Carl
Brett
Comparative Taphonomy and Taphofacies of Modern Siliciclastic and Carbonate
Shelf and Upper Slope Environments
Collaborative research with Eric Powell and Karla Parsons (Rutgers University),
Sally Walker (University of Georgia), and Ann Raymond (Texas A&M University);
research funded by two grants from NOAA
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Brett is a co-PI in a large scale collaborative research project (SSETI:
Shelf and Slope Experimental Taphonomy Initiative) that involves documentation
of preservation processes of modern organisms. This is a is a long term
study; replicate samples of molluscan shells, crabs, urchins, and wood
have been deployed at varied sites, in the Gulf of Mexico and in the Bahamas.
The objective of this project is to retrieve and analyze the samples after
periods of about one, two, five, and ten year intervals.
During cruises in 1993 and 1995 a series of samples were deployed in varied
siliciclastic substrate environments in the Gulf of Mexico (specific settings
include submarine brine pools, to deep reef settings, muddy continental
shelf, upper continental slope and petroleum seep environments). These
experimental sites range from 280 to approximately 1870 feet deep in the
Gulf off the Texas coast. Using the research submarine Johnson Sea-Link,
a first suite of samples was recovered in 1995 and analyzed after two
years of exposure.
A similar suite of samples was also deployed in 1993 and 1994, in varied
carbonate environments, ranging from shallow shelf (50`) and patch reef,
to shelf edge escarpment, and deep (800`) carbonate slope environments
in the vicinity of Lee Stocking Island (LSI) in the Bahamas. Samples have
been retrieved and analyzed from all of the LSI sites in the summers of
1994, 1995, and 1996, to document the taphonomic effects of exposure of
shell and other materials to various carbonate environments after one
and two years.
After a single year, over 100 species of epibionts, including algae, foraminifera,
bryozoa, serpulid tube worms, tunicates, oysters, and rare brachiopods
were observed on experimental shells. The species richness, diversity,
and % coverage of encrusters (including those of skeletonized forms preservable
in the fossil record) peaked at about 250` and declined markedly in deeper
water samples.
In addition, taphonomic study indicates that whereas shallow water shells
are highly altered by breakage, corrosion, and bioerosion, shells from
most sites deeper than about 500` are in almost pristine conditions with
only scattered encrusting serpulids, and small foraminifera after two
years.
Wood blocks display a very different pattern, being selectively colonized
by wood-boring bivalves and gribbles in shallow water but unselectively
riddled at deepest sites.
Overall, the project is anticipated to continue for about five more years
pending funding to retrieve the last sets of samples. Brett will work
with Powell and colleagues closely over the next five years in developing
this project, analyzing the data, and interpreting the results obtained
from these several environments.
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References
Brett, C. E., 1996, Taphonomy. Macmillan Encyclopedia of Earth Sciences.
Brett, C. E. and Baird, G. C., 1986, Comparative taphonomy: a key to paleoenvironmental
interpretation using fossil preservation. Palaios 1, 207-227.
Heise, L. M., Raymond, A., Walker, S. E.., Parsons, K. M. and Brett, C.
E., 1995. Taphonomy of wood, cones, and seeds in marine carbonate environments:
an experimental study.
Geological Society of America Abstracts with Programs 27 (6): A134.
Parsons, K. M. and Brett, C. E. 1991. Taphonomic processes and biases
in modern marine environments: An actualistic perspective on fossil assemblage
preservation. In Donovan, S. K., ed., The Processes of Fossilization.
Belhaven Press. London, p. 22-65.
Parsons, K. M., Powell, E. N., Brett, C. E., Walker, S, E., and Callender,
W. R. 1996, Taphonomic variability in molluscs on the Bahamas shelf margin
and slope: Depositional environment vs. shell type after one and two years.
Geological Society of America Abstracts with Programs. v. 28, p. A-364.
Powell, E. N., Brett, C. E. and others. 1995. The SSETTI program (shelf
and slope taphonomy initiative): description and progress report. Geological
Society of America Abstracts with Programs 27 (6): A 133.
Walker. S., Brett, C. E., Parsons, K. M., and Powell, E. N. 1995. Molluscan
experimental taphonomy: shelf-slope trends for epi- and endobionts on
gastropod shells. Geological Society of America Abstracts with Programs
27 (6): A 132.
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