Impact of carbon cycle changes in the Pacific Southern Ocean on climate shifts during the past 3.5 million years (IMPAC3.5T)

Physical, dynamical and biological processes in the Southern Ocean have a strong leverage on deep-ocean carbon storage, the stability of the Antarctic ice sheet, marine productivity and global climate. The drivers and impact of these numerous processes, particularly in the Pacific sector of the Southern Ocean, on long-term climate shifts and warmer-than-present climate conditions are insufficiently known for both the geological past and the future. In this project, we investigate the nature and interplay between deep carbon storage and ocean-atmosphere CO2 exchange in the Pacific Southern Ocean for two past climate states that provide extremes of how this ocean region might have impacted global climate and atmospheric CO2 levels: first, interglacial climates and warmer-than-present climates (i.e., the Mid-Pliocene Warm Period (MPWP), Marine Isotope Stage (MIS) 11 and 31) and second, glacial climate conditions of the last 1.7 million years and glacial intensification associated with the Mid-Pleistocene Climate Transition (MPT). The main objectives of the project are to a) quantify changes in Pacific Southern Ocean outgassing (or uptake) of CO2 as driver of atmospheric CO2 changes during these key intervals, b) estimate past changes in the ocean carbon storage in the deep Pacific Southern Ocean and link them with associated ocean-atmosphere CO2 exchange in that region, and c) assess possible mechanisms driving both. The basis of our paleo-reconstructions is the exceptional (temporally highly resolved and carbonate-rich) sedimentary record drilled at IODP Site U1541 in the central South Pacific Ocean. The project aims at achieving its goals through high-resolution reconstructions of surface ocean pCO2 levels based on planktonic foraminiferal B isotope analyses that are referenced against existing and emerging atmospheric CO2 records. These are combined with a multi-proxy assessment of concomitant variations in deep-ocean carbon storage (via foraminiferal U/Ca levels and (Cibicidoides) pore densities), surface ocean hydrography (via foraminiferal Mg/Ca ratios and stable oxygen isotopes), the deep-ocean carbonate system and sea ice extent (both via sedimentary census counts). The project provides much needed constraints on the oceanic mechanisms governing past atmospheric CO2 variations that are being reconstructed from ice cores and marine sediments alike. In addition, it significantly expands our current understanding of past marine carbon cycle dynamics during long-term (possibly irreversible) climate shifts and warmer-than present climates, specifically during the high-CO2 world foreseen for the future of our planet.