Seawater, composition

The failure to identify the necessary authigenic silicate phases in sufficient quantities in marine sediments has led oceanographers to consider different approaches. The current models for seawater composition emphasize the dominant role played by the balance between the various inputs and outputs from the ocean. Mass balance calculations have become more important than solubility relationships in explaining oceanic chemistry. The difference between the equilibrium and mass balance points of view is not just a matter of mathematical and chemical formalism. In the equilibrium case, one would expect a very constant composition of the ocean and its sediments over geological time. In the other case, historical variations in the rates of input and removal should be reflected by changes in ocean composition and may be preserved in the sedimentary record. Models that emphasize the role of kinetic and material balance considerations are called kinetic models of seawater. This reasoning was pulled together by Broecker (1971) in a paper called "A kinetic model for the chemical composition of sea water."... 

Baker, J.H., De Groot, P.M. and Roedder, E. (1983) Proteozoic seawater-felsic volcanics interaction, W.Bergslagen, Sweden. Evidence for high REE mobility and implications for 1.8 Ga seawater compositions. Contr. Mineral. Petrol, 82, 119-132. 

I apply these computational methods to various aspects of the Earth system, including the responses of ocean and atmosphere to the combustion of fossil fuels, the influence of biological activity on the variation of seawater composition between ocean basins, the oxidation-reduction balance of the deep sea, perturbations of the climate system and their effect on surface temperatures, carbon isotopes and the influence of fossil fuel combustion, the effect of evaporation on the composition of seawater, and diagenesis in carbonate sediments. These applications have not been fully developed as research studies rather, they are presented as potentially interesting applications of the computational methods. 

Residence time and reactivity are strongly correlated through equation (7.2.9). This is true for seawater composition since Whitfield and Turner (1979) showed a rather good correlation between oceanic residence times and seawater-crustal rock partition coefficients which are taken as a measure of element reactivity in the ocean. Actually, a better estimate of reactivity is given by oceanic suspensions, so Li (1982) suggested to use pelagic clay-seawater concentration ratios as a proxy to partition coefficients. 

The use of synthetic media is preferable to natural filtered and/or sterilised media, as trace substances are expensive to remove from natural waters. In addition to this, the reproducibility of assays is improved if synthetic media are used. Different artificial seawater compositions have been used in toxicity testing. From the media investigated by our group, (see Table 7.1.2) the ASTM substitute ocean water [66] gave the best results for growing microalgae. 

THE WHOLE PICTURE INTEGRATED MODELS OF SEAWATER COMPOSITION... 

Marine chemists have taken increasingly more sophisticated approaches towards modeling seawater composition. The goal of these models is to understand the biogeochemical controls on seawater composition well enough that the effects of future perturbations can be predicted. As described next, the first modeling efforts were based on a series of reactions that were assumed to reach equilibrium the next efforts took a steady-state approach as the composition of seawater was thought to have been relatively constant over time. 

The Whole Picture Integrated Models of Seawater Composition 547... 

Improvements in analytical techniques have made possible reconstruction of ancient seawater composition from fluid inclusion trapped in marine halites. This has forced marine chemists to accept that the major ion composition has changed significantly— at least over the past 500 million years. Since marine halites older than 500 million years are rare, little is known about the major ion composition of seawater prior to the Phanerozoic eon. Thus, current modeling effiarts are directed at simulating changes in seawater composition over the Phanerozoic. 

Table I shows the composition of some natural seawaters around the world. This wide seawater composition variation affects the quality of the product, i. e. the product salinity. This, of course, in the unlikely case that all we had to contend with in the RO conversion were the composition variations. In fact, the other variables such as the number and the nature of microorganisms, the amount, size, and nature of suspended solids and their variability, presence or absence of pollutants, each and all of which can be affected by the prevailing atmospheric condltionspresent, are much more consequential to trouble-free operations than the seawater composition variations.

In our simulations, we assumed that seawater compositions during the Neoproterozoic era would have been similar to present-day seawater, except for high Fe(II) concentrations. We arbitrarily assigned our hypothethical ocean a Fe(II) concentration of 0.01065 m, the same concentration as Ca to maintain a charge balance, we removed an equivalent amount of Na (from 0.48610 to 0.46480m) (Fig. 5.6). 

One major concept applicable to problems dealing with the behavior of carbonic acid and carbonate minerals in seawater is the idea of a "constant ionic media". This concept is based on the general observation that the salt in seawater is close to constant in composition, i.e., the ratios of the major ions are the same from place to place in the ocean. Seawater in evaporative lagoons, pores of marine sediments, and near river mouths can be exceptions to this constancy. Consequently, the major ion composition of seawater can generally be determined from salinity. It has been possible, therefore, to develop equations in which the influences of seawater compositional changes on carbonate equilibria can be... 

Care must be taken, however, in pore waters and special areas such as coastal waters, carbonate banks, and lagoons where significant deviations from normal seawater concentration ratios can occur. As an aside, it should also be noted that the apparent constants are not accurate where seawater composition is altered. This composition change can be especially important in anoxic environments where extensive sulfate concentration changes occur. 

Figure 5.16. Average weight % Sr and Mg in various marine carbonate solids. (After Veizer, 1983). Calcite, > Aragonite. Range of values of Sr and Mg in aragonite and calcite is also shown, based on calculations using distribution coefficients of Chapter 3 and an average seawater composition. Compare with Figure 5.15.

Figure 8.2. Phase relations in the system Ca0-Mg0-HCl-H20-(CC>2). At A. 25°C, 0.001 kb B. 150°C, 1 kb C. 300°C, 3 kb. Mag, magnesite Dol = dolomite Calc = calcite Hydro-Mag = hydromagnesite. Saturation lines for brucite (a) and lime (b) are also shown. Average seawater composition, river water composition, A some subsurface water compositions. (After Bowers et al 1984.)...

The oceans at this time can be thought of as the solution resulting from an acid leach of basaltic rocks, and because the neutralization of the volatile acid gases was not restricted primarily to land areas as it is today, much of this alteration may have occurred by submarine processes. The atmosphere at the time was oxygen deficient anaerobic depositional environments with internal CO2 pressures of about 10-2-5 atmospheres were prevalent, and the atmosphere itself may have had a CO2 pressure near lO-25 atmospheres. If so, the pH of early ocean water was lower than that of modern seawater, the calcium concentration was higher, and early global ocean water was probably saturated with respect to amorphous silica (—120 ppm). Hydrogen peroxide may have been an important oxidant and formaldehyde, an important reductant in rain water at this time (Holland et al., 1986). Table 10.5 is one estimate of seawater composition at this time. 

Table 10.5. An example of early seawater composition. (After Garrels and Perry,...

It appears that the best description of modem seawater composition is that of a chemical system in a dynamic quasi-steady-state. Changes in composition may occur over time, but the system always seems to return to a time-averaged, steady-state composition. In other words, since 1.5-2 billion years ago, evolutionary chemical changes in the hydrosphere have been small when viewed against the magnitude of previous change. 

Seawater intrusion is usually limited to a distance of 1-2 km from the sea, but it varies. The depth at which seawater-induced salinization occurs increases with the distance of the well from the sea. The invading seawater may have direct hydraulic interconnection with the open sea, or it may originate from a stagnant compartment of seawater that was trapped during past hydrological conditions. The latter case will be recognizable because of the old age of the saline water and by deviations from the present seawater composition, reflecting seawater-rock interactions. 

A number of eclogites have both radiogenic neodymium isotopes and radiogenic strontium relative to bulk Earth. For these samples, some of the Sr/ Sr values exceed modem and Archean seawater compositions (see Chapter 6.17). Hence, in a subduction-type model, the radiogenic strontium isotopic composition of the eclogite protohth is not solely inherited from oceanic cmst but is likely to be a time-integrated response to Rb/Sr enrichment during hydrothermal alteration... 

Wallmann K. (2001b) Controls on the Cretaceous and Cenozoic evolution of seawater composition, atmospheric CO2 and climate. Geochim. Cosmochim. Acta 65, 3005-3025. 

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