| Summary: | Previous age-dating attempts in siliceous marine sediments by measuring the extent and trends of amino-acid racemization have yielded inconclusive results. This was largely due to unexplainable departures from the expected linear trends. But, by considering the complex environmental interactions that destroy, alter and add organic matter to marine sediments, an empirical model for amino-acid racemization in siliceous marine sediments can be developed. This model is based on the pattern of D/L ratio changes observed in total sediment hydrolyzates from several Southern Ocean piston-cores. High D/L ratios are typically measured in near-surface sediment samples and indicate bacterial contamination. With increasing sediment depth and age, several racemization trends often decrease to apparent minimums before increasing in a somewhat linear fashion. These D/L ratio decreases may indicate the formation of a protected "residue" consisting of free amino-acids and humic compounds complexed with clay minerals. The depth corresponding to these minimum D/L ratios may represent an empirical initial point for subsequent racemization in these sediments. This pattern is best displayed in the D/L ratio trends of aspartic acid, valine and phenylalanine, and to ~ lesser extent alanine and glutamic acid. Application of this model to the racemization trends in another preliminarily-dated Southern Ocean pist?n-core reveals a detailed sedimentary history. Three major units are delineated and at least two hiatuses are documented. Sedimentary contacts that coincide with apparent racemization reversals or rapid increases are interpreted as hiatuses that were overlooked in a previous study. A reinterpretation of racemization data for one D.S.D.P. core show that deviations also occur where there is a change from calcareous to siliceous sediments. This implies that the residue composition and clay-mineral interactions are major influences for racemization kinetics. In the D.S.D.P. study, the D/L ratio trend of the amino-acid leucine is likewise consistent with the empirical model. Furthermore, these kinetic and environmental interpretations appear valid for aspartic acid racemization in lacustrine sediments. Future racemization studies that attempt to further document this important aspect of amino-acid geochemistry should include sampling above and below all sedimentary contacts. Laboratory studies involving the characterization of preserved proteins, the exact nature of the postulated "residue" and the racemization kinetics in the presence of clay minerals may lead to the development of this methodology as a valid age-dating and correlation technique for both lacustrine and siliceous marine sediments.
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