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Contribution des sédiments laminés lacustres à l'étude des changements environnementaux holocène Approche couplée sédimentologie/géochimie à haute résolution Application à deux lacs nord alpins

Abstract : The high resolution study of laminated sediments from Lake Anterne (2063 m asl) and Lake Bourget (231.5 m asl), both located in northern French Alps, led us to reconstruct paleoenvironmental changes induced both by climate changes and human activities. Anthropogenic impacts upon lake catchment area, and in particular on erosion processes, alter the sensitivity of the sediment record to climatic variations. One of the challenges of this thesis is to disentangle the respective roles of these two factors in the changes observed throughout the study of lake sediments. The sedimentary archive of Lake Anterne covers most of the Holocene period. The evolution of erosion processes in the catchment area have been reconstructed from sedimentological analyses. A "source-to-sink" approach was applied on organic and mineral geochemical analyses. This methodological approach, coupled with quantitative indicators of erosion, allowed highlighting the evolution of soil cover. These results brought also information about the climatic or anthropogenic origin of changes in erosion patterns. During the early Holocene (9950-5450 cal. BP), a gradual evolution was recorded: establishment of vegetation and soil with pedogenesis processes (decarbonation and acidification), and then stabilization of the catchment linked to a well-developed vegetation cover. We evidenced anoxic conditions at the bottom of the lake during this period of stabilization. We interpret it as a consequence of slope stabilisation by a dense vegetation cover. A regressive story of soils, accompanied by a significant enhancement of erosion processes, began at ca. 5450 cal. BP, due to a climate reversal towards colder conditions. This regressive story was reinforced by anthropogenic impacts (deforestation, pastoral activity) during the Bronze Age (around 3400 cal. BP), the end of Iron Age-beginning of the Roman period (2400 cal. BP-150 AD) and the lower middle age (1000-1200 AD). Our interpretations were reinforced and enriched by crossing our data with climate reconstructions obtained at Anterne (temperature from chironomids) and in the region, and with paleovegetation (pollen, wood macrofossils) and archaeological data from surrounding areas. The record from Lake Le Bourget covers only the last 120 years. This archive was studied in the aim to reconstruct the evolution of the trophic level and the oxygenation at the bottom of the lake, in relation with human activities (discharge of sewage waters, agricultural activities) and climate evolution. The hydroclimatic variability at annual and pluriannual scales was also considered. Unlike Anterne, laminated sediments of Lake Le Bourget are varved. Therefore, a fair, accurate and high resolution dating was obtained. Our environmental reconstruction is based on sedimentological and geochemical analyses. The comparison between trophic level indicators and River Rhône detrital inputs highlighted the opposed effects of Rhône floods (a non-permanent tributary) on lake water quality (nutrient but also oxygen supply). Therefore, a special interest was also brought to the understanding of the factors controlling Rhône floods (anthropogenic climate/activities).
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https://tel.archives-ouvertes.fr/tel-00553247
Contributor : Charline Giguet-Covex <>
Submitted on : Thursday, January 6, 2011 - 8:18:59 PM
Last modification on : Friday, November 6, 2020 - 3:31:42 AM
Long-term archiving on: : Thursday, April 7, 2011 - 2:31:19 AM

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Charline Giguet-Covex. Contribution des sédiments laminés lacustres à l'étude des changements environnementaux holocène Approche couplée sédimentologie/géochimie à haute résolution Application à deux lacs nord alpins. Minéralogie. Université de Savoie, 2010. Français. ⟨tel-00553247⟩

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