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Dispersal and deposition of suspended sediment on the shelf off the Tagus and Sado estuaries, S.W. Portugal
Progress in Oceanography 42 (1998) 233–257
Dispersal and deposition of suspended sediment on the shelf off the Tagus and Sado estuaries, S.W. Portugal J.M. Jouanneaua,*, C. Garciab, A. Oliveirac, A. Rodriguesb, J.A. Diasb, O. Webera a
DGO, UMR-CNRS 5805, Avenue des faculte´s, 33405 Talence cedex, France b Instituto Hidrografico, rua das Trinas 49, 1296 Lisbon codex, Portugal c UCTRA, Universidade do Algarve, 8000 Faro Portugal
Abstract The Portuguese margin in front of the Tagus and Sado rivers is characterized by a narrow shelf incised by numerous canyons and by a large mud deposit. The two estuaries that feed this continental margin have distinct impact. The suspended particulate matter concentration values in the mouth of the Tagus are four times higher than in the Sado. During the summer the surface nepheloid layer is always larger than during the winter when it is restricted near the mouth of the estuary. This nepheloid layer may reach 30 km in length extending westward. The bottom nepheloid layer usually shows higher nephelometer values, and has a typical distribution: it is usually diverted southward in the direction of the Lisbon Submarine Canyon. We estimate the amount of suspended matter being discharged annually from the Tagus estuary to be between 0.4 and 1 ⫻ 106 t. The area covered by fine deposits is about 560 km2. Hence the thickness of sediments deposited annually should be between 0.07 and 0.18 cm. The sedimentation rates calculated from the 210Pb excess vary between 0.16 and 2.13 cm y−1 which correspond to the maximum rate. For a layer of 1 cm thick, 81,000 t of particulate organic carbon (POC) should be trapped. That would represent, with a minimum sedimentation rate between 0.07 and 0.18 cm y−1, an entrapment of 6000–15,000 t POC y−1. The trace metals content of box core samples clearly shows the anthropogenic impact in the uppermost level (5 cm thick) in the Tagus estuary and in all the sedimentary deposits (15 cm thick) on the shelf muddy area. Despite the narrowness of the shelf, a significant part of continental fluxes fails to reach the deep ocean. 1998 Elsevier Science Ltd. All rights reserved.
* Corresponding author. 0079-6611/98/$ - see front matter 1998 Elsevier Science Ltd. All rights reserved. PII: S 0 0 7 9 - 6 6 1 1 ( 9 8 ) 0 0 0 3 6 - 6
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1. Introduction The Portuguese margin in front of the Tagus and Sado rivers is characterized by a narrow shelf incised by numerous canyons (Vanney & Mougenot, 1981; Mougenot, 1988). The canyons of Lisbon, Cascais and Setu´bal extend into the continental shelf. The Tagus and Sado estuaries have been well studied in terms of their geochemistry and sedimentary budgets (Vale, 1981, 1990; Vale & Sundby, 1987; Caˆmara, Silva, Ramos, & Ferreira, 1986) but the study of the adjacent continent shelf is only just beginning. On the shelf proper, a major area of mud deposit is linked to the estuarine supplies of the Tagus and to a lesser extent to the Sado estuary, Dias (1987). The principal mud deposit off the Tagus is very well defined and extends out to the 70 m isobath. It may act, to an unknown extent, as a temporary site of deposition for suspended matter, which eventually escapes to the open sea. To investigate the ongoing processes occurring across the continental shelf and into deep environments DGO (University of Bordeaux 1) and the Instituto Hidrografico selected a restricted study area on the Iberian margin. The aims of this study were to understand the sedimentary dynamics associated with major rivers and give an approximate quantification of sediment budgets (sources, sinks and main transport paths) over the continental shelf. The area lies within parallels 38°10⬘N and 38°45⬘N. The eastern and western boundaries are the coastline and the shelfbreak, respectively (Fig. 1). For the purpose of this paper the area constitutes a particularly interesting zone for the following reasons: 1. It is fed by two important river systems, the Tagus and the Sado, whose basins drain populated and industrialized regions of Portugal. Hence inputs of anthropogenic origin can be recognized, thus permitting the characterization of the distribution and supply patterns on the margin; 2. The Sado is the smaller of the two rivers being only 175 km long and with minor tributaries. The Tagus, on the other hand, is the most important river of the Iberian peninsula, longer than 1000 km, and with some of its affluents as important as the Sado river; 3. Another favourable factor concerns the geomorphology. Its particular morphological features (which are related to geological structures) allow good characterization of terrestrial fluxes to the deeper domains. The margin being narrow with the shelfbreak less than 30 km offshore, and the continental slope being incised by canyons might be expected to provide favourable conditions for the rapid transfer of the suspended matter into deep water. Nevertheless, the presence of large mud deposits in the middle shelf region is not entirely consistent with such rapid transfer. It seems that the continental shelf may be acting as a depositional centre to some extent. This study has been supported by six cruises in both winter and summer conditions paying particular attention to interannual and seasonal variability. During these cruises, CTD stations and sampling of suspended particulate matter (SPM) and surficial sediments have been carried out.
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Fig. 1.
2. Regional setting 2.1. Study area The continental shelf off the mouths of the estuaries has a width ranging from 3 to 30 km, with the shelfbreak located at 140 m depth. On this continental shelf the submarine protofluvial deltas of the two studied rivers originate from the accumulated sediments discharges from the estuaries (the Sado delta is smaller and is confined to the outlet). On the outer continental shelf and slope three main submarine troughs: the Cascais Canyon, the Lisbon Canyon and the Setu´bal Canyon are related to geological features (Mougenot, 1988). 2.2. Hydrology and oceanography The Tagus is the principal source of fresh water input to this part of the Iberian Continental margin (Table 1). The river discharge normally shows a pronounced dry/wet season signal as well as large inter-annual variation. The mean annual aver-
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age discharge of the Tagus is 364 m3 s−1 (ranging between 80 and 720 m3 s−1, Loureiro, 1979; Loureiro & Macedo, 1986), with monthly average discharge varying from 1 to 2200 m3 s−1. In the Tagus estuary, the SPM concentration is considered to be low, with values ⬍ 30 mg l−1 (Caˆmara et al., 1986), but with a great variability in time and space, related to the discharge of this river. Vale (1981) recorded a maximum value of turbidity in the order of 500 mg l−1 in the upper estuary during spring tide movement of this turbid zone into the estuary mouth (Vale & Sundby, 1987). The average suspended sediment load discharged by the Tagus into the estuary is 4 ⫻ 105 t y−1 but may reach 1 ⫻ 106 t y−1 during those years when there are several flood events (Vale & Sundby, 1987). The annual mean discharge of the Sado river system is always less than 10 m3 −1 s (Loureiro, Nunes, & Bothelo, 1982, 1986). Turbidity values are typically lower than in the Tagus estuary (Table 1). Melo (1981) reports values between 4 mg l−1 (December) and 24 mg l−1 (April). Highest values are related to the phytoplankton spring bloom and not to terrigenous inputs of the SPM. These values reflect the minor contribution by this river of terrestrial materials to the continental shelf sedimentary deposits. Considering the sedimentological and morphological characteristics of the Sado estuary, Quevauviller (1987) concluded that the lower estuary traps the majority of the fine sediments restricting their transit out over the continental shelf. Current velocities range from 1.2 to 2 m s−1 for the Tagus and between 0.48 and 0.60 m s−1 for the Sado river (Lopes da Costa, 1984). On the Portuguese continental margin, the tide has a semi-diurnal regime (period of 12 h 25 min) and the tidal range varies between 1.5 m (neap tides) and 3.5 m (spring tides). Tidal currents are weak except in the estuaries. The swell and winds from the west and northwest are dominant, but can change seasonally. In winter, storm events are characterized by strong winds from the southwest, and in summer by persistent winds from the north, which favour upwelling (Wooster, Bakun, & McLain, 1976; Fiu´za, Macedo, & Guerreiro, 1982; Fiu´za, 1983). 2.3. Sedimentary cover The sedimentary cover of the shelf near the Tagus and Sado river mouths was virtually unknown until the mapping of surficial sediments was started in the 1970s with the SEPLAT Programme (carried out by the Hydrographic Institute of Portugal). Nevertheless, the sediment budgets (sources, sinks and main transport paths) and distribution patterns of suspended matter are still an open question. Sediment maps of this particular area of the Iberian margin show that there are some peculiar deposits covering the sea bottom. Their presence results from to the geomorphologic setting and the effects of sea level rise since the last glaciation. In fact, the sedimentary cover of the Iberian margin is very young (less then 20,000 years), and is composed of late Quaternary and Holocene sediments (Dias, 1987). The sedimentary deposits off the Tagus river are essentially muddy (Fig. 2). The area is protected against swell from the northwest and since the Tagus acts as an important source of fine particles, almost the entire continental shelf is covered by
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Fig. 2.
a large expanse of mud deposit. Coarse sandy deposits occur elsewhere on the shelf, but are restricted to the northern sector where there are several rocky outcrops. Sandy deposits occur on the inner shelf, where there are high energy littoral currents. There are large bodies of sandy and gravelly sands on the outer shelf, near the shelfbreak. These are a remnant of an old shoreline with an age of 苲 16,000 years B.P. that have not become covered by recent muddy deposits because of prevailing dynamics of the outer shelf (Dias, 1987). In this area, strong turbulence and currents either keep fine sediment in suspension or frequently resuspend it, so that fine material that is supplied from nearshore areas is transported out over the continental slope or beyond without becoming permanently deposited near the shelfbreak (Southard & Stanley, 1976). This part of the shelf is also affected by internal waves, especially during periods when water masses are stratified. As stated by Heathershaw, New, & Edwards (1987), the outer shelf in the vicinity of shelfbreak is likely to be an area of bed load parting or divergence. The sedimentary deposits covering the continental shelf adjacent to the Sado river are mainly sandy mud. Muddy sediment areas are restricted to the northern coast of
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the bay in front of Sesimbra and to the Setu´bal canyon (Monteiro & Moita, 1971; Moita & Quevauviller, 1986). The composition of sedimentary cover is dominated by terrigenous particles (where quartz prevails), and so can be classified as a lithoclastic sediment (with a carbonate percentage less than 30%). Higher percentages of carbonate (CaCO3) particles are to be found in the sandy deposits of the outer shelf, where they can exceed 50% wt/wt (Rodrigues & Matos, personal communication). If one considers the spatial variability of sediments, there is a decrease in the textural characteristics, with coarser material near the Tagus river mouth and the finer material at the deeper parts (more than 50 m deep). This reflects in part the dynamics of the Tagus discharge outflow.
3. Methodology From 1993 to 1995, six cruises were undertaken in the study area in order to evaluate seasonal and interannual variability in the transfer dynamics of particulate matter from continent to oceanic sedimentary systems. Table 2 provides a description of those summer and winter cruises, promoted by the Portuguese Hydrographic Institute and the University of Bordeaux I (France). 3.1. Field measurements CTD profiles were measured using a Hydropolytester/Nephelometer ZULLIG probe. One hundred and sixty-one surface water samples were collected and filtered with a direct pressure filtration system using three types of filters: (1) and (2) a mixed cellulose ester (Millipore and Sartorius; 47 and 142 mm diameter with 0.45 m pore size) for evaluation of SPM concentration, grain size analysis and optical observations; (3) a glass microfibre Whatman GF/F for quantification of the particulate organic carbon (POC) content of the SPM. Eighty sediment samples were collected with a Smith-McIntyre grab; 30 of these were subsampled to compare the sediment characteristics at 0–1 cm and 1–2 cm. These sediment samples were used to study the bottom sedimentary cover, the sediment/water interface and the heavy metal distribution. Cores sampled were also obtained with a box-corer or a minicorer MARK I, ranging from few centimetres to 30 cm long, in order to study the sediment/water interface. 3.2. Laboratory analysis Sediment grain size analyses were performed by the classical physical method of sieving and the use of settling columns. Sediments were classified according to their gravel ⫹ sand/silt/clay ratios according to Shepard’s classification (Shepard, 1954). All filters were dried (T ⬍ 30°C) and the 47 mm filters were weighed for SPM concentration. The 142 mm filters were examined under a binocular magnifying glass ( ⫻ 100) to identify the principal organic/inorganic components of the SPM. The
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grain size was analysed using a MALVERN 3600 E laser diffraction instrument. The POC contents of sediment and water samples were determined using the Strickland & Parsons (1972) method as adapted by Etcheber (1981). Its content was measured with a LECO CS–125 equipment. The CaCO3 content was determined by a gasometric method showing a fairly low CaCO3 relative variation ( ⬍ 2%) in duplicate measurements. Major and trace elements were analyzed by X-ray fluorescence spectrometry on dried and pulverized samples of bulk sediment. Analyses were conducted using a Philips 21PW 1500-10 analyser, which allows multi-element analysis. Inter-element effects were reduced by means of equations taken from Lachance & Trail (1966). Results were calibrated against international synthetic rock standards of known composition provided by USGS and ANRT. To define sources, REE and trace elements were determined on selected samples (10) by ICP–MS. Sub-samples weighing about 10 mg were dissolved for 12 h in pressurized PTFE beakers at a temperature of about 125°C in a ⬇ 6N equimolar mixture of [HF ⫹ HClO4 ⫹ HNO3]. Elemental concentrations were determined using an ICP–MS Elan 5000 P.E.-Sciex. MESS 2 and BCSS 1 standards from NRCC have been systematically analyzed, and enabled us to achieve precision better than 10% for all elements. Accuracy was found to be 5% at 10 l l−1. The instrument was operated under standard settings in nearly clean room conditions (US-class 10,000). In five box-cores, 210Pb (46.5 keV) measurements were done on dried bulk sediment samples, using a high resolution gamma-spectrometer with a semi-planar detector (Intertechnique EGSP 2200-25-R). 210Pb excess have been calculated from: 210Pb total ⫺ 226Ra (226Ra is counted from its daughters: 214Bi and 214Pb). The sample (6– 10 g) was counted for 10–20 h.
4. Results 4.1. Suspended particulate matter distribution The nephelometric distributions show no major interannual differences. But seasonal variability of estuarine inputs is clearly demonstrated (Figs. 3 and 4), similar for instance to that observed in the Bay of Biscay off the Gironde estuary (Allen & Castaing, 1973). Despite the low values of surface SPM concentrations (1–20 mg l−1) the surface turbidity in both seasons, always shows higher nephelometric values of 5–8 ftu near the estuarine mouths (1 ftu,—Formazine Turbidity Unit— 苲 1.7 mg l−1). In summer, the surficial turbid layer is more extensive (it may reach 30 km in length) and is always westward orientated. The bottom turbid layer is characterized by higher values of nephelometry (6–8 ftu near the coast) and shows a southward trend towards the Lisbon Canyon. In winter there is strong mixing of the water column and both layers are located near the estuary mouth. The turbid plume associated with the Sado river is always restricted to its outlet, while the Tagus dominates the hydrological distribution of the continental shelf.
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Fig. 3.
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Fig. 4.
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During the winter survey (November–December 1994) and summer survey (June– July 1995) typical hydrological sections (Fig. 5) off the Tagus and Sado outlets show different patterns. In the summer, turbidity (Fig. 6) defines a surficial water mass with estuarine characteristics that stretches out over the shelf to the shelfbreak, while a benthic nepheloid layer (BNL) develops at the level of the shelfbreak (section B– D—Tagus and B—Sado). During the winter, a well defined difference develops near the shelfbreak between the turbid layers at the surface and at the bottom, which become separated by clear waters. The BNL is always more developed than the surface nepheloid layer and can reach a thickness of 150 m (section B–D—Tagus). Some intermediate turbid layers related to the shelfbreak may spread out into the open sea, and are directly related to the water stratification, as have been noted previously along other margins e.g. by McCave (1972) and Ruch, Mirmand, Jouanneau, & Latouche (1993).
Fig. 5.
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Fig. 6.
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4.2. Particulate organic carbon distribution in SPM The analysis of the POC distribution in the surface waters near the Tagus and Sado river mouths shows the importance of those rivers in the overall contribution of nutrients to the continental shelf. Normally the waters near the Tagus adjacent shelf have POC values two orders of magnitude greater than Sado. During winter [Fig. 7(A)], as expected, there is a decrease in the POC content (less than 150 g l−1), which is related to lower biological production and the great percentage contribution of the terrigenous fraction. In summer [Fig. 7(B)], SPM is richer in POC ( > 200 g l−1) associated not only with the river nutrient inputs but also with upwelling which induces a bloom, observed in the vicinity of the Roca Cape (waters with lower temperature). 4.3. Mean grain size At the surface, the mean grain size of SPM (Fig. 8), during all cruises, shows a tendency to increase in diameter towards the shelfbreak. Whatever the season, the estuarine water mass shows a dominant mode at 5.4 m grain size that mainly corresponds to fine terrigeneous material observed on the filters. On the other hand, the oceanic waters show a multimodal distribution with the coarser peaks differing with the season. These different distributions may be related to the variation of the biogenic components of the SPM, as it has been shown previously off the Gironde estuary (Weber, Jouanneau, Ruch, & Mirmand, 1991). The optical observation made by Garcia (1997) shows that, in these oceanic waters, the CaCO3 phytoplankton (foraminifera), the zooplankton (copepods) and organic detritus are dominant, and contribute the coarser modes in the grain size distribution. 4.4. Water/sediment interface 4.4.1. Particulate organic carbon POC values are generally higher on the Tagus margin than on the Sado margin. Nevertheless on the Tagus margin, two opposite gradients can be observed from a maximum centre (5%) located in the northwest of the mud area, at 100 m depth, as shown in Fig. 9, while on the Sado continental shelf there is a simple gradient extending out from the estuary mouth. At the shelfbreak the POC contents have an average of 1% and the minimum values (0.5%) were measured in coarse sediments. The grain size factor explains partially the POC pattern (Fig. 10). There is a clear decrease in POC content with the increase of the median grain size. This trend is particularly well defined on the Tagus continental shelf, although there are some exceptions to this relationship. It seems that there is a dominant hydrodynamic control superimposed on the estuarine source proximity. Thus, the westward extension of the surface nepheloid layer seems to be the main factor inducing the high carbon content in surficial sediments. The distribution of the POC vs depth (Fig. 11) does not show a clear decrease as may be expected from results from other similar environments. The samples collected
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Fig. 7.
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Fig. 8.
Fig. 9.
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Fig. 10.
Fig. 11.
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from the vicinity of the Tagus canyon at depths of 350 and 450 m had POC contents as high as those of samples from 150 m on the shelf. There are two hypotheses to explain this pattern related to the origin of this fresh organic matter. First, if the organic material is of estuarine or continental origin then, the high values measured in the canyon are probably the result of a downward transport in a BNL. Alternatively, if there is no relationship between the sources of the organic matter at 150 and 350 m, then the high values observed at the head of the canyon probably result from an increase in primary production related to upwelling events in surficial water.
4.4.2. Trace metals on sediments Because of their origin and behaviour in sedimentary dynamics, Zn, Pb and As have been measured in surficial sediment samples. They are good markers of present day human activity, as shown by de Bettencourt (1990), concerning the As contamination of the Tagus estuary and Vale (1990) and Paiva, Jouanneau, Araujo, Weber, Rodrigues, & Dias (1997) concerning other heavy metals. Trace metal contents are higher on the continental shelf adjacent to Tagus river than off the Sado, as a result of the different roles of the estuaries as sources of material to sedimentary deposits: the Tagus acts as a main source while the Sado has a limited and weak impact on the nearby oceanic systems. Normalized to aluminium, Zn, Pb and As show a decrease by a factor of 3 between the highest values, near the estuary mouths, and the lowest values close to the shelfbreak. To illustrate this distribution the CaCO3 Pb/Al2O3 data are presented in Fig. 12. Fig. 12 suggests that the continued removal of these trace metals occurs between the estuaries and the shelfbreak. This is confirmed by the poor relationship established between the CaCO3 and trace metal content and grain size, indicating that removal processes are preponderant over grain size effects. Thus trace metals may act as good markers of estuarine outputs and of estuarine impacts on the margin. The trace metal distributions do not show strictly the same patterns as the POC. Even if one observes a trace metal content decrease with increasing water depth, it appears that, trace metals originate mainly from the BNL and not from the surficial plume, as suggested by the southward gradient of metals contents. Then it could be considered that supply of the terrigenous components on the margin is principally southward in direction of Cascais and Lisbon canyons via a benthic process. This behaviour is the opposite of that of POC, which is more related to the surface nepheloid layer. Results of ICP–MS analysis CaCO3 (Table 3) confirm these observations. Results are expressed by the enrichment factor (normalized to Sc) in relation to the Tagus (T–1). It can be noted, in relation to As, Se, Ni, Mo, that the Sado shelf forms a specific domain where contributions from the Tagus estuary are in a minority. Nevertheless, we note that the present day contamination can be also recognized in the fine deposits of this part of the shelf, since for other elements, such as Pb or Zn, values are similar in the both areas and the Sado estuary acts as an additional source of heavy metals (Quevauviller et al., 1986).
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Fig. 12.
4.5. Vertical distribution Two box cores were collected in the Tagus estuary, one in the outer part (T1), the other upstream of Lisbon (T58). Significant differences were measured between these two sites, so we have chosen the sample located close to the inlet (T1) to give us an estuarine reference (the background of estuarine sediments) in the vertical study of the continental shelf sediments. From the distribution of Zn (normalized to Al) shown in Fig. 13, it is clear that there is an enrichment (by a factor of 2) in the surface levels from those values considered as geochemical background. Moreover, this enrichment extends down through the upper 5–6 cm despite this layer being coarser (mean grain size 17 m) and having a higher carbonate content (8%). The deeper layers of the sediment samples have a constant heavy metal content and the sediments are more homogeneous (mean grain size of 12 m and CaCO3 content of 6%). One conclusion of this vertical distribution is that there is strong evidence for contemporary anthropogenic contamination and the role being played by estuaries in supplying these elements to the SPM on the adjacent continental shelf.
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Fig. 13.
On the shelf, sedimentological and geochemical analysis was carried out on two box cores collected in the mud patch at 100 m depth (Fig. 14). It can be noted that, although the sediments are quite different (T20 is finer than T21), the vertical distributions of metals show no evidence of vertical gradients. So we can conclude that bioturbation is very active in the muddy deposits of the shelf, and secondly that present day sedimentation rates are very high. These conclusions are confirmed by radionuclide profiles (Fig. 15). Notwithstanding the disturbance induced by bioturbation 210Pb-excess profiles allow the evaluation of the sedimentation rate in this estuarine area to be 0.16 cm y−1. This value is similar to that obtained by Carvalho (1995) in the Tagus estuary who reported sedimentation rates ranging from 0.03 to 0.30 cm y−1. The 210Pb profiles are assumed to be dominated by sedimentation, not bioturbation. Away from the estuary mouth, profiles of radionuclides were measured in four cores, collected from depths ranging from 50 m depth (T14) to 100 m depth (T21, T23 and T25). In general, all of them were severely affected by bioturbation, and so, the estimates of the sedimentation rate based on 210Pb excess must be considered as maximum values. The four samples provided estimates of the sedimentation rate ranging between 0.16 and 2.13 cm y−1, with a Db (mixing rate) ranging between 2.01 ⫻ 10−4 cm2 y−1 and 3.37 ⫻ 10−8 cm2 y−1. In the extreme case one could consider that the present day sedimentation processes are efficiently removing the total supply on this margin.
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Fig. 14.
5. Discussion 5.1. Is the Portuguese margin fed by present day sediment supply and what are the most favourable conditions for the transfer of SPM to the shelf? The two estuaries that feed this continental margin have distinct impacts. The SPM values in the mouth of the Tagus are four times higher than in the Sado. This confirms the previous observations of Vale & Sundby (1987) and Loureiro, Nunes, & Bothelo (1982), and reflects the relative size of the two rivers’ drainage basins. Moreover, in the Sado the majority of the riverborne matter is trapped within the estuary itself, so the Tagus estuary is the main source of sediment for the adjacent shelf. On the Tagus shelf, the transfer of SPM occurs preferentially in two nepheloid
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Fig. 15.
layers, a surficial and a bottom layer. The magnitude of the transfer through the nepheloid layers depends on seasonal conditions. It should be noted that all three years of this study were classified as dry years, so we did not have a typical winter season. Hence we have only characterized the annual and interseasonal variability of the turbid plumes during dry years. During the summer the surface nepheloid layer is always larger than during the winter when it is restricted to near the mouth of the estuary. This nepheloid layer may extend westwards for 30 km during summer. This can be explained by two separate factors; first, the effects of northerly winds that are typical during this time of the year in enhancing the extension of the plume; and secondly, the increase of biogenic contribution all over the shelf, which is probably related to increases in the
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input of nutrients from the estuary, and to the remnants of the spring bloom enhanced by upwelling events (see POC distribution maps). A rough estimate of the stock of POC in the upper metre of the water column during the successive cruises (Plutur 2–6) shows that the summer standing stock (158 g l−1) is double that of winter (84 g l−1). The higher values found in this bottom layer can be explained if we take into account the bottom sediment distribution in this part of the shelf. As previously mentioned, the predominant sediments in this zone are sands and muddy sands. The high values of the currents measured in this zone, in the order of 45 cm s−1, are sufficient to prevent deposition of sediments and even to resuspend them. With the analysis of the mean grain size distribution maps and the optical observations results, we can identify periods of both higher and lower dispersion of terrigenous material in different seasons, although this material is still always dominant. This indicates that the Tagus river acts especially as a supplier of terrigenous particles, while the great majority of the ‘continental’ organic matter is retained inside the estuary. On the Sado shelf, turbid plumes do not have such sharp differences between their seasonal and interannual distributions. In this area we have always detected a plume with low nephelometer values in the order of 1 ftu, regardless of the season. 5.2. Sedimentological evidence of present day trapping on the margin The grain size analysis of SPM coming from the Tagus estuary has a median range of about 7–15 m (this study), similar to the fine deposits found on the shelf (Dias, 1987). This fact, together with the thickness of the muddy deposits (maximum of 25 m, integrated over the last 20,000 years, Rodrigues & Matos, 1994) indicates that sedimentation rates are high over the muddy deposit off the Tagus estuary. The values of the sedimentation rates calculated here, from the 210Pb excess vary between 0.16 and 2.13 cm y−1 assuming bioturbation is negligible (i.e. Db ⫽ 0) hence they correspond to the maximum possible rate. If we consider that the estimated amount of suspended matter discharged from the Tagus estuary is between 0.4 and 1 ⫻ 106 t y−1 (Vale & Sundby, 1987) and that is all deposited on the area of fine deposits which covers about 560 km2, then the annual thickness of deposited sediments should be between 0.07 and 0.18 cm. Moreover, since the boundaries to the general spreading of maximum BNL ( > 2 ftu) coincides with the limits of the mud patch, the implication is that the muddy patch is directly nourished by suspended particles of this layer (and vice versa). These particles can be resuspended during winter, as it is shown in nephelometric profiles. The terrestrial origin of the settled particles is evident from their levels of metallic contamination (Pb, As, Zn, etc.). The pattern of contamination shows that highest values occur where the sedimentation of BNL particles is highest: on the submarine delta front near the Lisbon submarine canyon. On the shelf this contamination decreases with distance from the estuary mouth. The results from the box core samples clearly show that the anthropogenic impact is in the uppermost level (5 cm thick) in the Tagus estuary and in all the sedimentary
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deposits (15 cm thick) on the shelf muddy area. The homogeneous vertical distribution of metallic elements in the samples can be explained by the intense biological activity that should be enhanced by fresh supplies of nutrients. Visual core observation and X-ray analyses (Fig. 16) indicate the high intensity of mixing in the surficial sediment, revealing a mottled facies without physical structures with numerous burrows. The POC distribution shows evidence of the high productivity in this part of the Portuguese margin. Taking into account that the mean concentration of POC in the fine deposits of the shelf (muds to silty sands) is 1% and that these muds cover around 560 km2 to a depth of 1 cm, then 81,000 t of POC are estimated to be trapped there. This would represent, with a minimum sedimentation rate between 0.07 and 0.18 cm y−1, an entrapment of 6000–15,000 t POC y−1. The origin of this amount of POC can be questioned. The POC distribution in the sedimentary cover cannot be explained by the river supply alone. Sediments with lower POC values occur near the estuary mouth, probably because the sediments are coarser, the sedimentation rate is low and the local resuspension processes and further mineralization. The highest POC value measured was on the outer shelf off Cape Raso, and was not directly related to the muddy depocenter. It is our belief that the POC pattern in sediments
Fig. 16.
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could reflect physical oceanographical processes on this margin such as vortices induced by the coastal irregularity, or upwelling events near Cape Raso.
6. Conclusions The six cruises realized over three years on the Portuguese margin in front of the Tagus and the Sado estuaries allowed use of hydrological, sedimentological and geochemical approaches to study the sedimentation on this narrow margin affected by a relatively high continental input. The magnitude of present day supply towards the oceanic domain by a benthic nepheloid layer is demonstrated. Despite the narrowness of the shelf, a large part of continental fluxes does not reach the deep ocean. Only where the heads of canyons seem to act as a sink is there a potential pathway for material to reach the deep ocean. From this study, it appears that the organic carbon storage on the shelf is dominant over export to the deep ocean. In addition to the importance of direct supplies originating in estuaries, canyons also contribute to the organic carbon flux on the shelf, as by favouring upwelling events, the additional nutrients enhance higher productivity over the shelf.
7. Acknowledgements This study has been supported by the contract No. MAS2-CT93-0069 (OMEX programme) of the EU. We thank G. Lavaux for ICP–MS measurements. We are very grateful to Dr D. Eisma and Dr G. Liebereit and the editors who greatly improved the manuscript.
8. References Allen, G.P., & Castaing, P. (1973). Suspended sediment transport from the Gironde estuary (France) onto the adjacent continental shelf. Marine Geology, 14, 47–53. de Bettencourt, A.M.M. (1990). Especiac¸a`o e biogeoquimica do arsenio no estuario do Tejo. The`sis Universidade de Evora. Caˆmara, A.S., Silva, M.C., Ramos, L., & Ferreira, J.G. (1986). Estudo ambiental do estuario do Tejo. Divisao do estuario do Tejo em zonas homogeneas. Secretario do Estado do Ambiente e Recursos Naturais. RPT 5. Carvalho, F.P. (1995). 210Pb and 210Po in sediments and suspended matter in the Tagus estuary, Portugal. Local enhancement of natural levels by wastes from phosphate ore processing industry. The Science of The Total Environment, 195, 201–215. Dias, J.M.A. (1987). Dinaˆmica sedimentar e evoluc¸a˜o recente da plataforma continental portuguesa setentrional. Ph.D. thesis, Universidade de Lisboa. Etcheber, H. (1981). Comparaison des diverses me´thodes d’e´valuation des teneurs en matie`res en suspension et en carbone organique particulaire des eaux marines du plateau continental aquitain. Journal Recherches des Oceanographiques, 2, 37–42.
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J.M. Jouanneau et al. / Progress in Oceanography 42 (1998) 233–257
Fiu´za, A.F.G. (1983). Upwelling patterns off Portugal. In Coastal upwelling: its sediment record (pp. 85– 98). New York: Plenum Press. Fiu´za, A.F.G., Macedo, M.E., & Guerreiro, M.R. (1982). Climatological space and time variation of the portuguese coastal upwelling. Oceanologica Acta, 5, 31–50. Garcia, C. (1997). Dispersao e deposic¸ao da materia particulada transportada em suspensao para a plataforma continental adjacente aos rios Tejo e Sado. Tese de Mestrado, Universidade de Lisboa. Heathershaw, A.D., New, A.L., & Edwards, P.D. (1987). Internal tides and sediment transport at the shelfbreak in the Celtic Sea. Continental Shelf Research, 7, 485–518. Lachance, G.R., & Trail, R.J. (1966). A practical solution to the matrix problem in X-ray analysis. Journal Methodology Canadian Spectroscopy, 11, 43. Lopes Da Costa, Y. (1984). Mare´s no estuario do Sado. Anais do Instituto Hidrografico, 5, 23–33. Loureiro, J.M. (1979). Curvas de durac¸a`o dos caudais medios diarios no rio Tejo. Technical report, Servico Hidraulico (DGRAH), Lisbon. Loureiro, J.M., & Macedo, M.E. (1986). Bacia hidrogra´fica do rio Tejo. In Monografias hidrolo´gicas dos principais cursops de a´gua de Portugal continental (pp. 281–337). Direcc¸a˜o Geral dos Recursos e Aproveitamentos Hidra´ulicos. Loureiro, J.M., Nunes, F.M., & Bothelo, F.O. (1982). Monografia hidrologica do rio Sado. Ministerio Habitac¸ao Obras Publicos e Transportes. Lisboa, Relatorio. Loureiro, J.M., Nunes, F.M., & Bothelo, F.O. (1986). Bacia Hidrogra´fica do rio Sado. In Monografias hidrolo´gicas dos principais cursos de a´gua de Portugal continental (pp. 411–464). Direcc¸a˜o Geral dos Recursos e Aproveitamentos Hidra´ulicos. McCave, I.N. (1972). Transport and escape of fine-grained sediment from shelf areas. In Shelf sediment transport: processes and pattern (pp. 225–248). Stroudsburg, PA: Dowden, Hutchinson and Ross. Melo, F. (1981). Estudo da populac¸a˜o fitoplanto´nica no canal norte do estua´rio do Sado. Esta´gio de licenciatura, Universidade de Lisboa. Moita, I., & Quevauviller, P. (1986). Les diffe´rents facie`s se´dimentaires de la plateforme continentale nord alentejanaise. comparaison avec l’ensemble continental. Boletin Informac¸ao Sociedade Geologico Portugal, 13, 30. Monteiro, J.H., & Moita, I. (1971). Morfologia e sedimentos da plataforma continental e vertente continental superior ao largo da peninsula de Setu´bal (pp. 301–330). Direc¸ao Geral Minerais e Servic¸o Geologico. Mougenot, D. (1988). Ge´ologie de la marge portugaise. The`se Doct. d’Etat. Univ. Paris VI. Paiva, P., Jouanneau, J.M., Araujo, F., Weber, O., Rodrigues, A., & Dias, J.M.A. (1997). Elemental distribution in a sedimentary deposit on the shelf off the Tagus estuary (Portugal). Water Air and Soil Pollution, 99, 507–514. Quevauviller, P. (1987). Etude ge´omorphologique, se´dimentologique et ge´ochimique du littoral de Gale´ et de l’estuaire du Sado (Portugal). The`se 3 e`me Cycle, Univ. Bordeaux I. Quevauviller, P., Jouanneau, J.M., Latouche, C., & Pera, T.R. (1986). Trace metals elements of finegrained sediments of the Sado estuary (Portugal) In Proceedings of the International Conference ‘Chemicals in the Environment’ (pp. 569–577). Lisbon, July 1986. Rodrigues, A., & Matos, M. (1994). Distribuc¸ao sedimentar do plataforma continentale portuguese entre Sines e Ericeira. 1st Symposio Margem Continental Iberica Atlantica. Abstract, 48. Ruch, P., Mirmand, M., Jouanneau, J.M., & Latouche, C. (1993). Sediment budget and transfer of suspended sediment from the Gironde estuary to Cap Ferret canyon. Marine Geology, 111, 109–119. Shepard, F.P. (1954). Nomenclature based on sand–silt–clay ratios. Journal Sedimentary Petrology, 24, 151–158. Southard, J.B., & Stanley, D.J. (1976). Shelf-break processes and sedimentation. In Marine sediment transport and environmental management (pp. 351–377). New York: J. Wiley and Sons. Strickland, J.D.H., & Parsons, T.R. (1972). Determination of particulate carbon. In A practical handbook of seawater analysis (pp. 207–211). Ottowa: Fisheries Research Board of Canada. Vale, C. (1981). Input of suspended particulate matter in the Tagus estuary during the flood of February 1979. Recursos Hidricos, 2, 37–45. Vale, C. (1990). Temporal variations of particulate metals in the Tagus river estuary. The Science of the Total Environment, 97–98, 137–154.
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Vale, C., & Sundby, B. (1987). Suspended sediment fluctuations in the Tagus estuary on semi-diurnal and fortnightly time scales. Estuarine Coastal and Shelf Science, 25, 495–508. Vanney, J.R., & Mougenot, D. (1981). La plate-forme continentale du Portugal et les provinces adjacentes: analyse ge´omorphologique. Memo´rias Servic¸os geolo´gicos de Portugal (Vol. 28). Weber, O., Jouanneau, J.M., Ruch, P., & Mirmand, M. (1991). Grain size relationship between suspended matter originating in the Gironde estuary and shelf mud-patch deposits. Marine Geology, 96, 159–165. Wooster, W.S., Bakun, A., & McLain, D.R. (1976). The seasonal upwelling cycle along the eastern boundary of the North Atlantic. Journal of Marine Research, 34, 131–141.
Dispersal and deposition of suspended sediment on the shelf off the Tagus and Sado estuaries, S.W. Portugal J.M. Jouanneaua,*, C. Garciab, A. Oliveirac, A. Rodriguesb, J.A. Diasb, O. Webera a
DGO, UMR-CNRS 5805, Avenue des faculte´s, 33405 Talence cedex, France b Instituto Hidrografico, rua das Trinas 49, 1296 Lisbon codex, Portugal c UCTRA, Universidade do Algarve, 8000 Faro Portugal
Abstract The Portuguese margin in front of the Tagus and Sado rivers is characterized by a narrow shelf incised by numerous canyons and by a large mud deposit. The two estuaries that feed this continental margin have distinct impact. The suspended particulate matter concentration values in the mouth of the Tagus are four times higher than in the Sado. During the summer the surface nepheloid layer is always larger than during the winter when it is restricted near the mouth of the estuary. This nepheloid layer may reach 30 km in length extending westward. The bottom nepheloid layer usually shows higher nephelometer values, and has a typical distribution: it is usually diverted southward in the direction of the Lisbon Submarine Canyon. We estimate the amount of suspended matter being discharged annually from the Tagus estuary to be between 0.4 and 1 ⫻ 106 t. The area covered by fine deposits is about 560 km2. Hence the thickness of sediments deposited annually should be between 0.07 and 0.18 cm. The sedimentation rates calculated from the 210Pb excess vary between 0.16 and 2.13 cm y−1 which correspond to the maximum rate. For a layer of 1 cm thick, 81,000 t of particulate organic carbon (POC) should be trapped. That would represent, with a minimum sedimentation rate between 0.07 and 0.18 cm y−1, an entrapment of 6000–15,000 t POC y−1. The trace metals content of box core samples clearly shows the anthropogenic impact in the uppermost level (5 cm thick) in the Tagus estuary and in all the sedimentary deposits (15 cm thick) on the shelf muddy area. Despite the narrowness of the shelf, a significant part of continental fluxes fails to reach the deep ocean. 1998 Elsevier Science Ltd. All rights reserved.
* Corresponding author. 0079-6611/98/$ - see front matter 1998 Elsevier Science Ltd. All rights reserved. PII: S 0 0 7 9 - 6 6 1 1 ( 9 8 ) 0 0 0 3 6 - 6
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1. Introduction The Portuguese margin in front of the Tagus and Sado rivers is characterized by a narrow shelf incised by numerous canyons (Vanney & Mougenot, 1981; Mougenot, 1988). The canyons of Lisbon, Cascais and Setu´bal extend into the continental shelf. The Tagus and Sado estuaries have been well studied in terms of their geochemistry and sedimentary budgets (Vale, 1981, 1990; Vale & Sundby, 1987; Caˆmara, Silva, Ramos, & Ferreira, 1986) but the study of the adjacent continent shelf is only just beginning. On the shelf proper, a major area of mud deposit is linked to the estuarine supplies of the Tagus and to a lesser extent to the Sado estuary, Dias (1987). The principal mud deposit off the Tagus is very well defined and extends out to the 70 m isobath. It may act, to an unknown extent, as a temporary site of deposition for suspended matter, which eventually escapes to the open sea. To investigate the ongoing processes occurring across the continental shelf and into deep environments DGO (University of Bordeaux 1) and the Instituto Hidrografico selected a restricted study area on the Iberian margin. The aims of this study were to understand the sedimentary dynamics associated with major rivers and give an approximate quantification of sediment budgets (sources, sinks and main transport paths) over the continental shelf. The area lies within parallels 38°10⬘N and 38°45⬘N. The eastern and western boundaries are the coastline and the shelfbreak, respectively (Fig. 1). For the purpose of this paper the area constitutes a particularly interesting zone for the following reasons: 1. It is fed by two important river systems, the Tagus and the Sado, whose basins drain populated and industrialized regions of Portugal. Hence inputs of anthropogenic origin can be recognized, thus permitting the characterization of the distribution and supply patterns on the margin; 2. The Sado is the smaller of the two rivers being only 175 km long and with minor tributaries. The Tagus, on the other hand, is the most important river of the Iberian peninsula, longer than 1000 km, and with some of its affluents as important as the Sado river; 3. Another favourable factor concerns the geomorphology. Its particular morphological features (which are related to geological structures) allow good characterization of terrestrial fluxes to the deeper domains. The margin being narrow with the shelfbreak less than 30 km offshore, and the continental slope being incised by canyons might be expected to provide favourable conditions for the rapid transfer of the suspended matter into deep water. Nevertheless, the presence of large mud deposits in the middle shelf region is not entirely consistent with such rapid transfer. It seems that the continental shelf may be acting as a depositional centre to some extent. This study has been supported by six cruises in both winter and summer conditions paying particular attention to interannual and seasonal variability. During these cruises, CTD stations and sampling of suspended particulate matter (SPM) and surficial sediments have been carried out.
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Fig. 1.
2. Regional setting 2.1. Study area The continental shelf off the mouths of the estuaries has a width ranging from 3 to 30 km, with the shelfbreak located at 140 m depth. On this continental shelf the submarine protofluvial deltas of the two studied rivers originate from the accumulated sediments discharges from the estuaries (the Sado delta is smaller and is confined to the outlet). On the outer continental shelf and slope three main submarine troughs: the Cascais Canyon, the Lisbon Canyon and the Setu´bal Canyon are related to geological features (Mougenot, 1988). 2.2. Hydrology and oceanography The Tagus is the principal source of fresh water input to this part of the Iberian Continental margin (Table 1). The river discharge normally shows a pronounced dry/wet season signal as well as large inter-annual variation. The mean annual aver-
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age discharge of the Tagus is 364 m3 s−1 (ranging between 80 and 720 m3 s−1, Loureiro, 1979; Loureiro & Macedo, 1986), with monthly average discharge varying from 1 to 2200 m3 s−1. In the Tagus estuary, the SPM concentration is considered to be low, with values ⬍ 30 mg l−1 (Caˆmara et al., 1986), but with a great variability in time and space, related to the discharge of this river. Vale (1981) recorded a maximum value of turbidity in the order of 500 mg l−1 in the upper estuary during spring tide movement of this turbid zone into the estuary mouth (Vale & Sundby, 1987). The average suspended sediment load discharged by the Tagus into the estuary is 4 ⫻ 105 t y−1 but may reach 1 ⫻ 106 t y−1 during those years when there are several flood events (Vale & Sundby, 1987). The annual mean discharge of the Sado river system is always less than 10 m3 −1 s (Loureiro, Nunes, & Bothelo, 1982, 1986). Turbidity values are typically lower than in the Tagus estuary (Table 1). Melo (1981) reports values between 4 mg l−1 (December) and 24 mg l−1 (April). Highest values are related to the phytoplankton spring bloom and not to terrigenous inputs of the SPM. These values reflect the minor contribution by this river of terrestrial materials to the continental shelf sedimentary deposits. Considering the sedimentological and morphological characteristics of the Sado estuary, Quevauviller (1987) concluded that the lower estuary traps the majority of the fine sediments restricting their transit out over the continental shelf. Current velocities range from 1.2 to 2 m s−1 for the Tagus and between 0.48 and 0.60 m s−1 for the Sado river (Lopes da Costa, 1984). On the Portuguese continental margin, the tide has a semi-diurnal regime (period of 12 h 25 min) and the tidal range varies between 1.5 m (neap tides) and 3.5 m (spring tides). Tidal currents are weak except in the estuaries. The swell and winds from the west and northwest are dominant, but can change seasonally. In winter, storm events are characterized by strong winds from the southwest, and in summer by persistent winds from the north, which favour upwelling (Wooster, Bakun, & McLain, 1976; Fiu´za, Macedo, & Guerreiro, 1982; Fiu´za, 1983). 2.3. Sedimentary cover The sedimentary cover of the shelf near the Tagus and Sado river mouths was virtually unknown until the mapping of surficial sediments was started in the 1970s with the SEPLAT Programme (carried out by the Hydrographic Institute of Portugal). Nevertheless, the sediment budgets (sources, sinks and main transport paths) and distribution patterns of suspended matter are still an open question. Sediment maps of this particular area of the Iberian margin show that there are some peculiar deposits covering the sea bottom. Their presence results from to the geomorphologic setting and the effects of sea level rise since the last glaciation. In fact, the sedimentary cover of the Iberian margin is very young (less then 20,000 years), and is composed of late Quaternary and Holocene sediments (Dias, 1987). The sedimentary deposits off the Tagus river are essentially muddy (Fig. 2). The area is protected against swell from the northwest and since the Tagus acts as an important source of fine particles, almost the entire continental shelf is covered by
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Fig. 2.
a large expanse of mud deposit. Coarse sandy deposits occur elsewhere on the shelf, but are restricted to the northern sector where there are several rocky outcrops. Sandy deposits occur on the inner shelf, where there are high energy littoral currents. There are large bodies of sandy and gravelly sands on the outer shelf, near the shelfbreak. These are a remnant of an old shoreline with an age of 苲 16,000 years B.P. that have not become covered by recent muddy deposits because of prevailing dynamics of the outer shelf (Dias, 1987). In this area, strong turbulence and currents either keep fine sediment in suspension or frequently resuspend it, so that fine material that is supplied from nearshore areas is transported out over the continental slope or beyond without becoming permanently deposited near the shelfbreak (Southard & Stanley, 1976). This part of the shelf is also affected by internal waves, especially during periods when water masses are stratified. As stated by Heathershaw, New, & Edwards (1987), the outer shelf in the vicinity of shelfbreak is likely to be an area of bed load parting or divergence. The sedimentary deposits covering the continental shelf adjacent to the Sado river are mainly sandy mud. Muddy sediment areas are restricted to the northern coast of
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the bay in front of Sesimbra and to the Setu´bal canyon (Monteiro & Moita, 1971; Moita & Quevauviller, 1986). The composition of sedimentary cover is dominated by terrigenous particles (where quartz prevails), and so can be classified as a lithoclastic sediment (with a carbonate percentage less than 30%). Higher percentages of carbonate (CaCO3) particles are to be found in the sandy deposits of the outer shelf, where they can exceed 50% wt/wt (Rodrigues & Matos, personal communication). If one considers the spatial variability of sediments, there is a decrease in the textural characteristics, with coarser material near the Tagus river mouth and the finer material at the deeper parts (more than 50 m deep). This reflects in part the dynamics of the Tagus discharge outflow.
3. Methodology From 1993 to 1995, six cruises were undertaken in the study area in order to evaluate seasonal and interannual variability in the transfer dynamics of particulate matter from continent to oceanic sedimentary systems. Table 2 provides a description of those summer and winter cruises, promoted by the Portuguese Hydrographic Institute and the University of Bordeaux I (France). 3.1. Field measurements CTD profiles were measured using a Hydropolytester/Nephelometer ZULLIG probe. One hundred and sixty-one surface water samples were collected and filtered with a direct pressure filtration system using three types of filters: (1) and (2) a mixed cellulose ester (Millipore and Sartorius; 47 and 142 mm diameter with 0.45 m pore size) for evaluation of SPM concentration, grain size analysis and optical observations; (3) a glass microfibre Whatman GF/F for quantification of the particulate organic carbon (POC) content of the SPM. Eighty sediment samples were collected with a Smith-McIntyre grab; 30 of these were subsampled to compare the sediment characteristics at 0–1 cm and 1–2 cm. These sediment samples were used to study the bottom sedimentary cover, the sediment/water interface and the heavy metal distribution. Cores sampled were also obtained with a box-corer or a minicorer MARK I, ranging from few centimetres to 30 cm long, in order to study the sediment/water interface. 3.2. Laboratory analysis Sediment grain size analyses were performed by the classical physical method of sieving and the use of settling columns. Sediments were classified according to their gravel ⫹ sand/silt/clay ratios according to Shepard’s classification (Shepard, 1954). All filters were dried (T ⬍ 30°C) and the 47 mm filters were weighed for SPM concentration. The 142 mm filters were examined under a binocular magnifying glass ( ⫻ 100) to identify the principal organic/inorganic components of the SPM. The
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grain size was analysed using a MALVERN 3600 E laser diffraction instrument. The POC contents of sediment and water samples were determined using the Strickland & Parsons (1972) method as adapted by Etcheber (1981). Its content was measured with a LECO CS–125 equipment. The CaCO3 content was determined by a gasometric method showing a fairly low CaCO3 relative variation ( ⬍ 2%) in duplicate measurements. Major and trace elements were analyzed by X-ray fluorescence spectrometry on dried and pulverized samples of bulk sediment. Analyses were conducted using a Philips 21PW 1500-10 analyser, which allows multi-element analysis. Inter-element effects were reduced by means of equations taken from Lachance & Trail (1966). Results were calibrated against international synthetic rock standards of known composition provided by USGS and ANRT. To define sources, REE and trace elements were determined on selected samples (10) by ICP–MS. Sub-samples weighing about 10 mg were dissolved for 12 h in pressurized PTFE beakers at a temperature of about 125°C in a ⬇ 6N equimolar mixture of [HF ⫹ HClO4 ⫹ HNO3]. Elemental concentrations were determined using an ICP–MS Elan 5000 P.E.-Sciex. MESS 2 and BCSS 1 standards from NRCC have been systematically analyzed, and enabled us to achieve precision better than 10% for all elements. Accuracy was found to be 5% at 10 l l−1. The instrument was operated under standard settings in nearly clean room conditions (US-class 10,000). In five box-cores, 210Pb (46.5 keV) measurements were done on dried bulk sediment samples, using a high resolution gamma-spectrometer with a semi-planar detector (Intertechnique EGSP 2200-25-R). 210Pb excess have been calculated from: 210Pb total ⫺ 226Ra (226Ra is counted from its daughters: 214Bi and 214Pb). The sample (6– 10 g) was counted for 10–20 h.
4. Results 4.1. Suspended particulate matter distribution The nephelometric distributions show no major interannual differences. But seasonal variability of estuarine inputs is clearly demonstrated (Figs. 3 and 4), similar for instance to that observed in the Bay of Biscay off the Gironde estuary (Allen & Castaing, 1973). Despite the low values of surface SPM concentrations (1–20 mg l−1) the surface turbidity in both seasons, always shows higher nephelometric values of 5–8 ftu near the estuarine mouths (1 ftu,—Formazine Turbidity Unit— 苲 1.7 mg l−1). In summer, the surficial turbid layer is more extensive (it may reach 30 km in length) and is always westward orientated. The bottom turbid layer is characterized by higher values of nephelometry (6–8 ftu near the coast) and shows a southward trend towards the Lisbon Canyon. In winter there is strong mixing of the water column and both layers are located near the estuary mouth. The turbid plume associated with the Sado river is always restricted to its outlet, while the Tagus dominates the hydrological distribution of the continental shelf.
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Fig. 3.
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Fig. 4.
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During the winter survey (November–December 1994) and summer survey (June– July 1995) typical hydrological sections (Fig. 5) off the Tagus and Sado outlets show different patterns. In the summer, turbidity (Fig. 6) defines a surficial water mass with estuarine characteristics that stretches out over the shelf to the shelfbreak, while a benthic nepheloid layer (BNL) develops at the level of the shelfbreak (section B– D—Tagus and B—Sado). During the winter, a well defined difference develops near the shelfbreak between the turbid layers at the surface and at the bottom, which become separated by clear waters. The BNL is always more developed than the surface nepheloid layer and can reach a thickness of 150 m (section B–D—Tagus). Some intermediate turbid layers related to the shelfbreak may spread out into the open sea, and are directly related to the water stratification, as have been noted previously along other margins e.g. by McCave (1972) and Ruch, Mirmand, Jouanneau, & Latouche (1993).
Fig. 5.
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Fig. 6.
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4.2. Particulate organic carbon distribution in SPM The analysis of the POC distribution in the surface waters near the Tagus and Sado river mouths shows the importance of those rivers in the overall contribution of nutrients to the continental shelf. Normally the waters near the Tagus adjacent shelf have POC values two orders of magnitude greater than Sado. During winter [Fig. 7(A)], as expected, there is a decrease in the POC content (less than 150 g l−1), which is related to lower biological production and the great percentage contribution of the terrigenous fraction. In summer [Fig. 7(B)], SPM is richer in POC ( > 200 g l−1) associated not only with the river nutrient inputs but also with upwelling which induces a bloom, observed in the vicinity of the Roca Cape (waters with lower temperature). 4.3. Mean grain size At the surface, the mean grain size of SPM (Fig. 8), during all cruises, shows a tendency to increase in diameter towards the shelfbreak. Whatever the season, the estuarine water mass shows a dominant mode at 5.4 m grain size that mainly corresponds to fine terrigeneous material observed on the filters. On the other hand, the oceanic waters show a multimodal distribution with the coarser peaks differing with the season. These different distributions may be related to the variation of the biogenic components of the SPM, as it has been shown previously off the Gironde estuary (Weber, Jouanneau, Ruch, & Mirmand, 1991). The optical observation made by Garcia (1997) shows that, in these oceanic waters, the CaCO3 phytoplankton (foraminifera), the zooplankton (copepods) and organic detritus are dominant, and contribute the coarser modes in the grain size distribution. 4.4. Water/sediment interface 4.4.1. Particulate organic carbon POC values are generally higher on the Tagus margin than on the Sado margin. Nevertheless on the Tagus margin, two opposite gradients can be observed from a maximum centre (5%) located in the northwest of the mud area, at 100 m depth, as shown in Fig. 9, while on the Sado continental shelf there is a simple gradient extending out from the estuary mouth. At the shelfbreak the POC contents have an average of 1% and the minimum values (0.5%) were measured in coarse sediments. The grain size factor explains partially the POC pattern (Fig. 10). There is a clear decrease in POC content with the increase of the median grain size. This trend is particularly well defined on the Tagus continental shelf, although there are some exceptions to this relationship. It seems that there is a dominant hydrodynamic control superimposed on the estuarine source proximity. Thus, the westward extension of the surface nepheloid layer seems to be the main factor inducing the high carbon content in surficial sediments. The distribution of the POC vs depth (Fig. 11) does not show a clear decrease as may be expected from results from other similar environments. The samples collected
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Fig. 7.
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Fig. 8.
Fig. 9.
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Fig. 10.
Fig. 11.
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from the vicinity of the Tagus canyon at depths of 350 and 450 m had POC contents as high as those of samples from 150 m on the shelf. There are two hypotheses to explain this pattern related to the origin of this fresh organic matter. First, if the organic material is of estuarine or continental origin then, the high values measured in the canyon are probably the result of a downward transport in a BNL. Alternatively, if there is no relationship between the sources of the organic matter at 150 and 350 m, then the high values observed at the head of the canyon probably result from an increase in primary production related to upwelling events in surficial water.
4.4.2. Trace metals on sediments Because of their origin and behaviour in sedimentary dynamics, Zn, Pb and As have been measured in surficial sediment samples. They are good markers of present day human activity, as shown by de Bettencourt (1990), concerning the As contamination of the Tagus estuary and Vale (1990) and Paiva, Jouanneau, Araujo, Weber, Rodrigues, & Dias (1997) concerning other heavy metals. Trace metal contents are higher on the continental shelf adjacent to Tagus river than off the Sado, as a result of the different roles of the estuaries as sources of material to sedimentary deposits: the Tagus acts as a main source while the Sado has a limited and weak impact on the nearby oceanic systems. Normalized to aluminium, Zn, Pb and As show a decrease by a factor of 3 between the highest values, near the estuary mouths, and the lowest values close to the shelfbreak. To illustrate this distribution the CaCO3 Pb/Al2O3 data are presented in Fig. 12. Fig. 12 suggests that the continued removal of these trace metals occurs between the estuaries and the shelfbreak. This is confirmed by the poor relationship established between the CaCO3 and trace metal content and grain size, indicating that removal processes are preponderant over grain size effects. Thus trace metals may act as good markers of estuarine outputs and of estuarine impacts on the margin. The trace metal distributions do not show strictly the same patterns as the POC. Even if one observes a trace metal content decrease with increasing water depth, it appears that, trace metals originate mainly from the BNL and not from the surficial plume, as suggested by the southward gradient of metals contents. Then it could be considered that supply of the terrigenous components on the margin is principally southward in direction of Cascais and Lisbon canyons via a benthic process. This behaviour is the opposite of that of POC, which is more related to the surface nepheloid layer. Results of ICP–MS analysis CaCO3 (Table 3) confirm these observations. Results are expressed by the enrichment factor (normalized to Sc) in relation to the Tagus (T–1). It can be noted, in relation to As, Se, Ni, Mo, that the Sado shelf forms a specific domain where contributions from the Tagus estuary are in a minority. Nevertheless, we note that the present day contamination can be also recognized in the fine deposits of this part of the shelf, since for other elements, such as Pb or Zn, values are similar in the both areas and the Sado estuary acts as an additional source of heavy metals (Quevauviller et al., 1986).
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Fig. 12.
4.5. Vertical distribution Two box cores were collected in the Tagus estuary, one in the outer part (T1), the other upstream of Lisbon (T58). Significant differences were measured between these two sites, so we have chosen the sample located close to the inlet (T1) to give us an estuarine reference (the background of estuarine sediments) in the vertical study of the continental shelf sediments. From the distribution of Zn (normalized to Al) shown in Fig. 13, it is clear that there is an enrichment (by a factor of 2) in the surface levels from those values considered as geochemical background. Moreover, this enrichment extends down through the upper 5–6 cm despite this layer being coarser (mean grain size 17 m) and having a higher carbonate content (8%). The deeper layers of the sediment samples have a constant heavy metal content and the sediments are more homogeneous (mean grain size of 12 m and CaCO3 content of 6%). One conclusion of this vertical distribution is that there is strong evidence for contemporary anthropogenic contamination and the role being played by estuaries in supplying these elements to the SPM on the adjacent continental shelf.
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Fig. 13.
On the shelf, sedimentological and geochemical analysis was carried out on two box cores collected in the mud patch at 100 m depth (Fig. 14). It can be noted that, although the sediments are quite different (T20 is finer than T21), the vertical distributions of metals show no evidence of vertical gradients. So we can conclude that bioturbation is very active in the muddy deposits of the shelf, and secondly that present day sedimentation rates are very high. These conclusions are confirmed by radionuclide profiles (Fig. 15). Notwithstanding the disturbance induced by bioturbation 210Pb-excess profiles allow the evaluation of the sedimentation rate in this estuarine area to be 0.16 cm y−1. This value is similar to that obtained by Carvalho (1995) in the Tagus estuary who reported sedimentation rates ranging from 0.03 to 0.30 cm y−1. The 210Pb profiles are assumed to be dominated by sedimentation, not bioturbation. Away from the estuary mouth, profiles of radionuclides were measured in four cores, collected from depths ranging from 50 m depth (T14) to 100 m depth (T21, T23 and T25). In general, all of them were severely affected by bioturbation, and so, the estimates of the sedimentation rate based on 210Pb excess must be considered as maximum values. The four samples provided estimates of the sedimentation rate ranging between 0.16 and 2.13 cm y−1, with a Db (mixing rate) ranging between 2.01 ⫻ 10−4 cm2 y−1 and 3.37 ⫻ 10−8 cm2 y−1. In the extreme case one could consider that the present day sedimentation processes are efficiently removing the total supply on this margin.
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Fig. 14.
5. Discussion 5.1. Is the Portuguese margin fed by present day sediment supply and what are the most favourable conditions for the transfer of SPM to the shelf? The two estuaries that feed this continental margin have distinct impacts. The SPM values in the mouth of the Tagus are four times higher than in the Sado. This confirms the previous observations of Vale & Sundby (1987) and Loureiro, Nunes, & Bothelo (1982), and reflects the relative size of the two rivers’ drainage basins. Moreover, in the Sado the majority of the riverborne matter is trapped within the estuary itself, so the Tagus estuary is the main source of sediment for the adjacent shelf. On the Tagus shelf, the transfer of SPM occurs preferentially in two nepheloid
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Fig. 15.
layers, a surficial and a bottom layer. The magnitude of the transfer through the nepheloid layers depends on seasonal conditions. It should be noted that all three years of this study were classified as dry years, so we did not have a typical winter season. Hence we have only characterized the annual and interseasonal variability of the turbid plumes during dry years. During the summer the surface nepheloid layer is always larger than during the winter when it is restricted to near the mouth of the estuary. This nepheloid layer may extend westwards for 30 km during summer. This can be explained by two separate factors; first, the effects of northerly winds that are typical during this time of the year in enhancing the extension of the plume; and secondly, the increase of biogenic contribution all over the shelf, which is probably related to increases in the
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input of nutrients from the estuary, and to the remnants of the spring bloom enhanced by upwelling events (see POC distribution maps). A rough estimate of the stock of POC in the upper metre of the water column during the successive cruises (Plutur 2–6) shows that the summer standing stock (158 g l−1) is double that of winter (84 g l−1). The higher values found in this bottom layer can be explained if we take into account the bottom sediment distribution in this part of the shelf. As previously mentioned, the predominant sediments in this zone are sands and muddy sands. The high values of the currents measured in this zone, in the order of 45 cm s−1, are sufficient to prevent deposition of sediments and even to resuspend them. With the analysis of the mean grain size distribution maps and the optical observations results, we can identify periods of both higher and lower dispersion of terrigenous material in different seasons, although this material is still always dominant. This indicates that the Tagus river acts especially as a supplier of terrigenous particles, while the great majority of the ‘continental’ organic matter is retained inside the estuary. On the Sado shelf, turbid plumes do not have such sharp differences between their seasonal and interannual distributions. In this area we have always detected a plume with low nephelometer values in the order of 1 ftu, regardless of the season. 5.2. Sedimentological evidence of present day trapping on the margin The grain size analysis of SPM coming from the Tagus estuary has a median range of about 7–15 m (this study), similar to the fine deposits found on the shelf (Dias, 1987). This fact, together with the thickness of the muddy deposits (maximum of 25 m, integrated over the last 20,000 years, Rodrigues & Matos, 1994) indicates that sedimentation rates are high over the muddy deposit off the Tagus estuary. The values of the sedimentation rates calculated here, from the 210Pb excess vary between 0.16 and 2.13 cm y−1 assuming bioturbation is negligible (i.e. Db ⫽ 0) hence they correspond to the maximum possible rate. If we consider that the estimated amount of suspended matter discharged from the Tagus estuary is between 0.4 and 1 ⫻ 106 t y−1 (Vale & Sundby, 1987) and that is all deposited on the area of fine deposits which covers about 560 km2, then the annual thickness of deposited sediments should be between 0.07 and 0.18 cm. Moreover, since the boundaries to the general spreading of maximum BNL ( > 2 ftu) coincides with the limits of the mud patch, the implication is that the muddy patch is directly nourished by suspended particles of this layer (and vice versa). These particles can be resuspended during winter, as it is shown in nephelometric profiles. The terrestrial origin of the settled particles is evident from their levels of metallic contamination (Pb, As, Zn, etc.). The pattern of contamination shows that highest values occur where the sedimentation of BNL particles is highest: on the submarine delta front near the Lisbon submarine canyon. On the shelf this contamination decreases with distance from the estuary mouth. The results from the box core samples clearly show that the anthropogenic impact is in the uppermost level (5 cm thick) in the Tagus estuary and in all the sedimentary
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deposits (15 cm thick) on the shelf muddy area. The homogeneous vertical distribution of metallic elements in the samples can be explained by the intense biological activity that should be enhanced by fresh supplies of nutrients. Visual core observation and X-ray analyses (Fig. 16) indicate the high intensity of mixing in the surficial sediment, revealing a mottled facies without physical structures with numerous burrows. The POC distribution shows evidence of the high productivity in this part of the Portuguese margin. Taking into account that the mean concentration of POC in the fine deposits of the shelf (muds to silty sands) is 1% and that these muds cover around 560 km2 to a depth of 1 cm, then 81,000 t of POC are estimated to be trapped there. This would represent, with a minimum sedimentation rate between 0.07 and 0.18 cm y−1, an entrapment of 6000–15,000 t POC y−1. The origin of this amount of POC can be questioned. The POC distribution in the sedimentary cover cannot be explained by the river supply alone. Sediments with lower POC values occur near the estuary mouth, probably because the sediments are coarser, the sedimentation rate is low and the local resuspension processes and further mineralization. The highest POC value measured was on the outer shelf off Cape Raso, and was not directly related to the muddy depocenter. It is our belief that the POC pattern in sediments
Fig. 16.
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could reflect physical oceanographical processes on this margin such as vortices induced by the coastal irregularity, or upwelling events near Cape Raso.
6. Conclusions The six cruises realized over three years on the Portuguese margin in front of the Tagus and the Sado estuaries allowed use of hydrological, sedimentological and geochemical approaches to study the sedimentation on this narrow margin affected by a relatively high continental input. The magnitude of present day supply towards the oceanic domain by a benthic nepheloid layer is demonstrated. Despite the narrowness of the shelf, a large part of continental fluxes does not reach the deep ocean. Only where the heads of canyons seem to act as a sink is there a potential pathway for material to reach the deep ocean. From this study, it appears that the organic carbon storage on the shelf is dominant over export to the deep ocean. In addition to the importance of direct supplies originating in estuaries, canyons also contribute to the organic carbon flux on the shelf, as by favouring upwelling events, the additional nutrients enhance higher productivity over the shelf.
7. Acknowledgements This study has been supported by the contract No. MAS2-CT93-0069 (OMEX programme) of the EU. We thank G. Lavaux for ICP–MS measurements. We are very grateful to Dr D. Eisma and Dr G. Liebereit and the editors who greatly improved the manuscript.
8. References Allen, G.P., & Castaing, P. (1973). Suspended sediment transport from the Gironde estuary (France) onto the adjacent continental shelf. Marine Geology, 14, 47–53. de Bettencourt, A.M.M. (1990). Especiac¸a`o e biogeoquimica do arsenio no estuario do Tejo. The`sis Universidade de Evora. Caˆmara, A.S., Silva, M.C., Ramos, L., & Ferreira, J.G. (1986). Estudo ambiental do estuario do Tejo. Divisao do estuario do Tejo em zonas homogeneas. Secretario do Estado do Ambiente e Recursos Naturais. RPT 5. Carvalho, F.P. (1995). 210Pb and 210Po in sediments and suspended matter in the Tagus estuary, Portugal. Local enhancement of natural levels by wastes from phosphate ore processing industry. The Science of The Total Environment, 195, 201–215. Dias, J.M.A. (1987). Dinaˆmica sedimentar e evoluc¸a˜o recente da plataforma continental portuguesa setentrional. Ph.D. thesis, Universidade de Lisboa. Etcheber, H. (1981). Comparaison des diverses me´thodes d’e´valuation des teneurs en matie`res en suspension et en carbone organique particulaire des eaux marines du plateau continental aquitain. Journal Recherches des Oceanographiques, 2, 37–42.
256
J.M. Jouanneau et al. / Progress in Oceanography 42 (1998) 233–257
Fiu´za, A.F.G. (1983). Upwelling patterns off Portugal. In Coastal upwelling: its sediment record (pp. 85– 98). New York: Plenum Press. Fiu´za, A.F.G., Macedo, M.E., & Guerreiro, M.R. (1982). Climatological space and time variation of the portuguese coastal upwelling. Oceanologica Acta, 5, 31–50. Garcia, C. (1997). Dispersao e deposic¸ao da materia particulada transportada em suspensao para a plataforma continental adjacente aos rios Tejo e Sado. Tese de Mestrado, Universidade de Lisboa. Heathershaw, A.D., New, A.L., & Edwards, P.D. (1987). Internal tides and sediment transport at the shelfbreak in the Celtic Sea. Continental Shelf Research, 7, 485–518. Lachance, G.R., & Trail, R.J. (1966). A practical solution to the matrix problem in X-ray analysis. Journal Methodology Canadian Spectroscopy, 11, 43. Lopes Da Costa, Y. (1984). Mare´s no estuario do Sado. Anais do Instituto Hidrografico, 5, 23–33. Loureiro, J.M. (1979). Curvas de durac¸a`o dos caudais medios diarios no rio Tejo. Technical report, Servico Hidraulico (DGRAH), Lisbon. Loureiro, J.M., & Macedo, M.E. (1986). Bacia hidrogra´fica do rio Tejo. In Monografias hidrolo´gicas dos principais cursops de a´gua de Portugal continental (pp. 281–337). Direcc¸a˜o Geral dos Recursos e Aproveitamentos Hidra´ulicos. Loureiro, J.M., Nunes, F.M., & Bothelo, F.O. (1982). Monografia hidrologica do rio Sado. Ministerio Habitac¸ao Obras Publicos e Transportes. Lisboa, Relatorio. Loureiro, J.M., Nunes, F.M., & Bothelo, F.O. (1986). Bacia Hidrogra´fica do rio Sado. In Monografias hidrolo´gicas dos principais cursos de a´gua de Portugal continental (pp. 411–464). Direcc¸a˜o Geral dos Recursos e Aproveitamentos Hidra´ulicos. McCave, I.N. (1972). Transport and escape of fine-grained sediment from shelf areas. In Shelf sediment transport: processes and pattern (pp. 225–248). Stroudsburg, PA: Dowden, Hutchinson and Ross. Melo, F. (1981). Estudo da populac¸a˜o fitoplanto´nica no canal norte do estua´rio do Sado. Esta´gio de licenciatura, Universidade de Lisboa. Moita, I., & Quevauviller, P. (1986). Les diffe´rents facie`s se´dimentaires de la plateforme continentale nord alentejanaise. comparaison avec l’ensemble continental. Boletin Informac¸ao Sociedade Geologico Portugal, 13, 30. Monteiro, J.H., & Moita, I. (1971). Morfologia e sedimentos da plataforma continental e vertente continental superior ao largo da peninsula de Setu´bal (pp. 301–330). Direc¸ao Geral Minerais e Servic¸o Geologico. Mougenot, D. (1988). Ge´ologie de la marge portugaise. The`se Doct. d’Etat. Univ. Paris VI. Paiva, P., Jouanneau, J.M., Araujo, F., Weber, O., Rodrigues, A., & Dias, J.M.A. (1997). Elemental distribution in a sedimentary deposit on the shelf off the Tagus estuary (Portugal). Water Air and Soil Pollution, 99, 507–514. Quevauviller, P. (1987). Etude ge´omorphologique, se´dimentologique et ge´ochimique du littoral de Gale´ et de l’estuaire du Sado (Portugal). The`se 3 e`me Cycle, Univ. Bordeaux I. Quevauviller, P., Jouanneau, J.M., Latouche, C., & Pera, T.R. (1986). Trace metals elements of finegrained sediments of the Sado estuary (Portugal) In Proceedings of the International Conference ‘Chemicals in the Environment’ (pp. 569–577). Lisbon, July 1986. Rodrigues, A., & Matos, M. (1994). Distribuc¸ao sedimentar do plataforma continentale portuguese entre Sines e Ericeira. 1st Symposio Margem Continental Iberica Atlantica. Abstract, 48. Ruch, P., Mirmand, M., Jouanneau, J.M., & Latouche, C. (1993). Sediment budget and transfer of suspended sediment from the Gironde estuary to Cap Ferret canyon. Marine Geology, 111, 109–119. Shepard, F.P. (1954). Nomenclature based on sand–silt–clay ratios. Journal Sedimentary Petrology, 24, 151–158. Southard, J.B., & Stanley, D.J. (1976). Shelf-break processes and sedimentation. In Marine sediment transport and environmental management (pp. 351–377). New York: J. Wiley and Sons. Strickland, J.D.H., & Parsons, T.R. (1972). Determination of particulate carbon. In A practical handbook of seawater analysis (pp. 207–211). Ottowa: Fisheries Research Board of Canada. Vale, C. (1981). Input of suspended particulate matter in the Tagus estuary during the flood of February 1979. Recursos Hidricos, 2, 37–45. Vale, C. (1990). Temporal variations of particulate metals in the Tagus river estuary. The Science of the Total Environment, 97–98, 137–154.
J.M. Jouanneau et al. / Progress in Oceanography 42 (1998) 233–257
257
Vale, C., & Sundby, B. (1987). Suspended sediment fluctuations in the Tagus estuary on semi-diurnal and fortnightly time scales. Estuarine Coastal and Shelf Science, 25, 495–508. Vanney, J.R., & Mougenot, D. (1981). La plate-forme continentale du Portugal et les provinces adjacentes: analyse ge´omorphologique. Memo´rias Servic¸os geolo´gicos de Portugal (Vol. 28). Weber, O., Jouanneau, J.M., Ruch, P., & Mirmand, M. (1991). Grain size relationship between suspended matter originating in the Gironde estuary and shelf mud-patch deposits. Marine Geology, 96, 159–165. Wooster, W.S., Bakun, A., & McLain, D.R. (1976). The seasonal upwelling cycle along the eastern boundary of the North Atlantic. Journal of Marine Research, 34, 131–141.