Social structuration in medieval rural society based on stable isotope analysis of dietary habits and mobility patterns: San Juan de Momoitio (Biscay, north Iberian Peninsula)

A total of 93 individuals, of which 34 were infants, from San Juan de Momoitio graveyard (9th to 12th centuries) were analyzed to explore social changes in a peasant community in the northern Iberian Peninsula within the historical evolution of the Middle Ages. The study consisted of human palaeodiet characterization and mobility patterns based on δ13C, δ15N, δ18O, and 87Sr/86Sr isotope analyses. Carbon and nitrogen values showed a staple diet derived from husbandry based on the consumption of animal protein (pig, poultry and sheep/goat) and cereals (millet and wheat). The increase in the consumption of millet rather than wheat during the High Middle Ages is noteworthy. The change in dietary pattern coincided with the transformation of the landscape from dispersed farmsteads to the foundation of a village, leading to landscape reorganization. This new situation resulted in the gradual abandonment of San Juan de Momoitio church and the graveyard. Oxygen and strontium isotopes show that most individuals were of local origin except for only two individuals. Strontium isotopes also recorded the population distribution after the village foundation and the subsequent landscape arrangement, reflecting the relative movement of individuals towards higher hillsides. Additionally, the oxygen isotopes of teeth recorded climate changes throughout the time of the settlement, suggesting variation from warmer climate conditions during the first occupation period towards colder climate conditions in the last period. Nitrogen and oxygen isotope composition also contributed to a better understanding of breastfeeding and weaning practices affecting infant peasants in the Middle Ages.