Annual Growth Patterns and Interspecimen Variability in Mg/Ca Records of Archaeological Ostrea edulis (European Oyster) from the Late Mesolithic Site of Conors Island

Abstract

Annual growth patterns in marine mollusc shells are valuable indicators of the condition of marine ecology through time. In archaeological contexts, the mollusc's time of death (i.e. the last season of growth) is an indicator of human exploitation patterns throughout the year, enabling the reconstruction of when and how often gathering occurred as well as when sites were occupied. Both pieces of information, growth rate and season of death, are vital for understanding exploitation pressure(s) in the past, and building baselines for modern environmental policies that secure sustainable marine resources. Previously, these parameters have been determined by incremental growth-line or isotopic analyses, which are time consuming and often expensive techniques, thus restricting sample size and the overall robustness of palaeoecological interpretations. Here, we apply Laser Induced Breakdown Spectroscopy (LIBS) to produce elemental maps (Mg/Ca) with the potential to trace and display growth patterns quickly, and at a reduced cost. We further compare the elemental maps with the results obtained from incremental growth-line analysis to provide a structural context for the geochemical data, and demonstrate the utility of an integrated methodological approach. Our pilot study was undertaken on 12 European oysters (Ostrea edulis, Linnaeus, 1758) from the Late Mesolithic shell midden at Conors Island, Co. Sligo in the Republic of Ireland. Our LIBS analysis enabled us to accurately and quickly determine repeating growth patterns, which were often in agreement with the annual growth increments visible through the microscopic analysis. Based on this comparative dataset, including structural and geochemical patterns, the Late Mesolithic site of Conors Island had been occupied throughout the year. Moreover, our analyses highlight the applicability of LIBS to determine prehistoric seasonality practices as well as biological age and growth at an improved rate and reduced cost than was previously achievable.

Sclerochronological and geochemical study of the carpet shell Ruditapes decussatus in archaeological contexts: A potential tool for season of collection and coastal paleo-temperature

“…Moreover, additional chemical analysis (e.g. mapping) of growth dependent elements such as Manganese, Magnesium, Strontium or Barium could help in the interpretation of growth marks and temporal frameworks (Langlet et al 2006, Poulain et al 2015, Hausmann et al 2019.…”

Using Mg/Ca Ratios from the Limpet Patella depressa Pennant, 1777 Measured by Laser-Induced Breakdown Spectroscopy (LIBS) to Reconstruct Paleoclimate

“…As can be seen in Figure 7, the two LAN545 series measured under the same experimental conditions showed the same trend through time, reporting a high correlation (R 2 = 0.78) between them. Overall, this indicates that, while patterns can be interpreted with confidence, some intrinsic variability is inherent in using the LIBS technique, as well as a local heterogeneity in Mg distribution that is not only related to temperature dependence [58,61,69,70].…”

“…Determining if Mg/Ca ratios derived from P. depressa can be used as a suitable paleothermometer to reconstruct paleoclimate conditions and seasonality would have significant implications for future archaeological and paleoenvironmental investigations. Mg/Ca ratios were measured using laser-induced breakdown spectroscopy (LIBS), one of the most novel techniques for measuring element/Ca ratios on shells due to significantly easier sample preparation and lower measurement times [57,58,[68][69][70]. A calibration-free LIBS (CF-LIBS) technique [71] was applied in order to estimate the molar concentrations of both chemical elements.…”