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

Czerski, Dorota and Adatte, Thierry and Humane, Sumedh (2017) Sedimentary record of the Naleshwar Lake (Maharastra, India): a witness of climate change and influence of human activity during the last 100 years. In: Swiss Geoscience Meeting, 17.11.2017-18.11.2017, Davos, Switzerland.

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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 a lake in Maharastra, central south India. The aim is to provide a reconstruction of the recent history of the small reservoir 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. The Naleshwar Lake is located on a basement composed of metamorphic sedimentary rocks. Forest and some 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 cause important yearly water level fluctuations with a consistent variation of the surface covered by the lake. In 1914 a dam was built in order to constitute a reservoir, control the water level and extract it for irrigation. When the lake level drops during the dry season, the exposed lakeside portions rich in nutrients are used for cultivation. Pesticides and fertilizers are spread on the soil and this, combined with the other human activities, may infer a signature in the lake sediments. The monsoon plays a very important role for the existence of lakes in the region. Some studies illustrate how lake sediments can record the Holocene climate variations linked to the monsoonal system. Our project takes into consideration a much shorter period of time, nevertheless, variations related to the climate seasonality could still be observed. The analyses were concentrated on two one-meter cores sampled with a gravity corer in the deepest part of the lake (4.9 meters depth). The cores were analyzed with a resolution of one centimeter and 0.5 centimeters in the upper 20 centimeters of the cores. A comparison with soil profiles and rock samples collected around the lake was carried out, in order to better understand the record of the lacustrine sediments. Thus, a precise chronostratigraphy was carried out for the first time on central India lakes using the method of 137Cs activity within the sediments. The 137Cs 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 for this level, which could be natural or human induced. The increase of phosphorus in the upper part of the core reflects an eutrophication of the basin, as a result an increment of primary productivity in the lake is observed. Both TOC (%) and Ntot increase towards the top of the core, this combined with a higher HI shows an enhancement of the algae proliferation in the basin. Moreover, the organic carbon isotope record also shows a slight increase in the last part of the record, confirming the increase in productivity and the subsequent depletion of lighter carbon isotope in the basin. The data mainly revealed an allochtonous source of organic matter to the lake. The HI and OI records showed a vascular plant origin or a very degraded organic matter. However, the TOC/Ntot ratio showed an OM signature of lacustrine origin. This can be explained with the results of the soil profiles around the lake, which show similar values. Intense bacterial activity within the soil and subsequent leaching during the rainy season represent in fact a good explanation. 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. Unfortunately no pyrite framboids were observed at SEM. 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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