Geoscience Reference
In-Depth Information
an empty snail shell. Yet another clue. The organic matter in the mud also
holds clues. It is created from dead organisms either in the water overly-
ing the mud or in the mud itself. If we're lucky, this organic matter can
tell us something about the organisms once living in the environment.
The mud particles themselves react with chemical constituents in ocean
water as the particles settle to the seafloor. The mud reacts further after
it's in place on the seafloor, and as the sulfide and methane accumulate.
Even more clues. Overall, the mud is awash with clues. Our challenge,
as we shall see as the story unfolds, is to appreciate which clues are there
and to understand how they relate to the oxygen content of the atmo-
sphere. So Vernadsky was right, the clues are in the mud, but under-
standing them is not always easy.
OK, so let's grab a bunch of old sedimentary rocks and get started. We
go to the rock store and ask the lady at the counter for rocks from, let's
say, both the shallow and deep parts of the ocean. (hy not? A compari-
son might be interesting.). Let's get samples from every 10 million years
at ten places from around the globe (we want to ensure good coverage),
from the birth of the planet 4.55 billion years ago until 2.5 billion years
ago (the end of the Archean Eon, and the focus of the current chapter).
That's lots of rocks, but what the heck, better too many than too few. The
lady's jaw drops, and she opens the storage room door wide. We immedi-
ately see the problem. The shelves are mostly bare. As much as the shop-
keeper would love to help, the rocks just aren't there. In fact, there's noth-
ing on the shelves but a few small mineral grains from before about 4.0
billion years ago.
5
The first 500 million of years or so of Earth history is
basically not represented in the rock record. We find a few rocks from
3.8 billion years ago, some more at 3.5, some at 3.2, 3.0, 2.9, 2.7, and
some even younger, with rather better coverage up to 2.5 billion years
ago. But, in general, the early geologic record is very poor. The problem
is that the very tectonic processes we discussed in
chapter 1
—the subduc-
tion, mountain building, weathering, and the constant recycling of rocks
at the Earth surface—all these processes that help make Earth a terrifi-
cally habitable planet also wreak havoc on the geologic record. And the
older the rocks, the more likely they have been lost to weathering or oth-
erwise been caught in and been transformed by the macerations of Earth.
We thank the lady at the rock store, take what we can get, and we go
on our way. We pause under the shade of a nearby tree and see what we
