Jean Trichet

Texture of Microbial Sediments Revealed by Cryo-Scanning Electron Microscopy

ABSTRACT Textures of modern lacustrine stromatolites on Kiritimati (Line Islands, Central Pacific Ocean), and of buried layers in the stromatolitic carbonate sediments from French Polynesian atoll lakes (kopara), have been studied using cryo-scanning electron microscopy (SEM equipped with a freeze-drying sample preparation device). This study confirms that microscopic three-dimensional organic networks built through reorganization of polysaccharide fibers inherited from sheaths of dead cyanobacteria, and from other extracellular polymer secretions, are common components of microbial sediments, of which they may form the framework, In addition to this role in sediment cohesion and formation of microstructure, the organic framework appears to be involved in carbonate precipitation with n the stromatolites, through chemical (nucleating), steric, and hydrodynamical controls. The role of the dead organic constituents of the stromatolites is not only confirmed in micrometer-size crystal precipitation but extended to the post mortem internal mineralization of cyanobacterial filaments, and to the formation of peloids that evolve into spherulites. Bacterial, including nannobacterial, carbonate bodies, and carbonate-impregnated cyanobacterial sheaths are also shown to form in the stromatolites studied. All these carbonate precipitation processes may cooperate in lithification of a given sediment.

Role of the mineral matrix during kerogen pyrolysis

Abstract Much is known about the important interactions existing, during pyrolysis of rocks, between minerals and the hydrocarbon compounds coming from the cracking of organic matter (kerogens). Clay minerals have been shown to be the most active in “retaining” the heaviest hydrocarbon compounds and in promoting their coking as the temperature rises. Under pyrolysis conditions of the Rock-Eval type, this paper investigates the interactions between different pure clay minerals and the hydrocarbons coming from the cracking of an organic matter of marine origin (type II). For this, different methods were used to analyse the volatile compounds issuing from pyrolysis (hydrocarbons, hydrogen, CO2, CO) and the carbon residues coming from the hydrocarbon compounds retained on the minerals. These methods included gas chromatography, mass spectrometry, Raman spectroscopy, electron microscopy and oxidation with temperature programming. The results show that: (i) The amount of hydrocarbons “adsorbed” increases with the specific surface area of the argillaceous minerals investigated. (ii) Montmorillonite of type Ca2+ (Camp Berteaux montmorillonite) reveals “catalytic” activity which enhances the formation of light hydrocarbons, with a predominance of aromatics to the detriment of the heaviest adsorbed compounds. In this case, the carbon residue formed on the mineral locally reveals the appearance of “pre-graphitic” structures. (iii) The same is not true for other minerals, such as illite and attapulgite. Despite their high retention power, such minerals do not show any catalytic activity, and the carbon residues they form seem to have a less polycondensed structure (easier combustion of these residues, higher H/C ratio).

Role of a cyanobacterial cover on structural stability of sandy soils in the Sahelian part of western Niger

Microbiotic soil crusts, mostly formed by cyanobacteria, are widespread on the surface of fallow land in western Niger. They lie adjacent to completely bare soils. We have investigated the structural stability of these soils by testing aggregate breakdown under fast-wetting, slow-wetting and mechanical breakdown. The tests were effected on aggregates ranging from 3 to 5 mm in size. These experiments were completed by micromorphological examinations under light and scanning electron microscope. For all tests, the fragment size distribution and the mean weight diameter (MWD) revealed the great stability of aggregates from soils with a cyanobacterial cover (MWD 1.82 to 3.10 mm), compared to those from soils devoid of cyanobacterial cover (MWD 0.25 to 1.26 mm). Fast- and slow-wetting of microbiotic soil crust material induced a lesser disaggregation of aggregates compared to mechanical breakdown. On the contrary, fast-wetting and mechanical breakdown of aggregates from soils devoid of cyanobacterial cover induced a greater disaggregation than slow-wetting. Microscopic examination of microbiotic crusts revealed an intricate network of filamentous cyanobacteria and extracellular polymer secretions, which binds and entraps mineral particles on the soil surface. Organo-mineral aggregates ranging from 0.10 to 0.12 mm were observed. Below the superficial crusts, filaments and residual organic matter bind tightly soil particles, thus giving a compact structure. The great stability of aggregates of soil with cyanobacterial cover is likely related to the binding and gluing effect of cyanobacteria and derived organic matter. This is consistent with the positive correlation between MWD values and organic carbon content. The present results thereby confirm the resistance to erosion of soil with microbial cover as indicated by field measurements.

Christmas Island lagoonal lakes, models for the deposition of carbonate{evaporite{organic laminated sediments

The atoll of Christmas Island (now known as Kiritimati) in the Kiribati Republic (Central Pacific) lies at about 28N in the intertropical convergence zone. Much of the surface area of the atoll (ca. 360 km 2 ) is occupied by numerous lakes in which carbonate, evaporite (calcium sulfate, halite) and organic layers are deposited. Observations suggest that deposition of these different laminae is controlled by climatic and biologic factors. It is thought that periodic climatic variations, such as El Nino- Southern Oscillations (ENSO) events which bring heavy rainfall to the atoll, result in the succession of the precipitation of carbonate minerals (during periods after dilution of hypersaline waters by heavy rains), followed by evaporitic minerals (carbonate, calcium sulfate, halite) when salinity increases through evaporation. Thick (up to 5 cm) microbial (essentially cyanobacterial) mats develop continuously on the lake bottom surfaces providing the sediment with an important (total organic carbon 2-5%) organic contribution in the form of an internal, geometrically structured, network in which the authigenic minerals precipitate. The high bioproductivity of these microbial populations is reflected in low d 13 C values of sedimentary organic carbon (214 to 217‰), interpreted as being the result of high atmospheric CO2 demand (Geochim. Cosmochim. Acta, 56 (1992) 335). The well-laminated organic layers present in the sediment profile result from the death and burial of microbial populations at the time of severe climatic events (storms, heavy rainfall). These lagoonal lakes provide a model for the deposition of carbonate and organic matter in an evaporitic environment. The high ratio of deposited carbonate vs. sulfate 1 chloride, when compared to low ratio in evaporitic salinas, results from both a lack of limitation of calcium, magnesium and carbonate ions (in a carbonate reef environment) and active processes of high-Mg calcite precipitation (organomineralization). q 2001 Elsevier Science B.V. All rights reserved.

Kopara in Polynesian atolls: early stages of formation of calcareous stromatolites

Widespread modern calcareous stromatolites known as kopara in the vernacular language on the Tuamotu Archipelago have been found accreting on the bottom of shallow (generally < 1 m depth) lakes located on the rims of these French Polynesian atolls. They are flat, gelatinous sediments, several tens of centimetres thick, with lamination comprised of vertically alternating red organic-rich, and white carbonate-rich, millimetre-thick laminae. They originate from benthic microbial communities mainly composed of cyanobacteria, generally dominated by the genus Phormidium. The living cyanobacteria occupy only the top 1–3 cm of the deposit, the red organic matter below being made of remnants of the dead microbes, essentially of polysaccharide microfibrils inherited from the sheaths and arranged into a three-dimensional network enclosing pores ranging from a few tens of angstroms to a few micrometres wide. The carbonates are predominantly high-Mg calcite (9–19 mole% MgCO3) which precipitated as micron-size bunches, within the pores, on walls of the organic network. It is sometimes accompanied by aragonite (less than 22% of the total mineral fraction when present), and lesser high-Mg calcite, allochthonous bioclasts, which occasionally form up to few centimetres-thick detrital intercalations. The pore waters are variable mixtures of freshwater (rain or ground water) and sea water from the ocean or lagoon which are feeding waters of the lakes. Their chemistry deviate from a straight mixture gradient with respect to Ca2+, Mg2+, and alkalinity under the influence of local calcification and bacterial processes (e.g. ammonia production, sulphate reduction) within the sediments. It is hypothesized that the calcification is initiated at carboxylic sites on the walls of the polysaccharide network, the pores acting as confining organic compartments with increased internal supersaturation. Lamination is suggested to be due to the combined effects of the stratification of the microbial activity of the deposits, and alternations of fresh and saline periods within the lakes. Lacustrine environments on oceanic atolls are found to be important habitats of modern microbialite formation.

Photosynthesis versus Exopolymer Degradation in the Formation of Microbialites on the Atoll of Kiritimati, Republic of Kiribati, Central Pacific

Aragonitic microbialites, characterized by a reticulate fabric, were discovered beneath lacustrine microbial mats on the atoll of Kiritimati, Republic of Kiribati, Central Pacific. The microbial mats, with cyanobacteria as major primary producers, grow in evaporated seawater modified by calcium carbonate and gypsum precipitation and calcium influx via surface and/or groundwaters. Despite the high aragonite supersaturation and a high photosynthetic activity, only minor aragonite precipitates are observed in the top parts of the microbial mats. Instead, major aragonite precipitation takes place in lower mat parts at the transition to the anoxic zone. The prokaryotic community shows a high number of phylotypes closely related to halotolerant taxa and/or taxa with preference to oligotrophic habitats. Soil- and plant- inhabiting bacteria underline a potential surface or subsurface influx from terrestrial areas, while chitinase-producing representatives coincide with the occurrence of insect remains in the mats. Strikingly, many of the clones have their closest relatives in microorganisms either involved in methane production or consumption of methane or methyl compounds. Methanogens, represented by the methylotrophic genus Methanohalophilus, appear to be one of the dominant organisms in anaerobic mat parts. All this points to a significant role of methane and methyl components in the carbon cycle of the mats. Nonetheless, thin sections and physicochemical gradients through the mats, as well as the 12C-depleted carbon isotope signatures of carbonates indicate that spherulitic components of the microbialites initiate in the photosynthesis-dominated orange mat top layer, and further grow in the green and purple layer below. Therefore, these spherulites are considered as product of an extraordinary high photosynthesis effect simultaneous to a high inhibition by pristine exopolymers. Then, successive heterotrophic bacterial activity leads to a condensation of the exopolymer framework, and finally to the formation of crevice-like zones of partly degraded exopolymers. Here initiation of horizontal aragonite layers and vertical aragonite sheets of the microbialite occurs, which are considered as a product of high photosynthesis at decreasing degree of inhibition. Finally, at low supersaturation and almost lack of inhibition, syntaxial growth of aragonite crystals at lamellae surfaces leads to thin fibrous aragonite veneers. While sulfate reduction, methylotrophy, methanogenesis and ammonification play an important role in element cycling of the mat, there is currently no evidence for a crucial role of them in CaCO3 precipitation. Instead, photosynthesis and exopolymer degradation sufficiently explain the observed pattern and fabric of microbialite formation.

Nitrogen fixation by microbial crusts from desiccated Sahelian soils (Niger)

Cyanobacterial crusts developing on the sandy and loamy soils of fallow lands in the Sahel (Niger) were investigated for their potential to fix nitrogen. Three sites were selected in this arid environment, differing in sediment type and species composition. In the sandy sites heterocystous nitrogen-fixing cyanobacteria were present, whereas the loamy site did not contain such species. All sites showed light-dependent nitrogenase activity, starting within 2 h after re-wetting of the desiccated crust samples. Inhibition of photosystem II caused a decrease of nitrogenase activity in the samples with heterocystous cyanobacteria, but was stimulatory in the non-heterocystous crust. The results suggest that cyanobacterial crusts may be important for the improvement of the soil by enriching it with nitrogen.

Interaction of uranium and organic matter in uraniferous sediments

Abstract Conventional transmission electron microscopy (lattice fringes and dark field techniques) was used for determining the structure and microtexture of some Precambrian organic matter. The samples came from Cluff (Saskatchewan, Canada) and Oklo (Gabon) and contain uranium with organo-metallic bonding (uranium was shown to be present by energy dispersive X-ray analysis carried out in the C.T.E.M.). Despite their algal origin, these materials show a high oxygen content. This strong degree of oxidation inhibits the parallel molecular orientation usually produced in carbonaceous products as coalification progresses. Progressive heat-treatment to 3000°C produces microporous carbon (50–100 A). It is, however, partially transformed into graphite in a manner similar to anthracites and non-graphitizable carbons heat-treated under pressure (5 kbars). It is favoured by pore flattening, due to pressure, which introduces a long-range, preferred orientation parallel to the flattening plane. Conversely, it is partially prevented by cross-linking due to oxygen. Comparison with materials of higher plant origin (e.g. from Arlit, Niger) suggests a possible mechanism of uranium fixation.

Relationship between Zn–Pb ore and oil accumulation processes: Example of the Bou Grine deposit (Tunisia)

Abstract The “Bahloul formation” of Cenomanian-Turonian age combines qualities of a potential source rock for oil and of a rich lead-zinc mineralization in the Bou Grine area, on the border of the Lorbeus massif diapir (Tunisian Atlas). Analysis of 46 core samples from 7 boreholes drilled within or outside the mine area revealed the richness in organic matter of the Bahloul formation (∼ 4–5% TOC), its low maturity ( T max ∼ 423°C) and marine planktonic origin (HI ∼ 600 mg HC g −1 TOC). Abnormally high values of the Oil Production Index (up to about 30%) indicate the presence of migrated oil in the mine and in samples from the northeast boreholes. This information is confirmed by other observations: high relative amounts of hydrocarbons in the bitumen extracts, high Pr/Ph ratios and high amounts of light naphthenic hydrocarbons. The abundance of the latter components observed in chromatograms where n -alkanes are nearly absent may indicate the development of a biodegradation process during migration. However, this biological activity may have exerted the greatest intensity at the expense of indigenous organic constituents, in the mine area, where considerable amounts of hopanes and steranes were observed. All these data supported by geological observations allow us to propose the following genetic model for the Bou Grine deposit. The Bahloul formation which associates high amounts of organic matter, zinc and lead on a regional scale (Zn + Pb ∼ 0.5–2%) and a high porosity, could have served as a source for the migrated oil and for the metals necessary to the mineralization, as well as a conduit for the oil and metal-bearing brines. An intense biological activity developed at the expense of the organic content of the “Bahloul formation” and of the sulfate from the adjacent Triassic diapir may have produced the hydrogen sulfide responsible for ore deposition.

On the appearance of cyanobacterial calcification in modern stromatolites

Abstract In August 1991 calcified (magnesian calcite) filamentous cyanobacteria ( Scytonema mirabile ) appeared on the surface of a modern lacustrine stromatolite (Lake R2, Rangiroa Atoll, Tuamotu Archipelago, French Polynesia). Previously, the topmost photosynthetic layer of this sediment was devoid of such microbial biomineralisers. The calcified cyanobacteria are accompanied by magnesian calcite grains which are evocative of bacterial precipitates. Although the appearance of cyanobacterial calcification might be related to environmental changes in the lake (in particular a decrease in water salinity), the persistence of non-calcified specimens of a filamentous cyanobacteria ( Phormidium crossbyanum ), which was once a dominant species of the lake benthos, shows that calcification and environmental conditions might be only indirectly related. Thus, attempts to use the fossil record of calcareous cyanobacterial skeletons to infer changes in the chemical composition of past oceans cannot be made without due regard of biological processes like the appearance of new competitors.