1. Carbonate compensation depth
(CCD): Thermodynamics
Radwan, Omar
201306050
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2. 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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3. 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
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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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5. 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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6. 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 1.88×10−5
CaCO3 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 from
V◦ is the mineral molar volume
ρ is mineral density
β is the coefficient of volume compressibility
at constant temperature
α is the coefficient of volume expansion
at constant pressure`
Klein et al., 1993
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7. o Effect of ion concentration
o Effect of pressure
o Effect of temperature
o Effect of amount of Dissolved Co2
Thermodynamics of Carbonate Dissolution
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8. Effect of Ion Concentration
CaCO3 (solid) ⇔ Ca2++ CO3
2-
• 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+][ CO3
2-]/k’sp
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9. = [Ca2+][ CO3
2-]/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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10. 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) ⇔ Ca2++ CO3
2-
V= V Ca+VCO3 - VCaCO3
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11. • 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 CO3
2- prefer the phase
occupying the least volume, calcite becomes more soluble with
pressure (depth)
Effect of Pressure
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12. 12

13. 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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14. • 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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15. Effect of Temperature
• The solubility of calcite and aragonite increases with decreasing
temperature
Mackenzie and Lerman, 2006 15

16. Applications for CCD
Mineral Formula Formula wt Density Crystal System Gf, 298 −log Ksp
Calcite CaCO3 100.09 2.71 Trigonal −1128842 8.30
aragonite CaCO3 100.09 2.93 Orthorhombic −1127793 8.12
Vaterite CaCO3 100.09 2.54 Hexagonal −1125540 7.73
Mackenzie and Lerman, 2006 16

17. Applications for CCD
Southard, 2007
MIT OCW 17

18. 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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19. Applications for CCD
CaCO3(solid) + H2O + CO2 ⇔Ca2++ 2CO3
2-+ 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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20. 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 20

21. 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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22. 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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