1. 1.Acid-base review Carbonate system in seawater 2.Carbonate sediments Dissolution / preservation 3.Pore water evidence of respiration-driven dissolution Pore water carbon isotopes ( 13 C and 14 C of DIC)

2. % CaCO 3 vs. water depth “lysocline” – onset of dissolution “calcite compensation depth” – dissolution rate = rain rate

3. Farrell and Prell

4. Why study benthic carbonate chemistry? Carbonate burial balances continental weathering CaCO 3 records (concentration, accumulation rate) reflect changes in ocean circulation and ocean chemistry, including: - atmospheric CO 2 - upper ocean productivity Carbonate saturation state and carbonate dissolution influence isotopic and elemental records of ocean chemistry and temperature Understand the long-term fate of fossil fuel CO 2

5. Stumm, W. and J. Morgan (1981) Aquatic Chemistry Broecker, W.S. and T.-H. Peng (1982) Tracers in the Sea Zeebe, R.E. and D. Wolf-Gladrow (2001) CO 2 in Seawater: Equilibrium, Kinetics, Isotopes Butler, J.N. (1982) Carbon Dioxide Equilibria and Their Applications

6. Carbonate system - aqueous speciation: CO 2 (g) CO 2 (aq)(solubility) CO 2 (aq) + H 2 O H 2 CO 3 (carbonic acid) H 2 CO 3 HCO 3 – + H + (bicarbonate ion) HCO 3 – CO 3 –2 + H + (carbonate ion)

7. Solid – solution interactions: CaCO 3 Ca +2 + CO 3 –2 (dissolution / precipitation) (calcification) CO 2 + H 2 O “CH 2 O” + O 2 (photosynthesis / respiration) Weathering of continental crust: CaCO 3 + CO 2 + H 2 O => Ca +2 + 2HCO 3 – (Cation,Al)silicate + H 2 CO 3 + H 2 O => HCO 3 – + H 4 SiO 4 + cation + (Al)silicate 2CO 2 + 11H 2 O + 2NaAlSi 3 O 8 => 2Na + + 2HCO 3 – + 4H 4 SiO 4 + Al 2 Si 2 O 5 (OH) 4

8. Solid – solution interactions: CaCO 3 Ca +2 + CO 3 –2 (dissolution / precipitation) (calcification) CO 2 + H 2 O “CH 2 O” + O 2 (photosynthesis / respiration) Weathering of continental crust: CaCO 3 + CO 2 + H 2 O => Ca +2 + 2HCO 3 – (Cation,Al)silicate + H 2 CO 3 + H 2 O => HCO 3 – + H 4 SiO 4 + cation + (Al)silicate 2CO 2 + 11H 2 O + 2NaAlSi 3 O 8 => 2Na + + 2HCO 3 – + 4H 4 SiO 4 + Al 2 Si 2 O 5 (OH) 4 CO 2 addition drives CaCO 3 dissolution

9. Acid – base equilibrium review (S&M ’81): HA H + + A – Define a dissociation constant, K, in terms of activities in solution: where K is a fn. of T and P. Use a mixed equilibrium constant when pH is determined as {H + } and HA and A - as concentrations.

10. A monoprotic acid: Given the equilibrium constant: and defining a concentration condition: C T = [A - ] + [HA] Algebra, to calculate concentrations in solution ([A], [HA]) in terms of C T, K, and {H}

11. Combine: [HA] K ’ HA + [HA]{H + } = C T {H + } Species concentrations in terms of C T, K (T,P), and {H + }

12. Example: Use these relationships to solve for the pH of a pure solution of HA Four unknowns: [HA], [A-], [H+], [OH-]; need four equations. Two equilibrium constants: K W = [H + ] x [OH - ] (≈ 10 -14 ) One concentration condition: C T = [A - ] + [HA] One proton condition (charge balance eqn) [H + ] = [A - ] + [OH - ] Exact solution (Stumm and Morgan Table 3.6): [H + ] 3 + [H + ] 2 K’ HA - [H + ](C T x K’ HA + K W ) – (K’ HA x K W ) = 0

13. Stumm and Morgan

14. Think of the “proton condition” in terms of species with excess or H +, relative to added material. So for HA added, HA  H + + A - the proton condition is: H + = OH - + A - “Excess” protons = “missing” protons (can also think of this in terms of electroneutrality)

15. or for NaA added (conjugate base) NaA  Na + + A - HA + A - = C T = Na H + + HA = OH - “Excess” protons = “missing” protons Or, in terms of electroneutrality: Na + + H + = A - + OH - (HA + A - ) + H + = A - + OH - HA + H + = OH -

16. Closed-system dissolved carbonate equilibria Define H 2 CO 3 * as = CO 2 (aq) + H 2 CO 3 (H 2 CO 3 * is mostly CO 2 (aq) (>99.7% at 25C) H 2 CO 3 * HCO 3 – + H + HCO 3 – CO 3 –2 + H + H 2 O H + + OH – Kw = [H + ][OH - ]

17. The concentration condition: C T = H 2 CO 3 * + HCO 3 – + CO 3 –2 (C T a.k.a.  CO 2 or DIC)  CO 2 is independent of T and P

18. Define Alkalinity (acid-neutralizing capacity) Alk = OH – + HCO 3 – + 2CO 3 –2 + B(OH) 4 - - H + (+ HPO 4 -2 + 2PO 4 -3 + H 3 SiO 4 - +NH 3 +…) (- HSO 4 - - HF – H 3 PO 4 - …) (a.k.a. “titration alkalinity”) The excess of bases (proton acceptors, pK > 4.5) over acids (proton donors, pK < 4.5) Alk is also independent of T and P In addition, Alk is independent of CO 2 addition / removal Measured titration alkalinity can be used to constrain the charge balance of a solution (thus it is an alternative to the proton condition)

19. Alk = OH – + HCO 3 – + 2CO 3 –2 + B(OH) 4 - - H + Zeebe and Wolf-Gladrow Bicarbonate dominates the alkalinity of seawater

20. Alk = OH – + HCO 3 – + 2CO 3 –2 + B(OH) 4 - - H + Zeebe and Wolf-Gladrow

21. Consider the carbonate system as a function of pH, as we did for the monoprotic acid, “HA” Explicit approach: 5 species (unknowns) H 2 CO 3 *, HCO 3 –, CO 3 –2, H +, OH – 3 equilibrium equations K 1, K 2, Kw 1 concentration condition C T 1 proton condition Alk Solution is 4th order in [H + ] (Stumm and Morgan, table 3.6) Any two cabonate system parameters fix the values of all the rest: C T, Alk, H 2 CO 3 * (or pCO 2 ), HCO 3 –, CO 3 –2, pH (readily measured)

22. Stumm and Morgan, Table 4.2 * * *

24. Graphical approach (Bjerrum plot): Not adequate for solubility calculations, but gives a good mental picture. First look at generic monoprotic acid “HA”, then at carbonate system. For HA, pK = 6, C T = 10 -3 mol/L Construct the plot in Stumm and Morgan, Figure 3.3

25. Stumm and Morgan, Figure 3.3 Kw = 10 -14 = [H][OH]; [H] = [OH] @ pH = 7 C T = 10 -3 mol/L pK = 6

26. pK = pH = 6

27. C T = 10 -3 mol/L pK = 6 (page 6)

28. C T = 10 -3 mol/L pK = 6

32. Carbonate system Bjerrum diagrams (S&M figure 4.1) Freshwater, T = 25C Ct = 10 -3 M pK’ 1 = 6.3 pK’ 2 = 10.25 Seawater (T = 10C) Ct = 2.3 x 10 -3 M pK’ 1 = 6.1 pK’ 2 = 9.3 pK’ B = 8.8 Bt = 4.1 x 10 -4 M

33. Fresh water

34. Seawater

35. How does [CO 3 –2 ] respond to changes in Alk or DIC? C T = [H 2 CO 3 *] + [ HCO 3 – ] + [CO 3 –2 ] ~ [ HCO 3 – ] + [CO 3 –2 ] (an approximation) Alk = [OH – ] + [HCO 3 – ] + 2[CO 3 –2 ] + [B(OH) 4 - ] – [H + ] ~ [HCO 3 – ] + 2[CO 3 –2 ] (a.k.a. “carbonate alkalinity”) So (roughly): [CO 3 –2 ] ~ Alk – C T C T ↑, [CO 3 –2 ] ↓ Alk ↑, [CO 3 –2 ] ↑ (Alk constant)(DIC constant)

36. Alk = OH – + HCO 3 – + 2CO 3 –2 + B(OH) 4 - - H + Zeebe and Wolf-Gladrow

37. Photsynthesis: C T ↓ H 2 CO 3 * HCO 3 – + H + HCO 3 – CO 3 –2 + H +

39. Stumm and Morgan pH contours as a function of Alk and DIC

40. CO 3 -2 contours as a function of Alk and DIC