Isotopic composition of gypsum hydration water in deep Core SG-1, western Qaidam basin (NE Tibetan Plateau), implications for paleoclimatic evolution

Abstract

The oxygen and hydrogen isotopic compositions of gypsum hydration water can be useful for determining the isotopic composition of the original brine from which gypsum precipitated. However, relatively few long-term and continuous records of the stable isotope geochemistry of gypsum hydration water in arid regions have been reported. We measured the δ18O and δD of primary gypsum hydration water from a 938.5m-long deep core (SG-1) in the western Qaidam Basin to study the mechanisms that contributed to gypsum formation and to reconstruct potential paleoclimatic change. The measured δ18O and δD ranged from −4.21‰ to 8.69‰ and from −72.77‰ to 49.73‰, respectively. The linear relationship between δ18O and δD indicates that meteoric water was the original source of the gypsum hydration water. The gradient of 5.39 for the δ18O and δD plots is lower than that of global meteoric water, suggesting that paleo-lakewater evaporated and became a CaSO4-rich brine leading to gypsum deposition. The evaporation/precipitation (E/P) ratio played an important role in determining δ18O and δD. The oscillations noted in the δ18O and δD of the gypsum hydration water imply that: (a) there was a long-term and stepwise aridification after ~2.2Ma in the western Qaidam Basin; and (b) there were three increasingly dry phases at 2.2–1.2Ma, 1.2–0.6Ma, and 0.6–0.1Ma, with two cold and dry events at ~1Ma and ~0.6Ma. Global cooling, especially during the Mid Pleistocene Climate Transition event (MPT), may have been the primary cause of the aridification recorded in core SG-1 in the Asian inland.