Vertical sheep mobility along the altitudinal gradient through stable isotope analyses in tooth molar bioapatite, meteoric water and pastures: A reference from the Ebro valley to the Central Pyrenees
Although the frequency of pastoral activities involving vertical sheep mobility has decreased over the last century, this is a herding strategy still used in the Ebro basin, where animals move from overwintering valley locations up to the Pyrenees from late spring to early autumn. Such practice allows herders to avoid the worst climatic conditions, seasonally balancing the great contrast between ecological zones in this region, from dry lowland Mediterranean steppe to wet mountain subalpine grasslands. As recent regional archaeological works have suggested, the altitudinal movement of flocks may have begun with the first early Neolithic groups settled in this territory. Here we investigate through stable isotope analyses one of the last flocks that still performs this activity. Sheep specimens were analyzed by sequential analyses (δ13C and δ18O) in bioapatite of tooth enamel, allowing detection of seasonal changes. Tooth series are interpreted according to rainfall distribution, seasonal patterns in δ 18O of meteoric water, vegetation changes and δ13C values in pastures along the altitudinal gradient in the area. Vertical movements in sheep sequential series are recognized by an inverse relationship between δ13C and δ18O values. Monthly δ18O values in meteoric water obtained in valley and mountain locations describe the same type of seasonal oscillation, with high values during the warm months and low values during the cold months. Pastures analyzed along the altitudinal gradient showed a decrease in δ13C values with altitude, linked to the seasonal availability of precipitation and vegetation differences among locations. These results define a new analytical and conceptual framework for the interpretation of archaeological samples in this region.