Clay minerals and geochemistry record from northwest Mediterranean coastal lagoon sequence: Implications for paleostorm reconstruction

Sedimentary Geology 228 (2010) 205–217

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Sedimentary Geology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s e d g e o

Clay minerals and geochemistry record from northwest Mediterranean coastal lagoon sequence: Implications for paleostorm reconstruction Pierre Sabatier a,b,⁎, Laurent Dezileau b, Louis Briqueu b, Christophe Colin a, Giuseppe Siani a a b

Université Paris-Sud, Laboratoire des Interactions et de la Dynamique des Environnements de Surface, CNRS/INSU UMR 8148 Orsay, France Université Montpellier 2, Geosciences Montpellier, CNRS/INSU, UMR 5243, Montpellier, France

a r t i c l e

i n f o

Article history: Received 10 July 2009 Received in revised form 21 April 2010 Accepted 27 April 2010 Available online 6 May 2010 Editor: G.J. Weltje Keywords: Paleostorm Mediterranean lagoon Clay mineralogy Geochemistry Sediment Late Quaternary

a b s t r a c t The paleostorm history of coastal lagoon environments (Pierre Blanche Lagoon, central part of the Gulf of Lions) has been established by integrating clay mineralogy and geochemical analysis. Clay mineralogy combined with bulk major and trace elements concentrations allow for the definition of four different end members: the Mosson drainage basin, the sandy barrier, the biogenic and the anthropic components. The two main sedimentary sources of the lagoonal system are the Mosson drainage basin which has a high concentration of smectite and Al2O3, and the sandy barrier characterized by high contents of illite, chlorite, SiO2 and Zr. Smectite/(illite + chlorite), SiO2/Al2O3 and Zr/Al2O3 ratios can be used to reconstruct the past storm history of the Pierre Blanche Lagoon. Our results indicate that the sensitivities of the clay mineralogy and geochemistry proxies are different. These sensitivities could be related: 1) first, to the comparison between different grain size for geochemistry and for clay mineralogy; 2) second, to the complexity of the mix (four end-members for the geochemistry and two end-members for the clay minerals); and 3) third, by the storm's ability to transport sediment of different sizes. © 2010 Elsevier B.V. All rights reserved.

1. Introduction The reconstruction of the recurrence and intensity of past storms in coastal areas is an important topic of study due to the recent concentration of resources and the population in this area (Pielke and Landsea 1999; Turner et al., 2006; Lionello et al., 2006). The effects of climate change on extreme events are difficult to assess due to many forms of nonlinearity and long term memory. The lack of instrumental long-term series prevents us from clearly demonstrating whether hurricane activity may increase in relation to climate change (Webster et al., 2005; Emanuel 2006; Landsea et al., 2006). Regional climate simulations have been used to investigate the variations of precipitation and the cyclonic activity in the Mediterranean region. Lionello and Giorgi (2007) showed that the reduction of cyclone activity observed in future scenarios could be responsible for the negative change in precipitation along the southern and eastern Mediterranean coast, while a positive change occurs in northern areas in relation to increased strength of midlatitude storm tracks. It is therefore important to study past storm activities to better understand the possible regional and local long-term trends of these events, as they relate to past climate conditions. Geological data offer opportunities to reconstruct a long-term record of intense events well beyond the observational record to ⁎ Corresponding author. Laboratoire UMR 8148 CNRS-Université de Paris-Sud Bât. 504, 91405 ORSAY Cedex, France. E-mail address: [email protected] (P. Sabatier). 0037-0738/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.sedgeo.2010.04.012

identify how cyclone activity has responded to past shifts in climate (Nott, 2004; Frappier et al., 2007). The Gulf of Lions shore line is characterized by many coastal wetlands that are a result of the interaction between a process of shore line regularization by migrations of sandy barriers due to the sediment transfer through littoral hydrodynamics and a filling of these areas by fluvial and marine inputs (Certain et al., 2004; Raynal et al., 2009). This study focused on the Palavasian lagoonal system, characterized by fairly high sedimentation rates, more particularly on the Pierre Blanche Lagoon where land-falling storms have previously been identified (Dezileau et al., 2005; Sabatier et al., 2008). Grain-size distributions were used for palaeoclimatic reconstruction from deep-sea sediments to provide valuable information on provenance and dispersal of sediments with end-member identification (Prins et al., 2002; Weltje and Prins, 2003). Usually, reconstruction of paleostorm events in coastal environments is also made by identifying the recurrence of overwash coarse-grained deposits and associated fauna contents (Liu and Fearn 1993, 2000; Collins et al., 1999; Donnelly et al., 2001a,b; Nott 2004; Donnelly, 2005; Donnelly and Woodruff, 2007; Scileppi and Donnelly, 2007; Sabatier et al., 2008). However, in such types of shallow water environments, endmember identification via grain size distribution was difficult in relation to the ability of each overwash event, each associated with a different intensity, to transport coarse-grained sediment at a fixed distance in the lagoon (Woodruff et al., 2008). A recent study of the Shelby Coastal Lake used other proxies such as stable isotope

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compositions of organic matter (δ15N and δ13C), where sand laminae/ transported shells are indistinguishable or absent, to understand hurricane history on the Holocene time scale (Lambert et al., 2008). In this study, we used clay minerals and geochemistry to characterize the main end-member of the lagoonal system to identify marine deposits related to paleostorms. Both proxies have been traditionally applied in palaeoclimatology to reconstruct palaeoenvironmental changes linked, for example, to Asian monsoon intensity (Liu et al., 2004; Boulay et al., 2005; Colin et al., 2006), the El Niño–Southern Oscillation (Lamy et al., 2001) or to cold Atlantic events (BoutRoumazeilles et al., 2007). The aim of this study was to characterize the sensitivity of clay mineral and bulk geochemistry for providing evidence of past storm events, to define new proxies in sediment cores and to reconstruct the paleostorm land-falling history of this Mediterranean coastal lagoon. 2. Geological setting The Palavasian lagoonal complex is located west of the Rhone delta (about 50 km) in the central part of the Gulf of Lions, which is south of France (Fig. 1). This area consists of several small lagoons with shallow

water depths (b1 m) that are bordered to the south by a narrow sandy barrier and to the north by calcareous Mesozoic hills. This wetland complex is now crossed by the artificial Rhône–Sète navigation channel, which was constructed in the 18th century (NW–SE in lagoonal system, Fig. 1). The sandy barrier is formed by sediment supply from the Rhône River to the east, transported by a westward coastal drift. These alongshore-oriented sand spits prograding toward the southwest therefore originated exclusively from the Rhône River watershed (Raynal et al., 2009). In some places, the sandy barrier is less than 60 m wide and 3 m high above the average sea level; this implies an extreme sensitivity to high energy events, enabling a temporary but strong marine influence during storm events. This is commonly highlighted by traces of over-wash fans and ancient temporary inlets (Dezileau et al., 2005; Sabatier et al., 2008). This coastal area displays a classical microtidal littoral zone with a maximal tide excursion of less than 50 cm. The Mosson River drains an area of 370 km2 and is composed of three main channels: Coulazou, Lassédéron and Mosson (Fig. 1). Most of the sediments transported by rivers and supplied to the studied lagoonal system are carried during flash flood events. The average

Fig. 1. (a.) Map of the western Mediterranean Sea and the central part of the Gulf of Lions (South of France) within the light rectangle the of Palavasian lagoonal system. Concentration of clay mineral (b.) and selected major elements (c.) in suspended sediment collected in the Mosson River during a flood event and in sediment from the sandy barrier. The core PB06 sample was collected in the Pierre Blanche Lagoon (PBL).

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flow of the Mosson River is 1.2 m3/s, but it can reach the values of up to 258 m3/s, as measured during the 3 December 2003 flash flood events (DIREN, 2009). The Mosson drainage basin is mainly composed of Mesozoic (limestone) and Cenozoic (conglomerate, carbonaceous sandstone and clay) sedimentary rock with Quaternary deposits (flood plain and deltaic terrigenous series).

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et al., 2002), and then the PCA calculation was performed, as summarized by Tolosana-Delgado et al. (2005). All statistical calculations were conducted with “R” software using the package “compositions” (van den Boogaart and Tolosana-Delgado, 2008). The multivariate data analysis makes it possible to interpret data in terms of the source influence of samples.

3. Materials and methods 4. Sedimentological and chronological framework 3.1. Sample location A 7.9-m-long piston core (PB06) was collected in the Pierre Blanche Lagoon in March 2006 (Fig. 1) with the UWITEC© gravity coring platform (University of Chambery). In this study, we focused on the first 1.30 m of this core. In addition, eight samples were collected in the muddy channel deposit from the Mosson drainage basin during the dry season (data not shown). Suspended sediments were also sampled, with sediment trap during flash flood events (September 2005) at three different locations (four samples, Fig. 1). Moreover, fine sediments were picked up from different sedimentary rocks present in the Mosson drainage basin. Samples were also collected from the sandy barrier in front of the Pierre Blanche Lagoon located in the southern part of the Palavasian system.

A lithological description of the PB06 core based on grain size, sedimentary structure and fauna content allows identification of different facies interpreted in terms of lagoon depositional environments (Sabatier et al., 2010). In a core transect that included PB06, Sabatier et al. (2008), Dezileau et al. (2009) and Sabatier (2009) recognized three main historical storm events. A comparison of 210Pb, 137 Cs and 14C chronology and historical accounts suggests that the three identified storm events took place in 1742, 1848 and 1893 A.D. (i.e. around 100, 60 and 34 cm depth Fig. 2) and these events allow us to specify PB06 chronology. The ages of these paleostorms, considered as instantaneous events, are in good agreement with the average sedimentation rate of 2.7 mm/yr, as defined by Sabatier et al. (2010),

3.2. Analytical methods Core PB06 was split, photographed, logged in detail (noting all physical and biogenic sedimentary structures and vertical facies successions), and divided into 1-cm-long vertical sections prior to analysis. In this study, the upper 1.3 m of sediment in this core were sampled every 2–3 cm to analyze clay mineral content as well as major and trace elements. Clay minerals were identified by X-ray diffraction (XRD) using a PANalytical diffractometer at the Laboratoire IDES (Université de Paris XI) on oriented mounts of non-calcareous clay-sized (b2 µm) particles. The oriented mounts were obtained following the methods described in detail by Colin et al. (1999). Three XRD trials were performed, each proceeded by air-drying, ethylene-glycol solvation for 24 h, and heating at 490 °C for 2 h. Identification of clay minerals was made mainly according to the position of the (001) series of basal reflections on the three XRD diagrams. Semi-quantitative estimates of peak areas of the basal reflections for the main clay mineral groups of smectite (including mixed-layers) (15–17 Å), illite (10 Å), and kaolinite/chlorite (7 Å) were carried out on the glycolated curve using the MacDiff software (Petschick, 2000). Relative proportions of kaolinite and chlorite were determined based on the ratios of the 3.57/ 3.54 Å peak areas. Major and trace elements analyses were conducted by flow injection ICP-MS using a Sciex Perkin Elmer ELAN 5000a at the Service d'Analyse des Roches et des Minéraux (SARM, Nancy, France) using the method described by Carignan et al. (2001). Samples were powdered (300 mg) and fused in Pt crucibles along with 900 mg of ultra-pure LiBO2 at 980 °C in an automatic tunnel oven on a rail over a period of about 60 min. After cooling to room temperature, the fusion glass was dissolved in a HNO3 (1 mol/l)-H2O2 (∼0.5% v/v)-glycerol (∼10% v/v) mixture to obtain a dilution factor of 333 relative to the amount of samples fused. Each element has a different range of uncertainties (Carignan et al., 2001), with an average between 1–5% for major elements and 5–10% for trace elements. Different strategies of multivariate data analysis are used to interpret a database from sediment samples. Principal component analysis (PCA) is a technique that transforms a large number of variables (concentration of elements) into a smaller number of independent variables to visualize the relation between the first set of variables and objects of interest (sediment samples). The major elements and loss of ignition (LOI) from the PB06 core were subjected to “centered logratio transformation” (clr-transformation, Aitchison

Fig. 2. 1.3 m of the PB06 core with X-ray and clay mineral analyses contents (%) obtained on the carbonated-free, b2-µm-sized fractions. Smectite and illite are dominant (up to 75% of the total clay minerals). Illite and chlorite co-vary and are inversely correlated to smectite contents. Kaolinite contents do not vary significantly with time. Shaded areas mark the main variations. The ranges of the end-member values for the different clay minerals are plotted as a straight line for the Mosson River and a dotted line for the sandy barrier. Grey bands are paleostorm events previously identified and dated at 1742, 1848 and 1893 by Sabatier et al. (2008) and Dezileau et al. (2009).

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for the last 700 yrs B.P. Therefore the 1.3 upper meter of this core represented the last 480 years B.P. 5. Results 5.1. Mineralogy results From the suspended sediment collected during a flood event at the Mosson River (n = 4) in the three main channels (Coulazou, Lassédéron and Mosson), smectite (73%–81%) was found to be the dominant clay mineral with an average content of 77%. Illite (8%–14%) and kaolinite (8%–13%) were less abundant with a similar average content of about 10%. Chlorite was only present as trace compound with content less than 3% (Fig. 1). For the sandy barrier sediment (n = 9), illite (45%–59%) was the dominant clay mineral, with an average content of 52%; chlorite (17%–26%) and smectite (8%–28%) were less abundant with average contents of 20% and 18%, respectively. Kaolinite (6%–16%) is a minor component at an average content of 10% (Fig. 1). The relative abundances of the main clay mineral groups in core PB06 are reported in Fig. 2. In core PB06, values of clay mineral contents are included between the two end-members defined above (i.e., the Mosson drainage basin and the sandy barrier, Fig. 2). This core is predominantly characterized by high contents of smectite (20%– 64%) and illite (19%–50%) and low contents of chlorite (5%–24%) and kaolinite (8%–17%). The average percentages of these contents are 49% for smectite, 28% for illite, 10% for chlorite and 12% for kaolinite (Fig. 2). Clay minerals can be subdivided into three groups along PB06. Changes in illite and chlorite content displayed similar variations with two main increases in concentrations between 46 and 70 cm and between 92 and 110 cm. Generally, smectite content is inversely correlated to illite and chlorite contents. Kaolinite contents do not vary significantly with depth. 5.2. Geochemistry results Bulk sediment from the Mosson drainage basin, sandy barrier and core PB06 consisted mainly of SiO2, CaO, Al2O3 and Fe2O3 with low concentrations of MgO, K2O, Na2O, TiO2, P2O5 and MnO (Table 1). Suspended sediments collected in the Mosson River during a flood event were characterized by a relatively high concentration of Al2O3 and Fe2O3 while materials from the sandy barrier consisted mainly of SiO2 and CaO (N70%, Fig. 1). Fig. 3 displays variations of seven selected major and trace elements through time. Content variations of these elements can be subdivided into three groups in core PB06. SiO2, Na2O and Zr exhibit similar variations with three layers of high concentrations. In general, trends in Fe2O3 and Al2O3 are inversely correlated to those of SiO2, Na2O and Zr. In addition, CaO and Cu concentrations display two different trends that are not related to those of the first two groups. The major elements and loss of ignition (LOI) from the PB06 core were subjected to PCA (clr-transformation, Aitchison et al., 2002). The first principal component (CP1) explains 63% of the variance and shows high positive loadings for Na2O, SiO2, K2O and MnO and high negative loadings for P2O5, CaO, LOI and MgO (Fig. 4a). PC2 explains 17% of the variance and gives high positive loadings for Na2O, P2O5 and CaO and negative loadings for MnO, K2O, Al2O3, Fe2O3, TiO2 and MgO. Trace elements were selected to trace the origin of sediment, with lithogenic signature (Th, Rb, Ni, Zr, Hf, La, Ta, Nb and Ba), biogenic source (Sr, N1000 ppm into aragonite, b200 ppm into calcite) and related to the pollution (Cd, Cu and Pb). REEs exhibit the same variability. Therefore, only two REEs were used (Th and La) to not “swamp” the analysis (Pe-Piper, et al., 2008). PC1 (81% of the variance) showed high positive loadings for Cu, Pb, Cd and Sr and negative loadings for all of the other elements. The PC3 (4% of the

variance) was chosen instead of PC2 (12% of the variance) to better discriminate elements with a negative loading on PC1 (Fig. 4b). Thus PC3, displayed positive loadings for Zr, Hf and Ba and weaker negative loadings for Ta, Th, La, Ni, Rb and Nb. Samples from storm layers (32–36, 54–64 and 93–109 cm) had positive PC1 scores in major elements PCA (Fig. 4a), and they had negative PC1 and positive PC3 scores in trace elements PCA (Fig. 4b). Samples from the upper centimeters of the core PB06 (0–27 cm), which presented a high concentration of Cu (Fig. 3) had positive PC2 and negative PC1 scores in major elements PCA (Fig. 4a) and display a high positive PC1 scores in trace elements PCA (Fig. 4b). Samples from depths of 18–30 and 38 cm also exhibited correlations with CaO and Sr variables (Fig. 4a,b). The other depths (40–50, 66–89 and 111–123 cm) were characterized by negative PC2 scores in major elements PCA (Fig. 4a) and negative PC1 and PC3 scores in trace elements PCA (Fig. 4b). 6. Discussion The interpretation of clay mineral and detrital geochemical records requires knowledge of the potential source areas, as well as the modes and intensities of the transport processes involved (Gingele et al., 2001; Boulay et al., 2003; Liu et al., 2007). An effect of global sea level fluctuations on the sedimentation of the Mediterranean coastal lagoon cannot be invoked on the Late Holocene timescale. In addition, the change from chemical to physical erosion on land cannot occur on a centennial timescale (Thiry, 2000; Egli et al., 2001). Consequently, for modern sediments of the Pierre Blanche Lagoon, mineralogical variations are induced by fluctuations in source sediments. 6.1. Sediment sources To understand the cause of mineralogical variations in the PB06 core, it is necessary to document the origin and source areas of the minerals present in the Pierre Blanche Lagoon. The clay mineral composition significantly varies from the Mosson drainage basin, with high concentration of smectite (73%–81%) to the sandy barrier which is characterized by high illite (45%–59%) and chlorite (17%–26%) contents. Smectite is derived mainly from the erosion of smectite-rich Cenozoic conglomerates (Vitrollien, 55%–70%) and associated muddy channel deposits, outcropping in the upper part of the Mosson drainage basin (BRGM, 1967). In general, illite is related to strong physical erosion and moderate hydrolysis conditions on land (Chamley, 1989). In the Gulf of Lions, the clay mineral distribution has been studied since 1990 (Courp and Monaco, 1990; Giresse et al., 2004; Bout-Roumazeilles et al. 2007). The main sediment source in the central part of the Gulf of Lions, is the Rhône River receives its detrital materials from the Alps where illite and chlorite are mostly found (N75%). In the Mediterranean Sea, eolian dust, supplied from North Africa, could account for 20% of the sediment deposition on the Gulf of Lions margin, as previously reported by Zuo et al. (1997) and Guerzoni et al. (1997). African dust can reach the central part of the Gulf of Lions by SW–NE transport, mostly during the winter when a large atmospheric depression system develops between the Canary Islands and the Iberian Peninsula (Rodríguez et al., 2001; Bout-Roumazeilles et al. 2007). The mineralogy of the dust transported from North Africa is mostly characterized by quartz and feldspars, which are associated with kaolinite and illite (Guerzoni et al., 1999). Another clay mineral typical of the arid and semi-arid climate is palygorskite, commonly used as an indicator of the presence of African dust (Coudé-Gaussen et al., 1987; Molinaroli, 1996; Caquineau et al. 1998; Bout-Roumazeilles et al. 2007). The occurrence of palygorskite was identified in some Mediterranean areas with, for example, an average concentration of 5% in Holocene sediments of the Alboran Sea (BoutRoumazeilles et al. 2007). In low sedimentation rate areas, such as

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Table 1 The Mosson drainage basin (channel deposits, suspended sediment), sandy barrier and PB06 concentrations of major and selected trace elements. Each element has a different range of uncertainties (Carignan et al., 2001), with uncertainties for major and trace elements averaging between 1–5% and 5–10%, respectively. Location

Mosson channel deposit

Mosson suspended sed.

Sandy barrier

PB06

Sample

BV 1 BV 2 BV 3 BV 4 BV 5 BV 6 BV 7 BV 8 BV 9 Mau1 Mau2 Cou Mpail E4 EO HP 0 3 6 9 12 15 18 21 24 27 30 32 34 36 38 40 42 46 50 54 58 62 64 66 68 70 72 74 78 82 85 89 93 97 99 101 103 105 107 109 111 115 119 123

SiO2

Al2O3

Fe2O3

MnO

MgO

CaO

Na2O

K2O

TiO2

P2O5

PF

Total

Ba

Cd

Cu

Hf

La

Nb

Ni

Pb

Rb

Sr

Ta

Th

Zr

%

%

%

%

%

%

%

%

%

%

%

%

ppm

ppm

ppm

ppm

ppm

ppm

ppm

ppm

ppm

ppm

ppm

ppm

ppm

41.6 39.7 32.0 44.0 38.9 41.1 44.0 51.6 39.5 36.7 35.3 36.5 33.8 65.1 40.2 57.8 33.8 33.7 33.9 31.6 29.4 29.8 30.3 30.5 29.4 27.5 32.2 33.6 36.6 34.7 32.8 32.4 32.4 33.3 35.5 37.5 36.0 37.4 38.3 31.8 30.5 31.7 33.2 33.1 33.5 34.4 33.9 36.9 40.4 42.9 44.0 45.4 44.3 44.7 40.8 40.5 38.0 35.4 33.2 28.6

2.4 1.4 2.2 1.6 1.5 2.1 2.4 3.7 2.7 7.2 11.6 10.5 10.5 4.6 3.2 3.3 9.3 9.1 9.8 9.5 9.0 9.1 9.3 9.6 8.9 7.8 9.3 8.4 8.3 8.7 8.9 9.4 10.0 10.4 10.4 10.0 10.1 10.0 9.7 9.5 9.3 10.0 10.7 10.9 10.6 10.0 10.2 10.4 11.1 10.4 9.8 9.7 9.9 9.2 10.4 11.0 11.6 10.9 10.3 9.4

1.2 1.0 1.4 2.0 1.8 1.1 0.9 2.2 1.3 2.8 4.4 4.1 4.0 0.9 1.1 0.5 3.6 3.6 3.9 3.7 3.5 3.5 3.6 3.7 3.5 3.1 3.5 3.2 3.1 3.2 3.4 3.5 3.7 3.8 4.0 3.9 3.9 3.9 3.7 3.6 3.5 3.8 4.0 4.0 3.9 3.9 4.0 4.0 4.2 4.1 3.7 3.7 3.8 3.6 4.1 4.3 4.4 4.2 4.0 3.6

0.0 0.0 0.0 0.1 0.0 0.0 0.0 0.0 0.0 0.1 0.1 0.1 0.1 0.0 0.1 0.0 0.0 0.0 0.1 0.0 0.0 0.0 0.0 0.0 0.0 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1

0.4 0.4 0.7 0.4 0.4 0.7 0.3 0.6 0.5 1.0 1.2 1.1 1.0 0.6 0.7 0.3 1.6 1.6 1.6 1.6 1.7 1.7 1.6 1.7 1.6 1.5 1.6 1.5 1.5 1.6 1.7 1.7 1.8 1.8 1.9 1.8 1.8 1.8 1.8 1.9 1.9 1.9 1.9 1.9 1.8 1.8 1.6 1.7 1.7 1.7 1.6 1.6 1.7 1.5 1.7 1.7 1.7 1.7 1.7 1.7

28.6 32.0 34.9 28.8 31.3 29.9 27.6 21.2 29.0 22.8 20.4 20.0 22.9 15.0 28.8 19.5 20.8 21.8 22.2 22.9 24.9 24.2 23.9 22.6 24.0 27.2 23.6 23.8 23.3 23.6 23.7 22.8 22.2 19.5 20.5 19.8 20.5 19.2 19.2 20.5 21.3 21.0 20.0 19.7 20.1 21.2 20.9 19.8 16.6 17.1 17.1 17.3 17.6 17.9 18.0 18.2 18.1 19.5 22.1 25.8

0.1 bL.D. 0.1 bL.D. 0.1 0.1 0.1 0.2 0.1 0.3 0.2 0.1 0.1 1.1 0.8 0.9 0.4 0.4 0.4 0.3 0.4 0.4 0.3 0.3 0.3 0.3 0.4 0.5 0.7 0.6 0.5 0.5 0.5 0.4 0.5 0.6 0.6 0.6 0.6 0.4 0.4 0.4 0.4 0.4 0.4 0.5 0.4 0.4 0.5 0.7 0.7 0.8 0.8 0.8 0.6 0.6 0.4 0.4 0.4 0.3

0.3 0.2 0.3 0.3 0.3 0.4 0.7 0.9 0.5 1.1 1.3 1.2 1.1 1.3 0.7 1.2 1.4 1.5 1.5 1.4 1.3 1.4 1.4 1.5 1.4 1.2 1.5 1.4 1.4 1.5 1.4 1.5 1.5 1.7 1.7 1.7 1.7 1.8 1.7 1.6 1.5 1.6 1.7 1.8 1.7 1.6 1.6 1.7 1.9 1.9 1.8 1.8 1.9 1.8 1.8 1.9 1.9 1.8 1.6 1.4

0.2 0.1 0.1 0.1 0.1 0.2 0.1 0.2 0.2 0.4 0.6 0.6 0.6 0.2 0.5 0.1 0.5 0.5 0.5 0.5 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.4 0.4 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.4 0.5 0.5 0.5 0.5 0.5 0.4

0.1 0.1 0.1 0.0 0.2 0.1 0.1 0.2 0.1 0.3 0.2 0.2 0.1 0.1 0.1 0.0 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1

25.3 26.0 28.8 23.6 24.9 25.0 23.6 19.0 26.1 27.3 25.4 26.5 26.4 11.4 23.4 16.2 27.0 26.4 26.3 27.3 28.6 28.6 28.1 28.7 29.0 29.7 26.7 25.7 23.9 25.0 25.7 26.1 27.2 27.3 25.8 23.2 23.6 23.4 23.1 28.9 30.9 29.2 27.5 27.4 26.6 25.0 25.5 23.2 21.9 19.3 19.4 18.7 18.5 18.4 20.9 22.0 23.5 25.7 25.2 28.3

100.2 100.9 100.6 100.9 99.5 100.8 99.9 99.9 100.1 99.8 100.6 100.9 100.6 100.2 99.6 99.7 98.7 98.7 100.3 99.0 99.5 99.4 99.2 99.3 98.7 99.0 99.4 98.8 99.3 99.6 98.7 98.5 99.9 98.7 100.8 99.1 98.8 98.7 98.6 98.7 100.1 100.2 100.1 99.9 99.3 99.0 98.8 98.8 99.2 98.7 98.7 99.6 99.0 98.5 98.9 100.9 100.4 100.1 99.1 99.7

57.4 53.9 49.5 70.9 50.9 80.1 105.3 120.4 92.6 160.1 180.2 170.1 158.6 226.3 131.9 194.2 154.5 147.6 157.9 152.3 142.9 143.7 140.5 132.9 129.5 112.0 151.1 160.7 186.0 175.7 167.7 160.7 161.2 175.0 179.2 193.4 195.2 192.4 203.5 170.6 155.8 165.9 167.8 171.5 171.1 168.7 164.8 180.0 211.1 237.0 230.1 223.5 212.1 262.0 226.2 203.7 184.9 188.9 164.8 140.8

bL.D. bL.D. bL.D. bL.D. bL.D. bL.D. bL.D. 0.5 bL.D. 0.5 0.5 0.6 0.5 bL.D. bL.D. bL.D. 0.6 0.5 0.8 0.8 0.7 0.6 0.6 0.5 0.4 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.2 0.2 0.3 0.2 0.3 0.3 0.3 0.4 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.2 0.2 0.2 0.3 0.2 0.2 0.3 0.3 0.3 0.3

35.7 15.9 19.4 9.9 71.2 17.3 19.4 46.1 43.8 63.0 82.4 94.9 92.0 b L.D. b L.D. b L.D. 71.6 62.5 83.9 102.2 103.2 98.6 80.7 68.5 55.4 29.2 25.8 19.6 16.1 18.0 18.6 20.0 21.3 25.8 20.9 19.6 20.4 22.5 21.1 22.7 22.4 23.0 23.7 24.8 23.0 22.2 21.0 22.5 22.6 19.9 16.5 15.0 13.3 16.5 17.6 18.9 20.0 19.6 18.2 17.8

2.4 0.8 0.9 0.8 0.7 1.6 1.3 2.0 2.2 3.5 3.2 3.8 3.1 1.1 4.2 0.7 2.7 2.3 2.5 2.4 2.1 2.2 2.3 2.0 1.9 2.1 2.6 2.6 3.4 2.9 2.7 2.6 2.3 2.8 2.7 2.8 2.7 2.8 3.0 2.4 2.3 2.5 2.4 2.4 2.5 2.7 2.6 2.9 3.3 3.5 3.8 4.2 2.9 3.5 3.3 3.2 2.8 2.5 2.7 2.1

14.1 10.6 14.1 14.9 13.9 12.2 11.5 14.7 13.4 23.1 31.3 35.2 29.4 10.4 26.1 5.5 24.7 22.6 25.0 24.2 22.9 23.1 24.0 23.6 22.9 19.2 24.4 22.5 22.9 23.8 24.8 24.2 24.8 28.0 25.4 24.5 24.8 23.8 25.4 24.1 23.3 25.2 25.9 26.4 25.5 25.7 26.6 27.3 28.0 26.7 25.8 25.0 21.3 25.2 26.6 26.5 27.2 26.0 25.1 22.9

3.6 2.3 2.6 2.2 1.8 3.3 2.2 3.8 3.7 8.4 12.3 13.2 12.1 3.1 8.3 1.6 10.0 9.2 9.9 9.8 9.1 9.4 9.8 9.6 9.3 7.5 9.5 8.6 8.5 8.9 9.2 9.2 9.6 10.9 10.0 9.6 9.8 9.6 9.8 9.9 9.5 10.4 11.0 11.2 11.0 10.3 10.5 10.9 11.0 10.5 9.8 9.6 8.8 10.1 10.6 10.5 10.8 10.6 10.1 9.1

12.1 9.5 14.0 16.4 14.8 25.4 11.0 21.4 13.5 27.1 40.6 39.8 35.0 7.2 11.2 6.7 35.5 33.7 35.8 35.1 34.8 34.4 35.1 36.0 37.8 29.2 35.9 33.7 32.7 33.1 34.7 36.8 38.8 43.3 38.8 36.6 37.8 39.0 40.6 40.7 38.7 39.6 41.9 42.7 41.7 38.4 38.6 40.2 40.9 39.1 39.6 37.8 33.8 42.3 39.6 40.3 40.1 40.2 36.7 34.1

16.3 12.9 20.0 15.8 15.6 25.6 19.5 103.4 131.9 35.6 40.1 38.2 33.7 7.7 6.6 7.5 81.4 48.9 68.4 79.1 75.2 72.9 72.4 62.5 59.8 41.5 40.4 31.1 26.3 28.0 27.9 28.7 29.5 33.3 27.5 26.2 24.7 24.3 25.3 27.9 26.5 32.4 29.2 32.2 29.9 33.5 28.4 28.8 27.6 23.7 21.1 19.3 16.7 21.1 23.7 24.6 28.5 28.2 28.5 26.5

15.3 9.2 13.2 11.4 13.3 18.7 27.9 36.2 23.3 56.1 76.5 69.3 63.7 44.9 27.9 37.7 84.3 72.1 85.5 84.1 81.0 81.8 81.4 79.8 76.5 61.2 80.7 75.1 76.4 78.9 79.2 81.5 85.7 97.1 88.9 84.9 88.9 87.9 90.2 89.3 84.3 92.7 98.0 101.0 97.4 89.1 92.3 93.8 104.4 95.3 87.7 87.5 77.2 87.0 95.1 98.7 104.7 101.2 92.2 81.3

116 117 115 93 139 140 119 268 123 144 123 105 109 350 738 546 491 396 416 501 709 724 558 562 785 876 551 730 759 739 721 649 607 502 488 518 625 496 505 589 560 498 420 409 388 429 411 388 330 351 361 365 344 408 365 338 304 430 466 604

0.3 0.2 0.2 0.2 0.2 0.3 0.2 0.3 0.3 0.7 1.1 1.2 1.0 0.4 1.0 0.1 0.8 0.7 0.9 0.8 0.8 0.8 0.8 0.8 0.7 0.6 0.8 0.7 0.8 0.8 0.8 0.8 0.8 1.0 0.9 0.8 0.8 0.9 0.9 0.8 0.8 0.9 0.9 1.0 0.9 0.9 0.9 0.9 1.0 0.9 0.9 0.9 0.8 0.9 0.9 0.9 0.9 0.9 0.9 0.8

2.9 1.8 2.2 1.7 1.7 2.2 2.0 3.0 2.9 6.4 8.9 8.9 8.3 2.6 5.7 1.6 7.3 6.6 7.4 7.1 6.8 6.9 7.1 6.8 7.1 5.6 7.1 6.7 6.7 7.0 7.1 7.2 7.3 8.0 8.2 7.6 7.6 7.7 7.6 7.1 6.7 7.4 7.7 7.9 7.6 7.7 8.2 8.4 8.9 8.9 8.2 7.8 7.2 8.3 8.7 8.5 8.5 8.8 7.7 6.9

98.4 32.5 36.9 30.3 28.2 61.9 49.2 76.2 88.8 138.2 119.9 144.3 117.3 38.2 164.4 26.5 97.7 93.7 92.6 91.2 81.6 84.0 86.7 78.5 81.0 77.5 98.2 100.7 129.5 107.2 101.0 97.8 87.7 99.2 100.5 106.2 105.0 106.5 112.5 89.8 86.9 88.9 85.9 84.8 85.8 99.8 98.6 108.3 121.0 133.1 139.4 161.3 115.6 128.6 123.5 118.2 97.9 97.8 96.7 77.6

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the Corsican lakes, palygorskite can account for more than 25% of the clay size fraction (Robert et al., 1984). Traces of palygorskite were also identified in the Mont Blanc ice sheet (De Angelis and Gaudichet, 1991). Nevertheless, XRD data have provided no evidence of palygorskite in core PB06, suggesting a low contribution of eolian material in the Pierre Blanche Lagoon. Even if scanning electron microscope observations were not realized on PB06 core samples to confirm this assumption, we can calculate the eolian material contribution with its estimated flux in the studied area. The mean eolian flux of 8 g m−2 yr−1 was estimated by Guerzoni et al. (1999), and mean terrigenous fluxes of around 600 g m−2 yr−1 were calculated for core PB06 (Sabatier et al., 2008). Therefore, the eolian flux contributions to the terrigeneous fraction is about 1.3%, suggesting that this source provides a negligible contribution to lagoonal sedimentation. Fig. 5 displays illite % versus smectite % in the Mosson drainage basin (grey), in the sandy barrier (dark) and in core PB06 sediments. Samples from core PB06 have been separated into two groups corresponding to normal sedimentation and to layers influenced by previously identified paleostorm events (Fig. 2). Samples from PB06 are reported in a linear model between these two end-members, suggesting that no other major sedimentary sources influence the clay size sedimentation of the Pierre Blanche Lagoon. PCA of major and trace elements on bulk sediment from PB06 showed that four groups of samples were influenced by different geochemistry end-members while clay minerals (less than 2-µm fractions) allowed the definition of two fine sediment sources (Fig. 4). For major elements (Fig. 4a), samples with high positive PC1 scores indicate a strong influence of marine component (sandy barrier) as defined above, while a highly negative PC2 score was correlated with the Mosson drainage basin. Moreover, surface sediment samples (b30 cm) showed highly negative PC1 scores that indicate either a strong influence of phosphate or an important biogenic component with a lot of shells. For trace elements (Fig. 4b), samples with highly positive PC1 scores (upper b20 cm) indicate a strong influence of pollution-related minor elements (Cd, Cu, Pb). Samples with positive PC3 scores were correlated with Zr and Hf, which are mostly present in heavy minerals (zircon) and are not present in the sedimentary formation of the Mosson watershed, but they are one of the components of the sandy barrier. Samples characterized by highly negative PC3 scores are related to Th and Ni elements, generally associated with clay minerals that are mainly transported by the Mosson River. One source is characterized by Al2O3 and Th. These elements are generally associated with clay minerals that are mainly transported by the Mosson drainage basin during flood events. Another source is defined by SiO2, Na2O, Zr and Hf, which are abundant in quartz, sodium feldspars and heavy minerals present in the sandy barrier (the presence of albite was determined by XRD analysis, data not shown). Biogenic content variations are evident in lagoonal sediment by shell abundances with high concentrations of CaO and Sr. Finally, domestic, agricultural and industrial effluents have been released, partly treated, for the last several decades. The high levels of phosphate and metallic elements (Cu, Pb and Cd) in the surface sediment (Fig. 3) are the result of the anthropic component. With respect to the sedimentation rate (2.7 mm/yr), this component was initiated at the beginning of the twentieth century. To characterize the end-members, geochemical analyses were performed on the different components of the system, including the Mosson drainage basin and barrier samples (Fig. 6). The results obtained from bulk sediment of core PB06 are also displayed but are separated into two groups that were previously identified by clay minerals as normal lagoonal sedimentation and layers influenced by storm events (Fig. 6). Samples from the sandy barrier appeared to be a mix between quartz (SiO2), sodium feldspars (Na2O), carbonated shells (aragonite with high concentrations of CaO and Sr) and

213

Fig. 3. 1.3 m of the selected PB06 geochemistry data. SiO2, Na2O and Zr co-vary in contrast to Fe2O3 and Al2O3. CaO and Cu contents vary in different phases. Grey bands are paleostorm events previously identified and dated at 1742, 1848 and 1893.

aluminosilicate minerals (illite, chlorite and probably mica). Samples from the Mosson channel deposit were composed of aluminosilicate mineral end-member (i.e., clay samples from the Mosson drainage basin were mainly characterized by smectite) and limestone (calcite, CaO and low Sr in comparison to shell contents). Suspended sediments collected during flood events were intermediate between clay from the Mosson drainage basin (smectite) and material from channel deposits (with a carbonate component). Core PB06 is mainly characterized by sediments transported by flood events and by carbonated shells with some layers influenced by the sandy barrier during storm events (Fig. 6). Therefore, the mineralogical components of this system are: 1) quartz, sodium feldspars, illite, chlorite and probably mica from the marine end-member that was ultimately derived from the Rhone River; 2) limestone and clay minerals, mainly smectite from the Mosson drainage basin; 3) carbonated shells due to the biogenic productivity inside the lagoon and 4) the anthropic component with phosphate and some metal elements. 6.2. Clay mineralogy and geochemistry variations of paleostorm record The Mosson drainage basin and the sandy barrier represent the main sources of sediment in the Palavasian lagoonal system. They are characterized by different mineralogical and geochemical signatures. Sabatier et al. (2008) showed that the strong marine influence in this area occurs during storm events, when wind and wave energies break up the barrier and trigger a landward material transfer from the barrier to the lagoon. Clay mineralogy and geochemistry display

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Fig. 4. (a.) Biplots of PC1 and PC2 loadings and sample scores for ten major elements and LOI. Numbers represent the depth of the different samples in the PB06 core. PCA was performed on centered logratio transformations of samples. Four different groups of samples were identified in relation to the variable. (b.) Biplots of PC1 and PC3 loadings and sample scores for thirteen selected trace elements. The same groups of compositions were identified.

significant variations between these two major sedimentary sources and can, therefore, be used to reconstruct the marine influence in the Pierre Blanche Lagoon that could be linked to past storm events. Considering that smectite and illite–chlorite reveal a distinct temporal evolution and that kaolinite abundance does not exhibit significant changes, it is possible to use the smectite/(illite + chlorite) ratio to describe the mineralogical variations within the clay size fraction. This mineralogical ratio can be used to reconstruct the history of paleostorm events. In core PB06, the smectite/(illite+ chlorite) ratio ranges from 0.4 to 2.4. Variations of this ratio show low values in comparison with previously identified storm events (Fig. 7). Moreover, two other storm events could be identified at depths of 44 and 120 cm (Fig. 7), but this evidence was found at just one point and geochemical data were absent at these depths. The relatively higher ratios observed during the rest of the time indicate the dominance of Mosson River sediment inputs. We also suggest that minerals with a high concentration of SiO2 and Zr could correspond mainly to the silt and sand fraction

Fig. 5. Illite versus smectite from samples from the Mosson drainage basin (grey) and the sandy barrier (dark), as well as from the PB06 core separated into two groups corresponding to: 1) normal lagoonal sedimentation and 2) layers influenced by previously identified paleostorm events (Fig. 2).

present on the littoral barrier (quartz and heavy minerals such as zirconium) while Al2O3 is the major component of clay minerals, mostly transported from the watershed. In core PB06, SiO2/Al2O3 and Zr/Al2O3 ratios increased in relation to storm events (Fig. 7). Good correlations between smectite/(illite + chlorite), SiO2/Al2O3, Zr/Al2O3 ratios and paleostorm events previously identified (Sabatier et al., 2008) suggest that these proxies determined in different size fractions of the sediment can be used to reconstruct the past storm history. The sensitivities of the clay mineralogy and geochemistry proxies are different. For example, in the 1848 event, the less intense

Fig. 6. Three-dimensional (3D) diagram of SiO2, Al2O3 and Sr. The diagram specifies the different end-members. All of the samples from the studied system are displayed, and samples from PB06 are separated into two groups corresponding to normal sedimentation and to layers influenced by paleostorm events (Fig. 4). Six samples of shells were analyzed (Cerastoderma glaucum, Hydrobia acuta and Abra ovata) and have an average Sr content of 1500 +/− 100 ppm (with 56% of CaO and 0% of SiO2 and Al2O3).

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considered the most catastrophic storm (around 100 cm in depth), but its smectite/(illite + chlorite) ratio suggests that this event was less intense than the storm in 1848 (with a 64-cm depth). Two other low values of the smectite/(illite + chlorite) ratio could be interpreted as paleostorm events at 44 and 120 cm of depth (Fig. 7). The values of this ratio are almost equal to the values of the most powerful hurricane, but this interpretation is not supported by geochemical data. Moreover, these events are recorded at just one point suggesting that if these two points are not artefacts, they recorded less intense events than those previously identified. Therefore, we propose that clay minerals cannot be used as an indicator of land-falling intensity, rather they may be a more sensitive proxy for identifying less intense storm events, when sand layers are indistinguishable or absent in the studied area. 7. Conclusion Clay mineralogy and bulk geochemical analyses of the Pierre Blanche lagoonal system were conducted to determine sediment sources of this coastal area and to test these proxies for their effectiveness in reconstructing paleostorm events that affected the north-western parts of the Mediterranean Sea. Fig. 7. Smectite/(illite + chlorite), SiO2/Al2O3 and Zr/Al2O3 ratios for the PB06 cores. Grey bands are paleostorm events previously identified.

paleostorm (Sabatier et al., 2008; Dezileau et al., 2009) is better identified than the other two storms by the smectite/(illite + chlorite) ratio, whereas geochemistry ratios present the smallest peak. This difference in sensitivity could be related to the complexity of the mix (two end-members for the clay mineral distribution and four endmembers for the geochemistry) and to the comparison between different grain sizes (in bulk for geochemistry and in fraction of less than 2-µm for clay mineralogy). In addition, there is a possible improved ability of a storm event to transport sediment of different sizes from the barrier to the lagoon. We can assume a constant morphology of the sandy barrier for the last 500 yrs (Sabatier, 2009). During a main storm event, wind and wave energies are strong enough to transport coarse sediment as silt and sand (with high concentration in quartz and heavy minerals with high ratios of SiO2/ Al2O3 and Zr/Al2O3) to the central part of the lagoon. However, if the intensities of wind and wave energies are not powerful enough to transport coarse sediment (silt or sand), the storm can just trigger a landward transfer of fine-grained sediment (clay fraction). Moreover, if the breaking up of the sandy barrier occurs far from the core location, coarse material cannot be transported over large distances while clay may record this influence over more distal locations. In both cases, the record of this event is better identified by a decrease in the smectite/(illite + chlorite) ratio, whereas geochemistry ratios seem to better record the most powerful events. 6.3. Clay minerals as indicators of storm intensity? Clay mineralogy distributions through time clearly show an increase of illite content as a consequence of paleostorms (Fig. 5) in relation to its high concentration in the sandy barrier. In the PB06 core, the relative contribution of illite within layers identified as storm events could be interpreted as an indicator of land-falling intensity. Nevertheless, Woodruff et al. (2008) associated the local flooding intensity to the ability for each overwash event to transport coarsegrained sediment across a fixed distance in the lagoon. Clay minerals are the smallest sediment component and can thus be exported landward across large distances during a storm event while coarser materials such as sand and silt need more energy to be transported over the same distance. The storm of the 4 September 1742, which was recorded in many city archives around the Aigues–Mortes Gulf, is

(1) The Pierre Blanche lagoonal clay fraction (b2 µm) contains four main clay mineral groups. Smectite and illite account for more than 75% of the clay assemblage. Chlorite and kaolinite contents are of secondary importance. Illite and chlorite covary (in core PB06) are mainly present in the sandy barrier and are inversely correlated with smectite, which is abundant in the Mosson drainage basin. Kaolinite contents do not present significant variations. (2) Major and trace element concentrations allow us to define four main components in the Palavasian lagoonal system. The first source was the sediment from the Mosson drainage basin transported during floods. The second end-member defines mineralogical components of the sandy barrier and comes from the Rhone River via littoral drift. The third component is mainly due to carbonate shell accumulation in relation to biogenic productivity inside the lagoon. The last end-member of this system is the anthropic component for the surface sediment (b30 cm). (3) The main sources of the lagoonal sedimentation are the Mosson drainage basin and the sandy barrier, which contribute to a landward transport of sand and silt materials during storm events. The important discrepancies of clay minerals and geochemistry between these two end-members allow for the tracing of land-falling events. Our results indicate that the sensitivities of the smectite/(illite + chlorite) and geochemistry (SiO2/Al2O3, Zr/Al2O3) ratios, are different in relation to the intensity of storm events. However, good correlations between these ratios and paleostorm events suggest that these proxies can be used to reconstruct past storm histories. Acknowledgments This research was undertaken in the framework of the ECLICA project financed by INSU (ACIFNS “Aléas et Changement Globaux”, coord.: L. Dezileau) and the INTEMPERIES project (“Appel à projets 2008 du Conseil Scientifique de l'Université Montpellier 2, coord.: L. Dezileau). Our two reviewers Nathalie Fagel and Zhifei Liu are thanked for comments on the manuscript. The authors are also grateful to Anis Bel Haj Mohamed and Estelle Ricard from the IDES Laboratory (CNRS/INSU UMR 8148 — University of Paris XI), who helped us to process clay mineral analyses by DRX. The authors also wish to thank IFREMER Palavas for allowing storage of the cores in their cold room. The GLADYS platform (www.gladys-littoral.org) and

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