Réf. Giguet-Covex & al. 2012 - A

Référence bibliographique complète

GIGUET-COVEX, C., ARNAUD, C., ENTERS, D., POULENARD, J., MILLET, L., FRANCUS, P., DAVID, F., REY, P.-J., WILHELM, B., DELANNOY, J.-J. 2012. Frequency and intensity of high-altitude floods over the last 3.5 ka in northwestern French Alps (Lake Anterne). Quaternary Research, Vol. 77, 1, 12–22. http://dx.doi.org/10.1016/j.yqres.2011.11.003 [Etude en ligne]

Abstract: In central Western Europe, several studies have shown that colder Holocene periods, such as the Little Ice Age, also correspond to wet periods. However, in mountain areas which are highly sensitive to erosion processes and where precipitation events can be localized, past evolution of hydrological activity might be more complicated. To assess these past hydrological changes, a paleolimnological approach was applied on a 13.4-m-long sediment core taken in alpine Lake Anterne (2063 m asl) and representing the last 3.5 ka. Lake sedimentation is mainly composed of flood deposits triggered by precipitation events. Sedimentological and geochemical analyses show that floods were more frequent during cold periods while high-intensity flood events occurred preferentially during warmer periods. In mild temperature conditions, both flood patterns are present. This underlines the complex relationship between flood hazards and climatic change in mountain areas. During the warmer and/or dryer times of the end of Iron Age and the Roman Period, both the frequency and intensity of floods increased. This is interpreted as an effect of human-induced clearing for grazing activities and reveals that anthropogenic interferences must be taken into account when reconstructing climatic signals from natural archives.

Mots-clés

Grain size - Itrax core scanner - Flood frequency - Extreme precipitation events - Climatic changes - Land use

 

Organismes / Contact

• EDYTEM, Université de Savoie, CNRS Pôle Montagne, 73376 Le Bourget du Lac, France
• CARRTEL, INRA, Université de Savoie, Campus universitaire, 73376 Le Bourget du Lac, France
• Laboratoire de Chrono-Environnement, UMR 6249 CNRS, UFR Sciences et Techniques, Université de Franche-Comté, 25030 Besançon cedex, France
• Institut national de la recherche scientifique, Centre Eau, Terre et Environnment, Québec (Qc), Canada G1K 9A9
• GEOTOP, Geochemistry and Geodynamics Research Center, CP 8888, Montréal, QC, Canada H3C 3P8
• GEOPOLAR, Institute of Geography, University of Bremen, Germany
• Aix-Marseille Univ, CEREGE, UMR 6635, 13545 Aix en Provence cedex 4, France

Analytical results were acquired in the framework of the scientific programs Aphrodyte and Pygmalion, founded by the CNRS program Eclipse and the French National Research Agency (ANR BLAN07-2_204489), respectively. Radiocarbon dating was performed by the national facility LM14C in the framework of the INSU ARTEMIS call-for-proposal. (…)

 

(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

Temperature/precipitation

 

Torrential floods

 

 

Pays / Zone

Massif / Secteur

Site(s) d'étude

Exposition

Altitude

Période(s) d'observation

 

 

Lake Anterne

 

2063 m

 

 

(1) - Modifications des paramètres atmosphériques

Reconstitutions

 

Observations

 

Modélisations

Results of observational and modeling studies show, in the context of global warming, an increase of intense precipitation events in many regions of the globe (Easterling et al., 2000 and Palmer and Räisänen, 2002). Theoretically, the rise of greenhouse gases would increase precipitation events of high intensity due to a change in the atmospheric moisture transport capacity (Trenberth, 1999 and Allen and Ingram, 2002).

Hypothèses

 

 

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

 

 

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

Reconstitutions

 

Observations

 

Modélisations

 

Hypothèses

 

 

Sensibilité du milieu à des paramètres climatiques

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

 

 

 

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

Reconstitutions

Results

[See the study]

Interpretation

Lithology

Matrix-supported sediment deposits: The characteristics of this type of deposit (high sorting values and absence of a clayey layer at the top) suggest that the energy for the transport is provided by the sediment weight. According to the classification of Mulder and Cochonat (1996), this type of deposit may be triggered by a liquefied or a fluidized flow of reworked sediment. Consequently, they were associated with gravity-reworked sediments due to slope failure of the delta foreset (Arnaud et al., 2002 and Beck, 2009). Seismic activity seems to be the best explanation for the initiation of these collapses. Four gravity-reworked sediment deposits were associated with high magnitude historically known earthquakes (> 7 MSK on the Medvedev–Sponheuer–Karnik seismic intensity scale) (Arnaud et al., 2002) which were used as chronostratigraphical markers in this study. The presence of these deposits mainly after 50 BC, during a period when the sedimentation rates became higher, shows that the delta reached then a critical morphology making it more sensitive to seismic destabilization (Strasser et al., 2007 and Beck, 2009).

Grain-supported sediment deposits: The well-sorted base (f4) for these deposits suggests that sediments were transported by a water current (Mulder and Alexander, 2001). Grain-supported sediment deposits were thus interpreted as the result of hyperpycnal turbidity current triggered by floods in the catchment (Arnaud et al., 2002). In such a case, there is a direct link between current velocity and grain size of sediment transported (Pidwirny, 2008). Past and ongoing catchment-scale monitoring of sediment transfer processes confirm the assumption of flood-triggered deposits and show that sediment transport to the lake mainly occurs during rainfall events and not during the seasonal snowmelt (Enters et al., 2009).

Laminated deposits: Long-term monitoring of sediment transfer to the lake showed that the deposition of laminated sediments is also triggered by precipitation events (Enters et al., 2009). Moreover, these monitoring efforts did not indicate any substantial contribution of snowmelt upon sediment transfer within the catchment nor into the lake. The rate of snowmelt probably is not high enough to generate sufficient stream power necessary for the erosion and transportation of particles. Laminations at Lake Anterne do not reflect a seasonal sedimentation pattern but rather individual flood deposits < 5 mm triggered by precipitation events as the grain-supported sediment deposits. Coarser sediments composing the dark gray laminae would represent the maximum stream power reached during the event (Cockburn and Lamoureux, 2008). These laminae are deposited in the bottom of the lake through hyperpycnal turbidity currents (Mulder and Alexander, 2001). Light-colored laminae, made of clay and fine silt, reflect the settling of fine sediments when the velocity of inflows decreases. Units representing fine-texture laminae are not necessarily associated with sedimentation below an ice cover in winter.

Indicator of flood intensity: Particle-size distributions of the laminated facies show at several depths the presence of the coarsest mode (450–700 μm), which is also observed in most (87%) of the grain-supported sediment deposits. Some flood deposits from the laminated facies have thus been triggered by precipitation events at least as intense as the ones responsible for some thicker flood deposits (grain-supported sediment deposits). Therefore, the thickness of flood deposits is not a reliable indicator for flood intensity in our system. The duration of precipitation events as well as the availability of easily erodible material in the catchment may also have influenced the flood-deposit thickness signal (Blass et al., 2003 and Bøe et al., 2006). As a consequence, the fraction of particles coarser than 306 μm in laminated sediments (< 5 mm-thick doublets) as well as in the facies 4 of grain-supported sediment deposits (> 5 mm grain-supported deposits) was chosen as an indicator of flood intensity.

High-temporal resolution indicator of flood intensity: Because calcium is prevalent in the coarsest facies of laminated and grain-supported thick sediment deposits (dark gray lamina and facies 4, respectively), the calcium record in laminated sediments can be used as a continuous high-resolution proxy of flood intensity in this part of the core. However, calcium content, resampled with the same discontinuous sampling as for grain-size measurements, matches only moderately well with the coarsest fraction (> 306 μm) measured in laminated sediments (n = 84, r = 0.57, p < 6.10− 7) in the upper 9.4 m. The authors attribute this moderate correlation to sampling-triggered systematic bias related to sediment deformations, linked to piston coring. Furthermore, grain-size measurements were made on samples taken from the core surface, while calcium content was analyzed with the core scanner on the U-channel surface. At the same depth, laminated sediments analyzed for grain size and calcium are not the same. Therefore, both methods cannot be perfectly compared. Furthermore, there is also a little calcium in other facies. Thus the absence of coarse particles (particles > 306 μm = 0%) can correspond to different calcium values (between about 270 and 590), which decreases the correlation coefficient between calcium content and the coarsest fraction. Below 9.4 m depth (360 BC), the relationship disappears, which suggests a change in sediment source.

Flood frequency and intensity: Flood frequency signals present variations at both low and high frequencies. Over the last 3.5 ka, two long-lasting periods of 500 and 790 yr of high flood frequency are indicated by laminated doublets < 5 mm and by thick flood deposits > 5 mm (period A, AD 1380–1880; and period E, 350 BC–AD 440). They correspond to the Little Ice Age (LIA) and the end of Iron Age followed by the Roman Climatic Optimum (RCO), respectively. The high flood frequencies of these two periods are associated with rather different climatic conditions in the Alps (e.g., Holzhauser et al., 2005 and Büntgen et al., 2011). Six shorter periods (between 90 and 250 yr) of high flood frequency are also recorded by thin and/or thick flood de-posits. Their ages of occurrence are (b) AD 950–1160, (c) AD 760–850, (d) AD 630–730, (f) 800–550 BC, (g) 1200–1110 BC and (h) 1460–1320 BC. In spite of some discrepancies, probably linked to dating, seven of these eight phases correspond to wetter periods in the Alps. These wetter periods are mainly characterized by higher lake levels in the Jura and Prealpine mountains (Magny, 2004) and also by advances of the Aletsch glacier advances in the Swiss Alps (Holzhauser et al., 2005) and by Rhone detrital inputs in Lake Le Bourget (Arnaud et al., 2005, Jacob et al., 2009 and Debret et al., 2010). At least for the last 1255 yr, these periods of high flood frequency tend to associate with cold summers (Büntgen et al., 2006). Cold climatic conditions seem thus to favor the generation of flood deposits in Lake Anterne.

To investigate the relationship between the occurrence of floods of high intensity and temperature, proxies of flood intensity discussed above (grain size of grain-supported sediment deposits and of laminated sediments and Ca) were compared with previously published chironomid-inferred July air temperatures at Lake Anterne (Millet et al., 2009). It is important to note that this method of temperature reconstruction failed for the last hundred years, corresponding to the global warming, due to fish introduction (Millet et al., 2009). This approach allowed comparison of proxy data independently of potential discrepancies in the age-model. Although the resolution of the temperature record is lower compared to other proxy data, we observe that floods of higher energy generally occur during periods of warmer temperatures. This relationship is well-marked between 0 and 540 cm (AD 690–2007), while below this depth it becomes less clear. This is due to three exceptional samples for which the July temperature presents an opposite relationship with the flood intensity. This trend of intense precipitation events mainly occurring during warmer climatic conditions is also confirmed by comparison between the calcium content and the tree-ring-based temperature reconstruction in the Swiss and Austrian Alps (Büntgen et al., 2006). Both curves present very similar trends.

Discussion

Flood-frequency record: climatic control: Conceptually, climatic changes can influence the flood frequency through different factors affecting the sediment mobilization, transport and availability. These factors are: (i) the summer/autumn precipitation, which directly controls the sediment mobilization and transport; (i) the soil stability, which modifies the catchment sensitivity to erosion processes; and (iii) the length of lake ice-free and of catchment snow-covered periods. The soil stability is partly linked to climatic changes through the vegetation cover: warm conditions favor vegetation development and consequently soil stability. On the contrary, cold conditions decrease the soil stability, which increases the sediment availability and makes easier the sediment mobilization during precipitation events. The length of lake ice-free and of catchment snow-cover influence the duration of potential flood record. During warm periods, the longer ice-free lake and the shorter snow-cover periods increase the probability to have floods recorded in lake sedimentation.

In case of Lake Anterne, the highest flood frequencies are observed during cold periods. This implies that there is no positive correlation between the duration of ice/snow free period and the probability of flood occurrence. Wet and cold conditions, through the rise of rainfall and the reduced soil stability, respectively, more likely explain the record of high flood frequencies. Nonetheless, given the good relationship between high flood frequency and wet periods known in the region, summer/autumn precipitation events are considered as the main factor driving our flood-frequency signal. Changes in characteristics of vegetation and soil cover probably serve mainly to amplify the flood-frequency signal.

Flood frequency record: land-use effects: Over the last 3.5 ka, one period contradicts the climatic interpretation of the present proxies. The period E between 350 BC and AD 440 is characterized by high flood frequencies during a climate that generally was relatively dry and probably warm. A brief departure from dryness occurred between AD 140 and AD 250, when a higher lake level in Jura and Prealpine Massifs increased flooding episodes in Bernese Alps and a small advance of the Aletsch Glacier all occurred (Magny, 2004, Holzhauser et al., 2005 and Schulte et al., 2009, respectively). Nevertheless, the high flood frequency recorded in Lake Anterne during the overall period E cannot be related only to climatic conditions but more probably to human land use. The combination of anthropogenic and climatic effects is hypothesized as responsible for the very high flood frequency recorded between AD 140 and AD 250. Increased presence of grazing herds probably reduced the efficiency of rainfall infiltration due to soil compaction that, in turn, accelerated surface runoff and attacked the protecting herb mat of the soil, leading to its destabilization. Moreover, sediment susceptibility to mobilization is enhanced by clearings and/or fire management for pasture maintenance.

In the Austrian and Swiss Alps, Iron Age (800–60 BC) and Roman periods (60 BC–AD 440) are well known for deforestations and/or grazing activities at high altitude (Schmidt et al., 2002, Heiri and Lotter, 2003, Heiri et al., 2003 and Schmidt et al., 2007). Furthermore, a recent palynological investigation of a peat deposit in the vicinity of the study site shows an increase of clearings and pastoral activities during the Roman Period, with a drastic decline of Alnus viridis (green alder) and an increase of Plantago (plantain) pollen (David, 2010). Despite an intensive archaeological field campaign in 2008 and 2009, only two dwelling remnants were discovered in the catchment of the lake (Rey, 2009 and Rey, 2010). Charcoal taken in one of these structures at two different stratigraphic positions was dated by 14C. The deepest sample was dated to 385–205 BC (2230 ± 30 14C yr BP) and the uppermost one to AD 85–245 (1835 ± 30 14C yr BP). Furthermore, shards of Gallo-Roman ceramic were found. These results denote human presence in the catchment area at least during the end of Iron Age and the Roman Period. This sparse evidence support the hypothesis of a possible anthropogenic impact on vegetation capable of causing soil destabilization and increased surface runoff that could have contributed to the flood-frequency signal.

Intensity versus frequency of floods: This study shows that flood frequency increases during cold periods. However, floods of high intensity occur preferentially during warm periods. The positive relationship between summer temperature and the occurrence of floods of high intensity is particularly clear in the < 5-mm flood-deposit record (i.e., Ca signal). During the last 1250 yr, periods with summer temperature anomalies around the mean of − 1°C, represented by the LIA between AD 1390 and 1530 and later decades between AD 1790 and 1880, are characterized by both high flood frequencies and floods of high intensity. However, floods were not as frequent and intense as during periods of warm conditions. This result suggests that mild temperatures, neither very cold nor very warm, are favorable to humid conditions with occasional intense precipitation events.

Over the last 3.5 ka, periods E (350 BC–AD 440), D (AD 630–730), and C (AD 760–850) represent times of both high flood frequency and floods of high intensity. During the dry and probably warm period E, representing the end of Iron Age and the Roman Period, this association of frequency and intensity probably is explained by human activities that have increased the flood frequency. These human activities probably also affected the flood intensity through accelerating surface runoff and increasing associated stream velocities. However, the relatively good relationship with the summer temperature for the last 1250 yr suggests that human activities are not the main factor controlling the occurrence of high-intensity floods during this period. Periods C and D of high flood frequencies, only recorded by thin flood deposits and thick flood deposits, respectively, also record floods of high intensity. In these two cases, the presence of both frequent floods and floods of high intensity is not well understood. It would be necessary to have a detailed reconstruction of summer temperatures in the Alps to interpret the interactive environmental patterns during these periods. Likewise, the discrepancy between the records of thin and thick flood frequencies during these periods is not well explained. It could reflect differences in sediment availability in the catchment and/or in precipitation event duration.

The flood record underlines the complexity of interactions between temperature, precipitation, human activities, and soils and erosion processes. In order to better understand the flood intensity and frequency patterns, flood deposits thicker than 5 mm were classified according to their thickness, depending on flood-event duration, intensity and sediment availability, and the proportion of sediment within the 306–2000 μm particle range. The sediment fractions between size classes were determined from the 50th and 90th percentiles of the dataset. The grain-size limit between the thickness classes corresponds to a separation between two different flood-deposit patterns underlined by a bi-plot: the 95th percentile of particle size distribution vs. the flood deposit thickness (Giguet-Covex et al., 2011). For each class, a probability of flood occurrence, expressed in percent, was calculated for warm and cold periods and for the period of suspected human impact (350 BC–AD 440). Flood deposits representing high and low intensity, between 0.5 and 6.3 cm thickness, present contrasting probabilities of occurrence in warm and cold climatic conditions. This classification confirms that floods of high intensity, which are rare, have a higher probability of occurrence during warm periods. On the contrary, floods of lower intensity have a high chance to occur, mostly in cold periods. For the few relatively thick flood deposits, there is no very clear climatic pattern. However, we note that thick floods of low intensity do not exist, which underlines the role of runoff intensity in processes triggering these flood deposits. During periods of strong human activities and characterized by warm and dry climate, the thickest flood deposits representing intermediate intensities and the thinnest flood deposits representing high intensities are less numerous than in warm periods. However, the thickest flood deposits of high intensity are more numerous. These observations confirm the importance of sediment availability, soil stability and runoff intensity on flood generation during times of human impact.

These results thus imply the coexistence of two climate-dependent flood responses to environmental change on the same site and they show the complex response of erosion patterns to climatic changes in mountain areas. A model of precipitation events associated with the different flood types recorded in Lake Anterne, and mainly based on flood intensity, is proposed (Merz and Blöschl, 2003). According to this model, floods of high intensity correspond to flash floods. These floods are generated by extreme rainfall triggered by convective storms only occurring in summer (June, July, August; Merz and Blöschl, 2003). The higher probability of a small catchment, such as Anterne, to encounter floods linked to severe convective storms (Schmocker-Fackel and Naef, 2010), explains the sensitivity of our record to these events and their absence in sediments of Lake Le Bourget. Floods of high frequency but low intensity, rather recorded in cold periods in Lake Anterne, are associated with floods of large spatial extent. Generation of long-duration rain floods in Anterne catchment is probably promoted in case of human pressure, making easier for sediment mobilization. The model built in our small mountain area thus suggests that warm periods are more favorable to the occurrence of thunderstorms. Consequently, they are expected to increase in the context of global warming. This assumption is supported by a simulation showing an increase of severe convective storms over Europe (Sander and Dotzek, 2010). A general decrease of annual average precipitation and increase of extreme precipitation events is also noted for some years and is predicted for the future in Western Europe in the context of global warming (Easterling et al., 2000, Christensen and Christensen, 2003, Brunetti et al., 2004 and Fuhrer et al., 2006). All these studies suggest that, on average, drier conditions are associated with a higher probability of extreme precipitation events during anomalously warm periods, a conclusion that is supported by the present results for the last 3.5 ka.

Studies in Switzerland and Italy suggested that changes in flood and precipitation patterns might be triggered by modifications of atmospheric circulation (Brunetti et al., 2004 and Schmocker-Fackel and Naef, 2010). Such relationships cannot be determined from the present data. However, future meteorological studies based on models or historical data, as well as intensive on-site monitoring, will improve understandings of the complex relationships between changes in flood patterns and atmospheric circulation regimes.

Observations

 

Modélisations

 

Hypothèses

 

 

Paramètre de l'aléa

Sensibilité des paramètres de l'aléa à des paramètres climatiques

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

Frequency and intensity of floods

In mountain areas, torrential floods are relatively frequent and cause considerable damages. Furthermore, these areas are particularly sensitive to climatic changes as shown by the comparison of mean temperature at high-elevation sites and at global scale (Beniston et al., 1997). The question of global warming effects on flood activity in mountains appears thus as an important issue. Since the 1980s an increase in the frequency of extreme precipitation events has been observed in the European Alps (Beniston et al., 1997 and Rebetez et al., 1997). However, the lack of studies covering long time periods precludes such an evolution to be attributed to global warming. Furthermore, it is difficult to simulate extreme precipitations with GCMs and RCMs (General and Regional Circulation Models), because of the complexity of the topography in mountain areas. In particular, uncertainties of scenarios increase for summer precipitation of high intensity (Frei et al., 2003 and Frei et al., 2006). Therefore, historical and natural archives represent interesting sources of information to better understand past and future evolution of flood and precipitation events (Rico et al., 2001, Benito et al., 2004, Schulte et al., 2009 and Bussmann and Anselmetti, 2010).

Lakes are widely spread over mountain landscapes and act as natural traps of catchment erosion products. Therefore, the occurrence of erosive events such as torrential floods may be recorded in the stratigraphy of alpine lake sediments over long time periods (Leeman and Niessen, 1994, Nesje et al., 2001 and Bøe et al., 2006). However, the climatic interpretation of changes in flood deposit frequency and thickness is not always straightforward. In particular, most European areas have encountered periods of human occupation, which affected vegetation, soil stability and thus the relationship between climate and erosion processes (Lanci et al., 2001, Dapples et al., 2002 and Schmidt et al., 2002).

In a previous study covering the entire Holocene (Giguet-Covex et al., 2011), the authors showed that the Lake Anterne catchment became highly sensitive to climate-triggered changes in erosion patterns since 3.4 ka. They argued this was due to the intensive use of alpine mid-altitude area for grazing since the Bronze Age. (…)

Lake Anterne is a small alpine lake (0.12 km²) located at 2063 m asl in the northern French Alps. Its catchment (2.55 km²) is currently covered by meadow vegetation except in the south and east, which are characterized by steep slopes mainly formed by easily erodible rocks (calc schists and black shales). The lake is frozen during 6 to 7 months each year and the southern slopes are rarely snow-free before July. Therefore, lake sediments only record erosion fluxes related to precipitation events occurring from early summer to mid-autumn. The absence of terrestrial debris fans shows that debris flows are not an important geomorphologic process in the catchment. However, debris-flow events cannot be totally excluded for the past.

The current study aims at distinguishing detrital deposits linked to “normal precipitation” and/or snowmelt and the ones triggered by extreme heavy rainfall events, focussing on the last 3.5 ka in the light of a chironomid-inferred July air-temperature reconstruction (Millet et al., 2009) from the same lake sediment sequence. As the mobilization and transport of coarse sediments toward lakes reflect an increase of stream velocity and discharge (Campbell, 1998, Francus et al., 2002, Bøe et al., 2006 and Parris et al., 2009), grain size was used as an indicator of paleoflood intensity. In order to have a high-resolution continuous record of flood intensity, a proxy of coarse particles was determined from geochemical measurement obtained with a core scanner. Relationships between flood frequency and intensity, land-use history and climate are also examined by comparison with other archives of climate and anthropogenic activities.

A composite core (ANT-07) was retrieved in the deepest part of the lake in 2007. In the present study the authors focus on this time period, corresponding to the uppermost 13.4 m of core ANT-07. This core was correlated to core ANT-01 taken in 2001 (Arnaud et al., 2006 and Millet et al., 2009) using lithological descriptions and in particular thick flood and gravity-reworked sediment deposits, as previously described by Arnaud et al. (2002). It was hence possible to compare Chironomid-inferred July air temperatures (Millet et al., 2009) obtained from core ANT-01 with the flood-intensity record presented in this study.

Flood deposits (thicker than 5 mm, i.e. visually recognizable) as described by Arnaud and others (2002) were visually identified and documented in terms of stratigraphic location and thickness. Flood deposit frequency was then computed as a running sum over a 100-yr time window as estimated by our age–depth model. These thick flood deposits are intercalated with finer laminae couplets (dark and light layers) that were also counted for the first 10 m of the core. The frequency of these couplets was also determined using a running sum over a 10-yr time window.

Grain-size measurements were performed by laser diffraction using a Malvern Mastersizer 2000G covering a theoretical range from 0 to 2000 μm. A discontinuous sampling step strategy (between 1 and 30 cm) was applied in laminated sediments due to the occurrence of thick instantaneous deposits. Each sample integrates 1-cm sediment depth. In addition, grain-size measurements were performed on the coarse basal layer of each flood deposits thicker than 5 mm. The sorting index was calculated with the statistical equation developed by Trask (1930): (P75/P25)1/2.

High-resolution calcium analyses were conducted following a sampling step of 100 μm on the uppermost 10 m of core ANT-07 and of 200 μm between 10 and 13.4 m. These high-resolution XRF analyses were performed only on laminated sections (< 5 mm), using an ITRAX core scanner (30 kV, 35 mA, 10 s sampling time) (Croudace et al., 2006). XRF measurements were resampled to build a data set with a single chemical value for each year and were then smoothed with a running mean over a 10-yr interval.

The age–depth model relies on twenty 14C AMS measurements performed on terrestrial macroremains, of which four come from core ANT-01 and were stratigraphically correlated to the ANT-07 sediment core (Fig. 1C, Table 1). The age–depth model was then constructed by fitting a smooth spline curve using the “CLAM” program (Blaauw, 2010) developed under the mathematics software “R” version 2.12.2 (R Development Core Team, 2011). The used calibration curve was Intcal09 (Reimer et al., 2009). Lead pollution was also integrated to improve our age–depth model. Lead concentrations were measured at the Activation Laboratories (Ancaster, Ontario, Canada) using the Ultratrace protocol (near total digestion with HF, HClO4, HNO3 and HCl followed by ICP-MS measurements). The anthropogenic lead flux was then calculated using the method described by Arnaud et al. (2004) and considering a natural background of 19 ppm, which corresponds to average concentration of the pre-anthropogenic period between 2000 and 3500 cal yr BP.

 

(4) - Remarques générales

Detailed sedimentary and geochemical characterization of late Holocene deposits in an alpine lake provides a long-term record of flood event frequency and intensity. This study documents temporal changes in floods in the northwest European Alps since 3.5 ka. Results show that floods are more frequent during relatively cool and wet periods, although extreme floods are more frequent when warmer summer climate conditions prevail. Although the origin of this pattern is not completely understood at present, these results constitute an important source of information to better understand past and future changes of hydrological patterns and future natural hazard management in alpine catchments in a context of global warming.

Our results also suggest that anthropogenic activities such as clearing and grazing contributed to the complexity of flood signals through impacts on vegetation cover and soil properties. Effects of these activities were mainly observed on flood frequency during the end of the Iron Age and during the Roman Period. Human land-use activities probably increased the sensitivity of the sediment record to floods triggered by precipitation events of low intensity.

 

(5) - Syntèses et préconisations

 

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