Pathways for abiotic organic synthesis at submarine hydrothermal fields

Abstract

Arguments for an abiotic origin of low-molecular weight organic compounds in deep-sea hot springs are compelling owing to implications for the sustenance of deep biosphere microbial communities and their potential role in the origin of life. Theory predicts that warm H2-rich fluids, like those emanating from serpentinizing hydrothermal systems, create a favorable thermodynamic drive for the abiotic generation of organic compounds from inorganic precursors. Here, we constrain two distinct reaction pathways for abiotic organic synthesis in the natural environment at the Von Damm hydrothermal field and delineate spatially where inorganic carbon is converted into bioavailable reduced carbon. We reveal that carbon transformation reactions in a single system can progress over hours, days, and up to thousands of years. Previous studies have suggested that CH4 and higher hydrocarbons in ultramafic hydrothermal systems were dependent on H2 generation during active serpentinization. Rather, our results indicate that CH4 found in vent fluids is formed in H2-rich fluid inclusions, and higher n-alkanes may likely be derived from the same source. This finding implies that, in contrast with current paradigms, these compounds may form independently of actively circulating serpentinizing fluids in ultramafic-influenced systems. Conversely, widespread production of formate by ΣCO2 reduction at Von Damm occurs rapidly during shallow subsurface mixing of the same fluids, which may support anaerobic methanogenesis. Our finding of abiogenic formate in deep-sea hot springs has significant implications for microbial life strategies in the present-day deep biosphere as well as early life on Earth and beyond.

Keywords: abiotic organic synthesis; fluid–vapor inclusions; formate; hydrothermal systems; methane.

Fig. 1.

Plots of measured Mg vs. (A) CH₄, (B) ΣCO₂, (C) ΣHCOOH, and (D) ΣCO₂ + ΣHCOOH concentrations for Von Damm vent fluids. Mg content is used as an indicator for seawater mixing; solid lines denote conservative dilution of the near-endmember composition (blue circles) with seawater (yellow stars), whereas dashed lines indicate species concentrations that result from nonconservative mixing in elevated Mg fluids (green symbols). Select δ¹³C_CO2 values are plotted in B near corresponding samples. Uncertainties (2σ) not shown are smaller than symbols.

Fig. S1.

Bathymetry of the Von Damm hydrothermal field located on the western flank of the Mid-Cayman Rise, with locations of fluid sampling indicated with circles.

Fig. S2.

Plot of measured Mg vs. measured (A) H₂, (B) C₂H₆, (C) C₃H₈, and (D) Cl. Conservative behavior in Cl, C₂H₆, and C₃H₈ as well as CH₄ in mixed fluids (green symbols; in the text) during mixing between near-endmember fluid (blue circles) and seawater (yellow stars) suggests that the Von Damm vent field is fed by a single-source fluid originating in the high-temperature reaction zone below the seafloor. Nonconservative H₂ behavior occurs at two mixed fluid vents: Old Man Tree (115 °C) and Shrimp Hole (21 °C). Uncertainties (2σ) not shown are smaller than symbols.

Fig. S3.

Plot of maximum sampled temperature and seafloor pressure conditions at the Von Damm vent field (blue circle). The curve represents the two-phase boundary of seawater (22). To attain measured temperatures, Von Damm fluids would have cooled by at least 150 °C after phase separation.

Fig. 2.

Chemical affinity for the production of HCOO⁻ from ΣCO₂ and H₂ in Von Damm mixed fluids. White symbols indicate a thermodynamic drive for reaction (positive affinity) as written based on conservative dilution of the near-endmember ΣHCOOH composition (blue circle). Green symbols denote affinity calculated with actual mixed fluid ΣHCOOH contents. Thermodynamic equilibrium is defined as affinity = 0 ± 5 kJ/mol (light blue shading). Symbol shapes correspond to those in Fig. 1.