Preservation Of Biosignatures Research Articles

Subsurface formation of oxidants on Mars and implications for the preservation of organic biosignatures

Hydrogen peroxide can form through the interaction of pyrite and anoxic water. The oxidation of pyrite results in the precipitation of sulfates and iron oxides, high redox potentials (~ 1000 mV) and acidic pH (3–4). The oxidative potential of the resultant solution may be responsible for the oxidation of organic compounds, as observed in the subsurface of the Rio Tinto Mars analog. On Mars subsurface migration of groundwater interacting with volcanogenic massive pyrite deposits would have mobilized acidic and oxidizing fluids through large portions of the crust, resulting in the widespread deposition of sulfates and iron oxides. This groundwater could have leached substantial volumes of aquifer material and crustal rocks, thereby erasing any organic compounds possibly down to depths of hundreds of meters. Therefore, the preservation of organic biosignatures must have been severely constrained in the portions of the ancient Martian crust that were exposed to aqueous processes, calling for a redefinition of the future targets in the search for biomolecular traces of life on Mars.

Preservation of biosignatures in metaglassy volcanic rocks from the Jormua ophiolite complex, Finland

Evidence of microbially-mediated alteration of basaltic glass is preserved in originally glassy basalts (rims of pillow lavas, hyaloclastite breccias, and chilled margins of dykes) from the well-preserved 1.95 Ga Jormua ophiolite complex (JOC) of Northeastern Finland. Although textural evidence of microbial alteration is commonly observed in relic glass from recent oceanic crust and some ophiolites, these textures have been destroyed during greenschist to lower amphibolite facies regional metamorphism and deformation of the JOC. However, another robust biosignature is found in the generally depleted δ 13C values of disseminated carbonate extracted from originally glassy basalts, relative to crystalline samples. The same distribution of δ 13C values is well documented in samples from recent oceanic crust as well as ophiolites of Phanerozoic age. This characteristic contrast in the δ 13C values of disseminated carbonate is interpreted to result from microbe-induced fractionation during oxidation of organic matter. X-ray mapping of initial alteration zones has identified residual carbon associated with highly-concentrated S that is unrelated to carbonate. We attribute these biosignatures to microbially-mediated alteration of originally glassy material prior to ophiolite emplacement.