Atmospheric dust load

Global simulations of the modern dust cycle are capable to reproduce the first-order pattern of dust transport and deposition under modern climate conditions.

The estimates of the global dust emissions for present climate published in the literature span a very large range, from about 100 Mt/yr to about 3000 or even 5000 Mt/yr on a global scale (IPCC 2001, Tegen et al., 2002a), and 800-1700 Mt/yr according to one of the most recent estimates assembling the largest compilation of quantitative observations of the modern global dust cycle (Tegen et al., 2002a).

The IPCC 2001 estimates the mineral (soil) dust emission in ~2150 Mt/yr on global scale, with very high spatial and temporal variability. The estimate for the dust emission in the Southern Hemisphere is less than 1/5 the emission estimated for the Northern Hemisphere (about 350 and 1800 Mt/yr respectively).

These uncertainties are linked to the scarcity of global datasets used to determine the model input parameters and to validate model simulations: observations around the globe are very scarce and they are often representative of a limited period of time. Local observations are often extrapolated to yield a global estimate even if they come from specific regions having conditions that are not necessarily typical of all dust-source regions. Moreover, there are many desert areas in the world that are still too poorly studied.

1 A recent workshop held in Germany (Max Plank Institute for Biogeochemistry, Jena, 2-4 May 2002) has brought together dust experts coming from many disciplines (Geology, physics, remote sensing, etc.) in order to formulate strategies for using existing datasets and creating new ones to improve model parametrization and evaluation. Some of the issues of the present thesis work have been presented, and an inter-comparison project for paleoclimate simulation is perspected [Tegen et al., 2002b].

2.4.2 Common characteristics of source terrains identified by satellite observations

In general terms, the regions providing the bigger dust fluxes at present time are primarily those with little or no ground cover, easily wind-erodible soils and associated to seasonal wetness (Mahowald et al., 1999).

A worldwide geographical mapping of major atmospheric dust sources has been recently provided by Prospero et al. (2002) on the basis of data from TOMS (Total Ozone Mapping Spectrometer) sensor on NIMBUS-7 satellite for a period of 13 years (1980-1992). The authors evidenced that the major sources for long-range transported dust are located in arid regions and are centered over topographical lows or on lands adjacent to topographical relief. Dry lake beds, relics of extensive lakes in the past, are a good example since lacustrine sediments are characteristically fine-grained, but also glacial outwash plains, riverine floodplains, alluvial fans and all areas where the recent geomorphological history has favoured the concentration of fine-grained material and the creation of large areas with low surface roughness are preferential sources (Tegen et al., 2002a).

Despite the arid (from semi-arid to hyper-arid) conditions of these environments at present time, these sources had either a relatively recent (Pleistocene) pluvial history or are associated with water features, such as ephemeral rivers and streams, alluvial fans, playas and saline lakes. Chemical weathering is enhanced by the abundance of water, and liquid transport is an efficient mechanism for production of small particles, separated from the soil or from the primary rock and carried to a depositional basins or an alluvial plain where, after drying, become mobilizable by wind (Prospero et al., 2002). Vegetation cover and soil crusting are also two important factors that can sensibly lower or suppress dust emission.

Sand dune systems do not appear good sources for long range transported dust (Prospero et al., 2002).

This is not paradoxical, since they are relatively coarse grained, from tens to several hundred micrometers (Lancaster, 1995), and are already impoverished in the fine fraction. The coarse sandy particles have a high settling velocity in air and therefore are not carried more than few hundred kilometers away by winds; however, the role of sandy particles in generation of dust particles by saltation is crucial (Gillette et al., 1982).

Terrains with a recent history of aridity therefore, appear much more active sources than old arid sandy areas.

2.4.3 Source regions at global scale

The major global dust sources have been identified by Prospero et al. (2002) through TOMS Absorbing Aerosol Index (AAI). The authors constructed a world map, reported in Fig. 2.2, on the

basis of the long term frequency of occurrence (FoO) distribution² for each source, and therefore is not representative of a particular period of the year. The sources, in fact, show a large variability and characteristic geometries in function of the seasons.

The principal sources for dust that can potentially be transported long-distance are located in the Northern Hemisphere, in particular in North Africa, in the Middle East, in central Asia and in the Indian subcontinent.

For the Southern Hemisphere, observations evidence that it is devoid of major dust sources impacting on large areas. This observations are consistent with concentration of dust-derived Al element in southern-ocean waters, that is much lower than in northern oceans (Measures and Vink, 2000) and with the flux of aeolian materials in deep sea sediments (Rea, 1994).

Interestingly, TOMS observations (Prospero, 2002) have evidenced that many sources are associated with areas where human impacts are well documented (e.g. the Caspian and Aral Seas, South-Western North America, the loess-lands in China), but the largest and most active sources however are located in remote areas without human activity.

Therefore, the present-day dust mobilization seems to be dominated by natural sources on global scale.

2.4.4 Principal dust “hot spots” in the Southern Hemisphere

The theme of this thesis work is the transport of mineral dust in the Late Quaternary towards the Antarctic continent, therefore it is useful to focus on the most recent issues coming from satellite observations on the present-day dust sources for the Southern Hemisphere.

This paragraph is inspired to the observations of Prospero et al. (2002), from which the three figures here reported (Australia, South Africa and South America) have been taken.

2 TOMS AAI frequency of occurrence (FoO) distribution is expressed in Prospero et al. (2002) as days per month when the AAI equals or exceeds 0.7.

Fig. 2 .2 : Global dust sources identified through TOMS AAI (Prospero et al., 2002).

Australia

In the Australian continent (Fig. 2.3) continuous dust deflation is detected in the Great Artesian Basin feeding Lake Eyre, that today constitutes a large playa³. The most active dust area is located North-East of present-day Lake Eire, corresponding to the pluvial Lake Dieri (ancestral Lake Eire).

Interestingly, the large Simpson Desert does not appear as persistent source for dust but only an occasional region for large dust events. There are only other minor active regions within the continent.

At first glance, it could seem surprising that such a large and arid continent is devoid of major sources for long-range transported dust.

The authors found an explication for this in the flat topography of the continent, and the lack of renewal of small particles (§ 2.2.1).

3 Playas are shallow, short-lived lakes that form where water drains into basins with no outlet to the sea and quickly evaporates. Playas are common features in arid (desert) regions and are among the flattest landforms in the world (USGS Geologic Glossary).

Fig. 2.3: Australian dust sources expressed as TOMS AAI FoO distributions, from Prospero et al., 2002.

South Africa

For South Africa (Fig. 2.4), a continuous source for dust is located in Botswana in the region centered at 21ºS, 26ºE.

Dust activity is centered over the western end of the Makgadikgadi Depression, occupied during the Pleistocene by a great lake, the Palaeo-Makgadikgadi (Goudie, 1996).

A second small but persistent source is centered at 16ºE, 18ºS over the Etosha Pan, Northern Namibia, at the extreme northwest of the Kalahari basin. During the Pleistocene also the Etosha Pan basin was occupied by a large lake (Goudie, 1996).

South America

For South America (Fig. 2.5) three dust source regions can be observed.

The first is located in the Bolivian Altipiano, around ~20°S and 68°W, in an arid intermountain basin situated at about 3750-4000 m of altitude that includes two of the largest salt flats (salars) of the world. A large part of the Altipiano was occupied in the Pleistocene by a lake, whose sediments are exported today by the strong winds blowing over the region.

The second major dust source area in South America is located in Argentina along the eastern flanks of the Andes (27-34ºS, 67-70ºW). Dust activity is centered in an intermountain area between the Andes (West) and the Sierra de San Luis - Sierra de Cordoba (East). It can be observed that the most active area lies in the western part of this region.

Fig. 2.4: South African dust sources expressed as TOMS AAI FoO distributions, from Prospero et al., 2002.

Finally, a large dust source region is located between 38ºS and 48ºS, and includes the Southern Pampas and Northern Patagonia, semiarid to arid region spanning from the eastern flanks of the Andean Cordillera to the Atlantic coast. Within this vast region a particular active area is located further south, close to Santa Cruz (46º-48ºS), but dust emission here has been attributed mostly to anthropogenic activities reducing vegetation cover and activating wind erosion over large areas of the province.

Fig. 2.5: South America dust sources expressed as TOMS AAI FoO distributions, from Prospero et al., 2002.

2.5 The Last Glacial Maximum