Geoscience Reference
In-Depth Information
24
Marine BaSO
4
(Paytan et al.,
2004)
22
20
18
16
Neogene
Paleogene
Cretaceous
14
0
20
40
60
80
100
120
140
Figure 9.8
Sulfur isotope compositions of marine barites, BaSO
4
, throughout the Cretaceous and the Tertiary
in
34
S reflect strong variations in the burial rates of sedimentary pyrite and global changes in the
redox system of the ocean.
δ
mineral oversaturated in the surface ocean but undersaturated at depth. As for most sulfates,
sulfur isotope fractionation upon barite precipitation is very small. The obvious advantages
of this mineral over more common evaporites are its clearly marine origin and its shown
immunity to diagenetic reworking. By about 125 Ma, the
34
S decreased by 4 per mil,
oscillated up and down for most of the Cretaceous and the Paleocene, and sharply increased
to modern marine-sulfate values at about 55 Ma (22
δ
); it has stayed nearly constant ever
since. There is probably no correlation with OAEs. Although such large variations must
reflect variable burial rates of sedimentary pyrite, the fast rate of change, in particular at
55 Ma, suggests either very large sulfur fluxes or fairly low sulfate concentrations in the
ocean.
9.2 The rise of atmospheric oxygen
9.2.1 The 2.1 Ga crisis
Charles Lyell's uniformitarianism states that “the present is key to the past,” which asserts
that fundamentally the same geological processes operate today as in the distant past. In
modern language, we could simply state that the Earth has long since reached some form of
steady state. Although this view proved helpful to elucidate some geological observations,
the memory of the first geological times did not just disappear overnight. If there is a
