Dual-bivalve proxy analysis of shellfish harvest intensity, and palaeotemperture in Powell River, British Columbia

Abstract

This thesis investigated the seasonality and intensity of shellfish harvesting and past sea surface temperature in Powell River, British Columbia, in the traditional territory of the Tla’amin First Nation. This is the first study to use two species of bivalve Saxidomus gigantea and Leukoma staminea as proxies for past cultural practices and environment. This is also the first study to use high-resolution stable oxygen isotope sclerochronological analysis for interpreting past subsistence and settlement patterns in the Powell River region. Seven archaeological shell midden sites were examined to interpret shellfish harvest intensity using sclerochronological measurements of 644 shells from seven shell middens. The results showed that overall, there a consistent, and intense pattern of shellfish harvesting for both species. The only notable site-level difference is at the village site DlSd-11, where the S. gigantea shells examined showed a higher relative harvest intensity whereas the L. staminea shells showed a lower relative harvest intensity. High-resolution stable oxygen isotope sclerochronological analysis was used to examine three live-collected L. staminea shells from the Sechelt Inlet in Sechelt, British Columbia. The results showed that the species L. staminea can be used to interpret the season of capture, although palaeotemperature calculations are affected by changes in salinity. The calculated temperature of the live-collected L. staminea shells had a range of 5.1-18.9℃ whereas the measured temperature was 5.7-20.4℃. Five S. gigantea and five L. staminea shells from the defensive lookout site DlSd-1 in Powell River, British Columbia were analyzed for seasonality. The results showed a difference between species for the interpreted season of collection. All five of the S. gigantea were collected during the spring. Two of the L. staminea shells indicated collection in the spring, one shell indicated either spring or summer collection, one shell indicated summer collection, and the last shell indicated an autumn or winter collection. The combination of ẟ18Oshell data, with other lines of archaeological evidence, and ethnohistorical records indicate that the site was inhabited during the spring and likely used throughout the year by the ancestral Tla’amin. The dual proxy analysis showed that the use of two provides insight into the overall history of shellfish harvesting. In this case, the use of two species shifts the interpretation from a single season of site use to year-round by looking at both species of bivalves. Further, the growth increment analysis affirmed a consistent pattern of intensive shellfish harvest in this region.