Showing papers by "Valérie Daux published in 2016"

French summer droughts since 1326 CE: a reconstruction based on tree ring cellulose δ 18 O

Abstract: . The reconstruction of droughts is essential for the understanding of past drought dynamics and can help evaluate future drought scenarios in a changing climate. This article presents a reconstruction of summer droughts in France based on annually resolved, absolutely dated chronologies of oxygen isotope ratios (δ18O) in tree ring cellulose from Quercus spp. Samples were taken from living trees and timber wood from historic buildings at two sites: Fontainebleau (48°23′ N, 2°40′ E; 1326–2000 CE) and Angouleme (45°44′ N, 0°18′ E; 1360–2004 CE). Cellulose δ18O from these sites proved to be a good proxy of summer climate, as the trees were sensitive to temperature and moisture availability. However, offsets in average δ18O values between tree cohorts necessitated a correction before joining them to the final chronologies. Using the corrected δ18O chronologies, we developed models based on linear regression to reconstruct drought, expressed by the standardized precipitation evapotranspiration index (SPEI). The significant correlations between the SPEI and cellulose δ18O (r ≈ −0.70), as well as the verification of the models by independent data support the validity of these reconstructions. At both sites, recent decades are characterized by increasing drought. Fontainebleau displays dominantly wetter conditions during earlier centuries, whereas the current drought intensity is not unprecedented in the Angouleme record. While the δ18O chronologies at the two studied sites are highly correlated during the 19th and 20th centuries, there is a significant decrease in the correlation coefficient between 1600 and 1800 CE, which indicates either a weaker climate sensitivity of the tree ring proxies during this period, or a more heterogeneous climate in the north and the south of France. Future studies of tree ring isotope networks might reveal if the seasonality and spatial patterns of past droughts can explain this decoupling. A regional drought reconstruction based on a combination of the two sites shows good agreement with historical records of grape harvest dates in France, which provide another proxy of summer climate.

Are the oxygen isotopic compositions of Fitzroya cupressoides and Nothofagus pumilio cellulose promising proxies for climate reconstructions in northern Patagonia

Abstract: Tree ring δ18O chronologies from two native species (Fitzroya cupressoides and Nothofagus pumilio) in northern Patagonia were developed to assess their potential for paleoclimate reconstructions. The five annually resolved cellulose δ18O chronologies (two for F. cupressoides and three for N. pumilio) are located on the Andes along the steep west-to-east precipitation gradient. Over the common 60 years long interval, the five site-δ18Ocell chronologies exhibit a strong common signal as indicated by the significant mean intercorrelation (r = 0.61, p < 0.05) and the high percentage (65%) of total variance explained by the first empirical orthogonal function. Although correlation analyses reveal that the two mean species-δ18Ocell chronologies are mainly modulated by December–May temperature, the N. pumilio chronology shows a greater sensitivity to record temperature variations (r = 0.57, p < 0.05). The δ18Ocell of N. pumilio contains a regional temperature signal representative of a large area in southern South America under the influence of the Southern Annular Mode. This study indicates that δ18Ocell in N. pumilio is a promising proxy to reconstruct past variations in temperature in South America south of 38°S.

Oxygen isotope fractionation between bird eggshell calcite and body water: application to fossil eggs from Lanzarote (Canary Islands).

Abstract: Oxygen and carbon isotope compositions of fossil bird eggshell calcite (δ18Ocalc and δ13Ccalc) are regularly used to reconstruct paleoenvironmental conditions. However, the interpretation of δ18Ocalc values of fossil eggshells has been limited to qualitative variations in local climatic conditions as oxygen isotope fractionations between calcite, body fluids, and drinking water have not been determined yet. For this purpose, eggshell, albumen water, and drinking water of extant birds have been analyzed for their oxygen and carbon isotope compositions. Relative enrichments in 18O relative to 16O between body fluids and drinking water of +1.6 ± 0.9 ‰ for semi-aquatic birds and of +4.4 ± 1.9 ‰ for terrestrial birds are observed. Surprisingly, no significant dependence to body temperature on the oxygen isotope fractionation between eggshell calcite and body fluids is observed, suggesting that bird eggshells precipitate out of equilibrium. Two empirical equations relating the δ18Ocalc value of eggshell calcite to the δ18Ow value of ingested water have been established for terrestrial and semi-aquatic birds. These equations have been applied to fossil eggshells from Lanzarote in order to infer the ecologies of the Pleistocene marine bird Puffinus sp. and of the enigmatic giant birds from the Pliocene. Both δ13Ccalc and δ18Ocalc values of Puffinus eggshells point to a semi-aquatic marine bird ingesting mostly seawater, whereas low δ13Ccalc and high δ18Ocalc values of eggshells from the Pliocene giant bird suggest a terrestrial lifestyle. This set of equations can help to quantitatively estimate the origin of waters ingested by extinct birds as well as to infer either local environmental or climatic conditions.

Water and carbon stable isotope records from natural archives: a new database and interactive online platform for data browsing, visualizing and downloading

Abstract: . Past climate is an important benchmark to assess the ability of climate models to simulate key processes and feedbacks. Numerous proxy records exist for stable isotopes of water and/or carbon, which are also implemented inside the components of a growing number of Earth system model. Model–data comparisons can help to constrain the uncertainties associated with transfer functions. This motivates the need of producing a comprehensive compilation of different proxy sources. We have put together a global database of proxy records of oxygen (δ18O), hydrogen (δD) and carbon (δ13C) stable isotopes from different archives: ocean and lake sediments, corals, ice cores, speleothems and tree-ring cellulose. Source records were obtained from the georeferenced open access PANGAEA and NOAA libraries, complemented by additional data obtained from a literature survey. About 3000 source records were screened for chronological information and temporal resolution of proxy records. Altogether, this database consists of hundreds of dated δ18O, δ13C and δD records in a standardized simple text format, complemented with a metadata Excel catalog. A quality control flag was implemented to describe age markers and inform on chronological uncertainty. This compilation effort highlights the need to homogenize and structure the format of datasets and chronological information as well as enhance the distribution of published datasets that are currently highly fragmented and scattered. We also provide an online portal based on the records included in this database with an intuitive and interactive platform ( http://climateproxiesfinder.ipsl.fr/ ), allowing one to easily select, visualize and download subsets of the homogeneously formatted records that constitute this database, following a choice of search criteria, and to upload new datasets. In the last part, we illustrate the type of application allowed by our database by comparing several key periods highly investigated by the paleoclimate community. For coherency with the Paleoclimate Modelling Intercomparison Project (PMIP), we focus on records spanning the past 200 years, the mid-Holocene (MH, 5.5–6.5 ka; calendar kiloyears before 1950), the Last Glacial Maximum (LGM, 19–23 ka), and those spanning the last interglacial period (LIG, 115–130 ka). Basic statistics have been applied to characterize anomalies between these different periods. Most changes from the MH to present day and from LIG to MH appear statistically insignificant. Significant global differences are reported from LGM to MH with regional discrepancies in signals from different archives and complex patterns.

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.