FOCUS: Estimated wheat yields during the emergence of agriculture based on the carbon isotope discrimination of grains: Evidence from a 10th millenuim BP site on the euphrates

The analysis of carbon isotope discrimination (Δ) in crop seeds from archaeological sites may help to assess yield during early agriculture. Wheat was among the first crops to be domesticated in the Near East. Here, a yield model based on Δ in grains from present-day crops of durum wheat (Triticum durum Def.) is developed using a wide range of genotypes, most of which were cultivated in the Eastern Mediterranean basin (Syria). A strong positive linear regression (r2=0•72) between yield on a logarithmic basis and grain Δ was observed. Initial estimates of ancient wheat yields were then obtained by fitting the Δ values of archaeological grains to the above relationship. The yield estimates were subsequently corrected to take into consideration the two main differences between ancient and modern crops not accounted for by the Δ of ancient samples: Atmospheric CO2 levels and grain/total above-ground biomass ratio. The model was then used to estimate yields of naked wheat (Triticum durumlaestivum) attained at Tell Halula, an early agricultural site, ranging from Middle Pre-Pottery Neolithic B (PPNB, c. 9550 BP calibrated) to Late Neolithic (Pre-Halaf, c. 8630 BP calibrated). This is the earliest archaeological settlement in the Middle Euphrates region (Rakka Governorate, Syria) at which the cultivation of domesticated naked wheat has been reported. Productivity, calculated from Δ of wheat grains found at this site, was relatively high, with an average (mean ± S.D.) of 1•56 ± 0•28 Mg ha-1. Although these yields are well below those attained in this region by present-day wheat crops under irrigation (e.g. 3•83 Mg ha-1, mean value for 1987-1996 period at Rakka Governorate), they are higher than those achieved for wheat under the current rainfed conditions of this semi-arid region (1•05 Mg ha-1) and suggest natural growing conditions were wetter than those than are prevalent today. In addition, our estimates for Tell Halula suggest that Neolithic agricultural practices may have produced greater yields than those to be expected from stands of wild cereals, which would have favoured the transition from gathering to cultivation.