Although recent hydrothermal experiments imply that abiogenic methane
(CH4) generation from hydrothermal reduction of CO2 can occur, evidence
from natural systems was still lacking. Based on the chemical and
isotopic equilibrium signatures of low-temperature fumarolic gas
discharges, we are able to provide hard evidence for its natural
occurrence, namely in three subduction-related bi-phase hydrothermal
systems of the Mediterranean, whose temperatures range from 260 to 470
degrees C. The attainment of equilibrium and the time spans of recent
volcanic dormancy allowed us to calculate minimum rates for chemical and
isotopic equilibration. These are significantly higher than those
previously reported and might be due to the presence of a saturated
water vapor phase in the investigated systems. The fact that nature
provides conditions enabling relatively fast production of hydrocarbons
from CO2 strongly supports the concerns that were recently raised from
laboratory experiments. These address the use of the carbon isotope
composition of reduced carbon in Archean sediments as a tracer of early
life and the occurrence of CH4 on extraterrestrial planets as a
bioindicator. In view of the potential role of abiogenic CH4 as a
precursor of life, we also present an estimate of abiogenic hydrothermal
CH4 fluxes throughout the Archean. It is not expected that these fluxes
exceeded 80 Mt/yr during the past 4.0 Ga. This, however, would have been
enough to facilitate HCN production on the prebiotic Earth. (C) 2007
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