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 trans port from the Nile River and its delta, but recent studies have also sugges ted a certain contribution of clay fraction from streams to the continental shelf and slope off Israel. In the present study we focused on clay fracti on deposition along the inner (similar to 40-1000 m water depth) shelf of I srael in order to substantiate and quantify its different sources. For this purpose detailed XRD mineralogical determinations as well as chemical comp ositions 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%), kaolin ite (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 expa ndable than those of the Nile, which are derived mainly from weathered basa lt. The Nile-derived component is depleted in the marine clay fraction alon g the Israel coast, evidenced by a decrease of IS expandability and the Fe2 O3/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 large st coastal plain stream flows to the sea. A northward increase of carbonate s in the clay fraction, and particularly of the bioskeletal component is in dicated by the ratios of MgO and Sr to CaO. The contribution of the local s treams to the shallow shelf marine clay fraction is estimated to be about 5 0%. 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 S cience B.V. All rights reserved.