Manganese and methane in hydrothermal plumes along the East Pacific Rise, 8°40′ to 11°50′N

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 11°50′N, using a combination of physical and chemical measurements. Our survey included the two major ridge segments north and south of the Clipperton Transform Fault at about 10°10′N, both limbs of the overlapping spreading centers (OSC's) at 9°03′N and 11°45′N, and a 30-km 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 CH 4) measurements, although the chemical measurements were more sensitive. The least active sections were the southern third of the northern segment from 10°20 to 52′N and the OSCs, especially the OSC at 11°45′N. Plumes there had weak Mn and CH 4 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 4 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 correlated positively and linearly with one another, suggesting that the segment was fed by relatively uniform end-member fluids with a mean CH 4 Mn molar ratio of 0.075. The southernmost section surveyed, from 8°42′ to 9°08′N, closely resembled the northern segment. The rest of the southern segment fell into three sections with different CH 4 Mn ratios: 9°39 to 53′N with CH 4 Mn as high as 10, 9°08 to 39′N with CH 4 Mn of 0.51, and 9°53′ to 10°07′N with CH 4 Mn of 0.85. The section with the highest CH 4 Mn was the site of the volcanic eruption, which produced high-temperature, low-salinity, gas-rich vent fluids carrying abundant bacterial particles. The high CH 4 concentrations are clearly associated with the volcanic eruption, but the origin of the CH 4 is unclear.