Geoscience Reference
In-Depth Information
Detrital uraninite
and pyrite
Redbeds
3
2.5
2
1.5
1
0.5
0
-0.5
-1
2200
2250
2300
2350
2400
2450
Age (millions of years)
Figure 8.2. Changes in the mass independent (mIF) sulfur isotope signal through the
Huronian supergroup. Also shown are the stratigraphic levels where red beds and detrital
uraninites and pyrites are found. gray areas mark glaciations. sulfur isotope data kindly
provided by James Farquhar.
Farquhar's sulfur isotopes. Recall from the last chapter that the mass-
independent distribution of sulfur isotopes was the rule in the Archean
Eon, and that these fractionations were likely formed under very low
concentrations of atmospheric oxygen. One of the beautiful aspects of
sulfur isotopes is that most marine rocks have some type of sulfur spe-
cies that we can analyze. This will usually be pyrite, but in some cases it
can also be sulfate minerals. Marine rocks are almost continuously rep-
resented in the time around the GOE, so sulfur isotopes from the sulfur
species associated with these rocks should yield a far more complete
oxygen history.
So, let's return to the rocks of southern Ontario and measure sulfur
isotopes. This, in fact, was done by Dominic Papineau of Boston Col-
lege and also by James Farquhar and his group. In addition, James
and his colleagues analyzed rocks from South Africa of similar age, from
the so-called Transvaal Supergroup. Taken together, the results show
that the mass-independent sulfur isotope signal switches to the “nor-
mal” mass-dependent signal between the last of the uraninites and the
transition marks the GOE, at least as it influenced the mass-independent
sulfur isotope signal. Our best understanding, then, is that about 2.3 to
2.35 billion years ago, oxygen concentrations rose to greater than 0.001%
