Geoscience Reference
In-Depth Information
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
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
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