Eolian Transport

Wind can be considered a very low density, low viscosity fluid that is capable of flowing and bringing about sediment transport. ^[1] The principles involved in entrainment and transport of particles by wind (eolian transport) are similar to those for water; however, wind’s low density and viscosity cause the threshold values for wind entrainment and transport to be quite different. ^[1] The density of water is more than 700 times that of air, which means wind must move at far higher velocities than water to exert equivalent drag and lift forces on a grain. Despite this disadvantage, wind can still move substantial quantities of sediment - but it is almost entirely limited to sand and dust.

Grain Sizes and Transport Mechanisms

Wind commonly transports particles of fine-sand size and smaller only. ^[1] Sand-size particles move by traction (surface creep) and saltation, and dust-size particles move by suspension. ^[1] Transport takes place at relatively high wind velocities, and the flow is commonly turbulent, characterised by eddies of various sizes moving with different speeds and directions. ^[1]

Impact Threshold and Seeding

Entrainment of grains by wind action can be strongly affected by the impact of moving grains hitting the bed. ^[1] At a value of wind velocity below the critical velocity needed to initiate grain movement, grain motion can be started and propagated downwind by throwing grains onto the bed - a process referred to as seeding. ^[1] This lower threshold for grain movement is called the impact threshold. ^[1]

The concept of the impact threshold explains a common field observation: during a wind storm, grain movement can continue to spread across a sand surface even where the local wind velocity is not quite sufficient to start movement on its own. Once a few grains are set in motion by the strongest gusts, their impact on the bed knocks other grains into motion - effectively amplifying the entrainment well beyond what the mean wind speed would predict.

Dustloads and Long-Distance Suspension

Suspended loads carried by wind are called dustloads. ^[1] Upward diffusion in unstable, buoyant air masses at an advancing front has been known to carry dust clouds rapidly to heights of hundreds or even thousands of metres during volcanic eruptions. ^[1] Material carried to such great heights may remain in suspension for long periods of time and subsequently be spread over a very broad area, including the ocean basins. ^[1] In fact, the very fine grained component of deep-sea pelagic sediments is believed to be largely of windblown origin. ^[1]

The aeolian link to deep-sea sediments is significant for palaeoclimate reconstruction: the mineralogy and isotopic composition of windblown dust in deep-sea sediment cores can be used to identify the source region of that dust, and changes in the dust flux record changes in the intensity of continental aridity and wind patterns through time.

Eolian Bedforms

Like water transport, eolian transport results in deposition of bedforms ranging in size from ripples a few centimetres high to gigantic dunes hundreds of metres high. ^[1]

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