Deep Seawater

Certain equilibria are modified for example, the precipitation of calcium carbonate does not take place at a great depth (the metal is not covered by a scale film), which explains the lesser corrosion resistance of certain metals and alloys in deep seawater. 

The development of several applications at the sea bed level such as bathyscaphs, small experimental submarines and submarine detection beacons has triggered studies of the corrosion of metals in deep waters. Their results are reported below. 

Several smdies have been undertaken since 1965 to investigate the corrosion resistance of aluminium up to a depth of 2000 m in the Pacific Ocean [14]. These tests have shown that the corrosion resistance in deep waters is comparable to that observed at the surface. The forms of corrosion are the same. Pitting depth on 5000 series alloys is of the same order of magnitude whatever the depth [15]. On other alloys (3000 and 6000 series), it can be higher. The role of oxygen has been put forward as an explanation for these differences. 

Duration of test (d) Depth (m) Water Pitting depth ( j.m) Sediments 

More recent tests up to a depth of 5000 m in the Atlantic Ocean [16] and up to 3000 m in the Indian Ocean [17] have shown that the corrosion resistance of aluminium does not depend on the immersion depth. Several exploration devices have been made of aluminium such as the Aluminaut made in 7079 T6 [18]. 

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It follows that deep seawater contains nutrients from two sources. First, it may contain nutrients that were present with the water when it sank from the surface. These are called preformed nutrients." Second, it may contain nutrients derived by the in situ remineralization of organic particles. These are called oxidative nutrients. 

Nakashima et al. [719] detail a procedure for preliminary concentration of 16 elements from coastal waters and deep seawater, based on their reductive precipitation by sodium tetrahydroborate, prior to determination by graphite-furnace AAS. Results obtained on two reference materials are tabulated. This was a simple, rapid, and accurate technique for determination of a wide range of trace elements, including hydride-forming elements such as arsenic, selenium, tin, bismuth, antimony, and tellurium. The advantages of this procedure over other methods are indicated. 

Figure 2.14. The log of dissolution rate in percent per day versus the log of (1-fi). A = whole Indian Ocean sediment dissolved in deep-sea sediment pore water B = whole Pacific Ocean sediment dissolved in Atlantic Ocean deep seawater C = whole Atlantic Ocean sediment dissolved in Long Island Sound seawater (Morse and Berner, 1972) D = > 62 pm size fraction of the Indian Ocean sediment dissolved in Atlantic Ocean deep seawater, E = the 125 to 500 pm size fraction of Pacific Ocean sediment dissolved in Atlantic Ocean deep seawater F = 150 to 500 pm Foraminifera dissolved in the Pacific Ocean water column. (After Morse, 1978.)...

It should be kept in mind that, in spite of these major variations in the CO2-carbonic acid system, virtually all surface seawater is supersaturated with respect to calcite and aragonite. However, variations in the composition of surface waters can have a major influence on the depth at which deep seawater becomes undersaturated with respect to these minerals. The CO2 content of the water is the primary factor controlling its initial saturation state. The productivity and temperature of surface seawater also play major roles, in determining the types and amounts of biogenic carbonates that are produced. Later it will be shown that there is a definite relation between the saturation state of deep seawater, the rain rate of biogenic material and the accumulation of calcium carbonate in deep sea sediments. 

As previously mentioned, the primary processes responsible for variations in the deep sea C02-carbonic acid system are oxidative degradation of organic matter, dissolution of calcium carbonate, the chemistry of source waters and oceanic circulation patterns. Temperature and salinity variations in deep seawaters are small and of secondary importance compared to the major variations in pressure with depth. Our primary interest is in how these processes influence the saturation state of seawater and, consequently, the accumulation of CaC03 in deep sea sediments. Variations of alkalinity in deep sea waters are relatively small and contribute little to differences in the saturation state of deep seawater. 

Saturation State of Deep Seawater with Respect to CaC03... 

In the present ocean calcium carbonate formation is dominated by pelagic plants (coccolithophores) and animals (foraminifera, pteropods, and heteropods). Examples are presented in Figure 4.13. Although benthic organisms are important in shoal water sediments, and for dating and geochemical studies in the deep sea sediments, they constitute only a minor portion of the calcium carbonate removed from deep seawater. Shoal water carbonates are discussed in detail in Chapter 5. 

The saturation state of deep seawater with respect to calcite and aragonite on a large scale needs to be determined within an order of magnitude more accuracy than it is now known. 

Open Ocean Dissolution Experiments. The first direct studies of calcium carbonate dissolution in deep seawater were made by Peterson (41) and Berger (42). Peterson suspended spheres of Iceland spar calcite, held in pronged plastic containers, at various depths in the Central Pacific Ocean for four months. The amount of dissolution was determined by weight loss, which was small relative to the total weight of the spheres. On the same mooring Berger suspended sample chambers, which consisted of... 

Wada, E., and Hattori, A. (1972). Nitrite distrihution andnitrate reduction in deep seawaters. Deep Sea Res. 19, 123-132. 

The turnover time and fluxes of DOC into the ocean are obtained by comparing the reservoir size and radiocarbon age. The ocean inventory of DOC is —680 Gt, and nearly all of this carbon resides in the deep sea, where concentration profiles and radiocarbon values are constant with depth. DOC ages by —1,000 yr as deep seawater moves from the Atlantic to the Pacific Basin, but even in the Atlantic, DOC radiocarbon values are significantly depleted relative to dissolved inorganic carbon (DIC) (Druffel et al., 1992). DOC persists in seawater through several ocean... 

Figure 3 Map of Nd-isotope variability in ferromanganese deposits. The map shows systematic geographic variability with lowest values in the North Atlantic, highest values in the Pacific, and intermediate values elsewhere. Arrows illustrate general movement of deep water, and show that the contours generally follow deep-water flow. Shaded fields delineate regions where the Fe -Mn and deep seawater data differ by >2Efjd units (after Albare(c)de and...

Neodymium-isotope ratios in deep seawater show the same general geographical pattern as in Fe-Mn nodules and crusts. The global geographical pattern is illustrated in the Fe-Mn nodule-crust map (Figure 3), and by some depth profiles (Figure 5). The North Atlantic is characterized by... 

These observations, taken together, indicate that the neodymium-isotope ratios of intermediate and deep seawater are imprinted mainly in the Atlantic and Pacihc oceans. The circum-Antarctic is fed by both and its intermediate neodymium-isotope ratio reflects those of the Atlantic and Pacihc water sources. Indian Ocean intermediate and deep water is fed primarily by the circum-Antarctic and tends to retain a circum-Antarctic neodymium-isotope ratio. 

Figure 13 Nd abundance versus (a) salinity and (b) silicate in deep seawater. Nd concentrations do not show the same well-behaved characteristics as Nd-isotope ratios with salinity and silicate in global deep water. Mixing envelopes are shown between North Atlantic and Pacific end-members, and the circum-Antarctic, South Atlantic, and Indian Ocean samples fall outside of it. Plotted data are from >2,500 mb si, except two Drake Passage data from 1,900 m and 2,000 m (Nd data sources Piepgras and Wasserburg, 1980, 1982, 1983, 1987 Spivack and Wasserburg, 1988 Piepgras and Jacobsen, 1988 Bertram and Elderfield, 1993 Jeandel, 1993 Shimizu et al., 1994 Jeandel et ah, 1998). Where salinity or silicate were not available in the publication, they were estimated from Levitus (1994),...

The salt dissolved in seawater has remarkably constant major constituents (Table 15.2). Cl", SO4", Mg, K, Ca, and Na" dominate sea salt. Their ratios one to another are veiy constant. This constancy does not extend to all the trace components (see Table 6.1), especially not to the biolimiting elements that are removed from the surface seawater by organisms. For nonbiolimited elements, the ratio of the element to the total salt (e.g., chlorinity) in both surface and deep seawater samples is unchanged. 

As shown by equation 20, the phosphorus concentration dissolved in the sea is controlled by (1) the upwelling rate of deep seawater, (2) the fraction of particles falling to the deep sea that survive oxidation, (3) the phosphorus content of average river water, and (4) the rate of continental runoff. 

M.S. Musumeci for the NEMO Collaboration Mechanical Structures for a Deep Seawater Neutrino Detector. In VLVi/T Workshop Proc., pages 153-156, Amsterdam, 2003. Proceedings available at http / /www. vlvnt. nl /proceedings... 

There are two basic types closed-cycle and open-cycle. In a closed-cycle system, warm surface seawater and cold deep seawater are used to vaporize and condense the working fluid such as ammonia, which then drives the turbine generator in a closed loop. In an open-cycle system, surface seawater is flash-evaporated in a vacuum chamber, and the resulting low-pressure steam drives a turbine-generator. Cold seawater is then used to condense the steam after it has passed through the turbine. The open cycle, therefore, can be configured to produce fresh water as well as electricity. 

It follows that deep seawater contains nutrients from two sources. First, it may contain nutrients that were present with the water when it sank from the surface. These are called "preformed nutrients". Second, it may contain nutrients derived by the in situ remineralization of organic particles. These are called oxidative nutrients. The oxidative nutrients can be estimated from the RKR equation. From this model, we might expect the four dissolved chemical species (O2, CO2, NO3, PO4) to vary in seawater according to the proportions predicted. The key to understanding these remineralization reactions is the parameter Apparent Oxygen Utilization (AOU), defined as ... 

Metabolic pathways and organic acid turnover rates in deep seawater and sediment... 

PROBABLE FATE photolysis not subject to considerable direct photolysis, occurs slowly, sunlight photolysis in surface water at 40 deg latitude in summer has a reported half-life of 450 years oxidation oxidized by hydroxy radicals after volatilization hydrolysis not important process, first-order hydrolytic half-life 3.4 years volatilization very rapid volatilization can be hindered by adsorption if organics are present sorption high potential for adsorption by organic materials biological processes high potential for bioaccumulation very little, if any biodegradation due to volatilization and adsorption evaporation half-life from 5.4m deep seawater 11-22 days half-life from a model river 4.2 hr predicted in atmosphere, reacts with photochemically produced hydroxyl radicals with an approximate vapor phase half-life of 18.5 days. 

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