Benthic phosphorus and iron budgets for NW-African slope sediments; biogeochemical processes and the importance of bioturbation

Abstract

Abstract. Despite intensive research on the different domains of the marine phosphorus (P) cycle during the last decades, frequently discussed open questions still exist especially on controlling factors for the benthic behaviour of P and its general distribution in sediment-pore water systems. Steady state or the internal balance of all relevant physical and (bio)geochemical processes are amongst the key issues. In this study we present and discuss an extended data set from surface sediments recovered from three locations on the NW-African continental slope. Pore water data and results from sequential sediment extractions indicate a close relationship between the benthic cycles of P and iron. Accordingly, most of the dissolved phosphate must have been recently released by microbially catalyzed reductive dissolution of iron (oxhydr)oxides. However, rates of release and association of P and iron, respectively, are not directly represented in profiles of element specific sediment compositions. Steady-state based transport-reaction modelling results suggest that particle mixing due to active bioturbation, or rather a physical net downward transport of P associated to iron (oxyhydr)oxides, is an essential process for the balance of the inspected benthic cycles. This study emphasizes the importance of balancing analytical data so as to provide for a more comprehensive interpretation of all relevant processes.