ON THE OCCURRENCE OF FERROMANGANESE MICRONODULES FROM THE SEDIMENTS OF THE BENGAL-FAN - A HIGH TERRIGENOUS SEDIMENT INPUT REGION

Studies of 13 samples (10 surface sediments and 3 cores) from across t he middle fan region of the Bay of Bengal reveal the presence of micro nodules (area 0.22 million km(2)) over the entire bay. These micronodu les have two dominant morphological forms: botryoidal in the eastern b ay and spheroidal in the central and the western regions of the bay. T he abundance of the micronodules was found to be higher in the eastern region of the bay. In the cores, the micronodules were generally foun d to decrease in abundance down-core. The core from the eastern region had micronodules down to deeper levels than the western side. Todorok ite is the manganese mineral present in the micronodules. Associated m ineral phases are quartz, feldspar and phillipsite. These micronodules have high Mn (19.20-32.13%), rather low Fe (1.20-1.83%), Ni (0.10-1.1 4%), 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 distri butions and the O/Mn ratio in the sediments, remobilization of Mn from deeper levels and its precipitation at the upper surfaces of the sedi ment 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 a ffect their growth.