Geochemistry of lacustrine carbonate rocks in southwestern Qaidam: Implications of silicate weathering and carbon burial triggered by the uplift of the Tibetan Plateau

Abstract

Lacustrine carbonates are known to be one of the responses of the hydrosphere to aridification in terrestrial ecosystems. >8000 km2 of carbonate-rich deposits, up to 3000 m in thickness, developed in the western part of the Cenozoic Qaidam Basin. The detailed depositional mechanisms of these regional long-scale carbonates and the uplift linkage of the Tibetan Plateau are yet to be explored. Thus, in this study, the fine-grained analysis of carbonate-rich cores from the Pliocene Upper Youshashan Formation of the Qaidam Basin was performed using major, trace element, isotope, and lipid biomarker chemistry and statistical and correlation analyses. As per the results, it was found that carbonate minerals had an average content of 37.2%, consisted mainly of Fe-dolomite (Fe-dol) and calcite, and presented contents that were significantly and positively correlated with the carbonate oxygen isotope value. As the highly crystalline silicate debris flowing from the Altyn Tagh mountains accumulated in the lake basin, it formed high-content clay minerals and silicate, which coexisted through strong silicate weathering and released N, S, K, Na, and trace elements, providing the material basis for the carbonate richness and eutrophication of the fresh–saline lake ecosystem. Chemical weathering, aquatic organism burgeoning, and lake alkalinization coupled and promoted each other, which resulted in the widespread precipitation of carbonate rocks in the study area. Geochemical data suggest that carbonate minerals precipitated in sub-oxidized waters, while the organic matter was buried in reducing conditions during the early diagenetic stage. Further studies showed that sulfate-reducing bacteria mediated the mechanism of lacustrine dolomite genesis. After the freshwater silicate debris flow entered the lake, the sulfate-reducing bacteria consumed terrestrial organic matter to produce dolomite in a slightly salty water environment. In contrast, Fe-dolomite is produced by sulfate-reducing bacteria consuming the mixed organic matter from aquatic and early terrestrial sources in saline water. Although the uplift of the Tibetan Plateau has led to the aridification of the Eurasian continent, It has been proven to cause considerable burial of organic and inorganic carbon.