Alain Préat

Carbon isotope excursions and sea-level change: implications for the Frasnian–Famennian biotic crisis

New carbon and oxygen isotope data from carbonates spanning the Frasnian–Famennian (F–F) boundary in the Guilin area, South China, show a broad positive δ13C rise and fall, with sharp, short-lived negative δ13C events; this pattern is comparable to that in Europe and North America. The integration of the isotope stratigraphy with high-resolution sequence stratigraphy corroborates the onset of the positive δ13C excursion during a third-order sea-level fall in the latest Frasnian. This can best be explained through increased burial of Corg during the sea-level fall, brought about by increased organic productivity caused by increased continent-derived nutrient flux to the ocean due to enhanced weathering through the proliferation of land plants in the Devonian. This scenario would have resulted in anoxic and eutrophic conditions over epicontinental seas and blooms of cyanobacteria, creating a highly stressful and fragile ecosystem for oligotrophic normal-marine benthic organisms and leading to their massive decline. The global third-order sea-level fall near the end of the Frasnian may have led to gas hydrate dissociation (giving the negative δ13C events), and caused wild climatic fluctuations. The subsequent short-term events of sea-level rise, anoxia and eutrophication in the latest part of the F–F transition would have placed additional environmental stresses on the already weakened biota, leading to their further demise.

Iron microbial communities in Belgian Frasnian carbonate mounds

SummaryThe Belgian Frasnian carbonate mounds occur in three stratigraphic levels in an overall backstepping succession. Petit-Mont and Arche Members form the famous red and grey “marble” exploited for ornamental stone since Roman times. The evolution and distribution of the facies in the mounds is thought to be associated with ecologic evolution and relative sea-level fluctuations. Iron oxides exist in five forms in the Frasnian mounds; four are undoubtedly endobiotic organized structures: (1) microstromatolites and associated forms (blisters, veils...), possibly organized in “endostromatolites”; (2) hematitic coccoids and (3) non dichotomic filaments. The filaments resemble iron bacteria of theSphaerotilus-Leptothrix “group”; (4) networks of dichotomic filaments ascribable to fungi; (5) a red ferruginous pigment dispersed in the calcareous matrix whose distribution is related to the mound facies type. The endobiotic forms developed during the edification of the mounds, before cementation by fibrous calcite. The microbial precipitation of iron took place as long as the developing mounds were bathed by water impoverished in oxygen.

Sur l’origine bactérienne et fongique de la pigmentation de l’Ammonitico Rosso (Jurassique, région de Vérone, Italie du nord)

Resume L’etude de 14 coupes de l’Ammonitico Rosso Veronese (Callovien a Tithonique) du plateau de Trente montre une tres grande variete de facies allant du pelagique a la plate-forme. Malgre cette grande diversite dans les bathymetries, les facies rouges s’observent a plusieurs niveaux. Les microfacies sont fort semblables a ceux observes dans d’autres calcaires rouges du Paleozoique et Mesozoique avec une abondance de bioconstructions hematitiques. Nous pensons donc que l’origine de la pigmentation de ces facies est semblable et l’attribuons a la presence de ferrobacteries. Deux differences notables sont toutefois observees : la presence ici de manganese en quantite non negligeable et l’existence de mattes bacteriennes et de Fungi imperfecti in situ preserves dans les matrices. Ces mattes representeraient un volume appreciable, voisin de 20 %. Leur excellent etat de preservation serait lie au faible taux de sedimentation, tant des facies pelagiques que des niveaux de hardgrounds.

Bacterial origin of the red pigmentation in the Devonian Slivenec Limestone, Czech Republic

SummaryThe deep-red lenses of the Pragian Slivenec Limestone have been extensively quarried for ornamental purposes since the XIIth century. Petrographic microscope observations indicate that the hematite stainings of the limestone follow ten different patterns. They range from massive non-directional filling of cavities to mineralized films and microstromatolites. Numerous ironrich endolithes are observed. Some could be derived from bacterial or lichen perforations and some related to ferric bacteria. Infiltration along welded calcite crystals, regular mineralized films and microstromatolites suggest a ferric bacterial origin for the pigment. This is confirmed by scanning microscope examinations of polished sections, that show hematite concentrations along micrometric filamentous sheaths.

Microfacies, sequence stratigraphy and clay mineralogy of a condensed deep-water section around the Frasnian/Famennian boundary (Steinbruch Schmidt, Germany)

Abstract A multidisciplinary analysis (microfacies, sequential stratigraphy and clay mineralogy) was made on Frasnian/Famennian (F/F) boundary strata of the Steinbruch Schmidt section in Western Germany. Three major microfacies are recognised. Their succession records a shallowing-upward evolution from deep, quiet and poorly oxygenated environments, below the storm wave base, to environments influenced by current activities close to the storm wave base. The Kellwasser Horizons correspond to the deepest microfacies. The shallowest microfacies correspond to fine-grained calcareous tempestites or turbidites coming from a distant shelf of northwest Germany. The sequential pattern through the F/F boundary shows the succession of seven systems tracts. Two sequence boundaries are located just above the Lower Kellwasser Horizon and at the F/F boundary itself. These are underlined by hardgrounds suggesting time gaps. The Kellwasser Horizons correspond to sea-level highstands and the overlying beds record a transition from lowstand to transgressive systems tracts. Illite and kaolinite are the dominant clay minerals associated with mixed layers and traces of chlorite. Illite abundance is maximal during Kellwasser Horizons. Illite and kaolinite were probably inherited from a highly weathered source area although part of the illite is diagenetic. Kaolinite is the second most abundant clay mineral and is particularly well represented (up to 50%) between the Kellwasser Horizons. An unusual clay assemblage of illite and mixed layers is associated with a bentonite layer. Kaolinite increases during times when thin tempestites or turbiditic microbioclastic layers come from a distant shelf during sea-level falls. The kaolinite percentage reaches its maximum at the top of the lowstand systems tract. The high percentage of kaolinite suggests a hot–wet climate and could be related to global warming.

Chapter 13 Neoproterozoic sequences of the West Congo and Lindi/Ubangi Supergroups in the Congo Craton, Central Africa

Abstract The focus of this chapter is the West Congo Supergroup in the West Congo Belt (WCB), which extends along the western margin of the Congo Craton from Gabon in the north to northern Angola in the south, and the Lindi/Ubangi Supergroup of the Lindian and Fouroumbala – Bakouma Basins exposed on the northern margin of the craton. In both regions, up to two distinct diamictite horizons have been recognized, the younger of which is often associated with carbonate rocks. Geochronological constraints are generally rather poor, many of the deposits lack modern sedimentological analysis, and the glacial versus non-glacial genesis of the diamictites is a matter of debate in the literature. However, recent studies suggest a periglacial influence of diamictite deposition, particularly for the sequences in the WCB. The stratigraphy of the various basins is described, available geochemical and geochronological information collated, and recent work regarding the periglacial nature of the diamictites discussed. Finally, an updated chronostratigraphic correlation between the basins is presented. However, much more work is required, particularly in the Neoproterozoic basins on the northern margin of the Congo Craton, and more accurate geochronological constraints are required before the Neoproterozoic palaeogeography and depositional environments of the western and northern Congo Craton can be fully understood.

Palaeozoic clay mineral sedimentation and diagenesis in the Dinant and Avesnes Basins (Belgium, France): relationships with Variscan tectonism

Abstract Clay mineral investigations have been performed on more than 500 limestones and shales sampled in Lower Devonian (Emsian) to Lower Carboniferous (Namurian) outcrops in the Dinant and Avesnes Basins (Ardenne Massif, NW Europe). Clay mineral data have been placed in the palaeoenvironmental and structural histories documented by previous lithological, stratigraphical, palaeontological, diagenetic and tectonic contexts. The clay associations are dominated by illite and chlorite derived partly from the erosion of land masses surrounding the marine domain. The geothermal gradient estimated from correlation with conodont colour alteration index ranges between 40 and 70°C/km. A diachronous northwards migration of the diagenesis/metamorphism interface links to uplift caused by Late Carboniferous compressional folding and overthrusting. Associated clay minerals include smectite, locally preserved from diagenetic changes mainly by early pore closure, that reflect lagoonal or quiet offshore marine conditions. Smectite and subordinate kaolinite abundances decrease upwards during the Devonian in three successive intervals suggesting alternations of sub-arid to drier climates. The local occurrence of corrensite (ordered chlorite–smectite mixed-layer) is attributed to the moderate diagenetic transformation of pre-existing smectite

Iron Oxidation and Deposition in the Biofilm Covering Montacuta ferruginosa (Mollusca, Bivalvia)

The shell of the bivalve Montacuta ferruginosa is covered with a rust-colored biofilm. This biofilm includes filamentous bacteria and protozoa encrusted with a mineral, rich in ferric ion and phosphate. The aim of this research was to study two possible microbial iron precipitation pathways in the biofilm, namely, microbial iron oxidation and microbial degradation of organic Fe(III) complexes. The iron-oxidizing activity was assayed spectrophotometrically by monitoring the formation of the dye Wurster blue in biofilm extracts. Iron-oxidizing activity was effectively detected in extracts obtained by oxalic acid treatment of biofilm fragments. Extracts obtained without oxalic acid treatment, heated extracts, or extracts supplemented with HgCl 2 did not show any activity. This suggests that an iron-oxidizing factor (IOF), possibly an enzyme, coprecipitated with the mineral. Additional information gathered by using sodium dodecyl sulfate-polyacrylamide gel electrophoresis, gel-filtration chromatography, and UV spectrophotometry indicate that the IOF would be a small peptide or glycopeptide (1,350 Da). Microbial degradation of organic Fe(III) complexes was assayed with biofilm fragments incubated in a medium containing ferric citrate. Analysis of the supernatants after various intervals revealed that the complex was degraded by living microorganisms much faster than in the heat-killed negative controls. We conclude that ferric iron precipitation in the biofilm may proceed by way of microbial Fe(II) oxidation as well as microbial degradation of organic Fe(III) complexes.The shell of the bivalve Montacuta ferruginosa is covered with a rust-colored biofilm. This biofilm includes filamentous bacteria and protozoa encrusted with a mineral, rich in ferric ion and phosphate. The aim of this research was to study two possible microbial iron precipitation pathways in the biofilm, namely, microbial iron oxidation and microbial degradation of organic Fe(III) complexes. The iron-oxidizing activity was assayed spectrophotometrically by monitoring the formation of the dye Wurster blue in biofilm extracts. Iron-oxidizing activity was effectively detected in extracts obtained by oxalic acid treatment of biofilm fragments. Extracts obtained without oxalic acid treatment, heated extracts, or extracts supplemented with H(g)Cl2 did not show any activity. This suggests that an iron-oxidizing factor (IOF), possibly an enzyme, coprecipitated with the mineral. Additional information gathered by using sodium dodecyl sulfate-polyacrylamide gel electrophoresis, gel-filtration chromatography, and UV spectrophotometry indicate that the IOF would be a small peptide or glycopeptide (1,350 Da). Microbial degradation of organic Fe(III) complexes was assayed with biofilm fragments incubated in a medium containing ferric citrate. Analysis of the supernatants after various intervals revealed that the complex was degraded by living microorganisms much faster than in the heat-killed negative controls. We conclude that ferric iron precipitation in the biofilm may proceed by way of microbial Fe(II) oxidation as well as microbial degradation of organic Fe(III) complexes.

Les ostracodes au passage Eifelien/Givetien a Glageon (Avesnois, France)

Fifty-six ostracod species are recognized in the Hanonet Formation (Upper Eifelian) and in the lowermost Trois-FontainesFormation (Lower Givetian) of the Glageon Quarry (Southern border of the Dinant Basin, Avesnois, Northern France). The ostracods belong to the Eifel ecotype sensu Becker (in Bandel & Becker, 1975) and two assemblages are recognized: 1. assemblage III characterizes a relatively quiet marine environment below wave base on the mid-ramp setting (Hanonet Formation); 2. assemblage II characterizes a shallow marine environment above wave base on the inner-ramp (lower part of the Trois-Fontaines Formation). Comparatively with other outcrops from the Southern border of the Dinant Basin, the sea level fluctuations were of very low amplitude near the Eifelian/Givetian boundary at Glageon. The study of sediments in Glageon is consistent with the data from ostracods.

Stable Iron Isotopes and Microbial Mediation in Red Pigmentation of the Rosso Ammonitico (Mid-Late Jurassic, Verona Area, Italy)

The iron (Fe) isotopic composition of 17 Jurassic limestones from the Rosso Ammonitico of Verona (Italy) have been analyzed by Multiple-Collector Inductively Coupled Plasma Mass Spectrometry (MC-ICP-MS). Such analysis allowed for the recognition of a clear iron isotopic fractionation (mean -0.8 per thousand, ranging between -1.52 to -0.06 per thousand) on a millimeter-centimeter scale between the red and grey facies of the studied formation. After gentle acid leaching, measurements of the Fe isotopic compositions gave delta(56)Fe values that were systematically lower in the red facies residues (median: -0.84 per thousand, range: -1.46 to +0.26 per thousand) compared to the grey facies residues (median: -0.08 per thousand, range: -0.34 to +0.23 per thousand). In addition, the red facies residues were characterized by a lighter delta(56)Fe signal relative to their corresponding leachates. These Fe isotopic fractionations could be a sensitive fingerprint of a biotic process; systematic isotopic differences between the red and grey facies residues, which consist of hematite and X-ray amorphous iron hydroxides, respectively, are hypothesized to have resulted from the oxidizing activity of iron bacteria and fungi in the red facies. The grey Fe isotopic data match the Fe isotopic signature of the terrestrial baseline established for igneous rocks and low-C(org) clastic sedimentary rocks. The Fe isotopic compositions of the grey laminations are consistent with the influx of detrital iron minerals and lack of microbial redox processes at the water-interface during deposition. Total Fe concentration measurements were performed by Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) (confirmed by concentration estimations obtained by MC-ICP-MS analyses of microdrilled samples) on five samples, and resultant values range between 0.30% (mean) in the grey facies and 1.31% (mean) in the red facies. No correlation was observed between bulk Fe content and pigmentation or between bulk Fe content and Fe isotopic compositions. The rapid transformation of the original iron oxyhydroxides to hematite could have preserved the original isotopic composition if it had occurred at about the same temperature. This paper supports the use of Fe isotopes as sensitive tracers of biological activities recorded in old sedimentary sequences that contain microfossils of iron bacteria and fungi. However, a careful interpretation of the iron isotopic fractionation in terms of biotic versus abiotic processes requires supporting data or direct observations to characterize the biological, (geo)chemical, or physical context in relation to the geologic setting. This will become even more pertinent when Fe isotopic studies are expanded to the interplanetary realm.