Methane and hydrogen in East Pacific Rise hydrothermal fluids


  • J. A. Welhan,

  • H. Craig


Recently discovered hydrothermal vents at 21°N on the East Pacific Rise are discharging turbid waters at up to 400°C; mixtures of the plumes with ambient seawater contain significant amounts of dissolved H2 and CH4as well as He. The first grab samples of these waters were diluted 50-100 foldbut they contained as much as 20 × 10−5 cc(STP) of H2 and 2 × 10−5 cc(STP) of CH4 per gram of water. H2/CH4 ratios in the vents increase with temperaturea result that is tentatively attributed to chemical equilibrium and/or the redox state of the individual waters. The phase diagram for the NaCl-H2O "surrogate-seawater" system shows that liquid-vapor separation may take place prior to dischargeand mixing of a vapor phase with entrained cooler sea-water would profoundly alter original concentrations of volatiles as well as dissolved salts. H2 and CH4 ratios to basalt-derived helium are respectively about 550 and 70 in these waters. The total fluxes from the world-ocean ridge systemestimated from the He-3 fluxare of the order of 1.3 × 109 m³/y for H2 and 1.6 × 108 m³/y for CH4. The CH4 flux so calculated is sufficient to replace the deep-sea methane in ∼ 30 yearsimplying a very rapid bacterial consumption rate below the thermocline.