The iron "redox battery" in sandy sediments: Its impact on organic matter remineralization and phosphorus cycling

Permeable sandy sediments cover 50-60 % of the global continental shelf and are important bioreactors that regulate organic matter (OM) turnover and nutrient cycling in the coastal ocean. In sands, the dynamic porewater advection can cause rapid mass transfer and variable redox conditions, thus affecting OM remineralization pathways, as well as the recycling of iron and phosphorus. In this study, North Sea sands were incubated in flow-through reactors (FTRs) to investigate biogeochemical processes under porewater advection and changing redox conditions. We found that the average rate of anaerobic OM remineralization was 12 times lower than the aerobic pathway, and Fe(III) oxyhydroxides were found to be the major electron acceptors during 34 days of anoxic incubation. Reduced Fe accumulated in the solid phase (expressed as Fe(II)) before significant release of Fe²⁺ into the porewater, and most of the reduced Fe (~96 %) remained in the solid phase throughout the anoxic incubation. Fe(II) retained in the solid phase, either through the formation of authigenic Fe(II)-bearing minerals or adsorption, was easily re-oxidized upon exposure to O₂. Excessive P release (apart from OM remineralization) started at the beginning of the anoxic incubation and accelerated after the release of Fe²⁺ with a constant P/Fe²⁺ ratio of 0.26. After 34 days of anoxic incubation, porewater was re‑oxygenated and > 99 % of released P was coprecipitated through Fe²⁺ oxidation (so-called "Fe curtain"). Our results demonstrate that Fe(III)/Fe(II) in the solid phase can serve as a relatively immobile and rechargeable "redox battery" under dynamic porewater advection. This Fe "redox battery" is characteristic for permeable sediments and environments with variable redox conditions, making Fe an important player in OM turnover. We also suggest that P liberated before Fe²⁺ release can escape the "Fe curtain" in surface sediments, thus potentially increasing net benthic P efflux from permeable sediments under variable redox conditions.