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Lab 10: The Ocean Part I: Seafloor Sediments Classification of Marine Sediments Marine sediments fall into the following four general categories, based on their origin (how they formed): 1. Terrigenous (also known as lithogenous): Inorganic sediments derived from land, consisting of rock and mineral fragments. 2. Biogenous: Organic sediments composed of the hard remains (shells, bones, teeth) of marine plants and animals. 3. Hydrogenous: Seafloor deposits formed in place by chemical reactions within seawater. Also sometimes called authigenic sediments, which means self-forming. 4. Cosmogenous: Sediments consisting of extraterrestrial debris, such as tiny meteorites and interplanetary dust that filters down through the Earth’s atmosphere. By weight and volume, the first two categories above account for the vast majority of ocean bottom sediments. Terrigenous and biogenous sediments are the subject of this lab. The following sections provide some additional background detail on these two sediment classes. Terrigenous Sediments Terrigenous sediments are usually sub-classified by either grain size or mineral type, or both. The table shows a standard grain size classification scheme, the Wentworth scale:
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Lab 10: The Ocean As might be expected, larger-grained, heavier terrigenous particles settle to the bottom first, and therefore are concentrated closest to the source (land). Finer grained, lighter particles, like silts and clays, remain suspended in the seawater for a longer period of time (sometimes months or years), and therefore can settle to the ocean bottom at much greater distances from the continents. This differential in settling according to particle size provides a natural sorting mechanism for terrigenous sediments, dependent on distance from the continental source. A well-sorted sediment means all the sediment is approximately the same size, like a bag of pennies. A poorly-sorted sediment means sediment is multiple sizes, such as a bag with quarters, nickels, dimes, and pennies mixed together in the same bag. Sediment mineral types are also variable, depending on the source. Deposits off mountainous coasts differ from those off coastal plains and river valleys, and are also different from sediments derived from glacial terrains, such as Greenland and Antarctica. A significant component of deep ocean bottom sediment is a reddish or brownish mud consisting of clay with a high iron content. This red clay achieves its rusty color through very slow oxidation of iron by dissolved oxygen in the seawater as the particles slowly settle to the bottom. Red clay is typically only found at depths greater than about 4,500 m (15,000 feet). Biogenous Sediments Biogenous deposits, known as oozes, are sub-classified into two principal groups according to their biochemical makeup. 1. Siliceous oozes: Remains of organisms whose hard shells or skeletons are composed of silicon dioxide (SiO2, also called silica), which is essentially the chemical makeup of ordinary glass. The two most dominant types of marine organisms producing siliceous oozes are microscopic single-celled algae known as diatoms, and microscopic single- celled protozoans (animal-like) called radiolarians. 2. Calcareous oozes: Remains of organisms whose hard shells or skeletons are composed of calcium carbonate (CaO3), which is also the chemical makeup of coral reefs, chalk, and our own bones). Many types of marine life produce calcareous deposits, but one of the most common and widespread is a singlecelled animal-like protozoan called oraminifera. Siliceous oozes may be found at any depth, but calcareous oozes are generally found only at depths shallower than the calcium carbonate compensation depth, or CCD, which varies in depth as a function of latitude, but averages about 4,500 m (15,000 feet). Below this threshold depth, calcium carbonate readily dissolves, which precludes the accumulation of significant quantities of calcareous sediments. If calcareous sediment layers are found in bottom sediments where the water depth is greater than the CCD, it indicates that those deposits must have accumulated when that part of the ocean bottom was shallower, and that the calcareous sediment was subsequently covered up by another sediment layer before exposure to seawater at that depth could dissolve it.
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