15 Nov 2023
 | 15 Nov 2023
Status: this preprint is open for discussion.

A new method for amino acid geochronology of the bivalve shell Arctica islandica

Martina L. G. Conti, Paul G. Butler, David J. Reynolds, Tamara Trofimova, James D. Scourse, and Kirsty E. H. Penkman

Abstract. The bivalve mollusc Arctica islandica can live for hundreds of years, and its shell has provided a valuable resource for sclerochronological studies and geochemical analyses for understanding palaeoenvironmental change. Shell specimens recovered from the seabed need to be dated in order to aid sample selection, but existing methods using radiocarbon dating or crossdating are both costly and time-consuming. We have investigated amino acid geochronology (AAG) as a potential alternative means of providing a less costly and more efficient rangefinding method. In order to do this, we have investigated the complex microstructure of the shells, as this may influence the application of AAG. Each of the three microstructural layers of A. islandica have been isolated and their protein degradation examined (amino acid concentration, composition, racemisation and peptide bond hydrolysis). The intra-crystalline protein fraction was successfully extracted following oxidation treatment for 48 h, and high temperature experiments at 140 °C established coherent breakdown patterns in all three layers, but the inner portion of the outer shell layer (iOSL) was the most appropriate component due to practicalities. Sampling of the iOSL layer in Holocene shells from early and late ontogeny (over 100–400 years) showed that the resolution of AAG is too low in A. islandica for within-shell age resolution. However, analysis of 19 subfossil samples confirmed that this approach could be used to establish a relative geochronology for this biomineral throughout the whole of the Quaternary. In the Late Holocene the temporal resolution is ~1500–2000 years. Relative dating of 160 dredged shells of unknown age were narrowed down using AAG as a range finder, showing that a collection of shells from Iceland and the North Sea covered the Middle Holocene, Late Holocene, post-medieval (1171–1713 CE) and modern day. This study confirms the value of A. islandica as a reliable material for rangefinding and for dating Quaternary deposits.

Martina L. G. Conti et al.

Status: open (until 30 Dec 2023)

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Martina L. G. Conti et al.

Martina L. G. Conti et al.


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Short summary
The mollusc Arctica islandica can survive for hundreds of years and its annual growth captures environmental conditions, so each shell provides a detailed climatic record. Dating is essential for sample selection, but radiocarbon and crossdating are time-consuming and costly. As an alternative, amino acid geochronology was investigated in the three aragonitic layers forming the shells. This study confirms the value of AAG as a method for rangefinder dating Quaternary A. islandica shells.