Evaluating bulk carbon and nitrogen isotope compositions of acidic hot spring and mudpot sediments as biosignatures

Carbon (C) and nitrogen (N) isotopes have been widely used as biosignatures to study the origin of organic materials. However, bulk δ¹³C and δ¹⁵N analyses have been underutilized in hydrothermal settings, making relevant comparisons to early Earth environments and other planetary materials difficult. Therefore, in this study we examined if bulk isotope analysis can be successfully used to detect a microbial signature in hydrothermal environments similar to where life might have begun on Earth. We analyzed concentrations and isotope compositions of C and N within acidic hydrothermal sediments in Iceland and the United States. The measured bulk δ¹³C of the hot spring and mudpot sediments was higher in barren Icelandic sites (−25.6 to −14.5 ‰) compared to lower values in more forested sites of the United States (−26.5 to −21.0 ‰). 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 negative ranges of bulk δ¹³C were indicative of life processes occurring in-situ (microbial activity) and in the surrounding area (production of plant biomass). The higher δ¹³C of the Icelandic sediments were consistent with smaller C isotope fractionations associated with thermophile (microbial) activity, wherein more negative bulk δ¹³C values corresponded to increasing input of allochthonous plant matter in the United States. Conversely, the bulk δ¹⁵N results (−18.4 to +3.7 ‰) were less useful in differentiating biosignatures because of overlapping δ¹⁵N values between various N sources such as allochthonous plant matter, volcanic/hydrothermal gases, and microbial processes.