In arid environments such as the Southwestern U.S., eolian processes cause soil erosion, transport mineral dust, and deposit the dust on surfaces where it may infiltrate. Dusts entering leaf canopies can directly provide nutrients to plants, a process which may bypass and dwarf the flow of nutrients from deep soils. Dust storms result in immediate, intense visibility hazards, as well as in less intense degradation of visibility over wide areas. The suspension of dust particles in the air has significant and complex effects on the passage of solar radiation (and reflected light) through the atmosphere. All of these effects vary by regions and by seasons, and today's regime may be significantly different from that of historical and prehistoric periods.
Below, we provide some background information about airborne dusts in the Southwestern U.S. and about their seasonal and regional fluxes and sources. The information is relevant to current investigations of ecosystems, transmission of radiation and climate forcing, and changes in atmospheric visibility.
Mineral dusts as soil builders and plant nutrients - - Mineral dusts transported by the atmosphere are a major source of the material that constitutes soils in many places (Marchand, 1970). Plants can take substantial fractions of their tissue-building nutrients directly from mineral dusts that fall onto leaf canopies and soil surfaces (Graustein and Armstrong, 1983), and may be dependent on dust for this reason. In arid, dust-producing areas, especially the Southwest, eolian processes have contributed to soil development so that the established indigenous flora may be dependent upon such soils. If such soils lose vegetation by natural climate change or are disturbed by human activity, they become vulnerable to wind erosion. Investigations of modern and past rates of dust deposition thus consider the time required to establish (replenish) shallow soils in the arid Southwest. Studies of deposition rates of (a) background dust from high-elevation snowpacks, and (b) larger-flux dust deposition sampled at lower altitudes (both described below) bear on the concepts of eolian processes as soil builders and nutrient suppliers.
|Figure 1. Eolian dusts are the major source of many mineral-derived nutrients to certain plant communities. Calcium and other elements enter the system as dusts, through folial contact and through deposition onto forest litter. Such cycling minimizes the flow of nutrients to plants from deep soil horizons. Photo by Warren Hamilton.|
Visibility considerations - - The National Park Service and collaborating agencies work on the causes, status, and amelioration of degraded visibility in the Southwest. Some of the research measures and monitors, at a network of sites, the fraction of total extinction of incoming sunlight and skylight that is caused by atmospheric particulates. Also measured is the role of mineral dust in that partial extinction (Malm et al, 1994 and 1996; Cooperative Institute for Research in the Atmosphere and National Park Service NatureNet; Air Waste Management Assn., 1994). The IMPROVE program (Interagency Monitoring of Protected Visual Environments) monitors, at a wide network of sites, many chemical constituents of the atmospheric load for a selected particle-size range collected by filters (Coop. Inst., 1996). The Grand Canyon Visibility Transport Commission (1996) of the Western Governors' Association assesses the causes of and possible remedies for visibility deterioration in one region of the Southwest (recommendations to EPA). Vasconcelos et al. (1996) have proposed that air masses affecting the interior of the Southwestern U.S. may originate in or pass through urban areas of Southern California.
Effects of atmospheric dusts on radiation absorption and insolation - - The different minerals that make up the particles of airborne dusts have different abilities to absorb the radiation that comes directly from the sun, or that is reflected back upwards from the surface of the earth. The different minerals also have different abilities to absorb various spectral bands ("colors") of the radiation that strikes them. These differences are related to the different energies (strengths) of the chemical bonds that connect the atoms in the orderly crystal networks of the minerals. Technically, these energies are described in physics by means of the molecular orbital and ligand field theories. Practically, they determine how much incoming sunlight is prevented from reaching the surface of the earth and is used instead to heat the atmosphere via the energy absorbed by the mineral grains of dust. Also, these energies determine how much reflected light is retained by the atmosphere instead of being lost to space. Other workers are interested in the mineral identity of dusts for its relevance to climate forcing models. (Sokolik and Toon, 1996; Tang, 1997). Iron and other metals of the transition element group have the most distinctive energy absorption properties. Iron oxide minerals (hematite, limonite, magnetite) and several types of micas and clays are rich in these metals and are also common in mineral dusts, and therefore give dusts distinctive absorption properties. The Southwestern U.S., with its extensive red- and brown-colored surfaces, may be expected to be a large source for eolian uptake of such minerals.
Agriculturally generated dusts - - Provisions of the Farm Bill of 1996 have focussed attention on the agriculturally-generated component of atmospheric dusts, which are believed in some regions such as the western and southeastern U.S. to be responsible for a major fraction of PM-10 atmospheric particles (Tegen and Fung, 1995; pers. comm., M. Molnau, Idaho State Climatologist, and Idaho Climate). The Northwest Columbia Plateau Project (1996, Stetler, 1994), largely funded by the U.S. Dept. of Agriculture (Natural Resources Conservation Service), studies PM-10 dusts in this agricultural region, assesses differences between modern and pre-cultivation levels of atmospheric dusts and their deposition, and recommends amelioration strategies. Busacca et al. (1996, pp. 47-54) have begun work on determination of long-term (including pre-cultivation period) deposition rates of dusts in the Columbia Plateau region by studies of sediments in lake basins not fed by sediment-bearing streams.
Figure 2. Dust plume rising to 1,500 m elevation in San Joaquin Valley, Calif., s.e. of Bakersfield, December 1977. Winds associated with this event exceeded 300 km/hr. Erosion damage was extreme, and the event is associated with an epidemic of valley fever in an area extending hundreds of km to the north, from northward transport of dust containing the fungus that causes the disease (Wilshire et al., 1996; photo by Sam Chase)|
Dust source studies- - Some past studies of dusts have succeeded in revealing various types of information about the source terranes of atmospheric dusts. The techniques used have included chemical composition, mineral identity and mineral-suite associations, and isotopes. For example, Reheis and Kihl (1995) used major-oxide analyses and x-ray diffraction of different size fractions to show that he composition of clay-size particles is similar across southern Nevada and California, whereas the composition of silt particles is more variable and may reflect both local and far-travelled dust. Hinkley (1974) showed that the Sierra Nevada receives dust of local origin for some months of the year, and dust from the interior of the Southwestern U.S. in other months. Source studies of other regions are by Hovan et al. 1997, Nakai et al. 1993, Schuetz and Sebert, 1987; Schuetz and Rahn, 1982, who focussed on dust from Saharan Africa and its areas of deposition. Grousset et al. (1992) demonstrated Patagonian sources for the Antarctic peninsula by means of radiogenic isotopes. Muhs et al. (1990) showed that islands of the Caribbean and Atlantic have African sources for their soils.
Seasonality of dust deposition - - Atmospheric load and deposition fluxes of mineral dusts vary seasonally, in almost all places. Source regions and composition of dust also vary by season. From standpoints of visibility and air quality, of nutrient supply to growing plant communities, and of influence of dusts on climate, weather, and insolation, knowledge of the seasonal regime of mineral dusts is important. Data of the IMPROVE program, the NOAA Air Monitoring Program (Meyers et al., 1990), and of the National Dry Deposition Network (NOAA) provide chemical information that give information about seasonality. For some of the Southwest the five-year study of Reheis and Kihl has provided basic information on annual variation. Bach et al. (1966) found that dust storms in the Mojave Desert are most common during the winter and spring; this also applies to Owens (dry) Lake (Reheis, in press) in the northernmost part of the Mojave. However, summer dust storms are also common on the Colorado Desert in southeastern California (Bach et al., 1996).
Secular change of dust regimes - - Several studies have determined that regimes of dust transport and deposition have changed on the time scale of the most recent millenia, centuries, and decades. The "dust bowl" years of the present century are well known. The very great intensity of dust deposition (100 times present) during the Late Glacial Maximum of the most recent ice age (about 14,000 years Before Present [B.P.]) has been documented in ice cores from high latitudes of the northern hemisphere (Alley, 1996). Studies by Dean (1997) of Minnesota lake sediments deposited in the last 10,000 years have shown major changes in dustiness and in dust sources in central North America. Moreover, a study by Matsumoto and Hinkley (1994) suggests that major changes in amounts and compositions of polar dusts during the period 6-9 kyBP reflects a time of worldwide alteration of dust regimes (since 6,000-9,000 years B.P.), the dusts have been more calcic, less potassic dusts, and sea salts may have become more important in the dust-salt mix of the atmospheric load). A very recent study by Moulin et al. (1997) has shown that the very large masses of mineral dusts exported from Saharan Africa to the North Atlantic and the Mediterranean basin vary strongly from year to year because of climatic variability (precipitation and regional atmospheric circulation) defined by the North Atlantic Oscillation. The differences are so great within cycles of a few years that it is difficult to assess the importance of anthropogenic influences (desertification and land-use changes).
The worldwide background dust and the "aging" phenomenon - - It has been proposed that there is a low-concentration atmospheric background dust load that has a common particle size distribution and common chemical (and mineral) composition on a regional, hemispheric, or even global scale (Ram and Gayley, 1994; Wake et al. 1994; Hinkley et al., 1997). The particle size distribution, and possibly the chemical and mineral compositions of the background dust, apparently converge to certain ranges as air masses "age", over periods during which there is no addition of new particulate material from sources on the ground. Such a background dust may at certain times (when atmosphere is not heavily loaded by local sources) dominate dust deposition even in typically dusty regions such as continental interiors.
Figure 3. The figure shows one of the chemical characteristics (potassium to calcium ratio, K/Ca) of broad-region atmospheric background dust. This dust has been identified not only in areas free from special local dust sources where local sources are distant or suppressed (polar regions, high coastal Alaskan mountains), but at high altitude in dusty regions of Central Asia, where local dusts are significant. The ratios of other rock- and soil-forming elements (not plotted here) support the identification of the background dust (Hinkley et al., 1997, in press)|
USGS. Dust Studies in the Southwestern U.S. - - The USGS is developing plans to study mineral dusts in the Southwest from the points of view of uptake and generation (including modelling), mass flux and rates of deposition, sources, and composition (especially mineralogy) and interaction with incoming solar, and reflected, radiation. The studies will emphasize (a) palaeoclimate (dust regimes in historical and prehistoric times), (b) processes (the mechanisms of uptake, transport, and deposition of dusts), and (c) monitoring of modern-day fluxes, compositions, and source regions of dusts. Contacts: Todd Hinkley, Richard Reynolds, Marith Reheis, Daniel Muhs.
Owens Lake - - a world-class source of salt-bearing dusts - - Possibly the greatest or most intense human-disturbed dust source on earth is Owens Lake, in Owens Valley east of the Sierra Nevada in east-central California. Since being drained as part of a Los Angeles water supply system in the early part of the century, the lake bed has been the source of high-particle-concentration dust storms, made up of unusually fine, unusually-alkaline particles. Dust from the lake bed certainly alters the visibility over a broad area on a seasonal basis, and probably has altered the soil properties over large areas to degrees depending on distance and direction from the source. Information on the dust regime at Owens Lake can be found in Reheis, 1997 and in "Owens (Dry) Lake, California: A Human-Induced Dust Problem" in this web workshop.
Gill (1996) summarizes the broad worldwide problem of the generation of dusts from dessicated lakes. Another prominent example, the Aral sea, one of the large natural lakes of Central Asia (Uzbekistan-Kazakhstan, Aral Sea Photo, Central Asia Research and Remediation Exchange), has shrunk dramatically during the late decades of the 20th century, surely in part due to interception of irrigation water from the rivers that feed it, and possibly in part due to cyclical fluctuations that may have tectonic causes. The contraction has left exposed tracts of land that serve as regional sources of alkaline dusts, yielding a situation analogous in some ways to that of Owens Lake.
The five-year southwestern dust deposition study of Reheis and Kihl - - The extended study of dust deposition rates, types, sources, and temporal changes by Reheis and Kihl (1995) largely forms the foundation for the proposed work on dusts in the Southwestern U.S.
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