Biological pump variations in the northeastern Indian Ocean since the last glaciation: evidence from coccolithophores

The biological pump (BP) constitutes a key component of the global carbon cycle by mediating the export of organic carbon from the surface ocean to the deep sea. However, long-term variability of BP strength in the northeastern Indian Ocean (NEIO), particularly within the strongly stratified Bay of Bengal (BoB), remains inadequately constrained due to the complex interplay between monsoon forcing and freshwater-induced stratification. Here, we present a high-resolution reconstruction of BP variability over the last ∼ 53 kyr based on primary productivity (PP) estimates derived from quantitative analyses of coccolithophore assemblages in a sediment core from the southern BoB. Coccoliths are abundant and well preserved throughout the record, with assemblages dominated by Florisphaera profunda , Gephyrocapsa oceanica , and Emiliania huxleyi . The reconstructed PP record exhibits pronounced glacial–interglacial (G–IG) variability, characterized by reduced productivity during the Last Glacial Maximum, followed by a progressive increase during the deglaciation and a peak in the Early Holocene. Principal component analysis of productivity records across the Indian Ocean (IO) identifies G–IG climate variability as the dominant mode of basin-scale productivity change while distinguishing secondary atmospheric and hydrological controls on regional productivity. Comparison with PP records from other sectors of the IO further highlights marked spatial heterogeneity in BP responses to basin-scale climatic forcing. In the NEIO, productivity was primarily controlled by G–IG changes in upper-ocean stratification and nutrient availability driven by sea level fluctuations and freshwater forcing, whereas secondary variability reflects the distinct atmospheric and hydrological expressions of the Indian Summer Monsoon. Our findings indicate that regional hydrographic processes governed the expression of basin-scale climatic forcing, distinguishing the freshwater-controlled NEIO from the wind-driven upwelling systems of the western tropical IO.