Benthic foraminiferal response to variations in surface water productivity and oxygenation in the northern Arabian Sea

Abstract

A benthic foraminiferal record of 120,000 year obtained from the base of the Oxygen Minimum Zone (OMZ) at a site in the northern Arabian Sea was studied. Quantitative benthic foraminiferal data were compared with palaeoproductivity indices (C org and Globigerina bulloides) and indices for bottom water oxygen concentration (Pteropod Preservation Index, Mo/Al, V/Al and Mn/Al). The benthic foraminiferal record revealed two distinct assemblages that show variations in the precession frequency band. A high diversity, low equitability assemblage proliferates in isotopic stages 1, 4 and 5, and has been interpreted to reflect relatively oxygenated bottom water conditions. This assemblage shows a strong covariance with minima in summer monsoon productivity. A low diversity, low equitability fauna, typically with a few species showing high dominances, is considered to reflect low bottom water oxygen conditions related to eutrophic surface water conditions. Dominating species in this assemblage are bolivinids, buliminids and globobuliminids. This assemblage dominates during precession-driven maxima in summer surface water productivity, and also during isotopic stages 2 and 3. We suggest that an intensified and colder glacial winter (NE) monsoon led to increased winter production, which superimposed on the precession-driven changes in surface water productivity, resulted in overall poorly oxygenated bottom water during glacial stages 2 and 3. A promising proxy for tracing winter productivity in the northern Arabian Sea is the planktonic foraminifer Globigerina falconensis. The benthic foraminiferal species Rotaliatinopsis seminvoluta in particular, is prolific under glacial conditions.