La matière organique des sols (MOS) : un héritage difficile mais fructueux

Interface entre géosphère et biosphère, les sols doivent, de manière immédiatement perceptible, la MO qu'ils renferment aux végétaux, voire aux animaux qu'ils supportent et/ou abritent. La présence de MO au sein des sols et son importance sont connues de longue date. Cette importance a même été très exagérée avec la théorie de l'humus qui, vers la fin du 18edébut du 19e siècle, supposait que les végétaux tiraient de ce substrat toutes leurs substances nutritives, carbone compris. En 1840, dans son traité « la chimie organique et son application en agriculture et en physiologie », Liebig rejette cette théorie et promeut des concepts plus proches de la réalité. Ramenée à une place plus modeste, la MOS est cependant bien prise en compte lors de l'invention de la pédologie moderne, attribuée au géologue russe Dokouchaev, à la fin du 19e siècle. Dépêché en Ukraine en 1877, pour en étudier les sols, Dokouchaev a fondé la reconnaissance, dans les profils qu'il y a étudiés (des chernozems), de trois horizons superposés se distinguant avant tout de bas en haut, par leur richesse en matière organique

l'Histoire thermique de la plate-forme liasique du district du Tazekka (Taza, Maroc oriental) : Etude par pyrolyse rock-eval de La matière organique associée aux minéralisations Pb-Zn.

L'étude de la matière organique associée aux minéralisations Pb-Zn de la plate-forme liasique du district du Tazekka par pyrolyse Rock-Eval, révèle qu'elle est allochtone et très mature (stade de métagenèse). Elle est essentiellement formée de pyrobitumes. Son abondance relative au niveau des gîtes minéralisés contraste avec la pauvreté quasi-générale du niveau porteur et des faciès stériles. En effet, des teneurs en COT (Carbone Organique Total) supérieures à 1% et parfois même dépassant 2% COT ont été fournis par des échantillons provenant des gîtes de Sidi Abdellah (2,67%), Bou Khalifa et Asdi Ben Zerhla

Pétrographie et géochimie comparées des pellets phosphatés et de leur gangue dans le gisement phosphaté de Ras-Draâ (Tunisie). Implications sur la genèse des pellets phosphatés

International audienceLes séries phosphatées tunisiennes, d' âge Paléocène-Eocène inférieur, sont constituées d'alternances de strates richement phosphatées (P2O5 ≥ 18%) et de strates intercalaires peu riches en P (P2O5 ≤ 2%). Les strates phosphatées sont constituées d'un sédiment argilocarbonaté servant de matrice (ou exogangue) à des grains phosphatés, arrondis, de taille comprise majoritairement entre 100 et 500 μm, encore appelés pellets, et à des grains cylindriques d'origine fécale. L' étude des pellets phosphatés et de leur exogangue dans les strates riches et des sédiments intercalaires peu riches en P dans le gisement de phosphates de Ras-Draâ (Bassin de Gafsa-Metlaoui, Tunisie) a réuni des observations microscopiques et diverses analyses chimiques des phases minérales présentes au sein des pellets et des sédiments (matrices et sédiments intercalaires). Le constituant apatitique majeur des grains est la carbonate-fluorapatite, ou francolite, dont une formule structurale, déterminée sur un échantillon représentatif, est la suivante: (Ca4,63Mg0,13Na0,22)(PO4)2,51(CO3)0,48)(OH0,77F0,23). Les constituants des sédiments encaissant ces grains (matrice) et des strates intercalaires sont de nature minéralogique essentiellement argileuse, carbonatée ou siliceuse (opale ou quartz). Les observations microscopiques et géochimiques de ces différentes phases conduisent à reconnaître que les pellets sont des corps étrangers au sein de leur matrice et de composition tout à fait indépendante de celle de leur exogangue. Les éléments fécaux cylindriques et les pellets arrondis qui en seraient dérivés par fragmentation et érosion, auraient été produits par des poissons, au sein de fèces riches en P et en matière organique et résistants vis à vis de la dispersion mécanique. Cette dernière propriété a valu à leur contenu de n' être pas dispersé au cours de la sédimentation et des remaniements sédimentaires et d' évoluer, diagénétiquement, dans un environnement réducteur

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

Carbon isotope evidence for sedimentary miliacin as a tracer of Panicum miliaceum (broomcorn millet) in the sediments of Lake le Bourget (French Alps).

International audienceWe here report on the determination of the carbon isotopic composition of miliacin (olean-18-en-3βol methyl ether), extracted from the sediments of Lake le Bourget (French Alps). It is compared to the δ13C of miliacin extracted from Panicum miliaceum (broomcorn millet, a C4 plant) and Chionochloa sp. (a C3 plant). The δ13C of sedimentary miliacin (-21.5 ‰) is very close to that of miliacin extracted from bran (-23 ‰) and seeds (-23.5 ‰) of P. miliaceum and significantly different from that of Chionochloa sp. (-33 ‰). These results provide additional support for the use of sedimentary miliacin as a tracer of broomcorn millet, a C4 cereal cultivated since the Bronze Age around Lake le Bourget. These findings illustrate the potential of this compound to reconstruct past agriculture from lake sediment archives. Finally, considering the high abundances of miliacin in the bran of P. miliaceum this compound could have been wind-transported to the sediment during threshing and winnowing on the lake shore

Organic Geochemistry of the Cenomanian-Turonian Bahloul Formation Petroleum Source Rock, Central and Northern Tunisia

International audienceTotal organic carbon (TOC) determination, Rock-Eval pyrolysis, extractable organic matter content (EOM) fractionation, gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS) analyses, were carried out on 79 samples from eleven outcrop cross sections of the Bahloul Formation in central and northern Tunisia. The TOC content varied between 0.23 to 35.6%, the highest average values (18.73%, 8.46% and 4.02%) being at the east of the study area (at Ain Zakkar, Oued Bahloul and Dyr Ouled Yahia localities, respectively). The Rock-Eval maximum pyrolysis temperature (Tmax) values in the 424-453°C range delineated a general east-west trend increase in the organic matter (OM) maturity. The disparity in hydrogen index (HI) values, in the range 114-824 mg hydrocarbons (HC) g−1 TOC, is relevant for the discrepancy in the level of OM preservation and maturity among localities and samples. The n-alkane distributions, maximizing in the C17 to C20 range, are typical for a marine planktonic origin, whereas pristine/phytane (Pr/Ph) average values in the 1-2 range indicate an oxic to suboxic depositional environment. Pr/n-C17 and Ph/n-C18 ratios in the 0.38-6.2 and 0.68-3.25 range, respectively, are consistent with other maturity indicators and the contribution of specific bacteria to phytol as a precursor of isoprenoids. The thermal maturity varies between late diagenesis to main-stage of petroleum generation based on the optic and the cis-trans isomerisation of the C29 sterane [20S/(20S+20R) and 14β(H),17β(H)/(14β(H),17β(H)+14α(H),17α(H)), respectively] and the terpane [18α(H)22,29,30-Trisnorneohopane/(18α(H)22,29,30-Trisnorneohopane+17α(H)22,29,30-Trisnorhopane): Ts/(Ts+Tm)] ratios. The Bahloul OM is represented by an open marine to estuarine algal facies with a specific bacterial contribution as revealed by the relative abundance of the ααα-20R C27 (33-44%), C28 (22-28%) and C29 (34-41%) steranes and by the total terpanes/total steranes ratio (1.2-5.33). These results attested that the Bahloul OM richness was controlled both by an oxygen minimum zone induced by high productivity and restricted circulation in narrow half graben structures and around diapirs of the Triassic salt

Evolution of organic matter indicators in response to major environmental changes: the case of a formerly cutover peatbog (Le Russey, Jura Mountains, France).

To assess whether the biochemical characteristics of peat can provide clues for past ecosystem changes or not, a study was carried out combining elemental analysis, micro-morphological counts and neutral monosaccharide determination of peat organic matter (OM) and the dominant living plants from a formerly cut-over peat bog in the Jura Mountains. Peat profiles (up to 50 cm depth) from two distinctive zones (bare peat, FRA and a regenerating stage, FRC) were compared with the reference profile (FRD) taken from an unexploited area of the bog. The results show contrasting OM composition along the profiles. In the upper sections of the FRD and FRC profiles, high C/N ratios and sugar contents (in the same range as in the source plants) and the large predominance of well preserved plant tissues indicate good preservation of primary biological inputs. In contrast, in peat from the FRA profile and deeper levels of the FRC profile, lower C/N ratios, lower amounts of sugars and a predominance of amorphous OM and mucilage suggest more extensive OM degradation. These features delineate a clear threshold between an uppermost "new" regenerating peat section and an "old" catotelm peat below. Nevertheless, even in the latter, the sugar contents remain relatively high (>80 mg/g) compared with other organic and mineral soils. Analysis of typical peat-forming plants and of bulk peat and fine grained fractions allowed identification of the following source indicators: xylose and arabinose for Cyperaceae; rhamnose, galactose and mannose for mosses; and ribose (and to a lesser extent, hemicellulosic glucose) possibly for microbial synthesis

Biomarker signatures of former soil uses. A case study from the catchment area of Lake Aydat (Massif Central, France).

International audienceThe 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

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

Données nouvelles sur le contenu organique des dépôts phosphatés du gisement de Ras-Draâ (Tunisie) New data on the organic matter associated to phosphatic ores of the Ras-Draâ deposit (Tunisia)

International audienceL'étude de la matière organique (MO) associée aux sédiments phosphatés a été conduite sur des fractions séparées – grains phosphatés (pellets) et leurs matrices – de strates riches en phosphate du gisement de Ras-Draâ. Cette étude a réuni des analyses géochimiques élémentaires C, N, S, des analyses par pyrolyse Rock-Eval (RE) et des extractions des substances humiques (SH). Les données géochimiques fournies par l'analyse CNS et par la pyrolyse RE indiquent : (i) des teneurs en carbone organique total (COT) plus élevées dans les matrices (atteignant 4,00 %) que dans les pellets (≤ 1,62 % ) ; (ii) la présence, dans les deux fractions, d'une MO se rattachant à la lignée II, planctonique marine, plus ou moins oxydée ; (iii) une faible évolution diagénétique subie par la MO dans chacune des fractions (Tmax, globalement < 430 °C). Les données quantitatives de l'extraction et du fractionnement des SH en acides fulviques, humiques et humine, indiquent une forte abondance de composés humiques extractibles (CH) dans les pellets (CCH not, vert, similar 70 % de la somme du COT des fractions séparées COTfr) et une présence occasionnelle, en faibles quantités, de CH extractibles dans les matrices. Cette situation exclut que ces pellets se soient constitués de façon authigénique au sein de leur matrice, ce qui est pleinement conforme à la possibilité, récemment formulée, qu'il s'agisse de fèces de poissons. Abstract The study of the organic matter (OM) content of phosphatic sediments from the Ras-Draâ deposit, Tunisia, was carried on the two separated lithological fractions constituting the bulk sediments, namely phosphatic grains (pellets) and their associated matrices. The geochemical characterization of the OM present in pellets and in their matrices by CNS elemental analysis and RE pyrolysis indicates that: (i) the TOC content is higher in matrices (where it reaches 4.00%), than in pellets in the same strata where it does not exceed 1.62%; (ii) the presence of more or less oxidized marine planktonic OM in both fractions; (iii) a low degree of diagenetic evolution of the OM in both fractions (RE Tmax globally < than 430 °C). The chemical extraction of the humic substances (HS) from both fractions followed by the separation of fractions according to the IHSS procedure, systematically indicates a higher abundance of extractable humic compounds (HC) in the pellets (CHC not, vert, similar 70% of the sum of TOC in the separated fractions, TOCfr) and a variable but lower extraction yield in matrices (CHC not, vert, similar 18% TOCfr). This significant difference between both fractions excludes the possibility that pellets formed authigenically from, and within, their matrix. This is consistent with recent findings suggesting that these pellets could be fish feces