Limpets and Mussels from Tierra del Fuego
Alejandro León Cristóbal is a doctoral researcher from the University of La Rioja (Spain). He does research in marine mollusc shells, ethnoarchaeology, oxygen isotope analysis, Palaeolithic Archaeology and Mesolithic-Neolithic Transition. The main objective in his doctoral thesis is to improve the understanding of the last hunter-gatherer-fisher societies of the Cantabrian Region. To achieve this, Alejandro employs a dual approach, both theoretical, using historical and ethnographic sources, and practical, through the use of oxygen isotope analysis and LIBS on archaeological and modern shells. He has been at other research centres for training, such as ICArEHB (Faro, Portugal), ICTA-UAB (Barcelona, Spain) or CADIC (Ushuaia, Argentina. In the latter, Alejandro worked closely with the research group led by Francisco Zangrando. As a result of this collaboration, Alejandro has been working with the SeaFront team with modern mussel shells and archaeological limpet shells with the aim of applying LIBS to them.
The modern mussels were collected during his research stay in Cambaceres Bay, located on the island of Tierra del Fuego in southern Argentina. Ten specimens were taken from the inner bay and ten from the outer bay, with the aim of later comparing the results obtained at both locations.
The archaeological samples come from the Casa Grande site, also located in Cambaceres Bay (Tierra del Fuego, Argentina). This site, still not fully excavated, is in an archaeological area of particular interest for the study of the indigenous tribal groups that inhabited the island of Tierra del Fuego. Around 40 limpets were collected here, divided into 4 layers. To obtain a meaningful comparison between the different chronologies of each layer, 10 specimens were selected from each one, with the aim of observing (or not) different collection seasons depending on the chronological period during which the site was inhabited.
The objectives of this stay were to better understand the internal structure of archaeological shells and estimate their seasonal shell collection. The results revealed little variability in Mg/Ca values. This is positive, since annual temperatures in Tierra del Fuego (Argentina) also show very limited fluctuations, with a predominantly cold climate owing to its geographical position. Moreover, most of the selected limpets and mussels were successfully analysed with LIBS, demonstrating that the method is efficient for both archaeological and modern samples.
Overall, the findings show several warm and cold phases within a single limpet shell, thereby fulfilling the aim of roughly estimating the molluscs’ growth range. In broad terms, and pending confirmation through stable oxygen isotope analysis, the data indicate that cold periods dominate at the beginning of the edges of the analysed Nacella shells. In other words, low temperatures marked the final phase of limpet growth.
In comparison, the results for the modern mussels were especially revealing in the specimens collected from Inner Cambaceres, showing a very clear correlation with their capture time in late spring. Stable oxygen isotope analysis will be carried out on these samples in future work to compare the inferred seasonality.
These promising results obtained will allow for a better understanding of the formation of the Middle Holocene shell middens present in the Argentine region of Tierra del Fuego. Furthermore, the results obtained will contribute to generating new knowledge that will be reflected in future scientific publications and congresses. The three-weeks stay in Germany has been very productive for the Alejandro’s training in the LIBS methodology, as well as in obtaining promising results for his archaeomalacological research.
