The Iron “Redox Battery” in Sandy Sediments: Its Impact on Organic Matter Remineralization and Phosphorus Cycling

38 Pages Posted: 16 Sep 2022

See all articles by Zhe Zhou

Zhe Zhou

Tongji University

Susann Henkel

affiliation not provided to SSRN

Sabine Kasten

affiliation not provided to SSRN

Moritz Holtappels

affiliation not provided to SSRN

Abstract

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 as the major electron acceptors during 34 days of anoxic incubation. Abundant reduced Fe in the solid phase (expressed as Fe(II)) was measured before extensive Fe2+ release into 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 “Fe2+ curtain”). Our results demonstrate that Fe(III)/Fe(II) in the solid phase can serve as relatively immobile and rechargeable “redox battery” under dynamic porewater advection. Due to frequent oscillation of redox conditions, the Fe “redox battery” is characteristic for permeable sediments and plays an important role in coastal OM turnover. We also suggest that P liberated before Fe2+ release can escape the “Fe2+ curtain” in porewater advection, thus potentially increasing net benthic P efflux from permeable sediments under variable redox conditions.

Keywords: permeable sediments, advection, redox cycling, Iron, Organic matter, Phosphorus

Suggested Citation

Zhou, Zhe and Henkel, Susann and Kasten, Sabine and Holtappels, Moritz, The Iron “Redox Battery” in Sandy Sediments: Its Impact on Organic Matter Remineralization and Phosphorus Cycling. Available at SSRN: https://ssrn.com/abstract=4220846 or http://dx.doi.org/10.2139/ssrn.4220846

Zhe Zhou (Contact Author)

Tongji University ( email )

Susann Henkel

affiliation not provided to SSRN ( email )

No Address Available

Sabine Kasten

affiliation not provided to SSRN ( email )

No Address Available

Moritz Holtappels

affiliation not provided to SSRN ( email )

No Address Available

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