the Middle Valley: Etouvie (Antoine, 1997), and one in the Acon tributary (Antoine, 1997; Munaut and Defgnée, 1997). A recent study in Bourdon, in the middle Somme, is also taken into account (Beaumont et al., 2023).

<figure><img src="image_34.png" /><figcaption>Figure 1. Geological context. a = Cretaceous chalk ; b = Holocene peat and alluvium ; c = Paleogene sand; d = Holocene marine sediment; e = Pleistocene loess; f = stratigraphic transect.</figcaption></figure>

# 3 RESULTS AND DISCUSSION

## 3.1 Weichselian Pleniglacial to Lateglacial: fluvial morphodynamic under contrasting climatic conditions

The chalk bedrock, shaped by the last major phase of incision of the Somme valley dating from the Early Weichselian period (Antoine and Limondin-Lozouet, 2024), is covered by a layer of chalk and flint gravels within a sandy-limestone matrix. The gravel layer was deposited in a network of braided or highly mobile meandering paleochannels, typical of periglacial flow regimes during the Pleniglacial period (Fig. 2).

At the end of the Pleniglacial period, calcareous silts with chalk granules of fluvio-aeolian origin were deposited on the valley floor and subsequently reworked into small channels within a tundra landscape (~20,000 cal BP at Morcourt, Garcia et al., 2025).

The Oldest Dryas has never been identified in the Somme basin (Antoine, 1997; Antoine et al., 2012). The absence of this stage in the stratigraphic sequence suggests that there was a gap of several thousand years between the Pleniglacial and Bølling deposits, indicating locally that an erosive phase occurred as observed in a thin unit of chalk gravel deposits in Conty (Antoine et al., 2012) or a lack of sedimentation.

The Bølling is characterised by an enrichment of organic matter in carbonate silt and a finer grain size at the Morcourt and Conty sites. Peat accumulations are locally observed in some abandoned channels. The development of soils and vegetation stabilise the fluvial system in anabranches, thereby drastically limiting erosion on the slopes (Antoine et al., 2012).

The climatic deterioration of the Older Dryas is only evidenced at Conty by a very thin layer of calcareous silts (5 cm thick). The deposit results from the flooding of the fluvial system and is composed of sediments derived from gelifraction and slope soil erosion (Antoine et al., 2012).

The temperate Allerød interstadial is reflected in the organic enrichment of organic silts to peaty deposits in the abandoned riverbeds due to the stabilisation of the fluvial system that gradually evolved to a single meandering river. This is evidenced in Morcourt, Conty, the Acon valley, and Tirancourt (Antoine, 1997; Antoine et al., 2012; Garcia et al., 2024, 2025).
The climatic crisis of the Younger Dryas resulted in the concentration of water flows in a wide, deep and highly dynamic channel (100 m wide at Morcourt, Garcia et al., 2025). It is probable that a significant proportion of the previously accumulated peat was eroded, particularly in the downstream watercourses.

<figure><img src="image_35.png" /><figcaption>Figure 2. Evolution patterns of the Somme basin valley floor since the Weichselian Lateglacial</figcaption></figure>

## 3.2 Early Holocene: the onset of peat clogging under favourable climatic influence

The rapid increase in temperature and rainfall at the very beginning of the Holocene enabled runoff to occur on slopes and plateaus that were sparsely wooded. This led to an increase in the flow of water towards the valley floor and a reduction of the sediment inputs, causing incision of fluvial deposits.

In the valley floor, brown-black clayey silts, rich in organic matter (hydromorphic soils), develop before the onset of peat formation along all transects within the basin (Fig. 2). This diachronic unit develops within small, shallow lateral channels (Antoine and Ducrocq, 1997). The processes of evapotranspiration, forest soil infiltration, and the retention of large amounts of water within the peat (~80% of the mass) play an important role in reducing water flow. The presence of woody remains (tree trunks) at the base of the peat (Renancourt, Tirancourt) indicates a rapid rise in the water table, and in peat accumulation. In a landscape that is still predominantly forested, the human footprint is inconspicuous (Antoine, 1997; Garcia et al., 2024). Peat clogged the entire valley floor to a thickness of several metres (3 on average) in a relatively stable environment and climate until about 5500 cal BP at Morcourt. In the middle Somme and Selle valleys, the last preserved peat dates from about 7000 cal BP.

## 3.3 Peat accumulation disturbances from the Middle Holocene: climatic or anthropogenic influence?

From the Middle Holocene onwards, erosive channels were established within the basin. In the middle Somme valley and in the lower Selle valley, the channels were filled with calcareous tufa deposits, whereas in the smaller tributaries and the upper valley, the infilling was mainly silty. At least two phases of erosion were identified between 7800 and 7500 cal BP, then around 5000 cal BP in the middle Somme valley and locally also a third around 3500 cal BP in the upper Somme valley. The intensification of fluvial dynamics played a role in the flooding of the valley floor and the transport of a multitude of detrital elements (molluscs, eroded peat pebbles and tufa) and silty sediments produced by anthropogenic soil erosion. The reactivation of fluvial dynamics can be attributed to two key factors: the phases of increased precipitation recorded from the Late Holocene (Mayewski et al., 2004); and the increased runoff linked to the human induced opening up of the landscape (decrease from 60 to less than 20% of arboreal pollens (Garcia et al., 2024) by the reduction of evapotranspiration and soil infiltration.
Human activity in the upper Somme valley (dams) appears to have contributed to the supporting of a high-water table level and to have favoured the formation and preservation of peat until the Low Middle-Age, prior to the fossilisation of the system by a lowering of the water table, causing surface peat deterioration and favoured the deposition of silty colluvium. In the middle Somme and in the Selle valleys, no peat appears to be younger than 4900 cal BP. The calcareous tufa, which overhanging the present-day valley floor by 2-3 m, is the result of a notable lowering of the alluvial water table level which may have led by the extensive drainage for peat excavation (at least from the 12th to the 20th centuries) and peat degradation.

## 4 CONCLUSION

In the Somme basin, modifications of fluvial and environmental dynamics respond to climatic forcing until the end of the Early Holocene. Subsequently, from the Middle Holocene (Neolithic) onwards, fluvial dynamics are both influenced by human actions (land clearance, water levels management) and amplified by climate (increased rainfall, soil erosion). During the Late Holocene, human activities became more pronounced and profoundly altered the hydrological functioning of the valley floor (peat extraction, dams), leading to the loss of peat or its fossilisation under colluvial layers resulting from slope soil erosion. The environmental response is variable depending on the zone of the basin.

## 5 ACKNOWLEDGMENT

The authors would like to express appreciation for the support of the sponsors of the Archeofen project: the Somme Departmental Council (CG 80) and the Water Agency Artois Picardie. This manuscript was reviewed by Laurent Deschodt and Axel Beauchamp.

## 6 REFERENCES

Antoine, P., 1997. Modifications des systèmes fluviatiles à la transition Pléniglaciaire-Tardiglaciaire et à l'Holocène : l'exemple du bassin de la Somme (Nord de la France). Géographie physique et Quaternaire, 51, 1, 93-106.

Antoine, P., Ducrocq, T., 1997. Les tourbes du bassin de la Somme : relations avec les systèmes fluviatiles et les occupations préhistoriques au Tardiglaciaire er à l'Holocène. Les Cahiers de Géographie Physique, 11, 12-27.

Antoine, P., Fagnart, J.-P., Auguste, P., Coudret, P., Limondin-Lozouet, N., Ponel P., 2012. Synthèse des données : évolution des environnements de la vallée de la Selle au Tardiglaciaire et au début de l'Holocène et relations avec les occupations Préhistoriques. Quaternaire, 5, 127-147.

Antoine, P., Limondin-Lozouet, N., 2024. Late Middle Pleistocene (MIS 10-6) glacial-interglacial records from loess-palaeosol and fluvial sequences from northern France: a cyclostratigraphic approach. Boreas, 53, 476-503.

Beaumont, L., Garcia, C., Antoine, P., Limondin-Lozouet, N., Brasseur, B., Dabkowski, J., 2023. Les tufs calcaires holocènes de la moyenne vallée de la Somme (Nord de la France) : répartition spatiale, chronostratigraphie et implications paléogéographiques. Géomorphologie. relief, processus, environnement 29, 4.

Garcia, C., Antoine, P., Ducrocq, T., Bacon, J., Beaumont, L., Coutard, S., Dabkowski, J., Brasseur, B., 2024. Mise en place des tourbières alcalines et modifications de la dynamique fluviatile dans la moyenne vallée de la Somme (France) à l'Holocène. Quaternaire, 35, 2.

Garcia, C., Brasseur, B., Bacon, J., Saulnier, Copard, S., Gauthier, C., Mathieu, L.-A., Michaelis, D., Mokadem, F., Antoine, P., 2025. Lateglacial and Holocene palaeoenvironmental evolution of alkaline peatlands in the Somme valley (France): between climate and anthropogenic forcing. Boreas, 54, 34-59.

Mayewski, P., Rohling, E., Stager, J., Karlén, W., Maasch, K., Meeker, L., Meyerson, E., Gasse, F., Van Kreveld, S., Holmgren, K., Lee-Thorp, J., Rosqvist, G., Rack, F., Staubwasser, M., Schneider, R., Steig, E., 2004. Holocene Climate Variability. Quaternary Research, 62, 243-255.

Munaut, A.-V., Defgnée, A., 1997. Biostratigraphie et environnement végétal des industries du Tardiglaciaire et du début Holocène dans le bassin de la Somme. Actes du 119ème Congrès national des sociétés historiques et scientifiques : « Le Tardiglaciaire en Europe du Nord-Ouest », Amiens. Ed. C.T.H.S., 27-37.

Ruppel, M., Väliranta, M., Virtanen, T., Korhola, A., 2013. Postglacial spatiotemporal peatland initiation and lateral expansion dynamics in North America and northern Europe. The Holocene, 23, 1596-1606.
# EVOLUTION OF SEDIMENTARY DEPOSITION ENVIRONMENTS FROM THE IRON AGE TO THE PRESENT DAY IN CUL-DE-LOUP BAY (NORMANDY, FRANCE): AN ILLUSTRATION OF INTERTIDAL ENVIRONMENT VULNERABILITY UNDER SIGNIFICANT HUMAN IMPACT

Benjamin LE LOREC<fnref n="1" />, Wendoline GRÉGOIRE<fnref n="1" />,<fnref n="2" />, Anne MURAT<fnref n="1" />,<fnref n="2" />, Régis GALLON<fnref n="1" />,<fnref n="2" />, Yann MÉAR<fnref n="1" />,<fnref n="2" />, Emmanuel POIZOT<fnref n="1" />,<fnref n="2" />, Bernadette TESSIER<fnref n="3" />, Laurent DÉZILEAU<fnref n="3" />, Lucille FURGEROT<fnref n="3" />, Yohann POPRAWSKI<fnref n="3" /> and Benoit LAIGNEL<fnref n="3" />

Corresponding author: benjamin.lelorec@univ-lille.fr

**Abstract:** Located in the northeast of the Cotentin Peninsula (Normandy, France), Cul-de-Loup Bay is currently undergoing significant hydro-sedimentary changes, marked by silting and coastal erosion. To better understand the future evolution of this back-bay environment, a study was conducted to analyze sedimentary deposits dating back to the Iron Age (2750 BP). A total of 11 surface cores were collected for grain-size, and geochemical analyses, and radiocarbon (¹⁴C) dating. Nine sedimentary facies grouped into five chronostratigraphic units have been distinguished. These sedimentary archives have recorded the impact of anthropogenic infrastructures on sediment dynamics, leading to a sudden retreat of depositional environments. Currently, sea level rise is exacerbating these transformations, enhancing the contrast between internal areas subject to rapid silting and more open coasts undergoing accelerated retreat, thereby revealing the vulnerability of such embayment.

**Keywords:** Coastal Environment, Paleoenvironment, Sea level rise, Elementary geochemistry, Organic matter

## 1 INTRODUCTION: THE STUDY AREA AND OBJECTIVE PRESENTATION

Cul-de-Loup Bay (CLB) is located at the northeastern tip of the Cotentin Peninsula (Normandy, France). It is a "back-bay" type environment (Dubois, 2000) covering an area of 3.5 km<fnref n="2" /> and is semi-open to the Baie de Seine (Fig. 1a). The coastline was artificially modified early on with the construction of the Hougue dike in the 17th century, which was built on a former tombolo linking the town of Saint-Vaast-la-Hougue to a granitic spur to the east (Hougue) (Fig. 1b). This seawall protects the bay from dominant northwesterly swells. However, swells from the east-southeast (30% of the year) reach the area. The CLB is an intertidal bay, subject to macrotidal dynamics (maximum tidal range 7 m). The northeastern part of the bay, sheltered behind the Hougue dyke, is characterized by mudflats, evolving toward salt marshes. The rest of the bay, in the center and SE area is occupied by an intertidal sandy platform getting progressively muddier toward the east (i.e., the dyke). CLB is a socio-economic (oyster farming, fish farming, shellfish gathering, tourism, and urban development) and ecological (ZNIEFF 1 and 2, Natura 2000) area of interest. For several years, the bay has been undergoing significant hydro-sedimentary changes, including silting in the back-bay area and coastal erosion (destruction of anthropogenic structures, formation of micro-erosion cliffs, and retreat of sediment ridges). This article is part of the PROTEC project ("D'une alerte DCE à un Projet de Territoire: Anse du Cul-de-Loup, CNAM-Intechmer/AESN"), which aims to understand future changes in the bay by reconstructing paleo-environments and the chronostratigraphic framework. Additionally, this project contributes to the Holocene study of coastal systems in the English Channel (Tessier et al., 2022). As part of the research, 11 surface cores were collected, described, and analyzed in the laboratory. Grain-size, elemental geochemical, and radiocarbon (¹⁴C) dating results helped identify nine sedimentary facies grouped into five chronostratigraphic units, which serve as archives of depositional environment changes. This work highlights both the impact of human activity and sea level rise while underscoring the fragility of back-bay intertidal zones.

## 2 MATERIALS AND METHODS

Manual interface coring was conducted at eleven stations distributed throughout the study area (Fig. 1b): seven in the open bay (stations 1, 2, 4, 5, 6, 7), two in the inner salt marsh at the foot of the dyke (stations 8, 9), and two on an erosive microcliff incising a lower salt marsh (one at the top, station 10, and one at the base, station 11). After opening the cores, the 'Archive' half-section was photographed and used for X-ray

<footnote n="1">¹Conservatoire National des Arts et Métiers, Intechmer, Boulevard de Colignon, 50100 Cherbourg-en-Cotentin.</footnote>

<footnote n="2">²Université de Caen Normandie, Laboratoire Universitaire des Sciences Appliquées de Cherbourg, 145 Chemin de la Crespinière, 50100 Cherbourg-en-Cotentin, France.</footnote>

<footnote n="3">³Université de Caen Normandie, Morphodynamique Continentale et Côtière, 24 rue des tilleuls, 14000 Caen.</footnote>
imaging, which was performed using Ifremer's CT scanner. The 'Work' half-sections were described in detail, with a focus on sediment color, visible granulometry, bioturbation, and apparent laminations, before being photographed. A total of 177 samples, collected at 5 cm intervals, were analyzed for grain-size using a laser diffraction particle size analyzer (Horiba LA-960, 0.011–2000 µm) and for elemental composition using X-ray fluorescence spectrotometry (Xsort SPECTRO, Ametek). The results were calibrated using the "Enviro-H" method, which is tailored for rocks and was adjusted through ICP-OES/MS analysis of 60 marine samples (Baux et al., 2019). Twenty-three elements were quantified, ranging from aluminum (the lightest element) to uranium (the heaviest). For this study, two elements were selected: potassium (K), an indicator of terrigenous phyllosilicates (Reynolds, 1963), and organic bromine (BrOrg), a marker of marine organic matter after the removal of interstitial bromides (Mayer et al., 1981). Additionally, 0.05 g of each subsample was used to measure total organic carbon (TOC) via infrared spectrometry (LECO CS 744) after decarbonation with phosphoric acid ($H_3PO_4$). Eleven radiocarbon ($^{14}$C) dates were obtained using the AMS method on bulk benthic foraminifera and whole shells (bivalves or gastropods) at the IsotopTech Zrt laboratory (Hungary). Radiocarbon ages were calibrated using the MARINE20 method (Heaton et al., 2020), with the marine reservoir effect measured by Tisnérat-Laborde et al. (2010), estimated at 640 ± 50 years around the island of Tatihou. For cores 10A and 11B, the MARINE20 calibration could not be applied because the sediments were too recent; only the reservoir age correction was directly applied to the raw results. To compensate for the lack of dating in the most recent sediments, accumulation rates were estimated based on the transition level identified in multiple cores. This level is dated between 1972 and 1980, as it contains plastic fragments and coincides with a period of intense anthropogenic activity visible in historical aerial imagery. Since the cores were collected in 2023, sediment accumulation rates were calculated using both 1972 and 1980 as reference years, and the average of these values was retained. In parallel, a cartographic analysis was conducted using archives of aerial photographs (1920–2011) provided by the IGN (Institut National Géographique, https://remonterletemps.ign.fr) to highlight historical changes in the study area. This analysis included both anthropogenic structures (oyster farms, dikes) and natural features (Pebble ridge, channels and limit of the salt marsh). Georeferencing accuracy was verified using geological substrate control points.

<figure><img src="image_36.png" /><figcaption>Figure 1. (a)Geographic location of Cul-de-Loup Bay. (b) High-resolution bathymetric and topographic map of the study area. The map also displays sediment cores and geomorphological features include the pebble ridge and the Hougue dike.</figcaption></figure>
# 3 RESULTS AND DISCUSSION

## 3.1 Sedimentary facies

Based on grain-sizes, colors (referenced using the Munsell color chart, Nickerson, 1940), bedding, and biological indicators (roots, burrows, shells), confirmed on X-ray imaging, 9 sedimentary facies were identified (A, B, C, D, E, E', F, G, T) (Fig. 2a). The results of particulate geochemistry further allowed the differentiation of depositional environments (Figs. 2b and 2c).

<figure><img src="image_37.png" /><figcaption>Figure 2. (a) Synthetic log of sedimentary facies organisation. Variations in TOC concentrations as a function of or K (b) or BrOrg (c). On b, colors refer to the sedimentary facies on a, and symbol shapes to core numbers (Fig. 1a for location).</figcaption></figure>

The K/COT ratio defines the marine vs continental signature: K/COT less than 3 signs marine sediment, more than 3, continental sediment. The BrOrg/COT ratio characterizes the degree of marine influence in a transitional "marine to continental" depositional environments such as estuaries or embayments like the CLB. A ratio greater than 45 characterizes maximum marine influence (i.e., the external area), a ratio between 25 and 45 corresponds to intermediate marine influence, and a ratio lower than 25 indicates minimal (moderate) marine influence (i.e. the internal area). However, the BrOrg/COT ratio has limitations, as BrOrg and COT are concentrated in fine-grained fractions, and thus absent in coarser fractions (from sands onward) that are usually dominant in the external marine-dominated areas.

The sedimentary facies T is present in 7 cores over 11 (Fig. 2a). It is characterized by very coarse-grained bioclastic and lithoclastic particles, as well as plastic waste, and by an erosive base. This facies is probably present as well at the base of cores 5B and 7A, characterized by gravel layers as revealed by X-ray images.

The upper sections of cores 1A, 2A, and 3A (Fig. 2a) are characterized by facies A. It consists of sandy-silty sediments enriched with shell debris and lacking visible structures in X-ray images. The absence of fine-grained sediments, and thus of low BrOrg concentration, limits the assessment of the degree of marine influence. However, the four samples containing BrOrg are at the BrOrg/COT = 45 threshold (Fig. 2c). They correspond to sandy deposits in a highly marine-influenced outer environment. In these same cores, two distinct facies were identified below facies T (Fig. 2a). Facies B, in core 1A and 2A consists of silty-sandy-silty sediments with distinct laminations. Although the K/COT ratio (<3) confirms a marine signature of this facies, the BrOrg/COT ratio suggests moderate marine influence (<25) (Fig. 2b and 2c). Facies B signs deposition in a low-energy marine environment (internal areas). This interpretation is corroborated by binocular magnification observations, which revealed an abundance of wood fragments and agglutinated foraminifera, particularly the species *Trochammina inflata* (Montagu, 1808), typical of vegetated salt marsh environments (Rodriguez-Lazaro et al., 2013). Facies D, found at the base of core 3A (Fig. 2a), has a very similar sandy-silty grain-size than the overlying facies A. It is enriched in shells and exhibits numerous laminations identified by X-ray imaging, similar to facies B. Geochemical results are similar to those described for facies B (K/COT <3, BrOrg/COT <25). Facies D is thus non-continental and was deposited in a moderately marine-influenced environment.

At the top of cores 4B, 8B, and 9B, facies E' (Fig. 2a) consists of silty sediments significantly disturbed by the presence of roots. The marine character of this sediment is confirmed by a K/COT ratio lower than 3 (Fig. 1b), while the BrOrg/COT ratio, ranging from 25 to 45 (Fig. 2c), suggests an intermediate degree of marine
influence. At the base of these cores, facies E is distinguished by its fine-grained, silty texture and ochre color. Geochemical analyses indicate a strictly continental signature (K/COT > 3, Fig. 2b) with very low values in COT and BrOrg. It is interpreted as loessic deposits, primarily formed during periglacial periods (Giles et al., 2017).

Facies F, identified at the top of cores 5B, 6A, and 7B (Fig. 2a), is characterized by sandy-silty grain-size, and bioturbation. The marine signature of facies F is confirmed by geochemical data. The K/COT ratio is lower than 3 (Fig. 2b), and the BrOrg/COT ratio is greater than 45, indicating a strong marine influence (Fig. 2c) featuring external (outer) environments.

Cores 10A and 11B feature facies G at their top and facies C at their base (Fig. 2a). Facies G exhibits a tri-modal grain-size distribution and is heavily bioturbated by roots and crab burrows. The sediment is silty-sandy, but root implantation generates a medium sand fraction. Facies C is characterized by a sandy-silty texture and is marked by numerous laminations. Both facies exhibit similar geochemical ratios. The K/COT ratio is less than 3 (Fig. 2b), highlighting the marine signature, while the BrOrg/COT ratio varies between 25 and 45, indicating an intermediate degree of marine influence (Fig. 2c). Binocular magnification observations revealed the presence of small marine gastropods and bivalves.

<figure><img src="image_38.png" /><figcaption>Figure 3. (a) Synthetic log of sedimentary units and photographs. The sampling altitude (marine elevation) of the cores is indicated next to the name.</figcaption></figure>

## 3.2 Chronostratigraphic units

Five main chronostratigraphic units (from oldest to most recent: U1, U2a, U2b, U3, and U4) have been distinguished on the basis of the combined analysis of photographs and X-ray radiographs, grain-size, and geochemistry data, as well as information on age and accumulation rates. U1 is characterized by facies E, U2a includes facies B and D, U2b is defined by facies C, and U3 corresponds to facies T. Finally, U4
encompasses all surface facies (A, E', and F), except for facies G, which represents the advanced degradation of facies C by current fauna (crab burrows and roots).

Unit U1 (Fig. 3a) is interpreted as loess deposits of periglacial origin, making it the oldest sediment identified in the bay. The absence of datable organic material for radiocarbon dating has prevented a precise determination of its age. An optically stimulated luminescence (OSL) dating (Cordier, 2010) is underway at the CIRAM laboratory on samples collected from a depth of 65 cm in core 8B. Unit U1 is primarily located in the most internal areas of the CDL (Fig. 3b), where root traces suggest more recent reworking during the development of the vegetated salt marsh. Unit U2a corresponds to a depositional environment of sandy-clayey intertidal flat (sand) to salt marsh type (mud), intersected by tidal channels with sandy grain size. Age data in unit U2a range from the beginning of the Iron Age, here 2650 BP, to 659 BP. The west/east profile described by cores 1A, 2A, 3A, and 6A helps to better understand the paleogeography of this environment. In the western part (Fig. 3b), all analyses and the low sediment accumulation rate (0.06 cm/year) recorded from core 2A describe a salt marsh environment with weak marine incursions (only during the highest spring tides). The eastern part of the profile (center of the bay) has similar geochemical characteristics (internal environment) but with coarser grain-size and likely higher sedimentation rate, as indicated by the radiocarbon age in core 3A. These cores are characteristic of an upper tidal flat environment, less vegetated than the salt marsh, and submerged during spring tides. Sedimentation rate is higher and channel migration is rapid (alternation of sand and mud).

Unit U2b is more recent, but the uncalibrated dates do not provide an absolute age, so they are used here for informational purposes. They indicate an age after 1950 at the top of core 10A and 197 to 83 years BP from the base to the top of core 11B (Fig. 3a). A diachronic analysis of aerial images (1940 to 2023) reveals that the erosion of the microcliff sampled by cores 10A and 11B began between the 1970s and 1980s, concomitantly to the landward migration of a pebble ridge making a ling of tombolo (Fig. 3b). By 1990, the tombolo reached the present location of cores 10A and 11B. Radiocarbon dating obtained from core 11B shows a sedimentation rate comparable to the present one, estimated at 0.64 cm/year (Figure 3a). Unit U2b thus corresponds to a salt marsh environment subject to a more pronounced marine influence than the older salt marshes located at the bottom of the bay (U2a) and that develop behind the ridge.

Unit U3 is characterized by the very coarse-grained facies T. Its development is associated to an intensification of anthropogenic activities in the cove. This hypothesis is supported by (1) the presence of plastic waste collected from this level, dated by packaging typology to the early 1980s, and (2) the diachronic analysis of historical aerial images showing an intensification of oyster farming activities between 1970 and 1980 (from 0.07 km² to 0.7 km²). U3 thus formed in a context marked by the expansion of oyster farming areas, the reclamation of the internal salt marshes, and the construction of dikes.

Unit U4 is related to the present-day sedimentation and is characterized by very variable accumulation rates, reaching up to 1.22 cm/year close to the channels and decreasing significantly in the internal areas of the bay (Fig. 3a) (0.06 cm/year). Sedimentation of U4 has followed that of U3 since the 1980s. According to a west-east transect, the environments evolve from an external sandy to an intermediate sandy-clayey intertidal flat, both strongly influenced by marine dynamics. Finally, to the east, a vegetated internal salt marsh sheltered back to a tombolo develops. This environment is subject to daily tidal fluctuations in the channelized parts, the higher vegetated areas being rarely submerged (Fig. 3b).

### 3.3 Evolution of depositional environments

The overall results allowed for the reconstruction of the depositional environments before and after U3, which corresponds to a sudden change and reconfiguration of the environments in the bay (Fig. 4). Currently, the western part of the bay is characterized by a sandy platform fed by marine sediments due to a combined action of littoral drift and tidal currents (flood tide). This dynamic allows the intrusion of sandy sediments into the bay and the establishment of a sandy-muddy gradient from west to east, until the dissipation of currents and the formation of a mudflat typical of a "bay bottom" environment. It also extends from north to south along the eastern coastline. The most internal sector (northeast), particularly well protected due to the presence of a tombolo and a dike (constructed in 1973) is characterized by mudflat and densely vegetated salt marsh (Fig. 4a). The geographical distribution of pre-U3 environments (from the Iron Age, around 2650 cal BP and 1970-1980) shows the same arrangement (sandy tidal flat, upper tidal flat, and then salt marsh from west to east). However, the extent of the sandy intertidal zone was more restricted than it is now; it did not extend as far into the bay and at the same time the mudflat was not limited only to the eastern part but extended westward, occupying a large portion of the bay. The salt marshes were also much more widespread, covering the entire bay bottom and extending into the eastern part (Fig. 4b). This configuration reflects depositional environments with lower hydrodynamic conditions than present-day ones.

The development of anthropogenic activities, marked by U3, deeply impacted the sedimentary dynamics of the bay and led to a reconfiguration of the depositional environments. The gravel and pebbles found in the cores collected to the east in the bay bottom, are attributed to human paving, which allowed access to the
oyster farms in the muddy flat. It is assumed that this unit is present throughout the bay. The eastward displacement of high energy environments is linked to (1) the establishment of oyster farming parks, now occupying the center of the bay, and (2) a narrow access path to the inner bay bottom. This passage accelerates water masses (tidal currents and littoral drift), promoting the penetration of currents into the bay. This evolution leads to an increase of sediment deposition in the internal areas (1.22 cm/year), and a retreat of gravel ridge and of the sandflat-mudflat limit inducing the erosion of underlying sediments. The recent acceleration of the gravel ridge retreat (from 0.6 m/year to 1.2 m/year between 2000 and 2020) is, however, associated with the acceleration of the sea level rise (from 3 mm/year to 5 mm/year between 2000 and 2020 (IPCC, 2021)).

<figure><img src="image_39.png" /><figcaption>Figure 4. Reconstruction of depositional environments: (a) present-day and (b) during the Iron Age.</figcaption></figure>

## 4 CONCLUSION

This study highlights the sedimentary and morphological transformations that have occurred in Cul-de-Loup Bay since the Iron Age, in response to significant human influence combined with sea level rise. Human activities, such as the intensification of oyster farming and the construction of dikes, have altered hydrodynamics, depositional environments, and morphologies. The retreat of the pebble ridge by reducing back-bay environments and diminishing its role as a natural barrier, exposes the vegetated salt marshes to increased erosion. Sea level rise reinforces the recently established configuration by intensifying the erosion of exposed areas. In calmer areas, sea level rise promotes sediment infilling by creating new accommodation space, which favors sediment accumulation from eroded sectors. Thus, it acts as a key factor in the reorganization of depositional environments. These disturbances exacerbate the vulnerability of back-bay environments to future changes.

## 5 ACKNOWLEDGEMENTS

The authors would like to thank the Agence de l'Eau Seine-Normandie (AESN) and the Conservatoire National des Arts et Métiers (CNAM) for their support in the PROTEC project. We sincerely thank Samuel and Joseph Costard from Actimer, as well as Yann Joncourt (AESN), for their valuable assistance during the coring campaigns. We are also grateful to the students from Intechmer for their help in the field. This paper was reviewed by Pascal Le Roy.

## 6 REFERENCES

Baux, N., Murat, A., Faivre, Q., Lesourd, S., Poizot, E., Méar, Y., Brasselet, S., Dauvin, J.-C., 2019. Sediment dynamic equilibrium, a key for assessing a coastal anthropogenic disturbance using geochemical tracers: Application to the eastern part of the Bay of Seine. Continental Shelf Research, 175, 87-98.

Cordier, S., 2010. Optically stimulated luminescence dating: procedures and applications to geomorphological research in France. Géomorphologie : relief, processus, environnement, 16, 21-40.

Dalrymple, R. W., Zaitlin, B. A., Boyd, R., 1992. Estuarine facies models; conceptual basis and stratigraphic implications. Journal of Sedimentary Research, 62, 1130-1146.

Giles, D.P., Griffiths, J.S., Evans, D.J.A., Murton, J.B., 2017. Chapter 3 Geomorphological framework: glacial and periglacial sediments, structures and landforms. Geological Society London, Engineering Geology Special Publications, 28, 59-368.

Heaton, T.J. et al., 2020. Marine20—The Marine Radiocarbon Age Calibration Curve (0-55,000 cal BP). Radiocarbon, 62, 779-820.
Masson-Delmotte, V., Zhai, P., Pirani, A., Connors, S.L., Péan, C., Berger, S., Caud, N., 2021. Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press.

Mayer, L.M., Macko, S.A., Mook, W.H., Murray, S., 1981. The distribution of bromine in coastal sediments and its use as a source indicator for organic matter: Organic Geochemistry, 3, 37–42.

Nickerson, D., 1940. History of the Munsell Color System and Its Scientific Application. Journal of the Optical Society of America, 30, 575-586.

Pinot, J-P., 1998. La gestion du littoral. Géographie physique et Quaternaire. Institut Océanographique, Paris, 759 p.

Reynolds, R.C., 1963. Potassium-rubidium ratios and polymorphism in illites and microclines from the clay size fractions of Proterozoic carbonate rocks. Geochimica et Cosmochimica Acta, 27, 1097–1112.

Rodriguez-Lazaro, J., Pascual, A., García, B.M., 2013. Recent benthic foraminifers as indicators of the sedimentary dynamics of the Tina Mayor and Tina Menor estuaries (S Bay of Biscay, N Spain). Journal of Marine Systems, 109–110, S213-S232.

Tessier, B., Reynaud, J. Y., Trentesaux, A., Le Bot, S., Grand, E. G., Gregoire, G., Méar, Y., 2022. Evolution of sediment wedges along the French coasts of the English Channel, a synthesis. Tidalites 2022 International Congress Abstracts.

Tisnérat-Laborde, N., Paterne, M., Métivier, B., Arnold, M., Yiou, P., Blamart, D., Reynaud, S., 2010. Variability of the northeast Atlantic sea surface Δ14C and marine reservoir age and the North Atlantic Oscillation (NAO). Quaternary Science Reviews, 29, 2633–2646.
# THE DYNAMICS OF SILICA SEDIMENT AT THE ECUSED SITE IN THE BAIE DE SEINE BASED ON THE COVA METHODOLOGY: (1) GLAUCONITE TRACED BY ARSENIC

Anne MURAT¹,², Emmanuel POIZOT¹,², Gwendoline GRÉGOIRE¹,², Valentin LE GOFF², Pierre WEILL³, Christoph MENSENS³, Léo PANCRAZZI³, Clément FRIGOLA² and Yann MÉAR¹,²

1 Conservatoire National des Arts et Métiers, INTECHMER, Bd de Collignon, 50110 Tourlaville, France

2 Normandie University, UNICAEN, LUSAC, EA4253, 60, Rue Max-Pol Fouchet, CS 20082 Cherbourg, France

3 Normandie University, UNICAEN, CNRS, M2C, Esplanade de la Paix 14000 Caen, France

Corresponding author: anne.murat@lecnam.net

**Abstract:** The English Channel, and specifically the Baie de Seine, is one of the maritime areas hosting the most human activities in Europe. The ECUSED project focuses on the cumulative impact of marine aggregate extraction activities ("Granulats Marins du Havre" concession) and dumping of dredged sediment by the Normand harbours ("Grands Ports Maritimes") for the maintenance of navigation channels (MACHU site), which are concentrated at the outlet of the Seine estuary. To achieve this goal, we used an innovative methodology (ELCOVA) on 114 surface sediments samples according to a regular grid of 2 stations per km². Samples were separated by sieving into 16 particle size fractions and elemental geochemical analyses were carried out on the separated granulometric fractions using X-ray fluorescence spectrometry. The present work focuses on identifying the movements of the various grain sizes naturally present at the site. The mineral considered is glauconite, an alumino-silicate, traced in the elemental analysis by arsenic. The study zone reveals a complex history in terms of granulometry, geography and time. We identify two sources of material, underwater erosion, river input, transports on the seabed and in suspension and a dynamic equilibrium sedimentary state. An essential point of the results obtained is that each granulometric fraction has its own history. Grains belonging to the same sediment sample do not come from the same sources, have not been transported in the same direction, and have not been deposited at the same time under the same energy conditions. It is possible to find grains of the same size and mineralogy from different sources in the same surface sample. This directly challenges the view of sedimentologists on sedimentation.

**Keywords:** Sediment dynamics, Glauconite, COVA methodology, Baie de Seine.

## 1 INTRODUCTION

The English Channel is a shallow epicontinental basin characterised by slow litho-bioclastic sedimentation established since the last transgression. Sediment distribution is controlled by tidal currents, with waves only interfering effectively in coastal areas. As the Bay of the Seine is relatively sheltered from long ocean swells, the distribution of surface sediments is directly linked to the action of tidal currents (Larsonneur, 1971; Gentil et Cabioch, 1997). Loose sediments generally form a thin layer. The siliceous sedimentary stock comes from contributions during regressive phases by various periglacial agents: fluvial and eolian transport, mudflows, sheets of cryoclasts from basement rocks and glacial rafts. The characteristics of this terrigenous material reflect a long evolution with successive contributions and repeated reworking (Auffret et al., 1975).

Assessing the cumulative impacts of human activities on the environment remains a major challenge for marine ecosystem conservation and management (Foley et al., 2017; Stelzenmüller et al., 2020). The English Channel, and specifically the Baie de Seine is among one of the maritime areas hosting the most human activities in Europe (Marmin et al., 2014; Dauvin et al., 2020). Among these activities, marine aggregate extraction and the dumping of dredged material from ports and waterways have increased in recent years, representing a challenge to coastal zone management (Le Bot et al., 2010; Duclos et al., 2013; Marmin et al., 2014). The cumulative sedimentary impacts of aggregate extraction and dredged material dumping, however, remain largely unknown, mainly due to a lack of methods allowing to trace sedimentary dynamics. The ECUSED project focuses on the cumulative impact of marine aggregate extraction activities ("Granulats Marins du Havre" concession) and dumping of dredged sediment by the Normand harbours ("Grands Ports Maritimes") for the maintenance of navigation channels (MACHU site), which are concentrated at the outlet of the Seine estuary.

Geochemistry is most often used to trace the origins of particles in total sediment (Baux et al. 2022). In coastal environments, the results obtained depend on the granulometric variability of the samples, which essentially reflects the energy conditions of the environment studied. By analysing grain size fractions, we can bypass this granulometric variability. It is then possible to access the spatial variability of the composition of grains of the same size, i.e. the geographical sources of these grains (Peuziat 2022; Grégoire et al., in
press). The elemental chemical composition of grain size fractions enables us to obtain two types of information: (i) identification and quantification of the different grain types: quartz, calcium carbonates, zircon... and (ii) the spatial variability of each grain type for each grain size fraction. We have developed a new methodology COVA (Composition Variation Analysis) that combines: (i) elemental analysis of sediment fractions and (ii) a sampling strategy organised according to a regular grid, with a sampling density adapted to the research objective. This approach provides 'ground truth' data that complement predictive numerical models which are limited when confronted with sediment heterogeneity and the complex hydrodynamic forcing characteristic of the Baie de Seine.

We use the term origin to designate the general conditions of particle formation. For example, it is accepted that quartz comes from the erosion of granites, and calcium carbonates from current to sub-actual marine biogenic production. The term source refers to a specific geographical location in the study area. A quartz grain sampled on a beach, that was initially crystallised in a granitic magma (origin), may have been brought to the beach from offshore by cross-shore transport (source). The study of spatial variability in the chemical composition of a single fraction provides information on the sources of the different types of grain that make it up. It is possible to identify the relationships that exist or do not exist between these different sources (mixing gradient between two sources, for example), thus obtaining information on the transport dynamics of these grains.

Sediment dynamics result from the dynamic of the water mass (tide and swell) modulated by a pre-existing morphology and applied to sedimentary grains likely to be in motion. This work focused on identifying the movements of the different grain sizes naturally present at the site. The mineral considered is glauconite, an alumino-silicate, traced in the elemental analysis by arsenic (Mumford et al., 2012; Baux et al., 2019).

# 2 MATERIALS AND METHODS

## 2.1 Study area

The study area (43 km²) is located in the Baie de Seine off the Seine estuary (Fig. 1) and was chosen because two distinct anthropogenic activities were taking place only 4 km apart. The Granulats Marins du Havre (GMH) concession is in the northern part, and at the time of the mission in April 2022, extraction started approximately 3 months before. Extractions of between 10,000 and 50,000 m³.yr⁻¹ are planned. The Machu dredge disposal area is located south of the study area, on the southern edge of the Banc de Seine. It consists of 6 series (labelled A to F) of boxes that will be used successively over a period of about 10 years. Each series is used for 2 to 3 years, during which deposits accumulate to a thickness of several metres and are then abandoned. At the time of the mission (2022), serie F has been used between 2017 to 2019 before being abandoned. The C serie has been in operation since 2020. The deposited sediments come from dredging carried out by the Grand Port Maritime de Rouen (GPMR), which oversees navigation in the Seine estuary up to the port of Rouen. The sediments amount to 5 million m³.yr⁻¹.

<figure><img src="image_40.png" /><figcaption>Figure 1: Location of the ECUSED study area, the Granulats Marins du Havre (GMH) concession and the Machu dredge deposit area.</figcaption></figure>
## 2.2 Methodology

The ECUSED mission (April 2022) collected surface sediment at 114 stations on a regular grid of 2 stations per km², with a higher station density in areas influenced by anthropogenic activities (Fig. 2A).

The samples were separated by sieving into 16 particle size fractions, ranging from less than 50 µm to 2000 µm (log scale of 1.26) (NF ISO 3310-1, 2016). Elemental geochemical analyses were carried out on the separated granulometric fractions using X-ray fluorescence spectrometry. Sixty marine sediment standards were used to calibrate the analytical method.

The granulometric mean is based on dataset obtained using a Horiba laser particle size analyser with a measuring range from 0.01 µm to 2000 µm. Residual tidal currents were obtained using the MARS 2D (Model for Application at Regional Scale in 2D) numerical model. This model simulates the horizontal movement of oceanic water masses to study and predict the dynamics of coastal and estuarine waters (Lazure and Dumas, 2008).

<figure><img src="image_41.png" /></figure>

**Figure 2: Physical data: 2A, location of the 114 sediment samples collected and granulometric mean variability; 2B, bathymetry of the study area, blue star for swell station (2D) and red star for position of tidal currents rose (2C); 2C, tidal currents rose; 2D, swell data, Hmo from Cerema CANDHIS 07605-Le Havre2 buoy at -17m depth, recorded between 10/01 and 05/03/2010.**

# 3 RESULTS AND DISCUSSION

## 3.1 Results

The natural bathymetry of the study area ranges from -21 m in the north to -14 m at the top of the Banc de Seine, a relief bounded by the -15 m isobath (Fig. 2B). In the extreme south-east, the Parfond corresponds to the paleo-valley of the Seine (Fig. 1). Dredged sediments from the F serie of boxes have reached the -12 m bathymetric level.

In this area of the Baie de Seine, tidal currents are northeasterly during the flood and south-westerly during the ebb, generally reaching 0.4 m.s⁻¹ with a maximum of 0.8 m.s⁻¹ in the Parfond (Fig. 2C). The dominant swells come from the northwest and the northeast (Fig. 2D).
The map in Fig. 2A shows the granulometric mean variability of the ECUSED zone. The sediments are sandy, with a refinement gradient from north (very coarse sands at -21 m) to south (fine to medium sands on the Banc de Seine). The finest sediments (silty sands) are found in the serie of active MACHU boxes C.

Glauconite is a specific clay that forms at the water-sediment interface in a marine environment, forming grains within the sandy particle size range. Glauconite is generally assumed to occur within fine-grained marine (Amorosi, 1997) outer shelf and slope deposits, at an average depth between about 50 m to 500 m (Odin and Matter, 1981) under warm climatic conditions (Banerjee et al., 2020). These formation conditions do not correspond to the current environment of the Baie de Seine. Here it is allochthonous detrital (Amorosi, 1997) derived and eroded from the Mesozoic chalk and glauconitic sandstone of the Bassin Parisien. The marine origin of glauconite means that it remains stable in the marine environment and is enriched in modern sands. Figure 3 shows the geographical variability in the amount of arsenic (As), the quantitative tracer of glauconite, for nine granulometric fractions. Each fraction has its own concentration scale, with a maximum value ranging from 79 ppm for the coarsest fraction to 15 ppm for the finest. Interpretation of the results is essentially based on the geographical position of the samples with the highest glauconite content for each fraction examined.

The two coarsest fractions, 800 - 1000 µm and 630 - 800 µm, could not be analysed at certain stations in the south of the zone due to insufficient material quantity.

For the 800 - 1000 µm fraction, the zone of maximum concentration is located on the northern flank of the Banc de Seine, slightly north of the -15 m isobath. This zone is considered as source 1 of glauconite grains, all the more so as glauconite grains >1000 µm were also identified. There is no glauconite grains at the northern and southern ends of the studied zone.

From 800 to 315 µm, the zone of maximum concentration moves south-eastwards, widening at the top of the Banc de Seine. Each finer fraction moves further, being transported upslope. This « shallower = finer » refinement gradient indicates the prevailing action of the tide on the swell (Larsonneur. 1971; Levoy and Larsonneur, 1992; Guillen and Hoekstra, 1996; Baux et al., 2019). Each fraction finds a depth on a slope where it reaches dynamic equilibrium. The F and C series become increasingly distinct because the sediments deposited there do not contain glauconite of these sizes.

Below 315 µm, a new source of glauconite grains appears in the south-east. The black circle on the 250 - 315 µm map identifies it as source 2. This source 2 probably corresponds to sub-actual inputs from the Seine River. For the 250 - 315 µm fraction, the extension zone progresses to south and west, bypassing the F and C series.

For the 200 - 250 µm fraction, the extension restricts to the west and reaches the F series of boxes, a 2m morphological obstacle which the glauconite grains can cross only by suspension. As the C boxes series is still free of arsenic, except in the north, kinetics must be used to interpret this difference. The use of the two series of boxes has been delayed by two years. There are two possible and compatible hypotheses: (i) the active C series is constantly receiving deposits that cover any glauconite grains deposited from suspended load, (ii) suspension occurs occasionally, for example during a storm, when swell and tidal currents combine to allow transport, and only the F series of boxes and the northern part of the C series can record these events.

For the 160-200 µm fraction, the regression area is wider to the west, south, and even north. Most of the source 1 zone no longer contains glauconite grains. The source 2 zone is dominant, but despite possible suspension, the area of extension remains limited. This may be because the very small amount of glauconite grains in suspension makes it unidentifiable in the deposits. Analysis of the deposits in the C series of boxes shows that some of the boxes, with no geographical connection between them, contain glauconite grains of this size. It is likely that some of the dredged areas are glauconite grains sources.

The last fraction in which glauconite grains can be identified is 125 - 160 µm. The extension is limited to the source 2 zone, which shows that even if some of the glauconite grains of this size are transported in suspension and dispersed, some are still transported in contact with the seabed.

## 3.2 Discussion

The study of the glauconite grains in the ECUSED zone reveals a complex history in terms of grain size, geography, and time. Two sources of material were identified.

**Source 1** contains coarse glauconite grains (> 800 µm) that were not transported under the current energy conditions. It corresponds to a deposition that occurred several thousand years ago. Based on seismic data, Benabdellouahed et al. (2013) interpreted the Banc de Seine as consisting of glacial alluvial deposits from the Seine River. Part of these fluvial terraces, located on the northern flank, is not covered by modern sedimentary deposits and, for certain granulometric fractions, is even undergoing erosion.
<figure><img src="image_42.png" /></figure>

Figure 3: Geographical variability of As concentration (ppm), the quantitative tracer of glauconite, for nine granulometric fractions. The black line on the 315 - 400 microns map indicates the position of source 1 and arrows the south-eastward movement of the glauconite grains. The black dot on the 250 - 315 and 125 - 160 microns maps represents the position of source 2. The white oval on the 250 - 315 and 200 - 250 microns maps highlights the evolution of the F series of boxes.

The 800 to 200 µm fractions are transported on the seabed by tidal currents up the northern slope of the Banc de Seine, with finest fractions being transported further south-east. Each granulometric fractions finds its zone of dynamic equilibrium and remains there. This dynamic equilibrium may have existed for thousands of years. The south-eastward transport may seem surprising, given that the dominant tidal currents are those from the north-east flow (Fig. 2C). Auffret and d'Ozouville (1986) reached to the same conclusion of a south-easterly transit and areas of underwater erosion based on side-scan sonar data. An analysis of the tide roses for different parts of the study area shows that currents with velocities greater than 50 cm.s⁻¹ flow to the south-east. It is probably these higher energy currents that make this transport possible. From 250 µm,
glauconite grains can be transported in suspension during high-energy events (exceptional tides or swells) and dispersed much further and in other directions.

**Source 2** consists of glauconite grains arriving at the study site already sorted (315 - 125 µm) from the south-east. It is possible that their transport dynamics are related to the current flooding of the Seine. Our data do not allow any further interpretation.

## 4 CONCLUSION

The COVA method makes it possible to: (i) interpret the history of a mineral in detail, (ii) distinguish bedload and suspension transport of particles, (iii.) identify the directions of transport in contact with the seabed, and (iii) reconstruct the sedimentary dynamics of a particular grain type in the present coastal environment on the basis of a single sampling campaign. An essential point of the results obtained is that each granulometric fraction has its own history. Grains from the same sediment sample do not come from the same sources, have not been transported in the same direction, and have not been deposited at the same time under the same energy conditions. It is possible to find grains of the same size and mineralogy from different sources in the same surface sample. This directly challenges the view of sedimentologists on sedimentation and explains the difficulties encountered by hydrosedimentary models in reconstructing bedload transport.

## 5 ACKNOWLEDGMENT

This research project was conducted under the "RIN Recherche Emergent" co-financed by the Normandy region and the European Union (ERDF). Sediment samples were obtained during the ECUSED cruise. The authors would like to thank the "Flotte Océanographique Française" and the vessel crew of the N/O "Côtes de la Manche". This paper was reviewed by Sophie Le Bot and Nicolas Tribovillard.

## 6 REFERENCES

Amorosi, A., 1997. Detecting compositional, spatial, and temporal attributes of glaucony: a tool for provenance research. Sedimentary Geology, 109, 1-2, 135-153.

Auffret, J.-P., Hommeril, P., Larsonneur, C., 1975. La Mer de la Manche, modèle de bassin sédimentaire épicontinental sous climat tempéré. Xe Congrès International de Sédimentologie, Nice, Thème 5, 25-32.

Auffret J.-P, d'Ozouville L., 1986. Apports de l'imagerie fournie par le sonar à balayage latéral à la connaissance de la dynamique sédimentaire en Baie de Seine. In: IFREMER Conference Proceedings, La Baie de Seine (GRECO-MANCHE Workshop), Université de Caen, 24-26 Avril, 201-209.

Banerjee, S., Choudhury, T.R., Saraswati, P.K., Khanolkar, S., 2020. The formation of authigenic deposits during Paleogene warm climatic intervals: a review. Journal of Palaeogeography, 9, 27 pp.

Baux, N., Murat, A., Faivre, Q., Lesourd, S., Poizot, E., Méar, Y., Brasselet, S., Dauvin, J.-C., 2019. Sediment dynamic equilibrium, a key for assessing a coastal anthropogenic disturbance using geochemical tracers: Application to the eastern part of the Bay of Seine. Continental Shelf Research, 175, 87-98.

Baux, N., Murat, A., Poizot, E., Méar, Y., 2022. An innovative geostatistical sediment trend analysis using geochemical data to highlight sediment sources and transport. Computers & Geoscience, 26, 263-278.

Benabdellouahed, M., Dugué, O., Tessier, B., Thinon, I., Guennoc, P., 2013. Évolution pléistocène de la Seine fluviatile préservée en Baie de Seine. Quaternaire, 24, 267-277.

Dauvin, J.-C., Raoux, A., Pezy, J.-P., Baux, N., Niquil, N., 2020. The Bay of Seine: A Resilient Socio-Eco-System Under Cumulative Pressures. In: Evolution of Marine Coastal Ecosystems under the Pressure of Global Changes (Ed. H.-J. Ceccaldi, Y. Hénocque, T. Komatsu, P. Prouzet, B. Sautour, and J. Yoshida), Springer International Publishing, Cham, 95-109.

Duclos, P.-A., Lafite, R., Bot, S.L., Rivoalen, E., Cuvilliez, A., 2013. Dynamics of Turbid Plumes Generated by Marine Aggregate Dredging: An Example of a Macrotidal Environment (the Bay of Seine, France). Journal of Coastal Research, 29, 25-37.

Foley, M.M., Mease, L.A., Martone, R.G., Prahler, E.E., Morrison, T.H., Murray, C.C., Wojcik, D., 2017. The challenges and opportunities in cumulative effects assessment. Environmental Impact Assessment Review, 62, 122-134.

Gentil F., Cabioch L., 1997. Carte des peuplements macrobenthiques de la baie de Seine et Manche centrale sud. Editions Station Biologique de Roscoff, Université Paris VI et C.N.R.S.- U.P.R. 9042, carte A3 et notice 18 pp.

Grégoire, G., Murat, A., Garland, T., Shute, P., Krieche, R., Gallon, R., Frigola, C., Méar, Y., Nivet, T., Le Moël, M., In press. Construction process of the Langue de Barbarie in Sénégal, based on the identification
and quantification of fluvial and marine sources: application of the new ELSA geochemical methodology. Estuaries and Coasts.

Guillén, J., Hoekstra, P., 1996. The "equilibrium" distribution of grain size fractions and its implications for cross-shore sediment transport: a conceptual model. Marine Geology, 135, 15-33.

Larsonneur, C., 1971. Manche Centrale et Baie de Seine : géologie du substratum et des dépôts meubles. Thèse doctorat d'Etat, Univ. Caen, 394 pp.

Lazure, P., Dumas, F., 2008. An external-internal mode coupling for a 3D hydrodynamical model for applications at regional scale (MARS). Advances in Water Resources, 31, 233-250.

Le Bot, S., Lafite, R., Fournier, M., Baltzer, A. Desprez, M., 2010. Morphological and sedimentary impacts and recovery on a mixed sandy to pebbly seabed exposed to marine aggregate extraction (Eastern English Channel, France). Estuarine, Coastal and Shelf Science, 89, 221-233.

Levoy, F., Larsonneur, C., 1992. Notice d'impact en vue de travaux de protection contre la mer (Basse-Normandie). Presses Universitaires de Caen, 76 pp.

Marmin, S., Dauvin, J.-C., Lesueur, P., 2014. Collaborative approach for the management of harbour-dredged sediment in the Bay of Seine (France). Ocean & Coastal Management, 102, 328-339.

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Peuziat, B., 2022. Ensamblage de la zone intertidale du littoral rocheux de Seine-Maritime: origines et évolutions diachroniques (Normandie, France). Thèse de l'Université de Normandie, 438 p.

Stelzenmüller, V., Coll, M., Cormier, R., Mazaris, A.D., Pascual, M., Loiseau, C., Claudet, J., Katsanevakis, S., Gissi, E., Evagelopoulos, A., Rumes, B., Degraer, S., Ojaveer, H., Moller, T., Giménez, J., Piroddi, C., Markantonatou, V., Dimitriadis, C., 2020. Operationalizing risk-based cumulative effect assessments in the marine environment. Science of The Total Environment, 724, 138118.
# HYPOTHESIS ON THE KARSTIFICATION/DOLOMITIZATION OF THE BARREMIAN RESERVOIR IN PARENTIS BASIN

Alexandre ORTIZ¹, Aloys BAUDESSON², Christine FLEHOC¹, Catherine GUERROT¹, Xavier LOPEZ², Olivier DOUEZ¹, Gabrielle RUMBACH²,³,

¹BRGM – Bureau de Recherches Géologiques et Minières

²Vermilion-Rep SAS

³ES-Geothermie

Corresponding author: a.ortiz@brgm.fr

**Abstract:** The Parentis sub-Basin, located southwest of Bordeaux (France), has been known since the 1950s as a prolific oil reservoir. It lies at the eastern end of the Bay of Biscay. Several fields are exploited in the onshore part of the basin, the largest being at Parentis-en-Born, discovered by ESSOREP in 1954 and exploited by VERMILION REP since 2006. The field is an east-west anticlinal structure, formed during various geodynamic episodes in connection with halokinesis of the Triassic and Hettangian series. One of the reservoirs exploited is a limestone/dolomite dating from the Barremian to the Upper Jurassic characterized by a matrix porosity of 14% and a permeability of 100 mD over an average thickness of 200 metres.

Numerous studies and acquisitions (3D seismic, 122 boreholes) have been carried out by ESSOREP and VERMILION REP exploration geologists to characterise the limestone/dolomite reservoir, particularly regarding the origin of the dolomitization and porosity. Models of emplacement suggest post-sedimentary dolomitization by reflux and epigenic karstification linked to early tectonic bulges.

Our study aims to use modern methods and concepts to gain a better understanding of the major stages in the development of this reservoir in relation to the geodynamic events. It is based on a detailed study of cores and thin sections. A total of 160 samples were taken from 7 boreholes to conduct a diagenetic study (petrography, cathodoluminescence and initial results on strontium isotopes).

The macroscopic study of the cores showed the presence of numerous mud losses (no-recovery zones), often correlated with faults (striations observed), which were previously interpreted as epigenic karstification. Numerous veins were observed in the fracture zones, composed of calcite, saddle dolomite and anhydrite. Microscopically, the paragenesis shows primary dolomitization prior to burial (euhedral to subhedral dolomite), followed by phases of circulation of magnesium-rich hydrothermal fluids (saddle dolomite - ‘non-planar’) and sulphates (anhydrite).

Initial interpretations suggest that the void zones encountered in the boreholes are linked to the upwelling of deep-seated fluids sourced by Triassic/Lower Jurassic evaporites and dolomites, resulting in hypogenic karstification. These episodes would be linked to the resumption of the anticline during the Pyrenean phase.

**Keywords:** Aquitaine Basin, parentis, hypogene, thin-section, saddle dolomite.

## 1 INTRODUCTION

The Parentis sub-basin is the eastern termination of the V-shaped bay of Biscay and recorded several structuration phases related to geodynamic events, (i) an initial phase of Permian-Triassic rifting culminated in the deposition of thick Keuper evaporites (Bourrouilh et al., 1995; Brunet, 1994; Desegaulx and Brunet, 1990; Mathieu, 1986), (ii) a second rifting phase took place between the latest Jurassic and the early Cretaceous with a Neocomian interval of reduced subsidence, (iii) after the late Albian, the basin underwent post-rift thermal subsidence (Mathieu, 1986; Roca et al., 2011), (iv) finally from Late Cretaceous to late Oligocene the basin was inverted in response to the Pyrenees mountain belt building (Brunet, 1994; Ortiz et al., 2020). This latest phase is responsible for the creation of the Parentis field anticline. The maximum burial phase take place during Late Cretaceous time (Vermilion, pers. comm.) and the Parentis field shows 1500 meters of Tertiary sediments.

The Parentis field, an east-west anticline, (Fig. 1A) is located in the Parentis sub-basin (Ardaens, 1992; Biteau et al., 2006; Enjalbert, 1957; Mathieu, 1986; Mathieu et al., 1984; Vajk and Walton, 1956). The structural style, mostly inherited from the Lower Cretaceous, is influenced by the mobility of the Keuper salt (Trias), with simple anticlines. The maximum relief of the structure is in the eastern part of the field, while the western part shows a more moderate dip of the anticline (Fig. 1B). Seismic images show that this structure is
located directly above a Triassic saliferous bulge (Mathieu et al., 1984; Mediavilla, 1987). In addition, the major fault appears to be rooted in this level.

The reservoirs are composed of a succession of limestone and dolomite levels of Lower Aptian, Barremian, Neocomian and Upper Jurassic ages. The main cover of the reservoirs is the marly Upper Aptian. Several types of reservoirs are present, classified according to their degree of porosity and permeability. The most prolific corresponds to very porous and permeable rock in the dolomitic zones of the field. These favorable characteristics are linked to intense fracturing associated with significant dissolution (karst, very vacuolated rock, even cavernous rock, etc.) (Ardaens, 1992).

<figure><img src="image_43.png" /><figcaption>Figure 1. A) Location of the Parentis field in the Parentis Basin (modified from Biteau et al., 2006), B) Isobath of the Top Barremian (meters) with the associated faults, location of the studied wells.</figcaption></figure>

Numerous hypotheses exist about the formation of this prolific reservoir, based on both dolomitization and karstification. They have been outlined in various internal ESSO-REP reports and in publications (Ardaens, 1993, 1992; Arents, 1978; Chueca, 1959; Todd, 1989). Several processes (dolomitization, karstification, faulting, compaction, oil migration) have been described, but the hierarchy of these processes and their relative contribution to the formation of the reservoir are still under discussion, and no consensus has been reached in the literature. Our study aims to characterize the establishment of these karstifications over time and understand the origin and sourcing of the dolomitization.

## 2 MATERIALS AND METHODS

The Parentis oil field has been studied since its discovery in 1954. A total of 122 wells have been drilled since then and many cores are available (Fig. 1B). The initial work focused on describing the core data (sedimentology, structure, karstification) from the Vermilion core storage (STC Boussens) while also examining all the thin sections (covered) available.

To understand the reservoir's formation and modification over geological time, 160 samples were collected from 7 wells, spanning from the western to the eastern part of the field. Additionally, 140 new thin sections were prepared for optical microscopy observations and cathodoluminescence characterizations. These two methodologies enabled the construction of a paragenesis and a hierarchy of the cements and veins associated with structural features. Geochemical isotopy analysis (δ13C ‰, δ18O ‰, ⁸⁷Sr/⁸⁶Sr) were conducted to constrain the composition and age of the parent fluids. In this paper, only the strontium isotopy will be presented, as the other geochemical analyses are still ongoing.

## 3 RESULTS

The first results on this study focus on two wells located in the central part of the anticline structure (Figs. 1B, 2). These results are based on different scales of description and different methods, ranging from core analysis to thin sections and from cathodoluminescence to strontium isotopy.

### 3.1 Core and thin section observations

The sedimentary sections (Fig. 2A) reconstructed from the original core description sheets have provided a better understanding of the distribution of lithologies and of the area of no-recovery (associated with mud loss). These sedimentary sections (Fig. 2A) synthetise lithology, sample location, areas of no-recovery, and the locations of faults and fractures from core observation, the 3D seismic model (Vermilion, pers. comm.) and the literature (Arents, 1978). The correlation between reservoirs is also indicated. Numerous occurrences of no-recovery areas are visible on the core boxes (indicated by the grey color on Fig. 2A).

The PS11 well (Fig. 2A) shows such areas in the R3 and R4 reservoirs. Where the core is available, fractures and faults are observed (between 2105 m and 2130 m in PS11, Fig. 2B-C). Faults from the 3D seismic data from Vermilion are also present in this interval. The PS30 well (Fig. 2A) also shows an area of no-recovery in the R4 and R5 reservoirs (between 2410 m and 2470 m in PS30, Fig. 2A). The 3D seismic
model shows faults in this interval. In PS30, adjacent to this fracture zone, evidence of fluid circulations is visible (Fig. 2D).

<figure><img src="image_44.png" /><figcaption>Figure 2. A) Synthetic sedimentary logs of PS11 and PS30 wells, with simplified lithologies, no-recovery, samples location, faults (from core observation, Vermilion seismic model and literature, Arents, 1978). B to D) samples, showing fault expression (striated fault) and fluid circulations with white saddle dolomite veins on cores. EA) Sample pictures showing stylolite and calcite vein. EB) Thin section (PLA) with three dotted</figcaption></figure>
rectangles for EC, ED, and EE zoom, EC, ED, EE) cathodoluminescence showing dolomite (D-Subhedral), stylolite post-dolomitization and two calcite cements (C1-Bright Yellow Blocky Calcite and C2-Dull to Light Yellow Blocky Calcite). FA) Thin section (PLA) with dotted rectangles for FB. FB) Cathodoluminescence showing relation between dolomite (D1), saddle dolomite (SD), calcite (C2) and oil. GA and HA) Thin section (PLA) with dotted rectangles for GB and HB. GB and HB) Cathodoluminescence showing primary dolomite cut by two veins filled by saddle dolomite (SD1) and calcite (C2). IA) Thin section (PLA) with dotted rectangles for IB. IB) Cathodoluminescence showing primary dolomite (D) cut by anhydrite vein (ANH). JA) Sample pictures showing two crosscut veins and dotted rectangle for JB and JC. JB and JC) PLA and cathodoluminescence images, with dolomitize mixed siliciclastic-carbonate rocks with two veins of saddle dolomite (SD1 and SD3) attesting for multiple fluids circulations. K) Strontium isotopy values of saddle dolomite and anhydrite from veins (from sample collected in PS11 and PS30) on the curve modified form McArthur et al. (2012).

The areas of no-recovery appear to be linked with intervals where faults and large veins are observed. Additionally, no stratigraphic correlation exists between dolomitization and no-recovery between the PS11 and PS30 wells. No classic epigenic karstification features (such as speleothems, clay and sand karst-infill) were observed in these two wells.

In thin section and cathodoluminescence images, several minerals and cementation features were observed, reflecting the complex history of reservoir formation. The original limestones (packstone to grainstone) show dolomitization (Fig. 2EC, 2ED, 2EE 2GA, 2GB, 2HA, 2HB, 2IA, 2IB, 2JA, 2JB, 2JC) with euhedral to subhedral dolomite which may be full or partial. Several veins and geodic mineralizations were observed at sample and thin section scales (Fig. 2D to 2JC). Three major minerals are represented: saddle dolomite (SD) (Fig. 2D, 2GA, 2GB, 2 HA, 2HB, 2JA, 2JB, 2JC), anhydrite (ANH) (Figures 2IA, 2IB), and calcite (C) (Fig. 2EA, 2EB, 2EC, 2ED, 2GB, 2HB, 2JC). Saddle dolomite, with its undulatory extinction in PLA and its anhedral (non-planar) shape is known to be hydrothermal form of dolomite (Machel, 1987; Radke and Mathis, 1980). Saddle dolomite can be found with anhydrite and/or calcite (Fig. 2G). At least three generations of saddle dolomite veins are observed (Fig. 2JC). Anhydrite veins can be related linked with saddle dolomite veins or be alone (Fig. 2IA, 2IB). Calcite cements show two generations which are distinguished by their luminescence (C1 and C2), these cements can be found in veins (with saddle dolomite) (Fig. 2GB, 2JC) and filling vuggy porosity (Fig. 2EC, 2EE) in saddle dolomite or primary dolomite (Fig. 2FA, 2FB). Figure 2FA shows also oil impregnation crosscutting all the cements.

In terms of chronology, figure 2EE shows a concentration of euhedral dolomite in stylolite which occurs due to differences in solubility between calcite and dolomite. This primary dolomitization (D-Euhedral, Subhedral) predates compaction due to burial (maximum burial occurred during late Cretaceous, Vermilion Pers. Comm.; Brunet, 1994; Mathieu, 1986). Then, several veins cross the horizontal stylolites (Fig. 2EB, 2FA, 2EE) and thus postdate the compaction phase. These veins are formed by both hydrothermal (saddle dolomite) and sulphate rich (anhydrite) minerals. These veins are all located next to the fault, and the degree of veins occurrences decrease as you move away from the accidents. Oil migration could occur after all these episodes of fluids.

## 3.2 Strontium isotopy

Strontium isotopy has been performed on the saddle dolomite and anhydrite (veins) to identify the origin of the fluids reponsible for the large modification of the reservoir properties. Four anhydrites and seven saddle dolomites were analyzed. Anhydrite ⁸⁷Sr/⁸⁶Sr values shows a variability from 0.707595 ± 0.000010 to 0.707636 ± 0.000009. Saddle dolomite ⁸⁷Sr/⁸⁶Sr values shows a variability from 0.707732 ± 0.000009 to 0.707933 ± 0.000010. Theses values are plotted on the ⁸⁷Sr/⁸⁶Sr global marine signal curve from McArthur et al., (2012) (Fig. 2K). At the first order, the ⁸⁷Sr/⁸⁶Sr value, which reflect the age of the chemical stock (magnesium and suflur), could correspond to several geological time (Figure 2K), including Palaeogene, Upper Cretaceous and Trias-Hettangian. In the sedimentary history of the Parentis basin, no evidence of large evaporites and dolomite have been described in Paleogene and Upper cretaceous. The Triassic ("Dolomie de Carcans") and Hettangian-Sinemurian ("Zone a anhydrite") sediments are rich in magnesium (dolomite) and sulfur (anhydrite) (Fig. 2K) (Ferrer et al., 2012; Mathieu, 1986). These levels could be strong candidates for the sourcing of theses hydrothermal circulations.

# 4 FIRST CONCLUSIONS ON THE KARSTIFICATION/DOLOMITIZATION

Initial work on two wells in the Parentis field (PS 11 and PS 30) has revealed diagenetic transformations that could lead to reservoir modifications, i) dolomitization of the limestones (partial or full) with euhedral and subhedral dolomite, ii) compaction ( stylolite), iii) faulting and fluid circulation near the fracture zones, leading to veins and geodic mineralizations (saddle dolomite, anhydrite, calcite). Several no-recovery areas were identified, all of which are located near these fracture zones. No classic epigenic karstification (speleothem, clay and sand pockets) was observed in these two wells.

The first results on ⁸⁷Sr/⁸⁶Sr indicate a possible Triassic/Lower Jurassic origin for the magnesium and suflur rich minerals. Anticlinal formation during Pyrenean inversion (Tertiary) helped connecting the Barremian
reservoir with the lower part of the basin through these faults. These hydrothermal fluids could have led to hypogenic karstification, responsible for these large areas of no-recovery, which are the only evidence of voids created by karstification.

These preliminary results need to be confirmed by studying the other wells mentioned and the 3D seismic data to confirm this initial hypothesis about karstification/dolomitisation in the Barremian reservoir at the Parentis field scale.

## 5 ACKNOWLEDGMENT

This work is being carried out under a CICo agreement between BRGM and VERMILION REP SAS. This work has benefited from state aid managed by the "Agence Nationale de la Recherche" (ANR) as part of the future investment programme for the IMAGINE project under the reference "ANR-21-ESRE-0043". The authors and editors would like to thank also the reviewers, Michel Séranne and Jean-Jacques Biteau, who helped improving the paper.

## 6 REFERENCES

* Ardaens, R., 1993. Champ de Parentis, Bilan des études géologiques. Rapport interne ESSO/REP.

* Ardaens, R., 1992. Le champ de Parentis. Réservoir carbonaté complexe et paléokarst. Pétrole et techniques, 372, 34-42.

* Arents, C., 1978. Ré-interprétation géologique du Champ de Parentis. Rapport interne ESSO/REP.

* Biteau, J.-J., Le Marrec, A., Le Vot, M., Masset, J.-M., 2006. The Aquitaine basin. Petroleum Geoscience, 12, 247-273.

* Bourrouilh, R., Richert, J., Zolnaï, G., 1995. The North Pyrenean Aquitaine Basin, France: Evolution and Hydrocarbons. AAPG Bull, 79, 831-853.

* Brunet, M.F., 1994. Subsidence in the Parentis Basin (Aquitaine, France): Implications of the Thermal Evolution, in: Mascle, A. (Ed.), Hydrocarbon and Petroleum Geology of France. Springer, Berlin, pp. 187-198.

* Chueca, A., 1959. Répartition des caractéristiques et facteurs influençant la récupération finale dans un réservoir carbonaté hétérogène. 5th World Petroleum Congress, New York, USA, paper WPC-8118.

* Desegaulx, P., Brunet, M.-F., 1990. Tectonic subsidence of the Aquitaine basin since Cretaceous times. Bulletin de la Société géologique de France, 8, 295-306.

* Enjalbert, H., 1957. Parentis. Revue géographique des Pyrénées et du Sud-Ouest, 28, 35-59.

* Ferrer, O., Jackson, M.P.A., Roca, E., Rubinat, M., 2012. Evolution of salt structures during extension and inversion of the Offshore Parentis Basin (Eastern Bay of Biscay). Geological Society London Special Publications, 363, 361-380.

* Machel, H.-G., 1987. Saddle dolomite as a by-product of chemical compaction and thermochemical sulfate reduction. Geology, 15, 936-940.

* Mathieu, C., 1986. Histoire géologique du sous-bassin de Parentis. Bulletin des centres de recherches exploration-Production Elf-Aquitaine, 10, 33-47.

* Mathieu, C., Modiano, T., Mure, E.P., 1984. Bassin de Parentis Synthèse géologique et géophysique, bilan pétrolier. Rapport interne Elf Aquitaine.

* McArthur, J.M., Howarth, R.J., Shields, G.A., Zhou, Y., 2012. Chapter 7- Strontium isotope stratigraphy. Geologic Time Scale, 1, 211-238.

* Mediavilla, F., 1987. La tectonique salifère d'Aquitaine. Le bassin de Parentis. Pétrole et techniques, 335, 35-37.

* Ortiz, A., Guillocheau, F., Lasseur, E., Briais, J., Robin, C., Serrano, O., Fillon, C., 2020. Sediment routing system and sink preservation during the post-orogenic evolution of a retro-foreland basin: The case example of the North Pyrenean (Aquitaine, Bay of Biscay) Basins. Marine and Petroleum Geology, 112, 104085.

* Radke, B.M., Mathis, R.L., 1980. On the Formation and Occurrence of Saddle Dolomite. Journal of Sedimentary Research, 50, 1149-1168.

* Roca, E., Muñoz, J.A., Ferrer, O., Ellouz, N., 2011. The role of the Bay of Biscay Mesozoic extensional structure in the configuration of the Pyrenean orogen: Constraints from the MARCONI deep seismic reflection survey. Tectonics, 30, TC2001.
# MASW-2D SEISMIC AS A TOOL FOR INVESTIGATING THE ARCHITECTURE OF HOLOCENE COASTAL DEPOSITS: EXAMPLES IN THE NORTH OF FRANCE (BOULONNAIS AND FLEMISH COASTAL PLAIN)

Jean-Yves REYNAUD<fnref n="1" />, Rachid OUCHAOU<fnref n="1" />, Alain TRENTESAUX<fnref n="1" />, Roa SEIF DINE<fnref n="1" />, Nicolas ROBIN<fnref n="2" />, Olivier RAYNAL<fnref n="2" />, Franz ROHMER<fnref n="3" />, Thomas BUCHWALD<fnref n="4" /> and Pierre ANTOINE<fnref n="4" />

Corresponding author: jean-yves.reynaud@univ-lille.fr

**Abstract:** A 24-channel seismic display recording on a DaQLink is used to carry out tomographies of P and S waves down to 40m deep in the Holocene deposits of the coastal Flemish plain west of Dunkirk and the Slack estuary in the Boulonnais. The flat topography along the profiles collected along a road, on the beach, or in farmed fields, makes it possible to avoid complex processing of topographic correction. The results, validated by a number of cores, provide details on the morphology of the incision at the bottom of Holocene transgressive sediments, either infilling estuarine valleys or onlapping bedrock fault scarps. Vertical and horizontal heterogeneities in shear-wave velocity, well expressed in the upper layers, may reflect lateral changes in lithology or water content.

**Keywords:** MASW-2D, seismic tomography, Holocene, incised valleys, coastal plain, France.

## 1 INTRODUCTION

Coastal plains and lowlands are at stake in the context of global warming, which cause coastal erosion and salt intrusion, and the growing pressure on groundwater resources. While lidar and GNSS surveys help assessing the submersion hazards, stratigraphic data on the shallow subsurface are often limited by the availability of sediment cores, thousands of which are necessary to reconstruct the 3D heterogeneities of the related estuarine and deltaic depositional systems and their evolution through the Holocene. In this short paper, we report on a seismic survey held on two sites of Northern France, the Flemish Coastal plain and the Slack River estuary, as part of two PhD projects (Ouchaou, 2023; and Buchwald & Antoine, 2025).

The Flemish Coastal plain is the westernmost part of the Holocene deposits that fringe the Southern Bight of the North Sea from Northern France to Denmark. It results from overfilling of a network of incised valleys by transgressive, dominantly sandy sediments primarily supplied by the Rhine-Schelde system. Hundreds of cores have accounted for the mapping of the valley incision down to 30m below the present-day coastline (Fig. 1A).

<figure><img src="image_45.png" /><figcaption>**Figure 1** – A: Isobath map of the base of Quaternary deposits of the Western Flemish Coastal plain (Desoutter, 2020, after Sommé, 1977 and Sommé et al., 2004). Red and green patterns indicate possible pre-Holocene deposits. B: Satellite view of the study area, showing the new cores and the two seismic profiles. A paleovalley has its thalweg 500m to the East of the profiles.</figcaption></figure>

<footnote n="1">¹ Laboratoire d'Océanologie et Géosciences (LOG), UMR 8187, Univ. Lille, CNRS, Univ. Littoral Côte d'Opale, France.</footnote>

<footnote n="2">² Centre Européen de Formation et de Recherches en Environnements Méditerranéens (CEFREM), UMR 5110, Univ. Perpignan Via Domitia, CNRS, France.</footnote>

<footnote n="3">³ GEOREVA, Rue du Puisatier, 35310 Cintré, France.</footnote>

<footnote n="4">⁴ Laboratoire de Géographie Physique (LGP), UMR 8591, Univ. Paris 1, Univ. Paris Est Créteil, France</footnote>
Based on a dense grid of new cores collected as part of a geotechnical survey prior to the southward extension of Dunkirk container port, the stratigraphy of the valley fill was recently specified (Desoutter, 2020). This provided the opportunity of calibrating two seismic profiles performed along and across the western border of one of those valleys (Fig. 1B).

The Slack River estuary, in the Boulonnais Coast, is a much smaller system (Fig. 2A) but the depth of incision is similar, as evidenced by a few boreholes (Fig. 2B). However, the nature of the valley flanks, either terraced fluvial banks or fault scarps, remains to be determined from a better subsurface mapping.

<figure><img src="image_46.png" /><figcaption>Figure 2 - A: Location of the Slack valley on a DEM of the Boulonnais coast (green: elevation below 10m asl; red: up to 50m asl; offshore: light blue above 30m bsl, deep blue: down to 50m bsl). B: Geological map of Marquise (1:50000). Yellow: Holocene dunes. Blue to green: Jurassic to Cretaceous. Location of the two seismic profiles S1 and S2 along drillholes 6-1, 6-2 and 7-4.</figcaption></figure>

## 2 MATERIALS AND METHODS

Two seismic profiles of 650m and 750m were collected W of Loon-Plage (respectively L1 and L2, Fig. 1B), and two others of 230m and 1350m in the Slack valley (respectively S1 and S2, Fig. 2B), using a 24-channel acquisition performed on a DaQLink with 5m interval between receivers. Seismic source was an ABS hammer plate with integrated trigger and a 11kg (for Loon-Plage) or 4kg hammer (for the Slack). Shots were located between receivers. The profiles L1 and L2 were acquired in non-cultivated meadows, the profile S1 along an asphalt road, and the profile S2 along the beach. The flat topography along L1 to S1 profiles was such that no topographic correction was necessary. The S2 profile was acquired during two successive days, from S to N. The first day was windy and rainy, bringing about a lesser quality recording of the first 200m. Therefore, during the second day the profile was acquired higher on the beach, and impinging the coastal barrier at its northern tip. It was thus necessary to integrate the elevation model for processing the P-wave data of this profile. The data were processed using Pickwin© and Crossrefa© modules of the Seislmager© suite software. Both time arrival of P waves and S waves were picked on the wiggles, and the primary mode of surface-wave dispersion, in order to create P-wave and S-wave (MASW-2D) tomographies. The MASW-2D analysis makes it possible to track low-velocity horizons below high-velocity horizons, which is not possible with a classic P-wave refraction processing. The tomographies revealed vertical and lateral changes in velocity that could be compared with the lithologies in cores, and with velocities expected for such lithologies based on a literature review (Tab. 1).

<table><caption>Table 1 - Values of P- and S-velocities for sediments based on Christensen & Mooney (1995), Telford et al. (1990), Lucius et al. (2007), Foti et al. (2013), Coe et al. (2018).</caption><thead><tr><th rowspan="2">Lithology</th><th colspan="2">P-wave velocity (m.s<sup>-1</sup>)</th><th colspan="2">S-wave velocity (m.s<sup>-1</sup>)</th></tr><tr><th>Non-saturated</th><th>Saturated</th><th>Non-saturated</th><th>Saturated</th></tr></thead><tbody><tr><td>Gravel</td><td>300-1200</td><td>1700-2500</td><td>150-600</td><td>200-800</td></tr><tr><td>Sand</td><td>200-900</td><td>1500-2000</td><td>100-350</td><td>150-600</td></tr><tr><td>Silt</td><td>150-500</td><td>1400-1900</td><td>50-200</td><td>100-300</td></tr><tr><td>Clay</td><td>100-400</td><td>1450-1800</td><td>50-150</td><td>50-200</td></tr></tbody></table>
## 3 RESULTS AND DISCUSSION

All profiles show a progressive increase at depth of P and S velocities. The profiles in Loon-Plage show an overall seaward deepening of the surface horizons with P velocities below 2500m.s⁻¹ (Fig. 3). This limit delineates the boundary between saturated, non-cohesive sediments, and rocks (Tab. 1). In the surveyed area, this limit is found around 20-25m, coinciding in the cores with the bottom of Quaternary deposits above the Ypresian claystone (Fig. 3A). The MASW-2D tomography shows that this boundary, less well defined, has S-wave velocities around 300 m.s⁻¹, which is less than expected in saturated gravels and sand, but better coincides with fine-grained deposits (silts, plastic clays). This could correspond to the Eemian clayey Loon Formation, or overlying loessic hillslope facies identified in the cores (Desoutter, 2020). The ups and downs of the valley floor expressed on the S-wave tomography, and not imaged on the P-wave tomography, can be questioned. If not resulting from inversion issues, the lows could point to gullies or tributaries of the main paleovalley, as they are visible only on the NS profile. Internal lithological heterogeneities of the Holocene infill are restricted to thin clay deposits (the non-recovery intervals in the cores) and are not evidenced in both tomographies. No grain-size data along the cores are available to discuss the overall increase at depth of velocity, that likely reflects the increasing compaction.

<figure><img src="image_47.png" /><figcaption>Figure 3 – A: P-wave tomography of profile L2, Flemish coastal plain (location in Fig. 1B). South is to the left. B: S-wave tomography of the same profile obtained by MASW-2D. The lithology of intersected cores is reported: yellow-sand, green-silt, brown-clay, black-peat. The clay at the bottom is the bedrock (Ypresian claystone). Blue: penetrometer profile.</figcaption></figure>

The two profiles obtained in the Slack estuary allow to discuss further the link to lithology, geometry of the valley floor, and also water saturation. The S2 profile shows P-wave variation in depth that is consistent with identification of the non-saturated (coastal barrier / beach berm) to saturated transition, around 1500 m.s⁻¹. The rise of rock-type P-wave velocity values (around 3000 m.s⁻¹) between 50 and 200m along the profile is consistent with the outcropping of Jurassic bedrock at the Petite Pointe aux Oies cape. Similarly, the abrupt rise of the bedrock up to 20m beneath the northern tip of the profile is consistent with the expected scarp created by the Slack fault at the northern border of the valley. There is an overall northward deepening across the valley fill of the 3000 m.s⁻¹ boundary, supposedly matching the deepening of the valley floor towards the fault. However, the grain-size changes tracked in the boreholes cannot be evidenced from neither the P-wave nor the S-wave tomography. The S-wave velocity of the corresponding deposits (sand with interstratified layers of gravel to rock fragments) has a wide range in the saturated zone (150 to 800 m.s-1), possibly explaining local velocity inversions where the coarse layers occur (see borehole 6-1, Fig. 4B).
<figure><img src="image_48.png" /></figure>

Figure 4 – A: P-wave tomography of profile S2, Slack valley (location in Fig. 2). South is to the left. B: S-wave tomography of the same profile obtained by MASW-2D. The grain-size profile of intersected boreholes is reported (coarser to the right).

The S1 profile (Fig. 5) is close to the southern valley border, as indicated by a Jurassic outcrop 300m to the South (Fig. 2B). The borehole 7-4 intersects the Jurassic bedrock (well cemented sandstone and limestone) at a P-wave velocity of 4500 m.s⁻¹, a value consistently higher than for the Ypresian claystone beneath the Flemish plain. This boundary gradually rises to the South, across possible terraces, as evidenced by the 7-3 boreholes (Fig. 5A). By contrast, the S-wave tomography has a dominantly flat or staircase layering, suggesting the profile was too short for a geometric assessment of the intersect layers. The values of S-wave velocities below 150 m.s⁻¹ in the 5 upper meters are consistent with unsaturated fine-grained sediments corresponding to the saltmarsh and eolian dune covers.

<figure><img src="image_49.png" /></figure>

Figure 5 – A: P-wave tomography of profile S1, Slack valley (location in Fig. 2). South is to the left. Boreholes labelled as in the geological map (Fig. 2B). Inferred lithology: grey-Jurassic limestones with sandstone levels; yellow-Quaternary sand with gravel (7-3) or shelly layers (7-4). B: S-wave tomography of the same profile obtained by MASW-2D.

4 CONCLUSION

These experiments confirm that the bottom of the coastal paleovalleys can be detected with refraction and surface wave analysis of relatively short displays powered with simple hammer. A core control is necessary,
however, to discuss the results. With such control, details of the bottom geometry can be revealed, such as scarps or gullies. More work would be necessary to locate precisely the velocity shifts associated to changes in lithology of the valley fill. The average velocities fall within the span of velocities determined for the expected sediments, with a strong control of compaction and water saturation.

## 5 ACKNOWLEDGMENT

This work was funded by the LOG, CEFREM and SFR Campus de la Mer. The authors would like to thank Noé Frisch for his help on the field, GEOREVA for time spent in processing the Boulonnais data, and Bernadette Tessier for her careful review of the manuscript.

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Buchwald, T., Antoine, P., 2024. Historique des travaux et perspectives de recherche sur deux séquences pléistocènes originales du Boulonnais: Wimereux Pointe aux Oies et Wissant Carrière du Fart (littoral du Pas-de-Calais, France). Géomorphologie: relief, processus, environnement, 30, 27-47.

Christensen, N., Mooney, W., 1995. Seismic Velocity Structure and Composition of the Continental Crust: A Global View. Journal of Geophysical Research, 100, 9761-9788.

Coe, J.T., Brandenberg, S.J., Ahdi, S., Kordaji, A., 2018. Geophysical methods for determining the geotechnical engineering properties of earth materials. California Department of Transportation Report Number CA-17-2111. 305 p.

Desoutter, S., 2020. Gravelines-Loon-Plage, Volume 2/3 – Études. Rapport de diagnostic D128113, INRAP Report, 256p.

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# CONTAMINATION OF PYRENEES MOUNTAIN STREAMS BY AGRICULTURAL SYNTHETIC FIBRE ROPES AND A FIRST ASSESSMENT OF THEIR POTENTIAL EFFECT ON AQUATIC AND TERRESTRIAL GASTROPODS

Laurent SEURONT<fnref n="1" />,<fnref n="2" />,<fnref n="3" />

laurent.seuront@cnrs.fr

**Abstract:** Despite the ever-growing amount of work devoted to plastic pollution on a global scale, we are still lacking information on the extent and effects of microplastic pollution in mountain terrains and foothills ecosystems worldwide. The present study was specifically triggered by the nearly ubiquitous presence of synthetic fibre ropes in two otherwise pristine valleys of the foothills of the central Pyrenees. In this context, the present work (i) assessed the presence of fibres in the riverbank sediments of two streams of the Pyrenees foothills, and (ii) inferred the potential effects on the local aquatic and terrestrial gastropods fauna on behavioural assays conducted on representative species of Pyrenees springs and streams, i.e. *Ancyclus fluviatilis* and *Radix balthica*, and two terrestrial gastropods, the endemic snail *Obscurella nouleti* and the widespread grove snail *Cepaea nemoralis*. Fibres were present in all samples considered but notably at relatively low concentrations. Other common microplastic items such as fragments, film and foam were remarkably consistently absent from all the samples considered in the present work. These results indicate that the Nistos and Ourse streams are only marginally impacted by plastic pollution in general and by plastic pollution related to agricultural activities in particular. However, the polyethylene fibres studied contained three low molecular weight phthalates which when incorporated into an inert gel, induced systematic behavioural avoidance in all species tested. To avoid underestimating the role of Nistos and Ourse streams as potential microplastic sinks, further work is needed to assess the nature and amount of microplastic items (including fibres) that may have been trapped in foothill soil and in stream sediment surface depending on their shape, density and polymer and/or advected downstream. Similarly, the observed consistent avoidance of rope leachates questions the potential long-term effects of the chemical compounds contained in the synthetic ropes that are extensively used in the agricultural sector and found nearly everywhere in the foothill landscapes.

**Keywords:** Plastic pollution, synthetic fibres, phthalate, riverbank sediment, behavioural assay.

## 1 INTRODUCTION

In an era of ever-growing anthropogenic pressure on both terrestrial and aquatic ecosystems, plastic pollution is increasingly acknowledged as a pervasive environmental problem due to its persistent behaviour and increased resistance to biodegradation; see e.g. MacLeod et al. (2021), Li et al. (2021) and Richard et al. (2024) for recent reviews. Specifically, due to plastic abuse and the worldwide struggle to cope with the increasing influx of plastic waste, this material has become one of the most ubiquitous sources of both contamination and pollution of the Anthropocene (Geyer et al., 2017). Plastic wastes are accumulating on landfills and beaches worldwide as well as in the most remote terrestrial and oceanic locations (Napper et al., 2020; Bergmann et al., 2022; De-la-Torre et al., 2024), threatening both terrestrial and aquatic environments, the economy and human well-being on a global scale (Amelia et al. 2021).

Though much attention has focused on the impacts of plastic waste on the world's oceans, remote and mountainous systems are increasingly acknowledged as areas that are significantly affected by plastic waste. Microplastics (MPs, i.e. plastic items in the 5 mm to 1 µm size range) are introduced into remote aquatic ecosystems, especially mountain terrains and foothills, including high-altitude ecosystems, via the degradation, fragmentation and subsequent transport of plastic products that accumulate in various mountainous regions — including Mount Everest (SkyNews, 2019) — due to the lack of proper waste disposal and recycling facilities (Free et al., 2014; Zhang et al., 2016). Rain and snow are, however, considered to be the primary drivers of microplastics (MPs) deposition in high altitude ecosystems, together with the wide range of synthetic fibres released by the clothing and equipment of hikers, mountaineers and other tourist activities, which are transferred by the wind into the main water body; see Padha et al. (2022) for a review. Synthetic fibre ropes (typically made of polyethylene and polypropylene) are also used in the agricultural sector, notably for binding bundles to be picked up by automatic balers and binders or similar

<footnote n="1">¹CNRS, Univ. Lille, Univ. Littoral Côte d'Opale, IRD, UMR 8187 - LOG - Laboratoire d'Océanologie et de Géosciences, Station Marine de Wimereux, F-59000 Lille, France</footnote>

<footnote n="2">²Department of Marine Resources and Energy, Tokyo University of Marine Science and Technology, 4-5-7 Konan, Minato-ku, Tokyo, 108-8477, Japan</footnote>

<footnote n="3">³Department of Zoology and Entomology, Rhodes University, Grahamstown 6140, South Africa</footnote>
machines. A common practice in rural areas of the Central Pyrenees foothills is to use these ropes to demarcate agricultural plots, hiking path and private properties, and are subsequently observed in various state of degradation in a range of landscapes (Fig. 1).

Research on remotely located mountainous MP-polluted ecosystems is, however, still in its early stage, very fragmented and MP abundance reported in the literature should be considered with caution given the diversity of potential sources, as well as both the sampling and extraction methods considered (Padha et al., 2022) for a review. Despite a drastic increase in the number of publications assessing microplastic pollutions in rivers over the last decade (see D'Avignon et al. (2022), their Fig. 2), there is still a critical lack of information on (i) MP abundances in mountain terrains and foothills ecosystems worldwide, (ii) the links between anthropogenic activities and the type of MPs, and (iii) the potential ecotoxicological consequences toward specific aquatic organisms. In this context, this preliminary study aims at assessing (i) the presence of plastic fibres in the riverbank sediment of two streams of the Pyrenees foothills where synthetic fibre ropes are extensively used, and (ii) the potential effects that these fibres may have on the local aquatic and terrestrial gastropods were investigated based on behavioural assays on representative species of Pyrenees springs and streams, i.e. *Ancylus fluviatilis* and *Radix balthica*, and two terrestrial gastropods, the endemic snail *Obscurella nouleti* and the widespread grove snail *Cepaea nemoralis*. Gastropods have been specifically targeted because the Pyrenees are a biodiversity hotspot for land snails and freshwater molluscs, with many endemic, specialised and relict species which reflect the region's uniquely varied climate, geology and range of microhabitats. In addition, freshwater gastropods play critical ecological roles through nutrient cycling, biofilm control, energy transfer, and habitat modification, influencing both biotic and abiotic components of their environments and their terrestrial counterparts serve as decomposers (hence contribute to nutrient cycling, soil health, and plant regulation), herbivores, prey species, and bioindicators, influencing ecosystem stability and biodiversity.

<figure><img src="image_50.png" /><figcaption>Figure 1. Illustration of the nearly ubiquitous presence of synthetic fibre ropes in the rural areas of the Central Pyrenees foothills (in the present case the Nistos valley, but similar observations hold for our second study site, the Barousse valley) where they are used in (A) in the vicinity of agricultural facilities to restrict access, to demarcate (B) hiking paths and (C) grazing plots, and are subsequently found in various state of degradations (E,F), even in remote and pristine areas (D). These ropes were consistently blue, though some colour variations do exist (F, inset) likely due to differences in manufacturers and/or the duration of exposure to environmental conditions.</figcaption></figure>

## 2 MATERIALS AND METHODS

### 2.1 Study sites

The Nistos stream and the Ourse River are located in the foothills of the Central Pyrénées (Fig. 2). The 18.5 km long Nistos stream is a sub-tributary of the Garonne River via the Neste River, and the 25.4 km long
Ourse river is sensu stricto formed at the confluence of the 'Ourse de Ferrère' and the 'Ourse of Sost' streams at Mauléon-Barousse and is a tributary of the Garonne River (Fig. 2C). Five sampling locations were considered, and were consistently chosen for the lack of vegetation between the shoreline and the water's edge as the presence of vegetation typically acts as natural buffer that increases retention and accumulation of microplastics within sediments compared to bare or sparsely vegetated areas (Tasseron et al., 2024), hence could lead to confounding results. Given the acknowledged role of flow velocity in microplastic deposition and accumulation on riverbanks and within sediments (Owowenu et al., 2023; Munz et al., 2024) two stations were selected along the Nistos stream, based on a visual assessment of the nature of their flow: the first one was characterized by a steep river slope with lots of rock and boulders creating highly turbulent flows (station N₁, Fig. 2C) and the second one by a nearly flat bottom with gently flowing water over a soft sediment bottom (station N₂, Fig. 2C). The four stations considered along the River Ourse (O₃ and O₄) and along each of its two tributaries (O₁ and O₂; Fig. 2C) were consistently characterized by water flowing over a gently sloping rocky bottom. All samples were taken in August 2022.

## 2.2 Sediment sampling

The sediment samples were consistently collected in an area between the shoreline and the water's edge in August 2023. At each site, five sediment samples were randomly selected at a depth of 2 cm, using a metal frame with dimensions of 15 x 15 cm and a height of 2 cm and a stainless-steel shovel. Each sample was gently mixed with a metal rod within a glass beaker and subsequently placed in an aluminum foil, frozen and transported to the laboratory for further analyses.

<figure><img src="image_51.png" /><figcaption>Figure 2. Localization of (A, B) the study area in the Central Pyrénées, and (C) the sampling stations along Nistos stream and the Ourse river and its two tributaries, the 'Ourse de Ferrère' and 'Ourse de Sost' streams.</figcaption></figure>

## 2.3 Samples treatment

Each sediment sample was dried at 50 °C for 72 h and 30 g of each sample was investigated and subsequently stored in a clean polyethylene bag (Sigma Aldrich Z162965). MPs were subsequently extracted by density separation with a hypersaline solution (300 g NaCl l⁻¹) prepared with filtered water (Whatman GF/C, 47 mm of diameter and 1.2 µm of pore size). The supernatant was subsequently GF/C filtered, the filters transferred to a clean glass Petri dish and sealed with Parafilm, pending microscopic examination. The number of fibres on each filter was counted, averaged over the five replicate samples for each sampling location, and expressed in fibres kg⁻¹ of dry sediment. Note that the filters were also examined for other MPs items such as fragments, film and foam.

## 2.4 Polymer assessment and additive content

Polymer type and additive content were assessed from light (n = 3) and dark (n = 3) blue fibres randomly sampled in the Nistos and Barousse valley (see inset of Fig. 1F), and polymer type was assessed from a sub-sample (n = 18) of fibres collected in the riverbanks of stations N₁ and N₂, and O₁, O₂, O₃, and O₄. Fibre
polymer types and additive content were assessed respectively using Fourier transform infrared (FTIR) spectroscopy and gas-chromatography coupled with mass spectrometry (GC-HRMS).

## 2.5 Behavioural assays

Despite the growing awareness of the ubiquity and toxicity of plastic compounds and their leachates — i.e. the cocktail of chemical compounds released by plastic items in the environment (Delaeter et al., 2022) — in aquatic (both freshwater and marine) and terrestrial systems (see e.g. Gunaalan et al. (2020), Delaeter et al. (2022), Nam et al. (2022), Omidoyin and Jho (2024) and Junior et al. (2024) for recent reviews), we are still critically lacking information on how leachates from agricultural fibre ropes may impact local invertebrates, in particular aquatic and terrestrial gastropods.

To assess the potential biological effect of synthetic fibres, the above-mentioned light and dark blue ropes were used in a behavioural assay specifically designed to assess if the additives contained in these fibres were susceptible to induce a behavioural response in the 4 species considered in this study. The small (i.e. 5-8 mm) freshwater, air-breathing limpet *A. fluviatilis* typically occurs in oxygen-rich fast-running waters, while the 10 to 20 mm pulmonate snail *R. balthica* is characteristic of slow-moving waters with abundant vegetation such as lakes, ponds and the numerous water troughs scattered in the Nistos and Barousse valley. The conical shell (12-15 mm in length) *O. nouleti* is often found in natural or artificial shaded limestone substrates where it is found on boulders, cliffs and screes, but it is also a forest species which is very present on the trunks of smooth-barked trees, in particular beech (*Fagus sylvatica*). In contrast, the rounded shell *C. nemoralis* (15-25 mm in shell diameter) is nearly ubiquitous in temperate Europe where it is typically found in woodland edges, hedgerows, and grasslands with shaded, humid conditions.

An avoidance assay was designed in a purpose-built arena made of Lego® tiles, with a central section separated from the outer section of the arena by a 3.2-mm deep and 7.8-mm wide trench (Fig. 3). This trench was filled by a control Ficoll gel made from GF/C filtered natural Nistos stream water and Ficoll gels made from leachate solutions of light and dark blue ropes. The behavioural assay consisted of placing one organism in the central section of the experimental arena and assess its ability to cross the gel-filled trench. Leachate solutions were prepared by immersing 0.3 g (ca. 10 cm) of either light or dark blue agricultural fibre ropes in one litre of aerated control stream water for 24 h. The light and dark blue ropes used to prepare the rope leachate solutions were respectively the light blue rope #1 and dark blue rope #1 described in Table 1. Control and leachate gels were prepared by the addition of a high-molecular weight inert polymer (Ficoll 400® PM400, Sigma Aldrich) at a concentration of 20% Ficoll (w/v). Note that the dimensions of the experimental arenas were tuned to the size of the test organisms, i.e. the surface of the central part of the experimental was consistently 4.5 to 5 times greater than the surface of the locomotory foot of the organisms (Fig. 3A).

<figure><img src="image_52.png" /><figcaption>**Figure 3.** Schematic representation of the three types of Lego® tiles experimental arenas specifically designed to run behavioural assays on *Ancylus fluviatilis* (A), *Radix balthica* and *Obscurella nouleti* (B) and *Cepaea nemoralis* (C). Scale bar = 15.6 mm.</figcaption></figure>

For each assay, the behavioural responses of 20 individual of each species were assessed in response to control Ficoll gel, and Ficoll gels obtained from leachate solutions of light and dark blue ropes. No significant differences existed between the size of the organisms used in the control and the leachate experiments, i.e. *A. fluviatilis* (5.5 ± 0.1 mm; mean ± SD), *R. balthica* (10.2 ± 0.2 mm), *O. nouleti* (12.1 ± 0.3 mm) and *C. nemoralis* (14.2 ± 0.2 mm; mean ± SD). Each behavioural assay was run with one single test organism that was placed in the centre of the experimental arena, and its behaviour was recorded for 30 minutes (or eventually stopped if the organism left the experimental arena in less than 30 minutes) from the top with a GoPro camera placed 40 cm above the experimental arenas at a frame rate of 1 image per minute for the slow-moving *A. fluviatilis*, and 1 image every 5 seconds for *R. balthica*, *O. nouleti* and *C. nemoralis*. Three experimental arenas were considered simultaneously, consistently with a control Ficoll gel and Ficoll gels
obtained from leachate solutions of light and dark blue ropes. Between each behavioural assay, the
experimental arenas were first carefully rinsed with 95% (v/v) acetone, and twice with GF/C filtered natural
Nistos stream water. All behavioural assays were conducted in June 2024, and all individuals were returned
to their natural environment after the behavioural experiments took place.

**2.6 Statistical analyses**

Multiple comparisons between sampling sites were conducted using the non-parametric Kruskal-Wallis test (KW test hereafter), which is more robust than a (parametric) analysis of variance given the small sample size (n = 5) considered in the present work. A subsequent multiple comparison procedure based on the Dunn test was then used to identify distinct groups of measurements.

**3 RESULTS AND DISCUSSION**

**3.1 Synthetic fibres are consistently present in stream sediment, but at relatively low concentrations**

Synthetic fibres have been found in all the samples, were consistently blue and made of polyethylene. These fibres ranged from to 0.12 to 2 mm in length, and their concentrations ranged from 2.35 to 16.46 fibres per kg of dry sediment (Fig. 4). No significant differences in fibre lengths were found between sites (KW test, *p* > 0.05). In contrast, fibre concentrations C significantly differ between sites (KW test, *p* < 0.05), with C(N₁) = C(O₁) = C(O₂) = C(O₃) = C(O₄) < C(N₂) (Dunn test, *p* < 0.05). Their concentrations were notably ca. 3-fold higher in station O₂, which — in contrast with the other 4 sampling stations — was characterised by a nearly flat bottom with gently flowing water. The observed increase in fibre concentration in riverbank sediment of station N₂ is consistent with the acknowledged role of flow velocity in microplastic deposition and accumulation on riverbanks and within sediments (Owowenu et al., 2023; Munz et al., 2024). It is also stressed that the concentrations of fibres observed in the present work are drastically (i.e. up to several orders of magnitude) lower than microplastic concentrations reported in the literature even from remote Himalayan and Andean streams; see e.g. Yang et al. (2021) and Canchari and Iannacone (2024). In this context, it is stressed that in sharp contrast with other studies, plastic items such as fragments, film and foam have never been found in the riverbank sediments of the 6 stations investigated in the present work. These results indicate that the Nistos and Ourse streams are only marginally impacted by plastic pollution in general and by plastic pollution related to agricultural activities in particular. Beyond the relatively limited extent of the agricultural activities held in these valleys, these results may also lie in the relative isolation of the Nistos and Barousse valleys which are both dead ends, their distance from ski resorts and the related reduced tourist activities.

<figure><img src="image_53.png" /><figcaption>Figure 4. Synthetic fibre concentration C (fibres per kilogram of dry sediment) in the riverbank sediments of the Nistos (stations N₁ and N₂) and Ourse (stations O₁, O₂, O₃ and O₄) streams.</figcaption></figure>

**3.2 Polymer type and additive content**

Both light and dark blue ropes were consistently made of polyethylene, contained only plasticizers (i.e. the low molecular weight phthalates Diethyl phthalate (DEP), Diisobutyl phthalate (DIBP) and Dimethyl phthalate (DMP)), are all acknowledged as endocrine disruptors, and for their reproductive and developmental toxicity, neurotoxicity, hepatotoxicity and nephrotoxicity and carcinogenicity; see e.g. Praveena et al. (2018) and Dutta et al. (2020) for reviews. The relative contents of these compounds differed between light and dark blue ropes (Tab. 1). Specifically, dark blue ropes (n = 3) only contained DIBP and DMP. The former was 33-, 198- and 276-fold more abundant than the latter. In contrast, light blue ropes contained either (n = 2) DEP and DIBP or (n = 1) DEP, DIBP and DMP. In any case, the proportion of these additives were highly variable as stressed for dark blue ropes. Though the resolution of this specific issue lies well beyond the scope of the
present work, further work is needed to decipher the origin of the observed variability in plasticizers content, in particular relation to the origin of their manufacturer (i.e. different manufacturers may have different additive recipes) and the impact of weathering processes (e.g. UV, wind and rain) on additive content. It is finally notable that no evidence of UV stabilisers nor antioxidants, which respectively protect the ropes against degradation to UV exposure and prevent polymer oxidation and brittleness over time.

<table><caption>Table 1. Plasticizer content of light and dark blue rope samples, expressed by the surface area (arbitrary units, Al) of the chromatogram peaks of each additive normalized by the sample size (µm), i.e. Al/µm. DEP: Diethyl phthalate, DIBP: Diisobutyl phthalate, and DMP: Dimethyl phthalate.</caption><thead><tr><th>Rope type</th><th>Sample #</th><th>DEP</th><th>DIBP</th><th>DMP</th></tr></thead><tbody><tr><td rowspan="3">Dark blue</td><td>1</td><td>-</td><td>746</td><td>3</td></tr><tr><td>2</td><td>-</td><td>769</td><td>4</td></tr><tr><td>3</td><td>-</td><td>408</td><td>13</td></tr><tr><td rowspan="3">Light blue</td><td>1</td><td>317</td><td>649</td><td>-</td></tr><tr><td>2</td><td>406</td><td>82</td><td>6</td></tr><tr><td>3</td><td>111</td><td>124</td><td>-</td></tr></tbody></table>

### 3.3 Behavioural assays

All the individuals of each species of gastropod considered in the present work (*Radix balthica*, *Obscurella nouleti* and *Cepaea nemoralis*) consistently started moving within the first minute after having been placed in the centre of the experimental arena. In contrast, only 14 *Ancylus fluviatilis* moved during the behavioural assays conducted with Ficoll gel obtained from GF/C filtered stream water. The proportion of motile *A. fluviatilis* was slightly higher in the behavioural assays conducted with Ficoll gels from leachate solutions of light and dark blue ropes, where 16 and 17 individuals moved.

<figure><img src="image_54.png" /><figcaption>Figure 5. Behavioural assay conducted on *Ancylus fluviatilis* on grey Lego® tiles with a 3.2-mm deep trench gell-free (white area; A), or filled with a 20% (w/v) Ficoll gel made with (B) GF/C filtered stream water (green stippled area), (C) light blue rope leachate solution (orange stippled area) and (D) dark blue rope leachate solution (red stippled area). The characteristic behaviour exhibited by *A. fluviatilis* in each experimental conditions is illustrated by the trajectories of five distinct individuals. Scale bar = 15.6 mm.</figcaption></figure>
R. balthica, O. nouleti and C. nemoralis did not exhibit any specific behaviour upon the encounter of the 3.2-mm deep trench which they consistently crossed and subsequently resumed their basal crawling activity (Fig. 5A). It is nevertheless stressed that a relatively small proportion of them (i.e. respectively 20%, 15% and 10% in R. balthica, O. nouleti and C. nemoralis) exhibited a positive thigmotactic behaviour, i.e. a tendency to follow the topographic discontinuity related to the surface-to-drop-off transition or the surface-to-wall transition respectively when entering and exiting the trench). This observation is in sharp contrast with the behaviour exhibited by A. fluviatilis which consistently exhibited a positive thigmotactic behaviour when entering and exiting the trench (Fig. 6A). Though the resolution of the differences in the observed thigmotactic responses in A. fluviatilis (Fig. 5A) and R. balthica, O. nouleti and C. nemoralis (Fig. 6A) lies well beyond the scope of the present work, they may be related to the size of the so-called topographic discontinuity relative to the size of the organisms considered. A 5 to 6 mm long A. fluviatilis as those considered in the present work is typically 2 to 2.5 mm high, which makes the 3.2 mm topographic discontinuity related to a surface-to-drop-off transition or a surface-to-wall transition of the trench a much stronger stimulus than for the other three species. This hypothesis is consistent with the relatively small proportion of R. balthica, O. nouleti and C. nemoralis (i.e. 20%, 15% and 10% in R. balthica, O. nouleti and C. nemoralis) exhibiting a thigmotactic behaviour when compared to the 100% observed in A. fluviatilis.

<figure><img src="image_55.png" /><figcaption>**Figure 6.** Behavioural assay conducted on *Radix balthica* on grey Lego® tiles with the 3.2-mm deep trench gell-free (white area; A), or filled with a 20% (w/v) Ficoll gel made with (B) GF/C filtered stream water (green stippled area), (C) light blue rope leachate solution (orange stippled area) and (D) dark blue rope leachate solution (red stippled area). The characteristic behaviour exhibited by *R. balthica* in each experimental conditions is illustrated by 5 trajectories. The black arrows indicate the presence of a thigmotactic behaviour (i.e. movement towards or away from the stimulus induced by a physical contact) and the white arrows the U-turn performed following a tentacle contact with the surface of a leachate gel. Scale bar = 15.6 mm.</figcaption></figure>

In accordance with previous work conducted on the intertidal species *Littorina littorea*, *L. saxatilis*, *Phorcus lineatus* and both *Steromphala cineraria* and *S. umbilicalis* (Seuront, unpublished data), none of the four species considered in the present work exhibit any avoidance behaviour towards the control Ficoll gel, which they consistently crossed without noticeably modifying the nature of their movements (Figs. 5B and 6B). These observations drastically differ from the results obtained in response to both light and dark blue rope leachate gels, where no individual ever crossed the gel-filled trench (Figs. 5C-D and 6C-D). Instead they consistently avoided the gel, with *R. balthica*, *O. nouleti* and *C. nemoralis* typically exhibiting a prodding of tentacles in the vicinity of the gel-air interface (i.e. a repeated touching of the surface with the tips of the cephalic tentacles) — as opposed to the normal tentacular motion exhibited by in the absence of foam which is that of side-to-side sweeping — and as such remained confined within the central section of the experimental arena trench (Figs. 5C-D and 6C-D). Note that *R. balthica*, *O. nouleti* and *C. nemoralis* consistently retracted their tentacles upon contact with the surface of a leachate gel — a response that was never observed when their tentacles touched the surface of a leachatre-free gel — and subsequently
exhibited a U-turn in the opposite direction of motion. This U-turn behaviour has also been observed in A. *fluviatilis* (Fig. 5C-D), but the resolution of the observations was not high enough to assess a putative tentacular response to leachate gels.

## 4 CONCLUSION

The present work was triggered by the nearly ubiquitous presence of blue synthetic ropes in the various landscapes of two valleys of the foothills of the Central Pyrénées (see Fig. 1), and the related questions of (i) their putative (i) accumulation in riverbank sediments and (ii) toxicity towards both local aquatic and terrestrial gastropods. Fibre fragments 0.12 to 2 mm in length were consistently present in the riverbank sediments of the Nistos and Ourse streams running in these valleys, but at relatively low concentrations (i.e. in the range of 2 to 16 fibres per kg of dry sediment). These results, together with the absence of other microplastic plastic items such as fragments, film and foam in the riverbank sediments of the 6 stations investigated in the present indicate that the Nistos and Ourse streams are only marginally impacted by plastic pollution in general and by plastic pollution related to agricultural activities in particular. It cannot be excluded, however, that microplastic items (including fibres) may have been trapped in stream sediment surface depending on their shape, density and polymer and/or advected downstream, leading to an underestimation of Nistos and Ourse streams as potential microplastic sinks. Though the resolution of this specific issue lies well beyond the scope of this preliminary work, it warrants the need for further work to assess the nature and concentration of microplastic items in riverbank sediment, stream sediment surface and stream water, and more generally in the soil of these foothill valleys. In this context, the non-ambiguous avoidance of rope leachates shown by the behavioural assays conducted in the present work questions the potential long-term effects of the chemical compounds contained in these synthetic ropes that are extensively used in the agricultural sector and found nearly everywhere in the foothill landscapes.

## 5 ACKNOWLEDGMENT

Leena M.Y. Seuront is acknowledged for her help in sediment sampling, and Dr. Dorine Seuront for her patience and understanding about my nearly irrepressible inclination to carry out scientific work during what should have been *stricto sensu* a holiday. Nicolas Spilmont and Mathilde Schapira are acknowledged for their suggestions and comments on a previous version of this work.

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RHYTHMITES PRESERVED IN INTERTIDAL FLAT SUCCESSIONS
OF THE HYPERTIDAL SANTA CRUZ – CHICO RIVER ESTUARY
(SOUTHERN PATAGONIA, ARGENTINA)

Bernadette TESSIER<fnref n="1" />, Thibaud LORTIE<fnref n="1" />, José I. CUITIÑO², Jean-Yves REYNAUD<fnref n="3" />, Roberto A. SCASSO⁴, Sebastián M. RICHIANO², Léo PANCRAZZI<fnref n="1" />, Edward SALAMEH<fnref n="1" />, Michel CONDOMINES<fnref n="5" />, Laurent DEZILEAU<fnref n="1" />, Malala LEKANDER⁶

Corresponding author: bernadette.tessier@unicaen.fr

**Abstract:** Rhythmites observed and cored in the hypertidal (12 m tidal range) Santa Cruz – Chico River estuary (Southern Patagonia, Argentina) are described. The estuary is made of two estuarine arms (the Chico River arm; the Santa Cruz River arm), both converging into an intermediate subtidal basin connected to the Atlantic Ocean. Tidal dynamics are dominant along the low fluvial discharge Chico River arm. In contrast, the water discharge of the Santa Cruz River is much higher so that the influence of tidal dynamics quickly drops down a few kilometers upstream. Extensive sandy to muddy tidal flats develop along the outer portion of the two arms. Upper intertidal sediment successions frequently reveal well-developed rhythmic layering, resembling tidal rhythmites. Cores (1m long) were retrieved in these successions, one from the tide-dominated Chico River arm, one from the tide-influenced Santa Cruz River arm. Core analyses show that the Chico River sediment succession is made of a fairly regular stacking of typical semi-diurnal sand-mud couplets and neap-spring-neap tidal rhythmites, and represents a 3-year continuous record. A seasonal modulation is probably superimposed to the tidal signal. Semi-diurnal tidal couplets are visible on the Santa Cruz arm succession, but the fortnightly cyclicity is altered, due to the combined influence of fluvial dynamics, westerlies-induced wave agitation and aeolian sand transport. 210Pbexc measurements confirm for both cores very high sedimentation rates. Although 137Cs data are difficult to exploit due to the very low fallout in the Southern Hemisphere, they could suggest that the successions are several decades old, meaning that the studied tidal flats are no longer aggrading.

**Keywords:** Tidal rhythmites, estuary, cores, hypertidal, fluvial influence, Patagonia.

# 1 GENERAL INTRODUCTION AND STUDY SITE PRESENTATION

Upper intertidal sediment successions within tide-dominated estuaries frequently contain well-preserved tidal rhythmites, recording basically semi-diurnal to neap-spring-neap cycles, eventually annual cycles in the uppermost part of the successions. Such rhythmic sediment records provide valuable information on hydrosedimentary processes and sedimentation rates (Tessier, 2023).

The rhythmic successions described here were deposited in the Santa Cruz - Chico River estuary (SCCR), located in South Patagonia, Argentina (Fig. 1A). The SCCR is a complex ria-type system combining hypertidal dynamics, high fluvial discharge, and wind-, wave-induced processes (Piccolo & Perillo, 1999; Pancrazzi et al., 2024; Tessier et al., 2024). In the details, the estuary comprises two arms, the Santa Cruz River arm and the Chico River arm, both converging into a 30 km – long subtidal basin connecting the system to the Atlantic Ocean through a 2 km wide narrow inlet (Fig. 1A). Tidal range reaches up to 12 m at Punta Quilla, located 4.5 km upstream of the inlet (Servicio de Hidrografía Naval: www.hidro.gov.ar). The tidal regime is semi-diurnal with very low diurnal inequality. Tidal currents are commonly faster than 1.5 m s<fnref n="1" />, reaching up to 4 m s⁻<fnref n="1" /> in the inlet. The flood is generally the dominant current, but the ebb is dominant in the downstream half of the intermediate basin. Fluvial discharge has little influence on the intermediate basin water levels and bottom velocities which are instead controlled by tidal dynamics. Along the low fluvial discharge (mean annual 30 m<fnref n="3" /> s⁻<fnref n="1" />) Chico River arm, tidal dynamics are dominant, with a tidal limit located at about 20-25 km from its mouth. In contrast, tidal dynamics quickly drops down a few kilometers upstream the Santa Cruz River arm due to a much higher water discharge (mean annual 750 m<fnref n="3" /> s⁻<fnref n="1" />). Moreover, water outbursts regularly occur (every 2-4 years) resulting in higher discharge peaks reaching up to 2500 m<fnref n="3" /> s⁻<fnref n="1" /> (Pasquini & Depetris, 2011). Sediment discharge is accordingly higher from the Santa Cruz River, especially for sand fraction but suspended sediment concentration is much higher in the Chico River tidal channels (Depetris et al., 2005). The Patagonian climate is characterized by intense and dry winds blowing almost permanently from the west. In the estuary, they provoke substantial sand transport on the tidal flats at low tide, and an important agitation on the water surface. Mean temperature in winter season (from May to September) is frequently below 0°C, with extreme less than -10 °C in July. It is around 20°C in summer (Nov.- Feb.) (Servicio Meteorológico Nacional Argentina: www.smn.gob.ar).

<footnote n="1">¹ UMR M2C CNRS, Univ. Caen-Rouen, France; ² CONICET-CENPAT, Puerto Madryn, Argentina;</footnote>

<footnote n="3">³ UMR LOG CNRS, Univ. Lille, France; ⁴ IGEBA CONICET, Universidad de Buenos Aires, Argentina ;</footnote>

<footnote n="5">⁵ UMR Geosciences Montpellier, France ; ⁶ Rio Grande, Tierra del Fuego, Argentina</footnote>
Along the edges of the outer portion of both the Chico and Santa Cruz River arms, extensive sand- to mud-flats develop.

<figure><img src="image_56.png" /><figcaption>Figure 1 – A: The Santa Cruz – Chico River estuary, in South Patagonia, Argentina (in Tessier et al., 2024). Red circle black dots indicate core location; B: zoom on the core C1 site (Google Earth image) sheltered behind a gravel barrier at the mouth of the Chico River arm; C: cross-section of the cored succession; D: zoom on the core C4 site (Google Earth image) 14 km upstream of the Santa Cruz River arm mouth; E: cross-section of the cored succession. White arrow for North on A, B and D.</figcaption></figure>

## 2 MATERIALS AND METHODS

In the uppermost intertidal part of the flats, below salt marshes, sediment sections made of heterolithic facies comprising sand-mud couplets are frequently exposed thanks to cutbanks along secondary tidal channels or creeks (Fig.1B to E). Some of these successions show clear cyclic patterns suggesting tidal rhythmites. Salt marsh successions may display as well very regular bedding corresponding likely to annual rhythmites (Tessier, 1998; Archer, 2013). In order to define the origin of the upper intertidal rhythmites, cores were retrieved manually, using PVC tube, at two sites on top of upper intertidal bankss: 1) at the mouth of the tide-dominated Chico River arm (Fig.1B, C). The site is sheltered behind a small gravel barrier (Core C1 – 114 cm); 2) at about 14 km upstream of the mouth of the tide-influenced Santa Cruz arm (Fig.1D, E). The site is fairly exposed to wind and wave agitation (Core C4 – 85 cm). On the basis of Sentinel-2 image examination, the elevation of the two coring sites can be set at about 10-10,5 m above lowest tide level, and the frequency of their inundation by tides is around 16% (cf. principle in Salameh et al., 2024).

Work on cores comprise sediment facies description, grain-size analysis (1 cm interval), and bed thickness measurements. 210Pbexc and 137Cs measurements were also performed.

## 3 RESULTS

**Upper intertidal rhythmite description:** Grain-size along both cores has a very narrow range centered around very fine sand (Fig. 2). High frequency fluctuations are related to the heterolithic nature of the successions. No significant trends are observed from base to top. Core C1 is slightly finer-grained along the upper 25 cm, only the surface being coarser. Along Core C4, the bottom is slightly finer, and the top is very slightly coarser with a better sorting featured by a narrower grain-size range.

Cores C1 and C4 both show sand-dominated beds alternating with mud-dominated beds – mdb (red arrows on Figs. 2, 3). Thickness of «mdb-to-mdb» packages ranges from a few mm to a few cm. Some of the packages clearly contain sand-mud couplets, of varying thickness and shape (examples: zoomed areas outlined in light blue, Fig. 2A-1 and 2B-1). These packages are typical tidal rhythmites recording neap-spring-neap (semi-lunar – or fortnightly) cycles. Other packages do not clearly show sand-mud couplets, and mdbs look amalgamated (examples: zoomed areas outlined in pink, Fig. 2A-2 and 2B-2). Such packages are well expressed in C1. In C4, mdbs are locally poorly discernible.

**Rhythmite thickness measurements (Fig. 3A, 3B):** The only individual sediment units present along the whole successions are the mdb-to-mdb packages, sand-mud couplets being discernible discontinuously. Thus, thickness measurements were applied only on the mdb-to-mdb packages. Thickness evolution shows a cyclic pattern (smoothed red curves on Fig. 3A, B) with successive thickening and thinning. The cyclic
pattern is especially well expressed along core C1 (Fig. 3A), with an average number of 7 mdb-to-mdb packages per cycle. Along core C4 a cyclic evolution is barely detected (Fig. 3B). Cycles contain between 4 and 9 packages.

<figure><img src="image_57.png" /><figcaption>Figure 2 – Core C1 (A) and core C4 (B) photography and grain-size evolution (heatmap). Red arrows indicate mud-dominated beds (mdb). On A and B: Zoom 1 on mdb-to-mdb packages with distinct sand-mud couplets; Zoom 2 on mdb-to-mdb packages without distinct sand-mud couplets and with amalgamated mdbs. fslt, mcslt: fine, medium to coarse silt; vf, f, m, c: very fine, fine, medium, coarse (sand).</figcaption></figure>

210Pbexc and 137Cs measurements (Fig. 3C): In core C4, 210Pbexc values are very small and without consistent variation with depth. 137Cs is present with maximum values between 15 and 55 cm depth. For core C1 there is no significant 210Pbexc, and no sign of 137Cs.

4 DISCUSSION

According to their elevation of 10 to 10,5 m, only spring tides can reach the sites, and not all spring tides, only those reaching at least this elevation. Present-day high tide level variations (for the period 2021-2024) show typical semi-annual cycles (equinox-solstice-equinox), comprising ca. 12 semi-lunar cycles (Fig. 4). Above an elevation of 10 m (purple curve on Fig. 4), less than 10 spring tide periods on average, in some case only 7, over 12, are effective.

The average number of 7 mdb-to-mdb packages per cycle found along core C1 approaches the number of spring tide periods during which the site can be inundated. Based on this comparison with present-day tidal data, we thus assumed that all mdb-to-mdb packages in core C1 represent the sedimentary record of semi-lunar cycles, and the cycles of increasing-decreasing thickness of mdb-to-mdb packages are semi-annual. Semi-lunar cycles are necessarily incomplete as the site is not inundated by neap tides. This interpretation leads us to conclude that C1 succession, characterized by a coherent cyclic signal of thickness evolution and the absence of obvious discontinuity along the core, represents a 3-year (Y1 to Y3 on Fig.3A) sedimentary record. Within this record, semi-lunar cycles with amalgamated mdbs occur during one half of years 2 and 3 (Fig.3A, pink rectangles). Therefore, we suggest that their occurrence could be of seasonal origin. However, the reason explaining amalgamation, i.e., the deposition of successive highly mud-dominated tidal “couplets” and that are not individually distinguishable, is matter of debate. Deposition of successive thick mud beds
during very high suspended fluvial load period, better preserved when spring tides are not the strongest, i.e., close to neap tides, could be an explanation. Homogenization of mud-dominated couplets due to freezing during winter can as well be evoked. We could indeed observe during a field survey in September ice-crystal cracks on the upper tidal flats along the Chico River estuary, as well as a very high density fracturing of tidal bedding due to these cracks in cross-section.

<figure><img src="image_58.png" /><figcaption>Figure 3 – Thickness evolution of mdb-to-mdb packages (red arrows) along core C1 (A) and core C4 (B). Red numbers: number of packages per cycle (smoothed red curve). Y on A for year (see text for explanation). Pink rectangles on A indicate mdb-to-mdb packages with amalgamated mdb; C: Results of 137Cs (blue) and 210Pbexc (red) measurements on C1 and C4.</figcaption></figure>

<figure><img src="image_59.png" /><figcaption>Figure 4 – Present-day high tide level variations for the period 2021-2024 at Punta Quilla (data from at the Hydrographic Navy Service of Argentina). Purple curve: high levels above 10 m (above lowest tide level), green rectangles: solsticial minimum.</figcaption></figure>

Compared with C1, the signal in core C4 is more difficult to decipher. The succession obviously contains individual tidal couplets (semi-diurnal couplets), as well as amalgamated mdbs, or at least thick mud dominated layers, that could sign very high suspended fluvial load episodes. However, mdb-to-mdb packages are not easily individualized along the whole succession, so that thickness measurements were difficult. The presence of discontinuities is probable but impossible to discern. The site is exposed to waves and eolian sand supply induced by permanent westerlies, and is influenced by the highly fluctuating fluvial
discharge and water levels of the Santa Cruz River. We thus believe that C4 succession records a very disturbed tidal signal.

According to the tidal signal, the sedimentation rate along C1 can be estimated around 30 cm/yr. This very high rate of deposition, typical of tidal rhythmite successions, is confirmed by the extremely low 210Pbexc values resulting from dilution by the high sediment flux. 210Pbexc values along C4 support similar very high rate. It is probably much lower than that estimated along C1, according to the combined wave and tide signal it contains, with discontinuities, but it certainly approaches a few cm per year. The apparent lack of 137Cs in core C1 could simply result, as the low 210Pb excesses, from an extreme dilution due to the very high sedimentation rate. Moreover, 137Cs fallout in the Southern Hemisphere was much less than in the Northern Hemisphere. For instance, very low values are detected in lagoonal cores in Australia, with undefined peaks (Pfitzner et al., 2004). Based on the values of 137Cs measured in surface sediment of a core from the Rio de la Plata River, Argentina (Di Gregorio et al., 2007), and assuming a similar 210Pbexc flux, values lower than 0.06 mBq/g of 137Cs can be anticipated for C1, which are close to the detection limit and hardly measurable. The higher 137Cs activities in C4 would then result from the lower sedimentation rate. Alternatively, the lack of 137Cs in core C1 could suggest that this succession was deposited before the 1960s, and the 137Cs "peak" in C4 could correspond to the 1963 atmospheric maximum. Although, on the field, C1 and C4 cross-sections were looking "fresh", satellite image comparison shows the very high stability of the channel and flat system position, especially on the core C1 site, during the last 40-50 years at least, signifying that this tidal flat is probably no longer active.

## 5 CONCLUSIONS

The two studied rhythmite successions well reflect the contrast in hydrodynamics and deposition processes that characterize the tidal flats along the edge of the Santa Cruz-Chico River estuary. The succession cored in the sheltered flat at the mouth of the tide-dominated Chico River estuary arm contains well expressed tidal rhythmites representing a 3-year record composed of neap-spring-neap and semi-annual cycles. A seasonal modulation is probably recorded through the deposition and preservation of thick mud beds during half of year, and related to very high suspended fluvial load. The succession cored along the tide-influenced Santa Cruz River arm obviously contains tidal beddings but fortnightly and semi-annual cycles are hardly detectable. The site, although subject to tidal dynamics, is exposed to the intense westerlies and induced agitation, giving rise to the record of an altered tidal signal. 137Cs measurements did not reach in concluding weather the studied successions were deposited very recently or date back a few decades or more. Satellite data comparison does not evidence sedimentological and morphological changes of the studied sites since at least 50 years, indicating that tidal flat aggradation is very poorly active nowadays.

## 6 ACKNOWLEDGMENT

The present study is part of the Hypertide project, funded by the ECOS-SUD - MINCYT program, and of the IRP Hypertide funded by CNRS-INSU. The authors thank Robert B. Dalrymple and Sergio Longhitano for their careful and helpful review. They would like to express as well their gratitude to Alain Trentesaux, Nicolas Tribovillard and Jean-Yves Reynaud, the head-organizers of the successful French Congress of Sedimentology, ASF Lille 2024.

## 7 REFERENCES

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# BENEATH THE VENDÉE DUNES: FORMER COASTAL SYSTEMS, INTERFACE BETWEEN MARSH AND FORESHORE REVEALED BY GROUND-PENETRATING RADAR AND CORE-PENETRATION TESTING

Julie Billy¹, Vivien Baudouin², Manuel Garcin³ and Gildas Noury³

¹BRGM, Bureau de Recherche Géologique et Minières, F-33600 Pessac, France

² BRGM, F-44323 Nantes, France

³BRGM, F-45060 Orléans, France

Corresponding author: j.billy@brgm.fr

**Abstract:** The combined use of ground-penetrating radar and geotechnical testing by CPT in Pays-de-Monts (Vendée, France) effectively characterizes the coastal geosystem. The results reveal the internal structure and soil behavior type of coastal dunes and their underlying sedimentary units, highlighting distinct features beneath the dune system at two neighboring sites. At La Parée Grollier, the dunes migrated above former foreshore and marsh deposits, and the study reveals the sub-surface foreshore-marsh transition. At La Pège, a clay-rich paleo-channel was identified which seems connected to the modern beach/dune system. These findings emphasize the value of subsurface data for effective coastal management considering their specific features.

**Keywords:** Sandy coastal geosystem, Internal architecture, Geotechnical test, Soil behavior type, Paleo-environment.

## 1 INTRODUCTION

In the context of coastal evolution and territorial adaptation to climate change, understanding coastal geosystems has become increasingly essential. Generally, sandy coasts are primarily characterized by their geomorphology, focusing only on the surface features of the geosystem. Current tools such as LiDAR, satellite imagery, and cameras are commonly used to monitor shoreline variations and the morphological evolution of foreshore, beach, or dune systems. However, knowledge of coastal geosystems, particularly their internal architecture and intrinsic properties, remains limited or poorly understood. As a result, their specific characteristics and geological spatial variability are rarely integrated into site management, or even less, into the development of future adaptation strategies. The aim of this work is to improve understanding and investigation methods for sandy coastal geosystems in order to highlight their specific characteristics, especially beneath dunes systems.

The Pays-de-Monts coastal dune system is located on the Atlantic coast in the Vendée region, western France (Fig. 1A). This area is a 27-km-long and 0.5-2.3-km-wide Holocene sandy dune system (e.g., Ters, 1956) composed of four generations of vegetated dunes (with increasing elevation inland), separated by large slack areas and bordered to the east by low-lying marshes (<2 m NGF). From inland to the beach, the dune system comprises: (i) highly digitate overlapping parabolic dunes (18–25 m high; D₁); (ii) digitate parabolic dunes (10–13 m high; D₂); (iii) an irregularly-shaped series of dune (7–14 m high; D₃); and (iv) longitudinal dunes connected to the modern beach (4–10 m high; D₄). This work focuses on two specific sectors: La Parée Grollier (D₁-4 dune types; Fig. 1A) in the northern part of the area and La Pège (D₃-4 dune types; Figure 2A) in the southern part. The aim of this study was to investigate if these dunes located close to each other have similar internal structures and to determine the nature of the underlying material beneath the dunes. Even though these areas are close to each other, are they similar? And furthermore, what are these dunes based on?

The Pays-de-Monts coastal geosystem is an emblematic study site for the BRGM's coastal research team. An innovative, multi-geophysical approach was developed and deployed in this area during 2016–2017 (SIBLES project). This approach combines continuous, multiscale, and non-destructive methods, including terrestrial seismic surveys (MASW), ground-penetrating radar (GPR), and electrical resistivity tomography. One of the key findings of previous work in La Parée Grollier area (Billy et al., 2020) is that the depth of Eocene limestone bedrock (fractured to highly fractured) reach -10 to -15 m NGF. GPR (Billy et al., 2014; Robin et al., 2023) and geotechnical testing (cone penetration test; e.g. Campo et al., 2023; Zhang et al., 2018) both demonstrate significant potential for characterizing the coastal geosystem in detail. Here we present a coupled approach GPR-CPT that has been experimented (ongoing SIBLES2 project) to characterize the coastal geosystem, and specifically the dunes as well as the underlying units down to the limestone bedrock.
## 2 MATERIALS AND METHODS

Ground-penetrating radar is based on the propagation and reflection of transmitted electromagnetic pulses to image sedimentary structures (Neal, 2004). The system used for the project is a Mala ProEx with two antennae centered at a frequency of 250 and 500 MHz, respectively. Data were processed using REFLEXW V10.5 (i.e. time-zero drift, background removal, band-pass filtering, gain, topographic correction) and interpreted following principles of radar stratigraphy (Neal, 2004) using DELPH RoadMap (V5). A constant radar velocity is estimated at 0.122 m/ns (Billy et al., 2020) and used for the time-depth conversion.

The cone penetration testing (CPT) is a standardized geotechnical test (ISO 22476-1, TE1 class). It measures soil resistance to the penetration of an instrumented cone that advances at a constant speed of 2 cm/s. Sensors measure two pressures (in MPa) every 2 cm: 1) the cone tip resistance, Qc, and 2) the lateral friction exerted by the soil on the sides of the cone, fs. Based on these data, parameters can be calculated to assess the properties of the soil traversed, such as the friction ratio, Rf (=100 * fs/qc, %), which provides an indication of soil granulometry, and relative density, Dr (%), representing grain packing (similar to porosity). Based on established charts (e.g., Robertson, 1990), these values can be used to classify soils based on their behavior (Soil Behavior Type, SBT 1-9, e.g. dense sands, soft clays, etc.). Existing cores and Log (from the French national BSS database) are also used to support the interpretation of soil classes identified through CPT. Only part of the data set is presented here: 4 and 8 CPTs for La Parée Grollier and La Pège, respectively. Each of these 12 sounding was performed up to the refusal (50 MPa).

## 3 RESULTS AND DISCUSSION

The Pays-de-Monts geosystem is basically defined by three main units: 1) fractured limestone (U~Basal~), overlain by 2) fluvio-marine deposits and 3) dune systems. The coupled GPR-CPT approach is suitable to characterize these two sedimentary cover units and to provide additional information on them.

### 3.1 Foreshore and marsh interface at La Parée Grollier

The internal structures of dunes at La Parée Grollier are well imaged using GPR. The parabolic dunes (U~D2~ dune type, in Fig. 1B) are primarily characterized by eastward-dipping reflectors (yellow dotted-line: U~D2a~, U~D2b~). This internal architecture reveals the former shape of the parabolic sandy dune and its past migration pattern towards the east (inland) before its fixation by vegetation (maritime pine forest) during the 19th century. CPT analyses, based on established charts, indicate that parabolic dunes are defined by soil behavior domain SBT 7: "dense sand to gravel" (yellow dots in Fig. 1D), and measurements are sparsely scattered.

<figure><img src="image_60.png" /><figcaption>Figure 1. A. Location map of the studied areas; B. Example of an interpreted GPR profile and C. Qc curves (CPT) at La Parée Grollier, and D. interpretation of depositional environments based on the soil behavior type chart, illustrating parabolic dunes, fluvio-marine and marsh deposits.</figcaption></figure>
Beneath this parabolic dune (U<sub>D2</sub>), GPR reveals a non-uniform underlying deposit that can be split in three domains, from beach (0 m) to land (~2 km distance from the shoreline):

i. From the beach to 850 m inland, the deposits are characterized by westward-dipping oblique reflectors (Fig. 1B, U<sub>FM</sub>). They are interpreted as a continuum of shoreface, foreshore and beach deposits (from -10 m to +2 m NGF). U<sub>FM</sub> illustrates the progradation pattern of this coast and the successive seaward sediment accumulation.

ii. From 850 m to 1 km, a mound-shape structure is observed on GPR profiles (Fig. 1B, U<sub>T</sub>). This unit is spatially restricted, but it also observed on profiles further north. U<sub>FM</sub> is adjoined to U<sub>T</sub>. It is interpreted as the boundary between fluvio-marine (U<sub>FM</sub>) and marsh deposits.

iii. From 1 to 2 km, a unit is identified on GPR profile based on its basal discontinuity. It consists of a 2 m thick unit characterized by a transparent facies (U<sub>M</sub>).

CPT analyses indicate that units under the dunes can be grouped into two populations. U<sub>T</sub> and U<sub>FM</sub> have a similar soil behavior domain, SBT 6, defined as "clean sands to silty sands" interpreted as foreshore to beach deposits (blue dots in Figure 1D). Like for parabolic dune SBT, measurements are sparsely scattered, and friction ratio (median: 0.6%) and spread are similar for both populations. However, foreshore to beach deposits have lower average normalized peak resistances than dune (median ~200 vs ~350). CPT Measurements at the depth of the unit U<sub>M</sub> (1 to -1 m NGF, green curve with low Qc values; Figure 1C) reveal a third population corresponding to the domain SBT 4, defined as "silt mixtures: clays silt and silty clay". These sediments are clearly distinguishable from the other layers, with Qtl and Fr medians at around 27 and 3.5% respectively. U<sub>M</sub> is hence interpreted as marsh deposits. Finally, CPT sounding refusals show the boundary between sediment and limestone (U<sub>Basal</sub>, Figure 1B), which is consistent with the depth and spatial variability of the limit determined by geophysics (Billy et al., 2020).

At La Parée Grollier, the parabolic dunes had migrated above foreshore and marsh deposits. Consequently, the current location of the marsh boundary identified using maps, photographs or LiDAR data does not show the real foreshore/marsh transition, contrary to the GPR-CPT approach.

## 3.2 Filled channel under the dune at La Pège

To the south, at La Pège (Fig. 1A), the dune system is less developed inland and can be narrowest (~40 m, Fig. 2A). Here, dunes are a natural barrier between the sea and touristic areas (parking area, campsite, etc.), as well as critical infrastructures such as water treatment plant. CPT soundings were realized on this narrow part of the dune along a north-west to south-east transect (pink dots; Fig. 2A).

A. Location map

<figure><img src="image_61.png" /></figure>

B. NW-SE longitudinal CPT profile (Qc curves, MPa)

<figure><img src="image_62.png" /></figure>

Refusal: Sounding were performed to refusal

C. Soil Behavior Type (SBT) chart

<figure><img src="image_63.png" /></figure>

**Figure 2.** A. Location map of La Pège; B. Qc curves of each CPT, along a NW-SE longitudinal profile and C. Analysis of CPT data following the soil behavior type chart.
Analyses highlight that the sand dunes have similar soil behavior than those described at La Parée Grollier. The CPT behavior of the fluvio-marine deposits is quite similar to that of the dunes, with both deposits corresponding to the SBT 7 domain: “dense sand to gravel”, but the fluvio-marine deposits appear to be coarser, i.e. with a lower Fr (0.3%; Figure 2C). Intercalated between the dune (U<sub>D4</sub>) and fluvio-marine deposits, a 1-2 m thick layer is identified on curves with low Qc values (CPT 234, 235, 202, 204 and 338; Fig. 2B). This unit corresponds to the SBT 3 domain: “clay to silty clay”. This clayed layer is observed along a 400 m-long transect (from CPT 234 to 338; Fig. 2A) and is absent on CPT 236 and 339, at the north and the south, respectively. These observations suggest that the clay layer could extend over a distance of 500 to 600 m (NW-SE orientation). This unit is also recorded on CPT and in cores more distant from the longitudinal dune (white dots, Fig. 2A), and seems to be related to the low topography inland corresponding to a paleo-channel. Thus, this clay layer is interpreted as a part of a filled former channel (e.g. former meander). Whereas the existence and the location of the paleo-channel beneath the dune system was previously unknown, our study shows a connection between the paleo-channel and the beach/dune system. In a context of coastal erosion and shoreline retreat, a better knowledge of the sub-surface geology is important because this layer is highly susceptible to appear on the beach in case of coastal retreat due to its altitude (-1 to 1.5 m NGF).

## 4 CONCLUSION

This study highlights the effectiveness of combining ground-penetrating radar and cone penetration testing for a detailed characterization of sandy coastal geosystems, particularly in complex dune environments. First, this study shows that sandy coasts can present notable singularities, even if they are spatially close and characterized by comparable dune morphologies. Our results reveal the internal structure of dunes and those of the underlying sedimentary units, providing valuable insights into stratigraphic transitions, and previously unknown features of the geosystem such as paleo-channel.

At La Parée Grollier, the coupling of GPR and CPT shows that i) the dune system, fluvio-marine and marsh deposits have distinct soil behavior types; ii) the eastward dune migration has covered foreshore/beach and marsh deposits, and iii) that the effective underlying location of the marsh-foreshore transition is beneath the dune system.

At La Pège, the study revealed the existence of a filled paleo-channel beneath the dune system, establishing a previously unknown link between former fluvial deposits and modern coastal landforms. This finding is particularly significant in the context of shoreline retreat and coastal erosion, as the clay-rich paleo-channel may become exposed, and thus have an impact on coastal sediment dynamics.

The integration of GPR and CPT provides a robust methodological framework for distinguishing sedimentary units and understanding their properties. While GPR offers high-resolution imaging of internal structures, CPT complements it by quantifying geotechnical properties and identifying sedimentary domains. Our results highlight the importance of incorporating subsurface geological data into coastal management strategies, particularly for adapting to global changes such as rising sea levels or coastal erosion.

## 5 ACKNOWLEDGMENT

This paper is a contribution of SIBLES2 research project co-funded by the BRGM, French Geological survey, and the Pays de La Loire Regional direction of the environment, DREAL. The authors would like to thank Bastien Ruaud and Emeric Gautier for their field assistance.

## 6 REFERENCES

Billy, J., Baudouin, V., Portal, A., Deparis, J., Bitri, A., Garcin, M., 2020. An Innovative Approach for a Comprehensive Characterization of Coastal Dune Systems through Internal Architecture and the Associated Intrinsic Geophysical Properties. Journal of Coastal Research, 95, 387–391.

Billy, J., Robin, N., Hein, C.J., Certain, R., FitzGerald, D.M., 2014. Internal architecture of mixed sand-and-gravel beach ridges: Miquelon-Langlade Barrier, NW Atlantic. Marine Geology, 357.

Campo, B., Bruno, L., Amorosi, A., 2023. Sedimentary facies characterization through CPTU profiles: An effective tool for subsurface investigation of modern alluvial and coastal plains. Sedimentology, 70, 1302–

Neal, A., 2004. Ground-penetrating radar and its use in sedimentology: Principles, problems and progress. Earth-Science Reviews, 66, 261–330.

Robertson, P.K., 1990. Soil classification using the cone penetration test. Canadian Geotechnical Journal, 27, 151–158.
Robin, N., Billy, J., Nicolae Lerma, A., Castelle, B., Hesp, P.A., Rosebery, D., Fauny, C., Deparis, J., Marieu, V., Bouchet, C., Miot da Silva, G., 2023. Natural remobilization and historical evolution of a modern coastal transgressive dunefield. Earth Surface Processes and Landforms, 48, 1064–1083.

Ters, M., 1956. La Vendée littorale : étude de géomorphologie. Oberthur, Paris, 578 p.

Zhang, X., Lin, C.-M., Dalrymple, R.W., Gao, S., Canas, D.T., 2018. Use of the Cone Penetration Testing (CPT) method to interpret late Quaternary tide-dominated successions: A case study from the eastern China coastal plain. Continental Shelf Research, 161, 49–57.
# IMPACTS OF DIKE REMOVAL ON CARBON BALANCES IN ESTUARINE INTERTIDAL ZONES

Camille DÉCULTOT¹, Nicolas CHEVALIER² and Lionel DENIS¹

¹ UMR 8187 Laboratoire D'Océanologie et de Géosciences – Université de Lille – CNRS – Université du Littoral Côte d'Opale – Station Marine de Wimereux – 28 Avenue Foch – 62930 WIMEREUX

² UMR 8187 Laboratoire D'Océanologie et de Géosciences – Université du Littoral Côte d'Opale– CNRS – Université de Lille – 32 Avenue Foch – 62930 WIMEREUX

Corresponding author: camille.decultot@univ-lille.fr

**Abstract:** Polders are modified intertidal zones that have been drained and separated from tidal inundation by dikes, for agricultural or urbanization purposes. Policy makers are considering removing or breaching the sea defenses of polders in response to growing concerns over increased flooding risks. As intertidal zones are known for their blue carbon sequestration capacities, there is a rising interest in the evolution of this ecosystem service following the rewilding of polders. Based on literature review, this paper outlines a general timetable for saltmarsh restoration steps, first focusing on Eastern England, then comparing this with other locations. Regardless of the location, the process follows a sequence starting with geomorphological evolution, followed by ecological colonization and finally reaching biogeochemical equilibrium. For example, in Eastern England, restored saltmarshes are expected to resemble natural ones within approximately a hundred years.

**Key-words:** Realignment management, Polders, Saltmarshes, Carbon sequestration, Ecosystem equilibrium.

## 1 INTRODUCTION

Coastal zones are recognized for the numerous ecosystem services they provide (Millennium Ecosystem Assessment, 2005): serving as feeding sites for coastal avifauna (Hughes, 2004), acting as water purification zones (Hasani et al., 2017), functioning as nurseries for fish species (Martinho et al., 2007) and sequestering carbon (Bulmer et al., 2020; Mazarrasa et al., 2023). However, a quarter of the global area of these zones has been lost over the two past centuries, primarily due to the artificialization of coastal territories (Nellemann et al., 2009; Mcleod et al., 2011). For example, polders are the result of drainage and dike embankment upon tidal estuaries. Even though polders have addressed agricultural and demographic challenges of past centuries, current concerns about climate change are emerging (Goeldner-Gianella, 2013). When high tidal coefficients and elevated river flows occur simultaneously, flooding is expected upstream of these artificially altered areas. As a solution to this increasing risk, the restoration of polder zones through the removal or breaching of dikes is being considered in local management plans. The resilience of these rewilded estuarine ecosystems would allow both tidal and river flows without threatening urbanized areas. However, some ecological consequences are anticipated after reintroducing tidal inundation to these lands. Specifically, as estuaries are capable of sequestering carbon within their sediments, a scientific concern arises: what are the impacts of dike removal on the carbon balance in estuaries?

## 2 ESTUARINE MORPHOLOGY AND CARBON FLUXES

Estuaries can be divided into three geomorphological zones based on their topographic levels and inundation frequency. The topographically lowest zone is the subtidal sandy channel, which is submerged most of the time. Next in the succession is the tidal flat, a sandy to muddy area (the latter referred to as the *slikke*) that is inundated at each high tide. Finally, the saltmarsh is a supra-tidal, muddy and vegetated zone that connects the estuary to the terrestrial domain (Fig. 1), and can be flooded by exceptionally high tides. Within the saltmarsh, three sub-zones can be distinguished, ranging from the most to the least frequently submerged: low, mid, and high marsh. Specific plant communities are characteristic of these zones, including *Aster tripolium*, *Halimione portulacoides*, *Halimione pedunculata*, *Limonium vulgare*, *Puccinellia maritima*, *Salicornia* spp., *Spartina* spp. and *Suaeda maritima* (Fig. 1). This typical saltmarsh vegetation plays a central role in the carbon sequestration capacity of saltmarshes. As plant communities capture inorganic carbon through photosynthesis, when their products are buried in the upper sediments, they constitute the autochthonous carbon input (Mason et al., 2023) (Fig. 1). Additionally, as the tide submerges vegetated saltmarsh zones, particles settle thanks to vegetation trap, thereby creating a second carbon input, characterized as allochthonous (Mason et al., 2023) (Fig. 1). However, biological processes of mineralization, such as biomass respiration or sub-surface methanogenesis, result in carbon outputs from the ecosystem (Adams et al., 2012; Mason et al., 2023). When the burial of carbon in the sub-surface
sediments exceeds the outputs in this estuarine ecosystem, the carbon then sequestered is referred to as "blue" carbon (Nellemann et al., 2009; Wang et al., 2021).

<figure><img src="image_64.png" /><figcaption>Figure 1: Conceptual model of carbon exchanges across the topography of a tidal estuary (modified from Mason et al., 2023).</figcaption></figure>

# 3 EXAMPLE OF THE EAST COAST OF ENGLAND

## 3.1 Geomorphological evolution

The removal of dikes impacts the sediment structure of the estuary, with the primary factor being the modification of the marsh elevation. Numerous studies report a consistent trend: for several years following dike removal, the surface elevation change - defined as the rise due to accretion minus the fall due to erosion or compaction - increases rapidly (Wolters et al., 2005; Adams et al., 2012). Then, as accretion slows down over the following years or decades, the elevation stabilizes (Fig. 2). Depending on the location and estuarine dynamics, the stabilized surface elevation resembles that of a natural saltmarsh within 10 to 30 years after dike removal (Fig. 2) (Wolters et al., 2005; Garbutt et al., 2006; Adams et al., 2012; Burden et al., 2019).

<figure><img src="image_65.png" /><figcaption>Figure 2: Surface elevation change in the restored saltmarshes of the Blackwater estuary depending on the years since dike removal (data compiled from Wolters et al., 2005; Garbutt et al., 2006; Adams et al., 2012).</figcaption></figure>

## 3.2 Ecological evolution

Once the surface reaches a suitable elevation, pioneer vegetation begins to establish, and this development can take between 1 and 10 years (Reading et al., 2002; Spencer et al., 2008). The main cited pioneer species in Europe are *Salicornia europaea*, *Scirpus maritimus*, *Spartina alterniflora*, *Spartina anglica*, *Spartina maritima*, and *Puccinellia maritima* (Langlois et al., 2003; Mossman et al., 2012; Spencer et al., 2008; Stoorvogel et al., 2024; Carneiro et al., 2024). After the colonization by pioneer species, more complex
saltmarsh vegetation communities begin to establish within 15 to 20 years, with species such as *Aster tripolium*, *Atriplex portulacoides*, *Limonium vulgare*, *Spergularia media* and *Suaeda maritima* (Reading *et al.*, 2002; Garbutt *et al.*, 2006; Garbutt & Wolters, 2008; Hughes *et al.*, 2009). However, achieving a complete vegetative cover and species richness comparable to that of a natural saltmarsh may take up to 100 years in a restored area (Garbutt & Wolters, 2008). Furthermore, depending on the site's characteristics (sediment type, salinity and seedbank), the vegetation succession can differ in terms of species and time, even in sites that are close to each other (Langlois *et al.*, 2003; Wolters *et al.*, 2008; Garbutt & Wolters, 2008). Nevertheless, the increase in vegetation cover always leads to further stabilization and compaction of sediments, hence facilitating the establishment of other plant species (Langlois *et al.*, 2003; Brand *et al.*, 2012). Thus, geomorphological and ecological equilibriums appear to be highly interdependent.

### 3.3 Biogeochemical evolution

Biogeochemical parameters of the sediments also respond to dike removal. The Carbon Accumulation Rate (CAR), whose determination is based on the evolution of elevation change (Burden *et al.*, 2019), is positively correlated with the accretion rate (Poppe & Rybczyk, 2021). CAR increases rapidly after restoration and then gradually decreases until stabilizing approximately thirty years after dike removal (Fig. 3) (Burden *et al.*, 2019). While CAR is a crucial factor for the burial of both autochthonous and allochthonous carbon, it varies independently of organic carbon contents in the upper sediments (Poppe & Rybczyk, 2021). Despite the rapid recolonization by pioneer vegetation (Fig. 3), the organic carbon content in the upper sediments of restored saltmarshes can take beyond 80 years to stabilize and approach levels found in natural saltmarshes (Fig. 3) (Burden *et al.*, 2019). Field studies in the same geographical area (Eastern England) report similar restoration timelines (Mossman *et al.*, 2012). This timing is likely related either to the duration required for vegetation to diversify and fully colonize the area (Garbutt & Wolters, 2008) or to the slow biogeochemical stabilization of sediments (Mossman *et al.*, 2012).

Burden *et al.* (2019) suggested that the restored state becomes comparable to a natural saltmarsh a hundred years after dike removal, at which point the soil carbon pool in both systems is similar, even if a restored saltmarsh ecosystem does not fully perform the same functions as a natural one (Fig. 3).

<figure><img src="image_66.png" /><figcaption>**Figure 3:** Schematic timetable of the evolution of key parameters in the restoration of saltmarshes in eastern England (based on data from Reading *et al.*, 2002; Wolters *et al.*, 2005; Garbutt *et al.*, 2006; Spencer *et al.*, 2008; Garbutt & Wolters, 2008; Hughes *et al.*, 2009; Adams *et al.*, 2012; Mossman *et al.*, 2012; Burden *et al.*, 2019).</figcaption></figure>

### 4 DIKE REMOVALS IN OTHER LOCATIONS

A study in the upper Bay of Fundy (New Brunswick, Canada), showed that the sedimentation rate increase was the first change following dike removal (Virgin *et al.*, 2020). Subsequently, pioneer vegetation began to spread rapidly in the first year of restoration and covered nearly the entire site within 8 years. The authors estimated that, in the following years, more typical saltmarsh plant communities would develop on the restored site (Virgin *et al.*, 2020). In the USA, Craft *et al.* (2003) monitored the development of saltmarsh ecosystems, with results partially aligning with those from the eastern coast of England. Sedimentation was the first parameter to evolve, and primary producers were established 5 to 15 years after restoration. Carbon pool levels were nonetheless lower than those of natural saltmarshes after approximately 30 years post-
restoration. Poppe and Rybczyk (2021) also conducted geomorphological monitoring of a restored saltmarsh on the eastern coast of the USA. Their findings indicated that accretion and carbon accumulation rates were high in the early stages of restoration (with accretion rates reaching up to 1.57 cm/year), and that carbon soil profiles were predicted to resemble those of natural saltmarshes several decades after restoration. Another study conducted in Texas, USA, showed that after 4 years of restoration, the density of above-ground biomass was comparable to that of a natural saltmarsh. However, the organic carbon pool in the sediments of the restored site was lower than that of a natural saltmarsh (Rezek et al., 2017).

In New South Wales (Australia), a drained and urbanized saltmarsh site has been restored with hydrological rehabilitation and plant transplantations. These efforts led to an equivalence of below- and above-ground organic carbon stocks between the restored and natural sites within 20 years of restoration (Santini et al., 2019). Moreover, another study conducted in southern Portugal showed that transplanting pioneer species from a natural to a restored saltmarsh may lead to a faster and more successful colonization of the restored site (Carneiro et al., 2024).

All things considered, the restoration processes observed in the USA and Canadian cases follow a similar sequence to those in England, albeit at a faster rate. Furthermore, the Australian and Portuguese cases illustrate that restoration can be facilitated, and subsequently accelerated.

## 5 CONCLUSION

Dike removal is a practice undertaken in many countries, however, studies combining carbon accumulation and vegetation development following de-embankment are scarce. The studies conducted in eastern England, Canada, USA and Australia, however, exhibit a similar pattern. Indeed, a high accretion rate is observed immediately after breaching, which correlates with a high rate of carbon accumulation. Nonetheless, carbon stock in restored saltmarshes does not align with that of natural saltmarshes for several decades. This is generally attributed to i) the growth and establishment of saltmarsh communities and ii) geomorphological adjustments within the restored site. While the restoration process appears to follow a similar pattern in all cases, the timescale can widely vary, and be accelerated by plant transplantation. Overall, the assessment of restoration success is highly site-dependent, and comparing timelines across different restoration sites remains challenging.

## 6 ACKNOWLEDGEMENTS

This work received financial support from the program CPER IDEAL and the "Fonds de dotation Nausicaà". This study is part of project CABESTAN of the exploratory research program FairCarboN and received government funding managed by the Agence Nationale de la Recherche under the France 2030 program, reference ANR-22-PEXF-0007.

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# THE IMPACT OF PLOUGHING ON SEDIMENT MINERALIZATION RATES IN THE SALT MARSHES OF THE SOMME ESTUARY

Lionel DENIS, Léna KOCYLA and Gwendoline DUONG

UMR 8187 LOG, Université de Lille- CNRS - Université du Littoral Côte d'Opale, Station Marine de Wimereux, 59000 Lille, France

Corresponding author : lionel.denis@univ-lille.fr

**Abstract:** Salt marshes are particularly popular with gatherers on foot, who harvest halophilic plants such as glasswort (Salicornia sp.) and annual sea-blite (Suaeda maritima). However, some other plants present in these rich and diverse ecosystems tend to replace the exploited species, such as the invasive English cord-grass (Spartina anglica). As a result, professionals are authorised to limit the spread of S. anglica through winter tillage within their concessions. Despite the recognition of these salt marshes as carbon sinks (blue carbon), no study to date has examined the impact of these activities on carbon budgets.

In this context, we investigated the intensity of carbon mineralisation by measuring CO2 flux at the sediment-atmosphere interface using dark benthic chambers. We compared exchanges across similar plant covers in the salt marshes (bare sediment, *Salicornia* sp., *Suaeda maritima*, *Spartina anglica*) in three adjacent areas: 1) ploughed during winter 2022-23, 2) ploughed during winter 2023-24 and 3) unploughed/control.

The results revealed a clear alteration in the carbon exchange mechanisms within these systems following tillering operations, with a drastic reduction in mineralization intensity in the ploughed areas. Furthermore, the more recent the ploughing, the lower the intensity of carbon mineralization compared to the unploughed control. Post-ploughing resilience was also observed, although the sediments did not return to a biogeochemical functioning comparable to that of unploughed areas, even after 1.5 years without ploughing. These results highlight the considerable impact of ploughing on the functionality of these salt marshes in terms of carbon cycling. The next step will be to include primary production processes under light conditions in order to quantify the full influence of tillage on carbon budgets in salt marshes undergoing such management practices.

**Keywords:** Tillage, salt marshes, *Spartina anglica*, benthic chambers, carbon dioxide fluxes, carbon mineralization, blue carbon.

## 1 INTRODUCTION

Despite a huge tidal forcing, along the French coast of the Eastern English Channel, the estuaries undergo accretion processes of fine particles due to lower hydrodynamics and terrigenous inputs. Consequently, specific habitats such as mudflats and salt marshes are located on both sides of the main channel, the latter constituting vegetated estuarine intertidal zones. Of major ecological and geochemical importance, due to their role as a buffer between the fluvial and oceanic domains and the diversity of conditions found there, these salt marshes are particularly popular with gatherers on foot for harvesting halophilic plants such as glasswort (*Salicornia* sp.), sea aster (*Tripolium pannonicum*) and annual sea-blite (*Suaeda maritima*). Although these plants grow naturally, their exploitation by professionals in Somme estuary is limited to four concessions located near Le Croyo (right bank) and La Pointe du Hourdel (left bank) (Figure 1), where these resources are collected by hand. However, other plants are present in these rich and diverse ecosystems, and tend to take over from the glasswort, such as the annual sea-blite (*Suaeda maritima*) or the invasive English cord-grass (*Spartina anglica*). In different parts of the world, several management practices aim to eradicate or to limit the invasion of the English cord-grass in salt marshes, among which mechanical control (hand removal, tillering, digging, disking, crushing), cutting or mowing, biological substitution or biological control by herbivory, smothering, heat treatments, herbicide use (Hedge et al., 2003, Roberts & Pullin, 2008). Even if the efficiency of these practices widely differs from one site to another, the temporality and regularity of the treatments, the combination of different practices, the adaptation to the size of the invasive patch(es) are of major importance (Li & Zhang, 2008; Gao et al., 2009). In the Somme estuary, each year, tillage is authorised and carried out in some areas of the concessions during the winter period (generally between 1st December of year n and 28th February of year n+1 - areas determined, controlled and followed by an environmental management structure) using a tractor equipped with a rotary harrow at the front and a rotavator at the rear (depth of tillage < 25cm).

While numerous studies have addressed the effectiveness of management practices, their impact on the diversity and functionality of salt marshes remains largely unexplored. However, it is urgent to quantify these
effects, especially considering that salt marshes are recognized as important carbon sinks (Endo & Otani, 2019, Macreadie et al., 2019). Plants in estuarine salt marshes fix atmospheric CO2 as organic matter during photosynthesis, accumulating it in their tissues during growth and also burying it in the soil through their roots and the detritus produced by tissue loss (e.g., leaves). Consequently, salt marsh may capture more carbon than they release and store it in sediments for extended periods. These ecosystems are known as blue carbon ecosystems (Lovelock and Reef, 2020), emphasizing the positive contribution of these intertidal areas to carbon budgets (Artigas et al., 2015).

As a first step in quantifying carbon budgets, this study aims to characterize and quantify the role of tillage in carbon mineralization rates across different parts of salt marshes, by measuring carbon dioxide exchanges at the water-atmosphere interface.

# 2 MATERIALS AND METHODS

## 2.1 Sampling strategy

The research was conducted at Le Croyo concession, where carbon dioxide exchanges at the sediment-air interface (during emersion period and in the dark) were compared across three adjacent areas (without mowing): an unploughed area (control), an area ploughed in winter 2022-23 (and not subsequently), and an area ploughed in winter 2023-24 (Fig. 1).

<figure><img src="image_67.png" /><figcaption>Figure 1. Area studied in the Somme estuary, along the eastern English Channel (a), between Le Croyo and Saint-Valery-sur-Somme (b), with concessions located along Le Croyo (in pink) and Le Hourdel (in green)((c) the reference numbers of concessions and total surface in hectares (ha) are given for each zone - adapted from Becuwe et al., 2024). The 3 areas studied (control/unploughed, ploughed in winter 2022-23, and ploughed in winter 2023-24) are located less than 150 m distance from each other, in the easternmost concessions (d).</figcaption></figure>

Four field campaigns were carried out in June and October 2024, during which carbon dioxide fluxes were measured in each area. For each area (control, ploughed in 2022-23, and ploughed in 2023-24), four sub-areas were pre-defined to represent the main plant species/ sediment types present in each location: i. Unvegetated bare sediments, ii. Sediments with *Salicornia* sp., iii. Sediments with *Suaeda maritima*, iv. Sediments with *Spartina anglica*.

In addition, triplicates measurements were made for each of the sub-areas (within 1m²), to account for small-scale spatial variability.

## 2.2 CO2 fluxes at the sediment/atmosphere interface

We measured CO2 exchanges at the sediment-atmosphere interface with the LI-COR 8100A device (LI-COR, Lincoln, USA), a semi-automatic dark benthic chamber equipped with an IRGA (Infra-Red Gas Analyser). The device consists of an opaque 'benthic chamber' (≈20cm i.d.), which is placed on a metallic base that is gently inserted 2 cm into the sediment. Once closed, the benthic chamber forms a hermetically sealed enclosure in which the temporal change of CO2 concentrations, the water vapour content and air temperature, as well as the humidity and temperature of the substrate (measured with additional probes: GS1 Ruggedized Soil Moisture Sensor (Campbell Scientific, Edmonton, Canada) and Omega 6000-09TC soil temperature probe (Omega, Stamford, USA)) are automatically recorded. These measurements allow for
the calculation of dry CO2 concentrations, enabling accurate quantification of CO2 fluxes through a known sediment/air surface area (318 cm²).

After preliminary tests, a standard measurement protocol was consistently applied. This involved placing an opaque PVC cover over the base (already embedded in the sediment) for at least 10 minutes before starting the measurements, allowing photosynthetic organisms to complete the chemical phase of photosynthesis. Once this time elapsed, the cover was removed, the air is renewed by vigorous stirring, and the benthic chamber was placed on the base before starting the measurement process. The procedure consisted of 1 minute of homogenisation and purging of the pump and tubing systems (with the chamber open) followed by 3 minutes of measurements with the chamber hermetically closed. Back in the laboratory, fluxes were calculated from the slope of the CO2 dry concentration versus time, obtained while the chamber was closed (considering an initial dead-band of 30 seconds) using the SoilFluxPro software (LI-COR, Lincoln, USA), which automatically accounts for all corrections (humidity, temperature, etc.). Given the temperature difference between sampling dates and times, we standardised CO2 flux data to a temperature of 20°C using a Q10 value of 2 (Boscolo-Galazzo et al., 2018). This calculation assumes that the intensity of a process doubles when the temperature rises by 10°C, as long as the temperatures remain within the physiological limits of the microorganisms implied.

## 2.3 Statistical analysis

Basic statistical analyses (T-tests, Pearson correlation), along with nested ANOVA, were carried out in R using the Stats package (version 4.2.1).

# 3 RESULTS AND DISCUSSION

## 3.1 General and temperature characteristics

The field experiments were carried out during low tide on 3 and 13 June, and 4 and 23 October, with the characteristics of the tidal cycle listed in Tab. 1.

<table><caption>Table 1. Dates, hours and environmental characteristics during the field campaigns.</caption><thead><tr><th rowspan="2">Dates</th><th rowspan="2">Tide coefficient</th><th rowspan="2">Low-tide hour</th><th colspan="2">Measurements</th><th rowspan="2">Sediment temperature Min / Max</th><th rowspan="2">Areas studied</th></tr><tr><th>Start</th><th>End</th></tr></thead><tbody><tr><td>03/06/24</td><td>73</td><td>16h52</td><td>14h46</td><td>17h48</td><td>19.4 / 22.1°C</td><td>control, ploughed 2023-24</td></tr><tr><td>13/06/24</td><td>46</td><td>12h04</td><td>10h53</td><td>12h21</td><td>16.4 / 20.8°C</td><td>ploughed 2022-23</td></tr><tr><td>04/10/24</td><td>87</td><td>08h29</td><td>08h15</td><td>11h09</td><td>4.6 / 14.4°C</td><td>control, ploughed 2023-24</td></tr><tr><td>23/10/24</td><td>63</td><td>11h16</td><td>10h37</td><td>11h43</td><td>11.1/15.5°C</td><td>ploughed 2022-23</td></tr></tbody></table>

Significant differences were observed in the temperature of surface sediments within the benthic chamber on all dates. Overall, temperatures were significantly lower in October than in June (t-test, n=72, p<0.001***). However, the primary variation occurred throughout the day (depending on the amount of sunshine), with temperatures increasing by up to +9.8°C during the morning of 4 October. On 13 June, the average temperature (±standard deviation) was 18.78 ± 1.64°C, significantly lower than the 21.13 ± 0.98°C on 3 June (t-test, n=36, p<0.001***). The coldest temperatures (as low as 4.6°C) were recorded early in the morning on 4 October (8.76 ± 3.39°C), coinciding with exceptionally low temperatures for the first half of October (including four days with the second-lowest daily temperatures in Abbeville in the last 50 years, ranging from 6.0 to 7.1°C below the seasonal norms for 1981-2010, according to Météo-France). In contrast, measurements taken in the late morning of 23 October showed less variability (13.02± 1.39°C).

## 3.2 Raw and normalised CO₂ fluxes

As chemically-mediated processes, and consequently dissolved exchanges and mineralization rates, are sensitive to temperature, we normalized the measurements to an ambient temperature of 20°C. However, the general pattern observed is very similar to that of the raw (non-standardized) flux values (Fig. 2).

In June, normalized carbon dioxide exchanges ranged from very low values, around 0.68 ± 0.40 µmol.m⁻².s⁻¹ for bare sediments ploughed in 2023-24 to 49.55 ± 14.31 µmol.m⁻².s⁻¹ for sediments with *Suaeda maritima* ploughed in 2023-24. Furthermore, sediments with *Suaeda maritima* exhibited greater variability compared to other sediment types. The trend was similar across bare sediments and those with *Salicornia sp.*, *Spartina anglica* and *Suaeda maritima*. Except for the measurements taken from *Suaeda maritima* after ploughing in 2023-24, the control area consistently recorded the highest carbon dioxide exchanges, while sediments ploughed in 2023-24 showed the lowest values, with sediments ploughed in 2022-23 yielding intermediate values. This indicates a marked decrease in mineralization intensity from the unploughed/control sites, to those ploughed in 2022-23, and finally those ploughed in 2023-24.