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 background. CH 4 / 3 He ratios were found to be 5-10 times greater than previously reported values for unsedimented ridges. An average δ 13 C value of -32.7±0.8‰ was determined for CH 4 concentrations > 10 nM and CH 4 /Mn ratios were determined to be 2.2. Hydrogen concentrations as high as 183 nM were measured; however most were < 10 nM. Microbial oxidation of methane was negligible (