Origin of modern dolomite in surface lake sediments on the central and western Tibetan Plateau

Abstract

Modern dolomite was found in the sediments of 20 lakes on the central and western Tibetan Plateau during lake surveys, in August to November of 2012–2015. We analyzed the mineralogy of authigenic carbonates and the chemical composition of lake water to investigate the environmental factors affecting dolomite formation and to determine the mechanisms of dolomite formation. X-ray diffraction analysis suggested that the dolomite content ranged from 3% to 10%, with a mean of 5%, and that the majority of the dolomite in the surface lake sediments was Ca-dolomite. Dolomite formed in a wide range of salinity environments. There was a positive linear relationship between salinity and Mg/Ca ratio, whereas the pH and sulfate (SO42−) concentration are not significantly correlated with Mg/Ca ratio. High Mg/Ca ratio, high salinity, high sulfate concentration, and high pH are important environmental factors in favor of dolomite precipitation. Evaporation and microbial activity are potential mechanisms for dolomite formation in the central and western Tibetan Plateau. The positive relationship between δ18O and δ13C indicates that the dolomite formed in lakes has a long residence time and undergoes continuous evaporation. The crystal morphologies of dolomite were scattered euhedral grains with diameters of 2–10 μm, as well as sub-spherical and spherical grains (<2 μm) which were present either as dispersed crystals or were aggregated into dolomite clusters. Dolomite crystals were identical both in shape and size to those of microbial dolomite previously discovered in modern lake environments and culture experiments. The dolomite precipitation temperature range calculated using the low-temperature microbial equation was close to the measured temperature, further indicating that microbial activity may be involved in dolomite formation. This study provides natural locations with different water salinities and other environmental factors for dolomite precipitation, which has important significance for studying the mechanism of dolomite formation.