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R. Shankar ¹, A. R. Karbassi ¹

Affiliations
1 Department of Marine Geology, Mangalore University, Mangalagangotri 574 199 (DK), IN

Abstract

A geochemical study of surficial sediments of the New Mangalore Port has shown that there is no anthropogenic input of base metals (Cu, Zn, Pb, Mn and Fe). In comparison with the adjoining inner shelf sediments, Port sediments seem to have a higher proportion of detrital particles and hence relatively less organic matter. Manganese is found predominantly as adsorbed species. Other base metals are present in the lithogenous fraction. Organo-metallic association, reported by several workers for sediments, is not found in the Port sediments. Magnetic susceptibility of the sediments (Xm = 16.6 x 10^-6 emu/g) is higher than that for shelf sediments. This is because of deposition of detrital particles of magnetic minerals in the nearshore area. The susceptibility values are not high enough to suggest any particulate pollution due to the Kudremukh iron ore loading operations at the New Mangalore Port.

Keywords

Geochemistry, New Mangalore Port, Base Metals, Pollution.

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R. Shankar ¹, A. R. Karbassi ²

Affiliations
1 Department of Geology and Geophysics, University of Edinburgh, West Mains Road, Edinburgh EH93JW, GB
2 Institute of Environmental Studies, Tehran University, PO Box 14155-6135, Tehran, IN

Abstract

Sedimentological and SEM studies have been carried out on surface and sub-surface samples of outer shelf sands off Mangalore, west coast of India. The outer shelf sediments can be described as coarse sands, moderately sorted, nearly symmetrical to positively skewed and mesokurtic. Multi·group discriminant analysis of the data shows that the sands were deposited in a beach environment during Pleistocene when the sea level was lower than at present. SEM studies reveal mechanically formed features like  'v' pits and curved grooves, which also corroborate the beach environment of deposition. The' v' pits and grooves have been smoothened due to chemical etching of the grains as the latter were in contact with sea water subsequent to the Pleistocene-Holocene transgression. Similar sand in cores obtained from east of the palaeobeach indicates its extension beneath Recent sediments. The thickness of palaeobeach sand increases landward. The cores studied have recorded tbe marsh Get black colour and abundance of vegetative matter), beach (coarse sands) and inner shelf (clayey silt) environments that prevailed at the core site as the sea transgressed. It is proposed that the outer shelf sands were not covered by Recent sediments because of the insufficient supply of sediments from the small, west-flowing rivers of Peninsular India, entrapment of sediments in estuaries, and onshore transport of sediments in the West coast region.

Keywords

Palaeobeach, Mangalore, West Coast, Pleistocene-Holocene, Sedimentology.

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A. R. Karbassi ¹, R. Shankar ², B. R. Manjunatha ²

Affiliations
1 Institute of Environmental Studies, Tehran University, P.O. Box 14155-6135, Tehran, IR
2 Department of Marine Geology, Mangalore University, Mangalagangotri - 574 199, IN

Abstract

A sediment core collected from the shelf (50 m water depth) off the Mulki-Pavanje river mouth has been studied for down-core variations of base metals, magnetic susceptibility (x_m) and partitioning of metals in various accumulative phases of sediment, to understand the impact of late Quaternary sea level fluctuations on shelf sedimentation processes.

The 192 cm-long core may be demarcated into two zones: the lower (140 to 192 cm) dark green to greenish black clayey silt with abundant plant debris; and the upper (0 to 140 cm) light green to dark green silty sand. Sediments of the lower zone are characterised by higher Cu, Zn, Pb, Co, Mn, Fe, Al, x_m and organic matter contents and higher metal/Al ratios, but lower Ca, and Pb/Al and Mn/Al ratios when compared to the upper zone indicating textural control on the geochemistry of sediments. Although Mn is high in the lower zone, the low Mn/Al ratios suggest that a part of Mn has been remobilised under anoxic-sulphidic conditions. Higher Pb/Al ratios in the upper zone may not imply mobilisation of Pb, but may be suggestive of anthropogenic input of this element from the use of leaded petrol in modern times.

Geochemical and textural variations of these sediments suggest changes in the depositional environments: Sediments of the lower zone were deposited in a marshy environment when the core site was close to the shore during the late Pleistocene low stand of sea level. The late Quaternary sea level rise led to a shift in the depositional environment - from marsh to a still stand beach environment- when silty sands were deposited. Although the core site has been in the shelf since the beginning of Holocene, there has not been much sedimentation not only because of the negligible quantity of sediment supplied by the small west-flowing rivers but also due to the rapid rise in sea level during early Holocene. The reconstructed depositional history of the core is in consonance with the recently proposed sea level curve for the western continental margin of India.

Keywords

Geochemistry, Late Quaternary, Sea level Fluctuations, western Continental Shelf, Mulki, Karnataka.

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