Biogeochemistry of the Earth System
QMS512 2015
Lecture 5
Dr Zanna Chase
16 June 2015
Lecture 5: Inorganic carbon chemistry
Outline
–  Inorganic carbon speciation in seawater- conceptual overview
–  Inorganic carbon speciation in seawater- the equations
–  Measurements in the inorganic carbon system
–  Sensitivity of carbon speciation to environmental variables
–  Processes affecting inorganic carbon in the ocean- photosynthesis &
respiration, gas exchange, carbonate precipitation and dissolution
IMAS
2
The ocean contains a large amount of carbon,
which can exchange rapidly with the atmosphere
(~60 x more C than the atmosphere).
IPCC AR5 WG1
Some questions to ponder
1.  Why does the ocean hold so much carbon?
2.  What form/species is the carbon in the ocean?
3.  Could the ocean hold more carbon? How much
more?
4.  Might the ocean hold less carbon? Why would it?
5.  What happens to ocean chemistry as carbon is
absorbed?
Ocean Carbon
Inorganic (~99%) Bicarbonate (HCO3-­‐) → 90% Carbonate ion (CO32-­‐) → 9% Dissolved CO2 (CO2(aq)) → 1% Organic (1%) ParDculate organic carbon (organisms) Dissolved organic carbon (decayed organisms) Carbonic acid (H2CO3) → 0.001% “If you can see it, it doesn’t matter!”
Marine Inorganic Carbon System
Dickson, A.G., Sabine, C.L., ChrisDan, J.R. (Eds.), 2007. Guide to best pracDces for ocean CO2 measurements. PICES Special PublicaDon 3. IOCCP Report No. 8, 191 pp. Marine Inorganic Carbon System
pCO2 w Dickson, A.G., Sabine, C.L., ChrisDan, J.R. (Eds.), 2007. Guide to best pracDces for ocean CO2 measurements. PICES Special PublicaDon 3. IOCCP Report No. 8, 191 pp. Marine Inorganic Carbon System
pCO2 a Air-­‐sea flux pCO2 w Dickson, A.G., Sabine, C.L., ChrisDan, J.R. (Eds.), 2007. Guide to best pracDces for ocean CO2 measurements. PICES Special PublicaDon 3. IOCCP Report No. 8, 191 pp. SURFACE OCEAN PCO2- SARMIENTO AND GRUBER CH 8
9
Air-Sea CO2 Flux
•  Reds: flux from sea to air
•  Blues: flux from air to sea
flux = k(pCO2w – pCO2a)
Takahashi et al 2009
Annual flux represents influence of biology driving undersaturation,
upwelling driving supersaturation, and wind-speed facilitating the
transfer across the interface
Marine Inorganic Carbon System
Alkalinity ≈ [HCO3-­‐]+ 2*[CO32-­‐] Dickson, A.G., Sabine, C.L., ChrisDan, J.R. (Eds.), 2007. Guide to best pracDces for ocean CO2 measurements. PICES Special PublicaDon 3. IOCCP Report No. 8, 191 pp. Marine Inorganic Carbon System
Total alkalinity (TA) = [HCO3-­‐] + 2[CO32-­‐] + [OH-­‐] + [B(OH)4-­‐] -­‐ [H+] ± minor consFtuents Dickson, A.G., Sabine, C.L., ChrisDan, J.R. (Eds.), 2007. Guide to best pracDces for ocean CO2 measurements. PICES Special PublicaDon 3. IOCCP Report No. 8, 191 pp. Marine Inorganic Carbon System
pH = -­‐ log{H+} Dickson, A.G., Sabine, C.L., ChrisDan, J.R. (Eds.), 2007. Guide to best pracDces for ocean CO2 measurements. PICES Special PublicaDon 3. IOCCP Report No. 8, 191 pp. Marine Inorganic Carbon System
ΣCO2 species à DIC Dickson, A.G., Sabine, C.L., ChrisDan, J.R. (Eds.), 2007. Guide to best pracDces for ocean CO2 measurements. PICES Special PublicaDon 3. IOCCP Report No. 8, 191 pp. Marine Inorganic Carbon System
pCO2, TA, DIC, pH Any 2 of the 4 parameters can be used to calculate the other 2 Dickson, A.G., Sabine, C.L., ChrisDan, J.R. (Eds.), 2007. Guide to best pracDces for ocean CO2 measurements. PICES Special PublicaDon 3. IOCCP Report No. 8, 191 pp. Inorganic carbon speciation and pH
In the following we will step through the equations
behind this graph, starting with a simple acid-base
system and moving onto the full carbonate chemistry
Aquatic Acid-Base Reactions
Nomenclature:
HBa is the acid (hydrogen ion donor)
Ba- is its conjugate base
H+ is the hydrogen ion
K’ is the (apparent) equilibrium constant
Weak acid: one that does not completely dissociate in water
Aquatic Acid-Base Reactions continued
combining the two equations above (do this!):
Aquatic Acid-Base Reactions continued
combining the two equations above (do this!):
pH = -log10[H+] so…
[H+] = 10(-pH)
→ Bjerrum plot of the concentration of
the species for conditions:
• 
• 
K’ = 10-6
BaT = 10-2 mol/kg
Matlab code used to generate Bjerrum plot
Question: What is the pH of this solution?
ie, what’s the pH of a 10-2 mol/kg solution of HBa, given K = 10-6?
In order to answer this, we need the electroneutrality constraint:
0=Σ+ - Σ[H+] = [Ba-]
Some more about the Bjerrum plot
at pH < pK, the acid dominates
→ Bjerrum plot for conditions:
•  K = 10-6
•  BaT = 10-2 mol/kg
at pH > pK, the base dominates
pH = pK,
acid and basic species are equal
pK = -log10[K]
Summary aquatic acid-base reactions now
including water for completeness
Reactions
Equilibria
Species mole fractions
HBa = [H+] + [Ba-]
ka= [H+][Ba-]/[HBa]
[HBa]/BaT= [H+]/([H+] + ka)
H2O = [H+] + [OH-]
kw= [H+][OH-]/[H2O]
[Ba-] /BaT = ka/([H+] + ka)
Conservation
Electroneutrality
BaT = [HBa]+[Ba-]
0=Σ+ - Σ[H+] = [Ba-] (ignore water)
[H+] = BaTka/([H+] +ka)
[H+]2 + ka [H+] – BaTka = 0 (derive this!)
Recall:
[H2O] = = 1
pkw = 14
pH = -log10[H+] = 7
solve by finding roots of quadratic equation
[H+] = [Ba-] + [OH- ] (with water)
[H+] = BaTka/([H+] +ka) +kw/[H+]
solve by iteration
Aquatic acid-base reactions: seawater carbon
K0
Henry’s law
CO 2 (g)⇔ CO 2
K1
K2
CO 2 + H 2 O⇔ HCO + H ⇔ CO32− + H 2+
−
3
+
[HCO−3 ][ H + ]
K =
[CO 2 ]
recall:
HCO3- bicarbonate
CO32- carbonate
*
1
K 2* =
2−
3
+
2 equilibrium equations
[CO ][ H ]
[HCO−3 ]
DIC≡ Σ[CO 2 ]+[HCO−3 ]+[CO32− ]
CA =[HCO−3 ]+2[CO32− ]
1 carbon mass balance equation
DIC is ‘dissolved inorganic carbon’
1 charge mass balance equation (alkalinity)
CA is carbonate alkalinity
carbon speciation as a function of pH
Aquatic Acid Base Reactions seawater carbon cont
The equilibrium constants describing these reactions are:
K0
=
[CO2]/fCO2
K1
=
[H+] [HCO3-] / [CO2]
K2
=
[H+] [CO3=] / [HCO3-]
The constants are determined experimentally and are T and S (and pressure)
dependent.
Ulf Ribesell
Ulf Ribesell
The master variables of the carbonate system in seawater are:
DIC, Alk, pCO2 and pH.
DIC and Alk are conservative, invariant with T,S,P
Any 2 parameters are sufficient to calculate all components.
DIC deep ocean value: 2280 umol/kg
ALK deep ocean value: 2380 umol/kg
For example
measure DIC and [H+] (ie pH) can determine the other components as follows:
104 µmol kg-1 (5%)
1818 µmol kg-1 (87%)
272 µmol kg-1 (13%)
Conditions: pH = 8.1 and DIC = 2100 µmol kg-1; atm pCO2 = 365 µatm at Salinity = 35
and T = 25C
Alkalinity: Charge Balance for seawater
Ulf Ribesell
Ulf Ribesell
Some useful approximations:
DIC ≈ [HCO3-] + [CO3=]
Alk ≈ [HCO3-] + 2[CO3=]
Combine these approximations to get the following
approximations (good to ~10%):
[CO3=] ≈ Alk - DIC
[HCO3-] ≈ 2DIC - Alk
[CO3=] ≈ Alk - DIC
Another approximation:
[HCO3-] ≈ 2DIC - Alk
temperature
dependent: increases
with increasing
temperature
- 2
3
23
K 2 [HCO ]
pCO 2 =
K 0 ⋅ K1 [CO ]
K 2 (2 ⋅ DIC − Alk)
pCO 2 ≈
K 0 ⋅ K1 Alk − DIC
2
So partial pressure of CO2 in water increases with increasing temperature,
and increasing DIC and decreases with increasing alkalinity
Programs to calculate carbon system:
-  CO2SYS
-  seacarb
-  csys
-  swco2
-  CO2calc
-  ODV
-  mocsy
e.g. given 2 of pCO2, pH, DIC or Alkalinity, as well as pressure,
temperature, salinity, calculate full carbon speciation at equilibrium
You will get to play with CO2SYS during the lab
Vary temperature
Alkalinity = 2300 uM
DIC = 2100 uM
Salinity = 35 g/kg
Vary Salinity
Alkalinity = 2300 uM
DIC = 2100 uM
Temp = 10C
Vary DIC hold all else constant
As DIC increases, pCO2 increases and pH decreases
DIC (Dissolved Inorganic Carbon) = [HCO3-] + [CO3=] + [CO2]
Vary alkalinity hold all else constant
As alkalinity increases, pCO2 decreases and pH increases
Alkalinity = charge imbalance of major irons (negative)
Alkalinity ≈ [HCO3-] + 2[CO32-]
How and why does carbon vary in the ocean?
Summary major effects on the carbonate system
CO2 invasion has no impact on
alkalinity
Photosynthesis removes DIC and
slightly increases alkalinity (sunlit
surface water)
Respiration adds DIC and slightly
removes alkalinity (dark
subsurface)
Calcification removes alkalinity and
DIC in ratio of 2:1 (sunlit surface
mostly)
DIC = dissolved inorganic carbon
= sum of all carbon species
Carbonate dissolution adds
alkalinity and DIC in a ratio of 2:1
(mostly at depth)
Summary
• 
Carbon dioxide reacts with seawater and this is responsible for the large storage of
carbon in the ocean
• 
The seawater inorganic carbon system is represented by a set of equilibrium acidbase reactions, the total DIC (dissolved inorganic carbon) and the charge balance
(alkalinity). These determine seawater pH and the relative abundance of carbon
species
• 
Gaseous CO2, pCO2, is the key for exchange of carbon with the atmosphere
• 
pCO2 increases with increasing temperature and decreases with increasing alkalinity
• 
If any two of DIC, Alkalinity, pH, and pCO2 are measured, the inorganic carbon
system can be fully specified
• 
Calculations are typically done using software packages, such as co2sys
• 
Invasion and evasion of CO2, organic matter production and remineralisation, and
carbonate precipitation and dissolution are important ocean processes affecting the
inorganic carbon system