Geoscience Reference
In-Depth Information
churnings of Earth. As we shall see in
chapter 7,
these gases have played
a critical role in regulating the concentrations of atmospheric oxygen
during some periods of Earth history.
hat about the other oxygen removal pathways, such as the oxida-
tion of organic carbon and pyrite in sedimentary rocks, that we talked
about before? These are equivalent to the drain in our bathtub example.
Experiments have shown that the oxidation rates of both pyrite and
organic carbon may indeed depend on atmospheric oxygen concentra-
tion, with more rapid oxidation at higher oxygen levels. This is a poten-
tial negative feedback on oxygen drawdown. If oxygen gets too low,
then the rate of removal by the oxidation of organic carbon and pyrite
decreases. As a irst-year graduate student at Yale, I saw this point de-
bated by Karl Turekian, a professor at Yale, and Bob Garrels,
7
whom
we briefly met earlier in the chapter. Karl Turekian was a gladiator, an
inspirational scientist who thrived on confrontation. Bob Garrels was a
geochemistry sage, soft spoken and mild mannered. Garrels was giving
the talk, and he made the point that oxygen concentration likely acted
as a negative feedback on the weathering rates of organic carbon and
pyrite. Karl then leaped out of his chair and savoring a fight, he roared
that this couldn't be right. Weathering rates are controlled by geologi-
cal uplift rates and the rates at which new rock is brought into the zone
of weathering at Earth's surface. Once there, he continued, all of the
organic carbon and pyrite will be oxidized before the weathered rock is
carried into rivers and transported back out to sea. Feeling the tempera-
ture rise, I was tempted to climb under my desk. Garrels, on the other
hand, was the picture of Buddhist calm; he smiled and said, “well, that's
your opinion Karl,” and continued with his talk.
In fact, Karl, as usual, made a good point. If the organic matter and
pyrite is completely oxidized during weathering and transport down-
river on the way to the sea, then there will be no oxygen feedback on the
rates of organic matter and pyrite oxidation. Indeed, the significance
of this feedback may well depend on the actual oxygen content of the
atmosphere. Let's go back to the rock outcrop we discussed earlier; this
is the one where we observed the weathered rock at the surface, and
where the pristine rock was uncovered only after some digging. Instead
of just observing the rock, let's collect samples at regular intervals from
the weathered rock surface and well into the fresh rock. We take these
