布朗大学同位素地球化学Carbonisotope

Ribulose-1,5-biphosphate carboxylase/oxygenase (Rubisco) for C3 plants (trees, aquatic plants and algae): “Rubisco” for short Phospoenol pyrurate carboxylase (PEP Carboxylase) for C4 plants (tropical grasses, sedges etc)

Key isotopic effects of steps leading to CO2 fixation in C3 plants
Process Isotope Discrimination Symbol effect (‰) (Δ) 1.0044 4.4 a
Diffusion of CO2 in air through the stomatal/pore
Dissolved inorganic carbon (DIC)

In equilibrium with atmospheric CO2, d13C values of DIC vary with pH, as carbonate speciation changes, because different carbonate species (CO2(g), CO2(aq), HCO3-, CO32-) have different d13C values
The Global Carbon Cycle
Carbonate system

CO2(g) ↔ CO2(aq)↔H2CO3*
KCO2
aH 2CO3* pCO2
3
101.46

H2CO2(aq) ↔
H++HCO3-(aq)
K1
aHCO aH aH 2CO3*
106.35

Objectives

Understand the systematics of carbon isotopic fractionation in inorganic and organic systems Know the basis of carbon isotope application in study of carbon cycling and plant ecology
Net C3 fixation with respect to Ci/Ca
Carbon isotope Part I
Topics for this section

Carbon isotopic fractionation in natural water systems Carbon isotope variation during photosynthesis in land plants (C3, C4, CAM plants)
-2
Common pH range in nature
-3
H2CO3*
6.35
HCO3-
10.33
CO32-
-4
log ai
-5
OH H+
-
-6
-7
-8 0 2 4 6 8 10 12 14
pHBiblioteka In most natural waters, bicarbonate ion is the dominant charged carbonate species in the alkalinity definition

HCO3- ↔ H++ CO32-(aq)
K2
aCO 2 aH
3
aHCO
3
1010.33
Bjerrum plot showing the activities of inorganic carbon species as a function of pH for a value of total inorganic carbon of 10-3 mol L-1.
Carbonate 13C fractionation at different temperature
Kinetic C isotope fractionation during photosynthesis

CO2+H2O → CH2O+O2 (large kinetic fractionation) Two key enzymes

Characteristics of Carbon

Carbon has two stable isotopes: 12C=98.89%; 13C=1.11% Carbon plays a key role in the biosphere, inorganic/organic carbon also exists in a diversity of compounds with different oxidation states CH4: CH3-CH3: Diamond/graphite: CO: CO2, H2CO3, HCO3CO32-, CaCO3, CaMg(CO3)2 -4 -3 0 +2 +4

The d13C values often show similar sequence as the oxidation states:
d13Ccarbonate > d13Cgraphite > d13CCH4
Carbon Isotope standards
Standards VPDB (primary std) NBS-16 NBS-17 NBS-18 NBS-19 NBS-20 NBS-21 NBS-22
Carbonate CO2 CO2 Carbonate Marble Limestone Graphite Oil
δ13C 0 -41.48 -4.41 -5.00 +1.92 -1.06 -28.10 -29.63
Natural variations in
13 d C
values
Excluding carbonate in carbonaceous chondrite, the terrestrial range of d13C values exceed 100 ‰