MICROBIAL PROMOTION OF CARBONATE MINERAL PRECIPITATION IN MICROBIALITES: A CASE FOR THE IMPORTANCE OF IRON REDUCTION (Invited Presentation)

Precipitation of early diagenetic cements, particularly carbonate minerals, is critical to the formation and preservation of microbialites. Precipitation and dissolution, although chemical processes, commonly are induced by biological processes of the mat community. Microbial processes such as photosynthesis, sulfate reduction, and metal reduction commonly alter the concentrations of bicarbonate ions, hydroxyl ions, or both, potentially affecting carbonate saturation. These processes can also indirectly affect calcium concentrations. Finally, microbes can lower kinetic barriers to precipitation by altering porewater compositions or acting as heterogeneous precipitation nuclei. Mat preservation can therefore be controlled by the microbial mat layer that promotes carbonate precipitation, unless precipitation is otherwise driven by environmental forcing.

There has been a vigorous debate over the relative importance of different metabolisms in promoting carbonate precipitation, in part because of difficulty reconciling theoretical, experimental, and field-based or historical observations. Indeed, under circumstances likely to be encountered in many modern microbial mats, precipitation ultimately induced by iron reduction could be localized primarily in zones dominated by other metabolisms. The mineral products of cementation during this process also may be prone to early alteration and thus difficult to identify. In particular, because many experimentally mineral compositions are metastable at low temperature, they could act as precursors to calcite, siderite and even dolomite. We suggest that iron reduction is likely to be a significant and underappreciated contributor to early carbonate cementation.