Soil Profile Modification, Genesis, Chronology and Paleo-environmental Interpretations from Paleosols in a Multi-episode Aeolian Section in Western Rajasthan

A 12 meter deep section in the dune-sandy plain terrain of western part of Thar desert was investigated for aggradation history, paleoenvironment and diagenesis. Based on optical luminescence dating, this study extends the record of aeolian dynamism in the Thar Desert to ~160 ka with successive sand aggradations at 100-90, ~60, 27 and 17–13 ka. The aggradation episodes were separated by marked breaks, mostly of 30–40 ka duration. Only a part of this experienced stability, pedogenesis and calcrete development and suggested climate amelioration. From the form of calcrete and the likely time for its formation, the duration of climate amelioration phase could be of the order of 10 ka for chalky and 15–20 ka for the hard nodule calcretes. Truncated calcrete profiles provided evidence of denudation prior to subsequent aeolian aggradation. These, besides the dynamism related to physiographic position, add further complexity in interpretation and it is quite possible that occasional episode is missed out. Nodule form is characteristic feature of calcretes. Chalky nodules reflect an early stage in the formation of hard, dense nodules that also involve some dissolution and re-precipitation of calcite mass. The process was accompanied by degradation of host minerals and formation of palygorskite even in these rather young calcretes. The δ¹³C values of calcretes lie in a narrow range and are typical of a C4 vegetation ecosystem. The δ¹⁸O values of calcrete are typical of those from slightly to moderately evaporated monsoon-sourced water.