STROMATOLITE BIOGENICITY: AN UPDATE (Invited Presentation)

Stromatolites (laminated, lithified accretionary structures with domed, columnar, or conical shapes) are commonly interpreted to have been built layer by layer by microbial mats. Thus, stromatolites would constitute biosignatures easily observable in the field and potentially represent the oldest record of life on Earth. It is clear that some, perhaps most, stromatolites on Earth were formed with biologic input. However, if one steps back from the classic interpretation, stromatolites displaying robust evidence for biogenicity are somewhat rare, as post depositional alteration typically removes most traces of microbial activity (if it was there in the first place). If one is searching for evidence of life, it is essential to identify criteria that point towards a biogenic origin for stromatolites, whether on Earth or elsewhere (e.g., Mars).

Along those lines, recent work has demonstrated that certain key features would point towards a biogenic origin for stromatolites, if present: 1) detrital grains trapped beyond the angle of slide indicate the presence of a mat with the ability to trap grains; 2) if grains are too small to image, detrital patterns can be revealed via magnetic susceptibility trends along lamina, where biogenic structures have a more even distribution vs. laminar angle and abiogenic structures follow angle of slide relationships; 3) where discernable, the comparison of local features potentially under microbial control vs. regional features under larger scale control may point towards biogenicity; 4) the presence of bubble molds indicating that gas was trapped in a mat, with no obvious abiogenic counterpart.

If a stromatolite lacks clear evidence for biogenicity, it is important to note that its biogenicity assessment does not default to abiogenic, but rather, should be considered ambiguous until additional information points one way or the other...something to consider as the Mars rover Perseverance makes its way through the lacustrine deposits of Jezero Crater, an environment that, if on Earth, could host stromatolites.