Mars-analog calcium sulfate veins record evidence of ancient subsurface life

Sean McMahon, J. Parnell, Philippe Reekie

Research output: Contribution to journalArticlepeer-review


Ancient veins of calcium sulfate minerals (anhydrite, bassanite and gypsum) deposited by subsurface aqueous fluids cross-cut fluviolacustrine sedimentary rocks at multiple localities on Mars. Although these veins have been considered an attractive target for astrobiological investigation, their potential to preserve biosignatures is poorly understood. Here, we report the presence of biogenic authigenic pyrite in a fibrous gypsum vein of probable Cenozoic emplacement age from Permian lacustrine rocks in Northwest England. Pyrite occurs at the vein margins and displays a complex interfingering boundary with the surrounding gypsum suggestive of replacive authigenic growth. Gypsum-entombed carbonaceous material of probable organic origin was also identified by Raman spectroscopic microscopy in close proximity to the pyrite. Spatially resolved ion microprobe (SIMS) measurements reveal that the pyrite sulfur isotope composition is consistently very light (δ34SVCDT = –30.7 ‰). Comparison with the sulfate in the vein gypsum (δ34SVCDT = +8.5 ‰) indicates a fractionation too large to be explained by non-biological (thermochemical) sulfate reduction. We infer t 24 hat the pyrite was precipitated by microorganisms coupling the reduction of vein-derived sulfate with the oxidation of wall-derived organic matter. This is the first evidence that such veins can incorporate biosignatures that remain stable over geological time, which could be detected in samples returned from Mars.
Original languageEnglish
Pages (from-to)1212-1223
Number of pages12
Issue number10
Early online date24 Sep 2020
Publication statusPublished - 9 Oct 2020


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