STAGES AND RATE OF THE GRAVEL SHATTERING PROCESS BY SALTS IN DESERT REG SOILS

Gravel shattering by sets is a common Process in gravelly young to mature desert Reg soils. The shattered gravel is found on the surface and in the B and C horizons down to a maximum depth of 50 cm. Field studies on the process of salt weathering in deserts are scarce. Therefore, the objectives of this study are to present the stages of development of the gravel shattering process by salts in desert Reg soils and to discuss factors influencing the rate of development under natural conditions prevailing in the Negev Desert. The studied Reg soils were developed on two alluvial fan chronosequences in the Negev desert: the fan of Nahal Ze'elim and the fan of Nahal Shehoret. The development of the shattering process can be divided into five stages (A-E) based on chemical, physical and micromorphological features of the Reg soils. Stages A-D (very young yo young Reg soils) represent the early Holocene to the present, some 14,000 years; salinity values are between 0.27 - 16 mmho/cm and the percentage of the shattered gravel is between 20% and 70%. The shattered gravel, however, still maintains its original shape. Stage E (mature Reg soil) represents the middle Pleistocene to early Holocene, or 500,000 years. Salinity values at this stage reach 62 mmho/cm. Although part of the gravel does not keep its original shape and starts to disperse in the profile, the percentage of shattered gravel is only 75-80%. It was found that the process of salt shattering in Reg soils does not depend on concentrations or mineralogy of the salts but on the micro-environmental conditions of the soil profile. The process requires extreme and rapid variations of temperatures and moisture conditions, such as found close to the soil surface, to cause salt crystallization in a confined system. With time the sealing of the soil surface, through the development of desert pavement and a gravel-free B horizon, results in a change in the soil micro-environment. This change impedes the process irrespective of the amounts, concentrations and types of salt minerals which exist in the profile. The fact that in extreme arid regions the gravel shattering process develops primarily close to the Reg soil surface, suggests that it can be used to identify buried Reg soils and to reconstruct climatic and micro-environmental conditions of paleosols.