Geoscience Reference
In-Depth Information
We start where we left off in the last chapter. That is, with evidence for
low-oxygen conditions dominating through the Archean Eon (punctu-
ated by some apparent “whiffs” of oxygen near the end). Preston Cloud,
like Dick Holland before him, came to this same conclusion (without
the whiffs though). But, Cloud went a step further. If atmospheric oxy-
gen concentrations were low in the Archean, Cloud asked, when did
they begin to rise? To answer this question, Cloud appealed to his ex-
perience scrambling through the rocks just north of Lake Huron in
southern Ontario while working at the University of Minnesota
(ig.
8.1
). The rocks here are part of a large sequence of geologic formations
known as the Huronian Supergroup, which range in age from about
2.5 to 2.2 billion years and thus span the age of interest. Cloud noted
that the older of these rocks, with ages from about 2.4 to 2.5 billion
years,
1
contained ancient river deposits with detrital uraninite and pyrite,
similar to the rocks from South Africa we discussed in the last chapter.
However, walking up into younger rocks, the uraninite and pyrite dis-
appeared as did any sign of banded iron formations (BIFs).
Cloud also noted a particularly intense red staining in some of the
sandstones found higher in the sequence (see
ig. 8.1
), above where de-
trital uraninite and pyrite were last observed. These rocks are known as
red beds, and the red coloration reflects iron in its oxidized form. How-
ever, these rocks are very different from the BIFs we saw earlier. In many
cases, red beds are formed on land or in shallow waters, and while the
red coloration is prominent, these red layers are not supercharged with
iron, unlike those found in BIFs.
2
Piecing all this evidence together, Preston Cloud first, and Dick Hol-
land later, argued for a substantial increase in atmospheric oxygen con-
centrations around 2.3 to 2.4 billion years ago. The uraninite and pyrite
disappeared as they were completely oxidized away in elevated levels
of atmospheric oxygen. Furthermore, in Cloud's view, BIFs stopped
depositing as the increased levels of oxygen swept into the deep ocean,
oxidizing the ferrous iron away to rust. The red beds, in contrast, formed
as a direct result of weathering on land in an oxygenated atmosphere.
Holland dubbed this transition to higher oxygen levels the “great oxida-
tion event,” or GOE for short. The GOE represents a quantum shift in
the oxygen content of the atmosphere. This is a big deal.
