Variations in hydrothermal methane and hydrogen concentrations following the 1998 eruption at Axial Volcano

During the February response cruise to Axial Volcano, methane and hydrogen concentrations within the plume were significantly elevated above backgroun d. CH4/He-3 ratios were found to be 5-10 times greater than previously repo rted values for unsedimented ridges. An average delta(13)C value of -32.7 /- 0.8 parts per thousand was determined for CH4 concentrations > 10 nM and CH4/Mn ratios were determined to be 2.2. Hydrogen concentrations as high a s 183 nM were measured; however most were < 10 nM. Microbial oxidation of m ethane was negligible (< 0.001 day(-1)) during this sampling period. Howeve r, microbial hydrogen oxidation was sufficient to explain decreased H-2 con centrations measured at 'older' downstream stations.