Methane and hydrogen in East Pacific Rise hydrothermal fluids

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 H 2 and CH 4 as 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 H 2 and 2 × 10 −5 cc(STP) of CH 4 per gram of water. H 2 /CH 4 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‐H 2 O "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. H 2 and CH 4 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 × 10 9 m³/y for H 2 and 1.6 × 10 8 m³/y for CH 4 . The CH 4 flux so calculated is sufficient to replace the deep‐sea methane in ∼ 30 yearsimplying a very rapid bacterial consumption rate below the thermocline.