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MISSION DISPATCH - DAY 2 Today's Weather - images courtesy of NOAA & RSMAS Dispatch by Dr. Karla Parsons-Hubbard July 25, 2001 Taphonomic analysis of shells can provide more information about environments of deposition and fossil assemblages than the use of community characteristics (species composition, abundance) alone. Dead shells are 
altered by the physical, chemical and biological processes that are
different for each marine environment in which they are found. As an
example, in shallow environments with relatively high water energy, physical
processes (abrasion, breakage) would be expected to be the cause of
alteration and destruction of shells; shells in deeper environments would be
expected to be affected more by chemical and biological processes. The
rates at which these processes alter shells should be different in each
environment; shells should show evidence of these processes that can be
tied to a unique environment.
These characteristics form what is referred
to as a taphomic signature that should be recognizible in fossil shell
assemblages.
Thus far on this cruise we have started compiling data from shells placed at a shallow (about 300 ft. ) marine site called Parker Bank for up to seven years and a deeper (1,800 ft) marine site known as Garden Banks. Parker Bank, visited on July 22, is a collapsed carbonate bank formed in association with a salt diapir. It appears that chemical alteration (primarily dissolution) of the shells is more dominant at Garden Banks; Parker Bank appears to have more abrasion, which can be attributed to physical processes/alteration. Events that can cause physical changes in the shells include storms, bottom currents, or disturbance by other organisms. 
Biological alteration (shell boring and encrustation) is also abundant in
the shallow site and is almost entirely absent in shells from the deeper
site. One of the most important questions for paleontologists is how long it
takes for shells to be completely destroyed and thus never become
fossilized. Our work here in the Gulf of Mexico is the first study designed
to look at rates of shell alteration over a very long period of time, at a
number of different environments, and in water too deep to reach by a SCUBA
diver. Thus we must use tools like the R/V JOHNSON-SEA-LINK.
Studying the alteration of shell material over a relatively long periods of time in a wide range of modern environments will allow us to make better and more accurate reconstructions of ancient environments of deposition from their fossil assemblages. We need to know about rates and processes that alter and destroy shells today so that the present will be a more useful key to the past.  | |
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