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Re: Solhofen lagoon

On Thu, 2 Nov 1995, David Brez Carlisle wrote:

> The lagoon cannot have been totally anoxic and stagnant, or  else
> there would be no fossils of marine animals.  It is likely that
> the surface layers were oxygenated and su pported a copious
> algal flora, but the deeper parts were anoxic and stagnant, like
> the depths of several water bodies, including the Black Sea.  Any
> unfornate creature that dived too deeply would be in trouble, but
> most fossils are almost certainly remains of creatures that died
> from other causes and then sank into the depths from the
> oxygenated surface layers, after death.

Neither can this be the full story for many of the marine animas 
preserved are benthic organisms (ie. bottom dwellers) eg. Limulids, 
various crustaceans and a small crinoid - the most abundant of solnhoffen 
fossils. This suggests that the boundary between oxic and anoxic water 
(which can be very sharp) was BELOW the sediment - water interface at 
least some of the time. The absence of any infaunal animals (animals 
living in the sediment such as bivalves) further suggests that the oxic - 
anoxic boundary was not far below the sediment surface (probably a few 
milimetres). Of course there would have been environmental fluctations 
when the boundary would have risen up into the water column. It would be 
at these times most of the fossils would be preserved.

Adam Yates.