Background, Significance and Approaches of Research
Evaluation of the impact of mineral aerosol (or “dust”) on the marine ecosystem requires a good knowledge about the dust cycle from emission to deposition and about the ultimate bioavailability of micronutrients after deposition into seawater. In this chapter, Section 1 will briefly introduce the emission mechanisms of dust and the factors affecting the emission process. Section 2 presents our current understanding on the dust cycle in the Southern Ocean from emission to deposition. Section 3 will discuss chemical properties of dust including the elemental composition and solubility that could affect the role of mineral dust as micronutrients supplier. In the Section 4, we will elaborate the questions we face for the moment and present the research strategies we have taken to improve our understanding on the supply of bioavailable micronutrients by atmospheric dust from continental sources to the Southern Ocean. Section 5 will introduce the content of following chapters.
Dust Emission processes
Mineral dust is emitted from soil surface by the aeolian erosion in arid or semiarid regions. Emission of mineral dust from soil surface to atmosphere could be driven by three modes (Figure 3): direct aerodynamic resuspension (Sweeney and Mason, 2013; Macpherson et al., 2008; Kjelgaard et al., 2004; Loosmore and Hunt, 2000), saltation bombardment, and aggregates disaggregation (Shao, 2008; Shao et al., 1993; Gomes et al., 1990). Under the concept of direct aerodynamic resuspension, dust particles are lifted directly from the surface by aerodynamic forces. However, for particles smaller than 20 µm, the cohesive force becomes more important and inhibits the direct resuspension of particles (Shao, 2008). Hence, dust emission due to the aerodynamic direct resuspension is generally occurred in low-magnitude but is highly frequent (Lee and Tchakerian, 1995). When the wind stress exceeds a minimum friction velocity that is known as the “threshold friction velocity”, soil particles could be driven in saltation. The collision between the saltating particles and soil surface induces disaggregation of saltating soil aggregates and dust coating on saltating sand particles, or leads to dust emission by breaking the bindings between dust particles of soil surface (Gillette, 1981; Gomes et al., 1990; Shao et al., 1993). The former phenomenon is known as “disaggregation” mechanism and the later is known as “saltation bombardment” (or “sandblasting”). Size of dust particles over the source regions generally varies from ~0.1 to 200 µm diameter with three lognormal modes. The first mode is large particles between 20-200 µm preexisting in soil. Particles in the second mode correspond to particles within 2-20 µm under the form of aggregate. The third mode is submicron mode, which is maximum in number distribution and is produced under more energetic condition (Alfaro et al., 1997; Gomes et al., 1990).
The complex processes of dust emission depends on multiple environmental factor, such as the wind stress, the presence of non-erodible elements like vegetation and rocks, surface roughness, soil texture, soil moisture, soil mineralogy, and aggregate structure of surface soil (Gillette, 1978; Gillette, 1979). In modeling studies, dust emission flux depends generally on the wind friction velocity (Marticorena and Bergametti, 1995; Shao et al., 1993). The presence of non-erodible elements decreases the dust production by preventing the soil surface from wind erosion and attenuating the wind momentum (Marticorena and Bergametti, 1995; Gillette, 1979). Soil texture classifies the soil particles according to the particle size distribution, and controls the availability of dust particles that are dominated by particles in clay fraction (diameter < 2 µm) and silt fraction (2~50 µm). Particles in very coarse fraction (1.0~2.0 mm) and gravel fraction (> 2 mm) are too large to be set in motion by wind. Soil containing less sand (50~1000 µm), and larger soil aggregates exhibits relatively higher threshold friction velocity (Gillette et al., 1980). Depending on the soil composition and soil texture, mineral particles can combine with organic and inorganic materials to form an aggregate structure with pore space. The soil aggregate structure strongly influences the resistance to wind erosion (Singer and Shainberg, 2004). The resistance to disruption of soil aggregates depends on the soil physical conditions such as soil moisture, aging history and chemical composition (Bronick and Lal, 2005). For example, different types of clay response differently to wetting and drying cycles, clay particles can separate from other particles during swelling and distribute more uniformly over the sand grains (Singer et al., 1992). Higher soil moisture may strengthen the cohesion forces between the soil particles with water content, thus increases the threshold velocity (Gillette et al., 1982; Ishizuka et al., 2008; Kim and Choi, 2015).
Overall, dust emission is a highly complex process controlled by a series of environmental factors, resulting high variability of dust emission in both spatial and temporal scales (Mahowald et al., 2005).
Sources, Transport and Deposition of Mineral Dust to the Southern Ocean
Distribution and contribution of dust sources in the Southern Ocean
Distribution of dust sources
Dust sources generally include desert or semi-arid desert areas, ephemeral dry lake or riverbeds, and human-disturbed land surfaces (Mahowald et al., 2003; Ginoux et al., 2012; Prospero et al., 2002). As shown in Figure 4, major dust sources in global scale are located in the following regions (Ginoux et al., 2012; Prospero et al., 2002; Li et al., 2008):
1. North Africa, such as Tunisia, Northeast Algeria, Eastern Libyan Desert, Egypt, Sudan, Niger, Lake Chad Basin;
2. Middle East, such as Arabian Peninsula;
3. Asia, such as Gobi desert, Tarim Basin and Takla Makan desert in China, Indian subcontinent, Pakistan Basins;
4. North America, such as Mojave desert in western United States;
5. Australia, such as the region around the Lake Eyre Basin;
6. Southern Africa, such as Makgadikgadi depression and pans in Botswana, Etosha pan in Namibia;
7. South America, including Patagonia desert, Bolivian Altiplano.
Comparing to the dust sources in Northern Hemisphere, Southern Hemisphere sources are much smaller. According to the model-based estimations of Ginoux et al. (2004) and Li et al. (2008), North Africa and Asia contributes 65% and 25% of the global emission, respectively, whereas the Southern Hemisphere contributes only 10% of total dust emission.
Figure 5 : Potential dust sources estimated based on MODIS Deep Blue product in a) summer (December, January, and February) in the South America; b) spring (September, October, and November) in the Southern Africa; c) summer in the Southern Africa; d) spring in the Australia; e) summer in the Australia. Source: combined with Figures 12, 13, and 14 in Ginoux et al. (2012).
In Southern Hemisphere, Southern America, Southern Africa and Australia are the three continental regions providing mineral dust deposited to the Southern Ocean. Both the global satellite data analysis of Prospero et al. (2002) and Ginoux et al. (2012) have studied the distribution of dust sources in the Southern Hemisphere. Prospero et al. (2002) used Total Ozone Mapping Spectrometer (TOMS) data to retrieve dusty days, while the study of Ginoux et al. (2012) was based on MODerate resolution Imaging Spectroradiometer (MODIS) Deep Blue aerosol products. Given that the a major uncertainty of TOMS retrivals is the sub-pixel contamination by cloud (Torres et al., 2002), the identification of dust sources in the cloudy Southern Hemisphere is particularly difficult when using TOMS data. The MODIS deep blue retrievals used by Ginoux et al. (2012) may better investigate the global dust sources distribution and also provide higher resolutions in small-scale features. Figure 5 illustrates the distribution of dust sources, as well as seasonal variability if available, in the three continental regions of Southern Hemisphere identified by Ginoux et al. (2012) based on MODIS deep blue data.
Figure 5a illustrates the identified dust sources in the South America. Although in the paper of Ginoux et al. (2012) only results in austral summer were available for South America, identified source regions are in agreement with the modeling simulation of Li et al. (2008). Main dust sources in South America include four regions: the Patagonia Desert (locations 1~8 in Figure 5 a), the western Argentina (part of location 8), the Atacama Desert of Chile (location 12) and the Bolivian Altiplano (location 13). Modeling study of Johnson et al. (2010) captured most source regions except the Bolivian Altiplano. Dust sources in South America are generally associated with river basins, glacial activities and salt lakes (Ginoux et al., 2012; Prospero et al., 2002). For example, in Patagonia, dust sources in locations 1 and 4~6 are associated with river basins. Locations 2 and 3 are linked to glacial lakes.
Patagonia accounts for more than 70% of the total dust emission from South America according to the model simulation of Li et al. (2008). Johnson et al. (2010) even estimated that 95% of South American mineral dust originates from Patagonia. Gaiero et al. (2003) measured the dust deposition flux at three coastal sites (latitudes 38°S, 43°S and 45°S) in north and central Patagonia and found different seasonal patterns of dust fallout. Generally, dust activity in Patagonia is more frequent in summer, although some dust events are observed in winter, as confirmed by the modeling study of Johnson et al. (2010).
Dust sources in Southern Africa include the Namib Desert along the western coast (location 1 in Figure 5b and c), the Great Escarpment of Namibia (location 12), the Kalahari Desert (location 10) including the Makgadikgadi Pan (location 9) and Etosha Pan (location 11), and the Karoo Desert (location 2~4). In Namib Desert, the dust sources are associated with dry riverbeds and saltpans instead of the Sand Sea in the south Namib Desert (Eckardt and Kuring, 2005). The Namib Desert is active during most seasons, as shown in Figure 5b and c. The Great Escarpment of Namibia, the Kalahari Desert, and the Karoo Desert are more active in summer. Dust materials in the Kalahari Desert are supplied by sediment inflows and the lake inundation activities (Bryant et al., 2007). Silt deposits are widespread on the Great Escarpment of Namibia due to the local weathering detritus and dust deposition from Kalahari Desert (Eitel et al., 2001). The easterly winds in summer activate the dust emission and result in a stronger dust emission in summer than other seasons (Figure 5b and c). Dust activity in the Karoo Desert is associated with ephemeral lakes and land disturbance due to the long history of human occupation (Meadows, 2003; Botha et al., 2008).
Figure 5d and e show the dust sources in Australia in austral spring and summer. The region around the Lake Eyre Basin (location 4) including the Simpson Desert (location 5) are the most active and the largest dust sources in Australia. Australian dust sources are generally associated with hydrologic activities or land use. In the Lake Eyre Basin, aeolian deposits and hydrologic activities contribute respectively 37% and 60% of dust plumes (Bullard et al., 2008). In addition, Bullard et al. (2007) demonstrated that the abrasion of weathered sands with a clay coating is most important dust production mechanism in the Simpson Desert. The dust sources in the northern Australia (location 3~11) are more active in spring, particularly the regions around the Lake Eyre Basin, while dust sources in southeastern Australia (location 1~2) are more active in summer. Higher dust concentration in summer in the southeastern Australia is also confirmed by the field observations at Cape Grim (Prospero, 1996; Ginoux et al., 2001; Zender et al., 2003).
In addition to the three continental regions above, several minor dust sources contribute to the dust input into the Southern Ocean. Bhattachan et al. (2015) suggested that the ice-free McMurdo Dry Valleys in Antarctica could be a potential source of mineral dust to the Southern Ocean. Annual melting sea ice around the Antarctic reserves mineral dust deposits in winter and releases accumulated dust materials when melts down (Winton et al., 2014; Edwards and Sedwick, 2001).
Table of contents :
Chapter 1 Background, Significance and Approaches of Research
1. Dust Emission processes
2. Sources, Transport and Deposition of Mineral Dust to the Southern Ocean
2.1. Distribution and contribution of dust sources in the Southern Ocean
2.1.1. Distribution of dust sources
2.1.2. Contribution of dust sources in the Southern Ocean
2.2. Dust transport and deposition in the Southern Ocean
3. Mineral Dust as Micronutrient Supplier
3.1. Elemental composition of mineral dust
3.2. Bioavailability of trace elements in dust
3.2.1. Factors controlling the solubility of micronutrients in mineral dust: the case of iron
3.2.2. Common methods of elemental solubility estimation
4. Research Topics and Strategies
4.1. Research topics
4.2. Research Strategies
4.2.1. Long-term dust concentration measurements in Patagonia
4.2.2. Spatial heterogeneity of source dust elemental compositions
4.2.3. Some aspects of the solubility of continental dust
Chapter 2 Long-term dust concentration measurements in Patagonia
2. Materials and methods
2.1. Aerosol sampling location and methods
2.2. Elemental analysis
2.3. Chemical compositions of the crustal fraction of the aerosol
2.4. Air mass back trajectories
2.5. Wind simulation and meteorological records
3. Results and discussion
3.1. Chemical composition of the dust fraction
3.2. Atmospheric concentration of sea salt and mineral dust
3.3. Seasonal pattern of the aerosol concentration
3.4. Meteorological dependence of seasonal dynamics of dust concentrations and emission
Chapter 3 Spatial Heterogeneity of source dust compositions
2. Study area
2.1. Patagonia Desert
2.2. Namibia: Namib Desert and Kalahari Desert
3. Materials and methods
3.1. Soil-derived aerosol generation
3.2. Soil sample collection
3.3. Elemental analysis
3.4. Principal component analysis of compositional data
3.5. Accumulation factor and enrichment factor of dust relative to parent soil
4. Results and discussion
4.1. Elemental composition of soil and aerosol
4.1.1. Element concentration of topsoil and soil-derived dust in Patagonia and Namibia
4.1.2. Spatial variation of elemental composition in regional scale
4.1.3. Robust principle component analysis
4.2. Variation of elemental composition from bulk soil to aerosol
Chapter 4 Contribution to Bioavailability Study of Mineral Dust from Patagonia and Namibia
2. Materials and methods
2.1. Mineral aerosol samples
2.2. Dissolution experiments of aerosol sample
2.3. Centrifugation separation of suspension
2.4. Chemical analysis
3. Results and discussion
3.1. Comparison of solubility values between centrifugation and filtration
3.2. Variation of solubility with elements and its dependence on pH
3.3. Dependence of solubility on types of dust sample