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Amino acid racemization kinetics in wood; Applications to geochronology and geothermometry

By
N. W. Rutter
N. W. Rutter
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C. K. Vlahos
C. K. Vlahos
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Published:
January 01, 1988

The geochemistry of amino acids in fossil wood materials appears to be applicable to geological problems such as correlation, relative-age dating, and paleothermometry of sedimentary deposits in the northern Yukon Territory, Canada (Rutter and Crawford, 1984). Activation energies and Arrhenius frequency factors were calculated for the racemization reaction of several bound amino acids (asp, ala, glu, leu). These parameters were obtained by determining elevated temperature rate constants for the bound amino acids isolated from modern and fossil Picea glauca (white spruce). The ratios of dextro to levro stereoisomers (D/L ratio) obtained for bound aspartic acid were found to be the most reliable and yielded values of 18.4 ±2.4 Kcal/mol and 27.6 ±3.0 yr−1 for activation energy and Arrhenius frequency factor (ρnA), respectively. Slight differences in kinetic parameters were obtained between fossil and modem wood replicates. Aspartic acid also yielded results correlatable to studies performed on Sequoiadendron giganteum (Engel and others, 1977). These findings suggested that species specific effects may not be significant for proteinaceous material found within wood matrices.

Extrapolation of a first-order rate constant for bound aspartic acid in the fossil Picea sp. yielded a value of 9.75 × 10−7 yr−1. This constant was derived from the extent of racemization of the dated sample (>53,000 yr B.P.). Rate constants were similarly determined for various fossil localities in the northern Yukon. These rate constants ranged from 9.75 × 10−7 yr−1 to 3.24 ± 0.2 × 10−6 yr−1. As this reaction is temperature dependent, estimations of paleotemperatures that the fossil samples had experienced were calculated. The values obtained (−49°C ± 30°C) were unrealistic since the racemization does not appear to follow simple reversible first-order kinetics. More reasonable results were obtained (−19°C) if the assumption used for calculation was based on the presence of free aspartic acid (complete protein hydrolysis). The apparent rate of racemization of free aspartic acid is characteristically lower than the apparent rates of racemization of protein-bound amino acids.

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GSA Special Papers

Dating Quaternary Sediments

Don J. Easterbrook
Don J. Easterbrook
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Geological Society of America
Volume
227
ISBN print:
9780813722276
Publication date:
January 01, 1988

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