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 proto-dolomite layer with weak cation ordering precipitated in Lake Sayram, Central Asia, during the Early Holocene. The 12-cm-thick layer, with abundant proto-dolomite (mean 50 wt% and maximum=81 wt%), consists of euhedral and rhombohedral grains. The similarδ18O values of proto-dolomites and ostracods at the same depth demonstrate that the former are authigenic. Morphologic and isotopic features of these proto-dolomites are distinct from those of microbial dolomites, suggesting an abiotic origin. We infer that this proto-dolomite layer is the product of a warming and arid climate, which is supported by regional climatic records. This study provides evidence for previous studies that dolomite can precipitate directly in “dolomite seas” at ambient temperature.