Geology Reports⌕ Search

Geology topics

D. Blunt

Publications and source records attributed to D. Blunt.

4 recordsLinked to original sources
USGS1981

Age estimations based on amino acid racemization: Reply to comments of J.F. Wehmiller

Determining geologic ages of fossils by amino acid racemization techniques is often difficult because of the uncertainties in assumptions about diagenetic temperatures. Two kinetic model methods have been employed. Method 1, used by us, assumes that racemization of amino acids in the bivalve mollusk Saxidomus giganteus from Willapa Bay, Washington, follows linear kinetics. Ages are calculated by means of first-order kinetic equations. Method 2, used by Wehmiller, involves an empirical non-linear kinetic model Method 1 is simpler in concept and more easily applied. Wehmiller claims that ambiguities in paleotemperature arise when method 1 is used and that these ambiguities can be reconciled by the use of method 2. We show that application of method 1 can also provide reasonable temperature histories and leads to age estimates that are consistent with the geologic history of the sedimentary deposits at Willapa Bay.

Geochimica et Cosmochimica Acta↗
USGS1981

Geochemistry of amino acids in sediments from Clear Lake, California

By studying the geochemistry of amino acids, we attempt to clarify uncertainties in the radiocarbon chronology and in correlations of ash beds and pollen spectra in lacustrine sediment from Clear Lake, California. Two amino acids, aspartic acid and alanine, are considered in detail. Relative concentrations of aspartic acid decrease with depth, a result likely due to diagenesis and to preferential adsorption and hydrolysis in clay. Relative concentrations of alanine show the reverse effect with depth, probably due, in part, to the generation of alanine from other amino acids during diagenesis. The aspartic acid racemization rate calibrated by radiocarbon yields anomalously low age and temperature estimations for Clear Lake when kinetic assumptions from a study in Lake Ontario are used. However, the “apparent” alanine racemization rate correlates well with the alanine racemization rate determined for sediments from Lake Biwa, Japan. Our age assessment based on alanine supports a time-depth curve obtained from oak pollen and oxygen isotopes for Clear Lake representing a time span of about 130,000 yr.

California↗