Tino Christopher J, Stüeken Eva E, Arp Gernot, Böttcher Michael Ernst, Bates Steven M, Lyons Timothy W
Department of Earth and Planetary Sciences, University of California, Riverside, California, USA.
School of Earth and Environmental Sciences, University of St. Andrews, St. Andrews, Scotland, United Kingdom.
Astrobiology. 2023 Oct;23(10):1027-1044. doi: 10.1089/ast.2022.0114. Epub 2023 Jul 26.
Discrepancies have emerged concerning the application of sulfur stable isotope ratios as a biosignature in impact crater paleolakes. The first δS 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 δS fractionations >21‰ as indicative of a biological origin, and studies of δS at analog impact crater lakes on Earth have followed the same approach. We performed analyses (including δS, 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 δS properties define a biosignature. The differences in δS 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 δS variations >21‰ at the Miocene Ries impact crater are tenuous. This discourages the use of δS 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.
关于硫稳定同位素比值作为撞击坑古湖生物标志的应用，已经出现了一些差异。来自火星盖尔陨石坑的首批δS数据显示出约75‰的范围，这被归因于一种非生物机制。然而，地球上古代环境的生物地球化学研究通常将δS分馏>21‰解释为生物起源的指示，并且对地球上类似撞击坑湖泊的δS研究也采用了相同的方法。我们对诺德林格里斯撞击坑的多种岩性进行了分析（包括δS、总有机碳重量百分比和扫描电子显微镜成像），重点关注热液蚀变的撞击角砾岩和相关的沉积湖相充填层序，以确定δS特性是否定义了一种生物标志。主体岩性之间的δS差异可能是由热化学硫酸盐还原、微生物硫酸盐还原、热液平衡分馏或它们的任何组合造成的。尽管有丰富的样本和目前仅适用于地球分析的仪器精度，但在中新世里斯撞击坑，基于δS变化>21‰而断言生物成因性是站不住脚的。这使得在没有包括完整地质、生物地球化学和结构背景以及对替代假说的全面认识等独立检验的类似环境中，不鼓励将δS用作生物标志。
