Aragonite saturation

A significant fraction of the CO2 injected into the atmosphere as a result of fias-sil fuel burning is now dissolving into the surfece ocean. This is causing a decline in seawater pH and O,. A recent modeling effort, shown in Figure 15.13, predicts a precipitous rise in the aragonite saturation horizon by the year 2100, with surfece waters in... 

Aragonite saturation horizon prediotions for the year 2100. Vaiues mapped are A[C03 ] = [CO ] n situ - PO ]aragonite saturation hore positive A[C02-]a indicates supersaturation and... 

I I Extremely Marginal Aragonite Saturation ([Pg.745]

P002 =517ppmv. flarag = aragonIte saturation state. See Eq. 15.8 for a definition of fl. Source After Gulnotte, J. M., et al. (2003). Coral Reefs 22, 551-558. (See companion website for color version.)... 

A reason that there has been so much controversy associated with the relation between the extent of carbonate dissolution occurring in deep sea sediments and the saturation state of the overlying water is that models for the processes controlling carbonate deposition depend strongly on this relation. Hypotheses have ranged from a nearly "thermodynamic" ocean where the CCD and ACD are close to coincident with calcite and aragonite saturation levels (e.g., Turekian, 1964 Li et... 

Chen (1982) pointed out that excess CO2 has caused the aragonite saturation depth in the North Pacific Ocean to rise close to the sea surface. 

Figure 6.10. Aragonite saturation state versus DIC (dissolved inorganic carbon = SCO2) in Checker Reef porewaters (lower points) as opposed to overlying seawater (upper points). (After Tribble, 1990.)...

Feely R.A. and Chen C.-T. A. (1982) The effect of excess CO2 on the calculated calcite and aragonite saturation horizons in the northeast Pacific. Geophys. Res. Lett. 9, 1294-1297. 

Figure 8, A detailed profile of calcite and aragonite saturation states and sediment marker depth in the Northwestern Atlantic Ocean, (Percentages are estimates of the amount of calcite dissolution which must occur to produce a given marker level.)...

Another problem in comparing Berger s (42) results to those of other experiments is that all H2O2 treated pteropod samples suspended below the aragonite saturation depth completely dissolved. Morse (57) found that if the slowest possible... 

Stability. AH calcitic and dolomitic limestones are extremely stable compounds, decomposing only in fairly concentrated strong acids or at calcining temperatures of 898°C for high calcium and about 725°C for dolomitic stones at 101.3 kPa (1 atm). A very mild destabilizing effect is caused by C02-saturated water, as described in the preceding section on solubihty. Aragonite, however, is not as stable as calcite. In sustained contact with moisture,... 

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. 

The saturation state of aragonite (Fig. 24.5), on the other hand, is affected little by temperature. Aragonite remains supersaturated by a factor of about ten (one log unit) over the gamut of analyses. The supersaturation probably arises from the effect of orthophosphate, present at concentrations of about 100 mg kg-1 in Mono Lake water orthophosphate is observed in the laboratory (Bischoff et al., 1993) to inhibit the precipitation of calcite and aragonite. 

Fig. 24.5. Saturation indices of gaylussite ( , ) and aragonite (o, ) calculated for Mono Lake water in various years from analyses in Table 24.2. Open symbols ( , o) represent values calculated for 0 °C and solid symbols ( , ) show those for 25 °C.

Most water masses are not at equilibrium with respect to either calcite or aragonite. The degree to which a water mass deviates from equilibrium for a particular mineral type can be expressed as its degree of saturation (fi), which is defined as ... 

The saturation state of seawater can be used to predict whether detrital calcite and aragonite are thermodynamically favored to survive the trip to the seafloor and accumulate in surfece sediments. Any PIC or sedimentary calcium carbonate exposed to undersaturated waters should spontaneously dissolve. Conversely, PIC and sedimentary calcium carbonate in contact with saturated or supersaturated waters will not spontaneously dissolve. Typical vertical trends in the degree of saturation of seawater with respect to calcite and aragonite are shown in Figure 15.11 for two sites, one... 

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. 

Depth in meters of the (a) aragonite and (b) calcite saturation horizons (fi = 1) in the global ooeans. Source-. After Feeley, R. A., et al. (2004). Science 305(5682), 362-366. (See oompanion website for oolor version.)... 

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.)... 

Figure 1. Saturation of lake water with respect to calcite. Double line (C for calcite, A for aragonite) represents the solubility product as a function of temperature...

Increase in water depth will shift the equilibrium and between 3.000 and 4.000 m the ocean starts to become unsaturated with respect to aragonite and calcite due to pressure effects122-124. In contrast, the majority of river and lake waters are unsaturated at all depths, because precipitation of CaC03 will readily take place the moment saturation is exceeded. The solubility products are92 ... 

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