AMINO ACID COMPOSITION AND DIAGENESIS IN THE SHELLS OF TEREBRATULID BRACHIOPOD BOUCHARDIA ROSEA (SW ATLANTIC)

Amino acids extracted from shells of marine invertebrates may aid taxonomic identification but may undergo specific diagenetic changes during their post-mortem history. We analyze dated shells of the brachiopod Bouchardia roseato determine intra-specific and diagenetic variability in composition of amino acids.

We analyzed 57 dated shells of Bouchardia rosea. These specimens come from surficial assemblages on the inner shelf of tropical southern Brazil. Age of individual shells was determined using the racemization rate of L-Ile to D-Ale calibrated with radiocarbon dates. Previous analysis showed that these shells represent time-averaged accumulations of modern to 3000-year-old specimens. In addition, two live-collected and two Pleistocene specimens were analyzed.

Amino acid composition was determined by High Performance Liquid Chromatography (HPLC) using post-column derivitization with OPA. Thirteen common amino acids are well resolved using this method. Live-collected specimens contain ~8 nM of amino acids/mg of shell. Gly is by far the most abundant amino acid, at 48% of total concentration. The remaining 12 amino acids; Asp, Thr, Ser, Glu, Ala, Val, Met, Ile, Leu, Tyr, Phe and Arg each represent less than 10% of the total concentration. Shells younger than 50 years show a large range in absolute concentration: 2 nM to 7 nM/mg, while the Pleistocene shells contain ~ 2.5 nM/mg. This indicates that rate of loss varies and may be rapid until a threshold is reached. Individually, Glu (a stable amino acid) shows no trend through time. Most Ser (unstable) is lost within ca. 50 yr. Thr (also unstable) shows a smaller decrease over a time scale of ca. 500 yr. In general diagenetic change appears to occur within hundreds of years; the Pleistocene shells not differing signifcantly from thousand-year-old shells. Principal Component Analysis on amino acid concentrations shows that the main axes of variation correlate with taphonomic conditon (PC1: r=0.55, p<0.001) and age (PC2: r=-0.63, p<0.001).

The diagenesis of amino acids in brachiopod shells appears to follow specific pathways based on age and on taphonomic processes. Therefore amino acid composition may be used to aid studies on the processes of time-averaging and taphonomy, but caution should be used when using amino acids of empty shells for taxonomic description.