Abstract
Thirty suspended and bottom sediment samples from the Narbada and Tapti river—estuarine systems of western India were separated into magnetic, clay and silt—sand fractions, the Al, Fe, Mn, Zn, Cr, Co, Ni and Cu concentrations of which have been determined by atomic absorption spectrophotometry. On an average, the silt—sand fraction accounts for ⩾90% of the suspended matter and bottom sediments. Of the rest, clays are more abundant than the magnetic fraction. For all fractions there are no major differences, in the region of investigation, in mineralogy between the estuarine suspended and the bottom sediments. The magnetic fraction is enriched in all metals (except Al) by a factor of 1.2–4 as compared to the other two fractions. The clays, in turn, are enriched in all metals by a factor of 1.4 ± 0.14 as compared to the silt—sand. The metal/Al ratios in the clay as well as in the silt—sand fractions do not show any variations beyond ± 25% between the freshwater end-member of the estuaries and the coastal Arabian Sea, thus indicating that the effect of estuarine processes on the inorganic solids are within this limit. In the magnetic fraction the metal/Al ratios vary by as much as an order of magnitude, whereas the corresponding metal/Fe ratios do not vary by more than a factor of 2. The clay and silt—sand fractions of sediments from the open shelf and slope regions of the Arabian Sea are enriched in Ni and Zn and depleted in Mn by a factor of 2 as compared to the estuarine suspended and bottom sediments. These enrichments are attributed to the reducing nature of the open marine environment. The inter-elemental correlations in the silt—sand fractions are good, the correlation coefficients (for 28 observations) ranging from 0.60 to 0.95 (the highest value is for Fe-Al). The suspended fluxes of metals from the Narbada and Tapti Rivers to the adjacent shelf and to the deep sea (via clays) are calculated. On average ∼5% of the total flux to the shelf region reaches the deep sea.
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