Flux of Particulate Elements in the North Atlantic Ocean Constrained by Multiple Radionuclides

Sinking particles strongly regulate the distribution of reactive chemical substances in the ocean, including particulate organic carbon and other elements (e.g., P, Cd, Mn, Cu, Co, Fe, Al, and ²³² Th). Yet, the sinking fluxes of trace elements have not been well described in the global ocean. The U.S. GEOTRACES campaign in the North Atlantic (GA03) offers the first data set in which the sinking flux of carbon and trace elements can be derived using four different radionuclide pairs ( ²³⁸ U ^:234 Th ^;210 Pb: ²¹⁰ Po; ²²⁸ Ra: ²²⁸ Th; and ²³⁴ U: ²³⁰ Th) at stations co-located with sediment trap fluxes for comparison. Particulate organic carbon, particulate P, and particulate Cd fluxes all decrease sharply with depth below the euphotic zone. Particulate Mn, Cu, and Co flux profiles display mixed behavior, some cases reflecting biotic remineralization, and other cases showing increased flux with depth. The latter may be related to either lateral input of lithogenic material or increased scavenging onto particles. Lastly, particulate Fe fluxes resemble fluxes of Al and ²³² Th, which all have increasing flux with depth, indicating a dominance of lithogenic flux at depth by resuspended sediment transported laterally to the study site. In comparing flux estimates derived using different isotope pairs, differences result from different timescales of integration and particle size fractionation effects. The range in flux estimates produced by different methods provides a robust constraint on the true removal fluxes, taking into consideration the independent uncertainties associated with each method. These estimates will be valuable targets for biogeochemical modeling and may also offer insight into particle sinking processes.