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International audience; Field investigations that help clarify local sedimentary processes involved in the migration of alternate bars as a consequence of flood events are lacking. A simple approach combining scour chains, stratigraphy and frequent bathymetric surveys is proposed to connect the dynamics of free migrating alternate bars present in disconnected channels of large sandy-gravelly rivers with their sedimentary products and vice versa. The results show that the spatial distribution of bars before a flood partly governs the scour and fill processes and that the sediment transport rates vary significantly on a single cross-section. This can be due to preferential axes of the migration of the bars determined by their location on the crosssection, the bank direction and the discharge. The approach allows the reconstruction of local sedimentary processes involved in alternate bar migration by combining maximum scour depths reached during a flood with frequent channel bed topography surveys and post-flood stratigraphy. It is also possible to distinguish deposited and preserved sediments compared with sediments by-passed during the flood.

International audience; Understanding past reactions of ecosystems under natural and anthropic constraints is of crucial importance to anticipate the consequences of the current global changes. Unraveling natural and anthropic impacts on environments requires the reconstruction of ancient land-uses. As a part of a larger project aiming at developing new molecular biomarkers in soils that could be specific of their overlying vegetation, we have analyzed the neutral lipid content of soils developed under distinct types of plants. Soils were sampled around Lake Aydat, in Central France, of which the catchment is covered with pastures/meadows, forests and moors. The ketone/ester lipid fraction from soils under pastures or meadows contains a series of 14 pentacyclic triterpenyl acetates with a large structural diversity (Fig. 1). This discovery motivated an extensive phytochemical literature survey (240 references) that showed restricted potential sources of triterpenyl acetates. -amyrin, taraxeryl, glutinyl, -amyrin, and multiflorenyl, acetates are found in very distinct taxa and are thus poorly informative. Conversely, lupeyl, bauerenyl, isobauerenyl, taraxasteryl and -taraxasteryl acetates appear more specific since they are majoritary reported in Asteraceae. Pichierenyl, isopichierenyl and gammacerenyl acetates seem to be the more specific ones since they are produced by a single known species, Picris hieracioides L. (hawkweed oxtongue, Asteraceae).

International audience; Anticipating the consequences of future global changes on natural systems requires the understanding of the responses of past ecosystems under climatic and anthropic constraints. As natural archives, soils are particularly relevant since they potentially preserve indications of their previous uses. Such uses are known to have irreversible impacts on soils, in terms of bulk chemical properties, soil nutrients availability, soil structure and local vegetation communities, at least at the historical scale (e.g. [1]). However, little is known about the persistence of lipid imprints through time in soils. The aim of this study is to compare the lipid imprint of three soils that suffered distinct land use histories in order to evaluate the possibilities offered by lipids to preserve information on past land uses. Three soils were sampled in the catchment of Lake Aydat (Central France) and then subsampled into 2 cm thick slices. Two of these soils remained under the same overlying vegetation (pasture: S-p, and forest: S-f) for the last 60 years. They constitute "reference" soils. These soils are compared to a soil formerly covered by a pasture/grassland 60 years ago and now by a conifer forest (S-pf). Bulk organic matter (OM) characterisation was performed by Rock-Eval pyrolysis. Lipids were extracted by ASE using DCM:MeOH and then separated into neutral, acidic and polar compounds on aminopropyl bonded silica. The neutral fraction was submitted to further separation by flash chromatography using solvents of increasing polarity. Lipid fractions were then identified by gas chromatography-mass spectrometry. Rock-Eval results suggest that, in the three soil profiles, the time of turnover and OM stabilisation increases with depth, with a global tendency toward increasing oxidation with depth. Combined with the absence of aromatic compounds, this indicates that lipids were not strongly affected by alteration. Degradation can thus not be invoked to explain variations in molecular content between the three soil profiles. Several classes of compounds were tested as potential tracers of former land uses: n-alkanes, tricyclic diterpenes, wax esters, FAMEs, steroids and triterpenes (e.g. [2], [3]). Although the most abundant n-alkane homologue is clearly defined in reference soils (n-C29 in S-f and n-C31 S-p), there is no shift from n-C31 to n-C29 in S-pf. Tricyclic diterpenoids, that are conifer markers, are present all along the S-f profile and are totally absent in S-p. These compounds are only present in the upper part of S-pf. Methoxyserratenes follow tricyclic diterpenes, although they also appear in low amount at the base of S-pf. Reversely, triterpenyl acetates are detected in S-p but are totally absent from S-f. Most of detected triterpenyl acetates are present all along the S-pf profile. This study highlights the variable potential of soil molecular biomarkers to record former land-uses, at least on decadal scales. The most specific and resistant families, such as pentacyclic triterpenes and tricyclic diterpenes, provide the most useful information but did not allow observing an obvious vertical zoning of molecular imprints in S-pf. This could result from mixing processes such as leaching, bioturbation and root penetration that affected this soil. Finally, criteria based on the presence/absence of given compounds appear more powerful than compound relative abundances. References [1] Hurtt, G.C., Frolking, S., Fearon, M.G., Moore, B., Shevliakova, E., Malyshev, S., Pacala, S.W., Houghton, R.A., 2006. The underpinnings of land-use history: three centuries of global gridded land-use transitions, wood-harvest activity, and resulting secondary lands. Global Change Biology 12, 7, 1208-1229. [2] Lavrieux, M., Jacob, J., Le Milbeau, C., Disnar, J.R., Zocatelli, R., Bréheret, J.G., Masuda, K., 2011. First detection of triterpenyl acetates in soils: sources and potential as new palaeoenvironmental biomarkers. 25th International Meeting on Organic Geochemistry, Interlaken, Switzerland. [3] Le Milbeau, C., Lavrieux, M., Jacob, J., Zocatelli, R., Disnar, J.R., 2011. Methoxyserratenes as discriminant biomarkers for soils developed under conifer forests. 25th International Meeting on Organic Geochemistry, Interlaken, Switzerland.

International audience; The history of human land use vs climate impacts on natural environments is of direct relevance in the perspective of future climate change and increasing human pressure on ecosystems. Although these impacts were discrete before agriculture and breeding development, they became critical after the industrial revolution, due to the massive anthropization of terrestrial surfaces. Natural archives such as soils and sediments potentially preserve information about past land uses. As a matter of fact, soils are reputed to retain the signature of their ancient uses [1] and lacustrine sediments integrate the evolution of environmental conditions that prevailed in the catchment basin through time.