Isoleucine epimerization in the two main shell layers of the bivalve Arctica islandica has been studied in heated modern samples and fossil samples. The derived Arrhenius parameters are similar to previously reported data on calcareous organisms. Reactions taking place in heated modern shell are essentially in agreement with diagenetic processes taking place during natural aging of Arctica shells at ambient temperatures. Hydrolysis rate exceeded epimerization rate of hound isoleucine over the entire temperature range from 0-160 C, suggesting preferential epimerization of terminal isoleucine. The two shell layers showed a significant difference in isoleucine epimerization rate. The lower epimerization rate of the inner layer is ascribed to a lower hydrolysis rate, possibly due to a lower water content in the inner layer. Accordingly, shell layering should be taken into consideration when fossil shells are being used for dating Quaternary sedimentary deposits and sampling strategies should be developed to restrict analyses to a single structural layer.

John-Erik Haugen, Section for Chemical Analysis, National Institute of Public Health, Geitmyrsveien 75, N-0462 Oslo 4, Norway;
Hans Petter Sejrup, Department of Geology, Section B, Allégaten 41, 5007, Bergen, Norway.