Preservation of clay-bearing geochemical biosignatures in Mars analogue sedimentary rocks over billion-year timescales

Oxia Planum, the landing site for ESA’s ExoMars Rover, Rosalind Franklin, hosts widespread layered Fe/Mg phyllosilicate-bearing deposits of Noachian age, evidence of a potentially long-lived aqueous paleoenvironment in a deltaic to fluvio-lacustrine setting. Fluvio-lacustrine environments have moderate to high preservation potential for organic matter, due to rapid sedimentation and subsequent burial. As a result, these are also some of the environments that, over the course of Earth history, have preserved biosignatures on billion-year timescales. Microbial activity and capture within sediments can result in geochemical and mineralogical anomalies, including localised elemental enrichments. These provide a means of detecting evidence of past life in concert with molecular biosignatures. Microbial mats especially can alter the geochemistry of surrounding sediments, producing spatially constrained variations that persist over geological timescales. Investigating such biosignatures in sedimentary environments analogous to those recorded at Oxia Planum is essential for informing future rover observations and measurement strategies.

We examine clay-bearing sedimentary facies with well-preserved microbially induced sedimentary structures (MISS), including (1) the 1.0 - 1.1 Ga Clachtoll and Diabaig formations in northwest Scotland— a  package of fluviolacustrine and estuarine sedimentary rocks deposited under fluctuating redox conditions; and (2) the 2.7 Ga Tumbiana Formation (Pilbara Craton, Western Australia), which records deposition in a shallow lacustrine environment that received input from basaltic volcanism. We present elemental distributions, redox sensitive trace element behaviour, and mineralogical variations in preserved microbial mat structures and compare these to neighbouring sediments with no microbial influence. Using a combination of raman spectroscopy and elemental mapping, we show elemental enrichments linked to biology, such as iron, manganese, and potassium, coincide with clay-rich organic matter bearing areas within the sediment, indicating that ~1 – 2.7 Ga microbial mats can preserve distinct geochemical biosignatures in association with clay-bearing lithologies. The spatial association between centimetre-millimetre sized sedimentary structures observable at outcrop scale and sub-millimetre geochemical anomalies highlights the importance of integrating imaging and geochemical datasets to support biosignature interpretations.