Anomalous boundary scavenging in the Middle Atlantic Bight: evidence from 230Th, 231Pa, 10Be and 210Pb

Abstract

Boundary scavenging processes result in the deposition of particle-reactive chemical substances in ocean margin sediments at rates much greater than their average rate of deposition over the entire ocean. Sediments and samples collected by time-series sediments traps were studied to examine the nature and extent of boundary scavenging in the Middle Atlantic Bight (MAB) region of the NW Atlantic. Here, new results are presented for 230Th, 231Pa and 10Be and compared to older findings, based on studies of 210Pb, where it appeared that the normal enhancement of particle-reactive trace-metal deposition at ocean margins was absent. In contrast to the behavior of 210Pb, deposition rates of 230Th, 231Pa and 10Be exceed their local rates of supply, in some cases by more than an order of magnitude. Enhanced deposition of these tracers is clearly occurring in the MAB. An unusual feature of boundary scavenging, however, is that the pattern of enhanced deposition follows the order Be > Th > Pa > Pb, whereas the expected order be Be > Pb ≥ Pa > Th. Furthermore, unsupported 231Pa/230Th activity ratios are consistently less than their production ratio of 0.093, in contrast to ratios typically found at other ocean margins in the range of 0.2–0.3. Export from the region of fine-grained manganese-rich particles is postulated to explain the low fluxes of 210Pb and the low 231Pa/230Th ratios. Other trace elements which have high affinities for adsorption to Mn oxides may, like Pb and Pa, have much lower rates of deposition in the MAB than in other, more typical, ocean-margin environments.