Multiple drivers of production and particle export in the western tropical North Atlantic

Abstract

To assess the impacts of Amazon River discharge, Saharan dust deposition, N 2 ‐fixation and mixed‐layer deepening on the biological carbon pump, sediment traps were moored from October 2012 to November 2013 at two sites in the western tropical North Atlantic (49°W,12°N/57°W,12°N). Particle exports interpreted along with satellite‐ and Argo‐float data show peak fluxes in biogenic silica (31 mg m −2 d −1 ) and organic carbon (25 mg m −2 d −1 ) during the fall of 2013 that were ten to five times higher than any time earlier during the year. These high export fluxes occurred in tandem with high surface chlorophyll a concentrations associated with the dispersal of the Amazon River plume, following retroflection into the North‐Atlantic‐Counter‐Current. High fucoxanthin fluxes (> 80 μ g m −2 d −1 ) and low δ 15 N‐values (−0.6‰) suggest a large contribution by marine diatom‐diazotrophic‐associations, possibly enhanced by wet Saharan dust deposition. During summer, the Amazon River plume resulted in high mass fluxes at 57°W that were enriched in biogenic silica but weakly influenced by diazotrophic‐associations compared to the fall event at 49°W. High carbonate‐carbon fluxes (17 mg m −2 d −1 ) dominated a second single event at 49°W during spring that was likely triggered by mixed‐layer deepening. Rain‐ratios of BSi/C carb amounted to 1.7 when associated with high export fluxes linked to the Amazon River plume. Compared to an annual average of 0.3, this indicates a more efficient uptake of CO 2 via the biological pump compared to when the plume was absent, hence supporting earlier observations that the Amazon River plume is important for ocean CO 2 sequestration.

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