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Sediments. Sediments. Sediments may be classified by particle size, origin, location or color. The position and nature of sediments provide important clues to the Earth’s recent history. Sediments.
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Sediments • Sediments may be classified by particle size, origin, location or color. • The position and nature of sediments provide important clues to the Earth’s recent history
Sediments • Sediment is made up of particles of organic and inorganic matter that accumulates in a loose, unconsolidated form. • The particles originate from the weathering and erosion of rocks. • Most of the ocean floor is being slowly dusted by a continual rain of sediment.
Sediments • Sediments accumulate at varying rates on the ocean floor, from a few centimeters to the thickness of a dime every thousand years. • The colors of marine sediments are quite striking. • Sediments of biological origin are white or cream colored
Sediments • Deep sea clays range from tan to chocolate brown. There is a dominance of red clay sediments resulting from the iron in them rusting. • Some nearshore sediments contain decomposing organic material and smell of hydrogen sulfide, but most are odorless.
Classifying Sediments • Particle size is frequently used to classify sediments
Particle Size • In general the smaller the particle the easier it can be transported by streams, rivers, and currents. • Sediments are sorted by size as they are transported. The coarser grains tend not to move as fast as the finer grains. • Most marine sediments are composed of the finer sediments.
Sediment Size • If the sediment is composed of particles of one size we say that the sediment is well-sorted. • If the sediment contains a mixture of sizes we say that the sediment is poorly-sorted. • Sorting is a function of the energy of the environment.
Sediment Size • Well-sorted sediments occur in an environment where the energy fluctuates very little. • Poorly-sorted sediments are found in environments where the energy fluctuates over a wide spectrum. • The mixture of rubble at the base of a rapidly eroding sea cliff is a good example of poorly-sorted sediments.
Sediment Origin • Another way to classify sediments is by origin. • Terrigenous (Lithogenous) sediments are the most abundant in the marine environment. • These are sediments which are derived from the continental land mass.
Sediment Origins • The rocks of the Earth’s crust are made up of inorganic, crystalline minerals. • The most common continental rock is granite the source of quartz and clay, two of the most common minerals in marine terrigenous sediments.
Sediment Origins • It is estimated that about 15 billion metric tons of terrigenous sediments are transported in rivers to the sea each year. • There is an additional 100 million metric tons transported from land to ocean as fine airborne dust and volcanic ash each year.
Sediment Origin • Biogenous sediments are the second most abundant in the marine environment. • These sediments represent the siliceous (SiO2) and calcareous (CaCO3) exoskeletons of former living marine organisms.
Sediment Origins • Biogenous sediments are most abundant where ample nutrients encourage high biological productivity. They are usually abundant near the continental margins. • Over millions of years organic molecules in these sediments can form oil and natural gas.
Sediment Origin • Biogenous sediments cover a larger percent of the area of the ocean floor, however terrigenous sediments dominate in total volume.
Sediment Origin • Hydrogenous (Authogenic) sediments have precipitated directly out of the water column. • Under certain conditions chemical reactions may take place in the water column which cause certain minerals to for precipitates which settle in place.
Sediment Origin • Although they usually accumulate very slowly, rapid deposition of hydrogenous sediments can take place. • The deposition of salts from the evaporating Mediterranean occurred relatively quickly.
Sediment Origin • The most common hydrogenous sediments are the manganese nodules which litter some deep seabed. • Hydrogenous sediments also include phosphorite nodules found along some continental margins.
Sediment Origin • Cosmogenous Sediments • These are sediments of extraterrestrial origin. • Scientist believe that coamogenous sediments are from two sources, interplanetary dust that constantly falls into the top of our atmosphere and impacts by asteroids and comets.
Sediment Origin • The highest concentration of cosmogenous sediments occur when large volumes of extraterrestrial matter arrive all at once • This happens rarely, only when the Earth is hit by a large asteroid or comet. Very few examples of this kind of event are known.
Sediment Origin • However geologist believe that 65 million years ago an asteroid that was 10 kilometers (6 miles) in diameter struck the earth on what is now the northeastern coast of Yucatan.
Sediment Distribution • If one looks at the sediments in the ocean around the world there are several things that stand out. • There is a lack of radiolarian deposits in much of the deep North Pacific. • The South Pacific and Indian Ocean Floors are covered by calcareous oozes.
Sediment Distribution • The broad, deep, relatively old Pacific contains extensive clay deposits, much of which has come from air airborne dust. • As we might expect the poorly sorted glacial deposits are found only at the high latitudes.
Economical Importance of Marine Sediments • The study of ocean sediments has revealed a rich source of mineral deposits. • In 1966 it was estimated that 31% of the world’s crude oil and 25% of the natural gas were extracted from sedimentary deposits of continental shelves and rises • Offshore hydrocarbons currently generate $115 billion a year.
Economic Importance of Marine Sediments • Sand and gravel valued at more than $400 million were taken from the ocean. • Commercial mining of manganese nodules has been considered. • They contain substantial amounts of iron and other industrially important chemical elements.
Collecting Sediments • Simple mechanical devices are employed when taking bottom sediment samples from a ship. • To insure the sample is informative we would like to distort them as little as possible. We also want to protect them from being washed or otherwise altered during the long journey up to the ship.
Collecting Sediments • Sampling of surface sediments is most frequently done with a dredge or a grab sampler. • Dredges are especially useful for coarse sediment, such as nodules or rocks. • Grab samplers have jaws that close when they reach the bottom. They are more useful for finer sediment.
Collecting Sediments • Deeper sections of sediment can be sampled using corers. These often resemble very large darts hanging from cables. • The “needle” of the dart is a long hollow pipe, called the barrel.
Collecting Sediments • The barrel is designed to minimize distortion as it plunges into the sediments. • The barrel usually has a liner which is removed along with the sample it contains.
Collecting Sediments • Several hundred kilograms of weights may be added to help push the barrel into the bottom.
