Rapid sedimentation

Growth faulted deltaic areas are highly prospective since they comprise of thick sections of good quality reservoir sands. Deltas usually overlay organic rich marine clays which can source the structures on maturation. Examples are the Niger, Baram or Mississippi Deltas. Clays, deposited within deltaic sequences may restrict the water expulsion during the rapid sedimentation / compaction. This can lead to the generation of overpressures. 

In practice, sedimentation is an important property of colloidal suspensions. In fonnulated products, sedimentation tends to be a problem and some products are shipped in the fonn of weak gels, to prevent settling. On the other hand, in applications such as water clarification, a rapid sedimentation of impurities is desirable. 

Figure 5. An example of the use of concentrations to assess changes in sediment mass accumulation taken from McManus et al. (1998). The upper panel shows the measured °Thxs, calculated from measured °Th concentrations by correction for detrital °Th, and for the effects of age using 5 0 stratigraphy (see Appendix). Because the supply of °Th to the sediment is a constant, low °Th represent times of rapid sediment mass accumulation. The calculated mass flux is shown in the lower panel. Dramatic increases in mass flux are observed during all but one of the Heinrich events, shown by the gray bands.

Other techniques are used for shorter time scales, including the measurement of the 226Ra Ba ratio in barite extracted from sediments (Paytan et al., 1996). This technique has a time scale of about five thousand years. Alternately, assessments of rapid sedimentation and bioturbation on time scales of days to centuries require shorter half-life nuclides such as 210Pb, 228Th, 234Th, and 222Rn. 

Extremely rapid sedimentation of paint pigment has been obtained with the following composition ... 

Sites 1251 and 1252 deal with the slope basin to the east with rapid sedimentation... 

Following consumption of dissolved O2, the thermodynamically favored electron acceptor is nitrate (N03-). Nitrate reduction can be coupled to anaerobic oxidation of metal sulfides (Appelo and Postma, 1999), which may include arsenic-rich phases. The release of sorbed arsenic may also be coupled to the reduction of Mn(IV) (oxy)(hydr)oxides, such as birnessite CS-MnCb) (Scott and Morgan, 1995). The electrostatic bond between the sorbed arsenic and the host mineral is dramatically weakened by an overall decrease of net positive charge so that surface-complexed arsenic could dissolve. However, arsenic liberated by these redox reactions may reprecipitate as a mixed As(III)-Mn(II) solid phase (Toumassat et al., 2002) or resorb as surface complexes by iron (oxy)(hydr)oxides (McArthur et al., 2004). The most widespread arsenic occurrence in natural waters probably results from reduction of iron (oxy)(hydr)oxides under anoxic conditions, which are commonly associated with rapid sediment accumulation and burial (Smedley and Kinniburgh, 2002). In anoxic alluvial aquifers, iron is commonly the dominant redox-sensitive solute with concentrations as high as 30 mg L-1 (Smedley and Kinniburgh, 2002). However, the reduction of As(V) to As(III) may lag behind Fe(III) reduction (Islam et al., 2004). 

The profile of Mg2+ in Figure 8.25 indicates downward diffusion of this constituent into the sediments. Mass balance calculations show that sufficient Mg2+ can diffuse into the sediments to account for the mass of organogenic dolomite formed in DSDP sediments (Baker and Bums, 1985 Compton and Siever, 1986). In areas of slow sedimentation rates, the diffusive flux of Mg2+ is high, and the pore waters have long residence times. Dolomites form under these conditions in the zone of sulfate reduction, are depleted in 13c, and have low trace element contents. With more rapid sedimentation rates, shallowly-buried sediments have shorter residence times, and dolomites with depleted 13C formed in the sulfate-reduction zone pass quickly into the underlying zone of methanogenesis. In this zone the DIC is enriched in 13C because of the overall reaction... 

They do not rapidly sediment (settle out) from solution under the influence of gravity,8 but they often do in an ultracentrifuge.9... 

Bolto and Dixon advocated a system which involves a simple pipeline for the adsorption reaction, in which the magnetized resin and raw water are pumped through a pipe at such a flow rate that turbulence results and the floes are broken apart so that the reaction is complete when the mixture leaves the pipe. The slurry flows into a settling vessel, whereupon the magnetic floes reform and rapid sedimentation ensues. 

Preparing slurries in aqueous solutions is rarely effective because most powdered materials undergo rapid sedimentation. This occurs immediately after the solid and liquid... 

N. G. Anderson, Nature, 181 45 (1958). Rapid Sedimentation of Proteins through Starch. 

Alongi, D. M., Pfitzner, J., Trott, L. A., Tirendi, F., Dixon, P., and Klumpp, D. W. (2005). Rapid sediment accumulation and microbial mineralization in forests of the mangrove Kandelia candel in the Jiulongjiang estuary, China. Estuarine Coastal Shelf Sd. 63, 605—618. 

The rate of organic matter input (i.e., sedimentation) and the availability of SOl control the rate of SO4 reduction in sediments. Sulfate is rarely limiting in marine systems except in brackish estuarine waters and with depth in sediments where SOl has been depleted. Sulfate reduction rates in sediments can span several orders of magnitude, but typical near-shore rates in the upper 5-10 cm of marine sediments are often 50-500 nmol cm d (Skyring, 1987). Rates in sediment with unusually rapid sedimentation rates can be reach 2,000 nmol ml d (Grill and Martens, 1987). Sulfate reduction in salt marshes and microbial mats can reach rates as high as 4,000 nmol ml d and 14,000 nmol ml -d respectively (Canheld and Des Marais, 1991 Hines et al., 1999). Anaerobic CH4 oxidation supports a large fraction of the SO4 reduction in some marine sediments (Table 6). 

Callender (2000) extensively studied the geochemical effects of rapid sedimentation in aquatic... 

Callender E. (2000) Geochemical effects of rapid sedimentation in aquatic systems minimal diagenesis and the preservation of historical metal signatures. J. PaleoUmnol. 23, 243-260. 

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