Geoscience Reference
In-Depth Information
the fates of nitrogen and phosphorus, two of the most important nu-
trients whose availability controls rates of primary production in the
oceans. Let's start with phosphorus. In fact, most geochemists generally
view phosphorus availability as the most important factor controlling
the rates the primary production in the oceans.
12
With this in mind,
my fellow Yale PhD student Ellery Ingall, now at the Georgia Institute
of Technology in Atlanta, made an astounding discovery during his dis-
sertation research. He found that both modern and ancient sediments
deposited in anoxic settings, like the Black Sea, contained much less
phosphorus than did sediments deposited under water columns contain-
ing oxygen.
13
This observation leads to additional feedbacks on atmo-
spheric oxygen concentrations. Therefore, in light of Ellery's observa-
tions, more expansive anoxic conditions should result in less phosphorus
removal from the water column into marine sediments. We already learned
that anoxic conditions enhance the burial of organic carbon and also
pyrite in some cases, but with less phosphorus removal from the water
into these sediments there should be more phosphorus available to fuel
still higher rates of primary production. This would result in even higher
rates of organic carbon burial and even higher rates of oxygen libera-
tion to the atmosphere. Hence, the phosphorus cycle acts to enhance
the negative feedback we just discussed on the role of anoxia in keeping
oxygen levels from getting too low.
We see the same enhanced effect when oxygen levels rise. With higher
oxygen levels, there is a reduction in the expanse of anoxic ocean water.
This is accompanied by an increase in the expanse of oxygenated waters
with a consequent increase in the removal rate of phosphorus from
these oxygenated waters into the deposited sediments. The increase in
the phosphorus removal rate will decrease the amount of phosphorus
in ocean waters, which should reduce rates of primary production. Re-
duced rates of primary production should lead to reduced rates of or-
ganic carbon burial and, therefore, reduced rates of oxygen liberation
to the atmosphere. This keeps oxygen concentrations from getting too
high. These elegant ideas were first presented by Ellery, together with
Philippe van Cappellen, another of my PhD student contemporaries
working with Bob Berner.
With all this discussion of phosphorus, we must not forget about
nitrogen. We took a brief look at the nitrogen cycle in the last chapter
