Geoscience Reference
In-Depth Information
the rock we have just the concentration of molybdenum in the rock and
maybe some other chemical species to give us some clues to the chem-
istry of ancient waters. We don't have the water from which the molyb-
denum was derived. Therefore, in the end, we can only estimate its con-
centration in the ancient oceans. This story is more or less the same for
all of the indicators we use to understand the chemistry of the ancient
ocean. We constantly get better at reading the geologic record, but the
clues are rather few, and our interpretations are subject to big uncertain-
ties. We could answer lots of questions with a time machine.
Our topic at hand is the aftermath of the GOE. In fact, to understand
what came after the GOE, we need to go back and look in even more
detail at what came before. I know we talked about the GOE in the last
chapter, and what came before in
chapters 6
and
7,
but we need to look
for small things that we may have overlooked. Things that may be dif-
ficult to determine from our reading of the geologic record. So, let's use
our time machine and travel to the time just before the GOE. We put
on our rubber boots (and an oxygen mask for good measure) and wade
through ancient rivers. We're looking for pyrite. As revealed in
chap-
ter 7,
before the GOE some pyrite settled as pebbles, unoxidized, into
ancient riverbeds. But, very few of these rivers have survived, and we
want to determine if pyrite transport in rivers was common or rare.
Could some of the pyrite have been oxidized? Did only the big pieces
of pyrite survive oxidation, while the smaller ones oxidized away? If we
could go back in time, we could follow the pyrite from its origin in
rocks undergoing weathering on land to its final resting place.
This would allow us to decide how the pyrite was cycled and recycled.
We could see how much of the pyrite liberated during weathering on
land was transported through rivers, into the sea, and back to sediments
again, with these sediments themselves ultimately turning back to stone.
If pyrite recycled this way, without oxidation in a low-oxygen atmo-
sphere, then it would accumulate to higher and higher concentrations in
sedimentary rocks as volcanic gases continued to deliver more and more
sulfur to the surface of Earth. Such cycling would likely be true for other
oxygen-sensitive minerals as well. This sequence of events makes good
sense in a very low-oxygen atmosphere, but was this the way it happened?
We could also look at the fate of organic matter as it was weathered
from ancient shales because these rocks house most of the organic matter
