Enhanced scavenging ofPb andPo by processes associated with the East Pacific Rise near 8°45′N

In 1977 and 1981 a hydrothermal plume was detected at the East Pacific Rise (EPR) near 8°45′N and at MANOP Site M, 25 km east of the EPR, by anomalous²²²Rn and Mn concentrations. In 1981, samples were also taken for²¹⁰Pb,²¹⁰Po and²²⁶Ra analyses to determine if enhanced scavenging of these elements occurred in the plume. At both the ridge crest and at Site M, the ²¹⁰Pb/²²⁶Ra ratios range from 0.09 to 0.35, which are among the lowest values ever measured. It appears that removal of the²¹⁰Pb is occurring by processes operating at or near the seafloor. There is also significant ²¹⁰Po/²¹⁰Pb disequilibrium at both locations, which appears to increase away from the seafloor towards the plume (as indicated by elevated Mn concentrations). An in-situ water column scavenging process is suggested by correlation of [²¹⁰Pb-²¹⁰Po] and Mn concentrations in the plume. The residence time of the²¹⁰Po is only about 1 year, which is close to its residence time in surface waters but quite short compared to typical deep sea values. Thus rapid scavenging of the polonium in the plume and relatively rapid settling of particles from the plume is suggested.