Iron oxide mineralogy and stable iron isotope composition in a Gleysol with petrogleyic properties
Iron oxide mineralogy and stable iron isotope composition in a Gleysol with petrogleyic properties
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DOI：
10.1007/s11368-011-0402-z
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发表时间：
2012-04
影响因子：
3.6
通讯作者：
T. Mansfeldt;S. Schuth;W. Häusler;F. Wagner;S. Kaufhold;M. Overesch
中科院分区：
文献类型：
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作者：
T. Mansfeldt;S. Schuth;W. Häusler;F. Wagner;S. Kaufhold;M. Overesch
目的在地下水位波动和氧化还原条件变化的土壤中，铁氧化物的性质知之甚少。本研究的目的是（a）表征铁氧化物的矿物组成和（B）确定在时间和空间上具有急剧氧化还原梯度的土壤中与稳定铁同位素比率的关系。（Petrogleyic）位于德国西北部，由氧化质土壤层组成（Ah 0-15，Bg 15-35，CrBg 35-70 cm）发育于全新世河流壤土之上，具有还原地貌特征（2Cr层，+70 cm）。在28个月的过程中，实地测量包括地下水位，土壤氧化还原电位，土壤溶液的分析监测。固体铁相进行了研究，室温和cryogenic 57 Fe穆斯堡尔谱，稳定的铁同位素组成的多个收集器电感耦合等离子体massspectrometry.Results and discussionThe地下水位范围从-83厘米以下+8厘米以上的土壤表面（中位数-27厘米）。2 Cr层出现永久还原条件，溶解Fe浓度为44.8 mg L-1（中位数）。氧化条件的持续时间按CrBg < Bg < Ah的顺序增加。总铁从50（Ah）到316（Bg）增加到412 g kg−1（CrBg），在2Cr层中最低（7 g kg−1）。在AH层中，铁水石（占总Fe的51%）比针铁矿（24%）占主导地位。相反，纳米针铁矿占主导地位的Bg（94%）和CrBg（86%）的视野。菱铁矿中的铁在CrBg层中占7%。铁同位素组成显示δ 57 Fe值范围为+0.29‰（Ah层）至-0.30 ‰（Bg层）。与上覆的CrBg（δ 57 Fe = − 0. 19 ‰）和Bg层相比，2Cr层具有较高的δ 57 Fe值（+0. 22 ‰）。一旦形成，针铁矿与水铁矿相比保持稳定，因为它不太可用于微生物介导的还原溶解。表层土壤中高δ 57 Fe值主要是由于还原条件后立即通气期间快速的水铁矿沉淀。相反，低δ 57 Fe值的富铁层（Bg，CrBg）促进吸附溶解铁与轻同位素组成的针铁矿在毛细上升。贫铁底土（2Cr）的δ 57 Fe值较高，与硅酸盐结合态铁有关，而与铁氧化物的溶解和沉淀无关。
PurposeProperties of Fe oxides are poorly understood in soils with fluctuating water tables and variable redox conditions. The objective of this research was to (a) characterize the mineralogical composition of Fe oxides and (b) determine the relationship to the stable Fe isotope ratio in a soil with temporally and spatially sharp redox gradients.Materials and methodsThe lowland Gleysol (Petrogleyic) is in Northwest Germany and consists of oximorphic soil horizons (Ah 0–15, Bg 15–35, and CrBg 35–70 cm) developed from Holocene fluvial loam overlaying glaciofluvial sand with reductomorphic properties (2Cr horizon, +70 cm). Field measurements during the course of 28 months included the monitoring of groundwater table, soil redox potential, and analysis of the soil solutions. Solid Fe phases were studied by room temperature and cryogenic57Fe Mössbauer spectroscopy, and stable Fe isotope compositions by multiple collector inductively coupled plasma mass spectrometry.Results and discussionThe groundwater table ranged from −83 cm below to +8 cm above soil surface (median −27 cm). Permanent reducing conditions occurred in the 2Cr horizon with dissolved Fe concentrations of 44.8 mg L−1(median). The duration of oxidizing conditions increased in the order CrBg < Bg < Ah. Total Fe increased from 50 (Ah) over 316 (Bg) up to 412 g kg−1(CrBg) and was lowest in the 2Cr horizon (7 g kg−1). Ferrihydrite (51% of total Fe) was dominant over goethite (24%) in the Ah horizon. Conversely, nanogoethite dominated both the Bg (94%) and CrBg (86%) horizons. Iron in siderite amounted to 7% in the CrBg horizon. Iron isotope compositions yielded a range ofδ57Fe values from +0.29‰ (Ah horizon) to −0.30‰ (Bg horizon). In contrast to the overlying CrBg (δ57Fe = −0.19‰) and Bg horizons, the 2Cr horizon is characterized by a relatively highδ57Fe value of +0.22‰.ConclusionsLasting water saturation and frequent reducing conditions lead to the enrichment of goethite in subsoil. Once formed, goethite remains stable compared to ferrihydrite because it is less available for microbial mediated reductive dissolution. Highδ57Fe values in the topsoil primary result from fast ferrihydrite precipitation during aeration immediately after reducing conditions. In contrast, the lowδ57Fe values of the Fe-rich horizons (Bg, CrBg) promote adsorption of dissolved Fe with a light isotope composition onto goethite during capillary rise. The highδ57Fe value of the Fe-poor subsoil (2Cr) is related to silicate-bound Fe rather than dissolution and precipitation of Fe oxides.