1. Carbonate Solubility
Solubility
Dissolution mechanisms
Dissolution rate expressions
Saturation state in the ocean
T, P, and CO2 release
In situ [CO3–2] and the saturation horizon
Sedimentary evidence of carbonate dissolution
Kinetic and/or thermodynamic controls

2. Carbonate Solubility
CaCO3 <=> Ca+2 + CO3–2
Define satauration state as:
Ώ = IAP / Ksp
With IAP (ion activity product)
= [Ca+2][CO3–2] for the solution,
and with the apparent solubility product:
for a solution in equilibrium with solid CaCO3 (T,P,S)

3. K’sp depends on mineralogy (calcite < aragonite < Mg-calcite,…)
as well as T and P
At equilibrium:
Ώ = IAP / Ksp = [Ca+2][CO3–2] = 1
When Ώ > 1, solution is supersaturated
When Ώ < 1, solution is undersaturated

4. Often, we take [Ca+2] as a constant function of salinity,
and express Ώ in terms of the saturation carbonate ion concentration
so, if [Ca+2] constant,
Define DCO3–2 as ([CO3–2] in situ - [CO3–2]sat)

5. Controls on saturation state:
Pressure and temperature effects on carbonate solubility
Solubility higher in deeper and colder water
Pressure, temperature, and respiration effects on [CO3–2] in situ
[CO3–2] lower in deeper, colder, and “older” water

6. in situ saturometer
pH sensor downstream of a cartridge filled with crushed carbonate; cycle pump, observe pH increase as carbonate dissolves.
Ben-Yakov et al., 74

7. Profile of saturation carbonate ion concentration, based on in situ saturometer (crosses) and calcite lysocline (circles); strong increase in solubility with increasing depth.
Broecker and Peng

8. Calcite solubility as fn. of T and P Ingle, 1975
Broecker and Peng

9. Use observed K’sp in 0-15 cm interval of in situ pore water profiles (Alk + pH => [CO3–2], Ca+2) to estimate solubility.
Sayles, 1985

10. Solubility vs depth for Sayles’ pore water data consistent with lab solubility studies

11. Solubility vs depth comparison.
Broecker and Peng

12. What is the relationship between degree of undersaturation and
dissolution rate?
CaCO3 <=> Ca+2 + CO3–2
Define the mass-normalized dissolution rate:
“k*” is the reaction rate (%/day) ((A/V)*k)
“n” is the reaction order
“*” to account for % CaCO3 in bulk sediments, and for surface area
Can express dissolution rate in terms of carbonate ion concentration
R = k* ([CO3–2]sat - [CO3–2] in situ )n
Where [CO3–2]sat is the saturation carbonate ion concentration (T,P)

13. Morse and Arvidson - High order kinetics consistent with control
of dissolution by one of the surface processes
(adsorption, migration, reaction, migration, desorption)
Hales and Emerson, Sayles and Martin
Shallow depth at which porewaters reach saturation implies low-order
kinetics; otherwise final approach to saturation would take much
longer (deeper).

14. An unresolved puzzle:
Laboratory dissolution studies (Keir, 1980) and
theoretical arguments (Morse) suggest that
k* ~ 1000 / day
and
n = 4.5
strongly non-linear
fast dissolution
really fast at low Ώ
But ~ all porewater dissolution studies give
k* ~ / day (102 to 103 slower)
n = 3, or even 1
(Hales and Emerson, 1997)
when n=4.5 is assumed

15. Recalculate with new constants: n = 3.2
Reassess solubiliy too: n = 1.3
Hales and Emerson

16. Why do we care?
Influences of dissolution rate law include:
vertical profile of dissolution in sediments
impact of CO2 release by benthic decomposition
total dissolution rate
shape of lysocline

17. Saturation state in the ocean
Influence of T, P, and CO2 release
In situ [CO3–2] and the saturation horizon

18. Pressure, temperature, and respiration effects on [CO3–2] in situ .
[CO3–2] lower in deeper, colder, and “older” water.
Aging dominates.

19. How does [CO3–2] respond to changes in Alk or DIC?
CT = [H2CO3*] + [ HCO3–] + [CO3–2]
~ [ HCO3–] + [CO3–2] (an approximation)
Alk = [OH–] + [HCO3–] + 2[CO3–2] + [B(OH)4-] – [H+]
~ [HCO3–] + 2[CO3–2] (a.k.a. “carbonate alkalinity”)
So (roughly):
[CO3–2] ~ Alk – CT
CT ↑ , [CO3–2] ↓ Alk ↑ , [CO3–2] ↑

20. Photosynthesis
CO2 + H2O => “CH2O” + O2
D SCO2 = -1
D Alk = 0
So D [CO3–2] = D Alk - D SCO2 = 0 – (-1) = +1
 [CO3–2] increases, Ώ increases.
Respiration
 “CH2O” + O2 => CO2 + H2O
D SCO2 = +1
So D [CO3–2] = D Alk - D SCO2 = 0 – 1 = -1
 [CO3–2] decreases, Ώ decreases.

21. Zeebe and Wolf-Gladrow

22. Calcification
 Ca+2 + CO3–2 => CaCO3
D SCO2 = -1
D Alk = -2
So D [CO3–2] = -2 – (-1) = -1
 [CO3–2] decreases, Ώ decreases.
Dissolution: 
CaCO3 => Ca+2 + CO3–2
D SCO2 = +1
D Alk = +2
So D [CO3–2] = 2 – 1 = +1
 [CO3–2] increases, Ώ increases.

23. % CaCO3 vs. water depth “lysocline” – onset of dissolution
“calcite compensation depth” –
dissolution rate = rain rate

24. Takahashi and Broecker ’80; GEOSECS data

26. in situ
Aragonite
Calcite saturation

27. High aragonite/calcite ratio in sinking flux:
Another puzzle:
NPac sediment traps – loss of carbonate above the aragonite saturation horizon.
High aragonite/calcite ratio in sinking flux:
0.2 to 20, most > 1
Betzer et al., 1984

28. High-solubility phase(s)? Magnesian-calcite
Carbonate flux in NAtl sediment traps – loss of carbonate above the aragonite saturation horizon.
High-solubility phase(s)? Magnesian-calcite
Milliman et al., 1999

29. Sedimentary evidence of carbonate dissolution
Kinetic and/or thermodynamic controls

30. Sediment evidence for dissolution?

31. Here, core-top aragonite distribution seems to match the depth of the aragonite saturation horizon.

32. Farrell and Prell

34. Adelseck 1977 – lab study of selective dissolution of planktonic foraminifera
Assemblage change only after substantial carbonate dissolution; not a sensitive indicator

35. Berger et al., 1982 – Selective dissolution in EqPac
Differential dissolution: a range of susceptibility
Thermodynamics? (different solubilities?)
Kinetics? (different area/g, crystallinity?)

36. % CaCO3
% Coarse
% whole plankt.
Peterson and Prell (1985) – “composite dissolution index” in core-top sediments of the eastern equatorial Indian Ocean
% benthic
% radiolaria
% whole G menardii

37. Peterson and Prell – a “composite dissolution index” compared with water column saturation state.
What assumption(s) underlies downcore (paleo) application of CDI?

38. Lohmann – size-normalized shell weight.
As individuals grow, the mass:size relationship is consistent.
Within a size class, negative deviations from expected relationship reflect dissolution.
(Shells thin (and lose mass), without a significant change in size.)

39. Size-normalized shell weight suggests dissolution starting well above the calcite saturation horizon.
Downcore (paleo) application thwarted by…?