1During the Quaternary period, fluvial systems of NW Europe, such as the Rhine River, Thames River or Somme River, evolved into incised valley systems, characterized by fluvial terraces, as a response to variations of base level related to climate oscillations, sea- level fluctuations and tectonic movements (Blum & Tornqvist, 2000; Antoine et al., 2000, 2007; Bridgland, 2000). After the Last Glacial Maximum, estuaries formed in these incised fluvial systems as a response to climate warming and induced sea-level rise (Long et al., 1998; Allen, 2000).
2The currently macrotidal estuary of the Seine River illustrates this Quaternary evolution (Lefebvre et al., 1974; Huault et al., 1975; Lautridou et al., 1999; Antoine et al.,2007). The Holocene alluvial deposits of the estuary have largely been studied since the 1940’s (e.g. Dubois & Dubois, 1943; Porcher, 1977; Sebag, 2002). There have been only a few attempts to summarize the Holocene evolution of the prism, and these generally emphasize only part of the system: the marine estuary (Lautridou et al., 1999), the middle estuary (Lefebvre et al., 1974; Delsinne, 2005) or the fluvial estuary (Sebag, 2002).
3We suggest here a synthetic Holocene evolution of the entire system, based on summarised evolutions of five sites distributed across the estuary: SE part of the Seine Bay, Seine outlet, Marais Vernier, Heurteauville and Anneville (fig. 1). We have, therefore, reviewed all available data for each site in order to define some units of evolution. We then compared each site to the others, to obtain a broader view of the system evolution and the related forcing factors.
Fig. 1: Presentation of the Seine estuary (DEM modified after Van Vliet‑Lanoë, unpublished data), also called the Lower Seine Valley, and the studied sites.
The figure to the upper right represents the extension of the Seine palaeochannel in the Seine Bay and its connection with the palaeochannel of The Channel (modified after Antoine et al., 2007).
4The Seine River, one of the main French rivers, drains the northern part of the Paris Basin. It ends as a macrotidal estuary (7,5 m during spring tide; Avoine, 1981), where water flows into the Channel between Le Havre and Honfleur (fig. 1).
5The macrotidal estuary, also named Lower Seine Valley, has a temperate climate, with a mean annual temperature of between 10 and 11ºC and a mean annual rainfall of between 700 and 1100 mm.
6Quaternary climate oscillations, induced sea-level variations and the tectonic uplift of the Paris Basin have modified the Seine River grade. As a response, the river channel has laterally migrated and the river has vertically incised into the chalk substratum (Lefebvre et al., 1994; Lautridou et al., 1999; Antoine et al., 2007). The Quaternary evolution of the Seine River is linked to that of the Channel River, as the Seine River was a tributary of The Channel River during cold phases (fig. 1; Lericolais et al., 2003).
7The current landscape of the Lower Seine Valley is characterized by (1) several sinuous and hypertrophied meanders; (2) eight stepped terraces, describing an interval of 1 million years, the lowest-elevated ones now lying under the Holocene estuarine prism (Rouen I and Rouen II); (3) some cut‑off meanders occupied by wetlands, such as the Marais Vernier (+3 m NGF, French General Levelling) (Lefebvre, 1988; Lautridou et al., 1999; Antoine et al., 2007). At the onset of the Holocene period, the Lower Seine Valley was a deeply incised valley characterized by the low-elevated Rouen I and Rouen II terraces (Porcher, 1977). The Rouen I terrace corresponded to the lowest-elevated one (-14 m NGF at Rouen; Porcher, 1977) and incised the Rouen II terrace to form a narrow curvilinear surface. The Rouen II terrace had an elevation of -6 m NGF at Rouen (Porcher, 1977) and described a wide curvilinear surface. These terraces created depressions that, as a result of climate warming and induced sea-level rise, were filled up with terrestrial and estuarine material.
8All radiocarbon dates obtained for each site and used in the section below are summarized in Appendix 1. Radiocarbon dates of marine shells were corrected for a marine reservoir effect of 242 ± 57 yrs (reservoir age described for the English Channel; Harkness, 1983). The radiocarbon dates were calibrated using Calib 5.0. (Stuiver et al., 2005) and expressed as years cal BC/AD (cal. BC/AD; 2σ).
9Garnaud and co-authors (2003a; 2003b; 2005) have described four sedimentary units (BS1 to BS4), based on lithological description (cores), palaeobiological description (pollen) and chronological evidence (14C dates).
10Seismic profiles indicate that Unit BS1, Pre‑Boreal/Boreal in age (9250 ± 100 yrs 14C BP / 7990 ± 40 yrs 14C BP), lies directly on top of the Mesozoic substratum (Auffret and Ozouville, 1986). It is an organic‑rich facies, comprising a white clay deposit with millimetric calcareous concretions and flint fragments, and peat layers. There are asynchronous erosive contacts between units BS1 and BS2 (9250 ± 100 to 7990 ± 40 yrs 14C BP) and between units BS2 and BS3 (6530 ± 100 to 4010 ± 100 yrs 14C BP). Unit BS2, Atlantic in age, is composed of homogenous bluish-grey clay with numerous plant fragments (halophytic plants) and some local sand layers. Unit BS3 describes a wide time span, ranging from the end of the Atlantic to 1260 ± 60 yrs 14C BP. It is composed of a planar to sub-planar silty-sand laminated deposit and some shell layers, one of them dated 6820 ± 100 yrs 14C BP. Unit BS4 corresponds to the sand cover seen today and systematically covers BS3.
11From 8720 – 8280 cal. BC, a swamp environment replaced the former floodplain (BS1). Then, a marine back-barrier marsh formed (BS2). Pollen data indicate the proximity of the shoreline and the shell accumulations indicate some marine incursions (overwash episodes). From 4880 - 4340 cal. BC far from the coast and from 2050 - 1460 cal. BC near the coast, the area was entirely opened to marine influences (event marked by an erosive contact) with the deposition of tidally-influenced material (BS3). From 1220‑1470 cal. AD (Subatlantic period), some evidence of nearby human activities was described (pollens of cereal plants). Today, the current dynamic of the area is marked by sand accumulation (BS4).
12Delsinne (2005) described eight units (E1 to E8) and five discordant surfaces (Es1 to Es5) on an N-S section of the Seine outlet Holocene infilling, based on seismic profiles, lithological description (cores), palaeobiological description (pollen) and chronological evidence (14C dates).
13Unit E1 describes the marl-calcareous Cretaceous substratum. Surface Es1 represents the substratum topography, marked by a palaeo-valley to the north and an adjacent shelf to the south. Unit E2, covering unit E1, comprises gravel, deposited during the Lateglacial (Lefebvre et al., 1974). Surface Es2 is discordant, the unit E3 reflectors are onlapping on Es2. Unit E3, dating from the end of the Boreal to the Atlantic period (8570 ± 40 and 8020 ± 50 yrs 14C BP), is composed of fine material in the incision (locally as deep as -20 m NGF) and peat layers at the edge of the incision. Above E3, there is an erosive surface, Es3, at the edge of the incision and a channelling‑erosive surface, Es4, at the incision. Unit E4, dating from the Atlantic to Subatlantic period (6470 ± 130 to 2770 ± 30 yrs 14C BP), is composed of green fine sand to the south of the profile, while to the north it consists of gravel, pebble and medium shell sand. Unit E5, dating from the same period as E4, consists of sandy-mud deposits typical of channel deposition. Units E6, E7 and E8, Subatlantic in age (605 ± 30 yrs 14C BP), have a sandy-gravel composition. The Es5 discordant surface delineates Unit E7 from E8.
14From 7130 – 6710 cal BC (Boreal period), the area shows a transgressive event marked by Es2. Then (end of Boreal to Atlantic period), a depositional environment marked by alternating slikke and schorre (E3) formed in the paleo-valley. From 5070 – 4390 cal. BC, a tidally‑influenced estuarine environment (E5) formed in the main incision, while some barriers formed, in particular to the south of the incision (E4). During the historical period, bars formed on top of the tidally‑influenced estuarine environments (E6). Even more recently, the evolution of the estuary outlet is marked by the filling up of the main channel (E8), inducing the dredging of the channel and the deposition of related material in the Kannik area since the 1970’s (E7).
Fig. 2: Presentation of the SE Seine Bay (A) (Garnaud, 2003a) and the Seine outlet (B) (Delsinne, 2005) sites.
Here the lithological, chronological and/or seismic data of the key cores that formed the basis of the reconstructed evolution are represented. The schematic view of the Seine outlet infill is redrawn from Delsinne (2005).
15Several authors have described the Holocene infilling of the area (Direction des Mines, 1949; Elhai, 1963; Huault, 1980; Huault & Lefebvre, 1983a; Huault, 1985; Frouin et al., 2006, 2007a, 2007b, 2009). We have identified three surfaces (MVs1 to MVs3) and seven units (MV1 to MV7), based on lithological description (cores), palaeobiological (diatom and pollen) and chronological evidence (14C dates).
16Unit MV1, located at the bottom of the fill, corresponds to gravel, deposited during the Lateglacial (Lefebvre et al., 1974). Surfaces MVs1, MVs2 and MVs3 describe the pre-Holocene topography. They have different shapes and elevation, MVs3 being the lowest-elevated surface (-17.5 m NGF) and MVs1 the highest-elevated one (+2 m NGF). Unit MV2, which is observed on top of MVs3, is composed of sand and clay material of terrestrial origin (colluvial fan deposit; Lefebvre, 1998). Its age is uncertain, as there is no chronological evidence. Unit MV3, ranging from the Boreal (7730 ± 50 yrs 14C BP) to the Atlantic in age, is composed of fine material, estuarine in origin, that alternates with peat layers. Like MV2, MV3 is only observed above MVs3. Unit MV4, deposited above Mvs2 and Atlantic in age, is composed of fine material, terrestrial in origin. Unit MV5, deposited above Mvs2 and Atlantic in age, is composed of laminated fine material, estuarine in origin. Unit MV6, ranging from the end of the Atlantic (6170 ± 40 yrs 14C BP) to the present time in age, mainly corresponds to a thick peat layer. We have here distinguished two sub-units: MV6a, where fine material of both estuarine and terrestrial origins is intercalated within the thick peat layer; and MV6b, which is made of peat only. Unit MV7, Subatlantic in age, is composed of fine material, terrestrial in origin and containing freshwater diatom species.
Fig. 3: Presentation of the Marais Vernier site (Huault & Lefebvre, 1983a; Frouin et al., 2007a; Frouin et al., 2007b, Frouin et al., 2009).
Here the lithological and chronological data of the key cores that formed the basis of the reconstructed evolution are represented. Keys can be found on figure 2.
17Before 7020-6100 cal. BC (Boreal period), the area was characterized by the deposition of colluvial material in an abandoned fluvial channel (MV2). From 7020- 6100 cal. BC (end of Boreal to Atlantic period), estuarine material supplied a swamp environment (MV3) that formed above MVs3, while colluvial material still deposited (MV4) above MVs2. Then tidal environments are recorded everywhere in the area; in unit MV5 they alternate with peaty environments. From 5880-3640 cal. BC (Atlantic period), a wide peatland developed in the area (MV6b). Locally, the environment has been disturbed by the formation of tidal channels (between 4230 and 1780 cal. BC; MV6a). The peatland is still active today. It is affected by the Seine river flood (MV7) and human activities (e.g. building of the Dutch dike, drainage of the peaty area, culture).
18We have used two sets of data: (1) a drilling survey carried out in the 1940’s, which only looked at the upper part of the fill (Direction des Mines, 1949); (2) lithological and palaeobiological (pollen and diatom) descriptions, and chronological evidence (14C dates) of core STH1 drilled in the 1980’s (Huault, 1986; Huault, 1988). Based on the core description, we identified six units. Considering the 1940’s data, this number may decrease to four units.
19Unit H1, located at the bottom of the fill, corresponds to gravel, deposited during the Lateglacial (Lefebvre et al., 1974). Unit H2, Atlantic in age, is composed of fine material, in which there are some salinity changes based on the diatom content. There are freshwater species at its bottom and marine species thereafter. Unit H3, with a bottom date of 6570 ± 250 yrs 14C BP, corresponds to a peat layer. Unit H4, dating from the end of the Atlantic period to the beginning of the Subboreal period, is composed of fine material, deposited in a marine to brackish environment, based on the diatom content. Unit H5, with bottom dates from around 4315 ± 170 yrs 14C BP and 4390 ± 90 yrs 14C BP, corresponds to the upper peat layer. According to the 1940’s data, the thickness of the upper peat layer ranges from 0.29 to 6.7 m. It is therefore possible that H4 corresponds to the formation of tidal channels in the area that would have locally disturbed the peat accumulation, as in the Marais Vernier. In that case, there will be a sub‑unit describing the peat accumulation with some intercalated fine material and a sub‑unit describing only a peat accumulation. Unit H6, which is only locally observed on top of the upper peat layer, corresponds to the upper fine material, and its thickness ranges from 0.1 to 1.5 m.
20During the Atlantic period and before 5990-4970 cal BC, there was a shift from a freshwater depositional environment to a brackish intertidal one (H2). From 5990‑4970 cal. BC, peat accumulated in the area (H3). At the end of the Atlantic period, a tidal environment or a tidal channel occurred (H4). From 3490‑2480 cal. BC, there is no record of fine material deposition and peat accumulated until the present day (H5). Some evidence of human activities is found in the pollen record of this unit (pollen of crops). The clastic deposit observed on top of the upper peat layer is attributed to floodplain deposits (H6).
21Several authors have described the Holocene fill of the area (Huault and Lefebvre, unpublished data; Sebag, 2002). We have identified here four main units based on lithological description (cores), palaeobiological description (pollen and diatom) and chronological evidence (14C dates) along a north‑south profile in the Anneville meander.
22Unit A1, located at the bottom of the fill, is composed of sand and gravel. Unit A2, Atlantic in age, is composed of clastic material with a varying content of organic matter. Its palaeobiological content is marked by fresh‑water species locally disturbed by some brackish ones, especially at the top of the unit. Unit A3, with a bottom date of 6175 ± 100 yrs 14C BP, is composed of organic-rich material. We have distinguished a sub-unit A3a, made of an intercalated fine material having mainly freshwater species, from a sub-unit A3b only made of organic rich material (peat and organic-rich clay). Unit A4, with a bottom date of 2660 ± 125 yrs 14C BP, is composed of clastic material having some brackish species and a terrestrial origin.
Fig. 4: Presentation of the Heurteauville (A) (Huault, 1986; Huault, 1988) and Anneville (B) (Huault & Lefebvre, unpublished data; Sebag, 2002) sites.
Here the lithological and chronological data of the cores and drill-holes that formed the basis of the reconstructed evolution are represented. Keys can be found on figure 2.
23From the Atlantic period and prior to 5340-4850 cal. BC, a freshwater marsh evolved into a brackish one (A2). From 5340-4850 cal. BC, organic-rich material began to widely accumulate in the area (A3), indicating a peaty environment to the north of the profile (A3a) and a swampy environment to the south, where fluvial channel(s) developed (A3b). From 1110 - 410 cal. BC, a floodplain environment replaced the peaty and swampy environments along the profile.
24In this section we compare all the sedimentary units described previously to identify the main phases of the Holocene evolution of the Lower Seine Valley (fig. 5).
25The Holocene fill lies directly above the substratum SE of the Seine Bay and above gravel (E1, MV1, H1 and A1) in the other locations. Gravel is used as a chronological reference in the area to determine the base of the Holocene fill, as it is said to have been deposited during the Lateglacial (Lefebvre et al., 1974). There are very few data available on the Lateglacial of the Lower Seine Valley. However, fluvial processes are expected and regional pollen data indicate a steppe-like landscape during the Older and Younger Dryas and a forested land-scape during the Allerød (Huault, 1972).
26The oldest dated deposits from the Holocene period are found seaward of the Lower Seine Valley: pre‑Boreal at the SE part of the Seine Bay (BS1, peat dating from 8720-8280 cal. BC), Boreal at the Seine outlet (E3, peat dating from 7080-6710 cal. BC) and at the Marais Vernier (MV2, peat dating from 7020-6100 cal. BC). The depositional environment ranges from continental (SE part of the Seine Bay and Marais Vernier) to estuarine (Seine outlet), the Seine outlet being the first environment to record the influence of the sea-level rise on its evolution. Before 7020-6100 cal. BC, there was the first influence of the sea-level rise on the Marais Vernier evolution, with estuarine supplies (MV3). Both the Seine outlet and the Marais Vernier are marked by a deep vertical incision (elevation < -14 m NGF, elevation of Rouen I) that has been filled up by estuarine clastic material alternating with some local peat layers. There is some evidence of the influence of sea-level rise at the SE part of the Seine Bay with a vegetation change to halophytic plants (BS2). Meantime, there is no record of that interval of time landward of the Lower Seine Valley and above the altitude of the deep vertical incision (MV4 at Marais Vernier, Heurteauville, and Anneville).
27From around 6000-5000 cal. BC, the deposits show evidence of an increasing influence of sea-level rise on depositional environment across the entire Lower Seine Valley (units BS2-BS3, E3, MV5, H2 and A2). The influence is marked by estuarine depositional environments (SE part of the Seine Bay, Seine outlet, Marais Vernier, Heurteauville, Anneville). The sea level is high enough to reach the altitude of the second level of pre- Holocene topography (elevation > -6 m NGF, elevation of Rouen II) common to every site; they have all been flooded.
28Following the flooding event of the Lower Seine Valley, landward and seaward sites evolved differently; the Marais Vernier is likely to mark a limit between the two evolutions. Downstream of the Marais Vernier, there are open-tidal environments with evidence of channel migration and erosion processes (BS3 and E5) and barrier formation (E4). Upstream of the Marais Vernier, there are peat accumulations (MV6a, H3, H5 and A3a). During the period when this peat accumulated, local tidal or fluvial channels disturbed the peatland environment at every upstream location across the Lower Seine Valley (MV6b, H4, A3b).
29At the onset of the Subatlantic period (around 1150 - 400 cal. BC), there is a return to a terrestrial clastic sedi- mentation at Anneville (A4) and some local floodplain deposits upstream of the Marais Vernier (MV7 and H6), while open-tidal conditions still remain downstream. Meantime, there is evidence of human occupation and modification of the landscape in the area. Pollen data indicate the developing of agricultural and deforestation practices from 3350-2890 cal. BC onwards (Huault, 1979; Huault & Lefebvre, 1983b; Huault, 1986; Allard et al., 2004). During the historical period, the Dutch dyke in the Marais Vernier was built in the 17th century and the Seine outlet dredged (E7).
30Based on the comparison of these units, we have (1) different periods of time recorded across the Lower Seine Valley, as the result of the pre-Holocene topography; (2) some broad change of sedimentation recorded across the entire Lower Seine Valley; (3) different forcing factors related to the evolution of the entire system or to a solely site.
Fig. 5: Holocene evolution of the Lower Seine Valley.
We summarised all the units identified at each site to describe the evolution of the Lower Seine Valley. The oldest Holocene records were observed downstream of the Marais Vernier. Around 6000-5000 cal. BC, all the sites show the influence of the sea-level rise; it characterizes the flooding of the entire system. After the flooding, sites downstream of the Marais Vernier present tidal environments, while sites upstream of the Marais Vernier indicate a shift from minerogenic to organogenic environments. Some tidal or fluvial channels developed in some upstream sites and a return to clastic sedimentation occurred at Anneville after 1100 cal. BC.
31According to the comparison discussed above, we can describe four main phases to relate the Holocene evolution of the Lower Seine Valley (fig. 5). Here we summarize these phases to emphasize the forcing factors related to the described evolution.
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Phase 1: retrogradational shifting of the shoreline (prior to 6000 cal. BC)
32During that interval, the sea-level rise had a progressive influence on depositional environment. The first changes to estuarine conditions were recorded where a low- elevated incision was described (Lefebvre et al., 1974; Delsinne, 2005; Frouin et al., 2007a). This incision (-14 m NGF at Rouen and -17.5 m NGF at the Marais Vernier) results from a base-level modification linked to climato-eustatic changes. Lautridou and co-authors (1999) say the formation of this incision resulted from a climato-eustatic change recorded during the Last Glacial period (sea-level drop from -70 to -120 m). However, as shown by recent works on fluvial systems of NW Europe (Antoine et al., 2007; Bridgland & Westaway, 2008), this vertical incision may have formed later, i.e. during Lateglacial climato-eustatic changes. The particular pre-Holocene topography of the Lower Seine Valley has affected the way it has evolved during the Holocene. All sites that do not present the deep vertical incision were mainly affected by climatic changes that, in turn, resulted in terrestrial supplies (Frouin et al., 2007b; Frouin et al., 2009).
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Phase 2: maximum flooding (around 6000-5000 cal. BC)
33During this interval, the sea level rose sufficiently highly to flood both the deep vertical incision and the other parts of the pre-Holocene topography (level common to all sites), giving a broad change to estuarine conditions across the entire Lower Seine Valley. The marine component (sea level and tides) then became the principal forcing factor that guided the evolution of the system.
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Phase 3: aggrading sequence (between 5000 and 1100 cal. BC)
34During this interval, the Lower Seine Valley had a wider accommodation and the sea-level rise rate declined world-wide (e.g. Pirazzoli, 1991; Mörner, 1995). The nature of the depositional environments then resulted from the distance to shoreline and ranged from open- tidal environment (clastic tidal sedimentation), through semi-isolated ones (peat accumulation with local tidal or fluvial channel) to isolated ones (only peat accumulation). Downstream of the Marais Vernier, depositional environments were primarily influenced by the tide (tidal lamination, tidal channel migration). Tidal channel migration resulted in erosion of some parts of the filling, giving an incomplete record of that time interval or masking any record of the influence of other forcing factors in these areas. Upstream of the Marais Vernier, depositional environments were influenced by local hydrological conditions (water-table level and climate changes) that allowed peat accumulation locally disturbed by tidal or fluvial processes.
35That period of aggradation also marked a period of landscape stabilization, where man became sedentary (Neolithic period; Sebag, 2002; Allard et al., 2004). He then started to influence and modify the environment.
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Phase 4: progradational shifting of the shoreline (from 1100 cal. BC)
36During this interval, there was a return to clastic conditions of deposition landward of the system. That change might be related to the increasing influence of human activities (culture of cereals, deforestation etc.; Huault & Lefebvre, 1983a; Huault, 1986; Huault, 1988; Sebag, 2002). During the historical period, there were more examples of the human influence on the evolution of the Lower Seine Valley (construction of a dike, dredging, etc…) that probably masked the influence of some forcing factors or even enhanced some others (e.g. storms events; Sorrel et al., 2009).
37We have here summarized some 50 years of studies on the Holocene evolution of the Lower Seine Valley. Climato-eustatic changes that happened during the Last Glacial period (Lautridou et al., 1999) or the Lateglacial period (Antoine et al., 2007; Bridgland & Westaway, 2008) resulted in a base-level modification that, in turn, gave a variable pre-Holocene topography (low-elevated incision) and a gravel bed, which is used as a chronological reference. At the onset of the Holocene period, the Lower Seine Valley was a deeply incised valley system like numerous other fluvial systems of NW Europe. The created accommodation was then filled up as the sea level rose.
38We have compared the Holocene evolution of five different sites located across the Lower Seine Valley. It enabled us to identify four main phases of evolution. First, there was a retrogradational shifting of the shore- line prior to 6000 cal. BC. Only depositional environments of low-elevated areas were affected by the influence of the sea-level rise, while the higher areas were affected by climatic changes. The pre-Holocene topography was then a primary forcing factor controlling the evolution of the system.
39Then, there was a maximum flooding event around 6000 – 5000 cal. BC. All depositional environments of the Lower Seine Valley were affected by the influence of the sea-level rise. An aggrading system followed between 5000 and 1100 cal. BC. Seaward environments suggest open-tidal conditions, while landward ones show peat accumulation, sometimes disturbed by the formation of tidal or fluvial channels. Seaward environments were thus controlled by tide, while landward ones were primarily controlled by local hydrological conditions. Finally, there was a progradational shifting of the shoreline from 1100 cal. BC. Seaward environments again show open- tidal conditions, while landward ones have clastic deposits related to floodplain deposition. During that interval, Man greatly modified his environment and landscape, becoming the primary forcing factor.
40The synthesis indicates that there is no system that has recorded the entire Holocene period. To obtain a full record, we need to consider different locations across the system. All studies carried out on the Lower Seine Valley have added some useful 14C dates to the database, and this has allowed us to compare the presented evolution. These dates can then be used to define the regional sea-level curve and determine the rates at which the Lower Seine Valley filled up. We will then be able to fully compare the Lower Seine Valley evolution to other systems of NW Europe.
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