From estuary to ocean : expression of local and global forcing factors in the sedimentary records of the Loire River dynamics from the Middle Holocene

• Main Author: Durand, Matthieu
• Other Authors: Bio-Indicateurs Actuels et Fossiles (BIAF), Université d'Angers (UA), Université d'Angers, Hélène Howa, Meryem Mojtahid, Grégoire Marie Maillet
• Format: Doctoral or Postdoctoral Thesis
• Language: French
• Published: HAL CCSD 2017
• Subjects: Loire estuary; Bay of Biscay; Human settlements; Paleohydrology; Sea level; Holocene climate; Sedimentary facies; Benthic foraminifera; Aménagements humains; Niveau marin; Faciès sédimentaires; [SDU.STU]Sciences of the Universe [physics]/Earth Sciences; North Atlantic; North Atlantic oscillation
• Online Access: https://theses.hal.science/tel-02076974; https://theses.hal.science/tel-02076974/document; https://theses.hal.science/tel-02076974/file/these_DURAND.pdf

The aim of this work is to reconstruct changes in the hydro-sedimentary dynamics of the Loire River according to global and regional climatic variations since the Middle Holocene (~ 7 ka), in a general context of slowing down of sea-level rise. The impact of the recent human settlements undertaken in the internal Loire estuary since the beginning of the 20th century is also considered. To achieve this objective, the studied sedimentary sequences were collected along a land-sea continuum from the internal estuary, to the incised paleovalleys off the present-day river mouth, until the northern part of the Bay of Biscay (South Brittany). The imbrication of both temporal and spatial dimensions makes the originality of this research on the scale of the Loire River, but also its complexity. In order to deconvolute the various recorded signals, we used a multiproxy approach combining sedimentological, micropaleontological (benthic foraminifera) and geochemical analyses, within a chronological framework constrained by 14C and 210Pb/137Cs dating. Our results show that the Middle to Late Holocene slowing down of sea-level rise plays a major role in shaping the estuarine landscape and in the channelization of the terrigenous flow towards the ocean. At the same time, this terrigenous flow is modulated on a larger scale by internal and external climatic forcing (e.g., North Atlantic Oscillation, solar forcing) controlling humidity over the Loire River catchment area, the occurrence of storms and ocean surface circulation. The most recent parts of our estuarine sedimentary sequences record the impact of human settlements since the beginning of the 20th century modifying significantly the morphology of the inner estuary and the location of the main river channel. L’objectif de ce travail est de reconstituer les changements de la dynamique hydrosédimentaire de la Loire en fonction des variations climatiques globales et régionales depuis l’Holocène Moyen (~7 ka), dans un contexte de ralentissement et de stabilisation de la ...