Lacustrine organic fluxes and paleoclimatic variations during the last 15 ka : lac du Bouchet (Massif Central, France)

To assess the influence of climatic changes on organic lacustrine sedimentation, two cores recovered from the centre of the Lac du Bouchet were studied by petrographical (palynofacies) and geochemical methods. Only core LDB H was used for estimation of the organic fluxes. The variation of these fluxes with climo-stratigraphic periods showed : low organic fluxes during the Lateglacial, an increase at the beginning of the Holocene, a minimum at the end of the Atlantic period resulting from the climatic cooling, and a maximum at the end of the Sub-Boreal related to the installation of the present climatic conditions. (Résumé d'auteur

Response of littoral chironomid community and organic matter to late glacial lake level and environmental changes at Lago dell'Accesa (Tuscany, Italy).

International audienceThis study focuses on the response of lacustrine littoral chironomid communities to late glacial changes in limnological, environmental and climate conditions in the Mediterranean context. Late glacial chironomid (Diptera: Chironomidae) assemblages, organic petrography and geochemistry were analysed in a sediment core from the littoral zone of Lago dell'Accesa (Tuscany, Italy), where the lake-level fluctuations and the vegetation history have been previously reconstructed. Comparison of the chironomid stratigraphy to other proxies (pollen assemblages, organic petrography and geochemistry, lake-level) and regional climate reconstruction suggested the predominant influence of lake-level changes on the littoral chironomid fauna. The main lowering events that occurred during the Oldest and the Younger Dryas were followed by higher proportions of taxa typical of littoral habitats. A complementary study of organic matter suggested the indirect impact of lake-level on the chironomids through changes in humic status and habitat characteristics, such as the type of substrate and aquatic macrophyte development. Several chironomid taxa, such as Glyptotendipes, Microtendipes and Cricotopus type patens, were identified as possible indicators of low lake-level in the late glacial records. Nevertheless, this study suggested that parallel analyses of organic matter and chironomid assemblages may be needed to circumvent misinterpretation of littoral chironomid assemblage stratigraphy. There was a weak response of the chironomid assemblages to small lake-level lowerings that corresponded to the Older Dryas and Preboreal oscillations. A higher level of determination, e.g. to the species group level, may be necessary to increase the sensibility of the indicators to lake-level changes

Organic carbon sources and transformations in mangrove sediments : a Rock-Eval pyrolysis approach

International audienceA Rock-Eval pyrolysis study was carried out on sedimentary cores and leaf and woody tissue of vascular plant species from the mangroves of French Guiana. These forests develop on moving mudbanks and have a lifetime limited to few decades before being eroded. Our main purpose was to complete the understanding of carbon cycling in this specific environment using a method that allows monitoring the depth evolution of sources and transformation of organic matter (OM) within a tight depth span. Leaf and woody tissue showed clearly distinct Rock-Eval parameters. However, these parameters strongly varied with OM decomposition, rapidly limiting their efficiency as mangrove plant tracers. The decrease in values of TPS2 (pyrolysis temperature corresponding to the maximum release of hydrocarbons) with depth clearly showed a line between a pedogenetic layer enriched in autochthonous OM and marine sediments enriched in allochthonous OM. This allochthonous OM originated from the particulate discharge of the Amazon River, which is partly deflected northwestwards by the current of the Guianas. Within the soil layer, decay processes induced a strong dehydrogenation and slight oxidation of OM, the two processes occurring simultaneously. The deeper layers were characterized by a very strong increase in OICO2 (quantity of pyrolysed CO2 relative to total organic carbon), beneath both the young and the senescent forests. This trend resulted rather from an increasing content of authigenic carbonate than from increasing concentration of oxidized allochthonous debris. Carbonate can form as a result of the more anoxic conditions prevailing in buried layers vs. the upper suboxic ones. Beneath the senescent mangrove, a greater increase in OICO2 probably reflected greater carbonate precipitation, resulting from the decomposition of higher amounts of O

La sédimentation organique lacustre en zone tropicale au cours des 36 000 dernières années

L'étude pétrographique (palynofaciès) et géochimique (pyrolyse Rock Eval) de la matière organique sédimentaire d'une carotte prélevée dans un lac de cratère de Madagascar révèle une sédimentation organique de type tourbeux avant 36 ka, entre ca 28 et 15 ka et entre 6 et 0 ka, mise en place au cours de périodes d'assèchement. Entre ca 36 et 28 ka, elle se caractérise par une sédimentation lacustre (phytoplancton dominant) et des migrations de végétation du bassin versant vers le lac lui-même. Des apports de matière organique allochtone témoignent de l'établissement d'un couvert végétal sur le bassin versant et du lessivage de sols, lors de périodes humides. Les variations d'écosystèmes et de peuplement sont à mettre en relation avec l'évolution du régime hydroclimatique depuis 36 ka, en termes de périodes sèches et humides. (Résumé d'auteur

La sédimentation organique lacustre en zone tropicale au cours des 36 000 dernières années (lac Tritrivakely, Madagascar)

L'étude pétrographique (palynofaciès) et géochimique (pyrolyse Rock Eval) de la matière organique sédimentaire d'une carotte prélevée dans un lac de cratère de Madagascar révèle une sédimentation organique de type tourbeux avant 36 ka, entre ca 28 et 15 ka et entre 6 et 0 ka, mise en place au cours de périodes d'assèchement. Entre ca 36 et 28 ka, elle se caractérise par une sédimentation lacustre (phytoplancton dominant) et des migrations de végétation du bassin versant vers le lac lui-même. Des apports de matière organique allochtone témoignent de l'établissement d'un couvert végétal sur le bassin versant et du lessivage de sols, lors de périodes humides. Les variations d'écosystèmes et de peuplement sont à mettre en relation avec l'évolution du régime hydroclimatique depuis 36 ka, en termes de périodes sèches et humides. (Résumé d'auteur

La sédimentation organique lacustre en zone tropicale au cours des 36 000 dernières années (lac Tritrivakely, Madagascar)

L'étude pétrographique (palynofaciès) et géochimique (pyrolyse Rock Eval) de la matière organique sédimentaire d'une carotte prélevée dans un lac de cratère de Madagascar révèle une sédimentation organique de type tourbeux avant 36 ka, entre ca 28 et 15 ka et entre 6 et 0 ka, mise en place au cours de périodes d'assèchement. Entre ca 36 et 28 ka, elle se caractérise par une sédimentation lacustre (phytoplancton dominant) et des migrations de végétation du bassin versant vers le lac lui-même. Des apports de matière organique allochtone témoignent de l'établissement d'un couvert végétal sur le bassin versant et du lessivage de sols, lors de périodes humides. Les variations d'écosystèmes et de peuplement sont à mettre en relation avec l'évolution du régime hydroclimatique depuis 36 ka, en termes de périodes sèches et humides. (Résumé d'auteur