Geoscience Reference
In-Depth Information
CHAPter 9
Earth's Middle Ages: hat Came after the GOE
Isn't it everyone's dream to travel in a time machine? Well, maybe not
everyone's, but most geologists I know would love to get their hands on
one. Of relevance for the story here, we could directly test with a time
machine if our ideas about the history of atmospheric oxygen are cor-
rect. We've pieced this story together from our reading of the geologic
record, but as mentioned before, this record gives an imperfect view of
the past. We pick up a rock, a piece of ancient sediment that was once
mud on the seafloor; maybe it's been heated, altering the minerals or
the organic matter, or maybe much later fluids have penetrated the rock,
altering its chemistry. The rock represents the sum of all the processes
that have influenced it during its long history on Earth, and not just
those we are most interested in.
Also, we can only surmise how the signals we are interested in were
frozen in place within the rock. For example, we understand from mod-
ern environments how molybdenum finds its way into the sediment (we
used molybdenum in
chapter 7
to record “whiffs” of oxygen into the
atmosphere). We know this because we can measure the concentration
of molybdenum in the water overlying sediments today. We can trace
its path from here and into sediments, and thus understand which pro-
cesses act to control this path. We can make budgets and test our under-
standing with models and experiments. Indeed, this kind of work forms
the basis for our reading of the geologic record. But, when we look at
