Basins restricted

Evaporites are sedimentary chemical deposits composed of minerals precipitated from brines (seawater) concentrated by evaporation. Like carbonates, evaporites are bom within their depositional environment (James and Kendall, 1992). Evaporite precipitation and preservation are primarily determined by a number of factors the degree of brine concentration, the brine composition, the hydrology (degree of basin restriction or isolation, and... 

In tlie previous section we showed tliat because tlie stmcture space is very sparse tliere have to be many sequences tliat map onto tlie countable number of basins in tlie stmcture space. The kinetics here shows tliat not all tlie sequences, even for highly designable stmctures, are kinetically competent. Consequently, the biological requirements of stability and speed of folding severely restrict tlie number of evolved sequences for a given fold. This very important result is schematically shown in figure C2.5.4. 

Evaporite deposition is a much more episodic process and thus difficult to quantify. Because seawater is significantly undersaturated with respect to common evaporitic minerals, like gypsum and halite, evaporites are only formed when restricted circulation develops in an ocean basin in which evaporation exceeds precipitation. A geologically recent example is the Mediterranean Sea of 5-6 Myr ago. At this time excess evaporation exceeded the supply of ocean water through shallow inlet(s) from the Atlantic Ocean. As salinity increased, first CaS04, then NaCl precipitated. Over time, salt deposits 2-3 km thick formed. This thickness represents about 40 desiccations of the entire... 

Flow in the Pennsylvanian Fountain formation, a sandstone aquifer that underlies the Lyons and is separated from it by an aquitard complex, was more restricted because the formation grades into less permeable dolomites and evaporites in the deep basin. Groundwater in the Fountain recharged along the Front Range and... 

Evaporite deposition also occurred in barred basins whose shallow sills restricted water exchange with the open ocean. Several modes of formation have been proposed for these basinal evaporites as illustrated in Figure 17.10. First, if sea level dropped... 

Abyssal clays are found in greater abundance on the western side of the Atlantic Ocean than on the eastern side. This is due to bottom topography that restricts the flow of North Atlantic Deep Water and Antarctic Bottom Water to the western side of the basin. The lower temperature of the western waters causes the CCD to be somewhat shaUower than on the east side of the basin as calcite solubility increases with decreasing... 

Equalization basins may be designed to equalize flow, concentrations, or both [27]. For flow equalization, the cumulative flow is plotted vs. time over the equalization period, which is usually 24 hours. The maximum volume above the constant-discharge line is the equalization volume required. The basin may also be sized to restrict the discharge to a maximum concentration of a critical pollutant. For example, if the maximum effluent from an activated sludge unit is 20 mg/L BOD5, the maximum allowable effluent from the equalization basin may be computed and thereby provide a basis for sizing the unit. Novotny and England [44]... 

The range of fusinites studied here is relatively small, being restricted to samples from thick fusain lenses in bituminous coals of Carboniferous age taken from two coal basins in the northern hemisphere. The present data, therefore, do not justify any general statement about the origin of fusinites. It can be stated, however, that the properties noted for the fusinites studied are consistent with the view that they were subjected to some kind of pyrolysis to perhaps 400-600°C. before incorporation in the organic sediment. 

Other Applications of the Multiple-Core Approach. The bulk of this chapter has dealt with the specific application of multiple-core methodology to questions of atmospheric Hg deposition. Whole-basin Hg accumulation rates for seven lakes, calculated from multiple sediment cores, were used in a simple mass-balance model to estimate atmospheric fluxes and Hg transport from catchment soils. This approach can be used to answer other limnological questions, and the model is not restricted to Hg or atmospheric deposition. 

Much of our current understanding of these impacts arose from research on wildlife and human populations within the Great Lakes basin. Despite being banned in North America more than 25 years ago, the toxicological risks from PCB exposure are still a present-day concern. For example, current PCB concentrations in fish in the Great Lakes are sufficiently large as to drive the need for fish consumption advisories for sport fish and to restrict commercial fisheries. 

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