Detailed phosphorus geochemistry of sediments from the equatorial proto-atlantic at demerara rise during oceanic anoxic event 2.

摘要

Oceanic anoxic events (OAE) are associated with increased organic matter burial and possibly caused by major changes in marine nutrient cycling. Phosphorus (P) limits biological productivity on geologic timescales, thus detailed P geochemistry may provide insight into the role of nutrients in the formation of these organic-rich deposits. P geochemical records that the span the OAE across the Cenomanian-Turonian boundary (CTB, ~94 Ma), known as OAE 2, are rare, and detailed P geochemical records are usually limited to relatively shallow settings. In this study, a sequential extraction (SEDEX) technique was employed to evaluate the sedimentary distribution of P (oxide-associated, authigenic, detrital and organic) in sediments mainly consisting of laminated black shales spanning the CTB/OAE 2 interval at a sample resolution of ~2-5 cm collected from Demerara Rise during Ocean Drilling Program Leg 207. Intermediate (Site 1260, 2549 m) and deep-sea (Site 1258, 3292 m) water depths were compared to assess variations in P distribution across the CTB, with paleo-water depths of ∼500 to 1000 m respectively. Diagenetic and redox conditions result in alterations to the sedimentary distribution of P in ancient sediments. Most notable is the effect of "sink-switching" whereby organic P is converted to authigenic and/or oxide-associated phases. Sequential extraction enables an examination of the dominant pathways of P removal from the ocean thus providing insight into marine P mass balance. All phases experience a decrease during OAE 2 compared to pre-event values. The high resolution sampling of this study demonstrates that P speciation and concentrations are dynamic throughout the section at both sites as revealed by their marked downcore variability. Preact decreases markedly for both sites within the OAE 2 interval; while the largest decrease is observed in the Site 1260 record. Both sites reveal a mean Corg/Preact ratio near Redfield values pre-event increasing by several orders of magnitude above Redfield ratios during OAE 2, indicative of P regeneration via loss (relative to C) from sediments. Corg/Preact ratios generally decrease after the event. The combined effect supports decreased P burial efficiency, most notably at Site 1260 during low-oxygen conditions. Instability of the water column over Demerara Rise driven by variability in sea-surface temperatures (Forster et al., 2007) during OAE 2 could promote water mass mixing and upwelling of P from bottom waters into the photic zone. Here the impacts of diagenetic remobilization of P through the critical OAE 2 interval is seen as a shift from P burial to P recycling. Combined with epsilonNd (Martin et al, 2012), the P records support a circulation-controlled North Atlantic nutrient trap. This oceanographic regime likely enhanced primary productivity sustaining a productivity-anoxia feedback mechanism.