22
B. Fractionation Mechanisms of Selected Elements
FONTES and GONFIANTINI (1967) determined a decrease of 15%0 in
deuterium in the chemically bound water of gypsum, which is in equilibrium with ocean water.
Equilibrium has not been proved in all of the above cases, but these
examples of chemical exchange reactions demonstrate how many possibilities exist for hydrogen isotope fractionations.
c) Kinetic Isotope Effects
Appreciable hydrogen isotope fractionation seems possible in biochemical processes, e.g., during photosynthesis (KRICHEVSKY et ai., 1961)
and during bacterial production of molecular hydrogen and methane.
II. Carbon
1. Introductory Remarks
Carbon is one of the most abundant elements in the universe, but it
occurs in the earth as a trace element. The average carbon content of the
crust and the mantle probably lies in the range of several hundred parts
per million. Besides playing a key role in the biosphere, inorganic carbon
is important because it shows a diversity of components, e.g., diamond,
carbonates, carbon dioxide. This distribution of more oxidized carbon
compounds of inorganic origin and of more reduced carbon in the
biosphere is an ideal situation for the search for naturally occurring
isotope fractionations.
Carbon has two stable isotopes:
12C = 98.89% (reference mass in modern compilations)
I3C = 1.11 % (NIER, 1950).
The naturally occurring variations of the carbon isotope composition is greater than 10% (neglecting meteoritic carbonate). Heavy carbonates with 6-values of more than + 20 and light methane of around
- 90%0 have been reported in the literature (see also Fig. 9).
Since the very first measurements of NIER and GULBRANSEN (1939)
and MURPHEY and NIER (1941), we know that the carbonates concentrate I3C relative to organic compounds.
A great many I3C/2C mass-spectrometric determinations have been
made in the last 20 years; many of these are unpublished because they
are of commercial interest for petroleum industries. The first - and one
of the best - summaries of the carbon isotope variations of the different
carbon compounds has been given by CRAIG (1953). Two recent compilations have been written by SCHWARCZ (1969) and DEGENS (1969).
B. Fractionation Mechanisms of Selected Elements
FONTES and GONFIANTINI (1967) determined a decrease of 15%0 in
deuterium in the chemically bound water of gypsum, which is in equilibrium with ocean water.
Equilibrium has not been proved in all of the above cases, but these
examples of chemical exchange reactions demonstrate how many possibilities exist for hydrogen isotope fractionations.
c) Kinetic Isotope Effects
Appreciable hydrogen isotope fractionation seems possible in biochemical processes, e.g., during photosynthesis (KRICHEVSKY et ai., 1961)
and during bacterial production of molecular hydrogen and methane.
II. Carbon
1. Introductory Remarks
Carbon is one of the most abundant elements in the universe, but it
occurs in the earth as a trace element. The average carbon content of the
crust and the mantle probably lies in the range of several hundred parts
per million. Besides playing a key role in the biosphere, inorganic carbon
is important because it shows a diversity of components, e.g., diamond,
carbonates, carbon dioxide. This distribution of more oxidized carbon
compounds of inorganic origin and of more reduced carbon in the
biosphere is an ideal situation for the search for naturally occurring
isotope fractionations.
Carbon has two stable isotopes:
12C = 98.89% (reference mass in modern compilations)
I3C = 1.11 % (NIER, 1950).
The naturally occurring variations of the carbon isotope composition is greater than 10% (neglecting meteoritic carbonate). Heavy carbonates with 6-values of more than + 20 and light methane of around
- 90%0 have been reported in the literature (see also Fig. 9).
Since the very first measurements of NIER and GULBRANSEN (1939)
and MURPHEY and NIER (1941), we know that the carbonates concentrate I3C relative to organic compounds.
A great many I3C/2C mass-spectrometric determinations have been
made in the last 20 years; many of these are unpublished because they
are of commercial interest for petroleum industries. The first - and one
of the best - summaries of the carbon isotope variations of the different
carbon compounds has been given by CRAIG (1953). Two recent compilations have been written by SCHWARCZ (1969) and DEGENS (1969).