SCAVENGING OF TH-234 AND PHOSPHORUS REMOVAL FROM THE HYDROTHERMAL EFFLUENT PLUME OVER THE NORTH CLEFT SEGMENT OF THE JUAN-DE-FUCA RIDGE

Dissolved and total Th-232 (t1/2 = 24.1 days) profiles were measured o ver the hydrothermal field located within the North Cleft segment of t he Juan de Fuca Ridge. Above the hydrothermal effluent plume, which ro se approximately 200 m from the seafloor, greater-than-or-equal-to 90% of the Th-234 was dissolved, and the total Th-234 activity was in equ ilibrium with its parent uranium. This is typical of deep water for wh ich the residence time of dissolved Th-234 with respect to particle up take is greater than 200 days. Within the effluent plume, however, as little as 30% of the Th-234 was in the dissolved state and the residen ce time of dissolved Th-234 with respect to particle uptake was then o f the order of only a few weeks. The flux (J(Th)) of dissolved Th-234 to the particle phase was about 300 disintegrations per minute (dpm) c m-2 yr-1. The total Th-234 was still approximately in equilibrium with U, indicating that the residence time of the Th-234-bearing particles with respect to sinking was greater than 100 days. Particulate P, V, and As, while insignificant above the plume, attained concentrations a s high as 60, 0.65, and 0.45 nmol/L within the plume. The particulate [p]/[Th-234] ratio and the Th-234 flux (J(Th)) are used to calculate a removal flux of P resulting from hydrothermal scavenging of approxima tely 50 mug cm-2 yr-1 or approximately 5.0 x 10(6) g p yr-1 per kilome ter of active ridge crest. Similarly, 1.0 x 10(5) g km-1 of V and As a re removed per year from the water column by hydrothermal scavenging o ver this region. Extrapolation to a global scale suggests that ridge c rest scavenging removes less-than-or-equal-to 10% of these elements fr om the ocean.