Deposition of Aeolian Dust on the Ocean by the Atmosphere

Aeolian dust deposition over the oceans provides a biogeochemical link between climate change and terrestrial and marine ecosystems (Ridgwell, 2002). A major natural source of new iron to open ocean surface waters is continentally derived aeolian dust, which supplies about three times the

Figure 7: Contemporary annual mean dust deposition rate (Ginoux et al., 2001 (For colour version, see Colour Plate Section)).

fluvial input (Duce and Tindale, 1991). In situ iron-fertilization experiments have been conducted in both the equatorial Pacific [IronEx I (Martin et al., 1994) and IronEx II (Coale et al., 1996) and Southern Ocean (SOIREE (Boyd and Law, 2001))]. On all three occasions, raising the iron level in the water by a few nanomoles per liter produced a significant increase in phytoplank-ton biomass. During IronEx II, the increase was at least an order of magnitude. Iron-limited high nutrient low chlorophyll (HNLC) regions comprise approximately 30% of the world ocean and include the Southern Ocean (de Baar et al., 1995). The majority of iron deposition to the ocean occurs in the NH and is principally associated with dust export from the major arid zones such as the Sahara and Taklamakan Deserts (Fig. 7). The North Atlantic and North Pacific Oceans receive 48% and 22% of global iron deposition to the oceans, while the Indian Ocean (principally in the Arabian Sea) receives 18% and the Mediterranean Sea 4%. The South Atlantic and South Pacific Oceans receive only 4% and 2%, respectively, while the polar regions in both hemispheres also receive very low iron inputs, with the Arctic receiving only 0.9% and the Antarctic 0.5% (Gao et al., 2001).

The subantarctic Southern Ocean is an HNLC region, and it has been suggested that deep mixing and the availability of iron limit primary production. Australian mineral dust is high in iron content and can be transported over the Australian sector of the subantarctic Southern Ocean, particularly during the austral spring and summer when dust storm frequency in southern Australia is maximal. Recently, Gabric et al. (2002) reported evidence for a coupling between satellite-derived (SeaWiFS) aerosol optical thickness and chlorophyll concentration in the upper ocean. The coupling was evident at monthly, weekly, and daily timescales. The shorter timescale coupling supports the hypothesis that episodic atmospheric delivery of iron is stimulating phytoplankton growth in this region.

Long-term climate variability may also be related to variation in dust deposition rates. Evidence for a possible influence on the glacial-interglacial variability of atmospheric CO2 comes from the observed changes in dust deposition, recorded in the 420 kyr Vostok Antarctic ice core (Petit et al., 1999). The concentration of dust contained within the ice exhibits a series of striking peaks against a background of relatively low values. Intriguingly, the occurrence of these peaks correlates with periods of particularly low atmospheric CO2 values. It has been hypothesized that enhanced dust supply to this region during the last glacial could have driven a more vigorous oceanic biological pump with consequent draw-down of atmospheric CO2 (Martin, 1990).

Numerical models of the global carbon cycle have since demonstrated that realistic increases in the strength of the biological pump in the Southern Ocean are unable to explain glacial atmospheric CO2 mixing ratios as low as ~190ppm. However, of the total ~90ppm deglacial rise in atmospheric CO2, the initial 40-50 ppm occurs extremely rapidly (within just kyr) and up to 10 kyr before the collapse of the NH ice sheets. Predictions of a carbon cycle model that explicitly accounts for the biogeochemical cycling of Fe in the ocean, confirm that changes in the aeolian supply of Fe to the Southern Ocean may be at least partly responsible for these particular features of the CO2 record (Watson et al., 2000).

Interestingly, it has also been proposed that aeolian delivery of iron can also influence the oceanic sulfur cycle and the oxidation of DMS in the remote marine atmosphere. Zhuang et al. (1992) report that over 50% of the total iron present in remote marine aerosols is in the soluble Fe(II) form, which is readily available to phytoplankton. The photoreduction reaction that produces Fe(II) in aerosols also produces the hydroxyl radical, which is required for the oxidation of atmospheric DMS to MSA, and ultimately the formation of CCN.

The fact that atmospheric Fe fluxes appear to play an important role in ecosystem dynamics in many locations underscores the interwoven nature of the links between climate change, the biogeochemical cycles of carbon, nitrogen, and sulfur, and the potential for the oceans to sequester carbon.

Was this article helpful?

0 0

Post a comment