| Iron oxides are common and widespread compounds in nature. These compounds are either oxides, hydroxides or oxides-hydroxyl, collectively referred to be iron oxides. The formation of iron oxide (Fe3+) is mainly composed of the aerobic weathering of magmatic rocks in the terrestrial and marine environment and the redistribution process of every area on the surface of the earth. It contains the processes in which wind/water erosion from pedosphere then removes mechanically to hydrosphere and atmosphere, and in which the iron oxides decompose because of reduction, then Fe2+migrating, oxidizing and settling down in the new environment. In the soil environment, Pedogenesis controls the direction and process of the formation and evolution of iron oxides. The initial reaction of the evolution of iron oxide minerals is via the dehydration and the oxidative decomposition of iron-bearing mineral (mainly Fe2+silicate minerals). At first, Fe releases from primary mineral, existing in the form of amorphous state hydrated ferric oxide (e.g. ferrihydrite). It dehydrates, crystallizes, and ages, then transforms to a more stable goethite or hematite and is preserved in the stratum when the condition changes. The process of mineral transforming, meanwhile records the environmental information because characteristics of composition, structure, physical properties and assemblages of these iron minerals to some extent are the fingerprints of the environment condition they formed.The laterite in south of China is the important accumulation date back to late Cenozoic and the product under the specific climatic environment. Therefore, the laterite reserves the richly environmental information of evolution, which becomes the good information carrier of the global and regional environmental change. The iron oxide minerals in the laterite are the product in the process of pedogenesis, which is a function of soil occurrence and development. Through the systematic research on the iron oxide minerals in the laterite, we can obtain the pattern of the environmental change in the southern China since Quaternary. The iron oxide minerals in the soil are composed of goethite, hematite, magnetite, maghemite, lepidocrocite, ferrihydrite and so on, in which the goethite and hematite are the most common ones. The hematite forms because of the internal dehydration of ferrihydrite. Under the wet condition, the action of water makes the oxidizing condition underdeveloped and benefits the accumulation of organic matter. Under this circumstance, Fe3+is easily be chelated by organic acid, which is not in favor of the dehydration of ferrihydrite and obstructs the formation of hematite. Under the hot with alternative wet and dry conditions, ferrihydrite precipitates because of the quick decomposition of organic material, prejudicing the chelation between Fe3+and organic acid. Much higher temperature will promote the dehydration of ferrihydrite which leading to the formation of hematite. On the contrary, much more organic matters are capable to bond Fe under the wet and cool soil environment, which restrains the evolution that Fe turning into the ferrihydrite. Even if the ferrihydrite forms, the lower temperature may delay its dehydration process, restraining the evolution of hematite and benefitting the formation of goethite. Therefore, the values of hematite and goethite will change with the annual average temperature and water content.Goethite is a most extensive ferric oxide mineral forming in the soil formation and can occur and exist in all kinds of environment, including temperate to tropical regions. Therefore, goethite can be selected as a proxy of soil formation with no area limitation. The Fe partly replaced by Al with the amount between0-33mole percent has been previously confirmed. The level of this replacement mainly depends on the soil weathering conditions. Therefore, Al substituting Fe into the goethite structure as an isomorphism is regarded to be decipher of the geochemical process when sediments weathering. The precise occupation and amount of Al in the goethite can provide important information for the degree of weathering. In addition, Al can also limit the size of the goethite crystal growth. Combined influences of particle size and amount of Al will control the color of goethite. Based on the previous research, Al goethite usually has low displacement quantity when it formed in aqueous environment, moderate acid soil and calcareous environment, while having high amount of Al substitution when it forms in the non-aqueous environment and high degree of weathering. During the formation of goethite, the Al that enters into the structure of goethite will increase with the decrease of pH and increase of active Al. Therefore, the replacement degree of Al isomorphism can reflect environmental condition when it forms and be the indicator of pedogenesis.Base on the theory of the genetic mineralogy, the formation of iron oxide in the laterite is strictly restricted to depositional environment, especially hematite, only forming under the strong weathering and dry and hot environmental condition. This fits the depositional climate environment of tropical and subtropics laterite in the south. Meanwhile, formation of goethite in the laterite is also controlled by certain environmental condition. Therefore, the characteristics of the assemblages of iron oxide minerals and changing of the content in the laterite must reveal certain regulation of the environmental change. In the paper the laterite sediments from Xuancheng, Anhui province are chosen to be a case study. To trace the climatic environment during pedogenesis and clarify the relationship between the environmental formatting in the middle to lower reaches of the Yangtze River and the global climate change, the heat treatment of the sodium hydroxide (NaOH), the based method of experiment to concentrate the iron oxide minerals in the clay samples, was used combined with the modern equipment analyzed technique (including X-ray diffraction(XRD), Mossbauer spectrum(MS) and high resolution transmission electron microscopes(HRTEM)) to study the assemblages of iron oxide minerals and its relative proportion along the section in vertical. This is important to the restoration and rebuilding of the environment in the laterite depositional process and enriches their instructed evidence in southern of China. The main research content is as follow.(1) The iron oxide minerals in the laterite sample is of low content and low degree of crystallinity. Meanwhile, lots of fine quartz particles exist in specimen, which affacts the results of qualitative and quantitative analysis of these iron oxide minerals in soil samples. The repeated experiments show that the usages of hydrogen peroxide (H2O2), decomposing the organic matter, is benefit to the soil particle dispersing fully in the aqueous solution. After24hours of sedimentation quartz granule almost can be completely separated. The soil samples treated by the NaOH can destroy the structure of kaolinite and play fewer roles in iron oxide minerals, which can reach the goal of further concentrating the iron oxide minerals. The super-fine suspended particles which can not precipitate by centrifuge, however, can be obtained by flocculated method using saturated sodium hydroxide (NaCl) solution. These flocculated materials are evidenced to contain larger concentration of iron oxide. Because the particle size of these iron oxides is too small, it can’t be used to measure the Mossbauer spectrometry (MS). The laterite samples separated after4hours of sedimentation can obtain useful samples for MS analysis.(2) High Resolution Transmission Electron Microscopy (HRTEM) is an effective method to indentify the iron oxide minerals by measuring the lattice finger images via the professional software of TEM. The results show the iron oxide minerals are goethite, hematite and ferrihydrite. These can also be confirmed by the results of X-Ray Diffraction (XRD) which show goethite and hematite are the main iron oxide minerals in soil samples. The iron oxide minerals in the laterite sample are very tiny, and the iron oxide minerals contain a lot of Al isomorphism substitution phenomenon. These make the patters of MS show the paramagnetic double peaks under the room temperature (300K) because of goethite and fines hematite, which is difficult to distinguish with the Fe overlap of spectral lines in the silicate mineral structure. Under the low temperature condition (liquid nitrogen temperature below,≤78K), the results of MS reflect goethite and hematite show their own absorption spectrum peak, respectively, and can be distinguished by fitting parameter. Ten laterite samples were tested by MS and results collectively show that all the laterite samples present2-3groups of magnetic sextet peaks and a paramagnetic double-peak. Using professional software to fit and evaluate the patters of MS confirms that2-3groups of magnetic sextet peaks are the absorption peak of goethite, hematite and ferrihydrite, and the paramagnetic double-peak is contributed to the Fe3+and Fe2+in the structure of silicate minerals. The results from MS are agreed well with the result of HRTEM and XRD.(3) In the tropical and subtropical areas in southern of China, where the climatic environment is relatively wet and warm; The vegetation develops widely and chemical weathering and pedogenisis act prosperous. The primary minerals are weathered with the help of organic matters and the soluble component (e.g. K, Na, Ca etc.) within the minerals are largely leached. However, Fe and Al concentrate in this process, which makes up of the foundation of indicating the climatic variation using these iron oxide minerals. The sum of the goethite and hematite(Gt+Hm) quantized by the X-ray diffraction technique(XRD) on the laterite samples is within the range of total Fe measured by the X-Ray Fluorescence, demonstrating that method is feasible and the data is reliable. The vertical change of the Gt+Hm in this section shows a decreasing trend from bottom to top, which reflects that the weathering weakened gradually in Xuancheng profile. It is a conclusion that the pedogenic climatic environment in the middle to lower reaches of the Yangtze River changs from muggy to warm generally.(4) The goethite exists generally in the soil under moist/warm and cool conditions and with higher oxidation potential relatively. However, the hematite exists mostly in the highly weathered soil of tropics and subtropics as well as dry and strong oxidizing environment. The iron oxide minerals data in the Xuancheng laterite profile from the XRD and MS show that the content of geothite decreases from the bottom to middle part of the setion; then follows by the increased trend upwards. On the contrary, the content of hematite is the least in the bottom, then reaches the maximum in the middle of the section, and fluctuates or relatively lessens at the top. It is clear that the Xuancheng laterite profile experiences the climate change from moist/hot to hot and wet-dry cycling to warm according to the vertical distribution characteristics of the assemblage and the content of the goethite and the hematite in the profile.(5) The degree of the replacement of Al isomorphism in the goethite of the soil reflects the intensity of soil weathering. The values of Al substitution in the goethite in Xuancheng section were calculated using XRD methods. The results show the values of Al substitution in the goethite range from10.80mole%to25.18mole%with mean value of19.04mole%, suggesting the method mentioned above is feasible and the data is reliable based on the result according well with the effective range (0-33mole%). Overall, Al in the goethite has high amounts of substitution, indicating a strong weathering in this laterite section. According to the variation of A1substitution amount in profile, it reveals the higher humidity and relatively stronger weathering in the lower part of this section; intensively alternative dry and wet with highly weathered leaching in the middle part of this section; cool and relatively moist accompanied by relatively weak weathering in the upper part of this section. Fluctuations of substitution amount of Al in the goethite in this section have a good agreement with the fluctuations of ocean oxygen isotope (δ18O) in glacial-interglacial period in Quaternary, indicating climate changes of laterite profile of Xuancheng show the global characteristic and could be the response to global climate change.(6) According to assemblages and proportions of the iron oxide minerals qualified and quantified from the results of XRD and MS, three parts can be divided from bottom to top:(1) from P18to P15with the thickness and age intervals between10-～7m and621-424ka B.P., respectively, the climatic environment changes from wet and hot to alternative dry and wet cycles gradually accompanied by stronger weathering environment.(2) from P14to P9with the thickness and age intervals between～7-3.3m and424to180ka B.P., respectively, the climatic environment is characterized by hot and intensively alternative dry and wet cycles with strong weathering and leaching.(3) From P8to P1with the thickness and age<3.3m and<180ka B.P., respectively, the climatic environment shows temperature turns cold gradually, the precipitation decreases, the soil keeps relative dry and the weathering is relatively weak. The climatic environment interpreted from this study agrees well with the previous evidence of geochemical, molecular fossils and particle-size analysis. Comparing the iron oxide mineral data in this research with the previous results, it is find that the climatic environment change is not only affected by the global climate event but also by the strong effect the regional monsoon in the laterite section of Xuancheng or even the whole middle to lower reaches of Yangtze River since middle Pleistocene. |
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