February 1st, 2011
Biological productivity in vast regions of the oceans is known to be limited by the supply of iron, an essential nutrient to marine organisms, impacting carbon export to the ocean floor. The atmospheric source of iron originates from desert mineral dust aerosols and is converted to a bio-available form of soluble iron during transport. Little is known of the magnitude and variability of soluble iron deposition (SFeD) and the character of its transport. We have used the GFDL Global Chemical Transport Model to examine the emission of mineral dust (~3.5% Fe) during Saharan desert dust storms; the chemical processing of iron to a soluble form during transport; and the subsequent dry surface deposition and precipitation scavenging deposition to the North Atlantic Ocean.
The following figure presents the seasonal SFeD magnitude (color shading) and variability (contour lines). The bulk of the North Atlantic Ocean receives its maximum soluble iron deposition during summer while the greatest deposition in the northeast Atlantic is found during March through May coincident with the observed spring phytoplankton bloom. The largest seasonal flux (> 5 µmol m-2 day-1; > 450 µmol m-2 season-1) occurs in the subtropics during summer when dust is transported westward off Africa by steady trade winds as seen by the relatively low (< 30%) variability. During winter and spring large year to year variability (> 60%) is seen in the central Atlantic from north of the Caribbean to the west of Europe. Regionally, a near absence of SFeD during some years and large episodes during other years produces very high variability (> 75%) as depicted in the central north Atlantic.
The nature of dry and wet deposition can be quite different. The figure below depicts a time series of daily dry and wet SFeD during July and March for a grid box off the coast of West Africa. During July there are five distinct pulses of SFeD (up to 14 µmol m-2 day-1) associated with intermittent dust outbreaks and wet removal in the Intertropical Convergence Zone (ITCZ). In March, however, the ITCZ precipitation maxima is located south of the grid box so that SFeD is controlled by comparatively slow evolving surface dry removal containing two broad lower magnitude peaks.
Note that the short-term episodes of SFeD during precipitation events (July) can be significantly larger than the monthly mean rate (straight line). Analysis shows this to be true throughout the entire Atlantic Ocean. However, it remains unclear what the effects of episodic natural soluble iron fertilization are on the initiation and growth of phytoplankton blooms and air-sea CO2 exchange.
Manuscript: Moxim, Walter, Song-Miao Fan, and Hiram Levy II (2011), The meteorological nature of variable soluble iron transport and deposition within the North Atlantic Ocean basin, J. Geophys. Res., 113, D03203, doi:10.1029/2010JD014709.