Composition of clays along the continental shelf off Israel: contribution of the Nile versus local sources

The eastern Mediterranean sedimentary regime is dominated by sediment transport from the Nile River and its delta, but recent studies have also suggested a certain contribution of clay fraction from streams to the continental shelf and slope off Israel. In the present study we focused on clay fraction deposition along the inner (similar to 40-1000 m water depth) shelf of Israel in order to substantiate and quantify its different sources. For this purpose detailed XRD mineralogical determinations as well as chemical compositions of the clay (<2 mu m) fraction of marine and continental sediments were used. The clay mineral composition of Israeli stream sediments is variable, with no distinct spatial or temporal trend but within a limited assemblage. This clay assemblage is made up of illite-smectite (IS) phases (70-90%), kaolinite (5-25%) and illite (<10%) and is similar to that of the Nile. However, it is clearly distinguished by the type of the IS phases, which are derived mainly from Mesozoic outcrops and recent sediments/soils and are less expandable than those of the Nile, which are derived mainly from weathered basalt. The Nile-derived component is depleted in the marine clay fraction along the Israel coast, evidenced by a decrease of IS expandability and the Fe2O3/Al2O3 ratio, and an increase in the K2O/Al2O3, MgO/Al2O3 and CaO/ Al2O3 ratios. The depletion is well pronounced north of Tel Aviv, where the largest coastal plain stream flows to the sea. A northward increase of carbonates in the clay fraction, and particularly of the bioskeletal component is indicated by the ratios of MgO and Sr to CaO. The contribution of the local streams to the shallow shelf marine clay fraction is estimated to be about 50%. A minor contribution of desert dust is identified by the relative increase in kaolinite in the marine sediments south of Tel Aviv. (C) 2000 Elsevier Science B.V. All rights reserved.