Microbial Mechanism Of Dolomite Formation In Qinghai Lake Sediment, Tibetan Plateau, NW China

The controversy about the mechanism of dolomite formation has lasted more than two centuries, yet the dolomite problem still remain unsolved. The microbial model for dolomite formation has been proposed decades ago but still can not be applied to the massive precipitation environment in ancient geological history. Since the microbial model for dolomite formation were proposed based on hypersaline or lagoon system, formation of dolomite in inland slightly saline lake has no experimental evidence to support the microbial model of dolomite. Meanwhile, the argument around dolomite formation has focused more on whether the existence of sulfate ions could be a inhibitor to dolomite formation since dolomite precipitation are found in more environments where sulfate ions are in high concentration value. The objective of this study was to investigate the formation mechanism of dolomite found in the sediments of Qinghai Lake, an inland saline lake (salinity 12.5g/L) on the Tibetan Plateau, NW China. A 5-m sediment core was collected and dolomite was found in the sediments with certain characteristic morphology suggesting a biogenic origin. Pore water chemistry indicated that dolomite was oversaturated and sulfur isotopic analysis performed on co-existing pyrite supported a biogenic origin. Dolomite precipitation experiments were set up in the laboratory to simulate and confirm the biogenic origin of the dolomite. Dolomite was precipitated using sulfate reducing bacteria (SRB) Desulfotomaculum ruminis detected in the Qinghai lake sediments and halophilic bacteria Halomonas marina. X-ray diffraction and scanning electron microscopy detected biogenic dolomite using both bacteria. These results deepen our current understanding of dolomite precipitation models and have important implications for the mechanisms of dolomite formation in slightly saline inland lake environments.