Résultat de la recherche
Platform-induced clay-mineral fractionation along a northern Tethyan basin-platform transect: implications for the interpretation of Early Cretaceous climate change (Late Hauterivian-Early Aptian)
High-resolution clay-mineral analyses were performed on upper Hauterivian to lower Aptian sediments along a platform-to-basin transect through the northern Tethyan margin from the Neuchâtel area (Switzerland), to the Vocontian Trough (France) in order to investigate links between climate change, carbonate platform evolution, and fractionation patterns in clay minerals during their transport.
During the Hauterivian, the northern Tethyan carbonate platform developed in a heterozoan mode, and the associated ramp-like topography facilitated the export of detrital material into the adjacent basin, where clay-mineral assemblages are dominated by smectite and kaolinite is almost absent, thereby suggesting dry-seasonal conditions. During the Late Hauterivian Balearites balearis ammonite zone, a change to a more humid climate is documented by the appearance of kaolinite, which reaches up to 30% of the clay fraction in sediments in the Vocontian Trough. This prominent change just preceded the Faraoni Oceanic Anoxic Event and the onset of the demise of the Helvetic Carbonate Platform, which lasted to the late early Barremian.
From the Late Barremian onwards, the renewed growth of the northern Tethyan carbonate platform in a photozoan mode and the associated development of a marginally confined platform topography fractionated the clay-mineral assemblages exported into hemipelagic settings: kaolinite particles were preferentially retained in proximal, platform settings, due to their size and their relatively high specific weight. In the inner platform environment preserved in the Swiss Jura, an average of 32% of kaolinite in the clay fraction is observed during the latest Barremian–earliest Aptian, whereas clay-mineral assemblages of coeval sediments from deeper depositional settings are dominated by smectite and show only minor amounts of kaolinite.
This signifies that besides palaeoclimate conditions, the morphology and ecology of the carbonate platform had a significant effect on the distribution and composition of clay assemblages during the Late Hauterivian–Early Aptian along the northern Tethyan margin.
Interactions between environmental change and shallow water carbonate buildup along the northern Tethyan margin and their impact on the Early Cretaceous carbon isotope record
The evolution of the Early Cretaceous, northern Tethyan carbonate platform was not only influenced by changes in sea level, detrital influx, and surface water temperature but also by changes in trophic levels. We distinguish between phases of carbonate production dominated by oligotrophic photozoan communities and by mesotrophic and eventually colder-water heterozoan communities. Superimposed on this bimodal trend in platform evolution were phases of platform demise for which we provide improved age control based on ammonite biostratigraphy. The initial phase of these episodes of platform demise corresponds in time to episodes of oceanic anoxic events and environmental change in general. On the basis of a comparison between the temporal changes in an Early Cretaceous, ammonite-calibrated δ13C record from southeastern France and coeval changes in the platform record, we suggest that the history of carbon fractionation along the northern Tethyan margin was not only influenced by changes in the oceanic carbon cycle such as in the rate of production and preservation of organic and carbonate carbon and in the size of the oceanic dissolved inorganic carbon reservoir, but it was also influenced by the above-mentioned changes in the ecology and geometry of the adjacent carbonate platform. Phases of photozoan carbonate production induced positive trends in the hemipelagic carbonate δ13C record. Phases of heterozoan carbonate production pushed the δ13C system toward more negative values. Platform drowning episodes implied an initial increase in δ13C values, followed by a longer-term decrease in δ13C values.
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.
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.
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.
Biostratigraphy, sedimentology and sequence stratigraphy of the latest Hauterivian – Early Barremian drowning episode of the Northern Tethyan margin (Altmann Member, Helvetic nappes, Switzerland)
Durant le Crétacé inférieur, les changements paléoceanographiques sont enregistrés sur la marge nord-Téthysienne par des fluctuations du mode de production des carbonates ainsi que des épisodes d’ennoiement de plate-forme. Le Membre d’Altmann, qui affleure en Europe centrale, dans les nappes plissées et charriées du domaine Helvétique, est le résultat d’un de ces ennoiements de la plate-forme carbonatée se déroulant durant l’Hauterivien tardif et le Barrémien précoce. Principalement constitué de couches fortement condensées riches en glauconie et phosphate, le Membre d’Altmann, qui peut être relié à des changements paléocéanographiques majeurs durant le Crétacé inférieur, n’a pour l’instant été que faiblement daté. De nouvelles datations biostratigraphiques basées sur des ammonites nouvellement trouvées, ainsi que sur une réévaluation de celles précédemment publiées, permettent de précisément dater cet épisode d’ennoiement de plate-forme. Celui-ci débute dans la zone d’ammonites à Pseudothurmannia seitzi (Hauterivien tardif) et se termine dans la zone à Coronites darsi (dernière zone du Barrémien inférieur). Ces nouvelles datations, couplées avec des interprétations en stratigraphie séquentielle, permettent de mieux appréhender le déroulement de cet épisode d’ennoiement de plate-forme qui se développe en deux temps : une première étape correspond à une importante phase de transgression marine durant l’Hauterivien terminal, accompagnée par une réduction importante de la production carbonatée. Une seconde étape est enregistrée durant la fin du Barrémien précoce par la formation d’un fond induré phosphaté associé à un bas niveau marin, suivie par une remontée rapide du niveau marin et le dépôt de sédiments de rampe carbonatée externe associé à une forte rétrogradation de la plateforme. De plus, la majeure partie du Barrémien précoce est condensée dans ce fond induré phosphaté associé à un bas niveau marin de second ordre. Le début de cet épisode d’ennoiement de plate-forme semble être lié à l’événement d’anoxie océanique du niveau Faraoni, tandis que durant le Barrémien inférieur, la phosphatogenèse semble être le résultat d’un important vannage des fonds océaniques durant une période d’eutrophie marine., During the Early Cretaceous, major palaeoceanographic changes are mirrored on the northern Tethyan carbonate platform by changes in the carbonate factory and by platform drowning. The Altmann Member of the central European, northern Alpine Helvetic thrust and fold belt, contains the sedimentary record of one of these drowning events which occurred during the Late Hauterivian – Early Barremian. It consists mainly of highly condensed beds, which are rich in glaucony and phosphates. The Altmann Member was hitherto only poorly dated. New ammonite findings and a re-evaluation of existing ammonite fauna allow to precisely date this drowning episode, starting in the Pseudothurmannia seitzi biozone (latest Hauterivian) and lasting until the Coronites darsi biozone (latest Early Barremian). These new age dates, coupled with sequence stratigraphic interpretations allow to better understand the unfolding of the drowning episode, which proceeded in two stages: The first stage consisted in an important phase of marine transgression during the latest Hauterivian, during which carbonate production was highly reduced; the second stage is recorded during the latest Early Barremian by an important sequence boundary, which is associated with a phosphatized hardground, followed by rapid sea-level rise and the deposition of outer ramp sediment associated with the backstepping of the platform. Almost the whole early Barremian is likely to be condensed in this phosphatized hardground, which is associated to a second order sea-level lowstand. The onset of the drowning event is linked to the Faraoni oceanic anoxic event, whereas during the Early Barremian, phosphatization might be the result of important winnowing during a period of highly eutrophic conditions.
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.
New data on the age of the installation of Urgonian-type carbonates along the northern Tethyan margin : biostratigraphy of the Chopf Member (Helvetic Alps, eastern Switzerland)
Le membre du Chopf correspond à un horizon glauconieux déposé dans la partie distale de la plate-forme helvétique. La découverte récente d'ammonites permet de dater précisément cet horizon de la partie inférieure de la zone à Gerhardtia sartousiana (Barrémien supérieur moyen). Cette nouvelle datation permet de donner un âge maximal à la formation du Schrattenkalk dans cette région. Cet âge est cohérent avec ceux qui sont obtenus dans les autres parties de la Téthys occidentale pour l'installation des faciès urgoniens. Cette nouvelle datation permet aussi d'apporter des précisions sur la calibration des courbes du δ13C du Barrémien supérieur., The Chopf Member is a glauconitic, phosphate-bearing succession that occurs in the distal part of the Helvetic Alps (eastern Switzerland). The recent discovery of age-diagnostic ammonites within this horizon allows for its attribution to the lower part of the Gerhardtia sartousiana zone (middle Late Barremian). This new age corresponds to a maximal age for the onset of the Schrattenkalk Fm. in this area, and is coeval with the onset of the Urgonian facies in other parts of the western Tethyan realm. This new age allows also for a more precise dating of Late Barremian δ13C curves.