Aeolian transport

The area of actual erg and dune formation is delimited by the 150 mm/yr isohyet. This precipitation boundary appears to have shifted strongly in the recent past. Between 20,000 and 13,000 yr BP, the southern limit of active dune formation in the Sahara desert was 800 km south of its present position and most of the now sparely vegetated Sahelian zone was an area of active dune formation at that time. These dunes, mostly of the longitudinal type, are now fixed by vegetation, but their aeolian parentage is still obvious from their well-sorted material. A similar story can be told for the Kalahari sands. Overgrazing in recent times has reactivated aeolian transport in many regions with sands. 

Wind (aeolian) transport (relocation by wind) can also occur and is particularly relevant when catalyst dust and coke dust are considered. Dust becomes airborne when winds traversing arid land with httle vegetation cover pick up small particles such as catalyst dust, coke dust, and other refinery debris and send them skyward. Wind transport may occur through suspension, saltation, or creep of the particles. 

Trace elements are delivered to the ocean by atmospheric, or aeolian, processes in both particulate and soluble forms. Most of the aeolian particles entering the ocean are less than 10 pm in size and are referred to as aerosols. Aeolian transport of particles occurs when winds, such as the Trades, pick up small particles from the land s surface and carry them over the ocean. Some trace elements, such as mercury, have a high enough vapor pressure that they are present as atmospheric gases. Still others are ejected during volcanic eruptions in either particulate or gaseous form. 

Much of the trace metal demand of plankton in the open ocean may be supplied by the aeolian transport of dust. If trace metals limit plankton growth, the factors that control the transport of dust will also ultimately control the ocean s biological pump and, hence, atmospheric CO2 levels. This suggests the existence of powerful feedbacks among the processes that control global climate. For example, shifts in climate that lead to regional... 

As shown in Figure 14.8, kaolinite concentrations are highest in tropical and equatorial latitudes, particiflarly off the western coasts of North Africa and Australia (>40%) and the northeastern coasts of Australia and South America (30%). The first two are the result of aeolian transport by the Trade Winds from the Saharan and Australian deserts, respectively. The other two are the result of river input from the eastern Australian continent and the Amazon River. 

The production of illite from chemical weathering occurs at all latitudes. It dominates the clay mineral assemblage in the North Atlantic and North Pacific Ocean, particularly at 40° reflecting aeolian transport by the westerlies (Figure 14.11). In the southern hemisphere, the input of illite by the westerlies is diluted by a large input of authigenic montmorillonite in the South Pacific and Indian Oceans and in the South Atlantic by a large input of kaolinite. 

A global map of quartz abundance is given in Figure 14.12. In this case, the contribution of quartz is presented as the contribution to the bulk sediment from which biogenic carbonate and silica have been removed. This map is very similar to the global distribution of dust presented in Figure 11.4, reflecting the importance of aeolian transport for this detrital silicate. 

On the early Earth, ions were mobilized from volcanic rocks by chemical weathering. Rivers and hydrothermal emissions transported these chemicals into the ocean, making seawater salty. These salts are now recycled within the crustal-ocean-atmosphere fectory via incorporation into sediments followed by deep burial, metamorphosis into sedimentary rock, uplift, and weathering. The last process remobilizes the salts, enabling their redelivery to the ocean via river runoff and aeolian transport. In the case of sodium and chlorine, evaporites are the single most important sedimentary sink. This sedimentary rock is also a significant sink for magnesium, sulfate, potassium, and calcium. 

The abyssal clays are composed primarily of clay-sized clay minerals, quartz, and feldspar transported to the siuface ocean by aeolian transport. Since the winds that pick up these terrigenous particles travel in latitudinal bands (i.e., the Trades, Westerlies, and Polar Easterlies), the clays can be transported out over the ocean. When the winds weaken, the particles fell to the sea siufece and eventually settle to the seafloor. Since the particles are small, they can take thousands of years to reach the seafloor. A minor fraction of the abyssal clays are of riverine origin, carried seaward by geostrophic currents. Despite slow sedimentation rates (millimeters per thousand years), clay minerals, feldspar, and quartz are the dominant particles composing the surface sediments of the abyssal plains that lie below the CCD. Since a sediment must contain at least 70% by mass lithogenous particles to be classified as an abyssal clay, lithogenous particles can still be the major particle type in a biogenous ooze. 

In the South Pacific, the CCD is deep enough to permit the preservation of calcareous oozes except in the center of the basin, which as a result is covered by abyssal clays. The relatively rapid supply of hydrogenous sediments prevents the accumulation of calcareous oozes on the East Pacific Rise. In the North Pacific, abyssal clays dominate as this is the location where the CCD is shallowest. Aeolian transport is the source of the clay minerals that make up these deposits. 

Some component of the terrestrial POM must be extremely nonreactive to enable a higher burial efficiency as compared to autochthonous POM. A possible candidate for this nonreactive terrestrial POM is black carbon. This material is a carbon-rich residue produced by biomass burning and fossil fuel combustion. Some black carbon also appears to be derived from graphite weathered from rocks. It is widely distributed in marine sediments and possibly carried to the open ocean via aeolian transport. 

The input of terrestrial DOM via rivers and aeolian transport was discussed in Chapter 23.3. Riverine concentrations of DOC range from 2 to 20mgC/L. In contrast, little or no terrestrial DOM is detectable in seawater, leading to the cmrent consensus that most is removed close to its point of input. In some estuaries, removal is associated with flocculation reactions promoted by the large increase in ionic strength that occms when river water mixes with seawater. In other estuaries, DOC exhibits conservative behavior, leaving marine chemists with a mystery as to how and where DOC is removed. 

At 20 °C, K = 10 - and so water of pH=8.1 in equilibrium with atmospheric O2 (p02 — 0.21 atm) has pe = 12.5. This conforms to surface conditions, but the pe decreases as the O2 content diminishes with depth. The oxygen minimum is particularly well developed beneath the highly productive surface waters of the eastern tropical Pacific Ocean, where there is a large flux of organic material to depth and subsequently considerable oxidation. The O2 becomes sufficiently depleted i.e., hypoxia) that the resulting low redox conditions causes NOs to be reduced to N02 - Aeolian transport of nitrate to Chesapeake Bay can lead to low O2 conditions. Similarly, intermittent hypoxia develops in parts of the Gulf of Mexico due to the riverine transport of nutrients derived from agricultural uses in the Mississippi catchment. 

The availability of nitrogen in the euphotic zone is an important, potentially limiting factor for productivity and the biological sequestration of carbon in the ocean. There are three principal routes by which new nitrogen makes its way into the euphotic zone of the Atlantic Ocean Physical transport of nitrate, nitrogen fixation by diazotrophic organisms, and aeolian transport and deposition. 

Silica may be transferred to the site of silcrete development by wind and/or water. Quartz dust, for example, may be transported considerable distances from desert areas by the wind (Goudie and Middleton, 2001). Similarly, plant phytoliths, sponge spicules and diatoms can be subject to aeolian transport (Clarke, 2003). All other transfers rely upon silica transport in solution as undissociated monosilicic acid, either as the monomer H4Si04 or the dimer H6Si207 (Dove and Rimstidt, 1994). Organic or inorganic complexes may also be formed. 

Additional support for the airborne transport of PAH is suggested by the results on the global distribution of PAH reported here. Thus, airborne transport of combustion-generated PAH from the New York City area to the sediments along the transect studied (Figure 5) is quite likely, especially since aeolian transport of land-derived material in this region has been documented (30). 

In contrast to the fluvial transport regime the aeolian transport is highly efficient for the deposition of terrigenous matter in the deep sea. 

These compounds may be introduced into the marine environment either fluvially or by aeolian transport of dusts and aerosols over continents and oceans (Simoneit, 1977a Simoneit and Eglinton, 1977). 

Because of concern about its environmental toxicity, the use of Cd has decreased in recent years. A few studies of lake sediments and snowpacks have documented the rise in Cd pollution as a result of the industrial revolution and its sharp decrease since the 1970s (Fig. 2) [3-5]. According to the data of Fig. 2, the present inputs of Cd to remote northern regions through aeolian transport of aerosols are near their pre-industrial value. As the industrial use of Cd declines, much of the remaining Cd pollution results from its presence as a contaminant in Zn-containing materials. 

Variations both or grain size and mineral density result in separation of minerals and rock fragments during aqueous and aeolian transport of sedimentary material. Such transport may affect lanthanide abundance patterns in the resulting sedimentary rock because of the widely variable patterns in the constituent minerals. The two most important effects are... 

The size of particle entrained determines the mode of aeolian transport (Bagnold, 1941), either as suspension, saltation, or creep (Figure 16.3). 

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