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Variation in human breastfeeding and weaning practices is subject to changing social pressures and norms, as well as individual agency. This paper presents a case study from the Yukisma Mound (CA‐SCL‐38), a prehistoric site in the San Francisco Bay Area, which was used as a ceremonial and cemetery space by the ancestral Ohlone Indians between 780 and 230 cal years bp. Stable carbon and nitrogen isotope values from bone collagen of 201 individuals, including 26 subadults, are analysed. The Weaning Age Reconstruction with Nitrogen isotope analysis package in R is used to model expected δ15N values of infant bone collagen, against which measured δ15N values reveal individual variation in weaning practices. Infants from this burial sample were exclusively breastfed until approximately 0.8 years (maximum density estimation (MDE), marginal probability = 0.0456), and completely weaned by around 4.4 years (MDE, marginal probability = .0409). Range of variation in adult diets is used as a baseline of comparison, in addition to mean δ15N values of adult females, to better consider supplementary food options, and to avoid the assumption of a homogeneous maternal diet. Comparison with weaning data from contemporaneous regional sites, drawn from studies using tooth dentin of individuals who survived childhood, reveals that introduction of complementary foods and cessation of breastfeeding occurred somewhat later at the Yukisma Mound. This contrast suggests prolonged supplementation with breastmilk for children who expressed nutritional stress or disease. Variation in weaning trajectories is further interpreted using δ13C values to identify individuals with elevated δ15N values due to supplementation with high trophic level foods (e.g., freshwater fish and marine foods). Changes in weaning practices are demonstrated by comparison of this sample to ethnohistoric accounts of the Ohlone. This study provides an example of collaborative research with descendent populations and of making the most of available materials.

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Previous stable isotope research on Middle-to-Late Holocene (6600 cal BP-present) hunter-gatherers of Central California has documented significant regional variation in human paleodiets between the San Francisco Bay Area and Central Valley. In general, this geographically-patterned dietary variation tracks greater consumption of marine food resources in the Bay Area and greater consumption of freshwater and terrestrial food resources in the Central Valley. Using stable carbon and nitrogen isotope analysis of bone collagen, stable carbon isotopes of bone bioapatite, and a large series of AMS dates (5420-2975 cal BP), we examine human paleodiets in 238 burials from CA-CCO-548 (Marsh Creek), located near the Sacramento-San Joaquin Delta, in Central California. Isotopic results support archaeofaunal and paleobotanical findings regarding the dietary importance of terrestrial C 3 -resources (e.g., artiodactyls, small seeds, acorns) and freshwater fish. The results further indicate that marine resources were of minor importance and could have been acquired from brackish areas near the mouth of the San Francisco Bay estuary located 9 km away or through trade. Although carbon and nitrogen isotopes of bone collagen suggest continuity in the sources of dietary protein over time. Carbon isotopes of bone bioapatite demonstrate a significant temporal shift toward greater consumption of C 3 plant resources, such as acorns and small seeds, consistent with resource intensification models. The paleodiet of the CA-CCO-548 population is similar to contemporaneous groups located to the south and northwest, but is distinct from groups within the Delta and San Francisco Bay Area.

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