Réf. Debret & al. 2010 - A

Référence bibliographique complète

DEBRET, M., CHAPRON, E., DESMET, M., ROLLAND-REVEL, M., MAGAND, O., TRENTESAUX, A., BOUT-ROUMAZEILLE, V., NOMADE, J., ARNAUD, F. 2010. North western Alps Holocene paleohydrology recorded by flooding activity in Lake Le Bourget, France. Quaternary Science Reviews, 29, 2185–2200. [Etude en ligne]

Abstract: A 14-m long piston core was retrieved from Lake Le Bourget, NW Alps (France), in order to provide a continuous record of flooding events of the Rhone River during the Holocene. The selection of the coring site was based on high resolution seismic profiling, in an area with limited mass wasting deposits and accumulated proximal Rhone River inter- and underflow deposits. The age-depth model of this core is based on (i) 14 AMS radiocarbon dates, (ii) radionuclide dating (137Cs) and (iii) the identification of historical data (flood events, eutrophication of the lake). The sedimentary record dates back to 9400 cal BP, and includes a thin mass wasting event deposited around 4500 cal BP. A multi-proxy approach was used to track the evolution and origin of clastic sedimentation during the Holocene, in order to identify periods of higher hydrological activity in the catchment area. Spectrophotometry was used to detect fluctuations in clastic supply and the study of clay minerals (especially the Illite crystallinity index) allowed locating the main source area of fine grained clastic particles settling at the lake after flood events. This dataset highlights up to 12 periods of more intense flooding events over the last 9400 years in Lake Le Bourget and shows that the main source area of clastic particles during this period is the upper part of the Arve River drainage basin. This part of the catchment area drains several large glaciers from the Mont-Blanc Massif, and fluctuations in Rhone River flood supply in Lake Le Bourget is interpreted as resulting essentially from Mont-Blanc Glacier activity during the Holocene. The comparison of clastic sedimentation in Lake Le Bourget with periods of increasing land use and periods of Alpine glacier and mid-European lake level fluctuations, suggest that the core LDB04 clastic record in Lake Le Bourget is a continuous proxy of the Holocene hydrological history of the NW Alps.

Mots-clés

 

 

Organismes / Contact

Laboratoire de Morphodynamique Continentale et Côtière, Université de Rouen, UMR CNRS/INSU 6143, Department of Geology, 76821 Mont-Saint-Aignan Cedex, France

ISTO – Université d’Orléans, UMR 6113 1A rue de la Ferollerie 45071 Orléans Cédex 2, France and Université François Rabelais UMR 6113 Parc de Grandmont 37200 Tours, France

Geological Institute, ETH Zürich, CH-8092 Zürich, Switzerland

Géosciences Azur, Université de Nice UMR6526, Parc Valrose, 28 Av. de Valrose, BP2135 06108 Nice cedex 2, France

LGGE, Université de Grenoble, UMR 5183, 54, rue Molière, 38402 - Saint Martin d’Hères cedex, France

PBDS Laboratory, UMR 8110 CNRS, University of Lille 1, 59 655 Villeneuve d’Ascq, France

LGCA Laboratory, Université Joseph Fourier, Maison des géosciences, BP 53, 38041 Grenoble cedex 9, France
EDYTEM Laboratory, UMR 5204, University of Savoie, CISM, Campus Scientifique, F-73376 Le Bourget du Lac Cedex, France

 

(1) - Paramètre(s) atmosphérique(s) modifié(s)

(2) - Elément(s) du milieu impacté(s)

(3) - Type(s) d'aléa impacté(s)

(3) - Sous-type(s) d'aléa

 

Glaciers, Paleoydrology (lake level fluctuations), Paleo-erosion, Clastic sedimentation in lake

Floods

 

 

Pays / Zone

Massif / Secteur

Site(s) d'étude

Exposition

Altitude

Période(s) d'observation

Northern French Alps

Rhone basin upstream Lake Le Bourget

Lake Le Bourget, Arve river, Mont-Blanc massif…

 

 

Back to 9400 cal BP

 

(1) - Modifications des paramètres atmosphériques

Reconstitutions

 

Observations

 

Modélisations

 

Hypothèses

 

 

Informations complémentaires (données utilisées, méthode, scénarios, etc.)

 

 

(2) - Effets du changement climatique sur le milieu naturel

Reconstitutions

A multi-proxy study was used to analyse one of the longest sedimentary records of flood deposits in the Alps. The continuity of the cores sampled throughout the last 9400 years allowed tracking the sources of detritism and studying the Holocene climatic variability recorded by the lake. The study of clay fractions made it possible to precisely locate the sources of detritism upstream of Sallanches, in the Mont-Blanc range. This source remained the only component of detritism recorded in the lake sediments throughout the Holocene. Thus, in spite of the distance (150 km), the lake record is closely linked to the glacier fluctuations in the Mont-Blanc massif, because they undergo the same hydrological forcing. Because of the specific geomorphological relationship between Lake Le Bourget and the Rhone River during the Holocene, human activities may have amplified the erosion of the catchment during periods of climate deteriorations, but could not affect the timing of the flood events. Consequently, the evolution of clastic sedimentation recorded in core LDB04 represents a reliable proxy for Holocene paleohydrology. Agreement between the 12 phases of detritism in Lake Le Bourget and different lake records, and with other Alpine glaciers, shows that the Mont-Blanc range underwent climatic oscillations with significant variations in the hydrological cycle. The variations of detritism recorded in Lake Le Bourget are in agreement with Alpine climatic oscillations, indicating that the hydrology of this part of the Alps reflects a regional climatic pattern.

Observations

 

Modélisations

 

Hypothèses

 

 

Sensibilité du milieu à des paramètres climatiques

Informations complémentaires (données utilisées, méthode, scénarios, etc.)

Lake Le Bourget receives a fraction of the discharge of the Rhone River when it leaves its partly-ice-covered alpine watershed (Chapron et al., 2002; Revel-Rolland et al., 2005). The abundance of the detrital fraction in Lake Le Bourget sediments was therefore used to reconstruct the past flooding activity of the Rhone River during the Little Ice Age (Chapron et al., 2002) and over the last 7500 years (Arnaud et al., 2005). Two limitations can be identified in this latter study performed on distal Rhone River flood deposits: 1) the magnetic susceptibility (MS) signal used to track the mineral discharge was very low in Mid-Holocene sediments, precluding a fine interpretation of the detrital signal; and 2) this study did not allow disentangling the relative contributions of climate and land use on the terrigenous signal. In the present paper, we address both those problems by studying a new longer core retrieved in more proximal Rhone River flood deposits with a new proxy subjected to spectrophotometry, which gives better resolution than MS; and through the discrimination of the source area of the detrital fraction of sediment by performing a mineralogical study of lacustrine, fluvioglacial and fluvial sediments.

Colorimetric measurements are commonly used in palaeoceanographic studies as a rapid, high resolution tool of sediment characterisation (Chapman and Schakleton, 2000). In Lake Le Bourget, the sediment was shown to be a two-phase mixture made of autochthonous bio-induced calcite and allochthonous mostly silicate-bearing minerals (Chapron, 1999; Arnaud et al., 2005; Arnaud, 2005). In such a case, the L* parameter, i.e. the gray-scale value of the sediment colour, can be used as a simple proxy of the carbonate/silicate ratio (Chapman and Schakleton, 2000) and therefore as a marker of detritism. The direct relationship between the carbonate/silicate ratio and the L* value is further confirmed and calibrated based on a low-resolution series of major element measurements.

Reconstructing the intensity of detritism can yield information about the drainage basin’s hydrological activity and therefore about variations in precipitation regimes, provided proof can be found that no change occurred through time in the source of clastic particles. This underscores a major uncertainty concerning the causes of changes in detrital fluxes which may be simplified into “climate” vs. “human” impact on erosion (Desmet et al., 2005). To control this over simplification, the authors still take care that internal processes, like lake level variations, did not affect the detritism record. They address this problem by looking at the origin of Lake Le Bourget’s fine grained detrital fraction from the angle of its mineralogical fingerprint and comparing it to the suspended sediment load of the main tributaries and the evolution of detrial input during the time. They test the regional significance of their paleohydrological record by comparing it with the reconstructed Holocene paleohydrological framework of the NW Alps based on Alpine glacier and lake level changes. As Lake Le Bourget is located more than 150 km downstream from its glacier-covered watershed, they discuss the potential causal link between glacier fluctuations and paroxysmal phases of Rhone River flooding activity.

Core LDB04 was retrieved in 2004 at 100 mwater depth in the northern part of Lake Le Bourget with a 3 m long UWITEC piston coring device operated from a platform. A 14 m continuous composite series was established from two series of overlapping 3 m long sections, according to 1) sedimentological properties measured every 5 mm with a multi sensor track (gamma density, P wave velocities and magnetic susceptibility) and 2) the identification of several marker horizons (thick flood deposits; mass wasting deposits) on spilt cores. Sediment water content and undrained peak shear strength were punctually measured at the same depths in the core by weighing 2 cm3 of sediment before and after drying in an oven at 105°C, and using a pocket vane shear testing device perpendicular to bedding. In addition, recent lacustrine sediment samples were retrieved in 2005 at the water sediment interface of a recent ice-contact lake in front of the Mer de Glace glacier (Mont-Blanc massif) and at the top of a short sediment core from Lake Anterne (Aiguilles Rouges massif) retrieved in 1999 (cf. Arnaud et al., 2002).

Fluvial sediment samples: Recent fluvial sediment samples were collected in 2005 from flood deposits resulting from significant snow melt events at the end of the winter in the catchment areas of the Arve and Giffre rivers. These samples where taken from river oxbows and river bed depressions in areas trapping flood events with suspended sediment load. Four samples were also obtained from sediment filters installed at the outlet of subglacial streams directly at glacier fronts or at the outlet of recent proglacial lakes in the Mont-Blanc massif. These samples therefore represent the fine grained suspended sediment load of the upper Arve River resulting from soil and bedrock erosion by heavy rainfall, snow melt events and high altitude warm based glaciers.

Clastic sediment analyses: The sediment magnetic susceptibility on this core was measured with a sampling interval of 5 mm. Lacustrine and fluvial sediment clay mineralogy was documented by X-ray diffraction. The error margin on the quantitative evaluation of minerals resulting from the interpretation is evaluated to 5%. Sampling for clay mineral determination was carried out every 25 cm in core LDB04. This protocol was also applied to the 26 samples taken within the catchment area of the Arve River. In addition to clay mineral identification, the illite crystallinity index (ICr) was documented. This index corresponds to the measurement of the illite peak width around the mid-height of the diffraction peak at 10A (Kübler, 1964) and gives an indication of the degree of bedrock metamorphism. The major element content in core LDB04 was determined with a sampling interval of 25 cm and each sample includes a sediment thickness of 1 cm. The measurements were performed with the accuracy of 1.4%. Finally, a spectrophotometer was used on core LDB04 to measure the sediment reflectance intensity of visible wavelengths between 400 and 700 nm, at 10 nm intervals. Laboratory descriptions of core lithologies are supported by laser diffraction grain size measurements using a Malvern Mastersizer at Savoie University.

The sediment macroscopic analysis coupled with granulometric, reflectance, clay mineralogy, MS measurements and the estimation of carbonate content in core LDB04, allowed defining three sedimentary units. The age model in core LDB04 was established based on fourteen AMS 14C radiocarbon dates (10 dates in Units 1 and 2; 4 dates in Unit 3). Four additional dates were integrated in the age-depth model on the basis of stratigraphic correlations with the previously dated core LDB01 (Arnaud et al., 2005) retrieved in distal Rhone River flood deposits (Chapron et al., 2005). All the calibrated ages were computed using Intcal version 5.0.2, with the calibration curve taken from Reimer et al. (2004). Detailed dating was obtained over the historical period using radiometric markers (137Cs peaks corresponding to the AD 1986 Chernobyl accident and the 1964 maximum fallout caused by atmospheric nuclear bomb tests), the onset of biochemical varve formation due to lake eutrophication in ca. AD 1943 ±1 years (Giguet-Covex et al., 2009) and the recognition of three thick underflow flood deposits corresponding to well-documented catastrophic floods events on the Rhone River between AD 1732e1734 (Chapron et al., 1999, 2002).

 

(3) - Effets du changement climatique sur l'aléa

Reconstitutions

The ICr indicates that illite in Lake Le Bourget Holocene sediments essentially came from glacial and peri-glacial processes in the Mont-Blanc massif. To test this interpretation and to further investigate the relations between Lake Le Bourget clastic sedimentation, Mont-Blanc glacier activity and climate changes, the MS and L* signals from core LDB04 were compared to Alpine glacier and mid-European lake level fluctuations documented during the Holocene.

Instrumental and historical records of alpine glaciers: decadal scale

Before AD 1870, glacier front positions in the northern Alps (France and Switzerland) were deduced primarily from paintings or historical writings on the damage caused by advancing glaciers. These data show that alpine glaciers in the middle of the 19th century were from 0.8 to 1.6 km longer than today and underwent a major retreat during the 20th century. During this period (150 years), the clastic record (MS) of Lake Le Bourget was compared with the cumulative length of five large alpine glaciers documented by Vincent et al. (2005): (i) 3 glaciers from the Mont-Blanc massif (Bossons, Argentière and Mer de Glace) draining into the Arve River; (ii) the Trient Glacier located in the Mont-Blanc massif but draining into the Rhone River in Switzerland and (iii) the Grindelwald glacier in the Bernese Alps (Central Switzerland). The maximum extensions of these glaciers are generally contemporaneous but not perfectly synchronous because their length is not directly linked with climate, but also results from other specific parameters controlling their dynamics, such as size, exposure and geometry (Nye, 1965; Johannesson et al., 1989). Nevertheless, the general evolution of the length of these alpine glaciers highlights a similar pattern over the last 150 years. Clastic sedimentation was higher in Lake Le Bourget when these glaciers were either increasing or decreasing from three periods of culminating length (i.e. at the end of 19th century; between AD 1920 and AD 1931; between AD 1983 and AD 1995). The MS signal in Lake Le Bourget shows three significant peaks around AD 1880, AD 1930, AD 1950 and a small plateau between the 1980s and the 1990s. This suggests that since AD 1870, clastic supply in Lake Le Bourget has resulted from bedrock erosion associated with Mont-Blanc glacier fluctuations in the Arve River following the end of the Little Ice Age. If an anthropogenic imprint exist, it seems that it only impact the amplitude of the signal and not the timing. It seems that the flooding of the Rhone River into the lake results from climatic factors.

Alpine glacier and mid-European lake level fluctuations over 3500 years: secular scale

The comparison of Swiss glacier fluctuations and periods of higher lake levels in Western Europe during the last 3500 years (Holzhauser et al., 2005) with the continuous record of Rhone River flooding activity in Lake Le Bourget (the magnetic susceptibility signal) highlights periods where increased flooding activity matches with periods when glaciers where near their maximum extent and higher lake levels: between 2800 and 2400 cal BP; 1500–1300 cal BP; 1200–1050 cal BP; 900–800 cal BP and 700–220 cal BP. Periods of increased flood activity occurring between 2200 and 2000; 1950–1600 and 1050–950 cal BP, are, on the other hand, not contemporaneous to any clearly documented glacier or lake level fluctuations in the alpine range, but took place after wet periods associated with higher Rhone River torrential activity (cf. Arnaud et al., 2005): the so-called Iron Age Hydrological crisis (1200–1550 cal BP), the Roman Wet Period (between 2100 and 1900 cal BP) and the High Middle Age Wet Period (1500–1200 cal BP). Moreover, at coring site LDB01, Arnaud et al. (2005) have shown that peaks in magnetic susceptibility were also matching several transgressive phases recognized over the last 3000 years by Magny and Richard (1985) at the archeological site of Conjux (NW littoral platform) in Lake Le Bourget. These transgressive phases and enhanced clastic supply at site LDB01 coincided with periods of high water activity in the Upper Rhone River documented by Bravard (1996).

Over the last 3500 years, it is also important to note that a trend toward increasing clastic supply in Lake Le Bourget since ca. 2800 cal BP is also documented in the French pre-Alps (Lake Annecy; Nomade, 2005), in the French Western Alps (Lake Bramant, Guyard et al., 2007), in the Eastern Alps (Lake Constance, Wessels, 1998) and up to Scotland (Lake Lochnagar, Dalton et al., 2005). This period is in addition matching a transgressive trend in jurassian lakes (Magny et al., 2003). A large number of archaeological studies performed in the alluvial plain of the Rhone River have also reported that hydrological activity was weak in the river valley from the Neolithic period (ca. 5000 cal. BP) to the 1st Iron Age (Salvador et al., 1993; Bravard, 1996). These comparisons suggest that since the onset of the Iron Age (ca. 2800 cal BP), increased Rhone River flood activity in Lake Le Bourget has been above all sensitive to significant hydrological changes, favouring higher lake levels in mid-Europe and a higher torrential activity in the Rhone River.

Alpine glacier and lake level fluctuations during the Holocene: millennial scale

Few continuous reconstructions of glacier or hydrological activities during the Holocene are documented in the Alps and those available originate from western Europe lake level reconstructions (Magny, 2004) and proglacial environments from the French, Italian, Swiss and Austrian Alps (Dorthe-Monachon, 1988; Ballandras and Jaillet, 1996; Wessels, 1998; Leeman and Niessen, 1994; Maisch et al., 2000; Hormes et al., 2001; Deline and Orombelli, 2005; Joerin et al., 2006; Holzhausser, 2007; Hass et al., 1998). The synthesis of environmental changes in the Alps describes the first part of the Holocene (until 6000 cal BP) with very small glaciers. This period is known as the Holocene climate optimum. This implies that information on glacier activity during this period is limited and Holzhausser (2007) underlines the difficulties of establishing a continuous chronology, while Hormes et al. (2001) and Joerin et al. (2006) suggested several periods with smaller glaciers than today, based on statistical studies of radiocarbon dates from wood samples found in glacial and proglacial deposits.

Nevertheless, between 3500 and 9400 cal BP the L* signal in core LDB04 (used as a marker of detritism) highlight several periods of enhanced clastic supply. Comparisons with lake level and glacier reconstructions suggest that Rhone River flood activity in Lake Le Bourget between 3500 and 9400 cal BP has been above all sensitive to wet (and often cooler) periods in the Alps (favouring higher lake levels in mid-Europe). These wet periods were contemporaneous to glacier fluctuations, especially after the Holocene Optimum (a period known as the Neoglacial). Only one period of higher lake level from Magny (2004) is not clearly documented in this study: it correspond to a period dated between 4800 and 4850 cal BP, matching (within dating uncertainties) a glacier advance of the Miage glacier documented by Deline and Orombelli (2005) between 4600 and 4800 cal BP in the Mont-Blanc Massif and the Rotmoss II phase of glacier advance in the Alps (Maisch et al., 2000). This period is not well documented at site LDB04 in Lake Le Bourget because of the occurrence of a slump deposit dated to ca. 4500 cal BP. The L* and magnetic susceptibility signals above this slump deposit are, however, characterized by sharp but significant fluctuations, suggesting enhanced clastic supply between ca. 4375 and 4500 cal BP. As a working hypothesis, this slump deposit remoulding littoral sediments may have been triggered by abrupt lake level fluctuations during Mont-Blanc glacier fluctuations. Since lake level reconstructions in Lake Le Bourget are only covering the last 4300 yrs (Magny and Richard, 1985) this hypothesis still need to be confirmed.

Observations

 

Modélisations

 

Hypothèses

 

 

Paramètre de l'aléa

Sensibilité des paramètres de l'aléa à des paramètres climatiques

Informations complémentaires (données utilisées, méthode, scénarios, etc.)

 

 

 

 

(4) - Remarques générales

 

 

(5) - Syntèses et préconisations

 

Références citées :

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Arnaud, F., 2005. Discriminating bio-induced and detrital sedimentary processes from particle size distribution of carbonates and non-carbonates in hard water lake sediments. Journal of Paleolimnology 34, 519–526.

Arnaud, F., Revel, M., Chapron, E., Desmet, M., Tribovillard, N., 2005. 7000 years of Rhône river flooding activity in Lake Le Bourget: a High-resolution sediment record of NW Alps hydrology. The Holocene 15 (No. 3), 420–428.

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