Sedimentary and geochemical characterization and provenance of the Portuguese continental shelf soft-bottom sediments
Highlights
► Portuguese shelf sediments are highly heterogeneous in terms of grain size. ► The observed geochemical patterns document detrital, biogenic and authigenic inputs. ► Trace metal enrichments are particularly high in muddy samples. ► Some near-shore sediments show evidence of potential anthropogenic contamination.
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.
Section snippets
Physiography of studied area
The Atlantic Iberian continental shelf extends from the Gulf of Cadiz to the Galicia Bank for approximately 900 km in length, with an average width of about 45 km and an irregular steep slope plunging to the abyssal plain (Fig. 1). Shelf-break slope occurs approximately at 160 m depth. Reviews of the main physiographic and geomorphological features of the Portuguese part of the West Iberian Margin can be found in Vanney and Mougenot, 1981, Mougenot, 1989.
The Portuguese shelf has been divided into
Sampling
Sampling campaigns were conducted in 2007 and 2008. Samples of soft bottom shelf sediment were collected, with a 0.1 m2 Smith-McIntyre grab, along transects perpendicular to the coastline (latitudes between 41°51.8′N to 36°50.1′N and longitude 9°43.1′W to 7°24.7′W; Fig. 1). Water depth ranged from 13 to 180 m. The whole set of samples (142) was analyzed for grain-size and total organic matter and a subset of 21 samples was subsequently selected for geochemical analysis.
Analytical techniques
Grain-size analysis was
Grain-size distribution and total organic matter content
The spatial distribution of the surface sediments of the Portuguese continental shelf is shown in Fig. 1. The coarser deposits (ranging from gravel to coarse sand) occur mainly in the inner and mid-shelf of the northwestern sector, at depths between 20 and 80 m and immediately south of the Nazaré and Setúbal canyons. Fine and very fine sands are found along a continuous band in the near shore shelf and in the outer shelf of the northwestern sector and constitute the dominant sediment type in the
Shelf sediments geochemical signature
The results obtained in the scope of the present investigation reveal that the Portuguese shelf sediments are composed by variable proportions of three main components (terrigenous, biogenic and authigenic).
Conclusions
The grain size distribution and geochemical signature of the Portuguese shelf sediments are highly complex, reflecting strong differences in the sources (natural and anthropogenic), fluvial input, shelf morphology, hydrodynamism and biological activity.
Due to the high energetic hydrodynamic regime and large fluvial sediment supply, the northwestern and central sectors display a general seawards decrease in grain size, with discontinuous coarse relict clastic deposits dominating in the inner and
Acknowledgments
The first author benefited from a PhD grant (SFRH/BD/44231/2008) awarded by the Portuguese Foundation for Science and Technology (FCT — Fundação para a Ciência e Tecnologia). This work was supported by the research projects ACOSHELF (POCI/MAR/56441/2004–PPCDT/MAR/56441/2004) and MeshAtlantic (Atlantic Area Program 2009-1/110 MeshAtlantic). Cristina Sequeira is acknowledged by technical assistance with the geochemical analyses and Rui Marques in the sampling campaigns. Valuable comments from two
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2019, Estuarine, Coastal and Shelf ScienceCitation Excerpt :Nevertheless, sedimentological characteristics including grain size distribution and their statistics (e.g: mean, sorting) are widely used to evaluate the sediment distribution and movements with respect to direction, volume, and magnitude of governing forces (Gao and Collins, 1994; Wang et al., 2010). Further, geochemical studies of major oxides and trace elements also have been used to gain an understanding of sediment distribution, their value as indicators of mineral suites, provenance and processes (Taylor and McLennan, 1985; Cullers et al., 1988; McLennan et al., 1993; Garzanti and Andò, 2007; Garzanti et al., 2009; Martins et al., 2012; Botsou et al., 2015). Therefore, a combined study of sedimentological and geochemical characteristics of recent sediments are important in new findings on amount and pattern of sediment supply, the chemical value of sediments, climatic influences, and depositional environmental changes of coastal environments (Panagiotaras et al., 2012; Affouri et al., 2017; Moreira et al., 2018).