Are large sulfur isotope variations biosignatures in an ancient, impact-induced hydrothermal Mars analog?
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Discrepancies have emerged concerning the application of sulfur stable isotope ratios as a biosignature in impact crater paleolakes. The first in situ δ34S data from Mars at Gale crater display a ∼75‰ range that has been attributed to an abiotic mechanism. Yet biogeochemical studies of ancient environments on Earth generally interpret δ34S fractionations >21‰ as indicative of a biological origin, and studies of δ34S at analog impact crater lakes on Earth have followed the same approach. We performed analyses (including δ34S, total organic carbon wt%, and scanning electron microscope imaging) on multiple lithologies from the Nördlinger Ries impact crater, focusing on hydrothermally altered impact breccias and associated sedimentary lake-fill sequences to determine whether the δ34S properties define a biosignature. The differences in δ34S between the host lithologies may have resulted from thermochemical sulfate reduction, microbial sulfate reduction, hydrothermal equilibrium fractionation, or any combination thereof. Despite abundant samples and instrumental precision currently exclusive to Earth-bound analyses, assertions of biogenicity from δ34S variations >21‰ at the Miocene Ries impact crater are tenuous. This discourages the use of δ34S as a biosignature in similar environments without independent checks that include the full geologic, biogeochemical, and textural context, as well as a comprehensive acknowledgment of alternative hypotheses.
Tino , C J , Stüeken , E E , Arp , G , Böttcher , M E , Bates , S M & Lyons , T W 2023 , ' Are large sulfur isotope variations biosignatures in an ancient, impact-induced hydrothermal Mars analog? ' , Astrobiology , vol. Ahead of Print . https://doi.org/10.1089/ast.2022.0114
Copyright © 2023 the Authors. This work has been made available online in accordance with the Rights Retention strategy. This accepted manuscript is distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. The final published version of this work is available at https://doi.org/10.1089/ast.2022.0114.
DescriptionNASA Fellowship in support of Christopher J. Tino under Cooperative agreement number 80NSSC19K1739 issued through the NASA Office of STEM Engagement. Funding was provided to Timothy W. Lyons through the NASA Astrobiology Institute under Cooperative agreement number NNA15BB03A issued through the Science Mission Directorate and the NASA Interdisciplinary Consortia for Astrobiology Research (ICAR). Eva E. Stüeken acknowledges funding from an NERC Frontiers grant (NE/V010824/1).
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