Stable isotopes constrain the genesis of Thar Desert gypsum playas and reveal Holocene paleoenvironmental variability in Northwest India

Abstract

Numerous evaporative saline playa lakes exist within the Thar Desert in Northwest India. Some are active seasonally, whereas 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). The isotopic composition of gypsum hydration water preserves the δ¹⁸O and δD of paleolake water at the time of gypsum formation. This provides a way to understand the hydrologic balance in a part of the world where it is typically very difficult to obtain any paleoclimate records. We present paleohydrological records from two dry playas (Karsandi, Khajuwala) and one active playa (Lunkaransar) in the Thar Desert using the oxygen and hydrogen isotopic composition of gypsum hydration water. We present a theoretical model to explain differences in how the gypsum records water isotopic composition from perennial playas (consistent paleoclimate recorders) as opposed to seasonally fed or ephemeral playas (that reflect evaporated meteoric water inconsistently). Results suggest that enhanced direct precipitation, with associated higher groundwater and possibly fluvial sources, maintained active playa lake basins in the central Thar Desert for the Early through Middle Holocene. We also examine δ³⁴S_SO4, δ¹⁸O_SO4, and ⁸⁷Sr/⁸⁶Sr of the gypsum sulfate to explore the source and evolution of solutes in the Thar Desert playas. Results indicate that seasalt aerosols likely accumulated in aeolian sands during glacial dry periods and concentrated in playa deposits once a threshold level of moisture was reached in the Early Holocene. By the Late Holocene, after c. 4.4 ka BP, these water sources diminished and some playas were again covered by aeolian deposits. The Thar Desert gypsum deposits provide valuable insight into local moisture balance during a time period that featured important cultural transformation in the surrounding region, including the South Asian Neolithic agricultural societies around 8 ka BP, the full span of the Indus Civilization (5.3–3.3 ka BP), and periods of human occupation after 3 ka BP.