Late Holocene geochemical history inferred from Sambhar and Didwana playa sediments, Thar Desert, India: Comparison and synthesis

摘要

The Sambhar and Didwana playas from the eastern margin of the Thar Desert (annual precipitation of 300-450 mm) have been investigated for mineralogy, major and trace element geochemistry of near surface sediments. Geochemical data and evaporite mineralogy of sediments from the playa surface and shallow depths provide insight to the present-day geochemical processes and the different vertical phases of pore water activity. Detrital minerals from both the playas reflect igneous and metamorphic source rocks in their catchments. The zeolite type mineral, analcime (NaAlSi_2O_6 • H_2O) is assumed to be authigenic in origin, a product of chemical reaction between the hypersaline brine and detrital feldspars. The evaporite mineralogy is dominated by halite, calcite and dolomite. Major differences between the evaporite mineralogy of the surface sediments of two playas are the occurrence of trace amounts of the H_2O-bearing carbonate, trona (Na_3H(CO_3)_2 • 2H_2O), in the Didwana and K- and Mg-bearing sulphate and chloride minerals e.g. polyhalite (K_2MgCa_2(SO_4)_4 • 2H_2O), carnallite (KMgCl_3 • 6H_2O), and sylvite (KCl) in the Sambhar. Based on the distribution of major and trace elements, the surface sediments from both playas are divided into three geochemical zones. Fed by river and rainwaters, the playa surfaces show enrichment of insoluble cations and hydrolysates in the outermost zone and soluble oxides and cations in the innermost zone. Further, the shallow depth profiles are also divided into distinct geochemical zones based on the elemental ratios. The sub-recent zone Ⅰ reflects low chemical weathering and higher aeolian input, the middle zone Ⅱ indicates relatively higher weathering index and zone Ⅲ, enriched in evaporites, also shows low chemical weathering. Despite the fact that the Sambhar and Didwana are separated only by ～50 km, they show significant differences in terms of evaporite mineralogy and inorganic composition of surface sediments. We interpret this to be a manifestation of local variations in precipitation, inflow and aeolian influx.