Productivity Changes in the North-west Indian Ocean (Arabian Sea) over the last 55 kyr: Interplay between Monsoon and Upwelling

The North-west Indian Ocean (Arabian Sea) is the second most significant upwelling zone, contributing 90 Tg-Cyr^-1 to the atmosphere. This region is oceanographically dynamic with a reversal in summer and winter monsoonal wind direction, resulting in season-specific upwelling and runoff dynamics. For example, the southwest (SW) monsoon drives large-scale upwelling and high productivity in the Western and Northern Arabian Sea, while the Eastern Arabian Sea (EAS) experiences comparatively moderate upwelling (Singh et al., 2011). In contrast, northeast monsoon winds promote convective mixing off central India and contribute to maintaining the strong Oxygen Minimum Zone (OMZ) at ~150-1200 m (Wyrtki, 1973; Naqvi, 1991. Despite its importance, high-fidelity paleoclimate reconstructions from this region are sparse.

This study aims to reconstruct past surface ocean productivity as a function of wind-driven upwelling and or surface runoff, thereby providing new constraints on carbon dynamics in the Eastern Arabian Sea over the last 55-kyr. We used the surface-dwelling planktic foraminifera species Globigerinoides ruber for reconstructing sea surface temperature (Mg/Ca), sea surface salinity (δ¹⁸O), surface ocean productivity and nutrient supply (Ba/Ca, Cd/Ca). All samples were reductively cleaned following Boyle & Keigwin (1985), ensuring reliable Cd/Ca and Ba/Ca measurements.

The Mg/Ca-derived SSTs demonstrate ~2–3 °C cooling during the LGM, whereas the earlier stadials are punctuated by modest cooling, consistent with reduced SW monsoon intensity (Elderfield & Ganssen, 2000; Saraswat et al., 2005; Anand et al., 2008). The δ¹⁸O_sw derived salinity reaches its maximum during the LGM, reflecting weakened summer monsoon precipitation, enhanced evaporation, and a southward migration of the Intertropical Convergence Zone (ITCZ) (Ivanochko et al., 2005).

Our foraminiferal Cd/Ca ranges from 0.03 to 0.18 µmol/mol, and Ba/Ca from 1.0 to 2.7 µmol/mol, reflecting substantial variability in nutrient supply, freshwater discharge and export productivity over the last 55 kyr. The following interpretations are based on the preliminary observations. Both Cd/Ca and Ba/Ca covary, with pronounced peaks during HS4 and HS5 and more moderate increases during HS2-HS3, consistent with intensified winter monsoon-driven mixing and shoaling of the nutricline (Altabet et al., 2002; Ivanochko et al., 2005). The LGM further combines elevated Cd/Ca with low Ba/Ca and the highest salinities, indicating reduced freshwater discharge and weak SW monsoon rainfall, which lowered dissolved Ba inputs while suppressing upwelling, and limiting export productivity. In contrast, HS1 is characterised by decreasing salinity, elevated Ba/Ca and moderate Cd/Ca values, suggesting enhanced export productivity (Lea & Boyle, 1991) or freshwater discharge despite lower nutrient concentrations than during the LGM. This pattern indicates that HS1 productivity is mediated by mixing-driven nutrient inputs, but in a hydrographic context distinct from the highly saline, nutrient-rich but low-productivity LGM.

Together, this study demonstrates that EAS productivity was profoundly shaped by shifts in monsoon-driven upwelling and freshwater runoff, thereby reflecting the far-reaching influence of high-latitude climate perturbations on tropical ocean processes.