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?)

Mann et al., 1998, Nature

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?

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

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?

?

“Textbook” Global Carbon Cycle

Annual fluxes and reservoirs of C (Pg)

Carbonate rocks shown as isolated. Are they really?Kump, Kasting, and Crane, 2010, The Earth System

Perturbation

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

PerturbationPerturbation

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)

What about carbonate mineral weathering? Less clear how it may affect atmospheric CO2 concentrations

CaSiO3 + CO2 CaCO3 + SiO2 (metamorphism – slow source)

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)

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

For gas phases, can write a dissolution reaction:

(g) indicates gas partial pressure

(aq) indicates amount dissolved in water

CO2(g) CO2(aq)

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)

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

Solomon et al., (eds) IPCC report 2007

Black – fluxes and reservoirs - pre 1750 Red – Anthropogenic induced fluxes Includes weathering – but limited to silicate minerals

PerturbationPerturbation

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*

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

Keq =aH2CO3*

aCO2(aq)aH2O

aH2CO3*

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)

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