Ancient veins of calcium sulfate minerals (anhydrite, bassanite, and gypsum) deposited by subsurface aqueous fluids crosscut 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 (delta S-34(VCDT) = -30.7 parts per thousand). Comparison with the sulfate in the vein gypsum (delta S-34(VCDT) = +8.5 parts per thousand) indicates a fractionation too large to be explained by nonbiological (thermochemical) sulfate reduction. We infer that 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.
Bibliographical noteFunding Information:
This work was funded by the European Union’s Horizon 2020 Research and Innovation Programme under Marie Skłodowska-Curie grant agreement 747877. We acknowledge the Natural Environment Research Council for supporting the NERC Ion Microprobe Facility in Edinburgh and for supporting P.B.R.R.’s PhD studentship. The authors sincerely thank Mike Hall and John Craven for generous assistance with sample preparation and the operation of the ion microprobe; Alison McDonald and Andrey Gromov for technical assistance with Raman microscopy and analysis; Connor Brolly for assistance with the TOC analysis; and Stephen Phillips for originally collecting the vein sample. We also thank Diana Marosi for bibliographic assistance. We thank Tim Lyons and an anonymous reviewer for helpful comments that improved the manuscript.
© 2020 Mary Ann Liebert, Inc., publishers.
Copyright 2020 Elsevier B.V., All rights reserved.
- Mineral veins
- SULFUR ISOTOPE FRACTIONATIONS
- GALE CRATER
- Biosignature. Astrobiology 20, 1212–1223