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10
0
8
0
6
4
34 S
δ
2
Mantle
0
-2
-4
4000
3000
2000
1000
0
Age (Ma)
Figure 9.9
Evidence for mass-independent isotope fractionation of sulfur isotopes in sediments
(Farquhar et al. , 2000 ) . The value of
34 S is a measure of the deviation from the
normal mass-dependant isotope fractionation behavior. Mass-independent isotope fractionations
prior to
33 S
33 S-0.5
= δ
δ
2.45 Ga indicate photolysis of atmospheric SO 2 by ultraviolet radiation and
disproportionation of sulfur compounds into reduced and oxidized species. After that time, the
rise of atmospheric
O 2 led to the formation of the ozone layer at the base of the stratosphere
and protected SO 2 from UV photolysis.
P
is very near zero. It was shown by Farquhar et al. (2000) ( Fig. 9.9 ) that prior to 2.45 Ga,
the S isotopic abundances deviated very substantially from the mass-dependent conditions
or, in other words, that
33 S was very different from zero. The interpretation calls for
different sulfur and oxygen cycles over geological times. In the modern atmosphere, ultra-
violet radiation does not reach the lower atmosphere: it is absorbed by the ozone layer
located in the stratosphere at
25 km above the ground before SO 2 pressure becomes
significant. Modern atmospheric SO 2 is therefore oxidized to sulfate and is washed down
with precipitation, all the reactions being characterized by regular mass-dependent isotope
fractionations. In contrast, prior to 2.45 Ga, the oxygen atmospheric pressure was too low
to produce a significant ozone layer. As shown by experiments, mass-independent isotope
fractionation results from the breakdown of SO 2 by ultraviolet radiation (UV photolysis)
and its subsequent recombination as reduced and oxidized compounds. The UV photolysis
of Archean atmospheric SO 2 therefore produced a variety of compounds that do not fol-
low a mass-dependent fractionation behavior, hence making non-zero
33 S values a strong
marker of the ozone layer and therefore of the total oxygen pressure. It was recently found
that
36 S correlates with
33 S, which makes the case of mass-independent fractionation
even stronger.
The isotopic record of sulfur and nitrogen provides additional clues about the evolution
of the pre-Phanerozoic atmosphere:
34 S values
1. Average values of sulfur isotopes of sedimentary pyrite show mantle-like
δ
(
0) from the oldest samples well into the late Proterozoic (
600 Ma). This reflects the
 
 
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