Ocean calcium carbonate solubility

The solubility of calcite and aragonite increases with increasing pressure and decreasing temperature in such a way that deep waters are undersaturated with respect to calcium carbonate, while surface waters are supersaturated. The level at which the effects of dissolution are first seen on carbonate shells in the sediments is termed the lysocline and coincides fairly well with the depth of the carbonate saturation horizon. The lysocline commonly lies between 3 and 4 km depth in today s oceans. Below the lysocline is the level where no carbonate remains in the sediment this level is termed the carbonate compensation depth. 

North Atlantic to 500 m in the North Pacific. This reflects an increasing addition of CO2 to deep waters as meridional overturning circulation moves them from the Atlantic to the Indian and then to the Pacific Ocean. Thus, as a water mass ages, it becomes more corrosive to calcium carbonate. Since aragonite is more soluble than calcite, its saturation horizon lies at shallower depths, rising from 3000 m in the North Atlantic to 200 m in the North Pacific. 

Morse J.W. and Berner R.A. (1979) The chemistry of calcium carbonate in the deep oceans. In Chemical Modelling—Speciation, Sorption, Solubility, and Kinetics in Aqueous Systems (ed. E. Jenne), pp. 499-535. Amer. Chem. Soc., Washington, D.C. 

The effects of pressure on equilibria in the oceans within depth profiles have been studied mostly in relation to the problem of calcium carbonate saturation in this environment (Millero, 1969 Bemer, 1965 Millero and Bemer, 1972 Edmond and Gieskes, 1970). The early calculations by Owen and Brinkley (1941) concerning the effect of pressure upon ionic equilibria in salt solutions have been extended to studies of BaS04 solubility at different depths (Chow and Goldberg, 1960) and to the pressure dependence of sulfate associations (Fisher, 1972). 

The solubility of CaCOs in seawater has been studied extensively because of its great abundance in sedimentary rocks and the ocean. The equation for dissolution of pure calcium carbonate ... 

So with all of these variables—the nature of the salt, nature of the solution, amount of salt, and temperature of the solution—it seems that precipitation and solubility are fairly complicated. Things don t just immediately fall into place. We will nonetheless be revisiting these concepts in many different guises as we proceed because precipitation and solubility factor into a lot of chemical situations. For instance, seashells form when calcium excreted from a sea creature mixes with the carbonate in the water to form calcium carbonate. Eons of seashells collecting on the bottom of the ocean account for the composition of the chalk and limestone that finds its way to our blackboards and sidewalks. 

The water soluble inorganic calcium compounds, most commonly bicarbonate, Ca(HC03)2, leaches permanently to river waters and finally migrates to the ocean. This bicarbonate forms in the reactions of calcium carbonate with carbonic acid. 

Iron is the most abundant transition element in the Earth s crust and, in general, in all life forms. An outline of the distribution of iron in the Earth s crust is shown in Table 1.2. As can be seen, approximately one-third of the Earth s mass is estimated to be iron. Of course, only the Earth s crust is relevant for life forms, but even there it is the most abundant transition element. Its concentration is relatively high in most crustal rocks (lowest in limestone, which is more or less pure calcium carbonate). In the oceans, which constitute 70 percent of the Earth s surface, the concentration of iron is low but increases with depth, since this iron exists as suspended particulate matter rather than as a soluble species. Iron is a limiting factor in plankton growth, and the rich... 

The concentrations of inorganic carbon species in sea water are controlled not only by the chemical reactions outlined above (i.e., eqn [I]) but also by various physical and biological processes, including the exchange of CO2 between ocean and atmosphere the solubility of CO2 photosynthesis and respiration and the formation and dissolution of calcium carbonate (CaC03). 

Si02 is moderately soluble in water (5-75 mg in river water and 4-14 mg L in seawater). The products are then transported in river water to the oceans. There organisms such as foraminifera use calcium carbonate to make shells. Other organ-... 

This consumption of carbonate ion shifts the dissolution equilibrium to the right, increasing the solubility of CaC03. This can lead to partial dissolution of calcium carbonate shells and exoskeletons. If the amount of atmospheric CO2 continues to increase at the present rate, scientists estimate that seawater pH will fell to 7.9 sometime over the next 50 years. While this change might sound small, it has dramatic ramifications for oceanic ecosystems. 

If [CO3"] in the ocean decreases enough, organisms such as plankton and coral with CaC03 shells or skeletons will not survive. Calcium carbonate has two crystalline forms called calcite and aragonite. Aragonite is more soluble than calcite. Different organisms have either calcite or aragonite in their shells and skeletons. 

In this reaction, the calcium ions and bicarbonate are combined in an aqueous medium to form calcium carbonate, which is slightly soluble in water. The calcium carbonate formation in surface waters and its precipitation into the oceanic floor is important in the transference of carbon from surface to deep water [49]. 

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. 

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... 

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