1. Global terrestrial carbon estimation map ecosystem extents
assume C storage (t/ha) in:
vegetation
litter
soils
LGM terrestrial biosphere:
~ Gt smaller?
(~35-70% smaller?)
(Crowley, 1995)

2. 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)

3. 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 to right
cooling (slightly influences K1 & K2)
freshening (slightly influences K1 & K2)
alkalinity:DIC change

4. 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)

5. Conservative alkalinity
What else determines 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

6. The conservative alkalinity excess positive charge is balanced primarily by three non-conservative acid-base systems: DIC, boron, and water
Titration alkalinity
Moles of H+ equivalent to the excess of proton acceptors (bases) over proton donors (acids)
Alk  [HCO3-] + 2[CO32-] + [B(OH)4-] + [OH-] – [H+]
carbonate alk borate alk water alk
DIC therefore shifts to right as conservative alkalinity increases, providing more negative charges

7. DIC speciation and pH
H+ OH-
pH and DIC systems “move together” in terms of charge
DIC buffers pH changes
add strong acid: CO2 forms, consuming H+, hindering pH drop

8. 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
CaCO3
Dissolution: Alk:DIC 2:1
Precipitation: Alk:DIC 2:1
Organic matter
Respiration: Alk:DIC
Photosynthesis: Alk:DIC
CO2 gas
Invasion: Alk:DIC
Evasion: Alk:DIC
Increase Decrease
Lesser increase Lesser decrease

9. Carbonate system parameters
carbonate system can be reduced to four interdependent, measurable parameters:
DIC
alkalinity
pCO2 pH
full characterization requires measurement of only two

10. 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

11. Carbonate compensation Role of seafloor CaCO3: CaCO3 dissolves with:
low T
high P
low [CO32-]
DIC
removed
DIC
added
supersaturation
under-
saturation

12. Carbonate compensation
say, DIC added to deep ocean at start of glaciation
deep ocean equilibrium shifts to left and CO32- drops
seafloor CaCO3 dissolves, releasing Alk:SCO2 in 2:1 ratio
pushes equilibrium back to right until CO32- recovers
since initial SCO2 addition was simple rearrangement within ocean, whole ocean has net Alk:SCO2 gain
at sea surface, this shifts equilibrium to right (CO2 drops)