Quadruple sulfur isotope biosignatures from terrestrial Mars analogue systems

In this study, we present quadruple sulfur isotope values (QSI: S-32,S-33,S-34,S-36) measured in sediments from two sulfur-rich Mars analogue environments: i) the glacially-fed hydrothermal pools in Iceland (Kerlingarfjoll and Kverkfjoll), and ii) the Lost Hammer hypersaline spring from Axel Heiberg Island, Nunavut, Canada. The localities host different physical and geochemical characteristics, including aqueous geochemistry, volcanic input, temperature, pH and salinity. The delta S-34 values of sulfur compounds from the Lost Hammer hypersaline spring exhibit large fractionations typical of microbial sulfate reduction (MSR) with or without additional oxidative sulfur cycling and microbial sulfur disproportionation (MSD) ((34)epsilon(SO4)(-CRS) from -49.5 to -43.5 parts per thousand), contrary to the small S isotope fractionations reported for the Icelandic hydrothermal sites ((34)epsilon(SO4)(-CRS) from -9.9 to -0.7 parts per thousand). Lost Hammer minor S isotope values (Delta S-33 and Delta S-36), interpreted within the context of a sulfur cycling box model, are consistent with a biogeochemical S cycle including both MSR and MSD. In contrast, the small range in delta S-34 values within the Iceland hydrothermal pools are consistent with a large volcanic H2S flux and minimal biological S cycling. The minor S isotope values recorded in the hydrothermal pools, however, indicate further biogeochemical sulfur cycling. Our results demonstrate that contrasting physical and chemical characteristics between sites support different microbial S cycling processes, as recorded in the QSI sedimentary values. The QSI data and the derived models support the strong potential for QSI values to be used as biosignatures in the search for life in Martian S-rich environments. These results also suggests that extreme, metabolic energy-limited environments with low abiotic sulfur fluxes could be more likely to produce unequivocal biological QSI signals than those with more moderate conditions or abundant available energy. This finding carries significant implications for targeting sites on Mars for in situ measurements or future sample return missions. (C) 2021 Elsevier Ltd. All rights reserved.

Automated summary

In this study, sediment samples from sulfur-rich environments in Iceland and Canada were analyzed for quadruple sulfur isotope values, revealing different microbial sulfur cycling processes between the sites. The results suggest the potential for quadruple sulfur isotope values to serve as biosignatures for life in sulfur-rich Martian environments. The contrasting physical and chemical characteristics of the sites support different microbial sulfur cycling processes, with implications for targeting sites for future missions to Mars.