Desert Dust, Aerobiology and Human Implications

 In recent years, the natural environment and human health have become increasingly associated. De Longueville et al (2010) reveal that around 30% of global diseases can be attributed to environmental dynamics. As we have discussed already, aeolian activity in deserts because of the extreme erosivity and erodibility can result in extensive entrainment of fine particles, and in some cases, long-distance transport on various trajectories. Transoceanic and transcontinental (in the case of Asian deserts) dust loadings are now known to facilitate the dispersal of pathogens. The biogeographical range of pathogens is consequently increased in significant dust events and the considerable health implications of allergens and pathogens means such occurrences are being increasingly investigated (Kellogg and Griffin, 2006).  

There are a number of ways in which dust particles can affect humans and ecological patterns and processes. At the most basic of levels, mineral dust can have a significant effect on horizontal visibility and air quality. More worryingly, perhaps, evidence has been presented of the long distance transfer of aerosolised microbiota around the globe. Moreover, the concentrations of such microorganisms have been confirmed as higher during dust events, although the magnitudes of concentrations are still to be verified (Kellogg and Griffin, 2006). Aspergillus fumigatus, Aspergillus niger, Staphylococcus gallinarum and Gordonia terrae have all been found in African dust. There are various hypotheses linking African dust transport to episodes of coral reef morbidity and Caribbean basin mortality with the discovery of the fungus Aspergillus sydowii in Sahelian soil (Rypien, 2008). Many of the bacteria found are spore-formers, making them more resistant to desiccation, increasing the potential of successful long distance transport.

Global  transport of microorganisms and toxic compounds in clouds of desert dust (USGS)

With humans no doubt adding to the production of dust by breaking down crusts, removing vegetation, overgrazing, vehicular use and construction activity, the potential for dust-borne pathogens to be transported to regions of high population density or ecological significance is ever increasing. Study is this discipline is in its relative infancy but will surely receive more attention because of the threat posed to human societies. 

The Role of Dust in Biogeochemical Cycles

Here is an exploration of three interesting papers on dust and biogeochemical cycling. I will attempt to describe dust’s involvement in major biogeochemical cycles and then focus in on a specific example.  We have already discovered the powerful impact dust can have on the global climate system and the consideration of their impacts on biogeochemical cycling is imperative to enhance our understanding of earth systems science.

Harrison et al (2000) suggests that changing dust fluxes and depositional rates through time can have a significant consequence for productivity of marine and terrestrial ecosystems because of their nutrient carrying capacities. In turn, changes in biospheric productivity have a direct impact on atmospheric composition (e.g. CO₂ and N₂O) potentially resulting in the initiation of widespread and various feedback mechanisms.

Dust also makes up a god proportion of oceanic sediments downwind of major desert regions. In the North Pacific, for example, it is estimated that around 75-98% of sediments have aeolian origin. Marine ecosystems can thus be significantly affected by the mineral composition of dust loadings, most notably silicon, iron and phosphorus.

Like sediments, soils can also be strongly influenced by desert-originated dust flows. Once again, key nutrients are sourced from this aeolian activity and, as outlined in a previous post on Amazonian dust deposition, can considerably alter ecosystem equilibrium. Saharan dust is attributable to the addition of key trace species such as K, NH₄ and NO₂. Mineral dust is also particularly important where leaching predominates to maintain a healthy nutrient balance.

Whilst we can recognise the importance of aeolian dust in affecting global biogeochemical cycles, Lawrence and Neff (2009) show that it is also important to consider past changes in the global dust cycle. Ice-core and sediment data show dust deposition was greatest during glacial maxima. This evidence only adds to the complexity of changing feedbacks and resultant environmental changes. Geochemical fluxes resulting from dust deposition are most potent in close proximity to source regions, but have these source regions changed over time? Hsu et al (2009) reinforce this important aspect of changing geochemical flux by highlighting the reduction in the quantity of dust deposited with distance from the source. 

Journey to the dustiest place on Earth

A short report on a study by former UCL academics.

Bodélé study

The Bodélé Depression

The Bodélé Depression is a palaeolake basin situated in northern Chad. Digital elevation models have highlighted a series of palaeoshorlines once prominent as palaeolake Megachad’s borders. Megachad was formerly the largest lake in Africa covering in excess of 350,000 km² but has since dried up exposing a largely diatomite surface (Bristow et al, 2009). TOMS (Total Ozone Mapping Spectrometer) Aerosol Index data illustrates the Bodélé Depression to be a critical mineral aerosol source throughout the year (Washington and Todd, 2005). Entrainment of dust into the atmosphere from this major source region forms an important component of the Earth’s climate system because of its potential to influence, for example, the radiation budget. Extreme erodibility and strong surface winds account for the magnitude of the dust plumes over the Bodélé. Engelstaedter et al (2006) suggest that anthropogenic activity is also responsible for the great entrainment of dust. Firstly through land use changes such as through agriculture, mining, water management or vehicular activity, and secondly through our contributions to climate change which in turn modify dust emissions.

      African aerosol indices and Bodele dust (From UCL and NASA)

Dust from the Bodélé Depression is also known to influence biogeochemical cycling. Koren et al (2006) advise that 40 million tons of dust are transported to the Amazon from the Sahara each year. With a considerable proportion of Saharan dust accounted for from the Bodélé, it is clear a significant proportion of Amazonian dust is from this region. Furthermore, Koren et al state that the Amazon basin has a dependence on the mineral fertilisation from Saharan dust sources. Washout of nutrients in heavy rainstorms makes a steady supply of mineral essential to maintain rainforest equilibrium. 

Dust II

There is an increasing awareness in the importance of atmospheric dust loadings, and in particular the significant role they may play in climate change. The world’s deserts are major source regions of dust since the arid conditions and lack of vegetation enables deflation and entrainment of silt-sized sediment from a variety of materials (Middleton and Goudie, 2001). Human activities of grazing and vehicular use, for example, can increase the range of susceptible surfaces from which dust can be entrained.

Dust can influence climate in a number of ways. Goudie and Middleton (2001) advise that air temperatures may be affected through the absorption and scattering of solar radiation, marine primary productivity may be affected since dust may provide considerable quantities of iron, and changes in concentrations of condensation nuclei may influence cloud formation and in turn precipitation. Furthermore dust loadings may affect soil formation, calcrete formation, ocean sediment fluxes, and human health.

The Sahara provides the most substantial quantities of aeolian dust with emission estimates between 500 and 1000 Tg yr^-1 which makes up around 50% of the global total (Goudie, 2009). The Bodélé depression is the widely considered the most important source region in the Sahara and contributes a considerable proportion of the Saharan emissions. Saharan dust is comprised of a number of a number of different particles but is made up predominantly of SiO₂ and Al₂O₃. The dust flux generated in the Saharan region emphasises just how important geomorphologic processes are in the desert biome.

Entrained dust off the western coast of Africa, courtesy of NASA

Saharan dust has three main trajectories – over the North Atlantic to North and South America, northwards to southern Europe, and eastwards to the Middle East (Goudie and Middleton, 2001). The North Atlantic flux is the largest and large outbreaks can see dust transported to the Caribbean, US and Brazil. Dust concentrations have been recorded in Amazonia and corral reefs off Barbados, particularly noteworthy considering dust’s influence on nutrient dynamics and biogeochemical cycling of terrestrial and marine ecosystems.

So how does dust influence terrestrial and marine ecosystems? How will dust fluxes change in the face of climate change? Do humans significantly modify the extent of dust entrainment? How important are ‘dust hotspots’ such as the Bodélé depression? What are the other major source regions? Read on to find out………..!

Dust

A gentle introduction to the power of dust!

Distribution of Deserts

My area of focus over the next couple of weeks will be away from desert hydrology and on to geomorphology, starting more specifically with desert dust. Desert dust is a critical component of the climate system at both a local and global scale.

As a prelude to a number of posts on the subject, it is worth highlighting where the various desert regions are found, and hence where dust may come from.

The US Geological Survey uses the following map to describe the locations of the worlds major desert regions.

It is highly apparent that the majority of the deserts are found at 30 degrees latitude north and south of the equator. This coincides with the falling limb of the Hadley Circulation and the subtropical high-pressure belt. Persistent thermodynamic stability results in a suppression of vertical motion and hence minimal precipitation. Compartmentalisation of anticyclonic cells breaks up subtropical subsidence explaining why hyperaridity is not prevalent across the latitudes (Ahrens, 2009). Georgia Southwestern State University provide useful diagrams that help to illustrate how global circulatory patterns are responsible for the distribution of deserts.

There are exceptions to this general rule. The great deserts of Central Asia, for example, can be attributed to the shear distance from the sea and hence pronounced sources of water. Greater seasonal changes in the desert environment may be associated with these continental interiors (Goudie, 2002). Furthermore, polar deserts are found largely because of the freezing temperatures and minimal precipitation.

There can be a number of regional factors that influence the precise formation and nature of particular deserts, but these will be examined later through specific case studies related to individual deserts.



References

Ahrens, C. D (2009) Essentials of Meteorology, Brooks/Cole: Belmont

Goudie, A.S. (2002) Great Warm Deserts of the World, Oxford: Oxford University Press.