Using 234Th disequilibria to estimate the vertical removal rates of polycyclic aromatic hydrocarbons from the surface ocean

A novel 234Th-coupled approach was employed to deduce the vertical fluxes of polycyclic aromatic hydrocarbons (PAHs) through surface waters of harbor, coastal, shelf, and pelagic regimes. We found reasonable agreement between these surface ocean fluxes, based on direct measurements of mixed-layer properties, and fluxes deduced from underlying sediments. Fluxes decreased exponentially away from the northeastern U.S.A. Source-diagnostic molecular ratios indicate that atmospheric fall-out of combustion-derived PAHs from the continents is the dominant source. Despite higher unit area fluxes inshore, the open ocean represents a large integral sink for PAHs. For example, of the 60 metric tonnes of pyrene that we estimate to annually become sequestered in the western North Atlantic, only ˜ 10% is deposited in coastal waters while the remaining 90% is exported farther offshore on the continental shelf and to pelagic regimes.