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, N2‐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 μg m−2 d−1) and low δ15N‐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/Ccarb 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 CO2 via the biological pump compared to when the plume was absent, hence supporting earlier observations that the Amazon River plume is important for ocean CO2 sequestration.

Carbonate fluxes by coccolithophore species between NW Africa and the Caribbean: Implications for the biological carbon pump

“…Overall, the positive correlation between the UPZspp/LPZspp-CaCO 3 ratio and POC fluxes (Fig. 9d-all sites; Supplementary Table S7) reflects the enhanced coccolith-CaCO 3 production in the upper photic zone linked to higher productivity in shallower nutricline conditions, whether due to geostrophic wind forcing (i.e., sites CB and M1) or to the occurrence of transient surface blooms (site M4; see Guerreiro et al 2017Guerreiro et al , 2019Korte et al 2020).…”

“…This is also the case for the rare but very large-sized species Scyphosphaera apsteinii, included in the category "others," which produced up to 15 mg m À2 d À1 at site M4 (Supplementary Table S3). Still, and albeit their much lighter and smaller-sized coccoliths, G. flabellatus and F. profunda were the most important Korte et al 2017 andvan der Does et al 2020). The mass flux of unspecified particles (i.e., residual fraction) results from subtracting the weights of biogenic constituents (i.e., CaCO 3 , organic matter and biogenic silica) from the total mass and has been used as a proxy for dust deposition in the Atlantic Ocean (Jickells et al, 1998), including our study area (e.g., Fischer et al 2016;Guerreiro et al 2017;Korte et al 2017).…”

The Impact of the Amazon on the Biological Pump and the Air‐Sea CO2 Balance of the Western Tropical Atlantic

“…Particularly relevant is the role of diatoms. While we account for this in our parameterizations, we treat DDAs and normal diatoms the same way, even though DDAs have been hypothesized to be more efficient in this export (Korte et al., 2020). This means that we may tend to underestimate the contribution of DDAs to POM formation and EP.…”

Seasonality in Saharan Dust Across the Atlantic Ocean: From Atmospheric Transport to Seafloor Deposition

“…At the same time, the lower sediment traps tend to show higher fluxes than their upper counterpart during the same year (13M2, 13M4 and 14M4; Figure 3). For the sediment traps at M4, geochemical analyses revealed that lateral input of Amazon‐River sediments to the upper trap is minimal, if not absent (Van der Does, Pourmand, et al., 2018), while the lower trap at 13M5 was affected by downslope transport of sediment re‐suspended from the nearby Barbados margin, causing much higher deposition fluxes than expected for the overall downwind decreasing trend, including the large outlier in April 2013 (Figure 3; Korte et al., 2020).…”