Matthew C. Baddock

Preferential dust sources: A geomorphological classification designed for use in global dust-cycle models

Received 11 April 2011; revised 28 September 2011; accepted 4 October 2011; published 24 December 2011. [1] We present a simple theoretical land-surface classification that can be used to determine the location and temporal behavior of preferential sources of terrestrial dust emissions. The classification also provides information about the likely nature of the sediments, their erodibility and the likelihood that they will generate emissions under given conditions. The scheme is based on the dual notions of geomorphic type and connectivity between geomorphic units. We demonstrate that the scheme can be used to map potential modern-day dust sources in the Chihuahuan Desert, the Lake Eyre Basin and the Taklamakan. Through comparison with observed dust emissions, we show that the scheme provides a reasonable prediction of areas of emission in the Chihuahuan Desert and in the Lake Eyre Basin. The classification is also applied to point source data from the Western Sahara to enable comparison of the relative importance of different land surfaces for dust emissions. We indicate how the scheme could be used to provide an improved characterization of preferential dust sources in global dust-cycle models.

The Southern Kalahari: a potential new dust source in the Southern Hemisphere?

Most sources of atmospheric dust on Earth are located in the Northern Hemisphere. The lower dust emissions in the Southern Hemisphere in part limit the supply of micronutrients (primarily soluble iron) to the Southern Ocean, thereby constraining its productivity. Climate and land use change can alter the current distribution of dust source regions on Earth. Can new dust sources be activated in the Southern Hemisphere? Here we show that vegetation loss and dune remobilization in the Southern Kalahari can promote dust emissions comparable to those observed from major contemporary dust sources in the Southern African region. Dust generation experiments support the hypothesis that, in the Southern Kalahari, aeolian deposits that are currently mostly stabilized by savanna vegetation are capable of emitting substantial amounts of dust from interdune areas. We show that dust from these areas is relatively rich in soluble iron, an important micronutrient for ocean productivity. Trajectory analyses show that dust from the Kalahari commonly reaches the Southern Ocean and could therefore enhance its productivity.

Dust: Small‐scale processes with global consequences

Desert dust, both modern and ancient, is a critical component of the Earth system. Atmospheric dust has important effects on climate by changing the atmospheric radiation budget, while deposited dust influences biogeochemical cycles in the oceans and on land. Dust deposited on snow and ice decreases its albedo, allowing more light to be trapped at the surface, thus increasing the rate of melt and influencing energy budgets and river discharge. In the human realm, dust contributes to the transport of allergens and pathogens and when inhaled can cause or aggravate respiratory diseases. Dust storms also represent a significant hazard to road and air travel. Because it affects so many Earth processes, dust is studied from a variety of perspectives and at multiple scales, with various disciplines examining emissions for different purposes using disparate strategies. Thus, the range of objectives in studying dust, as well as experimental approaches and results, has not yet been systematically integrated. Key research questions surrounding the production and sources of dust could benefit from improved collaboration among different research communities. These questions involve the origins of dust, factors that influence dust production and emission, and methods through which dust can be monitored.

High Latitude Dust in the Earth System

Natural dust is often associated with hot, subtropical deserts, but significant dust events have been reported from cold, high latitudes. This review synthesizes current understanding of high-latitude (≥50°N and ≥40°S) dust source geography and dynamics and provides a prospectus for future research on the topic. Although the fundamental processes controlling aeolian dust emissions in high latitudes are essentially the same as in temperate regions, there are additional processes specific to or enhanced in cold regions. These include low temperatures, humidity, strong winds, permafrost and niveo-aeolian processes all of which can affect the efficiency of dust emission and distribution of sediments. Dust deposition at high latitudes can provide nutrients to the marine system, specifically by contributing iron to high-nutrient, low-chlorophyll oceans; it also affects ice albedo and melt rates. There have been no attempts to quantify systematically the expanse, characteristics, or dynamics of high-latitude dust sources. To address this, we identify and compare the main sources and drivers of dust emissions in the Northern (Alaska, Canada, Greenland, and Iceland) and Southern (Antarctica, New Zealand, and Patagonia) Hemispheres. The scarcity of year-round observations and limitations of satellite remote sensing data at high latitudes are discussed. It is estimated that under contemporary conditions high-latitude sources cover >500,000 km2 and contribute at least 80–100 Tg yr−1 of dust to the Earth system (~5% of the global dust budget); both are projected to increase under future climate change scenarios.

Design, construction and calibration of a portable boundary layer wind tunnel for field use

Wind erosion is a natural process that has formed landscapes but threatens sustainable agriculture in many locations. Wind tunnels have been used for several decades to study wind erosion processes. Portable wind tunnels offer the advantage of testing natural surfaces in the field, but they must be carefully designed to ensure that a logarithmic boundary layer is formed and that wind erosion processes may develop without interference from the tunnel structures. Although large portable tunnels often meet the aerodynamic criteria, their size and transportation requirements often limit the locations where they may be employed. We designed and built a self-contained portable wind tunnel that is easily transported on a tandem-axle trailer and pulled with a pickup truck. The wind tunnel uses a centrifugal blower, a flow-conditioning section with optional abrader material feed, and a 1 m tall and 0.5 m wide working section that can vary in length from 2 m to 6 m. The maximum wind velocity attainable is 18.7 m s-1 although a mid-height centerline velocity of 12.6 m s-1 is normally used for field testing of natural surfaces. Based on measured wind velocity profiles in the tunnel working section, a conservative estimate of boundary layer depth within the working section is 0.5 m. Even though no wind tunnel can truly duplicate the scale and variability of the forces that drive wind erosion, tunnels such as this one with deeply developed boundary layers offer reasonable estimates of dust emissions and erodibilities of natural surfaces. This wind tunnel has been used to test rangeland and cropped surfaces in several locations and has provided reliable and useable soil erodibility and dust emission data.

Geomorphology of the Chihuahuan Desert based on potential dust emissions

Abstract Please click here to download the map associated with this article. The Chihuahuan Desert of central northern Mexico and southern Arizona, New Mexico and Texas, USA, is a regionally significant dust ‘hot-spot’ in North America. Typical of other such hot-spots, this desert consists of a mosaic of geomorphological settings, each of which has a varying propensity for dust emission. Recently, a classification system of dust emission potential based on surface geomorphic characteristics that establishes a common framework for describing the land surface has been proposed. The classification is readily applicable to different dust source regions and designed to facilitate comparison of the relative potential dust contributions and emissivity of varying geomorphological environments in such regions. The map here (1:3,500,000) was compiled by applying the new classification to a base map of polygons from existing soil and landform maps that were produced by national government agencies. Within the study area, 11 of the 17 possible geomorphic classes were present, the most extensive being unarmoured, unincised high relief alluvial surfaces, which covered 43% of the area. As an example of how empirical dust source point data can be used with the classification, the satellite observed origins of dust plumes for 26 major wind erosion events from 2001 to 2009 were overlain on the map. Despite a total area of only 4%, ephemeral lakes were the source of 48% of the observed plumes. This map and the relationships derived from it provide the basis for developing equivalent maps in other dusty regions, and mark a step toward improving the representation and documentation of the strength of dust sources in numerical mineral aerosol models.

Pyrosequencing Reveals Bacteria Carried in Different Wind-Eroded Sediments

Little is known about the microbial communities carried in wind-eroded sediments from various soil types and land management systems. The novel technique of pyrosequencing promises to expand our understanding of the microbial diversity of soils and eroded sediments because it can sequence 10 to 100 times more DNA fragments than previous techniques, providing enhanced exploration into what microbes are being lost from soil due to wind erosion. Our study evaluated the bacterial diversity of two types of wind-eroded sediments collected from three different organic-rich soils in Michigan using a portable field wind tunnel. The wind-eroded sediments evaluated were a coarse sized fraction with 66% of particles >106 μm (coarse eroded sediment) and a finer eroded sediment with 72% of particles <106 μm. Our findings suggested that (i) bacteria carried in the coarser sediment and fine dust were effective fingerprints of the source soil, although their distribution may vary depending on the soil characteristics because certain bacteria may be more protected in soil surfaces than others; (ii) coarser wind-eroded sediment showed higher bacterial diversity than fine dust in two of the three soils evaluated; and (iii) certain bacteria were more predominant in fine dust (, , and ) than coarse sediment ( and ), revealing different locations and niches of bacteria in soil, which, depending on wind erosion processes, can have important implications on the soil sustainability and functioning. Infrared spectroscopy showed that wind erosion preferentially removes particular kinds of C from the soil that are lost via fine dust. Our study shows that eroded sediments remove the active labile organic soil particulates containing key microorganisms involved in soil biogeochemical processes, which can have a negative impact on the quality and functioning of the source soil.

Do MODIS‐defined dust sources have a geomorphological signature?

The preferential dust source (PDS) scheme enables large-scale mapping of geomorphology in terms of importance for dust emissions but has not been independently tested other than at local scales. We examine the PDS qualitative conceptual model of surface emissivity alongside a quantitative measurement of dust loading from Moderate Resolution Imaging Spectroradiometer (MODIS) Deep Blue Collection 6 for the Chihuahuan Desert. The predicted ranked importance of each geomorphic type for dust emissions is compared with the actual ranked importance as determined from the satellite-derived dust loading. For this region, the predicted variability and magnitude of dust emissions from most surface types present coincides with the observed characteristics demonstrating the significance of geomorphological controls on emission. The exception is for areas of low magnitude but persistent emissions such as alluvial surfaces where PDS overpredicts dustiness. As PDS is a good predictor of emissions and incorporates surface dynamics it could improve models of future dust emissions.

Coupling leeside grainfall to avalanche characteristics in aeolian dune dynamics

Avalanche (grainflow) processes are fundamental drivers of dune morphodynamics and are typically initiated by grainfall accumulations. In sedimentary systems, however, the dynamism between grainfall and grainflow remains unspecified because simple measurements are hampered by the inherent instability of lee slopes. Here, for the first time, terrestrial laser scanning is used to quantify key aspects of the grainfall process on the lee (slip face) of a barchan sand dune. We determine grainfall zone extent and flux and show their variability under differing wind speeds. The increase in the downwind distance from the brink of peak grainfall under stronger winds provides a mechanism that explains the competence of large avalanches to descend the entire lee slope. These findings highlight important interactions between wind speed, grainfall, and subsequent grainflow that influence dune migration rates and are important for correct interpretation of dune stratigraphy.

Identifying Sources of Aeolian Mineral Dust: Present and Past

Aeolian mineral dust is an important component of the Earth’s environmental systems, playing roles in the planetary radiation balance, as a source of fertilizer for biota in both terrestrial and marine realms and as an archive for understanding atmospheric circulation and paleoclimate in the geologic past. Crucial to understanding all of these roles of dust is the identification of dust sources. Here we review the methods used to identify dust sources active at present and in the past. Contemporary dust sources, produced by both glaciogenic and non-glaciogenic processes, can be readily identified by the use of Earth-orbiting satellites. These data show that present dust sources are concentrated in a global dust belt that encompasses large topographic basins in low-latitude arid and semiarid regions. Geomorphic studies indicate that specific point sources for dust in this zone include dry or ephemeral lakes, intermittent stream courses, dune fields, and some bedrock surfaces. Back-trajectory analyses are also used to identify dust sources, through modeling of wind fields and the movement of air parcels over periods of several days. Identification of dust sources from the past requires novel approaches that are part of the geologic toolbox of provenance studies. Identification of most dust sources of the past requires the use of physical, mineralogical, geochemical, and isotopic analyses of dust deposits. Physical properties include systematic spatial changes in dust deposit thickness and particle size away from a source. Mineralogy and geochemistry can pinpoint dust sources by clay mineral ratios and Sc-Th-La abundances, respectively. The most commonly used isotopic methods utilize isotopes of Nd, Sr, and Pb and have been applied extensively in dust archives of deep-sea cores, ice cores, and loess. All these methods have shown that dust sources have changed over time, with far more abundant dust supplies existing during glacial periods. Greater dust supplies in glacial periods are likely due to greater production of glaciogenic dust particles from expanded ice sheets and mountain glaciers, but could also include dust inputs from exposed continental and insular shelves now submerged. Future dust sources are difficult to assess, but will likely differ from those of the present because of global warming. Global warming could bring about shifts in dust sources by changes in degree or type of vegetation cover, changes in wind strength, and increases or decreases in the size of water bodies. A major uncertainty in assessing dust sources of the future is related to changes in human land use, which could affect land surface cover, particularly due to increased agricultural endeavors and water usage.