Carboxylated Organic Matter Influences Magnesium Uptake in Three Low-Temperature Dolomite Models

Abstract

Numerous geochemical models explain the formation of dolomite at low temperature, a controversial mineral due to its abundance in geological deposits but paucity in modern environments. Several of these models are based on environments where dolomite either forms in the modern or is hypothesized to have formed in the past– mixing zones, sabkhas, and alkaline lakes. In the last twenty years, results of field and laboratory studies have found that microorganisms can promote the precipitation of dolomite at low temperature, either through active metabolism or by providing a reactive surface for mineral nucleation. The work summarized here utilizes bench-scale laboratory batch experiments to study simplified geochemical environments associated with low temperature dolomite formation both containing and excluding synthetic carboxylated organic matter in the form of functionalized polystyrene microspheres. These environments were characterized by their pH, alkalinity, and salinity to form geochemical end members. In effect, this combines longstanding geochemical models of dolomite formation with a mechanism known to overcome kinetic barriers to dolomite formation at low temperature. Experiments did not demonstrate formation of significant amounts of dolomite, however, data suggest that while bulk precipitation products reflect the specific geochemical environments, the presence of carboxylated organic matter promotes the incorporation of magnesium into the precipitate across geochemical environments. The amount of magnesium in the precipitate on the microsphere surface appears to be a function of the Mg/Ca ratio of the bulk solution. Magnesium is observed both in precipitates forming on the microsphere surface and is more broadly distributed in the bulk precipitate when microspheres are present. These results suggest that carboxylated organic matter can sequester magnesium into precipitates, which could have implications for magnesium availability during diagenesis. Results from experiments studying the impact of carboxylated organic matter on dolomitization of calcium carbonate sediments in mixing, sabkha, and artificial fluids are inconclusive. These negative results potentially arise from short experimental time scales and scaling issues during analysis. Future low temperature dolomitization experiments should include advection in the experimental design to provide the mass transfer presumably necessary for significant dolomitization.