Artículos

 

Geochemical characterization of sediment cores from the continental shelf off the western rias area (NW Iberian Peninsula)

 

Caracterización geoquímica de núcleos de sedimento de la plataforma continental en el área de las rías occidentales (NW Península Ibérica)

 

Catarina Corredeira¹, M. Fátima Araújo¹, Ângela Gouveia¹ and J.M. Jouanneau²

 

¹ Instituto Tecnológico e Nuclear, Química Estrada Nacional 10, 2686-953, Sacavém, Portugal. * E-mail: catarina@itn.mces.pt

² DGO-UMR 5805 CNRS, Université Bordeaux I Av. des Facultés 33405 Talence Cedex, France.

 

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

 

Abstract

The present work aims to understand the origin of sediments deposited in the Galician coastal zone and continental shelf. Selected sediment cores were studied using different geochemical approaches: grain-size measurements, carbonate determinations and elemental analyses. Chemical analyses were carried out by multielemental techniques: energy dispersive X-ray fluorescence spectrometry (EDXRF) and instrumental neutron activation analysis (INAA). The grain-size distribution demonstrated that samples collected off the rias (Vigo and Pontevedra) contain higher percentages of silt and clay. The carbonate enrichment measured in some sediment samples with different locations and granulometries seems to indicate that the sediments might have distinct origins: biological fluvial productivity in sediments off the rias and marine biogenic activities for the deeper continental shelf sediments. Chemical analyses made it possible to distinguish different elemental sources: lithogenic, anthropogenic and biogenic. The downcore profiles of the elemental composition showed signs of a recent continental contamination for Zn and to a lesser extent for As, although there are no signs of exportation to the adjacent continental shelf. Other elements that can indicate anthropogenic activities were present in low concentrations compared with reference values. The Zr distribution determined in sediments collected along a straight line off Vigo Ria increases westwards, indicating an absence of recent exportation of this element from the continent.

Key words: sedimentary geochemistry, NW Iberian shelf, energy dispersive X-ray fluorescence spectrometry, instrumental neutron activation analysis, heavy metals.

 

Resumen

En el presente trabajo se busca entender el origen de los sedimentos depositados en la zona costera y la plataforma continental de Galicia. Se estudiaron núcleos de sedimentos usando diferentes métodos geoquímicos: medición del tamaño de grano, determinación de carbonato y análisis de elementos. Se realizaron análisis químicos mediante técnicas multielementales: espectrometría de fluorescencia de rayos X por energía dispersiva (EDXRF) y análisis por activación neutrónica instrumental (INAA). La distribución del tamaño de grano mostró que los sedimentos recolectados frente a las rías de Vigo y Pontevedra contenían procentajes mayores de limo y arcilla. El enriquecimiento de carbonato medido en algunas muestras de sedimento de diferentes sitios y su granulometría parecen indicar distintos orígenes de los sedimentos: productividad biológica fluvial para los sedimentos frente a las rías y actividad biogénica marina para los sedimentos de la plataforma continental. Los perfiles de la composición de elementos del núcleo indicaron contaminación continental reciente de Zn y, en menor grado, de As, aunque sin evidencias de exportación a la plataforma continental adyacente. Otros elementos que pueden indicar actividades antropogénicas se encontraron en concentraciones bajas en comparación con valores de referencia. La distribución de Zr en los sedimentos recolectados a lo largo de una línea recta enfrente de la Ría de Vigo aumenta hacia el oeste, lo que indica que este elemento no ha sido exportado recientemente desde el continente.

Palabras clave: geoquímica sedimentaria, plataforma ibérica noroccidental, espectrometría de fluorescencia de rayos X por energía dispersiva, análisis por activación neutrónica instrumental, metales pesados.

 

Introduction

The northern Portuguese coastal zone is characterized by the presence of large rivers draining highly populated and industrialized regions, with different types of lithologies and levels of anthropogenic influence (Araújo et al., 2002). To the north, on the Galician coast, the influence of several rias drastically changes the coastal morphology. This region is characterized by a large sinuosity and the presence of the western Galician rias: Vigo, Pontevedra, Arosa and Muros.

Two major deposits of fine sediments are located along the northwestern shelf. One extends off the Minho River along the Galician rias and the other is located in the middle shelf, north of the Douro River.

Recent published works (Araújo et al., 2002; Jouanneau et al., 2002; Oliveira et al., 2002) indicated that the main source of the fine sediments in the southern deposit was the Douro River. Geochemical studies carried out in different sets of sediments (Douro Estuary, Douro deposit, Minho Estuary and Galicia deposit), evidenced striking similarities between the Douro estuarine samples and the Douro and Galicia fine sediments deposited at the mid-shelf (Araújo et al., 2002). Nevertheless, the chemical composition of the Minho estuarine sediments demonstrates that the Minho sediment load does not have a major contribution to the Galicia mud field. Besides, other studies carried out in a northern area of the Galicia mud field (Rey-Salgado, 1993) showed that nowadays, there are no inputs of sediment from the north towards the Galician shelf. Thus, the integration of these results may indicate that the main source of the fine sediments deposited at the Galician shelf could be located southwards at the Douro river/deposit (Araújo et al., 2002). However, geochemical studies on the Minho/ Galicia system were based just on a few sediment samples collected from a confined region located to the south of the deposit, in front of the Minho Estuary, at the Portuguese shelf.

In the present work we are extending this studies to the north, to the area between the mouth of the Minho River and the Finisterra Cape in the northwestern Iberian coastal zone and shelf. This study aims to investigate the Minho/Galician shelf sedimentary dynamics based on the geochemical characterization of selected sections of sediment cores collected from the Galician rias area up to the mid-continental shelf. Chemical and grain-size analyses were used to determine the distribution of natural and anthropogenic metals (contaminants) in the region. Selected elements of the data set obtained allowed to infer about the origin of the fine deposited sediments and to assess the interactions between continental and marine sediments.

 

Materials and methods

Eleven sediment cores, with lengths ranging from 10 to 235 cm, were collected along the northwestern Iberian shelf (fig. 1) with corers and box-corers (Reineck), during the GAMINEX (June 1998) and Charles Darwin (CD110, December 1997) cruises.

Grain-size analysis and carbonate determinations were carried out on all the samples using a laser diffractometer (Malvern Master Sizer) and a Bernard calcimeter, respectively (Gaulot, 1999).

The uppermost levels of each core (approximately the first 25 cm) were dried at 60-80°C, and then ground to obtain a grain-size fraction smaller than 63 µm. The homogenized ground material was dried at 110°C for 24 h. Fractions between 0.2 and 0.3 g from all samples were weighed into clean polyethylene vials for instrumental neutron activation analysis (INAA). To determine specific elements (e.g., Cu and Pb), selected fragments were also analyzed by energy-dispersive X-ray fluorescence spectrometry (EDXRF). In this case, 2 g of homogenized ground sediment were mixed with an organic binder and pressed into pellets.

EDXRF measurements (Araújo et al., 2002) were made by using a Kevex EDX-771 Analyst System, with a rotating 16-position sample tray. A computer running the WinXRF/ ToolBox applications software, which commands the X-ray generator, the detection system and the multichannel analyzer, controls the spectrometer. Samples of standard reference materials were prepared to calibrate the spectrometer and to determine the accuracy and precision of the analytical procedure: SRM1646 Estuarine Sediment and SRM 2704 River Sediment from the US National Institute of Standards and Technology (NIST). Accuracy is usually better than 5% except for the elements that present strong spectral interference (e.g., As and Pb). Precision is better than 10% for most of the elements, the major differences deriving from counting statistics, for low elemental content or peak overlap.

INAA analyses were carried out via irradiation in the core grid of a research reactor. A y-ray spectrometer, consisting of a 150 cm³ coaxial Ge detector and a low-energy photon detector (LEPD) connected through Canberra 2020 amplifiers to an Acuspec B (Canberra) multichannel analyzer, was used. The analytical procedure, accuracy and precision values were identical to those described in Gouveia and Prudencio (2000). The certified values used were those presented in Govindaraju (1994). The accuracy was satisfactory. The error is lower than 5% for all the chemical elements determined. The precision has an error of (a) <3% for K and Fe; (b) between 3% and 5% for Zn, Hf and Th; (c) between 5% and 10% for Sc, Cr, Co, Rb, Cs, Ta and U; and (d) approximately 12% for Ba and 13% for Sb.

 

Results and discussion

Grain size and carbonate distribution

The size, collection depth and results of the mean grain-size distribution and carbonate content of the sediment cores sampled are listed in table 1. The grain-size analysis of the samples indicates that sediments are mainly composed of silt and clay, with the exception of cores collected at the western locations: KIGX 02, KIGX 05 and S300 (table 1, fig. 1). Cores collected at the mouth of the Pontevedra and Vigo rias (KIGX 11 and KSGX 24, at depths of 49 and 39 m, respectively) exhibit high percentages of the silt/clay grain-size fraction. Furthermore, the same cores have the highest carbonate concentration values: KIGX 11 = 26.1% and KSGX 24 = 13.4%. Previously, in studies focused on the northwestern Portuguese shelf (Araújo et al., 2002), the carbonate (or calcium) enrichment had been directly associated with coarse grain-size particles of biogenic origin, particularly in sediments collected at higher depths. Cores collected at the Minho/Galicia shelf (KIGX 05, KIGX 16, S300 and S130), with lower contents of fine sediments, also present high carbonate contents (1012.8%). This opposite behaviour might be an indication of the different origin of the two sediment groups. Carbonates in sediments collected at the mouth of the rias are probably the result of high biological productivity, due to upwelling processes that occur in this region (Prego, 1993). In previous studies carried out at the Vigo Ria, the carbonated enriched materials were concentrated near the mouth, falling rapidly into the inner ria and more slowly offshore (Rubio et al., 2000). For the coarser sediments collected at the mid-shelf (between 126 and 213 m depth), the carbonate enrichment is probably of a marine bio-genic origin, similar to that observed in sediments collected farther south.

Elemental distribution

The minimum and maximum concentration values determined in each core of some lithogenic (K and Zr) and anthropogenic (Cu, Zn, As and Pb) elements are listed in table 2. In general, concentration values for the lithogenic elements are comparable to those published in Salomons and Forstner (1984), except for the case of Zr. The Zr contents in the shelf sediments are very high (max. = 1176 mg kg^-1; reference value = 160 mg kg^-1), probably because of the presence of heavy minerals like zircon, frequently accumulated in the coarser grain-size fractions. At the Portuguese shelf, coarse sediments enriched in Zr are usually located in confined areas and concentrated in the coastal zone, indicating in this case, a continental contribution and a mineral fractionation by faster settling (Araújo et al. 2002). In the present study, the Zr spatial distribution along a straight line on the continental shelf off the Vigo Ria, shows a consistent increase in the Zr concentration westwards, directly correlated (R² = 0.80) with the grain size (fig. 2). This behaviour suggests that sediments have not been exported from the Vigo Ria recently. The origin and distribution of zircon minerals along the Iberian shelf has been a subject of discussion in the past. In a comprehensive study on the sedimentary dynamics of the Portuguese shelf, Dias (1987) attributed the zircon distribution to relict sediments, disseminated along the northern Portuguese shelf. Also, according to Cascalho and Carvalho (1993), defined areas of zircon-enriched sediments are widely spread along the northern shelf.

The spatial distribution of the elements that may have anthropogenic origin indicates enrichment in Zn, and to a lesser extent in As and Pb, exclusively for the sediments collected at the mouth of the rias. Particularly, sediments at the mouth of the Vigo Ria present higher concentrations values for Zn, reaching 225 mg kg^-1 (KSGX 24) in the surface layers of the core. This result indicates a recent anthropogenic contamination in the region, probably as a result of urban wastes and industrial effluents because of the presence of naval construction works and several industries settled in a highly-populated region. Geochemical studies (Prego and Cobelo-García, 2003) of sediments collected along the rias have also reported heavy metal contamination. Sediment composition from similar locations was reported by Vilas et al. (1995, in Prego and Cobelo-García, 2003) as having a maximum value of 201 mg kg^-1 for Zn in the outer area of the Vigo Ria. In the middle of this ria, the heavy industrial activity strongly affects the port zone and a high Zn contamination was observed (max. = 567 mg kg^-1), indicating that the industries are probably the main reason for the high Zn concentration values observed at the mouth of the Vigo Ria.

The shelf sediments do not exhibit any contamination in Cu, Zn, As and Pb (even those collected in the coastal area adjacent to the Vigo Ria: KIGX 27 and S130), since their concentrations are systematically lower than those reported by Prego and Cobelo-García (2003) as natural reference values for Galician marine sediments. These values suggest the absence of any direct influence of the adjacent continental area on the marine sediments.

In summary, the geochemical characterization of the sediment cores studied indicates that sediments deposited at the northern Iberian shelf have three different sources: lithogenic, anthropogenic and biogenic. The sediments collected at the mouth of the Vigo and Pontevedra rias show distinct geochem-ical characteristics, namely, high carbonate contents due to the high biological productivity, and heavy metal contamination (Zn, As and Pb) as a consequence of human activities (naval construction works, industries and urban wastes). Nevertheless, heavy metal contents in sediments collected at the shelf were within the background values. The gradient of the Zr and grain-size distributions observed in samples collected at the mouth and off the Vigo Ria, together with the heavy metal distribution on shelf sediments, clearly indicates that the Galician rias do not behave as a sediment source for the continental shelf. In general, the chemical composition (lithogenic and anthropogenic elements) of sediments collected from the mid-shelf is comparable to the composition determined in sediments collected along the NW Portuguese shelf.

 

Acknowledgements

The authors thank the scientific teams responsible for the sampling operations developed during the EU OMEX II-II program (MAST3-CT97-0076). This work was partially supported by a FCT (Portugal) research grant to the CRIDA project: Consequences of River Discharge Modifications on Coastal Zone and Continental Shelf (PLE/8/00).

 

References

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