Elemental Distribution in a Sedimentary Deposit on the Shelf Off the Tagus Estuary (Portugal)

The Tagus estuary, located at the western Iberian coast in front of Lisbon is the largest one in Portugal, where it drains highly populated and industrialised regions. The amount of sediment transported by this river is so great that the submarine delta is one of the largest in the Iberian margin, very well defined and reaching the 70m isobath. The geochemical pattern of the muddy deposit off the Tagus river has been determined by elemental analyses of bulk surficial sediments, collected at the Portuguese margin adjacent to the Tagus estuary. Measurements were carried out by energy-dispersive X-ray fluorescence spectrometry (EDXRF). Sediments were usually composed of fine particles (silts and clays) and were high in organic matter (measured as loss on ignition). Elemental concentrations for Zn and Pb clearly indicate an estuarine contamination probably associated with discharges from urban centers or due to the influence of the industries located downstream. Apparently the depletion on the Cu contents is caused by its release (soluble complexes) into the marine environment. Variations observed in the elemental distribution of the sediments off the Tagus with distance from the estuary seem to be dependent on the grain size distribution related to the dynamics of the sediment transport.