Amino Acid Racemization of Planktonic Foraminifera: Testing the Effects of Bleaching Pretreatment

Amino acid racemization (AAR) is a geochronological method that can be applied to Quaternary carbonate-based fossils. AAR uses the ratio of D- to L-configurations (D/L) of optically active amino acids in fossil remains to determine the time elapsed since the death of the organism. Pretreating samples with bleach to remove contaminants and to isolate the intra-crystalline fraction of amino acids has been shown to improve the reproducibility and precision of the technique for several taxonomic types because the intra-crystalline fraction is less prone to environmental influences for a better approximation of a closed system. In this study we investigate the effect of this pretreatment method on the D/L ratios in three species of planktic foraminifera (Globorotalia tumida, Pulleniatina obliquiloculata, Globorotalia truncatlinoides) from late Holocene ( 4-5 ka) deep sea sediments of similar environmental setting (Ocean Drilling Program Sites 1056, 1059, and 1062). We report results from aspartic acid (Asp) and glutamic acid (Glu) because they are among the most abundant amino acids in foraminiferal protein and are the best resolved chromatographically. We analyzed 43 samples, each with an average of nine replicates per sample and 10-20 individual tests per replicate. Comparing D/L ratios from bleached versus unbleached samples indicates that bleaching only slightly reduces the variability in D/L values within the same sample (i.e. same species from the same core intervals) by, on average, 3.2% and 1.7% for Asp and Glu, respectively. Bleaching did not reduce the rate that individual analyses were rejected based on previously established screening criteria. Furthermore, comparison of D/L ratios from the same species found at more than one site does not show statistical differences whether bleached or not. We conclude that bleaching does not appear to reduce the variability among replicates from the same sample from core intervals of similar age and environmental setting. Ongoing work is extending the method comparison down-core to investigate the potential of improvement of down-core trends in D/L ratios spanning the late to mid Pleistocene.