90
C. Variations of Stable Isotope Ratios in Nature
Assuming that the rate of photosynthesis has increased over several
million years and is related to a higher accumulation rate of the organic
material in the sediments, then a measurable isotope effect in the system
C02(atm) -
carbonate rocks - organic carbon
must have occurred. Such an "age effect" has not been proven with
certainty, but some indications favor it.
Several authors (e.g., COMPSTON, 1960; KEITH and WEBER, 1964; WEBER, 1967) have tried to find an "age effect" in limestones. However,
because of possible diagenetic alterations, limestones are not very suitable. It also seems almost impossible to demonstrate an age effect on
crude oils, because of the mobility of oils and their variability in composition.
Kerogen-like material present in shales seems more suitable for an
analysis of this type (WELTE, 1970).
Most interesting in this connection is the question of the carbon
isotopic composition during early Precambrian time. From the available
data we must conclude that at least during late Precambrian time the
13Cj 12 C ratios were not too different from today.
2. Hydrogen
Plants remove hydrogen from water and transfer it to organic compounds. Although many organic hydrogen atoms are exchangeable with
environmental water, once a carbon-hydrogen bond is formed in an
organism, the hydrogen seems to be no longer readily exchanged. Studies on the natural abundance of deuterium and hydrogen in organic
matter done by BOKHOVEN and THEEUWEN (1956) and RITTENBERG
(1963) made no attempt to relate the D/H ratios from organic matter to
the hydrogen source, e.g., environmental water.
More recent determinations by ZBOROWSKI et al. (1967), SCHIEGL and
VOGEL (1970), and SMITH and EpSTEIN (1970) showed that the plants are
depleted in deuterium relative to the environmental water.
SMITH and EpSTEIN (1970) gave fractionation factors from water hydrogen to whole plant hydrogen from 1.008 to 1.085, with the average
value of 1.055. Fractionation factors from whole plant to lipid ranged
from 1.060 to l.i41, with an average value of 1.092.
SCHIEGL and VOGEL (1970) attempted to correlate organic D/H ratios with waters precipitated from the atmosphere. They observed three
differen t fractionation effects:
1) a depletion in the deuterium content of several percent during the
conversion of water to organic matter in living plants,
2) a decrease in the D-concentration going from living plants to coal,
and
C. Variations of Stable Isotope Ratios in Nature
Assuming that the rate of photosynthesis has increased over several
million years and is related to a higher accumulation rate of the organic
material in the sediments, then a measurable isotope effect in the system
C02(atm) -
carbonate rocks - organic carbon
must have occurred. Such an "age effect" has not been proven with
certainty, but some indications favor it.
Several authors (e.g., COMPSTON, 1960; KEITH and WEBER, 1964; WEBER, 1967) have tried to find an "age effect" in limestones. However,
because of possible diagenetic alterations, limestones are not very suitable. It also seems almost impossible to demonstrate an age effect on
crude oils, because of the mobility of oils and their variability in composition.
Kerogen-like material present in shales seems more suitable for an
analysis of this type (WELTE, 1970).
Most interesting in this connection is the question of the carbon
isotopic composition during early Precambrian time. From the available
data we must conclude that at least during late Precambrian time the
13Cj 12 C ratios were not too different from today.
2. Hydrogen
Plants remove hydrogen from water and transfer it to organic compounds. Although many organic hydrogen atoms are exchangeable with
environmental water, once a carbon-hydrogen bond is formed in an
organism, the hydrogen seems to be no longer readily exchanged. Studies on the natural abundance of deuterium and hydrogen in organic
matter done by BOKHOVEN and THEEUWEN (1956) and RITTENBERG
(1963) made no attempt to relate the D/H ratios from organic matter to
the hydrogen source, e.g., environmental water.
More recent determinations by ZBOROWSKI et al. (1967), SCHIEGL and
VOGEL (1970), and SMITH and EpSTEIN (1970) showed that the plants are
depleted in deuterium relative to the environmental water.
SMITH and EpSTEIN (1970) gave fractionation factors from water hydrogen to whole plant hydrogen from 1.008 to 1.085, with the average
value of 1.055. Fractionation factors from whole plant to lipid ranged
from 1.060 to l.i41, with an average value of 1.092.
SCHIEGL and VOGEL (1970) attempted to correlate organic D/H ratios with waters precipitated from the atmosphere. They observed three
differen t fractionation effects:
1) a depletion in the deuterium content of several percent during the
conversion of water to organic matter in living plants,
2) a decrease in the D-concentration going from living plants to coal,
and