Polar Front System Variability and Its Control on Export Rain Ratio Over the Past 800 ka: Implication for Atmospheric p CO ₂ Changes

Little is known about long‐term variability in the organic carbon to carbonate carbon export rain ratio, despite its key role in the global carbon cycle. Here, we reconstruct glacial‐interglacial changes in the sedimentary rain ratio over the Southern Ocean, using micropaleontological (coccoliths and foraminifera) and geochemical (CaCO 3 , Total Organic Carbon (TOC), δ 13 C, C/N) records from sediment core MD04‐2718, located in the Polar Front Zone (PFZ, Indian sector), complemented with published data sets from across the Subantarctic Zone (SAZ). We show that sedimentary CaCO 3 primarily reflects the export of biogenic calcium carbonate by calcifying phytoplankton and zooplankton, while TOC captures the export of phytoplankton‐derived organic carbon. The sedimentary TOC/CaCO 3 ratio thus serves as a robust proxy for past variations in the balance between organic and inorganic carbon export and hence, of the export rain ratio. Our results indicate higher rain ratios during glacial periods, driven by enhanced organic carbon export, as colder conditions and intensified iron‐rich dust inputs stimulated diatom productivity. In contrast, lower rain ratios during interglacials reflect strengthened biogenic carbonate export, as warmer conditions and elevated macronutrient supply from reinvigorated Southern Ocean upwelling supported coccolithophore and foraminifera blooms. These shifts reflect an ecological seesaw between silicifying and calcifying phytoplankton modulated by changes in westerly wind intensity and the position of the Polar Front. The negative correlation between rain ratios from the SAZ‐PFZ and atmospheric pCO 2 suggests that enhanced organic carbon export during glacials likely promoted deep‐ocean carbon sequestration, modulating atmospheric CO 2 levels.