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Carbonate System and pH Why study the carbonate system? Why study the carbonate system? Involves carbonic acid – an example of an acid-base reaction Involves.

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Presentation on theme: "Carbonate System and pH Why study the carbonate system? Why study the carbonate system? Involves carbonic acid – an example of an acid-base reaction Involves."— Presentation transcript:

1 Carbonate System and pH Why study the carbonate system? Why study the carbonate system? Involves carbonic acid – an example of an acid-base reaction Involves carbonic acid – an example of an acid-base reaction pH of most water controlled by CO 2 pH of most water controlled by CO 2 Can be generalized to other systems: Phosphoric, Sulfuric, Nitric, Silicic etc. Can be generalized to other systems: Phosphoric, Sulfuric, Nitric, Silicic etc. Global warming – C is an important factor Global warming – C is an important factor Should think a bit about C distribution in the earth 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 ≈ Carbonate outcrops = ~ 20% of terrestrial ice-free earth surface Karst aquifers provide ~25% of world’s of potable water Karst aquifers provide ~25% of world’s of potable water Large amount of the global C – How much? Large amount of the global C – How much?

3 Global Carbon Reservoirs Carbonate minerals comprise largest global C reservoir 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 CO 2 concentrations Measured increase in atmospheric CO 2 concentrations Fossil fuel combustion, deforestation, cement production Fossil fuel combustion, deforestation, cement production Does this matter? Does this matter?

5 Global Temperatures (CO 2 induced?) Mann et al., 1998, Nature  Hockey stick: Controversial, but T appears to rise with anthropogenic CO 2  12 years since 2000 among the 14 warmest years on record  Does this correlation hold over longer time periods?

6 Atmospheric CO 2 vs T at Vostok CO 2 correlates with global temperatures at glacial-interglacial time scales CO 2 correlates with global temperatures at glacial-interglacial time scales ~10 o C variation in T ~10 o C variation in T Increase in atmospheric CO 2 since 1957 ≈ glacial- interglacial variations Increase in atmospheric CO 2 since 1957 ≈ glacial- interglacial variations From Falkowski et al., 2000, Science

7 Global Carbon Reservoirs Data from Falkowski et al., 2000, Science  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? ?

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

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

10 Weathering and the Carbon Cycle Silicate weathering and coupled calcite precipitation: Silicate weathering and coupled calcite precipitation: CaSiO 3 + 2CO 2 + H 2 O  Ca HCO SiO 2 Ca HCO 3 -  CaCO 3 + H 2 O + CO 2 CaSiO 3 + CO 2  CaCO 3 + SiO 2 (phytoplankton – rapid sink)  What about carbonate mineral weathering?  Less clear how it may affect atmospheric CO 2 concentrations CaSiO 3 + CO 2  CaCO 3 + SiO 2 (metamorphism – slow source)

11 Model Now – discussion of carbonate mineral weathering by carbonic acid Now – discussion of carbonate mineral weathering by carbonic acid CO 2 dissolves when it comes in contact with water CO 2 dissolves when it comes in contact with water The amount dissolved depends on fugacity of CO 2 The amount dissolved depends on fugacity of CO 2 At atmospheric pressure (low), assume f CO2 = P CO2 (analogous to low dissolved concentrations) At atmospheric pressure (low), assume f CO2 = P CO2 (analogous to low dissolved concentrations)

12 Multiple sources of CO 2 Multiple sources of CO 2 Atmosphere Atmosphere Respiration Respiration Remineralization of organic matter Remineralization of organic matter Dissolution of carbonate minerals Dissolution of carbonate minerals P CO2 may be much higher than atmosphere in certain environments P CO2 may be much higher than atmosphere in certain environments E.g. soil gas, vadose zone E.g. soil gas, vadose zone

13 For gas phases, can write a dissolution reaction: For gas phases, can write a dissolution reaction: (g) indicates gas partial pressure (g) indicates gas partial pressure (aq) indicates amount dissolved in water (aq) indicates amount dissolved in water CO 2(g)  CO 2(aq)

14 Equilibrium constant: Equilibrium constant: Here K H is Henry’s Law constant Here K H is Henry’s Law constant Henry’s law: the amount dissolved is constant at constant T and constant f Henry’s law: the amount dissolved is constant at constant T and constant f For atmospheric pressure of CO 2, consider f = P For atmospheric pressure of CO 2, consider f = P K H = f CO2(g) a CO2(aq)

15 K H = at 25ºC K H = at 25ºC = = E.g., about 3.5% of CO 2 in atmosphere is in surface layer of ocean E.g., about 3.5% of CO 2 in atmosphere is in surface layer of ocean Total ocean reservoir >> atmospheric reservoir Total ocean reservoir >> atmospheric reservoir Show in a minute K H is not used much Show in a minute K H is not used much

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

17 Once CO 2 is dissolved it reacts with the water: Once CO 2 is dissolved it reacts with the water: Here H 2 CO 3 * is the true amount of carbonic acid in the water Here H 2 CO 3 * is the true amount of carbonic acid in the water CO 2(aq) + H 2 O = H 2 CO 3 *

18 Where K eq = 2.6 x C Where K eq = 2.6 x C I.e., a H2CO3 < 0.3% of a CO2(aq) I.e., a H2CO3 < 0.3% of a CO2(aq) K eq = a H2CO3* a CO2(aq) a H2O a H2CO3* a CO2(aq) ≈

19 But… reaction kinetics fast: But… reaction kinetics fast: any change in a CO2(aq) immediately translates to change in a H2CO2 any change in a CO2(aq) immediately translates to change in a H2CO2 Two reactions are combined Two reactions are combined Dissolution of atmospheric CO 2 and hydration of CO 2(aq) Dissolution of atmospheric CO 2 and hydration of CO 2(aq)

20 Only need to consider the control of P CO2 on the amount of carbonic acid in solution: Only need to consider the control of P CO2 on the amount of carbonic acid in solution: Here H 2 CO 3 o is sum of m CO2(aq) and m H2CO3* Here H 2 CO 3 o is sum of m CO2(aq) and m H2CO3* CO 2(g) + H 2 O = H 2 CO 3 o  CO 2(aq) + H 2 CO 3

21 Can write an equilibrium constant for dissolution reaction: Can write an equilibrium constant for dissolution reaction: Whether H 2 CO 3 º is CO 2(aq) or H 2 CO 3 * doesn’t matter much because of fast kinetics Whether H 2 CO 3 º is CO 2(aq) or H 2 CO 3 * doesn’t matter much because of fast kinetics K CO2 = a H2CO3º P CO2(g)

22 K CO2 = = at 25 o C K CO2 = = at 25 o C Only about 3% of CO 2(g) present is H 2 CO 3 º Only about 3% of CO 2(g) present is H 2 CO 3 º Most of the H 2 CO 3 is as CO 2(aq) Most of the H 2 CO 3 is as CO 2(aq) We’ll see that the amount of H 2 CO 3 º is very important for water chemistry We’ll see that the amount of H 2 CO 3 º is very important for water chemistry

23 CO 2 units Units commonly reported as ppm by volume: ppmv Units commonly reported as ppm by volume: ppmv Current atmospheric concentration is 383 ppmv Current atmospheric concentration is 383 ppmv Pre-industrial concentation about 278 ppmv Pre-industrial concentation about 278 ppmv Annual variation about 6 ppmv Annual variation about 6 ppmv

24 Keeling Curve

25 Conversion from ppmv to partial pressure (e.g., atm) Conversion from ppmv to partial pressure (e.g., atm) Because CO 2 is 383 ppmv of 1 Atm Because CO 2 is 383 ppmv of 1 Atm 383/10 6 Atm 383/10 6 Atm Partial pressure = Atm = Atm Partial pressure = Atm = Atm Concentration typically given as Atm = Atm = 316 ppmv Concentration typically given as Atm = Atm = 316 ppmv

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


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