Reporting atmospheric CO2 pressure corrected results of stable carbon isotope analyses of cereals remains from the archaeological site of Penalosa (SE Iberian Peninsula): agricultural and social implications

The practice of agriculture across the Mediterranean basin has had significant social and economic consequences, including the development of social inequalities. To inform on plant water status and thus on agricultural management techniques and environmental conditions during the first half of the second millennium BCE, we measured stable carbon isotopes in a set of 280 seeds of Hordeum vulgare L. and Pisum sativum from the archaeological site of Penalosa (Banos de la Encina, southern Iberian Peninsula). The ranges in stable carbon isotope values for archaeological samples were wider than those observed for modern reference samples collected in 2014 and 2015, suggesting that archaeological samples experienced more varied water status conditions. This variability was associated with the location of the seeds within the site and could be a consequence of the cultivation of different plots and/or from temporal variations in local environmental conditions. For absolute comparisons of water status between modern and past samples, we introduced a novel correction to account for temporal changes in the concentration of atmospheric CO2 in addition to the usual adjustment that accounts for the variability in atmospheric CO2 stable carbon isotope values. This comparison showed that past samples had greater water availability than modern references, and thus, irrigation or intentional selection of naturally irrigated soils was practiced at the site.