Changes in carbon isotope discrimination in grain cereals from different regions of the western Mediterranean Basin during the past seven millennia. Palaeoenvironmental evidence of a differential change in aridity during the late Holocene

Abstract

Carbon isotope discrimination (∆) was determined for kernels of six-row barley and durum wheat cultivated in the western Mediterranean basin during the last seven millennia. Samples came from different archaeological sites in Catalonia (north-east Spain) and in the south-east of Spain (mainly eastern Andalusia). Samples from the present were also analysed. Mean values of ∆ for barley and durum wheat grains decreased slightly from Neolithic (7000-5000 BP) to Chalcolithic-Bronze (5000-3000 BP) and Iron ages (3000-2200 BP) both in Catalonia and in south-east (SE) Spain. Values were consistently lower in SE Spain than in Catalonia throughout these five millennia, which suggests that Catalonia was less arid than SE Spain in this period. Within a given region, current discrimination values for kernels of the same cereal species cultivated under rainfed conditions were lower than those of archaeological grains, which implies more arid conditions at present. Furthermore, an empirical relationship between ∆ of mature kernels and total precipitation (plus irrigation where applicable) during grain filling (r 2 ϭ 0.73, N ϭ 25) was established for barley, currently cultivated at different locations in the western Mediterranean basin in Spain. The resulting relationship was applied to the ∆ data for barley kernels from 10 archaeological sites in Catalonia and 10 sites in SE Spain, to estimate the precipitation during grain filling at the time the kernels were produced. For both regions, current climatic conditions are consistently more arid than those inferred for the Neolithic, Bronze and Iron ages. In addition, although Catalonia was estimated to have had consistently wetter conditions (about 20% more precipitation) than SE Spain throughout these millennia, differences in precipitation between these two regions have recently increased, with 79% more precipitation in Catalonia. Results indicate a more rapid increase in aridity in SE Spain than in Catalonia, probably produced during the last few centuries, and due to anthropogenic causes.Abbreviations ∆, stable carbon isotope discrimination; δ 13 C, ratio of CO 2 ; WUE, water-use efficiency; BP, before present; NE Spain, north-east of Spain; SE Spain, south-east of Spain; 13 C/ 12 C relative to PeeDee belemnite standard; p i /p a, ratio of intercellular to atmospheric partial pressure of VPD, vapour pressure deficit.

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.…”

“…The majority of work establishing modern reference data has been undertaken by Araus and colleagues (Araus and Buxo, 1993;Araus et al, 1997b;Ferrio et al, 2005) and has centred on the Mediterranean, especially Spain. 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).…”

“…As carbon stable isotope data from C 3 plants reflect moisture conditions, they can be expected to track changes in climate over time. In the first applications of this methodology to charred cereal remains from archaeological sites, Araus and colleagues (Araus and Buxo, 1993;Araus et al, 1997a) compared D values for wheat and barley kernels from southeastern and northeastern Spain dating from the Neolithic to the Iron Age (7,000-2,200 BP). They concluded that the southeast had been consistently drier than the northeast and that, although climate was generally wetter than today, there was a trend towards more arid conditions throughout the periods studied (see also Araus et al, 2003).…”

From Traditional Farming in Morocco to Early Urban Agroecology in Northern Mesopotamia: Combining Present-day Arable Weed Surveys and Crop Isotope Analysis to Reconstruct Past Agrosystems in (Semi-)arid Regions

“…Araus et al 1997;Bogaard et al 2007;Fraser et al 2011;Wallace et al 2013;Fiorentino et al 2015). Thus variation in crop stable isotope values offers a useful way to investigate the ecology of present and past farming systems.…”

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

“…At present, the only way to directly link climatic fluctuations and agricultural productivity in the past is through stable carbon isotope analysis of archaeological plant remains. This method is an established tool for identifying ancient environmental conditions for plant growth in arid to semiarid environments because δ 13 C values in cereals provide a drought-stress signal when the amount of water received during the grain-filling period is low (21)(22)(23)(24)(25)(26). Carbon fixation during photosynthesis leads to typical ranges of δ 13 C between −28‰ and −25‰ in modern barley, which is a C3 plant (27).…”