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  • 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
    Sedimentary phosphorus record from the Oman margin : New evidence of high productivity during glacial periods
    (2003-03-25)
    Tamburini, Federica
    ;
    Föllmi, Karl B.
    ;
    Adatte, Thierry 
    ;
    Bernasconi, Stefano M.
    ;
    Steinmann, Philipp
    The northern region of the Arabian Sea is one of the biologically most fertile regions of the world oceans, with present productivity rates varying between 150 and 2500 mgC/m2 × day [ Madhupratap et al., 1996 ]. This is related to the influence of the southwesterly summer monsoon which causes vigorous upwelling along the Oman margin. Upwelling ceases during northeasterly winter monsoon activity; productivity rates, however, remain relatively high (about 800 mgC/m2 × day), related to deep water mixing [ Madhupratap et al., 1996 ]. The goal of this study is to verify if during the last glacial period, a period in which winter monsoon conditions prevailed, productivity rates were similarly high. With an analysis of phosphorus phases, stable nitrogen isotopes, organic matter content, and bulk mineralogy of the upper 10 m of the cores of ODP Hole 724C (corresponding to the last 140,000 years, sample resolution is ∼5 kyr), we provide new evidence of high productivity during this last glacial period (marine isotopic stages 2, 3, and 4). This was probably related to the combined effect of (1) increased eolian input of iron-containing dust due to dryness on the adjacent continent and stronger winter monsoon, and (2) regeneration and diffusion of dissolved phosphorus from the sediments to the water column due to variations in the position and intensity of the Oxygen Minimum Zone. These findings suggest that there is no one-to-one relationship between summer monsoon activity and productivity, which emerges to be a quasi-persistent phenomenon across glacial and interglacial stages.