Carbon and sulfur isotope biosignatures in Mars-analogue hydrothermal environments

• Main Author: Moreras Martí, Arola
• Other Authors: Cousins, Claire Rachel, Zerkle, Aubrey L., UK Space Agency, Science and Technology Facilities Council (STFC)
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: University of St Andrews 2023
• Subjects: Kverkfjöll; Iceland
• Online Access: http://hdl.handle.net/10023/26844; https://doi.org/10.17630/sta/250

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 ...