QUATERNARY PALAEOCLIMATIC RECONSTRUCTION FROM MINERALOGY AND GEOCHEMISTRY OF THE SAMBHAR LAKE PLAYA SEDIMENTS, THAR DESERT, INDIA

Multiple phases of clastic- and evaporite- dominated deposition is revealed from a deep core (~23m) from the Sambhar lake playa located at the eastern fringe of the Thar desert in Rajasthan, India. Mineralogical and geochemical data have been used to identify evaporite facies reflecting complex interactions between playa hydrology and brine chemistry. Sharp mineralogical variations marked by relative dominance of carbonates and sulfates reflect fluctuations in salinity level induced by climate change. The complete absence of gypsum in the top 5m and dominance of carbonates in the bottom ~7m of the sequence are remarkable. The relative dominance of dolomite over calcite at certain levels coupled with high d¹⁸O values suggest high salinity conditions during which dolomitization of calcite by Mg-rich brines was favoured. Increased evaporative conditions would favor precipitation of gypsum thereby increasing Mg/Ca ratio and permitting the brine to act as a dolomitizing fluid. The upper ~5 m of the playa deposits are rich in organic matter suggesting high organic productivity. d¹³C values of the organic matter in the top 5 m shows rapid fluctuations (about -15 to - 24 ‰). Although such fluctuations could reflect relative abundance of C₃ versus C₄ or of terrestrial versus aquatic vegetation, such regional changes in vegetation type in the Thar desert during Late Holocene is not plausible. We believe that such variations in d¹³C values may reflect in situ variation of d¹³C due to lake level/salinity fluctuations. We observe a significant correlation between the d¹⁸O values and the MgO/(MgO+CaO) ratios of the carbonate fraction of the Sambhar Lake plays which supports our interpretation that the variation in the d¹⁸O values chiefly reflect changes in degree of brine evaporation i.e. in evaporation/inflow ratios. There is also a close correlation between the d¹⁸O values and geochemical ratios (Na/Al, Na/Ti, K/Na) and this strengthens our interpretations. Although poorly constrained chronologically at this stage, our data reconstructs the paleohydrological and paleolimnological conditions, induced by climatic fluctuations, in the Thar region for the last 25ka and may serve as the longest reconstruction so far in the Thar desert based on lacustrine sequences.