Atmosphere
79
al. (1972) have shown that the oxygen isotope composition of ancient
evaporite sulfates is not constant. However, more data are needed before
an "age-effect" may be established.
Sulfates from nonmarine environments range over a wide spectrum.
LONGINELLI and CORTECCI (1970) determined the b 18 0 of the dissolved
sulfate in river waters. Large isotopic variations in both oxygen and
sulfur take place along the course of the analyzed rivers in the direction
of a progressive enrichment in the heavy isotopes. Several factors seem
to be responsible for these variation, i.e., human and microbiological
activity.
c) Evolution of the b 34 S of Ocean Water Sulfate during
the Geologic Past
As has already been mentioned, the b-value of the ocean sulfate has
not been constant during the earth's history. The total range of variation
has been at least 20%0. HOLSER and KAPLAN (1966) have stated that the
possible implications of any such changes reach far beyond the significance of the change in sulfate itself and have suggested some implications.
NIELSEN (1965) has postulated that the normal condition of the ocean
is one where the sulfur input rate is equal to the output rate and where
b (input) = b (output). In order to change b (ocean) it is necessary to increase the output rate of bacterial sulfate reduction and/or the input rate
of sulfur to the ocean reservoir. A similar model has been given by
HOLSER and KAPLAN (1966). According to these authors, changes of
b(ocean) reflect changes in the total sulfate content of the ocean. These
authors postulated that between Cambrian and Permian times, this content should have increased by nearly a factor of 2 and that between
Permian and Recent time it should have decreased by some 30%0.
REEs (1970) developed a somewhat different model in which the process of evaporite formation plays a key role. With two mechanisms
competing for the removal of sulfate from ocean water, one of which
involves isotope fractionation (sulfide formation) and one of which does
not (evaporite formation), the net fractionation produced in the ocean
water reservoir will depend on the relative extent of these mechanisms.
VI. Atmosphere
The atmosphere consists of a mixture of a number of gases. The
chemical composition is quite simple, being made up almost entirely of
three elements: nitrogen, oxygen, and argon. Other elements and compounds are present in amounts that, although small, are nevertheless
significant in terms of the geochemistry of the atmosphere. The atmo-
79
al. (1972) have shown that the oxygen isotope composition of ancient
evaporite sulfates is not constant. However, more data are needed before
an "age-effect" may be established.
Sulfates from nonmarine environments range over a wide spectrum.
LONGINELLI and CORTECCI (1970) determined the b 18 0 of the dissolved
sulfate in river waters. Large isotopic variations in both oxygen and
sulfur take place along the course of the analyzed rivers in the direction
of a progressive enrichment in the heavy isotopes. Several factors seem
to be responsible for these variation, i.e., human and microbiological
activity.
c) Evolution of the b 34 S of Ocean Water Sulfate during
the Geologic Past
As has already been mentioned, the b-value of the ocean sulfate has
not been constant during the earth's history. The total range of variation
has been at least 20%0. HOLSER and KAPLAN (1966) have stated that the
possible implications of any such changes reach far beyond the significance of the change in sulfate itself and have suggested some implications.
NIELSEN (1965) has postulated that the normal condition of the ocean
is one where the sulfur input rate is equal to the output rate and where
b (input) = b (output). In order to change b (ocean) it is necessary to increase the output rate of bacterial sulfate reduction and/or the input rate
of sulfur to the ocean reservoir. A similar model has been given by
HOLSER and KAPLAN (1966). According to these authors, changes of
b(ocean) reflect changes in the total sulfate content of the ocean. These
authors postulated that between Cambrian and Permian times, this content should have increased by nearly a factor of 2 and that between
Permian and Recent time it should have decreased by some 30%0.
REEs (1970) developed a somewhat different model in which the process of evaporite formation plays a key role. With two mechanisms
competing for the removal of sulfate from ocean water, one of which
involves isotope fractionation (sulfide formation) and one of which does
not (evaporite formation), the net fractionation produced in the ocean
water reservoir will depend on the relative extent of these mechanisms.
VI. Atmosphere
The atmosphere consists of a mixture of a number of gases. The
chemical composition is quite simple, being made up almost entirely of
three elements: nitrogen, oxygen, and argon. Other elements and compounds are present in amounts that, although small, are nevertheless
significant in terms of the geochemistry of the atmosphere. The atmo-