A paleoclimatic perspective of triple oxygen isotopes from gypsum in Holocene Thar Desert playa lakes

The Thar Desert (NW India) has numerous evaporative saline playa lakes. Some are still active and others are dry and preserve up to several meters of sedimentary deposits. These deposits feature a variety of evaporite minerals, including the hydrated mineral gypsum (CaSO₄ 2H₂O). Assuming no secondary exchange, the isotopic composition of the gypsum hydration water preserves the δ¹⁸O, δ¹⁷O and δ D of palaeolake water at the time of gypsum formation. This method provides a way to understand the hydrologic balance in a part of the world where it is typically very difficult to obtain palaeoclimate records. Our 36-hour pan evaporation experiment on site shows that triple oxygen isotopes track changes in evaporative conditions, which vary diurnally due to fluctuating temperature and relative humidity, and appear to reflect night-time condensation. We present new palaeohydrological records from two dry playas (Karsandi, Khajuwala) and one active playa (Lunkaransar) in the Thar Desert using the triple oxygen and hydrogen isotopic composition of gypsum hydration water. Results show that a source of water maintained active playa lake basins in the central Thar Desert for much of the Holocene, either by enhanced direct precipitation and/or fluvial sources. The derived ¹⁷O-excess and d-excess data potentially enable modelling of past changes in relative humidity, once other parameters (windiness, evaporation/inflow, etc.) are set.