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Geochemical and 13C trends in sedimentary deposits of coastal Pondicherry region, East coast of India – Insights from a borehole study

Major, trace, rare earth elements and 13C isotope results of sediments collected up to the depth of 150 m below ground level (m bgl) have been interpreted in relation to the provenance, weathering and deposition conditions of a multi layered sedimentary formation of Pondicherry region located in East Coast of India. The samples fall in the coarse to medium grain size range (mean Mz: 0.29–1.32) and the sorting values (mean SD: 0.29–1.04) mostly infer very well sorted nature of sediments. Skewness (mean Ski: 0.09-0.99) shows the domination of very fine skew while the kurtosis (mean KG: 0.76–7.73) shows sediments from extremely lepto kurtic to platy kurtic nature. Micro-structural studies infer that sediments are derived from multiple sources and transported towards the littoral zone through the fluvial processes. Relatively depleted elemental concentrations with respect to upper continental crust (UCC) (elemental ratios < 1) are noticed in most of the sediments suggesting contribution of the weathered source rocks. Chondrite normalized distributions of the selected rare earth elements show uniform trends characterized by light rare earth element enrichment and heavy rare earth element depletion, which is a typical pattern of UCC exposed to weathering and erosion. Elemental ratios such as La/Sc (2.06–6.64), Th/Sc (0.66–4.13), Th/Cr (0.06-0.26) and Th/Co (0.46–1.54) and ternary plots (La-Th-Sc, Th-Hf-Co) indicate the contribution of felsic rocks that are inherently heterogeneous in nature. Observed positive Eu anomaly in Post Archean Australian Shale (PAAS) normalized patterns indicates contribution of plagioclase minerals. Th/U ratio (2.12–9.57) of the sediments reflects deposition under oxic condition (shallow marine) whereas deeper sediments reveal anoxic condition (deep marine), which is further confirmed by δ13CTIC variations (−11.4 to +5.12‰ vs. VPDB) and Eu analmoly. Cr/Th ratio and total REE content indicate that there is no/negligible input of volcanic ash to these sediments.