ALTERNATIVE REGIMES OF SURFACE AND CLIMATE CONDITIONS IN SANDY ARID REGIONS - POSSIBLE RELEVANCE TO MESOPOTAMIAN DROUGHT 2200-1900 BC

Ten feedback factors acting in sandy arid regions tend to create two a lternative regimes of surface and climate conditions: an equilibrium w ith Vegetated, Crusted soil, and Convective Precipitation, commonly in the 200-400 mm year(-1) range (VC-CP), and an equilibrium with a Salt ating-Sands surface and Extreme Aridity (SS-EA). The recently describe d 300-year long drought in Mesopotamia following a deposition of volca nic dust in 2200 B.C. over the region may be explained as a transition from a VC-CP to an SS-EA regime. The transition is attributed to crea tion of saltation-favoring conditions by the volcanic layer, brought a bout by the choking-off of the plants. After the destruction of the so il-crusts, the high albedo saltating-sands surface adversely affected probabilities for convective precipitation. The change again to the VC -CP conditions around 1900 B.C. may have been triggered by unusual pat terns of cyclonic activity that brought far-above-average precipitatio n to the region. (C) 1995 Academic Press Limited