Dr. John Wehmiller
Research interests/projects
The primary focus of our research program over the past 20 years has been
the study of amino acid racemization (AAR) as a dating method for
Quaternary fossil material. Racemization is the phenomonon of conversion
of so-called "left-handed" amino acids into their mirror image equivalent
"right-handed" forms. With time, the ratio of right to left-handed amino
acids grows from 0.0 to an equilibrium value of 1.0 (for most amino
acids). AAR is valued as a dating method because of the relative ease of
analysis (compared with many methods based on radioactive decay) and its
potential for extending the range of more quantitative dating methods such
as Carbon-14. A variety of environmental geologic problems rely upon
Quaternary dating methods, as evidenced by a major recent review report
supported by the U.S. Nuclear Regulatory Commission. As a diagenetic
reaction, however, racemization does not have a single rate constant -
instead the reaction depends not only on temperature, but also on sample
type, in addition to the many geochemical factors that can affect the
preservation state of any fossil material being analyzed for
geochronological purposes. The use of AAR for Quaternary geochronology
requires a variety of approaches, because the method is experimental and
has many inherent uncertainties. The time range over which AAR is
applicable is affected by temperature, varying between perhaps 100,000
years for tropical latitudes to perhaps as much as 5 million years in
polar latitudes. The major field area for our research has been the
Atlantic coastal plain but we have collaborated with others to study
coastal Quaternary sequences along the western coasts of North and South
America. Using fossils from these regions, we have studied AAR reactions
to test the following trends: a) degree of racemization with increasing
stratigraphic age, using samples of known relative or "absolute" age; b)
the degree of racemization in samples of the same age at different
latitudes (temperatures); c) the degree of racemization in samples of
different taxa that are the same age (or samples of the same taxa, with
different preservation characteristics). In addition to these
field-oriented studies, we conduct a variety of laboratory-oriented
geochemical and microscopic studies of molluscan shell carbonate and
organic geochemistry. These experiments are directly related to
understanding the entire living and fossil history of the interaction
between organic matrices and their associated biominerals.
Reference:
Wehmiller, J.F., Lamothe, M., and Noller, J.S. (1998). Comparison of
approaches to dating Atlantic Coastal Plain sediments, Virginia Beach,
VA., in: Sowers, J.M., Noller, J.S. and Lettis, W.R., Dating and
Earthquakes: Review of Quaternary geochronology and its applications to
paleoseismology. U.S. Nuclear Regulatory Commission, NUREG/CR 5562,
pp.4-3 to 4-31.
Other
information
Revised: January 26, 1999
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