Building confidence in shell: Variations in the marine radiocarbon reservoir correction for the northwest coast over the past 3,000 years

In many regions, fluctuations have occurred through time in the local C-14 activity of seawater. Evaluating these shifts and their effects on C-14 age estimates is difficult, and, as a result, archaeologists working in coastal settings tend to preferentially date chat-coal samples over shell. Our research on 18 charcoal-shell pairs front Puget Sound and Gulf of Georgia archaeological sites helps elucidate the spatial and temporal dynamics associated with marine reservoir effects in the Pacific Northwest. This analysis suggests that between 0 and 500 B.P. the regional correction value (DeltaR) is 400 years, which agrees with the modern value determined by Stuiver and others. Between 500 and 1200 B.P, however, DeltaR dips close to zero, possibly reflecting a decrease in offshore upwelling. Front 1200 to 3000 B.P., DeltaR returns to 400 years. These data are presented as a Puget Sound/Gulf of Georgia regional correction curve for the late Holocene, which local researchers may use to calibrate dates of marine shell. In addition, we detail our methods for constructing calibration curves and present guidelines for archaeologists working in other coastal settings to develop calibration curves for their regions.