Geoscience Reference
In-Depth Information
60
40
20
0
-20
-40
-60
0
0.5
1
1.5
2
2.5
3
3.5
4
Age (Ga)
Figure 9.10
34 S values in pyrites from sediments (compilation courtesy of Don Canfield) over
the geological ages. The values remain close to zero until
Evolution of the
δ
600 Ma, which suggests that until
then the bulk of oceanic sulfur was dominantly in the form of sulfide. The rise of
O 2 in the late
Proterozoic shifted sulfur speciation in seawater from sulfide to sulfate, yet the incoming fluxes
maintained the
P
34 S of this reservoir at mantle values. Sulfur isotopes in
δ
<
600-Ma-old pyrite are
substantially fractionated with respect to marine sulfate (Canfield, 2004 ) .
fact that oceanic sulfur is dominantly of mantle origin (volcanic eruptions, hydrothermal
activity). Because sulfur isotopes fractionate between sulfates and sulfides by about 20
per mil ( Chapter 3 ) , this record indicates the lack of competing reservoirs of sulfate and
sulfur ( Fig. 9.10 ) prior to the Phanerozoic. This suggests that, prior to the Phanerozoic,
most of the oceanic sulfur was not present as sulfate but was dominantly in the form of
sulfide delivered by magmas and hydrothermal activity and went to the sediments with
no isotopic fractionation.
2. The nitrogen isotopes of sedimentary kerogen show a strong increase in
15 N values
from slightly negative mantle-like values prior to 2.7 Ga to positive values (up to
δ
10)
over the late Archean, Precambrian, and Proterozoic. These data can be interpreted as
indicating that nitrification and denitrification did not operate in the early Archean the
way they operate today.
3. In contrast, the
+
13 C values of ancient kerogen are in the range of their modern
δ
counterparts.
9.2.2 The Snowball Earth and the emergence of Metazoans
A second major oxygen crisis leading to another major increase in atmospheric P O 2 took
place during the Late Proterozoic. The BIFs that had disappeared from the surface of
the Earth after 1.85 Ga returned on different continents between 800 and 600 Ma, at
localities such as Rapitan, Canada, and Urucum, Brazil, and give the first indication that
 
 
 
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