QUANTITATIVE METHODS FOR EVALUATING STROMATOLITES AS POTENTIAL MICROBIAL BIOSIGNATURES

Stromatolites are self-organized mesoscale structures that result from various forms of accretionary growth in the generally vertical direction. The processes that contribute to stromatolitic growth typically include gravitational settling of sedimentary particles, surface-normal precipitation of mineral grains and fibers, upslope or downslope movement (diffusion) of previously deposited materials, and random natural effects best considered as noise (Grotzinger and Rothman, 1996). Most of these processes are equally characteristic of both abiotic and living systems and thus provide little or no evidence for a biogenic origin. However, upslope diffusion of previously deposited materials is an energy-requiring process that is best attributed to microbial motion in response to an external stimulus such as sunlight (phototaxis). We use a partial differential equation developed for understanding problems encountered in atomic-scale deposition of metals (Smilaeur, Rost and Krug, 1999), where stromatolitic growth is undesirable. In the process being modeled (electron beam epitaxy), there is an uphill component caused by an edge effect at the atomic scale. Thus, models constructed using this SRK formulation incorporate an upslope diffusion term that is not present in traditional interface-growth equations. As upslope diffusion is a process that is readily attributable to life but not to any other non-vital agents at anything larger than the atomic scale, this parameter may provide a definitive test for biogenicity of some classes of stromatolitic structures.