L’évolution holocène de la basse vallée de la Marne (Bassin parisien, France) entre influences climatiques et anthropiques
Article mis en ligne le 18 novembre 2012
dernière modification le 29 juin 2015

par Yann
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Holocene evolution of the lower Marne valley (Paris basin, France) : mixed climatic and anthropogenic influences

En LIGNE : GEOMORPHOLOGIE


Yann Le Jeune, Chantal Leroyer et Jean-François Pastre, « L’évolution holocène de la basse vallée de la Marne (Bassin parisien, France) entre influences climatiques et anthropiques », Géomorphologie : relief, processus, environnement, 4/2012 | 2012, 459-476.


Résumé :

Le remplissage de fond de vallée du cours inférieur de la Marne a été étudié sur plusieurs sites. L’étude des séquences sédimentaires, les analyses palynologiques, les datations 14C et la prise en compte des données archéologiques ont permis de reconstituer l’évolution du fond de vallée durant l’Holocène. Le début de l’Holocène se caractérise par une incision importante dans les alluvions mises en place lors des périodes froides antérieures. Durant le Préboréal et le Boréal, le niveau de la rivière remonte progressivement, permettant le dépôt de sédiments organiques sur les rives alors que des limons calcaires laminés se déposent en eaux plus profondes. Les données palynologiques montrent des impacts faibles et locaux des populations mésolithiques sur l’environnement. Après une phase d’incision qui pourrait être liée à l’événement 8,2 ka, les dépôts atlantiques sont ensuite principalement représentés par des tufs carbonatés à oncolithes et des dépôts organiques. Les données polliniques montrent en parallèle le développement progressif des activités agricoles dans le bassin-versant pendant le Néolithique ancien. Cependant, jusqu’au début du Subboréal, le fond de vallée reste marqué par l’absence de dépôts détritiques. Les premiers dépôts significatifs de limons dans le fond de vallée de la Marne n’interviennent en effet qu’au cours du Subboréal, soit plus d’un millénaire après les premiers défrichements importants. Le renforcement des activités agricoles entraîne par la suite une érosion importante de la couverture loessique sur les pentes. Les chenaux, principaux et secondaires, sont remblayés par plusieurs mètres de limons. Cette évolution souligne ainsi le passage d’un contrôle climatique à un contrôle anthropique. Les résultats obtenus posent en outre les bases d’une séquence de référence pour les dépôts alluviaux holocènes dans le centre du Bassin parisien.

Abridged English version

In the context of rescue archaeology, many multidisciplinary studies have been initiated within the last twenty years in the lower valley of the Marne River to provide a better understanding of the relationships between alluvial sedimentary sequences, climate variability, anthropogenic impact and environmental changes during the Holocene (Pastre and Leroyer, 1997 ; Pastre et al., 2002 a and b ; Le Jeune et al., 2005 ; Pastre et al., 2006). The lower Marne valley, in the centre of the Paris Basin, is characterised by many large meanders incised in calcareous Tertiary deposits, often covered by loess (fig. 1). There is only weak uplift in this area (1 m/10 ka ; Jost et al., 2007). Incision by the Marne River and downstream gradient (0.17 m/km) are also limited, leading to strong lateral migration of the watercourse and a high potential for the preservation of fluvial deposits. Recent research has included mechanical auger survey and coring, palynological analysis, archaeological mapping and radiocarbon dating (tab. 1). The alluvial sequences were studied at three main sites, two of them located on convex meander banks (Changis-sur-Marne, fig. 2, and Neuilly-sur-Marne, fig. 4) and another straighter part of the valley (Vignely ; fig. 3). We used auger survey to construct simplified cross-sections across the valley bottom between the older terrace and the modern river bed. Some cores were taken at the Haute-Île site for sampling and radiocarbone dating.
Palynological analyses of 18 sequences, together with 49 radiocarbon dates, allow us to define the Holocene vegetation history of the lower Marne valley. These new results fit well within the regional framework that was previously defined for the Paris Basin (Leroyer, 1997 ; Leroyer et al., 2011). In addition to the interpretation of environmental evolution, the focus of this palynological synthesis was to identify the human impact on vegetation (fig. 5). Towards the end of the Mesolithic (early Atlantic, palynozone VI), the vegetational record shows the first human impact at some sites, with an increase of ruderal taxa and Poaceae and a decline of oak and hazel. This shows a pioneering human impact also observed in the Seine valley (Leroyer, 2004). At the beginning of the Neolithic (palynozone VII), the first cereal pollen appeared, ruderal and grassland taxa increased and arboreal pollen slightly declines. Theses agro-pastoral signatures are recorded when close to archaeological sites (Leroyer and Allenet, 2006 ; Leroyer, 2006). They seem to decrease during the middle Neolithic, as archaeological sites become rare, excepted for Neuilly-sur-Marne and Chelles. The end of the Neolithic (palynozone VIII) shows a global impact of agro-pastoral activities, even though there is no known archaeological site hereabouts. Clearings are well recorded in palynological sequences that are close to human occupation (Fresnes-sur-Marne, Lesches et Chelles ; Leroyer, 2003). The pollen data for the Bronze Age (second part of palynozone VIII and the beginning of zone IX) do not show any lowering of human impact, whereas archaeological sites are quite rare, except for the late Bronze Age. Thereafter, the Iron Age (palynozone IX) is recorded in only three sequences, as massive silty deposits reduce pollen preservation (Pastre et al., 2003). An emphasis on clearings and agro-pastoral activities seems to be the trend but we lack sufficient records to infer a regional strengthening of human impact, since that phenomenon is not always detected in other valleys of the Paris Basin (Leroyer and Allenet, 2006 ; Leroyer et al., 2009).
The comparison of the sequences observed in Changis-sur-Marne, Vignely and Neuilly-sur-Marne (fig. 6) leads us to offer a reconstruction of the general evolution of the lower Marne valley during the Holocene (fig. 7). Holocene sedimentation by the Marne River began after the incision of the valley bottom and local erosion of the previous Lateglacial deposits (sequence S6, N1 and #2, fig. 7). That incision is well known in the Paris Basin (Pastre et al., 2002a) and is recorded here at Neuilly-sur-Marne and Vignely. This erosion event is linked with post-glacial warming and development of vegetation in the valley (Pastre et al., 2002 a and b). The water level began to rise slowly during the Preboreal and Boreal periods, resulting in laminated calcareous and organic mud deposition in deep water, whereas organic deposits and peats were formed at the shallower riverside edges (sequence S5, #3, fig. 7). That sequence seems to continue into the early Atlantic. An incision event is then recorded between 7570±70 BP and 6205±35 BP (#4, fig. 7). That erosion led to local remobilisation of previous deposits and could be connected to the global cooling associated with the 7500 BP event (8.2 ka cal. BP ; Alley et al., 1997). Calcareous sedimentation, forming oncolithic tufa, occurred (sequence S4, #5, fig. 7), with organic deposits at the margins, until the beginning of the Subboreal period. That sequence coincides with increasing human impact on vegetation, as recorded in the palynological data, but there is no corresponding evidence recorded in the valley-bottom sediments. Another incision event followed (#6, fig. 7), apparently before 3800±70 BP (Vignely, fig. 3) and perhaps associated with destabilisation of the catchment following forest clearance and increasing agro-pastoral activities during the late Neolithic. The initial impact would perhaps have been to induce more rapid transfer of water from the slopes of the valley to the river (Rey et al., 2004), given that no significant clastic deposits are recorded in the valley bottoms. During the Subboreal, the sedimentation of the Marne River remained mainly organic, with initial organic silts associated with numerous wood fragment and mollusc shells (sequence S3, #7, fig. 7). The first macroscopic non-organic silty deposits appear later, ca. 3600 cal. BP (base of the S3b sequence, fig. 6), whereas they are observed around 4500 cal. BP in the upstream parts of tributaries (Orth et al., 2004) or in colluvial contexts (Brunet and Le Jeune, 2004). The time lag between significant deforestation, colluvial deposition, and clastic sedimentation in the valley bottom is already known from other valleys (Lang and Hönscheidt, 1999). It could be estimated here as one millennium between the late Neolithic impact on the plateau and the silt deposition recorded at Neuilly-sur-Marne. The Subboreal period appears to be the threshold between climatic and anthropogenic factors as the primary influence on the evolution of the Marne River. The deposits formed subsequently, during the Subatlantic, are mostly silty. Erosion of loess from the plateau accelerated in parallel with the increasing human occupation that can be determined from archaeological data (fig. 8). It brought massive deposits of clayey silts in the valley bottom (sequences S1 and S2, #8, fig. 7) and increased incidence of flooding. Related deposition led to the formation of secondary channels abutting the older terrace (#8 and #9, fig. 7). Massive silt deposition gave rise to the modern form of the valley bottom (#10, fig. 7), with the modern river channel entrenched into the Holocene deposits.


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