Atmospheric iron deposition and sea-surface dissolved iron concentrations in the eastern Atlantic Ocean

Abstract

Atmospheric iron and underway sea-surface dissolved (o0.2 mm) iron (DFe) concentrations were investigated along a north-south transect in the eastern Atlantic Ocean (27 N/16 W-19 S/5 E). Fe concentrations in aerosols and dry deposition fluxes of soluble Fe were at least two orders of magnitude higher in the Saharan dust plume than at the equator or at the extreme south of the transect. A weaker source of atmospheric Fe was also observed in the South Atlantic, possibly originating in southern Africa via the north-easterly outflow of the Angolan plume. Estimations of total atmospheric deposition fluxes (dry plus wet) of soluble Fe suggested that wet deposition dominated in the intertropical convergence zone, due to the very high amount of precipitation and to the fact that a substantial part of Fe was delivered in dissolved form. On the other hand, dry deposition dominated in the other regions of the transect (73-97%), where rainfall rates were much lower. Underway sea-surface DFe concentrations ranged 0.02-1.1 nM. Such low values (0.02 nM) are reported for the first time in the Atlantic Ocean and may be (co)-limiting for primary production. A significant correlation (Spearman's rho=0.862, po0:01) was observed between mean DFe concentrations and total atmospheric deposition fluxes, confirming the importance of atmospheric deposition on the iron cycle in the Atlantic. Residence time of DFe in the surface waters relative to atmospheric inputs were estimated in the northern part of our study area (1778 to 28716 d). These values confirmed the rapid removal of Fe from the surface waters, possibly by colloidal aggregation. r

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“…For example, Fe concentrations as low as 0.02 nM (20 times less than the present surface sample) have been documented in the eastern Atlantic Ocean (Sarthou et al 2003). Measuring Fe isotopes at such low con- centrations will obviously present a future challenge.…”

Dissolved iron in the vicinity of the Crozet Islands, Southern Ocean

“…Saharan desert) being in the northern hemisphere (Sarthou et al, 2003). However such atmospheric inputs are estimated to be small in the Southern Ocean ) although very little data exist for this region.…”

Estimation of Dust Downfall Time in Dusty Days using the Correlation between PM<sub>10</sub> and Sunphotometer Data

“…China is dominated by dry deposition, while Korea, Taiwan 20 and the eastern sea are dominated by wet deposition. As we move towards the south, wet deposition gains the relative dominance in a belt which is under the influence of tropical convergence and rainfall (Sarthou et al, 2003).…”

“…Plantation is an efficient method for trapping the dust. Rocks do the same job, but rocky lands maintain only 20% of the deposited dust (Goossens, 1995). 25…”