Saharan dust
Saharan dust is an aeolian mineral dust from the Sahara, the largest hot desert in the world. The desert spans slightly more than 9 million square kilometers, from the Atlantic Ocean to the Red Sea, from the Mediterranean Sea to the Niger River valley and the Sudan region in the south.
The Sahara is the largest source of aeolian dust in the world, with annual production rates of approximately 400-700 million tons/year, which is almost half of all aeolian desert inputs to the ocean. Saharan dust is often produced by natural process such as wind storms and doesn't appear to be heavily influenced by human activities.
In most cases marine bacteria and phytoplankton require small amounts of the micronutrient iron, which can be supplied by transport of Saharan dust. The dust delivered to the Atlantic Ocean and the Mediterranean Sea has a small percentage of dissolvable iron; however, since so much iron is supplied to the regions, even with a low soluble percentage, Saharan dust is a large source of iron to these regions. Factors that contribute to dust solubility are particle size, the mineral composition of the dust, the temperature of the water, and its pH. Organic molecules called ligands can also increase the solubility of iron and make it more accessible to organisms to use for primary production. Weathered deposits of Saharan dust are essentially the only source of clay in the Bahama islands that is used by the Lucayan people for making pottery.
Saharan dust has been found to travel to the Amazon basin, Scandinavia, Japan, and other regions. The dust supplied to the North Atlantic and the Mediterranean brings nutrients that help to boost primary production. For the Amazon basin, which is limited in phosphorus in much of the soil in the basin, Saharan dust is a main source of phosphorus. This dust has also affected ecosystems in the southeastern United States and the Caribbean by supplying limiting nutrients, and in some cases promoting soil development on land. Saharan dust has even been found on glaciers and studied to examine atmospheric circulation. Adverse effects of Saharan dust on human health can include respiratory difficulties as well as other adverse health conditions during dust storms in the surrounding regions.
Properties of Saharan dust
Physical properties
Saharan dust particles from a 2005 dust storm event were analyzed, and their diameter ranged from 100 nanometers to 50 micrometers. It appeared that most of the particles were coated in sulfates, with the average coating on the silicate particles 60 nanometers thick. In the atmosphere, particles can act as aerosols, which can deflect sunlight back out into space. The absorption of sunlight increases with smaller particle size. For the reflectivity of the particles, all samples varied between 0.945 and 0.955. Values close to 1 indicate that these particles are highly reflective. The size of Saharan dust particles is largely determined by the distance from their source. The first particles to leave the atmosphere and return to the surface will be the largest and coarsest particles. As the particles travel farther, more of the smaller particles will remain.Chemical properties
In samples of Saharan dust from 2005, the average composition of the dust particles was: 64% silicates, 14% sulfates, 6% quartz, 5% high calcium particles, 1% iron rich, 1% soot, and 9% other carbon rich particles. These samples found 17 different elements in the dust particles, which included sodium, manganese, aluminum, silicon, iron, cobalt, copper, potassium, and calcium.The dust supplied from the Sahara to the subtropical North Atlantic contains a large amount of iron compared with other sources of dust to the ocean. It also contains aluminum, which is not needed for primary production, but can be used as a marker of the dust's source.
Dust from the Sahara also supplies phosphorus and silica to surface waters. Dust has also been shown to carry sulfur; however, this is not well understood.
Biological properties
Saharan dust provides marine ecosystems with important nutrients. Iron is a necessary micronutrient for photosynthesis in marine primary producers such as phytoplankton. In parts of the Atlantic, dissolved iron is thought to limit the amount of photosynthesis that phytoplankton can carry out. In most of the dust brought to the ocean's surface, the iron is not soluble, and organisms require an organic molecule called a ligand to help dissolve the iron so that organisms can use it for photosynthesis.Microorganisms living on particles can be transported away from their original habitat when dust is picked up and blown away. Sometimes, these organisms survive, and can grow where the dust has landed, effecting local ecosystems. One example is Mont Blanc in the Alps, bordering France, Italy, and Switzerland, where snow-colonizing bacteria was found on dust particles. There have also been studies where bacteria from Saharan dust caused sickness in corals in the Caribbean.
Solubility and bioavailability
Factors impacting Saharan dust dissolution & iron solubility
While Saharan dust delivers a large amount of iron to the Atlantic Ocean and the Mediterranean Sea, only a small amount of that iron is soluble in water. The solubility of Saharan dust in the world's oceans and the iron it delivers depend on a variety of factors, including particle size, mineral composition, temperature, pH, and the presence or absence of organic matter.Particle size
Saharan dust transported over long distances is primarily composed of very small particles called aerosols. Smaller particles have a larger surface area per unit of mass than larger particles. Once Saharan dust settles into a body of water, the larger surface area increases the contact the dust has with surrounding water, and causes it to dissolve faster than larger particles would. This effect is described by a variant of the Kelvin Equation.Mineral composition
Minerals in aerosolized dust are typically modified in the atmosphere to be more soluble than material in soils. Some processes known to modify iron to more soluble forms in the atmosphere are acidic reactions and photochemistry. Iron-containing minerals such as clays, feldspars, and iron oxides are commonly found in Saharan dust. Clays in Saharan dust tend to show higher fractional solubility than iron oxides. Changes in the relative amounts of these minerals and other forms of iron in Saharan dust can alter the amount of the dust that will dissolve.Temperature and pH
The solubility of many salts and minerals increases with temperature. As a result, Saharan dust is typically more soluble in regions with higher temperatures.Determination of the solubility of metal-containing materials is helped by pH. At low pH, iron is typically more soluble than it is at higher pH. This pH effect has been directly observed with Saharan dust iron solubility, as the dust tends to be more soluble in acidic aerosols and rainwater than it is in the more basic surface ocean. This makes wet deposition containing Saharan dust an important delivery mechanism for soluble iron to the Mediterranean and the Atlantic.
Organic stabilization of aerosolized and dissolved iron
Free aqueous iron is not very stable in non-acidic conditions; it is inclined to oxidize, form an iron oxide, and precipitate out of solution. Some types of organic matter can help stabilize iron by binding to the iron and preventing the formation of relatively insoluble iron oxides. These organic molecules are called ligands. Different functional groups and heteroatoms in organic molecules contribute differently to the molecules iron-binding activity. Heteroatoms such as oxygen, sulfur, and nitrogen can increase an organic molecule's iron-binding capacity; the presence of organic matter containing O and/or S and/or N can increase the solubility of iron contained in aerosols. Carboxyl groups in particular have been noted for increasing ligand-like activity of organic matter in aerosols. Other functional groups known to contribute to ligand-like properties in aerosols include ethers, esters, and amines. Aerosols containing more of these ligands have higher percentages of soluble iron than aerosols that have less or no ligands. Saharan dust aerosols contain lower amounts of these ligands, which contributes to the low solubility of iron from Saharan dust. Saharan aerosol organic matter tends to contain more carbohydrate-like material, which does not tend to have strong ligand activity.Ligands in the surface ocean are varied in molecular structure and include compound classes such as porphyrins and siderophores. These molecules are generally produced by marine bacteria or phytoplankton to obtain metals in regions where metal concentrations are low. Other ligands in the ocean are produced as organic matter breaks down to form humic acids. These humic acids, as well as oxalate, malonate, and tartrate, have been shown to specifically increase the solubility of iron contained in Saharan dust.
Bioavailability of Saharan dust-derived iron
Generally, marine bacteria and phytoplankton require some form of dissolved iron to meet their iron needs. Saharan dust delivers a large amount of iron to the oceans, but most of this iron is insoluble. Therefore, it can generally be stated that factors that increase the solubility of Saharan dust subsequently increase the bioavailability of iron to these organisms. However, the concept of bioavailability is more nuanced than this statement implies.Organismal preferences for different forms of iron can be complex. In a study comparing two distinct bacterioplankton communities and their uptake of iron bound to different ligands, the two communities were found to use different forms of bound iron. In this study, organisms from an area with abundant iron seemed to prefer iron bound to ligands such as phaeophytin, but not ligands such as pheophorbide, while organisms from an iron-depleted region preferred inorganic unbound iron or iron bound to chlorin e6. In other cases, organisms have been documented to produce organic molecules which increase the bioavailability of iron as an iron-acquisition strategy. Other organisms, when subjected to grazing pressure, produce ligands which decrease the bioavailability of iron to themselves and other species of phytoplankton. The bioavailability of Saharan dust-derived iron, therefore, depends on the kinds of organisms present to use that iron and the form of iron available in solution.