No abstract

No abstract

This paper aims to contribute to the fluvial dynamic knowledge of the Lower Tagus during the late Holocene. Two cores were done in selected geomorphological positions at the alluvial plain. Each ten centimeters of sediments were submitted to sedimentological and geochemical analyses in order to identify human contamination by heavy metals, sedimentation rates and environmental changes in the main channel position. The main conclusions are: (1) The witness of the lateral migration of the fluvial channel; (2) The very irregular sedimentation rate, from 3mm per year to 0.1mm per year; (3) The diversified contamination of fluvial sediments and fluvial channels by heavy metals, namely Cu due to cooper sulphate vineyards and Pb due to petrol and Cr from the tanning industries.R E S U M E N: Este trabajo pretende contribuir al conocimiento de la dinámica fluvial del curso inferior del río Tajo durante el Holoceno final. Se han realizado dos sondeos en lugares seleccionados de la llanura aluvial. Muestras del sedimento obtenidas cada diez centímetros se han analizado con el fin de identificar la contaminación humana con metales pesados, las tasas de sedimentación y posibles cambios en la posición del cauce principal. Las conclusiones más importantes son: (1) La prueba de la existencia de migración lateral del cauce fluvial; (2) La gran irregularidad de la tasa de sedimentación, que varía entre 0.1 y 3 mm por año; (3) La variada contaminación de los sedimentos fluviales y de los cauces por metales pesados, especialmente Cu debido a la aplicación de sulfato de cobre en viñedos, Pb debido al petróleo, y Cr por las industrias de curt i d o s .

No abstract

A study of the Tagus River floodplain (Portugal) has been carried out using a variety of methods including sedimentological, geochemical and geochronological analyses, as well as geomorphological and hydrological studies, performed in order to characterize the flood sediments and the dynamics of the river during the Holocene. Until the 19th century, the Tagus was an anastomosed river, with multiple channels separated by large areas of floodplain; today, it is a single channel river with alternate bars, mainly as a result of anthropogenic modification. In order to study its behaviour during the Holocene, four cores ranging in length from 3.70 to 8.04 m were obtained from the floodplain and 232 samples were analysed. Detailed textural analysis was necessary owing to the lack of preservation of sedimentary structures in the cores. The sediments of the present-day geomorphological elements of the floodplain (bars, natural levees, crevasse-splay deposits, flood basin and abandoned channels) were also studied in order to compare their textural characteristics with those of the cored samples. Both the present analogues and core sediments were well discriminated using mean diameter versus standard deviation and average mean diameter versus average mud percentage graphical correlations. The textural parameters defined (sand/mud ratio, mean, standard deviation, skewness) and particularly the interparameter correlations, together with 12 14 C numerical ages of organic matter obtained, allowed the evaluation of: (i) sedimentological changes in the floodplain (channel migration, avulsion and crevasse-splay development); and (ii) the chronological evolution of the different energetic environments of the floodplain for the past 4 kyr. These approaches permitted the determination of sedimentation rates for the different alluvial plain environments. The highest sedimentation rates occurred in the flood basin and channelfill domains, with values ranging from 2.2 mm yr −1 to 4.7 mm yr −1 and the lowest in the channel (0.3 mm yr −1 ). Values from 0.8 to 1.6 mm yr −1 were recorded in sedimentary environments proximal to the channel, where several crevasse-splay episodes have been recognized. In the period common to the four cores, i.e. the past 4000 yr, the sedimentation rates decreased towards the present. In spite of increasing human intervention in the hydrographic basin during this time, the increasing aridity of the climate is considered to have outweighed the sediment availability, which resulted in a decreasing sedimentation rate.

This work deals with the geochemistry of the sediments from the Santarém-Entre Valas (SEV) borehole, in the middle Tagus alluvial plain in Portugal. Methods for background determination were used, notably the concentration-length and cumulative frequency methods. Both methods were in excellent agreement for the elements of interest, namely Au, As, Fe, U, Cu, Pb, Zn, Co, and Cr. With these results, five segments in the SEV borehole were identified to have higher than background values for at least two or more of these elements. The most important segment spans at least 1,500 years during the Early Holocene Warm Period, with almost all elements with anomalous values. This has been interpreted as the result of enhanced weathering of the host rocks upstream as a consequence of deglaciation since the Last Glacial Maximum and general wetter conditions known to have occurred in this region. No other segment of the core has shown a similar variety of elements with higher than background values, but the upper segments correlate with the particular mineralogy on the sediments due to the development of an estuarine environment at circa 3,000-4,000 cal. years BP. The presence of charcoal and charred organic matter during the beginning of the Bronze Age characterises the segment with the first high values of Cu, but apart from deforestation and land use that affected this region at the time, this anomaly is likely due to the chemical affinity of Cu to organic matter and not to anthropogenic activities. Finally, the upper meter of the core sediments, spanning 812 years, have a clear anthropogenic signal for Cu (also seen in other two boreholes) and systematic high values for Au (and Pb). Although Cu is surely an indication of secular agricultural practices (vineyard sulphatation), Au (and Pb) might be the natural accumulation from weathering of ancient mine wastes scattered in the Hercynian Massif upstream.