On the occurrence of ferromanganese micronodules from the sediments of the Bengal Fan: A high terrigenous sediment input region
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Studies of 13 samples (10 surface sediments and 3 cores) from across the middle fan region of the Bay of Bengal reveal the presence of micronodules (area 0.22 million km@u2@@) over the entire bay. These micronodules have two dominant morphological forms: botryoidal in the eastern bay and spheroidal in the central and the western regions of the bay. The abundance of the micronodules was found to be higher in the eastern region of the bay. In the cores, the micronodules were generally found to decrease in abundance down-core. The core from the eastern region had micronodules down to deeper levels than the western side. Todorokite is the manganese mineral present in the micronodules. Associated mineral phases are quartz, feldspar and phillipsite. These micronodules have high Mn (19.20-32.13%), rather low Fe (1.20-1.83%), Ni (0.10-1.14%), Zn (0.10-0.30%) and Co (0.01-0.04%), and their Mn/Fe ratio ranges from 10.55 to 25.42. The contents of Mn, Ni, Cu and Zn, and the Mn/Fe ratio decrease down-core. Based upon the variations in the abundance, morphology, Mn/Fe ratio of the micronodules, and the elemental distributions and the O/Mn ratio in the sediments, remobilization of Mn from deeper levels and its precipitation at the upper surfaces of the sediment column, in an oxic environment, is suggested. The results of the present study also suggest that a high rate of sedimentation is not a limiting factor for the formation of micronodules, although it might affect their growth.
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