Boundary exchange and scavenging of radionuclides in continental margin waters of the Middle Atlantic Bight: implications for organic carbon fluxes

Abstract

Continental margins are sites where energy dissipation from physical (e.g., waves currents and tides) and biochemical process (e.g., rates of photosynthesis and decomposition) are highest. As a consequence, continental margins have been considered as possibly important sites of organic matter production and export to the ocean interior. As part of the Ocean Margins Program, vertical profiles of 234Th, 210Pb, particulate organic carbon, suspended particulate matter concentrations, vertical fluxes of sinking particles and associated natural radionuclides (e.g., 234Th, 210Pb, 14C), and elemental composition (e.g., C, N) were measured. Boundary scavenging and exchange processes of natural radionuclides and organic matter were investigated across the continental margin in the Middle Atlantic Bight (MAB). Large deficiencies of 214Th with respect to production from 218U decay in the MAB down to 2300 m water depth imply rapid benthic nepheloid layer exchange processes not only in deeper water, but also in the upper water column of the continental slope. Calculated lateral fluxes of organic carbon, likely of episodic nature and related to the relative position of the Gulf Stream, are up to an order of magnitude larger than long-term vertical deposition rates in the sediments.