An estimate of the efficiency of the iron‐ and manganese‐driven dissolved inorganic phosphorus trap at an oxic/euxinic water column redoxcline

Abstract

Geochemical records suggest the ocean has undergone periods of at least partial deeper‐ocean anoxia or euxinia. Two counteracting feedback loops involving redox control of the dynamics of the phytoplankton nutrient dissolved inorganic phosphorus (DIP) might coexist, helping to stabilize the redox state of the atmosphere and oceans. This concept implies that, during deeper‐ocean anoxia, the DIP transfer from the deep anoxic into the oxic surface ocean is uninhibited by processes taking place at the redoxcline. This implicit assumption requires testing because iron (Fe) and manganese (Mn) dynamics at oxic/anoxic water column redoxclines have the potential to form a DIP trap, inhibiting DIP transport from anoxic deep into oxic surface waters. Using a time series data set of Fe, Mn, DIP, and dissolved oxygen distributions in the Eastern Gotland Basin of the Baltic Sea, we provide estimates of the efficiency of this Fe‐ and Mn‐driven DIP trap. This efficiency was estimated by calculating the ratios of (1) the downward flux of DIP adsorbed onto and/or coprecipitated into the settling authigenic Fe‐ and Mn‐rich particles just above the redoxcline and (2) the upward turbulent‐diffusive DIP flux across the redoxcline. Depending on the assumed particle densities, we find average ±1 SD trapping efficiencies of 0.38 ± 0.29 and 0.63 ± 0.45. The efficiencies are significant in that they seem to impact cyanobacterial dynamics in the central Baltic Sea. We discuss possible implications of the trapping mechanism for, and propose two hypotheses relating to the potential importance of Fe‐controlled DIP trapping at redoxclines during, ocean anoxic events.