carbon dioxide sources and sinks: respiration and photosynthesis co 2 + h 2 o ---h ---> org-c +...
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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