Sedimentation oceanic particle size

Prediction of Oceanic Particle Size Distributions from Coagulation and Sedimentation Mechanisms... 

Hunt, J.R. (1980). Prediction of oceanic particle size distribution from coagulation and sedimentation mechanisms. Particulates in Water Characterization, Fate, Effects and Removal, Kavanaugh, M.D. and Kekie, J.T. (eds). Advances in Chemistry Series No. 189. American Chemical Society, New Yorkpp. 243-257. 

Figure 14.3. Particle size distribution, (a) Discrete and continuous cumulative particle size distribution, (b) Discrete and continuous particle size distribution, (c) Volume distribution plotted in accordance with equation 3. (d) Particle size distributions at four depths in a calcareous sediment from west equatorial Pacific Ocean, 1°6.0 S, 161 36.6 E, box core No. 136, water depth 3848 m. (From Lerman, 1979.)...

In this chapter, mechanisms of particle removal are limited to coagulation and sedimentation. Predictions of size distributions are obtained that are in reasonable agreement with measured size distributions from oceanic waters and digested sewage sludge. Sensitivity of the predictions to fluid turbulence and fluid density presents a plausible explanation for zones of higher particle concentration observed in the oceanic water column. The analysis does not include zooplankton fecal pellet production, particle breakup, or dissolution, nor does it directly incorporate biological productivity. 

Most of the rocks that make up the upper crust of the earth lie hidden beneath layers of sediments, unconsolidated accumulations of particles derived from the weathering of minerals and rocks (see Fig. 44 and Textbox 45) (Keller 1957). Once formed, the particles are either carried away or moved by the wind, rain, and gravitational forces into the seas and oceans or, before they get there, into depressions in the land. There they accumulate in a wide range of shapes and sizes (see Table 49) (Rocchi 1985 Shackley 1975). 

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. 

As a river or stream slows, it loses some of its ability to keep solids in suspension, and they begin to settle out. Heavier particles such as pieces of gravel and sand settle out first, and lighter particles, such as pieces of silt, settle out later. As a river or stream reaches a lake or ocean, it slows enough that essentially all of its suspended materials are deposited on the lake or ocean bottom. This process is referred to as sedimentation or siltation. A distinction is sometimes made between these two terms depending on the size of particles deposited, hut they are often used interchangeably. 

Lai, D. and Lerman, A. (1975) Size spectra of biogenic particles in ocean water and sediments./. Geophys. Res., 80, 423-430. 

Esser and Turekian (1988) estimated an accretion rate of extraterrestrial particles in ocean bottom and in varved glacial lake deposit on the basis of osmium isotope systematics and concluded a maximum accretion rate of between 4.9 x 104 and 5.6 x 104 tons/a. The discrepancy between this estimate and those derived from helium can easily be attributed to the difference in the size of the cosmic dust particles under consideration. Cosmic dusts of greater than a few ten micrometers may not be important in the helium inventory of sediments because the larger grains are likely to lose helium due to atmospheric impact heating (e.g., Brownlee, 1985). Stuart et al. (1999) concluded from studies on Antarctic micrometeorites that 50- to 1 OO-qm micrometeorites may contribute about 5% of the total flux of extraterrestrial 3He to terrestrial sediments. Therefore, the helium-based estimate deals only with these smaller particles. 

The main removal process for oceanic components is via sedimentation and burial thus, the interaction of dissolved metals with particles in sea water is a major indication of their concentration and distribution in the world s oceans. In open ocean areas the particle cycle is driven by the biological production of particles in the surface layers, which after processes of mineralization and packaging reach the necessary size and density to fall to the ocean bottom. On the basis of this consideration, one can say that in the open ocean area the biogeochem-ical cycle of trace metals determining their distribution and speciation is frequently dominated by biological processes. In eoastal areas or particular geographical zones, other phenomena, e.g., inorganic precipitation, can take place. 

A fraction of the eroded mass made of small-size particles is transported by winds over subglobal distances, to be ultimately deposited in the open ocean and on other continents. Rates of sediment accumulation in the open oceans are much lower than for sediments in continental bodies of water, where in lakes the rates are typically of an order of 10° 1 mm yr"1 for terrigenous sediments and are higher for deposition of organic-matter-rich muds in areas of strong primary productivity. 

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