Artículos

 

Geochemical characterization of surficial sediments from the southwestern Iberian continental shelf

 

Caracterización geoquímica de los sedimentos superficiales del sudoeste de la plataforma continental ibérica

 

A. Machado^{1, 3}, F. Rocha¹*, M.F. Araújo², F. Vitali², C. Gomes¹ and J.A. Dias³

 

¹ Centro de Minerais Industrias e Argilas Universidade do Aveiro 3810 Aveiro, Portugal * E-mail: frocha@geo.ua.pt

² Instituto Tecnológico e Nuclear Química, E.N. 10 2686-953 Sacavém, Portugal.

³ CIACOMAR Universidade do Algarve Campus de Gambelas 8000 Faro. Portugal.

 

Recibido en junio de 2003;
aceptado en abril de 2004.

 

Abstract

The impact of trace elements (Ba, Nb, Zr, Y, Sr, Rb, Pb, As, Zn, Cu, Ni, Co, Cr, V, La, Ce, Nd, Sm, Eu, Tb, Yb and Lu) transported by the Guadiana, Tinto-Odiel and Guadalquivir rivers on sediments from the southwestern Iberian continental shelf has been studied. Shelf surficial sediments from three transects established right in front of the river estuaries were characterized by specific signatures relative to those trace elements. The Guadiana transect shows high Zr concentration values; the Tinto-Odiel transect shows high heavy metal (Pb, As, Zn and Cu) concentration values; and the Guadalquivir transect shows high Sr concentration values. Data from the statistical analysis indicate three distinct sources for trace elements: lithogenic, biogenic and anthropogenic.

Key words: trace elements, rare earth elements, fine sediments, continental shelf, Iberian Peninsula.

 

Resumen

Se estudió el impacto de los elementos traza (Ba, Nb, Zr, Y, Sr, Rb, Pb, As, Zn, Cu, Ni, Co, Cr, V, La, Ce, Nd, Sm, Eu, Tb, Yb y Lu) acarreados por los ríos Guadiana, Tinto-Odiel y Guadalquivir sobre los sedimentos del sudoeste de la plataforma continental ibérica. Los sedimentos superficiales de tres transectos establecidos enfrente de los estuarios de los ríos se caracterizaron por las huellas específicas en lo que involucra a aquellos elementos. El transecto de Guadiana muestra valores altos de concentración de Zr; el transecto de Tinto-Odiel muestra valores altos de concentración para los metales pesados (Pb, As, Zn y Cu); y el transecto de Guadalquivir muestra valores altos de concentración de Sr. Los datos del análisis estadístico señalan tres fuentes distintas para los elementos traza: litogénica, biogénica y antropogénica.

Palabras clave: elementos traza, tierras raras (REE), sedimentos finos, plataforma continental, Península Ibérica.

 

Introduction

Chemical composition of shelf sediments is highly dependent on factors such as climate, vegetation cover, geo-morphology, pedogenetic processes and soil nature, as well as river hydrodynamics, chemical activity and anthropogenic activity.

During the last decade, several studies have been conducted on the mineralogical composition of sea-bottom sediments, of particulate materials in suspension and on cores, which together with paleontological and geochemical data are used for paleoclimatic and paleoenvironmental reconstructions, analysis of sea-level changes, identification of stratigraphic correlations, past and present sources of the sediments and means of terrigenous supply, dispersion of pollution and, finally, to describe the present oceanic water masses (Drago et al., 2002; Machado et al., 1999a, 1999b, 2000a, 2000b, 2001a, 2001b, 2002; Martins et al., 2001a, 2001b, 2002; Oliveira et al., 1998, 2000, 2001, 2002; Vidinha et al., 1998, 2000, 2002).

The study area of the present work is located between Vila Real de Santo António (Portugal) and Cadiz (Spain), more precisely in the continental shelf off the Guadiana, Piedras, Tinto-Odiel and Guadalquivir estuaries, limited by the coastline and the 200-m bathymetric curve.

The low density geochemical survey of Portugal put in evidence (Ferreira, 2000) some geochemical patterns, related to features such as geological aspects (lithology, occurrence of particular mineralization, structural zones, metallogenic provinces), surficial environment conditions and anthropogenic influences (mining activities, agriculture, industrialized areas). Recent studies have shown that the Iberian Pyrite Belt is the main source of Ni, Cu, Cd, Zn and As found in sediments of the western Mediterranean basin (Elbaz-Poulichet et al., 2001). On the southwestern coast of Spain, some of the processes referred to act cooperatively in the Tinto-Odiel fluvial system, one of the most polluted areas of the world primarily due to the mining activities that have taken place for many years in the Pyrite Belt (Pérez et al., 1991; Cabrera et al., 1992; Nelson and Lamothe, 1993).

The part of the continental shelf studied receives considerable fluvial input from the Guadiana, Tinto-Odiel and Guadalquivir rivers, and to a lesser extent from the Gilao and Piedras rivers, all of them draining the Iberian Pyrite Belt.

The main goal of the present study is to investigate the distribution patterns of the trace elements analyzed, including the rare earth elements (REE), on the continental shelf sediments, in particular in their silt and clay fractions, using geochemical criteria recommended by Chamley (1989). Sediment sampling took place along three transects off the Guadiana, Tinto-Odiel and Guadalquivir estuaries and the geochemical anomalies displayed by the sediments analyzed were determined.

 

Materials and methods

The sector of the continental shelf under study is located in the southwestern Iberian Peninsula, between the mouths of the Guadiana and Guadalquivir rivers, and limited by the coastline and the 200-m bathymetric line. Sediment sampling was carried out in February 2001 (rainy period), on board the research vessel N.R.P. Andrómeda from the Portuguese Hydrographic Institute, using a Smith-McIntyre sampler, with a PVC core. For the present study, 20 samples were selected from the total collected at the transects referred to (fig. 1), 8 off the Guadiana estuary, 5 off the Tinto-Odiel estuary and 7 off the Guadalquivir estuary.

After rinsing with distilled water, samples were wet sieved through a 400 ASTM sieve (0.038-mm mesh). Sediment fractions <0.038 mm were dried in an oven at 60°C and gently disaggregated with a porcelain mortar.

The fine fractions (<38 µm) of the surficial sediments were analyzed using the X-ray fluorescence spectrometer (Philips, model PW 1400) of the Department of Geosciences laboratory at Aveiro University. Fourteen trace elements (Ba, Nb, Zr, Y, Sr, Rb, Pb, As, Zn, Cu, Ni, Co, Cr and V) were determined. Eight REE (La, Ce, Nd, Sm, Eu, Tb, Yb and Lu) were also determined, with an accuracy better than 5% (Gouveia and Prudencio, 2000), by neutron activation analysis using the nuclear reactor operating at the laboratory of the Instituto Tecnológico e Nuclear, in Sacavém (Portugal).

The data obtained were statistically treated by principal components analysis, using Statistica 5.0 software from StatSoft, and the parameters selected were mapped using Surfer 7.0 software from Golden Software.

 

Results

Table 1 shows the main previously analyzed (Machado et al., 2000a, 2000b, 2001a, 2001b) characteristics of the samples studied (concerning depth, lithology and mineralogy). The sediments consist mostly of fine- to medium-grain sand, sandy mud and mud, sometimes containing shell fragments.

According to Machado et al. (2000a), the fine fraction (<38 mm) presents a variable quantitative mineralogical composition: quartz (Qz) is the dominant mineral, phyllosilicates (Phyl), mostly mica/illite, K-feldspar (FK), plagioclase (Plag), dolomite (Dol), anhydrite and calcite (Cal) are the main minerals, while opal C/CT, anatase, siderite and pyrite are the accessory minerals identified.

Regarding the mineralogical composition of the <2 µm fraction (fig. 2), illite (I) is the predominant mineral, accompanied by kaolinite (K) and smectite (Sm). Chlorite (Cl) and illite-smectite (I-Sm) are also present, generally in very discreet amounts (Machado et al., 2001a, 2001b).

Table 2 presents all the geochemical data (Ba, Nb, Zr, Y, Sr, Rb, Pb, As, Zn, Cu, Ni, Co, Cr, V, La, Ce, Nd, Sm, Eu, Tb, Yband Lu) obtained for the three transects studied (Guadiana, Tinto-Odiel and Guadalquivir).

Table 3 shows both the minimum and maximum contents for each element analyzed in the three transects. As the main goal of this research was the identification of eventual geochemical anomalies found in the sediments, we also exhibit the values of the elementary composition of average shale (Turekian and Wedepohl, 1961; Mason and Moore, 1982), in regard to the trace elements analyzed.

In general terms, Zr, Sr, Pb, As, Zn, Cr and Cu (showing concentrations higher than those published for average shale) and Ba, Y, Rb, Ni, Co and V (showing concentrations lower than the average shale) show anomalous concentrations in the surficial sediments of the section of the Iberian shelf studied. Only Nb shows concentrations in accordance with the normal values. Martin and Meybeck (1979) indicated the increment of concentrations for elements such as Cu, Pb and Zn, whereas elements like Ba, Y, Rb, Ni, Co and V showed concentration values equivalent to the theoretical ones.

Figure 3 shows the distribution of Sr, Pb, Zn, Cu, As and Zr concentration values, which are very high when compared with those published for average shale. It is interesting to note that the highest Zr concentration values occur along the transect off the Guadiana estuary, whereas Sr concentrations are higher in the transect off the Guadalquivir estuary. On the other hand, Ba, Y, Rb, Ni, Co and V are present in lower concentrations (fig. 4), compared with those referred to by Turekian and Wedepohl (1961) and Mason and Moore (1982) for average shale. Figure 4 shows that the sediments of the Guadalquivir transect contain very low Ba, Y, Rb, Ni, Co and V contents, compared to those determined in the other two transects.

Also, when analyzing the REE concentrations in sediments and/or sedimentary rocks, it is common to normalize those values using the REE concentrations exhibited by an "average" sediment/sedimentary rock, such as the North American Shale Composite (NASC). In general, the surface sediments studied show four REE concentrations that are anomalous with regard to NASC values (table 4). In the cases of the Guadiana and Tinto-Odiel transects, the sediments contain REE concentrations above the average background values, whereas the samples from the Guadalquivir transect contain concentrations below the average background values.

 

Discussion

Guadiana transect

In the Guadiana transect sediments, Pb, Zn, As and Cu concentrations slightly increase with increasing shelf depth. This could be related to the decrease of sediment grain size and the existence of organic matter, which is associated with the presence of sulphite minerals (coming from the Iberian Pyrite Belt).

According to Ferreira (2000), positive anomalies are found of: Pb, related to the metasediments outcropping along the Iberian Pyrite Belt; Zn, related to the metasediments outcropping in the Southern Portuguese Zone and in the Central Iberian Zone; As, related to the Central Iberian Zone formations; and Cu, related to both the Iberian Pyrite Belt and the Southern Portuguese Zone.

The relatively high Zr concentrations found near the outlet of the Guadiana River are most probably due to the presence of Zr-bearing minerals, such as biotite derived from the erosion of the metamorphic rocks (schists, graywackes and shales) that are very well represented in the Guadiana hydrographic basin.

Also, along the Guadiana transect, Ba, Y, Rb, Ni, Co and V concentrations decrease with the distance to the shore. This particular behaviour could be considered a signature of the Guadiana River. On the other hand, the highest Nb and Cr concentrations were found in sediments off the Guadiana estuary.

Tinto-Odiel transect

Off the Tinto-Odiel estuary, the shelf surficial sediments are highly polluted by heavy metals, such as Cu, Zn and Pb, associated with sulphide minerals coming in the outflows from sources in the Iberian Pyrite Belt and in the mineral wastes of industrial plants located near the estuarine border (Ruiz, 2001).

The Tinto-Odiel estuary seems to play an important role in the distribution of Pb, Zn, As and Cu, the concentration of these elements becoming strongly reduced in the mid and outer zones of the continental shelf. Actually, the relatively high heavy metal concentrations in sediments of the Tinto-Odiel transect are most probably related to the mining activities taking place in the hinterland.

Guadalquivir transect

The concentrations of Ba, Y, Rd, Ni, Co and V are relatively high in sediments from the Guadalquivir transect. The concentrations of Sr increase with increasing distance to the shore, which is possibly related to the increase of carbonate content in the sediments. According to Machado et al. (2000b), the calcite content of biogenic origin exhibits a similar trend.

Also, the high Zn concentrations determined in the sediments from this transect may be related to the environmental accident that occurred at the Aznalcóllar zinc mine (Andalucia, Spain), on 25 April 1998. This metal is predominantly fixed in the finest fraction, i.e., the clay fraction of the sediment.

In general, the concentrations of all the above-mentioned trace elements decrease with depth. This pattern is probably related to a differential behaviour in the hydraulic processes of the particles of the mineral phases containing these elements. The Pb, Zn, As and Cu concentrations determined in the sediments are surely related to the Pyrite Belt and their values may be influenced by industrial and mining activities, with the higher concentrations near the estuarine mouths and the lower concentrations far from the coastline. Enrichment of these heavy metals (Pb, Zn, As and Cu) in the fine fraction of surfi-cial sediment, seems not to exceed about 30 m depth.

Multivariate statistical analysis

The application of the principal components analysis (fig. 5) to the chemical data obtained (REE and trace elements) shows that the chemical parameters analyzed are associated in four sets. It was possible to extract three factors, with eigenvalues exceeding 1.0, explaining 89% of the total variance of the data set:

• Factor 1 (explaining 64%) shows the first set of parameters: REE together with Zn, Cu, Pb, As, Cr, Ba and Y.

• Factor 2 (explaining 17%) shows the second set of parameters: Ni, Rb and Nb.

• Factor 3 (explaining 8%) shows Zr in opposition to Sr.

These distinct sets of chemical parameters may reflect distinct sources (lithogenic, biogenic and anthropogenic) of the elements, derived or not from the three estuaries considered.

The first set of chemical parameters is probably a signature of the Pyrite Belt, whereas the second set is related to the siliciclastic material. As can be seen from table 4, the three chemical elements forming the second set (Ni, Rb and Nb) show high positive correlations with Al. The discrimination performed by factor 3 may be a consequence of grain size sorting, Zr being present in heavy minerals (like zircon) from biotite-chlorite rich metasediments, whereas Sr is present in the biogenic components of the sediments.

Geochemical ratios

Table 5, containing the correlation matrix, shows the relationship between Al and trace elements.

According to Martin and Meybeck (1979), when analyzing the relationship between weathering and river transport, a fractionation factor is defined with regard to a conservative element, usually Al. Therefore, trace element concentration values were normalized, using a normative element (Al) that is associated with the main rock-forming minerals (like feldspars). This element that, as a rule, is enriched in the sediment fine fractions, is not significantly affected by anthropogenic alterations (Ackerman, 1980).

A way of identifying the contamination is to correlate the normalized element (Al) and the assumed pollutant. When a linear relationship does not exist between both it is possible to attribute this situation to a polluting effect; this is the case of Pb, As, Cu and Zn (fig. 6). On the other hand, without anthropogenic disturbance that relationship is practically linear, as is the case of Ba, Nb, Y, Rb and Ni (table 5).

In general terms, heavy metal concentrations are geologically controlled, their sources being the drainage basins of the rivers. However, in some samples belonging to the transect off the Tinto-Odiel estuary, it is possible to identify some anthropogenic contributions of Pb, As, Zn and Cu, whose concentrations are relatively higher not only than the respective average concentration of the sediments analyzed but also than the reference patterns (Turekian and Wedepohl, 1961; Mason and Moore, 1982). According to Martin and Meybeck (1979), some elements (like Br, Sb, Pb, Cu, Mo, Zn and Ni) show enrichment relative to Al, whereas some less mobilized elements (like REE, Co, Cr, Cs, Mn, Rb, Si, Th, Ti, U and V) show no natural enrichment and others (like B, Ba, Ca, K, Mg, Na and Sr) show relative depletion.

Aluminium seems to be related directly to Ba, Nb, Y, Rb and Ni and indirectly to Sr. The geochemical normalization, based on Al, for these trace element concentrations points to the lithogenic source of the elements referred to (fig. 7).

Figure 8 shows the distribution of the following trace elements in the sediment samples of the three transects: Ba, Nb, Y, Rb, Ni and Sr (all Al normalized).

REE distribution

Rare earth elements (REE) occur as trace elements in all types of rocks (sedimentary, igneous or metamorphic) and exhibit a tendency to be concentrated in accessory rock-forming minerals such as titanite, apatite, zircon, epidote, garnet and clay minerals. Because REE are not easily fractionated during sedimentation, sedimentary REE patterns may provide information about the sediment provenance.

The distribution of REE in the sediments of the three transects studied is shown in figure 9. Comparing the three transects, REE distribution patterns show lower REE contents in sediments from the Guadalquivir transect, which could be due to dilution by biogenic carbonate.

Particularly high REE contents exist in the sediments of the Tinto-Odiel transect positioned close to the coastline, the contents decreasing rapidly with depth. Differently, in sediments from the Guadiana transect, REE contents gradually decrease with depth. The deposit of REE-rich minerals such as heavy minerals and clay minerals close to the Tinto-Odiel and Guadiana coastlines might explain these variations.

Figure 10 shows the presence of a muddy formation located approximately between the bathymetric lines of 30 and 100 m depth, in a zone off and limited by the Guadiana and Guadalquivir estuaries. Plumes provenance from the three estuaries studied could also be identified, especially in the Guadalquivir. High organic matter concentration near the Guadalquivir outlet corresponds to fine-grained sediments, and may be related to the local hydrodynamic regime.

Figure 11 shows the shale-normalized REE plot of the surf-icial sediments (<38 µm fractions) in each transect (Guadiana, Tinto-Odiel and Guadalquivir estuaries): the concentration of the individual REE in the sediment divided by the concentration in the NASC reference material (Taylor and McLennan, 1988). In general terms, a negative anomaly of Ce does exist, whereas Sm, followed by La, are the more enriched REE in this fraction.

The behaviour of the REE in the transect off the Guadiana estuary reveals that Ce presents a weak negative anomaly compared to the other two transects; on the other hand, it presents higher concentrations of Sm. In the case of the Tinto-Odiel transect, REE distribution curves are characterized by higher concentrations close to the coastline, a negative anomaly for Ce, a positive anomaly for Eu and low REE contents in the sediments located far away from the coastline. The Guadalquivir transect is characterized by low REE contents, compared to the other two transects, and a pronounced negative anomaly for Ce.

Figure 12 exhibits the variation of LREE and HREE (light and heavy RRE, respectively), using the La/Yb curve (Haskin, 1971) and the Ce anomaly, expressed by the Ce/Ce* ratio (Boust, 1986; Ce* = 3 x Ce/[(2 x La) + Nd]) in the sediments studied. The La/Yb ratio exhibits a more pronounced LREE enrichment relative to HREE from east to west.

 

Final considerations

The geochemical characterization of surficial sediments from the southwestern Iberian continental shelf, comprising 22 trace elements (REE included) quantified along three transects off the Guadiana, Tinto-Odiel and Guadalquivir estuaries, allowed the distribution patterns characteristic of those transects to be established; hence, it was possible to define in those sediments signatures attributed to the influence of the rivers referred to.

For the transect off the Guadiana estuary, where Pb, As, Zn, Cu and Co anomalies were found, the concentration of these elements becomes slightly higher as the distance to the coastline increases, and a similar tendency is shown by the organic matter concentration. Marine clays commonly show enrichment, relative to river particulate matter, in Cu and As but not in Zn, Pb and Co. Therefore, the supply of fresh particulate and organic matter appears to play an important role in the fixation of those heavy metals.

For the transect off the Tinto-Odiel estuary, extremely high accumulations of Pb, Zn and Cu were found close to the river outlet; these heavy metal concentrations are most probably related to mining and other industrial activities.

For the transect off the Guadalquivir estuary, the concentrations of all the trace elements analyzed show the highest values both in the middle and outer shelf, to some extent due to grain size control. The relatively higher Sr and Ba concentrations are directly correlated to the relatively higher carbonate content in the sediments.

As a rule, REE contents in the shelf surficial sediments decrease with the distance to the coastline, a fact most probably related to the change of a prevailing detrital sedimentation near the coast to a relatively more dominant chemical sedimentation far from the coast. The strong Ce negative anomaly is indicative of a more hydrogenized environment in the sediment-seawater interface.

Concerning the distinction between sediments from the Guadiana, Tinto-Odiel and Guadalquivir transects with regard to REE contents, the lower values found in the Guadiana transect could be related to the lower clay fraction content in the sediments, reflected in the less fixation ability of REE by the sediments. The same situation was found for the Guadalquivir transect. On the other hand, the relatively higher REE contents found in sediments from the Tinto-Odiel transect could be related to the higher clay fraction contents in those sediments.

The Guadiana sediments present a low content of REE and show relatively flat patterns, arguing for the importance of detrital material in these samples. That is also the case for the Guadalquivir transect. The REE profiles of the samples from the Guadalquivir transect are extremely similar to each other. This homogeneity tends to indicate that these samples have the same origin. For Tinto-Odel, a higher REE concentration and a clear Ce anomaly are noticeable in samples 593 and 597. This may be related to an increase of authigenic minerals formed in the seawater, or to the occurrence of biogenic material. Curiously, the drop in quartz abundance between the Guadiana transect and both the Tinto-Odiel and Guadalquivir ones did not affect the REE profiles, whereas usually, a high quartz content dilutes the concentration of REE in sediments.

 

Acknowledgements

The present work was carried out within the framework of the following projects: SIRIA (Situagao de Referencia na Regiao Costeira Algarvia Influenciável pela Barragem de Alqueva); Ambiente e Defesa of the National Ministry of Defense and the Foundation of Portuguese Universities; EMERGE (Estudo Multidisciplinar do Estuário do Rio Guadiana), ODIANA and CRIDA (Consequences of River Discharge Modifications on Coastal Zone and Continental Shelf) (PLE/8/00). The first author thanks Carlos Galhano of the Nova University of Lisbon, the CIACOMAR research team of the University of Algarve, especially Ramón González, and the Foundation of Science and Technology (FCT).

 

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