Testing the effect of oxidizing pre-treatments on amino acids in benthic and planktic foraminifera tests

Abstract

Amino acid racemization (AAR) is a geochronological method that uses the ratio of D- to L-configurations in optically active amino acids from carbonate fossils to determine the time elapsed since the death of an organism. Although AAR techniques have been widely applied to foraminiferal tests, there have been limited dedicated assessments of the potential of isolating a bleach-resistant, intra-crystalline fraction of proteins to improve the reliability of AAR in this biomineral system. In this study, we evaluate the effect of two oxidative pre-treatments (hydrogen peroxide and bleach) on amino acid concentrations and D/L values in sub-modern benthic foraminifers ( Ammonia spp. and Haynesina germanica ) and well-preserved mid Holocene and mid Pleistocene planktic foraminifers ( Pulleniatina obliquiloculata , Globorotalia truncatulinoides , and Globorotalia tumida ). The oxidative pre-treatments successfully reduced the amino acid content of the foraminiferal tests to a residual fraction, and with the exception of Ammonia spp., neither pre-treatment substantially affected the relative proportion of individual amino acids. The bleaching pre-treatment does not consistently alter D/L values when compared to peroxide pre-treatment, but it does tend to reduce the subsample variability in D/L values, albeit only to a small degree in an inconsistent fashion. Therefore, we recommend that a relatively weak oxidative pre-treatment with 3% hydrogen peroxide is sufficient for foraminifera-based AAR applications. • Intra-crystalline fraction of amino acids in foraminiferal tests was isolated using oxidative pre-treatments • Difference in concentration between the inter- and intra-crystalline fractions of amino acids decreases with increased sample age • No consistent improvement in intra-sample variability when foraminiferal tests are treated with bleach • Pretreating foraminiferal tests with peroxide rather than bleach is recommended