Late Quaternary Environmental Conditions during Aeolian Activity Episodes in Southwestern Iran

Recent reactivation of aeolian activity in southern Iran has led to an investigation to determine its root cause. Climate change, as it has affected precipitation amounts and seasonal distributions, as well as changes in prevailing wind directions were investigated. Changes in these and the impact that these changes had upon vegetation, and surficial landscape process were also explored. In addition, the role of human activity was examined. Because such episodes have occurred in the past, the role of climate, and other impact factors in the past during similar episodes of dune activity were studied to compare modern aeolian activity with that which has occurred centuries and millennia ago. The modern context for this study is based upon the fact that within the last ۲۰ years dune fields that had been relatively stable, and were being exploited for agricultural purposes, had suddenly become unstable and subject to extensive reactivation. Vegetation that had stabilized many dunes died, and areas that had been planted and turned into agricultural areas were suddenly being inundated by sand. Study of the environmental factors suggests that a combination of significantly lower annual rainfall, combined with a shift to summer seasonality, when evaporation rates are higher, has reduced the effective precipitation in vast areas of southern Iran. The change in the nature of rainfall to events of much greater intensity, has resulted in increased surface runoff of rainfall, and reduced infiltration, greatly reducing ground water resupply. Illegal deep ground water pumping has lowered the water table beyond the reach of dune stabilizing plants, resulting in the death of many of these plants. A change in the direction of the prevailing winds during the last ۲۰ years has resulted in the destabilization of dunes as they seek to correct their structure in response to new strike and dip orientations. This has led to a change in their direction of movement and contributed to their re-expansion. Examination of dated stratigraphic records from playas in southern Iran and as far afield as the Sudan have provided evidence of such events throughout the Holocene. These records suggest that changes in the climate in southern Iran, can be linked to a northward movement of the Intertropical Convergence Zone, and a northwestward shift of the Indian Monsoon. These affected not only mountain slope erosion resulting in the production of vast quantities of sandy sediment sources for aeolian movement, but the streams generated by the intense rainfall events helped transport and disperse these sediments over broad areas to accumulate sources for aeolian transport. Records of sediment and pollen from many of these playasindicate that there was an increase in rainfall that resulted in increased stream flow, that not only formed shallow lakes in valley bottoms, and grasslands around them but accumulated valley floor sandy and silty alluvium for aeolian transport. Periodic episodes of drought during these Holocene wet periods are evidenced by the mobilization of sand and the formation of sand sheets and dunes. These data suggest that the current trend of aeolian dune expansion will be the norm for the foreseeable future under current climate predictions.

کلیدواژه ها:

Aeolian Sediments ، Dune Expansion ، Prevailing Winds ، Ground Water ، Summer Rainfall ، ITCZ ، Indian Monsoon ، Hipsithermal