CAN INCREASE IN ARIDITY TRIGGER RAPID SURFACE EROSION OF DIAPIRS AND INTENSIFY THE BRINE FLOW? (EVAPORATE KARST IN ZAGROS MOUNTAINS, IRAN)

Zagros Mts. host numerous salt diapirs, which differs in uplift rate and relief. Some diapirs are exposed to arid conditions; some are situated in higher altitude in less arid climate. While some diapirs are formed by vast surfaces build predominately by halite, at others, the rock salt is covered by up to 30 m thick surficial deposits (residuum of the rock salt dissolution), which even enable planting of crops thanks to recharge concentration at sinkholes bottoms.

High variability of environments enabled us to study the relationships and feedbacks between climate, surficial deposits and vegetation cover.

Erosion rates of various kinds of surficial deposits as well as of rock salt exposures were measured at selected sites for a period up to 10 years by plastic pegs as benchmarks. Recorded data were standardized to a horizontal surface. Rain gages and no tension lysimeters were placed at and below soil zone to estimate the subsurface (chemical) erosion rate. Amount of dissolved salt was also measured at basin scale.

While the rock salt exposures are eroded at rate 30-120 mm/year the thick surficial deposits covered by vegetation shows negligible erosion. Based on chemical analyses and flow rate measurements the salt exposures produce huge amounts of dissolved and clastic load, thus affecting the land use in vast surrounding of the diapirs. Based on infiltration tests on different types of surficial deposits, most of the rainwater will infiltrate, while overland flow predominates on rock salt exposures.

On some diapir surfaces the TDS of the soil and surface water is very low (80 mg/L). As most of the water evaporates from these surfaces the amount of high TDS waters generated by percolation in underlying rock salt is very low. This enable to distinguish (based on aerial imaginary) the alluvial fans in diapir surroundings, which will be likely heavily polluted by brines from those potentially interesting for groundwater abstraction.

Radiocarbon and U-series dating of calcretes capping some diapir surfaces indicates that diapirs reached largest extent during the Last Glacial. Since than, the original thick surficial deposits are undergoing erosion on many diapirs. During less arid periods, vegetation cover protected the diapir surfaces. Vegetation degradation may be triggered also artificially by grazing sheep or salt mining.