Stable Isotope Fractionation in a Cold Spring System, Utah, USA: Insights for Sample Selection on Mars

Stable delta C-13 isotope analysis at hot and cold springs suggests that rapid degassing overprints carbon isotopic biosignatures even when microbial activity produces biogenic textures in the minerals. Mineral precipitation and potential biosignature preservation are evaluated at a cold spring system in Ten Mile Graben, Utah, USA, with scanning electron microscopy, X-ray diffraction, and stable carbon isotopes. Putative biogenic mineral habits such as aragonite microspheres and botryoids, and biologic materials (EPS and diatom tests) are abundant in modern mats, but the delta C-13 values are between +2 parts per thousand and +7.8 parts per thousand, consistent with rapid CO(2)degassing reported by other researchers. Multiple factors, however, influence isotopic signatures of mineral precipitates in this spring system, including rapid degassing, preferential microbial uptake of light carbon isotopes via multiple carboxylation pathways, hydrocarbon-charged fluid, and other inherited isotopic signatures in the fluid, particularly from dissolution of older limestones; therefore, it is not likely that this narrow range of isotopic ratios definitively shows an abiotic signature. A fossil vent preserves biogenic mineral habits, but not microbial body fossils. This study highlights the need for novel biosignature detection methods-and an understanding of what an abiotic signature definitively is-as we prepare for sample caching of carbonate rocks by the Mars2020 mission and future sample return.

Automated summary

Stable carbon isotope analysis at hot and cold springs in Utah suggests that rapid degassing can mask biogenic mineral textures, even when microbial activity is present. Multiple factors, including rapid degassing and preferential microbial uptake of light carbon isotopes, influence the isotopic signatures of mineral precipitates in this spring system. Therefore, determining biogenic or abiotic signatures based on isotopic ratios is complex and may require novel biosignature detection methods.