Geoscience Reference
In-Depth Information
processes including seafloor subduction and continent-continent col-
lisions.
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Thus, the “rapid recycling” of these organic carbon-rich and
pyrite-rich rocks into the zone of weathering and oxidation would soon
generate a strong oxygen sink. The end result is that the strong oxygen
source represented by the initial burial of these rocks is tempered by
a strong oxygen sink from their subsequent oxidation. Conversely, if
there is a time of sediment deposition with low organic carbon and py-
rite contents, the influence of this weak oxygen source is dampened by
the weak oxygen sink generated from the rapid recycling and subse-
quent oxidation of this same sediment. This keeps oxygen from getting
too low. The end result of rapid sediment recycling is to keep atmo-
spheric oxygen concentrations from becoming either too high or too low.
Note that this mechanism of oxygen stabilization does not represent
a feedback on oxygen concentration; it doesn't depend on the oxygen
content of the atmosphere. Rather, it is a stabilizing mechanism based
on the coupling between sediment burial, tectonics, and weathering.
The feedbacks and controls described above relate to oxygen sinks.
We can also identify various feedbacks associated with oxygen produc-
tion. One important group of feedbacks relates to the influence of ocean
anoxia on oxygen regulation.
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We start this discussion by looking at
sediment from the deep part of the Black Sea. The Black Sea is the larg-
est anoxic basin in the world; below about 70 to 80 meters depth, the
water is oxygen free, and if you take water from below about 120 meters
depth, it stinks of sulfide, the product of sulfate reduction. Such sulfidic
basins are known as euxinic basins, after the Greek name for the Black
Sea,
Pontus Euxinus
.
The sediments depositing in the euxinic waters of the Black Sea are
devoid of animals. They are also finely laminated and full of organic
carbon and pyrite sulfur, much more than we find in ordinary mud set-
tling around the coast of Denmark or anywhere near the beaches you
might frequent on your summer holiday. Indeed, the burial rates of or-
ganic carbon and pyrite are enhanced when the water column becomes
euxinic.
Before we go further, however, there is an important detail that we
need to address. In fact, not all types of water-column anoxia lead to
the sort of euxinic conditions we find in the Black Sea. For example,
many lakes in the midwestern part of the United States where I grew up
