Prof. Xu Huifang NASA Astrobiology Institute, Department of Geoscience, University of Wisconsin
Catalysis plays pivotal role in many low-temperature geochemical processes. An enigma of sedimentary dolomite problem is a good case for demonstarting calalysis at the dolomite-water interafce. Major role of a catalyst is to lower activation energy and promote chemical reaction, such as, dolomite growth. Many hydrocarbon reservoirs are associated with sedimentary dolomite. Sedimentary dolomites or dolostones are generally abundant in the geological record, yet scarce or less common in certain geological time periods and Holocene sediments. The main factor inhibiting the nucleation and growth of dolomite is the strong hydration of the aqueous Mg2+ ion. The dehydration of surface Mg2+–water complexes is the key to dolomite crystallization. Recent results from our research group indicate that disordered dolomite can precipitate and growth in normal seawater in presence of dissolved sufide, polysaccharides, or microbial extracellular polymeric substances (EPS). A positive relationship between concentrations of dissolved sulfide, polysaccharides, or microbial EPS in a solution and amount of Mg in the precipitated Ca-Mg-carbonates has been established. Polysaccharides are the main components in the EPS. Atomistic modeling results indicate that the adsorbed hydrogen sulfide or polysaccharides can weaken the bonding between water molecule and surface Mg, therefore promote carbonate anion binding and dolomite crystallization. Our results from sedimentary carbonate rocks with oscillatory dolomite and limestone layers (ribbon rocks) and micro-laminations clearly demonstrate preferential dolomite precipitation in the organic-rich (i.e., biomat-rich) carbonate layers. Polysaccharides associated with the carbonates sediments (calcite, high magnesian calcite, and aragonite) take very important catalytic roles in the dolomite formation. The dolomitization could be penecontemporaneous process through interaction with ambient sea water in presence of the catalysts. No special “dolomitizing fluids” are required.