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

Global Carbon Reservoirs Carbonate minerals comprise largest global C reservoir Carbonate minerals comprise largest global C reservoir Low-T reactions cause fluxes between reservoirs Low-T reactions cause fluxes between reservoirs Data from Falkowski et al., 2000, Science

Lithospheric reservoir Karst distribution ≈ Carbonate outcrops = ~ 20% of terrestrial ice-free earth surface Karst distribution ≈ Carbonate outcrops = ~ 20% of terrestrial ice-free earth surface

Atmospheric Reservoir? 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 Not steady state Not steady state What are effects of transience? What are effects of transience?

Global Temperatures (CO 2 induced?) Mann et al., 1998, Nature  Hockey stick: Controversial, but T appears to rise with anthropogenic CO 2  13 years since 2000 among the 14 warmest years on record  Does this correlation hold over longer time periods?

Long-term Transience Atmospheric CO 2 vs T at Vostok 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

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 include fluxes in and/or out of carbonates? ?

“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

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

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) CaSiO 3 + CO 2  CaCO 3 + SiO 2 (metamorphism – slow source)

Weathering and the Carbon Cycle Carbonate weathering: Carbonate weathering: Forward reaction - dissolution in terrestrial settings Forward reaction - dissolution in terrestrial settings Reverse reaction - equally rapid in marine settings Reverse reaction - equally rapid in marine settings CO 2(g) + H 2 O + CaCO 3 ↔ Ca HCO 3 -

Modelling carbonate reactions 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)

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

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)

Equilibrium constant: Equilibrium constant: Here K H is Henry’s Law constant Here K H is Henry’s Law constant Henry’s law: at equilibrium, the amount dissolved is linear with f at constant T Henry’s law: at equilibrium, the amount dissolved is linear with f at constant T Again – at atmospheric pressures, f = P Again – at atmospheric pressures, f = P K H = f CO2(g) a CO2(aq)

K H = at 25ºC = K H = at 25ºC = f CO2 = 0.035f CO2 f CO2 = 0.035f CO2 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 But: total ocean reservoir >> atmospheric reservoir But: total ocean reservoir >> atmospheric reservoir Show in a minute K H is used in a different way from simple CO 2 dissolution Show in a minute K H is used in a different way from simple CO 2 dissolution

IPCC Global Carbon Cycle Solomon et al., (eds) IPCC report 2007  Compare reservoir size of oceans to atmosphere Perturbation

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

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

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 Dissolution of atmospheric CO 2 Hydration of CO 2(aq) Hydration of CO 2(aq)

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: Convention used in S & M – other books may differ Convention used in S & M – other books may differ CO 2(g) + H 2 O = H 2 CO 3 * Where: H 2 CO 3 * = CO 2(aq) + H 2 CO 3

Can write an equilibrium constant for dissolution reaction: Can write an equilibrium constant for dissolution reaction: Whether H 2 CO 3 * is CO 2(aq) or H 2 CO 3 doesn’t matter much because of fast kinetics Whether H 2 CO 3 * is CO 2(aq) or H 2 CO 3 doesn’t matter much because of fast kinetics K CO2 = a H2CO3* P CO2(g)

K CO2 = K H = = at 25 o C K CO2 = K H = = at 25 o C Only about 3% of CO 2(g) present is H 2 CO 3 * Only about 3% of CO 2(g) present is H 2 CO 3 *

CO 2 units Units commonly reported as ppm by volume: ppmv Units commonly reported as ppm by volume: ppmv Current atmospheric concentration is nearly 400 ppmv Current atmospheric concentration is nearly 400 ppmv Pre-industrial concentration about 278 ppmv Pre-industrial concentration about 278 ppmv Annual variation about 6 ppmv Annual variation about 6 ppmv

Keeling Curve

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

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