FY 1994

Scavenging of ²³⁴Th and phosphorus removal from the hydrothermal effluent plume over the North Cleft segment of the Juan de Fuca Ridge

Kadko, D., R.A. Feely, and G.J. Massoth

J. Geophys. Res., 99(B3), doi: 10.1029/93JB02952, 5017–5024 (1994)

Dissolved and total ²³⁴Th (t_1/2 = 24.1 days) profiles were measured over the hydrothermal field located within the North Cleft segment of the Juan de Fuca Ridge. Above the hydrothermal effluent plume, which rose approximately 200 m from the seafloor, >=90% of the ²³⁴Th was dissolved, and the total ²³⁴Th activity was in equilibrium with its parent uranium. This is typical of deep water for which the residence time of dissolved ²³⁴Th with respect to particle uptake is greater than 200 days. Within the effluent plume, however, as little as 30% of the ²³⁴Th was in the dissolved state and the residence time of dissolved ²³⁴Th with respect to particle uptake was then of the order of only a few weeks. The flux (J_Th) of dissolved ²³⁴Th to the particle phase was about 300 disintegrations per minute (dpm) cm⁻² yr⁻¹. The total ²³⁴Th was still approximately in equilibrium with U, indicating that the residence time of the ²³⁴Th-bearing particles with respect to sinking was greater than 100 days. Particulate P, V, and As, while insignificant above the plume, attained concentrations as high as 60, 0.65, and 0.45 nmol/L within the plume. The particulate [P]/[²³⁴Th] ratio and the ²³⁴Th flux (J_Th) are used to calculate a removal flux of P resulting from hydrothermal scavenging of ~50 µg cm⁻² yr⁻² or approximately 5.0 × 10⁶ g P yr⁻² per kilometer of active ridge crest. Similarly, 1.0 × 10⁻⁵ g km⁻¹ of V and As are removed per year from the water column by hydrothermal scavenging over this region. Extrapolation to a global scale suggests that ridge crest scavenging removes &#lt;=10% of these elements from the ocean.