Distribution and Estimation of Short Term Fall Out Rate of Atmospheric ¹⁰Be in Sambhar and Didwana Lake, Rajasthan, India

Ekka, Shail Vijeta (2014) Distribution and Estimation of Short Term Fall Out Rate of Atmospheric ¹⁰Be in Sambhar and Didwana Lake, Rajasthan, India. Masters thesis, Indian Institute of Science Education and Research Kolkata.

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Abstract

In recent years, cosmogenic radionuclide such as ¹⁰Be has proved to be a potential tracer to find out the sedimentation rate, paleoclimatic variation and past solar activity since its production rate varies with the latitude and short residence time in the atmosphere. ¹⁰Be which is produced in the atmosphere by spallation reaction reaches the earth’s surface through dry and wet deposits. Consequently, it gets incorporated into the sediment and ¹⁰Be from successive layers of sediments of lake can provide various information about the Earth surface processes. The concentration of ¹⁰Be is very low in the continental sediments. Therefore a long chemical pre-concentration procedure is being followed prior to accelerator mass spectrometry (AMS) measurement of ¹⁰Be on sediment samples. In this study sediment samples were collected from Sambhar and Didwana Lake of Rajasthan to estimate present day ¹⁰Be flux into these lakes. These lakes are situated in the eastern margin of the Thar Desert; where contribution of dry deposits of 10Be is expected to be higher than wet deposits. In the sediment ¹⁰Be can be present in the different phases such as exchangeable, organic, carbonate, oxide and residual. A part of sequential extraction method was followed here to extract only atmospheric ¹⁰Be from the sediment. Here ‘Be’ was pre-concentrated from water soluble, acid soluble (8N HCl + 30% H₂O₂) at room temperature and residual phase separately. About 1g of powdered sediment samples were taken for these phase separation. Prior to Be pre-concentration known amount of ⁹Be carrier solution was added into these phases. Using anion and cation exchange column Be was separated from these phases and then Be was precipitated as hydroxide from the separated solution. Finally Be hydroxide was converted to BeO after step heating to 900°C. Water and acid soluble phase together accounts 40 – 50 wt% in the Sambhar lake sediments whereas it is lower (20 – 30 wt %) in the Didwana lake sediments. The observed ¹⁰Be concentration in the water soluble phase (Batch 1) of the sediments is varying between 0.27 ± 0.08 to 1.29 ± 0.29 × 107 atoms/g. Whereas, in the acid soluble phase (Batch 2), ¹⁰Be concentration is higher and ranges from 5.73 ± 0.93 to 16.60±1.92 × 107 atoms/g. The results indicate that the 90±6% of atmospheric ¹⁰Be present in the acid soluble phase of the sediments. The percentage of water soluble phase of the sediment does not correlate with the ¹⁰Be concentration but ⁹Be shows a negative correlation. The percentage of water soluble phase of the sediment is positively correlated with the percentage of evaporite mineral present in the sediments. As the Sambhar is a hypersaline lake, evaporite minerals are common in the surface sediment. But the atmospheric 10Be as well as ¹⁰Be mainly derived from the catchment area of the lake does not get incorporated in the water soluble phase of the evaporite minerals.

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