Phosphorus cycling from the margin to abyssal depths in the proto-Atlantic during oceanic anoxic event 2

Phosphorus (P) burial during the Cenomanian/Turonian oceanic anoxic event (OAE 2, ∼ 93.55 Ma) was studied using sediment records from four sites in the proto-Atlantic Ocean. The sites represent a variety of depositional settings, from the continental margin and slope to the abyssal plain. Strong links between changing marine conditions during OAE 2 and P cycling were observed in black shale sections on the continental margin and slope. Sedimentary P contents and P MARs increased around the onset of OAE 2, which may reflect enhanced productivity and P burial, partly in the form of biogenic apatite (fish debris). During the main stage of OAE 2, phosphorus was efficiently regenerated from the sediment, resulting in P-depleted sediments and high organic C (C_org)/total P ratios. The increased availability of dissolved P may have helped to sustain high productivity during OAE 2. The termination of OAE 2 was associated with a strong recovery of P burial efficiency in the sediment. Sedimentary records from the two deep sites do not reveal such close links between different stages of OAE 2 and the characteristics of P cycling. Here, OAE 2 was a period with maximum deposition of relatively thin black shale intervals that alternate with more oxic deep sea sediments. The black shale layers record enhanced P regeneration from organic matter and, in some cases, P enrichment relative to the oxic sediments. Detailed chemical P fractionation was performed on sediment from all sites, but the results were found to mostly reflect diagenesis and sample handling artifacts. In these ancient sediments, total P concentrations and the C_org/total P ratio appear to be the most robust indicators of P cycling and regeneration.