Sedimentary and geochemical characterization and provenance of the Portuguese continental shelf soft-bottom sediments

Abstract

Grain-size and geochemical analyses for soft-bottom sediments from the Portuguese continental shelf are used to determine their provenance and to identify the main factors governing the sediment distribution patterns.

The deeper areas of the northwestern and central sectors of the Portuguese shelf and most of its southwestern sector are covered by fine and very fine sands. Coarse deposits are found in the inner and middle shelf of the northernmost sector and immediately south of the Nazaré and Setúbal canyons, while muddy sediments are restricted to areas located off the mouths of major rivers (Douro and Tagus mud patches). In contrast, the southern shelf corresponds to a low energy environment receiving most of its sediment supply from the Guadiana River. As a result, an expressive mud patch is developed off the Guadiana estuarine system.

The application of multivariate analysis to geochemical data allowed the classification of the studied samples into four main groups: (a) a group of mud deposits constituted by detrital clay minerals and authigenic iron-manganese oxyhydroxides, displaying high contents for organic matter and for most major and trace elements; (b) a group of biogenic carbonate-rich sands showing high CaO, Sr and U contents which is well represented throughout the entire Portuguese shelf; (c) a group of quart-rich sediments, with very high SiO₂ contents and moderate Al₂O₃, K₂O and TiO₂ contents, supporting a major terrigenous contribution and (d) a group of coarse sand samples with high As levels from off important rivers, pointing to anthropogenic contamination.

Trace metal enrichments observed in the muddy areas from both the western and southern coasts appear to result from natural causes and do not exceed the international sediment quality thresholds. There is, however, local evidence for As, Zn, Cr and Pb anthropogenic contamination in near-shore sediments associated with rivers draining regions characterized by urban, industrial or mine exploration activities, highlighting the potential environmental risk associated with such sites.

Introduction

Continental shelf systems have high economic and ecological importance and are the major sinks for sediments exported from land. Numerous recent investigations have shown that geochemical composition of modern continental shelf sediments depends on geological, biological and climatic factors and can be used to trace their origin and the environmental changes induced by natural and anthropogenic factors (Alagarsamy and Zhang, 2010, Borrego et al., 2004, García et al., 2004, Leblanc et al., 2000, Lin et al., 2002, Preda and Cox, 2005, Sánchez-García et al., 2010).

In most soft-bottom marine sediments, it is possible to recognize three different types of source components: (a) a lithogenic component, essentially composed by detrital particles derived from weathering of continental rocks; (b) a biogenic component consisting of skeletal remains and (c) a hydrogenous or authigenic component (clays, ferro-manganese oxyhydroxides), directly precipitated from seawater or produced by the reaction of sediment particles with seawater or through microbial activity (Schulz and Zabel, 2006 and references therein). The relative contribution of these inputs for shelf marine sediments is the dominant factor controlling their bulk chemical composition, which can therefore provide valuable insights into the mechanisms involved in sediment formation, transport, dispersal and deposition patterns, hydrodynamic regimes and lithology of the adjacent land areas (Abrantes and Rocha, 2007, Corredeira et al., 2009, Daesslé et al., 2004, Delgado et al., 2010, Jouanneau et al., 2008, Karageorgis et al., 2005, Liaghati et al., 2004, Machado et al., 2005, Nobi et al., 2010, Preda and Cox, 2005, Rubio et al., 2000, Sánchez-García et al., 2010, Stevenson, 2001). Recent studies have also shown that, in addition to natural processes, the input of contaminants, via industrial, mining and/or domestic wastes, can increase the concentration of toxic heavy metals in river sand causing an important land to sea transfer of contamination in marine and estuarine areas (Gonzalez et al., 2007, Jesus et al., 2010, Karageorgis et al., 2005, Mil-Homens et al., 2009, Nobi et al., 2010, Radakovitch et al., 2008, Roussiez et al., 2006). The calculation of enrichment factors (EFs) provides a powerful tool for assessing the impact of anthropogenic activities in marine sediments and the environmental quality of continental shelves (Alagarsamy and Zhang, 2010, Delgado et al., 2010, Liaghati et al., 2004, Mil-Homens et al., 2007, Nobi et al., 2010, Sánchez-García et al., 2010).

The main purpose of this study is to characterize the surficial shelf sediments of the Portuguese Continental Shelf in terms of grain-size and bulk geochemistry and to identify the main factors governing their distribution patterns.