The Thar Desert Calcretes: A Proxy for Understanding Late Quaternary Paleoclimate Shifts

Abstract

In the Thar Desert, India, calcretes occur in several geomorphic environments and have been classified based on their form, induration, chemistry and associations with parent material. These calcretes have been dated from late Neogene-Early Pleistocene (hardpan calcretes) to 5 ka (nodular calcretes). The calcretes consist of CaCO₃ (40–75%) with SiO₂, Al₂O₃ and Fe₂O₃ content in decreasing amounts. Integration of multiple data including textural, chemistry and soil micromorphology indicate that the calcrete nodules formed within the dunes are pedogenic in origin while those occurring in the interdunal depressions are largely groundwater formations. Hardpan calcretes formed on various types of bedrocks are geosols and complex in origin. Stable isotope data for the pedogenic calcretes of C and O vary from −3.8 to −0.2‰, and between −6.1 and −1.5‰ for ¹⁸O PDB values, groundwater calcretes (δ¹³C −2.0 and 2.5‰, and between −5 and −1‰ for δ¹⁸O) and hardpan calcretes yield slightly higher δ¹⁸O values (−5.9 to −1.3‰) than the eolian pedogenic carbonates indicating that the calcretes in the desert margin of the Thar Desert was formed near the surface supporting a thin veneer of soil column. The carbon isotope values signify both C3 and C4 vegetation, with increasing C4 vegetation since the late Quaternary period.