Solution chemistry seawater

The overall effect of the terrestrial weathering reactions has been the addition of the major ions, DSi, and alkalinity to river water and the removal of O2, and CO2 from the atmosphere. Because the major ions are present in high concentrations in crustal rocks and are relatively soluble, they have become the most abimdant solutes in seawater. Mass-wise, the annual flux of solids from river runoff (1.55 x 10 g/y) in the pre-Anthropocene was about three times greater than that of the solutes (0.42 x 10 g/y). The aeolian dust flux (0.045 X 10 g/y) to the ocean is about 30 times less than the river solids input. Although most of the riverine solids are deposited on the continental margin, their input has a significant impact on seawater chemistry because most of these particles are clay minerals that have cations adsorbed to their surfaces. Some of these cations are desorbed... 

It is appropriate at this point to discuss the "apparent" pH, which results from the sad fact that electrodes do not truly measure hydrogen ion activity. Influences such as the surface chemistry of the glass electrode and liquid junction potential between the reference electrode filling solution and seawater contribute to this complexity (see for example Bates, 1973). Also, commonly used NBS buffer standards have a much lower ionic strength than seawater, which further complicates the problem. One way in which this last problem has been attacked is to make up buffered artificial seawater solutions and very carefully determine the relation between measurements and actual hydrogen ion activities or concentrations. The most widely accepted approach is based on the work of Hansson (1973). pH values measured in seawater on his scale are generally close to 0.15 pH units lower than those based on NBS standards. These two different pH scales also demand their own sets of apparent constants. It is now clear that for very precise work in seawater the Hansson approach is best. 

In this chapter, we introduced the reader to some basic principles of solution chemistry with emphasis on the C02-carbonate acid system. An array of equations necessary for making calculations in this system was developed, which emphasized the relationships between concentrations and activity and the bridging concept of activity coefficients. Because most carbonate sediments and rocks are initially deposited in the marine environment and are bathed by seawater or modified seawater solutions for some or much of their history, the carbonic acid system in seawater was discussed in more detail. An example calculation for seawater saturation state was provided to illustrate how such calculations are made, and to prepare the reader, in particular, for material in Chapter 4. We now investigate the relationships between solutions and sedimentary carbonate minerals in Chapters 2 and 3. 

Keeney-Kennicutt W. L. and Morse J. W. (1985) The redox chemistry of Pu(V)02 interaction with common mineral surfaces in dilute solutions and seawater. Geochim. Cosmo-chim. Acta 49, 2577-2588. 

The yttrium concentration in seawater is 17ngkg (Zhang etal. 1994). The solution complexation of Y in seawater is similar to that of Tb, as opposed to the different reactivity with ligands on the particle surfaces (Liu and Byrne 1995). Hence, solution chemistry may help in explaining the distribution of these elements in the oceans, and this field of study might be applicable with regard to the biological effects of yttrium. 

The solution chemistries of group 11 elements (Cu, Ag, Au) in oxidation state I are similar. Cul, Agl and Aul are strongly complexed with Cl- and hydrolysis is insignificant. While Ag solely exists as Agl, Cu occurs dominantly as Cull in oxygenated seawater and oxidation number 111 may be important for Au. Cull chemistry is dominated by carbonate complexation, while Aulll speciation in seawater appears (tentatively) to be dominated by mixed-l and chlorohydroxy complexes. 

The group 17 elements (F, Cl, Br, 1 and At) exist with -1 and V oxidation numbers and the -1 state is predominant for the l hter elements. F- occurs in seawater as an approximately equimolar mixture of F- and MgF+. Cl and Br occur dominantly as unassociated Cl- and Br-. The predominant oxidation number of 1 is V. IV occurs as 103- and is, to a small extent, ion paired with Mg2+. 1- is found in seawater at substantially lower concentrations than 103-. Little is known about the solution chemistry of highly radioactive At. 

Forms of Plutonium in Seawater and Other Aqueous Solutions," to be published in Marine Chemistry (1983). 

Why Do We Need to Know This Material The techniques described in this chapter provide rhe tools that we need to analyze and control the concentrations of ions in solution. A great deal of chemistry is carried out in solution, and so this material is fundamental to understanding chemistry. The ionic compounds released into waterways by individuals, industry, and agriculture can impair the quality of our water supplies. However, these hazardous ions can be identified and removed if we add the right reagents. Aqueous equilibria govern the stabilization of the pH in blood, seawater, and other solutions encountered in biology, medicine, and the environment. 

A solution, still controversial, has been recently proposed. This is the loss of sulfate from seawater during hydrothermal circulation through mid-ocean ridges (Edmond et al., 1979). The flow of water through these systems is estimated to be about 1.4 x 10 L/yr, about 0.4% of the flow of rivers. However, sulfate is quantitatively removed, yielding a flux of 125 Tg S/yr, capable of balancing the river flux. The controversy is whether the chemistry involved in removing sulfate is the formation of... 

The above argument on the calculation of chemical composition of ore fluids, seawater-rock interaction experiments, and isotopic compositions of ore fluids clearly demonstrates that Kuroko ore fluids were generated by seawater-rock interaction at elevated temperatures. The chemistry of present-day hydrothermal solution venting from back-arc basins and midoceanic ridges (sections 2.3 and 2.4) also support this view. 

Wolery (1978) and Reed (1982, 1983) have indicated based on a computer calculation of the change in chemistry of aqueous solution and mineralogy during seawater-rock interactions that epidote is formed under the low water/rock ratio less than ca. 50 by mass. Humphris and Thompson (1978), Stakes and O Nell (1982) and Mottl (1983) have also suggested on the basis of their chemical and oxygen isotopic data of the altered ridge basalts that epidote is formed by seawater-basalt interaction at elevated temperatures (ca. 200-350°C) under the rock-dominated conditions. If epidote can be formed preferentially under such low water/rock ratio, the composition of epidote should be influenced by compositions of the original fresh rocks. 

Phase separation and segregation are occurring in some hydrothermal systems (Gamo, 1995) which modify the chemistry of initial hydrothermal solution. Von Damm and Bischoff (1987) and Butterfield et al. (1994) obtained high chloride concentration of more than twice of the seawater value for the hydrothermal solution at the North Cleft Segment and the South Cleft Segment of the Juan de Fuca Ridge. 

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