Abstract
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.
Highlights
Iron (Fe) is the most abundant, redox-sensitive element on earth’s surface
In organic matter (OM)-rich marine sediments where dissolved sulfide is readily available via sulfate reduction, chemical reduction of Fe(III) oxides by dissolved sulfide generally dominates over the microbial pathways since the former is thermodynamically more favored than the latter, whereas microbial Fe(III) reduction is more important in some other marine sediments where reactive Fe(III) is abundant and/or OM flux to the seabed is low to intermediate [1,13,14,15,16,17]
The objectives of this study are: (1) to reveal the effects of early diagenesis on reductive reactivity of Fe(III) oxides based on depth-dependent variability of the three kinetic parameters, m0, k9 and c; (2) to compare selective extraction and kinetic dissolution with regard to the reactivity of Fe(III) oxides
Summary
Iron (Fe) is the most abundant, redox-sensitive element on earth’s surface. Its redox cycling in marine sediments has a profound influence on cycling and fate of carbon, sulfur, phosphorus, and a variety of trace elements [1,2,3]. Secondary Fe phases (from weathering of Fe-bearing primary minerals) occur dominantly as Fe(III) oxides, hydroxides, and oxyhydroxides (collectively hereafter referred to as Fe(III) oxides) with a wide spectrum of mineralogy, crystallinity, morphology and chemical compositions. They usually display quite variable adsorption affinity and reduction reactivity [4,5,6]. Quantitative characterization of Fe(III) oxide reactivity is of significance for understanding Fe cycles and its influences on diagenesis of many other interlinked elements
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