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Carbonate compensation depth (CCD): Thermodynamics

Radwan, Omar201306050

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OBIECTIVES

• What is CCD?• Why do we study CCD?• What are the thermodynamics factors affecting Dissolution

of Deep-Sea Carbonates?• How can we use thermodynamics to understand phenomena

accompanying CCD?

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OUTLINE

• Introduction• Thermodynamics of Carbonate Dissolution

– Effect of pressure– Effect of ion concentration– Effect of temperature– Effect of amount of dissolved CO2

• Applications for CCD• References

4 Peterson and Prell,1985

Carbonate compensation depth: the depth at which the rate of carbonate dissolution on the seafloor exactly balances the rate of carbonate supply from the overlying surface waters.

What?

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Why?

• In the present-day World Ocean, the CCD level is:o a division between pelagic areas where the processes of ore-

formation occur and those where this process is either absent or very hindered

o a boundary separating pelagic red clays, which with time may become a raw material (for production of Al, for example) from carbonate sediments

• importance for studying paleoclimate and paleoceanography.

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Thermodynamics of Carbonate Dissolution

Mineral phase G◦f(kJ mol−1)

V◦(cm3mol−1)

ρ(gcm−3)

β(bar−1(

α(K−1)

CaCO3 calcite −1128.8±1.4 36.934 2.71 1.367×10−6 1.88×10−5CaCO3 aragonite −1127.8±1.5 34.15 2.93 1.55×10−6 5.53×10−5

Table shows some of the main thermodynamic and physical parameters for calcite and aragonite at Standard conditions for temperature and pressure.

G◦f is the Gibbs free energy of formation fromV◦ is the mineral molar volumeρ is mineral density β is the coefficient of volume compressibility at constant temperatureα is the coefficient of volume expansion at constant pressure`

Klein et al., 1993

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o Effect of ion concentrationo Effect of pressureo Effect of temperatureo Effect of amount of Dissolved Co2

Thermodynamics of Carbonate Dissolution

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Effect of Ion Concentration

CaCO3 (solid) Ca⇔ 2++ CO32-

• The apparent constant, K’sp, is related to thermodynamic constants, Ksp, via the total activity coefficients of Ca2+ and CO3

2-

• The saturation state of seawater with respect to the solid is sometimes denoted by the Greek letter omega, .

= [Ca2+][ CO32-]/k’sp

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= [Ca2+][ CO32-]/k’sp

• 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 = 1 then the system is in equilibrium and should be stable. – If >1 ; the waters are supersaturated, and the laws of

thermodynamics would predict that the mineral should precipitate removing ions from solution until returned to one.

– If <1, the waters are undersaturated and the solid CaCO3 should dissolve until the solution concentrations increase to the point where = 1.

Effect of Ion Concentration

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Effect of Pressure

• The most important physical property determining the solubility of carbonate minerals in the sea is pressure.

• The pressure dependence of the equilibrium constants is related to the difference in volume V, occupied by the ions of Ca2+ and CO3

2- in solution versus in the solid phase.

• The volume difference between the dissolved and solid phases is called the partial molal volume change, V:

CaCO3 (solid) Ca⇔ 2++ CO32-

V= V Ca+VCO3 - VCaCO3

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• The change in partial molal volume for calcite dissolution is negative, meaning that the volume occupied by solid CaCO3 is greater than the combined volume of the component of Ca2+ and CO3

2- in solution.

• Since with increasing pressure of Ca2+ and CO32- prefer the phase

occupying the least volume, calcite becomes more soluble with pressure (depth)

Effect of Pressure

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Effect of amount of Dissolved Co2

• The dissolution of carbonate in seawater is intimately related to the marine carbon dioxide (CO2) system.

• CO2dissolved in seawater exists in three inorganic forms: o CO2 (aq.) (aqueous CO2)o HCO3

- (bicarbonate ion)o CO3

2-(carbonate ion)

• HCO3 dominates (90%), followed by CO3

2-.CO2 represents only a few percent of the total dissolved inorganic carbon in seawater

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• It can also been seen from Figure that [HCO3

-] is relatively constant for average oceanic pH values.

This leads to a valuable rule of thumb; the concentration of carbonate ion, [CO3

2-], is inversely related to [CO2].

Effect of amount of Dissolved Co2

Barker, 2013

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Effect of Temperature

• The solubility of calcite and aragonite increases with decreasing temperature

Mackenzie and Lerman, 2006

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Applications for CCDMineral Formula Formula wt Density Crystal System Gf, 298 −log Ksp

Calcite CaCO3 100.09 2.71 Trigonal −1128842 8.30aragonite CaCO3 100.09 2.93 Orthorhombic −1127793 8.12

Vaterite CaCO3 100.09 2.54 Hexagonal −1125540 7.73

Mackenzie and Lerman, 2006

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Applications for CCD

Southard, 2007MIT OCW

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Applications for CCD

• Q: Why in the eastern part of the equatorial Pacific the CCD is located at a depth of 3400m, which is an extremely shallow level throughout most of the equatorial zone of the World Ocean?

• A: where biological productivity very high

• Q: why in the Cretaceous through to the Eocene the CCD was much shallower globally than it is today?

• A: due to intense volcanic activity during this period atmospheric carbon dioxide concentrations were much higher.

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Applications for CCD

CaCO3(solid) + H2O + CO2 Ca⇔ 2++ 2CO32-+ 2H+

• biological productivity:the higher the biological productivity, the shallower the CCD

• photosynthesis:in photosynthesis, plants take up CO2 from the environment

Zeebe and Wolf-Gladrow, 2009

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Applications for CCD• The depth of the lower boundary of the CCD depends on latitude. In

areas adjacent to polar areas, the depth of the CCD is shallowest:200‒150m for calcite and not more than 100m for aragonite.

Mackenzie and Lerman, 2006

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CONCLUSIONS

• The exact value of the CCD depends on the solubility of calcium carbonate which is determined by temperature, pressure and the chemical composition of the water - in particular the amount of dissolved CO2 in the water. o more soluble at lower temperatures and at higher pressures.o more soluble if the concentration of dissolved CO2 is higher.

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REFERENCES

• Barker, S., 2013. Dissolution of Deep-Sea Carbonates, in: Elias, S.A., Mock, C.J. (Eds.), Encyclopedia of Quaternary Science (Second Edition). Elsevier, Amsterdam, pp. 859–870.

• Bickert, T., 2009. Carbonate Compensation Depth, in: Gornitz, V. (Ed.), Encyclopedia of Paleoclimatology and Ancient Environments, Encyclopedia of Earth Sciences Series. Springer Netherlands, pp. 136–138.

• Klein, C., Hurlbut, J.C.S., Dana, J.D., more, & 0, 1993. Manual of Mineralogy, 21 edition. ed. Wiley, New York.

• Mackenzie, F.T., Lerman, A., 2006. Carbon Dioxide in Natural Waters, in: Carbon in the Geobiosphere — Earth’s Outer Shell —, Topics in Geobiology. Springer Netherlands, pp. 123–164.

• Peterson, L.C., and Prell, W.L., 1985. Carbonate dissolution in recent sediments of eastern equatorial Indian Ocean: Preservation patterns and carbonate loss above the lysocline. Mar. Geol., 64, 259–290.

• Schneider, R.R., Schulz, H.D., Hensen, C., 2006. Marine Carbonates: Their Formation and Destruction, in: Schulz, P.D.H.D., Zabel, D.M. (Eds.), Marine Geochemistry. Springer Berlin Heidelberg, pp. 311–337.

• Southard, John. 12.110 Sedimentary Geology, Spring 2007. (MIT OpenCourseWare: Massachusetts Institute of Technology), http://ocw.mit.edu/courses/earth-atmospheric-and-planetary-sciences/12-110-sedimentary-geology-spring-2007 (Accessed 12 May, 2014).

• Zeebe, R.E., Wolf-Gladrow, D.A., 2009. Carbon Dioxide, Dissolved (Ocean), in: Gornitz, V. (Ed.), Encyclopedia of Paleoclimatology and Ancient Environments, Encyclopedia of Earth Sciences Series. Springer Netherlands, pp. 123–127.

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