# River flooding on the French Mediterranean coast and its relation to climate and land use change over the past two millennia

Jean-Philippe Degeai, Philippe Blanchemanche, Léa Tavenne, Margaux

Tillier, Hervé Bohbot, Benoît Devillers, Laurent Dezileau

► To cite this version:

Jean-Philippe Degeai, Philippe Blanchemanche, Léa Tavenne, Margaux Tillier, Hervé Bohbot, et al. River flooding on the French Mediterranean coast and its relation to climate and land use change over the past two millennia. CATENA, 2022, 219, pp.106623. 10.1016/j.catena.2022.106623. hal-03947235

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1 River flooding on the French Mediterranean coast and its relation to climate and land use

2 change over the past two millennia

3

4 Jean-Philippe DEGEAI (a)*, Philippe BLANCHEMANCHE (a), Léa TAVENNE (a), Margaux

5 TILLIER (a, b), Hervé BOHBOT (a), Benoît DEVILLERS (a), Laurent DEZILEAU (c)

6

7 (a) ASM UMR5140, Université Montpellier 3, CNRS, MCC, 34000 Montpellier, France

8 (b) Ipso Facto scop, 13200 Arles, France

9 (c) M2C UMR6143, Université de Caen, CNRS, 14000 Caen, France

10

11 *corresponding author

12 E-mail address: jean-philippe.degeai@cnrs.fr (J.-P. Degeai)

13

14

15 ABSTRACT

16 River flooding triggered by extreme rainfall events is a major natural hazard that frequently

17 causes fatalities and damages in southeastern France. A 2000-year record of flooding from

18 the Hérault River basin was reconstructed using magnetic susceptibility data from lagoon

19 deposits, supported by documentary databases of floods in coastal river basins from

20 Languedoc over the last centuries. Flooding was particularly important between 800 and

21 1350 cal CE, i.e. during the warm climatic conditions of the Medieval Climate Anomaly.

22 Moreover, several periods of increased flooding occurred in the 2^nd$, 6^th$-7^th$, 16^th$, 18^th$ and

23 20^th$ centuries. Increased flooding was significantly correlated with higher air temperature in

24 the northwestern Mediterranean and with higher sea surface temperature in the Gulf of
25 Lions. The analysis of relationships between river flooding and atmospheric conditions
26 shows that autumn flood events in Languedoc were generally more frequent during the
27 positive phase of the East Atlantic pattern, which is associated with southerly to
28 southeasterly winds in the Gulf of Lions. Besides, increased flooding could also have been
29 controlled by forest decline and human activities after 500 cal CE. Between 1400 and 1800
30 cal CE, palaeobotanical data show an intensification of land use characterized by an
31 expansion of grasslands and cultivated areas in the coastal lowlands east of the Hérault
32 River. Forest clearings and the development of agriculture seem to have led to a relative
33 increase in river flooding, especially in the late Middle Ages and early modern period.

34

35 Keywords:

36 Flood; Coastal river basin; Lagoon sediment; Climate forcing; Vegetation change;
37 Northwestern Mediterranean

38

39

40 **1. Introduction**

41 River flooding in Europe is one of the most important natural hazards that resulted in over
42 4700 fatalities and more than 150 billions of euros (2013 values) of direct economic losses
43 over the period 1980–2013 (EEA, 2017). Climate change and human activity can impact the
44 variability of river flood frequency (e.g. Benito et al., 2015; Rogger et al., 2017). For instance,
45 enhanced heavy precipitation and land use changes can have led to an increase in the
46 number of very severe European floods over the last decades (EEA, 2017). However, the
47 estimation of the relative influence of natural and anthropogenic factors on flooding is a
48 challenging task (e.g. Machado et al., 2017; Munoz et al., 2018; Corella et al., 2019). In the
49 western Mediterranean, the spatial variation of flood variability in response to Holocene
50 rapid climate changes was modified by the different types of vegetation and soil processes
51 as well as by human land use (Fletcher and Zielhofer, 2013). Therefore, palaeoenvironmental
52 data at local and regional scales are crucial to understand the complex relationships
53 between river flooding, climate, vegetation, soil erosion and human activities.

54 In southeastern France, the coastal plain of Languedoc is affected by river floods during
55 heavy rainfalls produced by the transport of warm and moist air masses from the
56 Mediterranean Sea (Nuissier et al., 2008; Ricard et al. 2012). These floods represent a major
57 hazard on the French Mediterranean coast, while growing population and tourism industry
58 increase the expose and vulnerability of coastal societies to extreme climate events in this
59 area (Dezileau et al., 2011).

60 Sedimentary archives are particularly interesting to study the long-term variability of flood
61 frequency, as shown by records from lake sediments in the southern French Alps (Wilhelm et
62 al., 2012; Brisset et al., 2017). In Languedoc, sediments deposited in lagoons located near
63 river outlets have the potential to provide flood records over the last millennia (Berger et al.,
64 2010; Degeai et al., 2017). However, there are very few studies on long-term river flooding in
65 this area. These studies cover the second half of the second millennium CE and are based on
66 either documentary sources (Blanchemanche, 2009) or slackwater flood deposits (Sheffer et
67 al., 2008; Dezileau et al., 2014). Otherwise, the Holocene record of river floods and extreme
68 fluvial events in southeastern France established by Benito et al. (2015) from the calculation
69 of a cumulative probability density function of radiocarbon dates is not specifically focused
70 on coastal river basins, while the reconstruction of historical floods in the Lower Rhône
71 Valley undertaken by Pichard et al. (2017) for the last seven centuries from documentary
72 and instrumental sources is representative of the river regime of a large catchment that can
73 be influenced by floating ice.

74 Here we present a two millennia flood record established from lagoonal sediments
75 supported by historical flood records of coastal rivers in Languedoc. The specific objectives
76 are to (1) tentatively identify the periods of increased river flooding in the coastal plain of
77 Languedoc over the last two millennia, (2) characterize changes in vegetation cover and land
78 use from palaeobotanical data, (3) estimate the relative influence of natural and
79 anthropogenic factors on the variability of river flooding from the comparison of our data
80 with proxy records of environmental change.

81

82

83 **2. Environmental setting**

84 The coastal plain of Languedoc is located in the northwestern Mediterranean between the
85 southeastern piedmont of the Massif Central and the Gulf of Lions (Fig. 1). From West to
86 East, the Orb, Hérault, Mosson, Lez, Vidourle and Vistre rivers have lengths of 136, 150, 36,
87 28, 94 and 36 km and drain areas of 1514, 2560, 386, 173, 798 and 480 km², respectively
88 (Fig. 2).

89 The upstream part of these coastal rivers drains highlands composed of (1) Paleozoic
90 granites and schists on the southern edge of the Cévennes, (2) Mesozoic sedimentary rocks
91 in the Causses, (3) Paleozoic sedimentary rocks in the Lodève basin and the Minervois, and
92 (4) Paleozoic gneiss in the eastern part of the Monts de Lacaune. The source of the Hérault
93 River is located on the southern flank of Mont Aigoual, which exhibits the highest altitude
94 (1565 m asl) in the study area. The lowlands of the coastal plain of Languedoc along the
95 Mediterranean Sea are mainly composed of Cenozoic continental and marine deposits and
96 show elevation generally lower than 100 m. In this area, the mean longitudinal slope of the
97 six rivers is lower than 2‰. Several small massifs of Mezosoic limestone and dolomite are
98 presents in the coastal plain. Besides, late Cenozoic basic volcanic rocks outcrop mainly in
99 the Hérault and Orb catchments.

100 Mean annual precipitations range from ca. 600-800 mm in the coastal plain to 1000-2000 mm in the upper catchments, where maximum daily precipitation for the 10 and 100-year frequencies can be higher than 200 and 300 mm, respectively (ADEME, 2011). Instrumental data from stream gauges over the last 70 years show that the mean annual discharge of the Orb, Hérault, Mosson, Lez, Vidourle and Vistre rivers exhibits values of 23.5, 41.1, 1.1, 2.54, 7.58 and 3.6 m³·s⁻¹ at Béziers, Agde, Saint-Jean-de-Védas, Montpellier, Marsillargues and Le Cailar (sites 6, 12, 38, 40, 45 and 57 in Fig. 2), respectively (source: DREAL Languedoc-Roussillon, www.hydro.eaufrance.fr, stream gauging stations Y2584010, Y2372010, Y3142010, Y3204030, Y3464010 and Y3534010). River regime is controlled by rainfall and Mediterranean climate, with maximum discharges during autumn or winter, and minimum discharges during summer.

110 Drainage basins in Languedoc can undergo extreme rainfall events causing flash floods when 111 the warmer Mediterranean Sea provides moisture and heat to the lowest levels of the 112 atmosphere at the end of summer and during autumn (Nuissier et al., 2008; Duffourg and 113 Ducrocq, 2011). Flood discharges in the lower basin of the Hérault River are estimated to be 114 1650 and 3000 m³·s⁻¹ for 10-year and 100-year flood events, respectively (DDTM, 2014). At 115 the station of Béziers (site 6, Fig. 2), the 10-year and 100-year flood discharges of the Orb 116 River (1540 km²) are assessed at 1250 and 2500 m³·s⁻¹ (DDE, 2007), i.e. about 50 and 100 117 times higher than the mean annual discharge, respectively. For the smaller drainage basins 118 of the Mosson and Lez rivers (386 and 173 km², respectively), the 100-year flood discharges 119
120 at the stations of Saint-Jean-de-Védas and Montpellier (sites 38 and 40, Fig. 2) are estimated
121 at 615 and 900 m³·s⁻¹ (DDTM, 2013), i.e. about 350 and 550 times higher than the mean
122 annual discharge, respectively.

123 A study of documentary sources from 14 sites in the coastal river basins of Languedoc for
124 the period 1566–1958 showed that flood events were more frequent in the 1700s and the
125 first half of the 1900s (Blanchemanche, 2009; Berger et al., 2010). Before the late 16th
126 century, many gaps in the documentary sources preclude a robust reconstruction of river
127 flooding in Languedoc (Blanchemanche, 2009). However, the sediments deposited in Bagnas
128 Lagoon can provide a long-term record of fluvial activity in the Hérault River basin before the
129 19th century (Degeai et al., 2017). This microtidal coastal lagoon is located in the southern
130 part of Thau Lagoon about 3 km east of the Hérault River, which drains a 4 km width
131 floodplain showing a mean slope of 0.6‰ and elevation lower than 10 m between Saint-
132 Thibéry and Agde (Fig. 2, sites 15 and 12, respectively).

133 Bagnas Lagoon is a shallow body of water (<1 m depth) frequently flooded by the Hérault
134 River during floods with high or low return periods, as shown by maps of water height and
135 flooded areas for 10-, 100- and 1000-year flood events (Fig. 3). This suggests that Bagnas
136 Lagoon is highly sensitive to fluvial inputs from the Hérault River. Moreover,
137 palaeogeographical and geomorphic studies showed that the small basin of Bagnas Lagoon is
138 connected to the floodplain of the Hérault River through the Sept-Fontes corridor since at
139 least 2500 cal BP, while the archaeological site of La Motte indicates that the position of the
140 Hérault River channel remained approximately the same in this area since the Late Bronze
141 Age (Devillers et al., 2019). Hence, geomorphic changes due to the progradation of the
142 Hérault River delta should not have drastically impacted riverine input into Bagnas Lagoon
143 over the past three millennia. Besides, there are no large dams or reservoirs that could
144 significantly reduce sediment flux from the Hérault floodplain to Bagnas Lagoon.

145 The development of coastal barriers and lagoon systems along the coast of the Gulf of Lions
146 was facilitated by the deceleration of Holocene sea level rise, which started around 7500 cal
147 BP in the northwestern Mediterranean (Vacchi et al. 2016; Salel et al., 2020). Environmental
148 conditions of closed lagoon prevailed in Bagnas and Thau lagoons since at least the last two
149 millennia as indicated by palaeoecological, geochronological and geophysical data acquired
150 along the coastal barrier between the Cape of Agde and Sète (Fig. 2) (Tessier et al., 2000;
151 Court-Picon et al., 2010). Besides, sea level rise in the Gulf of Lions was lower than 0.5 m in
152 the past three millennia (Vacchi et al., 2016), which suggests that flooding in the Hérault
153 River basin was probably not significantly affected by sea level change over this period.

154

155

156 **3. Materials and methods**

157

158 **3.1. Flood database**

159 Several databases and documents were used to count the number of floods in five coastal
160 river basins of Languedoc (Orb, Hérault, Lez-Mosson, Vidourle and Vistre) over the past
161 centuries, and especially over the last decades that were not investigated in the study of
162 Blanchemanche (2009). The two small basins of the Lez and Mosson rivers were considered
163 as a single river basin, given that the Mosson River is a tributary of the Lez River.

164 The number of floods was counted for each river basin from flood events mentioned in: (1)
165 the database of the national reinsurance central fund (caisse centrale de réassurance, CCR),
166 which lists the administrative orders related to natural disasters such as flood events
167 (https://catastrophes-naturelles.ccr.fr/les-arretes); (2) the database of the national
168 collaborative platform of flood sites and marks (plateforme nationale collaborative des sites
169 et repères de crues, https://www.reperesdecrues.developpement-durable.gouv.fr); and (3)
170 the reports of flood risk prevention plan (plan de prévention des risques d'inondation, PPRI)
171 available for the municipalities of the French administrative division of the Hérault
department (https://www.herault.gouv.fr/Politiques-publiques/Environnement-risques-
172 naturels-et-technologiques/Risques-naturels-et-technologiques/Les-Plans-de-Prevention-
173 des-Risques-approuves/Dossiers-des-PPR-approuves-au-format-PDF) and the Gard
department (https://www.gard.gouv.fr/Politiques-publiques/Securite-et-protection-de-la-
175 population/Risques/Gestion-du-risque-inondation/Plans-de-Prevention-des-Risques-
176 Inondation-PPRI).
177

178 A total of 69 municipalities were investigated from these reports and databases (sites 1–69, Fig. 2): 11 for the Orb River, 22 for the Hérault River, 9 for the Lez-Mosson river system, 13 for the Vidourle River and 14 for the Vistre River. Only municipalities located along rivers flowing over Cenozoic sediments where elevation is lower than 100 m were retained to study river flooding in the coastal plain of Languedoc. Moreover, we have added flood events mentioned in the reports of the atlases of floodable areas in the Hérault (DIREN, 2007) and Lez-Mosson (DREAL, 2010) river basins, and in the list of historical floods of the Vidourle River established by the Association for flood prevention at Lunel (Association pour la prévention des inondations à Lunel, APIL, https://apillunel.wixsite.com/apil/historique-crues-vidourle). One flood event was counted when water overflowed the river banks, which led to flooding in the floodplain.
179
180
181
182
183
184
185
186
187
188

189 The oldest flood mentioned in the documents and databases used in this study occurred in
190 1220 (Table S1). However, the frequency of floods determined from these sources seems to
be underestimated before the 19th century when compared with the flood record
established by Blanchemanche (2009) from municipal archives over the period 1566–1958
(Fig. S1). Hence, the databases and documents mentioned above were used to assess the
frequency of river flooding in the coastal plain of Languedoc over the period 1800–2021.

195

## 3.2. Sediment core analysis

A 9 m sediment core (B1, 3°30'58.6"N, 43°19'08.4"E, Fig. 3) was retrieved from the
southeastern part of Bagnas Lagoon using a core drilling rig and a push-tube sampler with a
cutting shoe. Sediment was collected in plastic tubes in order to undertake sedimentological
and palaeobotanical analyses (magnetic susceptibility, particle sizing, carpology).

Magnetic susceptibility is a convenient method to detect terrigenous input from fluvial
activity and flooding in coastal environments of the western Mediterranean (Degeai et al.,
2015, 2020; Delile et al., 2015). In Bagnas Lagoon, sediments with higher magnetic
susceptibility were related to terrestrial inputs during flood events (Degeai et al., 2015). The
volume magnetic susceptibility (VMS) of core B1 was measured at 1-cm resolution using a
Bartington MS2 magnetic susceptibility meter and a MS2F probe with an operating
frequency of 0.58 kHz, a period of 1.1 s and a precision of $1 \times 10^{-5}$ (SI).

Particle size analysis by laser diffraction was undertaken in order to characterize the particle
size distribution (PSD) of sediments deposited in Bagnas Lagoon during flood episodes. A
total of 82 samples were measured using a Beckmann-Coulter LS13320 particle sizer
connected to a Fluid Module. The <1.5 mm fraction of samples was mixed in deionized water
then poured in the Fluid Module to obtain an obscuration of 7–12% in the optical cell. The
time of background and sample measurements was set to 90 s and sonication was applied
during the sample measurement to obtain a good dispersion of fine particles. Each sample
215 was measured twice to check the good repeatability of measurement using the statistics

216 recommended by the international standard ISO 13320-1. Relative standard deviations for

217 the median and for the differences between the quantiles q10 and q30 and between the

218 quantiles q70 and q90 were lower than 3 and 5%, respectively.

219

220 3.3. *Palaeobotany*

221 Palaeobotanical data can provide useful information on landscape evolution and its impact

222 on flooding in southern France (Wilhelm et al., 2012; Brisset et al., 2017). Vegetation

223 changes in Languedoc can be studied from pollen records of Bagnas Lagoon (Dolez, 2018),

224 Pierre Blanche Lagoon (Azuara et al., 2015) and core SETIF near Sète (Court-Picon et al.,

225 2010). The two latter are located 35 and 15 km northeast of Bagnas Lagoon, respectively

226 (Fig. 2). Besides, we undertook a seed analysis of 15 samples from core B1 in order to assess

227 changes in local vegetation around Bagnas Lagoon. For each sample, 250 g of sediments on

228 average were sieved at 177 µm. A total of 2569 seeds were counted and classified in three

229 categories of palaeoecological conditions representative of aquatic environment

230 (*Ranunculus aquatilis*, *Ranunculus aquatilis*-type, Characeae, *Potamogeton* sp., *Ruppia*

231 *maritima*, *Zannichellia palustris*, *Phragmites*, *Najas minor*), humid environment (Bryophytes,

232 *Aster* cf. *tripolium*, *Salicornia europaea*, *Spergularia* sp., *Suaeda maritima*, *Alisma* sp.,

233 *Alismataceae*, *Carex* sp., *Cyperaceae*-type, *Cyperus* cf. *longus*, *Eleocharis* sp., *Juncus* sp.,

234 *Typha* cf. *latifolia*, *Typha* sp.) and land ecosystems (Cupressaceae, Poaceae, cf. *Atriplex* sp.,

235 *Cerastium* type, *Corydalis* sp., *Linum* sp., cf. *Medicago* sp., *Polygonum aviculare*, *Reseda*

236 *phyteuma*, immature *Rubus* sp., *Stellaria media*, *Torilis* sp.). Two samples at 7.95 and 8.3 m

237 depth were excluded given the low number of seeds (<10).

238
239 **3.4. Geochronology**

240 The chronology of lagoonal sediments of core B1 was studied from 12 radiocarbon ages and

241 historical archives (Table 1, Fig. 4). Radiocarbon dating of marine shells (*Parvicardium*

242 *exiguum*), charcoal and wood debris was conducted at the Centre de Datation par le

243 Radiocarbone (CNRS and University of Lyon 1) using accelerator mass spectrometry (AMS).

244 The model of ¹⁴C reservoir age R(t) established for Bagnas Lagoon by Degeai et al. (2015) was

245 used to estimate the atmospheric ¹⁴C age of shells, which was calculated by subtracting R(t)

246 from the conventional radiocarbon age of shells. The atmospheric ¹⁴C ages of charcoal, wood

247 debris and shells were then calibrated using the IntCal20 calibration curve (Reimer et al.,

248 2020) and Calib 8.2 (Stuiver et al., 2021). Moreover, the chronology of the uppermost part of

249 B1 was estimated from historical maps, which show that land emergence around 0.5 m

250 depth occurred between 1774 and 1821 CE (Degeai et al., 2015).

251 An age model for lagoonal sediments below 0.5 m depth was established from 10

252 radiocarbon ages between 2 and 9 m depth and from the age of historical marker at 0.5 m

253 depth. The two radiocarbon ages at 0.6 and 0.75 m depth were not included in the age

254 model because the chronological marker at 0.5 m depth is more accurate to date the

255 uppermost part of B1. Depth was converted into age by linear regression using AnalySeries

256 2.0 (Paillard et al., 1996). Linear regression was performed on the mean age of land

257 emergence around 0.5 m depth and on the mean calibrated ages of 2-sigma ranges with the

258 highest probability apart from radiocarbon dates between 2 and 3 m depth, for which the

259 younger age ranges are inconsistent with chronological data from historical maps (Fig. 4).

260

261 **3.5. Statistics**
262 Correlation tests were used to evaluate the significance of relationships between river flooding in Languedoc and climate change or atmospheric circulation in the northwestern
263
264 Mediterranean. First the normality of the distribution of flood records was verified by using the D'Agostino-Pearson normality test in R software. This test was preferred to the Shapiro-
265
266 Wilk or Kolmogorov-Smirnov normality tests because the number of observations is higher
267
268 than 50 for each variable and some observations have equal values. It shows that VMS as well as the number of flood events determined from historical databases, which is a discrete
269 variable, are not normally distributed (Table S2). Therefore, Kendall and Spearman rank
270 correlation tests were used to assess the significance of the relationships between flood
271 series and climate records or atmospheric patterns. A relationship between two variables is considered significant at *p* < 0.01. The *p*-value were calculated for each correlation using the
272
273 Kendall and Spearman two-sided tests in R software.

274

275

276 **4. Results**

277

278 **4.1. Historical floods**

279 A total of 447 floods were counted in the coastal plain of Languedoc over the period 1800-
280 2021, with 94, 130, 70, 90 and 63 flood events in the Orb, Hérault, Lez-Mosson, Vidourle and
281 Vistre river basins, respectively (Table S1). In all basins, the 19th century was characterized by
282 low river flooding, whereas the frequency of floods increased during the 20th century (Fig. 5).
283 More specifically, flood events were more frequent in the first decade of the 20th century
284 and in the 1930s in most basins, whereas a general increase in the frequency of flooding
285 occurred during the second half of 20th century. The number of floods was particularly high
286 in the year 1907, with 9 flood events in the lower basin of the Hérault River and 2 or 3 floods
287 in the other basins. The last decade of the 20th century experienced a high frequency of
288 flooding in all river basins. For instance, a total of 17 and 19 floods occurred in the lower
289 basins of the Orb and Hérault rivers during the 1990s. Overall, the frequency of flooding
290 remained high in the two first decades of the 21st century, although decreases occurred in
291 the Orb, Hérault, Vidourle and Vistre river basins.

292 Seasonally, river flooding occurred mainly in autumn, with percentages of autumn floods at
293 50% for the Orb River, 68% for the Hérault River, 83% for the Lez-Mosson river system, 74%
294 for the Vidourle River and 73% for the Vistre River (Fig. 5). The lowest percentage of autumn
295 floods is found in the westernmost catchment of the Orb River. The season with the second
296 highest percentage of flood events is in winter for the Orb (23%), Hérault (23%), Vidourle
297 (21%) and Lez-Mosson (10%) river basins, and in summer for the easternmost basin of the
298 Vistre River (16%). The proportion of floods is lower than 10% for seasons not mentioned
299 above.

300

301 **4.2. Stratigraphy and sedimentation rates**

302 The sediments of core B1 are composed of gray, greenish gray or weak red sandy layers
303 interbedded with gray or greenish gray clay or silt from 9 to 7.5 m depth (Fig. 6A). Above 7.5
304 m depth, the sediments are mainly composed of (1) greenish gray clay between 7.5 and 6.5
305 m depth, (2) gray or red clay between 6.5 and 6 m depth, (3) olive yellow or red clay or silt
306 between 6 and 4.6 m depth, (4) red clay between 4.6 and 0.9 m depth with a dark organic-
307 rich layer around 2.5 m depth, and (5) reddish brown clay in the upper 0.9 m.

308 The age model of B1 and sea level change over the past millennia show that the sediments
309 between 8.5 and 0.5 m depth were deposited in a subaqueous environment from ca. 200 cal
310 BCE to 1800 cal CE (Fig. 6A). The sedimentation rates range from 1 to 24 mm·yr⁻¹ (Fig. 6B).

311 The highest rates (>10 mm·yr⁻¹) are found between 0.5 and 2.5 m depth, i.e. in the 18th century.

313

### 4.3. Magnetic susceptibility

315 The VMS of B1 ranges from -2 to 47 × 10⁻⁵ (SI) (Fig. 6C). It is generally very low in the lower half of B1 below 4.7 m depth, where values are lower than 5 × 10⁻⁵ (SI) apart from 4 levels with sharp VMS increases around 8.7, 8.1, 6.5–6 and 5.4–5.1 m depth (F1–F4). F1 and F2 show maximum values of VMS around 15 × 10⁻⁵ (SI), i.e. just above the mean value of VMS at 13 × 10⁻⁵ (SI), whereas F3 and F4 exhibit maximum values of ca. 50 and 30 × 10⁻⁵ (SI), respectively. Overall, the lagoonal sediments in the upper half of B1 are characterized by higher VMS. Three levels with relatively high values generally ranging from 20 to 40 × 10⁻⁵ (SI) can be distinguished at 4.7–3.8, 3.3–3 and 2.7–0.8 m depth (F5–F7), whereas VMS ranges between ca. 2 and 20 × 10⁻⁵ (SI) at 3.8–3.3, 3–2.7 and 0.8–0.5 m depth. Besides, levels F2–F6 show an internal variability characterized by the presence of two (F2–F4 and F6) or more (F5 and F7) peaks of VMS.

322 The age model of B1 (Fig. 6A) suggests that sediments with higher VMS were deposited before 200 cal BCE (F1) and around 150–200 (F2), 550–600 (F3), 650–700 (F4), 800–1350 (F5), 1500–1600 (F6) and 1650–1800 (F7) cal CE.

329

### 4.4. Grain size

331 The bottom part of core B1 between 9 and 7.5 m depth is composed of a succession of 10–20 cm thick layers of sand, clayey or silty sand and silty clay with a median ranging from 7 to 171 µm, and with clay, silt and sand contents of 4–46, 5–52 and 4–91%, respectively (Fig. 6B).
334

6D). Between 7.5 and 5 m depth, the sediments are mainly composed of clay, silty clay and

335

sandy clayey silt with a median varying from 6 to 20 µm, except for a coarser level of silty

336

clayey sand with a median of 67 µm at 5.8–5.7 m depth. These sediments are characterized

337

by an increase in the contents of clay (10–50%) and silt (38–72%), whereas the

338

concentration of sand decreases below 32% apart from the coarser level at 5.8–5.7 m depth

339

(51%). Above 5 m depth, the sediments are composed of only clay and silty clay with a

340

median of 6–9 µm, high contents of clay (37–51%) and silt (48–61%), and a very low content

341

of sand (<5%).

342

A PSD analysis coupled to the use of a C-M diagram based on the 99th percentile (C) and

343

median (M) highlights three types of depositional environment with specific sedimentary

344

processes (I–III) and seven subtypes of PSD (Fig. 7). Type I corresponds to low energy

345

environment characterized mainly by pelagic suspension with three subtypes of bimodal or

346

trimodal PSD: subtype I-a shows a major mode at 7–9 µm (fine silt) and a secondary mode at

347

20–30 µm (coarse silt), with frequencies of 3–4 and 2–3%, respectively; subtype I-b exhibits

348

two modes at 9–13 µm (fine silt) and 25–40 µm (coarse silt) with similar frequencies around

349

3%; subtype I-c shows a trimodal PSD characterized by a major mode at 9–13 µm (fine silt)

350

and two secondary modes at 20–30 µm (coarse silt) and 100–150 µm (fine sand), with

351

frequencies of 2.5–3, 1.5–2.5 and 1–2%, respectively. An increase in the content of coarse

352

silt and fine sand is probably due to an increasing influence of uniform suspension. Most

353

sediments with high VMS have a PSD corresponding to the subtype I-a.

354

Type II is associated with moderate energy environment characterized by uniform or graded

355

suspension. Two subtypes of PSD showing a major mode of fine sand but no medium or

356

coarse sand mode can be distinguished: subtype II-a exhibits a major mode at 100–150 µm

357

(fine sand) with frequencies of 5–8% and two secondary modes at 7–9 µm (fine silt) and 20–
358 30 µm (coarse silt) with frequencies around 1–2%, whereas subtype II-b shows a major mode at 130–170 µm (fine sand) with high frequencies of ca. 10–12% and secondary modes with very low frequencies (<1%) in the silt fraction. The deposits of subtypes II-a and II-b are related to uniform and graded suspension, respectively.

362 Type III is representative of high energy environment with particles transported by bottom suspension and rolling. C values are higher than 400 µm. Two subtypes of PSD have been distinguished: subtype III-a is similar to subtype II-b but with the appearance of secondary modes around 400 µm (medium sand) and 800–1000 µm (coarse sand), whereas subtype III-b shows mixed distributions with four modes around 10 µm (fine silt), 30 or 50 µm (coarse silt), 150 µm (fine sand), and 400 µm or 1100–1400 µm (medium or coarse sand), which could be indicative of complex depositional environments.

369

370 4.5. Carpology

371 Carpological data around 9 m depth, i.e. before 200 cal BCE from the age model of B1, show that the local vegetation around Bagnas Lagoon was mainly composed of aquatic (55%) and land (36%) plants (Fig. 6E). The latter indicate the presence of grassland ecosystems with ruderal species (Linum sp.). From ca. 200 cal BCE to 1400 cal CE (8.5–3.5 m depth), the vegetation was largely dominated by aquatic plants. A small increase in the abundance of seeds of plants growing in humid conditions (11%) such as Carex sp. and Typha sp. occurred around 1000 cal CE (4.4 m depth). Seeds of plants found in land or humid environments were then more abundant in the sediments deposited between 3.5 and 2 m depth, i.e. from ca. 1400 to 1750 cal CE, with maximal values of 16 and 62.5%, respectively. Land species such as Polygonum aviculare and Reseda phyteuma suggest the presence of ruderal habitats and cultivated fields at this time. A sharp decrease in the abundance of seeds of plants
382 growing in land or humid ecosystems was then recorded in sediments deposited between 2
383 and 1 m depth, i.e. during the second half of the 18th century, whereas the seed abundance
384 of aquatic species strongly increased to reach 89% at 1 m depth.

385

386

387 **5. Discussion**

388

389 **5.1. Lagoonal sedimentation and river flooding**

390 The sediments of lagoons in Languedoc can contain allochthonous and autochthonous
391 materials (Sabatier et al., 2010; Degeai et al., 2015). The former originates mainly from the
392 erosion and transport of particles of soil or sediment around the lagoon, whereas the latter
393 can be composed of organic matter, evaporite minerals (mainly calcite, gypsum, halite) and
394 biogenic calcium carbonate related to lagoonal fauna (e.g. mollusc shells, ostracod valves,
395 fish bones). Moreover, Saharan dust particles can represent a potential source of
396 allochthonous materials in the northwestern Mediterranean (Goudie and Middleton, 2001).

397 These different sources of materials could potentially explain the variability of VMS in B1.

398 Very low or negative magnetic susceptibility can be found in sediments with a high content
399 of diamagnetic material (e.g. quartz, calcium carbonate, halite, organic matter), whereas
400 higher values indicate the presence of paramagnetic, antiferromagnetic or ferrimagnetic
401 minerals (Dearing, 1999). In B1, the autochthonous component of lagoonal sediments is
402 mainly composed of diamagnetic material such as organic matter and mollusc shells, which
403 cannot have contributed to an increase in VMS. Hence, it is more likely that the relatively
404 high values of VMS in F1–F7 were due to allochthonous inputs into Bagnas Lagoon.
Concerning the allochthonous inputs of aeolian origin, observational data collected in the late 20th century show that the mean annual atmospheric mass flux in southeastern France was significantly lower than the mean annual suspended sediment flux in the Hérault River basin during flood events, which were estimated at 4 g·m⁻² (Guerzoni et al., 1999) and ca. 44 g·m⁻² (Petelet-Giraud et al., 2005), respectively. Dust concentration in modern sediments of Bagnas Lagoon should therefore not exceed 10%. Actually, dust concentration should be substantially lower than this value by also considering terrigenous inputs from the river draining the small catchment north of Bagnas Lagoon, and from dunes and beaches southeast of the lagoon (Fig. 3).

Relative changes in Saharan dust fluxes over the western Mediterranean during the last three millennia were estimated by Sabatier et al. (2020) from a proxy record of dust input based on iron content in sediments of Lake Bastani in central Corsica (Fig. 1B). The highest dust input was found in the 19th and 20th centuries. However, Sabatier et al. (2010) showed that the contribution of aeolian flux in the lagoonal sediments of Pierre Blanche Lagoon, which is located near Bagnas Lagoon (Fig. 2), was negligible (ca. 1.3%) since the mid-18th century. This suggest that dust inputs in Bagnas Lagoon should also have been negligible during the Late Holocene. Besides, quartz is the most abundant mineral in Saharan dust outbreaks over southern Europe (Guerzoni et al., 1997; Goudie and Middleton, 2001). This diamagnetic mineral cannot explain increases in VMS. Therefore, aeolian flux has probably not significantly contributed to VMS increases observed in B1 over the past two millennia. In fact, these increases were more probably caused by terrigenous inputs of allochthonous sediments around Bagnas Lagoon.

The provenance of allochthonous sediments causing VMS increases can be estimated by comparing the VMS values of B1 with the magnetic susceptibility of sedimentary sources
429 around this coastal lagoon. Samples collected in the catchment area show that the VMS of
430 terrigenous sediments from the Hérault River floodplain is substantially higher than that of
431 deposits originating from coastal dunes and beaches southeast of Bagnas Lagoon (Fig. 3),
432 with values ranging from 67 to 89 × 10⁻⁵ (SI) with a mean ± 1 standard deviation of 79 ± 8 ×
433 10⁻⁵ (SI) for the former, and from 1 to 7 × 10⁻⁵ (SI) with a mean ± 1 standard deviation of 3 ±
434 2 × 10⁻⁵ (SI) for the latter (Degeai et al., 2015). Hence, terrigenous supply from the Hérault
435 River should have been relatively low in the sediments below 4.7 m depth (excluding levels
436 F1–F4) and around 3 m depth (Fig. 6C).

437 By contrast, the sediments with higher VMS values (F1–F7) indicate that significant amounts
438 of paramagnetic, antiferromagnetic or ferrimagnetic minerals were episodically deposited in
439 Bagnas Lagoon. More specifically, paramagnetic and antiferromagnetic mineral supplies
440 from the catchment area during flood events could explain the relative increase in VMS of
441 levels F1–F7. Indeed, the sediments of the Hérault delta and lower floodplain contain
442 paramagnetic minerals such as illite, biotite, olivine and pyroxene (Pauc, 2005; Devillers et
443 al., 2019). Olivine and pyroxene minerals are supplied from Cenozoic basic volcanic rocks
444 that outcrop mainly on the western side of the Hérault River (Fig. 2). Moreover, lithic clasts
445 eroded from the Permian red shales of the Lodève Basin (Fig. 2) were observed in the
446 sediments of the lower floodplain of the Hérault River (Devillers et al., 2019). These red
447 shales contain hematite (Maillol and Evans, 1992), which is an antiferromagnetic mineral
448 with a magnetic susceptibility higher than that of paramagnetic minerals (Dearing, 1999).
449 The VMS of F1–F7 is lower than that of sediments from the Hérault floodplain (Fig. 6C),
450 probably because riverine inputs into Bagnas Lagoon was mixed with diamagnetic lagoonal
451 autochthonous material and/or with allochthonous sediments from dunes and beaches
452 southeast of the lagoon during flooding. For instance, F1 and F2 show a relatively low VMS
compared to that of F3–F7 (Fig. 6C). This could be due to the presence in F1 and F2 of a higher content of sand originating from the sandy coastal formations, which have a very low VMS (Fig. 3), whereas sedimentary layers with relatively high VMS (F3–F7) contain a lower content of sand (<10%) (Fig. 8A). Nevertheless, F1–F7 show similar PSD of subtype I-a when excluding the sand fraction (Fig. 8B). This suggests that the deposition of sediments in Bagnas Lagoon during river floods was mainly due to a process of pelagic suspension in low energy environment, which is coherent with the distal position of this lagoon east of the Hérault River floodplain (Fig. 3). Moreover, the median and 99th percentile of sediments with relatively high VMS in B1 are generally finer than those of sediments in the Hérault floodplain studied by Devillers et al. (2019) (Fig. 7), which confirms that the former should be considered as distal inputs from the Hérault River during flood events. By comparison, sediments with C-M values representative of subtype I-a PSD in the Rhône delta east of Bagnas Lagoon (Fig. 1B) were generally deposited in swampy distal floodplain influenced by flooding process (Arnaud-Fassetta, 2004).

## 5.2. Regional flood records

The mean of the calibrated age range of the lowest radiocarbon dating (362–39 cal BCE) used in the age model of B1 (Fig. 4) and the mean of the age interval of the chronological marker at 0.5 m depth (1774–1821) allows to estimate the variability of magnetic susceptibility and riverine inputs in Bagnas Lagoon between 200 cal BCE and 1800 cal CE (Fig. 9A), while historical data can be used to assess the frequency of floods of the Hérault River after 1800 cal CE (Fig. 9B). These historical data show a period of increased flooding in the lower basin of the Hérault River after 1900 cal CE (F8), in addition to the six periods of relatively high VMS identified in the sediments of Bagnas Lagoon between 200 cal BCE and
477 1800 cal CE (F2–F7) (Fig. 9A–B). The VMS record of Bagnas Lagoon and the frequency of

478 historical floods in the Hérault basin can be compared with other records of river flooding or

479 fluvial activity in coastal basins of the northwestern Mediterranean (Fig. 9).

480 The VMS record of core B1 was first compared to the 400-year flood record established by

481 Blanchemanche (2009) from documentary sources of 14 sites located in the lower basins of

482 the Orb, Hérault, Lez, Mosson, Vidourle and Vistre rivers (Fig. 9C). Stationary tests showed

483 that historical flood events in the coastal plain of Languedoc were not homogenously

484 distributed, with two periods of lower frequency of flooding in the 17th and 19th centuries

485 and two periods of increased flood frequency in the 18th and 20th centuries (Blanchemanche,

486 2009; Berger et al., 2010). Besides, the new record of historical floods based on 69 sites in

487 the river basins mentioned above shows also a lower (higher) frequency of flooding in the

488 19th (20th) century (Fig. 9D).

489 More specifically, the data of Blanchemanche (2009) show that river floods were more

490 frequent from the late 17th century to the late 18th century (ca. 1680–1785) (Fig. 9C). This

491 period is associated with higher VMS in B1 (F7), which suggests that riverine inputs into

492 Bagnas Lagoon by flooding from the Hérault River increased at this time (Fig. 9A). Otherwise,

493 the chronology of the period of lower VMS between F6 and F7 is coherent with a period of

494 low frequency of flooding in the coastal plain of Languedoc during the 17th century. Besides,

495 sedimentary archives in the Hérault basin between Cazouls-d'Hérault and Aspiran (sites 21

496 and 26 in Fig. 2, respectively) indicate that the general increase in VMS around 500 cal CE

497 was accompanied with an increase in runoff and fluvial sedimentation during the Late

498 Antiquity (Devillers and Provansal, 2003).

499 The VMS record and historical flood series of the Hérault River were then compared to flood

500 records in the lower Rhône basin (Fig. 1B), although hydrological regime and environmental
conditions in the middle and upper parts of this large catchment are different from those of Mediterranean coastal rivers. For the last millennium BCE and the first millennium CE, sedimentary archives in the Rhône delta show several periods of flood-dominated regime from 800 to 500 cal BCE, from 100 cal BCE to 75 cal CE, from 125 to 200 cal CE and from 400 to 700 cal CE (Arnaud-Fassetta et al., 2010) (Fig. 9E). F2 and F3–F4 occurred during the two latter, respectively. For the last 700 years, documentary sources and instrumental data collected in the Lower Rhône Valley revealed several periods of more frequent and severe flooding in 1450–1580, around 1700 and the late 1700s, and during most of the 20th century (Pichard et al., 2017), which were approximately synchronous with F6, F7 and F8 (Fig. 9A, E). The comparison of VMS and flood series in the Hérault basin with the intensity of fluvial activity in the Argens delta east of the Rhône River (Fig. 1B) shows a similar trend over the past two millennia. It was characterized by a general increase in flooding and fluvial activity after 500 cal CE (Fig. 9A–B, G). Peaks of fluvial activity in the Argens delta generally occurred during the periods of increased VMS (F3–F7) and flood frequency (F8) in the Hérault basin apart from the peak around 100 cal BCE, which matches the early phase of flood-dominated regime in the Rhône delta from 100 cal BCE to 75 cal CE (Fig. 9E).

Otherwise, flood series of coastal rivers from Catalonia in northeastern Spain (Fig. 1B) exhibit higher frequencies of flooding around 1600, in the late 1700s and since the late 1800s (Fig. 9H), which occurred at the end of F6, in the late part of F7 and during F8, respectively. Overall, these observations suggest that the VMS of core B1 is a good proxy record of flood variability and fluvial activity in the Hérault River basin. The increasing VMS of F1–F7 should be representative of periods characterized by increased flooding. However, F1–F7 are more likely to be representative of multiple floods on decadal to centennial timescales rather than
524 of single flood events, which are more efficiently detected from annually-resolved lake records with varved sediments (e.g. Corella et al., 2014; Amann et al., 2015).

526

527 5.3. *Impact of climate change*

528 Heavy precipitating systems at the origin of floods in coastal river basins from the
529 northwestern Mediterranean are generated by three controlling factors (Nuissier et al.,
530 2008; Duffourg and Ducrocq, 2011): (1) a relatively warmer Mediterranean Sea at the end of
531 summer and the beginning of autumn, (2) a southerly to easterly flow that transports the
532 warm and moist air masses towards the coast, and (3) an orographic barrier leading to the
533 convection of air masses, such as along the southeastern edge of the Massif Central (Fig. 1B).

534 The two first factors suggest that sea surface temperature (SST) and air temperature could
535 be important parameters in the triggering of river flooding along the northwestern
536 Mediterranean coast.

537 Correlations between flood series in Languedoc and climate records or atmospheric patterns
538 were tested for the season with the highest flood frequencies, which occurred in autumn
539 (Fig. 5). Changes in the frequency of autumn floods were very similar to those of annual
540 floods over the past three centuries (Fig. 5, 10A–E). The frequency of autumn floods in the
541 river basins of the coastal plain of Languedoc is significantly positively correlated at *p* < 0.01
542 with SST anomalies in the northwestern Mediterranean and with air temperature at
543 Montpellier since the 19th century (Fig. 10F–G, Table S3). These significant correlations show
544 that an increase in the frequency of river flooding in the coastal plain of Languedoc was
545 generally associated with warmer water off the French Mediterranean coast and with higher
546 air temperature in Languedoc at least since the 19th century.
547 Correlations between river flooding and atmospheric circulation were also tested using
548 indices from the Climate Prediction Center (CPC) of the National Oceanic and Atmospheric
549 Administration (NOAA), which are available from the year 1950 (Fig. 10H–K). The interannual
550 variability of atmospheric conditions over the North Atlantic and Europe is mainly related to
551 four teleconnection patterns identified by rotated principle component analysis of
552 instrumental data: the North Atlantic oscillation (NAO), the East Atlantic pattern (EA), the
553 East Atlantic/West Russian pattern (EAWR) and the Scandinavian pattern (SCAN) (Barnston
554 and Livezey, 1987; Craig and Allan, 2021). There is a significant positive correlation at *p* <
555 0.01 between the EA<sub>CPC</sub> index and the frequency of flooding in all river basins of the coastal
556 plain of Languedoc (Table S3), which means that increased flooding occurred preferentially
557 during the positive phase of the East Atlantic pattern over the last seven decades. The other
558 atmospheric indices are not significantly correlated at *p* < 0.01 with the frequency of flood
559 events, except for the positive phase of the Scandinavian pattern with increased flooding in
560 the Vidourle basin.

561 Overall, the significant positive correlations of flood frequencies with temperature records
562 and EA<sub>CPC</sub> index mentioned above agree with results from Josey et al. (2011), who showed
563 that higher sea surface heat flux and air temperature in the northwestern Mediterranean
564 are associated with higher positive values of the EA<sub>CPC</sub> index. More specifically, the East
565 Atlantic pattern is the only atmospheric pattern that have a significant correlation at the
566 95% confidence level with heat flux in the northwestern Mediterranean (Josey et al., 2011).
567 The East Atlantic pattern is linked to convective anomalies in the tropical Atlantic (Maidens
568 et al., 2021). The positive phase of the East Atlantic pattern is characterized by a low sea
569 level pressure anomaly centered in the eastern North Atlantic west of the British Isles (Fig.
570 1A) and by southerly to southeasterly 10 m winds over the northwestern Mediterranean
(Josey et al., 2011; Craig and Allan, 2021). This wind direction is more likely to trigger extreme rainfall events along the French Mediterranean coast (Nuissier et al., 2011), which can lead to river flooding. This is coherent with the correlation tests which show that river flood events in the coastal plain of Languedoc occurred most frequently during the positive phase of the East Atlantic pattern (Table S3).

Over the past two millennia, a composite record of Hérault River flooding was established from 200 cal BCE to 2000 cal CE by calculating z-scores of VMS of core B1 (200 cal BCE–1795 cal CE, Fig. 9A) and z-scores of the 5-year moving average of the annual number of historical floods of the Hérault River (1800–2000 cal CE, Fig. 9B), both sampled at 5-year intervals (Fig. 11A). This composite record of Hérault river flooding is significantly correlated at *p* < 0.01 (Table S3) with the proxy record of SST at KSGC-31 site in the Gulf of Lions (Fig. 1B) from Sicre et al. (2016) and Jalali et al. (2018) (Fig. 11A–B), which suggests that periods of increased flooding were preferentially associated with higher SST.

A long-term reconstruction of air temperature is not available in Languedoc over the past millennia. Hence, we have used a long-term reconstruction of temperature anomalies based on carbon isotope records from speleothems in caves in northern Spain (Martin-Chivelet et al., 2011) (Cobre, Kaite and Mayor caves, Fig. 1B). Indeed, there is a good correlation (*p* = 0.72, n = 26) between this temperature reconstruction (Fig. 11C) and the instrumental temperature record at Montpellier (Fig. 2, 11D) since the late 19th century. The flood record of the Hérault River is significantly positively correlated at *p* < 0.01 with the record of temperature anomalies in northern Spain (Table S3), which suggests that an increase in riverine inputs in Languedoc was associated with warmer air temperature. More specifically, F3–F4, F5, F6 and F7 occurred during warmer periods characterized by positive temperature anomalies around 550–700, 750–1350, 1500–1600 and 1700–1800 cal CE, respectively,
while F8 is associated with global warming since the early 20th century (Fig. 11A, C). More specifically, the long period of anomalously high flooding between 800 and 1350 cal CE (F5) coincided with the warm period of the Medieval Climate Anomaly, during which was found a tendency towards higher frequencies of high-intensity flood events in lakes from the western European Alps (Wilhelm et al., 2016).

In fact, it was assumed that an increase in the number of extreme rainfall events in the northwestern Mediterranean could be driven by the rise of moisture associated with higher temperature gradients between the Mediterranean Sea and the continent, and more specifically with increased summer temperatures (Corella et al., 2014; Wilhelm et al., 2022).

An increase in air temperature in the northwestern Mediterranean could therefore lead to more frequent extreme rainfalls, which, in turn, should cause an increase in flood frequency in the river basins of Languedoc.

### 5.4. Impact of vegetation and land use

Flooding and related runoff processes can be influenced by soil stability and sediment availability, which are partly linked to climate change through the vegetation cover (Giguet-Covex et al., 2012). Dense forests can lead to slope stabilization and limitation of erosional processes (Degeai and Pastre, 2009; Bajard et al., 2016). Forest density in Languedoc over the past millennia can be estimated using the percentage of arboreal pollen from the palynological record of Pierre Blanche Lagoon (Azuara et al., 2015) (Fig. 2, 11E). This percentage is significantly correlated at $p < 0.01$ with the flood record of the Hérault River (Table S3). This correlation suggests that forest decline would have contributed to increase river flooding in Languedoc, which would confirm the influence of vegetation on flooding. In fact, vegetation and soil cover can modulate flood frequency (Giguet-Covex et al., 2012).
619 Hence, in the case of the Hérault River, high flood frequencies could have been primarily
620 controlled by increases in large rainfall events and amplified by higher soil erodibility caused
621 by forest decline. Overall, the period of generally low (high) flooding before (after) 500 cal
622 CE was associated with dense (open) forest characterized by arboreal pollen percentages
623 higher (lower) than 70% (Fig. 11A, E).

624 Flood changes at the catchment scale can also be influenced by human land use such as
625 forest management, agricultural practices, artificial drainage and terracing (Rogger et al.,
626 2017). In southern France, forest clearings and/or fire management for the development or
627 maintenance of agro-pastoralism during the last millennia led to an increase in surface
628 runoff and soil erosion, while an intensification of grazing may have caused soil compaction
629 that reduced rainfall infiltration and enhanced surface runoff (Degeai and Pastre, 2009;
630 Giguet-Covex et al., 2012; Bajard et al., 2017; Brisset et al., 2017). Flood generation could
631 have been facilitated by this strengthening of surface runoff, which would increase the
632 sensitivity of sedimentary records to floods triggered by low intensity rainfall (Giguet-Covex
633 et al., 2012; Brisset et al., 2017). Therefore, climate-driven variability in flood frequency may
634 have been affected by land use. Potential changes in flood frequency in response to
635 increasing runoff caused by anthropogenic activities were spatially variable, because human
636 impact on landscape differed in time and intensity depending on the location (Brisset et al.,
637 2017).

638 The correlation tests between the flood record of the Hérault River and the content of
639 botanical species indicative of land use and land development in Languedoc show that river
640 flooding was significantly positively correlated at *p* < 0.01 with land plants and
641 anthropogenic pollen indicators (API) around Bagnas Lagoon and with cultivated taxa at
642 Pierre Blanche Lagoon over the past two millennia (Fig. 11F–G, Table S3).
643 The carpological data of core B1 are useful to identify land cover changes at a local scale

644 around Bagnas Lagoon. These data suggest that the environmental impact of human

645 activities around this coastal lagoon was relatively low between 200 cal BCE and 1400 cal CE,

646 then a significant increase in land vegetation and ruderal habitats occurred between 1400

647 and 1800 cal CE (Fig. 11F). Increased flooding in the 18th century (F7) was associated with the

648 maximum expansion of grasslands and cultivated fields around Bagnas Lagoon. Besides,

649 documentary sources indicate that pastoralism and forestry activities were practiced at least
650 since the late 13th century in this area (Martin, 1988).

651 In Languedoc, low concentrations of API were generally related to high percentages of

652 arboreal pollen before 300 cal CE (Fig. 11E, G). F2 occurred during this period of lower

653 impact of land use on vegetation, whereas F3 and F4 were associated with a decrease in

654 arboreal pollen percentages between 300 and 700 cal CE. This decrease was accompanied

655 with an increase in API at Bagnas Lagoon, but the concentration of cultivated taxa in the

656 Pierre Blanche Lagoon pollen record shows relatively low values (<5%). This spatial

657 difference could indicate that land use was more intense in the lower basin of the Hérault

658 River.

659 The first half of F5 between 800 and 1000 cal CE was associated with low API in both pollen

660 records. The second half of F5 (1000–1350 cal CE) and F6 (1500–1600 cal CE) were then

661 related to a low concentration of API at Bagnas Lagoon and relatively high contents of

662 cultivated taxa (10–20%) at Pierre Blanche Lagoon that suggest a more intensive land use in

663 the northern part of the Hérault basin at this time. F8 was associated with an increase in API

664 at Bagnas Lagoon and with a slight decrease in the abundance of cultivated taxa at Pierre

665 Blanche Lagoon.
666 The higher abundance of cultivated taxa at Pierre Blanche Lagoon between 1000 and 1800 cal CE is coherent with pollen data from core SETIF east of the Hérault basin (Fig. 2), which show an increase in grazing pressure, an intensification of cereal production and an expansion of grasslands as well as of chestnut and olive orchards in the Middle Ages and early modern period (Court-Picon et al., 2010). Around Bagnas Lagoon, the intensification of local land use between 1400 and 1800 cal CE began about four centuries after the sharp increase in the proportion of cultivated taxa observed at 1000 cal CE in the pollen record of Pierre Blanche Lagoon (Fig. 11F–G). The period of low frequency of flooding in the 19th century was also associated with a high percentage of cultivated taxa.

675 Otherwise, increased urbanization in coastal river basins from the northwestern Mediterranean could have led to a growing number of floods over the last decades (Barnolas and Llasat, 2007; Barrera-Escoda and Llasat, 2015; Barriendos et al., 2019). However, urbanized areas account for only 1% of the total area of the Hérault River basin and therefore should not have significantly impacted the frequency of flooding (CLE, 2005).

680 Besides, flood discharges of tributaries in the eastern part of the Hérault basin can be reduced by the dams of Salagou and Olivettes (Fig. 2), which were built in the 1960s and 1980s, respectively. Nevertheless, the impact of these two dams on flooding of the Hérault River is assumed to be negligible because they control only 4% of the catchment area (CLE, 2005).

685

686 5.5. *Climate vs. anthropogenic forcing*

687 Correlation tests suggest that climate conditions influenced flood variability in the Hérault River basin over the past two millennia (Table S3). In southeastern France, the intensification of land use was another important factor of increased flooding during the last millennia
690 (Wilhelm et al., 2012; Brisset et al., 2017). The relative importance of temperature
691 fluctuations and land cover changes due to human activities on the episodes of increased
692 flooding of the Hérault River was tentatively estimated by calculating an index based on the
difference between climate and land use records (see caption of Fig. 12 for further
693 explanation). A positive (negative) value of this index should indicate that climatic
694 (anthropogenic) factor was the dominant driver of river flooding. It shows that F2, F5 and F8
695 should have been mainly driven by climate change, whereas flooding related to F3, F4, F6
696 and F7 could have been enhanced by anthropogenic forcing (Fig. 12).
697
698 The low impact of land use on F2 is coherent with the high percentages of arboreal pollen
699 and low contents of API or land plants before 300 cal CE (Fig. 11E–G), although
700 palaeobotanical data from archeological sites between Saint-Thibéry and Pézenas (sites 15
701 and 18 in Fig. 2, respectively) show the development of vineyards and orchards during the
702 Roman period (Figueiral et al., 2010). By contrast, a dominant anthropogenic forcing for F3
703 and F4 (Fig. 12) agrees with the decrease in arboreal pollen percentages and increasing API
704 at Bagnas Lagoon between 300 and 700 cal CE (Fig. 11E, G). F5 occurred during the Medieval
705 Climate Anomaly, which was characterized by a long period of high temperature anomalies
706 in the northwestern Mediterranean between 750 and 1350 cal CE (Fig. 11C). Hence, F5 was
707 probably mainly caused by climate warming, as suggested by the forcing index (Fig. 12).
708 Negative values for F6 and F7 seem to indicate that the impact of human activities on
709 increased flooding of the Hérault River was relatively more important between 1500 and
710 1800 cal CE. In the early part of this period (ca. 1500–1600 cal CE), F5 was associated with a
711 high concentration of cultivated taxa (ca. 15%) in the Pierre Blanche Lagoon record (Fig.
712 11G). Moreover, the intensification of deforestation and land use (grazing, cultivated plants,
713 orchards) highlighted by the pollen record of core SETIF (Fig. 2) from the end of the Middle
714 ages to the beginning of the modern period was accompanied by a sharp increase in the

715 abundance of *Glomus fasciculatum* (Court-Picon et al., 2010), which is indicative of soil

716 erosion (Anderson et al., 1984; Corella et al., 2019). This suggests that a regional increase in

717 erosion rates occurred in response to intensive land use at this time, during which the

718 environmental impact of human activities seems to have been more important than during
719 the Roman period.

720 Between 1650 and 1800 cal CE, F7 was associated with different land use dynamics at local

721 and regional scales, with an increasing development of cultivated land around Bagnas

722 Lagoon and a decrease in the concentration of cultivated taxa at Pierre Blanche Lagoon,

723 respectively (Fig. 11F–G). More specifically, the maximum expansion of ruderal habitats and

724 cultivated fields around Bagnas Lagoon occurred in the 18th century, which suggests that

725 local anthropogenic forcing could have significantly contributed to increased flooding during
726 F7 (Fig. 11A, F).

727 The frequency of river floods was then relatively low in the 19th century (Fig. 11A), although

728 high percentages of cultivated taxa at Pierre Blanche Lagoon suggest an intensive land use in

729 Languedoc (Fig. 11G). The low frequency of flooding during this period could be due to lower

730 than average air temperature and SST (Fig. 11A–C), but also to the development of

731 engineering works such as embankment construction and channelization along the Hérault

732 River and its tributaries in the 19th century after the period of increased flooding in the 18th

733 century (Valarie and Coeur, 2004). Indeed, river engineering along the French

734 Mediterranean coast might lead to less frequent but more destructive flooding, as in the

735 Rhône delta (Arnaud-Fassetta, 2003) (Fig. 1B). However, the frequency of flooding has

736 significantly increased after the 19th century although embankments were constructed along
737 large sections of the lower Hérault River after the year 1907 (CLE, 2005), which was
738 particularly damaging with 9 flood events in the Hérault basin (Fig. 5, Table S1).

739 The relatively high positive values of forcing index suggest that F8 is most likely related to
740 climate warming (Fig. 12). This is in agreement with the increase in air temperature and SST
741 that occurred in the northwestern Mediterranean during the 20th century, and especially
742 since the 1950s (Fig. 11A–C). For the period 1961–2015, Ribes et al. (2019) observed an
743 increase in extreme daily rainfall events over the French Mediterranean area under a
744 warming climate, which has probably contributed to increase the flood frequency of F8. An
745 acceleration of global warming in the future could therefore lead to a dramatic
746 enhancement of the risk of flooding due to extreme rainfall in the coastal basins of the
747 northwestern Mediterranean.

748 Finally, the direct impact of local or regional land use on increased flooding in the Hérault
749 basin should have been relatively low in the 20th century. This is in line with the decrease in
750 the concentration of cultivated taxa and reforestation in Languedoc at this time (Fig. 11E, G),
751 which were probably caused by widespread land abandonment due to rural exodus (Azuara
752 et al., 2015). Nevertheless, anthropogenic forcing could have had an indirect effect on the
753 climate-driven increase in flood frequency of the Hérault River during the 20th century when
754 considering the impact of human activities on greenhouse gas emissions and global
755 warming.

756

757

758 **6. Conclusion**

759 Climate change and land use can impact river flooding in drainage basins from the
760 northwestern Mediterranean, but it is challenging to estimate the relative importance of
these two factors on flood frequencies. Databases and documentary sources were used to estimate the frequency of flood events in five river basins of the coastal plain of Languedoc in southern France over the last three centuries. In all river basins, an increase in flood frequencies during the 20th century was significantly correlated with higher air temperature and sea surface temperature in the northwestern Mediterranean. These higher frequencies of flooding were also significantly correlated with the positive phase of the East Atlantic pattern, which is characterized by southerly to southeasterly winds in the northwestern Mediterranean. The patterns of temperature and atmospheric circulation associated with more frequent flooding in Languedoc are coherent with environmental conditions and air-sea interactions that lead to extreme rainfall events along the French Mediterranean coast.
Over the past two millennia, the sedimentary archive of Bagnas Lagoon was used to assess flood variability in the Hérault River basin. Magnetic susceptibility was measured to identify episodes of higher riverine input in the lagoon, while particle size analysis was undertaken to study depositional processes. There is a good match between the intensity of magnetic susceptibility of lagoonal sediments and the flood frequency estimated from documentary sources. A proxy record of flooding in the lower basin of the Hérault River was established from the combination of magnetic susceptibility data and documentary sources. This record shows seven periods of increased flooding around 150-200, 550-600, 650-700, 800-1350, 1500-1600, 1650-1800 cal CE and after 1900 cal CE.

The comparison of the long-term record of flooding in the Hérault River basin with climate records in the northwestern Mediterranean reveals that the periods of increased flooding were also significantly correlated with an increase in air temperature and sea surface temperature over the past two millennia. More specifically, the major episode of increased flooding between ca. 800 and 1350 cal CE occurred during the warmer conditions of the
785 Medieval Climate Anomaly. Besides, the general increase in flooding after 500 cal CE was
786 contemporaneous with forest decline. The environmental impact of human activities such as
787 forest clearings for cultivation increased in Languedoc after this date. A more intensive land
788 use was likely to cause an increase in flood hazards, especially during the late Middle Ages
789 and early modern period, which were characterized by an intensification of agriculture and
790 pastoral activities that could have led to an increasing soil erosion. Moreover, carpological
791 data show an expansion of grasslands and cultivated areas around Bagnas Lagoon between
792 ca. 1400 to 1800 cal CE. It seems therefore that vegetation and land use changes impacted
793 river flooding in the Hérault River basin.

794 Future works should focus on the use of new sedimentary archives and flood records from
795 multiple sites in the northwestern Mediterranean in order to robustly estimate the influence
796 of land use on river flooding, because human impact on the environment is generally
797 characterized by heterogeneity in time and space. Besides, atmospheric patterns will need
798 to be investigated over long-time scales in order to better understand the relationship
799 between warmer climate conditions, wind circulation and increased river flooding along the
800 French Mediterranean coast over the past millennia.

801

802

803 Acknowledgments

804 This research was funded by the LABEX ARCHIMEDE (program "Investissements d'Avenir"
805 ANR-11-LABX-0032-01). Core B1, magnetic susceptibility and geochemistry were undertaken
806 by the ArcheoEnvironnement technical platform (CNRS UMR5140). Grain size analysis was
807 performed at the Geosciences Montpellier laboratory. Access to the Bagnas nature reserve
808 was authorized by the ADENA. The authors thank Dr. Dennis E. Dahms (Editor) and

809 anonymous reviewers for their constructive comments.

810

811

812

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1061 Captions of tables and figures

1062
1063 Table 1. Radiocarbon ages of B1. Percentages in brackets correspond to relative areas under probability distribution. Calibrated age ranges used in the age model of B1 are in bold.
1065

1066

1067 Fig. 1. Location of the study area. (A) Positive phase of the East Atlantic pattern for autumn

1068 (SON) as shown by sea level pressure anomalies in the North Atlantic and Europe

1069 corresponding to the third empirical orthogonal function (EOF3) calculated from sea level

1070 pressure data in Luterbacher et al. (2002) for the period 1500–1999 CE. (B) Location of the

1071 coastal plain of Languedoc (LAN.) in the northwestern Mediterranean and hydroclimate

1072 records cited in text.

1073

1074 Fig. 2. Geology of coastal river basins in Languedoc. 1: Valras-Plage, 2: Sérignan, 3:

1075 Portiragnes, 4: Sauvian, 5: Villeneuve-lès-Béziers, 6: Béziers, 7: Maraussan, 8: Lignan-sur-Orb,

1076 9: Cazouls-lès-Béziers, 10: Thézan-lès-Béziers, 11: Murviel-lès-Béziers, 12: Agde, 13: Bessan,

1077 14: Florensac, 15: Saint-Thibéry, 16: Nézignan-l' Évêque, 17: Castelnau-de-Guers, 18:

1078 Pézenas, 19: Montagnac, 20: Lézignan-la-Cèbe, 21: Cazouls-d'Hérault, 22: Usclas-d'Hérault,

1079 23: Campagnan, 24: Paulhan, 25: Bélarga, 26: Aspiran, 27: Tressan, 28: Le Pouget, 29: Canet,

1080 30: Pouzols, 31: Saint-André-de-Sangonis, 32: Gignac, 33: Aniane, 34: Palavas-les-Flots, 35:

1081 Villeneuve-lès-Maguelone, 36: Fabrègues, 37: Lattes, 38: Saint-Jean-de-Védas, 39: Lavérune,

1082 40: Montpellier, 41: Juvignac, 42: Grabels, 43: Le-Grau-du-Roi, 44: Saint-Laurent-d'Aigouze,

1083 45: Marsillargues, 46: Saturargues, 47: Gallargues-le-Montueux, 48: Villetelle, 49: Saint-

1084 Sériès, 50: Aubais, 51: Boisseron, 52: Junas, 53: Sommières, 54: Villevieille, 55: Salinelles, 56:

1085 Aigues-Mortes, 57: Le Cailar, 58: Vestric-et-Candiac, 59: Aubord, 60: Uchaud, 61: Bernis, 62:

1086 Milhaud, 63: Caissargues, 64: Bouillargues, 65: Rodilhan, 66: Nîmes, 67: Marguerittes, 68:

1087 Saint-Gervasy, 69: Bezouce. First year of use of dams reducing flood discharge available at

1088 https://www.barrages-cfbr.eu/-Midi-Pyrenees-.html.
1089
1090 Fig. 3. Maps of flooded areas in the lower basin of the Hérault River for flood events with different return periods and magnetic susceptibility of sediments around Bagnas Lagoon.
1091
1092 Magnetic susceptibility data from Degeai et al. (2015). Source of maps of flooded areas:
1093 Direction Départementale des Territoires et de la Mer de l'Hérault, Cartographie informative des surfaces inondables et des risques, TRI de Béziers / Agde (data available at https://carmen.developpement-durable.gouv.fr/115/TRI_BEZIER.map).
1094
1095
1096
1097 Fig. 4. Chronological markers of core B1 in Bagnas Lagoon. Probability distributions of radiocarbon ages in green. The part of probability distributions in dark green correspond to calibrated age ranges used in the age model of B1. Red bar represents the interval during which occurred the transition from subaqueous to subaerial environment in B1.
1098
1099
1100
1101
1102 Fig. 5. Annual flooding and percentage of floods per season in river basins of the coastal plain of Languedoc over the period 1800-2021.
1103
1104
1105 Fig. 6. Stratigraphic, chronological, sedimentological and botanical features of core B1 in Bagnas Lagoon. F1-F7: high inputs of Hérault River sediments into Bagnas Lagoon. VMS of Hérault River floodplain sediments and coastal dunes and beaches southeast of Bagnas Lagoon from Degeai et al. (2015). VMS of diamagnetic material from Dearing (1999).
1106
1107
1108
1109
1110 Fig. 7. C-M diagram and particle size distribution (PSD) of sediments from core B1 at Bagnas Lagoon. Depositional processes and maximum site of particles transported by uniform or graded suspension from Passega (1964) and Passega and Byramjee (1969). T1: pelagic
1111
1112
1113 suspension from Passega and Byramjee (1969), T2: pelagic suspension from Passega (1964).

1114 FSi: fine silt, CSi: coarse silt, FS: fine sand, MS: medium sand, CS: coarse sand. C-M values of
1115 samples of sediment in the floodplain of the Hérault River from Devillers et al. (2019).

1116
1117 Fig. 8. Mean particle size distributions (PSD) of deposits with relatively high volume magnetic
1118 susceptibility (F1–F7) in core B1 at Bagnas Lagoon. A: PSD including sand content; B: PSD
1119 excluding sand content.

1120
1121 Fig. 9. River flood records along the French Mediterranean coast over the past two millennia.
1122 F2–F8: periods of increased flooding in the lower basin of the Hérault River. Location of sites
1123 in Fig. 1B. References: (A, B, D) this study; (C) Blanchemanche (2009); (E) Arnaud-Fassetta et
1124 al. (2010); (F) HISTRPHONE database available at https://histrhone.cerege.fr; (G) Degeai et al.
1125 (2020); (H) Barrera-Escoda and Llasat (2015), Barriendos et al. (2019).

1126
1127 Fig. 10. Comparison of the frequency of autumn (SON) floods in the river basins of the
1128 coastal plain of Languedoc since 1800 with climate records and atmospheric indices. SST: sea
1129 surface temperature. Data sources: (A–E) this study; (F) Kaplan et al. (1998) and the
1130 International Research Institute (IRI) for Climate and Society of the Columbia Climate School
1131 (data available at

1132 https://iridl.ldeo.columbia.edu/SOURCES/.KAPLAN/.EXTENDED/.v2/.ssta/datafiles.html); (G)
1133 ADEME (2011) over the period 1874–2009 and Bulletin climatique mensuel sur la France
1134 (Météo France) over the period 2010–2021 (data available at

1135 https://donneespubliques.meteofrance.fr/?fond=produit&id_produit=129&id_rubrique=52);
1136 (H-K) Climate Prediction Center (CPC) (data available at

1137 https://www.cpc.ncep.noaa.gov/data/teledoc/telecontents.shtml).

1138

1139 Fig. 11. Comparison of the composite record of flooding in the lower basin of the Hérault

1140 River with temperature records in the northwestern Mediterranean and with

1141 palaeobotanical data in Languedoc. F2-F8: periods of increased flooding in the lower basin

1142 of the Hérault River. Location of sites in Fig. 1B. References: (A, F) this study; (B) Sicre et al.

1143 (2016), Jalali et al. (2018); (C) Martin-Chivelet et al. (2011); (D) ADEME (2011) over the

1144 period 1874-2009 and Bulletin climatique mensuel sur la France (Météo France) over the

1145 period 2010-2021 (data available at

1146 https://donneespubliques.meteofrance.fr/?fond=produit&id_produit=129&id_rubrique=52);

1147 (E) Azuara et al. (2015); (G) Dolez (2018) for Bagnas Lagoon and Azuara et al. (2015) for

1148 Pierre Blanche Lagoon.

1149

1150 Fig. 12. Box-plots of the dominant driver of river flooding in the Hérault basin. The forcing

1151 index was calculated by subtracting the mean of the z-scores of land plants and API at

1152 Bagnas Lagoon and of cultivated taxa at Pierre Blanche Lagoon (Fig. 11F-G) from the mean

1153 of the z-scores of SST in the Gulf of Lions and of temperature anomalies in northern Spain

1154 (Fig. 11B-C) for the time intervals of F2-F8, which correspond to periods of increased

1155 flooding in the lower basin of the Hérault River.

1156
<table><caption>Table 1 (two-column format)</caption><thead><tr><th>Laboratory code</th><th>Depth (cm)</th><th>Material</th><th><sup>14</sup>C age (BP)</th><th>Reservoir age R(t) (yr)</th><th>1 sigma calibrated age ranges</th><th>2 sigma calibrated age ranges</th></tr></thead><tbody><tr><td>Lyon-11181</td><td>60</td><td>Shell of <i>Parvicardium exiguum</i></td><td>1620 ± 30</td><td>1478 ± 53</td><td>1680–1710 (16.3%), 1719–1740 (12%), 1752–1763 (5.3%), 1799–1823 (12.9%), 1832–1892 (34.3%) and 1905–1940 (19.3%) cal CE</td><td>1667–1782 (41.3%) and 1796–1950 (58.7%) cal CE</td></tr><tr><td>Lyon-11180</td><td>75</td><td>Shell of <i>Parvicardium exiguum</i></td><td>1720 ± 40</td><td>1576 ± 59</td><td>1677–1710 (16.5%), 1719–1742 (11.5%), 1751–1765 (7.1%), 1774–1777 (1.4%), 1799–1823 (12.8%), 1832–1892 (31.7%) and 1906–1941 (18.9%) cal CE</td><td>1661–1788 (43.1%) and 1792–1950 (56.9%) cal CE</td></tr><tr><td>Lyon-11169</td><td>235</td><td>Wood debris</td><td>120 ± 30</td><td>0</td><td>1690–1712 (16.3%), 1718–1728 (7.7%), 1809–1825 (12.7%), 1831–1894 (50.7%) and 1904–1921 (12.6%) cal CE</td><td><b>1679–1741 (27.3%), 1752–1764 (2.6%)</b> and 1799–1941 (70.1%) cal CE</td></tr><tr><td>Lyon-11168</td><td>252.2</td><td>Wood debris</td><td>95 ± 30</td><td>0</td><td>1695–1724 (31%), 1812–1837 (27.8%), 1847–1849 (0.8%), 1868–1870 (2.3%) and 1878–1915 (38%) cal CE</td><td><b>1685–1732 (27.3%)</b> and 1805–1927 (72.7%) cal CE</td></tr><tr><td>Lyon-11167</td><td>265</td><td>Wood debris</td><td>170 ± 35</td><td>0</td><td>1666–1693 (20.2%), 1726–1784 (45.4%), 1795–1810 (10.6%) and 1918–1950 (23.9%) cal CE</td><td><b>1659–1708 (19%), 1720–1816 (45.9%), 1833–1891 (15.5%)</b> and 1906–1950 (19.6%) cal CE</td></tr><tr><td>Lyon-11179</td><td>275</td><td>Shell of <i>Parvicardium exiguum</i></td><td>1370 ± 35</td><td>1162 ± 49</td><td><b>1639–1694 (30.7%)</b>, 1725–1811 (50.7%), 1873–1877 (1%) and 1917–1950 (17.6%) cal CE</td><td>1524–1561 (5.1%), 1563–1572 (0.8%) and 1630–1950 (94.2%) cal CE</td></tr><tr><td>Lyon-11166</td><td>365</td><td>Charcoal</td><td>555 ± 35</td><td>0</td><td>1327–1348 (40.7%) and 1395–1420 (59.3%) cal CE</td><td>1308–1362 (47.5%) and <b>1387–1434 (52.5%)</b> cal CE</td></tr><tr><td>Lyon-11178</td><td>382.5</td><td>Shell of <i>Parvicardium exiguum</i></td><td>1350 ± 35</td><td>728 ± 49</td><td>1300–1328 (35.4%), 1335–1371 (41.7%) and 1377–1396 (22.9%) cal CE</td><td><b>1280–1417 (100%)</b> cal CE</td></tr><tr><td>Lyon-11177</td><td>480</td><td>Shell of <i>Parvicardium exiguum</i></td><td>1995 ± 30</td><td>690 ± 49</td><td>659–708 (47.9%), 714–716 (0.7%) and 721–773 (51.3%) cal CE</td><td><b>644–776 (82.3%)</b>, 784–838 (12.4%) and 841–878 (5.3%) cal CE</td></tr><tr><td>Lyon-11173</td><td>690</td><td>Shell of <i>Parvicardium exiguum</i></td><td>2240 ± 30</td><td>676 ± 49</td><td>433–469 (29.4%), 472–521 (40.3%) and 525–561 (30.3%) cal CE</td><td><b>405–607 (98.5%)</b> and 623–638 (1.5%) cal CE</td></tr><tr><td>Lyon-11172</td><td>805</td><td>Shell of <i>Parvicardium exiguum</i></td><td>2480 ± 35</td><td>662 ± 49</td><td>130–143 (6.5%), 156–193 (20%), 199–256 (44%) and 283–327 (29.4%) cal CE</td><td>81–98 (2.3%), <b>109–382 (96.9%)</b>, 387–392 (0.3%) and 396–402 (0.5%) cal CE</td></tr><tr><td>Lyon-11171</td><td>847.5</td><td>Shell of <i>Parvicardium exiguum</i></td><td>2780 ± 30</td><td>644 ± 49</td><td>346–315 (17.1%), 204–90 (69.5%) and 80–53 (13.4%) cal BCE</td><td><b>362–39 (98.5%)</b> cal BCE and 11 cal BCE–2 cal CE (1.5%)</td></tr></tbody></table>
<figure><img src="image_2.png" /><figcaption>Figure 1 (two-column format)</figcaption></figure>
<figure><img src="image_3.png" /><figcaption>Figure 2 (two-column format)</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>Figure 3 (two-column format)</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>Figure 4 (single column format)</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>Figure 5 (two-column format)</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>Figure 6 (two-column format)</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>Figure 7 (two-column format)</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>Figure 8 (single-column format)</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>Figure 9 (two-column format)</figcaption></figure>
<figure><img src="image_11.png" /><figcaption>Figure 10 (single-column format)</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>Figure 11 (single-column format)</figcaption></figure>
<figure><img src="image_13.png" /><figcaption>Figure 12 (single-column format)</figcaption></figure>