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  • Publication
    Accès libre
    The Cenomanian/Turonian anoxic event at the Bonarelli Level in Italy and Spain: enhanced productivity and/or better preservation?
    (2007)
    Mort, Haydon
    ;
    Jacquat, Olivier
    ;
    Adatte, Thierry 
    ;
    Steinmann, Philip
    ;
    Föllmi, Karl B. 
    ;
    Matera, Virginie
    ;
    Berner, Zsolt
    ;
    Stüben, Doris
    The upper Cenomanian pelagic sediments of Furlo in the northern Apennines, Italy, are characterized by a 1.5-m-thick organic-rich stratigraphic horizon called the Bonarelli Level, which represents the second major oceanic anoxic event in the Cretaceous (OAE 2). The Bonarelli Level is depleted in carbonates and consists essentially of biogenic quartz, phyllosilicates, and organic matter, with values of TOC reaching 18%. The age of the Furlo section is constrained by correlating its δ13C curve with that of the well-dated Pueblo (USA) and Eastbourne (UK) sections. The presence of all the planktonic foraminiferid zones and details of the OAE 2 δ13C excursion indicates a relatively continuous but reduced sedimentation rate across the Cenomanian/Turonian (C/T) boundary. Sediment and TOC mass accumulation rates have been calculated and suggest a sedimentation break in the upper Bonarelli Level. This may be an artifact of the diachronous FAD of the planktonic foraminiferid Helvetoglobotruncana helvetica and suggests that in some sections the δ13C curve may provide more reliable age control for dating the C/T boundary. In order quantitatively to explain the carbon isotope curve and the measured TOC mass accumulation rate, a simple dynamic model of the isotope effects of organic versus inorganic carbon burial was developed. In order to verify the consistency of the model we correlated the modeled output of the Furlo section with that of the Manilva section, in southeast Spain. The modeling shows that increasing productivity only partially explains the measured δ13C excursion and is not the only factor relevant to black shales deposition. Preservation may play a central role, especially in the later stages of OAE 2. Phosphorus and TOC accumulation patterns in the Bonarelli Level in both Furlo and Manilva suggest a similar process although other factors may also be involved.
  • Publication
    Accès libre
    Enrichment of redox-sensitive trace metals (U, V, Mo, As) associated with the late Hauterivian Faraoni oceanic anoxic event
    (2007)
    Bodin, Stéphane
    ;
    Godet, Alexis
    ;
    Matera, Virginie
    ;
    Steinmann, Philipp
    ;
    Vermeulen, Jean
    ;
    Gardin, Silvia
    ;
    Adatte, Thierry 
    ;
    Coccioni, Rodolfo
    ;
    Föllmi, Karl B. 
    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.
  • Publication
    Accès libre
    Phosphorus and the roles of productivity and nutrient recycling during oceanic anoxic event 2
    (2007)
    Mort, Haydon P.
    ;
    Adatte, Thierry 
    ;
    Föllmi, Karl B. 
    ;
    Keller, Gerta
    ;
    Steinmann, Philipp
    ;
    Matera, Virginie
    ;
    Berner, Zsolt
    ;
    Stüben, Doris
    Four sections documenting the impact of the late Cenomanian oceanic anoxic event (OAE 2) were studied in basins with different paleoenvironmental regimes. Accumulation rates of phosphorus (P) bound to iron, organic matter, and authigenic phosphate are shown to rise and arrive at a distinct maximum at the onset of OAE 2, with an associated increase in δ13C values. Accumulation rates of P return to pre-excursion values in the interval where the δ13C record reaches its first maximum. An offset in time between the maximum in P accumulation and peaks in organic carbon burial, hydrogen indices, and Corg/Preact molar ratios is explained by the evolution of OAE 2 in the following steps. (1) An increase in productivity increased the flux of organic matter and P into the sediments; the preservation of organic matter was low and its oxidation released P, which was predominantly mineralized. (2) Enhanced productivity and oxidation of organic matter created dysoxic bottom waters; the preservation potential for organic matter increased, whereas the sediment retention potential for P decreased. (3) The latter effect sustained high primary productivity, which led to an increase in the abundance of free oxygen in the ocean and atmosphere system. After the sequestration of CO2 in the form of black shales, this oxygen helped push the ocean back into equilibrium, terminating black shale deposition and removing bioavailable P from the water column.