"Age Effect" of Carbon Isotope Distribution
89
sions concerning the mode of formation and possible fractionations during migration of methane. Various attempts have been made in this
direction; three of these are mentioned. SACKETT (1968) argued that time,
temperature, and the structure of parent organic materials are the primary factors regulating the isotopic composition of methane. COLOMBO
et al. (1966, 1970) believed that they had demonstrated that during the
migration of methane the light isotope is preferentially enriched in the
highest zones of a gas field. Contradictory results have been obtained by
MA Yet al. (1968), who demonstrated a more rapid migration for 13CH4
than for 12CH4 with 12CH4 being better adsorbed than 13CH4.
e) Coal
Coals have a 13CrC distribution like that of modern land plants
(CRAIG (1953), WICKMAN (1953), COMPSTON (1960), COLOMBO et al. (1970)).
Brown coals have been extensively analyzed by FISCHER et al. (1968,
1970). COLOMBO et al. (1970) analyzed coal samples of different coaJification grades. No dependence of Jl3C on the facies of the coal has been
clearly demonstrated. No change with increasing grade of metamorphism appears to occur.
It is quite remarkable that the process of coal formation, which
represents an extensive diagenetic alteration, has not left any measurable
fractionation on the Jl3C of coals. A final answer to the problem of a
possible isotope fractionation during coalification processes cannot by
given yet.
f) "Age Effect" of Carbon Isotope Distribution
This complex problem has been discussed by WELTE (1970). A variation of the carbon isotope composition in the reservoirs
CO2(alm) -limestone - biosphere
during the geological past might have occurred. The detection of this
effect is very complicated because diagenetic alteration might have
changed the J 13 C oflimestones and organic materials so that they do not
represent the primary composition.
From the careful studies of RONOY (1959), TRASK and PATNODE
(1939), and WELTE (unpublished data), we know that the distribution of
organic carbon in the sediments shows several maxima and minima
during earth's history. WELTE (1970) has argued that these irregularities
in the distribution are not so much due to a different preservation of
organic material, but due to real variations in the intensity of photosynthesis. KEELING (1960) has demonstrated that different photosynthesis
rates due to climatic change between summer and winter produce a
variation of about 1%0 in the isotopic composition of atmospheric CO2.
89
sions concerning the mode of formation and possible fractionations during migration of methane. Various attempts have been made in this
direction; three of these are mentioned. SACKETT (1968) argued that time,
temperature, and the structure of parent organic materials are the primary factors regulating the isotopic composition of methane. COLOMBO
et al. (1966, 1970) believed that they had demonstrated that during the
migration of methane the light isotope is preferentially enriched in the
highest zones of a gas field. Contradictory results have been obtained by
MA Yet al. (1968), who demonstrated a more rapid migration for 13CH4
than for 12CH4 with 12CH4 being better adsorbed than 13CH4.
e) Coal
Coals have a 13CrC distribution like that of modern land plants
(CRAIG (1953), WICKMAN (1953), COMPSTON (1960), COLOMBO et al. (1970)).
Brown coals have been extensively analyzed by FISCHER et al. (1968,
1970). COLOMBO et al. (1970) analyzed coal samples of different coaJification grades. No dependence of Jl3C on the facies of the coal has been
clearly demonstrated. No change with increasing grade of metamorphism appears to occur.
It is quite remarkable that the process of coal formation, which
represents an extensive diagenetic alteration, has not left any measurable
fractionation on the Jl3C of coals. A final answer to the problem of a
possible isotope fractionation during coalification processes cannot by
given yet.
f) "Age Effect" of Carbon Isotope Distribution
This complex problem has been discussed by WELTE (1970). A variation of the carbon isotope composition in the reservoirs
CO2(alm) -limestone - biosphere
during the geological past might have occurred. The detection of this
effect is very complicated because diagenetic alteration might have
changed the J 13 C oflimestones and organic materials so that they do not
represent the primary composition.
From the careful studies of RONOY (1959), TRASK and PATNODE
(1939), and WELTE (unpublished data), we know that the distribution of
organic carbon in the sediments shows several maxima and minima
during earth's history. WELTE (1970) has argued that these irregularities
in the distribution are not so much due to a different preservation of
organic material, but due to real variations in the intensity of photosynthesis. KEELING (1960) has demonstrated that different photosynthesis
rates due to climatic change between summer and winter produce a
variation of about 1%0 in the isotopic composition of atmospheric CO2.