The field of geobiology encourages the investigation of the ways that life has shaped and been shaped by its environment. Of particular interest are geological processes that are affected or controlled by microbial activity. Amongst these processes, precipitation of calcium carbonate, stands out as both a critically important geological process and one to which microbial influence is frequently ascribed. Travertine is a form of carbonate that is deposited when geothermally heated subsurface waters, rich in CO2, emerge onto the surface and rapidly precipitate calcium carbonate (CaCO3), in the form of either calcite or aragonite (Ford & Pedley, 1996). Travertine hot springs are excellent locations to study the interactions between microbes and carbonate precipitation. The high temperatures characteristic of travertine hot springs generally preclude metazoan life. This is desirable from a geobiological perspective for several reasons. First, from a purely practical perspective, restricting the number of organisms in the environment makes it easier to understand and model the interactions between organisms and their environment. Secondly, geobiology is concerned with interactions between organisms and their environment in the ancient as well as the modern and the absence of multicellular life in hot springs allows for observations and results made in modern systems to be extrapolated into Archaean and Proterozoic systems, before the evolution and widespread distribution of metazoa. Further, it is widely believed based upon phylogenies constructed from the genes encoding the 16S rRNA that life arose around a hydrothermal system (Fouke et al., 2000). Examining the microorganisms inhabiting hot springs today may provide additional insight into the conditions surrounding the origin and evolution of life. Finally, travertine hot springs typically have mineral precipitation rates that can reach or exceed 1mm per day (Ford & Pedley, 1996; Fouke et al., 2000). This generates deposits of sufficient size for laboratory analysis within a single field season.

We will be examining the travertine system at Angel Terrace, part of the Mammoth Hot Springs Complex in Yellowstone National Park, WY. As part of our study, we will characterize the depositional morphology of the travertine, both on the large terrace scale (10-100m) and the individual terracette pools (0.1 - 5m) that make up the system. In particular, we will examine the formation of microbial mats (tentatively identified as Phormidium spp.) and their subsequent calcification into distinctive tufted structures. These structures are analogous to the Conophyton stromatolites of the Archaean and Proterozoic. By examining the environmental conditions in which these tufted mats occur and the microbial consortia assorted with them, we may be able to constrain the paleoenvironmental conditions at the time of deposition of the Conophyton stromatolites.