Evaluating Bulk Carbon and Nitrogen Isotope Compositions of Acidic Hydrothermal Sediments on Earth– Implications for Mars Astrobiology

Carbon isotopes have been widely used as unique biosignatures because biologic processes commonly result in distinctive negative delta ¹³C values (-35 to -10 ‰) within organic material on Earth. However, the bulk delta ¹³C analysis in Gale crater sediments on Mars revealed a wider range of values, from -25 to +56 ‰ (Franz et al., 2020), which suggests a more complex origin. To determine if bulk isotope analysis can be successfully used to detect a microbial signature in a martian analog environment, we analyzed concentrations and isotope compositions of C within hydrothermal sediments in Iceland (Námafjall, Krýsuvík, Hveragerdi) and the United States (Lassen, Valles Caldera, Yellowstone). The measured bulk delta ¹³C of the hot spring and mud pot sediments was higher in barren Icelandic sites (-25.6 to -14.5 ‰) compared to lower values in more forested sites of the United States (-26 to -21 ‰). These corresponded to lower C concentration in Iceland (0.13 to 0.55 wt.%) and higher C concentrations in the United States (0.04 to 4.78 wt.%). The distinctive ranges of bulk delta ¹³C values observed in the studied sites were indicative of life processes occurring in-situ (microbial activity) and in the surrounding area (production of plant biomass), wherein more negative bulk delta ¹³C values corresponded to increasing input of allochthonous plant matter in United States. By contrast, the higher delta ¹³C of the Icelandic sediments were consistent with smaller C isotope fractionations associated with microbial activity. Bulk delta ¹⁵N results were less useful in differentiating biosignatures because of overlapping delta ¹⁵N values between the various N sources, such as allochthonous plant matter, volcanic/hydrothermal gases, and microbial processes. The larger variability of bulk delta ¹³C values observed in Gale crater is to some extent controlled by carbonate minerals present with significantly higher delta ¹³C, which were not present in our analog sites. Consequently, future bulk delta ¹³C analysis on Mars may consider analysis of acidic sediments (with no carbonate present) as they have the highest potential of displaying a distinctive C isotope biosignature associated with possible martian life.