Geoscience Reference
In-Depth Information
perhaps even higher concentrations of oxygen. This excursion was fol-
lowed soon after by a crash in oxygen to very low levels and a return to
BIF deposition. One can surmise that these low levels of oxygen were,
in fact, a consequence of the Lomagundi excursion. It's easy to imagine
that when the massive amounts of organic carbon buried during the
excursion were brought into the weathering environment,
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they would
have represented a huge oxygen sink, drawing down levels of atmo-
spheric oxygen. We appear, then, to have a veritable seesaw in oxygen
concentrations, apparently triggered initially by the GOE.
Now back to Simon. If you recall, we were interested in any possible
evidence to support or refute the idea that with an increase in atmo-
spheric oxygen levels, sulfide expanded into marine waters, bringing
an end to BIF formation. As mentioned, rocks in northern Minnesota
and southern Ontario became our focus, and in particular, we looked
toward the Gunflint Iron Formation deposited about 1.9 billion years
ago. As just argued above, this BIF, and other BIFs from the same time,
appear to have deposited during a period of very low atmospheric oxy-
gen concentrations. But, what happened just after the Gunflint stopped
depositing? To tackle this problem, we contacted the effervescent Phil
Fralick from Lakehead University in Thunder Bay, Ontario. Phil prob-
ably knows the Gunflint Iron Formation better than anyone. During a
memorable trip, Phil took us throughout the region to look at both the
Gunlint and the sediments depositing just after. hat struck us in the
field was that the Gunflint gave way to a very black shale known as the
Rove Formation. This was encouraging because black shales were often
deposited in sulfidic waters.
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The rocks we saw in the field were great for understanding the rela-
tionship between the Gunflint and the Rove Formations, but not great
for doing chemistry. As we have learned, such rocks lay exposed to the
weather and rain, becoming oxidized in the process. Any pyrite, for ex-
ample, originally in the rocks may well have been oxidized away. Luck-
ily, Phil knew of fresh rocks, recently cored from underground, which
nicely covered the transition from the Gunflint Iron Formation to the
overlying Rove Formation. The core, it turns out, came from a curious
farmer with a drill rig who once or twice a year drills a core through the
rocks underlying his land in order to prospect for precious metals. hen
the cores turn up nothing of interest, he donates them to the Ontario
