Major ion ratios

A significant amount of seawater is trapped in the open spaces that exist between the particles in marine sediments. This fluid is termed pore water or interstitial water. Marine sediments are the site of many chemical reactions, such as sulfate reduction, as well as mineral precipitation and dissolution. These sedimentary reactions can alter the major ion ratios. As a result, the chemical composition of pore water is usually quite different from that of seawater. The chemistry of marine sediments is the subject of Part 111. 

This is why the salinity of seawater is nearly the same throughout the open ocean, varying by only a few parts per thousand. (As per Figure 3.3, 75% of seawater has a salinity between 34 and 35 %o.) The small degree of spatial variability is a consequence of geographic variations in the balance of evaporation versus precipitation in the surface waters. Recall that these surface waters are the source waters for intermediate and deep water masses. Since shifts in the relative rates of evaporation versus precipitation involve only addition or removal of water, the major ion ratios are unaltered. This is why the major ion ratios do not exhibit little if any spatial differences within the open ocean. 

Bar graphs showing the relative concentrations of the major ions in seawater and river water along with a comparison of some of the major ion ratios in the ocean and river water. The concentrations in the two solutions are listed in Table 2.3. 

The origin of saline formation water in sedimentary basins has been problematical since it was first recognized that basinal fluids typically contain dissolved solids in concentrations considerably in excess of seawater. Vast differences in major-ion ratios quickly dispelled early assiunptions that basinal fluids were connate and represented buried seawater (Chave, 1960). Since then, different mechanisms have been advocated to account for the composition of subsurface water, and indeed, different mechanisms probably operate in basins with different lithologies and different burial histories. In some cases saline formation water may evolve in near isochemical rock—water systems during burial, as increasing temperature and pressure induce reactions which transfer components from the solid to the dissolved state. At the other end of the spectrum, fluid bearing no resemblance to the interstitial burial water may be imported from another part of the basin, or even from outside the basin, for example, by meteoric recharge, and modified by rock—water interaction. 

Table 10-9 The major ions of seawater concentration at 35%o, ratio to chlorinity, and molar concentration"...

Examination of the mass spectrum of P2VPY taken during the maximum decomposition rate reveals the major decomposition products as methylpyridine (93 a.m.u.), protonated vinyl pyridine (106 a.m.u.), and protonated dimer (211 a.m.u.) with ion ratios 74 100 59 respectively. Trimeric and tetrameric protonated species (316 and 421 a.m.u.) are also observed but in relatively small amounts. Protonated ions, rather than the simple monomers and dimers observed for the decomposition of poly(styrene) by MS11, may be created by a mechanism similar to that reported for the decomposition of 2-(4-heptyl)pyridine12 in the mass spectrometer. 

Fig. 3.7. Dyes used in these experiments and the mass-to-charge ratio of the major ion from each that was observed or monitored. Reprinted with permission from G. J. Van Berkel el al. [88].

This standard seawater has since proven problematic because it is based on real seawater, whose conductivity is influenced by concentration variations in the nonconservative ions and subtle fluctuations in the ratios of the major ions. To eliminate these issues, a practical salinity scale (PSS-78) was adopted by international agreement in 1978. As a... 

This constancy in relative ion concentration was first postulated by Alexander Marcet in 1819 and, hence, is known as Marcet s Principle or the Rule of Constant Proportions. Formally stated, it says that regardless of how the salinity may vary from place to place, the ratios between the amounts of the major ions in the waters of the open ocean are nearly constant. ... 

Hydrothermal vents are another source of water entering the ocean. These vents are submarine hot-water geysers that are part of seafloor spreading centers. The hydrothermal fluids contain some major ions, such as magnesium and sulfete, in significantly different ratios than foimd in seawater. The importance of hydrothermal venting in determining the chemical composition of seawater is described in Chapters 19 and 21. 

The major ions are transported across the air-sea interface by the ejection of water droplets from the sea surfece. These droplets result from water turbulence at the sea surface that causes microscopic bubbling. Some of these bubbles burst, ejecting seawater into the atmosphere. Since not all of the salt ions are ejected to the same degree, bursting bubbles can alter the ion ratios in the remaining water. 

The other reason why the average salinity of seawater is 35%o lies in the fundamental chemistry of major ions. For example, the sevenfold increase in the Na /K ratio between river water and seawater (Table 21.8) reflects the lower affinity of marine rocks for sodium as compared to potassium. In other words, the sodium sink is not as effective as the one for potassium. Thus, more sodium remains in seawater, with its upper limit, in theory, being controlled by the solubility of halite. Likewise, the Ca /Mg ° ratio in seawater is 12-fold lower than that of river water due to the highly effective removal of calcium through the formation of biogenic calcite. 

The concentrations of the major inorganic ions in seawater are well known in estuarine and coastal areas as well as in interstitial waters anomalies in their constant ratios may occur. The major cations are Na+, Mg2+, Ca2+, K+ and Sr2+, the major anions Cl-, SCty2-, HCO3", B(OH) ", F" and Br". Ion pairs involving these elements and H+, OH", CO32-, POif3- and SiOz -. Under anoxic conditions the S2- - ion and bi- and polysulphides become important. A summary of the major ion speciation in seawater is given by Kester et al. (1975). 

The LD spectrum of the tridecapeptlde neurotensin (Figure 4), shows several differences. Here fragmentation results almost exclusively from small neutral losses. A protonated molecular ion is still one of the major ions observed (m/z = 1673) and some cationized (K+) fragment species are apparent as well as a small amount of cationized molecular ion (<5%). Resolution has begun to be degraded to some extent, partially due to the decrease in overall ion signal which necessitates transformation of fewer data points to maximize signal to noise ratio. 

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... 

For example, little is known about the isotopic composition of formaldehyde in the atmosphere. Formaldehyde is a chemical intermediate in hydrocarbon oxidation. The carbon (8 C) and hydrogen (8D) isotopic composition of atmospheric formaldehyde is analyzed using continuous flow gas chromatography isotope ratio mass spectrometry." Isotope ratios were measured using GC-IRMS (Finnigan MAT 253 stable isotope ratio mass spectrometer, single-sector field with electron impact ion source and multiple ion collection) with a precision of 1.1 and 50%(lo ) for 8 C and 8D, respectively. The accuracy of the online continuous flow isotope technique was verified by calibrating three aliquots of the gas phase standard via the offline dual inlet IRMS technique. The concentration of formaldehyde in ambient air was determined on IRMS major ion peak areas (i.e., mass 44 for 8 C and mass 2 for 8D)." ... 

The concentrations of the major ions are commonly expressed in mgL they are also reported in meq or peq L , which permits a check of the ionic balance of an analysis the sum of cations (S in eq L ) should equal the sum of anions (2 in eqL ). Dissolved silica is generally not ionized at pH values commonly found in rivers its concentration is usually expressed in mgL or in pmolL . Ionic contents can also be expressed as percent of S " or S (%Q), which simphfies the determination of ionic types. Ionic ratios (C,/C ) in eq eq are also often tabulated (Na /Cl, Ca " /Mg2+, Cl /SO, etc.). As a significant fraction of sodium can be derived from atmospheric sea salt and from sedimentary halite, a chloride-corrected sodium concentration is commonly reported (Na = Na -Cl (in rneqL )). The export rate... 

Ionic ratios and ionic proportion distributions also show clearly that all major ions, except potassium, can dominate in multiple combinations ionic ratios also range over two to three orders of magnitude, and they can be greater or less than unity for all except the Na /K ratio, in which sodium generally dominates. River compositions found in less than 1 % of analyses can be termed rare for analyses from Qi to gio and ggo to Q99,1 propose the term uncommon, from gio to Q25 and 075 to ggo, common, and between Q25 and 075, very common. An example of this terminology is shown in the next section (Figure 2) for dissolved inorganic carbon (DlC). 

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. 

The relative proportions of the major ions in seawater and average river water are quite different. Concentration of river water does not produce ocean water. If average river water were concentrated by evaporation, a variety of minerals would precipitate and the ratio among the elements would change. As... 

The oceans contain about 4.5 billion tons of dissolved uranium, almost a thousandfold of the reasonably assured and estimated terrestrial uranium resources in the western world 101). The concentration of uranium in sea water appears to be remarkably constant at about 3.3 pg/liter 120-122). Very recent measurements of uranium concentrations in sea water samples taken in the Arctic and South Pacific Ocean down to depths of more than 5000 m confirm this mean value 123). However, with increasing salinity of sea water a slight increase of uranium concentration is observed 124). The molar concentration of uranium in sea water is nearly 8 orders of magnitude lower than the total concentration of the major ions 125). Marine uranium displays no detactable deviation from the normal terrestrial U-235/U-238 isotope ratio, 03>126). 

Seawater has remarkably constant relative concentrations of major ions in all the world s oceans. For example the Na+ CF ratio changes by less than 1% from the Arabian Gulf to the Southern Ocean. In the oceans, bicarbonate ions (HCO3) and Ca2+ are biologically cycled (Section 6.4.4), causing vertical gradients in their ratios relative to the other major ions. However, the differences in the ratios to Na+ are small—less than 1% for calcium. 

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