Points of Origin
“Dust deposition on mountain snow cover has occurred throughout much of recent history as demonstrated by annual dust layers in high elevation ice cores, increasing with prolonged or intense drought and land disturbance in source regions” (Painter et al., 2007).
“The Great Basin, Colorado Plateau, Mojave, and Sonoran deserts of the southwestern United States are responsible for the majority of [dust] emissions in North America” (Neff et al., 2008).
“According to anecdotal evidence, historical research, and recent observations…the San Juan Mountains receive multiple dust deposition events annually in February through May, arriving before and during the snowmelt period” (Painter et al., 2007).
“There are a number of potential sources of dust to the San Juan Mountains including the deserts of the southwestern United States and desert sources in Asia that are known to contribute dust to the North American continent. Although the precise provenance of San Juan dust samples is difficult to determine, the physical and isotopic properties of dust can be used to substantially narrow the potential source regions” (Neff et al., 2008).
How do we know?
After testing, “these isotopic compositions are consistent with dust sources to the south and/or southwest of the study area. Satellite detection of dust plumes and atmospheric back-trajectory modeling for this region also link wintertime dust deposition in the San Juan Mountains to dust plumes that originate in the deserts of the southwestern United States, further supporting a dust source indigenous to western North America” (Neff et al., 2008).
“Although Asian dust periodically falls on the San Juan Mountains, the textural distribution of dust samples also provides strong evidence for a regional source of dust. Nearly 40% of the mass of dust sampled from the snowpack occurs in the 10-37 µm size class, 26% in the 37-63 µm size class and 17% in the 63-180 µm size class. The relatively large proportion of particles over 37 µm is evidence for particles that have been transported hundreds, rather than thousands, of kilometers. This result suggests that the dominant source of wintertime dust inputs to the San Juan Mountains is the western United States rather than far-travelled Asian dust, which would be much finer (that is, in the less than 10 µm size classes)” (Neff et al., 2008).
“With the exception of a single event with northwesterly flow (February 22, 2003) across southeast Utah, the remaining events had southwesterly flow across northeastern Arizona and northwestern New Mexico” (Painter et al., 2007).
“The researchers found that the dust affecting San Juan snow cover came not from the immediate vicinity but from the Colorado Plateau, including the Four Corners region where the borders of Utah, Colorado, New Mexico, and Arizona intersect. Since the late nineteenth century, the American Southwest has seen significant land-use changes, including the expansion of grazing, recreational use, and agriculture. These land-use changes have disturbed desert soils and increased windblown dust” (Dust Reduces Snow Cover in the San Juans, 2007).
Characteristics of Source Area
“The erosivity of the soil surface, and thus the potential impacts of aeolian [wind-driven] processes at the plant-interspace scale, depend on both the ability of the soil surface to resist erosion and the ability of the wind to reach the soil surface. Erosion resistance is determined by the strength of the soil and presence of surface protectors, such as rocks, plant litter, and physical and biological soil crusts. Rocks and plant litter too large to be moved by wind offer the greatest soil protection. Physical soil crusts – created by the binding together of silt and clay particles when wetted and then dried – protect soils, except when crusts are subjected to disturbance. Unless disturbed, these soils have an inherently higher resistance to erosion than soils dominated by coarser sand particles. Biological soil crusts, composed of cyanobacteria, lichens, and moss, stabilize soils by excreting mucilaginous material that binds soil surface particles together, thereby increasing soil aggregate size and increasing soil resistance to the shearing forces of wind” (Field et al., 2009).
“The type, cover, and arrangement of vegetation have the strongest influence on the ability of the wind to reach the soil surface. The patchy and dynamic nature of vegetation in dryland regions results in aeolian transport being highly heterogeneous in both space and time. The amount of material that is moved depends on the size of unvegetated gaps upon which the wind can act (generally excluding rocky or gravelly areas, referred to as desert pavement, and areas covered by physical or biological soil crust) and the height and density of the vegetation, which controls the size of the protected area downwind of individual plants. Although surface characteristics are important, the amount of horizontal flux depends largely on the structure of the ecosystem and the degree of connectivity between unvegetated gaps” (Field et al., 2009).
Potential Causes of Dust Events
“Many of the factors that drive wind erosion are, of course, greatly affected by soil surface disturbances. Grazing cattle crush biological and physical soil crusts and decrease vegetative cover, thereby increasing wind erosion. Offroad vehicles and military training activities also crush vegetation and impact plant-interspace surface characteristics, particularly biological and physical soil crusts. Fire can dramatically increase wind erosion, although fire may be less spatially extensive than grazing and recreational use. Burning vegetation (even by typical rangeland fires) releases different amounts of organic compounds, which, in turn, lead to different levels of water repellency in the soil, depending on various factors, such as vegetation type, soil properties, and fire intensity and duration. Fire-induced water repellency decreases the strength of interparticle wet-bonding forces by increasing the soil-water contact angle. This repellency enhances soil erodibility by causing a drop in threshold friction velocity, thereby increasing post-fire erosion” (Field et al., 2009).
“Another potential impact of grazing in arid-land soils is the disruption of biological soil crusts (BSC), which influence nutrient cycling and stabilize surface soils” (Neff et al., 2005).
“…during settlement of the western U.S., the intensification of human activities such as agriculture, grazing, and resource exploration in semiarid landscapes led to 500% greater dust deposition in the adjacent mountains. Furthermore, dust deposition in many regions could increase as a result of the increasing extent of arid lands and greater human activity in these areas” (Steltzer et al., 2009).
“Expansion and intensification of grazing, recreational use and agriculture over the past ~140 years has increased the dust emission from the Colorado Plateau and other desert regions of the western US” (Painter et al., 2007).
“A change in dust source over the past several decades may result from the increasing disturbance of southwestern desert surfaces by human activities that include urbanization, agriculture, livestock grazing, off-road vehicle use, use of dirt roads, water diversion from lakes, and military training. These activities increase dust emission from previously stable desert surfaces” (Reynolds et al., 2001).
“…dust deposited in the San Juan Mountains comes primarily from the Colorado Plateau. Precipitation in the fall 2005/winter 2006 on the Colorado Plateau was the lowest on record and contributed to the doubling of the number of deposition events over those 2003-2005. We conclude then that snow cover duration across the San Juan Mountains is reduced by 18-35 days due to the deposition of dust from the disturbed deserts of the Colorado Plateau and not from sources local to the mountain basins” (Painter et al., 2007).
“More than 50% of the conterminous United States experienced moderate to severe drought conditions in 2002, with record or near-record precipitation deficits throughout the western United States” (Cook et al., 2004).
“Drought abated in many areas by late 2002 to early 2003, but severe drought conditions have continued to affect the interior western United States throughout the 2004 summer” (Cook et al., 2004).