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 δ
13C 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 C
org/P
tot 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.
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The discussion is mainly focused on the Hauterivian and Barremian zonal schemes. For the Hauterivian Stage, the main changes concern the
Subsaynella sayni and
Balearites balearis Zones with the ...introduction of one horizon (
Subsaynella begudensis Horizon) and four subzones (
B. balearis,
Binelliceras binelli,
Crioceratites krenkeli,
Spathicrioceras seitzi subzones), respectively. For the Lower Barremian, the main amendments concern the introduction of two subzones in the
Taveraidiscus hugii auctorum Zone (
T. hugii auctorum and
Psilotissotia colombiana subzones) and of four horizons in the
Kotetishvilia compressissima Zone (
Holcodiscus fallax,
Nicklesia didayana,
Heinzia communis and
Subtorcapella defayae Horizons). The
Coronites darsi Zone is replaced by the
Moutoniceras moutonianum Zone in which two horizons occur (
C. darsi and
Heinzia caicedi Horizons). The base of the Upper Barremian is defined by the base of the
Toxancyloceras vandenheckii Zone and Subzone. The former
Hemihoplites feraudianus and
Martelites sarasini Zones are lowered in rank to subzones of the
Gerhardtia sartousiana and
Imerites giraudi Zones, respectively.
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The Aptian stage, between ∼113 and ∼121 million years ago (Ma), was punctuated by a succession of Oceanic Anoxic Events (OAEs), recording extreme global warmings, dramatic expansions of the ocean's ...oxygen minimum zones, along with perturbations to the biotic and carbon cycles. However, the chronology of the Aptian stage is poorly constrained, impacting the duration and timing of OAEs. Using a greatly expanded sedimentary composite record (380 m) of key outcropping sections in the Vocontian Basin (SE France) combined with available radiometric dates and correlations to a set of astronomical solutions, we provide a constrained absolute astrochronology of the Aptian stage. The 405 kyr (gVenus–gJupiter) eccentricity astronomical timescale indicates a minimal duration of ∼9.4 Myr for the Aptian stage and an age of 122.6 ± 0.3 Ma for the base of the Aptian, consistent with radioisotope dating. We find a deviation in the periodicity of gMercury–gJupiter eccentricity term in the mid-Aptian stage, at ca. 117.19 ± 0.3 Ma, that we ascribe as an expression of the resonance transition σ = (gMercury – gJupiter) – (sMercury – sVenus), in relation with a strong chaotic orbital motion of Mercury. Such a geological observation is supported by a concomitant resonant transition in the La2004 astronomical model.
•The tuned MS data indicates a minimal duration of ∼9.4 Myr for the Aptian stage.•Our revised Aptian duration appears concordant with numerical calibration.•A shift in the gMercury–gJupiter eccentricity term is observed at 117.19 ± 0.3 Ma.•Geological observation is supported by a resonant transition in astronomical model.
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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 delta13C curves.
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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.
This work is mainly an answer to the work of , on families Helicancylidae Hyatt, 1894 and Acrioceratidae Vermeulen, 2009. On the nomenclatural speaking, the non validity of the genus Helicancylus ...Gabb, 1869 and of the family Helicancylidae Hyatt, 1894 and, consequently, the validity of the family Acrioceratidae Vermeulen, 2004 are established. The second part of this work is devoted to the respect of former work and, contrary to the assertions from , it is shown that the stratigraphical positioning of the Upper Barremian level with small Barremites, realized by , is at least as precise as that proposed by Bert. The last part of this work highlights a lack of bibliographical references, fact which had already been highlighted () in a former work of .
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Ce travail est essentiellement une réponse à celui de
Bert (2009), sur la famille des Helicancylidae Hyatt, 1894 et sur celle des Acrioceratidae Vermeulen, 2009. Sur le plan nomenclatural, la ...non-validité du genre
Helicancylus Gabb, 1869 et de la famille des Helicancylidae Hyatt, 1894 et, conséquemment, la validité de la famille des Acrioceratidae Vermeulen, 2004 sont établies. La deuxième partie de ce travail est consacrée au respect des travaux antérieurs et, contrairement aux affirmations de
Bert (2009), il est démontré que le positionnement stratigraphique du niveau à petits
Barremites du Barrémien supérieur, réalisée par
Cotillon (1971), est au moins aussi précis que celui proposé par Bert. La troisième partie de ce travail met en évidence un manque de références bibliographiques, fait qui a déjà été mis en évidence (
Vermeulen et Lepinay, 2010) pour un travail antérieur réalisé par
Bert et al. (2009).
This work is mainly an answer to the work of
Bert (2009), on families Helicancylidae Hyatt, 1894 and Acrioceratidae Vermeulen, 2009. On the nomenclatural speaking, the non validity of the genus
Helicancylus Gabb, 1869 and of the family Helicancylidae Hyatt, 1894 and, consequently, the validity of the family Acrioceratidae Vermeulen, 2004 are established. The second part of this work is devoted to the respect of former work and, contrary to the assertions from
Bert (2009), it is shown that the stratigraphical positioning of the Upper Barremian level with small
Barremites, realized by
Cotillon (1971), is at least as precise as that proposed by Bert. The last part of this work highlights a lack of bibliographical references, fact which had already been highlighted (
Vermeulen and Lepinay, 2010) in a former work of
Bert et al. (2009).
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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
δ
13C and
δ
18O 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
δ
13C values within sediments from the middle Hauterivian
Subsaynella sayni ammonite zone to the Hauterivian–Barremian boundary;
δ
13C values remain essentially stable during the early Barremian. During the latest early Barremian and most of the late Barremian,
δ
13C values increase slowly (until the
Imerites giraudi zone) and the latest Barremian is characterized by a negative trend in
δ
13C values, with minimal values at the Barremian–Aptian boundary. During the earliest Aptian,
δ
13C mean values start to rise again and attain
+
2.25‰.
We interpret the evolution of the
δ
13C 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
δ
13C record towards more negative values and exerted a general attenuation on the
δ
13C record. From this may have come the general change from a heterozoan to a photozoan carbonate platform community, which influenced the evolution in
δ
13C values by increasing the export of aragonite and diminishing export of dissolved organic carbon into the basins.
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This work is mainly an answer to the work of Bert, Delanoy and Canut, on the genus Imerites Rouchadze, and we add also some points to the knowledge of this genus. An answer is given for the ...implication of one of the authors (J.V.) of this present work, about the origin of the genus Imerites Rouchadze, and some significant bibliographic references, not named in the 2009 work, are specified and placed in their context. The characteristics of the ornamental stage "Pseudoshasticrioceras" mainly defined by an oval and compressed section of the whorls and by regular ventrolateral clavi, well marked, which frame the venter in a typical way (Bert et al., p.181), don't allow to affirm the presence of this stage during the ontogeny of all species of the genus Imerites Rouchadze, contrary to what Bert, Delanoy and Canut (p.30, 31) wrote. The type-species of the genus Imerites Rouchadze is made clear by the application of the rules of the I.C.Z.N. code. The article 23, with the paragraph 23.1 of the I.C.Z.N. code, imply the senior synonymy of Imerites cristatus (Orbigny) on Imerites giraudi (Kilian), even if the Kilian's species is still the type species of the genus Imerites Rouchadze. The taxinomical validity of the species Imerites cristatus (Orbigny) is confirmed. Its morphological variability is clarified by the distinction of four referential morphotypes, Cristatus, Giraudi, Favrei and Raricostatum. Consequently, the assumption of a shape-dimensional dimorphism into the populations of the genus Imerites Rouchadze is rejected, awaiting irrefutable proof. A new species, Imerites stephaniae sp. nov., is described. It was collected in the Gerhardtia sartousiana Zone, in the uppermost part of the Gerhardtia provincialis Subzone, and it is a probable ancestor of the cogeneric species of the Imerites giraudi Zone.
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