Carbonate compensation depth (CCD): Thermodynamics_Radwan

1. Carbonate compensation depth (CCD): Thermodynamics Radwan , Omar 201306050 1

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? 2

3. OUTLINE Introduction Thermodynamics of Carbonate Dissolution Effect of pressure Effect of ion concentration Effect of temperature Effect of amount of dissolved CO 2 Applications for CCD References 3

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? 4

5. Why? In the present-day World Ocean, the CCD level is: a division between pelagic areas where the processes of ore-formation occur and those where this process is either absent or very hindered 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 . 5

6. Thermodynamics of Carbonate Dissolution Mineral phase  G◦f (kJ mol−1) V◦ (cm3mol−1) ρ (gcm−3) β (bar−1 ( α (K−1) CaCO 3 calcite −1128.8±1.4 36.934 2.71 1.367×10−6 1.88×10−5 CaCO 3 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 6

7. Effect of ion concentration Effect of pressure Effect of temperature Effect of amount of Dissolved Co 2 Thermodynamics of Carbonate Dissolution 7

8. Effect of Ion C oncentration CaCO 3 (solid) ⇔ Ca 2+ + CO 3 2- The apparent constant, K’ sp , is related to thermodynamic constants, Ksp , via the total activity coefficients of Ca 2+ and CO 3 2- The saturation state of seawater with respect to the solid is sometimes denoted by the Greek letter omega,  .  = [Ca 2+ ][ CO 3 2- ]/ k’ sp 8

9.  = [Ca 2+ ][ CO 3 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 CaCO 3 should dissolve until the solution concentrations increase to the point where  = 1. Effect of Ion C oncentration 9

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 Ca 2+ and CO 3 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: CaCO 3 (solid) ⇔ Ca 2+ + CO 3 2-  V= V Ca+VCO 3 - VCaCO 3 10

11. The change in partial molal volume for calcite dissolution is negative, meaning that the volume occupied by solid CaCO 3 is greater than the combined volume of the component of Ca 2+ and CO 3 2- in solution. Since with increasing pressure of Ca 2+ and CO 3 2- prefer the phase occupying the least volume, calcite becomes more soluble with pressure (depth) Effect of Pressure 11

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13. Effect of amount of Dissolved Co 2 The dissolution of carbonate in seawater is intimately related to the marine carbon dioxide (CO 2 ) system. CO 2 dissolved in seawater exists in three inorganic forms: CO 2 (aq.) (aqueous CO 2 ) HCO 3 - (bicarbonate ion) CO 3 2- (carbonate ion) HCO 3 dominates (90 %), followed by CO 3 2- .CO 2 represents only a few percent of the total dissolved inorganic carbon in seawater 13

14. It can also been seen from Figure that [HCO 3 - ] is relatively constant for average oceanic pH values. This leads to a valuable rule of thumb; the concentration of carbonate ion, [CO 3 2- ], is inversely related to [CO 2 ]. Effect of amount of Dissolved Co 2 Barker, 2013 14

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  G f , 298 − log K sp Calcite CaCO 3 100.09 2.71 Trigonal −1128842 8.30 aragonite CaCO 3 100.09 2.93 Orthorhombic −1127793 8.12 Vaterite CaCO 3 100.09 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. 18

19. Applications for CCD CaCO 3 (solid) + H 2 O + CO 2 ⇔Ca 2+ + 2CO 3 2- + 2H + biological productivity : the higher the biological productivity, the shallower the CCD photosynthesis : in photosynthesis, plants take up CO 2 from the environment Zeebe and Wolf- Gladrow , 2009 19

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 CO 2  in the water. more soluble at lower temperatures and at higher pressures. more soluble if the concentration of dissolved CO2 is higher. 21

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. 22

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