An Early Holocene Primary Dolomite Layer of Abiotic Origin in Lake Sayram, Central Asia

The “dolomite problem” is a long‐standing puzzle in sedimentology and mineralogy. Previous studies have shown that some dolostones are formed by microbes or in hydrothermal–burial environments. Here, we provide a different case in which an abiotic and Ca‐rich dolomite layer precipitated in Lake Sayram, Central Asia, during the early Holocene. The 12‐cm‐thick layer consists of abundant partially ordered dolomite crystals (mean > 50 wt% and maximum = 81 wt%) which have euhedral and rhombohedral shapes and weak cation ordering. Transmission electron microscopy reveals that even in a single crystal, compositions vary among different domains. Mineralogical and isotopic evidence suggests that these dolomites are of primary and abiotic origin. We infer that this dolomite layer might be the product of day and night temperature cycling of shallow lake water under local warming trend. This study provides evidence for abiotic dolomites forming at ambient temperature. Plain Language Summary Dolostone is common in ancient rocks but rare in modern marine sediments, and dolomite is notoriously difficult to synthesize without microbes at room temperature; this is known as the “dolomite problem”. In many studies, microbes have often been associated with dolomite, and synthesis by microbial mediation has succeeded. However, the mineralogy of microbial dolomites has been questioned and not all massive dolostones have been found with microbial structures. Hydrothermal–burial alteration is another promising solution, but recent studies have demonstrated that dolostones could also form at surface temperatures (