A model study of the relative influences of scavenging and circulation on ~(230)Th and ~(231)Pa in the western North Atlantic

摘要

The oceanic cycles of thorium-230 and protactinium-231 are affected by a number of processes, such as removal by adsorption to settling particles and transport by ocean currents. Measurements obtained as part of GEO-TRACES and earlier programs have shown that, in the North Atlantic, the activities of dissolved Th-230 (Th-230(d)) and Pa-231 (Pa-231(d)) at abyssal depths are lower near the western margin than in the basin interior. At least two factors could explain the lower Th-230(d) and Pa-231(d) near the margin: (i) intensified scavenging in benthic nepheloid layers (BNLs) extending a thousand meters or more above the seafloor; and (ii) ventilation by relatively Th-230(d)- and Pa-231(d)-poor waters emanating from the Deep Western Boundary Current (DWBC).Here a regional ocean circulation model with 'eddy-permitting' resolution (1/4 degrees) that incorporates Th-230 and Pa-231 is used in an effort to reproduce the observed distributions of Th-230 and Pa-231 in the western North Atlantic. In this model, Th-230 and Pa-231 removal from solution is governed by a prescribed distribution of particulate matter that is derived from a recent synthesis of nephelometer and transmissometer data. The model simulates a meandering Gulf Stream and a DWBC along the continental slope and rise, although noticeable differences with physical observations also exist. A model solution is found that explains most of the variance of Th-230(d) measurements (85%) and Pa-231(d) measurements (81%) from (pre-)GEOTRACES cruises. On the other hand, measurements of particulate Th-230 (Th-230(p)) and Pa-231(Pa-231(p)) are more difficult to reproduce, with the same solution accounting for only 49% (11%) of the Th-230(p) (Pa-231(p)) variance. Sensitivity experiments suggest that the low Th-230(d) and Pa-231(d) activities observed near the western margin are due to enhanced removal rates of both nuclides in BNLs rather than to deep water ventilation from the western boundary. The radionuclide activities present in the DWBC at its inflow location are also found to strongly influence the basin-scale distributions of Th-230 and Pa-231. Overall, our study points to BNLs as important sites of Th-230 and Pa-231 scavenging in the ocean and illustrates the difficulty to explain simultaneously radionuclide measurements in dissolved and particulate forms in the studied area.