Carbonate microbialites

Carbonate cementation in surface and within endolithic habitats, e.g., stromatolites, and intertidal low-latitude beachrock sediments, via growth of biofilms plays a key role in stabilizing and preserving these materials. While a range of chemical and biological factors influences carbonation, alkalinity generation by cyanobacteria activity is considered to be the primary driver in these contemporary geological systems. Though no single biogeochemical feature or process is responsible for carbonate precipitation in microbialites, recent work has highlighted the importance of microbialites in the consolidation/architecture of stromatolites well below the photic zone, but still within regions influence by tidal pumping, i.e., still under aerobic conditions. Cements in natural and synthesized beachrock, and in a Shark Bay stromatolite were characterised using X-ray fluorescence microscopy (XFM) and by using secondary electron and backscattered electron, scanning electron microscopy, suggesting that heterotrophic microbialite activity plays an important role in secondary, and perhaps tertiary, carbonate precipitation. Locally, dissolution of trapped and bound materials via boring by endolithic bacteria generates high concentrations of soluble calcium and some magnesium. In near surface environments, cyanobacteria (photosynthesis) creates alkaline microenvironments, which, when combined with high cation concentrations and inorganic carbon from heterotrophic activity, produces supersaturating conditions. The microbial community also aids cement formation through the generation of extracellular polymeric substances, which provide nucleation sites for carbonate mineral precipitation. Deeper within these materials, and far away from the photic zone, carbonate ‘recycling’ driven by heterotrophy may control mineral carbonation.