Saturday, 15 July 2006

Record of Holocene Environmental Changes in the Northern Fertile Crescent Inferred From Pedogenic Carbonate Laminations on Stones at Göbekli Tepe (SE Turkey).

Konstantin Pustovoytov, Univ of Hohenheim, Inst of Soil Science and Land Evaluation, Emil-Wolff-Str. 27, Stuttgart, 70599, Germany, Klaus Schmidt, German Archaeological Inst, Podbielskiallee 69-71, Berlin, 14195, Germany, and Heinrich Taubald, Univ of Tubingen/Inst of Geosciences, Wilhelmstr. 56, Tubingen, Germany.

Paleoclimatic reconstructions in arid regions are complicated by scarcity of materials routinely used to gain ecological proxy records (such as peats, lacustrine sediments etc.). Pedogenic carbonate (PC) can serve in many cases as a reasonable alternative solution, however it mostly provides a lower-resolution natural archive if compared, for example, with palinological sequences. This study represents the first attempt to enhance the resolution of PC record by examining continuous successions of micro-layers in secondary carbonate coatings on stones. We investigated PC laminations on stones at the Pre-Pottery Neolithic(PPN)(11.5-9 ka BP) (here and in the following calendar years) site Göbekli Tepe in Upper Mesopotamia (SE Turkey) as an indicator of local early-mid Holocene environmental changes. PC accumulated on the undersides of stones and other clasts of the cultural layer at the site to form coatings up to 5 mm thick. 14C age and stable isotopic composition of carbon and oxygen of thin (0.2-0.3 mm in thickness) lamina were examined. In two coating samples, 13C and 18O values in sequences of 16 and 25 micro-layers and 14C ages of the inner (oldest), middle and the outer (youngest) micro-layers were determined. In addition, the 14C ages of the oldest micro-layers in other 6 coating samples were measured. The 14C ages of the oldest, 0.2-0.3 mm thick coating micro-layers as well as the 13C values of coatings indicate that the formation of secondary carbonate essentially occurred in isotopic equilibrium with soil CO2 in terms of carbon isotopes. The radiocarbon ages of the oldest micro-lamina ranges between ca. 9 and 8.5 ka BP (uncalibrated) in six samples of coatings from PPNA (11.5-10.6 ka BP) structures and are distinctly younger than the – ca. 8 ka and 7.3 ka BP in a PPNB (10.6-8.8 ka BP) and a post-PPNB contexts respectively. This relationship provides chronological control to radiocarbon dates on PC microlaminae. Dates on the middle and outer (youngest) micro-layers in the two coatings suggest that the accumulation of PC terminated at ca. 4 ka BP most probably due to climate dessication. The rate of coating growth was on average substantially higher in the mid-Holocene (1.4-1.9 mm/ka) than in the early Holocene (0.6-0.7 mm/ka) which is attributable to more humid climatic conditions (“optimum”) during the mid-Holocene which gave way to the late-Holocene arid phase. The 13C and 18O values of PC show general increase from the early-Holocene to the mid-Holocene micro-lamina. There are two distinct phases in the 13C and 18O curves. The early-Holocene one, roughly from 10 to 6 ka BP, is characterised mainly by opposite trends in 13C and 18O. During this period, a progressive depletion in 13C took place over the first 1-2 ka and was followed by relatively stable minimum 13C values, whereas the 18O values were constantly increasing in general and reducing the amplitude of fluctuations. In the mid-Holocene phase, approximately between 6 and 4 ka cal BP, both 13C and 18O values were essentially higher compared to the early Holocene and the trends in the 13C curve are mostly parallel with the 18O curve. At the same time, there was a dramatic short-term negative 18O excursion in one of the coatings shortly before ca. 4 ka BP. A combination of the radiocarbon and stable isotope data suggest an amelioration trend at Göbekli Tepe during the early Holocene (ca. 10-6 ka BP) with continuously increasing temperatures and a relatively stable maximum of soil respiration rates or/and maximum C3-photosynthetic fractionation in the second half of the early Holocene. The signals of changes in humidity over the early Holocene are more difficult to detect but the moisture balance as a whole in the landscape is expected to have become more favourable for vegetation in later periods of the early Holocene. The mid-Holocene (ca. 6-4 ka BP) ecology of the site was supposedly characterised by the Holocene maximum of humidity and generally higher temperatures than in the early Holocene. Nevertheless, some occasional, short-term cooling events also cannot be excluded for the mid-Holocene. At around 4 ka BP, the climate conditions became relatively dry and never turned back to their early-mid Holocene state in terms of humidity. The proposed model of environmental development in the surroundings of Göbekli Tepe is consistent in its main features with a wide range of environmental proxy records from the eastern Mediterranean.

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