Shellfish, Seasonality, and Stable Isotope Sampling: δ¹⁸O Analysis of Mussel Shells From an 8,800-Year-Old Shell Midden on California's Channel Islands

ABSTRACT

Analyzing stable isotopes in mollusk shells allows archaeologists to address issues ranging from seasonality of harvest, to settlement and subsistence strategies, sea surface temperatures, and nearshore paleoecology. Studying California mussel shells from an ∼8,800-year-old shell midden on San Miguel Island, we investigate how mollusk growth rates influence sampling strategies for determining seasonality. Using a fully profiled shell as a standard to interpret more limited measurements (terminal growth band plus one) on 39 additional shells, we identified what appeared to be a multi-seasonal occupation. Sampling 20 of the 39 shells more intensively, however, changed our conclusions about season of harvest for 35 percent of the analyzed shells, producing data more consistent with other evidence for a short-term occupation of CA-SMI-693. Sampling strategies for determining seasonality from marine mollusks should carefully consider the ecology of individual species, particularly for fast-growing shellfish such as California mussels.

[Supplementary material is available for this article. Go to the publisher's online edition of Journal of Island & Coastal Archaeology for the following free supplemental resource(s): Appendix 1. Reported δ¹³C and δ¹⁸O and inferred temperature values for all isotopic determinations from forty analyzed California mussel shells from CA-SMI-693.]

Keywords:

• Early Holocene
• stable oxygen isotopes
• Mytilus californianus
• San Miguel Island

ACKNOWLEDGEMENTS

Our δ¹⁸0 samples reported were prepared by the senior author in the Coastal Archaeology and Human Ecology Lab, then analyzed in the Department of Geological Sciences Stable Isotope Lab at the University of Oregon (UO). We thank Jim Palandri and Ilya Bindeman for their help in analyzing the samples. We also thank Torben Rick, Tracy Garcia, Brendan Culleton, Douglas Kennett, and John Robbins for their intellectual contributions to our research, which was supported by the National Science Foundation (BCS 0917677 to Erlandson and Rick), the Edna English Trust, the National Parks Service, and UO. Finally, we thank DirectAMS for providing a free AMS ¹⁴C date from CA-SMI-693, the anonymous reviewers, and the editors of JICA for comments that significantly improved this article.