Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

P. Periakali ¹, S. Eswaramoorthi ¹, S. Subramanian ¹, P. Jaisankar ¹

Affiliations
1 Climate Change Research Unit, Department of Applied Geology University of Madras, Guindy Campus, Chennai 600 025, IN

Abstract

Fourteen sediment samples collected from Pichavararn 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 mechanisms controlling the behaviour of metals. Four factors accounted for around 75% variance. Fe, Mn, and Hg are associated with organic carbon and fine fraction content of the sediments and loaded onto factor I whlch accounted for 33.6% variance. This is due to the complexation of Fe, Mn, and Hg with organic matter and the adsorption of these organo-metallic compounds onto the finer sediments due to their high surface/volume ratio and presence of surface 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 detritai clay minerals. The association of CaCO₃, with this factor is not clearly understood. Factor 3 is loaded with organic carbon and coarse fraction of the sediments and it accounted for 13.3% variance. The association of organic carbon with coarse fraction of the sediments could be either due to the Ca²⁺ and Mg²⁺ ions induced adsorption of organic matter onto mineral grains, or due to the presence of coarse vegetal debris. The fourth factor accounted for 9.8% variance and is loaded with sedimentary sulphur; no metal is associated with this factor. This explains that though sulphate reduction occurs 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 oxidation by Mn oxides present in the sediments where upon FeS is converted to Fe hydrous oxide, and Cu and Hg are released to the water column.

Keywords

Geochemistry, Sediments, Pichavaram Mangroves, Trace Metals, East Coast of India.

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P. Periakali ¹, S. Eswaramoorthi ¹, S. Subramanian ¹

Affiliations
1 Climate Change Research Unit, Department of Applied Geology, University of Madras, Guindy Campus, Madras- 600025, IN

Abstract

The sequential extraction of core sediment samples of Pulicat lake for the determination of exchangeable, carbonate-bound. Fe-Mn oxides-bound, and organic-bound fractions of Fe and Mn were carried out to study the remineralization of organic matter in the sediments. Iron and manganese oxides are highly depleted in the upper portion (0-18 cm) of the core due to their reduction during organic matter oxidation. The model calculations based on the solid phase organic carbon depth profile indicate an average accumulation rate for organic carbon in the Pulicat lake sediments to be 13.5 gC/m².yr. The average remineralization rate is 12gC/m².yr. This is around 89% of the organic carbon deposited in thc sediments. The remineralization of organic carbon in this lake sediment could release 16 x 10³ tonnes of CO₂ /yr.

Keywords

Organic Geochemistry, Carbon, Sequential Extraction, Trace Metals, Core Sediments, Pulicat Lake, Tamil Nadu.

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