Evidence for life in the isotopic analysis of surface sulphates in the Haughton impact structure, and potential application on Mars

Abstract

The analysis of sulphur isotopic compositions in three sets of surface sulphate samples from the soil zone in the Haughton impact structure shows that they are distinct. They include surface gypsum crusts remobilized from the pre-impact gypsum bedrock (mean δ 34 S +31‰), efflorescent copiapite and fibroferrite associated with hydrothermal marcasite (mean δ 34 S −37‰), and gypsum-iron oxide crusts representing weathering of pyritic crater-fill sediments (mean δ 34 S +7‰). Their different compositions reflect different histories of sulphur cycling. Two of the three sulphates have isotopically light (low δ 34 S) compositions compared with the gypsum bedrock (mean δ 34 S +31‰), reflecting derivation by weathering of sulphides (three sets of pyrite/marcasite samples with mean δ 34 S of −41, −20 and −8‰), which had in turn been precipitated by microbial sulphate reduction. Thus, even in the absence of the parent sulphides due to surface oxidation, evidence of life would be preserved. This indicates that on Mars, where surface oxidation may rule out sampling of sulphides during robotic exploration, but where sulphates are widespread, sulphur isotope analysis is a valuable tool that could be sensitive to any near-surface microbial activity. Other causes of sulphur isotopic fractionation on the surface of Mars are feasible, but any anomalous fractionation would indicate the desirability of further analysis.

Evidence for biological activity in mineralization of secondary sulphate deposits in a basaltic environment: implications for the search for life in the Martian subsurface

“…Sulphur isotopic systems have been suggested as a potential means for searching for signs of life on Mars by Parnell et al . (2012), especially sulphates that are prolific on the Martian surface. The oxidation of sulphidic minerals to sulphate through a series of intermediate species represents an important energy-yielding pathway for endolithic micro-organisms.…”

Hyperspectral mapping of alteration assemblages within a hydrothermal vug at the Haughton impact structure, Canada

“…These microbial signatures are also preserved in the sulfates that formed through weathering of the sulfides (Parnell et al. ).…”

“…In addition, isotopic fractionations between sedimentary gypsum and hydrothermal marcasite and pyrite at Haughton are too high to have formed from abiotic processes and rather are likely the result of microbial activity during the hydrothermal system that was active within 10,000 yr of the impact event (Parnell et al 2010). These microbial signatures are also preserved in the sulfates that formed through weathering of the sulfides (Parnell et al 2012).…”