Résultat de la recherche
Enrichment of redox-sensitive trace metals (U, V, Mo, As) associated with the late Hauterivian Faraoni oceanic anoxic event
The Faraoni Level is a short-lived oxygen-deficient event that took place during the latest Hauterivian. In order to improve our understanding of the palaeoenvironmental conditions that occurred during this event, we have analysed the contents of several redox-sensitive trace elements (U, V, Mo, As, Co, Cd, Cu, Zn, Ni, Pb, Cr) from bulk limestone samples of late Hauterivian–early Barremian age from three reference sections. U, V, Mo and As show consistent and significant enrichments during the Faraoni event whereas the other redox-sensitive trace elements analysed here are not systematically enriched. In order to explain this discrepant behaviour, we propose that the Faraoni Level was deposited during a period of anoxic conditions near the sediment–water interface. The distinctive peaks in U, V, Mo and As contents are traceable throughout the three studied sections and represent a good correlation tool which helps to identify the Faraoni Level and its equivalents in the western Tethyan realm and outside of the Tethys. For example, a peak in U contents in upper Hauterivian sediments of the northwestern Pacific realm (ODP leg 185, site 1149) may well be an expression of the Faraoni event in this particular basin.
Palaeoceanographic and palaeoclimatic changes during the Late Hauterivian-Barremian and their impact on the northern Tethyan margin: a combined sedimentological and geochemical approach
The goal of this PhD study was to decipher the mechanisms responsible for changes in the carbonate platform factory accompanied by incipient drowning. For this purpose, a peculiar condensed level called the Altmann Member, which recorded the northern Tethyan margin drowning during the latest Hauterivian – Early Barremian, and which is now locked up in the Helvetic realm, was studied. Indeed, this horizon is situated at the verge of a major change in the carbonate platform production mode. On one hand, during the Hauterivian, heterotrophic organisms such as crinoids and bryozoans dominated the northern Tethyan carbonate platforms. On the other hand, phototrophic organisms such as corals, green algae and rudists dominated during the Late Barremian. This important transition and its documentation in the shelf and basinal realms of the Tethys offered the framework of this study. Helvetic realm: A first step was to establish as precisely as possible the time range of the Late Hauterivian – Barremian sediments in the Helvetic realm. Thanks to numerous ammonite findings, it was possible to determine that the Altmann Member spans from the Pseudothurmannia seitzi (latest Hauterivian) to the Coronites darsi (latest Early Barremian) ammonite zones. The onset of the Schrattenkalk Formation is dated by sequence stratigraphy correlation and by ammonite findings in the Chopf Member. This event is dated as belonging to the Gerhardtia sartousiana zone. Sedimentology and sequence stratigraphy studies helped to determine the unfolding of the Altmann Member drowning episode, which has proceeded in two steps. The first one is coeval with the Faraoni event, which was thus far only reported from basinal settings. The second step is coeval with the Barremian second-order sea-level rise occurring at the Early – Late Barremian transition. During the Early Barremian second-order sea-level lowstand, the Helvetic platform was submitted to winnowing currents leading to the formation of phosphatized crusts. Basin sections: In order to understand the link between carbonate platform factory changes and drowning events, geochemical studies were done on bulk-rock and belemnite samples from (hemi-) pelagic sections. These include redox-sensitive trace metals, phosphorus, carbon and oxygen isotopes. Four sections were chosen: the Angles section, SE France, which is the Barremian stratotype section; the Veveyse de Châtel – St. Denis section, Ultrahelvetic realm, Switzerland: the Fiume-Bosso and Gorgo a Cerbara sections, Umbria-Marche basin, Italy. These four sections offer a good coverage of the western Tethys and allow thus to establish general trends in the paleoceanographic conditions of the western Tethys at that time. Enrichments of redox-sensitive trace metals were used to trace oceanic anoxic events during the Late Hauterivian – Early Barremian, because numerous black-shale horizons are recognized in the corresponding rocks. Only one level, corresponding to the Faraoni Level, was identified as the result of an oceanic anoxic event. This result is confirmed by the C/P ratio that shows a positive shift associated to the Faraoni Level. Phosphorus burial rates were used to trace nutrient contents in the ocean during the Late Hauterivian – Barremian. They have resulted in a tripartite division of trophic conditions during the studied time interval. Thus, the middle Late Hauterivian is associated to mesotrophic conditions, the latest Hauterivian – Early Barremian to eutrophic conditions and the Late Barremian to oligotrophic conditions. Bulk rock carbon isotopes were investigated in order to obtain information about the carbon cycle and its link to the carbonate platform. It appears that the bulk-rock carbon isotope signal in basinal sections situated close to the northern Tethyan margin is strongly influenced by carbonate factory changes due to carbonate platform shedding into the basin. During the time of heterozoan-dominated platform growth, the pelagic carbon isotope signal is buffered by the shedding of calcite-dominated fragments and dissolved inorganic carbon. During the time of photozoan-dominated platform growth, the pelagic carbon isotope signal is pushed to heavier values due to the export of aragonite. Finally, in order to better understand the interactions between palaeoenvironmental changes and carbonate platform, an integrated approach, coupling numerous geochemical data as well as clay minerals, sea-level changes and others information, was done. This approach allows to conclude that changes in the northern Tethyan carbonate platform factory were driven by the overall nutrient content in seawater. During times of oligotrophic conditions, photozoan carbonate systems dominated the neritic realm whereas during times of meso-eutrophic conditions, heterozoan carbonate systems developed. Moreover, carbonate platform drowning events were linked to changes in ocean current pattern during time of sea-level highstand, together with high seawater nutrient levels. These changes have favored winnowing of platform-surface sediments and the deposition of phosphate-rich layers, and precluded carbonate platform growth along the northern Tethyan margin.
Evolution of the marine stable carbon-isotope record during the early Cretaceous : A focus on the late Hauterivian and Barremian in the Tethyan realm
In order to improve our understanding of the relationships between the late Hauterivian oceanic anoxic Faraoni event, contemporaneous platform drowning along the northern Tethyan margin and global environmental change in general, we established high-resolution δ13C and δ18O curves for the late Hauterivian and the entire Barremian stage. These data were obtained from whole-rock carbonate samples from the Veveyse de Châtel-Saint-Denis section (Switzerland), the Fiume-Bosso section and the nearby Gorgo a Cerbara section (central Italy), and the Angles section (Barremian stratotype, France). We observe an increase of 0.3‰ in mean δ13C values within sediments from the middle Hauterivian Subsaynella sayni ammonite zone to the Hauterivian–Barremian boundary; δ13C values remain essentially stable during the early Barremian. During the latest early Barremian and most of the late Barremian, δ13C values increase slowly (until the Imerites giraudi zone) and the latest Barremian is characterized by a negative trend in δ13C values, with minimal values at the Barremian–Aptian boundary. During the earliest Aptian, δ13C mean values start to rise again and attain + 2.25‰. We interpret the evolution of the δ13C record as resulting from the interaction between changes in the carbon cycle in the Tethyan basin and the adjacent platforms and continents. In particular, changes towards warmer and more humid conditions on the continent and coeval phases of platform drowning along the northern Tethyan margin may have contributed to enhance the oceanic dissolved inorganic carbon (DIC) reservoir which may have pushed the δ13C record towards more negative values and exerted a general attenuation on the δ13C record. From this may have come the general change from a heterozoan to a photozoan carbonate platform community, which influenced the evolution in δ13C values by increasing the export of aragonite and diminishing export of dissolved organic carbon into the basins.
Paleoceanographic changes during the early Cretaceous (Valanginian-Hauterivian): evidence from oxygen and carbon stable isotopes
We investigated Valanginian-Hauterivian bulk rock and belemnite samples from Vocontian Basin sections in southeastern France for their stable carbon and oxygen isotope signature. Firstly, because these sections permit calibration with high-resolution biostratigraphy based on ammonites and secondly because detailed isotope studies for the Hauterivian are lacking. The results show that delta(13)C values for bulk rock decrease during the late Valanginian-early Hauterivian with 1 parts per thousand and increase again during the late Hauterivian with 1 parts per thousand. The delta(18)O signal for bulk rock samples is mostly disturbed by diagenesis, The belemnites show carbon and oxygen isotope values that are lower and higher than bulk rock samples respectively. We explain this as the result of the belemnites recording a deeper water signal with lighter delta(13)C values and heavier delta(18)O values, implicating colder water at greater depth. The overall preservation of the belemnites is very good and permits the construction of a paleo-temperature trend. This trend shows warm deeper water temperatures during the late early Valanginian (15 degrees C) and progressively cooler temperatures during the late Valanginian and early Hauterivian (11 degrees C), During the late Hauterivian temperatures increase again (13 degrees C). We relate this cooling trend for deeper water to a second order sea level rise, which allowed for the exchange of cold Boreal and warm Tethyan water masses. The influx of cold nutrient rich water had a profound effect on carbonate producing biota along the northern margin of the Tethys during the Hauterivian leading to prolonged phases of condensation and platform destruction. During the early Hauterivian the carbonate system along the northern Tethyan margin shifted into a 'green water' mode of carbonate production. High rates of carbonate production under mesotrophic conditions, also observed in other parts of the world, meant that the global carbon cycle became buffered shown by the stable trends in carbon isotopes. (C) 2000 Elsevier Science B.V. All rights reserved.
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.