Seawater shallow

Figure 10. Sedimentation of LACSD sludge in seawater, shallow column (dilutions (dlj 500 1 (( ) 100 1 (A) 50 1)...

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... 

Barite and sphalerite tend to precipitate at lower temperature from the hydrothermal solution mixed with a large amount of cold seawater (but mixing ratio (seawater/hydrothermal solution) may be less than 0.2). These minerals precipitate on the seafloor and/or at very shallow subsurface environment. However, chalcopyrite tends to precipitate from high temperature solutions in ore bodies and/or at the sub-seafloor sediments. Usually shale which is relatively impermeable overlies the Besshi-type ore bodies. This suggests that hydrothermal solution could not issue from the seafloor and... 

All soils contain soluble salts, but their concentration is low. The salt content of most arid soils is, however, much higher. Salts in desert soils are usually derived from three main sources (1) deposition of wind-blown salt spray or dust (2) in situ weathering of salt-containing rocks or sediments, and (3) upward movement with the capillary flow from a shallow salty groundwater. Along the coastline, some salinization may occur through intrusion and flooding by seawater. 

Figure 8-10 shows the first 200 years of evolution of the concentrations at the same depths as plotted in Figure 8-9. The concentrations of both total carbon and calcium at a 500-centimeter depth decrease at first and then increase. This decrease occurs because I used starting values equal to seawater values. The waters were initially supersaturated and started out by precipitating calcium carbonate. This initial precipitation was overwhelmed at the shallower depths by the rapid addition of carbon as a result of respiration.

The evolution of the profiles of the isotope ratio is shown in Figure 8-12, which plots the profiles at various times in the calculation. Early in the calculation, isotope ratios at shallow depths have been driven more negative by the release of isotopically light respiration carbon, but little change has occurred at greater depths. As the evolution proceeds, the ratios at shallow depths become more positive as the result of the dissolution and diffusion of heavier carbon from both above and below. In the final steady state, after some 15,000 years, the isotope ratio is nearly constant at about -0.6 per mil at depths below 100 centimeters, rising rapidly to the seawater value, +2 per mil in the top 100 centimeters. The final values reflect a balance between the release of isotopically light carbon by respiration and the release of isotopically heavy carbon by dissolution, with the additional influence of the diffusion of isotopically heavy seawater carbon. 

Chan and Kastner (2000) examined pore fluids fi om ODP Sites 1039 (Fig. 16c), 1040, and 1042, outboard of Costa Rica, with a total range in 5T i of +22.2 to +39.3. At Site 1039 they observed a pronounced correlation between Li concentration and Li isotopes, such that the pore waters with the highest concentrations (approximately equal to seawater) had the lowest 5T i. Variations in data from shallow depths at Site 1039 were interpreted to reflect superimposed effects of alteration of volcanic ash and ion exchange between LL and NH/ (Chan and Kastner 2000). The former consumes Li from fluids, the latter releases, but the isotopic fractionation... 

Unfortunately, seawater is slightly compressible, so in situ density increases with increasing pressure. The rate at which pressure increases with increasing depth below the sea surface is nearly equal to 1 dbar per m. At 45° latitude, the actual rate is l.Oldbar/m from 0 to 2500m. Below 2500m, it increases to 1.02dbar/m. This rate increase is due to an increasing resistance to further compression. Because Earth is not a perfect sphere, the rate at which pressure increases with depth also varies with latitude. Thus, the depth at which 4500 dbar of pressure is attained is 4428 m at the equator and 23 m shallower at the poles (4405 m). Tabulated values of pressure as a... 

The thin-film model is the simplest and, therefore, most commonly used approach to estimate air-sea fluxes of gases. In this model, molecular diffusion is assumed to present a barrier to gas exchange in each of two layers. As illustrated in Figure 6.5, one layer is composed of a shallow region of the atmosphere that lies in direct contact with the sea surface. The second is a shallow layer of seawater tliat lies at the sea surface. These layers have depths less than 100 (am and, hence, are referred to as thin films. 

Shallow-water embayments provide a mechanism to isolate seawater so that evaporation can raise salt ion concentrations. Arid climates are required to ensure that the rate of water loss from evaporation exceeds the rate of water supply by rainfell, groundwater seeps, or river runoff. Seawater can be resupplied continuously via a type of antiestuar-ine circulation as illustrated in Figure 17.2 or episodically as a result of sea level change, plate tectonics, or very high tides and storm surges. 

Schematic longitudinal profile through a semi-isolated basin located in a hot, arid climate and separated from the open sea by a narrow portal. The sill depth, although shallow, is still great enough to permit some two-way flow of surface water. The lines show inferred seawater density (g/cm ) and the arrows show current directions. The pattern of evaporite deposition is based on the relationships between brine density and precipitate composition as shown in Figure 17.1, assuming that salt particles accumulate on the seafloor through the process of pelagic sedimentation. Source-. From Scruton, P. C. (1953). American Association of Petroleum Geologists Bulletin, 37, 2498-2512.

Thus, the Mediterranean Sea must have been refilled in an episodic fashion such that conditions favoring shallow-water evaporite deposition were rapidly reattained. Some geologists have proposed that this was achieved via periodic inflows of seawater from the Atlantic Ocean over the exposed Gibraltar Sill into a nearly dry Mediterranean Sea basin. This must have taken the form of a waterfeU hundreds of meters in height The episodic nature of this process is reflected in the repeating evaporite sequences foimd throughout the Messinian deposits. 

Because warm surface seawater is usually supersaturated with respect to calcium carbonate, abiogenic precipitation of calcite and aragonite does occur, at least when supersaturations are very high. These conditions are limited to shallow water where temperatures can get sufficiently high, namely coastal tropical seas. 

Particular attention has been focused on the toxic effects of aromatic hydrocarbons because these chemicals have proven highly carcinogenic to humans and marine life. Of greatest concern are the PAHs, which are toxic to the benthos at the ppb level. The most common compounds are shown in Figure 28.20 their structures are based on fused aromatic rings. These high-molecular-weight compoimds are very nonpolar and, hence, have low solubilities. Once in seawater, they tend to adsorb onto particles and become incorporated in the sediments. The toxicity of PAHs is enhanced by photochemical reaction with UV radiation. Photo-activated toxicity is especially problematic in shallow-water sediments, such as found in estuaries. 

---