Sediment abyssal plains

Whelan, J.K. 1977. Amino acids in surface sediment core of the Atlantic abyssal plain. Geochimica et Cosmochimica Acta 41 803-810. 

Table 14.3 Average % Composition of <2 xm Fraction in Sediments of the Abyssal Plains. Carbonate-Free Size...

The slowest growth rates are found in the Fe-Mn oxides that have formed predominantly by precipitation of solutes from seawater, being on the order of 1 to a few millimeters per million years. Because of slow formation rates, these hydrogenous precipitates tend to form only in areas where sedimentation rates are slow, such as the abyssal plains of the mid-Pacific Ocean, or where bottom currents are strong enough to prevent sediment accumulation, such as on submarine seamounts and plateaus. 

Figure 20.1 illustrates that the major sediment type on the abyssal plains are the abyssal clays with some local exceptions, including equatorial radiolarian oozes, manganese nodifles, and metalliferous sediments. 

The abyssal clays are composed primarily of clay-sized clay minerals, quartz, and feldspar transported to the siuface ocean by aeolian transport. Since the winds that pick up these terrigenous particles travel in latitudinal bands (i.e., the Trades, Westerlies, and Polar Easterlies), the clays can be transported out over the ocean. When the winds weaken, the particles fell to the sea siufece and eventually settle to the seafloor. Since the particles are small, they can take thousands of years to reach the seafloor. A minor fraction of the abyssal clays are of riverine origin, carried seaward by geostrophic currents. Despite slow sedimentation rates (millimeters per thousand years), clay minerals, feldspar, and quartz are the dominant particles composing the surface sediments of the abyssal plains that lie below the CCD. Since a sediment must contain at least 70% by mass lithogenous particles to be classified as an abyssal clay, lithogenous particles can still be the major particle type in a biogenous ooze. 

Latitudinal patterns in clay mineral distributions are pronounced in abyssal plain sediments as illustrated in Figures 14.8 through 14.11. These latitudinal bands reflect the... 

Because of the relative scarcity of lithogenous particles and fast seafloor spreading rates, metalliferous sediments are common around the East Pacific Rise and very high densities of manganese nodules are present on the abyssal plains, especially in the Southern Hemisphere. In these locations, the weathering products of volcanic detritus, such as montmorillonite, phillipsite, nontronite, and celadonite, are also found in great abimdance. 

Pelagic sedimentation Sedimentation that occurs at rates less than 1 cm/lOOOy. This is characteristic of sediment on the abyssal plains and mid-ocean ridges. 

The sediments of the abyssal plain of the central Black Sea region are mostly biogenic and are enriched with organic matter. The floor of the deepwater depression is covered with coccolith oozes. In peripheral zones, in addition, terrigenous lowly calcareous oozes and carbonate-free silts are observed. 

Marine sediments cover the ocean floor to a thickness averaging 500 m. The deposition rates vary with topography. The rate may be several millimetres per year in nearshore shelf regions, but is only from 0.2 to 7.5 mm per 1000 years on the abyssal plains. Oceanic crustal material is formed along spreading ridges and moves outwards eventually to be lost in subduction zones, the major trenches in the ocean. Because of this continual movement, the sediments on the seafloor are no older than Jurassic in age, about 166 million years. 

S0rensen, J., Jorgensen, K. S., CoUey, S., Hydes, D., Thomson, J., and Wilson, T. (1984). Depth localization of denitrification in a deep-sea sediment from the Madeira Abyssal Plain. Limnol. Oceanogr. 32, 758—762. 

De Lange G. J., Jarvis L, and Kuijpers A. (1987) Geochemical characteristics and provenance of late quaternary sediments from the Madeira abyssal plains N. Atlantic. In Geology and Geochemistry of Abyssal Plains (eds. P. P. E. Weaver and J. Thomson). Blackwell, London, pp. 147-165. 

Abyssal plains are relatively flat areas of the ocean basin with slopes of less than one part in a thousand. They tend to be found at depths of 13,000-16,000 ft (4,000-5,000 m). Oceanographers believe that abyssal plains are so flat because they are covered with sediments that have been washed off the surface of the continents for thousands of years. On the abyssal plains, these layers of sediment have now covered up any irregularities that may exist in rock of the ocean floor beneath them. 

Abyssal plains found in the Atlantic and Indian Ocean tend to be more extensive than those in the Pacific Ocean. One reason for this phenomenon is that the majority of the world s largest rivers empty into either the Atlantic or the Indian Oceans, providing both ocean basins with an endless supply of the sediments from which abyssal plains are made. 

Comparative elemental analyses of the upper and lower sections of two sediment cores collected on the MAP abyssal plain show that organic concentrations decreased at both locations from values of 0.93-1.02 wt% OC below the oxidation front to values 0.16-0.21 wt% within the surface oxidized layer (Fig. 12.9). PoUen abundances decreased in the same samples from about 1600 grains g below the oxidation front to zero above it. Overall, 80% of the organic matter and essentiaUy all of the pollen that has been stable for 140 000 y in the presence of porewater sulfate was degraded in the upper section of the MAP cores as a result of long-term exposure to dissolved O2. 

Profiles of the weight percent organic carbon (%OC) and pollen abundances down two sequences of the f-turbidite from the Madeira Abyssal Plain. Oxidized sediments are those that have been exposed to oxygen after thousands of years of burial. See text for significance of these results. From Cowie et al. (1995). 

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