Seawater carbonate concentration

The higher the pH of the seawater the greater will be the proportion of carbonate ions present (since as the pH increases, so the H concentration decreases, thereby moving the equilibrium to the right). It follows that at a surface under cathodic protection, the hydroxyl ion produced has the effect of increasing the local carbonate ion concentration. 

An increase in carbonate-ion concentration moves the equilibrium in favour of calcium carbonate deposition. Thus one secondary effect of cathodic protection in seawater is the production of OH , which favours the production of CO, , which in turn promotes the deposition of CaCOj. Cathodically protected surfaces in seawater will often develop an aragonite (CaCOj) film. This film is commonly referred to as a calcareous deposit. 

Under relevant conditions (neutral pH, presence of carbonate) the concentrations of dissolved trace metals are limited by the solubility of their solid hydroxides or carbonates. A prominent example is the solubility of Fe(III), which is very low (around 10 10 mol dm 3) under relevant conditions of fresh water or seawater at neutral pH [96]. Iron(III) occurs primarily as solid or colloidal iron hydroxide at neutral pH. 

In Chapter 5. we noted that COj readily dissolves in seawater to form several inorganic carbon species. TDIC is defined as the sum of the concentration of those species, i.e., the sum of the carbonate, bicarbonate, carbonic acid, and carbon dioxide concentrations. 

Sampling sites are also referred to as station locations. For water column work, depth profiles are constructed from seawater samples collected at representative depths. Temperature and salinity are measured in situ with sensors. Remote-closing sampling bottles deployed from a hydrowire are used to collect water for later chemical analysis, either on the ship or in a land-based laboratory. The standard chemical measurements made on the water samples include nutrients (nitrate, phosphate, and silicate), dissolved O2, and total dissolved inorganic carbon (TDIC) concentrations. 

Of particular concern are the impacts of seawater acidification on biocalcification and the burial rates of sedimentary carbon. Carbonate ion concentrations in the surface waters have already declined by 16%. Thus, it is not surprising that the abundance of tropical/subtropic planktonic foraminiferan species appears to have declined since the 1960s. This information was obtained by studying the rapidly accumulating sediments of the Santa Barbara Basins off the coast of California. 

Figure 11.1. A conceptual diagram of the size-composition continuum of organic matter in the ocean. The seawater concentration of total organic carbon (TOC) in various size fractions increases with decreasing size of particles, colloids, and dissolved molecules. The percentages of carbon characterized as specific molecules, such as amino acids and neutral sugars, decreases with decreasing size. Most of the organic carbon resides in the ocean as small molecules that have not been structurally characterized.

Care must be taken, however, in pore waters and special areas such as coastal waters, carbonate banks, and lagoons where significant deviations from normal seawater concentration ratios can occur. As an aside, it should also be noted that the apparent constants are not accurate where seawater composition is altered. This composition change can be especially important in anoxic environments where extensive sulfate concentration changes occur. 

Thus, the fossil fuel "neutralizing capacity of seawater is largely dependent on the carbonate ion concentration and that of marine sediments depends on the calcium carbonate which is near enough to the sediment-water interface to react with the overlying water. 

The calculation of the total ion molal carbonate ion concentration is more complex than for calcium because it is part of the carbonic acid system. The following reactions take place between CO and carbonic acid in seawater ... 

The Geochemical Ocean Section Program (GEOSECS) has produced data from which it is possible to profile the saturation state of seawater with respect to calcite and aragonite in the Atlantic and Pacific oceans. Representative north-south calcite saturation profiles for the Western Atlantic and Central Pacific oceans are presented in Figures 5 and 6 (based on 39). It was observed that the saturation state of seawater with respect to calcite at the CCD was close to constant ( 2 = 0.70 I" 0,05) except in the southern extremes (39). Broecker and Takahashi (31) have recently found that the carbonate ion concentration is close to constant at the FL, when appropriate corrections are made for pressure. The saturation state of seawater at the FL, calculated by the method presented in this paper, is 0.80 0.05. Berger (40) has presented profiles for Rq, FL, CCD and CSL (calcite saturation level) in the eastern and western Atlantic ocean (see... 

The numerator of the right side is the product of measured total concentrations of calcium and carbonate in the water—the ion concentration product (ICP). If n = 1 then the system is in equilibrium and should be stable. If O > 1, the waters are supersaturated, and the laws of thermodynamics would predict that the mineral should precipitate removing ions from solution until n returned to one. If O < 1, the waters are undersaturated and the solid CaCOa should dissolve until the solution concentrations increase to the point where 0=1. In practice it has been observed that CaCOa precipitation from supersaturated waters is rare probably because of the presence of the high concentrations of magnesium in seawater blocks nucleation sites on the surface of the mineral (e.g., Morse and Arvidson, 2002). Supersaturated conditions thus tend to persist. Dissolution of CaCOa, however, does occur when O < 1 and the rate is readily measurable in laboratory experiments and inferred from pore-water studies of marine sediments. Since calcium concentrations are nearly conservative in the ocean, varying by only a few percent, it is the apparent solubility product, and the carbonate ion concentration that largely determine the saturation state of the carbonate minerals. 

Figure 2 Saturation carbonate ion concentration for calcite at deep-water temperatures as a function of water depth (i.e., pressure). Curves are drawn for three choices of AF between Ca " " and CO ions when combined in calcite and when dissolved in seawater. The vertical black lines indicate the average CO concentration in the deep equatorial Pacihc and in NADW. The bold segments indicate the onset of dissolution as indicated by in situ experiments carried out in the deep North Pacihc (Peterson, 1966) and in the deep North Atlantic (Honjo and Erez, 1978). As indicated in the upper right-hand corner, estimates of AV cover a wide range.

Table 1 The calcite-saturation carbonate ion concentration in cold seawater and the slope of this solubility as a function of water depth based on a 1 atm solubility of 45 pimol CO kg and a AT of 40 cm mol . ...

Pearsou P. N. aud Pahner M. R. (1999) Middle Eocene seawater pH and atmospheric carbon dioxide concentrations. Science 284(5421), 1824-1826. 

The apparent constants are not those for standard seawater where seawater composition has been significantly altered. Composition changes can be especially important in anoxic environments where extensive sulfate concentration changes are produced. The carbonate ion concentration can be calculated from any two of the four parameters pH, total CO2 (TCO2), the partial pressure of CO2 (pcof) (c-g-> Morse... 

The uranium in seawater is present, depending upon the pH and carbon dioxide concentration, as UO2CO3, [U02(C03)2H20]2- or [U02(C03)3]4-. 

Spero, H.J., J. Bijima, D.W. Lea and B. E. Bemia (1997) Effect of seawater carbonate ion concentration on foraminiferal carbon and oxygen isotopes. Nature 390, 497-500. 

Culturing experiments revealed that the stable isotopic composition of planktic foraminiferal tests responds to changes in seawater carbonate ion concentration (Spero et al. 1997). Recently, this so-called carbonate ion effect (CIE) was applied to interpret deviations of values of planktic... 

The alkalinity and DIC data were used to calculate the carbonate ion concentration and pH of the culture medium during the second half of the first experiment, and throughout the second experiment (Fig. 3). These calculations used the program C02SYS (Lewis Wallace 1998) with the carbonate dissociation constants of Roy et al. (1993), the calcite solubility of Mucci (1983), and the assumption that the boron/salinity ratio of the culture system water was equal to the seawater ratio. Because much of the culture system water in both years was Instant Ocean, it may not be correct to estimate the total borate concentration from the whole-ocean boron/salinity relationship. However, trends in the concentrations of carbonate system species during each year will be independent of the actual absolute total borate concentration. 

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