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Signature climatique et anthropique dans les sédiments holocènes des lacs du Bourget et d'Anterne (NO des Alpes) – paléohydrologie et contamination au plomb

Abstract : The Holocene is the youngest Geologic period. It is an interglacial one, beginning 11500 years ago. The interest in studying this period is strengthened in the context of present-day human-triggered global warming. This period is also the one during which organised human societies have emerged and deeply modified the natural environment through agriculture and industrial activities. This thesis present a sedimentological and geochemical study of lake sediment cores (up to 13m-long) taken in two alpine lakes: Le Bourget and Anterne. Two distinct topics have been developed: the record of Rhône river floods in Lake Le Bourget and the record of human-induced lead contamination in both lakes. The particular setting of Lake Le Bourget, alternatively tributary and emissary of Rhône river makes it highly sensible to major outburst floods. The silicate sedimentary load of the Rhône presents a particular magnetic and geochemical fingerprint allowing to distinguish it from the carbonaceous autochthonous background sedimentation. In core LDB 01-I, covering the last 7200 years, the autochthonous vs. river-borne ratio in sediment has thus been measured using major elements and and magnetic properties following a sampling-step a les than 150 and 8 years, respectively. During the last 3000 years, the phases of enhanced detritism hence evidenced are in agreement with previous knowledge about the Upper Rhône hydrological activity (Bravard et al., 1992 ; Bravard, 1996). Before 2700 cal. BP, detrital fluxes are particularly weak and the detrital signal is made more complex to the influence of numerous levels of sand reworked from the surrounding sandy shores. Those episodes may be related to phases of higher lake-levels elsewhere identified (Magny, 2004). Finally, the detrital signal allows to evidence a long-term trend toward increasing frequency and/or intensity of floods. This trend should reflect the long-term Holocene trend enhanced moisture-carrying westerlies in response to increasing low-latitude insolation (Rimbu et al., 2003). The role the humankind and its increasing pressure onto soil stability is still difficult to assess. Nevertheless, mineralogical evidences should indicate an increasing part of freshly eroded material all along the Holocene rather than an increasing part of soil-derived material. Centennial- to millennial-scale oscillation are superimposed onto the long-term trend of the detrital signal and follows the changes in solar activity. This study hence confirms that the Holocene climate is paced by orbital parameters at a pluri-millenial scale and by solar activity at a pluri-centennial one (Bradley, 2003). Lake Antern is an high elevation lake located nearby the Mont Blanc Massif. The deep study a short core allowed to evidence a particular sedimentary behaviour made of annual laminas and thick instantaneous deposits interbedded within. Two different types of such deposits have been identified and associated to flood events and earthquakes, respectively (Arnaud et al., 2002). Lead-210 dating of these levels allowed to link the last one to historically-know earthquakes. Lead and thorium concentrations as well as lead isotopic compositon have been measured in short cores from both lakes. The acquisition of long cores from the same sites allowed to measure the “preanthropic” composition of the sediment and downstream to compute the concentration and isotopic composition of the contaminant lead. The signals hence obtained agree together whereas the lakes are distant and located in different settings. The comparison with a series from an ice-core taken on the Mont Blanc confirms the regional significance of these signals until the 80s'. Since that date, corresponding to a drop in the use of leaded-gasoline, two distinct source of lead are encountered at low and high elevations, suggesting different pathways of contamination. The long core taken in Lake Anterne allowed to obtain a 1800 years-long series of lead contamination. This series notably evidences a contamination peak during the Roman period, more intense than the one measured during the 1970s'. This peak is probably due to local mining activities.
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Contributor : Fabien Arnaud <>
Submitted on : Thursday, December 2, 2004 - 5:03:32 PM
Last modification on : Monday, January 4, 2021 - 9:20:07 AM
Long-term archiving on: : Wednesday, November 23, 2016 - 5:27:22 PM

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Fabien Arnaud. Signature climatique et anthropique dans les sédiments holocènes des lacs du Bourget et d'Anterne (NO des Alpes) – paléohydrologie et contamination au plomb. Géochimie. Université des Sciences et Technologie de Lille - Lille I, 2003. Français. ⟨tel-00007606⟩

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