Étude structurale et dynamique des fractions lipidiques et organiques réfractaires de sols d'une chronoséquence forêt/maÏs (Cestas, sud-ouest de la France)

Soil organic matter (OM) is an important pool of carbon on Earth. The OM stock depends on biologic factors, thus soil OM constitutes the global carbon cycle compartment the more sensitive to anthropic and climatic disturbances. In the course of biodegradation, OM represents an atmospheric CO2 source, hence a better knowledge of its structure and dynamic modelling is necessary and to forecast atmospheric CO2 variation, linked to environmental modifications. Different kinetic pools have been distinguished in soil OM which residence times vary from years to millennia. The more important residence time pools represent a stable stock of carbon poorly submitted to disturbance. The qualitative determination and quantitative estimation of this stock of carbon still has to be improved. During this work, we have studied the structure and dynamic of lipids, often considered as relatively stable, and of a chemically refractory fraction, obtained by a protocol including acid and basic hydrolyses. This fraction can present a resistance to biodegradation. Thus, we have studied two sandy soils (podzols) belonging to a chronosequence: a forest soil converted to maize cropping. Lipid composition determination in three particle-size fractions (sand, silt and clay) of these two soils, especially by GC-MS, makes possible to have access to substantial information on their source and dynamic. Spectroscopic methods (13C NMR, FTIR) combine to pyrolytic methods and chemical oxidations provide information on refractory organic matter (ROM) chemical composition and structure. Soil comparison has revealed the corn cropping influence on lipids and ROM. Moreover natural 13C abundance makes possible to obtain data on OM, lipids and ROM dynamic.La matière organique (MO) des sols est un réservoir important de carbone sur Terre. Le stock de MO dépend de facteurs biologiques, ainsi la MO des sols constitue le compartiment du cycle global du carbone le plus sensible aux perturbations anthropiques, mais aussi climatiques. Lors de sa biodégradation, la MO représente une source de CO2 atmosphérique, aussi une meilleure connaissance de sa structure et de sa dynamique est nécessaire afin de pouvoir modéliser et prévoir les variations de CO2 atmosphériques liées à des modifications du milieu. Différents pools cinétiques ont été distingués au sein de la MO des sols dont les temps de renouvellement varient de la dizaine d'année au millénaire. Les pools aux temps de résidence les plus importants forment un stock de carbone stable peu soumis aux perturbations. La détermination qualitative et l'estimation quantitative de ce stock de carbone reste encore à améliorer. Au cours de ce travail, nous nous sommes intéressés à la structure et à la dynamique des lipides, qui sont souvent considérés comme relativement stables, et à une fraction chimiquement réfractaire, obtenue à la suite d'un protocole comprenant des hydrolyses acides et basiques. Cette fraction est susceptible de présenter également une résistance à la biodégradation. Pour cela, nous avons étudié deux sols sableux (podzols) des Landes appartenant à une chronoséquence: un sol forestier ayant été converti à la culture de maïs. La détermination de la composition lipidique dans trois fractions granulométriques (sable, limon et argile), notamment par CG-SM, de ces deux sols a permis d'accéder à de nombreuses informations tant sur leur source que sur leur dynamique. Les méthodes spectroscopiques (RMN 13C, IRTF) associées à des méthodes pyrolytiques et à des oxydations chimiques ont fourni des renseignements sur la composition chimique de la matière organique réfractaire (MOR) et sa structure. La comparaison des deux sols a révélé une influence de la mise en culture du sol forestier sur les lipides ainsi que sur la MOR. De plus l'utilisation du marquage naturel par le carbone 13 a permis d'obtenir des données sur la dynamique tant de la matière organique du sol, que des lipides ou de la MOR

Organic signatures of fireplaces: Experimental references for archaeological interpretations

International audienceThe use of fire is a well-established human practice, at least from the Late Pleistocene. The variability in fuel type highlights complex practices regarding fire technology throughout this period. This contribution provides the organic signatures from fireplaces and is based on a set of experimental studies using different types of fuel, notably bone and/or wood. Soil layers affected by fire operation were compared with soils unaffected by heating and soils impregnated with unburned bone fat. The carbon content and lipid and bulk organic matter (OM) composition were determined through organic carbon measurement, gas chromatography–mass spectrometry (GC–MS), as well as TMAH (tetramethylammonium hydroxide)-assisted pyrolysis (py) coupled to GC–MS

Trace organic contaminants within solid matrices along an anthropized watercourse: Organo-mineral controls on their spatial distribution

International audienceAlthough numerous studies have determined significant contamination in terms of trace organic contaminant (TrOC) diversity and concentration, the occurrence of TrOCs within solid matrices as suspended solids and sediments flies under the radar. In this study, the occurrence of 35 TrOCs of various classes (i.e. pharmaceutical products and pesticides) was investigated in three compartments, namely dissolved phase, suspended particulate matter (SPM) and sediments, within an anthropized river in France. The sampling was performed to assess the spatial contamination dynamics and the impact of a major wastewater treatment plant (WWTP), under two contrasted hydrological conditions, i.e. base level and flood conditions. Solid samples were finely characterized (XRD, grainsize, TOC) in order to assess the impact of organic and mineral composition on the sorption extent of TrOCs. The study reveals that the clear spatial pattern of contamination in water samples, mostly generated by the effluent discharge of WWTPs, is less clear in solid matrices as the variability of the organo-mineral composition of such samples strongly impacts their favourability for sorption. Moreover, the flood event strongly impacted the sedimentary compartment, remobilizing fine and TrOC contaminated particles that were further found in suspended particulate matter. Lastly, the representativeness of contaminant 2 diversity and concentration within the solid matrices displayed more favourable insights for SPM

Fire management during Early Upper Palaeolithic : archaeological and experimental results from the Régismont-le-Haut Aurignacian campsite

International audienceArchaeological research and experimental work at the Aurignacian open-air site of Régismont-le-Haut (Southern France) has provided remarkable insight into prehistoric fireplace operation, use, and taphonomy. To date, 30 archaeological fireplaces (or fire-related structures) have been uncovered and our ongoing research focuses on deciphering their organization and function within the campsite. This communication presents field and lab results recently obtained through excavation, experimentation, and geoarchaeological and organic geochemical analyses. More specifically, micromorphological and organic matter (Curie point pyrolysis-GC-MC and GC-MS) analysis data are presented. The interest of this multiscale methodology and of solid experimental reference libraries (to create site-specific interpretive frameworks) are also highlighted. Types of fuel used to stoke fires, estimated duration, structure shapes, and surrounding activities are discussed and, according to those multiple observations, a first typology of the Régismont fire-related structures is proposed. It includes short term/low intensity wood fires, dedicated to restricted activities such as the knapping of a single core. Other fireplaces showing the use of an important quantity of bone as fuel, maintenance feature and sometimes refection seems to be devoted to longer and diversified activities. We believe that these different types of fireplaces exemplify a significant technical investement in fireplace management by the Aurignacians

Combining Rock-Eval® thermal analysis and thermochemolysis to evaluate the influence of forest management on soil organic matter quality and stability

International audiencehe effect of forest management practices on carbon quality is poorly documented. To assess changes in the quality and stability of soil organic carbon (SOC) of a temperate forest upon human activities, we investigated soil from forests (i) developed following natural regeneration after clearcutting 20 and 40 years ago, (ii) developed following afforestation on an abandoned crop area 40 years ago and (iii) in an area where regular clear-cut (with wood residues input) was conducted 40 years ago. Topsoil and subsoil layers were collected (0–20 cm and 50–80 cm). Soil organic matter (OM) was characterized by elemental analysis (total carbon and total nitrogen), thermal analysis (Rock-Eval®) and thermochemolysis (i.e., Py-GC/MS in the presence of tetramethylammonium hydroxide (TMAH)). In addition, a size fractionation to separate the labile coarse fraction (50–2000 μm) from the fine fraction (<50 µm) was performed. These fractions were analyzed by thermal analysis.Despite no measurable differences in carbon and nitrogen contents, the characterization of the OM by thermal analysis, and the relative quantification of OM compounds revealed differences in the composition in OM for the topsoil layers. The thermal analysis clearly distinguished sites with inputs of woody residues (higher HI) with a higher relative contribution of lignin and cutin/suberin compounds. However, the OM thermal stability seems mainly controlled by the organo-mineral interactions rather than chemical composition. Combination of Rock-Eval® thermal analysis and Py-GC/MS suggests that thermal stability cannot be used as an indicator of stability in specific contexts where pedogenetic processes are deeply modified by regular and extensive anthropogenic inputs of woody residues

Study of the composition of the macromolecular refractory fraction from an acidic sandy forest soil (Landes de Gascogne, France) using chemical degradation and electron microscopy

The composition of the insoluble, non-hydrolysable, macromolecular fraction from an acidic sandy forest soil (Cestas, Landes de Gascogne, S.W. France) was examined via chemical degradation and high resolution transmission electron microscopy (HRTEM). Chemical degradation was performed using four reagents (sodium persulfate followed by potassium permanganate, sodium perborate and ruthenium tetroxide). Large differences in the level of degradation of this refractory organic material (ROM) and in the nature and relative abundance of the GC-amenable reaction products were observed between these reagents. Chemical degradation supported the presence of condensed tannins in the ROM, previously suggested from NMR data, and showed the contribution of lipid-derived moieties condensed to the macromolecular refractory material. The contribution of black carbon was revealed with ruthenium tetroxide oxidation and by HRTEM observations that showed the presence of charcoal particles probably related to fire events. All the oxidation treatments point to the lack of a significant contribution of bacteria-derived moieties to the Cestas ROM. The different oxidants revealed different components of the ROM. Thus, the occurrence of suberin was better indicated by the perborate treatment, the contribution of moieties originating from the oxidative cross-linking of unsaturated acids by alkaline permanganate and RuO4, and the presence of condensed tannins and black carbon was only revealed by the RuO4 treatment.Financial support to this work was provided by Picasso program.Peer Reviewe

Fire management during Early Upper Palaeolithic : archaeological and experimental results from the Régismont-le-Haut Aurignacian campsite

International audienceArchaeological research and experimental work at the Aurignacian open-air site of Régismont-le-Haut (Southern France) has provided remarkable insight into prehistoric fireplace operation, use, and taphonomy. To date, 30 archaeological fireplaces (or fire-related structures) have been uncovered and our ongoing research focuses on deciphering their organization and function within the campsite. This communication presents field and lab results recently obtained through excavation, experimentation, and geoarchaeological and organic geochemical analyses. More specifically, micromorphological and organic matter (Curie point pyrolysis-GC-MC and GC-MS) analysis data are presented. The interest of this multiscale methodology and of solid experimental reference libraries (to create site-specific interpretive frameworks) are also highlighted. Types of fuel used to stoke fires, estimated duration, structure shapes, and surrounding activities are discussed and, according to those multiple observations, a first typology of the Régismont fire-related structures is proposed. It includes short term/low intensity wood fires, dedicated to restricted activities such as the knapping of a single core. Other fireplaces showing the use of an important quantity of bone as fuel, maintenance feature and sometimes refection seems to be devoted to longer and diversified activities. We believe that these different types of fireplaces exemplify a significant technical investement in fireplace management by the Aurignacians