Human impact and soil erosion during the last 5000 yrs as recorded in lacustrine sedimentary organic matter at Lac d'Annecy, the French Alps

Sedimentary organic matter has been systematically studied in an eight-metre long core from the centre of the Petit Lac (Annecy, French Alps). The palynofacies composition identifies different terrestrial organic sources including forest floors, soil-horizons and geological substratum. The amount of recycled organic matter derived from the geological substratum is estimated and subtracted from the other contributions from the catchment area. The palynological record indicates that the relative variations in organic sources are directly dependent on human land-use. From ca. 5000 to 1700 BP, the human impact on soil cohesion is very low and organic matter is mainly exported from the surficial forest floor. The Roman invasion (ca. 1700 BP) marks the most important ecological and hydrological change. From 1700 to 900 BP, the clearing of forests released deeper-soil components. This trend is increased after 900 BP with agriculture intensification, which resulted in a higher sedimentation rate. In all the periods, extreme events such as flood or intensive run-off are characterised by notable increases of organic matter from surface (5000 to 1700 BP) and deep (1700 BP to now) soils.     

Évolution thermique des séries sédimentaires de la Marge ardéchoise : étude pétrographique de la matière organique et des argilesThermal evolution of the sedimentary series of the Ardèche Margin: petrological studies of organic matter and clays

Two wells, Balazuc (BA1) and Morte-Mérie (MM1), located in a confined area (1200 m apart) and separated by the Uzer fault (a Liassic structure with a dip fault of 1300 m) were analysed using conventional methodologies and techniques (PRV, TAI, XRD, STEM) in order to compare the diagenetic evolution of clays and organic matter. The thermal convective process allows the circulation of hot fluids and the oxidation of organic matter. The conductive process allows the maturation of the organic matter, the expulsion of hydrocarbons and the deposit of pyrobitumes in the migration channels. To cite this article: L. Martinez et al., C. R. Geoscience 334 (2002) 1021–1028.     

Granulitic metamorphism in the Laouni terrane (Central Hoggar, Tuareg Shield, Algeria)

In the Laouni terrane, which belongs to the polycyclic Central Hoggar domain, various areas contain outcrops of formations showing granulite-facies parageneses. This high-temperature metamorphism was accompanied by migmatization and the emplacement of two types of magmatic suite, one of continental affinity (garnet pyroxenites and granulites with orthoferrossilite–fayalite–quartz), and the other of arc affinity (layered metanorites). Paragenetic, thermobarometric and fluid-inclusion studies of the migmatitic metapelites and metabasites make it possible to reconstruct the P–T–aH₂O path undergone by these formations. This path is clockwise in the three studied areas, being characterized by a major decompression (Tamanrasset: 10.5 kbar at 825 °C to 6 kbar at 700 °C; Tidjenouine: 7.5 kbar at 875 °C; to 3.5 kbar at 700 °C; Tin Begane: 13.5 kbar at 850 °C; to 5 kbar at 720 °C), followed by amphibolitization that corresponds to a fall of temperature (from 700 to 600 °C) and an increase in water activity (from 0.2–0.4 to almost 1).The main observed features are in favour of petrogenesis and exhumation related to the Eburnean orogeny. However, the lacks of good-quality dating work and a comparison with juvenile Pan-African formations having undergone high-pressure metamorphism, in some cases reaching the eclogite facies, do not rule out the possibility that high-temperature parageneses are locally due to Pan-African events.     

Exploration for concealed orebodies by the analysis of volatile organic compounds contained in surface rocks: Trèves Zn-Pb deposit (Gard, France)

A previous study (Disnar et al., 1986a) indicated significant volatile hydrocarbon compounds (nC₁ to C₅) in mineralized carbonate rocks of the Trèves Pb-Zn deposit (Gard Dept., France) as well as an accumulation of methane, ethylene and ethane in the hanging wall of the deposit. The present study analyzed rock samples taken from outcrops vertically above and at a distance from the deposit and identified three anomalous zones indicating the presence of concealed mineralization. The first zone is due to methane (radius about 3 km), the second, to alkenes (about 2 km) and the third, to alkanes, higher homologues of methane (about 1 km). These anomalies are interpreted as resulting from migration of these compounds through the sedimentary cover from the deposit where they could have been generated during mineralizing events, or during subsequent diagenesis. Their specific signature with regard to that given by samples taken at a distance from the orebodies can be attributed to the hydrothermal and biological processes responsible for the deposition of the ore.     

Evolution of organic matter indicators in response to major environmental changes: The case of a formerly cut-over peat bog (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.     

Effects of short-term ecosystem experimental warming on water-extractable organic matter in an ombrotrophic Sphagnum peatland (Le Forbonnet, France)

In a future warmer world, peatlands may change from a carbon sink function to a carbon source function. This study tracks changes in water-extractable organic matter (WEOM) after 1 year of in situ experimental warming using open top chambers (OTCs). WEOM was studied in the upper peat layers (0–10 cm) through analysis of water-extractable organic carbon (WEOC), stable C isotopic composition (δ¹³C), specific UV absorbance at 280 nm and sugar composition of cores taken from an open bog (DRY sites) and a transitional poor fen (WET sites). At the DRY sites, the impact of OTCs was weak with respect to WEOM parameters, whereas at the WET sites, the air warming treatment led to a decrease in peat water content, suggesting that the supply of heat by OTCs was used mainly for evapotranspiration. OTCs at the WET sites also induced a relative enrichment at the surface (0–5 cm depth) of aliphatic and/or aromatic compounds with concomitant decrease in WEOC, as a result of decomposition. On the contrary, WEOC and sugar content increased in the deeper peat layer (7.5–10 cm depth) probably as a result of increased leaching of phenolic compounds by roots, which then inhibits microbial activity. The different response to experimental warming at DRY and WET sites suggests that the spatial variability of moisture is critical for understanding of the impact of global warming on the fate of OM and the carbon cycle in peatlands.     

Occurrence of triterpenyl acetates in soil and their potential as chemotaxonomical markers of Asteraceae

As a part of a wider study aimed at determining new molecular biomarkers in soils and sediments that could be specific for distinct vegetation types with respect to unravelling past changes in land use, we analysed the neutral lipid content of soil developed in the catchment of a small lake in central France. The ketone/acetate lipid fraction of soil under pasture or meadow contained a series of pentacyclic triterpenyl acetates with wide structural diversity, most being reported in soil for the first time. The restricted number of potential plant sources of triterpenyl acetates (some produced by a single species) underlines the potential of triterpenyl acetates as new biomarkers to track past vegetation change in palaeoenvironmental studies, when found in natural archives such as soil, sediments or peat.     

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

Paléocène-Early Eocene phosphate outcrop in the Ras-Draâ deposit (Tunisia) comprises alternances of P-rich strata (P₂O₅ ≥ 18%) and P-poor ones (P₂O₅ ≤ 2%). In phosphate rich strata, P is concentrated in rounded grains—so-called pellets—(28% ≤ P₂O₅ ≤ 38%) embedded in a matrix—so-called exogangue—much poorer in P (P₂O₅ # 7%) than pellets. The study of pellets (whose size lies mainly between 100 and 500 μm), of their surrounding matrix and of poorly phosphatic sediments, interlayered between the phosphatic strata, has been performed by optical microscopy and various chemical analyses, ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectroscopy) and punctual chemical analyses by EDS (Energy Dispersive Spectroscopy). Mineralogical and chemical studies of the Tunisian phosphatic pellets show that the basic mineralogical component of these grains is carbonate-fluor-apatite, or francolite. The structural formula of a representative specimen of francolite in the basin is as follows: (Ca_4.63Mg_0.13Na_0.22)(PO₄)_2.51(CO₃)_0.48(OH_0.77F_0.23). The mineralogy of the constituents of the sediment surrounding pellets comprises carbonates, clays, silica (opal, quartz). Microscopic observations of Ras-Draâ phosphorites, added to geochemical results, establish that pellets are small bodies, allochtonous to their matrix and whose composition is independent of that of this matrix. Fecal cylindrical bodies and subspherical pellets, the latest being supposed to proceed from the fecal bodies by fragmentation, are considered to have been formed by fishes, as P-rich and organic matter-rich faeces. These faeces turn out to be resistant to mechanical dispersion and to chemical diffusion of soluble elements from and in direction of sea water. This closeness is responsible for the strong reducing conditions having prevailed inside and during the pellets diagenesis.