Sedimentary iron–phosphorus cycling under contrasting redox conditions in a eutrophic estuary

Phosphorus (P) is often a limiting nutrient within freshwater and estuarine systems, thus excess inputs of P from anthropogenic activities (dominantly agriculture) can induce eutrophication in receiving water bodies. The sequestration of P within estuarine sediments is controlled by sorption and precipitation processes, which are impacted by local redox conditions and burial environment. Despite the global spread of oxygen depletion in coastal marine systems, P burial under reducing conditions remains poorly understood. We investigated P cycling in relation to iron (Fe) redox chemistry in sediments from the eutrophic Peel-Harvey Estuary in Western Australia, using a combination of porewater analysis, sequential chemical P extractions, synchrotron-based micro-scale X-ray fluorescence mapping and Fe K-edge X-ray absorption spectroscopy, and PO₄³⁻ sorption experiments. The sediments represented redox regimes varying from strongly reducing, organic-rich sediments with little or no reactive Fe(III) (oxyhydr)oxides to oxygenated sediments that were enriched in reactive Fe(III) phases. Organic P and Fe-associated P were the main P burial phases, and the latter was quantitatively important even in sediments with an overall strongly reducing character. We attribute this to adsorption of P onto micro-scale Fe(III) oxyhydroxide enrichments and/or Fe-bearing clay minerals. The organic-rich sediments showed a strong decline in P contents with depth; P was released from organic matter and Fe phases but apatite precipitation was apparently inhibited in these sediments. Despite greater and stronger PO₄^{3 −} sorption capacity, the oxic sediments contained relatively little P due to a lack of the primary P source in marine sediments: organic matter. Our results provide detailed insight into P burial in dynamic estuarine sediments and show that micro-scale spectroscopic analyses greatly advance our understanding of P sequestration processes.