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Re: Solnhofen Taphonomy



Jerry Harris wrote:


>>Saline water is denser than freshwater (due to the amount of salts disolved
>>in it), and hypersaline water is denser than normal sea water. Therefore
>>the freshwater entering the lagoon will tend to stay separate from, and
>>flow over the much denser hypersaline water (cue horrible ascii diagram).
>>
>>
>>***------------------------------------------------
>>    *          Freshwater/Normal Saline   **
>>      *----------------------------------*   *
>>        *       Hypersaline             *      *
>>           *  *  *  *  *  *  *  *  *  *          *
>>
>>
>>Thus the benthic (bottom) conditions remain hypersaline despite freshwater
>>inflow.


>        Thanks for the refresher course!  8-)  Of course, since the waters
>won't mix, that presents problems for the excellent preservation of the
>fossils -- dead animals (of the terrestrial variety, washed in from
>wherever) will float a while before sinking to the anoxic, hypersaline
>bottom waters for preservation, and during that floating would undergo some
>decomposition.  We do see that in some of the _Archaeopteryx_ specimens,
>and some of the other vertebrates, but we also get the truly gorgeous ones
>that appear to have been buried the instant they died!  I doubt that the
>influx of stream waters was anoxic.  Hmmmm....a puzzle!  8-)  Is there a
>solution you can propose?  I'm at a loss (but then again, I'm no expert in
>taphonomy, either...)

Umm, errr, Weeeeell.

The hypocline (the boundary between hypersaline water and
normal/freshwater) is very fluid (groan!). It is affected by water
temperature, climate - atmospheric conditions (amount of evaporation),
freshwater/normal saline water imput -, currents (mixing) etc. Thus, during
times of high evaporation and low water imput, the hypocline can rise to
very shallow depths (possibly even reaching the surface? - although that is
a bit extreme). Thus organisms entering the lagoon during these times have
a high preservational potential (as represented by excellently preserved
specimens?). During times of lessened evaporation and increased water
input, the hypocline will sink to lower depths resulting in shoddy
preservation - or possibly no preservation at all - of organisms entering
the lagoon from above.

A fluctuating hypocline, plus attendant anoxia, has been invoked to explain
the mass death and excellent preservation of fish from the Cretaceous
Santanna Formation in Brazil. The fish from this deposit have muscle tissue
preserved down to the cellular level (Martill 1988). Also pterosaur wing
membranes have been preserved (Martill & Unwin 1989).

Assuming the pterosaurs were not swimming with the fish, we have to assume
that the bouyancy coefficent of dead organisms may need revising :-)

Chris

Martill, D.M. (1988) The preservation of fishes in concretions from the
Santanna Formation (Cretaceous) of Brazil. Paleontology, 31: 1-18.

Martill, D.M. & Unwin, D.M. (1989) Exceptionally well preserved pterosaur
wing membrane from the Cretaceous of Brazil. Nature, 340: 138-140.

cnedin@geology.adelaide.edu.au                  nedin@ediacara.org
-------------------------------------------------------------------
Many say it was a mistake to come down from the trees, some say
the move out of the oceans was a bad idea. Me, I say the stiffening
of the notochord in the Cambrian was where it all went wrong,
it was all downhill from there.