Zhou Zhe, Henkel Susann, Kasten Sabine, Holtappels Moritz
Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research, 27570 Bremerhaven, Germany.
Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research, 27570 Bremerhaven, Germany; Center for Marine Environmental Sciences - MARUM, University of Bremen, 28359 Bremen, Germany.
Sci Total Environ. 2023 Mar 20;865:161168. doi: 10.1016/j.scitotenv.2022.161168. Epub 2022 Dec 23.
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 Fe 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 O. Excessive P release (apart from OM remineralization) started at the beginning of the anoxic incubation and accelerated after the release of Fe with a constant P/Fe ratio of 0.26. After 34 days of anoxic incubation, porewater was re‑oxygenated and > 99 % of released P was coprecipitated through Fe 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 Fe release can escape the "Fe curtain" in surface sediments, thus potentially increasing net benthic P efflux from permeable sediments under variable redox conditions.
渗透性砂质沉积物覆盖了全球大陆架的50%-60%，是调节沿海海洋中有机物（OM）周转和养分循环的重要生物反应器。在砂质沉积物中，动态孔隙水对流可导致快速的质量传递和可变的氧化还原条件，从而影响OM的再矿化途径以及铁和磷的循环利用。在本研究中，将北海砂质沉积物置于流通式反应器（FTRs）中，以研究孔隙水对流和变化的氧化还原条件下的生物地球化学过程。我们发现，厌氧OM再矿化的平均速率比好氧途径低12倍，并且在34天的缺氧培养过程中，发现氢氧化铁（III）是主要的电子受体。在铁大量释放到孔隙水之前，还原态铁在固相（以Fe(II)表示）中积累，并且在整个缺氧培养过程中，大部分还原态铁（约96%）保留在固相中。通过自生含Fe(II)矿物的形成或吸附而保留在固相中的Fe(II)，在暴露于氧气后很容易被重新氧化。过量的磷释放（除了OM再矿化）在缺氧培养开始时就开始了，并且在铁以0.26的恒定P/Fe比释放后加速。经过34天的缺氧培养后，孔隙水被重新充氧，>99%的释放磷通过铁氧化共沉淀（所谓的“铁幕”）。我们的结果表明，在动态孔隙水对流条件下，固相中Fe(III)/Fe(II)可作为相对固定且可再充电的“氧化还原电池”。这种铁“氧化还原电池”是渗透性沉积物和具有可变氧化还原条件的环境的特征，使铁成为OM周转中的重要参与者。我们还认为，在铁释放之前释放的磷可以逃离表层沉积物中的“铁幕”，从而可能增加可变氧化还原条件下渗透性沉积物的底栖净磷通量。
