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

南大洋的物理、动力和生物过程对深海碳储存、南极冰盖的稳定性、海洋生产力和全球气候具有强大的影响力。这些众多过程，特别是在南大洋太平洋部分，对长期气候变化和比现在更暖的气候条件的驱动因素和影响，对于地质过去和未来都是不够了解的。在这个项目中，我们研究了南太平洋深层碳储存和海洋-大气二氧化碳交换之间的性质和相互作用，以及过去两个气候状态的相互作用，这两个气候状态提供了该海洋区域可能如何影响全球气候和大气二氧化碳水平的极端情况：第一，间冰期气候和比现在更暖的气候(即中上新世暖期(MPWP)、海洋同位素阶段(MIS11和31))和第二，过去170万年的冰川气候条件和与中-更新世气候转变相关的冰川加剧。该项目的主要目标是：(A)量化南太平洋二氧化碳放出(或吸收)的变化，作为这些关键时段大气二氧化碳变化的驱动因素；(B)估计南太平洋深海海洋碳储存过去的变化，并将其与该区域相关的海洋-大气二氧化碳交换联系起来；(C)评估推动这两种变化的可能机制。我们古重建的基础是在南太平洋中部IODP站点U1541钻探的特殊(时间上高分辨率和富碳酸盐)沉积记录。该项目旨在通过以现有和新出现的大气二氧化碳记录为参照的浮游有孔虫B同位素分析，通过高分辨率重建表层海洋二氧化碳水平来实现其目标。结合了对深海碳储量(通过有孔虫U/Ca水平和(Cibiidoids)孔密度)、表层海洋水文(通过有孔虫镁/钙比值和稳定氧同位素)、深海碳酸盐系统和海冰范围(两者都通过沉积普查计数)伴随变化的多指标评估。该项目为控制过去大气二氧化碳变化的海洋机制提供了亟需的约束，这些机制正在从冰芯和海洋沉积物中重建。此外，它大大扩展了我们目前对长期(可能不可逆转)气候变化和比目前气候变暖的过去海洋碳循环动态的理解，特别是在我们星球未来预见的高二氧化碳世界期间。

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.