Oceans major ions

Meybeck M, Ragu A (1996) River Discharges to the Oceans. An assessment of suspended solids, major ions, and nutrients. Environment Information and Assessment Rpt., United Nations Environment Programme. Nairobi, 250 p... 

Let us consider the dissolution-precipitation process in seawater in the following example. The normal concentrations of calcium and of carbonate in the near-surface oceanic waters are about [Ca2+] = 0.01 and [C032-] 2 x lO"4 M. The CaC03 in solution is metastable and roughly 2U0% saturated (1). Should precipitation occur due to an abundance of nuclei, TC032-] will drop to 10-4 M but [Ca2+] will change by no more than 2%. Therefore, the ionic strength of the ionic medium seawater will remain essentially constant at 0.7 M. The major ion composition will also remain constant. We shall see later what the implications are for equilibrium constants. 

Equation 1.2 assumes that the concentration of C is constant throughout the ocean, i.e., that the rate of water mixing is much fester than the combined effects of any reaction rates. For chemicals that exhibit this behavior, the ocean can be treated as one well-mixed reservoir. This is generally only true for the six most abundant (major) ions in seawater. For the rest of the chemicals, the open ocean is better modeled as a two-reservoir system (surface and deep water) in which the rate of water exchange between these two boxes is explicitly accoimted for. 

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. 

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. 

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 major source of solutes and solids to the ocean is via river transport. The only major ion with a direct source associated with hydrothermal input seems to be calcium. The hydrothermal input of DSi is also significant. Volcanic gases are presently contributing a minor amount of HCl and sulfur gases (H2S and SO2). Each of these sources is discussed next with primary focus on how terrestrial chemical weathering provides most of the major ion input the oceans. 

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... 

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. 

The major ions have two main escape routes from the ocean (1) incorporation into sediments or pore water and (2) ejection into the atmosphere as seasalt spray. This spray is caused by bursting bubbles that produce small particles, called aerosols, that range in diameter from 0.1 to 1000 pm. The annual production rate of seasalt aerosols is large, on the order of 5 x lO kg/y, but virtually all of it is quickly returned when the spray fells back onto the sea surfece. A small fraction (about 1%) is deposited on the coastal portions of land masses and carried back into the ocean by river runoff. As shown in Table 21.6, seasalts represent a significant fraction of dissolved solids in river runoff, especially for sodium and chloride. Due to the short timescale of this process, seasalt aerosol losses and inputs are considered by geochemists to be a short circuit in the crustal-ocean-atmosphere fectory. The solutes transported by this process are collectively referred to as the cyclic salts. ... 

What has happened to the bicarbonate and calcium delivered to the ocean by river runoff As described later, these two ions are removed from seawater by calcareous plankton because a significant fraction of their hard parts are buried in the sediment. In contrast, the only sedimentary way out of the ocean for chloride is as burial in pore waters or precipitation of evaporites. The story with sodium is more complicated— removal also occurs via hydrothermal uptake and cation exchange. Because the major ions are removed from seawater by different pathways, they experience different degrees of retention in seawater and uptake into the sediments. Another level of fractionation occurs when the oceanic crust and its overlying sediments move through the rock cycle as some of the subducted material is remelted in the mantle and some is uplifted onto the continents. 

As described in Chapter 21.7, a system of biogeochemical feedbacks act to stabilize the major ion composition of seawater. Some operate on short time-scale cycles, such as calcite compensation, and others operate over longer periods, such as the basalt-carbonate buffer. The linkages in the crustal-ocean-atmosphere fectory that act on the major ions also influence atmospheric CO2 levels and seawater s pH and alkalinity. 

Table 5), and several are now being used, or are potentially useful, for measuring key ocean elements. The most common use of direct potentiometry (as compared with potentiometric titrations) is for measurement of pH (Culberson, 1981). Most other cation electrodes are subject to some degree of interference from other major ions. Electrodes for sodium, potassium, calcium, and magnesium have been used successfully. Copper, cadmium, and lead electrodes in seawater have been tested, with variable success. Anion-selective electrodes for chloride, bromide, fluoride, sulfate, sulfide, and silver ions have also been tested but have not yet found wide application. 

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... 

Table 6.5. water Major ions that define salinity in ocean ... 

Salinity is defined here as the grams of dissolved solids (or inorganic dissolved compounds) per kg of seawater, or parts per thousand (or as a per thou-sand%o). Alternatively, it can be defined as the mg/L or mmol/L of the major ions (i.e., those present in concentrations above 1 ppm). The total concentration of dissolved solids ranges from 7,000 ppm for the Baltic Sea, to an average of 35,000 ppm in large oceans, and up to 40,000 ppm in regions where evaporation is high and inputs are low, such as the Red Sea. 

Interestingly, the relative proportions of the major ions are approximately constant throughout the oceans as observed in Table 6.5. These ions are not very reactive, and as such they are more permanent in solution. As discussed above, NaCl is the major compound present at a typical (although not constant) concentration of 500-560 mmol/L, which accounts for about 75% of the total dissolved solids. The rest of salinity comes mainly from Mg, Ca, and K chlorides and sulfates. 

Meybeck, M., and Ragu, A. (1995). River Discharges to the Ocean An Assessment of Suspended Solids, Major Ions, and Nutrients. United Nations Enviomment Programme, Nairobi. 245pp. 

A relatively constant major ion chemistry of the Precambrian ocean was postulated by Holland... 

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). 

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