Geoscience Reference
In-Depth Information
Phanerozoic
Proterozoic
Archean
10
8
6
GOE
4
2
0
-2
-4
0
500
1000
1500
2000
2500
3000
3500
4000
Age (millions of years)
Figure 7.2. Compilation of the mass-independent sulfur isotope signal through earth history
as expressed as
Δ
33
s (see text for details). Also shown are the major eons of earth history,
and the great oxidation event, or geO, which will be discussed in the next chapter. Data
compilation kindly made available by James Farquhar.
32
S and
33
S (33 - 32 = 1), a two mass unit difference between
32
S and
34
S
(34 - 32 = 2), and a four mass unit difference between
32
S and
36
S (36 -
32 = 4). In practice, if there is some process that fractionates isotopes,
the effect should be about half as large between
32
S and
33
S compared
to the effect between
32
S and
34
S. And, the effect between
32
S and
36
S
should be about twice as large as between
32
S and
34
S. hese patterns
are followed through almost all biological and geological processes that
fractionate sulfur isotopes. Such fractionations are known as mass de-
pendent, because the size of the fractionation depends on the mass dif-
ferences between the isotopes.
James saw something very different. In Archean rocks (and indeed
rocks older than about 2.3 to 2.4 billion years), the isotopes did not be-
have as expected. The fractionations were not mass dependent. Rather,
they deviated from these trends, generating signals that are described as
mass independent. This relationship is shown in
igure 7.2.
Plotted on
this figure is a parameter called Δ
33
S. This parameter simply designates
the difference between the fractionations that are expected based on
mass-dependent behavior and what is observed. If Δ
33
S is zero, the world
behaves as we expect, and indeed this is case after about 2.3 to 2.4 bil-
lion years ago. This transition to normal behavior will be the focus of
the next chapter, but I think we can all agree that something weird was
happening before 2.3 to 2.4 billion years ago. But what was it?
