LIBS on Mussels from Santa Rosa Island
As part of her doctoral research, Alexandria completed a research stay in Summer 2025 with the Emmy Noether Group SEAFRONT, working with the Laser-Induced Breakdown Spectroscopy (LIBS) system housed at the Leibniz-Zentrum für Archäologie (LEIZA; Figure 1). This visit builds on her ongoing work at UNR, where she integrates sclerochronology (shell growth band analysis) with stable oxygen (δ¹⁸O) and carbon (δ¹³C) isotope analysis of California Mussel (Mytilus californianus) shells. During her stay, she worked closely with postdoctoral researcher Rosa Arniz Mateos and Research Associate Danai Theodoraki, gaining practical experience applying LIBS to both modern and archaeological shell material. The skills developed during this collaboration contribute directly to her broader aim of refining environmental reconstructions and improving our understanding of human-environment interactions over approximately the last ~2000–3000 years.
The shells analyzed during this project were collected between 2015 and 2019 from Santa Rosa Island (SRI), located within California’s Northern Channel Islands. Sampling focused on both the northern (Dry Canyon) and southern (La Jolla Vieja) coasts of the island (Figure 2a and 2b), areas characterized by contrasting oceanographic conditions. These sites lie at the convergence of the cool California Current and warmer seasonal countercurrents, creating a dynamic marine environment that is well suited for investigating how environmental variability is recorded in shell growth.
In addition to modern specimens, select archaeological shells from nearby coastal sites, including CA-SRI-138 and CA-SRI-19 (Figure 2c and 2d), were also analyzed. These sites represent long-term occupations spanning different environmental zones of the island and provide an opportunity to connect environmental reconstructions with past human activity. The material examined therefore offers both a modern baseline and an archaeological record through which environmental signals can be interpreted.
The primary objective of this research stay was to evaluate the applicability of the LIBS system housed at LEIZA as a tool for environmental reconstruction, particularly in comparison to traditional stand-alone isotope-based sampling methods. To achieve this, Alexandria conducted high-resolution elemental mapping of mussel shells from both northern and southern collection areas and tested the method on archaeological specimens to assess its reliability across different preservation conditions. Additional analyses were carried out on Owl Limpet (Lottia gigantea) shells, to assess further extending the scope of her dissertation work to a multi-species analysis in addition to a multi-proxy one. This work confirmed a focus on mussels and formed a pilot framework for determining how best to apply LIBS to the distinct internal structure of California mussels, which uniquely exhibit both inner and outer calcitic growth layers (Figure 4).
The results demonstrate that individual mussel shells’ outer calcitic growth preserve clear seasonal variability, with alternating warm and cold phases visible in elemental (Mg/Ca) patterns (Figure 5a). This confirms that these shells can serve as reliable archives of environmental change. A particularly significant outcome of the study is the contrast observed between shell layers. The outer calcite exhibits a consistent and predictable environmental signal, whereas the inner calcite shows greater variability (Figure 5b), likely reflecting a stronger influence of biological processes separate from the targeted environmental signal. This distinction is critical for future analyses, as it identifies the outer calcite as the most reliable substrate for reconstructing past ocean conditions.
Once multi-proxy analyses of modern specimens are complete, these data will be used as a baseline for interpreting archaeological shell material. The archaeological sites to be examined encompass a period of important cultural developments within Island Chumash society, including shifts in settlement patterns, the emergence of territoriality, and the expansion of trade networks. By integrating environmental reconstructions with archaeological evidence, this research contributes to a more nuanced understanding of how coastal communities responded to changing marine conditions over time.
Overall, this research stay represents a successful pilot study in applying LIBS to both modern and archaeological shell materials. The results highlight the method’s potential for high-resolution environmental reconstruction and provide a clear pathway for future work. Alexandria plans to continue collaborating with LEIZA in the upcoming summer season, focusing on expanding her modern calibration dataset and applying refined methodologies to a broader sample of archaeological shells from SRI.
