Composition of aeolian dust in natural traps on isolated surfaces of the central Mojave Desert—Insights to mixing, sources, and nutrient inputs

doi:10.1016/j.jaridenv.2005.06.031

Journal of Arid Environments

Volume 66, Issue 1, July 2006, Pages 42-61

https://doi.org/10.1016/j.jaridenv.2005.06.031 Get rights and content

Abstract

The recognition and characterization of aeolian dust in soil contribute to a better understanding of landscape and ecosystem dynamics of drylands. Results of this study show that recently deposited dust, sampled in isolated, mostly high-ground settings, is chemically and mineralogically similar on varied geologic substrates over a large area (15 000 km²) in the Mojave Desert. The silt-plus-clay fraction (fines) on these isolated surfaces is closely alike in magnetic-mineral composition, in contrast to greatly dissimilar magnetic compositions of rock surfaces of vastly different lithologies, on which the fines have accumulated. The fines, thus, are predominantly deposited dust. The amounts of potential nutrients in the sampled dust are much more uniform than might be provided by direct, local weathering of bedrock or by dust locally derived from nearby weathered products. The compositional similarity of the dust on these surfaces is interpreted to result from mixing of fines in the atmosphere as well as in fluvial, alluvial, and lacustrine depositional settings prior to dust emission.

Introduction

Aeolian dust influences surficial and ecological processes in desert regions, where large amounts of dust are both sequestered and emitted over long periods of time (Goudie, 1978). In these settings, aeolian dust plays important roles in soil formation, soil hydrology, development of surfaces, distribution of biologic soil crusts, and nutrient status (e.g. Yaalon and Ganor, 1973; Wells et al., 1985, Wells et al., 1987; McFadden et al., 1986, McFadden et al., 1998, McFadden et al., 1987; Chadwick and Davis, 1990; McDonald et al., 1996; Belnap and Gillette, 1998; Shachak and Lovett, 1998; Reynolds et al., 2001a).

This paper reports primarily on the recognition and composition of aeolian dust on high surfaces, isolated from surrounding terrain, over an area of about 15 000 km² of the central Mojave Desert in the southwestern United States (Fig. 1). These isolated surfaces were sampled because the fine-grained sediment (fines) on them had no local alluvial source and thus must be primarily from dust. In the field, moreover, the fines at different sites were similar in color and magnetic susceptibility in contrast to variable characteristics of underlying bedrock. The principle goal of this study was to examine the compositional variation of dust in this setting over an area of diverse bedrock geology and geomorphology. Wells et al. (1982) previously interpreted fines in similar settings to be largely aeolian dust. The presence of aeolian dust in talus on steep dryland slopes (Whitney and Harrington, 1993; Blank et al., 1996) represents an analogous setting. Strong compositional similarity among distant sites would reflect the mixing of fines from disparate bedrock types. The mixing might occur in different ways, such as in surficial deposits that are dust sources and (or) in the atmosphere during dust transport. Compositional disparities might provide clues to dust sources and the physical processes, such as aerodynamic sorting of minerals, that control mineral-dust properties. Attention is also given to the potential nutrient loads of deposited dust because of their importance for biogeochemical dynamics.

Samples of fine-grained sediment on the isolated surfaces were collected from the surface down to only a few cm (typically 1–2 cm depth). These surfaces receive modern dust, but the time span represented by the entire sampled interval is not known and may vary among sites. It is likely that the sampled sediments represent dust deposited during at least the past few hundred years, as suggested by Reheis et al. (2002) for ‘old dust’ trapped in a comparable setting—vugs in volcanic rocks.

In this study we also compare dust collected on topographically isolated features with fine-grained sediment in surficial deposits at a few sites, most of them on alluvial fans, from approximately the same area to evaluate whether these different settings capture dust of similar or disparate properties. Significantly different properties might be attributable to proximity to local dust sources or to conditions causing spatial variability in dust deposition.

Section snippets

Setting

Sampled sites are in the central Mojave Desert, from Death Valley National Park southward to the Soda Lake area and the Cima volcanic field in the Mojave National Preserve (Fig. 1). This part of the Mojave is characterized by a variety of geologic surfaces that includes mountain ranges of diverse rock types, playas, ancient lake deposits, lava flows, sand-dune fields, alluvial fans, and riverbeds. In the northern part of the study area, Greenwater Valley and bounding ranges are dominantly

Sampling

We sampled fine-grained material suspected to be primarily aeolian dust at 10 sites that included the tops of peaks and isolated high parts of ridges, as well as the surface of a mesa (site Z-3) (Fig. 1; Table 1). We collected the upper ∼2 cm of fine-grained sediment found in pockets and crevices directly below summits. The sediment on the mesa was collected directly below rock clasts that formed a desert pavement; we also sampled a subjacent vesicular A soil layer. At most sites, fine-grained

Results

Magnetic properties of the fines on isolated surfaces are similar and, as a group, contrast greatly with the highly variable magnetic properties of the bedrock samples. Magnetite abundance in the fine fraction varies by about 2× (IRM, 1.99–3.90×10⁻² Am² kg⁻¹), hematite by 2.4× (HIRM, 1.28–3.12×10⁻³ Am² kg⁻¹). In contrast, IRM and HIRM in rocks vary by about 3450× and 330×, respectively (Fig. 2a; Table 2). Concentration-independent parameters (ARM/IRM, magnetic grain size; and S parameter, relative

Compositional similarities—evidence for mixing of fines

The overall compositional similarities of the dust on the isolated, widely separated, and geologically diverse surfaces are interpreted to reflect mixing of aeolian sediment from multiple sources. Such mixing likely has taken place in different environments—in the atmosphere as well as in fluvial, alluvial, and lacustrine depositional settings prior to dust entrainment. Observations of contemporary dust events and storms in the Mojave Desert indicate that many windstorms, even those of modest

Conclusions

Dust deposited on widely separated, isolated settings in the Mojave Desert are similar in mineralogic and chemical composition, regardless of underlying bedrock composition. These overall compositional similarities reflect mixing of fines from multiple sources. Such mixing likely has taken place in the atmosphere as well as in fluvial, alluvial, and lacustrine settings. Compositional variation in dust from these isolated sites is similar to that in fines from the mainly intermontane soil

Acknowledgments

We express deep gratitude to Harland Goldstein for invaluable assistance in sample and data management, to Nilah Mazza, Jiang Xiao, and Eric Fisher for physical property measurements, to Fred Luiszer for XRF analyses, and to Jeremy Workman and Ren Thompson for discussions about the geology of the Death Valley region. The manuscript benefited from reviews by Jayne Belnap, Harland Goldstein, Jason Neff, and two anonymous referees. This work was supported by the Earth Surface Dynamics Program of

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Composition of aeolian dust in natural traps on isolated surfaces of the central Mojave Desert—Insights to mixing, sources, and nutrient inputs

Abstract

Introduction

Section snippets

Setting

Sampling

Results

Compositional similarities—evidence for mixing of fines

Conclusions

Acknowledgments

Journal of Arid Environments

Geoderma

Geomorphology

Geomorphology

Applied Geochemistry

Geoderma

Catena

Geomorphology

Journal of Arid Environments

Quaternary Research

Quaternary Research

Catena

Geochimica et Cosmochimica Acta

Journal of Arid Environments

Quaternary Research

Soil-forming intervals caused by eolian sediment pulses in the Lahontan basin, northwestern Nevada

Geology

Magnetic susceptibility of soil: an evaluation of conflicting theories using a national data set

Geophysical Journal International

Dust storms and their geomorphological implications

Journal of Arid Environments

N and P fertilization on rangeland production in Midwest Argentina

Journal of Range Management