Sedimentary record of the Naleshwar Lake (India): a witness of climate change and human activity during the last 100 years

Czerski, Dorota and Adatte, Thierry and Humane, Sumedh K. (2019) Sedimentary record of the Naleshwar Lake (India): a witness of climate change and human activity during the last 100 years. In: INQUA2019 Dublin Ireland, 25th -31th July 2019.

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In the present study a multiproxy approach including sedimentological, mineralogical and geochemical analyses was carried out on two cores sampled in the Naleshwar lake (Maharastra, central south India). The small reservoir lake is located on a basement of metamorphic sedimentary rocks. Forest and small villages surround the area and the local people main livelihood consists in animal breeding, fishing and agriculture activities. The semi-arid climate affected annually by the monsoon causes important yearly water level fluctuations. The study aimed to provide a reconstruction of the recent history of the lake. First, the sedimentary record was analyzed, in order to correlate the weathering of the catchment and the detrital supply due to climate variations, seasonal or over a larger scale. Secondly, the anthropogenic influence on the lake sedimentation regime was evaluated. A precise chronostratigraphy was carried out for the first time on central India lakes using the method of 137Cs activity within the sediments. The dating showed that studied sediments approximately cover the last seventy years, with sedimentation rates of 1.7 cm/year. The data revealed very fine sediments with cyclical variations between a silty and clayey deposit, which could be mainly related to climate and seasonal variation (magnetic susceptibility, granulometry, mineral fraction (%)). In the upper part a positive excursion of clay minerals, iron and phosphorus is observed. Suggesting that some change in sedimentation or supplies is registered, this could be natural or human induced. The record shows an allochtonous source of organic matter to the lake. However, the increase of phosphorus in the upper part of the core reflects an eutrophication of the basin, with a resulting increment of primary productivity in the lake. TOC (%), Ntot and the organic carbon isotope increase towards the top of the core, the combined higher HI reflects an enhancement of the algae proliferation in the basin. Additionally, a uranium rich layer corresponding to the increase of primary productivity suggests the setting of anoxic/suboxic conditions in the lake basin. This observation is coherent with the resulting depletion of phosphorus towards the top of the core. Finally, the main weathering product of the catchment is kaolinite. This clay mineral is very common in warm and humid regions and is often a product of quartzitic rocks. The clay minerals as well as the CIA (around 85%) remain constant all over the record. To conclude, the recorded eutrophication could result from human activity and the surrounding cultivated fields. But except this, the sedimentological record seems to be related to natural variations as climate. Nevertheless, the sedimentary record shows a decrease of the organic carbon isotopic composition from bottom to top of the cores. This could be the result of the global and human induced environmental change.

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