Iron redox shuttling and uptake by silicate minerals on the Namibian mud belt

IF 4.5 1区 地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS Geochimica et Cosmochimica Acta Pub Date : 2024-10-19 DOI:10.1016/j.gca.2024.10.013
Paul Vosteen , Michael Kossack , Christoph Vogt , Chloe H. Andersen , Sonja Geilert , Matthias Zabel , Florian Scholz
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Abstract

Anoxic marine sediments represent an important source of bioavailable iron (Fe) to the ocean. The highest sedimentary Fe fluxes are observed in open-marine oxygen minimum zones where anoxic bottom waters are in contact with ferruginous surface sediments. Here, sedimentary Fe release, transport and re-deposition (i.e., Fe shuttling) may generate a lateral pattern of sedimentary Fe enrichment and depletion, which can be used to trace redox-related Fe mobility in the paleo-record. However, depending on the balance between terrigenous and authigenic (i.e., shuttle-related) Fe flux, the stability of bottom water redox conditions as well as post-depositional processes of mineral alteration, the sedimentary fingerprint of an Fe redox shuttle may be obscured and difficult to identify.
We investigated sedimentary Fe cycling along two transects across the Namibian mud belt with variable terrigenous sedimentation (23°S < 25°S) and during two seasons with opposing bottom water redox conditions (oxic in austral winter versus anoxic to sulfidic in austral summer). On both transects, substantial benthic Fe fluxes up to −50 µmol m−2 d−1 were inferred based on pore water profiles. The magnitude of these fluxes is comparable to those reported for other open-marine oxygen minimum zones. On the transect at 23°S, Fe source areas with ferruginous surface sediments were clearly separated from Fe sink areas with highly sulfidic surface sediments. Therefore, Fe redox shuttling was reflected by a lateral pattern of reactive Fe depletion and enrichment relative to the terrigenous background sedimentation. By contrast, on the transect at 25°S, benthic Fe fluxes were temporally and spatially more variable and surface sediments were ferruginous or only weakly sulfidic. Therefore, sedimentary Fe depletion and enrichment was less pronounced at 25°S. In the Fe sink area at 23°S, hydrogen sulfide was present at the sediment surface during both sampling campaigns and solid phase data suggest that Fe sulfide minerals represented the main burial phase of reactive Fe. By contrast, at 25°S excess Fe was associated with potassium (K) rather than reduced sulfur. While a differing sediment provenance cannot be ruled out entirely, combined evidence from pore water silica profiles, K to Fe stoichiometric relationships and electron microprobe images suggest that laterally derived excess Fe was incorporated into pre-existing and/or authigenic clay minerals during early diagenesis. Iron uptake by clay minerals may be supported by frequent redox oscillations and sediment mixing preventing preservation of Fe sulfide minerals and promoting Fe and K fixation in clay minerals. The burial fluxes of excess Fe associated with sulfide minerals at 23°S and silicate minerals at 25°S were similar. Our findings thus underscore that neoformation or alteration of silicate minerals can be important processes within the low-temperature marine Fe cycle.
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纳米比亚泥带上硅酸盐矿物对铁氧化还原的穿梭和吸收
缺氧海洋沉积物是海洋生物可利用铁(Fe)的重要来源。在缺氧底层水与铁锈质表层沉积物接触的开阔海洋最小含氧区,可观察到最高的沉积铁通量。在这里,沉积铁的释放、迁移和再沉积(即铁的穿梭)可能会产生沉积铁的横向富集和贫化模式,可用于追踪古记录中与氧化还原有关的铁的流动性。不过,根据原生铁和自生铁(即与穿梭有关的铁)通量之间的平衡,古记录中与氧化还原有关的铁移动的稳定性也会受到影响、我们沿纳米比亚泥带的两个横断面研究了沉积铁循环,这两个横断面的陆相沉积(23°S < 25°S)和两个季节的底水氧化还原条件截然相反(冬季缺氧,夏季缺氧至硫酸)。在这两个断面上,根据孔隙水剖面推断出大量的底栖铁通量,最高可达-50 µmol m-2 d-1。这些通量的大小与其它开阔海洋最低含氧量区的通量相当。在南纬 23°的横断面上,铁锈质表层沉积物的铁源区与高硫酸盐表层沉积物的铁汇区明显分开。因此,相对于土著背景沉积物而言,铁氧化还原穿梭反映为反应性铁的横向贫化和富集模式。相比之下,在南纬 25 度的横断面上,底栖铁通量在时间和空间上的变化更大,表层沉积物为铁锈色或仅为弱硫酸盐。因此,在南纬 25°,沉积铁的贫化和富集不那么明显。在南纬 23°的铁沉积区,两次取样过程中沉积物表面都存在硫化氢,固相数据表明硫化铁矿物是活性铁的主要埋藏相。相比之下,在南纬 25°,过量的铁与钾(K)而不是还原硫有关。虽然不能完全排除沉积物来源不同的可能性,但孔隙水硅石剖面、钾与铁的化学计量关系以及电子显微探针图像等综合证据表明,侧向衍生的过量铁在早期成岩过程中融入了原有的和/或自生的粘土矿物中。粘土矿物对铁的吸收可能得益于频繁的氧化还原振荡和沉积物混合,这阻碍了硫化铁矿物的保存,并促进了粘土矿物中铁和钾的固定。在南纬 23°,与硫化物矿物相关的过量铁的埋藏通量与在南纬 25°,与硅酸盐矿物相关的过量铁的埋藏通量相似。因此,我们的研究结果强调,硅酸盐矿物的新形成或改变可能是低温海洋铁循环的重要过程。
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来源期刊
Geochimica et Cosmochimica Acta
Geochimica et Cosmochimica Acta 地学-地球化学与地球物理
CiteScore
9.60
自引率
14.00%
发文量
437
审稿时长
6 months
期刊介绍: Geochimica et Cosmochimica Acta publishes research papers in a wide range of subjects in terrestrial geochemistry, meteoritics, and planetary geochemistry. The scope of the journal includes: 1). Physical chemistry of gases, aqueous solutions, glasses, and crystalline solids 2). Igneous and metamorphic petrology 3). Chemical processes in the atmosphere, hydrosphere, biosphere, and lithosphere of the Earth 4). Organic geochemistry 5). Isotope geochemistry 6). Meteoritics and meteorite impacts 7). Lunar science; and 8). Planetary geochemistry.
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