Glacial atmospheric CO2 lowering must be due
to greater storage in ocean
Modern surface pCO2
(Takahashi et al., 2002)
• at equilibrium, atmospheric pCO2 determined by Henry’s Law
• pCO2 = [CO2] / K0
• need mechanisms to lower [CO2] or raise K0 (solubility)
dissolved inorganic carbon (DIC):
SCO2 = [CO2] + [HCO3-] + [CO32-]
~1%
~90%
~10%
where CO2  CO2(aq) + H2CO3
Therefore we can lower [CO2] by:
• decreasing DIC
• shifting DIC equilibrium away from CO2 (‘to right’)
• cooling (slightly influences K1 & K2)
• freshening (slightly influences K1 & K2)
• alkalinity:DIC change
Temperature & salinity
(effects on K values only)
LGM temperature (colder)
• CO2 more soluble in cold waters (K0)
• DIC also shifts away from CO2 ([CO2])
• could account for -30 ppm
LGM salinity (saltier)
• CO2 less soluble in salty waters (K0)
• DIC also shifts toward CO2 ([CO2])
• could result in +10 ppmv
(Takahashi et al., 2002)
Alkalinity:DIC ratio also affects the speciation of DIC (at
constant T, S)
Electroneutrality
In any solution, the sum of cation charges must balance the
sum of anion charges
Conservative alkalinity
Excess of conservative cations over conservative anions
(conservative: no [ ] change with pH, T, or P)
Alk = S(conserv. cation charges) - S(conserv. anion charges)
= ([Na+] + 2[Mg2+] + 2[Ca2+] + [K+]…) - ([Cl-] + 2[SO42-]…)
 2350 meq/kg
The conservative alkalinity excess positive charge is
balanced primarily by non-conservative anions from three
systems: DIC, boron, and water
Alk  [HCO3-] + 2[CO32-] + [B(OH)4-] + [OH-] – [H+]
carbonate alk
borate alk
water alk
DIC therefore shifts to right (away from CO2) as conservative
alkalinity increases, providing more negative charges
DIC speciation and pH
1 .0
F R A C T IO N
0 .8
CO2
0 .6
HCO3
-
CO3
2-
0 .4
0 .2
0 .0
2
H+
4
6
8
pH
10
12
14
OH-
• pH, DIC, and B systems ‘move together’ in terms of charge
• DIC buffers pH changes
• add strong acid: CO2 forms, consuming H+, hindering pH drop
Conservative alkalinity and DIC together
• increase Alk/DIC: DIC shifts to right (pCO2 drops)
• decrease Alk/DIC: DIC shifts to left (pCO2 rises)
• add Alk/DIC at 1/1: very little change in DIC speciation
CO2 gas
Invasion: Alk:DIC
Evasion: Alk:DIC
Organic matter
Respiration: Alk:DIC
Photosynthesis: Alk:DIC
CaCO3
Dissolution: Alk:DIC 2:1
Formation: Alk:DIC 2:1
Increase
Decrease
Lesser increase Lesser decrease
cold
cold
upwelling
photosynthesis & stratification
Organic matter
Respiration: Alk:DIC
Photosynthesis: Alk:DIC
photosynthesis & stratification
upwelling
cold
upwelling
cold
cold
Takahashi et al. (2002)
Carbonate system parameters
• carbonate system can be reduced to four interdependent,
measurable parameters:
• DIC
• alkalinity
• pCO2
• pH
• full characterization requires measurement of only two
Some useful approximations
DIC  [HCO3-] + [CO32-]
Alk  carbonate alk = [HCO3-] + 2[CO32-]
Therefore:
[HCO3-]  2DIC – Alk
[CO32-]  Alk – DIC
And since:
pCO2 = K2[HCO3-]2 / K0K1[CO32-]
It follows that:
pCO2  K2(2DIC – Alk)2 / K0K1(Alk – DIC)
Using average surface water values:
1% increase in DIC gives ~10% increase in pCO2
1% increase in Alk gives ~10% decrease in pCO2