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  • Publication
    Accès libre
    Organic carbon deposition and phosphorus accumulation during Oceanic Anoxic Event 2 in Tarfaya, Morocco
    (2008)
    Mort, Haydon P.
    ;
    Adatte, Thierry 
    ;
    Keller, Gerta
    ;
    Bartels, David
    ;
    Föllmi, Karl B.
    ;
    Steinmann, Philipp
    ;
    Berner, Zsolt
    ;
    Chellai, E.H.
    With a multi-proxy approach, an attempt was made to constrain productivity and bottom-water redox conditions and their effects on the phosphorus accumulation rate at the Mohammed Plage section on the Tarfaya coast, Morocco, during the Cenomanian-Turonian Anoxic Event (OAE 2). A distinct δ13Corg isotope excursion of +2.5‰ occurs close to the top of the section. The unusually abrupt shift of the isotope excursion and disappearance of several planktonic foraminiferal species (e.g. Rotalipora cushmani and Rotalipora greenhornensis) in this level suggests a hiatus of between 40–60 kyrs at the excursion onset. Nevertheless, it was possible to determine both the long-term environmental history as well as the processes that took place immediately prior to and during OAE 2. TOC% values increase gradually from the base of the section to the top (from ~ 2.5% to ~ 10%). This is interpreted as the consequence of a long-term eustatic sea-level rise and subsidence causing the encroachment of less oxic waters into the Tarfaya Basin. Similarly a reduction in the mineralogically constructed ‘detrital index’ can be explained by the decrease in the continental flux of terrigenous material due to a relative sea-level rise. A speciation of phosphorus in the upper part of the section, which spans the start and mid-stages of OAE 2, shows overall higher abundances of Preactive mass accumulation rates before the isotope excursion onset and lower values during the plateau. Due to the probable short hiatus, the onset of the decrease in phosphorus content relative to the isotope excursion is uncertain, although the excursion plateau already contains lower concentrations. The Corg/Ptotal and V/Al ratios suggest that this reduction was mostly likely caused by a decrease in the available bottom oxygen content (probably as a result of higher productivity) and a corresponding fall in the phosphorus retention ability of the sediment. Productivity appears to have remained high during the isotope plateau possibly due to a combination of ocean-surface fertilisation via increased aridity (increased K/Al and Ti/Al ratios) and/or higher dissolved inorganic phosphorus content in the water column as a result of the decrease in sediment P retention. The evidence for decreased P-burial has been observed in many other palaeoenvironments during OAE 2. Tarfaya's unique upwelling paleosituation provides strong evidence that the nutrient recycling was a global phenomenon and therefore a critical factor in starting and sustaining OAE 2.
  • 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.
  • 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
    Evolution of the marine stable carbon-isotope record during the early Cretaceous : A focus on the late Hauterivian and Barremian in the Tethyan realm
    (2006-02-28)
    Godet, Alexis
    ;
    Bodin, Stéphane
    ;
    Föllmi, Karl B.
    ;
    Vermeulen, Jean
    ;
    Gardin, Silvia
    ;
    Fiet, Nicolas
    ;
    Adatte, Thierry 
    ;
    Berner, Zsolt
    ;
    Stüben, Doris
    ;
    Van de Schootbrugge, Bas
    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.