Processes Controlling Enrichment of Transition Elements in the Surficial Sediments of the Western Bay of Bengal

The processes leading to surficial enrichment of transition elements have been studied in continental slope and deep-sea sediments from the western Bay of Bengal (~500–4500 m water depth). The lower continental slope and rise (1800–3000 m) sediments are characterized by enrichment of Mn/Al ratios by 2–3 orders of magnitude and transition elements (Fe, Ni, Co, Cr and V) by a factor of 2–3 compared to the upper continental slope (500–1100 m) sediments. The occurrence of micronodules rich in Fe and Mn and elements associated with Mn-oxides, confirmed by scanning electron microscope and electron probe micro analysis, are responsible for the accumulation of transition elements in the lower continental slope sediments. The freshwater stratification, low carbonate productivity, fine grain size, low porosity, thin oxidized surficial layer and high sedimentation are conducive for diagenetic enrichment of Fe, Mn and elements associated with Mn-oxides in the lower slope sediments. The mid-depth oxygen minimum zone, resuspension and absence of oxidized surface layer enhance metal release to water column in the upper slope sediments. The oceanographic settings and high sedimentation rates control metal cycling in the Bay of Bengal sediments.