DISTANCE-IMPACTED GRAIN SIZE OF LOESS AND DUST RESULT IN THE FORMATION OF DIVERSE SOIL TYPES AROUND THE MEDITERRANEAN

Soils in the Mediterranean region are primarily controlled by incoming dust. Two main dust sources have been identified in this environment: (a) dune fields that contribute proximal, fining downwind, coarse to medium silt, and (b) distal sources, bringing only fine silt and clays from thousands of kilometers away in Sahara and Arabia. We hypothesize that this distance-dependent dust size have long term impacts on soil formation and soil diversity. We studied the silicate components in soils at similar geological position on top of carbonate mountain terrain in Malta, Crete and across south to north transect in Israel, i.e. across isohyets of 100 mm to 800 mm. Soils developed on similar landforms, but are impacted only by distal fine dust without local coarse silt sources as in the Upper Galilee, Crete, and Malta are thin and shallow lithosols. These soils are less productive. In contrast, proximity to a distinct source of coarse silt, as dune fields, results in thicker accretion of soil profiles suitable for intensive agriculture. Contrary to expectations, thick soils and well developed profiles are located near the desert at the semi-arid climatic region, whereas shallow, less developed soils are formed in more humid areas of the Mediterranean as the Upper Galilee or Crete. In Israel coarse fraction (20-60 µm) content decrease from ~40% to ~20% from southern (margin of desert) to northern areas. Clay content presents the opposite trend of ~25% to 50% at the south and north, respectively. The coarse fraction controls dust mass accumulation rates, and in turn, determines the thickness and degree of soil development around the Mediterranean. Precipitation amount and rock weathering rates are not the main factors in the process of soil formation in Mediterranean carbonate terrains while accretion rates of incoming dust are. In areas of >400 mm yr^-1, soils are thin, less developed in comparison to soils at the semiarid areas with less precipitation. Moreover, the mountainous area in the eastern Mediterranean and in isolated islands, without massive coarse silt (loess) influx, are practically bare of soils. This is not a matter of soil erosion or limited weathering, but of formation of cumulic soils controlled by the spatio-temporal distribution of silicate grain size and their influx rate.