Modelling the effect of boundary scavenging on Thorium and Protactinium profiles in the ocean
The "boundary scavenging" box model is a cornerstone of our understanding of the particlereactive radionuclide fluxes between the open ocean and the ocean margins. However, it does not describe the radionuclide profiles in the water column. Here, I present the transportreaction equations for radionuclides transported vertically by reversible scavenging on settling particles and laterally by horizontal currents between the margin and the open ocean. Analytical solutions of these equations are
... pared with existing data. In the Pacific Ocean, the model produces "almost" linear 230 Th profiles (as observed in the data) despite lateral transport. However, omitting lateral transport biased the 230 Th based particle flux estimates by as much as 50%. 231 Pa profiles are well reproduced in the whole water column of the Pacific Margin and from the surface down to 3000 m in the Pacific subtropical gyre. Enhanced bottom scavenging or inflow of 231 Pa-poor equatorial water may account for the model-data discrepancy below 3000 m. The lithogenic 232 Th is modelled using the same transport parameters as 230 Th but a different source function. The main source of 232 Th scavenged in the open Pacific is advection from the ocean margin, whereas a net flux of 230 Th produced in the open Pacific is advected and scavenged at the margin, illustrating boundary exchange. In the Arctic Ocean, the model reproduces 230 Th measured profiles that the uni-dimensional scavenging model or the scavenging-ventilation model failed to explain. Moreover, if lateral transport is ignored, the 230 Th based particle settling speed may by underestimated by a factor 4 at the Arctic Ocean margin. The very low scavenging rate in the open Arctic Ocean combined with the enhanced scavenging at the margin accounts for the lack of high 231 Pa/ 230 Th ratio in arctic sediments.