Atmospheric Deposition, Shelf Sediment Supply, Riverine Input, and Redox Conditions Control Dissolved Manganese in the Indian Ocean

Abstract

Dissolved manganese (dMn) is an essential bioactive element required for marine organisms. Redox condition determines its solubility and its solid phase removal from seawater. It displays a typical scavenging type profile in the Indian Ocean with an elevated concentration in the Oxygen Minimum Zone (OMZ) of the Bay of Bengal (BoB). The surface dMn decreases southward in the BoB, and its concentration gradient correlates well with salinity because of the enormous riverine influx. Reductive dissolution of Iron-manganese (Fe-Mn) oxyhydroxides-rich sediments brought by the Ganga-Brahmaputra rivers enriches dMn in the bottom waters of the shelf regions (∼25 nM), which gets advected to the open ocean through cross-shelf transport. The atmospheric input is the prominent source of dMn in the BoB. Transport of the Indonesian Through Flow waters supplies high dMn in the surface waters of the Central Indian Ocean Basin. Internal cycling seems to control the dMn distribution in the water column in addition to its external sources. Water column denitrification increases dMn in the OMZ waters of the BoB through the reductive dissolution of sinking Mn oxide particles under the prevailing suboxic conditions. The presence of two sub-surface peaks of dMn associated with nitrite maxima suggests active denitrification in the OMZ waters of the BoB, similar to the Arabian Sea. The interaction of circulating fluid with subducting Fe-Mn-rich crusts enriches the deep water dMn in the Java Sumatra region. Further, the hydrothermal activity over the Southeast and Central Indian Ridges contributes significantly to the dMn budget of the deeper waters.