Carbonate System and pH
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Transcript of Carbonate System and pH
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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
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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?
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Global Carbon Reservoirs
Carbonate minerals comprise largest global C reservoir
Data from Falkowski et al., 2000, Science
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Why the interest in C?Keeling Curve
Measured increase in atmospheric CO2 concentrations 1957-2011
Fossil fuel combustion, deforestation, cement production
Does this matter?
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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?
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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
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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?
?
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“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
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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
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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)
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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)
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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
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For gas phases, can write a dissolution reaction:
(g) indicates gas partial pressure (aq) indicates amount dissolved in
water
CO2(g) CO2(aq)
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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)
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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
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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
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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*
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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)≈
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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)
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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
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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)
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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
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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
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Keeling Curve
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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
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On board: Summarize all dissolution reactions Carbonic acid dissociation Controls on pH of water