Re: paleonet non-green plants and Mistaken Point

[Matthew E Clapham <clapham@xxxxxxx>]

The specific context at Mistaken Point is that the "ash" that buried the 
fossils is a fairly coarse crystal tuff (feldspar phenocrysts up to 2-3 mm 
in diameter).  A quick Stokes Law calculation suggests a settling velocity 
of nearly 3 cm/s for a 1 mm diameter crystal (25 cm/sec for a 3 mm 
crystal).  The finer-grained, true ash, is only found as resedimented 
turbidites which followed the crystal tuff fall by some period of time 
(often a fairly long period).

Matthew

At 15:21 10/8/02 -0400, you wrote:

>On Tue, 8 Oct 2002, Rod Savidge wrote:
>  This rate-of-sinking argument seems reasonable.  It also seems plausible to
>  me that it might take some time, following shock-wave killing of
>  sea-surface organisms, before initially upward ejected ashes would begin to
>  settle on the sea surface.   There must be measurements on this in relation
>  to recent eruptions??   Anyway, I'm sure it could be determined 
> experimentally.
>
>I'm not so sure about the rate-of-sinking argument ("high-density tuff
>particles will settle more quickly than low-density ediacarans"). Sinking
>velocity for tuff particles will be dominated by viscous drag due to their
>tiny sizes; they will settle out *after* particles that are large enough to
>be dominated by inertial drag -- even if those larger particles (here,
>Ediacarans) are somewhat less dense. We all know how long ash can stay
>suspended in the atmosphere without falling, and the case for water would
>be even more suspensory. A child's balloon, on the other hand, has
>extremely low density but will not stay in the air more than a few minutes
>(assuming no helium is involved).
>
>My apologies if I've missed a context-specific and cogent argument about
>the Mistaken Point scenario. Just trying to catch a potential taphonomic
>red herring before it's fried...
>
>  Peter A. Kaplan * Peter A. Kaplan * Peter A. Kaplan * Peter A. Kaplan
>  * Ph. D. Candidate * Department of Geology * University of Michigan *
>
>       1511 Pine Valley Blvd               UMMP, 1109 Geddes Road
>       Apartment 21             _______      Ann Arbor, MI  48109
>       Ann Arbor, MI 48104     / retep ;      phone: 734.764.0489
>       734.975.4331            ;______/         fax: 781.723.0267
>                        @..@  /
>                       (-==-)
>                      ( >__< )    pefty@aya.yale.edu
>                      ~~ ^^ ~~

Matthew E. Clapham
Department of Earth Sciences
3651 Trousdale Pkwy
University of Southern California
90089-0740
Phone:  (213) 821-6291
Fax: (213) 740-8801
clapham@usc.edu