Oxygen isotope analysis of California mussel shells: seasonality and human sedentism at an 8,200-year-old shell midden on Santa Rosa Island, California

Abstract

To evaluate coastal settlement and land use strategies among maritime hunter-gatherers, we analyzed oxygen isotope (δ18O) data from 131 marine carbonate samples from 21 California mussel (Mytilus californianus) shells obtained from a large ∼8,200-year-old shell midden (CA-SRI-666) on California’s Santa Rosa Island. Seasonal distributions of the isotopic data were assigned using a paleo-sea surface temperature model created by comparing modern sea surface temperatures (SST) to a fully profiled ∼8,200-year-old shell. For 20 additional shells, we used two sampling strategies to compare season-of-harvest inferences and explore whether the Early Holocene site occupants were sedentary. Estimated season-of-harvest differed by 35 % between the two sampling methods, corroborating recent isotope analysis of an 8,800-year-old shell midden on San Miguel Island. Shellfish appear to have been collected year-round at CA-SRI-666 from intertidal or subtidal water temperatures similar to modern SST in the vicinity of eastern Santa Rosa Island. The isotope results are consistent with other evidence from CA-SRI-666 that suggest that the site served as a residential base for relatively sedentary maritime people.