# Carbon Dioxide Sources and Sinks: Respiration and Photosynthesis

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

Gas Exchange [CO2] = Kh pCO2

Hydration of CO2

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

Systematic treatment of Equilibria

The first dissociation (K1)

The 2nd Dissociation Constant (K2)

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!

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

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

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

Lets solve using a dummy variable

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

Lets focus on TA and TCO2

Deffeyes’ Diagram pH 10 pH 8 pH 6

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

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

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

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

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

Distribution diagram for H3PO4

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

Distribution of Phosphate

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

NO2 vs PO4 plots NO2- PO43-

O2 vs PO4 plots O2 PO43-

General Phosphorus Cycle
Animals Plants photsynth. respiration Dissolved inorganic P

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