M.F. Araújo

Present day sedimentary processes on the northern Iberian shelf

Abstract The Galicia-Minho Shelf features two large mud patches, the Douro and the Galicia Mud patches. These are recent sediment bodies that have accumulated under a combination of conditions including: (1) abundant supplies of sediment; (2) morphological barriers that act as sediment traps; and (3) hydrographic conditions that favour the accumulation of fine sediment in these sinks. This paper describes the mechanisms controlling the deposition of the fine-grained sediment depositions and the processes that result in resuspension processes on the Galicia-Minho Shelf. Fine-grained sediments are provided from discharges from the river basins on the southern sector of the shelf, mainly the Douro and Minho rivers. Sediments are exported from river estuaries onto the shelf during episodic flood events. In contrast, most of the sediments originating from the Galician hinterland fail to contribute significantly to sedimentation on the shelf, because they are retained in the Galician Rias, which function as sediment traps. Sediments deposited on the shelf are frequently remobilized, particularly during southwesterly storms that coincide with downwelling conditions. Once in suspension the fine-grained sediments are transported northwards by the poleward flowing bottom currents and are eventually deposited on the Douro and Galicia Mud patches after a series of resuspension events. The locations of the two mud patches are strongly influenced by the shelf morphology. Fines already deposited on the mud patches are occasionally reintroduced into the system by large storm events. Some material from the Douro Mud patch and adjacent areas is re-deposited in the Galicia Mud patch. It is probable that sediments re-suspended from the Galicia Mud patch are carried off the shelf when storm events coincide with downwelling conditions.

Geochemical tracers of northern Portuguese estuarine sediments on the shelf

The northern Portuguese coastal zone is drained by large Iberian rivers, along which there is intensive industrial, agricultural and urban activity. Offshore of the two main river basins of the Douro and Minho, two fine-grained sedimentary formations can be characterised. A geochemical study of this sediment cover has been performed focussing in particular on the fine deposits and the adjacent estuaries. The chemical composition of superficial sediments collected from the northern Portuguese shelf and from the river basins of the Minho and Douro rivers was determined for major, minor and trace elements; the rare earth elements (REE) composition was also determined in selected samples. Statistical analyses of these data for the shelf sediments allow us to identify elements with similar behaviours and the different sediment types along the shelf, and to evaluate of their origins. Downcore profiles of the chemical composition of the fine sediments reveal no evidence for changes induced by recent human activities. To assess the influence of the rivers on the fine sedimentary formations, the elemental distribution patterns of selected sediments have been compared with the estuarine sediments. Lithogenic elements composition normalised to Al and the shale normalised REE distributions showed there are similarities between the Douro River sediments and the sediment composition of the Douro and Galicia mud patches. However, the Galicia mud field showed no relationship in its chemical composition to sediments derived from the Minho River. Nor was any evidence for heavy metal contamination originating from the Douro estuary detected in anthropogenic element/Al ratios in the shelf sediments. This may be because of processes associated with resuspension, complexation and bioturbation that occur during sediment transport and deposition. Nevertheless, the Douro River seems to be the main continental source of fine sediments being deposited on the northern Portuguese shelf.

Comparison of EDXRF and PIXE in the analysis of ancient gold coins

Abstract The metal contents of several ancient uncleaned gold coins, with compositions ranging from about 35% to 99% Au, were determined and compared by using two techniques: EDXRF and PIXE. These techniques have proved to be a powerful tool in numismatic investigations, due to their ability of rapid, simultaneous and nondestructive multielement analysis. The results obtained with the two techniques for coins where gold is present in concentrations above 94% showed to be in good agreement. However, they appeared to diverge for coins with a lower standard of fineness. In this case, the Au contents determined by PIXE are higher than those obtained by EDXRF. This may be due to the well known surface enrichment effect and to the circumstance that the protons used as incident beam in PIXE analysis have a shorter penetration depth than the primary X-rays generated by the XRF tube.

Quality assurance of X-ray spectrometry for chemical analysis

Abstract Three different techniques, energy dispersive X-ray fluorescence, total reflection X-ray fluorescence and particle induced X-ray emission were used to initiate an evaluation program on quality assurance (QA) procedures applied to X-ray spectrometry for chemical analysis. The use of standard methodologies to assure the statistical control of measurement data is the main objective of this work. Certified Reference Materials were used and up to 15 certified elements were analyzed to carry out the QA procedures. For the internal quality control, z -scores were calculated and control charts were produced. The plotted elemental data illustrate statistically controlled methodologies for the majority of the determinations. Even the cases where the control charts exhibit values out of control limits, the z -scores are below 3 in absolute value, indicating satisfactory results. Concerning external quality control the statistical methods applied showed that the results obtained for the three techniques are comparable, although some significant differences occur, mainly due to sample preparation. Therefore, the techniques are traceable to certified reference materials and the data gathered so far, enable to initiate a database for QA procedures.

Heavy metal assessment in sediments of the Ave River Basin (Portugal) by energy-dispersive X-ray fluorescence spectrometry

Anthropogenic activities have contaminated stream sediments around the world with toxic metals. In Portugal, the Ave river basin, draining to the northwestern Portnguese shelf, is heavily contaminated. The Ave river course and its tributaries are located in one of the most populated regions of the country with intensive agricultural and industrial activities, which effluents have been discharged directly into the local stream. This paper reports results obtained in the analysis of superficial sediments collected at the Ave estuary and along the river course within a distance of about 50 km during February 1993. In order to account for seasonal variabilities occurring in the estuary, stations located in this region were also sampled in August 1993. A total of 16 sediment samples were analysed as pressed pellets by energy-dispersive X-ray fluorescence spectrometry for 15 elements. The trend of the elemental contamination was evaluated as a function of the location, taking into account external factors that may influence that distribution (tributaries, industries, etc.) and anthropogenic contributions were identified. Comparison of our data with published values considered as the background for non-polluted sediments (average shale) were carried out by calculating enrichment factors. The latter proved that sediment contamination by Zn and Pb exists all over the river basin and also by Cr and Cu in two regions of the area studied. A general depletion in Ni was measured. No significant seasonal variabilities were detected.

Characterisation of Late Bronze Age large size shield nails by EDXRF, micro-EDXRF and X-ray digital radiography

In the present study six exceptional large size metallic nails, a dagger and a sickle from the Late Bronze Age archaeological site of Figueiredo das Donas (Central Portugal) have been analysed by EDXRF, micro-EDXRF and X-ray digital radiography for the study of material composition and technology of fabrication. The combination of these analytical and examination techniques showed that all artefacts are made of bronze with As, Sb and Pb impurities, and that the nails were most likely manufactured using the casting-on technique. These results reinforce the use of binary bronze by Late Bronze Age in the region, and the incorporation of new fabrication technologies that resulted from ancient spheres of interaction.

Orientalizing Artifacts from Fraga dos Corvos Rock Shelter in North Portugal

An unusual set of metallic artifacts with orientalizing character, attributed to a late period of the Late Bronze Age (LBA), was found in a shelter in Northern Portugal. Conventional and micro-energy dispersive X-ray fluorescence (XRF) analyses have been performed to determine the metal composition. The microstructures of some artifacts have also been studied under optical and scanning electron microscope (SEM). The results show that most of the items are made of a copper–tin alloy with low lead content and show a recrystallized grained microstructure.

Elemental composition of Demospongiae from the eastern Atlantic coastal waters

AbstractThe elemental composition (Al, Si, K, Ca, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn, Br, Rb, Sr, Zr and Pb) of 30 coastal and two deep-sea Demospongiae specimens, belonging to 19 different species and ...

Bromine enrichment in marsh sediments as a marker of environmental changes driven by Grand Solar Minima and anthropogenic activity (Caminha, NW of Portugal)

A sediment core collected in Caminha tidal marsh, NW Portugal, was used to assess bromine (Br) signal over the last ca. 1,700 years. The Br temporal variability reflects its close relationship with soil/sediment organic matter (OM) and also alterations in Br biogeochemical recycling in marsh environment. The highest Br enrichment in sediments was found during the Maunder Solar Minimum, a major solar event characterized by lower irradiance (TSI) and temperature, increased cloudiness and albedo. The obtained results suggest that those climate-induced changes weakened the natural mechanisms that promote Br biochemical transformations, driven by both living plants metabolism and plant litter degradation, with the ensuing generation of volatile methyl bromide (CH3Br). It seems that the prevailing climate conditions during the Maunder favoured the retention of more Br in marsh ecosystem, ultimately decreasing the biogenic Br emissions to the atmosphere. During the 20th century, the Br pattern in sediments appears to mirror likewise anthropogenic sources. The significant correlation (p<0.05) between Br/OM ratios and Pb contents in sediments after 1934 suggests a common source. This is most probably related with the rise, massive consumption and prohibition of leaded gasoline, where ethylene dibromide was added as lead scavenger to antiknock mixtures. More regionally, the concerted use of flame retardants on forest fire management, covering the 1980s through mid-1990s in the north of Portugal and Galicia, could be responsible for the observed increase of sediment Br (relatively to Pb) pool of this tidal marsh. Although man-made brominated compounds are being phased-out since the inception of the 1992 Montreal Protocol, the Caminha tidal marsh sedimentary record showed that Br levels only started to decline after 2002.

Microstructure Interpretation of Copper and Bronze Archaeological Artefacts from Portugal

In the present paper, a selection of micrographs showing some typical microstructures and corrosion layers developed in copper and bronze alloys from different archaeological contexts will be discussed. Metallurgic interpretation of the microstructures observed, based in the proper binary equilibrium phase diagrams is presented. Micro-EDXRF and SEM-EDS analysis were carried out to assess the alloy composition and to contribute to the understanding of the corrosion processes occurred during the long periods of burial of the archaeological metals. Examples of surface decuprification, strong intergranular corrosion, cuprite under green corrosion layers and copper redeposition indicate particular corrosion processes. The presence of copper oxides, sulphides inclusions, lead globules and porosities is also documented.