Carbon and sulfur isotope biosignatures in Mars-analogue hydrothermal environments

Abstract

The study of terrestrial environments that bear similarity to Mars provides valuable information for interpreting data from missions, including how to find evidence of relict life on the red planet. Stable isotope signatures evidencing microbial metabolic activity are commonly used as a biosignature tool. Here, an interdisciplinary study investigating two volcanic hydrothermal systems in Iceland is presented, with additional contextualisation through a comparison with a non-volcanic hypersaline spring. This thesis combines mineralogy, geochemistry, microbial community DNA, and stable isotope systems (carbon and sulfur) to analyse: i) the preservation of isotopic biosignatures in hydrothermal and hypersaline springs, and ii) the relationships between the biosignatures and the geochemistry of these environments.

Firstly, a characterisation of two Mars analogue hydrothermal environments in Iceland (Kerlingarfjöll and Kverkfjöll), reveals deep volcanic processes controlling the geochemistry of the hydrothermal pools. The volcanic processes create two very distinct pH environments, with Kerlingarfjöll circum-neutral and Kverkfjöll acidic, with distinct water geochemistry and mineralogy. The water geochemistry is found to be a key parameter controlling the microbial communities, based on pH differences and the different electron donors and acceptors available. Secondly, carbon isotope fractionations preserved as sedimentary organic carbon, are controlled in Kerlingarfjöll and Kverkfjöll systems by temperature. Low temperature pools favour carbon CO₂ fixation pathways that produce larger or more variable carbon isotope fractionations. Lastly, sulfur isotope values (δ³⁴S) recorded in the sediments are not conclusive as geochemical biosignatures in Kerlingarfjöll and Kverkfjöll sediments. This is due to abundant H2S with abiotic δ³⁴S values overwhelming biological δ³⁴S values. Conversely, when combining δ³⁴S with Δ³³S and Δ³⁶S as a Quadruple Sulfur Isotope system (QSI), two pools in Kerlingarfjöll show complex S-cycling combining biological and volcanic processes. Importantly, the non-volcanic hypersaline spring preserves larger fractionations in δ³⁴S and large Δ³³S values, typical of reduction and disproportionation of sulfur by microorganisms. The main environmental variables causing larger S isotope fractionations in the hypersaline spring are the salinity stress and the limitation of electron donors and acceptors in the environment.

Overall, this thesis improves the understanding of carbon and sulfur isotopes as biosignature tools for investigating hydrothermal and hypersaline environments in Mars, and opens the door for the use of QSI as a more robust biosignature for future missions.