Carbonate System and pH
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Why study the carbonate system?
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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
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Should think a bit about C distribution in the earth
Global Distribution of Karst
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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
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Data from Falkowski et al., 2000, Science
Carbonate
minerals
comprise
largest global C
reservoir
Why the interest in C?
Keeling Curve
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Measured
increase in
atmospheric CO2
concentrations
1957-2011
Fossil fuel
combustion,
deforestation,
cement
production
Does this matter?
Global Temperatures
(CO2 induced?)
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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
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From Falkowski et al., 2000, Science
CO2 correlates with
global temperatures at
glacial-interglacial
time scales
~10 oC variation in T
Increase in
atmospheric CO2 since
1957 ≈ glacialinterglacial variations
Global Carbon Reservoirs
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?
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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
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Perturbation
Perturbation
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Annual fluxes
and reservoirs
of C (Pg)
Carbonate
rocks shown
as isolated.
Are they
really?
Kump, Kasting, and
Crane, 2010, The Earth
System
IPCC Global Carbon Cycle
Perturbation
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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
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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)
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What about carbonate mineral weathering?
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Less clear how it may affect atmospheric CO2 concentrations
Model
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Now – discussion of carbonate mineral
weathering by carbonic acid
CO2 dissolves when it comes in contact
with water
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The amount dissolved depends on fugacity of
CO2
At atmospheric pressure (low), assume fCO2 =
PCO2 (analogous to low dissolved
concentrations)
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Multiple sources of CO2
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Atmosphere
Respiration
Remineralization of organic matter
Dissolution of carbonate minerals
PCO2 may be much higher than
atmosphere in certain environments
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E.g. soil gas, vadose zone
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For gas phases, can write a dissolution
reaction:
CO2(g)  CO2(aq)
indicates gas partial pressure
(aq) indicates amount dissolved in water
 (g)
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Equilibrium constant:
KH =
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Here KH is Henry’s Law constant
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aCO2(aq)
fCO2(g)
Henry’s law: the amount dissolved is constant
at constant T and constant f
For atmospheric pressure of CO2, consider
f=P
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KH = 10-1.46 at 25ºC
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= 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
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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
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Once CO2 is dissolved it reacts with the
water:
CO2(aq) + H2O = H2CO3*
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Here H2CO3* is the true amount of
carbonic acid in the water
Keq =
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aH2CO3*
aCO2(aq)aH2O
≈
aH2CO3*
aCO2(aq)
Where Keq = 2.6 x 10-3 @ 25 C
I.e., aH2CO3 < 0.3% of aCO2(aq)
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But… reaction kinetics fast:
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any change in aCO2(aq) immediately translates
to change in aH2CO2
Two reactions are combined
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Dissolution of atmospheric CO2 and hydration
of CO2(aq)
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Only need to consider the control of PCO2
on the amount of carbonic acid in
solution:
CO2(g) + H2O = H2CO3o  CO2(aq) + H2CO3
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Here H2CO3o is sum of mCO2(aq) and mH2CO3*
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Can write an equilibrium constant for
dissolution reaction:
KCO2 =
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aH2CO3º
PCO2(g)
Whether H2CO3º is CO2(aq) or H2CO3*
doesn’t matter much because of fast
kinetics
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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
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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
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Conversion from ppmv to partial pressure
(e.g., atm)
Because CO2 is 383 ppmv of 1 Atm
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383/106 Atm
Partial pressure = .000383 Atm = 10-3.41 Atm
Concentration typically given as 10-3.5 Atm
= 0.000316 Atm = 316 ppmv
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On board:
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Summarize all dissolution reactions
Carbonic acid dissociation
Controls on pH of water