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 hydrothe rmal plumes above these ridge segments are sites of intense scavenging removal of Pb-210. At Endeavour, the total Pb-210 activities within t he 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 acti vities are deficient with respect to the activity of the Po-210 daught er. These are perhaps the lowest Pb-210 activities ever measured in th e deep sea. The large gradient of Pb-210 between the plume and surroun ding deep water suggests that scavenging is focused into the plumes th rough horizontal transport. The implication, therefore, is that this p rocess 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, t he 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 volu me 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 1 0(5) y. The geochemical cycle of elements with ocean residence times m uch shorter than this (e.g., Pb and Th) will not be greatly affected b y hydrothermal scavenging. On the other hand, this process holds signi ficance for the geochemistry of other elements scavenged by hydrotherm al plumes, such as P and V, whose ocean residence times are > 10(4) y.