Abstract
Suspended matter supplied from the Ganges–Brahmaputra river system was sampled in waters of the adjacent shelf area in times of low suspended matter load (November/December 1997). The absolute amount of suspended matter ranges between >500 mg/l near the river mouth and <1 mg/l on the outer shelf. Chemical and mineralogical analyses of the suspended matter show changes in its composition with distance from the source. Compositional changes are based on grain size partitioning; quartz and heavy minerals are precipitating sooner, leaving the suspended matter from the outer shelf depleted in quartz, REE, Zr, Hf, Nb, Ta, W, U, Th, Y, and Ca. The other part of the compositional changes is based on the reaction of suspended matter with seawater. Manganese hydroxide is producing coatings on suspended matter grains, and adsorbing elements out of the seawater. As a consequence, enrichment of Mn, Zn, Ni, Cr, Co, Li, Cs, As, Sb, Pb, Tl, and Cd can be observed in the suspended matter from the outer shelf. Arsenic is accumulating on the inner shelf by a factor of 2. Chemical partitioning in sequential leaching experiments shows that arsenic does not follow the enrichment mechanism of manganese and other transition metals. More than 90% of the total arsenic is bound to nearly insoluble phases of the suspended matter (e.g., crystalline iron oxides, residual minerals). For comparison with suspended matter, adjacent surface sediments were sampled and analysed. In this sediment, the dominating phase is the one which settled down during the times of high suspended load, with more quartz and heavy minerals and consequent chemical composition. The enrichment of manganese oxide and adsorbed transition metals gets lost; because of anaerobic conditions in the bottom-near seawater, they dissolve from the sediment and get recycled in seawater. Arsenic is found in nearly insoluble parts of the sediment, as it is the case in the suspended matter
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