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Th e absence of sulphide discoloration in the lithographic stone
of Solhofen indicates clearly that conditions were not wh olly
annoxic. In presentday analogues of lagoons and lakes with anoxic
bottom waters the anoxia is almost always periodic, often annual.
Betweenwhiles, when the oxygenated water penetrates to the bottom,
the b enthos can flourish. In waters wh ere the bottom is
permanaently anoxic then the sediments becomes blackened with
sulphides. This has not h appened in the lithogrpahic beds, but
the evidence is clear that the bottom waters were soemtimes
anoxic. To me the most obvious conclusion seesm to be that during
long periods when the bottom co ntains oxygenated water then
pelagic creatures, and terrestrial cadavers, would not have
fossilized, only the benthos might have been; but during the
periods of anoxia there was no benthos to be fossilized but
only cadavers drifting down from above could be fossilized.
I cannot agree that the oxic.anoxic boundary could have been
a few millimeters be
below the water/sediment interface, because that too would have
led to sulphide discoloration. The oxic periods must have been
long enough for all sulphides to have been oxidised. In other
words there never was a permanent oxic/anoxic boundary.
>From: David Brez Carlisle