MAGNESIUM ISOTOPE RECORDS ASSOCIATED WITH DOLOMITE FORMATION IN A MODERN ALKALINE LAKE

The occurrence of dolomite in modern sediments of Deep Springs Lake (DSL), an alkaline playa lake in eastern California, provides an opportunity to study magnesium isotope systematics associated with dolomite formation. DSL is recharged by seasonal melt water from surrounding mountains and is fed by a network of springs discharging into the lakebed. We collected push cores of sediment from two lake bottom sites and five sites adjacent to springs. Pore water was collected by field-deployed peepers and centrifugation of subsampled sediment cores. Sediment was washed, dried, and homogenized before dissolution in weak acid. Magnesium isotope ratios (δ²⁶Mg) of pore waters adjacent to springs are isotopically enriched (~ -0.0‰ DSM). These spring pore water values are significantly heavier than silicate source material, suggesting loss of isotopically depleted Mg to carbonate minerals prior to discharge into DSL. Pore water δ²⁶Mg profiles vary <0.1‰ in the 40 cm below the sediment surface, indicating that little dolomite precipitation or dissolution is occurring within the sediment; this observation is consistent with the hypothesis that dolomite precipitates directly from DSL lake water. Sediment δ²⁶Mg is depleted from the pore water (~ -1.8‰ DSM). If this dolomite was derived from waters with δ²⁶Mg of spring pore waters, the fractionation factor for these terrestrial lacustrine dolomites is ~1.8‰, which is similar to previously reported fractionation factors measured in marine systems.