Calcite Ocean

Most natural water systems in contact with calcite (oceans, rivers, lakes, carbonate rock aquifiers) are, however, near equilibrium, and PCO2 dependence cannot be ignored. According to our model, the rate of backward reaction is a significant function of surface pH, and surface pH is determined by calcite equilibrium at the surface PCO2. At the relatively high pH, low PCO2 conditions of most natural waters, the surface pH is least well defined and may depend, in part, on the flux of CO2 between the surface and bulk fluid. 

Oceanic surface water is everywhere supersaturated with respect to the two solid calcium carbonate species calcite and aragonite. Nevertheless carbonate precipitation is exclusively controlled by biological processes, specifically... 

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. 

Figure 7.15 A simple ocean-atmosphere-continent system. Pressure of C02 enhances Ca release from the continental crust (which is assumed to be made of CaSi03) and controls the depth of calcite saturation. Calcite precipitation is therefore controlled by the hypsometric curve, equation (7.4.8), and Pco2-...

Solubility of calcite increases significantly with increasing pressure. In the present ocean which contains 10.3 x 10 3molkg I Ca2+, the depth zs of CaC03 saturation is given by... 

Figure 7.17 Same as Figure 7.16 for pCOl, seawater alkalinity A, runoff [Ca2+] and the fraction F of precipitated calcite which is preserved on the ocean floor. It takes a little less than 10000 years for runoff calcium to neutralize the excess dissolved COz, but calcite precipitation takes much longer to eliminate Ca and carbonate excess. 

According to Equation 48 calcite should precipitate from waters having a Mg/Ca ratio below a certain value, while dolomite should precipitate from waters having a Mg/Ca ratio above that critical value. This rule is obeyed under conditions of precipitation from very slightly supersaturated aqueous solutions like those occurring in certain areas of the ocean. Ocean water is close to equilibrium with both calcite and dolomite (53). 

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 ... 

In the present-day ocean, about half of the PIC exported to oceanic sediments consists of the remains of foraminiferans. These microorganisms are protozoans. They are widespread in the marine environment with some species having a pelagic lifestyle and others benthic. As shown in Figure 15.1a, their calcareous structures have the appearance of a chambered snail shell and are composed of the mineral calcite. Since this hard part is covered by tissue, it is technically a type of skeleton. These detrital remains are referred to as tests or forams. Among the present-day and extinct species of foraminiferans, considerable variation exists in the size, shape, and density of their tests. 

Saturation state of seawater, Cl, with respeot to (a) calcite and (b) aragonite as a function of depth. The dashed vertical line marks the saturation horizon. North Pacific profile is from 27.5°N 179.0°E (July 1993) and North Atlantio profile is from 24.5°N 66.0°W (August 1982) from CDIAC/WOCE database http //cdiac.esd.oml.gov/oceans/CDIACmap.html) Section P14N, Stn 70 and Section A05, Stn 84. Source From Zeebe, R.E. and D. Wolf-Gladrow (2001) Elsevier Oceanography Series, 65, Elsevier, p. 26. 

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. 

Based on thermodynamic considerations, sediments that lie at depths below the saturation horizon should have 0% CaCOj. This then explains why calcareous oozes are restricted to sediments lying on top of the mid-ocean ridges and rises and why the sediments of the North Pacific are nearly devoid of calcite and aragonite. (The low %CaCOj in the sediments of the continental margin is a result of dilution by terrestrial clay minerals.)... 

The model provided in Figure 20.1 is for an ocean basin whose abyssal plains all lie below the CCD. This most closely resembles the conditions in the North Pacific, whereas the rest of the ocean basins have a significant portion of their abyssal plains lying above the CCD, and, hence, contain some calcareous oozes. From a global perspective, calcareous oozes are more abundant than siliceous oozes. This is caused by two phenomena (1) all seawater is undersaturated with respect to opal, whereas all surface waters and 20% of the deep waters are saturated with respect to calcite, and (2) siliceous plankton are dominant only in upwelling areas. 

Abyssal clays are found in greater abundance on the western side of the Atlantic Ocean than on the eastern side. This is due to bottom topography that restricts the flow of North Atlantic Deep Water and Antarctic Bottom Water to the western side of the basin. The lower temperature of the western waters causes the CCD to be somewhat shaUower than on the east side of the basin as calcite solubility increases with decreasing... 

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