Geochemistry of Pichavaram mangrove sediments, southeast coast of India

Fourteen sediment samples collected from Pichavaram mangrove were analysed for grain size, organic carbon, sedimentary sulphur, calcium carbonate, Fe, Mn, Al, Cu, and Hg. Factor analysis was carried out to infer the mechanism s controlling the behaviour of metals. Four factors accounted for around 75 % variance. Fe, Mn, and tig are associated with organic carbon and fine fra ction content of the sediments and loaded onto factor 1 which accounted for 33.6% variance. This is due to the complexation of Fe, Mn, and Hg with org anic matter and the adsorption of these organo-metallic compounds onto the finer sediments due to their high surface/volume ratio and presence of surf ace charge. Factor 2 accounted for 18.6% variance and is loaded with CaCO3, finer fraction, Al, and Cu, implying contribution of Al and Cu from detrit al clay minerals. The association of CaCO3 with this factor is not clearly understood. Factor 3 is loaded with organic carbon and coarse fraction of t he sediments and it accounted for 13.3% variance. The association of organi c carbon with coarse fraction of the sediments could be either due to the C a2+ and Mg2+ ions induced adsorption of organic matter onto mineral grains, or due to the presence of coarse vegetal debris. The fourth factor account ed for 9.8% variance and is loaded with sedimentary sulphur; no metal is as sociated with this factor. This explains that though sulphate reduction occ urs in the surface sediments, it could not result in the burial of Fe, Cu, and Hg as their respective sulphides. This could be due to their back oxida tion by Mn oxides present in the sediments where upon FeS is converted to F e hydrous oxide, and Cu and Hg are released to the water column.