Sedimentary organic matter signature hints at the phytoplankton-driven Biological Carbon Pump in the Central Arabian Sea

Abstract. The Central Arabian Sea, a unique tropical basin is profoundly impacted by monsoon wind reversal affecting its surface circulation and biogeochemistry. Phytoplankton bloom associated with high biological productivity and particle flux occurs in the northern part of the central Arabian Sea due to summer monsoon-induced open ocean upwelling and winter convection. The core Oxygen Minimum Zone (OMZ) at the intermediate water depths is another important feature of the north-central Arabian Sea and fades southward. In this study, we have attempted to interlink how these factors collectively impact phytodetrital export to the sediment. Short sediment core top (1 cm) samples representing the recent particle flux signatures were analyzed from 5 locations (21° to 11° N; 64° E) in the central Arabian Sea. The C37 alkenone-based sea surface temperature (SST) proxy indicated cooler SST (27.6 ± 0.25 °C) in the north mostly due to upwelling (summer) and convective mixing (winter) and warmer (0.4 °C) in the south, which usually remains nutrient-poor. This trend was consistent with the satellite-derived average SST values (2017–2020). Lipid biomarker analysis suggested that dinoflagellates were the highest contributor as indicated in dinosterol and its degradative product dinostanol followed by brassicasterol, and C37 alkenone representing diatoms, and coccolithophores, respectively. The stations in the north (21–15° N) that largely experience periodic phytoplankton blooms and is influenced by the thick OMZ revealed the highest contents of organic matter, diatom frustules (diversity and abundance) dominated by large thickly silicified cells (e.g. Coscinodiscus and Rhizosolenia), and phytoplankton organic biomarkers, but lower zooplankton biomarkers (cholesterol and cholestanol). Whereas relatively smaller chain-forming centric (e.g. Thalassiosira) and pennate (e.g. Pseudo-nitzschia, Nitzschia, Thalassionema) diatom frustules along with lower phytoplankton biomarker contents were found in the south where zooplankton biomarkers and silicious radiolarians were more abundant. The probable impacts of the presence of the OMZ along the sampling transect on particle flux related to the phytoplankton community, zooplankton grazing along with other factors have also been discussed.