Dr. John Wehmiller

Professor, Appointed: 1985
Department of Geology
University of Delaware
Newark, DE 19716 USA
phone: (302)831-2926  fax: (302)831-4158  
e-mail: jwehm@udel.edu

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
Problems, comments, or suggestions to the webmaster.