Hydromagnesite precipitation in the Alkaline Lake Dujiali, central Qinghai-Tibetan Plateau: Constraints on hydromagnesite precipitation from hydrochemistry and stable isotopes

Abstract

The mineral hydromagnesite, Mg5(CO3)4(OH)2·4H2O, is a common form of hydrated Mg-carbonate in alkaline lakes, yet the processes involved in its formation are not well understood. This study focuses on Dujiali Lake, in the central Qinghai-Tibetan Plateau (QTP), which is one of the few environments on the earth's surface with extensive Holocene precipitation of hydromagnesite. The hydrogeochemistry of surface waters, and the mineralogical, stable isotope (δ13C and δ18O), and radiogenic isotope content of hydromagnesite deposits were analyzed to investigate formation mechanisms. The chemical composition of surface water around Dujiali Lake evolved from the rock-weathering-type waters of T1 (CaMgHCO3 water type) to more concentrated sodic waters of T2 (NaSO4Cl water type) due to evaporation. XRD results show that the mineralogical composition of samples is pure hydromagnesite. Analysis of oxygen isotopes in the hydromagnesite indicates that supergene formation with authigenic carbonate crystallization from evaporation water is the dominant precipitation process. Combined carbon-oxygen isotope analysis suggests atmospheric CO2 provided a carbon source for the precipitation of hydromagnesite. These findings suggest that hydromagnesite precipitation at Lake Dujiali is mainly inorganic in nature, and the greenhouse gas, CO2, is trapped and stored in the hydromagnesite directly from the atmosphere. AMS radiocarbon dating of samples indicates CO2 was sequestered between 5845 ± 30 to 6090 ± 25 cal a BP in the Dujiali Lake hydromagnesite deposit. The study contributes to improved understanding of hydromagnesite formation in modern and ancient playas.