A SYNTHESIS OF SCLEROCHRONOLOGICAL DATASETS TO IMPROVE SEASONAL AND ENVIRONMENTAL INTERPRETATIONS OF ARCHAEOLOGICAL CALIFORNIA MUSSEL SHELLS

Sclerochronological analysis of archaeological bivalve mollusc shells can improve our understanding of past human-environment interactions and paleoenvironmental variability along coastlines, though species- and site-specific variables can impact interpretations in uncertain or unexpected ways. We aimed to optimize the paleoenvironmental utility of an archaeologically important marine bivalve species from coastal North America, Mytilus californianus (California mussel). Abundant stable isotope (δ¹⁸O and δ¹³C) records exist from ¹⁴C-dated archaeological M. californianus shells, originally sampled to study human history at California’s Channel Islands. Here, we compile and analyze over 6000 δ¹⁸O and δ¹³C data points from M. californianus shells from Channel Islands middens spanning 9000 years. Synthesizing and analyzing previously collected sclerochronological data allowed for novel interpretations of spatial, temporal, and biological influences on shell chemistry in a species with cultural, ecological, and archaeological importance. Geographically, we found that M. californianus shells record statistically significant differences in δ¹⁸O_shell by site (ANOVA, F_5,2658 = 252.7, p < 0.001), with increasingly warmer signals from San Miguel Island to Anacapa Island. Temporal patterns include seasonally driven oscillations in individual δ¹⁸O_shell profiles throughout the study period and a negative shift in collective δ¹³C_shell through time. Lastly, we found shorter δ¹⁸O_shell-inferred seasonal cycles with increasing ontogenetic age and high variability in terminal-edge δ¹⁸O_shell values in shells from the same harvest date and location, indicating that ontogenetic patterns and sampling methodology can strongly influence seasonal and environmental interpretations. Our analysis contributes new insights into sampling and interpreting widely-studied archaeological M. californianus shells, which grow and record environmental information more irregularly than expected.