Manganese and methane in hydrothermal plumes along the East Pacific Rise, 88409 to 118509N

Abstract-In November, 1991, we surveyed the water column for hydrothermal plumes along 350 km of the East Pacific Rise axis from 8’40 ’ to 1 lo50 ’ N, using a combination of physical and chemical measure-ments. Our survey included the two major ridge segments north and south of the Clipperton Transform Fault at about lO”lO’N, both limbs of the overlapping spreading centers (OSC’s) at 9”03 ‘N and 1 lo45 ‘N, and a 30&m section of the next ridge segment to the south. 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 (dissolved Mn and CR) measurements, although the chemical measurements were more sensitive. The least active sections were the southern third of the northern segment from lo”20 to 52’N and the OSCs, especially the OSC at ll”45’N. Plumes there had weak Mn and CH4 signals and were barely detectable by physical methods. These axial sections 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 CH, and light attenuation, whereas the opposite was true on the southern segment, which was the site of a volcanic eruption at 9”45-52’N only seven months prior to our cruise. On the northern segment the four physical and chemical plume tracers correlate