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Carbon Dioxide

Carbon Dioxide. Sources and Sinks: Respiration and Photosynthesis CO 2 + H 2 O ---h n ---> Org-C + O 2 (photosynthesis) Org-C + O 2 -----> CO 2 + H 2 O (respiration) The CO 2 system is regulated by the acid/base chemistry of the carbonate system! (pH between 6.0 - 8.5)

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Carbon Dioxide

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  1. Carbon Dioxide • Sources and Sinks: Respiration and Photosynthesis • CO2 + H2O ---hn---> Org-C + O2 (photosynthesis) • Org-C + O2 -----> CO2 + H2O (respiration) • The CO2 system is regulated by the acid/base chemistry of the carbonate system! (pH between 6.0 - 8.5) • Formation of calcareous sediments (CaCO3) • It is time to do some serious chemistry

  2. Gas Exchange [CO2] = Kh pCO2

  3. Hydration of CO2

  4. Forms of CO2 in Water • Carbonic acid (H2CO3), Bicarbonate (HCO3- ), and Carbonate (CO32-). • H2CO3 = H+ + HCO3- = CO32- + 2H+ (4)

  5. Systematic treatment of Equilibria

  6. The first dissociation (K1)

  7. The 2nd Dissociation Constant (K2)

  8. Carbonate Mass and Charge Balance • Total CO2 = H2CO3 + HCO3- +CO32- • Total Alkalinity = HCO3- + 2CO32- • Another way of thinking about alkalinity is the charge of permanent cations - charge of permanent anions!

  9. Lets pull this all together • TCO2 = H2CO3 + HCO3- +CO32- • TA = HCO3- + 2CO32- • pH = -log(H+)

  10. Lets pull this all together • TCO2 = H2CO3 + HCO3- +CO32- • TA = HCO3- + 2CO32- • pH = -log(H+)

  11. 6 Equations, 8 Unknowns • Measure two variables to define the system. • pH • Total Alkalinity • pCO2 • We also need values for K1, K2, K**

  12. Lets solve using a dummy variable

  13. Carbonate Solubility • Carbonate (CaCO3) • Aragonite (CaCO3) • CaCO3 = Ca2+ + CO32- • Ksp vs. Ion Product • IP = [Ca2+][CO32-] • @ Equilibrium Ksp=IP • Ksp > IP precipitation • Ksp < IP dissolution • diss. produces 2 units of TA, 1 unit TCO2 • ppt. consumes 2 units of TA, 1 unit TCO2

  14. Lets focus on TA and TCO2

  15. Deffeyes’ Diagram pH 10 pH 8 pH 6

  16. How do we mess with the CO2 system • Carbonate precipitation or dissolution? • diss. produces 2 units of TA, 1 unit TCO2 • ppt. consumes 2 units of TA, 1 unit TCO2 • Photosynthesis or Respiration? • photo. Consume TCO2, TA = constant • respir. Produce TCO2, TA = constant • Evaporation/precipitation • Change in TCO2 = TA

  17. Deffeyes’ Diagram pH 10 CaCO3 pH 8 Physical Biological pH 6

  18. Deffeyes’ Diagram pH 10 pH 8 CaCO3 Physical pH 6 Biological

  19. CaCO3 Physical Biological Deffeyes’ Diagram pH 10 pH 8 pH 6

  20. Other Weak Acids? • H3PO4 Orthophosphate • Triprotic Acid • pK1 = 2.148, pK2 = 7.199, pK3 = 12.5

  21. Distribution diagram for H3PO4

  22. Characterization of Phosphorus Total P 0.45 m filter Particulate Dissolved oxidation Inorganic Organic Inorganic Organic Reactive Phosphate

  23. Distribution of Phosphate

  24. What is the cause of the phosphate distribution? • Lets look at other nutrients and oxygen as a clue.

  25. NO2 vs PO4 plots NO2- PO43-

  26. O2 vs PO4 plots O2 PO43-

  27. General Phosphorus Cycle Animals Plants photsynth. respiration Dissolved inorganic P

  28. General Phosphorus Cycle Animals Plants photsynth. respiration weathering Dissolved inorganic P fecal pellets bacteria Dissolved and detrital organic-P Sediments

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