Geochemistry of ferromanganese micronodules and associated Mn and trace metals diagenesis at high terrigenous depositional site of middle fan region, Bay of Bengal

The influence of sedimentation on the morphology, growth and geochemistry of the micronodules, and on Mn enrichment in the sediments of the Bay of Bengal has been evaluated. Mn accumulation is 307–219 mg cm2 kyr−1 in cores having 20–17 cm kyr−1 sedimentation rate, but drops to 34–17.7 mg cm2 kyr−1 in cores with 3–4 cm kyr−1 sedimentation rates. However, Mn-enrichment zone becomes thinner with increased sedimentation. The abundance of the micronodules increases, and they have been observed down to the deeper levels with higher contents of Mn Ni, Cu and Zn from the areas of the lower sedimentation. The results of the present study reveal that though the higher sedimentation may be conducive for higher Mn accumulation in the upper surfaces of the sediments, the growth, abundance and enrichment of Mn, Ni, Cu and Zn in the micronodules are mostly regulated by their residence time within Mn oxidation zone. The influence of residence time on the morphology and the internal structures of the micronodules are also evaluated.