Seawater constituents

Chapter 3 discusses the analysis of seawater constituents and reiterates the scientific problems that necessitate a number of seawater-based reference materials. 

Elements considered in seawater speciation calculations can be separated into major and minor components. Such a separation is possible because the vast majority of seawater constituents have concentrations so low that they do not significantly influence the activities of the major cations and anions in seawater. As such, the equilibrium behaviour of the major ions in seawater can be understood (calculated) independently of the numerous minor constituents and these results can then be applied to calculations involving individual minor constituents. 

Because of this behaviour, individual seawater constituents can be utilised for source apportionment studies in non-marine environments. For instance, an enrichment factor EF) for a substance X is defined as... 

Another practical consideration when dealing with the seawater carbonic acid system is that in addition to carbonate alkalinity, H and OH , a number of other components can contribute to the total alkalinity (TA). The seawater constituent that is usually most important is boric acid. Under most conditions, boric acid contributes — 0.1 mmol alkalinity it is usually taken into consideration when making calculations. Nutrient compounds, such as ammonium, phosphate, and silica, whose concentrations in seawater are highly variable, can also influence alkalinity. They must be taken into account for very precise work. In anoxic pore waters a number of compounds, such as hydrogen sulfide and dissolved organic matter, can be significant contributors to alkalinity (e.g., see Berner et al, 1970). 

This variability for the reported concentrations of mercury in open ocean waters may indicate that there are significant analytical diflBculties associated with the proper sample collection and the accurate measurement of mercury in seawater. These problems tend to override and preclude precise geochemical calculations and marine geochemical interpretations regarding the sources, sinks, and interactions of mercury in the oceans. These observational discrepancies for trace seawater constituents such as mercury can be resolved only through intercalibration programs and the use of standardized seawater samples. Such standards are not presently available for mercury concentrations at 100 ng Hg/1. or less in seawater. 

The distribution of salinity in surface waters of the ocean is presented in Fig. 1.1. Because the concentrations for many major seawater constituents are unaffected by chemical reaction on the time scale of ocean circulation, local salinity distributions are controlled by a balance between two physical processes, evaporation and precipitation. This balance is reflected by low salinities in equatorial regions that result from extensive rain due to rising atmospheric circulation (atmospheric lows) and high salinities in hot diy subtropical g5Tes that flank the equator to the north and south (20-35 degrees of latitude) where the atmospheric circulation cells descend (atmospheric highs). 

Seawater Constituent (mmol I-1) Hydrothermal fluids (mmoll-1) A (mmoll-1)... 

The carbon content of the oceans is more than 50 times greater than that of the atmosphere. Over 95% of the oceanic carbon is in the form of inorganic dissolved species, bicarbonate (HCOj) and carbonate (CO3-) ions the remainder exists in various forms of organic carbon (Druffel et al. 1992). Oceanic uptake of C02 involves three steps (1) transfer of C02 across the air-sea interface, (2) chemical interaction of dissolved C02 with seawater constituents, and (3) transport to the deeper ocean by vertical mixing processes. Steps 2 and 3 are the rate-determining processes in the overall transfer of C02 from the atmosphere to the ocean, and oceanic transport and mixing processes are the primary uncertainties in predicting the rate of oceanic uptake of C02. 

To obtain global estimates of photochemical fluxes, many investigators assume that the absorption due to CDOM, rtcDOMj dominates the absorption of all other seawater constituents in the ultraviolet, and thus that (3cdom(/1)/I] (2) 1.While this approximation is reasonable for many coastal waters, it is not clear that this approximation is valid for all oligotrophic waters. This approximation leads to the final expression for flux. 

Thus the formation of OH in seawater seems on careful examination to lead to very rapid oxidation of bromide, but this reactivity is in turn dissipated within a millisecond by reactions with other seawater constituents, apparently involving the carbonate system in a pH-dependent manner. This process, not the second-order self-decay due to Rll observed at high light intensities, undoubtedly predominates in natural sunlight. However, no reactions of the dibromide ion-radical with carbonate species have been reported, and the nature of this interaction is not clear. The simplest plausible reaction might be ... 

Two other source terms need Co be considered. First, there Is the possibility of dry or wet deposition of a chemical species to the water surface from the atmosphere. The rate of dry deposition depends on Che concentration of Che species In Che gas phase, the concentration In surface water, Che Henry s Law constant, and the reactivity of Che species with seawater constituents, as well as the wind speed and the aerosol In the air adjacent to the water surface. e rate of wet deposition depends on Che concentration of the species In the precipitation and Che frequency and duration of precipitation events. Little Information is currently available about these processes, although there Is some evidence that a rainstorm can Inject significant quantities of 2 2 upper... 

Group 14 elements (C, Si, Ge, Sn, Pb) have diverse speciation characteristics. C is partitioned dominantly between C032- and HC03-, while for both Si and Ge, uncharged forms are dominant (Si(OH)40 and Ge(OH)40) with lesser concentrations (< 15%) of SiO(OH)3- and GeO(OH)3-. The sparse data available for assessment of SnIV speciation indicate that Sn(OH)40 is dominant over a wide range of pH. The speciation of Pb is apparently unique among seawater constituents in that Pbll is partitioned between chloride complexes and carbonate complexes. [17] The latter are dominant above pH 7.85. 

TABLE 1—Major seawater constituents at (35 parts per thousand (%o) salinity) and [/]. 

Analyser systems are available commercially but may also be constructed from single components according to individual requirements. A typical system for the automated analysis of seawater constituents consists of a sampler, proportioning pump, the analytical manifold (a delay and reaction system), a flow-through spectrophotometer and a data acquisition system. Fig. 10-13 shows a 4-channel CFA system built and used in the Institute of Marine Research in Kiel (Germany). One example manifold (nitrate) including a flow-through-spectrophotometer is displayed in Fig.10-14. 

Because concentrations of chlorinated organics in seawater are extremely low (t)q)ically in the pg/L or fg/L range), their determination involves concentration steps over several orders of magnitude so as to increase the amounts of substance available for determination to a level compatible with the sensitivity of the analytical instrumenL At the same time they must be separated from other, interfering, seawater constituents present at much higher concentrations. 

Note For analyses of compounds more polar than CBs it is important to remove acetonitrile quantitatively. Complete removal is indicated by water droplets appearing on the inner wall of the solvent trap. Residual acetonitrile would seriously impede the extraction with n-hexane of more polar organic seawater constituents from the remaining water phase. 

Table 1. Artificial seawater constituents for a salinity of 35 after Kester et al. (1967).

---