Clay distribution over the landscape of Israel: From the hyper-arid to the Mediterranean climate regimes

Abstract

The current study evaluates clay distribution in modern surface sediments, mainly soils, to which desert dust is a major contributor. The mineralogical composition of the clay fraction of seventy seven samples was analyzed by X-ray diffraction. Twenty nine soil and dust samples were also analyzed for their bulk chemical composition. The samples are settled dust, soils developed on various sedimentary rocks, stream and lake sediments. They were collected along a climate gradient from hyper-arid to Mediterranean regimes in Israel. The purpose of the study is to decipher the main factors that control variable clay distribution along this gradient including annual precipitation, substrate type and topography. The common clay composition for most samples, of all sources, is illite–smectite (IS) > kaolinite > illite. Trace amounts of chlorite, palygorskite, goethite and quartz might be present. Pedogenic processes are recognized even under arid climate where loessial soils display kaolinite depletion and more illitic IS phases than dust. Two main processes shape clay composition under a Mediterranean climate regime, regardless of the parent material. A smectitization process occurs in leached, low-permeable clayey soils, and reaches optimum evolution with ~ 90% smectite, or smectitic IS. This evolutionary trend is also recognized in poorly-drained soils of catenary chains that are related to other soil types like terra rossa, hamra and pale rendzina. Kaolinitization and apparently illite pedogenic formation occur in well-drained red Mediterranean soils of the terra rossa and hamra types to the detriment of smectitic IS phases of parent materials. In the most evolved terra rossa and hamra soils kaolinite becomes the principal mineral and illite is significantly enriched. Pedogenic evolution of the clay fraction mineralogical composition affects bulk chemical composition of most soil types. Clay inheritance from the bedrock is rather limited in leached soils. It is widely recognized in pale rendzina soils by the presence of smectite, or smectite and palygorskite, derived from late Cretaceous or Eocene chalks, respectively. Streams' clay composition, like that of dust, is fairly uniform, as streams average various sources across their drainage basins. However, composition variations can be attributed to local contribution from certain sources.