Abstract
Abstract Atmospheric deposition contributes potentially significant amounts of the nutrients iron, nitrogen, and phosphorus (via mineral dust and anthropogenic aerosols) to the oligotrophic tropical North Atlantic Ocean. Transport pathways, deposition processes, and source strengths contributing to this atmospheric flux are all highly variable in space and time. Atmospheric sampling was conducted during 28 research cruises through the eastern tropical North Atlantic (ETNA) over a 12-yr period, and a substantial dataset of measured concentrations of nutrients and trace metals in aerosol and rainfall over the region was acquired. This database was used to quantify (on a spatial and seasonal basis) the atmospheric input of ammonium, nitrate, soluble phosphorus, and soluble and total iron, aluminum, and manganese to the ETNA. The magnitude of atmospheric input varies strongly across the region, with high rainfall rates associated with the intertropical convergence zone contributing to high wet deposition fluxes in the south, particularly for soluble species. Dry deposition fluxes of species associated with mineral dust exhibited strong seasonality, with the highest fluxes associated with wintertime low-level transport of Saharan dust. Overall (wet plus dry) atmospheric inputs of soluble and total trace metals were used to estimate their soluble fractions. These also varied with season and were generally lower in the dry north than in the wet south. The ratio of ammonium plus nitrate to soluble iron in deposition to the ETNA was lower than the N:Fe requirement for algal growth in all cases, indicating the importance of the atmosphere as a source of excess iron.
Highlights
Atmospheric deposition accounts for approximately 70% of the input of iron (Fe) to the open ocean (Jickells et al 2005) and is an important source of fixed reactive nitrogen (N) (Duce et al 2008; Krishnamurthy et al 2007; Zamora et al 2011) and soluble phosphorus (P) (Bristow et al 2010; Mahowald et al 2008; Ternon et al 2011)
A proportion of the nitrogen- and phosphorus-containing aerosols over the eastern tropical North Atlantic (ETNA) derive from the biomass burning products that are cotransported with Saharan dust (Chou et al 2008; Crutzen and Andreae 1990; Real et al 2010) and aerosols transported from Europe or northwest Africa (Savoie et al 1989)
The northern half of the ETNA is most strongly influenced by the remote North Atlantic (RNA) and SAH source regions, while, in the southern half, the relative importance of SAH, Southern African (SAF), SAB, and remote Southern Atlantic (RSA) source types varies with season
Summary
Atmospheric deposition accounts for approximately 70% of the input of iron (Fe) to the open ocean (Jickells et al 2005) and is an important source of fixed reactive nitrogen (N) (Duce et al 2008; Krishnamurthy et al 2007; Zamora et al 2011) and soluble phosphorus (P) (Bristow et al 2010; Mahowald et al 2008; Ternon et al 2011). We use a unique database of aerosol and rain samples collected during 28 research cruises over a 12-yr period to estimate atmospheric inputs of N, P, and trace metals, including Fe, to the ETNA.
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