Climatic interpretation of carbon isotope content of mid-Holocene archaeological charcoals from eastern Anatolia

The C-13/C-12 ratio of juniper charcoals from the archaeological site of Arslantepe, Malatya (Turkey) was analysed for the mid-Holocene period spanning from 3350 to 2000 years BC. In addition, modern juniper and deciduous oak specimens were collected during a botanical survey around Arslantepe in August 2008. After the evaluation of weather parameters, the modern C-13/C-12 ratio was compared with the archaeological one, to infer past seasonal precipitation relative magnitude patterns. Delta C-13 values obtained from these samples suggest that the present-day climate is, on average, drier than that recorded in the past. Comparing the Delta C-13 juniper record with the already published curve for archaeological charcoals of deciduous oaks from Arslantepe, it appears that the two curves show similar trends in the short period between ca. 2850 and 2500 BC, in which the curves are substantially parallel and the Delta C-13 difference between them is quite reduced, but they are shifted by ca. 50-150 years in the rest of the record. For the interpretation of the arboreal curves, however, it was necessary to consider the autoecology of modern plants and the source of the water used for photosynthesis, as the two selected trees show Delta C-13 curves with rather different patterns. Overall, C-13/C-12 contrasting ratios in juniper and deciduous oaks suggest that seasonality in rainfall distribution and aquifer recharge played a complex role. Thus, a consistent seasonal contrast in precipitation (similar to the one in recent times) took place at ca. 3350, 3100, and 2300 BC indicating that semi-arid conditions have occurred in the past, in alternation with periods characterised by minor differences in the distribution of annual precipitation. In particular, the period between ca. 2850 and 2500 BC may suggest more humid conditions in late winter/early spring. The combination of the two records suggests that 1) two periods of drought are found at ca. 3350-3100 and at ca. 2300-2200 BC and 2) that at present winter precipitation is lower than that recorded during the period 3350-2000 BC and summer aridity less or equal to that recorded ca. 2200 BC. Changes in precipitation seasonality and the complex and the peculiar hydrogeological situation hindered the interpretation of the isotope records. The use of two taxa having different autoecology and different growing season allows interpretation of the data in terms of changes in precipitation seasonality and of specific habitat in this complex and peculiar hydrological setting.