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

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 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.