Mao-Xu Zhu

Reactive iron and its buffering capacity towards dissolved sulfide in sediments of Jiaozhou Bay, China.

Reactive iron (Fe) oxides in marine sediments play a critical role in removal of free sulfide. In this study, 0.5 and 6 N HCl-extractable Fe, acid volatile sulfide (AVS), and pyrite were examined in sediments at three sites of eutrophic Jiaozhou Bay to investigate the interactions of sulfur and Fe and possible influences of eutrophication on free sulfide removal. The results indicate that formation and accumulation of AVS and pyrite are limited by low availability of labile organic matter, despite eutrophication of the bay water. Quick buffering of free sulfide proceeded mainly via consumption of 0.5 N HCl-extractable Fe (labile Fe), however, the consumption did not result in a depletion of the Fe pool. High residual buffering capacity enables a quick removal of free sulfide in porewater, and thereby it is difficult for sulfide to accumulate and to cause detrimental effects on benthic organisms at the present steady state. Significant effects of eutrophication on Fe and sulfur geochemistry is restricted only to the estuarine sediments which were subject to direct wastewater discharges, whereas no such effects were observed in other sediments of the bay.

Sulfur and iron diagenesis in temperate unsteady sediments of the East China Sea inner shelf and a comparison with tropical mobile mud belts (MMBs)

Redox cycling of iron (Fe) and sulfur (S) exerts profound influences on fates of numerous elements in coastal marine sediments. In this study, S and Fe cycling and its geochemical expressions in the East China Sea (ECS) inner shelf, a representative of temperate mobile mud belts (MMBs), were characterized and compared with tropical counterparts (the Amazon shelf and the Gulf of Papua). Fe and S speciation consistently points to the dominance of authigenic nonsulfidized Fe(II) phases (i.e. poorly crystalline clays (PCCs) and carbonates) and the prevalence of Fe redox cycling in the suboxic or weakly sulfidic regimes of the ECS-MMBs. High contents of authigenic magnetite may be a common diagenetic expression in all MMBs. Compared to the tropical MMBs, three main differences of diagenetic expressions in the ECS-MMBs are: (i) light 34Spyrite in the ECS-MMB vs. characteristically heavy 34Spyrite in the Amazon shelf MMBs; (ii) lower total reactive Fe (FeTR), total diagenetic Fe(II), and ratio of FeTR to total Fe in the ECS-MMBs; (iii) Fe(II) carbonates and PCCs are equally important sinks for nonsulfidized Fe(II) in the ECS-MMBs, whereas PCCs are the predominant sink in the tropical counterparts. These differences are ascribable to factors including low degradability of organic matter, small diffusion scales, less intense chemical weathering in the drainage basin and/or weaker reverse weathering in the ECS-MMBs. Despite the differences above, Fe and S diagenetic expressions that characterize the prevalence of Fe redox cycling in the unsteady suboxic regimes are shared by the ECS- and tropical MMBs.

Spatial distribution of organic and pyritic sulfur in surface sediments of eutrophic Jiaozhou Bay, China: Clues to anthropogenic impacts

Anthropogenic perturbations exert important impacts on sulfur geochemistry in marine sediments. In the study, chemical extraction was used to quantify four sulfur pools, i.e., pyrite, humic-acid sulfur (HA-S), fulvic-acid sulfur (FA-S), and residual organic sulfur (ROS), in surface sediments of eutrophic Jiaozhou Bay. Results show that riverine inputs are the main control on organic matter (OM) distribution in the sediments. OM enrichment in the eastern coast is mainly due to discharges of anthropogenic wastes. Spatial coupling of pyrite and FA-S vs. TOC points to the impacts of OM enrichment on formation and preservation of pyrite and FA-S. Poor spatial coupling of HA-S vs. TOC is due to low fractions of diagenetic OS in the pool. ROS is mainly from riverine inputs and anthropogenic OS has been superimposed on this pool. Spatial coupling among TOC, pyrite-S and FA-S is a sensitive indicator of anthropogenic impacts on benthic processes of the bay.

Reductive reactivity of iron(III) oxides in the east china sea sediments: characterization by selective extraction and kinetic dissolution.

Reactive Fe(III) oxides in gravity-core sediments collected from the East China Sea inner shelf were quantified by using three selective extractions (acidic hydroxylamine, acidic oxalate, bicarbonate-citrate buffered sodium dithionite). Also the reactivity of Fe(III) oxides in the sediments was characterized by kinetic dissolution using ascorbic acid as reductant at pH 3.0 and 7.5 in combination with the reactive continuum model. Three parameters derived from the kinetic method: m 0 (theoretical initial amount of ascorbate-reducible Fe(III) oxides), k′ (rate constant) and γ (heterogeneity of reactivity), enable a quantitative characterization of Fe(III) oxide reactivity in a standardized way. Amorphous Fe(III) oxides quantified by acidic hydroxylamine extraction were quickly consumed in the uppermost layer during early diagenesis but were not depleted over the upper 100 cm depth. The total amounts of amorphous and poorly crystalline Fe(III) oxides are highly available for efficient buffering of dissolved sulfide. As indicated by the m 0, k′ and γ, the surface sediments always have the maximum content, reactivity and heterogeneity of reactive Fe(III) oxides, while the three parameters simultaneously downcore decrease, much more quickly in the upper layer than at depth. Albeit being within a small range (within one order of magnitude) of the initial rates among sediments at different depths, incongruent dissolution could result in huge discrepancies of the later dissolution rates due to differentiating heterogeneity, which cannot be revealed by selective extraction. A strong linear correlation of the m 0 at pH 3.0 with the dithionite-extractable Fe(III) suggests that the m 0 may represent Fe(III) oxide assemblages spanning amorphous and crystalline Fe(III) oxides. Maximum microbially available Fe(III) predicted by the m 0 at pH 7.5 may include both amorphous and a fraction of other less reactive Fe(III) phases.

Characterization of iron diagenesis in marine sediments using refined iron speciation and quantized iron(III)-oxide reactivity: a case study in the Jiaozhou Bay, China

As a case study, refined iron (Fe) speciation and quantitative characterization of the reductive reactivity of Fe (III) oxides are combined to investigate Fe diagenetic processes in a core sediment from the eutrophic Jiaozhou Bay. The results show that a combination of the two methods can trace Fe transformation in more detail and offer nuanced information on Fe diagenesis from multiple perspectives. This methodology may be used to enhance our understanding of the complex biogeochemical cycling of Fe and sulfur in other studies. Microbial iron reduction (MIR) plays an important role in Fe(III) reduction over the upper sediments, while a chemical reduction by reaction with dissolved sulfide is the main process at a deeper (> 12 cm) layer. The most bioavailable amorphous Fe(III) oxides [Fe(III)am] are the main source of the MIR, followed by poorly crystalline Fe(III) oxides [Fe(III)pc)] and magnetite. Well crystalline Fe(III) oxides [Fe (III)wc] have barely participated in Fe diagenesis. The importance of the MIR over the upper layer may be a combined result of the high availability of highly reactive Fe oxides and low availability of labile organic matter, and the latter is also the ultimate factor limiting sulfate reduction and sulfide accumulation in the sediments. Microbially reducible Fe(III) [MR-Fe(III)], which is quantified by kinetics of Fe(II)-oxide reduction, mainly consists of the most reactive Fe(III)am and less reactive Fe(III)pc. The bulk reactivity of the MR-Fe(III) pool is equivalent to aged ferrihydrite, and shows down-core decrease due to preferential reduction of highly reactive phases of Fe oxides.

In situ, high-resolution DGT measurements of dissolved sulfide, iron and phosphorus in sediments of the East China Sea: Insights into phosphorus mobilization and microbial iron reduction

Dissolved sulfide, iron (Fe), and phosphorus (P) concentrations in sediments of the East China Sea were simultaneously measured in situ by diffusive gradients in thin films (DGT) technique. The results, by combination with solid-phase Fe speciation, were used to characterize the interplays of Fe, S and P. Diverse distributions of dissolved sulfide among the sites are attributable to highly dynamic diagenetic regimes and varying availability of labile organic carbon (OC). The DGT technique provided high-resolution evidence for coexistence of microbial iron reduction (MIR) and sulfate reduction in localized zones, and for Fe-coupled P mobilization. Measured Fe2+/P ratios suggest that Fe2+ reoxidatiion at the oxic zones can serve as an efficient scavenger of P. Empirical estimation indicates that MIR plays an important role in anaerobic OC mineralization in the sediments, which is a combined result of low availability of labile OC, high reactive Fe content, and unsteady diagenetic regimes.