Settling removal rates of PCBs into the Northwestern Atlantic derived from U-238-Th-234 disequilibria

Surface ocean export fluxes of polychlorinated biphenyls (PCBs) were deduced utilizing Th-234 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 Th-234-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 Th-234-derived surface 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 hydroxyl radical-mediated decomposition. Based on the limited data set, the largest fraction of each PCB is removed into the open ocean, although higher areal fluxes are seen closer to the coast. For example, we estimate that 14 000 mol yr(-1) of a tetrachlorobiphenyl (congener 52) were sequestered in the northwestern Atlantic Ocean in 1993-1994, with 75% of the flux being in the pelagic sector, 20% on the continental shelf, and only 5% in the 20-km-wide coastal region closet to the continental source.