Geoscience Reference
In-Depth Information
still holds the answer. As the model suggested, increased oxygenation
led to more pyrite weathering and a higher flux of sulfate to the ocean.
This led to more sulfide production from sulfate reduction, expanded
sulfidic conditions, and enhanced removal of Fe
2+
from solution. Even
so, overall, the flux of Fe
2+
to the ocean was apparently still greater than
the rates of sulfide production by sulfate reduction.
15
Thus, while sul-
fide could accumulate in those regions of the ocean where rates of sul-
fate reduction were high, in other regions, Fe
2+
dominated, generating
ferruginous waters. It appears, however, that there was enough sulfide
produced by sulfate reduction to react with a sizable proportion of the
dissolved Fe
2+
from the oceans. This meant that while ferruginous con-
ditions could still persist in large volumes of the anoxic ocean, the
concentrations of Fe
2+
were significantly lower than during times when
BIFs were actively forming; quite simply, with lower Fe
2+
concentra-
tions, there was not enough iron dissolved in the ocean to support active
BIF deposition. Thus, it appears that the Canfield Ocean model had it
right in general, while not in all the specifics.
Perhaps most important to the present discussion, however, is that
deep anoxic ocean waters appear to have been the normal state through
Earth's middle ages. This implies that atmospheric oxygen concentra-
tions were considerably lower than today. But how much lower? The
original ocean modeling that started me on this track suggested that
vast expanses of deep anoxic ocean water would develop as atmospheric
oxygen dipped to some 40% to 50% of present levels. Unfortunately,
the geologic record does not provide a more precise estimate. How-
ever, I believe that oxygen levels were much lower than this, probably
less than 10% of present levels. This assertion is based not so much on
the evidence we've discussed so far, but rather, on what comes in the
next two chapters.
