Dust deposition over the Dead Sea

Dust deposition over the Dead Sea has been studied for 3 years (1997–1999) using two collectors installed on a buoy anchored 3·5 km off-shore, south-east of Ein-Gedi. Deposition rates ranged between lows during winter (6·7–15·2 g m−2 year−1) and summer (11·4–24·7 g m−2 year−1) and highs in spring (35·7–120·7 g m−2 year−1) and autumn (39·1–158·3 g m−2 year−1). Most of the deposition was in the form of pulses, generated by dust storms. A gradual increase in yearly deposition was observed from 255 kg ha−1 in 1997, to 605 kg ha−1 in 1999. The particle-size distribution is distinctly unimodal, with the mode close to 10 μm and is not season related. This distribution suggests that the dust had been transported from medium to long range and differs markedly from that of land based collectors, where the distribution curve is bimodal, indicating the presence of material of local origin. The deposit consists of soluble salts, carbonates, quartz, and aluminosilicates, principally feldspars and clay minerals. Soluble cations and anions, including bromide, are all related to Dead Sea water, except for nitrates which are of anthropogenic origin. Some of the sulphate may be air-borne gypsum. Calcite contents vary between 5·2% and 33·1%, dolomite in the range of 1·5–14·8%. The calcite/dolomite ratio rises with the rise in the deposition rate. Apatite is present in the range of 1–5% and apparently is season related. Small amounts of phosphate appear to be related to the frequency of winds blowing from a phosphate-mining area about 45 km away. Clay minerals include smectite, kaolinite, illite and minor amounts of palygorskite and differ distinctly from that of North-African Harmattan dust. The overall mineral composition of dust over the Dead Sea shows no relation to west and north Saharan dusts and suggests an origin principally in the Negev, Egyptian and Lybian deserts.