Convergence order of upwind type schemes for transport equations with discontinuous coefficients

An analysis of the error of the upwind scheme for transport equation with discontinuous coefficients is provided. We consider here a velocity field that is bounded and one-sided Lipschitz continuous. In this framework, solutions are defined in the sense of measures along the lines of Poupaud and Rascle's work. We study the convergence order of the upwind scheme in the Wasserstein distances. More precisely, we prove that in this setting the convergence order is 1/2. We also show the optimality of this result. In the appendix, we show that this result also applies to other "diffusive" "first order" schemes and to a forward semi-Lagrangian scheme

Beyond Poisson-Boltzmann: Numerical sampling of charge density fluctuations

We present a method aimed at sampling charge density fluctuations in Coulomb systems. The derivation follows from a functional integral representation of the partition function in terms of charge density fluctuations. Starting from the mean-field solution given by the Poisson-Boltzmann equation, an original approach is proposed to numerically sample fluctuations around it, through the propagation of a Langevin like stochastic partial differential equation (SPDE). The diffusion tensor of the SPDE can be chosen so as to avoid the numerical complexity linked to long-range Coulomb interactions, effectively rendering the theory completely local. A finite-volume implementation of the SPDE is described, and the approach is illustrated with preliminary results on the study of a system made of two like-charge ions immersed in a bath of counter-ions

Invasion of a Sphagnum-peatland by Betula spp and Molinia caerulea impacts organic matter biochemistry. Implication for carbon and nutrient cycling.

Article AcceptedInternational audiencePeatland act as a sink of carbon (C) through the conservation of dead remains of plants. Under global changes triggered by human activities, it is not only the sink capacity of peatland that is in danger, but also the C already stored. Invasion of Sphagnum peatlands, mainly by Molinia caerulea and Betula spp, is a growing preoccupation. This study aims to assess the extent of the influence of such invasion on the biochemical characteristics of the peat. Elemental analysis and Rock Eval pyrolysis parameters were measured in 50 cm profiles collected in invaded and intact plots. The results showed that Oxygen Index (OI) can be used to detect new C substrate injection as such new materials have a lower OI than Sphagnum spp and Sphagnum peat. Differences in terms of water table level and nutrient status are suggested to be of great importance to determine the fate of surface and deep new C input. This study stimulates new investigation testing a possible priming effect triggered by the invading vegetation and using stochiomeric constrain as a theoretical framework

Insights into the origin of carbonaceous chondrite organics from their triple oxygen isotope composition

International audienceDust grains of organic matter were the main reservoir of C and N in the forming Solar System and are thus considered to be an essential ingredient for the emergence of life. However, the physical environment and the chemical mechanisms at the origin of these organic grains are still highly debated. In this study, we report high-precision triple oxygen isotope composition for insoluble organic matter isolated from three emblematic carbonaceous chondrites, Orgueil, Murchison, and Cold Bokkeveld. These results suggest that the O isotope composition of carbonaceous chondrite insoluble organic matter falls on a slope 1 correlation line in the triple oxygen isotope diagram. The lack of detectable mass-dependent O isotopic fractionation, indicated by the slope 1 line, suggests that the bulk of carbonaceous chondrite organics did not form on asteroi-dal parent bodies during low-temperature hydrothermal events. On the other hand, these O isotope data, together with the H and N isotope characteristics of insoluble organic matter, may indicate that parent bodies of different carbonaceous chondrite types largely accreted organics formed locally in the protosolar nebula, possibly by photochemical dissociation of C-rich precursors

Effects of a short-term experimental microclimate warming on the abundance and distribution of branched GDGTs in a French peatland

International audienceBranched glycerol dialkyl glycerol tetraethers (GDGTs) are complex lipids of high molecular weight, recently discovered in soils and increasingly used as palaeoclimate proxies. Their degree of methylation, expressed in the MBT, was shown to depend on mean annual air temperature (MAAT) and to a lesser extent on soil pH, whereas the relative abundance of cyclopentyl rings of branched GDGTs, expressed in the CBT, was related to soil pH. To date, only a few studies were interested in the application of the MBT and CBT proxies in peatlands. In order to validate the applicability of branched GDGTs as temperature proxies in these environments, it is essential to investigate the effect of temperature on branched GDGT-producing bacteria and especially on the speed of adaptation of these microorganisms to temperature changes. The aim of this work was to study the effects of in situ experimental climate warming on the abundance and distribution of branched GDGTs in a Sphagnum-dominated peatland (Jura Mountains, France). Branched GDGTs either present as core lipids (CLs; presumed of fossil origin) or derived from intact polar lipids (IPLs, markers for living cells) were analysed. Air temperature was experimentally increased using a passive warming system consisting of open mini-greenhouses (Open-Top Chamber - OTC). The effect of the OTCs was especially apparent in spring and summer, with (i) an increase in maximal air temperature of ca. 3°C during these two seasons and (ii) an increase in average air temperature of ca. 1°C in summer. Despite the short duration of the climate experiment (26 months), branched GDGT distribution was significantly affected by this temperature rise, with higher MBT values in the OTCs than in the control plots, supporting the empirical relationship between MBT and MAAT established from a large range of soils. The difference in branched GDGT-derived temperatures between control and OTC plots (2-3 °C) was in the same range as the increase in maximal (daytime) temperature induced by the OTCs in spring and summer, suggesting that branched GDGT-producing bacteria might be more active during the warmest months of the year. The OTC treatment had no significant effect on the abundance of branched GDGTs, which were mainly present as "fossil" CLs (70 to 85% of the total branched GDGT pool). Furthermore, no significant differences in branched GDGT distribution were observed between the CLs and IPLs, which both provided higher MBT and MAAT values for the OTCs. This suggests that the fossil pool of branched GDGTs has a very fast turnover (less than the 2 year duration of the experiment) at the peat surface and that branched GDGT distribution may rapidly reflect changes in environmental conditions (at least air temperature) occurring in peat environments

Inversion-based control of electromechanical systems using causal graphical descriptions

Causal Ordering Graph and Energetic Macroscopic Representation are graphical descriptions to model electromechanical systems using integral causality. Inversion rules have been defined in order to deduce control structure step-bystep from these graphical descriptions. These two modeling tools can be used together to develop a two-layer control of system with complex parts. A double-drive paper system is taken as an example. The deduced control yields good performances of tension regulation and velocity tracking

Inversion-based control of electromechanical systems using causal graphical descriptions

Causal Ordering Graph and Energetic Macroscopic Representation are graphical descriptions to model electromechanical systems using integral causality. Inversion rules have been defined in order to deduce control structure step-bystep from these graphical descriptions. These two modeling tools can be used together to develop a two-layer control of system with complex parts. A double-drive paper system is taken as an example. The deduced control yields good performances of tension regulation and velocity tracking

Organic matter sources and decay assessment in a Sphagnum-dominated peatland (Le Forbonnet, Jura Mountains, France). Impact of moisture conditions.

International audienceIn a context of climate change peatlands may switch from a sink to a source of carbon (C). The induced positive feedbacks are closely linked to C dynamics, and thus to the fate of organic matter (OM) in the underlying peat. Our aims were to determine how moisture conditions in a Sphagnum-dominated peatland affect the dynamics of diagenetic sensitive OM which is thought to be the most reactive to warming effects. The approach was based on the identification of combined bulk and molecular bioindicators of OM sources and decay of peat profiles (up to 50 cm in depth) from two moisture conditions of the Le Forbonnet peatland (Jura Mountains, France). The (xylose+arabinose)/(galactose+rhamnose+mannose) ratio derived from the analysis of neutral monosaccharides was used to obtain indications on the botanical origin of the peat and tended to indicate a greater contribution of Cyperaceae in the deepest parts of the peat. Most bioindicators showed that OM decay increased with depth and was higher in the driest conditions. In these conditions, decay was shown by a loss of diagenetic sensitive oxygen-rich OM, probably leached by water table fluctuations. Decay intensity was also shown by the high ribose and lyxose contents at the peat surface, where microscopic observations also revealed relatively large quantities of fungal hyphae. The sugars could have arisen from microbial synthesis, primarily protozoan and fungal activity. These results suggest that water level changes preferentially impact biochemical changes in diagenetic sensitive OM, believed to be sensitive to drought events

Tetramethylammonium hydroxide thermochemolysis for the analysis of cellulose and free carbohydrates in a peatbog

International audienceWe have compared TMAH thermochemolysis with the classical method using acid hydrolysis for carbohydrates analysis in a peat core. Even if TMAH thermochemolysis does not analyse hemicellulosic carbohydrates and discriminate each individual carbohydrate sensu stricto, it allows the analysis of a cellulose pool hidden to acid hydrolysis and the specific analysis of free and terminal carbohydrates. Simple direct comparisons of thermochemolysis data with data generated from acid hydrolysis cannot be done because of the different mechanisms involved in each process. TMAH thermochemolysis must be viewed and used as a pertinent and complementary method for the analysis of carbohydrates protected and trapped by the organic matter in complex environmental systems

Effects of short-term ecosystem experimental warming on water-extractable organic matter in an ombrotrophic Sphagnum peatland (Le Forbonnet, France)

International audienceIn a future warmer world, peatlands may change from a carbon sink function to a carbon source function. This study tracks changes in water-extractable organic matter (WEOM) after one year of in situ experimental warming using open top chambers (OTCs). WEOM was studied in the upper peat layers (0-10 cm) through analysis of water-extractable organic carbon (WEOC), stable C isotopic composition (δ13C), specific UV absorbance at 280 nm and sugar composition of cores taken from an open bog (DRY sites) and a transitional poor fen (WET sites). At the DRY sites, the impact of OTCs was weak with respect to WEOM parameters, whereas at the WET sites, the air warming treatment led to a decrease in peat water content, suggesting that the supply of heat by OTCs was used mainly for evapotranspiration. OTCs at the WET sites also induced a relative enrichment at the surface (0 to 5 cm depth) of aliphatic and/or aromatic compounds with concomitant decrease in WEOC, as a result of decomposition. On the contrary, WEOC and sugar content increased in the deeper peat layer (7.5-10 cm depth) probably as a result of increased leaching of phenolic compounds by roots, which then inhibits microbial activity. The different response to experimental warming at DRY and WET sites suggests that the spatial variability of moisture in is critical for understanding of the impact of global warming on the fate of OM and the carbon cycle in peatlands