Climate at the onset of western Mediterranean agriculture expansion: Evidence from stable isotopes of sub-fossil oak tree rings in Spain

Climate conditions during the early Holocene may have contributed to the spread of Old World agriculture from its area of birth, in the Fertile Crescent (Near East), west through the Mediterranean Basin. Reconstructing the specific environmental conditions existing in early agricultural settlements of the western Mediterranean may help to elucidate this point. The aim of this work was to gain information on past climate of one of the earliest agricultural settlements of the Iberian Peninsula, La Draga, a lacustrine site dating back to the 2nd half of the 8th millennium BP, in which post fragments of deciduous oak have been exceptionally well preserved in an anaerobic environment (sub-fossil wood). We studied the relationship between climate factors and carbon isotope discrimination (Δ¹³C) and oxygen isotope composition (δ¹⁸O) in wood α-cellulose from modern tree-ring series of Quercus humilis Mill. and Quercus petraea (Matt) Liebl. recovered from the area. Climatic responses observed in extant material were used to interpret the isotopic signal of wood α-cellulose extracted from sub-fossil wood. Results showed that water availability of late spring, early summer and September influences Δ¹³C and δ¹⁸O, which allows their use as proxies for palaeoclimatic reconstruction. Differences between sub-fossil and extant samples in Δ¹³C (19.35 vs. 18.02‰) and δ¹⁸O (26.32 vs. 29.28‰) records suggest slightly lower temperatures and higher plant water availability than at present during the establishment of agriculture at the site. These results seem to disprove the existence of an environmental limitation preventing continuation of hunting and gathering activities as a cause for the adoption of agriculture in this early agricultural site on the Iberian Peninsula.