6.11: Chapter Summary

Classification of Sediments. 

Sediments can be classified by their predominant grain size or by the origin of the majority of their particles. Grain size ranges are usually classified as gravel, sand, silt, and clay, in order of decreasing size.

Lithogenous Sediments. 

Lithogenous sediment particles are primarily chemically resistant minerals produced by weathering of continental rocks. They are transported to the oceans by rivers, glaciers, waves, and winds. Lithogenous sediments are deposited in thick layers beyond the mouths of some rivers, but many rivers trap sediments in their estuaries and deltas. Fine-grained dust, especially from deserts, can be transported by winds for thousands of kilometers before settling on the sea surface and eventually sinking to the seafloor. Some volcanic eruptions eject large quantities of ash that reach ocean sediment in a similar way.

Biogenous Sediments. 

Biogenous sediments are the remains of marine organisms. In most cases, the organic matter is decomposed before the hard inorganic parts are deposited and buried.

Some marine species, including species of phytoplankton and zooplankton, have calcareous or siliceous hard parts, both of which dissolve in seawater. Most siliceous diatom frustules dissolve very slowly and become included in sediments. The intricate siliceous shells of radiolaria generally dissolve before being buried in sediments in all but tropical waters, where radiolaria are abundant. Because the solubility of calcium carbonate in seawater increases with pressure, the rate of dissolution of calcareous hard parts increases with depth. Calcareous shell material has two forms: calcite and aragonite. The aragonite of pteropods is dissolved much more easily than the calcite of foraminifera. Pteropod oozes are present only in relatively shallow water, whereas foraminiferal oozes predominate at intermediate depths. Below the carbonate compensation depth (CCD), little or no calcareous material survives dissolution to be incorporated in the sediment. The CCD varies between oceans because of differences in dissolved carbon dioxide concentrations, and it also varies historically with climate changes.

Hydrogenous Sediments.

Hydrogenous sediments, which are less abundant than terrigenous (lithogenous) or biogenous sediments, are deposited by the precipitation of minerals from seawater. They include manganese nodules that lie on the sediments in areas of the deep ocean, phosphorite nodules and crusts on continental shelves beneath oxygen-deficient water, calcium carbonate deposits in a few shallow areas where water temperature is high and dissolved carbon dioxide concentration is low, and evaporites (salt deposits) in coastal waters with limited water exchange with the open ocean, low rainfall, and high evaporation rate. Hydrogenous sediments also include metal-rich sediments accumulated by the precipitation of minerals from water discharged by hydrothermal vents.

Cosmogenous Sediments. 

Cosmogenous sediment particles are fragments of meteorites or tektites created by meteorite impacts. They constitute only a tiny fraction of the sediments. Many are found as cosmic spherules.

Sediment Transport and Deposition. 

Large particles are deposited quickly, unless current speeds are high, and they are difficult to resuspend. Large particles collect close to river mouths, glaciers, and wave-eroded shores. Because orbital velocity in waves is higher than ocean current speeds, large particles can be transported in the nearshore zone but cannot be transported far from shore. Smaller particles may be carried long distances before being deposited.

Turbidity currents that resemble avalanches carry large quantities of sediment down continental slopes and onto the abyssal plains. Graded beds of turbidites are present in deep-ocean sediments, and they are common on the abyssal floor near the continents with passive margins.

Sediment Accumulation Rates. 

Sediments are mixtures of particles of different origins. Sediment characteristics are determined by the relative accumulation rate of each type of particle. Sediment accumulation rates range from about 0.1 cm per 1000 years in the deep oceans to more than 1 m per year near the mouths of some rivers.

Continental Margin Sediments. 

Because fine-grained sediments are transported off the continental shelf by currents, shelf sediments are generally sand- and silt-sized lithogenous particles. Relict sediments, deposited when sea level was lower and containing remains of terrestrial and shallow-water organisms, are present in some areas on the continental shelf.

Distribution of Surface Sediments. 

Deep-sea clays are present far from the continents on the abyssal plains. Radiolarian oozes occupy a band of high primary productivity that follows the equator, but not in the Atlantic, where the radiolarian shells are diluted with lithogenous sediment. Diatom oozes are present in high-latitude areas and in other upwelling areas where primary productivity is high and lithogenous inputs are low. Sediments dominated by terrigenous particles are deposited near the mouths of major rivers. Calcareous sediments dominate where the seafloor is shallower than the CCD and lithogenous inputs are low. Hydrothermal sediments are deposited in some areas on oceanic ridges.

The Sediment Historical Record. 

Sediments accumulate continuously, providing a record of the Earth’s history. The stratigraphic record can be complicated because sediment layers may be disturbed or chemically and physically altered by diagenesis. Stratigraphic studies of ocean sediments have revealed much of the Earth’s climate and tectonic history during the past 170 million years. For example, such studies have provided information about the impact of a large meteor that may have contributed to the extinction of the dinosaurs and other species 65 million years ago.