Polar Front System variability and its control on export rain ratio over the past 800 ka: implication for atmospheric pCO2 changes

Antarctic ice-core records reveal that over the past 800 000 years (800 kyr), atmospheric pCO₂ closely tracked global climate variability on orbital timescales, declining progressively to ~180 ppm during glacial periods and increasing rapidly by 50–100 ppm during glacial terminations. The Southern Ocean is widely recognized as a key regulator of atmospheric pCO₂ at these timescales, owing to its strong influence on global air–sea CO₂ exchange through coupled physical and biological processes. In particular, the marine biological pump plays a central role: while the production and export of organic carbon reduce surface-ocean and atmospheric pCO₂, the formation and export of biogenic carbonate increase surface-ocean pCO₂ and may partially offset this drawdown. Consequently, variations in the balance between organic and inorganic carbon export—commonly expressed as the export rain ratio—exert a first-order control on atmospheric pCO₂. Although this ratio is relatively well constrained in the modern ocean, its past variability remains poorly documented.Here, we reconstruct glacial–interglacial changes in the rain ratio across the Southern Ocean using a combination of micropaleontological (coccoliths and foraminifera) and geochemical (CaCO₃, total organic carbon (TOC), δ¹³C, C/N) records from sediment core MD04-2718 (1428 m water depth; 48°53.31′S, 65°57.42′E), located in the Polar Front Zone (PFZ, Indian sector), complemented by published records from the Subantarctic Zone (SAZ). We show that sedimentary CaCO₃ primarily reflects the export of biogenic carbonate by calcifying phytoplankton and zooplankton, whereas TOC records the export of phytoplankton-derived organic carbon. Accordingly, TOC/CaCO₃ ratios provide a robust proxy for past variations in the rain ratio.Our results indicate higher rain ratios during glacial periods, driven by enhanced organic carbon export associated with increased diatom productivity, as colder conditions and intensified iron-rich dust inputs prevailed. In contrast, lower rain ratios during interglacials reflect strengthened carbonate export, as warmer conditions and enhanced macronutrient supply from reinvigorated Southern Ocean upwelling, favored coccolithophore and foraminifera productivity. These patterns further suggest that glacial northward migration of the polar front system, combined with reduced sea-surface temperatures and expanded sea-ice cover, promoted an equatorward retreat of coccolithophores and a northward expansion of diatom-dominated ecosystems, with opposite trends during interglacial periods. Because increases in rain ratios generally coincide with declining atmospheric pCO₂, our results support a role for glacial–interglacial rain-ratio dynamics in the SAZ–PFZ — driven by shifts between silica- and carbonate-producing phytoplankton communities — in modulating the global carbon cycle.