1. Carbonate System and pH
Why study the carbonate system?
Involves carbonic acid – an example of an acid-base reaction
pH of most water controlled by CO2
Can be generalized to other systems: Phosphoric, Sulfuric, Nitric, Silicic etc.
Global warming – C is an important factor
Should think a bit about C distribution in the earth

2. Global Distribution of Karst
Karst ≈ Carbonate outcrops = ~ 20% of terrestrial ice-free earth surface
Karst aquifers provide ~25% of world’s of potable water
Large amount of the global C – How much?

3. Global Carbon Reservoirs
Carbonate minerals comprise largest global C reservoir
Data from Falkowski et al., 2000, Science

4. Why the interest in C? Keeling Curve
Measured increase in atmospheric CO2 concentrations
Fossil fuel combustion, deforestation, cement production
Does this matter?

5. Global Temperatures (CO2 induced?)
Hockey stick: Controversial, but T appears to rise with anthropogenic CO2
12 years since 2000 among the 14 warmest years on record
Does this correlation hold over longer time periods?
Mann et al., 1998, Nature

6. Atmospheric CO2 vs T at Vostok
CO2 correlates with global temperatures at glacial-interglacial time scales
~10 oC variation in T
Increase in atmospheric CO2 since 1957 ≈ glacial-interglacial variations
From Falkowski et al., 2000, Science

7. Global Carbon Reservoirs
Industrialization revolution: transfer fossil C to atmosphere
C in atmosphere, oceans, and terrestrial biosphere closely linked
Do these fluxes also includes fluxes in and/or out of carbonates? ?
Data from Falkowski et al., 2000, Science

8. “Textbook” Global Carbon Cycle
Annual fluxes and reservoirs of C (Pg)
Carbonate rocks shown as isolated. Are they really?
Perturbation
Perturbation
Kump, Kasting, and Crane, 2010, The Earth System

9. IPCC Global Carbon Cycle
Perturbation
Perturbation
Black – fluxes and reservoirs - pre 1750
Red – Anthropogenic induced fluxes
Includes weathering – but limited to silicate minerals
Solomon et al., (eds) IPCC report 2007

10. Weathering and the Carbon Cycle
Silicate weathering and coupled calcite precipitation:
CaSiO3 + 2CO2 + H2O  Ca+2 + 2HCO3- + SiO2
Ca+2 + 2HCO3-  CaCO3 + H2O + CO2
CaSiO3 + CO2 CaCO3 + SiO2 (phytoplankton – rapid sink)
CaSiO3 + CO2 CaCO3 + SiO2 (metamorphism – slow source)
What about carbonate mineral weathering?
Less clear how it may affect atmospheric CO2 concentrations

11. Model
Now – discussion of carbonate mineral weathering by carbonic acid
CO2 dissolves when it comes in contact with water
The amount dissolved depends on fugacity of CO2
At atmospheric pressure (low), assume fCO2 = PCO2 (analogous to low dissolved concentrations)

12. PCO2 may be much higher than atmosphere in certain environments
Multiple sources of CO2
Atmosphere
Respiration
Remineralization of organic matter
Dissolution of carbonate minerals
PCO2 may be much higher than atmosphere in certain environments
E.g. soil gas, vadose zone

13. CO2(g)  CO2(aq) For gas phases, can write a dissolution reaction:
(g) indicates gas partial pressure
(aq) indicates amount dissolved in water
CO2(g)  CO2(aq)

14. aCO2(aq) KH = fCO2(g) Equilibrium constant:
Here KH is Henry’s Law constant
Henry’s law: the amount dissolved is constant at constant T and constant f
For atmospheric pressure of CO2, consider f = P
KH =
fCO2(g)
aCO2(aq)

15. Show in a minute KH is not used much
KH = at 25ºC
= 0.035
E.g., about 3.5% of CO2 in atmosphere is in surface layer of ocean
Total ocean reservoir >> atmospheric reservoir
Show in a minute KH is not used much

16. IPCC Global Carbon Cycle
Perturbation
Perturbation
Black – fluxes and reservoirs - pre 1750
Red – Anthropogenic induced fluxes
Includes weathering – but limited to silicate minerals
Solomon et al., (eds) IPCC report 2007

17. CO2(aq) + H2O = H2CO3* Once CO2 is dissolved it reacts with the water:
Here H2CO3* is the true amount of carbonic acid in the water
CO2(aq) + H2O = H2CO3*

18. Keq = aH2CO3* aCO2(aq)aH2O aCO2(aq) ≈ Where Keq = 2.6 x 10-3 @ 25 C
I.e., aH2CO3 < 0.3% of aCO2(aq)

19. But… reaction kinetics fast:
any change in aCO2(aq) immediately translates to change in aH2CO2
Two reactions are combined
Dissolution of atmospheric CO2 and hydration of CO2(aq)

20. CO2(g) + H2O = H2CO3o  CO2(aq) + H2CO3
Only need to consider the control of PCO2 on the amount of carbonic acid in solution:
Here H2CO3o is sum of mCO2(aq) and mH2CO3*
CO2(g) + H2O = H2CO3o  CO2(aq) + H2CO3

21. Can write an equilibrium constant for dissolution reaction:
Whether H2CO3º is CO2(aq) or H2CO3* doesn’t matter much because of fast kinetics
KCO2 =
aH2CO3º
PCO2(g)

22. KCO2 = = at 25o C
Only about 3% of CO2(g) present is H2CO3º
Most of the H2CO3 is as CO2(aq)
We’ll see that the amount of H2CO3º is very important for water chemistry

23. CO2 units Units commonly reported as ppm by volume: ppmv
Current atmospheric concentration is 383 ppmv
Pre-industrial concentation about 278 ppmv
Annual variation about 6 ppmv

24. Keeling Curve

25. Conversion from ppmv to partial pressure (e.g., atm)
Because CO2 is 383 ppmv of 1 Atm
383/106 Atm
Partial pressure = Atm = Atm
Concentration typically given as Atm = Atm = 316 ppmv

26. On board: Summarize all dissolution reactions
Carbonic acid dissociation
Controls on pH of water