Textural and geochemical evidence for the provenance of aeolian sand deposits on the Aquitaine coast, SW France
S A M A N T H A E. S A Y E & K E N N E T H P Y E
Surface Processes and Modern Environments Group, Department of Geology, Royal Holloway, University of London, Egham, Surrey, TW20 OEX, UK
Abstract: The coastal dune system of the Aquitaine region, SW France, is the largest in Europe. At the present time the dunes are mostly stabilized by forest vegetation which is largely the product of dune stabilization schemes undertaken since the late 18th century. Much of the shoreline is currently eroding at rates of 1-2 ma ~, which are likely to increase if predictions of accelerated sea-level rise and increased storminess are correct. The sources of the beach and dune are poorly understood, and need to be identified in order to assess the sand budgets and likely dynamic response of the dune systems to changes in environmental forcing factors. This paper presents the results of an investigation to characterize the beach and aeolian sand in the region, and to identify its origin. The dune sands were found to be texturally and compositionally similar to the Quaternary Sable de Landes which occurs landward of, and partially beneath, the coastal dune belt. The results suggest that marine, aeolian and local fluvial reworking of the Sable de Landes has provided the main source of coastal dune sand, although marine erosion of Pleistocene deposits exposed in the coastal cliffs of the Medoc has made a secondary contribution.
The Aquitaine coast in SW France consists of a 250km long, almost straight and continuous sandy beach interrupted only in its middle by Arcachon Bay which is partially closed by the Cap Ferret spit (Fig. 1). The coast is dominated by the largest coastal dune complex in Europe, ranging in width from 0.2-10km, that encloses several large lakes. The dune system consists of foredunes and older inland dunes which have previously been mapped separately by the French Geological Survey, largely on the basis of morphology, as 'modern' and 'primary' dunes.
The predominant wind direction on the coast is essentially westerly (N280 °) and the consis- tent orientation of the dune axes parallel to the coast suggests that the wind climate has remained largely unchanged in the Aquitaine region during the last 3500 years (Froidefond & Prud'homme 1991).
In the northern part of the region, on the Medoc Peninsula, the dunes rest upon Holocene estuarine sediments associated with tributary valleys and terraces of the Gironde estuary. Further south the dunes overlie Pleistocene sand sheet sediments (the Sable de Landes) which form a generally flat, triangular area extending up to 100 km inland. These deposits are mainly fluvio-aeolian in origin, with discontinuous areas
of low relief continental dunes and aeolian sand sheets separated by, and interbedded with, fluvial deposits. These sediments are believed to have been transported to the area by rivers flowing from the Pyrenees under periglacial conditions during the late Pleistocene (Legigan 1979).
The coastal dunes are largely stabilized by coniferous forests at the present day, the result of dune stabilization schemes undertaken since the late 18th century (Barr6re 1992; Favennec 1996). Parabolic and barchanoid dune forms are both well represented. Parallel with the shore in many areas is a foredune ridge or regressive foredune platform which is also heavily managed at the present time (Fig. 2). The beaches and frontal dunes have experienced significant net erosion during the last 50 years, as indicated by the presence of fallen Second World War German military blockhouses which are now situated on the beach in many places (Fig. 3). Average historical erosion rates vary spatially from 1-2 ma -1 , with coastal progradation limited only to a relatively small number of areas (Froidefond & Prud'homme 1991; Aubi~ & Tastet 2000). Large transgressive dunes occur in only one or two places such as the famous Dune du Pyla situated at the entrance to the Arcachon basin (Fig. 4; Froidefond & Legigan 1985).
Pointe de Grave~._.~. _ Les Contines Soulac . . . . . . . . . . . . . . . UAmelic Pointe de Io Neqode Le Gurp Montetivet les Beins/" Hourtin - Carcens L.acanou ~-----__._ Correyre Pont du Hourbiel ~ ~ Le Porge Le Grand C r o h o t ~ l Cap Ferret--_... Pyla-- Plage de I0 Salie-Nord--_~_~ Biscarrosse Parent is-en-Bo rn------~ I Mimizen , ! Contis tes Boins~.~,.~N, Nt i St, Girons Ploge Mallets PIaQe----~
Soustons Piege Descasernes. Capbreton ~ Labenne - - Plage du Me t ro - - Plage la B a r r e - - Sable d'or Biarritz Plage Morbelto . ~ \ Bidart "2
N
0 i0 20 30 40 50km 1 1 1 1 I t
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m ..... = = : : ===:
, o
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St.deen de Luz Hendeye
• SAMPLE SITES
COASTAL DUNES
SABLES DES LANDES
:MARSHES
LAKES
Fig. 1. Map illustrating the distribution of the morphological units in the Aquitaine region and the location of the samples collected.
A number of previous studies have examined the morphostratigraphy and chronology of the dunes and associated deposits (Bressolier et al. 1990; Tastet & Pontee 1998; Clarke et al. 1999), but the sedimentary characteristics and provenance of the coastal sands remain poorly documented. In this study the textural and geochemical characteristics of the aeolian sedi- ments was investigated in order to provide information regarding sediment provenance,
and in particular to test the hypothesis that the fluvio-aeolian sand sheet (the Sable de Landes) is the main source of the dune sands.
Sampling rationale and methods
Sediment samples weighing 200-400g were collected at regular intervals along transects perpendicular to the coastline between Pointe de Grave at the mouth of the Gironde Estuary
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PROVENANCE OF AEOLIAN SAND DEPOSITS ON THE AQUITAINE COAST 175
Fig. 2. Highly managed foredune platform north of Cap Ferret.
and Hendaye near the French/Spanish border (Fig. 1). The sampling sites were located using a global positioning system (GPS) with a spatial resolution of approximately 50 m.
The mid-beach, upper beach and foredune sediments were sampled from the surface to a depth of 15 cm using a bulb planter. Only the beach was sampled south of the Adour River as the coastline is cliffed and dunes are largely absent. Further inland, shallow pits were dug to obtain samples from the modern and primary dunes, the Sable de Landes and the ftuvially reworked Sable de Landes up to 16km east of the coastline. These units were identified with the aid of topographic maps (Institut Geogra-
phique National, Paris 1:25 000) and geological maps prepared by the BRGM, the Bureau de Recherches Geologiques et Minieres (Mar- ionnaud 1972, Carte Geologique de la France 1 : 50 000). Samples were generally collected from a depth of 25cm below the surface in the modern and primary dune units but samples were taken at variable depths from the sand sheet depending on evidence of horizonation due to pedogenesis. The depth profiles were obtained to test a subsidiary hypothesis that surface area, geochemistry and surface texture features of the sand is influenced by weathering.
The modern and primary dunes were consid- ered as one morpho-sedimentary unit for the
Fig. 3. Coastal dune erosion at Cap Ferret, SW France illustrated by the distance between the shoreline and fallen German Second World War blockhouses now located on the beach.
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Fig. 4. Dune du Pyla a large transgressive dune encroaching upon forested modern/primary dunes.
purposes of sampling in this study since existing maps which distinguish the two units on the basis of morphology are almost certainly inaccurate (Tastet & Pontee 1998; Clarke et al. 1999). The fluvially reworked Sable de Landes and aeolian reworked Sable de Landes deposits were differ- entiated though not on the location map due to the dispersed and less extensive coverage of the former morphological unit. A sixth sampling category, 'other', used in this study included samples from the Pleistocene cliff exposure beneath the foredunes in the Medoc region.
Laboratory analytical methods
The grain size distributions of 98 samples were determined by dry sieving at 0.250 intervals. The mean, standard deviation (sorting) and skewness for each sample were obtained from the resulting particle size distribution data by the method of moments using a computer programme.
Textural and chemical fingerprinting of the aeolian sands and potential source sediments was undertaken using a standardized grain size fraction. This procedure was adopted primarily to minimize variation in these properties due to grain size. The size fraction chosen for analysis was 1.5-2.0~ as this is the predominant modal fraction present in most of the samples. The separated grains in this size fraction were cleaned by boiling in 18% hydrochloric acid for 30 minutes to remove carbonates, oxides, alumino-silicate and organic coatings.
X-ray fluorescence (XRF) spectrometry was used to determine the major oxide and trace element composition of 66 selected grain size
fractions. Approximately 5 g of each sample was ground using an agate mill and made into a pressed powder pellet with lithium borate back- ing. In order for the particles to bind together to form a pressed powder pellet it was necessary to add approximately 8 drops of 2% Moviol glue solution comprising polyvinyl acetate.
A Coulter SA 3100 BET instrument, which uses the nitrogen gas adsorption method, was used to measure the surface area of c. 10 g sub- samples from the unground size fraction used for the geochemical analyses. This instrument can measure surface area in the range 0.01 to >2000m 2 g-l . The precision (relative error) of the technique was ascertained by running four samples twice and was found to range from 0-1.25%. The accuracy of the technique, estab- lished by running certified standards of known surface area, was better than 5%.
Scanning electron microscopy (SEM) was used to examine the shapes and surface textures of the grains and to assist in interpretation of the surface area results. The samples were examined using the secondary electron mode on a JSM 5300 SEM with a Link Systems AN 10085 energy dispersive X-ray (EDX) analyser. The EDX was used to determine the composition of individual grains and to distinguish between quartz and feldspar grains which can appear very similar when the quartz is weathered. The EDX was also used on surface features to aid the visual differentiation between secondary silica and remains of clays that had not been removed during the cleaning process. Selected grains were embedded in epoxy resin, sectioned and polished to allow backscattered electron mode SEM examination of the grain outlines and internal structure in plan view.
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The mean grain of all the sampling units was found to lie within the range of medium sands (Table 1). The beach sediments show a wide range of mean grain sizes but on average are slightly coarser, less well sorted and more nega- tively skewed than the aeolian sediments. The examples of coarser beach sediments also show a tendency to be more poorly sorted and less negatively skewed than the finer beach sediments (Fig. 5).
The foredunes and modern/primary dune samples are both predominantly well sorted to very well sorted with near-symmetrical to nega- tively skewed grain size distributions, while the Sable de Landes and fluvially reworked Sable de Landes are moderately well sorted to poorly sorted with skewness values ranging from slightly positively skewed to very negatively skewed. The negative skewness values obtained for the dunes are in agreement with previous grain size analysis at Dune du Pyla by Vincent (1996).
The average mean grain size of the foredunes (1.71 :t: 0.08~) is relatively coarse in comparison with foredunes found in many other parts of Europe, and is in part a reflection of the coarse nature of the source material on the beach from which the fines have probably been winnowed by marine processes.
The modern and primary dune sands are slightly coarser but have similar average sorting to the foredune sands. They have a slightly wider range of skewness values and are intermediate between the foredunes and Sable de Landes samples in this respect. The Sable de Landes and reworked Sable de Landes samples have a broadly similar average mean size to the fore- dune and primary/modern dune samples but show poorer sorting.
Figure 6 shows the variation in mean size of the different sampling units as a function of geographical position between the northern Medoc (top of columns) and the Spanish border (bottom of columns). Although there are two or three localized areas of exceptionally coarse beach sediments, the mean size shows only limited longshore variation between Point de Grave and the Adour River Mouth. South of the Adour there is a marked fining trend towards the Spanish border. Very little along- shore variation in mean size is evident amongst the foredune, modern/primary dune and Sables de Landes samples.
Statistical analysis using the Mann-Whitney U-test indicated that the beach and foredune samples are texturally similar to the Sable de Landes samples, differing only in terms of mean size and sorting, respectively, at the 95% con- fidence level (Table 5). The mean size of the modern/primary dunes is not significantly differ- ent from that of the Sable de Landes at the 95%
BEACH V- FORE DUNE MODERN / --] SABLE DE i SABLE DE OTHER PRIMARY DUNES LANDES LANDES i
FLUVIALLY i REWORKED =
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" ' = " - • ~oo~ ~oo+ • ~ ~o6~ g. o zo8 '~ m O 0.4~ ~ O 0.4+ ~'~ m O 0.4J I 'm O ~ •
~ - ~ 4 - ~ ~ ~ ~ , J : L ~ _ _ r , : , 0 / , ~ - - ~ ~ 1 p
1 0 , 2 3 1 0 , 2 31 3 1 0 ~ ~ ~ 1 1 1 0 1 ~ 3 -~ 0 ~ M FAN G ~ I N S e E I MEAN GRAIN S IZE MEAN GRAIN S IZE / MEAN GRAIN SIZE MEAN GRAIN S I Z E M FAN GRAIN SIZE
(ph) J l (phi) I (phi) (phi) (phi) (phi)
BEACH FORE DUNE ] MODERN/ I SABLE DES ~ B L E DE OTHER PRIMARY DUNES [ LANDES LANDES
PROVENANCE OF AEOLIAN SAND DEPOSITS ON THE AQUITAINE COAST 179
MODERN / PRIMARY DUNES
SABLE DE LANDES SABLE DE LANDES - - FLUVIALLY REWORKED
I
I
M EAN GRAIN SIZE (phi) M FAN GRAIN SIZE (phi) M FAN GRAIN SIZE (phi) M EAN GRAIN SIZE [phi) M F.AN GRAIN SIZE [phi) MEAN GRAIN SIZE (phi)
Fig. 6. Mean grain size of samples from different morphological units in the Aquitaine region. NB The values are ordered north to south with samples from the same profile in depth sequence. The beach and the foredunes were sampled from the surface to a depth of 0.15 m and the sample depths of the other units are given to the right of the mean grain size bars. The mid and upper beach were sampled denoted by M and U respectively.
confidence level, although there are significant differences in sorting and skewness between the two groups. Values of the mean, sorting and skewness for the Sables de Landes and flu- vially reworked Sable de Landes are not statisti- cally different.
Chemical composition of the sands"
All the morphological units have very low cal- cium carbonate and heavy mineral contents and are very siliceous, containing greater than 90% silica. The percentage of the major oxides of aluminium, sodium, potassium and calcium are similar in foredunes, modern/primary dunes and the Sable de Landes sands (Table 2).
Figure 7 shows the silica to potassium oxide ratio for the samples from each morphological unit, plotted from north to south down the coast. Potassium oxide concentration is related to the presence of potassium feldspar, and thus the ratio can be used as a proxy to indicate the quartz:feldspar ratio and degree of weathering (Krauskopf & Bird 1995). It can be seen from Fig. 7 that values of the ratio are slightly higher in the foredunes than in the modern and pri- mary dunes, and are slightly higher in the fore- dunes on the central sections of the coast than in the northern and southern sections. Within the Sable de Landes, very high values of the ratio
are found in the highly weathered and podso- lized surface sand layers, with lower values at depth. Values within the fluvially reworked Sable de Landes deposits are broadly similar to those in the foredunes, while those in the 'Other' sampling category show a systematic increase towards the south.
Values for the average concentrations of selected trace elements in the different sampling units are shown in Table 3. There are no statis- tically significant differences in chemical compo- sition between the Sable de Landes and the reworked Sable de Landes. The foredune sands have significantly different (at the 95% con- fidence level) concentrations of aluminium oxide, strontium and zirconium compared with the Sables de Landes, whilst the modern/primary dunes differ significantly from the Sables de Landes in terms of aluminium oxide, potas- sium oxide, rubidium, strontium and zirconium (Table 5). As mentioned above, these differences may reflect a lower content of feldspars in the more weathered and podsolized Sable de Landes near-surface sediments. Careful examination of the data for individual samples indicates that there is a higher degree of similarity between the near surface samples from the foredunes and modern/primary dunes with the B horizons of the more pedogenetically modified Sable de Landes sediments.
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8iO' / K20 RATIO SIOz / K'zO RATIO SiO~ / KzO RATIO I / R SiO~ / KzO RATIO
Fig. 7. NiO2/K20 ratio for the 1.5 2 phi grain size fraction of sand samples from the different morphological units in the Aquitaine region. yB The values are ordered north to south with samples from the same profile in depth sequence. The foredunes were sampled from the surface down to a depth of 0.15 m and the sample depths for the other units are given to the right of the ratio bar.
The trace element yttrium is present in low concentrations in the foredune, the modern/ primary dune unit and the Sable de Landes to the north of Pyla and generally absent in the south. This corresponds with the presence of yttrium in the Pleistocene cliff at Le Gurp in the Medoc region, possibly indicating that erosion of these cliffs has locally contributed to the sediment supply. The chromium and zinc con- centrations demonstrate almost identical spatial distributions. The opposite trend was displayed by the copper concentrations with low concen- trations present in the samples situated from Pyla moving southwards and a general absence to the north of Pyla with the exception of the Pleistocene cliff at Le Gurp (GRA 1G) and the corresponding foredune sample (GRA 1C), supporting a localized sediment source. The signature of the Pleistocene cliff in the samples from the dune system in the north is also observed in the zirconium concentrations.
Surface area
The BET surface area values generally vary between 0.1 and 1.0m 2 g-1 (Table 4). The fore- dune and modern/primary dunes are uniform and similar with mean values of 0.496 m 2 g-1 and 0.439m 2 g-1 respectively. At any one sampling
site on the sand sheet the surface area increases with depth. The mean values of the surface horizons are 0.188m2g -1 and 0.315m2g -1 for the Sable de Landes and fluvially reworked Sable de Landes, respectively, and at depths below 0.3m the average values are 0.526m2g -1 and 0.630m 2 g-1 respectively. The values for the fore- dune and modern/primary dunes are inter- mediate between these values. At the 95% significance level there are no statistically sig- nificant differences between the Sable de Landes sands and the other sampling units in terms of surface area values. However, the P-values obtained from the Mann-Whitney U-test indi- cate that the highest degree of similarity is between the Sable de Landes sands and the flu- vially reworked Sable de Landes, and the lowest level of similarity is between the Sable de Landes sands and the foredune sands (Table 5).
There is little longshore variation in the sur- face area values for the foredune sands, suggest- ing homogeneity due to well-mixing by marine and aeolian processes (Fig. 8). Similarly, there is no systematic longshore variation in the values for the modern/primary dunes, Sable de Landes and reworked Sable de Landes sediments. Varia- tions in these units are evidently more related to depth of sampling, and thereby to degree of weathering and pedogenesis.
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PROVENANCE OF AEOLIAN SAND DEPOSITS ON THE AQUITAINE COAST 183
Table 4. Mean surface area values of the sample populations, determined by the nitrogen gas adsorption technique
Sample population Ns Mean surface a r e a (m 2 g 1)
Foredune 12 0.496 -t: 0.071 Modern/primary dunes 17 0.439 i 0.170 Mean <0.3m 14 0.456 ± 0.170 Mean >0.3m 3 0.363 ±0.177 Sable de Landes 21 0.383 + 0.388 Mean <0.3m 8 0.188 :t: 0.236 Mean >0.3m 13 0.526 + 0.465 Sable de Landes fluvially reworked 12 0.512± 0.381 Mean <0.3m 5 0.315 ± 0.246 Mean >0.3m 7 0.630 ~: 0.448 Other 4 1.545 ± 2.484 Mean <0.3m 1 0.111 :t: 0.000 Mean >0.3m 3 2.023 J: 2.808
Data refer to the 1.50 2.00~ grain size fraction. Ns is the number of samples used in surface area analysis. (:t:) denotes the population standard deviation (o-n).
Sur face t ex ture
SEM examinat ion revealed the samples are very largely composed of subangular to subrounded quartz grains, defined according to the visual comparat ive scheme of Powers (1953), with only a few percent of feldspar grains, regardless of the morphological environment . The Sable de Landes and the fluvially reworked Sable de Landes were petrographically very similar but significant differences in the surface texture were observed between samples f rom the pedogenic A and B horizons. The majori ty of the quartz grains within the A horizon of the Sable de Landes possess smooth surfaces with chemical etch pits which is indicative of a leached hor izon in a podzolic soil profile (Figs 9A & B). Leaching (eluviation) of the A hor izon causes dissolu- t ion of mineral grains, including quartz. Fine,
granular secondary silica, arising from the repre- cipitation of the mineral material lost f rom the A horizon, is easily identifiable on the quartz grains f rom the B hor izon samples (Figs 9C, D). These changes in grain surface texture probably explain the lowering of the surface area values in the A hor izon and the observed increase in the B horizon.
In the foredune samples the quartz grains displayed evidence of either secondary silica preservation within depressions on smooth grain surfaces with chemical etch pits (Fig. 9E) or freshly fractured surfaces (Fig. 9F). The chemical etch pits and the freshly fractured grains are indicative of recent marine reworking and exposure to mechanical abrasion/breakage processes. Fresh, angular quartz and feld- spar grains, possibly indicative of an addit ional source of sediment supply, were found to be rare.
Table 5. P-values derived from the Mann-Whitney U-test during significance testing of the textural and geochemical data between the Sable de Landes and the other morphological units in the Aquitaine region
Beach Foredune Modern/primary Sable de Landes dunes fluvially reworked
Fig. 8. Surface area of the 1.5 2.0 phi grain size fraction of sand samples from different morphological units in the Aquitaine region NU The values are ordered north to south with samples from the same profile in depth sequence. The foredunes were sampled from the surface to a depth of 0.15 m and the sample depths of the other units are given to the right of the surface area bars.
D i s c u s s i o n
The grain size, surface textural and chemical compositional data obtained in this study are consistent with the hypothesis that the main source of the coastal dune sands on the Aquitaine coast is fluvial, marine and aeoilan reworking of the Pleistocene Sable de Landes deposits. These sediments were ultimately derived from the Pyrenees but have experienced significant post- depositional weathering prior to reworking.
Reworking of the Sable de Landes sand sheet probably began while sea-level was over 100m below its present level during the last glacial maximum (Lambeck 1997). Pedogenesis of the Sable de Landes probably continued during its partial submergence due to sea-level rise, between 15000 and 6000 years ago (Lambeck 1997). Parts of the sand sheet were probably reworked during the Holocene transgression, with sand being pushed landwards as a trans- gressive marine-aeolian wedge in the manner originally proposed by Cooper (1958) for the Oregon coast.
Sea-level has been relatively stable at or near to the present level in this area since 5000 years ago. During the last few thousand years episodic aeolian activity is more likely to have reflected
variations in storminess. The Little Ice Age of the 17th to 18th centuries (cf. Lamb 1995) appears to correlate well with widespread aeolian activity in the Aquitaine region which is known to have occurred from historical accounts and maps (Tastet & Pontee 1998).
Although coastal erosion of the Pleisto- ene cliffs in the Medoc region has locally contributed to the sediment supply in recent centuries, there appears to be very limited supply of new sediment to the Aquitaine coast at the present day. Most of the shoreline is eroding, with landward transfer of sediment by aeolian processes and some infilling of embayments and estuaries. Beach and dune erosion is likely to continue for the foreseeable future, and may accelerate if the rate of sea-level rise increases or there is a significant increase in storminess. Such changes will bring increased pressure on the dunes and favour instability of the type which was widespread prior to stabilization measures in the late 18th and early 19th centuries.
C o n c l u s i o n s
• The morpho-sedimentary units sampled in this study are texturally and geochemically similar, supporting the hypothesis that the
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PROVENANCE OF AEOLIAN SAND DEPOSITS ON THE AQUITAINE COAST 185
Fig. 9. Secondary electron SEM micrographs of sand grains from the Aquitaine region: (A) & (B) Etched surface texture of quartz grain from a depth of 0.1 m in the A horizon of profile PYLA 1J, Fluvially Reworked Sable de Landes; (C) & (D) Secondary silica on quartz grain from a depth of 0.4rn in the BjR horizon of PYLA 1H, Fluvially Reworked Sable de Landes; (E) Secondary silica preserved in a depression on the surface of a quartz grain with smooth surface texture and chemical etch pits from PYLA 1A, foredune; (F) Freshly fractured quartz grain from PYLA 1A, foredune.
dune sand was derived from reworking of the Sable de Landes. Reworking was probably initiated by sea- level rise in the early Holocene and con- tinued to the present day. Erosion of the Pleistocene sedimentary cliffs in the Medoc region has provided an addi- tional localized source of sand.
• Variations in the chemical and surface area data sets obtained in this study appear to be due mainly to differences in weathering and pedogenesis with depth in deposits of vary- ing age.
• The BET surface area method provides a useful tool for quantitative comparison of the 3D surface textures of bulk sand samples.
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This work was partially supported by a NERC MRes Studentship to S. E. Saye and a Leverhulme Trust Grant to Professor K. Pye. The assistance of Professor J-P. Tastet of the Department of Geology and Oceanography at the University of Bordeaux I and of technical support staff at the Postgraduate Research Institute for Sedimentology, University of Reading, is also gratefully acknowledged.
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