Stromatolites as Biosignatures: Improving Confidence
Stromatolites are an important geological feature for tracking the earliest life on Earth. These sedimentary structures first appeared in the geological record ~3.5 billion years ago and have historically been used as a biosignature for life, often with some controversy (Grotzinger and Knoll, 1999). Stromatolites are loosely defined as laminated accretionary structures formed from the trapping and binding of sediment by microbial mats – picture green scum coating the seafloor and accreting wrinkled mounds of material. Huge numbers of single-celled photosynthesising organisms worked together like this for over a billion years before more complex life evolved. Upon being buried and lithified, evidence for these ‘microbial mats’ can be preserved for billions of years.
Carbonate stromatolites (Cambrian) near Saratoga Springs, New York. The wrinkly columnar structures can be seen from the top and the side – are these biological structures?
Image from: https://commons.wikimedia.org/wiki/File:Stromatolites_hoyt_mcr1.JPG
If all stromatolites were formed exclusively by microbial mats, astrobiology would be easy. The columnar structures – which can be up to 10 metres in diameter – would be a large signpost for single-celled microorganisms which are much harder to spot in rocks. However, wrinkly stromatolite mounds can also be formed by physical processes (like ocean waves moving sediment) and chemical reactions. Such methods independent of cellular life form ‘abiotic’ stromatolites – lookalikes which cannot be used as signatures for ancient life. Historically, researchers have relied on understanding the past environment stromatolites are found within to prove their origin (Allwood et al., 2007) – if they were deposited in warm shallow sea environments, it’s likely they were formed from green slime.
A recent paper I wrote along with Dr Dominic Papineau (Goodwin and Papineau, 2022) looks at microscopic biosignatures within stromatolites. We wanted to reduce the ambiguity of determining whether carbonate stromatolites are biological in origin or not. This has important implications for the search for life on other planets. Small outcrops of carbonates are known to exist on Mars (Ehlmann et al., 2008) and spotting stromatolites in this rock type would be easily achievable with robotic rovers, before verifying such structures with microscopic evidence. In our paper, we identify three major kinds of microscopic biosignatures: (1) primary microbial sedimentary textures – the wrinkly stromatolite structures themselves; (2) diagenetic organomineral assemblages – which are minerals formed from the reaction of organics left over from dead cells; and (3) stable isotope compositions – which record the preference of life for certain materials.
The hope is that by calibrating the above criteria, these can be used on a case-by-case basis to understand if stromatolites were formed from sticky algae binding sediment, or are just lookalike structures. Especially if wrinkly carbonates are found on Mars, we need tools to verify they’re life before getting too excited.
Allwood, A.C., Walter, M.R., Burch, I.W. and Kamber, B.S., 2007. 3.43 billion-year-old stromatolite reef from the Pilbara Craton of Western Australia: ecosystem-scale insights to early life on Earth. Precambrian Research, 158(3-4), pp.198-227, doi: 10.1016/j.precamres.2007.04.013
Ehlmann, B.L., Mustard, J.F., Murchie, S.L., Poulet, F., Bishop, J.L., Brown, A.J., Calvin, W.M., Clark, R.N., Des Marais, D.J., Milliken, R.E. and Roach, L.H., 2008. Orbital identification of carbonate-bearing rocks on Mars. Science, 322(5909), pp.1828-1832, doi: 10.1126/science.1164759
Goodwin, A. and Papineau, D., 2022. Biosignatures Associated with Organic Matter in Late Paleoproterozoic Stromatolitic Dolomite and Implications for Martian Carbonates. Astrobiology, 22(1), pp.49-74, doi: 10.1089/ast.2021.0010
Grotzinger, J.P. and Knoll, A.H., 1999. Stromatolites in Precambrian carbonates: evolutionary mileposts or environmental dipsticks?. Annual review of earth and planetary sciences, 27(1), pp.313-358, doi: 10.1146/annurev.earth.27.1.313