Origins of methane in hydrothermal systems

Abstract

Isotopic and compositional evidence indicates that the principal source of methane in most subaerial hydrothermal systems is from thermocatalysis of organic matter (thermogenesis). No firm evidence exists for abiogenic production at hydrothermal temperatures. However, abiogenesis represents the principal methane source in high-temperature rock-dominated mid-ocean-ridge hydrothermal systems, in which methane is derived from the rock itself. The importance of carbon-isotope equilibrium between CH 4 and CO 2 at hydrothermal temperatures (≤350°C) is in dispute, although in most systems studied, equilibrium at such temperatures appears unlikely. Carbon isotope compositions of methane in high-temperature systems vary from ∼ −50 to -15‰ vs. PDB, with the most negative values occurring in systems with high CH 4 abundances, i.e. where thermocatalysis is important. Carbon isotope compositions of coexisting CO 2 also decrease with increasing CH 4 abundance (or degree of thermogenic input). This coupled carbon isotope variation suggests that high-temperature oxidation and thermocatalytic breakdown of organic carbon compounds are important in many hydrothermal environments. The enrichment of 13C in methane from high-temperature mid-ocean ridge environments is consistent with isotopic equilibrium at very high temperatures (> 500°C) within the host rocks, and supports the concept of methane genesis within the rock prior to hydrothermal extraction. Unusual 13C-enriched methanes are also found in some subaerial geothermal systems, although a rock-derived origin remains speculative in these cases. In light of the existing evidence, it appears that the sources of methane (either thermogenesis from organic carbon or abiogenesis within rocks) are much more important in determining the relative abundance and carbon isotope composition of hydrothermal methane than any subsequent approach to chemical and/or isotopic equilibrium under hydrothermal conditions.