The Onset of Biogenic Calcium Carbonate Precipitation in Hypersaline Microbial Mats (Eleuthera, Bahamas)
The global carbon cycle is driven by redox processes at the lithosphere-biosphere interface. Microbial mats are key players of this element cycling. They are characterized by high turnover rates of biomass and influence the surrounding physicochemical microenvironment by their metabolic activities. Hence they can favor carbonate precipitation. However, precipitation within microbial mats can also be due to physicochemical carbonate oversaturation of the macroenvironment. The study of mechanisms of calcium carbonate precipitation in modern microbial ecosystems can provide valuable information for the interpretation of the rock record. Still today, processes leading to microbial carbonate formation, as well as the incorporation of key cations (e.g., Mg) in the crystal structure are not fully understood at the molecular level.
Big Pond, a hypersaline lake situated on Eleuthera Island (Bahamas), harbours a microbial mat, precipitating large amounts of CaCO3. The CaCO3 minerals form well-defined layers, which are preserved in the sediment record.
Single layer investigations, using X-ray diffraction, thin sections, isotopic signature (∂13C, ∂18O) and electron microscopy reveal that the precipitated minerals are a calcite-dolomite solid solution and show different macro- and micromorphologies. Mineralogical and microbiological investigations were complemented with physicochemical characterization of the environment.
Big Pond is a system that is highly oversaturated with respect to carbonates. However carbonate precipitation is strongly inhibited by inorganic molecules and EPS functional groups, chelating cations like Ca2+ and Mg2+. Although carbonate deposition in evaporitic systems is often reduced to evaporation/dilution events, hypersaline Big Pond represents a more complex biologically-induced or biologically-influenced precipitation scenario. Our data suggest that degradation of EPS by heterotrophic microbial activity (e.g., sulfate reducers) may reduce inhibition resulting in subsequent calcification of the EPS matrix. These data, as well as abiotic mineralization experiments, suggest a strong control of EPS functional groups (e.g., -COOH, - SH -OH, -NH2) on the mineralogy and the morphology of biogenic carbonate minerals. We believe that these functional groups catalyze the incorporation of Mg into carbonates, process that is otherwise kinetically inhibited under Earth surface physicochemical conditions (i.e., the so-called ‘dolomite problem’).
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009