Lateral Comparative Investigation of Stromatolites: Astrobiological Implications and Assessment of Scales of Control.

The processes that govern the formation of stromatolites--structures that may represent macroscopic manifestation of microbial processes and a clear target for astrobiological investigation--occur at various scales (local versus regional), yet determining their relative importance remains a challenge, particularly for ancient deposits and/or if similar deposits are discovered elsewhere in the Solar System. We build upon the traditional multiscale level approach of investigation (micro-, meso-, macro-, mega-) by including a lateral comparative investigational component of fine- to large-scale features to determine the relative significance of local and/or nonlocal controls on stromatolite morphology, and in the process, help constrain the dominant influences on microbialite formation. In one example of lateral comparative investigation, lacustrine microbialites from the Miocene Barstow Formation (California) display two main mesofabrics: (1) micritic bands that drastically change in thickness and cannot directly be traced between adjacent decimeter-scale subunits and (2) sparry fibrous layers that are strikingly consistent across subunits, suggesting the formation of sparry fibrous layers was influenced by a process larger than the length scale between the subunits (likely lake chemistry). Microbialites from the uppermost Triassic Cotham Member, United Kingdom, occur as meter-scale mounds and contain a characteristic succession of laminated and dendrolitic mesofabrics. The same succession of laminated/dendrolitic couplets can be traced, not only from mound to mound, but over 100 km, indicating a regional-scale influence on very small structures (microns to centimeters) that would otherwise not be apparent without the lateral comparative approach, and demonstrating that the scale of the feature does not necessarily scale with the scope of the process. Thus, the combination of lateral comparative investigations and multiscale analyses can provide an effective approach for evaluating the dominant controls on stromatolite texture and morphology throughout the rock record and potentially on other planets via rover-scale analyses (e.g., Mars).