Surface phytoplankton dynamics and biological pump variability driven by global climate cooling in the South Tasman Sea over the past 1960 kyr

Regional paleoenvironmental and paleoclimatic changes can be inferred from variations in surface ocean phytoplankton assemblages. This study reconstructs at a high temporal resolution the evolution of the phytoplankton structure and biological pump efficiency over the past 1960 kyr on the basis of the biomarker analyses of a long sediment core from the Ocean Drilling Project (ODP) Site 1170 in the South Tasman Sea. The Mid-Pleistocene Transition (MPT) notably enhanced regional phytoplankton abundance and biological pump efficiency. During the MPT, surface phytoplankton structure was strongly influenced by global cooling and increased Fe input from intensified terrestrial weathering, leading to pronounced phytoplankton blooms aligned with reduced atmospheric CO₂ levels. Both terrigenous herbaceous vegetation and aquatic lipid content increased significantly. On orbital timescales, correlations between diatom and coccolithophore contents and other climate indices, including Antarctic Circumpolar Current (ACC) strength, atmospheric CO₂, Fe MAR, and global ice volume (δ¹⁸O_B), were predominantly expressed on a 100-kyr cycle, whereas the Ca/Si ratio showed stronger correlations on a 40-kyr cycle. Furthermore, after the MPT, the relationship between surface phytoplankton structure, biological pump efficiency, and atmospheric CO₂ weakened, while the influence of physical process indicators such as ACC strength, global ice volume, and Fe MAR became increasingly pronounced. These results indicate that oceanic physical processes, including ACC strength and bottom-water ventilation, became the primary drivers of CO₂ dynamics after the MPT, diminishing the relative role of biological processes. This study provides a long-term record of surface phytoplankton structure and reveals the impact of Earth's climate cycles on the biological pump efficiency.