Intrinsic Factors that Affect Amino Acid Geochronology: Protein Diagenesis

Lisa L. Robbins

Use of alloisoleucine/isoleucine ratios as a geochronological tool has been the basis of many stratigraphic studies of the Quaternary. In a number of these studies, the amino acid dates have been remarkably good, allowing chronologies to be established and sequences checked, at least in limited geographic locations. However, in other studies the variability and scatter of dates have obscured or confounded interpretation of the geochronology. A number of extrinsic and intrinsic factors may contribute to this variability. One intrinsic factor may be the varying rates of epimerization and breakdown of the many proteins within the shell matrix of the organism that is being analyzed; the shell matrix of planktonic foraminifera contains at least two classes, or types, of protei s and up to 40 protein components. Individual proteins differ in stability based on differences of structure. This stability will have direct ramifications on racemization rates.

Traditionally, amino acid ratios have been analyzed from the remnants of the total organic material of shells, such as mollusk or planktonic foraminifera. To examine how the diagenetic breakdown of specific proteins may affect alloisoleucine/isoleucine ratios of the total organic matrix, different classes of proteins from fossil planktonic foraminifera were analyzed and compared. Although planktonic foraminifera were used as a model system, the molluscan shell is analogous.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.