Complex agriculture during the second millennium bc: isotope composition of carbon studies (δ13C) in archaeological plants of the settlement Cerro del Castillo de Alange (SW Iberian Peninsula, Spain)

Abstract

The isotope composition of fruits and seeds of Hordeum vulgare L. (barley), Triticum aestivum/durum (wheat) and Vicia faba var. minor (faba bean) from three chronological phases between 2200 and 1321 cal bc of the settlement Cerro del Castillo de Alange (SW Iberian Peninsula) have been studied. The δ13C values for cereals were between −24.40 and −20.39‰ (V-PDB), with a mean of −22.01‰, the discrimination (Δ) being 15.96‰. The legumes registered similar values, between −26.25 and −20.49‰ (mean = −22.59‰), with a differential for the period of 16.51‰. In both cases, a change was noted from the first phase, where water availability is clearly a limiting factor for plant development. In subsequent phases the growing conditions appear be wetter. By comparison, we measured samples of Quercus ilex-coccifera (oak) charcoal, which shows similar values throughout the series. This suggests that there was no significant climate shift to moister conditions that could explain the above results, but rather they were the consequence of a change in crop management. In addition, we measured samples of a current rainfed Triticum sp. (year 2014), which averaged Δ13C 15.56‰. The changes between the three phases could indicate the development of mixed models of exploitation that combine strategies based on the use of rainfed and potentially irrigated areas during the 2nd millennium bc. The implementation of such a cropping technique, taking advantage of the river banks, could be a response to the processes of climatic degradation that begin in the Middle Holocene. The situation of the Cerro del Castillo reservoir between the rivers of the Guadiana and Matachel rivers is consistent with the development of this type of practice indicated by the isotopic and archaeobotanical data.