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

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?

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

Why the interest in C? Keeling Curve Measured increase in atmospheric CO2 concentrations 1957-2011 Fossil fuel combustion, deforestation, cement production Does this matter?

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

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

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

“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

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

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

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)

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

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)

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)

Show in a minute KH is not used much 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

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

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*

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

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)

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

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)

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

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

Keeling Curve

Conversion from ppmv to partial pressure (e.g., atm) Because CO2 is 383 ppmv of 1 Atm 383/106 Atm Partial pressure = .000383 Atm = 10-3.41 Atm Concentration typically given as 10-3.5 Atm = 0.000316 Atm = 316 ppmv

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