Fine-Grained Sediment Spatial Distribution on the Basis of a Geostatistical Analysis: Example of the Eastern Bay of the Seine (France)

ARTICLE IN PRESS

Continental Shelf Research 26 (2006) 2335–2351 www.elsevier.com/locate/csr

Fine-grained sediment spatial distribution on the basis of a geostatistical analysis: Example of the eastern Bay of the Seine (France)

Y. Me´ara,Ã, E. Poizota, A. Murata, P. Lesueurb, M. Thomasc

aCnam/Intechmer, BP 324 F50103 Cherbourg Cedex, France bUMR CNRS 6143 M2C, Universite´ de Caen, Esplanade de la Paix, F14032 Caen Cedex, France cSchool of Applied Sciences, University of Glamorgan, Trefforest, Pontypridd, UK CF37

Received 16 December 2004; received in revised form 18 May 2006; accepted 12 June 2006 Available online 26 September 2006

Abstract

The eastern Bay of the Seine (English Channel) was the subject in 1991 of a sampling survey of superficial sediments. Geostatistic tools were used to examine the complexity of the spatial distribution of the fine-grained fraction (o50 mm). A central depocentre of fine sediments (i.e. content up to 50%) oriented in a NW–SE direction in a muddy coastal strip, in a very high energy hydrodynamical situation due to storm swells and its megatidal setting, is for the first time recognised and discussed. Within this sedimentary unit, the distribution of the fine fraction is very heterogeneous, with mud patches of less than 4000 m diameter; the boundary between these mud patches and their substratum is very sharp. The distribution of this fine fraction appears to be controlled by an anticyclonic eddy located off the Pays de Caux. Under the influence of this, the suspended material expelled from the Seine estuary moves along the coast and swings off Antifer harbour, towards the NW. It is trapped within this eddy because of the settling of suspended particulate matter. Both at a general scale and a local scale the morphology (whether inherited or due to modern processes) has a strong influence on the spatial distribution of the fine fraction. At the general scale, the basin-like shape of the area facilitates the silting, and the presence of the submarine dunes, called ‘‘Ridins d’Antifer’’, clearly determines the northern limit of the muddy zone. At a local scale, the same influence is obvious: paleovalleys trap the fine sediments, whereas isolated sand dunes and ripples limit the silting. This duality of role of the morphology is therefore one of the reasons why the muddy surface is extremely heterogeneous spatially. The presence of an important population of suspension feeding echinoderm, the brittle-star Ophiothrix fragilis Abildgaard, has led to a local increase in the silting, and to the modification of the physicochemical and sedimentological parameters. A complex relationship is shown to occur between the amount of fine fraction and the number of brittle-stars (ind. m2). Classical statistical methods are not appropriate to study the spatial distribution of the mud fraction, because the spatial component of the percentage of the distribution is not integrated in the analysis. On the other hand, this is the main property of the geostatistic concepts. The use of geostatistic tools within a strict and clearly identified procedure enables the proposal of an accurate cartography. Further application of the proposed protocol (based on a semivariographic study and a conditional

ÃCorresponding author. Tel.: +33233887341; fax: +33233887339. E-mail address: [email protected] (Y. Me´ar).

0278-4343/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.csr.2006.06.009 ARTICLE IN PRESS 2336 Y. Meár et al. / Continental Shelf Research 26 (2006) 2335–2351 simulation interpolation) for surficial sediments mapping will help explain spatial and temporal variations of fine-grained fraction. Then assessments of sedimentation and erosion stages allow highlighting signature of environmental processes. r 2006 Elsevier Ltd. All rights reserved.

Keywords: Geostatistics; Semivariogram; Sediment transport; Mud; Fine-grained content; English Channel; Bay of the Seine

1. Introduction point of the chart an estimation of the error (variance of the estimation) linked to the interpola- The spatial and temporal expression of the tion of the data. The information provided by variations affecting nearshore sea-beds are at the geostatistics is a fundamental asset compared to the origin of many methodological difficulties in other interpolation techniques (Aubry, 2000). the elaboration of cartographic documents. These Sediments are being studied increasingly for difficulties are partly solved by the development of monitoring the aquatic environment, because they Geographical Information Systems and of their can integrate contaminants and therefore provide associated numerical databases. One of the funda- valuable information on source, dispersion and mental problems, solved by the implementation of accumulation of pollutants. As the majority of the Geographical Information Systems, is the contaminants are usually associated with the fine comparison of maps. However, even when maps fraction of the sediment (Fo¨rstner et al., 1982), appear comparable, their correlation remains very quantification of the fine fraction is necessary (Birch low. This expresses the existence of various pro- and Taylor, 2000). blems related either to the creation of maps or to the This study gives an example of mapping the fine- information acquisition processes at sea. grained content (often called mud fraction) in the Geostatistics, whose fundamental basis was de- superficial sediments to assess the distribution of fined by Matheron (1963, 1965), characterises the muddy areas in the NE part of the Bay of the Seine. regular component of the variation of natural This study thus generalises the more limited objects (Kuzyakova et al., 2001). With recent observations by Dethleff et al. (1996) and by progresses in automatic calculations, the use of Spineanu (1998). geostatistics for the spatial analysis of environmen- Geostatistical methods were used, first, to inter- tal data has become extremely common both on polate the fine fraction variable and, second, to land and sea (Chang et al., 1998; Goovaerts, 1999; estimate errors arising from this process. The Atkinson and Lewis, 2000; Batista et al., 2001; Saito various stages which mark out the geostatistic step and Goovaerts, 2001; Caeiro et al., 2003; Leecaster, allow defining a robust protocol for the fine fraction 2003). mapping. Further application of the proposed Geostatistics include tools that establish maps protocol (based on a semivariographic study and a from the analysis of the spatial structure of data conditional simulation interpolation) for surficial (Davis, 1986), and not simply by the application of sediments mapping will help explain spatial and simpler methods (IDW, spline, etc.) whose para- temporal variations of fine-grained fraction. meterisation (number of neighbourhood, distances The work started within the framework of the of research, etc.) is the choice of the user and under French coastal studies program (PNEC: Pro- no physical considerations. Therefore it adds a new gramme National d’Environnement Coˆtier) ‘‘Bay dimension to the analysis. of the Seine’’, and will be used as a basis for the In marine sedimentology, the study of the spatial study of the variability of the deposition at various distribution is achieved by the creation of maps. To time scales. optimise their interpretation, maps must take into account the spatial structure (regionalised phenom- 2. Study area enon) of the studied parameter (regionalised variable) and allow the description of the environmental 2.1. Morphology parameters that influence this spatial distribution. Temporal study is only possible if the cartographic The Bay of the Seine, a wide shallow gulf, opens documents compared are established using the same to the English Channel between the Cretaceous method and if it is possible to propose for each chalk cliffs of the Pays de Caux to the East and the ARTICLE IN PRESS Y. Meár et al. / Continental Shelf Research 26 (2006) 2335–2351 2337

Palaeozoic peninsula of the Cotentin to the West. It offshore the Cap d’Antifer, namely ‘‘Ridins d’Anti- has a surface area of approximately 5000 km2 and a fer’’. Close to the coast, rocks locally disturb maximum water depth of 40 m (Fig. 1). This study the monotony of the seafloor (e.g. Plateau du concerns the eastern part of the Bay of the Seine, Calvados). limited to the West by the Meridian passing through Courseulles-sur-mer. The bathymetric contours are 2.2. Superficial sediments usually parallel to the shoreline but are disturbed by the existence of a 15 m deep depression, marked The seafloor of the Bay of the Seine is mainly by the 30 m isobath. This depression, namely ‘‘Le covered with coarse-grained sediment (Fig. 2), Parfond’’, orientated NW–SE, is the relic of the pre- including coarse sands, gravels and pebbles (Lar- Holocene palo-valley of the Seine River (Larson- sonneur and Hommeril, 1967; Larsonneur, 1971; neur, 1971; Auffret et al., 1980). It is bordered north Vaslet et al., 1978; Larsonneur et al., 1982). by the Banc de Seine, which consists of a succession Larsonneur (1972) and Avoine (1981) showed a of sand dunes. Other sand waves are observed progressive decrease of grain size from the stony

0°30′0′′W 0°0′0′′ 0°30′0′′E

UK 50°0′0′′N 50°0′0′′N

FRANCE

r fe ti R i n d i n s d ’ A

Antifer

Pays de Caux Dredging 30 m spoil Banc Le Havre de Seine 49°30′0′′N 49°30′0′′N

20 m Seine estuary Parfond 15 m P lateau Honfleur du Calva dos 10 m Trouville/Mer

Courseulles/Mer Cabourg km Datum : WGS84 0212.5 5 50 Ouistreham Projection : UTM (30)

0°30′0′′W 0°0′0′′ 0°30′0′′E

Fig. 1. The eastern Bay of the Seine: main geographical locations. Plateau du Calvados is composed of outcropping calcareous rocks, the Banc de Seine is a sand bank with a depth less than 15 m, Le Parfond is a trough, Ridins d’Antifer are a field of sand waves (i.e. submarine dunes). Black dots are for sample locations of superficial sediments. ARTICLE IN PRESS 2338 Y. Meár et al. / Continental Shelf Research 26 (2006) 2335–2351

Fig. 2. A present-day sedimentological map of the eastern Bay of the Seine (modified from Larsonneur and Hommeril, 1967, Larsonneur et al., 1982). This map shows evidence of the general tendency of decreasing grain-size from offshore toward the Seine estuary. seafloor in the open sea to the subtidal and fine deposits take the shape of diffuse patches. However, estuarine sediments of the Seine River. As is previous studies only cover the area of the mouth of common throughout the English Channel, fine the Seine or the adjacent seafloor near the Calvados sediments are found in sheltered bays and estuaries coast, in areas shallower than 10 m water depth. where the fine sediment can settle (Avoine, 1981, The coastal fringe of muddy sediments between 1986; Larsonneur et al., 1982). It also occurs in a the Seine estuary and Antifer Harbour has a fine narrow fringe between 13 and 20 m water depth fraction content ranging from 5% to 10% and was near the coast of the Pays de Caux, from the mouth described by Larsonneur and Hommeril (1967). of the Seine to the Antifer harbour (Crevel, 1984, Avoine (1981) mapped the same area but he showed 1986). a fine fraction content ranging from 10% to 25%. Recent studies (Lesueur and Lesourd, 1999; Crevel (1984, 1986) extended the studied area to the Lesourd, 2000; Lesourd et al., 2001, 2003a, b) Antifer harbour and showed spatial and temporary showed that the mud deposits tend to extend into fluctuations in the mud content. This coastal fringe the mouth of estuaries and that temporary fluid of fine sediments extended from the dredging spoil mud deposits often appear during high river flows of deposit of Octeville-sur-mer to the Cap d’Antifer, the Seine (Lesueur et al., 1997, 2003a, b; Garnaud where it seems to be bounded towards the North by et al., 2002). These permanent or temporary mud the long breakwater (3.5 km) of the harbour. Fine ARTICLE IN PRESS Y. Meár et al. / Continental Shelf Research 26 (2006) 2335–2351 2339 material is generally found trapped in the coarse- the Cap d’Antifer, this results in water flowing grained substrate in proportions varying from 0.4% parallel to the coast (Fig. 3). This flow meets to 7% (Dethleff et al., 1996; Spineanu, 1998; the estuarine water at the mouth of the Seine, and Ve´le´grakis et al., 1999). the currents are then deflected towards the open sea. Le Hir et al. (1986) claimed that the residual 2.3. Hydro-sedimentary processes circulation was the result of stratification related to freshwater supplied by the Seine river, combined 2.3.1. Water circulation with wind-induced circulation (Fig. 4). Thus under- The Bay of the Seine, as is the English Channel, is lining the complexity of the hydrodynamics in the a typical example of the macrotidal (even megatidal) eastern Bay of the Seine, these authors showed: (i) a setting, with spring tidal ranges up to 7.5 m. differential flow between surface and bottom Tidal currents: The coastal fringe of the Pays de waters, (ii) the importance of the wind and (iii) the Caux is sheltered from the general ebb flow coming existence of an anticyclonic eddy in front of the from the eastern English Channel. To the south of Pays de Caux.

Le Havre

Tide current in the oriental part of bay of Seine (From Larsonneur 1971)

DOS CURRE Trouville CALVA

Dives Flood tide Ouistreham (A) PM -4H

Le Havre ANTIFER CURRENT Le Havre T EN UR R VERHAULE C

Trouville Trouville

Dives Flood tide Dives Ebb tide Ouistreham (B) Ouistreham PM -1H (C) PM +4H

Fig. 3. Main tidal currents (Calvados current, Verhaule current and ANTIFER CURRENT) affecting the eastern part of the Bay of the Seine (modified from Larsonneur, 1971). (A) Flood tide: the flood tide starts off the Calvados coast, 5 h before high tide, with the Calvados current skirting along the coast to the Seine estuary. In the meantime, along the coast of the Pays de Caux, a NNE–SSW orientated current (‘‘ANTIFER CURRENT’’) is parallel to the coast. They deviate towards the E to penetrate the estuary. (B) Flood tide (1 h before high tide level): at the end of the flood tide, the Calvados current cannot go through the estuary anymore; it is then directed towards the North, moving along the coast of the Pays de Caux to the N (‘‘Verhaule Current’’). (C) Ebb tide (4 h after high tide level): during the ebb-tide, water masses moves toward the W and the Seine estuary empties. ARTICLE IN PRESS 2340 Y. Meár et al. / Continental Shelf Research 26 (2006) 2335–2351

0°30′0′′W 0°0′0′′E

Tidal currentsHaline circulation Wind currents Top Bottom

30 m 49°30′0′′N 49°30′0′′N

20 m 15 m

10 m

km Datum : WGS84 0204010 Projection : UTM (30)

0°30′0′′W 0°0′0′′E

Fig. 4. A synthesis of the main forcings on water circulation including winds, tides and haline circulations (modiﬁed from Le Hir et al., 1986). The size of the arrows is not proportional to the current velocities.

Waves: The most frequent waves are from NW 2.3.2. Sediment dynamics and NE, with amplitudes of about 1.5 m and with As is the entire English Channel, the Bay of the periods from 4 to 5 s. The storm swells are from W Seine is a tidal-dominated sedimentary environ- to SW, with an annual maximum wave height ment. Spring tidal currents are strong enough to (Hmax) of approximately 5 m (Lesourd, 2000). The move the surface sediments with grain size less most important waves for the redistribution of than 2 mm. suspended material are from NW (Larsonneur and However, the Seine, the largest river in the Hommeril, 1967). The effect of storms on the English Channel, flows into this tidal dominated turbidity of the bay can be more important than embayment. According to the model of Bowman the effects of the supply by the Seine river (Avoine and Iverson (1978), spring tides are the most et al., 1986; Le Hir et al., 2001) as was confirmed by important periods for the export of the estuarine Aminot et al. (1997) in shallow areas of the eastern particulate material. The so-called coastal river Bay of the Seine. (Dupont et al., 1986) therefore corresponds to the ARTICLE IN PRESS Y. Meár et al. / Continental Shelf Research 26 (2006) 2335–2351 2341 part of the Seine estuarine water, expelled in the the grab’s speed and decelerate before impact. The shape of surface plumes towards the North (Dupont grab samples that had only small quantities of et al., 1991; Brunet et al., 1996; Brylinski et al., 1996). sediment or had lost sediment in transit were Alternating tidal currents, parallel to the coast, rejected. Care was taken to avoid water sloshing govern the coastal waters and the sediment is across the water–sediment interface as the samples exported out of the Bay of the Seine towards the were lifted to the deck. In case of homogeneous North Sea through the Pas-de-Calais (Brylinski et al., sediment, the total sample was used. If the sample 1991). Dupont et al. (1991) highlighted the temporal showed a layering, the sediment of the grab was sub- persistence of this frontal structure that moves along sampled according to the structure. In the latter case, the coast of the Pays de Caux to the NE. Also, a only the superficial homogeneous deposit was used. transfer of fine sediments expelled by the Seine River The positioning of each sampling point was is carried towards the south-eastern part of the Bay carried out with a SYLEDIS SR3 radiopositioning of the Seine (Avoine, 1984; Lafite, 1990). system that has an accuracy of less than 10 m. Sand: Generally, in the Bay of the Seine, waves Positioning modifications accounted for the dis- are the most important factor for the bed-load tances between antenna and winch. transport of sand in water depths lower than ca. 7 m (Larsonneur, 1971). Bed-load transport is domi- 3.2. Laboratory analyses nantly directed from offshore to onshore, toward the Seine estuary, both on the Calvados coast and To reduce as much as possible the addition of along the coast of the Pays de Caux (Avoine, 1986). analytical errors or distortion, a strict procedure This transport was confirmed both by underwater was defined and followed in laboratory analyses. acoustic imagery (Auffret and D’Ozouville, 1986) The samples were filtered under water, on a 50 mm and by radioactive tracers (Avoine et al., 1986). mesh in order to obtain the percentage of fine Fine fraction: Due to a lack of measurements at fraction (o50 mm). To prevent the distortion of the the water–sediment interface, little is known about results, the various processes of the analysis the processes of transport, deposition and resuspen- (temperature of drying, quartering, suspension and sion of fine-grained sediments in the eastern Bay of duration of agitation) were strictly similar for all the Seine. It is, however, assumed that the move- samples. The volume of water evaporated during ment of suspended sediment is controlled by the drying phase was measured to account the combined action of tidal and wave-induced currents distortion of the results related to dissolved salts. and that optimum condition for transport probably occurs during stormy periods (Avoine, 1986). The 3.3. Data treatment maximum water depth for storm action is still unknown, but varies throughout the bay, between Until the late 1980s, geostatistics were primarily roughly 7 and 20 m. used to describe the spatial organisation of environmental parameters (semivariogram) and to predict 3. Methods the value (interpolation) of these parameters in non- sampled points (kriging or simulation). The com- 3.1. Field work plexity and the diversity of these tools can be, as Aubry (2000) stated, the origin of many problems in Two hundred samples of surface sediments were their use, weakening the credibility of the study. To taken (Fig. 1) in July 1991 during neap tides preserve all the power of the methods of carto- (Fluxmanche II cruise). During the previous months graphic estimation and description of environmen- (April–June), the flow of the Seine River was tal parameter characteristics, it appears necessary to characteristic of a period of low water level establish and to follow a strict procedure for the (200–300 m3 s1). implementation of geostatistics tools (Ouyang et al., The samples were collected using a Shipek grab, 2002). Such a procedure was applied to the study of which is a widely used sampler (Sly, 1981). This the distribution of the muddy areas in the eastern grab produces a sufficient amount of sediment for Bay of the Seine. In this study, the fine fraction of analyses in the laboratory. To avoid flushing the the sediment represents the regionalised variable. fine fraction by a shockwave as the grab reached The regionalised phenomenon gathers all the the bottom, a great deal of care was paid to control environmental parameters influencing the spatial ARTICLE IN PRESS 2342 Y. Meár et al. / Continental Shelf Research 26 (2006) 2335–2351 distribution of the fine fraction of sediment. The the data is made by the study of the anisotropy procedure for this study includes several stages as (Cheng et al., 2000; Wang et al., 2002; Caeiro et al., follows. 2003). Anisotropy means that the variance of the fine-grained fraction varies not only with distance 3.3.1. Gaussian transformation but also with direction. To conduct an anisotropy No assumption can be made about the geostatis- study, there must be enough information present. It tical interpolator method to be used to create the requires a significant number of data points, i.e. map. Gaussian methods assume the frequency higher than 100 (Webster and Oliver, 1992a, b; distribution data to be close to a theoretical Gascuel-Odoux and Boivin, 1994) or even 150 Gaussian distribution (Webster and Oliver, 1990; (Webster and Oliver, 1993). The anisotropy, re- Ouyang et al., 2002; Schnabel et al., 2004). In the vealed by one-way semivariograms and which was case of the fine fraction of sediment, many reasons calculated from a too small number of samples, can (non-adapted sampling scale to spatial sedimentary be regarded as none available (Goovaerts, 1997a). process scale, sampling of non-representative samples, laboratory analytical errors, etc.) can be the 3.3.4. Variographic study cause of the non-normality of the data set. So, it is The semivariogram (Matheron, 1965) is a basic first necessary to do an appropriate transformation tool for the estimation and mapping of regionalised to obtain a normally distributed data set. variables. It reveals the random and structured aspects of spatial dispersion. The experimental 3.3.2. Trend analysis semivariogram is defined as While acquiring environmental data, analytical 1 skews can be introduced. The temporally variable g^ðhÞ¼ Ef½Zðx þ hÞZðxÞ2g characteristic of some environmental parameters 2 (e.g. physicochemical, alternation of deposit and and is normally estimated by the method of sediment erosion phases, etc.) can also introduce moments (Journel and Huijbregts, 1978): skew into measurements (Gringarten and Deutsch, 1 XNh 2001). It is particularly true when environmental g^ðhÞ¼ ½Zðx þ hÞZðxÞ2, 2NðhÞ mechanisms operate on smaller time scales than the l¼1 duration of sampling. When skews have an orga- nised structure (spatial and/or temporal), they can where N(h) is equal to the number of pairs of values lead to a trend in the measurements, which makes in which the separation distance is equal to h, Z(x) the regionalised variable non-stationary; it is there- and Z(x+h) are the values of the experimental data fore appropriate to remove the trend. In the case of separated by the distance h. Practical details of this the fine fraction of sediment, there can be many calculation may be found in Isaaks and Srivastava reasons for the origin of a trend. Among these, some (1989). are natural (sampling time longer than natural The variographic study has been used to attribute environmental sedimentological variations, i.e. de- to each characteristic (distance, direction, cyclicity, position or erosion), man-made (inadequate techni- size, etc.) of the experimental semivariogram, some cal knowledge) or artefacts tied up with field and information relating to environmental structures laboratory procedures (grab or box corer; sieving or (Gringarten and Deutsch, 2001). It represents the laser diffractometer methods). modeler’s quantitative understanding of the spatial variability of the property of interest given the data. 3.3.3. Anisotropic analysis Variographic study has been conducted in two The spatial distribution of the fine fraction in a steps, i.e., the establishment of the experimental sediment is governed by complex mechanisms such semivariogram and the choice of a theoretical model as settling of suspended particles, accumulation by of adjustment. benthic organisms, erosion by hydrodynamic processes, and human influences (i.e. sediment dump- 3.3.5. Choices of the method of interpolation ing, dredging, fisheries, etc). Complexity and Stochastic simulation does not take into account interactions of these mechanisms can induce spatial any predetermined statistical law. It is preferred to variability of the sediment fine fraction in several kriging when the spatial variations of the field of ways. The description of this spatial difference in measurement must be conserved (Desbarats, 1996; ARTICLE IN PRESS Y. Meár et al. / Continental Shelf Research 26 (2006) 2335–2351 2343

Goovaerts, 1997b, 1999; Vanderborght et al., 1997; grids, are operations easily realised under the Arc/ Bellehumeur et al., 2000). It is a geostatistical tool Info Grid module. The Arcplot module was used to that does not try to minimise the error variance create maps. (such as the simple or ordinary kriging), but focuses on the reproduction of statistics such as histograms 4. Results and semivariograms, with respect to the values of the original data (Asselman, 1999). This procedure In order to study the statistical distribution of fine involves the two following steps: (i) generation of a fraction in the sediments, classes of 5% were used number of n simulations covering the study area, and represented in a frequency histogram (Fig. 5A). (ii) averaging of the n simulations to produce the The study of this histogram reveals several char- final map. acteristics of the distribution of the fine fraction in Each simulation produces a map, the number n is the Bay of the Seine. defined according several aspects: to be able to fit The fine-grained content in the superficial sedi- the results within a Gaussian distribution, to take ments varies from less than 1–55% (Fig. 5A). The into account the maximum of the total variance and maximum values are similar to those obtained by minimise the computational time. Avoine (1981), Crevel (1984, 1986), Garnaud (2003) Normality analysis and statistical tests were and Garnaud et al. (2003). For these authors, the carried out using the scientific computational soft- areas of maximum deposition are in very shallow ware R (http://www.r-project.org). In particular, R areas (o10 m): (i) off the coast of Calvados was used for the computation of classical statistics (Avoine, 1981; Garnaud, 2003), (ii) in the Seine (mean, variance, etc.), the study of the fine fraction mouth (Avoine, 1981; Lafite, 1990) and (iii) in a percentage data set for normality with the Shapiro littoral fringe extended from Octeville, where the test and Kolmogorov–Smirnov test. The various dredging spoils of Le Havre harbour occur, from geostatistical treatments were carried out using the NW of the Cap de la He` ve to the Antifer software GSTAT (Pebesma, 2000). It has in harbour (Crevel, 1984, 1986). particular been used for the semivariogram study However, all these well-known muddy areas are and the definition of the theoretical model used for not considered on the present study area, where the interpolation. The results, saved as raster data, samples containing the higher amounts of fine- were treated and analysed using Arc/Infos. This grained fractions are in areas further from the coast, last application permits quick and efficient manage- in water depth from 10 to 40 m. ment and display of all the different grids obtained The histogram of fine-grained sediment (Fig. 5A) after the interpolation process. In particular, shows a great heterogeneity and that only a small summations, computation of the mean on a set of number of samples have high mud content. Indeed,

0.20 0.7

0.6 0.15 0.5

0.4 0.10 Density Density 0.3

0.2 0.05 0.1

0.00 0.0 0 1020304050 -3 -2 -1 0 1 2 3 (A) Fine fraction percent (B) Fine fraction percent

Fig. 5. (A) Histogram of the fine-grained content of the sediment samples, showing the non-normality of the distribution. (B) Histogram of logarithmic transformation of the fine fraction content in the sediments (ZFF: transformed variable, FF for fine fraction or fine-grained content), showing a normal distribution. ARTICLE IN PRESS 2344 Y. Meár et al. / Continental Shelf Research 26 (2006) 2335–2351 only 46 samples among 200 show mud content some other transformation (Saito and Goovaerts, higher than the mean value of the studied zone 2001). (6%) and only 24 samples have a content of mud The number of analysed samples is found higher than 10%. sufficient (N ¼ 200) to allow a study of isotropy This heterogeneity also indicates the lack of on the study area which was conducted through the various percentage classes (20–22%; 24–30%; establishment of a semivariogram map. This omni- 34–42% and 44–54%). The juxtaposition of directional technique gives more precise results for these classes underlines the existence of intensely the direction of a possible anisotropy than those silted sedimentary areas, marked by a very sharp obtained with experimental semivariograms. The boundary. semivariogram map built from the transformed data The first observations based on the analysis of the ZFF, of approximately 2700 m offset, is shown in frequency of the fine fraction are confirmed by Fig. 7. It underlines anisotropy at the scale of the statistical parameters. The asymmetry (Skw ¼ 3.64) eastern Bay of the Seine, oriented in an ENE–WSW of the frequency distribution of the fine fraction direction (Fig. 7), which corresponds to the data and the result obtained (W ¼ 0:578) from the strongest gradients of silting. These will therefore Shapiro–Wilkcoxon test (95%) imply that data be separated in a perpendicular direction to the total transformation is needed to approach a normal anisotropic orientation, i.e. NNW–SSW. In com- distribution. Because of the abnormal distribution parison with previous works (Larsonneur and and positively skewed histogram of the fine-grained Hommeril, 1967; Avoine, 1981; Crevel, 1984, fraction, data were undergone logarithmic transfor- 1986), these results reveal the existence of a new mation to satisfy the assumption of normality. The orientation of the silted sand and gravel areas. In reciprocity of the transformation (i.e. reverse agreement with previous results, the experimental transformation) was then checked. semivariogram is established following a direction A test of normality (W ¼ 0:952), conducted on of research orientated WNW–ENE. This semivar- the transformed data, indicates a clear tendency to iogram (Fig. 6) shows a strong nugget effect (0.648), normality. The shape of the histogram representing which characterises sharp variations in the amount the frequency distribution of the standardised data of fine fraction over a short distance. These (Fig. 5B) confirms this tendency. The transformed variations (Gringarten and Deutsch, 2001) may variable, noted ZFF (fine fraction or fine-grained have either an artificial or natural origin, because content) is used for the geostatistic analysis. The they could stem from analytical errors, or under- semivariogram of the transformed data (Fig. 6) does lying morphological structure. In this study, no not show a continuous increase over the study area. explanation could be given for this nugget effect There is therefore no discernable trend in the data, (errors of analysis or the use of different analytical which permits the use of geostatistic tools without

5 Directional variogram with fitted model 0.5 4

1.2 3 0.0

1.0 Latitude 2

0.8 -0.5 1 semivariance

0 0.6 -0.5 0.0 0.5 Longitude 0.1 0.2 0.3 Distance in decimal degree Fig. 7. Variogram map for ZFF (transformed variable), showing a clear anisotropy with the maximum of continuity observed in Fig. 6. Directional semivariogram with a ﬁtted parametric model the direction of azimuth 551. g(h) (semivariance value) is

(solid line) of ZFF. The model type is circular with semivariance expressed in a 6 levels grey-scale. Latitude and longitude are g(h) ¼ 0.584514 Nug(0)+0.608174 Cir (0.2000192). expressed in decimal degrees. ARTICLE IN PRESS Y. Meár et al. / Continental Shelf Research 26 (2006) 2335–2351 2345 methods). The nugget effect of the semivariogram, fit for the present data set, because it is not within a however, highlights the sharp transition between the stationary domain (mean and variance are assumed muddy units and their sand/gravel substratum. It to remain constant across the scene) and no also underlines the existence of morphological complementary parameters can be added to explain phenomena with dimensions lower than the average a possible trend. sampling distance (4000 m). Co-kriging would be another geostatistical mod- The semivariogram (Fig. 6) also shows a cyclicity elling method. Co-kriging, which is kriging plus a (i.e. hole effect) characterised by the presence of covariate, allows consideration of other environ- undulations after the distance where the curve mental variables such as water depth. In the study reaches a sill; this attenuates gradually (i.e. dam- area, bathymetry is far from linear and could have pened cyclicity). According to Pyrcz and Deutsch been considered as a key factor for the reconstruc- (2003), such a cyclicity shows the existence of tion of the sea floor texture. However, no relation- groups of small muddy sedimentary structures ship between the water depth and percent of fine which are variable in size; its progressive attenua- fraction can be shown (Fig. 8). So, in this case, co- tion generally demonstrates: (i) an important kriging using water depth as a covariate cannot be variability in the distance between these regroupings considered as a valuable geostatistical model tool. and/or (ii) the fact that different sedimentary units Conditional simulation, a stochastic method, is are superposed. particularly adapted to this study (Pebesma and The choice of the interpolation method is Wesseling, 1998; Vann et al., 2002). By reproducing governed by the information emphasised within the total variance of the data, both quantitatively the data set. In the case of the Bay of the Seine, one (through the histogram) and spatially (through the of the most important results, found through the semivariogram), conditional simulation is a tool semivariogram analysis, is the presence of small particularly adapted to the description of variabil- muddy structures separated by variable distances. ities. Vann et al. (2002) underlined the poor quality As a consequence, the interpolation operator must of the results obtained through one single simula- be allowed to visualise these sedimentary units by tion compared to those obtained with the kriging; it their principal characteristics (i.e. fine-grained con- is necessary to proceed to a series of simulations, all tent, dimension, geographic localisation). different, to take into account the spreading of the Classical geostatistics interpolators, such as for group of variographic dots (Asselman, 1999). example ordinary kriging, simple kriging and In order to proceed with the majority of the data universal kriging (Matheron, 1963, 1965), are not variance values, 80 simulations of ZFF, were

-10

-20

-30 Water depth (meters) -40

-50 0 102030405060 Sediment fine fraction (%)

Fig. 8. Relationship between the percentage of fine-grained fraction (o50 mm) in the sediments and water depths. There appears to be no correlation between these two parameters. ARTICLE IN PRESS 2346 Y. Meár et al. / Continental Shelf Research 26 (2006) 2335–2351 calculated. This therefore represents 80 different has a very strong heterogeneity that is related to the distributions, which are, however, equiprobable fine sediments deposited on a rocky substrate. Its regarding the spatial distribution of the percentage origin can be linked to the ebb-tidal delta of the of fine fraction. The final document (Fig. 9), built Orne river: the fine estuarine sediments expelled to from the average of 80 simulations, can thus be the sea travel as suspended matter to settle in this regarded as robust. area under favourable conditions (i.e. easterly winds and ebb currents). Due to the chaotic surface of the 5. Discussion rocky bottom, the waves cannot erode completely these fine deposits. Two types of muddy lithofacies are distinguished The spatial distribution of the fine fraction in the in the study area: (i) heterolithic oxidised sediments, superficial sediments shows also the existence of a grey–beige in colour, with a high water content, wide central deposit that had not been precisely called ‘‘O’’ facies; (ii) a compact, reduced organic mapped before this study, in spite of the various mud, called ‘‘R’’ facies. investigations carried out in the area (Larsonneur In the open sea, sandy gravels are widely and Hommeril, 1967, Avoine, 1981; Crevel, 1986). predominant (‘‘O’’ facies) with a fine-grained An E–W muddy zone, approximately 150 km2 in content on average of 4%, but with extreme values area, extends off the Antifer harbour (where it ranging from 2% to 25%. A depocentre (‘‘O’’ connects with the coastal silted fringe), from facies) located off Courseulles-sur-mer between 5 approximately 20 m to more than 40 m water and 10 m water depths, is very marked. As described depths. The western termination of this muddy by Avoine (1981), its contours had never been zone is gradually directed towards the NW, giving specified during previous surveys. This depocentre to this central deposit a concave towards the N

Fig. 9. Spatial repartition of the fine-grained fraction (o50 mm) in the eastern part of the Bay of the Seine. This figure should be compared with the clay extension on the Fig. 2. ARTICLE IN PRESS Y. Meár et al. / Continental Shelf Research 26 (2006) 2335–2351 2347 configuration. The shape of this unit is related to the populations of O. fragilis used to live but have now presence of a field of sand waves approximately 5 m abandoned. This desertion can have several origins, high, named the ‘‘Ridins d’Antifer’’ and this W–E among which are: a too high content of fine orientated field limits the extension of the muddy sediments in the bottom, or local changes in zone towards the N. The location of this muddy hydrodynamics. A desertion of the area related to unit is closely linked to an eddy, which was a change in the physicochemical properties of described by Le Hir et al. (1986) and Cugier and the sediments under the action of the faeces of Le Hir (2002). This eddy traps a part of the O. fragilis should be significant, leading to an suspended matter expelled from the Seine estuary increase in sulphate reduction. towards the north, and spreads it along the coast of An additional complicating parameter is the the Pays de Caux. In this same zone, Cugier and Le heterolithic gravel bottom that contributes to the Hir (2002) stressed the existence of coastal down- variable trapping of the fine sediments in the inter- welling, created by south-westerly winds. These two granular voids of the surficial sediments. mechanisms may contribute to offshore transport of Without ignoring the morphological aspect over fine-grained sediment. Furthermore, the wide shal- all scales, i.e. from the sedimentary body to the low depression in this area, which reflects sub- infra-metric ripple scales, the fluctuations in the surface geology (i.e. soft unconsolidated Cretaceous rates of fine fraction deposition appear strongly sands), may favour settling of suspended particulate linked to the presence or the absence of O. fragilis matter. which can reach a density higher than 7000 ind. m2 The geostatistic approach revealed the extreme in this area of the Bay of the Seine (Davoult and spatial heterogeneity of the area, where the mor- Migne´, 2001). This species is known to be primarily phology of the sea bottom plays a role on two a suspension feeder (Vevers, 1952; Cabioch, 1968; different scales. At a general scale, it influences the Warner, 1971). While raising its arms to collect shape of the sedimentary unit; at a local scale, it is food, it is responsible for a reduction of the bottom responsible, to some extent, for its installation and current and thus favours the settlement of the finest its permanence. The detailed surface sediment particles, accentuated by the expulsion of mucus- analysis revealed that the most strongly silted zones rich faeces. This study shows that the relationship make up small mud patches (o4000 m wide), as between brittle-stars and fine-grained content of the previously stressed by the nugget effect of the sediment is complex (Fig. 10). According to the semivariogram (Fig. 6). These mud patches are result of a non-parametric Shapiro–Wilcoxon test located in various morpho-sedimentary settings. (P value: 0.0005 with a ¼ 0:05), two populations of Sediments sampled within paleovalleys or between samples can be distinguished. The first population hydraulic dunes, are oxidised muddy gravely sands consists of samples for which the number of brittle- (‘‘O’’ facies); whose fine-grained content remains stars is lower than 500 ind. m2 (Table 1). All of moderate (15–25%). Other mud patches with analogous sizes do not seem to be influenced by morphology, because they 8 are located on flat areas. The mud content within these zones is very variable (10–450%); this type of 7 mud patches corresponds to the highest (450%) 6 content of fine fraction. Some of them correspond to the ‘‘O’’ facies, but others to the ‘‘R’’ facies. The 5 sediments in areas of ‘‘O’’ facies are typically 4 2 brittle-stars (ind.m-2) floored by a high concentration (45000 ind. m ) Log of the number 3 of the brittle-star Ophiothrix fragilis (Abildgaard). This population of echinoderms, whose concentra- 5101520 tions had been reported by Davoult and Migne´ Sediment fine fraction (%) (2001), is never associated with ‘‘R’’ facies (i.e. black mud). However, detailed observations of sediments Fig. 10. Relationship between the fine-grained content and the number of brittle-stars Ophiothrix fragilis in ind. m2 in the from the latter show that they contain a very large superficial sediments. Triangles refer to samples taken off the amount of needles of O. fragilis. The black-mud Antifer harbour in the inset of Fig. 9, stars refer to samples taken areas can be thus interpreted as loci where dense elsewhere in the offshore part of the study area. ARTICLE IN PRESS 2348 Y. Meár et al. / Continental Shelf Research 26 (2006) 2335–2351

Table 1 these mud patches have also a qualitative varia- Percentage of fine fraction in the superficial sediments of the bility. Their spatial and qualitative complexities study area and the number of O. fragilis in the same samples correspond to interactions between various forcing Sample Fine fraction Depth (m) Number of parameters that control sedimentation of fine percent ophiures particles; they include: (i) hydrodynamics, (ii) (ind. m2) morphology and nature of sediments and (iii) benthic fauna. 91FM111 2.7 26.0 10 91FM124 5.0 29.0 25 In the Bay of the Seine and more generally in the 91FM68 5.4 23.0 42 English Channel, in the exposed areas, the fine 91FM69 5.1 22.5 48 sediments were until now regarded as of low 91FM117 5.7 41.5 74 frequency. However, on the contrary, even with a 91FM118 6.5 40.5 81 strong hydrodynamical energy relating to the 91FM122 6.2 26.0 89 91FM127 6.9 39.0 102 combined action of the tides and the storm swells, 91FM70 7.3 25.0 250 this work underlines the importance of fine-grained 91FM121 10.9 28.0 310 deposits associated to gravel and sands. 91FM149 7.4 31.0 1980 The use of geostatistical tools within a strict and 91FM142 8.6 32.5 2800 clearly identified procedure makes it possible to 91FM28 8.9 33.0 3200 91FM146 14.4 27.5 3500 propose a sound cartography of the fine fraction in 91FM26bis 14.2 31.0 3600 the superficial sediments of coastal seas. Only the 91FM145 16.3 30.0 4000 strength of geostatistics to produce variance maps 91FM23 8.4 30.0 4900 makes it possible to make temporal studies and 91FM144.2 23.8 31.5 5100 therefore to propose an assessment of deposition/ These samples were taken in July 1991 with a Shipek grab. To erosion rates for the key area of the Bay of the Seine evaluate the density of the O. fragilis population, only one bucket and the distal part of the Seine estuary system. These (20 20 cm) was taken for each sample. results will thus enable a reconsideration of the fine sediment budget within the central English Channel. these samples are located in the deeper and westward part of the study area. Here, the fine Acknowledgements fraction in the sediment is in the range of 4–12%. The second population corresponds to samples for This work was carried out under the framework which the number of brittle-starts is between 500 of the research programmes PNEC (French Na- and 1000 ind. m2, the fine fraction ranging from tional Coastal Environment Programme) supported 6% to 24%. The latter sediments were sampled off by CNRS-INSU and IFREMER. The crew of the Antifer harbour. For both populations, a correla- oceanographic research vessel Suroıˆt (IFREMER/ tion seems to be present (Fig. 10), but the number of GENAVIR) is acknowledged for its support during samples is too low to give a confident result. the field investigations. The authors would like to thank the two anonymous reviewers for all the 6. Conclusions helpful comments and contributions, and Pr. G. Evans (Southampton Oceanography Centre) The analysis of the geographical distribution of who helped to improve the final manuscript. the fine-grained content in superficial sediments of the eastern Bay of the Seine confirms and refines previous results on some muddy areas: the open sea References area, the Courseulles-sur-mer area and the coastal Aminot, A., Guillaud, J.F., Kerouel, R., 1997. La Baie de Seine: fringe along the Pays de Caux. It also reveals, for Hydrologie, nutriments et chlorophylle (1978–1994). In: the first time, a central muddy deposit of great Repe` re Oceán. Ifremer, Brest, France, p. 148. extent, offshore the Cap d’Antifer. Asselman, N.E.M., 1999. Grain-size trends used to assess the The geostatistic analysis enables the understand- effective discharge for floodplain sedimentation, River Waal, ing of the extreme complexity of this central deposit the Nederlands. Journal of Sedimentary Research 69 (1), 51–61. made up of a patchwork of small mud patches. The Atkinson, P.M., Lewis, P., 2000. Geostatistical classification for sedimentological analysis, combined with the mor- remote sensing: an introduction. Computers and Geosciences phological study of the sea bottom, showed that 26 (4), 361–371. ARTICLE IN PRESS Y. Meár et al. / Continental Shelf Research 26 (2006) 2335–2351 2349

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