Carbon Dioxide

Sources and Sinks: Respiration and Photosynthesis

CO2 + H2O ---h---> 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

Gas Exchange

[CO2] = Kh pCO2

Hydration of CO2

Forms of CO2 in Water

• Carbonic acid (H2CO3), Bicarbonate (HCO3- ), and Carbonate

(CO32-).

• H2CO3 = H+ + HCO3- = CO3

2- + 2H+ (4)

Systematic treatment of Equilibria

The first dissociation (K1)

The 2nd Dissociation Constant (K2)

Carbonate Mass and Charge Balance

Total CO2 = H2CO3 + HCO3- +CO3

2-

Total Alkalinity = HCO3- + 2CO3

2-

Another way of thinking about alkalinity is the charge of permanent cations - charge of permanent anions!

Lets pull this all togetherTCO2 = H2CO3 + HCO3

- +CO32-

TA = HCO3- + 2CO3

2-

pH = -log(H+)

Lets pull this all togetherTCO2 = H2CO3 + HCO3

- +CO32-

TA = HCO3- + 2CO3

2-

pH = -log(H+)

6 Equations, 8 Unknowns

Measure two variables to define the system.pHTotal AlkalinitypCO2

We also need values for K1, K2, K**

Lets solve using a dummy variable

Carbonate SolubilityCarbonate (CaCO3)

Aragonite (CaCO3)

CaCO3 = Ca2+ + CO32-

Ksp vs. Ion ProductIP = [Ca2+][CO3

2-]

@ Equilibrium Ksp=IPKsp > IP precipitationKsp < IP dissolutiondiss. produces 2 units of TA, 1 unit TCO2

ppt. consumes 2 units of TA, 1 unit TCO2

Lets focus on TA and TCO2

Deffeyes’ DiagrampH 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/precipitationChange in TCO2 = TA

Deffeyes’ DiagrampH 10

pH 8

pH 6

CaCO3

Biological Physical

Deffeyes’ DiagrampH 10

pH 8

pH 6

CaCO3

BiologicalPhysical

Deffeyes’ DiagrampH 10

pH 8

pH 6

CaCO3

BiologicalPhysical

Other Weak Acids?

H3PO4 Orthophosphate

Triprotic Acid pK1 = 2.148, pK2 = 7.199, pK3 = 12.5

Distribution diagram for H3PO4

Characterization of PhosphorusTotal P

0.45 m filter

Dissolved Particulate

Inorganic InorganicOrganic Organic

oxidation

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

PO43-

O2

General Phosphorus Cycle

Dissolvedinorganic

P

Plants Animals

respirationphotsynth.

General Phosphorus Cycle

Dissolvedinorganic

P

Plants Animals

Dissolved and detrital organic-P

Sediments

weathering

fecal pellets

respirationphotsynth.

bacteria