Calcium concentration ocean

The oceans at this time can be thought of as the solution resulting from an acid leach of basaltic rocks, and because the neutralization of the volatile acid gases was not restricted primarily to land areas as it is today, much of this alteration may have occurred by submarine processes. The atmosphere at the time was oxygen deficient anaerobic depositional environments with internal CO2 pressures of about 10-2-5 atmospheres were prevalent, and the atmosphere itself may have had a CO2 pressure near lO-25 atmospheres. If so, the pH of early ocean water was lower than that of modern seawater, the calcium concentration was higher, and early global ocean water was probably saturated with respect to amorphous silica (—120 ppm). Hydrogen peroxide may have been an important oxidant and formaldehyde, an important reductant in rain water at this time (Holland et al., 1986). Table 10.5 is one estimate of seawater composition at this time. 

Calculation of Calcium and Carbonate Total Molalities. Because of the constancy of composition of seawater, the total ion calcium concentration in seawater can be calculated, in "open ocean seawater samples, directly from high precision salinity measurements, using the relationship (14) ... 

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. 

How well can we presently determine the saturation-horizon depth (where D = 1) for calcite in the sea If we assume that we know the calcium concentration exactly, then the error in D is determined by the errors in and the measured carbonate ion concentration, [CO ]. Mucci (1983) was able to determine repeated laboratory measurements of the apparent solubility product, p, at 1 atm pressure to — 5%, and the pressure dependence at 4 km is known to 10%. These errors compound to 11% in the value of K sp (4 km). Carbonate ion concentrations in the sea are almost always calculated from Ax and Die. Being slightly conservative about accuracy of these values in ocean surveys ( 4p.eqkg for Ax and 2p.molkg for DIG they can be determined with errors about half these values if conditions are perfect), and assuming we know exactly the value of the... 

Table 11-1 lists the concentrations and their ratios to chlorinity considered to be representative of average seawater. With the exception of calcium, all oceanic chlorinity ratios studied show little or no variation with depth. The increase of the Ca IC values in deep waters of most oceans (on average 0.3-0.5 %, with maximum deviation in deep North Pacific waters by as much as 1.3 %) can be explained by (a) calcium extraction from surface waters by biological activity, (b) decomposition of organic material in deeper layers and (c) the increased solubility of calcium carbonate at the lower temperature and higher pressure. 

Chlorine. Nearly all chlorine compounds are readily soluble in water. As a result, the major reservoir for this element in Figure 1 is the ocean (5). Chloride, as noted earHer, is naturally present at low levels in rain and snow, especially over and near the oceans. Widespread increases in chloride concentration in mnoff in much of the United States can be attributed to the extensive use of sodium chloride and calcium chloride for deicing of streets and highways. Ref. 19 points out the importance of the increased use of deicing salt as a cause of increased chloride concentrations in streams of the northeastern United States and the role of this factor in the chloride trends in Lake Ontario. Increases in chloride concentration also can occur as a result of disposal of sewage, oil field brines, and various kinds of industrial waste. Thus, chloride concentration trends also can be considered as an index of the alternation of streamwater chemistry by human development in the industrialized sections of the world. Although chlorine is an essential element for animal nutrition, it is of less importance for other life forms. 

The moles X/moles P in average plankton is given by a, and b is the surface water concentration in phosphorus free water (water stripped of nutrients). In the case of P itself the surface ocean concentration is close to zero, while the deep Pacific has a concentration of 2.5 pM. For N, the N/P ratio of plankton is 16 and the surface water concentration is 0 pM. The predicted deep sea nitrate is 40 pM. The ratio of (deep)/(surface) is greater than 10. For calcium the Ca/P of... 

In addition to climate change, the increased atmospheric concentration of C02 is likely to have wide-spread ecological effects in various environments, since C02 is a physiologically active gas, in plants as well as animals. The acidic nature of C02 will also lead to changes in the chemistry of the ocean s surface, which is in equilibrium with the atmosphere. Once the shift in the oceanic chemical balance becomes significant, it will affect ecosystems. It has been shown, for example, that doubling C02 concentration in the atmosphere will reduce the rate of calcium carbonate deposition in coral reefs by 30-40% (Langdon et al., 2000). 

Tsunogai and Nozaki [6] analysed Pacific Oceans surface water by consecutive coprecipitations of polonium with calcium carbonate and bismuth oxychloride after addition of lead and bismuth carriers to acidified seawater samples. After concentration, polonium was spontaneously deposited onto silver planchets. Quantitative recoveries of polonium were assumed at the extraction steps and plating step. Shannon et al. [7], who analysed surface water from the Atlantic Ocean near the tip of South Africa, extracted polonium from acidified samples as the ammonium pyrrolidine dithiocarbamate complex into methyl isobutyl ketone. They also autoplated polonium onto silver counting disks. An average efficiency of 92% was assigned to their procedure after calibration with 210Po-210Pb tracer experiments. 

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. 

As noted above, it is likely that the calcium input fluxes to the oceans and the outputs fluxes are not always equal. According to Equation (5), this means that the seawater 5 Ca ratio can vary with time. The rapidity with which the seawater 5 Ca can change is dictated by the residence time of calcium in seawater. At present, the residence time is estimated to be about 1 million years (e g., Holland 1978). In the past, the residence time could have been larger or smaller, perhaps by as much as a factor of 5 or even 10, depending on the Ca concentration in seawater (Fig. 12) and the riverine, diagenetic and hydrothermal fluxes of calcium to the oceans. 

The CO2 concentration in the earth s atmosphere is ultimately governed by the calcium carbonate equilibrium in the ocean (e.g., Berner et al. 1983). If the oceans are in equilibrium with calcite, which is usually the case, then to a reasonable approximation, the PCO2 of the atmosphere is defined by the equilibrium ... 

Most cation exchange occurs in estuaries and the coastal ocean due to the large difference in cation concentrations between river and seawater. As riverborne clay minerals enter seawater, exchangeable potassium and calcium are displaced by sodium and magnesium because the Na /K and Mg /Ca ratios are higher in seawater than in river water. Trace metals are similarly displaced. 

Carbonate compensation The ocean s response to perturbations through shifts in its carbonate chemistry. These shifts require changes in the carbonate ion concentration that change the depth of the calcium carbonate compensation depth and hence lead to changes in the burial rate of carbon as biogenic calcium carbonate. 

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