GEOCHEMISTRY AND GEOBIOLOGY OF A DOLOMITIC ALKALINE PLAYA LAKE

The occurrence of fine-grained unlithified dolomite in modern sediments of Deep Springs Lake (DSL), a hypersaline alkaline playa lake in eastern California, provides an opportunity to study the geochemical and geobiological context of dolomite formation. DSL is in a closed basin with groundwater recharged by mountain streams. Groundwater emerges at several springs on the lake margin. Low lake levels allow the observation of numerous gas vents on the lake floor. Reduced gases including methane and hydrogen sulfide bubble out of the water filling the vents. A subsurface microbial origin for the methane is supported by carbon isotope ratios (δ¹³C) of dissolved methane (~-70‰ VPDB) and inorganic carbon (~+9‰ VPDB). Environmental 16S rRNA sequences of surface biofilms around the rims of the gas vents are similar to those of known phototrophic sulfide oxidizers. Hydrogen and oxygen isotopes of water from the freshwater springs feeding the lake, the water in the gas vents, and sediment pore water define a linear array with a slope of ~4.5, a trend typical of evaporation in a closed basin. Magnesium isotope ratios (δ²⁶Mg) of vent waters are isotopically enriched (~0.0 ‰ DSM) relative to silicate source material, indicating precipitation of carbonate minerals prior to discharge into DSL; carbonate cementation of alluvium is observed in many stream beds feeding the valley. That δ²⁶Mg pore water profiles vary <0.1‰ in the 40 cm below the surface of the lakebed indicates that little dolomite dissolution or precipitation is occurring in the sediment.

A hypothesis for dolomite formation at DSL is based on the geochemical and geobiological observations. When lake levels are high, submerged gas vents provide a flux of reduced gases to the oxic water column. Strong redox and salinity gradients between the vents and surrounding lake promote rapid cell turnover, and the gas flux is accompanied by a flux of microbes from the anaerobic subsurface community. The (metabolically inactive) microbial cell surfaces then nucleate dolomite crystal precipitation in the supersaturated brine. Such a process has recently been demonstrated in laboratory experiments, where high-density carboxyl groups on cell surfaces dehydrate Mg²⁺, making it available for carbonate precipitation.