The elemental composition of the deep particle flux in the Sargasso Sea

The elemental composition of the deep particle flux at 500 m, 1500m and 3200m was measured at the Oceanic Flux Program (OFP) time-series site in the northern Sargasso Sea from September 2000 to March 2015. The results clearly show that the deep particle flux is sourced from two components: an "extrinsic flux" component that is derived from and temporally coupled with recent surface export fluxes, and an "intrinsic flux" component that is derived from chemical scavenging and suspended particle aggregation within the deep water column. Elemental fluxes show a seasonal cycle that is closely tied to the overlying cycle in primary production. Fluxes of biogenic minerals and nutrient elements peak during the late winter-spring maximum in overlying primary production. A small late winter-spring peak in fluxes of non-biogenic elements suggests that intrinsic flux generation within the deep water column also covaries with overlying production and the extrinsic flux. Flux elemental composition also shows a marked seasonal cycle that reflects dilution of the intrinsic flux in the late winter and spring with fresh biogenic materials sourced from recent overlying production. Factor of four increases in lithogenic element fluxes and lithogenic element ratios indicate that the source of the deep lithogenic flux is not Saharan dust but North American margin sediments that have been resuspended and advected to the region via Gulf Stream recirculation. High rates of authigenic mineral precipitation accompany organic matter degradation in mesopelagic waters. The excess Mn flux (i.e. flux not supported by lithogenic minerals) increases by an order of magnitude between 500m and 1500 m, and the excess Ba flux increases by a factor of three. There is a major reorganization of element associations with flux carrier phases with increasing depth. In particular, the geochemical behavior and flux of P becomes increasingly independent of organic carrier phases and the geochemical behavior of Fe, Co, Ni, Cu and, to a lesser extent, V and Pb becomes increasingly controlled by manganese oxide scavenging. This article is part of a special issue entitled: "Cycles of trace elements and isotopes in the ocean - GEOTRACES and beyond" - edited by Tim M. Conway, Tristan Horner, Yves Plancherel, and Aridane G. Gonzalez.