Geochemical signatures of Late Holocene paleo-hydrological changes from Phulera and Pokharan saline playas near the Eastern and Western margins of the Thar Desert, India

Abstract

Stratigraphical, mineralogical, geochemical and optical dating methods were used to reconstruct paleo-hydrological changes in two playas (Phulera, 500 mm/a and Pokharan, 200 mm/a) in near extremum climatic regions of the Thar Desert. Sediment successions in shallow profiles from Phulera and Pokharan contain three and four stratigraphic units, respectively, each with characteristic geochemical properties. These units reflect changes in chemical weathering, detrital input, salinity and provide a measure of the changes in precipitation (i.e. monsoon) through time. Sediments from Pokharan suggest short rainfall events during ca. 6.6-4 ka, relatively stable fresh water (higher and persistent rainfall) regime during 4-2.3 ka, and a hyper saline (low rainfall) condition during 2.3-1.1 ka. Sediments at Phulera, record hyper saline (low rainfall) lacustrine conditions during < 2.3 ka to >1.4 ka. Higher abundance of gypsum in Pokharan (2.3-1.1 ka) and proto-dolomite in Phulera (2.3-1.4 ka) are nearly synchronous and reflect enhancement of salinity. Selenite crystals in Pokharan and large desiccation cracks in buried horizons at Phulera reflect desiccation of playas at ca. 2 ka. Both playas progressively became less saline after 1.4 ka. Given the regional nature of this record, these changes are attributed to fluctuation of the monsoon over the Indian sub continent.