Runoff-induced sediment yield over dune slopes in the Negev Desert. 2: Texture, carbonate and organic matter

Runoff-induced sediments were collected in the Hallamish dune field for fou r years (1990-1994). Runoff and consequently water-transported sediments we re generated on the dunes owing to the presence of a thin microbiotic crust . These sediments were analysed for their particle-size distribution and ca rbonate content. In addition, the organic matter content was calculated by measuring the chlorophyll content within the runoff. The results were compa red to the slope parent material, i.e. the crust and the underlying sand, a s well as to playa sediments, which are scattered within the Hallamish inte rdunal areas, and which were previously hypothesized to originate from runo ff-induced sediments. Higher amounts of fines (silt and clay) and carbonate characterize the foot slopes in comparison to the midslopes. Intermediate contents of fines (17 p er cent) and carbonate (8 per cent) characterized the sediments in comparis on to the fines (27 per cent) and carbonate (15 per cent) of the crust and to the fines (4 per cent) and carbonate (4 per cent) of the underlying sand . The runoff-induced fines and carbonate contents were significantly differ ent from those of the playas, suggesting that the playa flats do not origin ate from runoff-induced sediments. The sediments were enriched with organic matter. Organic matter which origi nates from the crust amounted to 0.3-0.4 per cent as compared to less than or equal to0.1 per cent in the bare sand. Nevertheless, the crust was found to be relatively resilient to water flow. Only 0.1-0.5 per cent of the cru st was annually eroded off the slope by water, with south-facing crusts sho wing higher resilience than north-facing crusts. The data may thus assist i n the evaluation of the crust's residence time. Copyright (C) 2001 John Wil ey & Sons, Ltd.