Transcript speciation of CO2 in seawater

Global terrestrial
carbon estimation
• map ecosystem extents
• assume C storage (t/ha) in:
• vegetation
• litter
• soils
LGM terrestrial biosphere:
~750-1500 Gt smaller?
(~35-70% smaller?)
(Crowley, 1995)
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 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)
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
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
DIC speciation and pH
1.0
FRACTION
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 and DIC 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
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
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
Role of seafloor CaCO3:
Carbonate
compensation
DIC
removed
DIC
added
CaCO3 dissolves with:
• low T
• high P
• low [CO32-]
undersaturation
supersaturation
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)