Siliceous microfossils in the surface sediments of the eastern Arabian Sea

Abstract

The global oceanic silica cycle is regulated by siliceous plankton such as diatoms, radiolarians, and silicoflagellates. Among these, diatoms play a major role in oceanic CO₂ sequestration as they are abundant in most oceanic regions. We studied the biogenic silica (BSi) and siliceous microfossil assemblages in the surface sediments of the eastern Arabian Sea. The BSi content ranged from 1.87 to 8.51% and the major contributors to it were diatoms, sponges (their spicules) and radiolarians, with minor contributions from silicoflagellates and siliceous dinoflagellates. The abundances of diatoms and sponge spicules showed a strong positive correlation with BSi and a negative correlation with sediment grain size. Our results indicated a high abundance of diatoms in the nutrient-rich coastal sediments, while the radiolarian contribution to BSi was high in offshore oligotrophic regions. Diatom abundance showed large spatial variation, and ranged from 0.69 × 10⁴ to 1.85 × 10⁶ valves/g. The diatom assemblages in the surface sediments differed significantly from those reported for the water column. The results of this study are considered to be of particular significance with respect to ongoing effects of climate change that already pose a threat to the Arabian Sea, such as silicate limitation and ocean acidification.