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(and other places as well) become seasonally anoxic in their deep wa-
ters. However, these lakes generally have low concentrations of sulfate,
so they can only support limited amounts of sulfate reduction. There-
fore, when these lakes go anoxic, dissolved iron (so-called ferrous iron;
there will be much more on this in
chapter 7)
rather than sulfide accu-
mulates. This situation was also very common in the oceans during cer-
tain times in the geologic past, as explored in subsequent chapters. If
you remember the oxygen-free waters of the coast of Chile discussed in
the last chapter, neither iron nor sulfide accumulates, but these waters
are instead dominated by nitrogen chemistry. Important though, is that
all types of water column anoxia lead to reduced rates of organic matter
decomposition (compared to when decomposition uses oxygen), lead-
ing in turn to enhanced concentrations of organic carbon in the sedi-
ments depositing from these waters.
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Pyrite burial, in contrast, is gen-
erally enhanced only under euxinic water-column conditions (when the
water column contains sulfide). In what follows, I will refer to anoxic
water columns in general, which could mean any of the types just ex-
plored, unless otherwise stated.
So, with this digression behind us, we can imagine an oxygen feed-
back based the expansion and contraction of marine anoxic conditions.
It is generally expected that the amount of anoxia in the deep waters
of the ocean would increase as the concentration of atmospheric oxy-
gen is lowered. This makes sense because with lower atmosphere oxygen
levels, less oxygen will be mixed and transported into the deep ocean.
However, with the Black Sea as an example, as anoxic conditions ex-
pand, the burial rates of organic carbon and pyrite sulfur (if the water-
column is euxinic) should also increase. The increased burial of organic
carbon and pyrite would generate a greater oxygen source to the atmo-
sphere and thereby increase the concentration of atmospheric oxygen.
This effect, however, has a limit. Elevated oxygen levels will, in turn,
reduce the extent of ocean anoxia; this reduces organic carbon and py-
rite burial rates and thus reduces the strength of the atmospheric oxygen
source. Such a chain of events provides a nice stabilizing mechanism
(negative feedback) for keeping oxygen concentrations from getting
either too low or too high.
If we dig deeper, we find that ocean anoxia produces additional feed-
backs on oxygen concentration. To understand these, we need to follow
