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Phosphorus and the roles of productivity and nutrient recycling during oceanic anoxic event 2

Haydon P. Mort^*,1, Thierry Adatte¹, Karl B. Föllmi¹, Gerta Keller², Philipp Steinmann^,3, Virginie Matera³, Zsolt Berner⁴ and Doris Stüben⁴

¹ Institute of Geology, University of Neuchâtel, Rue Emile Argand 11, Case postale 158, CH-2009 Neuchatel, Switzerland
² Department of Geosciences, Princeton University, Guyot Hall, Princeton, New Jersey 08544-1003, USA
³ Institute of Geology, University of Neuchâtel, Rue Emile Argand 11, Case postale 158, CH-2009 Neuchatel, Switzerland
⁴ Institut für Mineralogie und Geochemie, Universität Karlsruhe, 76128 Karlsruhe, Germany

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 ¹³C values. Accumulation rates of P return to pre-excursion values in the interval where the ¹³C record reaches its first maximum. An offset in time between the maximum in P accumulation and peaks in organic carbon burial, hydrogen indices, and C_org/P_react 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.

Key Words: phosphorus • feedbacks • biogeochemistry • Cenomanian-Turonian • oceanic anoxic event 2

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