Clams and climate: 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, the paleoenvironmental record reflects fluctuating marine conditions throughout the region. The effects of changes in regional climate patterns, as well as human responses to such change, however, can exhibit great variability locally. 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 while climate reconstructions for the Gulf of Alaska are not uncommon, regional climate reconstructions are often insufficient for archaeological research. Many climate reconstructions lack sub-annual resolution and cannot produce a detailed understanding of seasonal behaviors in human populations. Sclerochronological and sclerochemical analysis of shellfish remains from archaeological sites in the archipelago may provide additional paleoenvironmental information. 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, 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 circalunidian growth lines were counted between annual winter growth lines confirmed by oxygen isotope analysis. Clams collected from Alaska grew a total of 143±34 days while the Canadian clams grew 273±14 days. Additionally, oxygen isotope values were more positive from annual winter growth lines from Alaskan samples than Canadian samples. 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), which grew an average of 166±22, 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.