Stable isotopic evidence for the origin of salt lakes in the Thar Desert

There are a number of salt lakes in and around the Thar Desert, India. A few major ones are Sambhar, Kuchaman, Didwana in Rajasthan and Kharaghoda in Gujarat. There have been a number of hypotheses concerning the origin of these salt lakes. One of them is that these lakes are relicts of the Tethys Sea that existed in this region before the collision of the Indian and Eurasian plates around 70 Ma ago. We have tested this hypothesis by collecting more than 50 brine and fresh water samples from in and around these lakes and measuring their stable isotope ratios of oxygen (δ18O) and hydrogen (δD). Our results show no evidence for the above hypothesis of marine origin for these lakes. On the contrary, the isotopic data are consistant with the hypothesis that the water in these lakes are of meteoric origin and the salt is locally derived by the weathering of rocks in this region. A simple model calculation shows that these lakes behave as terminal lakes; they receive water during the short monsoon season (June-September) and evaporate during the rest of the year (there is no outflow and evaporation is equal to inflow). The model calculated saturation δ18O value for these lakes is in good agreement with the mean measured value indicating that these lakes are formed by accumulation and evaporation cycles fed by local precipitation.