Rare earth element distributions in the Arabian Sea reveal the influence of redox processes within the oxygen deficient zone

Abstract

The Arabian Sea is a dynamic system driven by intense monsoon cycling, water mass interactions, and a persistent oxygen deficient zone (ODZ). It has been an area of much interest due to high primary productivity with the potential to act as a major source of climatically important gases, nitrous oxide and dimethylsulfide, and a sink for fixed nitrogen. Trace metal geochemistry is influenced by complex physics and biology within the Arabian Sea, and by inputs from continental margins that surround it on three sides. Rare earth elements (REE) and manganese have been studied on a transect through the Arabian Sea during the southwest monsoon in 2007. Fractionation within the REE series (especially cerium) and comparison with manganese and previously published iron data show the importance of redox gradients within the ODZ and transport from the continental margin. Shale-normalized patterns of the entire series reveal the importance of aeolian deposition, but also reveal that some dust may have been significantly altered by weathering before deposition, with important implications for other metals. Hydrographic data, a slightly positive europium anomaly, and Fe and Mn data give evidence for hydrothermal input from the Gulf of Aden. Interactions between the hydrothermally-derived, shelf-driven, and aeolian deposition inputs make it difficult to ascertain the inputs directly, but there is supporting evidence for all three.