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Marine Chemistry II
Environmental Geochemistry DM Sherman, University of Bristol
Carbonate Equilibria and the pH of Seawater
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Atmosphere (748 as CO2)
Lithosphere (50 x 106 as CaCO3; 3870 as Oil/Gas/Coal)
Biosphere
(1600)
Oceans
(37000 as HCO3-)
Volc
anic
Em
issi
on
(0.0
48-0
.18)
CO2 Dissolution/Degassing (100) Respiration (60) Photosynthesis (120)
The Global Carbon Cycle (in 1015 g)
Soil (1200-1600) 0.2
Res
pira
tion
(60)
Rivers (0.6)
Speciation of H2CO3
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Open Carbonate System (fixed PCO2= 10-3.5 bar)
CO2(g) + H2O = H2CO3 pK = 1.46
p H2CO3 = pK1+ pPCO2 (If pPCO2 = 3.5 then, pH2CO3 = 4.94)
H2CO3 +H2O = H3O+ + HCO3- pK = 6.35
pHCO3- = pK2 + pH2CO3 - pH =11.29 - pH
HCO3- +H2O = H3O+ + (CO3)2- pK = 10.33
p(CO3)2- = pK3 + pHCO3- - pH = 21.62 - 2pH
Resulting pH of Rain...The charge-balance constraint [H+] = [HCO3-] +2[CO3-2] gives pH pHCO3- = 5.6
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Effect of CaCO3 (Calcite/Aragonite)
If the system is saturated in calcite, we need the additional equilibrium:
CaCO3(s) = Ca2+ + (CO3)2- pK = 8.48
pCa2+ = pK4- p(CO3)2- + 2pH
= -13.14 + 2pH
Charge-balance requires that
2[Ca] = 2[CO3-2] + [HCO3-] [HCO3-]
or pCa = pHCO3- + 0.301
pH of Seawater (Surface, pPCO2 = 3.5)When pCa = pHCO3 + 0.301, pH = 8.3
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Actually, its a bit more complicated..When we include all of the other acid-base and ion-pairing equilibria, we find that surface seawater should have a pH near 7.8.
The observed surface pH = 8.3 is a consequence of supersaturation of CaCO3 (SI = 1.2).
Presumably, supersaturation is favored by calcifying organisms as it is easier to grow shells under such conditions.
Predicted Changes in Oceanic pH (Caldeira & Wickett 2003)
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Biological Controls on Dissolved Carbonate
CO2 + 2H2O CH2O + O2
i) CO2 consumption by photosynthesis:
ii)CO2 release (mineralization) by respiration CH2O + O2 CO2 + 2H2O
(Note, the arrows indicate irreversible reactions!)
iii) Biomineralization of CO3-2 Ca+2 + CO3-2 = CaCO3(calcite)
CaCO3-Shelled ZooplanktonForaminifera
Pteropods
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CoccolithophoridsSingle-celled phytoplankton with cell walls made of CaCO3 (calcite). Large blooms dominate the ocean surface.
Map of near-surface marine calcite (from NASA)
pH of Seawater vs. Depth
Respiration decreases pH: CH2O + O2 CO2 + H2O CO2 + H2O = H+ + HCO3-
Dissolution of CaCO3: CaCO3 + H+ = Ca2+ + HCO3-
Photosynthesis increases pH: H+ + HCO3- = CO2 + H2O CO2 + H2O CH2O + O2
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Alkalinity and Total Dissolved Carbon
[Alk] = [HCO3-] + 2[CO3-2] + [OH-] - [H+]
Alkalinity is the capacity of a solution to neutralize added acid.
Carbonate Alkalinity is
[HCO3-] + [CO3-2] + [H2CO3]
Total Dissolved Inorganic Carbon (CO2) is
Effect of Photosynthesis
CO2 + H2O CH2O + O2
Photosynthesis will not affect alkalinity but will increase the pH:
H2CO3 = CO2 + H2O
H+ + HCO3- = H2CO3
H+ + HCO3- CH2O + O2
The pH increase will favor pptn of CaCO3:
Ca+2 + HCO3- = CaCO3 + H+
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Effect of Respiration
CH2O + O2 CO2 + H2O
Respiration will not affect alkalinity but will decrease pH:
CO2 + H2O = H2CO3
H2CO3 = HCO3- + H+
CH2O + O2 H+ + HCO3-
The pH decrease will favor dissolution of CaCO3:
CaCO3 + H+ = Ca+2 + HCO3-
Effect of CaCO3 precipitation
Ca+2 + CO3- CaCO3
Ca+2 + 2HCO3- CaCO3 + CO2 + H2O
HCO3- = H+ + CO3-
H+ + HCO3- = H2CO3
H2CO3 = CO2 + H2O
Hence, pptn of CaCO3 actually releases CO2, decreases the alkalinity, and decreases pH.
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Shallow vs. Deep Ocean
Shallow vs. Deep Ocean
Surface Ocean
Deep Ocean
Downwelling
Upwelling Sinking Particles
Sinking calcite shells and organic matter provides a one-way flux of C to the deep ocean.
Oxidation of organic C and dissolution of CaCO3 will increase CO2 in Deep Ocean.
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Effect of Biological Processes
CaCO3 pptn
Photosynthesis
Distribution of Carbonate Sediments
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Carbonate Depth Profile (Mid Atlantic)
Surface waters are super- saturated with CaCO3.
Deep waters are under- saturated with CaCO3.
Calcite Compensation Depth:
Dissolution Rate > Rain Rate
CaCO3 saturation Depth (Feely et al., 2004)
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Change in Alkalinity/TDC in Oceans
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Absorption of Atmospheric CO2 by the Oceans
CO2 + H2O + CO3-2 = 2HCO3-
H+ + HCO3- CH2O + O2
Ca+2 + 2HCO3- = CaCO3 + CO2 + H2O
CH2O + O2 H+ + HCO3-
CaCO3 + H+ = Ca+2 + HCO3-
Atmospheric CO2
Sinking of CH2O + CaCO3
Gas Absorption
Upwelling
Shallow Ocean: carbon fixing by photosynthesis.
Deep Ocean: mineralization by respiration.
SummaryEquilibrium with atmospheric PCO2 = 10-3.5 bar and saturation in CaCO3 buffers the pH of seawater to 8.3.
CO2 is sequestered in the deep ocean by the biological pump.
Surface waters are supersaturated in calcite; deep waters are undersaturated. The carbonate compensation depth reflects the competition between the kinetics of dissolution and the downward flux of particulate CaCO3.
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Box Model for OceansSurface Ocean
Atlantic
Southern
Indian Pacific
Primary Productivity