Geoscience Reference
In-Depth Information
rock type called banded iron formation (frequently just called BIF) in
early Earth oceans as evidence of an ocean chemistry different than to-
day's. We didn't give many details though.
Our goal now is to take a closer look at these banded iron formations
and to explore in more detail what they mean for early ocean chemistry
and atmospheric oxygen levels. To do this, we book a ticket to Perth,
Australia, rent a car, and take a trip some 1400 kilometers north to the
Karijini National Park (formerly the Hamersley Range National Park).
As we drive, we pay attention to the kangaroos; they dart out in front of
the car with no notice, and we keep a special lookout for road trains. We
see their dust far in the distance, but we pull over to the side in good
time and hold the steering wheel tight as they pass. They come fast, and
as far as I can tell, they don't alter their course for anyone or anything.
We also watch the gas gauge. hat are listed as towns on the map in this
part of Australia are gas stations. They are few and far between. Miss
one, and you might not make it to the next.
The Karijini National Park contains rocks that are described geologi-
cally as belonging to the Hamersley Basin. We enter the park and drive
to one of the deep gorges cut by rivers through the layers of sedimen-
tary rock. We descend to the bottom of the gorge (
plate 3
) and look
more closely at the rock. We are struck first by its colors: blood red, al-
ternating with light red, gray, and white layers. hen we look up and
all around; this layering is apparent in every direction. We also remem-
ber that we passed kilometer after kilometer of similar rocks as we drove
to the entrance of the gorge.
The red coloration in these rocks is due to a high concentration of
iron minerals. These rocks are examples of the banded iron formations
we've been talking about, and they reveal much about the chemistry of
the oceans at the time they were formed. First, there's their wide distri-
bution in both space and time.
4
Simply stated, they are a common rock
type from early Earth. Then there's the iron; these massive amounts of
iron are typically found in layers that may extend over great distances
of kilometers or more. Indeed, key insights into the chemistry of the
ocean and atmosphere come from understanding where all of this iron
originated and how it made its way to the seafloor.
To understand this, we need to know something about the chemis-
try of iron (Fe). There's pure metallic iron, we all know about that, but
