Progress in Oceanography 52 (2002) 277–297www.elsevier.com/locate/pocean

Geochemical tracers of northern Portuguese estuarinesediments on the shelf

M.F. Araujo a,∗, J.-M. Jouanneaub, P. Valerio a, T. Barbosaa, A. Gouveiaa,O. Weberb, A. Oliveira c,d, A. Rodriguesc, J.M.A. Diasd

a Departamento de Quımica, Instituto Tecnologico e Nuclear, E.N. 10, 2686–953 Sacavem, Portugalb DGO-UMR 5805 CNRS, Universite Bordeaux I, Av. des Facultes, 33405 Talence Cedex, France

c Instituto Hidrografico, Rua das Trinas 49, 1249–093 Lisbon, Portugald UCTRA, Campus de Gambelas, Univesidade do Algarve, 8000 Faro, Portugal

Abstract

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-grainedsedimentary formations can be characterised. A geochemical study of this sediment cover has been performed focussingin particular on the fine deposits and the adjacent estuaries. The chemical composition of superficial sediments collectedfrom 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. Statisticalanalyses of these data for the shelf sediments allow us to identify elements with similar behaviours and the differentsediment types along the shelf, and to evaluate of their origins. Downcore profiles of the chemical composition of thefine sediments reveal no evidence for changes induced by recent human activities. To assess the influence of the riverson the fine sedimentary formations, the elemental distribution patterns of selected sediments have been compared withthe estuarine sediments. Lithogenic elements composition normalised to Al and the shale normalised REE distributionsshowed there are similarities between the Douro River sediments and the sediment composition of the Douro andGalicia mud patches. However, the Galicia mud field showed no relationship in its chemical composition to sedimentsderived from the Minho River. Nor was any evidence for heavy metal contamination originating from the Douro estuarydetected in anthropogenic element/Al ratios in the shelf sediments. This may be because of processes associated withresuspension, complexation and bioturbation that occur during sediment transport and deposition. Nevertheless, theDouro River seems to be the main continental source of fine sediments being deposited on the northern Portugueseshelf. 2002 Elsevier Science Ltd. All rights reserved.

Keywords: continental shelf; rivers; sediments; elemental composition; multivariate statistical analysis; Iberian Shelf

∗ Corresponding author. Tel.:+351-21-994-6204.E-mail address: faraujo@itn1.itn.pt (M.F. Arau´jo).

0079-6611/02/$ - see front matter 2002 Elsevier Science Ltd. All rights reserved.PII: S0079 -6611(02 )00011-3

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Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 278

2. Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2812.1. Sampling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2812.2. Sample preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2812.3. Analytical methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2812.4. Statistical treatment of the data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 283

3. Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2833.1. Elemental composition of surface sediments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2833.2. Multivariate statistical spatial patterns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 283

3.2.1. Factor analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2833.2.1.1. Factor 1: Fine sediment fraction: Al, K, Rb and transition elements . . . . . . . . . . . . . . . . . . 2853.2.1.2. Factor 2: Coarse sediment fraction: Si, Ca and Sr . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2863.2.1.3. Factor 3: Ti and Zr (heavy minerals) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 286

3.2.2. Douro and Galicia deposits—Cluster analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2863.2.3. Core profiles—vertical distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 288

3.3. Origin of shelf sediments—Dispersal pathways of estuarine sediments . . . . . . . . . . . . . . . . . . . . 2893.3.1. Influence of the northern estuarine sediment composition in shelf sediments . . . . . . . . . . . . . . . 2903.3.2. Major (lithogenic) elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2913.3.3. Anthropogenic elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2913.3.4. Element/Al ratios . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2913.3.5. REE distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 292

4. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 294

1. Introduction

The North-western Portuguese continental shelf is a high energy environment, experiencing seasonalcoastal upwelling during summer and episodic downwelling events induced by atmospheric depressions,during winter. It is characterised by the outflows of large rivers that drain catchments that are highlypopulated and industrialised and have different lithologies and levels of anthropogenic influences.

The highly energetic hydrodynamic conditions lead to remobilisation of sediments, and so are probablyresponsible for the shelf being predominantly covered with sandy deposits (Dias & Nittrouer, 1984). Evenso three major deposits of fine-grained sediments occur along the western Iberian shelf (Fig. 1). Two ofthese deposits are located on the northwestern shelf as elongated features that are well defined and coverthe mid-shelf region below the 60 m isobath. Both of them extend to the north of the two main estuariesof the Douro and Minho rivers that drain the adjacent coastal zone. The third deposit is located furthersouth, offshore of the Tagus river basin, and covers the continental shelf from the river delta front out tothe shelfbreak. This is an extensive mud deposit and is directly linked to the estuarine inputs (Paiva,Jouanneau, Araujo, Weber, Rodrigues, & Dias, 1997; Jouanneau, Garcia, Oliveira, Rodrigues, Dias, &Weber, 1998a).

The main features of the shelf that define the circulation, transport and deposition of fine sediments arediscussed by Dias, Jouanneau, Gonzalez, Araujo, Drago, Garcia et al. (this volume) and Jouanneau et al.(this volume). The geomorphologic settings of each of these three depositional areas are distinct (see Fig.1). In the Minho/Galicia region, the Galician deposit extends northward, over a rather large, smooth area

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Fig. 1. Fine sedimentary deposits at the Portuguese shelf.

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of the Galician shelf. The Douro deposit is confined to the mid-shelf region in front of the Douro estuaryand is bounded on its western side by Cretaceous and Palaeocene outcrops. The deposit on the Portugueseshelf offshore of Tagus estuary is quite narrow and is confined by several canyons that intersect theouter shelf.

Our study was focused on the northwestern Portuguese coastal zone (between latitudes 41°05�N and41°52�N), which is drained by five major rivers: the Minho, Lima, Cavado, Ave and Douro. The dischargesfrom the Minho and Douro rivers are by far the most important; their basins are larger and their sedimentloads are greater. Dams built on these rivers during recent decades have reduced their discharges, even sothey remain the main sources for the continentally derived sediments that reach the coastal region. TheMinho River, at the northern Portuguese border, has a length of 300 km and drains a catchment area of17081 km2 (Loureiro & Macedo, 1986), which is both agricultural and industrialised. The Douro River is927 km long and has a catchment area of 97682 km2 (Loureiro, Macedo, Almeida, & Martins, 1986). Ittoo flows through an area that is both highly industrialised and intensively farmed. The lower reaches ofthe Douro River pass through a densely populated region, and Oporto, the second-largest city in Portugalis situated on it estuary.

During the last decade, studies aimed at understanding of the sedimentary dynamics along the Portugueseshelf have used various approaches including sedimentology, mineralogy and the geochemistry of bothsediments and suspended particulate matter (Araujo, Jouanneau, & Dias, 1994; Araujo & Gouveia, 1998;Cascalho & Carvalho, 1997; Dias, 1987; Drago, Oliveira, Magalhaes, Cascalho, Jouanneau, & Vitorino,1998; Magalhaes, Araujo, Cascalho, Drago, Oliveira, Jouanneau et al., 1995; Oliveira, Rodrigues, Jouan-neau, Weber, Dias, & Vitorino, 1998). Recent studies on the accumulation rates of recent sediments, usingthe 210Pb method and radiocarbon dating of deep cores collected from the Douro and Galicia deposits,have demonstrated that the mud patches are of modern origin and are still receiving terrigenous inputs(Drago, 1995; Drago et al., 1998, Jouanneau et al., this volume). However, until now there has been alack of information on the geochemistry of the shelf sediments, and in particular on the geochemicalprocesses that control the behaviour and transport of the riverine particles from estuaries out over the shelf.In the co-ordinated framework of research projects (including the OMEX I/II) we have studied the spatialand temporal elemental variations of the Northwestern Iberian shelf sediment cover, and of the main adjac-ent river basins.

We have examined the influence of these large Iberian rivers on the coastal marine environment, parti-cularly their role in contributing to the two fine-grained sedimentary deposits, off N.W. Portugal. Thegeochemical distribution patterns of some major, minor and trace elements constituents of sediments col-lected at the northern Portuguese shelf have been described by multivariate analysis. In addition the elemen-tal profiles of dated sediment cores are described and related to the local processes occurring over the shelfand in the sediment. The main phase-carrier of metals in coastal environments is the fine-grained fractionof sediments, so the metallic contents of fine sediments deposited on the shelf have been compared withthose exported from the main adjacent estuaries. Finally, the distributions of rare earth elements (REE)are discussed, since they are a particularly interesting group of elements that help to establish the pathwaysfollowed by marine sediments from their sources to their sites of deposition. According to Piper (1974)the major source of REE into the ocean is expected to be riverine inputs derived by erosion and weatheringfrom exposed continental formations. They may be transferred in bulk from the inland source to oceanicsediments.

Using these tools, we seek to understand the origins and the processes whereby the two large sedimentarydeposits have been formed in the middle shelf. Also we endeavour to assess the biogeochemical processesand pathways whereby river-borne materials become deposited on the northwestern Portuguese shelf.

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2. Methodology

2.1. Sampling

Seventy shelf sediment samples were collected from the uppermost 10 cm layer, along the inner, midand outer Portuguese shelf during the cruises GEOMAR92, PLAMIBEL III and CORVET 96 (Fig. 2).Box-cores samples penetrating to depths of 6 to 14 cm were also collected at some stations. Superficialshelf sediments were also collected using a Smith–McIntyre grab and a multicorer, which collects undis-turbed sediments. Riverine sediments were sampled along the river courses during two sampling campaigns,but mainly from the lower reaches and the estuaries. To account for seasonal variability, samples of super-ficial sediments were collected in February 1993 (SEDIMINHO I/93) and August 1993 (SEDIMINHOII/93). Sub-samples were taken from the central part of the grab to avoid metal contamination from thesampler. The sampling locations were selected to provide a set of samples representative of the variousdistinct regions of the drainage basins (e.g., the influence of the tributaries, urban centres, industrialactivities). Samples of the uppermost 10 cm of sediment were collected from the middle of the river witha sediment grab.

2.2. Sample preparation

Immediately after collection the sediments were deep-frozen. Cores were refrigerated and later split into1 cm horizons using a plastic spatula (to avoid metal contamination). Prior to analysis, samples were freeze-dried and then ground to give a grain size fraction �63 µm. This homogenised, ground sample was driedat 110°C for 24 h, before two sub-samples were taken for analysis. About 2 g were mixed with an organicbinder and pressed into pellets to carry out energy-dispersive X-ray fluorescence spectrometry (EDXRF).Fractions of between 0.2 and 0.3 g were weighed into clean polyethylene vials and irradiated for instrumen-tal neutron activation analysis (INAA). Details of the sampling preparation techniques have been describedby Araujo, Valerio and Jouanneau (1998) and Gouveia, Araujo and Dias (1993).

2.3. Analytical methods

The superficial sediment samples and subsamples from the cores were analysed by EDXRF (Energy-Dispersive X-Ray Fluorescence Spectrometry). INAA (Instrumental Neutron Activation Analysis) wasundertaken on selected estuarine and shelf sediments.

The EDXRF measurements were made using a Kevex Delta XRF Analytical System controlled by aDEC LSI 11/73 computer system, which provides a programmed control of the X-ray generator, detectionsystem and multi-channel analyser. The spectrometer included a high power rhodium anode X-ray tube,with secondary targets (Gd, Zr, Ge, Fe, Ti) and filters and a rotatable sample tray with 16 positions. TheX-rays emitted by the sample were collimated at 90° and 114° with the secondary target and the directexcitation geometries respectively, and measured with a Si(Li) detector with a resolution of 165 eV andan active area of 30 mm2. Sediment samples were irradiated using two monochromatic X-ray beams gener-ated by a Ge (15 kV, 2 mA) and an Ag (35 kV, 1.2 mA) secondary targets and the direct beam generatedat the Rh anode (4 kV, 0.18 mA) during 500 s, under low vacuum conditions. Corrections for matrixeffects were made by a calibration method (O’Reilly & King, 1986) that calculates the relation of theincoherent and coherent scattered radiation with the atomic number of the elements constituents of thesample. By using this analytical technique, we were able to determine the following elements: Al, Si, K,Ca, Ti, Cr, Mn, Fe, Ni, Cu, Zn, Rb, Sr, Zr and Pb. The accuracy and precision of the overall procedurewere estimated by analysing the standard reference materials MAG1 (US Geological Survey) and SRM2704(National Institute of Standards & Technology, USA). Accuracy was usually �5% and precision �10%.

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Fig. 2. Map showing sampling locations on the northern Portuguese shelf.

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Detection limits for the transition elements were typically ~10 mg/kg. In the case of the low Z elements(Z�16) EDXRF performed less favourably and detection limits were around 100 mg/kg.

INAA analyses were carried out to determine the REE contents (La, Ce, Nd, Sm, Eu, Tb, Yb, and Lu)by irradiation in the core grid of the Portuguese Research Reactor (Sacavem) at a neutron flux of 4.4·1012

n·cm�2 s�1 for 7 h; ϕepi/ϕth=1.4; ϕth/ϕfast=12.2. A γ-ray spectrometer consisting of a 150 cm3 coaxial Gedetector, and a low-energy photon detector (LEPD), connected through Canberra 2020 amplifiers to anAcuspec B (Canberra) multichannel analyser was used. This system has a FWHM of 1.9 keV at 1.33 MeV[coaxial Ge detector], of 300 eV at 5.9 keV and of 550 eV at 122 keV (LEPD). Different cooling timeswere used to determine radionuclides with different half-lives. Two standards from the Institute of Geo-physical and Geochemical Prospecting (IGGE) (sediments GSD-9 and GSD-11) were used as referencematerials. Recommended values of each element in the standards used to calculate the elemental concen-trations in the samples were taken from data published by Govindaraju (1989). The accuracy and precisionof the overall procedure are usually better than 5% RSD as described recently (Gouveia & Prudencio, 2000).

2.4. Statistical treatment of the data

The data set obtained for the superficial sediments consisted of 15 variables and 70 samples. Multivariateanalysis of these data was carried out by using the STATISTICA (v.5.1) software package. Initially, weused factor analysis to establish the spatial patterns exhibited by the elements measured in the sediments.Then hierarchical clustering was performed on the normalised data using Ward’s method (Massart, Vandeg-inste, Deming, Michotte and Kaufman, 1988: 488 pp.) to identify groups of samples with similar concen-tration patterns.

3. Results and discussion

3.1. Elemental composition of surface sediments

Table 1 lists the minimum and maximum concentrations of the 15 elements determined in the superficialshelf sediments samples (n=70). There were large variations in the major elemental composition of theshelf sediments, which were related to the different sediment types and their grain size spectrum. Thetransition element contents (Cr, Mn, Fe, Ni, Cu and Zn) of the sediments showed wide variation ranges,but were systematically depleted relative both to average shale contents (Forstner & Wittman, 1983: 486pp.) and to the average superficial contents of the rocks exposed to weathering (Martin & Meybeck, 1979).The Pb contents were variable and in many instances were higher than currently accepted backgroundlevels. In previous studies Araujo et al. (1994) and Drago, Araujo, Valerio, Weber and Jouanneau (1999)analysed cores collected in the same region and found local enrichments of Pb, but could not link theseto any anthropogenic sources. Nor could we identify any systematic patterns in the Pb distributions, whichwere erratic even when taking into account that they were measured at different locations, at differentdistances from the coast and at various depths. However, any enhanced Pb levels were always associatedwith the finer grain size fractions.

3.2. Multivariate statistical spatial patterns

3.2.1. Factor analysisAt first we established the geochemical distribution patterns along the entire shelf. The elemental distri-

bution is complex with large ranges of variation in the concentrations. To identify shelf sediment sources,we carried out a factor analysis using the STATISTICA (v.5.1) program. Previous studies have proved

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Table 1Elemental minimum and maximum elemental concentrations of the bulk shelf sediments

NW Portuguese shelf Average World’s

min - max shale surface rock

(present study) (Salomons & Forstner, 1984 ) (Martin & Meybeck, 1979)

Al % 1.60 - 7.92 8.0 6.93Si % 13.4 - 47.9 27.3 27.5K % 1.13 - 2.67 2.7 2.44Ca % 0.28 - 20.3 2.2 4.5Ti % 0.02 - 0.62 0.5 0.38Cr mg/kg 10 - 83 90 71Mn mg/kg 34 - 385 850 720Fe % 0.17 - 3.31 4.7 3.59Ni mg/kg 10 - 39 68 49Cu mg/kg 10 - 26 45 32Zn mg/kg 26 - 88 95 127Rb mg/kg 59 - 174 - 112Sr mg/kg 53 - 439 140 278Zr mg/kg 28 - 1300 160 -Pb mg/kg 10 - 50 20 16

that factor analysis is a valuable tool to recognise elements with similar behaviour in environmental andgeochemical studies (Bopp & Biggs, 1971; Shine, Ika, & Ford, 1995).

Based on the chemical elements associated with each factor and their relative importance along the shelf,the sediment distribution pattern can be drawn. The analysis showed that that three factors accounted for84% of the variance in the original data matrix. The results of the factors loadings are listed in Table 2

Table 2Results of factor analysis for the bulk shelf sediments data set (n=70) and percentage of variance explained for the individualfactor loadings

Factor 1 Factor 2 Factor 3

Al 0.94 0.11 0.15Si �0.48 0.81 �0.02K 0.85 0.06 0.03Ca �0.17 �0.92 �0.22Ti 0.57 0.15 0.75Cr 0.71 �0.40 �0.03Mn 0.59 0.06 0.69Fe 0.76 �0.55 0.25Ni 0.83 �0.21 0.05Cu 0.64 0.02 0.57Zn 0.92 �0.02 0.29Rb 0.86 0.32 0.22Sr �0.09 �0.94 �0.07Zr �0.16 0.13 0.94Pb 0.66 �0.16 0.39

Var (%) 45.2% 20.5% 17.8%

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and can be summarised as follows. Factor 1 represents the distributions of Al, K, Rb and the transitionelement group (Cr, Fe, Ni, Zn), all metals that are usually associated with the fine particles. Factor 2 relatedto the occurrence of coarse-sized particles, indicating there were two groups of particles with contrastingchemical characteristics: quartz particles (Si) and calcium carbonate particles (Ca). The high factor loadingsdemonstrate the spatial independence on the distribution on these types of particles. Factor 3 representsthe Ti and Zr distributions that are usually associated with heavy minerals. In Fig. 3(a–c) contoured mapsof each sample’s factor scores are presented, and their geochemical meaning is discussed in detail below.

3.2.1.1. Factor 1: Fine sediment fraction: Al, K, Rb and transition elements Factor 1 explains the finesediment particle distribution of the shelf superficial sediments, resulting from the high correlation betweenthe heavy metal concentrations and Al. The highest values of the factor scores occurred over the area ofthe middle shelf in the regions of the fine deposits and also at the coastal zone adjacent to the main estuaries(Fig. 3(a)). The Al distribution range on the shelf sediments is highly variable, strongly dependent on thegrain size and seems to be controlled by two main factors. In the coastal region the Al content is low,with concentrations of below 2%. These concentrations increase towards the west, reaching 6 to 7% onthe mid shelf. Usually, Al is mainly associated with finer sediment fractions (phyllosilicates), which remainin suspension longer and are readily re-suspended. Highest Al concentrations occurred within the area offine muddy deposits and also tended to increase towards the boundaries of the Douro deposit. This indicatesthat the rocky outcrops are having a geomorphologic influence, by acting as a barrier and so preventingfurther sediment dispersal. However, four coastal sediment stations, two of them off the Douro estuary(PL42 and PL46) and the other two off the Minho (PL2 and PL5) proved to be an exception from thisgeneral pattern, and the higher concentrations of Al observed were the result of the presence of fine-grainedsediments. The deposition of fine sediments at these particular locations may be indicative of the presenceof fine riverine particles in the marine environment. The trace metal content of sediments is a consequenceof not only natural weathering but also local influences, which reflect changes in the surrounding environ-ment and anthropogenic additions. Generally the low trace metal content of the sediments (Table 1) indi-

Fig. 3. Factor scores for the sampling area (n=70). (a) Relative influence of factor1 (fine particles: Al, transition elements, K andRb); (b) Relative influence of factor 2 (coarse quartz and carbonate particles); (c) Relative influence of factor 3 (Ti and Zr).

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cates that they are naturally occurring metals, which are usually strongly bonded to the mineral phase. Thisis corroborated by the high factor loadings between Al and K (and Rb), which are certainly a consequence oftheir dominant presence in finer fractions, most probably in micas, felspars and/or clay minerals, whichare abundant in that region of the shelf (Magalhaes, 1999).

3.2.1.2. Factor 2: Coarse sediment fraction: Si, Ca and Sr Factor 2 accounts for the Si distribution,and is highly negative correlated with the Ca and Sr: carbonate system (Fig. 3(b)). The maximum valuesof the factor scores occurred at the inner shelf where there were high Si concentrations. Silicon is generallythe dominant constituent of the sediments, but exhibits a large range of variation (13–36%), with the largestvalues being in the sediments collected at the inner shelf. These percentages decrease progressively towardsthe mid and outer shelf, and fell to their lowest values near the Oporto canyon region. In the mid shelfarea where the fine-grained deposits occurred the silicon distribution was nearly uniform. The Si enrichmentof sediments over the inner shelf may result from a preferential settling of the coarser quartz particles inthe inshore areas and/or the effects of specific local hydrodynamic conditions that result in the removal ofthe finer sediment particles.

The Ca distribution also reveals a considerable variation (~1–5 %), but contrary to the pattern observedfor Si, increases towards west. However, much high percentages of Ca (5–20%) were found in the sedimentscollected close to the Oporto canyon region at greater depth (200 m). This is interpreted as being a conse-quence of enhanced biogenic activity resulting in an enrichment of coarse carbonate particles (Libes, 1992:734 pp.) so Ca concentrations are higher whereas those of Al and Si are lower. Sediments deposited atgreater depths (enriched in carbonate biogenic particles) have much higher Ca/Sr ratios, which are probablycontrolled by water chemistry that regulating the solubility of CaCO3 and SrSO4. These compounds arethe most common components of these biomineral formations (Silva & Williams, 1991: 561 pp.). Theirsimilar chemical behaviour and the incorporation of Sr in the carbonate fraction explain the strong positivecorrelation between Ca and Sr.

3.2.1.3. Factor 3: Ti and Zr (heavy minerals) Fig. 3(c) shows the distribution of the third factor score,which accounts for the Ti and Zr distributions. This factor score mainly reflects the variations in Zr concen-trations (see factor loadings), although this association is most likely the result of increases in heavy min-erals that are limited in their transport and are not readily re-suspended.

The Zr content of the sediments shows a systematic enrichment within the inner and mid shelf zones,with zircon being concentrated in coarse grained sediments (Taylor & McLennan, 1985: 312 pp.) as aresult of heavy mineral fractionation, by faster settling. In effect, zircon minerals are widely spread alongthe northern shelf (Cascalho & Carvalho, 1993). Besides, some enhanced concentrations of Zr were detectedaround the edges of the fine-grained deposits, except along the west side of the Douro deposit in theneighbourhood of the Cretaceous and Palaeocene outcrops. These probably are relict sediments, dissemi-nated along the northern shelf (Dias, 1987).

3.2.2. Douro and Galicia deposits—Cluster analysisThe raw analytical data had been normalised to avoid artefacts in the classifications as a result of the

concentrations of the elements measured differing by orders of magnitude. The corresponding dendrogramaccording to complete linkage (Euclidian distance) and based on normalised data is displayed in Fig. 4.This defines four groups of sediment type, and the clusters formed can be ordered according to a sequence.The first cluster (a) from PL51 to PL26, on the left hand side of the figure, corresponds to the coarserinshore sediments with a high Si content (between 43 and 47.9%) and rather low Al content (1.60 to2.83%). The second cluster (b) from CV10 to G17, are samples collected at greater depths (close to theOporto canyon), which had a high Ca content (from 11.8 to 20.8%). The third cluster (c), from CV27 toG13, includes sediments of variable composition, but with intermediate content of Al and Si but high Ti

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Fig. 4. Dendrogram by complete linkage (Euclidian distance) based on normalised data, for 70 stations and 15 elements.

and Zr content. The fourth cluster (d), from CV37 to G1, includes sediments with the higher Al contentsfrom 5.16% to 7.92%. Based on these clusters, we have selected samples from the fourth group of sedimentsto calculate the average elemental composition of the fine sedimentary Galicia and Douro deposits (Table 3).

In general, the chemical composition of surface sediments is rather homogeneous along the Douro and

Table 3Chemical composition patterns of the fine sediments deposited at the northern shelf surface

Element Douro mud field Galicia mud fieldn = 17 n = 5

Al % 6.34±0.60 5.65±0.75Si % 27.40±2.28 28.60±1.44K % 2.20±0.19 2.10±0.16Ca % 2.59±0.94 2.54±0.80Ti % 0.38±0.04 0.37±0.05Cr mg/kg 61±19 69±8Mn mg/kg 282±29 238±10Fe % 2.80±0.21 2.70±0.14Ni mg/kg 30±5 27±3Cu mg/kg 20±3 19±3Zn mg/kg 75±8 69±8Rb mg/kg 155±7 142±10Sr mg/kg 175±38 177±24Zr mg/kg 242±57 302±50Pb mg/kg 35±6 38±7

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Galicia fine deposits for all the determined elements (major, minor and traces). Concentration values areidentical and variation ranges are for some elements even lower than the error of the analytical method.

3.2.3. Core profiles—vertical distributionTo determine the temporal variations in the elemental composition of the deposited sediments we have

investigated some of the elements on a fine time scale focussing in particular on those that have beenaccumulating as a result of anthropogenic activities.

Using selected dated sediment cores (Jouanneau et al., this volume) from the Galicia and Douro finesedimentary deposits, we have measured downcore profiles of these elements to look for signals of recenthuman activity. Chemical analyses of these cores revealed no significant downcore variation for the majorelements (Al, Si, K and Ca). Profiles of Ti, Fe and Mn are analogous. The Fe (Fig. 5(a)) contents increasedslightly in the deeper layers of the two mud patches, as a result of downcore changes in the redox conditions(Salomons & Forstner, 1984). Elements with similar chemical behaviour, such as K and Rb and Ca andSr, had identical downcore profiles indicating that there was a single dominant original source for all theseelements. The trace metals contents determined in the dated core fractions (Cr, Ni, Cu and Zn) weresystematically below the average content of shale, which is considered to be typical of natural backgroundconcentrations (Salomons & Forstner, 1984). Downcore variations in the trace elements were mostly withinthe scope of analytical error of the overall procedure and so seem not to be meaningful (Fig. 5(b)). Theseelements exhibited a similar trend for cores taken within the sedimentary deposits, although their concen-trations were even lower than for the inner shelf cores, which indicates trace metal contents are controlledby grain size. The downcore profiles of lead concentrations presented higher ranges of variation, whichwere above the “background values” (see Section 3.1). Comparable values had been previously determinedby Araujo et al. (1994) in a dated core (1400±70 years BP at 3.10 m depth) collected from the Douromud patch. The Pb concentrations in the deeper layers of the core (e.g. 1 m, 1.5 m, 2 m and 2.5 m) were

Fig. 5. Downcore profiles of the elemental distribution on the Galicia and Douro deposits. (a) Fe and Mn; (b) Cu and Zn.

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similar (33–44 mg/kg) to those determined in this study and were grain-size dependent. Contaminationwith anthropogenic Pb is most unlikely to be encountered at such depths, so these values can be taken asthe reference background value of the composition of fine sediments on the shelf.

However, contaminated river borne particles may be subjected to metal desorption, homogenisation andmixing with other materials during transport, in particular if the transport takes place over a long distance,i.e. whether the residence time of suspended material is relatively long, in a high energy environment.

Some complementary studies on the radiographic analyses of the sediment cores (Jouanneau et al., thisvolume) reveal the existence of a mixing zone at the upper layer of the cores extending down to a depthof 5–9 cm. This homogenised layer, which is the product of bioturbation and storm events, meant thataccumulation rates as measured by the 210Pb method, could only be determined for a few of the sedimentcores, and then only in fractions from beneath this mixing zone. Depending on the mixing conditions, themeasured accumulation rates varied from 0.1 to 0.4 cm.yr�1. A detailed discussion on these sedimentationrates, bioturbation and mixing processes all over the northern Portuguese shelf is provided by Jouanneauet al. (this volume).

Accepting these sedimentation rates implies that the sediments in mixing zone will have been depositedover a longish period of time (~10 to 100 years), which is probably sufficient to obscure any recentanthropogenic signal.

3.3. Origin of shelf sediments—Dispersal pathways of estuarine sediments

The components of the chemical contents of fine sediments deposited off estuaries often exhibit signsof continental inputs that may have either a geochemical (lithogenic) or an anthropogenic origin. The firstgroup of components, which are derived from the products of erosion and/or weathering of rocks, includeselements like K, Ca, Rb, Sr and Zr. The second group consists of metals, which although they occurnaturally are often enriched as a result of human activities, such as Cr, Ni, Cu, Zn and Pb (Forstner &Wittman, 1983).

Previous studies of sediment chemistry have identified contributions of the riverine and estuarine sedi-ments to distinctive coastal marine environments (Bertine & Goldberg, 1973; Caliani, Munoz, & Galan,1997; Chow, Bruland, Bertine, & Soutar, 1973; Cotte-Krief, Guieu, Thomas, & Martin, 2000; Elbaz-Poul-ichet & Leblanc, 1996; Ergin, Kazan, & Ediger, 1996; Jouanneau, Weber, Grousset, & Thomas, 1998b;Price, Brand, Pates, Mowbray, Theocharis, Civitarese et al., 1999; Puig, Palanques, Sanchez-Cabeza, &Masque, 1998; Windom, Schropp, Calder, Ryan, Smith, Burney et al., 1989).

In a study carried out in a southern region of the Portuguese shelf on the origins, dispersal and depositionof fine sediments, offshore from the Tagus estuary demonstrated that both anthropogenic and lithologicalinfluences of the river basin could be clearly identified in the sedimentary deposits (Paiva et al., 1997;Jouanneau et al., 1998a). Paiva et al. (1997) found the muddy deposits contained anthropogenic contami-nation of Zn (up to 300 mg/kg) and Pb (up to 120 mg/kg). Comparable values were reported by Vale(1990) from a gravity core sample at the muddy area outside the inlet channel. This contamination measuredon the shelf originated from human activities in the middle and lower estuary. Here, the narrow shelf andthe local circulation pattern results in rapid deposition of fine-grained sediments directly from the riverdischarge. However, the low Cu concentrations in the shelf deposits indicated that this element is releasedinto solution during transport. This decrease in the content of some metals has been attributed to theincreasing capacity of divalent ions in the first transition elements series (up to Cu and decreasing for Zn)to form metal ion soluble complexes, largely independent of ligands (Shriver, Atkins, & Langford, 1994:819 pp.). Additionally, a recent comparative study of some superficial sediments from the Douro and Tagusdeposits revealed that the Ca contents were systematically enriched along the Tagus deposit as a result ofthe lithological characteristics of the river basin. The measured Ca/Sr ratio values were for the northernfine sediments comparable to shales (Ca/Sr ~110) and much higher for the Tagus deposit (Ca/Sr ~220),

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reflecting differences in the source rocks for the fine sediments (Valerio, Barbosa, Araujo, & Jouanneau,2000).

3.3.1. Influence of the northern estuarine sediment composition in shelf sedimentsUsing a similar approach, we have compared the elemental distribution patterns (max–min) of the fine

sediments collected along both deposits with the elemental composition of the sediments transported bythe main rivers draining the Portuguese hinterland, the rivers Douro (Table 4(a)) and Minho (Table 4(b)).The REE distributions are also discussed in order to infer the provenance of the shelf fine sediments.

Table 4Elemental minimum and maximum elemental concentrations of the bulk sediments collected at the two mud fields and adjacent mainestuaries. (a) Douro; (b) Minho/Galicia

Element Douro River Douro Mud Field

min – max min – max

(a)Al % 5.6 – 7.3 5.3 – 7.4Si % 24.6 – 30.1 23.1 – 32.6K % 2.1 – 2.6 1.9 – 2.5Ca % 0.6 – 2.6 1.4 – 5.0Ti % 0.2 – 0.5 0.3 – 0.4Cr mg/kg 59 – 169 22 – 83Mn mg/kg 152 – 524 235 – 335Fe % 2.9 – 3.9 2.4 – 3.3Ni mg/kg 17 – 44 22 – 38Cu mg/kg 32 – 92 15 – 26Zn mg/kg 153 – 452 64 – 88Rb mg/kg 92 – 190 139 – 167Sr mg/kg 101 – 185 139 – 277Zr mg/kg 155 – 255 161 – 349Pb mg/kg 45 – 117 26 – 50

Element Minho River Galicia Mud Field

min – max min – max

(b)Al % 2.8 – 5.5 5.0 – 6.8Si % 33.1 – 37.8 26.9 – 30.6K % 1.6 – 2.7 2.0 – 2.3Ca % 0.1 – 0.5 1.9 – 3.9Ti % 0.1 – 0.3 0.3 – 0.4Cr mg/kg 15 – 54 59 – 77Mn mg/kg 54 – 267 222 – 248Fe % 0.3 – 1.8 2.5 – 2.9Ni mg/kg 10 – 24 23 – 31Cu mg/kg 10 – 16 14 – 23Zn mg/kg 10 – 52 61 – 78Rb mg/kg 90 – 164 129 – 151Sr mg/kg 43 – 83 150 – 212Zr mg/kg 39 – 171 262 – 360Pb mg/kg 13 – 25 27 – 47

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3.3.2. Major (lithogenic) elementsThe average elemental composition of most of the lithogenic elements (e.g.: Al, Si, K and Rb) in the

sediments from the Douro River is comparable to those of the adjacent mud field (Table 4(a)). The geologyof the northern Portugal is predominantly granitic, and so considerably depleted in carbonates, so both theDouro and Minho riverine sediments have low Ca contents (�1%) (Araujo, Marques, & Rocha, 2000). Inthe estuary the Ca content (~1–2.6%) increases significantly as a result of the mixing of materials with acontinental and a marine origin. Therefore, at the northwestern shelf, the Ca content is predominantly ofmarine biogenic origin. Similarities between the lithogenic elemental composition of sediments from theDouro and the adjacent mud patch indicate that the input of riverine particles dominates over the authigenicparticle formation.

In contrast, there are marked differences between the chemical composition of the fine sediments in theGalicia mud patch and that of the finer grain-sized Minho river sediments (Table 4(b)). The Minho riversediments have a relatively high Si content (low Al) but rather low concentrations in Ca, Mn, Fe, Sr andZr. The Minho sediments are predominantly coarse quartz particles (Araujo et al., 2000).

3.3.3. Anthropogenic elementsThere is a general decrease in trace element content of the sediments from the Douro estuary to the

Douro mud patch deposit because of the effects of complexation and mixing and/or dilution with authigenicparticles. In general, the trace element content of the shelf sediments indicates that they have been depletedby geochemical processes during the long time the sediments are held in suspension before they aredeposited (Table 4(a)).

For the Minho/Galicia system an opposite trend is observed. Apparently there is a general metal enrich-ment when comparing fine sediments from the Minho River and Galicia deposit (Table 4(b)). Withoutexception the maximum trace elements content of sediments from the Minho River are either identical orlower than the minimum values determined for shelf sediments. This consistent trend leads to the conclusionthat the sediments deposited at the Minho/Galicia region are not dominated by particles furnished by theMinho river sediment load.

3.3.4. Element/Al ratiosEven though our discussion has been based on the distributions of elements in fine-grained sediments,

differences in grain size distributions still occur. This becomes more relevant for the elemental distributionsfound in the Minho river sediments, in which trace metal distributions may be controlled by particle sorting.Therefore, we have used Al contents as a means of geochemically normalising the lithogenic and anthropog-enic contents. The average comparative values of the ratio between element/Al concentrations in sedimentsfrom the Douro and Minho estuaries and at the offshore deposits are shown for both the lithogenic (Table5(a)) and anthropogenic (Table 5(b)) elements.

The main differences between the ratios in sediments from the estuaries and mud fields are the enrich-ments in Sr and Zr towards the shelf; typically these are related to mixing with allochthonous materials.The Rb/Al ratios indicate that the sediments in both the Minho and Galicia deposits are remarkably relatedto the Douro estuary inputs.

Element/Al ratios show there are different trends for the Douro and Minho/Galicia systems, in the anthro-pogenic elements. For the Douro, the general trace metal decrease from the estuary towards mud field isprobably the result of chemical processes, dilution effects and/or mixing with authigenic materials. In theMinho/Galicia system there is a marked increase in the anthropogenic/Al ratios towards the mud field,indicating that again river borne particles are unlikely to be the main source of the fine sedimentary deposits.This becomes particularly evident if the averages of the ratios of the anthropogenic El/Al at the Minhoand Douro estuarine sediments and the El/Al shelf sediment composition are plotted as a function of the

292 M.F. Araujo et al. / Progress in Oceanography 52 (2002) 277–297

Table 5Average comparative values of the element/Al concentrations at the Douro and Minho estuaries and at the Douro and Galicia mudfields. (a) lithogenic elements; (b) anthropogenic elements

Douro est. (n=7) Mud field (n=17) Minho est. (n=12) Mud field (n=5)

(a)Rb/Al 25.1 → 24.5 33.4 ↘ 25.4Sr/Al 18.6 ↗ 27.8 16.5 ↗ 32.0Zr/Al 30.9 ↗ 38.8 19.7 ↗ 54.2

Douro est. (n=7) Mud field (n=17) Minho est. (n=12) Mud field (n=5)

(b)Cu/Al 7.6 ↘ 3.1 2.8 ↗ 4.3Zn/Al 33.8 ↘ 11.8 6.4 ↗ 12.7Pb/Al 10.9 ↘ 5.6 4.5 ↗ 9.0

distance from the coastal margin (Fig. 6). These plots display striking similarities between the element/Alratios for both deposits.

3.3.5. REE distributionTo facilitate data comparison, REE concentrations are usually represented normalised to the shale compo-

sition (Sholkovitz, 1990). Our results on the REE distribution have also been normalised, using the set ofvalues for the shales provided by Haskins and Haskins (1966).

The shale normalised REE distributional range for the selected Douro and Galicia superficial shelf sedi-ments are presented in Fig. 7(a) and (b) respectively, and can be compared with the normalised REEdistribution patterns for the sediments from the respective Douro and Minho estuarine sediments, shownFig. 8(a) and (b) respectively. No meaningful variations in the REE distribution patterns could be detectedbetween the two mud patches. The REE patterns are uniformly similar, and show a negative Eu anomalythroughout each sedimentary deposit, which is typical of most of sedimentary rocks (sandstones, mudstones,carbonates). However, an enrichment in the light rare earths La to Sm (LREE) is noticeable, which is alsoseen in the estuarine sediments. Previously, significant depletion in the heavy rare earths Gd-Lu (HREE)in river suspended loads and abyssal sediments have been reported when the data have been shale-normal-ised (Condie, 1991; Goldstein & Jacobsen, 1988).

The LREE enrichments in near shore sediments have been explained by the increasing stability of thesolvated heavier HREE forming stronger complexes with carbonate ions and being retained in solution.Some authors (Condie, 1991) have attributed the HREE depletion to incomplete dissolution of heavy min-erals during chemical analysis, but here the INAA used to analyse the sediments was carried out on solidsamples. The REE distribution patterns for the sediments of the Portuguese northern shelf are comparableto those presented by Sholkowitz (1990) for shelf and slope sediments with different origins, suggestinga constant REE fractionation in sediments, as previously suggested by Taylor and McLennan (1985).

The (La/Yb)n ratios calculated for the analysed sediments did not allow us to distinguish the sedimentaryenvironments. The samples collected at the Douro sedimentary deposit presented similar distributions inLREE and HREE, with (La/Yb)n ranging from 1.15 up to 1.32. The sediment samples collected at theMinho mud patch also exhibited comparable values, ranging from 1.25 up to 1.46. The variation range of(La/Yb)n obtained for the estuarine sediments was between 1.36 and 1.46, for the Douro estuarine sedimentsand between 1.04 and 1.55 for Minho estuary. These values are in agreement with those summarised bySholkovitz (1988), where the LREE enrichment factors ranged between 1.2 and 2. Also, according to

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Fig. 6. Variations of the metal/Al ratios as a function of the distance from estuaries (the d=0 km corresponds to the average of themetal/Al in several estuarine sediments).

Goldstein and Jacobsen (1988) the terrigenous input to the oceans from major rivers is heavy REE depletedrelative to shales.

However, some main differences were measurable related to the total REE in the sediments and the Eucontents. The total REE abundances in the Minho estuarine sediments were very low (�REE ~ 30–80),which is typical of quartz-rich sediments (Taylor & McLennan, 1985). It should be noticed that the �REEfor the Douro estuary were between 90 and 150, and for the mud fields were typically between 120 and150. Besides values of the ratio Eu/Eu∗, which signify either enrichment or depletion of Eu, indicate thatno negative Eu anomaly was observed for either the Douro or the Minho estuarine sediments (Fig. 8(a)and (b)). Moreover, there is a marked enrichment in Eu in the particular case of the Minho estuarinesediments (Eu/Eu∗=1.18). This positive Eu-anomaly together with the fairly low abundances of REE, havebeen reported as notable features of alkali-feldspars (Taylor & McLennan, 1985).

In summary, the shale-normalised patterns indicate that the REE distribution of the mud fields is highlycomparable to the one obtained for the Douro estuarine sediments, although the former resemble to theshale composition.

294 M.F. Araujo et al. / Progress in Oceanography 52 (2002) 277–297

Fig. 7. Ranges of the shale normalised REE distribution in shelf sediments. (a) Douro deposit; (b) Galicia deposit.

Fig. 8. Ranges of the shale normalised REE distribution in estuarine sediments. (a) Douro; (b) Minho.

4. Conclusions

Sediments deposited at the northern Portuguese shelf are dominantly composed by inorganic detritalcoarse particles with different origins: quartz particles resulting from chemical and physical weathering ofthe continent, along the inner shelf; carbonate marine particles mostly concentrated at the outer shelf andcoarse zircon relict particles distributed on the sediment cover particularly, in regions surrounding the finedeposits. These coarse materials, part of natural sediments are depleted in trace metals and dilute thesediment metal concentration.

The lithogenic elemental (Al, K, Rb and REE) distributions of fine sediments points out the remarkablesimilarities between the different sets of sediments: Douro estuary, Douro deposit and Galicia deposit. TheDouro River seems to have made a substantial contribution to the build up of the muddy deposit locatedin the middle of the adjacent shelf, although not by a direct process. The normalised trace metal distributionin the sediments provides evidence for the release of these elements into the marine environment. In fact,

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none of the anthropogenic heavy metals enrichment detected within the Douro estuary were detected onthe shelf.

Galicia mud field do not seem to be related to sediments exported from the Minho River, which arecoarse, mainly composed by quartz and highly depleted in trace metals and rare earth elements. Accordingto Rey Salgado (1993), currently there are no inputs of sediment from the north towards the Galicia shelf.Thus, taking into account the striking similar elemental composition of sediments at both mud patches,the main source of the fine sediments deposited at the Galicia studied area seems to be located to the south.

Acknowledgements

The authors would like to thank the Captain and crew of the R.V. Almeida Carvalho for their assistancein the cruises during which the samples were taken. We are indebted to our colleagues from the scientificteams of GEOMAR92, CORVET96 and SEDIMINHO 93 for all their enthusiastic helpful collaborationduring the sampling operations. This work was supported by JNICT (Portugal) research grantPMCT/P/MAR/706/90 and EU program OMEX II/II (MAST3-CT97-0076).

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