Theses defended

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2017

Matthieu DURAND - 19th December

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


Camilo ARIAS-RUIZ - 18th December

" High resolution climate records from modern and last interglacial periods derived from giant clam shells (Tridacnidae) in Sulawesi, Indonesia"
Giant clam shells (Tridacnidae family) were used to reconstruct climatic conditions during the last interglacial periods in Indonesia's Sulawesi Island. Geochemical analyzes on modern shells have shown that these organisms actually record the variation of the environment in which they live, in terms of: temperature, salinity, precipitation and nutrient inputs. The main results were the reproducibility of the geochemical signal in two different species, as well as the recording of the climatic anomalies related to the La Niña phenomenon in 2010. Uranium-series dating revealed reefs formed at the beginning of the last interglacial period (MIS-5) and the penultimate interglacial or MIS-7. The comparison between modern and fossil reconstructions gave the following main results: 1) colder average temperature conditions with reduced seasonality for both periods (beginning of MIS-5 and MIS-7) due to drier conditions.2) These parameters respond systematically to a decrease / increase of the sea level and testify the strong interaction between the atmospheric and ocean circulation in the region3) The interannual variability of SST / SSS presents oscillations in the same amplitude as today for the MIS-5 period, however for MIS 7, these oscillations are reduced around 50% compared to the current interglacial4) The Mg/Ca ratio has a strong taxonomic variation, additional studies are needed to remove the biological factor related to geochemical signal. 5) Ba/Ca present a potential use as tracers of upwelling processes and/ or variations in salinity. Giant clam shells (Tridacnidae family) were used to reconstruct climatic conditions during the last interglacial periods in Indonesia's Sulawesi Island. Geochemical analyzes on modern shells have shown that these organisms actually record the variation of the environment in which they live, in terms of: temperature, salinity, precipitation and nutrient inputs. The main results were the reproducibility of the geochemical signal.


Jassin PETERSEN - 3th April

"Development of paleo-oxygenation proxies : new insights into Mn/Ca ratios and pore patterns of benthic foraminiferal tests"
The main objective of this thesis was to investigate how the temporal and spatial variability of bottom water oxygenation is recorded in the benthic foraminiferal test, by the Mn/Ca ratio, and by the pore parameters. To achieve this objective, Ammonia tepida, a group of pseudocryptic species, of the Lake Grevelingen (Netherlands) was mainly studied. At this site, the depth gradient is accompanied by an increase in duration and intensity of seasonal hypoxia/anoxia which indirectly influences the Mn/Ca ratio and directly the porosity of A. tepida. Our study of the Oxygen Minimum Zone in the Arabian Sea suggests the presence of a significant diagenetic effect during the transition of a Mn/Ca signal from a living foraminifer to a fossil signal. In conclusion of this PhD research, the Mn/Ca signal of a benthic foraminiferal test seems to be influenced by three temporal factors: 1) the oxygenation of the benthic ecosystem and the position of the Mn2+ zone in the interstitial water, 2) the cable bacteria activity, and 3) the period of calcification, as well as by three spatial factors which determine the position of the foraminifer with respect to the Mn2+ zone during calcification 4) the microhabitat, 5) the vertical migration of the foraminifer and 6) the bioturbation of macrofauna. In the Lake Grevelingen, the porosity of A. tepida shows a relationship with oxygenation of the benthic ecosystem. The coupling between porosity and oxygenation could be rather direct, a greater porosity allowing more intensive gas exchanges. In contrast, the relationship between the Mn/Ca ratio and the oxygenation would be more indirect, being influenced in particular by the microbial activity. The simultaneous use of these two totally independent proxies seems promising, especially because these proxies react at different time scales.


Antoine BA - 28th February

"LiDAR waveform analysis and high resolution spectrometry remote sensing for sensitive spaces in a coastal environment"
Coastal dunes are ecosystems of high ecological interest as well as coastal defenses against marine-related hazards, especially in the current climate change and coastal erosion context. The understanding of dune morphology and the dynamics of its components allows to apprehend its functioning and enables to provide a pertinent and durable coastal zone management. The hyperspectral imagery is a technique allowing the passive observation of optical properties of a surface scene constituents. This method is particularly adapted for covering large geographical areas, with an appropriate spatial scale for observing the coastal features. The LiDAR (Light Detection And Ranging) remote sensing is an active method measuring the Earth’s topography. The full-waveform LiDAR, in addition to provide data on the surface elevation, allows to acquire data on the density and light diffusion power of an observed scene features. Therefore, simultaneous acquisition of both data types should improve the monitoring of Earth’s surfaces. In order to compare and process the data from both types of remote sensing method, a pre-processing step of geometric and radiometric corrections should be applied to the hyperspectral images and full-waveform LiDAR data. Following this step, two applications on the hyperspectral/LiDAR data coupling were developed for the estimation on the vegetation distribution over two coastal dunes systems on the island of Noirmoutier, Vendée, France. The first application built up a mapping methodology for the dune vegetation, from a habitat to a specie scale, in order to discuss the species distribution as well as assessing their impact on the dune morphology evolution. The second application, a complete procedure of vegetation habitats was developed, combining the chemical and the light diffusion properties of the dune surfaces.