Geology Club Seminar

Earth's earliest fossil assemblages are important for understanding the origins of life on Earth and, by analogy, how and where to search for signs of primitive life in the rock record of other planets. Greenland's Paleoarchean supracrustal belts contain Earth's oldest rocks and - despite the extensive metamorphism those rocks have been subjected to - are a prime astrobiology target. A recent study by Nutman et al. (2016) described putative conical and domical stromatolites1 (laminated accretionary structures formed by microbially mediated sedimentation) in the Isua supracrustal belt. The morphology, layering, mineralogy, chemistry and geological context of the structures were attributed to microbial mat formation in a shallow marine environment by 3,700 Ma, at the start of Earth's rock record.

However, we show that the "stromatolites" are more plausibly interpreted as part of an assemblage of deformation structures formed in carbonate-altered metasediments long after burial.

Importantly, we acquired a larger sample that allowed us to analyze deformation fabrics in the host rock, and enabled broader scale geochemical context development with PIXL (X-ray Lithochemistry instrument on Mars 2020 rover). The investigation of ISB structures serves as a cautionary tale in the search for signs of past life on Mars.