Sea level changes and the Indian Summer Monsoon influence on chemical weathering and sediment deposition on the Ninetyeast Ridge over the past 21 ka

The sediment core retrieved from the Ninetyeast Ridge (NER) provides valuable insights into the interplay between chemical weathering processes and sediment deposition patterns in the marine environment. This interplay is strongly influenced by fluctuations in sea level and variations in the Indian Summer Monsoon (ISM). In this study, geochemical analyses of elemental compositions suggest that the Last Glacial Maximum (LGM) (∼21–19 ka) was a period of enhanced chemical weathering, likely driven by lowered sea levels and intensified glacial or aeolian activity. Concurrently, high surface productivity and reduction in bottom-water oxygenation favored the accumulation of organic carbon in the sedimentary record. In contrast, the deglacial period (∼17–11 ka) was characterized by rapid sea-level rise and dynamic monsoon evolution. This interval generally exhibited oxic bottom-water conditions, except for the Bølling–Allerød (B/A) interstadial. During the B/A event, increased sedimentation rates coincided with elevated paleoproductivity and suboxic marine conditions. The Holocene epoch reflects a shift, as rising sea levels and intensified ISM activity promoted enhanced chemical weathering in source regions. Notably, the Holocene Climatic Optimum (HCO) stands out as a period when increased solar insolation and strengthened monsoon circulation within the Indo-Pacific Warm Pool contributed to bottom-water deoxygenation and increased marine productivity. These findings highlight the intricate coupling between sea-level fluctuations and monsoonal forcing in modulating chemical weathering regimes and deep-sea sedimentation dynamics in the Indian Ocean basin.