A highly diverse living benthic foraminiferal assemblage in the oxygen deficient zone of the southeastern Arabian Sea

Abstract

The exchange of seawater between the Arabian Sea and the Bay of Bengal, creates a strong seasonality in the southeastern Arabian Sea, including the oxygen deficient zone (ODZ) at intermediate depth. Here, we assess the effect of this strong seasonality on benthic foraminifera living within ODZ of the southeastern Arabian Sea. We delineate five benthic foraminiferal assemblages characterizing different dissolved oxygen levels. A distinct depth zonation in living benthic foraminiferal species is observed within the southeastern Arabian Sea ODZ. Cassidulina laevigata, Cassidulina carinata and Eponides umbonatus dominate shallow depth, hypersaline, oxygen rich, warm water environment with low organic matter (percent C sub(org)) availability. The upper slope intense suboxic zone with moderate percent C sub(org) abundance, is characterized by Bolivina seminuda, Hopkinsinella glabra and Eubuliminella exilis. A similar suboxic zone, but with high percent C sub(org) concentration on the middle slope, is represented by Rotaliatinopsis semiinvoluta, Hopkinsinella glabra and Epistominella exigua. The lower slope (approx. 1000-1500 m) assemblage is dominated by rectilinear species, namely Bulimina arabiensis, Bulimina elegans and Bolivina earlandi and represents a comparatively higher dissolved oxygen environment (1-2 mL/L) with high organic carbon at deeper water depths. Bolivina earlandi, Bulimina aculeata and Globocassidulina subglobosa dominate deep-water benthic foraminifera population, where bottom water is oxygen rich (greater than 2 mL/L) with plenty of organic matter and the lowest bottom water temperature and salinity. The living benthic foraminiferal abundance as well as diversity increases manifold within the southeastern Arabian Sea ODZ and may be attributed to the strong seasonality. The depth habitat of the dominant living benthic foraminifera has also been reported.