Changements paléoenvironnementaux et production carbonatée hémipélagique de la marge nord-ouest téthysienne durant le Valanginien (approches minéralogique, micropaléontologique et géochimique)

Le Valanginien (Crétacé inférieur, ~135 Ma) se caractérise à l échelle du globe, par une diminution drastique de la production carbonatée et une eutrophisation accrue de l écosystème marin. Le paroxysme des perturbations coïncide avec l enregistrement d une anomalie positive du 13C dans les sédiments carbonatés. L objectif de ce travail est de comprendre, à haute résolution (<100 ka), les interactions majeures existant entre le climat, l altération-érosion sur le continent et la productivité primaire du Valanginien, dans la bande intertropicale du domaine téthysien. Il repose sur une approche minéralogique (roche totale et fraction argileuse), micropaléontologique (nannofossiles calcaires et calpionelles) et géochimique (éléments chimiques et isotopes stables) des archives des marnes de la coupe d Angles (Bassin vocontien, SE de la France).Après avoir contrôlé le potentiel d altération diagénétique de chacun des traceurs, l étude simultanée à haute résolution de la production carbonatée primaire, des conditions trophiques vocontiennes ainsi que de l intensité de l érosion/altération sur les continents a permis d établir les changements paléocéanographiques, paléoenvironnementaux et paléoclimatiques du Bassin vocontien au Valanginien. Ils se matérialisent par une crise des principaux organismes producteurs de carbonates (nannofossiles calcaires et calpionelles), engendrée par une fertilisation accrue de la zone photique suite à une intensification de l altération/érosion du continent, dans un scénario global de climat à effet de serre.The Valanginian time interval (lower Cretaceous, ~135 Myr) is documented by a widespread eutrophication of marine ecosystems associated with a crisis of carbonate producing biota. The paroxysm of the perturbations coincides with a global positive carbon isotope excursion.The objective of this study is to explore the relationship between climate, continental weathering, and primary productivity in the northwestern Tethys. This work focuses on the Angles section, located in the hemipelagic realm of the vocontian Basin. This site offers the possibility for a high-resolution micropalaeontological (calcareous nannofossils and calpionellids) mineralogical (bulk rock and clay minerals) and geochemical (elemental geochemistry and stable isotopes) investigation. The high resolution study of the vocontian primary production and the trophic conditions associated with the continental weathering pattern allowed to provide new insight on the palaeoceanographic, palaeoenvironmental and palaeoclimatic changes of the vocontian Basin during the Valanginian. They are characterized by a crisis of the carbonate producing biota triggered by higher fertility conditions in the photic zone, through an increase continental runoff and erosion, in a global greenhouse climate scenario.ORSAY-PARIS 11-BU Sciences (914712101) / SudocSudocFranceF

Occurrence of photosynthetic microbial mats in a Lower Cretaceous black shale (central Italy): a shallow-water deposit

Cretaceous oceanic anoxic events (OAEs) were periods of high organic carbon burial corresponding to intervals with excellent organic matter (OM) preservation. This work focuses on the Urbino level, i.e., OAE1b, which is thought to be of regional extent. A detailed microscopical study of OM shows a dominance of microbial activity, characterized by a typical arrangement of exopolymeric substances (EPS) related to microbial mats, bacterial bodies, and some photosynthetic microorganisms, as shown by thylakoids. The latter lived where they have been found, i.e., at the sea bottom, which indicates that OM results from the diagenesis of benthic photosynthetic microbial mats, an interpretation supported through the comparison with a recent analogue. The exceptional preservation of such organic structures in OM points to the joint role of the selective and sorptive preservation pathways. These data and interpretation strongly differ from previous observations in OAE1b equivalents. They suggest that the Urbino level might be an atypical OAE of regional/local extent which was formed within the photic zone

Bacterial Activity and Preservation of Sedimentary Organic Matter: The Role of Exopolymeric Substances

Although exopolymeric substances (EPS) are associated with the microorganisms contributing to the production/degradation of sedimentary organic matter, their role in theses processes have so far never been mentioned. Using high-resolution microscopical tools (scanning and transmission electron microscopy, atomic force microscopy), fossil organic matter in the Miocene Monterey Formation (California) and Kimmeridgian laminites (France) has been compared with its present-day analogs, i.e., respectively sulphuroxidizing bacteria and cyanobacterial biofilms. This comparison shows that, particularly in the case of Kimmeridgian cyanobacterial mats deposited in a shallow back-reef environment, organic matter preservation is conditioned by exopolymeric substances secreted by bacteria. A model is proposed for the evolution through time of exopolymeric substances in relation to the mechanical constrains they have been exposed to, during lithification and diagenesis. This model is based on the microscopical observation of sulphuroxidizing bacteria and could explain the morphology of fossil organic matter usually referred to as "amorphous" in standard light microscopy. The highly hydrated nature of exopolymeric substances helps to protect organic matter from degradation and remineralization. These substances can be observed only in microscopy and are undetectable through organic geochemical methods, hence the need to combine these two methods in organic matter studies. Consequently, exopolymeric substances must be considered as an important contributing agent to organic matter preservation. These results confirm the complexity of the bacterial role in geoenvironments and add a new parameter in the productivity-vs-preservation debate

The late Hauterivian Faraoni oceanic anoxic event in the western Tethys: Evidence from phosphorus burial rates

In the uppermost Hauterivian sediments of the western Tethys, a short-lived anoxic event (Faraoni event) is documented both in the form of an interval enriched in organic matter (pelagic realm) and in a condensed interval enriched in glauconite and phosphate (shelf realm). This latter interval represents the onset of a drowning episode on the Helvetic carbonate platform along the northern tethyan margin that lasted throughout the early Barremian. This drowning episode marks a turning point in the way the platform carbonate factory functioned: during the Hauterivian carbonate production was dominated by heterozoans, whereas during the late Barremian a photozoan assemblage developed that is preserved in the so-called Urgonian limestone. The late Hauterivian Faraoni oceanic anoxic event is of particular interest because it is not accompanied by a major positive shift in δ13C unlike other oceanic anoxic events during the Cretaceous (Valanginian, early Aptian, Cenomanian–Turonian boundary). We have analyzed four (hemi-)pelagic sections with regards to their phosphorus content to better understand the palaeoceanographic conditions related to this anoxic event and the associated changes in the shallow-water carbonate factory. The sections are located in Angles (SE France), Fiume-Bosso and Gorgo a Cerbara (central Italy), and Veveyse de Châtel-St. Denis (west Switzerland). We calculated phosphorus mass accumulation rates by using a cyclostratigraphic approach in order to obtain an adequate age model. We observe a comparable and correlatable long-term trend for the four sections, which suggests that the phosphorus mass accumulation rates and temporal changes therein are representative for the western tethyan pelagic realm. The Faraoni event is marked by a minimum in phosphorus accumulation and a positive shift in the Corg/Ptot ratios, which is interpreted as a reflection of the decreased capacity of storing and preserving phosphorus in oxygen-depleted sediments. Moreover, the onset in the decrease in phosphorus accumulation coincides with a sea level rise, while the Faraoni level itself corresponds to a maximum flooding interval. This phase of sea-level rise may have been important in the establishment of marine connections between the boreal and tethyan realms and, as such, in the exchange of nutrient-enriched waters. The model for the origin of the Faraoni oceanic anoxic event proposed here incorporates these aspects together with a positive feedback loop generated by phosphorus regeneration and a negative feedback loop related to changes in the ocean oxygen cycle. The subsequent long-term changes in phosphorus burial rates during the Barremian suggest that the Faraoni event marks the onset of a long period of environmental instability with regards to platform growth, leading to periodic phases of eutrophication and drowning of the northern tethyan carbonate platform. This environmental crisis ended during the late Barremian with the onset of the deposition of the Urgonian limestone under oligotrophic conditions