Records of sedimentary dynamics in the continental shelf and upper slope between Aveiro–Espinho (N Portugal)

Journal of Marine Systems 96–97 (2012) 48–60

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Records of sedimentary dynamics in the continental shelf and upper slope between Aveiro–Espinho (N Portugal) Virgínia Martins a, b,⁎, Isabel Abrantes c, Carlos Grangeia a, Paula Martins a, Renata Nagai d, Sílvia H.M. Sousa d, Lazaro L.M. Laut e, João M. Alveirinho Dias f, João M. Dias g, Eduardo Ferreira da Silva a, Fernando Rocha a a

Aveiro University, Geosciences Department, GeoBioTec Research Centre, Portugal Aveiro University, Chemistry Department CESAM Research Centre, Portugal c ESEV Polytechnic Institute of Viseu, 3504-504 Viseu, Portugal d Oceanographic Institute, São Paulo University, Brazil e Natural Science Department, Federal University of the State of Rio de Janeiro, UNIRIO, Brazil f Algarve University, Campus de Gambelas, Faro, Portugal g CESAM, University of Aveiro, Department of Physics, Aveiro, Portugal b

a r t i c l e

i n f o

Article history: Received 31 October 2010 Received in revised form 25 November 2011 Accepted 3 February 2012 Available online 12 February 2012 Keywords: Multiproxies Fluvial input Sediment alongshore and cross-shelf transport processes Sediment sources and sinks Portuguese continental coast

a b s t r a c t The sedimentary unconsolidated cover of the Aveiro–Espinho continental shelf and upper slope (NW Portugal) records a complex interplay of processes including wave energy and currents, fluvial input, sediment transport alongshore and cross-shelf, geological and oceanographic processes and sediment sources and sinks. In order to study this record, a set of surface sediment samples was studied. Sediment grain size and composition, as well as the mineralogical composition (by XRD) of the fine (b 63 μm) and clay (b 2 μm) fractions and benthic microfaunal (foraminifera) data were analysed. Cluster analysis applied to the sedimentological data (grain size, sediment composition and mineralogy) allowed the establishment of three main zones corresponding to the: inner-, mid- and outer-shelf/upper slope. On the inner-shelf, the sedimentary coverture is composed of siliciclastic fine to very fine sand, essentially comprising modern (immature) terrigenous particles. The sediment grain size, as well as mineralogical and microfaunal composition, denote the high energetic conditions of this sector in which the alongshore transport of sand is predominantly southward and occurs mostly during the spring–summer oceanographic regime, when the main river providing sediments to this area, the River Douro, undergoes periods of drought. This effect may emphasize the erosive character of this coastal sector at present, since the Ria de Aveiro provides the shelf with few sediments. On the mid-shelf, an alongshore siliciclastic band of coarse sand and gravel can be found between the 40 m and 60 m isobaths. This gravelly deposit includes relic sediments deposited during lower sea-level stands. This structure stays on the surface due to the high bottom energy, which promotes the remobilization of the fine-grained sediments, and/or events of sediments bypassing. Benthic foraminifera density and “Benthic Foraminifera High Productivity” (BFHP) proxy values are in general low, which is consistent with the overall small supply of organic matter to the oceanic bottom in the innerand mid-shelf. However, the Ria de Aveiro outflow, which delivers organic matter to the shelf, leaves its imprint mainly on the mid-shelf, identifiable by the increase in foraminifera density and BFHP values in front of the lagoon mouth. The higher values of BFHP along the 100 m isobath trace the present position of an oceanic thermal front whose situation may have changed in the last 3/5 ka BP. This zone marks a clear difference in the density, diversity and composition of benthic foraminifera assemblages. Here, in addition, sediment composition changes significantly, giving rise to carbonate-rich fine to medium sand in the deeper sector. The low bottom energy and the small sedimentation rate of the outer-shelf contributed to the preservation of a discontinuous carbonate-rich gravel band, between the 100 m and 140 m isobaths, also related to paleo-littorals, following the transgression that has occurred since the Last Glacial Maximum. The winter oceanographic regime favours the transport of fine grained sediments to the outer-shelf and upper slope. The inner- and mid-shelf, however, have low amounts of this kind of sediment and the Cretacic carbonated complexes Pontal da Galega and Pontal da Cartola, rocky outcrops located at the mid- and outershelf, act as morphological barriers to the cross-shelf transport of sediments. Thus a reduced sedimentation

⁎ Corresponding author at: Aveiro University, Geosciences Department, GeoBioTec Research Centre, Portugal. Tel.: + 351 96 619 27 64. E-mail address: [email protected] (V. Martins). 0924-7963/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jmarsys.2012.02.001

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rate occurs in these deeper sectors, as indicated by the lower abundance of detrital minerals, which is compensated for the high sedimentary content of biogenic carbonates. The relatively high BFHP and Shannon Index values indicate water column stratification, high supply of organic matter and environmental stability, which provide favourable conditions for a diversified benthic fauna to flourish. These conditions also encourage authigenic chemical changes, favourable to glauconite formation, as well as illite and kaolinite degradation. Benthic foraminifera and clay mineral assemblages also reveal the effect of the internal waves pushing upward, and downslope losses of the sediments on the outer-shelf and upper slope. © 2012 Elsevier B.V. All rights reserved.

the aim of identifying the influence of present and past oceanographic/ climatic processes in shaping the characteristics of the surface sedimentary coverture of the continental shelf and upper slope from Aveiro to Espinho. The continental shelf between Aveiro and Espinho (41°N-40°31′ N), ranging from 38 km to 50 km in width, is characterised by a gentle sloping surface and bathymetric lines roughly parallel to the coast line (Fig. 1). It is delimited by the shelf-break at depths of around 160 m. The monotonous slope is interrupted by some geomorphological features, such as the Aveiro Canyon, indenting the shelf-break from a depth of 130 m, and rocky outcrops known as Pontal da Cartola and Pontal da Galega, which represent Cretacic carbonated complexes (Vanney and Mougenot, 1981).

1. Introduction

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In recent years various contributions have been made to our understanding of the current sedimentary dynamics on the North Iberian Continental Margin, which is characterized by a highly energetic hydrodynamic wave and tidal regime (e.g. Abrantes, 2005; Dias, 1987; Dias and Nittrouer, 1984; Dias et al., 2002a,b; Drago et al., 1998; Jouanneau et al., 2002; Magalhães, 1999; Oliveira et al., 2002; Thomsen et al., 2002; van Weering and McCave, 2002). However, the oceanographic/ climatic influence on the surface sedimentary coverture of the continental shelf and upper slope is still today poorly understood. As such, this work intends to contribute to an integrated analysis of sedimentological (textural, compositional, and mineralogical) and microfaunal data with

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Fig. 1. Location of the studied area. The sites where the surface sediment samples were collected are marked in the lower map (adapted from Vanney and Mougenot, 1981), where bathymetric curves as well as the rocky outcrops (the shaded areas) are also shown.

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V. Martins et al. / Journal of Marine Systems 96–97 (2012) 48–60

A brackish lagoon named “Ria de Aveiro”, which is separated from the coast by a sand bar and has a general NNE–SSW orientation, constitutes an important feature of this coastal area. The lagoon, which actually has its sand bar partially eroded by the sea, is a typical barbuilt estuary characterised by many branching fluvial channels which originated in, and evolved rapidly during, the Holocene (Rodrigues and Dias, 1989). Nowadays the lagoon covers an area of 66 km 2 at low tide, rising to 83 km 2 at high tide (Dias et al., 2000a). It receives fresh water mainly from the Vouga and Antuã rivers and exchanges water with the Atlantic through an artificial inlet. According to previous studies (Abrantes et al., 2006; Cunha et al., 2003; Lopes et al., 2008; Pato et al., 2008; Picado et al., 2010), this lagoon exports properties (including sediments) to the adjacent ocean, therefore influencing the local coastal dynamics. A great number of forcing mechanisms play an important role in the area under study, examples of which include nearshore processes, density gradients due to the lagoon discharges, oceanic fronts and shelf/slope exchanges, winds and topographical effects (Aveiro Canyon, Pontal da Cartola and Pontal da Galega reliefs; Fig. 1). The relative importance of some of these agents varies over a short time scale (Peliz et al., 2002). On the Iberian west coast the waves and tides are very energetic. Swells from the NW are dominant while W swells are also important (Costa, 1994). The average height of the waves varies seasonally (Vitorino et al., 2002a,b). In summer, the significant wave heights are 1–3 m with periods smaller than 10 s. During winter storms the waves' height often exceeds 7 m, with typical periods of 13 s, sometimes reaching 18 s (Costa, 1994; Vitorino et al., 2002a,b). During storms, the majority of waves reach heights of 6.5 m, but every 3–4 years more intense storms occur, causing waves of 9–12 m in height (Pita and Santos, 1989). According to these authors, the Portuguese coast is exposed to three storms per year on average, each taking about 4 days. The tide in the study area is semidiurnal: M2 is the main tidal harmonic constituent with an amplitude of around 1 m, followed by the S2 constituent, with an amplitude 30% lower (Marta-Almeida and Dubert, 2006; Sauvaget et al., 2000). The intensity of the semidiurnal tidal current is around 1–2 cm/s (Marta-Almeida and Dubert, 2006), with a pattern that is characterized by tidal ellipses with a parallel alignment to the coast in the deep sea (mostly longitudinal currents), suffering a spin on the path to the continental shelf (between the 1000 m and 200 m isobaths) and becoming almost transversal near the coast. The whole study area is essentially affected by the Portugal Current system, showing a dominant southerly flow with an average speed of about 1.6 cm/s (Martins et al., 2002; Perez et al., 2001), which induces southward sediment transport. However, the oceanic circulation and hydrology on the continental margin off Aveiro–Espinho exhibit strong seasonality, influenced as they are by the wind regime which is forced by the Azores anticyclone and the Iceland low pressure centre (Fiúza et al., 1982). Along the coast, northerly winds induce upwelling processes, which are more frequent and intense between the months of March/April and September/October (Bakun and Nelson, 1991; McClain et al., 1986; Wooster et al., 1976). During the upwelling events, cold and nutrient-rich Eastern North Atlantic Central Water (ENACW) rises on the shelf (Fiúza, 1983; Fiúza et al., 1982; Fraga, 1981). As a consequence, the upwelling induces a strong southward current (Portugal Coastal Current) along the edge of the continental shelf of the study area, with maximum speeds of approximately 40 cm/s (Peliz et al., 2002). The upwelling in this area is a very complex system, influencing the along-shore and cross-shore distribution of physical–chemical properties (Peliz et al., 2002). In winter, storm winds, which blow predominantly SW, induce an Ekman transport towards the coast and the development of downwelling events (e.g. Fiúza et al., 1982; Vitorino et al., 2002a,b). Consequently, the oceanic currents on the shelf flow north (Portugal Coastal Countercurrent), with an average

speed ranging from 0.1 to 0.3 m.s − 1 (Huthnance et al., 2002). These conditions result in a northward transport of sediments. Studies carried out by different authors (Abrantes et al., 1994; Cascalho, 2000; Dias, 1987; Dias and Nittrouer, 1984; Magalhães, 1999; Monteiro et al., 1982), focusing on surface sediment samples collected on the Portuguese Continental Shelf (including this area or nearby areas) identically characterize the main surface sedimentary deposits. The sedimentary coverture of the Iberian Margin is a consequence of complex oceanographic processes acting on the interface between the continent and the deep ocean and, in some places, the sedimentary column behaves as a climate archive (e.g. Bernárdez et al., 2008; Desprat et al., 2003; Naughton et al., 2007). Since climate fluctuations can affect the oceanographic processes and influence the sediment grain size and composition, as well as the foraminiferal assemblages (Bartels-Jónsdóttir et al., 2006 ; Diz et al., 2002; GonzálezÁlvarez et al., 2005; Martins et al., 2006a,b, 2007), this kind of data provides a significant contribution to our understanding of the record of oceanographic processes acting on the sedimentary dynamic. 2. Materials and methods A set of 98 surface sediment grab-samples from the continental shelf and upper slope region off Aveiro–Espinho (between 40°31′N and 40°58′N parallels; water depth from 10 to 700 m; Fig. 1) were studied. These sediments were collected in 1993 with a Smith-McIntyre grab by the Portuguese Hydrographical Institute under the program SEPLAT. 2.1. Sedimentological analyses For the grain size analysis, the dried sediment samples were homogenized and a portion of about 150–250 g was analysed. Fine fractions (b63 μm; silt plus clay particles) were separated by wet sieving through a 63 μm screen. The fractions b63 μm were integrally preserved, dried and weighed. The sedimentary fraction >63 μm was dry sieved through a battery of sieves 1 phi spaced. Each sediment fraction was expressed as a percentage of the total sediment dry weight. The sediment compositional analysis was performed by counting the number, per gram, of terrigenous, authigenic and biogenic particles of the sedimentary fraction > 63 μm using a light binocular microscope. The number per gram of old foraminifera (reworked and yellowish tests, filled with authigenic deposits and chemical precipitates) was also counted. These tests were named in this work as fossils of foraminifera. The mineralogical composition of the sediments was analysed in selected sediment fractions, namely in silt (b63 μm) and clay (b2 μm), by X-ray diffraction (XRD) techniques (CuKα-radiation). The mineralogical composition was determined both on non-oriented powder mounts, for the silt fraction, and on oriented aggregates for the clay fraction. The clay fractions were separated by sedimentation according to Stokes law, using 1% sodium hexametaphosphate solution to avoid flocculation. Semiquantitative determinations followed the method described in Martins et al. (2007). Further studies were carried out on difractograms based on XRD techniques of the clay fraction, in particular the determination of the crystallinity indexes of illite (Kübler index) and kaolinite. The Esquevin index also was determined (Esquevin, 1969). The Kübler index of illite “crystallinity” was determined, which measures changes in the shape of the first basal reflection of dioctahedral illite-muscovite at an XRD spacing of approximately 10 Å. According to Kübler (1967), this is based on the measurement of the width (in mm), at half the height, of the reflection (001) 10 Å made in the XRD patterns with the mounting effected in the natural state. The width of the reflection is related to the structural regularity, with a high illite crystallinity likely to display a symmetric and narrow reflection, unlike a low crystallinity which presents a large

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and irregular reflection. Thus the value of the index increases as the crystallinity of illite decreases. As the crystallinity of illite depends on the chemical composition, Esquevin (1969) proposed the use of the ratio of reflection intensities (002) to 5 Å and (001) to 10 Å, determined in a natural aggregate, to provide an initial estimate of the content of Al2O3/FeO + MgO in the octahedral layer of illite. Thus, the values of this ratio can be correlated with the different compositions of the solid phase of the octahedral sheet of the biotite, phengite, and muscovite or of the mixture of biotite + muscovite. The Esquevin index generally has ratios above 0.3 in aluminous illite, whereas in magnesium and ferriferous illite the values are typically below 0.3. The Kübler index combined with that of Esquevin allows the analysis of the evolution of illite crystallinity and its composition considering the degree of weathering and diagenesis. The estimation of the kaolinite crystallinity, which follows the ratio between the width measured at half-height of the (001) 7 Å peak and the height of this peak, in air-dry aggregates, was used, after the decomposition of chlorite and kaolinite 7 Å peaks (Aparicio and Galan, 1999; Oliveira et al., 2002). The ratios obtained are greater for the higher crystallinity of kaolinite. Clay minerals were used in this work since they are excellent tools for understanding sedimentary processes. They reproduce the climatic and geological characteristics of the continental source area, as well as the hydrodynamic conditions of the continental margin (Biscaye, 1965; Chamley, 1989; Chauhan and Gujar, 1996; Gingele and De Deckker, 2004; Gingele et al., 2001; Oliveira et al., 2002). 2.2. Foraminifera data The density of foraminifera tests was determined by the number of tests in the sedimentary fraction 63–1000 μm. A particular sediment fraction was chosen to avoid the overestimation of foraminifera density when the sediment is very fine, or under-estimation when the sediment is very coarse (Martins et al., 2010, 2011a,b). This particular choice of sedimentary fraction was considered suitable as the foraminifera adults typically have a similar dimension in the study area. To ideally characterize the composition of dead assemblages of benthic foraminifera, more than 300 (Fatela and Taborda, 2002) well preserved specimens were identified and quantified in the dried residue of the sand fraction (63–1000 μm) using a stereoscopic microscope. The suprageneric identification of benthic foraminifera was based on Loeblich and Tappan (1988). Species identification was based on the references cited by Martins and Gomes (2004). Samples with a small number (less than 100 tests) of foraminifera were not considered in statistical studies. Changes in species diversity were evaluated by the application of the Shannon-index (Shannon, 1948): H = −Σpi.ln(pi), where pi is the proportion of each species. Foraminifera are a useful tool to discriminate between environmental changes such as temperature and salinity variations, bottom currents and organic carbon flux (Martins et al., 2007; Murray, 1991; Sen Gupta, 1999). “Benthic Foraminifera High Productivity” proxy (BFHP) was applied according to Martins et al. (2006a,b, 2007) and Nagai et al. (2009). BFHP is based on the total percentage of species/taxa related to a high and sustainable supply of organic matter (e.g. Bernhard et al., 1997; Sen Gupta and Machain-Castillo, 1993), such as Bolivina/Brizalina spp., Cassidulina carinata, Stainforthia spp., Uvigerina spp., Valvulineria bradyana, Bulimina spp., Buliminella subfusiformis var. tenuata and Chilostomella ovoidea. 2.3. Statistical analysis Univariate and multivariate statistical analysis were applied, using Statistical@ package 7. Some data were subjected to cluster analysis: i) the main species of benthic foraminifera (with relative percentage ≥ 5% and with at least 3 occurrences in the studied

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samples), the most representative groups of species and Shannon index values, by applying the Pearson Correlation and ‘complete linkage’ method for data agglutination; ii) the studied stations were clustered by using the Euclidean distances and Ward's method on a standardized data matrix, based upon compositional, mineralogical and sediment grain size data for the sediments. Textural, compositional and mineralogical variables were also subjected to principal component analysis. 3. Results 3.1. Sediments texture, composition and mineralogy The surface sediment mean grain size varies between 0.05 and 3.75 mm in the study area (Fig. 2a). Superficial sediment cover for this region is dominated by sand particles (average 78%). Fine fraction content (b63 μm) is generally low [average 5% (3% silt + 2% clay)], increasing across the study area in a seaward direction (Fig. 2b). Compositional analysis of the sedimentary fraction >63 μm revealed that quartz is the predominant mineral on the inner- and mid-shelf, primarily related to the gravel deposit. Biogenic carbonated particles, including mollusc's bioclasts and foraminifera tests increase on the outer-shelf and upper slope (Fig. 2c). The abundance of biogenic particles is smaller in the sediments of the mid-shelf gravel deposits, around the ~50 m isobath. Benthic foraminiferal abundance (no. ind./gram) is highly variable in the study area, ranging from 0 to 46,500 specimens per gram of sand fraction. The abundance of fossilized foraminifera increase significantly along the outer-shelf, shelf-break and upper slope. Glauconite particles, including sedimentary grains and bioclast infill deriving mostly from glauconitized foraminifers, were also found in increased numbers on the outer-shelf and upper slope sector (Fig. 2d). The fine fraction (b63 μm) mineralogy consists mainly of calcite (max. 82%; aver. 33%), followed by quartz (max. 44%; aver. 26%), phyllosilicates (mica/illite, kaolinite and chlorite; max. 42%; aver. 16%), K-feldspar (max. 31%; aver. 8%), and plagioclase (max.17%; aver.8%). Other minerals were identified but in lower percentages, such as dolomite (max.33%; aver.4%), pyrite (max.4%; aver.1%), opal C/CT (max.12%; aver.3%) and aragonite (max.5%; aver.1%). Zeolites and amphibole minerals are very scarce in the study area. The distribution maps show that quartz, phyllosilicates and K-feldspars decrease westwards, occurring in higher percentages on the inner- and midcontinental shelf, whereas calcite presents an inverse pattern (Fig. 3a– d). The relative abundance of K-feldspars, as well as plagioclase, decreases westwards more abruptly than quartz (Fig. 3a, c). Dolomite is mainly found on the outer-shelf, near the dolomitic rocky outcrops. An increased occurrence of carbonates/detrital ratio exists on the outer-shelf and upper slope, expressing the dichotomy between the terrigenous transport and the biogenic component. The highest percentages of phyllosilicates were found in the shallower stations of the inner-shelf (Fig. 3b). The clay minerals identified in the study area were: illite, which was predominant (max.70%; aver.60%), followed by kaolinite (max.40%; aver.30%), smectite (b20%; aver.4%), chlorite (max.9%; aver.5%) and irregular mixed-layers of illite-chlorite and illite-smectite (b3%). Higher contents of illite occur mainly along the inner- and mid-shelf up to the 100 m isobath. Illite particles display lower crystallinity (higher values of the Kübler index) moving seawards, mainly at depths greater than 100 m (Fig. 4). Esquevin index values that vary between 0.3 and 0.9 attain higher values more frequently in the outer-shelf and upper slope sectors. The chlorite sediment content is higher along the inner-shelf up to the 50 m isobath (Fig. 4). Smectite content is in general lower on the inner shelf and tends to increase moving seaward (Fig. 4). The distribution pattern of kaolinite is more complex and heterogeneous (Fig. 4). Higher values for the kaolinite crystallinity index are observed on the inner- and mid-shelf, decreasing as you move seaward (Fig. 4).

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Fig. 2. Distribution maps for: (a) sediment mean grain size (mm), (b) silt-clay fraction content (%) in the studied area, (c) the logarithm of number per gram of biogenic carbonates, (d) glauconite particles in the sedimentary fraction 63–1000 μm, (e) Shannon index and (f) BFHP values. The range of values is presented in each map.

3.1.1. Statistical analysis of the sedimentary data 3.1.1.1. Principal Components Analysis (PCA). Textural, compositional and mineralogical data are included in Appendix 1. PCA analysis was applied with the aim of identifying patterns and highlighting similarities and differences in the textural, compositional and mineralogical data. The results of the extracted factors 1 and 2, which explain about 55% of the data variability, are presented in Fig. 5. Three main groups of variables can be considered. The groups 1 and 2 include variables with opposite trends, separating two main sectors of the study area: the inner- plus mid-shelf zone and the outer-shelf plus upper slope region. Group 1 includes carbonates, dolomite, smectite and Kübler index values. Group 2 is composed of quartz, K-

feldspars, plagioclase, phyllosilicates, illite and chlorite, which are coincidentally the variables that reach higher values on the innerand mid-shelf. Group 3 contains silt fraction, density of foraminifera, fossils of foraminifera and glauconite. Variables included in groups 1 and 3 attain higher values in the deeper sectors of the study area. 3.1.1.2. Cluster analysis in Q-mode. The standardized data matrix of sedimentological data (grain size, sediment composition and mineralogy, presented in Appendix 1) was submitted to cluster analysis in Q-mode using Euclidean distances and Ward's method to group the stations. Four groups of stations were established (Fig. 6). These groups allow the delimitation of three main zones in the study area which largely correspond to the inner-shelf, mid-shelf, and outer-

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Fig. 3. Distribution maps for (a) quartz, (b) phyllosilicates, (c) K-feldspars and (d) calcite. The range of values is presented in each map.

shelf plus the upper slope sectors. An elongated band oriented N-S and parallel to the coast line delineates an area located between ≈40 and 80 m. 3.1.2. Data analysis of the sedimentological data The nearshore sedimentary deposit, b30 m depth, is composed mostly of moderately to well calibrated fine/very fine siliciclastic sand (Figs. 3 and 7a). The silt-clay fraction is low (b5%) and primarily includes quartz, feldspars, and phyllosilicates, namely clay minerals, such as illite, kaolinite and chlorite (Fig. 4a, b, c; group 1 of Fig. 5). Illite (Fig. 4e; lower values of Kübler index) and kaolinite minerals (Fig. 4f) display high crystallinity. Quartz particles in this area are subangular, or immature (Fig. 7a), according to the terminology of Dias and Nittrouer (1984). Roughly parallel to the coast line, on the mid-shelf, at depths between ≈ 40 and 80 m, a band of siliciclastic sediments (Fig. 3) with higher mean grain size values (Fig. 2a), composed mostly of coarse sand and gravel fractions can be found. In these deposits the gravel fraction frequently reaches percentages > 50%, achieving occasionally ≈ 75%. Rounded particles of mature quartz (according to the terminology of Dias and Nittrouer, 1984) predominate in the coarser fractions of these deposits (Fig. 7b). The low carbonate contents, as well as low benthic foraminifera densities, are apparently in contrast to the high proportions of siliciclastic components, on the inner- and mid- shelf. This dichotomy is reflected in the results of the PCA analysis (Fig. 5; variables in group 1 are distinct compared to the variables in groups 2 and 3). Grain size on the outer-shelf and upper slope varies from medium to fine sand. Sand in these areas is poorly sorted and is rich in biogenic carbonated particles (including old foraminifera tests) and glauconite grains (Figs. 2c, d; 3c and 7c, d). Sediments from these areas have

in general a higher proportion of fine fraction (b63 μm; Fig. 2b) and smectite. Illite of these sediments is in general more aluminous and has lower crystallinity. Discontinuous, coarser sediment zones where gravel fraction can rise to about 20% can be also found in the outer shelf where the coarser fractions are often relics (Fig. 7 e, f). 3.2. Benthic foraminifera abundance and species composition Approximately 210 species of benthic foraminifera were identified and counted. The number of specimens per species and per sample, and the percentage of species are included in Appendix 2. Calcareous species dominate benthic foraminifera faunas. Agglutinated species (b40%) reach higher percentages on the mid-shelf in front of the Ria de Aveiro mouth. Miliolids (b16%), mainly represented by Quinqueloculina seminula and Quinqueloculina akneriana, also increase in abundance on the mid-shelf, in front of Ria de Aveiro mouth. The similarity in the distribution of the relative abundance of the most frequent species and groups of species was also evaluated by cluster analysis in R-mode. The percentage of species in 42 selected stations was considered in this analysis, as only a small number of foraminifera were found in samples collected at less than 55 m of water depth which was considered insufficient to represent assemblage composition (Fatela and Taborda, 2002). Thus cluster analysis results mostly include the faunal distribution in samples located at depths below 55 m. Shannon index values (varying between 1.7 and 3.4; Appendix 2) were also taken into account in this analysis. Two main clusters can be considered in the dendrogram of Fig. 8. Cluster 1 is composed of species which are more common and reach a higher relative abundance on the mid-shelf such as: Ammonia beccarii, Quinqueloculina seminula, Nonion fabum, Elphidiun crispum, Bulimina

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a)

b)

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Fig. 4. Distribution maps for (a) illite, (b) kaolinite, (c) smectite, (d) chlorite, (e) Kübler index highlighting illite crystallinity and (f) kaolinite crystallinity index. The range of values is presented in each map.

truncana, Cassidulina crassa, Buliminella subfusiformis var. tenuata, Cibicides spp., Planorbulina mediterranensis and agglutinated species. Cluster 2 consists of species/groups of species which are more common from the outer-shelf to upper slope such as: Cancris oblongos, Bulimina marginata, Cassidulina teretis, Trifarina bradyana, Uvigerina peregrina, Globocassidulina crassa rossensis, Nutalides umbonifera, Bolivinita quadrilatera, Bolivina/Brizalina spp. (namely Bolivina compacta, Bolivina seminuda, Bolivina dilatata, Bolivina ordinaria, Brizalina spathulata), Gyroidina umbunata, Bulimina elongata/gibba and Cassidulina carinata. Cluster 2 also contains the Shannon index value. Fig. 2e maps the Shannon index values which are noticeably higher in an extended band, parallel to the coast line, within the 50 m isobath, and in a

transverse belt, across the shelf, in front of the Ria de Aveiro, but mostly at water depths below 100 m. Very low Shannon Index values were found surrounding the Cartola rock outcrops. The highest BFHP values were found in an elongated band located around the isobath of 100 m, decreasing significantly in shallower waters, in the innerand mid-shelf (Fig. 2f). In fact, the 100 m isobath marks a significant change in the relative abundance of the most frequent species, which explains the establishment of clusters 1 and 2. The distribution pattern of other species, not considered in this cluster analysis, was studied by analysing the number of specimens per species and per sample, in all the studied stations. Table 1 shows the distribution pattern of the most frequently occurring species in the study area.

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55

Factor Loadings, Factor 1 vs. Factor 2 Rotation: Unrotated Extraction: Principal components 0,6

0,4

Carb

1.

2.

Dol

0,2

Factor 2

Sm

0,0

K-F

Kub

Il Qz

-0,2

Chl

-0,4

Glau.D

3.

Phyl Plag

For.F For.D

-0,6

Silt.F

-0,8 -1,0

-0,8

-0,6

-0,4

-0,2

0,0

0,2

0,4

0,6

0,8

1,0

Factor 1 Fig. 5. Results of principal components analysis for the extracted factors 1 and 2 which identifies three main groups of textural, compositional and mineralogical variables. Figure caption: Carb – carbonates (%); Dol – dolomite (%); Sm – smectite (%); Kub – Kübler Index values; Glau.D – density of glauconite grains (n.º/g); For.F. – density of fossils of foraminifera (n.º/g); For.D – density of well-preserved foraminifera (n.º/g); Silt.F – silt fraction (%); K-F – K-feldspars (%); Il – illite (%); Qz – quartz; Chl – chlorite (%); Phyl – phylossilicates; Plag – plagioclase.

4. Discussion

4.1. The inner- and mid-continental shelf

Cluster analysis of the sedimentological data (grain size, sediment composition and mineralogy) established three main zones in the study area corresponding to the inner-, mid-, and outer-shelf to upper slope regions (Fig. 6). The delimitation of these zones corresponds to the general characterization of the Portuguese continental shelf described by Dias and Nittrouer (1984). However, this analysis creates a single general zone from the outer-shelf, the shelf break and the upper slope (cluster 1 of Fig. 6) whilst further limiting an area on the mid-shelf related to coarser sediments (cluster 2 of Fig. 6). Sand-sized particles are in general the most common with no muddy-dominated sediments being found, however, the sediment grain size and composition change significantly within the study area. Particular features found in each sector are analysed below.

The grain size and composition of the inner-shelf sediments reveals that these deposits containing mostly fine/very fine sand and are composed essentially of modern particles (immature; Fig. 7a). In this sector the general low biological productivity and the high terrigenous component (including mostly minerals from group 2 of Fig. 5, see also Fig. 3) occurs due to active alongshore current transporting siliciclastic sediments. The high energetic conditions on the innershelf promote the sediment instability which inhibits the development of benthic foraminifera populations and other organisms with calcite shells. Approximately 2 × 10 6 m 3 of sediment per year are transported by the longshore current (Oliveira et al., 1982) which actively feed this nearshore deposit. The shelf mostly receives sediments from the River Douro, at the north, and the nearby Aveiro lagoon and the River Mondego, at the south. However, the Ria de Aveiro provides the shelf with small amounts of sediments (Abrantes and Rocha, 2007 ; Martins et al., 2009, 2011b). This lagoon exports mostly muddy particles, composed essentially of quartz and phyllosilicates, namely clay minerals (Martins et al., 2009). Nowadays, illite and kaolinite are the most abundant clay minerals in the sediments of the Aveiro lagoon (Rocha et al., 2000) and River Douro (Araújo et al., 2000). The distribution pattern of illite and chlorite trace the contribution of sediment from the adjacent continental land (from the erosion of plutonic and metamorphic outcropping rocks) to the ocean. These contributions are also identifiable by the higher crystallinity of illite and kaolinite. The lower values of the Esquevin index suggest the occurrence of more ferro-magnesian illites in this sector, indicating a lower chemical degradation of illite contained in nearshore sediments when compared with the other sectors, as this mineral evolves to an aluminous term in deeper areas. According to Dias (1987) and Magalhães (1999), the volume of sediment transported by the River Douro is significantly higher than that by the River Mondego. Thus sands are primarily provided by the River Douro (Abrantes et al., 2005) and are transported by the longshore current to the study area. This contribution was higher in the past than at present, as documented by the studies of the

Fig. 6. The distribution of four groups of stations established by Q-mode cluster analysis of the sedimentological data using Euclidean distances and “Ward's method” applied to a standardized data matrix of sedimentological data (grain size, sediment composition and mineralogy). Groups were assigned with a grey scale.

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a)

b)

c)

d)

e)

f)

Fig. 7. Photos of the sediments. a) Siliciclastic very fine sand from the inner-shelf with immature (subangular, hyaline) quartz particles; b) siliciclastic particles of coarse sand and gravel composed of mature quartz particles (rounded, frosted) and gravel bands from the mid shelf; c) and d) medium sand, rich in biogenic carbonated particles (including old foraminifera tests) and glauconite grains, from the outer-shelf and upper slope; e) and f) carbonated gravel particles from the outer-shelf.

formation process of the Ria de Aveiro and/or the recent processes of erosion of this coastal stretch (Dias, 2003, 2004, 2008; Dias et al., 1994). Over the last two decades the amount of sediments transported along the coast that originated from this river decreased due to the construction of dams in the River Douro, sand extraction activities and coastline protection jetties (Oliveira et al. 1982). Alongshore transport of sand is predominantly southward, mostly during periods of fair weather (Davies et al., 2002; Dias et al., 2002b). Thus the longshore current essentially involves the sediments transported through the inner shelf and is favoured by the spring–summer oceanographic regime, when the strengthening of northerly winds induces currents flowing towards the south, onto the continental shelf (Peliz et al., 2002). Storms that affect the study area during upwelling conditions can generate wave shear velocities of 3.5 cm/s, and layers of high turbidity several meters above the bottom, improving the southwards alongshore transport of sediments (Dias et al., 2002b; Vitorino et al., 2002b). During the winter, however, this oceanographic scenario changes, when south-westerly winds drive downwelling conditions. The prevailing downwelling conditions, combined with the high energy wave environment, promote the resuspension of fine sediments and their transport northwards and offshore to the quieter environment of the deeper sectors of the study area (Vitorino et al., 2002b). Thus during winter, the sediments provided by the Douro and Mondego rivers and the Aveiro lagoon can be transported by the poleward flow to the N-NW shelf areas or across the shelf to

deeper oceanic areas. The spring–summer season is in general dryer and the river flow is typically lower compared with the winter. The floods of the River Douro, that constitute the main source of sediments for this zone, occur mainly in winter (Dias et al., 2002b), when the poleward flow prevails on the shelf (Vitorino et al., 2002a). Under these conditions sediments provided to the shelf are essentially transported to northern areas, reaching the Minho-Galician shelf. This effect may accentuate the lack of sediments on the Aveiro shelf, contributing to the exacerbation of the erosive character of this coastal sector (Dias, 2003, 2004, 2008; Dias et al., 1994). The gravelly-sandy deposits of the mid-shelf, present between 40 m and 80 m deep, are characterized by high gravel content and are dominated by the terrigenous component (Fig. 7b). The existence of coarse material further from the coast, and deeper than the finer sediments of coastal deposits, is a factor resulting from anomalous processes that the current supply and distribution of particles cannot explain. These gravely deposits were accumulated in past highenergy environments and are probably related to the Holocenic location of the River Vouga estuary and paleo-littorals (e.g. Dias and Neal, 1990; Dias and Nittrouer, 1984) due to the sea level drop (of about − 140 m) during the Last Glacial Maximum and posterior transgression (Dias et al., 2000b). In any case, the actual strong hydrodynamic conditions do not allow the accumulation of large amounts of fine sedimentary particles overlying these gravelly deposits. The low-energy wave environment

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57

Tree Diagram for 28 Variables Complete Linkage 1-Pearson r 1,6

Linkage Distance

1,4 1,2 1,0 0,8 0,6 0,4

0,0

C.teretis N.umbonifera G.c.rossensis B.seminuda B.spathulata C.carinata B.ordinaria B.elongata/gibba G.umbonata Bolivina/Briz. B.quadrilatera B.dilatata U.peregrina T.bradyana Shannon Index B.marginata C.oblongos B.compacta P.mediterranensis Cibicides spp. B.s.tenuata C crassa B.truncana E.crispum Agglutinated N.fabum Q.seminulum A.beccarii

0,2

Cluster2

Cluster1

Fig. 8. Results of cluster analysis in R-mode looking at the most frequent species and groups of species of benthic foraminifera using Pearson correlation and complete linkage method. In this dendrogram two groups of variables can be separated – cut off occurs at 1.4.

prevalent during the summer has little effect on the sediment cover at mid-shelf depths. Even so, waves during winter and summer storms can promote erosion of fine grained sediments in this area (Vitorino et al., 2002b) but not of the coarser sediments. This justifies the preservation of these gravelly deposits in the surface coverture of the mid-shelf. The relatively high energetic levels of this sector are also reflected in the low abundance of foraminifera in spite of the relatively higher abundance found in shallower waters. In this sector higher frequencies of species can be found such as: A. beccarii, P. mediterranensis, E. crispum, C. ungerianus, C. ovoidea, Cassidulina minuta, B. subfusiformis var. tenuata, B. elongata/gibba, B. truncana, agglutinated species (e.g. Textularia sagittula) and miliolids (e.g. Q. seminula). The Ria de Aveiro is a depocenter of organic matter (Duarte et al., 2007), exporting it to Table 1 The distribution pattern of species not considered in the cluster analysis, analysing the original matrix of the number of specimens per species and per sample in 95 of the stations. On the mid to outer shelf (50–100 m)

Cassidulina minuta Elphidium complanatum Elphidium incertum Outer shelf and upper slope Amphicoryna scalaris (100–700 m) Bolivina difformis Brizalina subaenariensis Brizalina translucens Bulimina elegans Bulimina exilis Bulimina striata Cancris auricula Chilostomella ovoidea Cibicidoides pachyderma Epistominella exigua Favulina hexagona Fissurina fasciata carinata Hoeglundina elegans Hyalinea balthica Upper slope > 500 m Bolivina pseudolobata Ioanella tumidula Ubiquitous Neoconorbina parkerae

Gavelinopsis praegeri Hanzawaia nitidula Lenticulina clericii Melonis barleeanum Neolenticulina variabilis Palliolatella bradyiformis Sphaeroidina bulloides Stainforthia fusiformis Textularia aglutinans Textularia communis Textularia conica, Textularia deltoidea Textularia sagittula Uvigerina auberiana Uvigerina mediterranea Valvulineria bradyana Nonionella bradii Nuttallides umbonifera

the shelf and therefore contributing to an increase in the development of some species populations as indicated by the higher values of BFHP (Fig. 2f). This also benefits from the large diversified assemblages of benthic foraminifera fauna as suggested by the Shannon index values (Fig. 2e) for the mid-shelf in front of the lagoon's mouth, as compared to other areas. The zone around the 100 m isobath marks a clear difference in the density and relative abundance of the most frequent species of benthic foraminifera (a change identifiable by the different species included in clusters 1/2, in Fig. 8 and Table 1). This different pattern is due to the influence of the Ria de Aveiro, as the coastal region is more affected by the water exchange processes between the Aveiro lagoon and the adjacent ocean. During a tidal cycle, an average volume of 1.8 × 10 6m 3 of fresh water is discharged into the lagoon by the inflowing rivers (Moreira et al., 1993), while the average volume of marine water exchanged between the lagoon and the shelf is approximately 89 × 10 6m 3 (Dias et al., 2000b). Exiting the mouth of the lagoon, the plume water mass has salinities ranging between 24.0 and 35.8 and temperatures ranging from 20 to 25 °C in summer (Almeida et al., 2002), while during the winter temperatures decrease to 15 °C. This explains why the eurythermic and euryhaline species (identified considering the study of Murray, 1991) are more frequent at depths below 100 m (included in cluster 1 of Fig. 8; see also Table 1). 4.2. The outer-shelf, break-shelf and upper slope Along the outer-shelf to the upper slope off the Iberian Peninsula, water depths are significantly higher and tidal currents weaker than over other shelf areas, typically inducing some stratification with a deep permanent thermocline (Davies et al., 2002). The low-energy environments allow the accumulation of fine-grained material (silt + clay fractions) including organic matter (groups 1 and 3 of Fig. 5). The stability of environmental factors, such as temperature and salinity and the lower hydrodynamic conditions explain why foraminiferal density is higher and the stenothermic and stenohaline species (according to Murray, 1991) are more frequent (species included in cluster 2 of Fig. 8; see also Table 1). However the outer-

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shelf, break-shelf and upper slope sedimentary coverture consists mostly of medium to very fine sand, with variable calibration and with abundant biogenic particles (Fig. 7c, d). According to Dias (1987), these deposits have two distinct origins being a mixture of modern and relic particles. The glauconite occurs in most samples. Between 100 m and 150 m isobaths patched gravelly deposits can be found. They are composed of coarse sand and gravel fractions, where gravel can reach ≈20% and are carbonate rich (Figs. 2a, c; 3d). The gravelly particles of these deposits have a strong relic tendency and also have significant glauconite content (Fig. 7e, f) and as such they were related to paleo-littorals (Abrantes et al., 1994; Dias, 1987; Magalhães, 1999). Their characteristics indicate that these sediments were generated through dependence on old coastal areas, during periods when the relative sea level was near the continental break, probably during the Last Glacial Maximum. These sediments remained on the surface because since then the sediment supply to this area has been minimal. The reduced sedimentation rate in the deeper sector is indicated by the lower abundance of detrital minerals compensated by the high content in biogenic carbonates (Fig. 2c; group 3 of Fig. 5). This means a small dilution of bioclasts by terrigenous particles. Mostly fine grained particles are transported offshore (Dias et al., 2002b). As the inner- and mid-shelf are poor in mud, the terrigenous transport to deeper areas is reduced. Furthermore the offshore sediment exportation across the shelf is also limited by the reliefs of Pontal da Cartola and Pontal da Galega, which act as morphological barriers. The present terrigenous transport from shallow to deeper areas is favoured by the winter oceanographic regime. Under downwelling events, once re-suspended by the waves, sediments can easily be transported seaward across the shelf (Taborda, 1999). Associated with winter winds are long period (14–21 s) swell waves that give rise to significant bed-orbital velocities at typical shelf water depths of up to 200 m (Davies et al., 2002). These bed-orbital velocities, due to the occurrence of internal waves (Jeans and Sherwin, 2001a, b), should explain the higher abundance of old tests of foraminifera along the outer-shelf, shelf-break and upper slope (Fig. 7c, d). These materials may be reworked, resuspended, mixed with recent foraminiferal assemblages and redeposit again in the area. Internal waves (e.g. Drake and Cacchione, 1986; Heathershaw, 1985; Jeans and Sherwin, 2001a,b), downslope movements and along-slope currents (e.g. Bower et al., 1995; Pingree and Le Cann 1993) have floor polish effects, favouring remobilization or avoiding sediment deposition and promoting its transport to deeper or shallower areas. The downslope loss, for instance, is also corroborated by spots of lower illite concentrations in some zones (Fig. 4a) crossing the outer-shelf and the upper slope. The occurrence of species that generally inhabit lower bathyal areas, such as Nuttallides umbonifera and Oridorsalis umbonatus (Bremer and Lohmann, 1982; Murray, 1991), in the upper slope may also be an evidence of the upward effect of the internal waves in this sector. Biogenic productivity is enhanced in general by the spring– summer oceanographic regime (Fiúza et al., 1982; Wooster et al., 1976). Upwelling events cause a shift in the primary producer community away from nanoplankton and diatoms resulting in an increase in organic matter flux to the sea floor in the quietest areas of these deeper sectors. This enhancement also promotes an overall increase of benthic faunal density and diversity with a succession of species determined by differences in their trophic efficiency. The superficial layer of organically rich aggregate on the Iberian margin was noticed for instance by Davies et al. (2002). It is also identified by the generic high values of BFHP index, based on benthic foraminifera. The higher values of BFHP along the 100 m isobath trace the present position of an oceanic thermal front (Peliz et al., 2002). According to Martins et al. (2007), the position of this oceanic front migrated in the late Holocene, due to shifts of the wind patterns and the prevalence of long periods of upwelling or downwelling.

This may have occurred after the stabilization of the sea level, on the current quota, at about 3/5 ka BP, following the transgression that occurred since the Last Glacial Maximum (e.g. Dias et al., 2000b). The environmental conditions of the outer-shelf are favorable towards glauconite formation: high productivity (from upwelling); foraminifera tests as substrates and some clay precursor, suggested by the occurrence of smectite (Abrantes and Rocha, 2007). Glauconite is a well-ordered K- and Fe-rich mica-structure clay mineral which frequently occurs in association with bioclast infills and faecal pellet replacements, which also indicate that physicochemical conditions are appropriate in these micro-environments for glauconitization (groups 1 and 3 of Fig. 5). Smectite content is higher along the outer-shelf to the upper slope. This mineral may have been produced by continental weathering and may be a precursor of glauconite formation. The smaller size of smectite allows a longer suspension and facilitates its transport across the shelf, and its deposition in calmer deeper areas. As smectite sinks through sea water it adsorbs carbon-rich molecules into its intercrystalline surfaces, since this mineral's crystals bear a large charged surface area that are attractive for organic molecules (Tonle et al., 2003). In general, glauconite formation takes place at the watersediment interface where suitable substrate conditions are present, in a semi-confined and suboxic environment with abundant Fe supply (Kelly and Webb, 1999). Iron concentrations are high along the outer-shelf to the upper slope (Abrantes et al., 2005). Low oxic micro-environments are generated by the enhanced burial of organic carbon giving rise to higher oxygen consumption due to its aerobic degradation. The above mentioned adequate conditions for the glauconite formation may have originated as a consequence of marine flooding events which generated intervals of slow sedimentation and environmental quiescence (Dias, 1987). These settings allowed the deposition of some argillaceous sediment which remained in the appropriate physicochemical regime sufficiently long enough for the complex glauconitic structures to form (Kelly and Webb, 1999). This may also have favoured glauconite formation and evolution. Sediments of these sectors also provide evidence of higher chemical degradation, which is indicated by the lower content of K-feldspars, plagioclase and chlorite (more easily degraded mineral), more aluminous illite (higher Esquevin indexes), illite and kaolinite content with lower crystallinity (Fig. 4 e, f). 5. Conclusions The surface sediment grain size, compositional, mineralogical and microfaunal data clearly show latitudinal and bathymetrical changes, which are mainly governed by sedimentary sources and oceanic circulation and processes. The main sediment sources for the Aveiro–Espinho continental shelf sector are the River Douro, the Aveiro lagoon and the River Mondego. Inner-shelf fine sands are transported mainly by a southward longshore current and are largely provided by the River Douro. The inner- and mid-shelf are high energetic sectors, promoting sediment remobilization and transport. In the mid-shelf coarse sand and gravelly deposits can be found, accumulated in past high-energy environments, during lower sea level stands, probably related to the Holocenic location of the River Vouga estuary, and paleo-littorals surfaces. The present hydrodynamics are not strong enough to erode these coarser sediments but yet they are strong enough to avoid the deposition of fine-grained particles keeping coarser deposits on the surface. The Ria de Aveiro benefits the development of a diversified benthic foraminifera fauna in the mid-shelf. Higher values of BFHP were, however, found over the 100 m isobath signifying the occurrence of oceanic fronts in this area which benefit the deposition of finer sediments enriched in organic matter.

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Across the shelf, sediment flows to deeper areas are blocked by the rocky outcrops of Pontal da Galega and Pontal da Cartola. On the outer-shelf and upper slope, the less intense hydrodynamic conditions, strong water column stratification and the deposition of high amounts of organic matter favour benthic foraminifera population development. In these deeper zones low sedimentation rates and high organic matter flux allow the occurrence of some authigenic changes in the mineralogical particles: chlorite and illite chemical hydrolysis and glauconite formation. Internal waves, downslope movements and along-slope currents may leave behind a record in some areas. Upwards, the internal wave effect may explain the occurrence of species such as Nuttallides umbonifera and Oridorsalis umbonatus, which generally inhabit lower bathyal areas in the upper slope. Downslope losses may explain the lower illite concentration spots from the outer-shelf to the upper slope. Supplementary materials related to this article can be found online at doi:10.1016/j.jmarsys.2012.02.001.

Acknowledgements The authors would like to thank the reviewers for the comments and the Portuguese Hydrographical Institute for providing the samples. The authors also would like to thank Cristina Freitas and Paulo Miranda for the technical support. This work was financed by the FCT (Portugal) through the AdaptaRia Project (PTDC/AAC-CLI/ 100953/2008), co-funded by COMPETE/QREN/UE.

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