# Weichselian lateglacial and Holocene fluvial dynamics variability in the Somme basin: the evolution of an alluvial peatland

Chloé Garcia, Boris Brasseur, Agnès Gauthier, Lou-Anne Mathieu, Dierk Michaelis, Jérémy Bacon, Ségolène Saulnier-Copard, Pierre Antoine

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Chloé Garcia, Boris Brasseur, Agnès Gauthier, Lou-Anne Mathieu, Dierk Michaelis, et al.. Weichselian lateglacial and Holocene fluvial dynamics variability in the Somme basin: the evolution of an alluvial peatland. Jean-Yves Reynaud; Eric Armynot du Châtelet. Short Papers from the 19th French Congress of Sedimentology, Lille, 84, pp.55-58, 2025, 2-907205-83-8. 10.70665/MJEW6151. hal-05005410

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# WEICHSELIAN LATEGLACIAL AND HOLOCENE FLUVIAL DYNAMICS VARIABILITY IN THE SOMME BASIN: THE EVOLUTION OF AN ALLUVIAL PEATLAND

Chloé GARCIA<fnref n="1" />, Boris BRASSEUR<fnref n="1" />, Agnès GAUTHIER<fnref n="2" />, Lou-Anne MATHIEU<fnref n="2" />, Dierk MICHAELIS<fnref n="3" />, Jérémy BACON<fnref n="1" />,<fnref n="4" />, Ségolène SAULNIER-COPARD<fnref n="2" /> and Pierre ANTOINE<fnref n="2" />

Corresponding author: garcia.chloe@u-picardie.fr

**Abstract:** The Somme basin is one of the most extensive alkaline alluvial peatland systems in North-West Europe. The Weichselian Lateglacial and Holocene peat sequences from this basin have remarkable potential for palaeoenvironmental reconstructions, to understand the long-term influence of climate and anthropogenic forcing on fluvial dynamics. New field investigations carried out since 2021 are based on two high-resolution stratigraphic transects (120 boreholes), soil and geochemical characterisation, identification of pollen and plant macroremains, and 50 radiocarbon dates. The results, supplemented by a synthesis of the data from the various archaeological excavation programmes undertaken since the 1990s, highlight the response of fluvial and peat systems to climatic and anthropogenic environmental change, and provide a particularly robust and detailed outline of the evolution of the valley floor sedimentary infilling. The dynamics and nature of the deposits were determined by climatic changes until the mid-Holocene. Fluvial dynamics constrain the accumulation of peat, temperate periods were particularly favourable to its development: reduction of water flow, detrital impute and particle size, and the development of vegetation. The oldest peat deposits were confined to the abandoned river channels during the Bølling and the Allerød, then peat progressively filled the entire valley floor during the Early and Middle Holocene. A reactivation of fluvial dynamics is recorded in the basin from 7800 cal BP with the establishment of deep erosive channels. From this time onwards, environmental responses vary across the basin. Anthropogenic influences had an increasingly pronounced impact and varied within the basin: soil erosion on slopes, management of alluvial water tables (increased by dams or decreased by peat extraction), causing diachronic interruptions in peat accumulation processes and even peat degradation.

**Keywords:** alluvial peatland, river dynamic, chalk basin, climate variation, anthropisation.

## 1 INTRODUCTION

Peatlands are exceptional archives for palaeoenvironmental reconstruction. In the chalk valleys of north-western Europe, alluvial alkaline peatlands have recorded fluvial dynamics and past ecosystems. Since the Lateglacial, several initiation periods of peat formation have been recorded in Europe: either associated with favourable climatic conditions (rise in temperature, intensification of precipitation with the return of oceanic conditions), as during the Lateglacial temperate interstadials and during the first half of the Holocene. Or with human-induced modifications to the hydrography, as in the second half of the Holocene (Ruppel et al., 2013).

The Somme basin, with its remarkable historical archaeological records and existing stratigraphic data on the valley floor infillings, represents a particularly important case study for investigating the spatial impacts of climatic and anthropogenic changes and modifications on the river dynamics since the Lateglacial.

## 2 MATERIALS AND METHODS

Data on stratigraphy, chronology and palaeoenvironmental conditions from the Weichselian Lateglacial period onwards provide a framework for identifying patterns of change in the fluvial dynamics of the catchment. The data presented here originate from two recently studied stratigraphic transects: Tirancourt (Middle Somme Valley, Garcia et al., 2024) and Morcourt (Upper Somme Valley, Garcia et al., 2025).

In addition, the sites are compared with former studies throughout the basin (Fig. 1), including in the Selle tributary: in Conty (Antoine et al., 2012) and Renancourt (Antoine and Ducrocq, 1997), both situated within

<footnote n="1">¹ EDYSAN (Ecologie et Dynamiques des Systèmes Anthropisés) UMR 7058 CNRS - UPJV, UFR de pharmacie, 1 rue des Louvels, FR-80037, AMIENS, cedex 1</footnote>

<footnote n="2">² Laboratoire de Géographie Physique, Environnements quaternaires et actuels UMR 8591 CNRS- Univ. Paris 1 & UPEC, 2 rue Henri Dunant, FR-94320, THIAIS.</footnote>

<footnote n="3">³ Institute for Botany and Landscape ecology, University of Greifswald, Greifswald, Germany</footnote>

<footnote n="4">⁴ Ecologie des Hydrosystèmes Naturels et Anthropisés, UMR 5023 CNRS-Université Lyon 1, 3-6 rue Raphaël Dubois, Bâtiments Darwin C. & Forel, FR-69622, VILLEURBANNE.</footnote>
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_2.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_3.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

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