Geoscience Reference
In-Depth Information
Northwestern University, Fred Mackenzie at the University of Hawaii,
and Lee Kump, one of Bob Garrels's last PhD students and now at Penn
State University.
In fact, before Bob Berner began modeling atmospheric oxygen, Lee
Kump and Bob Garrels published an earlier modeling attempt, pro-
ducing a history of atmospheric oxygen concentrations through the lat-
est 100 million years of Earth history. This is a wonderful piece of work,
where oxygen liberation rates to the atmosphere were quantified from
the carbon and sulfur isotope record (as explored in
chapters 8
and
9)
,
and expressions were derived to quantify oxygen removal rates during
the weathering of carbon and sulfur-bearing rocks. The oxygen concen-
tration in the atmosphere became the kinetic balance between the rates
of oxygen production and consumption. In general, Kump and Garrels
found that oxygen concentrations were elevated when the isotope record
indicated high rates of oxygen liberation to the atmosphere and lower
when the isotope record indicated lower rates of liberation. This is per-
haps not too surprising, but the challenge, as beautifully embraced by
Kump and Garrels, was to figure out how all of these processes were
quantitatively linked and how oxygen concentrations could be extracted
from the isotope data.
Indeed, Bob's first entrance in oxygen modeling built solidly on this
work but added an important extra component, which is the idea of
rapid recycling. This idea was discussed in
chapter 5,
and the basic point
is that the most recently deposited sediments will also be the most prone
to weathering through processes like sea-level change or uplift of the
land. Thus, through rapid recycling, high rates of oxygen production
through the burial of organic-rich sediments will quickly lead to high
rates of oxygen consumption through the exposure of these organic-
rich sediments to weathering. From a modeling perspective, and as
explored in
chapter 5,
rapid recycling (along with a number of negative
feedbacks) helps to dampen oxygen changes. This is important because
the fluxes of oxygen through the atmosphere during organic carbon and
pyrite burial, and by weathering, are huge compared to the relatively
small amounts of oxygen in the atmosphere. Thus, all of the oxygen in
the present atmosphere is cycled through geologic processes of oxygen
liberation (organic carbon and pyrite burial) and consumption (weath-
ering) on a time scale of about 2 to 3 million years. This may seem long
