The mineral hydromagnesite, Mg 5 (CO 3) 4 (OH) 2 •4H 2 O, 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 (δ 13 C and δ 18 O), 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 (Ca-Mg-HCO 3 water type) to more concentrated sodic waters of T2 (Na-SO 4-Cl 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 CO 2 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, CO 2 , is trapped and stored in the hydromagnesite directly from the atmosphere. AMS radiocarbon dating of samples indicates CO 2 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.

obtained hydrochemical data from 1956 for various types of salt lake on the Qinghai-Tibet Plateau. Combined with analysis of the tectonic characteristics of the plateau, they discussed the hydrochemical characteristics of the lakes. In the Qaidam Basin salt lakes, different regions have slightly different sources in different periods. However, weathering of the surrounding rock and the circulation of deep water, volcanic hydrothermal water, and paleolake transitive-gathering water were found to be the principal sources (Vengosh et al.The origin of the abundant salt resources in the Gasikule salt lake in the northwest Qaidam Basin in China remains highly contested. We performed a detailed study of the hydrochemical characteristics and material sources of the Gasikule brines based on their chemical composition and hydrogen-oxygen isotopes, together with high-resolution remote sensing imagery of the stream water, surface brine, and intercrystalline brine. Stream waters within the catchment were found to be weakly alkaline and dominated by HCO 3 -, Cl -, and Na + ions. The Gasikule surface brine was also weakly alkaline and classified as chloride type. The average total dissolved solid (TDS) was 366.19 g L -1 ; Cland SO 4 2were the dominant anions, and Mg 2+ and Ca 2+ were the dominant cations. The intercrystalline brine was weakly acidic and classified as a magnesium sulfate subtype. The average TDS was 381.80 g L -1 ; Clwas the dominant anion, and K + and Mg 2+ were the dominant cations. The surface and intercrystalline brines were enriched in K + because of the annual formation of "dry salt lake" conditions. Using Enhanced Thematic Mapper (ETM) and Satellite Probatoire Pour l'Observation de la Terre 5 (SPOT5) remote sensing data, two (approximately) north-south trends in salinity in the eastern area of the Gasikule surface brine were identified (referred to as "water swell annular anomalies"). These two anomalies were found to be located (approximately) above an existing buried fault from which Ca-Cl type deep water recharged the surface brine. Three main sources were identified for the recharge of the surface and intercrystalline brines of the Gasikule salt lake: (a) stream water sourced from rainfall and melting ice; (b) leaching of Pliocene salt-bearing host rocks that contain sulfate minerals such as gypsum, mirabilite, and celestite at the northern end of the salt lake; and (c) Ca-Cl type deep water, associated with a buried fault.

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