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doi:10.2204/iodp.proc.336.110.2012

Overview

The current estimate of the quantity of desert dust that moves through Earth’s atmosphere each year ranges from 0.5 to 5.0 metric tons (Perkins, 2001). A majority (50%–75%) of this quantity is believed to originate from the Sahara and Sahel Deserts in North Africa (Moulin et al., 1997; Perry et al., 1997; Goudie and Middleton, 2001; Prospero and Lamb, 2003). In general, high-energy storm activities over deserts and other arid regions can mobilize significant quantities of soil into the atmosphere (Gillies et al., 1996; Qian et al., 2002). Desert soils originating from the vast desert landscape of North Africa can impact air quality in the Middle East, Europe, the Caribbean, and the Americas.

Source regions of dust in the Sahara include the Bodele depression and a region covering western Mali, southern Algeria, and eastern Mauritania (Goudie and Middleton, 2001; Middleton and Goudie, 2001). Lake Chad, located southwest of the Bodele depression, is a significant source of dust in this region because of the exposed and dry lake bed. In 1963, Lake Chad had a surface area of 25,000 km² that was reduced to ~1,350 km² by 1997 as a result of the current North African drought and anthropogenic activity (Coe and Foley, 2001). Although anthropogenic activities (deforestation and desertification) in the Sahara and Sahel Deserts are also believed to influence dust transport, analyses of data collected between 1980 and 1997 demonstrated year-to-year variation in the overall size of the desert regions but no longer term changes (Tucker and Nicholson, 1999).

North African annual rainfall rates are influenced by atmospheric systems such as the North Atlantic Oscillation (NAO) and the El Niño Southern Oscillation (ENSO). The NAO has been in a predominantly positive (northerly) phase over the North Atlantic Ocean since the late 1960s, which has corresponded with an overall decrease in rainfall over North Africa (Moulin et al., 1997). It has also corresponded with a general increase in the amount of desert soil being delivered to the Caribbean and Americas (Prospero, 1999). Compared to the overall trend in dust deposition noted in the Caribbean, some of the highest deposition rates have corresponded with major ENSO events (Prospero and Nees, 1986; Prospero and Lamb, 2003; Shinn et al., 2003). Although dust transport out of North Africa may move north into the North Atlantic and Europe and northwest into the Middle East at various times of the year, the most consistent transport is across the Atlantic to the Caribbean and Americas (Perry et al., 1997). Transatlantic dust transport generally occurs between latitudes 15° and 25°N (Graham and Duce, 1979). Latitudinal Saharan/​Sahel dust transport across the Atlantic is influenced by seasonal Hadley Cell shifts (Isard and Gage, 2001). In the Northern Hemisphere summer (June–October), dust transport is to the mid- to northern Caribbean and North America, and during the winter (November–May) transport is to the mid- to southern Caribbean and South America (Graham and Duce, 1979). On 7 January 1999, a “blood rain” deposited an estimated 47 metric tons of dust on the Island of Tenerife (Criado and Dorta, 2003). It has been estimated that 40 million metric tons of African dust is deposited in the Amazon Basin each year (Koren et al., 2006).

Negative impacts of dust movement include transport of toxins (i.e., agricultural and industrial emissions), harmful algal blooms, and long-range transport of pathogenic microorganisms (O’Malley and McCurdy, 1990; Barrie et al., 1992; O’Hara et al., 2000; Weir et al., 2004; Griffin et al., 2001, 2006; Lenes et al., 2001; Walsh and Steidinger, 2001; Garrison et al., 2006; Griffin, 2007). Dust clouds may contain high concentrations of organics composed of plant detritus and microorganisms (Griffin et al., 2002; Jaenicke, 2005; Griffin, 2007) and may pick up additional biological loads (fungal spores, bacteria, viruses, pollen, etc.) as the clouds move through and sandblast downwind terrestrial environments or pass over aquatic environments, where microbial-laden fine-sized aquatic sprays adhere to dust particles (Grini and Zender, 2004). All of these dust cloud constituents may negatively influence human health in downwind environments, with the greatest risk factors being frequency of exposure, concentration of and composition of particulates, and immunological status (Griffin, 2007). Of particular interest is the transport of dust-borne pathogenic microorganisms that may impact human and ecosystem health.

Desert dust research has demonstrated the ability of diverse groups of bacteria and fungi to survive long-range atmospheric transport in clouds of desert dust (Griffin, 2007). In dust source regions, a single gram of topsoil may contain 10⁹ and 10⁶ bacteria cells and fungal cells and spores, respectively, in addition to communities of protozoa and viruses (Whitman et al., 1998; Tate, 2000). In regard to both bacteria and fungi, the extent of diversity is probably much greater than previously reported given that the majority of published papers on the topic only report culture-based approaches and it is known that the given number of colonies isolated from any sample type typically represent <1% of what is actually present (Whitman et al., 1998).

It is obvious from a review of the scientific literature that dust storms can transport constituents such as toxic compounds and microorganisms that may negatively impact human and ecosystem health. What is surprising is that few (<40) studies to date have been conducted on this topic, and many of those are significantly dated. There is a clear need for studies geared at addressing this topic, and the field is wide open for many varied scientific disciplines (e.g., microbiology, toxicology, epidemiology, etc.).

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