The production of methane, hydrogen, and organic compounds in ultramafic-hosted hydrothermal vents of the Mid-Atlantic Ridge

Abstract

Both hydrogen and methane are consistently discharged in large quantities in hydrothermal fluids issued from ultramafic-hosted hydrothermal fields discovered along the Mid-Atlantic Ridge. Considering the vast number of these fields discovered or inferred, hydrothermal fluxes represent a significant input of H2 and CH4 to the ocean. Although there are lines of evidence of their abiogenic formation from stable C and H isotope results, laboratory experiments, and thermodynamic data, neither their origin nor the reaction pathways generating these gases have been fully constrained yet. Organic compounds detected in the fluids may also be derived from abiotic reactions. Although thermodynamics are favorable and extensive experimental work has been done on Fischer-Tropsch-type reactions, for instance, nothing is clear yet about their origin and formation mechanism from actual data. Since chemolithotrophic microbial communities commonly colonize hydrothermal vents, biogenic and thermogenic processes are likely to contribute to the production of H2, CH4, and other organic compounds. There seems to be a consensus toward a mixed origin (both sources and processes) that is consistent with the ambiguous nature of the isotopic data. But the question that remains is, to what proportions? More systematic experiments as well as integrated geochemical approaches are needed to disentangle hydrothermal geochemistry. This understanding is of prime importance considering the implications of hydrothermal H2, CH4, and organic compounds for the ocean global budget, global cycles, and the origin of life.

FIG. 1.

The MOR system showing the presently known and sampled hydrothermal sites. (Color graphics available at www.liebertonline.com/ast)

FIG. 2.

The MAR axis between 10°S and 45°N showing the known hydrothermal vent fields. Black circles represent basalt-hosted hydrothermal fields. Red circles represent ultramafic-hosted vent fields. (Color graphics available at www.liebertonline.com/ast)

FIG. 3.

Modified after Bradley and Summons (2010). Ranges of δ¹³C and δD detected in methane produced by a variety of sources. “Autotrophic” and “heterotrophic” is microbial methane. “Thermogenic” refers to cracking of biologically derived oils, while “geothermal” refers to cracking of high-molecular-weight organic compounds. The remaining are values observed at several locations where abiotic methane formation has been suggested: Canadian Shield gases (including Kidd Creek), the Oman ophiolite, Zambales ophiolite. The dark surface represents the range of values (from Table 2) measured for methane in fluids from the Rainbow, Lost City, Logatchev 1 and 2, Ashadze 1 and 2 ultramafic-hosted vent fields.

FIG. 4.

Carbon stable isotope ratios versus carbon number (Nc) for the n-alkane series detected in fluids from the Lost City field (triangles) and the Rainbow field (squares). C₉–C₁₄ (Konn et al., 2009) and C₁₆–C₂₀ (Konn et al., unpublished results). Analytical and sample collection methods are described by Konn et al. (2009).