Since their discovery in 1977, seafloor hot spring fluids have been widely proposed as a potential source of organic molecules necessary for early life to emerge and thrive on a Hadean–Archaean Earth ( 1 – 5), and for metabolic energy and fixed carbon in modern hydrothermal systems ( 6, 7). Widespread thermal degradation of subsurface organic matter may be an important source of organic production in unsedimented hydrothermal systems and may influence microbial metabolic strategies in cooler near-seafloor and plume habitats. This strongly implicates thermogenic interactions between upwelling fluids and microbial biomass or associated dissolved organic matter during subsurface mixing in crustal aquifers. Associated low-temperature fluids (<200 ☌) formed by admixing of seawater, however, are invariably enriched in CH 3SH (up to ∼10 −6 M) along with and low-molecular-weight hydrocarbons relative to high-temperature source fluids, resembling our observations from a sediment-hosted system. CH 3SH concentrations are uniformly low (∼10 −8 M) in high-temperature fluids (>200 ☌) from all unsedimented systems and, in many cases, suggestive of metastable equilibrium with CH 4 instead. Our data demonstrate that the distribution of CH 3SH is inconsistent with metastable equilibrium with inorganic carbon, indicating that production by abiotic carbon reduction is more limited than previously proposed. In an effort to constrain sources and sinks of CH 3SH in seafloor hydrothermal systems, we determined for the first time its abundance in diverse hydrothermal fluids emanating from ultramafic, mafic, and sediment-covered midocean ridge settings. Thiols are also presumptive ligands for hydrothermal trace metals and potential fuels for associated microbial communities. Putative CH 3SH synthesis by abiotic (nonbiological) reduction of inorganic carbon (CO 2 or CO) has been invoked as an initiation reaction for the emergence of protometabolism and microbial life in primordial hydrothermal settings. Simple alkyl thiols such as methanethiol (CH 3SH) are widely speculated to form in seafloor hot spring fluids.

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