User:Fillmann/sandbox

Draft:

Opening

Terrigenous sediments are transported through several physical processes, winds, rivers, or icebergs. The transportation method inherently allows controls of sediment grain size, shape, and  composition (Pye & Tsoar, 2009)

Rivers

Precipitation and glacial melt cause the initial weathering of rock and transport sediment grains through river/stream networks to the oceans. This process emphasizes the transport of smaller and lighter sediment grains over larger, heavier grains. The size and mass distribution are also dictated by the rate and turbulence of the flow (Schulz & Zabel, 2006). Generally, grains 30 µm or less are supported in a river’s suspension load, while large sizes are moved more slowly in the traction load (Schulz & Zabel, 2006).

Of course, the geographic location, and thereby the type of rock/soil being weathered, controls the compositions of the sediment grains. For example, Agricultural regions are far more prone to sediment/soil loss, and therefore supply nearby water networks with more sediment grains to transport. Regional climates control the intensity of the local water cycle and therefore control the relative amount of sediment transportations from one location to another. Generally, most sediment grains transported via this method are deposited in coastal zones, while only the smallest and lightest grains make it far enough offshore to be carried further by ocean currents (Schulz & Zabel, 2006). Transportation by river flow allows the formation of submerged sediment fans in nearshore continental zones (Schulz & Zabel, 2006, pg 35)

Wind

Wind transportation is most predominant in dry climates with sources of finer sediment grains. Transportation of various grain sizes is determined by the wind speed and ability to keep particles suspended (turbulence). Generally, grains are restricted to a size of 80 µm or less (Schulz & Zabel, 2006). Regional climatic wind patterns and the hydro-meteorological tendencies place further control on transportation potential. Though some source regions may meet the criteria for wind transportation, the prevailing wind patterns may not lead to the oceans. Regions like the Sahara and the Middle East are prone to direct wind transportation from land to ocean. Under certain conditions, suspended sediment grains act as cloud condensation nuclei and may be precipitated back on land or directly on the oceans. These sediments are crucial components in the development and organization of severe storms. Once entering the ocean, these very fine grains most likely see further transport by currents before being permanently deposited.

Icebergs

Glacial processes may or may not be fully involved in the transportation of sediments to the ocean. That is, some sediment, suspended in ice, is transported to the ocean entirely by glacial processes (glacial flow, caving, iceberg migration, and melting) (Schulz & Zabel, 2006). Otherwise, glacier transportation may comprise most of the process, carrying the grains from the source to the glacial terminus, where glacial melt takes over and the remainder of the transportation occurs through a liquid water medium. Whether in the northern or southern hemisphere, icebergs usually migrate to the mid-latitudes before melting completely, dispersing sediment along the way. Iceberg transport in the northern hemisphere is restricted to the North Atlantic due to limited iceberg sources (Schulz & Zabel, 2006). Sediments tend to not be chemically or physically, being encased and protected in ice. These sediments, called ice-rafted detritus (IRD), are then deposited in a wide variety of sizes and shapes (Schulz & Zabel, 2006). Iceberg transportation speed is entirely dependent on the ocean currents the ice is suspended in, while the deposition of the sediment (loosened from the ice due to melting) is entirely driven by when the sediment became encased in ice (Schulz & Zabel, 2006). Generally, the largest grains, up to meter-thick boulders (Schulz & Zabel, 2006), occur along the bottom of the glaciers where the glacier scraped across the underlying land as it flowed.

Size Controls

As previously noted, the transportation mechanism determines the size of the grains and the speed at which they are transported. Therefore, the composition of a sediment sample, with respect to grain size, is controlled by the most-predominant transportation method that created it. Weaker processes such as wind, tend to carry only the finest grains and generally transport them further and more rapidly than other processes. Rivers tend to move grains of larger sizes (along with fine grains) but at a slower rate. Fine grains are transported quickly in the suspension load, while larger grains move slowly in the traction load. River runoff usually does not have as much of a regional expanse as other processes. Glacial and iceberg transportation move the largest and heaviest grains, but this occurs at a variety of rates. Transportation fully by glacial flow is extremely slow, however, the transportation speed immediately increases after caving and melting. A sediment sample sourced primarily by icebergs tends to be very coarse, where a sample sourced by wind processes is very fine. Grain sizes and descriptions are commonly categorized with the Udden-Wentworth scale (Nichols, 2009).

Shape Controls

Grain geometry is largely influenced by the method in which it was transported to the oceans. Liquid water transportation methods tend to yield rounded smooth grains. This is driven by the combined effects of both impacts and further weathering/erosion while submerged. Wind transportation methods tend to yield rounded but unsmooth grains (Pye & Tsoar, 2009), here shape is only driven by impacts. While smoothing of aeolian grains does occur, this is a function of chemical weathering and is not related to the transportation mechanism. Lastly, glacial/iceberg transportation methods have almost no controls on grain shape; this is largely due to the lack of weathering or interaction between grains and anything else.

Deposition Controls

          The location of deposition can depend on several variables. Once introduced into the ocean, grain size and ocean turbulence determine when a grain is deposited, therefore also determining where the deposition occurs. Smaller, lighter grains tend to be transported across great geographical extents by ocean currents, though at slower speeds. Conversely, larger and heavier grains tend to sink rapidly and thereby are deposited near where they first entered the ocean. Otherwise, and as mentioned earlier, the deposition location depends on the transportation mechanism. River-sourced sediment is most common around coastlines. Aeolian grains, being the smallest are easily deposited everywhere. While, glacial/iceberg sediments are generally deposited between the high and mid-latitudes, with almost no deposition in the North Pacific.

Reference

1.

Schulz, H. D., Zabel, M. (2006). Marine Geochemistry. Springer.

2.

Nichols, G. (2009). Sedimentology and stratigraphy. Wiley-Blackwell. ProQuest Ebook Central https://ebookcentral.proquest.com.

3.

Pye K., Tsoar H. (2009) Characteristics of Windblown Sediments. In: Aeolian Sand and Sand Dunes. Springer. https://doi-org.ezproxy.proxy.library.oregonstate.edu/10.1007/978-3-540-85910-9_3.