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

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

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

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

Micromorphological and (bio)chemical organic matter changes in a formerly cutover peat bog : Le Russey, Jura Mountains, France.

Peatlands have been exploited since the Middle Ages for diverse uses, including fuel, animal bedding and growth substrate in horticulture and agriculture. In many countries, these traditional peatland uses are now redundant, but the sites are being reappraised for their specific biodiversity and suitability for long-term carbon storage

In situ thermochemical sulfate reduction during ore formation at the Itxaspe Zn-(Pb) MVT occurrence (Basque-Cantabrian basin, Northern Spain)

International audienceOrganic matter is thought to play a role in the genesis of many Mississippi Valley-type (MVT) deposits, acting as a reducing agent during thermochemical sulfate reduction (TSR). Although TSR is an extremely slow reaction at low temperatures (<100ºC), under favorable conditions it may supply the necessary reduced sulfur during ore formation. To test this hypothesis, the Itxaspe Zn-(Pb) MVT occurrence in the Basque-Cantabrian basin (Northern Spain) was studied. Sphalerite, the main ore phase, is generally found disseminated in Urgonian (Lower Cretaceous) carbonates, and in close relationship with solid bitumen. The bitumen source rock was very likely deposited in a marine marginal setting. Differences in composition of the bitumen samples are attributed to a fractionation during hydrocarbon expulsion and/or migration. The fluids involved in ore deposition were low temperature (Th ~130ºC), Na-Ca-Cl-(K-Mg)-type brines (salinities ~12.5 equiv. mass % NaCl). The origin of brine solutes (including sulfate) is related to the dissolution of Mesozoic evaporite units, although the contribution of evaporated seawater brines cannot be ruled out. The temperatures of ore deposition, the close relationship between the bitumen and ore phases, the presence of aromatic sulfur-bearing compounds and the 34S of sulfides and sulfates are consistent with an in situ TSR during ore formation in the Itxaspe Zn-(Pb) occurrence. Therefore, at least for small mineralizations like Itxaspe, our conclusion is that the necessary reduced sulfur can be supplied by TSR during ore genesis at the site of metal deposition

Organic matter accumulation and preservation controls in a deep sea modern environment: an example from Namibian slope sediments.

The Lüderitz upwelling cell is presently the most productive area of the Benguela current system and abundant organic matter (OM) accumulates on the adjacent slope sediments even at great water depth. OM from two cores taken on the slope and covering the last 280 kyear was analysed in terms of "petroleum quality" (Rock-Eval), chemical features (FTIR, EDS) and petrographic composition (light microscopy and TEM). These data indicate that the OM is more oxidized at 3606 m water depth than on the upper slope sediments (1029 m) although the petroleum quality of the OM throughout the deep-water core remains surprisingly high for hemipelagic deep-sea sediments (HI=200–400 mg/g). The petroleum quality of OM accumulated on the upper slope is consistently high: HI averages 450 mg/g. Two petrographic types of OM are distinguishable from microscopic observation, each ascribed to distinctive preservation mechanisms: (1) ‘Granular' amorphous OM, which dominates in the deep-water core, is formed by organo-mineral aggregates. Aggregation appears to be the primary preservation mode at this depth although is quantitatively limited (maximum TOC value of 4 wt.% of bulk sediment obtained through this process). The ultrastructure of the aggregates highlights an intimate association pattern between sedimentary OM and clays. (2) ‘Gel-like' nannoscopically amorphous OM (NAOM) largely dominates at 1000 m water depth and contains sulfur. Thus, early diagenetic sulfurization was probably involved in the preservation of this OM, but a contribution from the classical degradation–recondensation pathway cannot be ruled out. Moreover, selective preservation occurred at both sites but represents an insignificant part of the OM. Organic fluxes mainly control the occurrence and extent of sulfurisation at both water depths by determining the redox conditions at the sea floor. Aggregate formation is limited by both organic and mineral fluxes at the lower slope whereas OM supply is never limiting on the upper slope. Although consistently operating through time at both depths, preservation by organo-mineral association is limited by mineral availability and thus accounts for a relatively minor portion of the OM accumulated on this organic-rich slope. In the case of large organic fluxes, sulfurisation and/or degradation–recondensation is required to obtain TOC contents above 4 wt.% of bulk sediment in the area

Solubility of platinum in aqueous solutions at 25°C and pHs 4 to 10 under oxidizing conditions

Platinum has been found to be mobile under supergene conditions, including placers and weathering profiles. To elucidate the nature of Pt mobility in supergene environments, the dissolution of platinum metal was investigated under physicochemical conditions similar to those found in such environments. The solubility of platinum metal was measured at 25°C in several systems: Pt-K-HC8H4O4-H2O (pH 4.02), Pt-Na-HCO3-Cl-H2O (pH 6.40), Pt-Na-K-H2PO4-HPO4-H2O (pH 6.90), Pt-Na-HCO3-H2O (pH 8.30), Pt-Na-OH-H2O (pH 8.54), and Pt-Na-HCO3-H2O (pH 9.91). The redox conditions of these experiments were relatively oxidizing, with measured Eh values ranging from +280 to +590 mV. The ionic strength of the aqueous solutions did not exceed 0.30 (molal scale). The interpretation of the solubility results, in terms of the following reaction and its equilibrium constant, Pt(s)+nH2O left right double arrow Pt(OH)2−nn+nH++2e− served to identify the importance of the hydroxylated complex PtOH+ in the pH range (4 to 10) and to determine its stability constant at 25°C. Linear regression of the solubility data using the function log [Pt]−2pe=n pH+log Kn yielded a value of 1.01 ± 0.07 for n, the average ligand number, and −31.76 ± 0.55 for the thermodynamic equilibrium constant of reaction. The resulting stability constant (β1) of PtOH+ (Pt2+ + OH− left right double arrow PtOH+) is 24.91 ± 0.50, assuming the same value of the free energy of formation of Pt2+, ΔGfo(Pt2+) as that given by Glushko et al. (Thermodynamic Constants of Matter, Academy of Science, USSR, 1972). The range of values of ΔGfo(Pt2+) proposed to date is +185.63 to +258.74 kJ/mol. The value of Glushko et al. (1972) (+244.11 kJ/mol) appears to fit better with our measurements at pH 4 to 10 and with those of Wood (Wood S. A., “Experimental determination of the hydrolysis constants of Pt2+ and Pd2+ at 25°C from the solubility of Pt and Pd in aqueous hydroxide solutions,” Geochim. Cosmochim. Acta 55, 1759–1767, 1991) at pH 9 to 15.5. Finally, according to these new measurements of the solubility of platinum, the recommended values for Gibbs free energy (ΔGfo, in kiloJoule per mole) of the different aqueous species of Pt are +244.11 (Pt2+), −55.96 (PtOH+), and −234.48 (Pt(OH)2(aq)). The integration of data from the literature for chloride and sulfate complexes was used to calculate the speciation of platinum in seawater and in solutions with variable chlorinity (0.1, 1, and 3 mol/L NaCl) at 25°C. These calculations showed that in the absence of strong ligands (i.e., S2O32−, CN−), the transport of platinum in supergene environments primarily occurs in the form of PtOH+ (90%) and Pt(OH)2(aq) (9.7%). Chloride complexes (PtCl42− and PtCl3−) account for less than 1% of the dissolved platinum. This study clearly shows that the hydroxylated complexes (PtOH+ and Pt(OH)2(aq)) can play a very important role in controlling platinum transfer mechanisms in surface fluids (soils, placers, weathering profiles, etc.). Because the charged species PtOH+ is largely predominant, the mobility and transfer of platinum can also be affected by adsorption-desorption mechanisms onto oxides and hydroxides

Distribution of the organic matter in the channel-levees systems of the Congo mud-rich deep-sea fan (West Africa). Implication for deep offshore petroleum source rocks and global carbon cycle

International audienceThe Corinth Rift is superimposed on the Hellenic nappe stack that formed at the expense of the Apulian continental crust above the subducting African slab. Extension started in the Pliocene and the major steep normal faults that control the geometry of the present-day rift were born very recently, some 600 kyr ago only. They root into a shallow-dipping zone of microseismicity recorded near the base of the upper crust. The significance of this seismogenic zone is debated. Considering the northward dip of the zone of microseismicity, the depth of microearthquakes and their focal mechanisms, we observe a strong similarity with the northern Cycladic detachments in terms of expected pressure, temperature conditions and kinematics. We herein show (1) that the formation of the Corinth Rift can be considered a part of a continuum of extension that started some 30–35 Ma in the Aegean and that was recently localised in a more restricted area, (2) that the present-day structure and kinematics of the Corinth Rift can be explained with a series of decollements relayed by steeper ramps that altogether formed a mechanically weak, crustal-scale detachment, and (3) that the deformation, fluid behaviour and metamorphic features seen in the northern Cycladic metamorphic core complexes can be good analogues of the processes at work below the Corinth Rift

The fate of organic matter in mangrove sediments subject to variable environmental conditions (French Guiana).

International audienceDuring the four last years, a National research Programme for Coastal Environment (PNEC) was partly devoted to the littoral of French Guiana. Several field trips allowed to sample twinned sedimentary cores from both the mangroves and the unvegetated sediments, and various vascular plants specific to these swamps. An elemental, pyrolytic and optical study of total sedimentary organic matter (OM), completed with stable isotope and molecular analyses, was carried out on these sets of samples and coupled with a study of pore waters (physico-chemical properties and dissolved organic content) collected in the mean time, during contrasted seasons. The development and age of mangrove marshes and forests was assessed by parallel studies performed by ecologists