Seawater conservative behaviour

Thallium (Tl), which appears to exhibit conservative behaviour in seawater, has two potential oxidation states. As Tl1, thallium is very weakly complexed in solution. In contrast, Tl111 should be strongly hydrolysed in solution ([T13+]/[T13+]t — 10 20 5) with Tl(OH)3 as the dominant species over a very wide range of pH. The calculation of Turner et at. (1981) indicated thatTl111 is the thermodynamically favoured oxidation state at pH 8.2. Lower pH and p()2 would be favourable to Tl1 formation. Within the water column, pH can be considerably less than 8.2 and /)( )2 lower than 0.20 atm. In view of these factors, and the observation that redox disequilibrium in seawater is not uncommon, the oxidation state of Tl in seawater is somewhat uncertain. The existence of Tl in solution as Tl+, a very weakly interactive ion, would reasonably explain the conservative behaviour of Tl in seawater. However, the extremely strong solution complexation of Tl3+ suggests that Tl3+ may be substantially less particle reactive than other Group 13 elements (with the exception of boron). 

The major constituents in seawater are conventionally taken to be those elements present in typical oceanic water of salinity 35 that have a concentration greater than 1 mg kg excluding Si, which is an important nutrient in the marine environment. The concentrations and main species of these elements are presented in Table 1. One of the most significant observations from the Challenger expedition of 1872-1876 was that these major components existed in constant relative amounts. As already explained, this feature was exploited for salinity determinations. Inter-element ratios are generally constant, and often expressed as a ratio to Cl%o as shown in Table 1. This implies conservative behaviour, with concentrations depending solely upon mixing processes, and indeed, salinity itself is a conservative index. 

If the concentration of the measured component is, like salinity, controlled by simple physical mixing, the relationship will be linear (Fig. 6.3). This is called conservative behaviour and may occur with riverine concentrations higher than, or lower than, those in seawater (Fig. 6.3). By contrast, if there is addition of the component, unrelated to salinity change, the data will plot above the conservative mixing line (Fig. 6.3). Similarly, if there is removal of the component, the data will plot below the conservative mixing line (Fig. 6.3). In most cases, removal or input of a component will occur at low salinities and the data will approach the conservative line at higher salinity (Fig. 6.4). Extrapolation of such a quasiconservative line back to zero salinity can provide, by comparison with the measured zero salinity concentration, an estimate of the extent of removal (Fig. 6.4a) or release (Fig. 6.4b) of the component. 

Biological cycling not only removes some ions from surface waters, it also transforms them. The stable form of iodine (I) in seawater is iodate (IOf), but biological cycling results in the formation of iodide (F) in surface waters, because the production rate of the reduced species is faster than the rate of its oxidation. Biological uptake of IOf in surface water results in a nutrient-like profile, contrasting with the conservative behaviour of most halide ions, for example CP and Br . The biological demand for NCp also involves transformation. Phytoplankton take up NO3 and reduce it to the -3 oxidation state (see Box 4.3 Fig. 5.12) for... 

The distributions of Group 17 elements (Fig. 12.12) are most closely comparable to those of the weakly interactive Group 1 elements. F, Cl and Br are conservative elements. The halides interact strongly with a number of Periods 4 and 5 metals between Groups 8 and 15. However, these metals have seawater concentrations very much lower than H, Br, Cl- and F and, thereby, do not significantly influence the distribution and chemical behaviour of the latter elements. 

The chemistry of dissolved metals in seawater can be grouped into three classes, which describe the behaviour of the metal during chemical cycling. These classes—conservative, nutrient-like and scavenged—have been recognized by the shapes of concentration profiles when plotted against depth in the oceans. 

Only 11 elements can be considered major components of seasalt the cations sodium, potassium, magnesium, calcium and strontium, and the anions chloride, sulphate, bromide, hydrogen carbonate (carbonate), borate (borid acid) and fluoride. These major dissolved constituents (concentrations > 1 mg/kg in ocean waters) make up > 99 % of the soluble ionic species of seawater. The elemental ratios are relatively constant throughout the world ocean, and their concentrations change due to the addition or substruction of water only (concept of conservatism ). Therefore, it is possible to characterize the composition by determining only one constituent that is easy to measure and is conservative in its behaviour. An example is chlorinity (Cl, as defined in Section 11.2.4). 

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