Settling Removal Rates of PCBs into the Northwestern Atlantic Derived from 238U−234Th Disequilibria

Abstract

Surface ocean export fluxes of polychlorinated biphenyls (PCBs) were deduced utilizing 234Th as an indicator of the removal rates of particle-bound species from coastal, continental shelf, and pelagic surface waters. The fraction of the PCB settling flux that reaches the sediments (the ratio of bottom sediment import flux to 234Th-derived surface ocean export flux) decreased with both water column depth and with aqueous solubility of the congeners, presumably as a result of particle decomposition and/or compound desorption. A sharper decrease in 234Th-derived surface ocean fluxes away from the continental source was seen for congeners with increasing chlorination. This suggests that atmospheric washout followed by particle-mediated settling out of the surface ocean are the predominant processes affecting the PCBs, as opposed to atmospheric hydroxyl radical-mediated decomposition. Based on this limited data set, the largest fraction of each PCB is removed into the open ocean, although higher areal fluxes a...