81-7 The Potential Role of Microbes in the Formation of Reduction Spheroids: Co-occurrence of Organic Matter and Metals in Devonian and Mesoproterozoic Redbeds.

Abstract:

The hunt for life on other planets has broadened our understanding of paleoenvironments on Earth and Mars. By using Earth as an analogue for modelling Martian environmental processes, we can expand upon data received by remote sensing and rover missions. In 2024, the Perseverance rover detected mm-scale reaction fronts termed “leopard spots” within mudstones in Neretva Vallis, the river channel feeding Jezero Crater. The leopard spots are considered analogous to reduction spots/spheroids, a phenomenon ubiquitous across many terrestrial redbeds. These redbeds provide excellent analogues for Martian sedimentary environments, which are also strongly enriched in ferric iron. Combined with the identification of organics within both Earth redbeds and Neretva Vallis sedimentary rocks, this becomes compelling evidence of potential life on another planet (“potential biosignatures”). Indeed, it has long been suggested that reduction spheroids may be microbially mediated. Reduction spheroids are readily identified in visible light imagery by their “halo”, caused by the reduction of Fe, which makes them an easily identifiable target to search for on current and future missions.

This study aims to characterize reduction spheroids in the Old Red Sandstone (~400 Ma) at Dingwall, Scotland, and the Bay of Stoer Formation of the Torridon Group (~1200 Ma). Using multiple techniques—including SEM-EDS/-WDS and µXRF (major and trace elements, including transition metals), and Raman microspectroscopy (organic mapping and characterization)—this study provides datasets for comparison with the Martian “leopard spots”, enabling the development of a model that explains the formation of these features. Using each of these techniques, organic carbon has been identified to be associated directly with cores containing redox-sensitive elements, such as transition metals. The quantification of elements identified, such as Ni, Cu, Zn, Mn, and Cr, is compared to similar data sets from PIXL, the Perseverance’s rover µXRF instrument. The presence of localised concentrations of organic carbon, detected for the first time using Raman microspectroscopic mapping of reduction spheroids, may implicate dissimilatory metal-reducing microbial colonies in the formation of reduction spheroids. This is corroborated by increased presence of metals directly adjacent to organic matter in some cores from each locality. Perseverance PIXL XRF data corroborate the importance of such elements on Mars, and this comparison will serve as a basis for Mars Sample Return analyses, particularly of the core “Sapphire Canyon.”

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