Conservative elements in seawater

The chemical reactivity of minor elements in seawater is strongly influenced by their specia-tion (see Stumm and Brauner, 1975). For example, the Cu ion is toxic to phytoplankton (Sunda and Guillard, 1976). Uranium (VI) forms the soluble carbonate complex, U02(C03)3, and as a result uranium behaves like an unreactive conservative element in seawater (Ku et ah, 1977). 

To within a few percent, conservative elements in seawater have constant concentration salinity ratios. That is, their concentrations are not greatly affected by processes other than precipitation and evaporation the same processes that control salinity in the ocean. This definition is of course operational since the ability to determine the effect of biological and chemical processes on concentration depends on the accuracy and precision of the measurement method. Elements of high concentration tend to be conservative because they are relatively unreactive however, conservative elements are present in all concentration ranges because some of them are both low in crustal abundance and relatively unreactive. There are of course... 

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 relative constancy of major (and many minor) elements in seawater is referred to as the rule of constant proportions or Marcet s principle. These elements are considered to be conservative elements, whereby changes in their concentrations reflect the addition or loss of water through physical processes. The remaining elements in seawater are termed nonconservative because they remain in constant proportion due to biological or chemical processes. 

Because water is a universal solvent, at least some of virtually every element is present as a solute in seawater. As shown in Table 3.1, the most abundant substances in seawater are the major ions (Cl , Na", SO4 , Mg ", Ca ", and K" ). They are present in nearly constant proportions in the open ocean because their concentrations are largely controlled by physical processes associated with water movement, such as transport by currents, mixing via turbulence, evaporation, and rainfall. These solutes are also referred to as conservative ions. Most of the rest of the solutes in seawater are not present in constant proportions because their concentrations are altered by chemical reactions that occur faster than the physical processes responsible for water movement. These chemicals are said to be nonconservative. Though most substances in seawater are nonconservative, they collectively comprise only a small fraction of the total mass of solutes and solids in the ocean. 

In Chapter 4, we saw how conservative chemicals are used to trace the pathway and rates of water motion in the ocean. True conservative behavior is exhibited by a relatively small number of chemicals, such as the major ions and, hence, salinity. In contrast, most of the minor and trace elements display nonconservative behavior because they readily undergo chemical reactions under the environmental conditions found in seawater. The rates of these reactions are enhanced by the involvement of marine organisms, particularly microorganisms, as their enzymes serve as catalysts. Rates are also enhanced at particle interfaces for several reasons. First, microbes tend to have higher growth rates on particle surfaces. Second, the solution in direct contact with the particles tends to be highly enriched in reactants, thereby increasing reaction probabilities. Third, adsorption of solutes onto particle surfaces can create fevorable spatial orientations between reactants that also increases reaction probabilities. 

The horizontal and vertical distributions of trace metals in seawater are determined by their relative rates of supply and removal. In this section, we inspect a variety of concentration profiles and assess the processes responsible for determining their shapes. In the case of the vertical profiles, the trace metals can be classified into one of the following types (1) nutrient, (2) conservative, and (3) scavenged, with some elements exhibiting a mixture of these types. 

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. 

Oxidized iron in equilibrium with even the most unstable of iron minerals has a concentration of about 10 M in seawater at pH 8 (Stumm and Morgan, 1981), and oxidized forms of manganese are nearly insoluble at neutral pH. Due to their poor solubility, these elements are conserved in soils and sediments derived from... 

We define the major ions in seawater (Table 1.4) as those with concentrations greater than 10 pmol kg. Most of the major ions are conservative (exceptions are Sr +, HCO3 and COi ) and these ions make up more than 99.4% of the mass of dissolved solids in seawater. Na+ and Cl account for 86% and Na+, Cfr, SO4, and Mg + make up 97%. Conservative elements with concentrations less than 10 pmolkg are found in rows 5 and 6 of the periodic table where elements with lower crustal abundances occur. 

Classification of the chemical constituents of seawater into conservative, bioactive and adsorbed (Chapter 1) revealed much about the processes that control concentration distributions in seawater of the latter two categories, but little about the conservative elements. Concentrations of the elements that make up most of the salinity of the oceans provide clues to the mechanisms that control their sources and sinks. Thus, the chemical perspective of oceanography revealed by conservative element concentrations is about processes that occur at the ocean boundaries weathering reactions on land, authigenic mineral formation in marine sediments and reactions with the crust at hydrothermal areas. The amount of time some of the dissolved constituents remain in solution before they are removed chemically is very long, suggesting the possibility for chemical equilibrium between seawater and the minerals in the ocean... 

Key nuclear and physical transformations that and its longer-lived daughters undergo in the ocean and atmosphere are illustrated in Fig. 5.20. This cartoon is a simplification that excludes some physical sources and sinks, does not specify the chemical forms of the elements, and ignores daughters vith half lives less than a day. Like most seawater elements, is weathered out of continental rocks and carried by rivers to the ocean, where it occurs in a highly soluble dissolved form or in detrital sedimentary minerals. Because uranium is strongly complexed by CO3 ions, it is relatively inert to particle adsorption, is not readily used by marine biota, and behaves conservatively in seawater. Dissolved which does decay in... 

A direct geochemical check on the particle flux of organic matter from the euphotic zone determined by sediment traps is achieved by using the mass balance of thorium isotopes in surface waters. Since decay systematics and chemistry of the uranium series isotopes were introduced in Chapter 5, we will only briefly reiterate them here. is relatively unreactive in oxic seawater and exists in the ocean as a conservative element, i.e. the concentration normalized to salinity is everywhere the same to within measurement error. decays to which is very reactive to particles and has a relatively short radioactive half life of 24.1 d ... 

Scavenged species are all metals and their residence times in seawater are estimated to be a few hundred years, short in comparison with nutrient and conservative elements (Table 6.9). These rapid removal rates imply that river inputs are... 

The arsenic chemistry of the Krka estuary, Croatia, is very pristine that is, essentially unaffected by arsenic pollution from humans. Total dissolved arsenic concentrations in the estuary waters vary from 0.13 to 1.8 pgL-1 (Table 3.7 (Smedley and Kinniburgh, 2002), 525). The range of concentrations can be readily explained by low-arsenic (0.13 pg E1) river water simply diluting 1.8 pgL-1-arsenic seawater in various proportions (Smedley and Kinniburgh, 2002), (Seyler and Martin, 1991), 525. In this estuary, arsenic (in particular As(V)) behaves conservatively that is, the element is relatively unreactive in at least the short... 

An element that is relatively conservative through water-rock reaction is chlorine in the form of the anion chloride. Chloride is key in hydrothermal fluids, because with the precipitation and/or reduction of SO4 and the titration of HC03"/C03, chloride becomes the overwhelming and almost only anion (Br is usually present in the seawater proportion to chloride). Chloride becomes a key component, therefore, because almost all of the cations in hydrothermal fluids are present as chloro-complexes thus, the levels of chloride in a fluid efiectively determine the total concentration of cationic species that can be present. A fundamental aspect of seawater is that the major ions are present in relatively constant ratios—this forms the basis of the definition of salinity (see Volume Editor s Introduction). Because these constant proportions are not maintained in vent fluids and because chloride is the predominant anion, discussions of vent fluids are best discussed in terms of their chlorinity, not their salinity. 

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