Carbonate System and pH

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 1957-2011 • 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 CO2vsT 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. 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. For gas phases, can write a dissolution reaction: • (g) indicates gas partial pressure • (aq) indicates amount dissolved in water CO2(g) CO2(aq)

14. 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 aCO2(aq) KH = fCO2(g)

15. KH = 10-1.46 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. 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. Where Keq = 2.6 x 10-3 @ 25 C • I.e., aH2CO3 < 0.3% of aCO2(aq) aH2CO3* aH2CO3* Keq = ≈ aCO2(aq)aH2O 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. 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 aH2CO3º KCO2 = PCO2(g)

22. KCO2= 10-1.47 = 0.033 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 • 383/106Atm • Partial pressure = .000383 Atm = 10-3.41Atm • Concentration typically given as 10-3.5Atm = 0.000316 Atm = 316 ppmv

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