Protein diagenesis in Patella shells: Implications for amino acid racemisation dating

Abstract

The inter-and intra-crystalline fractions of Patella vulgata limpets recovered from archaeological sites in Northern Spain (covering Neolithic, Mesolithic, Magdalenian, Solutrean, and Aurignacian periods) were examined for amino acid composition and racemisation over time. The calcitic apex and rim areas of the shells were found to probably be composed of similar proteins, as the D/L values and amino acids were comparable and varied in the same way with increasing age; however, the mineral structures present in these areas differed. The aragonitic intermediate part of the shell showed a distinctly different amino acid composition and mineral structure. The main protein leaching from the inter-crystalline fraction occurred within the first 6000 yr after the death of the organismo In contrast, the intra-crystalline fraction -comprised of a different protein composition than the inter-crystalline fraction -appeared to behave as a closed system for at least 34 ka, as reflected by the lack of a significant decrease in the amino acid content; however, changes in the amino acid percentages occurred during this periodo The concentration of aspartic acid remained almost constant with age both in inter-and intra-crystalline proteins, and its contribution to the total amino acid content increased with age at the expense of other amino acids such as glutamic acid, serine, glycine and alanine. Temperature is thought to playa key role in the amino acid racemisation of P. vulgata and could explain why in the localities belonging to the Gravettian and Solutrean period, which formed during relatively cold conditions, D/L values were similar to those detected in shells from sites formed during the Magdalenian.

Impact damage and repair in shells of the limpet Patella vulgata

“…The shell consists almost entirely of calcium carbonate, with a small percentage of organic material and water. The microscopic structure is very complex and varies with species (Ortiz et al, 2015). Fig.…”

“…Each layer has a different structure -some layers are made from calcite and some from aragonite, and the orientation and arrangement varies from layer to layer including prismatic and lamellar arrangements of crystal units. The basic structure of M-layers is known as the first-order structure, with second and third orders being identified at finer scales (see for example Ortiz et al, 2015;Szabó and Koppel, 2015).…”

Radiocarbon-calibrated amino acid racemization ages from Holocene sand dollars (Peronella peronii)

“…Demarchi et al (2013) reported similar THAA abundance patterns in Patella vulgata, with Asp, Glu and Ala being the most abundant and geochronologically useful amino acids, although Asp is less abundant in these taxa than reported from P. vulgata. Change in the relative concentration can be an indication of closedsystem behaviour or remnant protein structure (Penkman et al, 2008;Demarchi et al, 2013;Ortiz et al, 2015). The relatively narrow confidence intervals on the relative amino acid abundance plot indicate consistency across the amino acids.…”

Converting A/I values (ion exchange) to D/L values (reverse phase) for amino acid geochronology

“…Longer hydrolysis times generally increase D/L values (Canoira et al, 2003), with the difference typically proportionally greater in younger samples, and for amino acids with the higher racemization rates, such as Asp. The influence of pre-treating with bleach to isolate the intra-crystalline fraction is less predictable, with some studies reporting an increase in D/L values (e.g., for Glycymeris, Demarchi et al, 2015; ostrich eggshell, Crisp et al, 2013), others a decrease (e.g., for Patella, Ortiz et al, 2015), and others no effect (e.g., for ostracodes, Bright and Kaufman, 2011).…”