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1.
峡东地区南华纪-寒武纪地层风化过程元素及Sr-Nd同位素演化特征及其地球化学意义 总被引：1，自引：0，他引：1
以峡东地区南华纪、震旦纪和寒武纪标准地层泥岩、冰碛泥岩、砂岩、灰岩和白云质灰岩及对应的风化土壤为研究对象, 分析了地层风化成土过程中不同元素的迁移行为, 根据剖面样品的Sr-Nd同位素组成变化, 探讨了其同位素体系的封闭性特征与应用意义.结果表明: (1) 不同岩性基岩在成土过程中的蚀变强度有明显的差异, 在相似地表条件下, 碳酸岩风化剖面的风化程度高于泥质岩和砂岩; (2) 通过对比稳定高场强Ti元素在基岩和风化剖面中的含量变化, 计算出土壤样品在风化过程中体积相对基岩发生的改变量, 进而计算出不同岩性基岩在风化过程中微量元素的绝对含量变化以探讨这些元素的活动规律.结果表明, 灰岩和白云质灰岩的风化剖面元素含量变化明显, 而在泥质岩风化过程中大多数元素保持了相对稳定, 说明沉积岩风化过程中元素的活动性特征明显地受到了原岩矿物组成的制约.风化过程中, 不同性质的元素的活动性差异明显, 其中亲硫元素(Cu、Zn、Pb、Mo) 和大离子亲石元素(Rb、K、Sr、Ba) 在不同岩性的风化剖面中均表现出明显的元素含量变化, 而高场强元素(Zr、Hf、Nb、Ta) 含量则相对稳定; (3) 泥质岩风化形成的土壤层REE含量变化较小, 而碳酸岩风化土壤层REE含量发生了明显下降, 且其风化形成的土壤表现出LREE和HREE相对于MREE的富集.无论是碳酸岩或泥质岩风化形成的土壤, 均出现了明显的Eu负异常和Ce的正异常, 但在其原岩中这些异常并不存在或不明显; (4) 基岩与土壤剖面间Sr同位素组成和Rb/Sr比值存在明显差异, Rb-Sr同位素组成发生了明显的开放.所形成土壤层的Sr同位素组成受到2种因素的约束: 原岩性质和外来组成的Sr同位素比值.因此在总体上, 风化土壤的Sr同位素组成已不能代表基岩的Sr同位素组成; (5) 沉积岩风化过程中, 碳酸岩和泥质岩形成的风化土壤基本保持了原岩的Sm-Nd同位素组成特点, 由其组成所获得的Nd同位素亏损地幔模式年龄等能反映其原岩信息, 而近源沉积形成的砂岩和含砾冰碛泥岩所形成的土壤, 其Nd同位素组成则存在不同程度的改变. 相似文献
2.
《Chemie der Erde / Geochemistry》2015,75(3):301-315
Sr and Nd isotopes were applied to 5 soil profiles from the Muravera area, in south-eastern Sardinia.All the soils, which have developed during the Quaternary on the Lower Paleozoic metamorphic basement except for one on Eocene carbonates, are located far from major sources of pollution. Therefore, they are suitable for testing pedogenic processes and geochemical evolution to benefit for environmental studies.The Sr isotopic ratios range largely (δ87Sr = 1.7–65.9‰), even in each soil profile. In particular, the observed increase of δ87Sr with depth in the most of the metamorphic rock-based soils can be accounted for by the downward decrease of Sr contributions from organic matter and Saharan dust, both displaying lower isotopic ratios than the soil bedrocks. The carbonate rock-based soil exhibits δ87Sr higher (1.7–18.1‰) than the bedrock, indicating a significant contribution of radiogenic Sr from the siliciclastic fraction of the soil, and probably from dust input. The Nd isotopic ratios are slightly variable through the profiles (ɛNd from −7.8 to −14.5), confirming little mobility of Nd and Sm during the pedogenesis. Among the minerals present in the soils, phosphates, albite, and calcite are those important in providing low radiogenic Sr and Nd to organic matter of the soils.Lastly, this isotopic study has in particular allowed for evaluating the potential proportion of contribution of Saharan dust to south-eastern Sardinia, thus corroborating the findings of other studies related to soils from the central-western Mediterranean. 相似文献
3.
Mirjam Kiczka Jan G. Wiederhold Jakob Frommer Stephan M. Kraemer Ruben Kretzschmar 《Geochimica et cosmochimica acta》2011,75(19):5559-5573
The chemical weathering of primary Fe-bearing minerals, such as biotite and chlorite, is a key step of soil formation and an important nutrient source for the establishment of plant and microbial life. The understanding of the relevant processes and the associated Fe isotope fractionation is therefore of major importance for the further development of stable Fe isotopes as a tracer of the biogeochemical Fe cycle in terrestrial environments. We investigated the Fe mineral transformations and associated Fe isotope fractionation in a soil chronosequence of the Swiss Alps covering 150 years of soil formation on granite. For this purpose, we combined for the first time stable Fe isotope analyses with synchrotron-based Fe-EXAFS spectroscopy, which allowed us to interpret changes in Fe isotopic composition of bulk soils, size fractions, and chemically separated Fe pools over time in terms of weathering processes. Bulk soils and rocks exhibited constant isotopic compositions along the chronosequence, whereas soil Fe pools in grain size fractions spanned a range of 0.4‰ in δ56Fe. The clay fractions (<2 μm), in which newly formed Fe(III)-(hydr)oxides contributed up to 50% of the total Fe, were significantly enriched in light Fe isotopes, whereas the isotopic composition of silt and sand fractions, containing most of the soil Fe, remained in the range described by biotite/chlorite samples and bulk soils. Iron pools separated by a sequential extraction procedure covered a range of 0.8‰ in δ56Fe. For all soils the lightest isotopic composition was observed in a 1 M NH2OH-HCl-25% acetic acid extract, targeting poorly-crystalline Fe(III)-(hydr)oxides, compared with easily leachable Fe in primary phyllosilicates (0.5 M HCl extract) and Fe in residual silicates. The combination of the Fe isotope measurements with the speciation data obtained by Fe-EXAFS spectroscopy permitted to quantitatively relate the different isotope pools forming in the soils to the mineral weathering reactions which have taken place at the field site. A kinetic isotope effect during the Fe detachment from the phyllosilicates was identified as the dominant fractionation mechanism in young weathering environments, controlling not only the light isotope signature of secondary Fe(III)-(hydr)oxides but also significantly contributing to the isotope signature of plants. The present study further revealed that this kinetic fractionation effect can persist over considerable reaction advance during chemical weathering in field systems and is not only an initial transient phenomenon. 相似文献
4.
Geochemistry of soils of King George Island, South Shetland Islands, West Antarctica: Implications for pedogenesis in cold polar regions 总被引：1，自引：0，他引：1
Fine fractions of soils on the Barton Peninsula, King George Island, West Antarctica have been forming during the last 6000 yr since the last deglaciation. Texturally, they are mostly composed of mineral and rock fragments with some volcanic ashes, which are also indicated by geochemical compositions representing for the nonclay silicate minerals and low values of chemical index of alteration. No significant changes are observed in major- and trace element abundances. Such geochemical characteristics suggest that chemical weathering of bedrocks on the Barton Peninsula seems insignificant and that the soils are composed of physically weathered mineral and rock fragments which are mixed with eolian additions of volcanic ashes and Patagonian dusts. Chondrite-normalized rare earth element (REE) distribution patterns of the Barton Peninsula soils are slightly different from those of bedrocks, indicating that the REE abundances and characteristics were influenced by eolian additions. Mixing calculations, which mass-balance the REEs, suggest that volcanic ashes blown from Deception Island were the major eolian contributor, followed by atmospheric dusts sourced from Patagonia, South America. Even in the warmer and humid climatic conditions in the maritime Antarctic region, the chemical weathering of bedrocks appears to be insignificant, probably due to the relatively short duration of weathering since the last deglaciation. 相似文献
5.
黑色页岩是富含有机质和硫化矿物的特殊沉积岩,但人们对其风化过程的元素活动性及矿物风化机制关注较少.为探讨不同地形位置的黑色页岩化学风化过程,采集了渝东北城口某山脊 (A)、近山顶 (B) 和沟谷 (C) 的下寒武统水井沱组黑色页岩风化剖面岩样,利用XRD、XRF和化学分析手段对采集样品的矿物成分、主量元素进行测定分析.元素和矿物的质量迁移系数 (τ) 和质量迁移通量 (Mj, flux) 的计算结果表明,黑色页岩风化过程中Ca、Mg和Na元素具有明显的贫化现象,近地表处存在Al元素的富集现象;矿物成分方面,黄铁矿和有机质氧化后形成的酸性水环境,造成方解石、白云石、斜长石等不稳定矿物溶解,并生成含水石膏、铁质氧化物、黏土矿物等次生矿物.不同赋存位置的黑色页岩风化程度有所差异,Na/K-CIA、K/Ca*-Al/Na、A-CN-K和A-CNK-FM图解显示:A剖面处于脱Ca过程的初级风化阶段,B剖面处于脱Ca、Na初期的初等-中等风化阶段,C剖面已发生脱Ca、Na过程,并伴随脱Si作用的中等-强烈风化阶段,结合不同风化指数 (如:CIA、CIW、PIA、MWPI等),得出各剖面的化学风化强弱程度依次为C＞B＞A. 相似文献
6.
7.
《Geochimica et cosmochimica acta》1999,63(23-24):4013-4035
The effect of organic matter during soil/water interaction is still a debated issue on the controls of chemical weathering in a tropical environment. In order to study this effect in detail, we focused on the weathering processes occurring in a small tropical watershed (Nsimi-Zoetélé, South Cameroon). This site offers an unique opportunity to study weathering mechanisms in a lateritic system within a small basin by coupling soil and water chemistry.The lateritic cover in this site can reach up to 40 m in depth and show two pedological distinct zones: unsaturated slope soils on the hills and/or elevated areas; and water-saturated soils in the swamp zone which represent 20% of the basin surface. The study present chemical analysis performed on water samples collected monthly from different localities between 1994–1997 and on soil samples taken during a well drilling in December 1997. The results suggest the existence of chemical and spatial heterogeneities of waters in the basin: colored waters flooding the swamp zone have much higher concentrations of both organic matter (i.e., DOC) and inorganic ions (e.g., Ca, Mg, Al, Fe, Th, Zr) than those from springs and groundwater from the hills. Nevertheless, these organic-rich waters present cation concentrations (Na, Ca, Mg, K) which are among the lowest compared to that of most world rivers. The main minerals in the soils are secondary kaolinite, iron oxi-hydroxides, quartz, and accessory minerals (e.g., zircon, rutile). We mainly focused on the mineralogical and geochemical study of the swamp zone soils and showed through SEM observations the textural characterization of weathered minerals such as kaolinite, zircon, rutile, and the secondary recrystallization of kaolinite microcrystals within the soil profile. Water chemistry and mineralogical observations suggest that hydromorphic soils of the swamp zone are responsible for almost all chemical weathering in the basin. Thus, in order to explain the increase of element concentration in the organic-rich waters, we suggest that organic acids enhance dissolution of minerals such as kaolinite, goethite, and zircon and also favors the transport of insoluble elements such as Al, Fe, Ti, Zr, and REE by chemical complexation. SiO2(aq) concentrations in these waters are above saturation with respect to quartz. Dissolution of phytholithes (amorphous silica) may be responsible for this relatively high SiO2(aq.) concentration. Al/Mg ratios obtained for the soil and the Mengong river waters show that a significant amount of Al does not leave the system due to kaolinite recrystallisation in the swamp zone soils. Geochemical data obtained for this watershed show the important contribution of vegetation and organic matter on chemical weathering in the swamp zone. Quantitatively we propose that the increasing amount in total dissolved solid (TDS) due to organic matter and vegetation effect is about 35%. In summary, this interaction between soils and waters occurs mostly in soils that are very depleted in soluble elements. Thus, the low concentration of major elements in these water is a direct consequence of the depleted nature of the soils. 相似文献
8.
The sources impacting the water chemistry of the Nakdong River (NR) in South Korea were investigated in order to examine the pollution mechanism including the fate and transport of the contaminants and how much such sources may affect its main channel of water resource. Water samples were collected between 2007 and 2008 from 8 sites along a 510 km downstream transect of the NR, and chemical and isotopic compositions of the water samples were evaluated to identify natural and anthropogenic sources contributing to the water chemistry of the NR. The results showed that the major ion concentrations were mainly controlled by chemical weathering that occurred in the watershed, in which a silicate weathering is more dominant than a carbonate weathering. The 87Sr/86Sr ratios of the water samples were in a range from 0.71043 to 0.71520 within those of the Mesozoic volcanogenic sedimentary rocks long developed in the watershed, thereby supporting the fact that the water chemistry is governed by a chemical weathering. The δ34S values varied in a narrow range of 1.8–3.1‰, regardless of spatial and seasonal variations. Mass balance calculations indicated that the contributions of chemical weathering and anthropogenic sources during summer time increased along a downstream transect, from 67.3 ± 1.3 to 73.6 ± 0.5% and from 6.0 to 15.7%, respectively. In contrast, the contribution of chemical weathering during winter time decreased from 82.7 ± 0.8 to 72.5 ± 0.3%, while anthropogenic contribution increased up to 22.2%. These results indicated that the water chemistry of the NR was mainly caused by a chemical weathering, followed by anthropogenic inputs and rainwater. This study will provide baseline information for comparing the water quality issue before and after the implementation of the Four Rivers Restoration Project of South Korea. 相似文献
9.
In recent years, a series of important progresses have been made in the aspect of magnesium isotopes behavior in weathering processes. These progresses are not only favorable to understand the change of the magnesium isotopic compositions in rivers, but also establish the foundation to further reveal the magnesium isotope geochemical cycle. The magnesium in rivers is both magnesium sink for weathering and magnesium source for the ocean. The Mg isotopic compositions in rivers are dominated by the magnesium sources and Mg isotope fractionations processes. The sources of magnesium in rivers originate mainly from draining rocks, as well as less contribution from the eolian deposition, groundwater, plant debris, and precipitation. The Mg isotope fractionations in rivers are mainly related to precipitation and dissolution of carbonate minerals, silicate mineral hydrolysis, adsorption on mineral or colloidal matter surface, and plant uptake. Generally, the contribution of carbonate minerals dissolution or precipitation is equal to add or reduce magnesium from carbonate endmember, which has a remarkably negative δ26Mg value. Based on the fact that most clay minerals are rich in 26Mg during nature silicate mineral hydrolysis, then it is possible to infer that residual weathering products enrich in 26Mg. However, there is no significant Mg isotope fractionation causing by the adsorption on mineral or colloidal matter surface during river water migration. For the plant uptake, the root prefers to have 26Mg, leading the plant itself rich in heavier Mg isotopic composition. In addition, formation of secondary minerals in rivers could also reflect the changes of chemical parameters in rivers (such as major elements, CO2 solubility, pH, etc.). Hence, Mg isotopic composition in rivers and associated isotope fractionations are not only the basis for the application of magnesium isotope to trace surface material cycle, but also have important significance for the further understanding the geochemical cycle of magnesium isotopes. 相似文献
10.
A small watershed (160 km2) located in the Massif Central (France) has been chemically, isotopically and hydrologically studied through its dissolved load, bed sediments and soils. This watershed is underlain by basaltic bedrock and associated soils in which the vegetation is dominated mainly by meadows.Dissolved concentrations of major ions (Cl, SO4, NO3, HCO3, Ca, Na, Mg, K, Al and Si), trace elements (Rb and Sr) and strontium isotopes have been determined for two different hydrologic periods on the main stream of the Allanche river and its tributaries.The major objectives of this study were to characterize the chemical and isotopic signatures of each reservoir occurring in the watershed. Changes in chemical and isotopic signatures are interpreted in terms of fluctuations of the different components inputs: rainwater, weathering products, anthropogenic addition.Water quality may be influenced by natural inputs (rainwater, weathering processes) and anthropogenic additions (fertilizers, road salts, etc.). Precipitation serves as a major vehicle for dissolved chemical species in addition to the hydrosystem and, in order to constrain rain inputs, a systematic study of rainwaters is carried out over a one year period using an automatic collector. Corrections of rainwater addition using chloride as an atmospheric input reference were computed for selected elements and the Sr/Sr ratio. After such corrections, the geochemical budget of the watershed was determined and the role of anthropogenic additions evaluated through the relationship between strontium isotopes and major and trace element ratios. Thus, 10% of Ca and Na originate in rainwater input, 40 to 80% in fertilizer additions and 15 to 50% in rock weatheringThe cationic denudation rates for this watershed are around 0.3 g s–1 km2 during low water discharge and 0.6 g s–1 km2 in high water stage. This led to a chemical denudation rate of 5.3 mm/1000 years.For solid matter, the normalization of chemical species relative to parent rocks shows the depletion or enrichment in soils and sediments. The use of K and Ca as mobile reference illustrates the weathering state of soils and sediments relative to parent rocks. This weathering state for bed sediments range from 15 to 45% for the K normalization and from 2 to 50% for the Ca normalization. For the soils, the weathering state ranges from 15 to 57% for the K normalization and from 17 to 90% for the Ca normalization. 相似文献
11.
Anne-Catherine Pierson-Wickmann Luc Aquilina Jérôme Molénat 《Geochimica et cosmochimica acta》2009,73(16):4688-4704
In natural river systems, the chemical and isotopic composition of stream- and ground waters are mainly controlled by the geology and water-rock interactions. The leaching of major cations from soils has been recognized as a possible consequence of acidic deposition from atmosphere for over 30 years. Moreover, in agricultural areas, the application of physiological acid fertilizers and nitrogen fertilizers in the ammonia form may enhance the cation leaching through the soil profile into ground- and surface waters. This origin of leached cations has been studied on two small and adjacent agricultural catchments in Brittany, western France. The study catchments are drained by two first-order streams, and mainly covered with cambisoils, issued from the alteration and weathering of a granodiorite basement. Precipitations, soil water- and NH4 acetate-leachates, separated minerals, and stream waters have been investigated. Chemical element ratios, such as Ba/Sr, Na/Sr and Ca/Sr ratios, as well as Sr isotopic ratios are used to constrain the relative contribution from potential sources of stream water elements.Based on Sr isotopic ratio and element concentration, soil water- and NH4 acetate leaching indicates (1) a dominant manure/slurry contribution in the top soil, representing a cation concentrated pool, with low 87Sr/86Sr ratios; (2) in subsoils, mineral dissolution is enhanced by fertilizer application, becoming the unique source of cations in the saprolite. The relatively high weathering rates encountered implies significant sources of cations which are not accessory minerals, but rather plagioclase and biotite dissolution.Stream water has a very different isotopic and chemical composition compared to soil water leaching suggesting that stream water chemistry is dominated by elements issued from mineral and rock weathering. Agriculture, by applications of chemical and organic fertilizers, can influence the export of major base cations, such as Na+. Plagioclase dissolution, rather than anthropogenically controlled soil water, seems to be the dominant source of Na+ in streams. However, Ca2+ in streams is mostly derived from slurries and manures deposited on top soils, and transferred into the soil ion-exchange pool and stream waters. Less than 10% of Na+, 5-40% of Sr2+ and 20-100% of Ca2+ found in streams can be directly derived from the application of organic fertilizers. 相似文献
12.
Geochemistry of the dissolved load of the Changjiang Basin rivers: Anthropogenic impacts and chemical weathering 总被引：12，自引：0，他引：12
B. Chetelat C.-Q. Liu Z.Q. Zhao Q.L. Wang S.L. Li J. Li B.L. Wang 《Geochimica et cosmochimica acta》2008,72(17):4254-4277
This study focuses on the chemical and Sr isotopic compositions of the dissolved load of the rivers of the Changjiang Basin, one of the largest riverine systems in the world. Water samples were collected in August 2006 from the main tributaries and the main Changjiang channel. The chemical and isotopic analyses indicated that four major reservoirs (carbonates, silicates, evaporites and agriculture/urban effluents) contribute to the total dissolved solutes. The overall chemical weathering (carbonate and silicate) rate for the Changjiang is approximately 40 ton/km2/year or 19 mm/kyr, similar to that of the Ganges-Brahmaputra system, and the basin is characterized by carbonate and silicate weathering rates ranging from 17 to 56 ton/km2/year and from 0.7 to 7.1 ton/km2/year, respectively. In the lower reach of the Changjiang main channel, the weathering rates are estimated to be 36 and 2.2 ton/km2/year for carbonates and silicates, respectively. It appears that sulphuric acid may dominate chemical weathering reactions for some sub-basins. The budgets of CO2 consumption are estimated to be 646 × 109 and 191 × 109 mol/year by carbonate and silicate weathering, respectively. The contribution of the anthropogenic inputs to the cationic TDS of the Changjiang is estimated to be 15-20% for the most downstream stations. Our study suggested that the Changjiang is strongly impacted by human activities and is very sensitive to the change of land use. 相似文献
13.
Geochemistry and provenance of bed sediments of the large rivers in the Tibetan Plateau and Himalayan region 总被引：1，自引：0，他引：1
Weihua Wu Hongbo Zheng Shijun Xu Jiedong Yang Hongwei Yin 《International Journal of Earth Sciences》2012,101(5):1357-1370
We investigated the geochemical characteristics of major, trace and rare earth elements and Sr–Nd isotope patterns of bed sediments from the headwaters and upper reaches of the six large rivers draining the Tibetan Plateau (the Jinsha River—Yangtze, Lancang River—Mekong, Nujiang River—Salween, Huang He—Yellow, Indus, and Yarlung Tsangpo—Brahmaputra). By using Ca/Al versus Mg/Al, La/Sc versus Co/Th, and 87Sr/86Sr versus εNd (0) binary differentiation diagrams of provenance, some typical contributors to the different catchment sediments can be identified. In the Three-River (the Jinsha, Lancang, and Nujiang Rivers) tectonomagmatic belt, acidic–intermediate-acidic volcanic rocks are very important provenance of sediments. Carbonate rocks and Permian Emeishan basalts are dominant in the Jinsha River. The Yellow River sediments have similar geochemical characteristics with loess in catchments. The Indus and Yarlung Tsangpo Rivers sediments are mainly from ultra-K volcanic rocks and Cenozoic granitoids widely distributed in the Indus–Yarlung suture. The intensity of chemical weathering in these river catchments is evaluated by calculating the chemical indices of alteration (CIA) of sediments and comparing them with bedrocks. The CIA values of the six river sediments are from 46.5 to 69.6, closing to those of bedrocks in the corresponding catchment, which indicates relatively weak chemical weathering intensity. Lithology, climate, and topography affect the chemical weathering intensity in these river catchments. 相似文献
14.
15.
Anthropogenic Pb accumulation in forest soils from Lake Clair watershed: Duchesnay experimental forest (Québec, Canada) 总被引：1，自引：0，他引：1
Mineral soil horizons (Ae, Bhf1, Bhf2, Bf, BC and C) were carefully collected from two podzolic soil profiles in the Lake Clair watershed (Québec) in order to assess anthropogenic trace metal accumulation. Petrographic and selective analyses were performed to establish the soil mineralogy and properties. Furthermore, a complete sequential extraction procedure has been applied to help understanding the complex chemical speciation of Pb in forest soils. Chemical speciation of Pb showed a strong vertical gradient: 85% of this metal is mainly partitioned in refractory minerals in the C-horizon whereas in the upper Bhf1 and Ae-horizons, less than 50% of Pb is associated with this fraction. In the Ae-horizon, for example, 35%, 30% and 12% of total Pb, respectively, is associated with the exchangeable, labile organic matter and amorphous Fe-Mn oxides fractions. The distribution of Pb and Cr in the studied forest soils mainly reflects progressive contamination of the watershed by anthropogenic atmospheric sources. The anthropogenic source is indicated by elevated Cr and Pb concentrations in the topsoil (Bhf and Ae) horizons and by strong negative correlation between 206Pb/207Pb ratios and total Pb concentrations. According to these isotopic values, penetration of anthropogenic Pb does not exceed 10 cm in both soil profiles. Below this depth, both Pb concentrations and isotopic ratios remain nearly constant and similar to values observed in pre-anthropogenic sediments from Lake Clair. These values are interpreted as the natural geochemical backgrounds of the watershed. Based on that behaviour, calculated anthropogenic Pb net inputs amounted to between 1.24 and 1.8 g/m2. 相似文献
16.
Jan G. Wiederhold Nadya Teutsch Stephan M. Kraemer Alex N. Halliday Ruben Kretzschmar 《Geochimica et cosmochimica acta》2007,71(23):5821-5833
Stable iron isotope ratios in three soils (two Podzols and one Cambisol) were measured by MC-ICPMS to investigate iron isotope fractionation during pedogenic iron transformation and translocation processes under oxic conditions. Podzolization is a soil forming process in which iron oxides are dissolved and iron is translocated and enriched in the subsoil under the influence of organic ligands. The Cambisol was studied for comparison, representing a soil formed by chemical weathering without significant translocation of iron. A three-step sequential extraction procedure was used to separate operationally-defined iron mineral pools (i.e., poorly-crystalline iron oxides, crystalline iron oxides, silicate-bound iron) from the soil samples. Iron isotope ratios of total soil digests were compared with those of the separated iron mineral pools. Mass balance calculations demonstrated excellent agreement between results of sequential extractions and total soil digestions. Systematic variations in the iron isotope signature were found in the Podzol profiles. An enrichment of light iron isotopes of about 0.6‰ in δ57Fe was found in total soil digests of the illuvial Bh horizons which can be explained by preferential translocation of light iron isotopes. The separated iron mineral pools revealed a wide range of δ57Fe values spanning more than 3‰ in the Podzol profiles. Strong enrichments of heavy iron isotopes in silicate-bound iron constituting the residue of weathering processes, indicated the preferential transformation of light iron isotopes during weathering. Iron isotope fractionation during podzolization is probably linked to the ligand-controlled iron translocation processes. Comparison of iron isotope data from eluvial and illuvial horizons of the Podzol profiles revealed that some iron must have been leached out of the profile. However, uncertainties in the initial iron content and iron isotopic composition of the parent materials prevented thorough mass balance calculations of iron fluxes within the profiles. In contrast to the Podzol profiles, the Cambisol profile displayed uniform δ57Fe values across soil depth and showed only a small enrichment of light iron isotopes of about 0.4‰ in the poorly-crystalline iron oxide pool extracted by 0.5 M HCl. This work demonstrates that significant iron isotope fractionations can occur during pedogenesis in oxic environments under the influence of organic ligands. Our findings provide new insights into fractionation mechanisms of iron isotopes and will help in the development of stable iron isotopes as tracers for biogeochemical iron cycling in nature. 相似文献
17.
陆地硅的生物地球化学循环研究进展 总被引：2，自引：0，他引：2
地球表层硅(Si)的生物地球化学循环与大气CO2浓度变化、大洋生物泵作用以及海岸带富营养化等过程密切相关,因此成为全球环境变化研究的核心问题之一。在地质时间尺度上,硅酸盐矿物的化学风化是地球表层所有次生Si的来源。陆地生态系统各次生Si库具有不同的形成机制和驱动因子,这导致各Si库的贮存量和循环周期存在明显差异。土壤Si库中的黏土矿物Si、溶解硅(DSi)和淀积在其他矿物表面的无定形Si都源自硅酸盐矿物的化学风化过程;植物生长过程中吸收土壤中的DSi形成生物Si,然后经微生物分解过程返还给土壤;地表径流将流域陆源Si以悬移质Si和DSi的形式输入河流、海洋。迄今,陆地不同形态Si库的大小及其对全球Si循环的贡献仍不确定。因此,在研究陆地Si的生物地球化学循环过程中,综合考虑各种地表过程及其耦合作用是非常必要的。 相似文献
18.
地壳风化速率研究综述 总被引：6，自引：1，他引：6
地壳风化速率研究的理论基础是质量守恒原理和溶液与矿物反应动力学法则。元素在风化过程中的行为受多种因素控制，主要包括基岩风化量、大气沉降量、径流量、生物的输出数量和人为输入量（如施肥）。硅酸盐矿物化学风化过程中，矿物与溶液之间总的化学反应速率是单个反应速率之和，其中涉及到 3个关键参数，即：酸中和能力（ANC）、基本阳离子/无机铝（BC/Al无机）比值和临界负荷（CL）。风化速率的研究主要采用四种方法，即PROFILE模型、基本阳离子损耗、元素输入-输出指数和Sr同位素比值等。PROFILE模型是一个稳定态的综合土壤化学模型，矿物的分解速率、矿物的暴露表面积、土壤水饱和度和土壤层厚度决定着该矿物的风化速率，总的风化速率为各种矿物的风化速率之和。元素损耗，主要是基本阳离子（Ca、Na、K和Mg）的损耗，假设Ti、 Zr和Nb在成土过程中含量稳定并不参与风化反应，那么对于给定的土壤层，化学风化损耗的基本阳离子可以通过比较土层与成土母质之间元素组成的差异来计算。输入-输出指数的假设前提是研究的流域处于稳定状态，一般认为输入指数是大气沉降，输出指数是河流搬运溶解部分、悬浮的非岩屑成因部分和生物营养净吸收部分。Sr同位素在生物和化学作用过程中并不分馏，不同生态系统阳离子场中Sr同位素组成是大气和矿物风化来源的Sr的混合物。 相似文献
19.
Boron isotopes geochemistry of the Changjiang basin rivers 总被引：1，自引：0，他引：1
B. Chetelat C.-Q. Liu J. Gaillardet Z.Q. Zhao Y.K. Xiao 《Geochimica et cosmochimica acta》2009,73(20):6084-6097
We report analyses of B isotopic compositions in water and suspended particulate matter collected in the Changjiang and its main tributaries. We showed that four sources control the dissolved boron budget; namely atmospheric deposition, evaporite dissolution, anthropogenic inputs and silicate weathering. The contribution of silicate weathering to the dissolved B load ranges from 40% to 50% for the Changjiang main channel and from 45% to 88% for the main tributaries. The isotopic composition of dissolved boron derived from silicate weathering range from −3‰ up to +9‰ suggesting that isotopic fractionation occurs during silicate weathering. The boron isotopic composition of suspended particulate matter range from −11.4‰ to −6‰. Boron derived from silicate weathering is preferentially carried out by the dissolved load which accounts for 30-96% of the total boron. We show that the isotopic compositions of both the dissolved load and suspended particulate matter are controlled by the competition between boron leaching and boron uptake into secondary phases. The first process is characterized by a loss of boron relative to the bedrock without apparent isotopic fractionation whereas the last one is associated to a large isotopic fractionation which enriches the dissolved boron in heavy isotope. 相似文献
20.
We report Li isotopic compositions, for river waters and suspended sediments, of about 40 rivers sampled within the Mackenzie River Basin in northwestern Canada. The aim of this study is to characterize the behaviour of Li and its isotopes during weathering at the scale of a large mixed lithology basin. The Mackenzie River waters display systematically heavier Li isotopic compositions relative to source rocks and suspended sediments. The range in δ7Li is larger in dissolved load (from +9.3‰ to +29.0‰) compared to suspended sediments (from −1.7‰ to +3.2‰), which are not significantly different from δ7Li values in bedrocks. Our study shows that dissolved Li is essentially derived from the weathering of silicates and that its isotopic composition in the dissolved load is inversely correlated with its relative mobility when compared to Na. The highest enrichment of 7Li in the dissolved load is reported when Li is not or poorly incorporated in secondary phases after its release into solution by mineral dissolution. This counterintuitive observation is interpreted by the mixing of water types derived from two different weathering regimes producing different Li isotopic compositions within the Mackenzie River Basin. The incipient weathering regime characterizing the Rocky Mountains and the Shield areas produces 7Li enrichment in the fluid phase that is most simply explained by the precipitation of oxyhydroxide phases fractionating Li isotopes. The second weathering regime is found in the lowland area and produces the lower δ7Li waters (but still enriched in 7Li compared to bedrocks) and the most Li-depleted waters (compared to Na). Fractionation factors suggest that the incorporation of Li in clay minerals is the mechanism that explains the isotopic composition of the lowland rivers. The correlation of boron and lithium concentrations found in the dissolved load of the Mackenzie Rivers suggests that precipitation of clay minerals is favoured by the relatively high residence time of water in groundwater. In the Shield and Rocky Mountains, Li isotopes suggest that clay minerals are not forming and that secondary minerals with stronger affinity for 7Li appear.Although the weathering mechanisms operating in the Mackenzie Basin need to be characterized more precisely, the Li isotope data reported here clearly show the control of Li isotopes by the weathering intensity. The spatial diversity of weathering regimes, resulting from a complex combination of factors such as topography, geology, climate and hydrology explains, in fine, the spatial distribution of Li isotopic ratios in the large drainage basin of the Mackenzie River. There is no simple relationship between Li isotopic composition and chemical denudation fluxes in the Mackenzie River Basin. 相似文献