HOLOCENE BIVALVE STABLE ISOTOPE RECORDS FROM ARCHAEOLOGICAL SHELL MIDDENS AS PROXIES FOR CLIMATE CHANGE AND RESOURCE COLLECTION STRATEGIES

Archaeological shell middens provide a unique insight into the historical record of resource collection strategies and regional environmental change. Here we present a 5000-year oxygen-isotope (δ¹⁸O) record from bivalves (Saxidomus giganteus) obtained from archaeological shell midden deposits at the oldest continuously occupied site of Namu, which provides the longest record of bivalve shell accumulation on the coast of British Columbia. Bivalve specimens from radiocarbon-dated midden strata were analyzed at high resolution to produce sinusoidal profiles representing annual and multi-annual climate δ¹⁸O cycles. The long-term δ¹⁸O record indicates a significant negative shift in δ¹⁸O values between 3600–2000 cal BP. Bivalve δ¹⁸O values produce anomalously high water temperatures, and thus suggesting that large seasonal variation in precipitation is the dominating factor controlling bivalve oxygen-isotope compositions in the Namu area. Saxidomus giganteus inhabit an estuarine-type environment, therefore δ¹⁸O values are more strongly affected by the large changes in precipitation with minor temperature effects. Therefore, the negative shift in δ¹⁸O values during the 3600–2000 cal BP period has been interpreted to be associated with a shift to increased precipitation levels. Concurrent with this period is an increased dietary reliance on shellfish and a significant decrease in pink salmon remains within the midden deposits. We propose that a significant change in precipitation was the driving factor behind the biological disturbance in the Namu region, which subsequently caused the human population to diversify its resource strategies and develop more peripheral sites. This study illustrates how a geochemical investigation of continuous archaeological records can provide a wealth of information regarding long-term environmental change during periods of occupation.