The inherent sampling and preservational biases of the archaeological record make it difficult to quantify prehistoric human diets, especially in coastal settings, where populations had access to a wide range of marine and terrestrial food sources. In certain cases, geochemical proxies such as stable isotope ratios may be utilized to provide robust estimates on the relative proportions of various food resources consumed by prehistoric populations. The Harkins Slough archaeological site (SCR-60/130) is an early to middle Holocene coastal assemblage located on Monterey Bay in central CA. The carbon (δ 13C) and nitrogen (δ 15N) isotope composition of human burials and associated archaeofauna excavated from the site were measured for input into a concentration-dependent isotope mixing model that is able to statistically discriminate among multiple (>3) food sources. The human burials segregate into two distinct groups, an early (∼7000 bp) and middle (∼4500 bp) Holocene population with significant dietary differences. Stable isotope analyses indicate a 70-84% marine food source contribution for the early Holocene group (EHG), but only a 48-58% marine dietary contribution for the middle Holocene group (MHG). Results also suggest that pinnipeds were an important marine food source for both groups. Modeling results are in agreement with archaeological evidence from southern CA, suggesting that early Holocene coastal populations were highly dependent on marine resources, with the ability to procure both littoral and pelagic species. Further, the use of terrestrial plant sources likely increased over time, with the feasible contribution range doubling from the EHG (4-30%, mean 19%) to MHG (10-52%, mean 38%). This trend is supported by regional archaeological evidence for the advent of technological innovations linked to intensive terrestrial plant processing beginning in the middle Holocene. Methodologically, this work demonstrated the utility of a concentration-dependent stable isotope mixing model in an iterative framework to provide feasible ranges of dietary contribution when the number of food sources is too high to allow a unique solution.
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