Middle to Late Holocene landscape changes and geoarchaeological implications in the marshes of the Dives estuary (NW France)

Quaternary International 216 (2010) 23–40

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Middle to Late Holocene landscape changes and geoarchaeological implications in the marshes of the Dives estuary (NW France) Laurent Lespez a, *, Martine Clet-Pellerin b, Robert Davidson c, Guillaume Hermier d, Vincent Carpentier e, Jean-Michel Cador f a

Universite´ de Caen-Basse-Normandie, Department of Geography, GEOPHEN-LETG UMR CNRS 6554, Esplanade de la Paix, BP 5186, 14032 Caen Cedex, France M2C-UMR CNRS 6143, Esplanade de la Paix, BP 5186, 14032 Caen Cedex, France Universite´ de Caen-Basse-Normandie, Department of Geography, GEOPHEN-LETG UMR CNRS 6554, Esplanade de la Paix, BP 5186, 14032 Caen Cedex, France d Universite´ de Rouen, Department of Geology, M2C-UMR CNRS 6143 – 76821 Mont-Saint-Aignan Cedex, France e INRAP Basse-Normandie-CRAHAM-FRE 3119 CNRS-Universite´ de Caen Basse-Normandie, Boulevard de l’Europe, 14540 Bourgue´bus, France f Universite´ de Caen-Basse-Normandie, Department of Geography , GEOPHEN-LETG UMR CNRS 6554, Esplanade de la Paix, BP 5186, 14032 Caen Cedex, France b c

a r t i c l e i n f o

a b s t r a c t

Article history: Available online 2 July 2009

Holocene palaeogeographic changes in the lower Dives valley, a small estuarine system in Normandy, France, have been investigated using a combination of facies interpretation from borehole cores, 14C dating and pollen analysis. The palaeoenvironmental record is broadly similar to that of other coastal and estuarine systems along the English Channel but exhibits specific features in comparison with large estuarine and coastal systems, especially as regards the narrow lower valley and its margins. The identified environments included uniform tidal (6800–5500, 2200–2000 cal. BP), freshwater fen (5500–3300 cal. BP), and drained floodplain (300–0 cal. BP). In contrast, landscape variability was high during the periods 3300–2300 and 1900–300 cal. BP in relation to gradients in environmental conditions from downstream to the upper reaches of the valley. A significant palaeogeographic change between ca. 3000 and 2200 cal. BP indicates that episodes of strong coastal hydrodynamic forcing, notably storm events, may have had dramatic consequences, with significant implications for human activities. The pollen diagrams show evidence of the opening up of woodland in order to develop pastures and agriculture on the edges of the freshwater marsh during the Bronze Age (4500–2800 cal. BP), although the settlement patterns remain unknown because of the sparse archaeological discoveries related to this period. During the Iron Age, increased land use in the valley bottom had a significant impact on the environment. From the Iron Age to the Late Roman period (2800–1500 cal. BP), the dramatic change caused by marine flooding and the large size of the tidal environment favoured the development of salt extraction sites from the coast to the inner estuary. From the Early Middle Ages, restricted tidal influences and the progressive silting of the valley bottom by overbank deposits enabled a lasting change, with the development of pastureland and the miscellaneous exploitation of the various environments. Finally, after the 17th century, the systematic reclamation and drainage of the wetland produced marked changes, inducing, in particular, a significant loss of landscape and ecological diversity. In this regard, the pattern of land use in the Middle Ages probably represents an example of sustainable landscape exploitation. Ó 2009 Elsevier Ltd and INQUA. All rights reserved.

1. Introduction Lower river valleys and estuarine environments represent a transition zone from the fluvial to the coastal domain. They are very sensitive to external forces, such as sea level rise, fluvial * Corresponding author. Tel.: þ33 0231566088; fax: þ33 0231566386. E-mail addresses: [email protected] (L. Lespez), [email protected] (M. Clet-Pellerin), [email protected] (R. Davidson), guillaume.hermier@ univ-rouen.fr (G. Hermier), [email protected] (V. Carpentier), [email protected] (J.-M. Cador). 1040-6182/$ – see front matter Ó 2009 Elsevier Ltd and INQUA. All rights reserved. doi:10.1016/j.quaint.2009.06.018

activity, and human interference, and are of economic and ecologic interest. There has been considerable research on the Holocene coastal and estuarine dynamics of the meso- and macrotidal areas of northwestern Europe, especially the coasts of the southern UK and the southern North Sea coast. These studies have identified patterns of infill in relation to sea level rise (Vos and van Heerigen, 1997; Baeteman, 1999; Allen, 2000; Beets and van der Spek, 2000; Long, 2001; Long et al., 2002; Waller and Long, 2003; Mrani-Alaoui, 2006; Massey et al., 2008), sediment compaction (Long et al., 2006a; Massey et al., 2006), and morphology and accommodation

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space (Baeteman, 2005a, b; Frouin et al., 2007b). Further studies have been carried out on lowland floodplains and on coastal mire vegetation (Brown, 1999; Waller, 1998; Waller et al., 1999). Studies on long-term environmental change specifically addressing the French coasts of the English Channel are few, and have mainly concerned the Seine estuary (Lesueur et al., 2003, Frouin et al., 2007a, b), the large tidal marshes of Mont-St-Michel (BonnotCourtois et al., 2002), and the small estuaries of the eastern English Channel (Anthony, 2000) and their adjacent dune coasts (Anthony, 2002). Among these studies, only Bonnot-Courtois et al. (2002) have attempted to reconstruct palaeogeographical evolution and environmental history in relation with the settlement patterns identified from archaeological and historical investigations. The Dives lower valley in Normandy, France (Fig. 1), provides an opportunity to analyse the complexity of the response of a small estuarine system to sea level, and climatic and anthropogenic changes during the Holocene. The aim of this research, which was conducted within the framework of a geoarchaeological programme (Carpentier et al., 2006; Lespez et al. 2008a), is to reconstruct the palaeogeographical evolution of the lower Dives valley and to define the roles of environmental and anthropogenic controls on the Holocene landscape dynamics. 2. Study area The lower Dives valley, 12 km long and 1–6 km wide, forms a coastal wetland comprising an estuary (Fig. 1). The coast experiences a mean tidal range of 5.5 m and a maximum tidal range of 7.8 m at Cabourg. A continuous dune belt 10–18 m-high borders the shoreline. Currently, the tidal influence is perceptible up to 20 km from the sea, in the Troarn-St Samson reach (Fig. 1), while the salt-freshwater mixing zone extends no more than 6.3 km from the sea (Germain, 1970). The wetland studied corresponds to the limits of the tidal influence. It has a surface of about 50 km2 within a catchment area of 1752 km2 completely confined in the sedimentary bedrock of the Paris Basin. The Dives River has an average discharge of 15 m3 s1 and its hydrologic regime is driven by the temperate oceanic climate of Normandy, characterised by seasonal wetland inundation from November to March. The bedrock underlying the lower Dives marshes consists of impermeable Callovian marls. It is bounded in the west by the steep slopes of the Pays d’Auge cuesta, whereas eastwards, the gentle bounding slopes correspond to Bathonian limestone overlying Callovian marls. In the centre of the lower valley, Robehomme hill, 33 m high, crops out as Callovian marls and divides the valley and the stream into two parts (Fig. 1). The elevation of the valley bottom is comprised between þ2.2 m NGF (mean sea level) and þ5 m NGF. The mean elevation is higher seawards (þ3 to 4 m NGF) than landwards (þ2 to 3 m NGF). In detail, the topography is flat, except along the meandering streams and the drainage ditches where natural or artificial levees are 0.5–2 m high, and downstream where small artificial mounds are 1–2 m high. The marsh has been completely drained since the 17th century to create new agricultural land (Carpentier et al., 2007). Consequently, the landscape today is mainly characterised by meadows, delimited by drainage ditches. 3. Previous work Geomorphological research in the lower Dives valley, and generally in the lower reaches of the small valleys of Normandy, has been sparse (Elhaı¨, 1963; Clet-Pellerin et al., 1977; Lespez et al., 2004, 2008a). Salimeh (1990) showed that the thickness of the Holocene sediments in the lower Dives valley was between 7 and 10 m landwards and 19–20 m seawards. Huault (1972) and Clet-

Pellerin (1981) identified interstratifications of clastic sediment and organic deposits. In the absence of radiocarbon dates, the chronological framework was deduced from pollen studies. In the lower valley, the basal formation recognised corresponds to Weichselian gravel deposits (Huault, 1972; Clet-Pellerin, 1981; Salimeh, 1990) as commonly identified in many of the valleys of Normandy (Lespez et al., 2008b). These coarse deposits are covered by grey silty clay (Unit 1: U1), attributable to the Boreal period (around 10,000– 8800 cal. BP) according to the pollen content (Huault, 1972; Clet-Pellerin, 1981). The high value of Chenopodiaceae (15–40%) indicates marine influence and a brackish environment probably fringed by a freshwater open marshland, while the valley sides were characterised by woodland dominated by pine and oak (Huault, 1972; Clet-Pellerin, 1981). These basal deposits are overlain by alternating peat beds and silty sand and sand deposits (U2). The organic deposits have a thickness comprised between 1 and 4 m and are discontinuous (Salimeh, 1990). This sequence is interpreted as a brackish marsh and estuarine channel environment fringed by freshwater marshland (Huault, 1972; Clet-Pellerin, 1981). From ca. 6000 cal. BP, data from the foregoing sources show a divergent pattern. Landwards, the development of freshwater marsh contrasts with the marine and brackish conditions that were predominant seawards. However, the data available are not accurate enough to provide a palaeogeographical reconstruction of the Middle and Late Holocene periods. Furthermore, the question of the human impact on the estuarine landscapes remained open in an area of Normandy replete with archaeological and historical data that indicate dense settlement since the middle Neolithic at ca 6700 cal. BP (Marcigny et al., 2007), as well as important changes in land exploitation during the last two millennia (Carpentier, 2007). In particular, the development of salt manufacturing was widespread from the Iron Age to the 3rd century AD (Fig. 1, Carpentier et al., 2006). From the 11th century AD, the historical archives of the Benedictine Abbey of Troarn confirm an intensive exploitation of the marshland (Carpentier, 2007). Reeds were intensively used as litter and roofing materials, as was extracted peat. The marshland was used for grazing of cattle and hay making (Carpentier, 2007). Thus, the lower Dives valley was particularly attractive during the Middle Ages, but its geographical evolution remains largely unknown. The last step of landscape development concerned the building of dikes downstream and the drainage of freshwater land. This started in the 11th–13th century AD, but after the 17th century, the techniques of diking having significantly improved and the value of pasture land having strongly increased, the reclamation of the saltmarsh became complete and numerous canals were created in order to drain the wetland (Carpentier et al., 2007). 4. Methods A systematic geomorphological survey was carried out in the valley bottom to reconstruct the depositional history of the infill. Auger holes and cores, correlated in three cross-sections (T1, T2West and East, T3), were sunk in order to understand the lateral and longitudinal pattern of the Holocene fill (Fig. 1). It was decided to investigate most of the landward reaches of the lower Dives valley, as the main objective was to understand the transition from fluvial–continental to marine sedimentation. Cross-section T1 has been used to describe the sediments at the limits of the current zone of tidal influence, and T3 at the limits of the brackish-freshwater mixing zone. T2 is an intermediate cross-section used to compare the evolution on both sides of Robehomme Hill. The crosssections were levelled to NGF (French mean sea level) using a differential global positioning system (Trimble 5700–5800). A total of 32 hand auger drillings (3–7 m deep) and seven cores (7– 10 m deep) were obtained with a percussion drilling machine. Each

L. Lespez et al. / Quaternary International 216 (2010) 23–40

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Fig. 1. Map of the lower Dives valley and locations of auger and core drillings. 1. Slope and plateau. 2. Coastal sand barrier. 3. Valley. 4. Canal network. 5. River. 6. Town and village. 7. Old Varaville road and causeway. 8. Auger (white dots) and core (black dots) drillings. 9. Core from previous studies: Huault, 1972 (large dot), Salimeh, 1990 (small dot). 10. Saltmaking sites, after Carpentier et al. 2006 (black star: La Vignerie).

core was described in the field and in the laboratory, and grain size analyses were carried out using a laser grain sizer (BeckmanCoulter, LS 200). These analyses concerned the sediments in the cores (110 samples), as well from the present sedimentary environments (14 samples) in order to characterize the general pattern of sedimentation along the lower valley. Suitable cores were used for palaeoecological analyses. Due to the generally poor preservation of the pollen in clastic deposits, the organic sediments of the Troarn (C0 and C2) and Petiville (C1) cores were selected for pollen analyses (109 samples extracted at 3–10 cm intervals). Despite the problem of preservation and of waterborne contamination, recent studies highlight the potential value of pollen data obtained in floodplain and coastal fens to reconstruct the environments of lower valley

bottoms and surrounding areas (Leroyer, 1997; Waller, 1998; Brown, 1999; Waller et al., 1999). The pollen sum was always evaluated using 300 and 500 grains. Pollen diagrams were plotted using the programmes TILIA 2.0 and TILIAGRAPH 2.0 (Grimm, 1992). As pollen diagrams constructed from peats beds situated in coastal lowland areas are prone to problems of interpretation (Waller, 1998; Waller et al., 1999), two complete pollen diagrams from cross-section T1 were used for a better understanding of the processes controlling vegetation development. One was obtained from a core located in the centre of the past marshland (Troarn-C2), and the other from its fringe close to the eastern side of the Dives valley (Troarn-C0). Diatom analysis was carried out on a silty sedimentary unit interstratified in the organic deposits in order to detect a tidal

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Table 1 Characteristics of samples from the lower Dives valley submitted for radiocarbon dating. 14

C BP (1 s)

Location

Lab. code

Core Drilling

Depth (m NGF)

Material

Measure

Dates

Troarn Troarn Troarn Troarn Troarn Troarn Troarn Troarn Troarn Troarn Troarn Petiville Petiville Petiville Petiville Petiville Petiville Petiville Basseneville Basseneville Basseneville Basseneville Basseneville Basseneville Basseneville Basseneville Varaville Dives/Mer

Erl-6457 Erl-6458 Erl-6460 Erl-6461 Erl-6462 Erl-6463 Erl-6464 Erl-7597 Erl-7598 Erl-7599 Erl-7600 Erl-7824 Erl-7825 Erl-8362 Erl-7827 Erl-7829 Erl-7831 Erl-7832 Erl-7837 Erl-7838 Erl-7836 Erl-7839 Erl-7840 Erl-7833 Erl-7834 Erl-7835 UL-2764 Erl-8785

C0 C0 C2 C2 C2 C2 C2 C5 C5 C5 C5 C1 C1 C1 C1 C2 C2 C2 C1 C1 C1 C1 C1 C3 C3 C3 C7 C4

0.45–0.40 0.86 to 0.81 1.53–1.48 0.23–0.19 0.67 to 0.62 1.71 to 1.76 2.83 to 2.87 0.71–0.69 0.34–0.32 2.09 to 2.10 4.78 to 4.83 1.46 to 1.43 0.65 to 0.63 0.58 to 0.60 1.69 to 1.71 1.77–1.75 0.97 to 0.99 1.95 to 1.97 0.11 to 0.16 2.47 to 2.53 4.08 to 4.10 5.02 to 5.04 5.78 to 5.80 1.73 1.17 to 1.19 2.42 to 2.48 0.97 to 0.91 1.58 to 1.56

Peat Peat Organic Peat Peat Wood Peat Peat Peat Peat Peat Organic Peat Peat Peat Organic Organic Organic Wood Wood Peat Peat Peat Organic Peat Organic Peat Organic

AMS AMS AMS AMS AMS AMS AMS AMS AMS AMS AMS AMS AMS AMS AMS AMS AMS AMS AMS AMS AMS AMS AMS AMS AMS AMS Conventional AMS

2219  52 3973  56 1749  52 2265  52 3190  55 4140  58 5336  62 2052  52 2268  45 4840  51 34079  468 1816  48 2541  47 4105  50 5588  58 2766  115 4383  52 5193  145 1692  51 5246  55 5530  65 6041  65 6800  84 2586  102 3048  54 5120  66 4030  90 3843  52

silt

silt

silt silt silt

silt silt silt

D13C %O

Calibrated dates (2 s)

28.6 28.7 27.0 28.8 28.4 28.7 28.1 31.5 31.3 29.3 26.3 29.0 30.9 28.9 29.7 27.6 29.2 27.8 28.0 30.8 29.5 29.5 29.2 26.6 30.1 29.1 – 29.1

2344–2117 4780–4242 1813–1547 2352–2151 3560–3270 4835–4522 6278–5950 2146–1895 2351–2153 5708–5336 – 1871–1616 2755–2468 4824–4446 6484–6288 – 5272–4845 6280–5658 1722–1419 6182–5915 6449–6206 7156–6734 7830–7508 – 3378–3079 5997–5663 4825–4259 4416–4095

Calibrated age according to Reimer et al. (2004).

origin. At least 300 diatom valves were identified in each sample. For each slide, at least 300 diatom valves were counted along transects under oil immersion at magnifications of 1000 and 1500. Diatom identification was carried out using the criteria of Germain (1981), Hustedt (1930), Peragallo and Peragallo (1897–1908), and Witkowski et al. (2000). After counting, the total surface on which the valves were enumerated was measured. This allows for the calculation of the size of the fossil population expressed as cells g1 of the dry weight of material (valves g1 dw). Taxa were organized into ecological groups according to the definitions given by Vos and de Wolf (1993). The chronology is based on the radiocarbon dating of 28 (Table 1) organic samples (peat, organic silt and wood). Twentyseven ages were obtained by the Accelerated Mass Spectrometry (AMS) method at the Physikalisches Institut of Erlangen (Germany), and the remaining one by scintillation methods at the Centre d’Etude Nordique (Laval University, Canada). The use of organic sediment from marshy deposits for dating may introduce complications due to the potential of reworking and contamination. The ages obtained show only one chronological inversion. In crosssection T2-West, the dating of a peat deposit at C2 at 1.95 to 1.97 m NGF gave an age of 5193  145 14C BP. This result is therefore younger than an upper peat deposit at 1.69 to 1.71 m NGF that gave an age of 5588  58 14C BP. However, the younger age shows an important standard deviation and may be evidence for reworking of peat deposits along a river or tidal channel, as often observed in coastal wetlands (Baeteman, 2005b). Further evidence for the reworking of organic and peat deposits is given by three dates with ages older than expected, and also with important standard deviations (T2-West, C2: 2586  102 14C BP; T2-East, C3: 2766  115 14C BP; T1-C5: 34079  468 14C BP). These ages were therefore excluded. Nevertheless, the rarity of these cases argues in favour of the consistency of the chronology of each core and of the cross-sections studied. The calibrated age ranges quoted in the text and in Table 1 (at two-sigma age range) were calculated using the Intcal04 curve (Reimer et al., 2004).

5. Results 5.1. Lithofacies and sequence organization The investigated Holocene sediments are 8–17-m thick from Troarn (landward) to Varaville (seaward). The grain size classification, based on the CM-diagram (Passega, 1957), provides evidence for five groups of sediments (Fig. 2). Eight types of facies were identified according to grain size, sedimentary structures and organic content. Their interpretation in terms of depositional environments is based on classic models (Reineck and Singh, 1980), regional studies of estuarine and coastal areas (Clet-Pellerin et al., 1977; Frouin et al., 2007a, 2007b; Billeaud et al., 2008), and comparison with modern sediments that characterize the estuarine environment of the Dives River (Fig. 2, Table 2). Three facies are characteristic of the fluvial (F1, F3) and freshwater environments

Fig. 2. CM-Image of the Holocene deposits.

L. Lespez et al. / Quaternary International 216 (2010) 23–40

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Table 2 The sedimentary facies and environments of the lower Dives valley. Facies

Description

M

C99

Grain size group

F1 F2

Coarse sand and gravel layers with pebbles lenses Fine to medium dark organic silt with plant fragments to silty peat Massive to weakly bedded fine grey silt

>500 mm <13 mm

5 cm 100 < x < 800 mm

Group 3

7 < x < 15 mm

<120 mm

Group 2

> 30 mm

120 < x < 2000 mm

Group 5

Alluvial channel Freshwater swamp with overbank sandy input Fine overbank silt in floodplain environment Tidal flat and tidal channel deposits

>30 mm

120 < x < 2000 mm

Group 4

Mudflat

<7 mm

<60 mm

Group 1

Upper part of the tidal flat schorre

15 < x < 30 mm

70 < x < 800 mm

Group 4

15 < x < 30 mm

70 < x <800 mm

Group 4

Lagoonal to swampy environment with sandy input Subtidal to fluvial environment

F3 F4

F5 F6 F7 F8

Fine to coarse grey sand deposits with shell debris (cerastoderma edule, Buccinum undatum.). Planar beddings and occasional cross-beddings can be observed. Silt to fine sand deposits, grey to blue in colour, with planar beddings. Fine silt and clay, blue in colour, with marine diatoms, plant fragments and roots Lamination of dark organic silt and well-sorted white fine sand deposit Weakly bedded medium silt to fine sand deposit

(F2). Three are interpreted as tidal and supratidal (F4, F5, F6). The interpretation of F6 as supratidal is confirmed by its diatom content (see section 5.2). The transition from a fluvial to a tidal environment is difficult to identify in view of the sedimentary characteristics of the infill in such a low energy and suspended load-dominated fluvial system. Therefore, based on the present day sedimentation, facies F8 is interpreted as lateral accretion within the meander belt and the proximal floodplain in the inner estuary. It forms a transition between fluvial and estuarine environments. The very low content in diatoms and pollen grains precludes a more accurate interpretation. The Holocene sequence is characterised by low lateral variability. Based on facies observations in the three crosssections, the sedimentary architecture was reconstructed. Eight sedimentary units were identified (U1–U8) and correlated from landward to seaward areas of the lower valley. 5.1.1. Cross-section T1 Cross-section T1 (Fig. 3) crosses the valley bottom between Troarn and Saint-Samson at the upstream limit of the tidal influence. It corresponds to the narrowest part of the valley bottom (1250 m). Presently, the Dives River is located on the right side of the valley. The centre and the left-sides have artificial channels (Canal Oursin, La Tranche´e), and a palaeochannel characterises the last left-side tributary of the Dives (Old Muance). The Holocene deposits are more than 9 m-thick. Six stratigraphic units have been identified with radiocarbon dating control. The bottom unit

Depositional environment

corresponds (U3) to weakly bedded blue-grey silts (F8), which indicate the transition from a fluvial to an estuarine environment. The estuarine environment was identified with certainty 1.5 km downstream in the Bures core (Huault, 1972). With respect to a minimum sedimentation rate of 1 mm yr1 estimated for the peat (U4), a 5-m-thick silty peat bed of (U4, F2) developed at the latest after 6800 cal. BP. It contains several layers with abundant macroscopic plant remains (mainly wood and reed). To the east, the infill corresponds to weakly bedded silty sand (U5, F8). This indicates rapid lateral facies changes across the valley. While peat was growing on one side, the channel remained on the other side. Between 4780-4242 cal. BP and 2344–2117 cal. BP at the latest, almost the entire valley bottom was characterised by peat. Only a narrow channel persisted to the east. A fine silty deposit (U6, F6) overlies these marshy floodplain deposits. It corresponds to lowenergy sedimentation by settling in two large tidal flat areas crossed by a tidal channel, and indicates the widespread return of tidal influences. The contact with the underlying unit (U4) is abrupt, and indicates erosional truncation. The three last ages (2352–2151, 2351–2153, 2344–2117 cal. BP) obtained from the upper surface of the peat deposits (U4) give a minimum age for the marine inundation. Thus, this tidal sedimentation began after 2352–2117 cal. BP. These deposits are overlain by organic silts (U7), which indicate a marshy freshwater environment influenced by the river (F2). They are dated from 2146–1895 to 1813–1547 cal. BP. The gradational contact with the underlying unit indicates a gradual

Fig. 3. Cross-section T1 (Troarn-St Samson) and lithology and facies association of core Troarn-C5.

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change in sedimentation. Finally, after 1813–1547 cal. BP, overbank silt filled the entire valley bottom (U8, F3). 5.1.2. Cross-section T2-West Cross-section T2-West (Fig. 4a) is 2 km-long and crosses the valley bottom between Petiville and Robehomme Hill, in the central part of the lower valley. Six sedimentary units have been distinguished. The bottom of the investigated deposits comprises gravel and cobbles in a greyish sandy matrix (F1) corresponding to Pleistocene deposits incised by the Holocene channels (Salimeh, 1990). The following deposition of blue-grey silt to sandy silt (U3) indicates the transition from a fluvial to an estuarine environment (F8 and F5) before 6484–6288 cal. BP. U3 is overlain, between ca. 6484–6288 and 2755–2468 cal. BP, by a thick peat bed (U4, F2), except in the east, where the channels of the Divette were active throughout this period. The peat bed contains several layers with macroscopic plant remains (mainly wood and reed) and indicates the development of a peat bog. The channel deposits (U5) are composed of laminated silty sediments, thin peat beds and sand layers. These resulted from a succession of high-energy channel deposition, suspension and settling during quiet flows, and temporary peat growth. As noted in the methods section, the thin peat beds in the Divette channel may be also be reworked peat in channel deposits. The main peat bed (U4) is overlain by a sandy silt

deposit with shell debris (U6, F5) that fills the entire valley bottom. The contact with the underlying unit (U4) is abrupt, and U4 has probably suffered erosional truncation. The age of 2755– 2468 cal. BP obtained from the upper surface of the peat (U4) indicates a minimum age for the tidal influence. From 18711616 cal. BP, organic silts (U7, F2) covered the valley. Finally, grey to brown overbank silt (U8, F3) progressively filled the valley bottom, except in some places where the sedimentation remained organic (F2). 5.1.3. Cross-section T2-East Cross-section T2-East (Fig. 4b) is 2 km-long and crosses the valley bottom between Robehomme Hill and Basseneville. Seven sedimentary units have been identifed. Core C1 gives the description of the bottom of the investigated deposits. At the base, a peat layer 0.3 m thick is observed (U2, F2). Its top is dated at 7830– 7508 cal. BP. It is overlain, before 7156–6734 cal. BP, by a silt layer (U3, F7) 0.5 m-thick. This succession indicates a transition from a freshwater marsh to a tidal environment that has also been identified in the Bures and Varaville cores (Huault, 1972; Clet-Pellerin, 1981). It is succeeded by a 1.5 m-thick peat layer (U4, F2) dated from 7156–6734 to 6449–6206 cal. BP, and shows the resumption of coastal peat bog development. From 6449 to 6206 cal. BP, this peat is overlain by sandy silt to sand deposits (U5,

Fig. 4. a. Cross-section T2-West (Petville-Robehomme) and lithology and facies association of core Petiville-C1. b. Cross-section T2-East (Robehomme-Basseneville) and lithology and facies association of core Basseneville-C1.

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F7 and F4), which occupied a large part of the valley. These deposits indicate a tidal flat crossed by an estuarine channel, identified by its coarser sediments. From 5997 to 5663 cal. BP, these sandy silts to sandy sediments are covered by a peat layer (U4, F2) at the base of Robehomme Hill. This succession indicates a restricted lateral floodplain fen whereas a large part of the valley bottom was mainly filled with silty sand and sand deposits (U5, F7 and F4). The fine deposits are weakly laminated while the coarser sand shows layers up to 15 cm-thick, but also comprises thin peat layers that are probably partly reworked. These characteristics indicate a large estuarine channel in the central part of the valley bottom. After 3378–3079 cal. BP, the valley bottom was mainly infilled with weakly to thinly laminated sandy silt deposits (U6, F4 and F6). Coarser sediments (silty sand, sand) with thin laminations (1– 2 mm) and layers up to 30 cm thick have been observed only along the present Dives channel. This indicates a narrower estuarine channel in the central part of a large estuarine tidal flat and saltmarsh environment. Finally, probably during the last hundred years, grey to brown silt associated with a floodplain environment sediment (U8, F3) has infilled the valley bottom. 5.1.4. Cross-section T3 This cross-section (Fig. 5) is located in the distal part of the lower valley, 4 km from the sea. It is 4 km-long and crosses the valley bottom between Varaville and Perriers-en-Auge. The data from the base of the Holocene fill come from previous work (Salimeh, 1990; Clet-Pellerin, 1981). A general chronological pattern is provided by the palynostratigraphy of one core and one radiocarbon age. Seven sedimentary units are distinguished. The coarse Pleistocene and the Early Holocene deposits (U1 and U2, F1) were followed by the deposition of sandy silt and silty sand (U3, F4) in a tidal environment with a main estuarine channel located in the eastern part of the cross-section. Then, silty peat (U4, F2) and intercalated sandy silt (F4, F5) were deposited in the valley. The organic deposits are 0.5–3 m-thick and are mainly comprised of strongly decomposed organic matter; however, they contain several layers with abundant reed remains. One sample obtained 1.5 m under the top of this

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deposit yielded an age of 4825–4259 cal. BP. Near the Dives and the Divette, the infill corresponds to silty sand to sand (U5), indicating channel deposits. A silty sand layer deposited in a tidal marsh (U6, F4) overlies these channel deposits. Finally, the valley bottom was covered by more or less organic silts (U7–U8, F and F2). 5.1.5. Sedimentation rates and longitudinal section The radiocarbon ages obtained from the organic sediment of the three cross-sections describing the Middle to Late Holocene infill of the Lower Dives valley are considered with regards to their depths in Fig. 6. A sedimentation rate of the infill may be estimated for cross-sections 1 and 2. In the central and inner parts of the estuary, the rate is moderate, about 2 mm yr1, during the Middle Holocene. After 5500 cal. BP, it decreases to 0.6 mm yr1, before a slight increase to 0.9 mm yr1 from around 3000 cal. BP. These results need to be considered with caution, given the strong possibilities of autocompaction of intercalated peat deposits (Massey et al., 2006; Long et al., 2006a, b). The longitudinal section is constructed in order to demonstrate the relationship between the stratigraphy in the landward and seaward parts of the valley (Fig. 7). According to the data available for the lower valley and the levelled cores, the topography of the profile shows a higher elevation (4–5 m NGF) seawards than landwards (2.2–3 m NGF). The section is located in the eastern part of the palaeovalley along the course of the Dives River. It is constructed using the available cores displaying the thickest sequence of the palaeovalley fill. It shows a stratigraphic parasequence (Cateneau et al., 2009) characteristic of the Holocene transgression, albeit with particular features. The bottom of the fill is described following previous work (Huault, 1972; Clet-Pellerin, 1981; Salimeh, 1990), while the upper part, above U3, is mainly reconstructed from the new data provided by this study. Eight Holocene sedimentary units are distinguished. The Pleistocene and Early Holocene deposits indicate the maximum regression surface. They are covered by silt to sandy silt (U3) corresponding to the transition from a fluvial to a widespread tidal environment. This unit appears particularly continuous from landwards to seawards between 6 and 5 m NGF, prior to ca. 6000 cal BP. This surface

Fig. 5. Cross-section T3: Varaville.

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Fig. 6. Age-depth model for the organic deposits of the lower Dives valley.

constitutes the maximum flooding surface and indicates the end of the transgression. Then, a silty peat bed 3 m–5 m thick (U4) was deposited in the landward part of the valley, whereas towards the sea sandy silt layers disrupt peat beds 0.5–2 m-thick. This unit constitutes the most important of the intercalated organic deposits.

It is locally incised by channels filled with silty sand and sand (U5). A widespread fine silty deposit (U6) overlies these marshy floodplain deposits and indicates a resumption of tidal influences. It corresponds to a significant event that disrupted the normal regressive succession. Landwards, it is progressively overlain by

Fig. 7. Longitudinal cross-section of the lower Dives valley.

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organic silt (U7), indicating a return to a marshy freshwater environment. Following this, overbank silt and silty sand has filled the valley bottom, thus indicating an increase in fluvial sediment supply, whereas, on the seaward side, tidal sedimentation continued to prevail before the building of the dikes. 5.2. Valley bottom environmental changes: the diatom data Diatoms analysis has been carried out on a silty sedimentary unit (U6) interstratified in the organic deposits in order to assess its tidal origin. This unit has been sampled in three cores Petiville-C1, Troarn-C0 and Troarn-C2 (Fig. 8a,b,c). The overlying organic silty layer (U7) has been sampled in Troarn-C2. The diatoms are rather scarce in samples from core C0 (Fig. 8a; counts range from 340,000 valves g1 dw in C0–24 sample to 4.5 million valves g1 dw in C0-25). This scarcity is due to a low productivity and/or to poor preservation of the frustules in the sediment. Diatoms in samples taken between 223 and 151 cm (U6) from core C2 are abundant (Fig. 8b). In this layer, the concentration varies between 7.2 and 31.4 millions valves g1 dw, in relation with a higher productivity and very good preservation. However, many frustules of benthic species bear traces of dissolution. At the top of this layer (U6, 155–145 cm), the diatom concentration decreases to 1.4 million valves g1 dw. Between 138 and 145 cm in the overlying unit (U7), the organic sediment has higher concentrations and contains 34.8 million valves g1 dw. In core PV C1 (Fig. 8c), the three examined samples (142–125 cm) are those that exhibit the highest concentrations of diatoms (from 30 to 47.8 million valves g1 dw) and the best preservation of the frustules. Sedimentation rate and productivity are high. This is due to the position of the core, in the centre of the estuary. In all the samples examined, the most abundant among benthic species are the marine/brackish epipelon. This ecological group is, certainly, the autochthonous group, and confirms the tidal influence in the inner and central parts of the estuary during the sedimentation of U6. Furthermore, the epiphytic species sporadically observed in some layers also indicate the proximity of halophilous plants, specific of saltmarshes. Nevertheless, planktonic or tychoplanktonic taxa show a high percentage of marine species (between 30 and 80%). As diatoms can be used to define the sources of suspended material (Dupont et al., 1994; Huault et al., 1994), these assemblages indicate the sedimentation of suspended particulate matter from the English Channel. The decrease in diatom concentration at the top of U6 can be attributed to the progressive filling of the estuarine channel and/or to a retreat of the shoreline. The samples from U7 and particularly Troan C2–13 sample (138– 131 cm) show dominant taxa living in freshwater. This indicates the gradual decrease in marine sediment supply and the seaward retreat of the tidal environment. However, the few recorded marine taxa show that the communication with marine water was still extant. 5.3. Valley bottom environmental changes: the pollen data 5.3.1. Pollen diagram of Troarn-C2 (Cross-section 1) The chronology of the pollen diagram of Troarn-C2 (Fig. 9) is covered by five radiocarbon ages. This is the most complete diagram in cross-section T1, and concerns vegetation changes from about 6800–1100 cal. BP. The chronology has been divided into four pollen zones. The minerogenic sediment underlying the peat layer contains few pollen grains, and precludes pollen analysis. The pollen concentration of the analyzed samples ranges from 8000 to 30,000 grains g1 of sediment. Zone Troarn-C2-1 (535–263 cm) has been divided into four sub-zones. At the base, sub-zones Troarn-C2-1a,b (535–365 cm)

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contain the highest percentage of AP (70–90%). This represents a woodland of deciduous species with Quercus accompanied by Corylus and Tilia in the dryland and with Alnus accompanied by Salix in the wetland. The sub-zones Troarn-C2-1c,d (365– 263 cm) are characterised by a slight decline in tree pollen (60– 80%). This is due to the slight drop in percentage of both Quercus and Alnus pollen types and an increase in spores of ferns and Cyperaceae in the wetland and Pteridium aquilinum in the dryland. Zone Troarn-C2-2 (263–230 cm) is characterised by a strong decline in both dryland and wetland tree pollen (10–20%). Fern spores increase considerably (70%), suggesting more open conditions and a contribution from sediments derived from riverbank erosion. In zone Troarn-C2-3 (230–140 cm), the AP percentage increases (30–40%) mainly with the dryland species (Quercus with Corylus). Alnus pollen experienced only a slight increase, while a marine/ brackish influence is indicated by an increase in Chenopodiaceae pollen (15–30%). Zone Troarn-C2-4 (140–65 cm) is characterised by a decline in dryland and wetland tree species. The spread of Cyperceae (30– 40%) and other marsh and aquatic vegetation indicate the development of freshwater fen. The presence of Cerealia and a rise in the ruderal assemblage are indications of human activities in the valley and on the slopes. 5.3.2. Pollen diagram of Troarn-C0 (Cross-section 1) The pollen diagram of Troarn-C0 (Fig. 10) has been constructed for comparison with the pollen data obtained in the centre of the valley bottom. Core C0 is located 700 m east of Troarn-C2 and is closer to the eastern side of the valley (500 m). The chronology of the pollen diagram of Troarn-C is covered by two radiocarbon ages. This diagram concerns vegetation change from 4780–4242 to ca. 1100 cal. BP and has been divided into five pollen zones. The minerogenic sediment underlying the peat layer contains few pollen grains, thus precluding pollen analysis. The pollen concentration of the analyzed samples varies from 30,000 to 150,000 grains g1 of sediment. Zone Troarn-C0-1 (300–200 cm) has been divided in two subzones. At the bottom, sub-zone Troarn-C0-1a (300–278 cm) is dominated by a high percentage of AP (70–80%). It represents woodland with deciduous species composed of Quercus in the dryland and Alnus in the wetland. Sub-zone Troarn-C0-1b (278– 200 cm) is characterised by a slight decline in tree pollen (60–70%) and an increase in fern spores and Cyperaceae pollen, thus suggesting wetter and more open conditions. The continuous presence of Cerealia and of a ruderal assemblage point to the onset of land use changes within the valley. Zone Troarn-C0-2 (200–163 cm) is marked by a decrease in tree pollen (20–30%) concerning mainly fen carr/woodland (Alnus). Furthermore, the high percentages of Cyperaceae (30–50%), of fern spores (10%), and of Poaceae (10%) indicate the development of unshaded open wetland and a rising water level. The increase in Cerealia and in the ruderal assemblage indicates increasing human activity along the fringes of the marshland and the valley sides. Zone Troarn-C0-3 (163–127 cm) is characterised by a slight increase in percentage of AP (35–45%). This is mainly due to a rise in tree species of dryland areas. Cyperaceae pollen (20 %) experienced a slight decrease, while the rise in saltmarsh communities (10–15%) indicates a marine/brackish influence. Zone Troarn-C0-4 (127–89 cm) has been divided into two subzones. Sub-zone 4a is dominated by herb pollen. Chenopodiaceae decline while the rise in Cyperaceae (30–50%) and other marsh and aquatic vegetation indicates the development of freshwater fen. The presence of Cerealia and an increase in the ruderal assemblage

Fig. 8. a. Simplified percentage diatom diagram of Troarn-C0 with taxa grouped according to their ecological affinities. b. Simplified percentage diatom diagram of Troarn-C2 with taxa grouped according to their ecological affinities. c. Simplified percentage diatom diagram of Petiville-C1 with taxa grouped according to their ecological affinities.

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Fig. 9. Percentage pollen diagram of Troarn-C2.

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indicate human exploitation of the valley floor and slopes. The presence of Plantago L. and high values of Poceae (20%) suggest the development of meadows.

Fig. 10. Percentage pollen diagram of Troarn-C0.

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5.3.3. Pollen diagram of Petiville-C1 (Cross-section T2-West) The pollen diagram of Petiville-C1 (Fig. 11) has been obtained from samples of core Petiville-C1 located in the centre of crosssection 2b. The chronology of the pollen diagram is given by four radiocarbon ages. It covers the vegetation change from 6484–6288 to ca. 1000 cal. BP., and has been divided into five pollen zones. The minerogenic sediment underlying the peat layer contains few pollen grains, and, therefore, no pollen analysis was carried out. The pollen concentration of the analyzed samples varies from 10,000 to 80,000 grains g1 of sediment (Fig. 11). Zone Petiville-C1-1 (400–350 cm) is dominated by a high percentage of AP (70–85%). It represents woodland with deciduous species composed of Quercus accompanied by Corylus, Tilia and Ulmus in the dryland and Alnus in the wetland. Zone Petiville-C1-2 (350–250 cm) has been divided into three sub-zones. Sub-zone Petiville-C1-1a (350–300 cm) contains the lowest percentage of AP (30%). The drop in tree species is due to the slight decrease in percentage of Quercus and Tilia and mainly to the decline in Alnus. This last observation is in agreement with an increase in Cyperaceae (20–30%), and Poaceae (10%), indicating the development of unshaded open wetland. In contrast, sub-zones Petiville-C2-1b,d (300–250 cm) are characterised by two successive stages of strong increase in Alnus, and decline in Cyperaceae (10%) and Poaceae (5%), thus indicating the temporary return to alder carr on the valley floor. Zone Petiville-C1-3 (250–140 cm) is dominated by Cyperaceae (30–60%), and Poaceae (20%) indicating the development of unshaded open wetland. The decline in tree pollen (30%) is attributable to a decline in Alnus (10%), and dryland species (10%). The presence of Cerealia and a rise in the ruderal assemblage are indicative of human activities in the valley. Zone Petiville-C1-4 (140–90 cm) is characterised by a slight decline in Alnus (5%) and a strong decrease in Cyperaceae pollen (15–20%), while a spread of fern spores (30%) is observed. These results indicate drier conditions in a valley floor still characterised by open landscapes. The presence of Cerealia and of a ruderal assemblage confirms the development of cultivated areas in the valley and on the valley slopes. Zone Petiville-C1-5 (90–40 cm) is characterised by a decline in both dryland and wetland tree species (20 to 10%). The spread of Cyperaceae (30 %) and other marsh and aquatic vegetation indicates the development of freshwater fen, and a rise in Chenopodiaceae (10%) the proximity of saltmarsh. Judging by the rise in Cerealia, in Plantago and in the ruderal assemblage, cultivation was certainly practised on the valley sides and along the marshland fringes.

6. Discussion 6.1. The palaeogeographic evolution of the lower Dives valley The cross and longitudinal sections show a highly variable Holocene sedimentary infill. The depositional history reveals a palaeogeographic evolution of the lower valley that may be divided into four major periods. These periods are not established on the basis of a rigid chronology because accurate radiocarbon age control is still missing for some periods and boundaries. Nevertheless, a schematic sequence of evolutionary stages of the lower Dives valley focused on six short and well-defined chronological windows may be established (Fig. 12).

Fig. 11. Percentage pollen diagram of Petiville-C1.

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6.1.1. Before ca. 6000 cal. BP Data from previous work provided evidence for a Pleistocene palaeovalley deeply encased in the Mesozoic bedrock (Salimeh,1990). The top of the Pleistocene deposits is comprised between 5 m NGF landwards down to 14 m NGF seaward. This surface constitutes the maximum regression surface in the Dives estuary, the silt to sandy silt (U3) corresponding to the transition from a fluvial to a widespread tidal environment. The absence of underlying basal peat precludes accurate dating of the initial marine inundation. The range of elevations obtained for the initial flooding of the valley, 14 to 11 m NGF (Huault, 1972; Clet-Pellerin, 1981), is lower than that observed on the continental shelf, at around 10 m NGF (Garnaud et al., 2003). Furthermore, the estimated ages for the palaeovalley are younger (Huault, 1972; Clet-Pellerin, 1981) than the age obtained on the continental shelf, which is ca. 7100–6500 cal. BP (Garnaud et al., 2003). This shows that the rising sea first invaded the area via the deeply incised palaeovalley, while the adjacent continental shelf experienced the onset of the marine invasion later. From ca. 8800– 6000 cal. BP, the U2 deposits indicate a brackish marsh and estuarine channel environment fringed by freshwater marshland. This may indicate shoreface stabilisation and in situ filling of the estuary. The last phase of this period (U3) is better known through the new research undertaken in the valley. Only cross-section T1 exhibits a persisting lateral peat bed, and the tidal environment appears particularly continuous between 6 and 5 m NGF, which corresponds to a time prior to ca. 6000 cal. BP (fig. 12a). 6.1.2. Ca. 6000–3300 cal. BP The initiation of peat formation started ca. 6800–6000 cal. BP in cross-sections T1 and T2. After ca. 6000–5500 cal. BP, peat accumulation (U4) became predominant on the valley floor. Between cross-sections T1 and T2, peat accumulation (1.3–4 m thick) largely occurred within fen carr/woodland dominated by alder. Locally, the development of unshaded open wetland in relation with channel shifting or water table rise introduced landscape diversity. This was accompanied by the narrowing of the main estuarine channels and their associated tidal environment. In the seaward part of the lower valley (cross-section T3), the deposits indicate a notable development of freshwater marshland on the fringes of a reduced brackish marsh and tidal channel environment. The rates of peat accumulation varied from 1.1 mm yr1 to 2 mm yr1. This corresponds to the range of valley organic sedimentation rates observed in Normandy for the Middle Holocene (Lespez et al., 2004, 2008a, b), but the rates are higher (1.6–2 mm yr1) in the thinnest sequence (<2 m), and lower (1.1 mm yr1) in the thickest landward sequence (>2.5 m). This difference in accumulation rates may be explained by peat compaction, which can result in an estimated reduction in thickness of approximately 50% for this kind of peat bed (Long et al., 2006a, b). Such compaction may generate an increase in subsidence of the landward areas where the peat bed sequences are thickest, and may explain the lowest elevations recorded in the inner part of the lower valley. The base of the peat occurs between 5 and 2 m NGF landwards (cross-section T1) and 2 and 1 m in the central and seaward parts of the lower valley (cross-sections T2–T3). The elevation of the most widespread zones of peat development is between 1.5 and þ1 m NGF and corresponds to a time slice between 4500 and 3300 cal. BP. Consequently, together with the Middle and Late Holocene period, the freshwater marsh environment was most extensive during this chronological window (Fig. 12b). 6.1.3. 3300 to 1900–1600 cal. BP This period is characterised by a dramatic change in the lower valley landscape. The diatom and pollen diagrams provide evidence for significant landward migration of the tidal influence

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Fig. 12. Schematic evolutionary stages of the lower Dives valley during the Holocene focused on six chronological windows.

in the inner part of the estuary, up to 20 km from the sea, and the geomorphological data attest to a considerable enlargement of the estuary. It is difficult to establish the timing of this change. There is no good relationship between the age and the altitude of the upper peat surface, which occurs between 1.1 m and þ 1.5 m NGF. A chronological control is provided by the dates obtained from the upper surface of the peat, which give a minimum age for the marine inundation. Tidal sedimentation began after 3378–3079 cal. BP in cross-section T2-East, at about 2755–2468 cal. BP in cross-section T2-West, and after 2353– 2117 cal. BP in cross-section T1 (Fig. 12c and 13). Given their abrupt contact with the transgressive unit (U5), the peat beds probably suffered erosional truncation. This is confirmed in cross-section T2-East by the low elevation of these beds (-1.1 m NGF) and by the thickness of the overlying deposits (2.5–3 m), whereas the truncation appears more subtle in cross-section T1 and T2-West. The surface of the peat bed shows the same range of elevations (þ0.2 to 0.6 m NGF). The thickness of overlying sediments is comparable

Fig. 13. The radiocarbon ages from directly underlying apparently marine/brackish sediments (in black) and from overlying sediments (in grey) in cross-sections T1 and T2-West (calibration according to Reimer et al., 2004).

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(around 1.5 m) and the three radiocarbon ages obtained are very similar (Fig. 13). In cross-section T1, the pollen diagrams show the abrupt demise of freshwater communities (Zones C2-2 and C0-2) followed by saltmarsh communities (Zones C2–3 and C0–3). This calls into question the presence of transitional communities, which constitute a key step in assessing vegetation successions associated with gradual inundation (Waller et al., 2006). The abrupt increases in fern spores on C2 diagrams suggest more open conditions, and a contribution from sediments derived from riverbank erosion. The contemporaneous development of unshaded open wetland hinged on a rising water level on the C0 diagrams is more convincing. These results reveal the presence of transitional communities. However, the absence of reedswamp communities following the rise in water level does not allow for a firm conclusion of gradational sedimentation (Waller et al., 2006). On the landward side, from cross-section T1 to T2-West, renewed organic sedimentation after tidal deposition indicates that the tidal effect did not permanently change the environmental conditions of the freshwater swamp. The peat and organic deposits started from 2146–1895 cal. BP in cross-section T1 and 1871–1616 cal. BP in cross-section T2-West (fig. 13). In summary, the data suggest a rapid change resulting in flooding of the whole valley as a result of enlargement of a backbarrier estuary. The migration of the tidal system landwards concerned the entire lower valley during a short chronological window, probably around 2200 to 2100 cal. BP (2nd–1st century BC; Fig. 12d). This change was probably longer in cross-section T2, where Robehomme Hill remained an island surrounded by tidal marshes from around 2755 to 1616 cal. BP. This change was a lasting one in the seaward areas and along the Dives River. 6.1.4. 1600–1000 cal. BP During the Late Roman and the Middle Ages, landscape development in the landward side of the lower Dives valley differed from that of the seaward side. In the landward part of the valley, organic silt is overlain by minerogenic sediments, and diatom data indicate the return of freshwater fen. The tidal sedimentation stopped completely after 2146–1895 cal. BP in cross-section T1 and after 1871–1616 cal. BP in cross-section T2-West. The decrease in marine inputs and the retreat of the tidal environment were gradual, according to the diatoms, the pollen, and the sedimentological observations. After 1813–1547 cal. BP, freshwater sedimentation ended in cross-section T1 and the size of floodplain swamps significantly decreased. The upper part of valley bottom experienced silt sedimentation mainly composed of overbank deposits. Towards the sea and along the Dives river channel, the marine influence still prevailed during a large part of the medieval period. The pollen diagram of Petiville indicates the proximity of saltmarsh until around the 10th century AD within the context of a linear rate of sedimentation. Cross-section T2-East shows the persistence of a large river channel influenced by the tide, while cross-section T3 shows the predominance of salt marsh and tidal environments (Fig. 12e). Protected by the coastal barrier, these environments remained more or less unchanged until diking started in the 11– 13th century AD (Carpentier et al., 2007). 6.2. Environmental adjustments to sea level and climatic control, and comparison with other coastal systems in the English Channel A comparison of the sedimentological, pollen and diatom data enable a discussion of the changes caused by fluvial activity, relative sea level and sediment budgets. The early Holocene palaeovalley inundation has been recorded with the same range of elevations and chronology in two major estuarine systems in Normandy: the Seine estuary (Huault, 1985; Lesueur et al., 2003; Frouin

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et al., 2007a) and the Mont-St-Michel Bay (Morzadec-Kerfoun, 2002). It is related to the rapid relative sea level (RSL) rise of 7– 5 mm yr1 during the Holocene transgression in the English Channel and southern North Sea (Baeteman and Declercq, 2002; Shennan and Horton, 2002). From 8800 to 6000 cal. BP, freshwater sedimentation prevailed locally, essentially for a short period within the tidal and saltmarsh environment. This change has been recorded along the southern part of the North Sea (Baeteman and Declercq, 2002) and in several areas along the southern English coast (Waller and Kirkby, 2002). It is related to a decline in the RSL rate, which dropped to 2–4 mm yr1 after about 7500 cal. BP (Waller et al., 1999; Baeteman, 2005a). Consequently, the rapid landward shift of the tidal environment ceased, and coastal barriers fronting estuaries and tidal basins became more or less stable (Baeteman and Declercq, 2002). The narrow Dives valley offered restricted accommodation space and this was particularly favourable to the early development of a coastal barrier. The sediment supply was essentially related to the marine transgression because the rivers of Normandy during this period supplied predominantly dissolved loads (Lespez et al., 2008a, b). From ca. 6000 cal. BP to ca. 3000 cal. BP, the replacement of a tidal environment by extensive freshwater swamp is in agreement with the research results obtained at a regional scale (Elhaı¨, 1963; Huault, 1972; Clet-Pellerin et al., 1977; Morzadec-Kerfoun, 2002; Lespez et al., 2004) and along the English Channel coasts. This period corresponded to one of widespread peat growth in the coastal plains bordering the English Channel (Baeteman, 1999; Waller and Long, 2003). A further slowdown in the rate of RSL rise (0.7–1 mm yr1) led to shoreface accretion and further stabilisation of the coastal barrier, particularly in the context of the narrow Dives estuary. Autocompaction probably increased the accommodation space that subsequently enabled thick accumulation of peat in the backswamps (Waller and Long, 2003). From ca. 3300 to 1600 cal. BP, the valley experienced a major marine inundation. Even if the timing is not identical, late Holocene marine flooding is widespread throughout southern England (Waller and Long, 2003), along the southern English Channel (Morzadec-Kerfoun, 2002) and in the North Sea coastal plains (Baeteman, 1999; 2005a; Mrani-Alaoui and Anthony, 2005; Mrani-Alaoui, 2006). The role of the RSL rise has been discussed (Waller et al., 1999; Morzadec-Kerfoun, 2002); however, RSL experienced a slow-down after 5500–5000 cal. BP (Baeteman and Declercq, 2002; Waller and Long, 2003). Baeteman (2008) showed, furthermore, that the rise and fall of RSL during the Late Holocene did not play a significant role in changes in the Belgian coastal plain. The onset of tidal migration landwards could have been the result of an imbalance in sediment budget compensation of the RSL rise (Baeteman, 1999) or the direct consequence of a breaching of the coastal barrier related to storm events (Waller et al., 2006; Billeaud et al., 2008). At the local and regional scales, there is evidence for strong hydrodynamic activities during this period. In the outer Seine estuary, one of the strongest hydrodynamic events of the Middle-Late Holocene period is dated at ca. 2700 cal BP (Sorrel et al., 2009). A similar event has been evoked to explain the dismantling of coastal barriers and tidal flats in the Mont-SaintMichel Bay at about 3000 cal. BP (Billeaud et al., 2008). Furthermore, this event corresponds to a pronounced climatic disruption recorded in northwest Europe (Magny, 1992; Van Geel et al., 1996; Barber et al., 2003) and experienced by numerous fluvial systems (Lewin et al., 2005; Pastre et al., 2001, 2005), including those of Normandy (Sebag, 2002; Lespez et al., 2008b). An increase in runoff in the hinterland and in energy of the fluvial system is attested by previous work in the river basin (Germain-Valle´e and Lespez, 2006) and in the neighbouring areas (Lespez et al., 2008b). These conditions probably accelerated the erosion of the estuarine channels as suggested for the Belgian coastal plain (Baeteman,

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2008). Therefore, the onset of the inundation of the lower Dives valley may have been related to high fluvial activity, shoreface erosion, coastal barrier breaching and/or long-lasting overtopping probably related to a period of enhanced storms around 3000– 2500 cal. BP. The landward migration of tidal channels and flats eroding the freshwater swamp was promoted by the autocompaction of peat, which resulted in surface lowering, and provided new accommodation space. This explains the dramatic geographical consequences of the increase in tidal influence. Since 2100–1600 cal. BP, renewed shoreface and coastal barrier stabilisation explain the progressive return of freshwater marshland in the landward areas. This was also promoted by the direct and indirect consequences of human activities. 6.3. Past human responses to paleoenvironmental changes and human impact on environment During the Middle Holocene (ca. 6800–4000 cal. BP), the environment remained completely wooded (Fig. 12a, b). The valley floor was characterised by fen carr/woodland dominated by alder, whereas the valley sides had mixed deciduous forest with hazel, oak and elm. Despite the importance of Middle and Late Neolithic settlement (ca. 6700–3000 cal. BP) in the neighbouring areas of the Plain of Caen (Marcigny et al., 2007), located only a few kilometres westwards, there is no evidence of human activities and associated landscape change during that period. The record in the lower Dives valley differs from that of many northwestern European wetlands where there is evidence for settlement and exploitation since the Neolithic period (Coles and Hall, 1998), including, at the regional scale, the Mont-Saint-Michel Bay (Lapporte et al., 2003) and the Seine valley (Giligny, 2005). The Dives valley pollen diagrams give very different impressions of the nature and intensity of human activity during the Bronze Age (4500–2800 cal. BP), and this is of particular archaeological significance. The pollen diagram of Troarn-C0 (Zone C0–1b) and PetivilleC1 (Zone C1–3) show evidence for the opening up of woodland in order to develop pastures and agricultural land after, respectively, 4844–4446 and 4835–4522 cal. BP. The pollen records testify to land use in the valley since the Bronze Age. Nevertheless, the persistence of the woodland or fen carr is well indicated by the pollen diagram of Troarn-C2, from the centre of the freshwater marsh. According to the pollen data, land use increased on the slopes and probably in a few small areas within the valley floor (Fig. 12c). The first occurrence of human exploitation appears consistent with the archaeological data available for the lower valley, which indicate the first settlement on the edge of the marshland (Carpentier, 2007), and more generally with the records of the main coastal marsh of southern England (Coles and Hall, 1998; Long et al., 2002). From the Iron Age to the Late Roman period (2800–1500 cal. BP), the dramatic change caused by marine inundation has had significant consequences for the development of human activities. The large size of the tidal environment explains the development of a salt manufacturing site from the coast up to cross-section T3 from the 5th century BC to the 3rd century AD (Fig. 1, Carpentier et al., 2006). This site probably comprised a multitude of seasonal workshops installed in the saltmarsh close to seawater as in other areas of Normandy (Bizien-Jaglin, 2002). They can be described on the basis of one archaeological excavation near the mouth of Dives River. The site of La Vignerie (Carpentier et al., 2006) is probably made up of such seasonal workshops within the salt marsh. The anthropogenic deposits are characterised by large fired clay residue resulting from the production of salt, because the technology consisted in treating salt brine in furnaces in order to evaporate the water and concentrate the salt. This technology implies widespread

exploitation of the sandy silt of the tidal flat (Carpentier, 2007). In the lower Dives valley, they are associated with natural or artificial mounds (locally named ‘‘Hogue’’), as in many Northwestern European coastal wetlands (Rippon, 2000; Behre, 2004; Meier, 2004; Vos and Gerrets, 2005). Since that time, the archaeozoological and pollen data available from La Vignerie testify to the grazing of cattle in salt meadows (Carpentier et al., 2006). Landward, the three pollen diagrams simultaneously show cereal cultivation and the development of ruderal communities ca. 2800–2350 cal. BP. These are related to human exploitation of the fringe of the saltmarsh, tidal environment, and valley sides. The farming of the valley slopes and the adjacent plateau was probably complete, as attested by the pollen diagrams of the valley, and by data from other sites in Normandy (Clet-Pellerin and Verron, 2004; Lespez et al., 2004, 2005). This is consistent with archaeological evidence identifying numerous domestic settlements from the Iron Age to the Roman periods (Carpentier, 2007). For the first time, humans may have played a predominant role in landscape development, and may have had significant environmental impact. Locally, the inwash of minerogenic material across the marshland is testified at La Vignerie (Dives/Mer) between the 5th and the 4th century BC (Lespez in Carpentier et al., 2006). More generally, the vertical accretion due to overbank sedimentation in the freshwater marshland is related to accelerated soil erosion and production of fine sediments resulting from anthropogenic influences. This change commenced in the Iron Age in Normandy (Lespez et al., 2008b), and has been widely observed in the Paris basin (Pastre et al., 2001) and more generally in northwest Europe. This is expressed later in the lower Dives valley because of the marine inundation during this period, and explains the rise in the rate of sedimentation observed over the last millennia (Fig. 6). Despite the lack of historical archives and archaeological data during the Late Roman and Early Middle Ages (5th–9th century AD), the palaeoenvironmental data highlight the persistence of human exploitation of the valley and its renewed freshwater marshland as in many European wetlands (Rippon, 2000). In the landward part of the valley, the rise in Cerealia, Poaceae and in the ruderal assemblage is an indication of cultivation and pastureland. Furthermore, the development of meadows within the freshwater swamp resulted in the progressive silting of the valley floor. During the Middle Medieval period (10–12th century AD), the historical archives of the Benedictine Abbey of Troarn confirm an intensive exploitation of the marshland (Carpentier, 2007), which was particularly dynamic and attractive because of its diversity and the rich natural resources it provided. The reeds were intensively exploited and the peat extracted in the landward part of the estuary, in relation with the persistence of the freshwater marshland, while the silted up areas of the valley floor were used for grazing of cattle from March to November according to the timing of fluvial inundation (Carpentier, 2007). On the seaward side, the landscape exhibited marked resilience, as attested by the persistence of the salt-making sites until the 12th century AD. Therefore, the inherited landscape lastingly depicts the geographical setting of human activities during the Middle Ages. The onset of significant environmental change is dated from the 11th and 13th centuries AD, and is related to the building of the dikes and causeways. This constitutes the first step in the reclamation of the tidal marsh and the complete draining of the freshwater wetlands (Fig. 12f). The lower Dives valley experienced a pattern of landscape development during the Late Holocene that is similar to that of numerous coastal and estuarine areas of western Europe, even though the timing of the changes appears late, particularly in comparison with the major coastal marshes of the English Channel and the North Sea (Rippon, 2000; Meier, 2004; Behre, 2004).

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7. Conclusions Small estuarine systems appear simple to understand. However, research has demonstrated spatial and temporal complexities for such coastal systems. The investigation of the lower Dives valley gives, for the first time, a chronological pattern of Holocene infill of a small estuarine system in western France. The lower Dives valley record is broadly similar to those of other coastal and estuarine systems bordering the English Channel, but presents particular features in comparison with large estuarine and coastal systems, especially with regards to the sensitivity of the marginal and narrow lower valley. During some periods, it experienced a uniform predominance of tidal conditions (6800–5500, 2200–2000 cal. BP), freshwater fen (5500–3300 cal. BP) or drained floodplain (300– 0 cal. BP). In contrast, landscape variability was high during 3300– 2300 and 1900–300 cal. BP, hinged on a longitudinal gradient in environmental conditions from downstream to upstream. The significant palaeogeographic change recorded between ca. 3000 and 2200 cal. BP indicates that episodes of strong coastal hydrodynamic activity may have had dramatic consequences. Nevertheless, it is also worth noting that some other strong events registered in the Seine estuary (Sorrel et al., 2009) have had limited impact on the lower Dives. Therefore, climate appears only as one among the many controlling factors in such a coastal system. From the Early Middle Ages, human control appears predominant, and the Dives valley landscapes were progressively modified by human activities. The lower Dives valley experienced a pattern of landscape development similar to that of numerous coastal and estuarine areas in northwestern Europe (Rippon, 2000; Meier, 2004; Behre, 2004). After the 17th century, the systematic reclamation and drainage of the wetland produced dramatic change, notably a significant loss in landscape and ecological diversity in this small estuary. The high diversity associated with the environment during the Middle Ages constitutes a potential alternative reference for sustainable landscape development. Finally, a more accurate chronology of the changes and a better understanding of the landscape changes on the seaward side of the valley are needed. Future research will focus on shoreline changes in order to understand the resilience of the coastal barrier (Long et al., 2006b), especially with regard to severe storms (Clarke et al., 2002). Acknowledgments This work was supported by a Research Programme funded by the Fond National pour la Science (ACI ‘‘Jeunes chercheurs’’ n 6079) and a ‘‘Prospection the´matique’’ from the Archaeological Services of Calvados (CG14) and Basse-Normandie (SRA). We are grateful to F. Fichet de Clairefontaine, F. Delacampagne and N. Coulthard. Many thanks to the reviewers and their constructive criticism of the manuscript, which have significantly improved the paper and to J. Brown and Norm Catto for their help with the English. References Allen, J.R.L., 2000. Morphodynamics of Holocene salt marshes: a review sketch from the Atlantic and Southern North Sea coasts of Europe. Quaternary Science Reviews 19, 1155–1231. Anthony, E.J., 2000. Marine sand supply and Holocene coastal sedimentation in northern France between the Seine estuary and Belgium. In: Special Publications of the Geological Society of London, vol. 175, pp. 87–97. Anthony, E.J., 2002. Long-term marine bedload segregation and sandy versus gravelly Holocene shorelines in the eastern English Channel. Marine Geology 187, 221–234. Baeteman, C., 1999. The Holocene depositional history of the Ijzer paleo-valley (western Belgian coastal plain) with reference to the factors controlling the formation of intercalated peat beds. Geologica Belgica 2 (3–4), 39–72.

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