Crop water availability in early agriculture: evidence from carbon isotope discrimination of seeds from a tenth millennium BP site on the Euphrates

Abstract

The analysis of carbon isotope discrimination (∆) in crop plant remains from archaeological sites may help to assess water availability for early agriculture. This study presents the analysis of ∆ in seeds of naked wheat (Triticum aestivum/durum), lentil (Lens orientalis/ culinaris), and flax (Linum sp.) found at the archaeological site of Tell Halula in the valley of the Middle Euphrates (Syria). This Neolithic site is the oldest in this region of the Fertile Crescent where the cultivation of domesticated plants has been reported, with seed remains ranging from 9550 to 8465 BP. Most of the seeds analysed showed ∆ values greater than 16 ‰, reaching 20 ‰ for some samples of flax. For wheat, ∆ values were much higher than those reported in present-day (1996) durum wheat crops cultivated under rainfed conditions in north-west Syria under environments with somewhat higher rainfall than Tell Halula. Similarly, grains of present-day (1997) barley cultivated in the archaeological site also showed lower values than those found in archaeological kernels. An empirical relationship between ∆ of mature kernels and total precipitation (plus irrigation where applicable) from heading to maturity (r 2 ⍧ 0.82, n ⍧ 11) was established for durum wheat, currently cultivated in different environments of the Mediterranean basin. The resulting relationship was applied to the data on ∆ of wheat fossil kernels from Tell Halula to estimate the accumulated water inputs during the time (about 6 weeks) the kernels were produced. Calculated water inputs for wheat during early agriculture were (over 110 mm) at least 5 times higher than current-day rainfall accumulated in Tell Halula during the same phenological period. These results strongly suggest that early agriculture wheat was cultivated at Tell Halula under much wetter conditions than are currently to be found in the area. The presence of flax and its very high ∆ values also support this conclusion. Whether such humid conditions during cultivation were due to moister conditions prevailing at this time, by planting in alluvial areas or by irrigation works is discussed.

Drought stress variability in ancient Near Eastern agricultural systems evidenced by δ 13 C in barley grain

“…Although in arid to semiarid environments other factors, such as crop density and soil salinity, may play a role in causing a drought stress signal in the plant, the strongest correlation of stable carbon isotope data in plants exists with water availability (27)(28)(29). Variability of δ 13 C measurements in archaeobotanical cereal grains occurs at different levels.…”

An investigation into the archaeological application of carbon stable isotope analysis used to establish crop water availability: solutions and ways forward

“…A reliable method for establishing whether artificial crop irrigation existed at an ancient settlement would therefore be invaluable. Initial investigations, especially by Araus and his group (Araus and Buxo, 1993;Araus et al, 1997aAraus et al, , 1997bAraus et al, , 1999 suggest that plant stable carbon isotope analysis has great potential in this respect, as it should, theoretically, be possible to infer from the carbon stable isotope composition of crops whether they received more water than was 'naturally' available, and therefore whether a system of artificial irrigation was in place. In order for this approach to be viable, it is first essential to distinguish between the effects on carbon isotope discrimination caused by irrigation and those of climate fluctuations.…”

“…They employed the reference values from modern crops mainly in two ways: as baseline data for relative statements about past climate conditions ('wetter/drier than today') (Araus et al, 1997a;Ferrio et al, 2003Ferrio et al, , 2005, or for establishing relationships between the Δ of different types of crops grown under the same environmental conditions. As we have seen above, such data are essential for identifying irrigated crops by comparison with other, probably non-irrigated, plant species from the same site (Araus et al, 1997b(Araus et al, , 1999Ferrio et al, 2005). Araus and colleagues (Araus et al, 1997b(Araus et al, , 1999Ferrio et al, 2003) also used modern D values from crops grown under a variety of known watering regimes to formulate non-linear regression equations for calculating total water input from Δ values.…”

“…As we have seen above, such data are essential for identifying irrigated crops by comparison with other, probably non-irrigated, plant species from the same site (Araus et al, 1997b(Araus et al, , 1999Ferrio et al, 2005). Araus and colleagues (Araus et al, 1997b(Araus et al, , 1999Ferrio et al, 2003) also used modern D values from crops grown under a variety of known watering regimes to formulate non-linear regression equations for calculating total water input from Δ values.…”

“…Few modern crop growing experiments are designed with archaeological samples in mind, and, for want of better alternatives, modern samples are often assembled from a number of sites which differ not only in water availability but also in other environmental parameters such as altitude (see, for example, Araus and Buxo, 1993;Araus et al, 1997bAraus et al, , 1999.…”