Intra-crystalline protein diagenesis (IcPD) in Patella vulgata. Part I: Isolation and testing of the closed system

B Demarchi,K Rogers,D.A Fa,C.J Finlayson,N Milner,K.E.H Penkman

doi:10.1016/j.quageo.2012.03.016

Abstract

This study successfully isolates a fraction of intra-crystalline proteins from shells of the marine gastropod Patella vulgata and assesses the suitability of these proteins for IcPD (Intra-crystalline Protein Diagenesis) geochronology. We discuss the mineralogical composition of this gastropod, investigated for the first time by X-ray diffraction mapping, and use the results to inform our sampling strategy. The potential of the calcitic rim and of a bulk sample (containing both apex and rim) of the shell to act as stable repositories for the intra-crystalline proteins during diagenesis is examined. The composition and the diagenetic behaviour of the intra-crystalline proteins isolated from different locations within the shell are compared, highlighting the necessity of targeting consistent sampling positions.We induced artificial diagenesis of both intra-crystalline and whole-shell proteins by conducting high-temperature experiments in hydrous environment; this allowed us to quantify the loss of amino acids by leaching and therefore evaluate the open- or closed-system behaviour of the different fractions of proteins. The results obtained provide further confirmation that patterns of diagenesis vary according to the protein sequence, structure, and location within or outside the intra-crystalline fraction. As Patella is frequently found in the fossil record, both in archaeological and geological contexts, the application of IcPD geochronology to this biomineral opens up the possibility to obtain reliable age information from a range of sites in different areas of the world.

Highlights

Isolating a closed system of proteins within biominerals enables the study of the diagenesis undergone in situ by the original protein fraction (Curry et al, 1991)
Conclusions on bleaching experiments Bleaching experiments performed on P. vulgata showed that both bulk shell powders and the calcitic rim retain a fraction of proteins which can be effectively isolated by a 48-h bleaching step, whilst inducing a negligible amount of racemisation
This paper has evaluated the potential of marine gastropods P. vulgata for intra-crystalline protein diagenesis (IcPD) geochronology

Summary

Introduction

Isolating a closed system of proteins within biominerals enables the study of the diagenesis undergone in situ by the original protein fraction (Curry et al, 1991). As protein breakdown within such a system should be solely dependent on time and temperature, the extent of degradation can be considered an accurate indicator of the thermal age of a sub-fossil sample. This has powerful implications for protein diagenesis/amino acid racemisation geochronology (AAR). An indigenous intra-crystalline fraction of proteins, behaving as a closed system with regards to diagenesis, has been reported in eggshells (Brooks et al, 1990) and has been isolated by a strong oxidation treatment in a range of other calcareous biominerals (Penkman et al, 2008, 2011; Demarchi, 2009; Hendy et al, 2012; Crisp et al, 2013; Tomiak et al, 2013). Ostracodes have been tested for their ability to retain an intra-crystalline fraction of proteins which can be isolated by strong oxidation; proteins in this biomineral are intimately associated with an abundant chitinaceous component, which results in a pool of proteins immune to chemical removal, hampering the effectiveness of the oxidation treatment (Bright and Kaufman, 2011)

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Conclusion