Microbial ecosystems provide a critical link between the short-term (?biological?) and the long-term (?geological?) carbon cycle. Microbial mats are an example of such ecosystems found at the surface of sediments. These mats are believed to resemble the earliest evidence of life that we can find in the rock record (up to 3.5 billion years old). The metabolism of microbes present in the mats can change the balance between mineral dissolution and precipitation at the interface between the lithosphere and the biosphere. Rock-forming microbial mats, especially the ones producing carbonates, are precisely situated at this interface. Therefore, they are ideal model systems to study the transformation of organic carbon into carbonate minerals through a combination of intrinsic (microbial processes) and extrinsic (the environment) factors.
The rock-forming ecosystems (called microbialites), like the model system studied in this proposal, provide the unique opportunity to study the early biogeochemical changes of the carbonates precipitated by the microbes near the surface of the mat. This process, called early diagenesis, is very important in forming of the rock record. The microbes are most active near the surface of the mat, but at depth they have the luxury of time and their metabolism, albeit very slow, can greatly change the properties of both the organic carbon and the carbonate minerals. We propose to investigate the coupling of geochemical and microbiological reactions that produce biogenic carbonate minerals. We chose to investigate a unique microbial mat system that is approximately one meter thick and comprised of hundred of layers of carbonate minerals separated by layers of organic carbon. We also will study the geologic changes of these minerals with depth, and coupled to this, the changes in chemical properties of the organic carbon. The result of the combined microbial-geochemical reactions is a layered carbonate rock that is very similar to the finely-laminated micritic stromatolites, which are typically found in the fossil record. Our goal is to determine the contribution of microbial and physicochemical processes to early diagenesis of microbial carbonates. This will shed light on which fraction of the initial carbonate precipitate remains preserved in the rock record from the initial precipitation. In other words, we will be able to determine the role of microbes and their products in the diagenesis of minerals directly. Using state-of-the-art techniques, study of the processes altering the sediments at depth represents a critical step in the understanding of the slow transition of these microbial carbonates toward the fossil record. Furthermore, this project provides insight of how microbial carbonates can store inorganic carbon (i.e., CO2) into a long-?residence time? carbonate reservoir (so-called carbon sequestration). It also opens a window on the past by revealing geomicrobial mechanisms that could preserve traces of life (microbial signatures) during early diagenesis and further in the rock record. These aspects are key features for understanding modern and past carbon cycles, development of early life as well as possible life on other planets.
