The Umm El Tlel sequence: A high resolution pedo-sedimentary record of climate-triggered environmental changes during the late Quaternary (OIS 5 to 1) in the Syrian desert
The pivotal role in human history of the drastic alteration of natural resources incites to firmly establish the synchrony between cultural changes and past environmental shifts. Here we report on a detailed reconstruction of environmental changes over the last 100 000 years in the Syrian desert using the benchmark prehistoric site of Umm El Tlel. This ten meter thick sequence forms the terminal stage of an endoreic filling originated from discontinuous aeolian, lacustrine, colluvial and alluvial accumulation concurrent to soil development. The natural deposits are interlayered with 70 occupation levels ranging from the Early Neolithic to the Middle Palaeolithic. C14 radiometric dating and TL ages on burnt flints provide a coherent series of time markers from ca. 10 kyr BP to 70 kyr BP. The micromorphological study of the microstratified sequence allows to establish a qualitative dynamic model for interpreting sedimentary processes and pedogenic transformations in terms of environmental conditions. Their palaeoclimatic significance is calibrated using palynological data. The end of OIS5 corresponds to the lower layers (IX & VIII) with development of an arboreal vegetation. The water-layered organo-mineral units with corroded calcareous clasts, gypsic-ghosts, carbonate-depleted brown soil fragments with fine aerosols, and sandy lenses indicate the stable conditions under regularly distributed local rainfall, low-evapotranspiration, weak storminess and occasional flooding. The abrupt transition to OIS4 (unit VII) corresponds to high energy erosion of the endoreic basin due to torrential rainy episodes under inceasing aridity. OIS 4 (VI3) is marked by lacustrine biochemical sedimentation with distinctive torrential phases synchronous to human occupation. The permanent availability of water resulting from the aquifer recharge during the previous wet phase has offered favourable conditions for humans. The following OIS 3 and 2 (VI2 to III) show recurrent interruption of the overall aridity synchronous to persisting aeolian erosion by torrential episodes under a stronger Mediterranean influence, and rainfall increase as marked by the alternation of palustrine layers and charred-rich organo-mineral deposits. The synchroneity of recurrent wild-fires and fin-dust fall of long distance origin with the humid phases seem to chronologically match distinctive climate shifts such as the Heinrich events. Violent aeolian deflation culminates during the last glacial maximum (II) with the extensive deposition of gypsic dunes formed from aeolian reworking of the basin. The transition to the first Holocene climatic optimum (I) shows humid pulses, synchronous to occupation, expressed by the substitution of Ca-carbonates to Ca-anhydrite, and accumulation of carbonate-depleted organo-mineral lenses.