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

K h =[H2CO3 ][CO2 ]

=1.5x10−3

[H2CO3 ]=1.5x10−3 * [CO2 ] so we lump these terms together

[H2CO3 ]+ [CO2 ]

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[H2CO3] + [CO2] = K * pCO2

Forms of CO2 in Water

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

(CO32-).

• H2CO3 = H+ + HCO3- = CO3

2- + 2H+ (4)

0

0.2

0.4

0.6

0.8

1

4 5 6 7 8 9 10 11

Fraction of each carbonate species

Seawater pH

H2CO3 HCO3-

CO32-

Systematic treatment of Equilibria

CO2 +H2O =H2CO3 Kh

H2CO3 =HCO3- +H+ K1

HCO3- = CO3

2- + H+ K2

The first dissociation (K1)

K1 =[HCO3

- ][H+ ]{[H2CO3] + [CO2 ]}

H2CO3 = HCO3- + H+ K1

The 2nd Dissociation Constant (K2)

HCO3- = CO3

2- + H+ K2

K2 =[CO3

2- ][H+ ]

[HCO3- ]

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+)

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[H2CO3] + [CO2] = K * pCO2

K1 =[HCO3

- ][H+ ]{[H2CO3] + [CO2 ]}

K2 =[CO3

2- ][H+ ]

[HCO3- ]

Lets pull this all togetherTCO2 = H2CO3 + HCO3

- +CO32-

TA = HCO3- + 2CO3

2-

pH = -log(H+)

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[H2CO3] + [CO2] = K * pCO2

K1 =[HCO3

- ][H+ ]{[H2CO3] + [CO2 ]}

K2 =[CO3

2- ][H+ ]

[HCO3- ]

6 Equations, 8 Unknowns

Measure two variables to define the system.pHTotal AlkalinitypCO2

We also need values for K1, K2, K*

Constants for the Cabonate Systemfrom: Millero, F. J. (1979), Geochimica et Cosmochimica Acta, 43, 1651-

1661.

A) K’ values at infinite dilution: lnK’ = A + B/T + C lnT

ACID A - B - C

H2O 148.9802 13874.26 23.6521B(OH) 3 148.0248 8966.90 24.4344H2CO3 290.9097 14554.21 45.0575HCO3- 207.6548 11843.79 33.6485Calcite 303.1308 13348.09 48.7537Aragonite 303.5363 13348.09 48.7573

B) K** values as a function of S 0/00 and temperature:lnK** = lnK’ + (Ao + A 1/T + A2 lnT)S1/2 + B0 S

ACID A0 A1 A2 B0 102

H2O -79.2447 3298.720 12.0408 -1.9813B(OH)3 0.5998 -75.25 -- -1.767H2CO3 0.0221 34.02 -- --HCO3- 0.9805 -92.65 -- -3.294Calcite 1.6233 -118.64 -- -6.999Aragonite same as calcite

C) Henry’s law constants for CO2. From Weiss (1974) Marine Chemistry 2, 203-205.

Κ∗ = -75.8793 + 4058.56/ +23.3585 ( ) +T Log T( 0.010213 - 1.027 10x -4 + 2.043 10T x -7T2) S

Lets solve using a dummy variable

[H2CO3 ]+ [CO2 ] = v* [H+]2

[HCO3- ] = v * K1 * [H+ ]

[CO32- ] = v* K1 * K2

TCO2 = v*{[H+ ]2 + [H+ ] * K1 +K 1 * K 2}

TA = v *{[H+ ]* K1 + 2 * K1 * K2}

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pCO2 = v*[H+]2

K *

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

TCO2 = v*{[H+ ]2 + [H+ ] * K1 +K 1 * K 2}

TA = v *{[H+ ]* K1 + 2 * K1 * K2}

Deffeyes’ Diagram

0

0.0005

0.001

0.0015

0.002

0.0025

0.003

0 0.0005 0.001 0.0015 0.002

Total CO2

Total Alkalinity

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/precipitationChange in TCO2 = TA

Deffeyes’ Diagram

0

0.0005

0.001

0.0015

0.002

0.0025

0.003

0 0.0005 0.001 0.0015 0.002

Total CO2

Total Alkalinity

pH 10

pH 8

pH 6

CaCO3

Biological Physical

Deffeyes’ Diagram

0

0.0005

0.001

0.0015

0.002

0.0025

0.003

0 0.0005 0.001 0.0015 0.002

Total CO2

Total Alkalinity

pH 10

pH 8

pH 6

CaCO3

BiologicalPhysical

Deffeyes’ Diagram

0

0.0005

0.001

0.0015

0.002

0.0025

0.003

0 0.0005 0.001 0.0015 0.002

Total CO2

Total Alkalinity

pH 10

pH 8

pH 6

CaCO3

BiologicalPhysical

Other Weak Acids?

H3PO4 Orthophosphate

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

P

O

OHHO

OH

Distribution diagram for H3PO4

-0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 2 4 6 8 10 12

pH

fraction

H3PO4(H2PO4)-(HPO4)2-(PO4)3-

Characterization of PhosphorusTotal P

0.45 m filter

Dissolved Particulate

Inorganic InorganicOrganic Organic

oxidation

Reactive Phosphate

Distribution of Phosphate

VV

VV

V

V

V

V

V

V

JJJJ

J

J

J

J

J

HHHH

H

H

H

H

H

800

700

600

500

400

300

200

100

0

0 5 10 15 20 25 30 35 40 45

0 0.5 1 1.5 2 2.5 3

Nitrate concentration μmol/kg

Temperature oC

Phosphate Concentration μmol/kg

Temperature

NO3-

PO43-

What is the cause of the phosphate distribution?

Lets look at other nutrients and oxygen as a clue.

NO2 vs PO4 plots

0

5

10

15

20

25

30

35

40

45

0 1 2 3

NO3Linear (NO3)

NO2-

PO43-

O2 vs PO4 plots

0

50

100

150

200

250

0 1 2 3

O2Linear (O2)

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