Geoscience Reference
In-Depth Information
3.5 Carbon
Carbon has two isotopes 12 and 13 with abundances of 98.89 and 1.11 percent, respec-
tively. The universally accepted standard of
13
C is the Pee Dee belemnite carbonate
(PDB). Carbon is less abundant than oxygen but nevertheless ubiquitous. The major
reservoirs are located in the mantle and in sedimentary carbonates. The different forms
of carbonate ions dissolved in the ocean (H
2
CO
3
,HCO
3
, and CO
2
3
) and atmospheric
CO
2
are minor reservoirs with respect to carbonates and mantle carbon but are particu-
larly important to us. Carbon can be in reduced (C, CH
4
, organic material) or oxidized
form (CO, CO
2
). It is under different forms in the mantle and the relative abundances of
these forms are not well constrained, but the “terrestrial” value of
δ
13
C should not be very
δ
different from
.
The core reaction of carbon isotope geochemistry is the following:
−
7
CH
4
+
2O
2
⇔
CO
2
+
2H
2
O
(3.39)
Contrary to all the substitutions we have seen so far, the two carbon molecules involved in
this reaction have very different configurations and therefore very different normal modes
tants and the products is very different. Isotope fractionation induced by such a reaction
is therefore particularly strong, especially at low temperatures: the
13
CofCO
2
is higher
than that of CH
4
by 80 per mil at 0
◦
C, 33 per mil at 200
◦
C, and by 10 per mil at 700
◦
C.
This is a strong indication that biological processes must have a profound impact on the
carbon isotope compositions of carbon-bearing systems. From left to right, the previous
chemical reaction produces oxidative energy and is a mockup of respiration, which liber-
ates CO
2
with high
δ
13
C. Solar energy is needed to activate the non-spontaneous reaction
from right to left: this is the essence of photosynthesis, which liberates oxygen and stores
low-
δ
13
C reduced carbon in plants. Why is it that the final products do not have the same
isotope compositions as the initial product such as suggested by the reaction? It is simply
that the reactions are much more complicated and involve repeated exchanges of carbon
inside the cell during which intermediate products are lost. For respiration, the excreted
CO
2
is isotopically buffered by the biological material.
In addition to fractionation between reduced carbon and CO
2
,
12
C/
13
C also fraction-
ates at low temperature between atmospheric CO
2
, the dissolved carbonate species, and
CaCO
3
carbonates with
δ
13
C being typically 10 per mil higher at 15
◦
C in calcite than in
atmospheric carbon dioxide.
The fractionation just referred to concerns the distribution of isotopes between phases
in equilibrium, but another type of fractionation (kinetic isotope effect or KIE) takes place
when species with different oxidation states and coordination (here CO
2
and CH
4
)fail
to achieve equilibrium, in particular during biological reactions. For example, carbonic
anhydrase enzyme found in mammal blood speeds up the conversion of CO
2
into an HCO
3
ion by five orders of magnitude, thus preventing bubbles of excreted CO
2
from forming.
Light isotopes having higher vibrational frequencies enter into reaction paths more often
than heavy isotopes and are therefore exchanged more readily. They are also bound less
δ
