Historical developments and recent advances in amino acid geochronology applied to loess research: examples from North America, Europe, and China

Amino acid geochronology provides important chronostratigraphic insight in the regional correlation and paleoclimatic evaluation of loess-paleosol sequences in the midwestern US, throughout western, central, and eastern Europe, and in China. In general, loess of the last four glacial cycles, corresponding to marine oxygen-isotope stages (OIS) 2-4, 6, 8, and 10, respectively, can be distinguished on the basis of alloisoleucine/isoleucine (A/I) ratios measured in fossil gastropod shells preserved in the loess. The racemization reaction is much slower in older samples, resulting in decreased temporal resolution. In the midwestern US and Europe, it is not possible to confidently subdivide the last glacial cycle on the basis of aminostratigraphic data. However, in China, where effective temperatures were higher during the Late Pleistocene, loess above and below L1SS1, the OIS-3 interstadial paleosol, can be distinguished using amino acid racemization data. With improvements in analytical methods, multiple amino acid D/L-enantiomers are now being measured using reverse-phase liquid chromatography. Aspartic acid racemizes at a higher rate than the traditionally measured alloisoleucine/isoleucine diastereomers and can be measured more rapidly. This development offers hope for better temporal resolution and enhanced stratigraphic subdivision of loess units than has been achieved previously. (C) 2001 Elsevier Science B.V. All rights reserved.