Estimation of the annual organic carbon yield related to carbonated rocks chemical weathering : implications for the global organic carbon cycle understanding.

The aim of this paper is to propose an initial estimation of the annual organic matter yield induced by chemical weathering of carbonates and shales, considering their global surface at outcrop and their organic matter content. The calculation also uses data on river fluxes resulting from carbonate rocks and shales weathering in major world watersheds, published by numerous authors. The results obtained from the studied watersheds have then been extrapolated to a global scale. Despite rather large uncertainty to such an approach, the calculated value of ca. 0.1 Gt implies that the annual organic carbon yield related to carbonates and shales chemical weathering might be a non-negligible component of the global carbon cycle. The individual contributions of different watersheds necessarily depend on the organic matter content of altered rocks. They are also obviously controlled by climatic parameters. The calculated yields do not constitute a direct supply to soils and rivers because of mineralisation when organic carbon is brought in contact with the atmosphere. Even so, the release of fossil organic matter would have implications for the global carbon cycle through the efficiency of the global chemical weathering as a carbon sink. Whatever the chosen hypothesis, the results of this study suggest that the recycled organic yield is a neglected component in the global organic carbon cycle assessment. Because it exists and, in addition, because it might represent a non-negligible carbon pool, fossil organic carbon deserves to be taken into account for a better evaluation of the organic stocks in soils and rivers presently only based on climatic data and current vegetal production

Relations between organic and terrigenous matter deposition in Holocene fluvial sediments: heterodox observations and conclusions.

International audienceAccumulation of organic matter in fens of fluvial valleys is often related to a low terrigenous matter delivery and to palaeoenvironmental conditions inducing low mechanical erosion. These assumptions come from the interpretation of contents in organic (MO) and mineral (MM) matters in sediments, expressed in percents, and then exactly anticorrelated. Calculation of mass accumulation rates of MO (TaMO) and MM (TaMM), expressed in g m−2 yr−1, shows that TaMO and TaMM generally are not anticorrelated and that high values of TaMO and TaMM could appear simultaneously. That expression of MO and MM accumulation makes it possible to precise the climatic and human impact on sedimentation

Microbial mat development and dolomite formation under pre-evaporitic conditions during the Atlantic in a temperate area: The Sarliève Lake (French Massif Central)

International audienceThe Sarliève marsh in the Limagne plain, in the heart of the French Massif Central, functioned as an endorheic lake during the Late Glacial and the Early and Middle Holocene. During the Late Boreal and the Atlantic it experienced drastic lowering of the water level as a result of dry and warm climatic episodes. Then, pre-evaporitic conditions triggered the deposition of sediment rich in organic matter (OM) and in carbonates including dolomite. Fifty-one samples from a ca. 1.8 m sediment core section covering the period were analysed using Rock-Eval pyrolysis and gas chromatography-mass spectrometry (GC-MS). Throughout the interval, the OM content remained notable to high [up to 13.35% total organic carbon (TOC)] and of good quality as indicated by low oxygen index (OI) values (<200 mg CO2 g−1 TOC) and high hydrogen index (HI) values (160-660 mg HC g−1) which, as a rule, increased with increasing TOC content. In contrast to the acid fractions, which sometimes contained notable proportions of n-C16 or n-C18 fatty acids (FAs; analysed as the methyl esters, FAMEs), the neutral fractions were almost devoid of low molecular weight compounds. The latter were probably biodegraded during early diagenesis. Conversely, high molecular weight compounds were abundant in both fractions and were dominated by n-alkanols, n-alkanes, steroids and hopanoids in the neutrals and even numbered FAs and hopanoids in the acid fractions. The hopanoids were dominated by regular bishomohopanoids, accompanied by 2-methylated bishomohopanoids, as well as by unidentified bishomohopanoids with methylation in either the D or E ring. These distributions, typical for bacteria, provide support for previous hypotheses on the contribution of microorganisms to the studied record and for providing conditions for the precipitation and growth of dolomite and other pristine carbonate minerals [Bréheret, J.G., Fourmont, A., Macaire, J.J., Négrel, Ph., 2008. Microbially mediated carbonates in the Holocene lacustrine deposits of the Marais de Sarliève (French Massif Central) testify to the evolution of a restricted environment. Sedimentology 55, 557-578]. The preservation of the compounds was probably ensured by persistent reducing conditions during diagenesis, despite variable climatic conditions and related changes in lake level and sedimentation rate. Comparable distributions of hopanoic acids and hopanols, as well as roughly parallel variation with depth in the corresponding components in both fractions, strongly suggest that all the hopanoids derive from the same microbial precursors, slight shifts in the acid/alcohol ratio being governed by limited changes in redox conditions during early diagenesis

Évolution morphologique et sédimentaire de la plaine alluviale d'un espace urbanisé (Tours, Indre-et-Loire, France)

http://racf.revues.org/1985National audienceUrban areas located in floodplains offer a great geo-archaeological potential for the comprehension of fluvial dynamics on long term and its relationship with land occupation, as they allow exploiting many survey data of the basement. In the floodplain of Tours, shared by the River Loire and Cher river, relations between human societies and environment are the subject of an interdisciplinary work within the frame of an archaeological program initiated in the late 1960's. A sedimentary database including 1309 surveys has been created then exploited in a geographic information system to produce geomorphological and geostatical models. The stratigraphy and chronology of the alluvial filling were also studied with surveys and 14C and OSL dating obtained in the course of archaeological prospections. The combined results provide a new reading of the morphological evolution of the floodplain of Tours from the Weichselian to nowadays.Les zones urbaines situées dans les plaines alluviales offrent un grand potentiel géo-archéologique pour la compréhension de la dynamique fluviale dans la longue durée et ses relations avec l'occupation du sol en contexte anthropisé, car elles permettent d'exploiter des très nombreuses données de sondages du sous-sol. Dans la plaine alluviale de Tours, partagée par la Loire et le Cher, les relations sociétés/milieux font l'objet de travaux interdisciplinaires autour d'un programme de recherches archéologiques initié à la fin des années 1960. Une base de données sédimentaires comprenant 1309 sondages a été constituée puis exploitée dans un système d'information géographique pour produire des modèles géostatistiques et géomorphologiques. La stratigraphie et la chronologie du remplissage alluvial ont également été étudiées avec des sondages et des datations 14C et OSL obtenues à l'occasion d'opérations archéologiques. Les résultats croisés permettent une lecture renouvelée de l'évolution morphologique de la plaine alluviale tourangelle du Weichsélien à nos jours

Contrasted morphosedimentary activity of the lower Kert River (northeastern Morocco) during the Late Pleistocene and the Holocene. Possible impact of bioclimatic variations and human action

International audienceFrom field observations in the lower Kert valley and 16 radiocarbon dating measurements, six alluvial units (UF1 to UF6) deposited since about 30,000 years BP have been identified, comprising 18 lithofacies and separated by six incision stages resulting in three alluvial terraces (T1 to T3). While the Late Pleistocene is mainly marked by sedimentary accretion (UF1 forming part of T1), the Holocene is marked by the alternation of major incision (I2, I3 and I4) and accretion (UF2 and UF3) stages, with roughly similar height ranges, between the current river level and polyphased T1 surface. During the Lower and Middle Holocene, this complex morphosedimentary evolution could have been the result of contrasted bioclimatic trends that did not appear during the Late Pleistocene. Human activities modified the earth surface conditions, intermittently during the Middle Holocene, and during the Upper Holocene, inducing new responses from the fluvial geosystem to the environment

Classification and mapping of anthropogenic landforms on cultivated hillslopes using DEMs and soil thickness data - Example from the SW Parisian Basin, France

International audienceThis study focuses on linear anthropogenic landforms of decametric width on cultivated hillslopes and their relations to soil thickness variability. The 16 ha study area shows a rolling topography supported by Cretaceous chalk of the SW Parisian Basin, France. Two types of landforms were identified: lynchets, similar to those described as soil terraces occurring on downslope field parts in other contexts, and undulations, linear, convex landforms that cut across fields. Accurate DEM construction and a detailed soil thickness survey were performed all over the study area. Soil samples were classified considering their location on specific types of anthropogenic landforms. The classification tree (CT) method was applied to assess whether lynchets and undulations can be discriminated through morphometric attributes (slope, curvature, profile curvature and planform curvature) and soil thickness (CTsoil) or through morphometric attributes only (CTtopo). The CT application establishes predictive classification models to map the spatial distribution of lynchets and undulations over the whole study area. The validation results of the CTsoil and CTtopo applications show model efficiencies of 83% and 67%, respectively. Both models performed well for lynchets. Errors arise mainly from difficulties in unequivocally discriminating gently convex undulations and undifferentiated surfaces, especially when soil thickness is not accounted for. Mean values of soil thickness are 1.08, 0.62 and 0.45 m in lynchets, undulations and undifferentiated areas, respectively. The general shape of the thickened soil is characteristic to each type of anthropogenic landform. Multi-temporal mapping of field border networks shows that undulations are linked to borders that were removed during the latest land consolidation. Lynchets are associated with current field borders. Lynchets and undulations, which cover 39% of the study area, define topographic indicators of human-induced soil accumulations. The method involves perspectives for efficiently mapping and quantifying the anthropogenically modified spatial variability of soil thickness on agricultural hillsides

Effet de l'évolution du parcellaire agricole sur la redistribution des sols et la morphologie des versants cultivés (exemple du sud-ouest du Bassin parisien)

L évolution historique des types et techniques de production agricole a causé celle de l organisation paysagère, via les parcellaires notamment. Les bordures de parcelles créent des discontinuités dans les processus d érosion-dépôt de sol. L objectif est ici de comprendre l effet du parcellaire et de son évolution sur la distribution actuelle des sols et la morphologie des versants. Un versant cultivé (16 ha) dans le SW du Bassin Parisien a été étudié. La distribution spatiale de figures morphologiques linéaires et de l épaisseur des sols a été analysée. Les deux types de figures identifiés correspondent à des épaississements de sols (dépôts) induits par des bordures de parcelles, pérennes depuis plusieurs siècles pour les premières, et disparues en 1967 (remembrement) pour les secondes. Ces épaississements ont été cartographiés grâce à une analyse statistique (CART) de la morphologie du versant. L étude de traceurs granulométriques et minéralogiques (SEDI) a permis d identifier les processus d érosion-dépôt impliqués. La solifluxion périglaciaire puis le ruissellement auraient affecté l ensemble du versant avant la pérennisation d un parcellaire. Puis, des processus hydriques et aratoires ont agi dans des unités (parcelles) fixées par des bordures : les plus pérennes (1000 ans) montrent les dépôts les plus marqués (banquettes). La distribution spatiale du 137Cs et sa conversion en taux d érosion (modélisation numérique) ont permis d évaluer l implication relative des processus hydriques (15%) et aratoires (85%) depuis 1954. Le remembrement parcellaire de 1967 s avère avoir favorisé l érosion des sols, convertissant des zones de rétention en zones sources (ondulations).The historical evolution of agricultural practices is associated with a concomitent evolution of landscape spatial organisation, especially through field-border networks. Field borders create discontinuities of soil erosion-deposition processes. The aim of this PhD is to understand the effects of field-border networks and their evolution on the present spatial distribution of soils and hillslope morphology. A cultivated hillslope (16 ha) of the SW Parisian Basin was studied. The spatial distribution of linear landforms and soil thickness was analyzed. The two types of identified landforms correspond to soil thickenings (deposition) induced by field borders, that have existed for several centuries in the case of the first ones, and disappeared during a land consolidation (1967) in the case of the second ones. These soil thickenings were mapped through a statistical analysis (CART) of the hillslope morphology. The study of granulometric and mineralogical tracers (SEDI) led to identify the involved soil redistribution processes. Periglacial solifluxion, followed by runoff, probably occured over the whole hillslope before the establishment of perennial field borders. Then, water and tillage processes occured within areas delimited by field borders. The more developed soil depositions (lynchets) are linked with the most perennial borders (i.e., 1000 yrs). The spatial distribution of 137Cs and its conversion into erosion rates (numerical modeling) allowed to assess the relative implication of water (15%) and tillage processes (85%) since 1954. The land consolidation that occured in 1967 rised soil erosion when converting soil retention areas to sources (undulations).TOURS-Bibl.électronique (372610011) / SudocSudocFranceF

Organisation territoriale et socio-économique au Néolithique final dans la région du Grand-Pressigny: caractérisation des provenances des matériaux céramiques

National audienceIn and around Le Grand-Pressigny (Indre-et-Loire, France), a petrographic study was implementedon 92 ceramic samples from the Final Neolithic sites of Le Petit-Paulmy and Bergeresse (Abilly).Analyses showed that the sediments used were extracted from local sources: Claise and Creusealluvium, local substrate and loessic silts. Quantitatively, the study showed that almost all the ceramicmaterials in Bergeresse and more than half of those in Le Petit-Paulmy come from the valley of theCreuse. The mineralogical compositions of three samples from Le Petit-Paulmy, including one ofunusual form, indicate sources from an exogenous region, the Massif Central (perhaps the alluviumof the Loire or the Allier). These results show the important role played by the valley of the Creusein the territorial organisation of Le Grand-Pressigny. This study therefore makes an originalcontribution to the debate on flint exploitation in this region. These ceramic data are expected to offernew interpretative models concerning the socio-economic organisation of local Neolithic communities.Dans la région du Grand-Pressigny (Indre-et-Loire, France), une étude pétrographique a été conduite sur 92 échantillons céramiques du Néolithique final provenant des sites du Petit-Paulmy et de Bergeresse (Abilly), respectivement situés dans la vallée de la Claise et de la Creuse. Les analyses ont montré que les sédiments utilisés provenaient majoritairement de sources locales, correspondant aux alluvions de la Claise, de la Creuse, au substrat local et à des limons loessiques. Quantitativement, l'étude a montré que presque tous les matériaux céramiques de Bergeresse et plus de la moitié de ceux du Petit-Paulmy proviennent de la vallée de la Creuse. Les compositions minéralogiques de trois échantillons du Petit-Paulmy, dont un concernant une forme particulière, indiquent des provenances exogènes, du Massif central (alluvions de la Loire ou de l'Allier). Ces résultats montrent l'importance du rôle joué par la vallée de la Creuse dans l'organisation territoriale de la région du Grand-Pressigny. L'étude des céramiques apporte une contribution originale dans le débat sur l'organisation socioéconomique de l'exploitation du silex du Grand-Pressigny et devrait permettre de proposer de nouveaux modèles interprétatifs