MANGANESE AND METHANE IN HYDROTHERMAL PLUMES ALONG THE EAST PACIFIC RISE, 8-DEGREES-40' TO 11-DEGREES-50'N

In November, 1991, we surveyed the water column for hydrothermal plume s along 350 km of the East Pacific Rise axis from 8 degrees 40' to 11 degrees 50'N, using a combination of physical and chemical measurement s. Our survey included the two major ridge segments north and south of the Clipperton Transform Fault at about 10 degrees 10'N, both limbs o f the overlapping spreading centers (OSC's) at 9 degrees 03'N and 11 d egrees 45'N, and a 30-bm section of the next ridge segment to the sout h. We found vigorous plumes along most of this ridge axis, in keeping with its magmatically robust cross-section, axial summit caldera, and shallow, magma-related seismic reflector. These plumes were detectable by both physical (temperature and Light attenuation) and chemical (di ssolved Mn and CH4) measurements, although the chemical measurements w ere more sensitive. The least active sections were the southern third of the northern segment from 10 degrees 20 to 52'N and the OSCs, espec ially the OSC at 11 degrees 45'N. Plumes there had weak Mn and CH4 sig nals and were barely detectable by physical methods. These axial secti ons were the only ones surveyed that lie deeper than 2600 m and appear to be magma starved. The most active sections on the northern segment gave stronger signals for Mn and temperature than for CH4 and light a ttenuation, whereas the opposite was true on the southern segment, whi ch was the site of a volcanic eruption at 9 degrees 45-52'N only seven months prior to our cruise. On the northern segment the four physical and chemical plume tracers correlated positively and linearly with on e another, suggesting that the segment was fed by relatively uniform e nd-member fluids with a mean CH4/Mn molar ratio of 0.075. The southern most section surveyed, from 8 degrees 42' to 9 degrees 08'N, closely r esembled the northern segment. The rest of the southern segment fell i nto three sections with different CH4/Mn ratios: 9 degrees 39 to 53'N with CH4/Mn as high as 10, 9 degrees 08 to 39'N with CH4/Mn of 0.51, a nd 9 degrees 53' to 10 degrees 07'N with CH4/Mn of 0.85. The section w ith the highest CH4/Mn was the site of the volcanic eruption, which pr oduced high-temperature, low-salinity, gas-rich vent fluids carrying a bundant bacterial particles. The high CH4 concentrations are clearly a ssociated with the volcanic eruption, but the origin of the CH4 is unc lear.