Implications for measuring seasonality in the marine bivalve, Saxidomus gigantea

Abstract Sclerochronological and sclerochemical analysis of shellfish remains from archaeological sites afford the opportunity to understand environmental change and its impacts on human populations through time. During the Late Holocene in the Gulf of Alaska (GOA), the paleoenvironmental record reflects fluctuating marine conditions throughout the region. In the Kodiak archipelago in the Gulf of Alaska, changing environmental conditions, population growth, technological transitions, and contact with other communities likely promoted the transition from needs-based maritime hunting and gathering to surplus-based, semi-permanent villages. The precise role of climate in this transition is understudied. Few paleoclimate reconstructions are available for the Kodiak archipelago and climate reconstructions for the GOA are often insufficient for archaeological research because they lack sub-annual resolution and cannot produce a detailed understanding of seasonal behaviors in human populations. Analysis of shellfish remains from archaeological sites in the archipelago may provide additional paleoenvironmental information for the Gulf of Alaska. Measuring and comparing the length of seasonal shell growth in select species of bivalves may complement stable oxygen isotope analysis, together providing a more precise paleoclimate reconstruction. This research utilizes the growth of Saxidomus gigantea (also known as S. giganteus), abundant both on modern and ancient coastlines to provide information about the length of its growing seasons. To measure seasonality, a total of 25 modern samples were collected from Alaska and British Columbia and the number of lunar-daily growth increments (LDGI) was counted between annual winter growth bands confirmed by δ18O maxima measured in sinusoidal time series. Clams collected from Alaska grew a total of 143 ± 34 days while British Columbian clams grew 273 ± 14 days. δ18O values and the number of LDGI per year were then compared to previous studies along the Pacific Northwest coast. δ18O values were more positive from annual winter growth bands from Alaskan samples (average value of 1.0‰) than British Columbian samples (average value of −0.2‰). Maximum δ18O values from winter growth generally increased with latitude (R2 = 0.67), as did the number of LDGI per year (R2 = 0.74). Minimum δ18O values did not appear to be significantly related to latitude (R2 = 0.42). This method was then applied to three archaeological samples collected from the Rice Ridge site (KOD-363), the Uyak site (KOD-145), and the Settlement Point site (AFG-105) from the Kodiak archipelago, Alaska, which grew an average of 166 ± 22 days, to confirm that these methods can be applied to archaeological samples through time to detect spatial and temporal changes in seasonality. These results suggest that changes in sea surface conditions and seasonality are detectable both spatially and temporally through detailed sclerochronological and sclerochemical analysis of shellfish remains from archaeological sites and offer the potential to reconstruct marine environmental conditions throughout the Holocene.