Geochemical variations of core sediments of Pichavaram Lagoon, southeast coast of Tamil Nadu, India: a provenance and paleoenvironmental study

The present study aims to identify the natural and anthropogenic imprints left by the accumulation of elements and to determine/identify the current level of enrichment of major oxides in the Pichavaram mangrove sediments. Pichavaram mangrove is a dynamic intertidal environment lying in between the Vellar and Coleroon estuaries in the Cauvery delta along the east coast of India. In the present study, results of texture studies reveal highly variable depositional environment in upper and lower portion of Core 1 and middle portion of Core 2. In both the cores, decrease in grain size is observed in the upper portion which may suggest that the sediments in the upper portion were deposited at low energy condition. Mn/Al, Mg/Al, Ca/Al, Ti/Al, P/Al and K/Al ratios are found to be low in the both the core samples. Ti elemental ratio is lower in both core samples indicating that Ti concentrations in marine sediments are the best indicators of lithogenous material flux to the marine environment. The concentration of various elements in the core can be summarized as Si > Al > Fe > Mn > Mg > Ca > Ti > P > K. The PIA values of Core 1 indicate that the sediments were derived from predominant weathering up to 40 cm and the remaining sediments derived from unweathered matrices. Plagioclase Index of Alteration (PIA) values of most of the downcore samples of Core 2 are found to be more than 40 suggesting the predominance of weathering activity. Low PIA values are observed at the depth of 54–70 cm indicating the moderately weathered nature of the sediments in the study area. Positive correlation between Fe and Mg and their negative relationship with silica indicates the presence of ferromagnesian debris in top of the sediments. Major oxide geochemistry indicates a heterogeneous and moderately weathered to unweathered source for the sediments of the present study area.