Stable isotopes as a tool for investigating changes in Holocene hunter-gatherer strategies along the northern California coast

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With NSF support, Dr. Ann Russell, Dr. Robert Bettinger, and Mr. Michael Kennedy will use stable oxygen and carbon isotopes (d18O and d13C) at the edges of California sea mussel (Mytilus californianus) shells collected from 14C-dated archaeological components to determine the season in which prehistoric northern California hunter-gatherers harvested this marginal marine resource over the last 8600 years. In addition, they will evaluate the relationship between seasonality of the mussel harvest and changes in marine conditions reconstructed from annual records of d18O and d13C. In recent years, there has been increased focus on the emergence of hunter-gatherer maritime adaptations and the role of climate change as a selective force in shaping cultural responses. Major cultural changes in southern and central California, such as the use of marginal resources as well as increasing economic and sociopolitical complexity, may have developed from resource stress related to changes in sea-surface temperature (SST) and marine productivity. Little is known about these aspects of hunter-gatherer evolution along the coast of northern California. Reconstruction of seasonal foraging patterns, as well as upwelling intensity and marine nutrient levels along the coast of northern California will provide critical information for understanding the emergence of marine resource intensification in this region.Previous research has shown that the record of stable isotopes (d18O and d13C) in shell calcite of mollusk shells provides important information on marine environmental conditions (SST, nutrient levels, and inferred marine productivity). Preliminary results confirm this and suggest that the approach of using both d18O and d13C of the terminal margin (growing edge) of Mytilus californianus (California sea mussel) shells is a valuable tool for determining the season of harvest.How intensively did hunter-gatherers utilize marginal marine resources (shellfish) throughout the year in northern California, and what role did marine climate play in the development of intensified marine foraging strategies? To address these questions, the research team will generate three seasonal records of d18O and d13C in mussel shell calcite from five 14C-dated archaeological components near Bodega Bay, California. These profiles will provide information on average marine environmental conditions, as well as the typical range of seasonal variation of SST and d13C at the time of occupation. In addition to these long-term records, the researchers will obtain isotopic data from the terminal margins of at least 20 additional shells per component to determine the season of harvest. An important aspect of the research will be to continue a modern calibration/verification of the use of coupled d18O and d13C from shell terminal margins to establish the season of harvest. Using the results of these studies in conjunction with other archaeological data, the research team will reconstruct changes in environmental conditions and in the adaptive strategies of maritime hunter-gatherers of northern California since 8600 BP. This research will permit a regional evaluation of annual foraging intensity over the last ~8600 years, and will provide a far more sophisticated examination of marine resource intensification along the northern California coast than previous research has offered.