MIDDLE TO LATE PLEISTOCENE DRASTIC CHANGE IN EOLIAN SILT GRAINS ADDITIONS INTO MEDITERRANEAN SOILS AT THE LEVANT'S DESERT FRINGE
The Negev desert loess (100–300 mm yr-1) is an ideal study site for identifying loess impact downwind. It was accreted during ~95-11 ka and characterized by a bimodal (3-8 µm, 50-60 µm) particle size distribution (PSD) with a clear up section coarsening. The main source of the coarse silt is the adjacent sand dunes. We hypothesize that soils located downwind to the loess in wetter Mediterranean regions accumulated finer silt grains concurrently to the Negev loess. To examine these hypotheses we studied 1-4 m thick late Pleistocene pedo-sedimentary sequences in Jerusalem, central Israel (550 mm yr-1) only 50 km downwind to the loess edge. We used PSD and mineralogy analyses with optically stimulated luminescence ages and Palaeolithic artifacts to study temporal changes in the sediments properties.
The sequences are divided into two regionally recognized units separated by a clear unconformity. The lower unit (>150 ka) lacks coarse silt and is composed of unimodal clay (3-5 µm) with chert clasts and lower-middle Palaeolithic artifacts. The upper unit (27-5 ka) is clay-loam with a bimodal PSD (3-5 µm, 30-45 µm); the quartz content, the amplitude and the mode of the coarse fraction increase up section.
We suggest that the coarse silt grains are a recent addition to the southern Levant Mediterranean region. In the middle Pleistocene the deposited dust was of mainly clay transported from afar. The coarse silts were deposited in this mountainous area downwind and coeval with of episodes of fast accretion of the loess, and generated by coeval eolian abrasion of sand grains in the upwind dunes. Similar to the Negev, the addition of coarse silts had resulted in the burial of drainage network, producing a net decrease in runoff and soil erosion rates. We stress here the importance of desert loess in determining soil composition and surficial hydrology in wetter areas located downwind.