AN ASSESSMENT OF THE EFFECT OF CHEMICAL SCAVENGING WITHIN SUBMARINE HYDROTHERMAL PLUMES UPON OCEAN GEOCHEMISTRY

Profiles of Pb-210 over the Endeavour and North Cleft Segments of the Juan de Fuca Ridge are used to model a time scale for the scavenging, by hydrothermal plumes, of reactive elements in seawater. The hydrothermal plumes above these ridge segments are sites of intense scavenging removal of Pb-210. At Endeavour, the total Pb-210 activities within the plume are as low as 8 dpm/100 1 and dissolved activities are as low as 3 dpm/100 1. At the North Cleft, which is characterized by higher particulate Fe concentrations, the total Pb-210 activities are 4.5 dpm/100 1, the dissolved activities are 1-2 dpm/100 1 and the Pb-210 activities are deficient with respect to the activity of the Po-210 daughter. These are perhaps the lowest Pb-210 activities ever measured in the deep sea. The large gradient of Pb-210 between the plume and surrounding deep water suggests that scavenging is focused into the plumes through horizontal transport. The implication, therefore, is that this process might impact the ocean on a scale larger than that local to the ridge crest. By coupling published measurements of particle flux from Endeavour with Pb-210 activities on particles trapped at that site, the total volume of seawater stripped of Pb-210 per year for that site was calculated to be 7.4 x 10(12) 1/y. Globally, the extrapolated volume flux of seawater stripped of reactive constituents is 5.7 x 10(15) 1/y, such that the entire ocean is processed in this manner in 2.4 x 10(5) y. The geochemical cycle of elements with ocean residence times much shorter than this (e.g., Pb and Th) will not be greatly affected by hydrothermal scavenging. On the other hand, this process holds significance for the geochemistry of other elements scavenged by hydrothermal plumes, such as P and V, whose ocean residence times are > 10(4) y.