Frédéric Levrault

Bio: Frédéric Levrault is an academic researcher. The author has contributed to research in topics: Climate change & Forest restoration. The author has an hindex of 1, co-authored 2 publications receiving 3 citations.

Rapid losses of intertidal salt marshes due to global change in the Gironde estuary (France) and conservation implications for marshland passerines

Abstract: We analysed coastline movements between 2000 and 2016 along the 24.5 km of the mesohaline region of the North bank of the Gironde estuary (France). This sector is identified as hosting the largest expanse of salt marshes of the estuary and as an important breeding and stopover site for different marshland passerines of conservation concern. Our results from the study area reveal an average shore retreat of 14.74 ± 0.50 m over the period, corresponding to a loss of 49.96 ha of intertidal wetlands (i.e. 2.04 ha per kilometer of coastline) and reaching on average of more than 30 m for 42% of the coastline. This erosion dynamic, explained by a significant perturbation of the estuary’s hydro-sedimentary dynamic (due to decreases in freshwater discharges and relative sea level rise) highlights the rapid disruption that can occur in estuarine eco-complexes in response to global change. Given the impacts that estuarine intertidal wetland losses have on carrying capacity for marshland passerines, experimental management approaches are being tested in the study area to compensate for losses already observed and to anticipate those expected. These approaches reveal in particular that partial reconnection of agricultural polders to tide influences with a regulation system for water ingress may allow interesting trade-off between maintaining polders with agricultural activities such as grazing and conservation plans for vegetation of intertidal salt marshes exploitable by marshland passerines.

Climate Variability in Southwest France During the Last 2000 Years : Proxy Calibration and Reconstruction of Drought Periods Based on Stable Isotope Records from Speleothems and Tree Rings

Abstract: The characterization of natural climate variability is important in order to understand the climate response to natural forcings and to identify anthropogenic influences. The aim of this thesis is to reconstruct climate changes in the southwest of France, a region which is characterised by recurrent drought periods, where high resolution proxy records of the last millennia were lacking.The reconstruction is based on multiple proxies from two continental archives: speleothems and tree rings. Their combination can make use of the strengths of each archive while compensating their weaknesses. There are two principal objectives: first, to gain a better understanding of the climatic and non-climatic influences on each proxy; and second, to reconstruct drought periods in the past.The oxygen isotopic composition (δ18O) of speleothem fluid inclusions and tree ring cellulose is controlled to a large extent by the δ18O of precipitation, which can serve as a tracer of the atmospheric circulation. In order to interpret these proxies in terms of climate, it is necessary to understand how the climate signal becomes recorded in the proxy, and which processes modify the original signal during the formation of the archive.Measurements of δ18O in precipitation, cave drip water, and fluid inclusions in modern speleothem samples from Villars Cave demonstrated that the isotopic composition of cave drip water corresponds to the pluri-annual average precipitation. The speleothem fluid inclusions, in turn, preserve the isotopic composition of the drip water. Based on this calibration, it is possible to reconstruct drip water isotope variability using fluid inclusions in a more than 2000 year old stalagmite, which has been dated by laminae counting, as well as U-Th and 14C measurements. Changes in the cave environment, e.g. the vegetation cover, are indicated by other proxies from the same stalagmite (stable isotopes in calcite and trace element concentrations), but these changes do not seem to impact the fluid inclusion δ18O significantly.The isotopic composition of tree ring cellulose from Quercus spp. in the study area is strongly influenced by climate conditions during the summer. However, non-climatic influences on the isotopic composition of cellulose are identified. They are linked to the age of the trees and to site hydrology, and must be accounted for in the sampling and analytical procedures. Crossdated cores from living trees and timber wood in historic buildings near Angouleme are used to build an annually resolved chronology of cellulose δ18O. Significant correlations with meteorological data enable a calibration and a reconstruction of drought periods since 1360 AD.Lastly, this thesis explores a novel approach of integrating oxygen isotope records from speleothem fluid inclusions and tree ring cellulose from closely located sites to reconstruct both high- and low-frequency variability of droughts in the past.

A multi-proxy approach to drought reconstruction

Abstract: In palaeoclimate reconstructions, the combination of proxy records measured in different climate archives is challenging because of the uncertainties associated with each proxy, but it can also help reduce some of these uncertainties. Here, we present a novel approach to combine speleothem and tree ring proxies for a drought reconstruction of the last 640 years: a fluid inclusion δ18O record from a stalagmite from Villars Cave (southwest France) and a tree ring cellulose δ18O record of Quercus spp. from the nearby Angouleme area. The δ18O of the fluid inclusions is taken as an estimate of the δ18O of the trees’ source water. Then, the cellulose and source water δ18O are used to calculate the leaf water isotopic enrichment, as well as relative humidity, which is the dominant controlling factor of this enrichment. The reconstructed long-term trends in relative humidity differ from a previously published reconstruction of moisture variability based on the tree ring record alone. Further measurements will be necessary to support either reconstruction. Nevertheless, this investigation demonstrates the great potential for combining isotope proxies from speleothems and tree rings to reconstruct both the low- and high-frequency variability of drought.