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Sedimentation rates and fluxes in the continental shelf mud fields in the Bay of Biscay (France)
Continental Shelf Research 21 (2001) 1383–1401
Sedimentation rates and fluxes in the continental shelf mud fields in the Bay of Biscay (France) P. Lesueura,*, J.-M. Jouanneaub, D. Boustc, J.-P. Tastetb, O. Weberb UMR CNRS 6143, Morphodynamique continentale et co#tie"re, Universite! de Caen, 24 rue des Tilleuls 14000 Caen Cedex, France b UMR CNRS 5805, De!partement de Ge!ologie et Oce!anographie, Universite! Bordeaux I, Avenue des Faculte!s, 33405 Talence Cedex, France c Laboratoire d ’e!tudes radioe!cologiques de la facade atlantique, IPSN, B.P. 10, 50130 Octeville, France Received 8 May 2000; received in revised form 27 October 2000; accepted 15 November 2000 a
Abstract Sedimentation rates in shelf mud fields of the Bay of Biscay (the Gironde shelf mud patches and ‘‘La Grande Vasi"ere’’) were investigated for a 100-yr time scale, using 210Pb geochronology of nine cores. The distribution of apparent sedimentation rates reveals a general trend along the terrigenous dispersal system from the Gironde outlet of: no or negligible (50.1 cm yr1) cumulative sedimentation in the inner zone of the mud fields; 0.1–0.2 cm yr1 in the outer zone where the supply is reduced; and a maximum value of about 0.5 cm yr1 in the depocentre of the main mud field. Except for the latter value, the 210Pb sedimentation rates are of the same magnitude as longer-term rates based on 14C, and although slightly higher, are in good agreement with an earlier estimation determined for the rate of accumulation of surficial sediments, using pollen analyses. These differences are interpreted as being due to an increase in the supply of fine-grained sediment over the last century (from 0.2 to 0.5 cm yr1), perhaps as a result of an intensification of estuarine suspended matter supply to the shelf. Compilation of a sediment budget shows that about one third of the annual Gironde solid discharge is trapped in the Gironde nearshore shelf mud fields. The mid-shelf muddy belt (i.e. ‘‘La Grande Vasi"ere’’) is the site of accumulation of fine-grained material from various terrigenous sources (mainly from the Gironde and Loire rivers). Two cores from the latter area yielded a maximum rate of sedimentation of 0.2 cm yr1. Mixing processes dominate in this open middle shelf, due to the periodic sediment removal by long-period swells and bioturbation. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Shelf; Mud; Radioisotope; Sedimentation rate; Sediment budget; Bay of Biscay
*Corresponding author. Tel.: +33-2-3156-5754; fax: +33-2-3156-5757. E-mail address: [email protected] (P. Lesueur). 0278-4343/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 2 7 8 - 4 3 4 3 ( 0 1 ) 0 0 0 0 4 - 8
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1. Introduction Evaluations of sedimentation rates in fine-grained deposits from coastal zones and shelves are important because of the association with such deposits of chemical pollutants, their impact on biological processes, and their relationship to stratigraphic considerations, including changes in sedimentary environments. Most of the recently published papers on shelf muds have focused on the large dispersal systems associated with subaqueous-deltas, i.e. Amazon, Ganges– Brahmaputra, Changjiang–Huanghe, Fly River (e.g. Kuehl et al., 1982, 1986; Kuehl et al., 1989; DeMaster et al., 1985; Alexander et al., 1991; Harris et al., 1993). Results from small dispersal systems are less common, partly because they are more complicated due to the relatively greater importance of non-steady state effects of storms, currents and benthic biological activity. This study examines the temporal and spatial variabilities in the rate of fine-grained accumulation on a shelf which experiences a weak continental sediment discharge (i.e. 1.5 106 t yr1, after Castaing and Jouanneau, 1987). Over the 10 last years, sedimentation of mud on the Gironde continental shelf has been the subject of several high resolution sedimentological analyses (Jouanneau and Weber, 1989; Jouanneau et al., 1989; Lesueur et al., 1989, 1991; Lesueur and Tastet, 1994). Nevertheless, nowhere on the entire French Atlantic shelf have any evaluation of sedimentation rates been made on the basis of excess 210Pb. Thus, there was an important gap between evaluations of accumulation rates on the Gironde estuarine intertidal sediments where such studies have been made, and continental margin deposits (Jouanneau et al., 1999). The aim of this paper is (1) to evaluate the sedimentation rates in the shelf mud fields, based on analyses of radioisotope data (i.e. excess 210Pb geochronology) over the last century, (2) to compare these sedimentation rates with those of the longer time interval of the latter part of the Holocene, determined on the basis of radiocarbon and pollen data and (3) to provide a contribution to the knowledge of the fine-grained sediment budget on the French continental margin off the Gironde estuary.
2. Environmental setting The shelf of the Bay of Biscay is a meso to macro-tidal (4–5 m regime maximum range) and storm wave-dominated environment (Arbouille, 1987). Shelly gravels to fine sands surround and form the substrate of two main open-shelf fields of fine-grained deposits: the Gironde shelf mud fields and ‘‘La Grande Vasi"ere’’ (Allen and Castaing, 1977). 2.1. The Gironde shelf mud fields According to McCave’s (1972) classification, the Gironde shelf mud fields are nearshore mud belts, which extend between 30 and 75 m water depth. From the sedimentological point of view, these fine-grained deposits form distinctive morpho-sedimentary units which overlie the sands and gravels covering the North Aquitaine continental shelf. These offshore mud fields, the WestGironde mud patch (WGMP) and the South-Gironde mud patch (SGMP), are located just seaward of the two main channels of the Gironde mouth (Fig. 1). The maximum thicknesses of the
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Fig. 1. Map of the continental shelf deposits of the Bay of Biscay (France), showing the location of the mud belts (from Allen and Castaing, 1977). Note the extensive mid-shelf muddy belt ‘‘La Grande Vasi"ere’’, around the 100 m water depth and the Gironde shelf mud fields, which appear as mud patches. Inset shows that the Gironde shelf mud fields are located around 50 m water depth, across shore in front of the two channels of the Gironde estuary. Mud thickness (isopachs) are in metres.
muddy deposits are about 4 m for the WGMP and 2 m for the SGMP. Their boundaries do not vary through time, although a transitory and somewhat undefined area of fluid muds may be recorded around them (Lesueur, 1992). Fluid mud patches are frequent shoreward of the WGMP, especially during river floods from the Gironde estuary and during spring, i.e. when the river output reaches a maximum (Castaing, 1981). Comparison of the silt- and clay-sized sediment being transported in the Gironde today and that of the mud field sediments indicates that the latter is largely an offshore segregation of suspended matter (SM) originating from this estuary (Weber et al., 1991; Parra et al., 1999). In addition, a small amount of silty and shelly fine sand originates from the shoreface from where it is reworked infrequently during storm events (Weber et al., 1991; Lesueur and Tastet, 1994). Internal structures of the deposits show a clear seaward pattern with the frequency of physical structures (i.e. interbedded sand/silt layers or traction marks) decreasing, whilst the bioturbation increases offshore (Jouanneau et al., 1989; Lesueur and Tastet, 1994). Synthesis of detailed studies of about 100 cores, based on sedimentary facies, shows that the muds also record storm events. The general stratigraphy of the mud deposits was established using radiocarbon dating and pollen analyses (Lesueur et al., 1989), and these showed that the mud fields appeared and then developed during the historical period (i.e. during the last 2000 years at the most).
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2.2. ‘‘La Grande Vasie"re’’ According to the classification of McCave (1972), ‘‘La Grande Vasi"ere’’ is a mid-shelf mud belt. All authors agree that it is really a patchwork of ill-sorted sands, in which the entrapped finegrained material may account for about 25% the total dry mass of the sediment (Berthois and Le Calvez, 1959; Glemarec, 1969; Barusseau, 1973; Pinot, 1974; Vanney, 1977). This extensive blanket, situated around 100 m water depth, extends along the shelf for more than 250 km, from the offshore area south of Britanny to the Rochebonne shoal (Fig. 1). The muddy sediment lies on sands and shelly gravels derived from fluvial deposits, reworked during the Holocene transgression (Allen and Castaing, 1977). These fine-grained deposits do not generally exceed a few decimetres in thickness, and the extent of this blanket varies according to the season (Pinot, 1974) and to hydrodynamical conditions (i.e. swells and storms).
3. Methods In order to estimate sedimentation rates, nine sites were investigated in June 1995, from different representative areas in the Gironde mud fields and from the southern part of ‘‘La Grande Vasi"ere’’. At each site, undisturbed box cores were collected, in which subsamples (PVC pipes, 10 cm diameter) were taken. The cores were opened, described and vertical slabs were removed for radiographic analyses. X-radiographs were used to control the sampling of the thin layers (1 cm), which were selected for radioisotope analysis. Water content was determined by weight loss after oven drying: water content (W in %)=(water weight/wet sediment weight 100). The silt and clay fractions were determined using a Malvern 3600E laser diffraction particle sizer (in the following results, grain-size is expressed as median grain diameter, Mf in mm). 5–10 subsamples were recovered in a vertical profile from the topmost 30 cm of the cores. Radioisotopic measurements were made using a semi-planar germanium detector (EGSP 220025-R from EURYSIS Mesures) coupled to a multichannel (8000 channels) analyser. Dry homogenized samples were packed in 9.5 ml petri dishes. Radioisotope activity was calculated using appropriate calibration coefficients taking into account the gamma ray intensity and the detector efficiency. 210Pb and 137Cs activities were determined by the direct measurements of their gamma decay energies at 46.5 and 661.6 keV, respectively. Excess 210Pb was calculated from total 210 Pb activity and supported 210Pb activity calculated from 214Pb at 295.2 and 352.0 keV, and from 214Bi peak at 609.3 keV. Measurements were carried out at least 2 months after packing of samples, to allow the secular equilibrium to be established between 226Ra and its granddaughters 214 Pb and 214Bi. The use of the natural radionuclide 210Pb (half-life=22.3 yr) is a well established technique to determine sedimentation rates during at least the past century (Koide et al., 1972; Guinasso and Schink, 1975; Nittrouer et al., 1979). Excess 210Pb apparent sedimentation rates were estimated based on a one-dimensional, two-layer model, i.e. negligible mixing below the surface layer (Nittrouer et al., 1983/1984). The 137Cs (half-life=30.1 yr) was secondarily considered, when significant (>0.1–0.2 dpm g1). 137Cs is considered as chemically mobile in the sediment, and is liable to diffuse via pore fluids. 137Cs activities in muds on the Bay of Biscay continental shelf are generally low, and do not generally show pronounced subsurface maxima corresponding to
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known events (i.e. the 1963 maximum in the atmospheric radioisotope bomb-produced fallout, or the 1986 Chernobyl peak, which is not obvious in southwestern France). They have been used to constrain 210Pb-based sedimentation rates and to investigate deep mixing beneath the surface mixed layer (see Alexander et al., 1991).
4. Results 4.1. Sedimentation rates in the Gironde shelf mud fields Seven cores were analysed from the Gironde shelf mud fields (Fig. 2): three cores from a proximal area (i.e. water depth of 40 m or less) and four cores from a distal area (i.e. water depth >40 m). Excess 210Pb activity in the topmost 10 cm of the surficial sediment constantly increases from very low values as 0.1–1 dpm g1 dry weight in less than 40 m water depth, to 2–5 dpm g1 between 40 and 60 m, and to 6–7 dpm g1 for sites beyond 70 m water depth. 4.1.1. The proximal area (30 to circa 40 m water depth) In the inner zone of the two mud fields (WGMP and SGMP), the sediment is homogeneous and is disturbed by only a minor amount of bioturbation. This mud alternates with coarser-grained layers (i.e. silts and sands). As a result, the water content is rather variable (655W5110%), depending on the grain-size.
Fig. 2. Distribution of muds fields on the Aquitaine continental shelf (isopachs of the muds in metres). Location of the box cores collected for 210Pb analyses at variable water depths and variable distances to the Gironde estuary.
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Fig. 3. Selected cores from the Gironde shelf mud fields, Bay of Biscay. Sketches of X-radiograph description: contacts, primary physical structures and biological structures. KI 95151, 95132 and 95149 (Gironde shelf mud fields): median grain-size. KI 95136 and 95142 (‘‘La Grande Vasi"ere’’): median grain-size after sieving at 200 mm; diamonds (KI 95142) symbolize the 563 mm fraction in the total bulk sediment.
Radioisotopic data from three cores collected in this area are in a good agreement (Fig. 4). The core KI 95151 (39 m water depth) is representative of the proximal area. It typically exhibits an alternation of very dark gray (2,5 Y3/0 in Munsell colour) clayey muds (Mf ¼ 9216 mm) and interbedded coarse silt or fine sand layers (Mf ¼ 22240 mm) with shell fragments (Fig. 3). At the top of the core, sandy silts (Mf ¼ 41 mm) lie on an erosional surface which is overlain by a shell lag. As in the other cores from this area (Fig. 4B), excess 210Pb activity is only present in the upper few centimetres of this core. It is impossible to estimate a sedimentation rate in sediments which show such marked lithological changes. At the very most, the rate would be 50.1 cm yr1 (KI 95148, Fig. 4C). In this proximal area of the mud fields, the 137Cs is at most at the limits of detection (i.e. 0.1 dpm g1). 4.1.2. The distal area (40–75 m) Four cores were analysed in the distal area of the WGMP. The sediment is typically an olivegray (5Y4/1–5Y4/2) clayey silt–silty clayey mud. A fine sand layer is sometimes present in the upper centimetres of the cores. The core KI 95131 (WGMP, 44 m), is composed of a clayey silt mud (Mf ¼ 14218 mm), with a few interlayered fine silts (Mf ¼ 20227 mm) which include small shell fragments. There is no
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Fig. 4. Profiles of excess 210Pb activity in the box cores from the proximal area of the Gironde mud fields. (A) Core KI 95130 (36 m, WGMP); (B) Core KI 95151 (39 m, WGMP); (C) Core KI 95148 (40 m, SGMP).
superficial silt layer in this core. A few long and thin burrows, mainly vertically oriented, are perceptible on the radiographs. The radioisotope profile (Fig. 5A), show no surface mixed layer (SML) and permits the plotting of a straight line to estimate the logarithmic decrease of the excess 210Pb activity. This yields an apparent sedimentation rate of 0.13 cm yr1. The core KI 95132 (WGMP, 49 m) is from the central part of the mud field (Fig. 3). It is composed of silty clays (Mf ¼ 11218 mm), with only a few interlayers of fine silts (Mf ¼ 22226 mm). Only one slight discontinuity is perceptible at a depth of about 22–25 cm and the bioturbation, although present, consists of small diffuse horizontal burrows. Neither the radiographs nor the radioisotopic data indicate a SML at the top of the core KI 95132 (Fig. 5B). The logarithmic decrease of excess 210Pb activity indicates an apparent sedimentation rate of 0.56 cm yr1 (i.e. 0.48 g cm2 yr1, assuming an average dry bulk density of 0.85 g cm3). However, this is a high value for a shelf which is not associated with a major dispersal system, and deep mixing must be suspected. Consequently, cross-checking with data of 137Cs was undertaken. Assuming a sedimentation rate of 0.56 cm yr1, the theoretical level of maximal penetration of 137Cs in the sediment is estimated to be 25 cm, based on the appearance of this bomb-produced radionuclide in ca. 1952. Whereas 137Cs has a measured penetration level between 17 cm (0.18 dpm g1) and 21 cm (0.05 dpm g1), it is not detectable at 26 cm; this indicates that the previous value, although a maximum, approximates to the actual sedimentation rate. Core KI 95133 (58 m) is a dark gray (5Y 4/1) silty clay. The grain-size is not very variable (Mf ¼ 15221 mm) and the water content remains between 65% and 80%. There is abundant but diffuse bioturbation with scattered fine shell debris of bivalves. The excess 210Pb data from this core show a complex SML in the topmost 5–10 cm (Fig. 5C). Below this SML, the profile of this core suggests a logarithmic decrease to a near background level. Based on decreasing 210Pb activity between 15 and 25 cm, the apparent sedimentation rate would be 0.3 cm yr1. However, at this site, the 137Cs has been detected down to a depth of
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Fig. 5. Profiles of excess 210Pb activity in the box cores from the distal area of the West Gironde mud patch (WGMP). (A) Core KI 95131 (44 m); (B) Core KI 95132 (49 m); (C) Core KI 95133 (58 m); (D) Core KI 95149 (74 m). When representative, comparison between the predicted depth (PD) and the measured depth (MD) of 137Cs penetration is noted.
approximately 30 cm, when the maximum theoretical penetration (estimated from 210Pb data and including the SML) should be of only approximately 24 cm. Thus, the presence of 137Cs indicates that mixing dominates in the sediment; this has extended to a depth of 30 cm during approximately the last forty years and the excess 210Pb data give here a maximum rate. The core KI 95149 is representative of the most distal zone of the WGMP (74 m). It consists of a dark (2,5 Y4/0) clayey mud (Mf ¼ 8213 mm) with a high water content (955W5115%). In the greater water depth of the WGMP, the mud contains large burrows in which gastropods (Nassarius sp., Nassarius semistriatus) are found (Fig. 3). A single erosional surface occurs at 17 cm.
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Below a SML of 5 cm, the excess 210Pb profile of the core KI 95149 (Fig. 5D) indicates an apparent sedimentation rate of about 0.11 cm yr1 (0.08 g cm2 yr1). This value suggests that 137Cs extends down to a theoretical depth of 10 cm. Actually 137Cs activity is present (0.18– 0.30 dpm g1) in the upper 12 cm and is not detectable below 14 cm; thus it may be assumed that a value of 0.1 cm yr1 is close to the actual sedimentation rate at this site. 4.2. Fine-grained sedimentation rates in ‘‘La Grande Vasie"re’’ Excluding the southern zone of Rochebonne where mud is likely to accumulate, sampling showed that it is difficult to define accurately this supposedly large ‘‘mud field’’ (Fig. 6). The superficial sediment is composed of sands and gravels which contain a variable, but low content of silt and clay (510–30% of the total bulk samples). Two long box cores were selected from the muddier parts of these ill-sorted sediments, for 210Pb analysis (Fig. 6). The core KI 95142 (100 m water depth) is representative of the surficial sediments from this open mid-shelf mud belt. The deposits are organized into a decimetre-scale fining upward sedimentary sequence. The lower part of this core is composed of gravel and reworked coarse shelly fragments and the topmost 10 cm are composed of muddy fine sands (Fig. 3). Thus, the silt and clay content increases from 14% at the base to 26% at the top of the core. For radioisotopic analysis, the sediment of the core KI 95142 was sieved at 63 mm and only the fine-grained fraction was analysed. Using the excess 210Pb profile (Fig. 7A), a sedimentation rate of 0.26 cm yr1 may be estimated between 9 and 20 cm sediment depth. In fact, when the maximum predicted depth of 137Cs activity would be of 17 cm, it remains significant (i.e. up to 0.13 dpm g1) down to 25 cm. Thus, the radiochemical profile is affected by deep mixing of the whole sampled sedimentary column, and the calculated rate is a maximum estimate of the actual accumulation rate. Results from KI 95136 core (94 m water depth, south of Rochebonne shoal) shows similarities with those of the previous core, although the sediment (fine-sand) contains more mud. Radiography revealed subtle horizontal burrows and tracks at the top of the core, and distinct sand-infilled vertical burrows near its base (Fig. 3). A surface mixed layer, where excess 210Pb and 137Cs activities are constant, extends down to a depth of 9 cm (Fig. 7B). The fine-grained sedimentation rate is estimated at 0.22 cm yr1 (i.e. 0.18 g cm2 yr1). The observed depth of 137Cs penetration (between 13 and 21 cm) is in this case in good agreement with the predicted depth of penetration (19–20 cm), and the present sedimentation rate of fine-grained material seems to be close to the apparent rate.
5. Discussion 5.1. The sedimentation rates in the mud fields 5.1.1. The Gironde shelf mud fields Spatial distribution of 100-yr time scale fine-grained sedimentation rates on the shelf off the Gironde estuary are summarized (Fig. 8). In cases where the effects of mixing on the rate of
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Fig. 6. The southern part of the mid-shelf muddy belt ‘‘La Grande Vasi"ere’’: location of sampling sites of the superficial sediments in June 1995. Dots are grab stations, stars are the location of the two cores that were examined for 210Pb in this work. Values are % of 563 mm material (silt+clay). Fine-grained sediment does not exceed 30% of the total bulk sediment, apart from south Rochebonne shoal, where up to 60% silt and clay are locally measured. Contours correspond to the assumed limits of muds in the superficial sediments and heavy dotted line the isovalue 20% silt+clay (after Vanney, 1977). Note the southwestern location of the WGMP, and the absence of fine-grained sediments in the area located between these mud fields.
accumulation could not be resolved, maximum rates of sedimentation were assumed to be correct. According to their geographical distribution, these estimations appear to be consistent. Although sedimentation occurs at the inner sites close to the source (SM of Gironde origin) the local dynamics (i.e. bottom tidal currents and very important storm action) both prevent this
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Fig. 7. Profiles of excess 210Pb activity in the cores from the mid-shelf muddy belt ‘‘La Grande Vasi"ere’’. (A) Core KI 95142 (open shelf); (B) Core KI 95136 (south Rochebonne shoal). Comparison between the predicted (PD) and the measured (MD) depths of 137Cs penetration.
Fig. 8. Summary of 100-yr time scale sediment rates in the Gironde shelf mud fields. Rates are maximum estimates. Note the consistance of the data obtained from 210Pb analysis (underlined values), with accumulation rates yielded on the basis of other markers (14C, palynology) from the upper metre of the muds (see the text). The latter are asterisked at 43 m in the SGMP; 45, 49 and 64 m in the WGMP (after Lesueur, 1992). Normal characters are total finegrained sedimentation rates (cm yr1). Italics are sediment accumulation rates (g cm2 yr1) of assumed estuarine particles (i.e. 530 mm).
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sedimented mud from long-term total preservation, and furthermore remobilize earlier sediments which originate partly from the substrate of the mud fields. These processes are recorded within the deposits: (1) X-radiographs of cores show sedimentary structures and interbedded mud and fine sand layers (Lesueur and Tastet, 1994); (2) radioisotopic data indicate that sediments from this area are devoid of both 210Pb and 137Cs. Thus, the 100-yr time scale sedimentation rates have never reached 0.1 cm yr1 in this proximal area that corresponds to the inner zone of the mud fields. This study shows that modern silty-clay sedimentation has merely a temporal (i.e. seasonal) presence in the shallowest-water area of the WGMP, as suggested earlier (Jouanneau et al., 1989). As a consequence, it is only beyond a minimal water depth of approximately 40 m that the balance between supply and local dynamics allows preservation of the deposited material. The greatest sedimentation rates (>0.2 cm yr1) occur at depths between 40 and 60 m, with the maximum value (0.56 cm yr1) in about 50 m water depth, in the WGMP. This zone was previously considered as the depocentre of the fine-grained sedimentation on the Gironde shelf (Lesueur et al., 1996). Moreover, studies of the seasonal hydrographic regime in the Bay of Biscay showed that in winter time (i.e. the season of major output of SM) a thermo haline front is located along the mid-shelf, between Brittany and Arcachon (Castaing et al., 1999). The result of this oceanographic structure is that suspended particles are trapped around a depth of 50–60 m on the Gironde shelf, where a bottom nepheloid layer extends across the shelf, from the estuary to the WGMP. The same phenomenon occurs around the 100 m isobath along ‘‘La Grande Vasi"ere’’ (Jouanneau et al., 1999). A similar explanation of a hydrodynamic barrier, concentrating suspended sediment, has been used in order to explain high deposition rates on the eastern Brazilian continental shelf (Patchineelam and Smoak, 1999). The decrease in rate of accumulation further offshore at depth around 50–60 m, is interpreted as being due to an offshore depletion in the flux of the suspended matter. Such an across-shelf decrease in sedimentation rate with distance from source is a typical phenomenon, which has been already described in most other sites of deposition of fine-grained sediment on continental margins (e.g. Nittrouer et al., 1984/1985). A comparison of the data presented here with the sedimentation rates estimated over the longer term (i.e. for time scale >100 yr) has been undertaken for various stations from the Gironde mud fields. The results of longer term rates of accumulation are based upon different recognized chronological markers in long cores, including radiocarbon ages and pollen analyses (Lesueur et al., 1996). These rates were estimated as variable and ranged from 0.1 to 0.4 cm yr1. Comparison of data (i.e. 210Pb versus 14C) from similar locations shows rates of the same order of magnitude (Fig. 8). However, where excess 210Pb sediment accumulation rates have been estimated at their highest values (i.e. 0.56 cm yr1 in the central zone, 50 m water depth), 14C sedimentation rates are estimated as between 0.25 and 0.32 cm yr1 (Lesueur et al., 1996). Such a disparity between smaller long term rates and larger short term rates (i.e. 100-yr scale) seems to be common (e.g. Saddler, 1981; Maldonado et al., 1983). Nevertheless, both 14C and pollen data suggest an increase in sedimentation rate in both the topmost deposits and in those of the central zone of the mudfields. The appearence of Pinus pinaster pollen marks the beginning of the intense afforestation of Les Landes in the mid-19th century (Marambat, 1992); although this is not detectable in the cores recovered on the inner shelf, it is noticeable in the topmost decimetres of the cores from the distal area (Lesueur et al., 1996). At the site of the core KI 95132 (50 m water depth), this pollen was recognized down to almost 70 cm beneath the sediment surface (Lesueur,
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1992), indicating a sedimentation rate of approximately 0.5 cm yr1. This value is similar to the estimated excess 210Pb rate of 0.56 cm yr1 from the same site (Fig. 8). In summary, various data suggest an increase in rates of fine-grained accumulation during at least the last century, indicating an increase of supply of fluvial suspended matter from the Gironde onto the shelf. Lesueur et al. (1996) suggested that such an increase could be the consequence of natural causes, such as infilling and morphological changes of the estuary, which were enhanced by human activities since the mid-XIXth century or more recently (including acceleration of soil erosion and/ or management of the estuarine area). The result has been the downstream shift in the zones of maximum sediment influx and deposition (Allen et al., 1979) and a suspected concomitant increase of SM expelled onto the shelf during the last century (Castaing, 1981). 5.1.2. ‘‘La Grande Vasie"re’’ Cores recovered from surficial deposits of this large open shelf muddy field display some evidence of reworking by periodic erosion due to physical agencies, and by biological processes (large but indistinct burrows provide evidence of biological mixing). An estimated fine-grained sedimentation rate of 0.26 cm yr1 for the open shelf site (core KI 95142) is very excessive. A rate of 0.1 cm yr1 over the surface area of ‘‘La Grande Vasi"ere’’ (9.103 km2), and this is only considered the 530 mm material of estuarine origin (Weber et al., 1991), implies a rate of mass accumulation that exceed the fluvial sediment discharge which reaches the shelf (i.e. 2.5 106 t yr1, Jouanneau et al., 1999). Of particular interest, is the studied site near Rochebonne shoal (core KI 95136) where mud preferentially accumulates, with estimated rate of about 0.2 cm yr1. This is a high value, as this open shelf area is far away (i.e. >100 km) from any fluvial SM sources. However, recent geochemical studies of metallic elements have shown that it is the result of the cumulative effect of both the Gironde and the Loire rivers in this part of ‘‘La Grande Vasi"ere’’ (Jouanneau et al., 1998). Hence, near Rochebonne, the accumulating sediment probably includes fine-grained material from both the two largest suppliers of SM on the Atlantic coast of Europe (Castaing and Jouanneau, 1987). 5.2. Comparison with sedimentation rates in other shelf mud settings In the last 20 years, there have been numerous studies on sedimentary processes on the continental margins. Among the latter, those concerned with fine-grained deposits of shelves which are not associated with major rivers and deltas are scarce in the literature (Table 1). Nittrouer et al. (1984/1985) proposed that on a 100-yr time scale, sedimentation rates associated with major rivers (i.e. >108 t yr1) range from centimetres per year to millimetres per year, while rates associated with small dispersal systems (i.e. 5108 t yr1) are generally millimetres per year or less. Despite the low river sediment discharges from to the Gironde estuary (1.5 106 t yr1 after Castaing and Jouanneau, 1987), estimated sedimentation rates for fine-grained sediments on this shelf range from 50.1 to 0.5 cm yr1; these are far from negligible values, by comparison with other continental shelves associated with larger suppliers. Despite the small dimensions of the Gironde shelf mud fields, the observed across shelf trends (i.e. proximal area/distal area) in sedimentary structures and associated sedimentation rates are similar to the across shelf zonation of fine-grained deposit associated with higher dispersal systems, of subaqueous parts of deltas (i.e. topset/foreset/bottom set deposits).
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Table 1 A review of sedimentation rates deducted from excess 210Pb chronology in shelf muds deposits linked to major dispersal systems (upper part of the table), shelf mud fields associated with small dispersal systems (lower part of the table) Delta and associated shelf
Area
Sedimentation rate (cm yr1)
Reference
Changjiang (China) Huanghe–Changjiang (China)
Inner shelf Offshore Topset deposits Foreset deposits Bottomset Deposits Topset/inner shelf Foreset/mid shelf bottomset/outer shelf Inner shelf Mid shelf Delta front Prodelta Distal area Proximal area Middle prodelta Distal prodelta
1–5 0.3 0.1–0.2 0.4–0.9 0.2–0.4 50.1 2–10 0.1–2 0 2–8 1.7 0.8 4.1 1.1 0.39 0.09 0.25 0.12 0.06
Yang and Milliman (1983) DeMaster et al. (1985) Alexander et al. (1991)
Amazon (Brazil) Ganges–Brahmaputra (India) Fly River (New Guinea) Ebro (Spain)
Kuehl et al. (1982) Kuehl et al. (1986) Kuehl et al. (1989) Harris et al. (1993)
Diaz et al. (1996)
Continental shelf
River
Water depth (m)
Sedimentation rate (cm yr1)
Reference
Washington North California North California Oregon New England Jinhae Bay (S Korea) Eastern Brazil
Columbia r. Russian r. Eel r. Klamath r. Outer shelf Nakdong r.
60–120 50–125 50–100 60–150 10–40
0.3–0.7 0.15–0.24 0.1–0.15 0.06 0.02–0.05 0.4
Nittrouer et al. (1979) Demirpolat (1991) Leithold (1989) Kulm et al. (1975) Bothner et al. (1981) Park et al. (1995)
Pardo, Mucuri Doce, Caravelas Minho Douro
10–40
0.2–0.8
Patchineelam and Smoak (1999)
65–120 65–130
0.1 0.16–0.55
Eastern Spain (Valencia shelf)
Ebro
20–130
0.1–0.3
Gulf of Lion (S France) Aquitaine (SW France) Bay of Biscay (W France)
Rhone
50–100
0.15–0.63
Carvalho and Ramos (1990) Carvalho and Ramos (1990), Drago et al. (1994) Nittrouer et al. (1985), Maldonado et al. (1983), Palanques and Drake (1990) Zuo et al. (1991)
Gironde
35–75
50.1–0.56
This paper
Gironde+Loire
100
40.2
This paper
N Portugal N Portugal
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5.3. Contribution to the sediment budget on the continental shelf of the Bay of Biscay An attempt was made to calculate the amount of fine-grained sediment brought by the Gironde and trapped in the offshore shelf mud patches. The amount of material 530 mm, which is characteristic of the fluvial supply is the basis of this budget. The estimated sedimentation rates have been applied to various areas of the mud fields, according to (i) water depth, (ii) geometry of the different muddy bodies and (iii) sedimentological characteristics (i.e. grain size, silt+clay and water contents). In each area, the accumulation rate (g cm2 yr1) was multiplied by the surface area (cm2) and by the dry bulk density (g cm3). The total sum yields a value of 5 105 t yr1 as the rate of sediment accumulation in the Gironde shelf mud fields, on a 100-yr time scale. This total flux corresponds to about 13 of the total sediment output of Gironde origin. This result is in agreement with other previous flux evaluations: Jouanneau and Weber (1989) claimed that 30% of the fluvial solid discharge is deposited in this mud patch, and Lesueur (1992) estimated that about 27% of the Gironde SM outputs was trapped in the Gironde shelf mud fields (34 of the accumulated amount in the WGMP). According to the present estimation, 73% of this particulate matter is trapped in the WGMP and only 21% in the SGMP (the remained being deposited in the smaller mud patches). This is consistent with the data on the present outputs, as Ruch et al. (1993) estimated that only 20% of estuarine particles are discharged to the shelf from the southern channel. Lastly, this work complements the data on previous budgets of sedimentary fluxes to the slope and rise along on the French continental margin (Jouanneau et al., 1999). These first estimations of sediment accumulation rates in the Aquitaine shelf mud fields give values between 50 and a maximum of about 400 mg cm2 yr1. Much of the remaining sediment accumulates in the inner coastal bays of Charentes, or farther away on the northwestern shelf, or escapes to the slope and rise (Table 2). Table 2 A synthesis of our present knowledge of fine-grained sediments from the Gironde estuary to the continental shelf, slope and rise in the Bay of Biscay. Sediment accumulation rates in the shelf mud fields. This work complement previous evaluations: sedimentation rates on intertidal areas from the Gironde estuary (from Grousset, personal communication; Fregard, 1994; Lesourd, 1996) and on mud flats from the Charentes bays (from Gouleau et al., 2000); sediment budget on continental slope and rise (from Jouanneau et al., 1999) Muddy area
Sedimentation rate (cm yr1)
Fine-grained accumulation rate (mg cm2 yr1)
La Grande vasi"ere (southern zone) West Gironde mud patch Proximal area (540 m) Depocentre (circa 50 m) Distal area (60–75 m) South Gironde mud patch (inner zone) Tidal flats Gironde estuary Charentes bays Continental slope and rise Shelf break 2500–3000 m water depth
0.2–0.26
150
40.1 0.3–0.5 0.1–0.2 0.1
450 300–400 50–100 450
2–2.5 0.25–1
Nd Nd
Nd Nd
>10 2
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6. Conclusions The first excess 210Pb measurements of superficial muddy sediments from the French Atlantic shelf give encouraging results for the evaluation of rates of sedimentation during the past century. The major conclusions of this paper are: (1) Sedimentation rates in the Gironde shelf mud fields as determined by 210Pb measures are roughly comparable to those obtained by other means (i.e. 14C and palynology). They are maximum values, because they cannot be corrected for perturbation by biological and physical processes. Sedimentation rates, ranging between 1 and 5 mm yr1, are similar to values obtained on other shelves associated with small to medium dispersal systems. (2) The highest rate obtained (close to 0.5 cm yr1) is high for a shelf associated with a small river system. This high value corresponds to the mean of the nearshore mudfield, at the depocentre of the fine-grained sedimentation. In this zone, the main part of the Gironde SM is trapped by a winter thermo haline front. Lower rates (0.1–0.3 cm yr1) estimated for the outer zone of the muddy deposits are the result of progressive depletion in sediment supply with distance. Either there is no sediment accumulation, or very low rates (i.e. 40.1 cm yr1) in the inner zone of the mud patches (water depth 540 m), due to resuspension of estuarine fluid muds by heavy swell and storms. (3) The highest sedimentation rate, of about 0.5 cm yr1 at 50 m water depth in the WGMP, is in good agreement with an earlier value, based on palynology. It is consistent with the hypothesis of an increase of fine-grained sedimentation in the central zone of the Gironde shelf mud fields, during at least the last century. The corollary of this observation is that there has been an increase of the Gironde output during this period. Infilling of the estuary, climatic changes and human activities have jointly affected the supply of river-borne suspended discharge to the shelf. (4) It is estimated that about 13 of the present day total discharge of SM from the Gironde estuary is trapped in the Gironde shelf mud fields. (5) In the mid-shelf mud belt, ‘‘La Grande Vasi"ere’’, fine-grained sedimentation rates are overestimated (0.2 cm yr1), when the studied sites are distant (i.e. >100 km) from continental sources (Gironde and Loire rivers). In this zone, relict shelly sands are mixed with the fine-grained material, by biological activity and by storms and reworking of the sediment is generally the dominant process in ‘‘La Grande Vasi"ere’’.
Acknowledgements The cores and sediments were collected during the DEPOVASE VI cruise (June 1995); the authors are grateful to the crew from the board of R.V. ‘‘Co# te d’Aquitaine’’ (CNRS/INSU). Analyses were carried out in the labs of University Bordeaux I (DGO), whose personnel is acknowledged. The authors are indebted to Y. Baron (GEA Marine Nationale Cherbourg) for having provided some of the gamma analyses, J.L. Reyss (CFR CEA/CNRS Gif/Yvette) for the intercalibration exercise of gamma analyses on some samples, S.A. Kuehl (VIMS Gloucester Point) who provided some constructive remarks on the first writing of this work, G. Evans (Southampton Oceanography Centre) who helped to improve the final manuscript, and the two
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referees. Part of this research was supported by the EC-funded Human Capital and Mobility Network (CHRX-CT94-0541), Coastal Environments (E-Atlantic: Physical Processes).
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Sedimentation rates and fluxes in the continental shelf mud fields in the Bay of Biscay (France) P. Lesueura,*, J.-M. Jouanneaub, D. Boustc, J.-P. Tastetb, O. Weberb UMR CNRS 6143, Morphodynamique continentale et co#tie"re, Universite! de Caen, 24 rue des Tilleuls 14000 Caen Cedex, France b UMR CNRS 5805, De!partement de Ge!ologie et Oce!anographie, Universite! Bordeaux I, Avenue des Faculte!s, 33405 Talence Cedex, France c Laboratoire d ’e!tudes radioe!cologiques de la facade atlantique, IPSN, B.P. 10, 50130 Octeville, France Received 8 May 2000; received in revised form 27 October 2000; accepted 15 November 2000 a
Abstract Sedimentation rates in shelf mud fields of the Bay of Biscay (the Gironde shelf mud patches and ‘‘La Grande Vasi"ere’’) were investigated for a 100-yr time scale, using 210Pb geochronology of nine cores. The distribution of apparent sedimentation rates reveals a general trend along the terrigenous dispersal system from the Gironde outlet of: no or negligible (50.1 cm yr1) cumulative sedimentation in the inner zone of the mud fields; 0.1–0.2 cm yr1 in the outer zone where the supply is reduced; and a maximum value of about 0.5 cm yr1 in the depocentre of the main mud field. Except for the latter value, the 210Pb sedimentation rates are of the same magnitude as longer-term rates based on 14C, and although slightly higher, are in good agreement with an earlier estimation determined for the rate of accumulation of surficial sediments, using pollen analyses. These differences are interpreted as being due to an increase in the supply of fine-grained sediment over the last century (from 0.2 to 0.5 cm yr1), perhaps as a result of an intensification of estuarine suspended matter supply to the shelf. Compilation of a sediment budget shows that about one third of the annual Gironde solid discharge is trapped in the Gironde nearshore shelf mud fields. The mid-shelf muddy belt (i.e. ‘‘La Grande Vasi"ere’’) is the site of accumulation of fine-grained material from various terrigenous sources (mainly from the Gironde and Loire rivers). Two cores from the latter area yielded a maximum rate of sedimentation of 0.2 cm yr1. Mixing processes dominate in this open middle shelf, due to the periodic sediment removal by long-period swells and bioturbation. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Shelf; Mud; Radioisotope; Sedimentation rate; Sediment budget; Bay of Biscay
*Corresponding author. Tel.: +33-2-3156-5754; fax: +33-2-3156-5757. E-mail address: [email protected] (P. Lesueur). 0278-4343/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 2 7 8 - 4 3 4 3 ( 0 1 ) 0 0 0 0 4 - 8
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1. Introduction Evaluations of sedimentation rates in fine-grained deposits from coastal zones and shelves are important because of the association with such deposits of chemical pollutants, their impact on biological processes, and their relationship to stratigraphic considerations, including changes in sedimentary environments. Most of the recently published papers on shelf muds have focused on the large dispersal systems associated with subaqueous-deltas, i.e. Amazon, Ganges– Brahmaputra, Changjiang–Huanghe, Fly River (e.g. Kuehl et al., 1982, 1986; Kuehl et al., 1989; DeMaster et al., 1985; Alexander et al., 1991; Harris et al., 1993). Results from small dispersal systems are less common, partly because they are more complicated due to the relatively greater importance of non-steady state effects of storms, currents and benthic biological activity. This study examines the temporal and spatial variabilities in the rate of fine-grained accumulation on a shelf which experiences a weak continental sediment discharge (i.e. 1.5 106 t yr1, after Castaing and Jouanneau, 1987). Over the 10 last years, sedimentation of mud on the Gironde continental shelf has been the subject of several high resolution sedimentological analyses (Jouanneau and Weber, 1989; Jouanneau et al., 1989; Lesueur et al., 1989, 1991; Lesueur and Tastet, 1994). Nevertheless, nowhere on the entire French Atlantic shelf have any evaluation of sedimentation rates been made on the basis of excess 210Pb. Thus, there was an important gap between evaluations of accumulation rates on the Gironde estuarine intertidal sediments where such studies have been made, and continental margin deposits (Jouanneau et al., 1999). The aim of this paper is (1) to evaluate the sedimentation rates in the shelf mud fields, based on analyses of radioisotope data (i.e. excess 210Pb geochronology) over the last century, (2) to compare these sedimentation rates with those of the longer time interval of the latter part of the Holocene, determined on the basis of radiocarbon and pollen data and (3) to provide a contribution to the knowledge of the fine-grained sediment budget on the French continental margin off the Gironde estuary.
2. Environmental setting The shelf of the Bay of Biscay is a meso to macro-tidal (4–5 m regime maximum range) and storm wave-dominated environment (Arbouille, 1987). Shelly gravels to fine sands surround and form the substrate of two main open-shelf fields of fine-grained deposits: the Gironde shelf mud fields and ‘‘La Grande Vasi"ere’’ (Allen and Castaing, 1977). 2.1. The Gironde shelf mud fields According to McCave’s (1972) classification, the Gironde shelf mud fields are nearshore mud belts, which extend between 30 and 75 m water depth. From the sedimentological point of view, these fine-grained deposits form distinctive morpho-sedimentary units which overlie the sands and gravels covering the North Aquitaine continental shelf. These offshore mud fields, the WestGironde mud patch (WGMP) and the South-Gironde mud patch (SGMP), are located just seaward of the two main channels of the Gironde mouth (Fig. 1). The maximum thicknesses of the
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Fig. 1. Map of the continental shelf deposits of the Bay of Biscay (France), showing the location of the mud belts (from Allen and Castaing, 1977). Note the extensive mid-shelf muddy belt ‘‘La Grande Vasi"ere’’, around the 100 m water depth and the Gironde shelf mud fields, which appear as mud patches. Inset shows that the Gironde shelf mud fields are located around 50 m water depth, across shore in front of the two channels of the Gironde estuary. Mud thickness (isopachs) are in metres.
muddy deposits are about 4 m for the WGMP and 2 m for the SGMP. Their boundaries do not vary through time, although a transitory and somewhat undefined area of fluid muds may be recorded around them (Lesueur, 1992). Fluid mud patches are frequent shoreward of the WGMP, especially during river floods from the Gironde estuary and during spring, i.e. when the river output reaches a maximum (Castaing, 1981). Comparison of the silt- and clay-sized sediment being transported in the Gironde today and that of the mud field sediments indicates that the latter is largely an offshore segregation of suspended matter (SM) originating from this estuary (Weber et al., 1991; Parra et al., 1999). In addition, a small amount of silty and shelly fine sand originates from the shoreface from where it is reworked infrequently during storm events (Weber et al., 1991; Lesueur and Tastet, 1994). Internal structures of the deposits show a clear seaward pattern with the frequency of physical structures (i.e. interbedded sand/silt layers or traction marks) decreasing, whilst the bioturbation increases offshore (Jouanneau et al., 1989; Lesueur and Tastet, 1994). Synthesis of detailed studies of about 100 cores, based on sedimentary facies, shows that the muds also record storm events. The general stratigraphy of the mud deposits was established using radiocarbon dating and pollen analyses (Lesueur et al., 1989), and these showed that the mud fields appeared and then developed during the historical period (i.e. during the last 2000 years at the most).
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2.2. ‘‘La Grande Vasie"re’’ According to the classification of McCave (1972), ‘‘La Grande Vasi"ere’’ is a mid-shelf mud belt. All authors agree that it is really a patchwork of ill-sorted sands, in which the entrapped finegrained material may account for about 25% the total dry mass of the sediment (Berthois and Le Calvez, 1959; Glemarec, 1969; Barusseau, 1973; Pinot, 1974; Vanney, 1977). This extensive blanket, situated around 100 m water depth, extends along the shelf for more than 250 km, from the offshore area south of Britanny to the Rochebonne shoal (Fig. 1). The muddy sediment lies on sands and shelly gravels derived from fluvial deposits, reworked during the Holocene transgression (Allen and Castaing, 1977). These fine-grained deposits do not generally exceed a few decimetres in thickness, and the extent of this blanket varies according to the season (Pinot, 1974) and to hydrodynamical conditions (i.e. swells and storms).
3. Methods In order to estimate sedimentation rates, nine sites were investigated in June 1995, from different representative areas in the Gironde mud fields and from the southern part of ‘‘La Grande Vasi"ere’’. At each site, undisturbed box cores were collected, in which subsamples (PVC pipes, 10 cm diameter) were taken. The cores were opened, described and vertical slabs were removed for radiographic analyses. X-radiographs were used to control the sampling of the thin layers (1 cm), which were selected for radioisotope analysis. Water content was determined by weight loss after oven drying: water content (W in %)=(water weight/wet sediment weight 100). The silt and clay fractions were determined using a Malvern 3600E laser diffraction particle sizer (in the following results, grain-size is expressed as median grain diameter, Mf in mm). 5–10 subsamples were recovered in a vertical profile from the topmost 30 cm of the cores. Radioisotopic measurements were made using a semi-planar germanium detector (EGSP 220025-R from EURYSIS Mesures) coupled to a multichannel (8000 channels) analyser. Dry homogenized samples were packed in 9.5 ml petri dishes. Radioisotope activity was calculated using appropriate calibration coefficients taking into account the gamma ray intensity and the detector efficiency. 210Pb and 137Cs activities were determined by the direct measurements of their gamma decay energies at 46.5 and 661.6 keV, respectively. Excess 210Pb was calculated from total 210 Pb activity and supported 210Pb activity calculated from 214Pb at 295.2 and 352.0 keV, and from 214Bi peak at 609.3 keV. Measurements were carried out at least 2 months after packing of samples, to allow the secular equilibrium to be established between 226Ra and its granddaughters 214 Pb and 214Bi. The use of the natural radionuclide 210Pb (half-life=22.3 yr) is a well established technique to determine sedimentation rates during at least the past century (Koide et al., 1972; Guinasso and Schink, 1975; Nittrouer et al., 1979). Excess 210Pb apparent sedimentation rates were estimated based on a one-dimensional, two-layer model, i.e. negligible mixing below the surface layer (Nittrouer et al., 1983/1984). The 137Cs (half-life=30.1 yr) was secondarily considered, when significant (>0.1–0.2 dpm g1). 137Cs is considered as chemically mobile in the sediment, and is liable to diffuse via pore fluids. 137Cs activities in muds on the Bay of Biscay continental shelf are generally low, and do not generally show pronounced subsurface maxima corresponding to
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known events (i.e. the 1963 maximum in the atmospheric radioisotope bomb-produced fallout, or the 1986 Chernobyl peak, which is not obvious in southwestern France). They have been used to constrain 210Pb-based sedimentation rates and to investigate deep mixing beneath the surface mixed layer (see Alexander et al., 1991).
4. Results 4.1. Sedimentation rates in the Gironde shelf mud fields Seven cores were analysed from the Gironde shelf mud fields (Fig. 2): three cores from a proximal area (i.e. water depth of 40 m or less) and four cores from a distal area (i.e. water depth >40 m). Excess 210Pb activity in the topmost 10 cm of the surficial sediment constantly increases from very low values as 0.1–1 dpm g1 dry weight in less than 40 m water depth, to 2–5 dpm g1 between 40 and 60 m, and to 6–7 dpm g1 for sites beyond 70 m water depth. 4.1.1. The proximal area (30 to circa 40 m water depth) In the inner zone of the two mud fields (WGMP and SGMP), the sediment is homogeneous and is disturbed by only a minor amount of bioturbation. This mud alternates with coarser-grained layers (i.e. silts and sands). As a result, the water content is rather variable (655W5110%), depending on the grain-size.
Fig. 2. Distribution of muds fields on the Aquitaine continental shelf (isopachs of the muds in metres). Location of the box cores collected for 210Pb analyses at variable water depths and variable distances to the Gironde estuary.
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Fig. 3. Selected cores from the Gironde shelf mud fields, Bay of Biscay. Sketches of X-radiograph description: contacts, primary physical structures and biological structures. KI 95151, 95132 and 95149 (Gironde shelf mud fields): median grain-size. KI 95136 and 95142 (‘‘La Grande Vasi"ere’’): median grain-size after sieving at 200 mm; diamonds (KI 95142) symbolize the 563 mm fraction in the total bulk sediment.
Radioisotopic data from three cores collected in this area are in a good agreement (Fig. 4). The core KI 95151 (39 m water depth) is representative of the proximal area. It typically exhibits an alternation of very dark gray (2,5 Y3/0 in Munsell colour) clayey muds (Mf ¼ 9216 mm) and interbedded coarse silt or fine sand layers (Mf ¼ 22240 mm) with shell fragments (Fig. 3). At the top of the core, sandy silts (Mf ¼ 41 mm) lie on an erosional surface which is overlain by a shell lag. As in the other cores from this area (Fig. 4B), excess 210Pb activity is only present in the upper few centimetres of this core. It is impossible to estimate a sedimentation rate in sediments which show such marked lithological changes. At the very most, the rate would be 50.1 cm yr1 (KI 95148, Fig. 4C). In this proximal area of the mud fields, the 137Cs is at most at the limits of detection (i.e. 0.1 dpm g1). 4.1.2. The distal area (40–75 m) Four cores were analysed in the distal area of the WGMP. The sediment is typically an olivegray (5Y4/1–5Y4/2) clayey silt–silty clayey mud. A fine sand layer is sometimes present in the upper centimetres of the cores. The core KI 95131 (WGMP, 44 m), is composed of a clayey silt mud (Mf ¼ 14218 mm), with a few interlayered fine silts (Mf ¼ 20227 mm) which include small shell fragments. There is no
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Fig. 4. Profiles of excess 210Pb activity in the box cores from the proximal area of the Gironde mud fields. (A) Core KI 95130 (36 m, WGMP); (B) Core KI 95151 (39 m, WGMP); (C) Core KI 95148 (40 m, SGMP).
superficial silt layer in this core. A few long and thin burrows, mainly vertically oriented, are perceptible on the radiographs. The radioisotope profile (Fig. 5A), show no surface mixed layer (SML) and permits the plotting of a straight line to estimate the logarithmic decrease of the excess 210Pb activity. This yields an apparent sedimentation rate of 0.13 cm yr1. The core KI 95132 (WGMP, 49 m) is from the central part of the mud field (Fig. 3). It is composed of silty clays (Mf ¼ 11218 mm), with only a few interlayers of fine silts (Mf ¼ 22226 mm). Only one slight discontinuity is perceptible at a depth of about 22–25 cm and the bioturbation, although present, consists of small diffuse horizontal burrows. Neither the radiographs nor the radioisotopic data indicate a SML at the top of the core KI 95132 (Fig. 5B). The logarithmic decrease of excess 210Pb activity indicates an apparent sedimentation rate of 0.56 cm yr1 (i.e. 0.48 g cm2 yr1, assuming an average dry bulk density of 0.85 g cm3). However, this is a high value for a shelf which is not associated with a major dispersal system, and deep mixing must be suspected. Consequently, cross-checking with data of 137Cs was undertaken. Assuming a sedimentation rate of 0.56 cm yr1, the theoretical level of maximal penetration of 137Cs in the sediment is estimated to be 25 cm, based on the appearance of this bomb-produced radionuclide in ca. 1952. Whereas 137Cs has a measured penetration level between 17 cm (0.18 dpm g1) and 21 cm (0.05 dpm g1), it is not detectable at 26 cm; this indicates that the previous value, although a maximum, approximates to the actual sedimentation rate. Core KI 95133 (58 m) is a dark gray (5Y 4/1) silty clay. The grain-size is not very variable (Mf ¼ 15221 mm) and the water content remains between 65% and 80%. There is abundant but diffuse bioturbation with scattered fine shell debris of bivalves. The excess 210Pb data from this core show a complex SML in the topmost 5–10 cm (Fig. 5C). Below this SML, the profile of this core suggests a logarithmic decrease to a near background level. Based on decreasing 210Pb activity between 15 and 25 cm, the apparent sedimentation rate would be 0.3 cm yr1. However, at this site, the 137Cs has been detected down to a depth of
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Fig. 5. Profiles of excess 210Pb activity in the box cores from the distal area of the West Gironde mud patch (WGMP). (A) Core KI 95131 (44 m); (B) Core KI 95132 (49 m); (C) Core KI 95133 (58 m); (D) Core KI 95149 (74 m). When representative, comparison between the predicted depth (PD) and the measured depth (MD) of 137Cs penetration is noted.
approximately 30 cm, when the maximum theoretical penetration (estimated from 210Pb data and including the SML) should be of only approximately 24 cm. Thus, the presence of 137Cs indicates that mixing dominates in the sediment; this has extended to a depth of 30 cm during approximately the last forty years and the excess 210Pb data give here a maximum rate. The core KI 95149 is representative of the most distal zone of the WGMP (74 m). It consists of a dark (2,5 Y4/0) clayey mud (Mf ¼ 8213 mm) with a high water content (955W5115%). In the greater water depth of the WGMP, the mud contains large burrows in which gastropods (Nassarius sp., Nassarius semistriatus) are found (Fig. 3). A single erosional surface occurs at 17 cm.
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Below a SML of 5 cm, the excess 210Pb profile of the core KI 95149 (Fig. 5D) indicates an apparent sedimentation rate of about 0.11 cm yr1 (0.08 g cm2 yr1). This value suggests that 137Cs extends down to a theoretical depth of 10 cm. Actually 137Cs activity is present (0.18– 0.30 dpm g1) in the upper 12 cm and is not detectable below 14 cm; thus it may be assumed that a value of 0.1 cm yr1 is close to the actual sedimentation rate at this site. 4.2. Fine-grained sedimentation rates in ‘‘La Grande Vasie"re’’ Excluding the southern zone of Rochebonne where mud is likely to accumulate, sampling showed that it is difficult to define accurately this supposedly large ‘‘mud field’’ (Fig. 6). The superficial sediment is composed of sands and gravels which contain a variable, but low content of silt and clay (510–30% of the total bulk samples). Two long box cores were selected from the muddier parts of these ill-sorted sediments, for 210Pb analysis (Fig. 6). The core KI 95142 (100 m water depth) is representative of the surficial sediments from this open mid-shelf mud belt. The deposits are organized into a decimetre-scale fining upward sedimentary sequence. The lower part of this core is composed of gravel and reworked coarse shelly fragments and the topmost 10 cm are composed of muddy fine sands (Fig. 3). Thus, the silt and clay content increases from 14% at the base to 26% at the top of the core. For radioisotopic analysis, the sediment of the core KI 95142 was sieved at 63 mm and only the fine-grained fraction was analysed. Using the excess 210Pb profile (Fig. 7A), a sedimentation rate of 0.26 cm yr1 may be estimated between 9 and 20 cm sediment depth. In fact, when the maximum predicted depth of 137Cs activity would be of 17 cm, it remains significant (i.e. up to 0.13 dpm g1) down to 25 cm. Thus, the radiochemical profile is affected by deep mixing of the whole sampled sedimentary column, and the calculated rate is a maximum estimate of the actual accumulation rate. Results from KI 95136 core (94 m water depth, south of Rochebonne shoal) shows similarities with those of the previous core, although the sediment (fine-sand) contains more mud. Radiography revealed subtle horizontal burrows and tracks at the top of the core, and distinct sand-infilled vertical burrows near its base (Fig. 3). A surface mixed layer, where excess 210Pb and 137Cs activities are constant, extends down to a depth of 9 cm (Fig. 7B). The fine-grained sedimentation rate is estimated at 0.22 cm yr1 (i.e. 0.18 g cm2 yr1). The observed depth of 137Cs penetration (between 13 and 21 cm) is in this case in good agreement with the predicted depth of penetration (19–20 cm), and the present sedimentation rate of fine-grained material seems to be close to the apparent rate.
5. Discussion 5.1. The sedimentation rates in the mud fields 5.1.1. The Gironde shelf mud fields Spatial distribution of 100-yr time scale fine-grained sedimentation rates on the shelf off the Gironde estuary are summarized (Fig. 8). In cases where the effects of mixing on the rate of
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Fig. 6. The southern part of the mid-shelf muddy belt ‘‘La Grande Vasi"ere’’: location of sampling sites of the superficial sediments in June 1995. Dots are grab stations, stars are the location of the two cores that were examined for 210Pb in this work. Values are % of 563 mm material (silt+clay). Fine-grained sediment does not exceed 30% of the total bulk sediment, apart from south Rochebonne shoal, where up to 60% silt and clay are locally measured. Contours correspond to the assumed limits of muds in the superficial sediments and heavy dotted line the isovalue 20% silt+clay (after Vanney, 1977). Note the southwestern location of the WGMP, and the absence of fine-grained sediments in the area located between these mud fields.
accumulation could not be resolved, maximum rates of sedimentation were assumed to be correct. According to their geographical distribution, these estimations appear to be consistent. Although sedimentation occurs at the inner sites close to the source (SM of Gironde origin) the local dynamics (i.e. bottom tidal currents and very important storm action) both prevent this
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Fig. 7. Profiles of excess 210Pb activity in the cores from the mid-shelf muddy belt ‘‘La Grande Vasi"ere’’. (A) Core KI 95142 (open shelf); (B) Core KI 95136 (south Rochebonne shoal). Comparison between the predicted (PD) and the measured (MD) depths of 137Cs penetration.
Fig. 8. Summary of 100-yr time scale sediment rates in the Gironde shelf mud fields. Rates are maximum estimates. Note the consistance of the data obtained from 210Pb analysis (underlined values), with accumulation rates yielded on the basis of other markers (14C, palynology) from the upper metre of the muds (see the text). The latter are asterisked at 43 m in the SGMP; 45, 49 and 64 m in the WGMP (after Lesueur, 1992). Normal characters are total finegrained sedimentation rates (cm yr1). Italics are sediment accumulation rates (g cm2 yr1) of assumed estuarine particles (i.e. 530 mm).
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sedimented mud from long-term total preservation, and furthermore remobilize earlier sediments which originate partly from the substrate of the mud fields. These processes are recorded within the deposits: (1) X-radiographs of cores show sedimentary structures and interbedded mud and fine sand layers (Lesueur and Tastet, 1994); (2) radioisotopic data indicate that sediments from this area are devoid of both 210Pb and 137Cs. Thus, the 100-yr time scale sedimentation rates have never reached 0.1 cm yr1 in this proximal area that corresponds to the inner zone of the mud fields. This study shows that modern silty-clay sedimentation has merely a temporal (i.e. seasonal) presence in the shallowest-water area of the WGMP, as suggested earlier (Jouanneau et al., 1989). As a consequence, it is only beyond a minimal water depth of approximately 40 m that the balance between supply and local dynamics allows preservation of the deposited material. The greatest sedimentation rates (>0.2 cm yr1) occur at depths between 40 and 60 m, with the maximum value (0.56 cm yr1) in about 50 m water depth, in the WGMP. This zone was previously considered as the depocentre of the fine-grained sedimentation on the Gironde shelf (Lesueur et al., 1996). Moreover, studies of the seasonal hydrographic regime in the Bay of Biscay showed that in winter time (i.e. the season of major output of SM) a thermo haline front is located along the mid-shelf, between Brittany and Arcachon (Castaing et al., 1999). The result of this oceanographic structure is that suspended particles are trapped around a depth of 50–60 m on the Gironde shelf, where a bottom nepheloid layer extends across the shelf, from the estuary to the WGMP. The same phenomenon occurs around the 100 m isobath along ‘‘La Grande Vasi"ere’’ (Jouanneau et al., 1999). A similar explanation of a hydrodynamic barrier, concentrating suspended sediment, has been used in order to explain high deposition rates on the eastern Brazilian continental shelf (Patchineelam and Smoak, 1999). The decrease in rate of accumulation further offshore at depth around 50–60 m, is interpreted as being due to an offshore depletion in the flux of the suspended matter. Such an across-shelf decrease in sedimentation rate with distance from source is a typical phenomenon, which has been already described in most other sites of deposition of fine-grained sediment on continental margins (e.g. Nittrouer et al., 1984/1985). A comparison of the data presented here with the sedimentation rates estimated over the longer term (i.e. for time scale >100 yr) has been undertaken for various stations from the Gironde mud fields. The results of longer term rates of accumulation are based upon different recognized chronological markers in long cores, including radiocarbon ages and pollen analyses (Lesueur et al., 1996). These rates were estimated as variable and ranged from 0.1 to 0.4 cm yr1. Comparison of data (i.e. 210Pb versus 14C) from similar locations shows rates of the same order of magnitude (Fig. 8). However, where excess 210Pb sediment accumulation rates have been estimated at their highest values (i.e. 0.56 cm yr1 in the central zone, 50 m water depth), 14C sedimentation rates are estimated as between 0.25 and 0.32 cm yr1 (Lesueur et al., 1996). Such a disparity between smaller long term rates and larger short term rates (i.e. 100-yr scale) seems to be common (e.g. Saddler, 1981; Maldonado et al., 1983). Nevertheless, both 14C and pollen data suggest an increase in sedimentation rate in both the topmost deposits and in those of the central zone of the mudfields. The appearence of Pinus pinaster pollen marks the beginning of the intense afforestation of Les Landes in the mid-19th century (Marambat, 1992); although this is not detectable in the cores recovered on the inner shelf, it is noticeable in the topmost decimetres of the cores from the distal area (Lesueur et al., 1996). At the site of the core KI 95132 (50 m water depth), this pollen was recognized down to almost 70 cm beneath the sediment surface (Lesueur,
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1992), indicating a sedimentation rate of approximately 0.5 cm yr1. This value is similar to the estimated excess 210Pb rate of 0.56 cm yr1 from the same site (Fig. 8). In summary, various data suggest an increase in rates of fine-grained accumulation during at least the last century, indicating an increase of supply of fluvial suspended matter from the Gironde onto the shelf. Lesueur et al. (1996) suggested that such an increase could be the consequence of natural causes, such as infilling and morphological changes of the estuary, which were enhanced by human activities since the mid-XIXth century or more recently (including acceleration of soil erosion and/ or management of the estuarine area). The result has been the downstream shift in the zones of maximum sediment influx and deposition (Allen et al., 1979) and a suspected concomitant increase of SM expelled onto the shelf during the last century (Castaing, 1981). 5.1.2. ‘‘La Grande Vasie"re’’ Cores recovered from surficial deposits of this large open shelf muddy field display some evidence of reworking by periodic erosion due to physical agencies, and by biological processes (large but indistinct burrows provide evidence of biological mixing). An estimated fine-grained sedimentation rate of 0.26 cm yr1 for the open shelf site (core KI 95142) is very excessive. A rate of 0.1 cm yr1 over the surface area of ‘‘La Grande Vasi"ere’’ (9.103 km2), and this is only considered the 530 mm material of estuarine origin (Weber et al., 1991), implies a rate of mass accumulation that exceed the fluvial sediment discharge which reaches the shelf (i.e. 2.5 106 t yr1, Jouanneau et al., 1999). Of particular interest, is the studied site near Rochebonne shoal (core KI 95136) where mud preferentially accumulates, with estimated rate of about 0.2 cm yr1. This is a high value, as this open shelf area is far away (i.e. >100 km) from any fluvial SM sources. However, recent geochemical studies of metallic elements have shown that it is the result of the cumulative effect of both the Gironde and the Loire rivers in this part of ‘‘La Grande Vasi"ere’’ (Jouanneau et al., 1998). Hence, near Rochebonne, the accumulating sediment probably includes fine-grained material from both the two largest suppliers of SM on the Atlantic coast of Europe (Castaing and Jouanneau, 1987). 5.2. Comparison with sedimentation rates in other shelf mud settings In the last 20 years, there have been numerous studies on sedimentary processes on the continental margins. Among the latter, those concerned with fine-grained deposits of shelves which are not associated with major rivers and deltas are scarce in the literature (Table 1). Nittrouer et al. (1984/1985) proposed that on a 100-yr time scale, sedimentation rates associated with major rivers (i.e. >108 t yr1) range from centimetres per year to millimetres per year, while rates associated with small dispersal systems (i.e. 5108 t yr1) are generally millimetres per year or less. Despite the low river sediment discharges from to the Gironde estuary (1.5 106 t yr1 after Castaing and Jouanneau, 1987), estimated sedimentation rates for fine-grained sediments on this shelf range from 50.1 to 0.5 cm yr1; these are far from negligible values, by comparison with other continental shelves associated with larger suppliers. Despite the small dimensions of the Gironde shelf mud fields, the observed across shelf trends (i.e. proximal area/distal area) in sedimentary structures and associated sedimentation rates are similar to the across shelf zonation of fine-grained deposit associated with higher dispersal systems, of subaqueous parts of deltas (i.e. topset/foreset/bottom set deposits).
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Table 1 A review of sedimentation rates deducted from excess 210Pb chronology in shelf muds deposits linked to major dispersal systems (upper part of the table), shelf mud fields associated with small dispersal systems (lower part of the table) Delta and associated shelf
Area
Sedimentation rate (cm yr1)
Reference
Changjiang (China) Huanghe–Changjiang (China)
Inner shelf Offshore Topset deposits Foreset deposits Bottomset Deposits Topset/inner shelf Foreset/mid shelf bottomset/outer shelf Inner shelf Mid shelf Delta front Prodelta Distal area Proximal area Middle prodelta Distal prodelta
1–5 0.3 0.1–0.2 0.4–0.9 0.2–0.4 50.1 2–10 0.1–2 0 2–8 1.7 0.8 4.1 1.1 0.39 0.09 0.25 0.12 0.06
Yang and Milliman (1983) DeMaster et al. (1985) Alexander et al. (1991)
Amazon (Brazil) Ganges–Brahmaputra (India) Fly River (New Guinea) Ebro (Spain)
Kuehl et al. (1982) Kuehl et al. (1986) Kuehl et al. (1989) Harris et al. (1993)
Diaz et al. (1996)
Continental shelf
River
Water depth (m)
Sedimentation rate (cm yr1)
Reference
Washington North California North California Oregon New England Jinhae Bay (S Korea) Eastern Brazil
Columbia r. Russian r. Eel r. Klamath r. Outer shelf Nakdong r.
60–120 50–125 50–100 60–150 10–40
0.3–0.7 0.15–0.24 0.1–0.15 0.06 0.02–0.05 0.4
Nittrouer et al. (1979) Demirpolat (1991) Leithold (1989) Kulm et al. (1975) Bothner et al. (1981) Park et al. (1995)
Pardo, Mucuri Doce, Caravelas Minho Douro
10–40
0.2–0.8
Patchineelam and Smoak (1999)
65–120 65–130
0.1 0.16–0.55
Eastern Spain (Valencia shelf)
Ebro
20–130
0.1–0.3
Gulf of Lion (S France) Aquitaine (SW France) Bay of Biscay (W France)
Rhone
50–100
0.15–0.63
Carvalho and Ramos (1990) Carvalho and Ramos (1990), Drago et al. (1994) Nittrouer et al. (1985), Maldonado et al. (1983), Palanques and Drake (1990) Zuo et al. (1991)
Gironde
35–75
50.1–0.56
This paper
Gironde+Loire
100
40.2
This paper
N Portugal N Portugal
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5.3. Contribution to the sediment budget on the continental shelf of the Bay of Biscay An attempt was made to calculate the amount of fine-grained sediment brought by the Gironde and trapped in the offshore shelf mud patches. The amount of material 530 mm, which is characteristic of the fluvial supply is the basis of this budget. The estimated sedimentation rates have been applied to various areas of the mud fields, according to (i) water depth, (ii) geometry of the different muddy bodies and (iii) sedimentological characteristics (i.e. grain size, silt+clay and water contents). In each area, the accumulation rate (g cm2 yr1) was multiplied by the surface area (cm2) and by the dry bulk density (g cm3). The total sum yields a value of 5 105 t yr1 as the rate of sediment accumulation in the Gironde shelf mud fields, on a 100-yr time scale. This total flux corresponds to about 13 of the total sediment output of Gironde origin. This result is in agreement with other previous flux evaluations: Jouanneau and Weber (1989) claimed that 30% of the fluvial solid discharge is deposited in this mud patch, and Lesueur (1992) estimated that about 27% of the Gironde SM outputs was trapped in the Gironde shelf mud fields (34 of the accumulated amount in the WGMP). According to the present estimation, 73% of this particulate matter is trapped in the WGMP and only 21% in the SGMP (the remained being deposited in the smaller mud patches). This is consistent with the data on the present outputs, as Ruch et al. (1993) estimated that only 20% of estuarine particles are discharged to the shelf from the southern channel. Lastly, this work complements the data on previous budgets of sedimentary fluxes to the slope and rise along on the French continental margin (Jouanneau et al., 1999). These first estimations of sediment accumulation rates in the Aquitaine shelf mud fields give values between 50 and a maximum of about 400 mg cm2 yr1. Much of the remaining sediment accumulates in the inner coastal bays of Charentes, or farther away on the northwestern shelf, or escapes to the slope and rise (Table 2). Table 2 A synthesis of our present knowledge of fine-grained sediments from the Gironde estuary to the continental shelf, slope and rise in the Bay of Biscay. Sediment accumulation rates in the shelf mud fields. This work complement previous evaluations: sedimentation rates on intertidal areas from the Gironde estuary (from Grousset, personal communication; Fregard, 1994; Lesourd, 1996) and on mud flats from the Charentes bays (from Gouleau et al., 2000); sediment budget on continental slope and rise (from Jouanneau et al., 1999) Muddy area
Sedimentation rate (cm yr1)
Fine-grained accumulation rate (mg cm2 yr1)
La Grande vasi"ere (southern zone) West Gironde mud patch Proximal area (540 m) Depocentre (circa 50 m) Distal area (60–75 m) South Gironde mud patch (inner zone) Tidal flats Gironde estuary Charentes bays Continental slope and rise Shelf break 2500–3000 m water depth
0.2–0.26
150
40.1 0.3–0.5 0.1–0.2 0.1
450 300–400 50–100 450
2–2.5 0.25–1
Nd Nd
Nd Nd
>10 2
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6. Conclusions The first excess 210Pb measurements of superficial muddy sediments from the French Atlantic shelf give encouraging results for the evaluation of rates of sedimentation during the past century. The major conclusions of this paper are: (1) Sedimentation rates in the Gironde shelf mud fields as determined by 210Pb measures are roughly comparable to those obtained by other means (i.e. 14C and palynology). They are maximum values, because they cannot be corrected for perturbation by biological and physical processes. Sedimentation rates, ranging between 1 and 5 mm yr1, are similar to values obtained on other shelves associated with small to medium dispersal systems. (2) The highest rate obtained (close to 0.5 cm yr1) is high for a shelf associated with a small river system. This high value corresponds to the mean of the nearshore mudfield, at the depocentre of the fine-grained sedimentation. In this zone, the main part of the Gironde SM is trapped by a winter thermo haline front. Lower rates (0.1–0.3 cm yr1) estimated for the outer zone of the muddy deposits are the result of progressive depletion in sediment supply with distance. Either there is no sediment accumulation, or very low rates (i.e. 40.1 cm yr1) in the inner zone of the mud patches (water depth 540 m), due to resuspension of estuarine fluid muds by heavy swell and storms. (3) The highest sedimentation rate, of about 0.5 cm yr1 at 50 m water depth in the WGMP, is in good agreement with an earlier value, based on palynology. It is consistent with the hypothesis of an increase of fine-grained sedimentation in the central zone of the Gironde shelf mud fields, during at least the last century. The corollary of this observation is that there has been an increase of the Gironde output during this period. Infilling of the estuary, climatic changes and human activities have jointly affected the supply of river-borne suspended discharge to the shelf. (4) It is estimated that about 13 of the present day total discharge of SM from the Gironde estuary is trapped in the Gironde shelf mud fields. (5) In the mid-shelf mud belt, ‘‘La Grande Vasi"ere’’, fine-grained sedimentation rates are overestimated (0.2 cm yr1), when the studied sites are distant (i.e. >100 km) from continental sources (Gironde and Loire rivers). In this zone, relict shelly sands are mixed with the fine-grained material, by biological activity and by storms and reworking of the sediment is generally the dominant process in ‘‘La Grande Vasi"ere’’.
Acknowledgements The cores and sediments were collected during the DEPOVASE VI cruise (June 1995); the authors are grateful to the crew from the board of R.V. ‘‘Co# te d’Aquitaine’’ (CNRS/INSU). Analyses were carried out in the labs of University Bordeaux I (DGO), whose personnel is acknowledged. The authors are indebted to Y. Baron (GEA Marine Nationale Cherbourg) for having provided some of the gamma analyses, J.L. Reyss (CFR CEA/CNRS Gif/Yvette) for the intercalibration exercise of gamma analyses on some samples, S.A. Kuehl (VIMS Gloucester Point) who provided some constructive remarks on the first writing of this work, G. Evans (Southampton Oceanography Centre) who helped to improve the final manuscript, and the two
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referees. Part of this research was supported by the EC-funded Human Capital and Mobility Network (CHRX-CT94-0541), Coastal Environments (E-Atlantic: Physical Processes).
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