GEOF236 CHEMICAL OCEANOGRAPHY (HØST 2012) Christoph Heinze

1. GEOF236 CHEMICAL OCEANOGRAPHY (HØST 2012) ChristophHeinze University of Bergen, Geophysical Institute and Bjerknes Centre for Climate Research Prof. in Global Carbon Cycle Modelling Allegaten 70, N-5007 Bergen, Norway Phone: +47 55 58 98 44 Fax: +47 55 58 98 83 Mobile phone: +47 975 57 119 Email: christoph.heinze@gfi.uib.no DEAR STUDENT AND COLLEAGUE: ”This presentation is for teaching/learning purposes only. Do not useany material ofthispresentation for any purpose outsidecourse GEOF236, ”Chemical Oceanography”, autumn 2012, Universityof Bergen. Thankyou for yourattention.”

2. Sarmiento&Gruber 2006 Chapter 8: Carbon cycle, part 2

3. Ocean is vital for governing atmospheric CO2: reservoir size! K.K. Liu et al., 2010 Reservoirs and fluxes in GtC resp. GtC/yr

4. Mean annual pCO2 difference surface ocean –atmosphere: Takahashi et al., 2002 Source: Sarmiento&Gruber (2006)

5. In the ocean CO2 is highly reactive due to the ability of seawater to disscociate weak acids: Sea water – buffer system! 1. Dissociation step: CO2 + H2O HCO3- + H+ CO2 + H2O + CO32-(1+x)HCO3- + (1-x)CO32- + (1-x)H+ 2. Dissociation step: CO32- + H+ HCO3- CO2 : HCO3- : CO32- 1 : 100 : 10 Zeebe & Wolf-Gladrow, 2001

6. How can we quantify the different dissociation reactions? E.g. we would like to know how much [HCO3-], [CO32-], and [H+] would we have in seawater at a specific pCO2 (CO2 partial pressure). Black board

7. Alkalinity concept using simplified sea water (excursion following Wolf-Gladrow, D., et al., 2007, Total alkalinity: The explicit conservative expression and its application to biogeochemical processes, Marine Chemistry, 106, 287-300) Based on a simple solution consisting of: NaCl (0.59767 moles per kilogram solution) and a small amount of NaHCO3 (0.00233 moles per kilogram solution) to distilled water. Equilibrate solution with air of pCO2= 295 μatm. This is a rough approximation of sea water but much simpler. Ingredients: NaCl, NaHCO3, CO2, and H2O. The simplified seawater includes now: H2O, H+, OH-, Na+, Cl-, CO2, HCO3-, and CO32- The observed conditions now are: [Na+]=0.6 mol kg-1 DIC=2 mmol kg-1 pH=8.2 T=25°C The equilibrium constants for the dissociation of water Kw and the 2 carbonic acid dissociation steps K1, K2 are given through respective pK=-log10(K) values, or K=10-pK. pKw=13.2, pK1=5.9, pK2=9.9

8. Alkalinity concept using simplified sea water: One arives at the following values: H+ = 6.3 ∙ 10-3μmol kg-1 OH- = 10 μmol kg-1 CO2 = 8.3 μmol kg-1 HCO3- = 1660 μmol kg-1 CO32- = 331μmol kg-1 Wolf-Gladrow et al. (2007)

9. Alkalinity concept using simplified sea water: Wolf-Gladrow et al. (2007) Titration of the system (1 kg of solution) with HCl (1 mol kg-1) → decrease in pH, see Fig. Most of the H+ added is used up to convert CO32- → HCO3-, and HCO3- → H2CO3 The H2CO3 is mostly available as CO2 ans H2O. At initial pH of 8.2, CO32- and HCO3- are the main proton acceptors in the solution. Total alkalinity Alk is approximately a measure of the amount of these proton acceptors or – more precisely – The amount of protons which can be accepted by these acceptors: Alk ≈ [HCO3-] + 2 ∙ [CO32-]

10. Alkalinity concept using simplified sea water: Wolf-Gladrow et al. (2007) Total alkalinity Alk is approximately a measure of the amount of these proton acceptors or – more precisely – The amount of protons which can be accepted by these acceptors: Alk ≈ [HCO3-] + 2 ∙ [CO32-] Alk is measured by acidic titration which also affects the concentrations of other proton acceptors and proton donors such as OH- and H+ . Thus Alk is defined as: the excess of proton acceptors over proton donors. At the alkalinity equivalence point (where Alk = 0) the pH here is about 4.3 (see fig.), thus the Alk of the initial sample could be estimated as the amount of HCl to be added to reach this pH value.

11. More comprehensive definitions of Alk (Dickson, 1981):: Source: Wolf-Gladrow et al. (2007)

12. After: Sarmiento&Gruber (2006)

13. pCO2 pH Harvey, 1955, The chemistry and fertility of seawater, Cambridge University Press.

14. Global mean dissolved inorganic carbon and alkalinity (horizontal averages): Source: Sarmiento&Gruber (2006)

15. Dissolved inorganic carbon DIC and alkalinity Alk along the ocean conveyor belt (salinity normalised values): Source: Sarmiento&Gruber (2006)

16. Property/property plot including DIC: Source: Sarmiento&Gruber (2006)

17. Opposing effects on temperature on solubility K0 and carbonic acid dissociation K1, K2: After: Sarmiento&Gruber (2006)

18. A thought experiment on the interplay of temperature, dissolved inorganic carbon DIC, and surface CO2 partial pressure pCO2: After: Sarmiento&Gruber (2006)

19. Equilibration time for CO2 gas exchange for a ca. 40m thick ocean surface layer: ∂[A]w/∂t = (kw/zml) ∙ ([A]a – [A]w); Solution: [A]w(t) = [A]w(t0) + ([A]a – [A]w(t0)) ∙ exp [-(kw/zml) ∙ Δt], Δt=t-t0 [A]a: equilibrium concentration for solubility equilibrium with atmosphere [A]w: concentration in water kw: gas exchange coefficient zml: mixed layer depth Characteristic time scale for CO2 gas exchange, IF CO2 WOULD NOT REACT WITH OTHER CONSTITUENTS IN WATER: T = zml/kw= 40m/20 cm h-1 = 8 days THEREFORE THIS IS THE WRONG VALUE!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! IN REALITY: CO2 or its hydrated form H2CO3* has mostly to react with carbonate to form bicarbonate: H2CO3* + CO32- → 2 HCO3- See Sarmiento&Gruber (2006, p. 330) for a derivation of the real time scale: T = ∂DIC/∂[H2CO3*] ∙ (zml/kw) ≈ 20 ∙ (zml/kw) i.e. 160 days ca. ½ year !!!!

20. Buffer factor or “Revelle factor”: • How large is the • relative changeofseawater CO2partialpressure • relative to a • relative change in DIC • after full equilibrium? • RE = (DIC/pCO2) ∙ (∂pCO2/∂DIC), • so this a sensitivity describing how much the seawater pCO2 changes in a sepcified volume for small changes in DIC in this specified volume • after the system is again in full equilibrium. • There are two ways of deriving the buffer factor (see Maier-Reimer and Hasselmann, 1987, Clim.Dyn.): • Variational method using total differentials. • Differential buffer factor. • (excursion, go to Maier-Reimer and Hasselmann, 1987)

21. the buffer factor increases with pCO2 Zeebe and Wolf-Gladrow, CO2 in seawater:Equilibrium kinetics, isotopes. Elsevier Oceanography Series. 2001

22. the buffer factor decreaes with temperature Zeebe and Wolf-Gladrow, CO2 in seawater:Equilibrium kinetics, isotopes. Elsevier Oceanography Series. 2001

23. Buffer factor or “Revelle factor”: Buffering gets better with increasing temperature. Buffering gets worse with increasing pCO2. Sabine et al., Science, 2004

24. Seasonal amplitude of sea surface CO2 partial pressure pCO2: After Sarmiento & Gruber (2006)

25. Seasonal amplitude of sea surface CO2 partial pressure pCO2 – the temperature component: After Sarmiento & Gruber (2006)

26. Seasonal amplitude of sea surface CO2 partial pressure pCO2 – the biological component component: After Sarmiento & Gruber (2006)

27. How do different processes imprint themselves onto DIC and Alk? 1 bring deep water sample to surface ocean of 20°C Organic matter production: The major factor for changing TA is precipitation/dissolution of CaCO3: CaCO3solid ↔ Ca2+ +CO32- 2 account for organic matter production 3 account for CaCO3 production After: Sarmiento&Gruber (2006)

28. pCO2 variations in subtropical gyres AfterSarmiento & Gruber (2006)

29. What vertical gradient makers exist in the ocean – the carbon pumps: Source: Sarmiento&Gruber (2006), p. 342 Source: Sarmiento&Gruber (2006)

30. Carbon pumps SOLUBILITY Heinze, C., E. Maier-Reimer, and K. Winn, 1991, Glacial pCO2 reduction by the World Ocean - experiments with the Hamburg Carbon Cycle Model, Paleoceanography, 6, 395-430.

31. Source: Sarmiento & Gruber (2006)

32. Carbon pumps Source: Zeebe & Wolf-Gladrow, 2001

33. How do the different carbon pumps influence the vertical DIC profile? Thought experiment on left diagram. Sarmiento&Gruber (2006) introduce the gas exchange pump instead of the solubility pump. This accounts for the inflow/outflow of carbon at the surface with communication to the atmosphere when either Corg production decreases the surface pCO2 or CaCO3 production increases the surface pCO2. Soft-tissue pump: Carbonate pump: Gas exchange pump:

34. Global mean profiles of the three main carbon pumps, plotted accumulatively: Source: Sarmiento & Gruber (2006)

35. Source: Sarmiento & Gruber (2006)

36. What is the main conclusion drawn by this analysis?

37. What is the main conclusion drawn by this analysis? The interior structure of DIC (or the “gradients of CO2 in the ocean) in the ocean is caused by biologial effects, not by gas exchange!

38. Simulation with and without biota, and observations: Look at the gradients! with biota without biota observations Source: Maier-Reimer and Hasselmann, 1987

39. TCO2 from a model (HAMOOC4) Pre-industrial 1751 Today 2004 Atlantic Ocean

40. TCO2 from a model (HAMOOC4) Pre-industrial 1751 Today 2004 Pacific Ocean

41. The actual TCO2 and the anthropogenic TCO2 have completely different patterns. Only the ”actual” can readily be observed.

42. Source: Broecker&Peng, 1982

43. Carbon pumps Heinze, C., 1990, PhD