Evaluating the biosignature potential of ammonium in Proterozoic red beds and implications for the search for life on Mars

Over the past two decades, it has become increasingly apparent that early Mars may once have been warmer, wetter and more habitable for microbial life than it is today, which has spurred discussions about potential biosignatures that may be preserved in Martian sediments. An impediment to this line of research is the pervasive oxidation of Mars’ surface due to photochemical oxidants that have likely destroyed remnants of organic matter. Here, we investigate whether nitrogen (N) transferred from biomass to phyllosilicate minerals during diagenesis can be preserved in oxidized mudrocks. We investigate two sequences of terrestrial Proterozoic red beds, namely the Sibley Group (1.4 Ga) in Canada and the Stoer Group (1.2 Ga) in Scotland, and we find enrichments in authigenic N in the range of several tens of ppm in both units. The highest concentrations (ca. 100 ppm on average) are found in the most desiccated red beds of the Stoer Group, concurrent with enrichments in potassium (K). We discuss similarities and differences between the two sets of rocks with regards to salinity, pH, biological productivity and K-metasomatism, and we conclude that the ideal mechanism for the preservation of biogenic N in red beds may be in-situ release of ammonium from microbial mats into the clay substrate, possibly facilitated by early diagenetic, biologically induced illitization. Illite and smectite have been observed on Mars, and experiments suggest that Martian waters contained moderate amounts of dissolved K. Hence, it is conceivable that a similar K and N enrichment process could have occurred as to what we document for the Proterozoic, preserving evidence of life that may have survived to the modern day.