subantarctic Zone, are key components in Pleistocene climate change on orbital to (sub-)millennial timescales. Variations in ventilation and biogeochemical characteristics of bathyal and abyssal water masses, as well as changes in the stratification between those deep and mesopelagic waters play a major role in the marine carbon cycle and hence atmospheric CO₂ concentrations on geological timescales. The realisation of IODP Expedition 383 DYNAPACC to the subantarctic South Pacific closed a critical gap in available sedimentary records from the high-latitude Southern Hemisphere, in particular for the Pacific. We use those sites in combination with piston cores from R/V Polarstern campaigns to study changes in water mass patterns and their physical and chemical signatures in the Subantarctic Zone and northern section of the ACC across the last 1.2 Ma BP. We measured benthic and planktic foraminiferal oxygen and carbon isotopes to reconstruct physical and ventilation characteristics, with one focus on Lower Circumpolar Deepwater (LCDW) at the intersection to Antarctic Bottom Water (AABW). Our results provide an abyssal δ¹⁸O and δ¹³C South Pacific water mass signature over last 1.4 Ma BP with with suborbital- to millennial-scale resolution, allowing to differentiate different CDW, AABW source waters and their subsequent mixing. Complementary XRF scanning-derived Zr/Rb ratios are used to relate observed variations to abyssal bottom current strength changes in the Antarctic Circumpolar Current domain. Observed offsets between Ross Sea-derived AABW and our sites imply mixing with other source waters and higher glacial isolation from proximal Antarctic bottom water sources than previously thought.Generally, glacial epibenthic δ¹³C minima correlate with Antarctic CIrcumpolar Current strength reductions. Suborbital- to millennial-scale δ¹³C changes vary in phase with atmospheric CO² changes as recorded in the EPICA Dome C ice core back to 800 ka BP, implying a critical role of the bathyal to abyssal Pacific Southern Ocean in the Pleistocene marine carbon cycle.