首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 640 毫秒
1.

Grain size of eolian deposits from the Loess Plateau in China has been widely used to reconstruct the history of the East Asian winter monsoon. However, the grain size of bulk samples is only partially indicative to the strength of the winter monsoon because post-depositional weathering processes have significantly changed the grain size of original eolian particles. Here, non-weathered loess samples were separated into eight different particle fractions, and major chemical elements were determined in order to establish a geochemical indicator of original eolian grain size. The results show that SiO2 and Al2O3 contents and the SiO2/Al2O3 ratio in different fractions vary regularly with grain size, and that a good linear relation exists between the SiO2/Al2O3 ratio and grain size for the fractions <50 μm. Because Al and Si are among the most stable elements and pedogenic processes in the Loess Plateau cannot affect the SiO2/Al2O3 ratio, this index can be used to reflect the grain size of original eolian particles. Application of this index in the Weinan and Luochuan loess sections of the last climatic cycle shows that SiO2/Al2O3 is in good agreement with median grain size (Md) in the loess units. On the contrary, SiO2/Al2O3 has documented a series of fluctuations in the soil units that are not clearly indicated by the grain-size changes of bulk samples.

  相似文献   

2.
本文对黄土高原和天山黄土区表土进行系统的岩石磁学和粒度测试分析,探讨了表土磁性特征及其环境意义,结果表明表土中的强磁性矿物均为磁铁矿和磁赤铁矿,弱磁性矿物为赤铁矿和纤铁矿或针铁矿,黄土高原黄土地层中的磁赤铁矿至少有部分属于风积成因.黄土高原表土中磁化率与频率磁化率呈良好的正相关,气候作用是主导黄土高原表上磁化率增强的主...  相似文献   

3.
Chinese loess–palaeosol sequences are well known for their records of monsoonal climatic variations. However, the modern processes of dust accumulation and soil formation remain poorly understood. A high‐resolution investigation on modern soils, including the measurement of magnetic susceptibility, particle‐size distribution, total Fe, total organic carbon, CaCO3 content, and optical stimulated luminescence (OSL) dating was carried out on the Zhouyuan loess tableland in the southern Loess Plateau. The results indicate that modern cinnamon soils (luvisols) have developed on contemporarily accumulated aeolian dust during the Holocene. The aeolian loess accumulated during the Younger Dryas was identi?ed in the top part of the Malan Loess that underlay the modern soil by OSL dating and proxy climatic data. It indicates that the Malan Loess accumulated during the last glaciation (marine isotope stages 2–4) does not serve as the parent material for the modern soils. Pedogenesis of the soils started with the increased precipitation and soil moisture that have occurred on the loess tableland since the early Holocene. Precipitation‐driven pedogenesis and organic activities are responsible for the leaching of CaCO3, decomposition of mineral dust and the production of clay and ferromagnetic minerals. Drier intervals have interrupted soil formation several times, and therefore pro?les with multiple soils have been developed at many sites on the loess tableland. At places where soil erosion was relatively strong, either a single soil or welded soils are preserved in the Holocene pro?les. This does not necessarily mean, however, that modern soils over the plateau have been developed without interruption under a constantly warmer, moister climate. This is signi?cant for understanding the surface processes and climatic variation during the formation of the numerous palaeosols over the Loess Plateau in the Quaternary. Copyright © 2003 John Wiley & Sons, Ltd.  相似文献   

4.
Particle size analysis for samples from three last glacial cycle loess-palaeosol profiles along a northwestsoutheast transection on the Chinese Loess Plateau has been carried out. The paleoclimatic significance of grain size indices has been obtained by analyzing the fraction content variations and their comparisons with global ice volume and solar radiation variations. The results show that (i) paleoclimatic significance of the grain size indices of loess-palaeosol deposit is different with grain size fraction content and sampling points in Chinese Loess Plateau; (ii) the sub-coarse grain fraction is a good proxy index of East Asia winter monsoon strength and therefore can be used to detect the global climate changes; (iii) the content of sub-fine and fine grain fractions is influenced by both the input of sub-coarse grain fraction and pedogenesis; (iv) the sub-coarse fraction exhibits a negative relationship with the sub-fine and fine fractlon. Project supported by the 9.5 major project of Chinese Academy of Sciences and the open grant of the State Key Laboratory of Loess and Quaternary Geology, Chinese Academy of Sciences.  相似文献   

5.
China's Loess Plateau was formed under special conditions. The tectonic movement, topographical characteristics, and monsoon patterns combined to create a favourable environment for the accumulation of thick loessic deposits. The Loess Plateau itself is part of the ‘Monsoon Triangle’ of China, a region very susceptible to climatic changes. Throughout the Upper Pleistocene the palaeoenvironment on the Loess Plateau alternated from steppe, to deciduous forest and coniferous forest, in response to shifts in the atmospheric circulation. Three monsoon patterns appear to be indicated: (1) a full glacial monsoon pattern (18000–15000 yr BP) which induced a cold and dry climate favouring loess accumulation in steppe conditions; (2) an interglacial monsoon pattern (last interglacial and Holocene) in which a warm humid climate prevailed with deciduous forests, leaving palaeosols interbedded within the loess sequence; and (3) a transitional or interstadial monsoon pattern (50 000–23 000 yr BP) in which the climate was cold and humid in the Loess Plateau, encouraging the development of coniferous forest.  相似文献   

6.
From Yulin, which is located in the transitional zone between the Loess Plateau and the Mu Us desert, to Weinan in the southernmost part of the Loess Plateau, 9 loess sections were studied. Grain size analyses show that the advance-retreat changes of the deserts in northern China may be the most important factor controlling changes in the sand particle percentage of the loess-soil sequences during the last glacial-interglacial period. It is thus suggegted to use the sand grain content of loess deposits as a proxy indicator of desert variations. Applying this indicator to the last glacial loess deposits in the northwestern part of the Loess Plateau reveals that there were many millennial-scale cycles of the desert environments. Project supported by the National Natural Science Foundation of China (Grant No. 49525230)  相似文献   

7.
Rock magnetism research on eolian deposits from the Chinese Loess Plateau shows that magnetite, maghe- mite and hematite are the main magnetic minerals in loess-paleosol sequences and red clay se-quences[1―8], and finds that the magnetic susceptibility increase is positively correlated to the content of ultra-fine minerals and relatively strong pedogene- sis[7―15]. This offers significant information in studying the Late Cenozoic paleoclimatic history of Chinese Loess Plateau[1,5―8,10―22…  相似文献   

8.
The magnetic susceptibility has been used as a quantitative or semi‐quantitative proxy for reconstructing the summer monsoon intensity in the Chinese Loess Plateau based on extensive studies on climatic or/and environmental mechanisms producing the magnetic susceptibility signatures. However, the precise nature of the link between past climates and the susceptibility signatures has remained uncertain primarily due to lack of our understanding in the ?nalizing and preserving processes of the signatures. This paper attempts to examine the reliability or acceptability of this summer monsoon proxy from non‐magnetic perspectives of soil‐forming processes. We chose nine sections along two transects: one across the western part of the Chinese Loess Plateau and another across the eastern part. Several conclusions can be drawn from our analytical data. First, clay translocation within the S1 palaeosol pro?les, as indicated by ?eld‐observed clay coatings on ped faces in Bt and Bk horizons and demonstrated by laboratory‐analysed clay contents, must have moved some of the magnetic minerals downward so that the susceptibility re?ects only the post‐translocation distribution of the magnetic‐susceptibility‐producing minerals. Second, the best‐developed palaeosol S1S3 at most of the sections studied is not expressed by the magnetic susceptibility because this palaeosol developed in underlying coarse loess (L2) and coarse textures tend to lower the susceptibility. Third, carbonate concentration is normally negatively correlated with the magnetic susceptibility or simply suppresses the magnetic susceptibility peak when the susceptibility enhancement exceeds the carbonate dilution effect. To conclude, extreme caution must be observed when using magnetic susceptibility signatures to retrieve high‐resolution records of the last interglacial palaeoclimate in the Chinese Loess Plateau. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

9.
The Mangshan loess on China’s Central Plain, located on the transitional zone between the uplifting Loess Plateau and the subsiding North China Plain, is a proximal sandy loess transported from the fanhead of alluvial fan in the lower reaches of the Yellow River and has recorded the coupling effect of the tectonics and climate over the last 200 ka. An abrupt environmental change indicated by the abrupt rise of deposit rate in the late penultimate glaciation, about 150 ka BP, took place; that is, the Yellow River downcut and moved eastwards through the Sanmenxia Gorge and transported abundant materials from the Loess Plateau to form paleosol S1 with a thickness of 15.7 m and loess L1 with a thickness of 77.3 m. The loess-paleosol sequence at Mangshan has not only recorded detailed climate responses of this area to the East Asian monsoon, but also reflects the tectonogenetic environmental effect caused by the trunk stream of the Yellow River cutting through Sanmenxia Gorge into sea. Project supported by the National Natural Science Foundation of China (Grant No..49572132).  相似文献   

10.
The magnetic susceptibility (MS) of Chinese loess showing a general proportional relationship to pedogenic grade has been widely recognized and used for reconstruction of paleoclimate by Quaternary scientists. The in-situ pedogenic enhancement of ferrimagnetic content is normally believed to be the main reason for the increase of susceptibility in soil units. However, this pattern of high magnetic susceptibility in palaeosols, and low values in loess, are not replicated in some loess deposits. Siberian loess deposits display a completely opposite susceptibility behavior: high values in loess and low values in palaeosols. This inverse relationship has been explained by the idea that magnetic susceptibility is reflecting the magnitude of an aeolian ferrimagnetic component of consistent mineralogy, the grain size of which is related to average wind velocity. Our magnetic study of Siberian samples in this paper suggests that there are notable differences in magnetic properties between Siberian loess and developed palaeosols, not only in magnetic grain-size and concentration but also in magnetic mineralogy. This evidence is difficult to explain fully through variation in wind strength alone, but implies that the low magnetic susceptibility values in the Siberian paleosol units are a reflection, at least in part, of the alteration of the ferrimagnetic content by post-depositional processes. The Loess Plateau is a very arid area where potential evaporation is always higher than precipitation; pedogenesis occurs under dry oxidising conditions. The Siberian Kurtak region is located on the edge of the tundra where it is always wet and saturation during interglacials will lead to a reducing pedogenic environment. Ferrimagnetic minerals under this condition will be destroyed, resulting in lower magnetic susceptibility. Therefore, great care should be taken when using susceptibility values for paleoclimatic reconstruction.  相似文献   

11.
中更新世以来青藏高原RH孔和洛川黄土记录的比较研究   总被引：2，自引：1，他引：1  
薛滨  王苏民 《湖泊科学》1997,9(2):123-128
根据若尔盖盆地ＲＨ孔的研究成果,具体地将该孔的２１个阶段和洛川黄土记录进行对比,分析其异同点,探讨了高原不同降升阶段自身环境特点及其对邻近西北干旱区的影响。  相似文献   

12.
We have conducted detailed rock magnetic experiments on samples from loess unit 8 (L8) and paleosol unit 8 (S8) in Jingbian, Yichuan and Duanjiapo loess sections along an N-S transect in the Chinese Loess Plateau. Major rock magnetic results are as follows: (i) An increase of high field susceptibility (χh) in the same level of loess or paleosol from north to south is observed, suggesting an enhancement of pedogenesis. (ii) The low field susceptibility(χL) in loess unit L8 is almost the same in three sections. In contrast, the χL of paleosol unit S8 in Yichuan is highest, and the χL of Duanjiapo is lower than that in Yichuan section, suggesting that there is not correlation between the χL and the degree of pedogenesis in loess-paleosols. (iii) With the increasing of χL, both the contents of the superparamagnetic (SP) and the ferrimagnetic grains in loess-paleosol increase, however, the enhancement of magnetic susceptibility is probably dependent more on the increase of the ferrimagnetic concentration than on a change in the grain size. (iv) The content of the maghemite in loess unit 8 increases from the northern to the southern part of the Chinese Loess Plateau, and is positively correlated with the pedogenesis of the loess.  相似文献   

13.
Based on the geological records, the palaeomonsoon in eastern desert region of China is divided into three major evolution stages, i.e. summer monsoon prevailing stage of last interglacial period (130-70kaB.P.), winter monsoon prevailing stage of last glacial period (70-10 kaB. P.), and unstable summer monsoon prevailing stage of postglacial period (10 kaB. P. to present) and further divided into several substages. The conversion between summer monsoon and winter monsoon in the region is dominated by the sudden change process. The north limit of summer monsoon in the region retreated to the north limit of sandy loess zone of the Loess Plateau in the last glacial period from the Mazong Mts. -Ulan Bator of last interglacial period, then it entered Shandan-Yabrai region in the optimum period of the Holocene, and finally it retreated to the present extended line from north piedmont of the Yinshan Mts to Hulun Buir. This shows that the summer monsoon caused by East Asian monsoon circulation tends to be weakened fluctuationally. However, the factors affecting the monsoon vicissitudes are complex, so special attention should be paid to the study of the short-period climatic fluctuations of the Holocene.  相似文献   

14.
The loess-soil sequences in northern China provide a near continuous record of Quaternary paleoclimate. The pedogenetic intensity of the sequences is closely linked with the variations of the East Asian summer monsoon. In this study, 2181 samples from the Changwu and Xifeng loess sections are analyzed and two high-resolution paleo-weathering timeseries of the last 1.2 Ma are generated, using the ratio of CBD extractable free Fe2O3 (FeD) versus the total iron (FeT). This new index is compared with micromorphological features, low-frequency magnetic susceptibility, frequency-dependent magnetic susceptibility, and the Rb/Sr ratio[5,6]. The results suggest that the FeD/FeT ratio is able to better reflect the degree of soil development. Since the chemical weathering of loess in the Loess Plateau region mainly depends upon the summer precipitation and temperature under modern climate condition, which are closely associated with strength of summer monsoon, and the chemical weathering intensity of loess primarily reflects the variations of the summer monsoon circulation.  相似文献   

15.
The dominant magnetic minerals and carriers of magnetic signals within the Chinese Loess Plateau are magnetite, maghemite, hematite, and goethite. In this study, we investigated the provenance and evolution of magnetic minerals during loess pedogenesis, using X-ray diffraction (XRD) and optical and electron microscopy, including field emission scanning electron microscopy (FESEM) and high-resolution transmission electron microscopy (HRTEM). Our results reveal that single- and multiphase mineral assemblages among magnetic minerals in the loess-paleosol sequence have been formed. Partial oxidation of coarse eolian magnetite has occurred in the desert source area and the oxidation degree is enhanced after deposition of the dust upon the Chinese Loess Plateau. This mode of origin resulted in a microtexture consisting of an inner magnetite core surrounded by a hematite rim, and strongly affected the magnetic characteristics of the loess. Goethite coexists with hematite in the loess and paleosol, and nanometer-scale hematite is formed upon goethite rims via dehydration. Our study provides direct mineralogical evidence of the magnetic record and paleoclimatic implications of the loess-paleosol sequence of the Chinese Loess Plateau. Supported by National Natural Science Foundation of China (Grant Nos. 40772032 and 40573054) and National Basic Research Program (Grant No. 2007CB815603)  相似文献   

16.
The concentrations of Rb and Sr, and magnetic susceptibility in loess and paleosol samples from the Luochum profile have been measured. The loess units deposited in different geological periods display a very similar pattern of Rb and Sr distribution while paleosol units exhibit a dramatic increase in the Rb/Sr ratio, ranging from 20% to 120% in increase amplitude. Owing to different geochemical behavior of the two elements, Rb appears to be immobile while Sr appears to be mobile in the processes of weathering and pedogenesis. So variations of the Rb/Sr ratio in the loess-paleosol sequences could reflect intensities of weathering and pedogenesis, thus recording the relative wind strength of the East Asian summer monsoon circulation. This could be supported by the high degree of cornlation between the Rb/Sr ratio and the magnetic susceptibility. A continuous record of the Rb/Sr ratio in the Luochuan profile over the last 800 ka bears a striking resemblance to the δ18 O curve of the deep sea sediments and is in accordance with the SPECMAP chronology. Such similarity between the terrestrial and the deep sea records suggests that variability in global ice volume is a primary dynamic factor controlling long-term changes of the East Asian summer monsoon intensity. Project supported by the National Natural Science Foundation of China (Grant No. 49725307).  相似文献   

17.
Developing an accurate chronological framework is always a key issue in paleoclimatic studies. Magneto- stratigraphy has been a routine tool for such purposes. However, complexities arise for inter-profile correla- tions of magnetostratigraphy due to effects of the lock-in process. One good example is the “mystery” of the mismatching of stratigraphic locations of the Matuyama/Brunhes boundary (MBB) (occurred at ~780 ka) recorded in Chinese loess and marine sedi- ments. Tauxe et al.[1] con…  相似文献   

18.
Carbonates in loess-red clay sequences consist mainly of calcite and dolomite. The EDTA analysis of carbonates in different size fractions and magnetic susceptibility reveal that calcite is a sensitive index of summer monsoon. The chemical analysis of carbonates and calcite from an 8.1 Ma loess-red clay sequence at Chaona on the Chinese central Loess Plateau shows that the evolution of the Asian summer monsoon experienced four stages, namely 8.1―5.5 Ma, 5.5―2.8 Ma, 2.8―1.5 Ma and 1.5―0 Ma, with increasing intensification and fluctuation, suggesting a possible combining impacts of uplift of the Tibetan Plateau and global changes on the Asian summer monsoon.  相似文献   

19.
Two Red Clay profiles near Xi’an and Xifeng were investigated in an attempt to determine magnetostratigraphic and palaeoclimatic records. The results show that aeolian dust accumulation and the related East Asia palaeomonsoon system had begun by 6.5 Ma, and it is deduced that the Tibetan Plateau had reached a significant elevation at that time. The late Tertiary palaeoclimatic history of the Red Clay as reflected by magnetic susceptibility is reconstructed during the period of 6.5–2.5 Ma. Stepwise increase in susceptibility of aeolian dust accumulation appears to have a close correlation to the uplift processes of the Tibetan Plateau. The remarkable increase of aeolian dust accumulation at 3.2 Ma appears to be due to the influence of global ice volume on the East Asia monsoon. Palaeomonsoon variation during the late Tertiary as recorded in the Red Clay sequences from the Chinese Loess Plateau can be regarded as the product of a number of interacting factors, such as uplift of the Tibetan Plateau, solar radiation, global ice volume, etc. Project supported by the National Natural Science Foundation of China and the Foundation of Xi’an Laboratory of Loess and Quaternary Geology, Chinese Academy of Sciences.  相似文献   

20.
黄土岩石磁学参数是古气候研究中的重要指标,其中磁化率应用最为广泛,并在黄土高原地区取得重大进展,其受控于成壤作用的变化机制也被普遍接受.然而在黄土高原外缘的新疆地区,磁化率的变化机制仍不明确,导致磁化率的古气候意义在该区存在较大争议.本文选取塔里木盆地南缘具有精确年代控制的典型黄土剖面（羊场剖面）开展岩石磁学和高分辨率磁化率研究,利用交叉小波分析方法并结合剖面粒度、矿物及元素特征对该地区磁化率变化机制进行初步探讨.结果显示,羊场剖面的岩石磁学性质主要由粗颗粒软磁性矿物所控制,同时也表现出一定的顺磁性特征.根据载磁矿物和磁化率变化特征可将剖面进一步划分为两个阶段：阶段Ⅰ（8.5~2.5 ka）,载磁矿物以亚铁磁性的磁铁矿为主,磁化率值整体较高;阶段Ⅱ（2.5~0.2 ka）,亚铁磁性矿物依然占据主导地位,但硬磁性矿物和以黄铁矿为代表的顺磁性矿物相对增多,磁化率值显著降低.相关性研究和交叉小波分析表明：阶段Ⅰ磁化率与粗颗粒组分的变化具有一致性,符合"风速论"模式;阶段Ⅱ磁化率不仅与粗颗粒组分具有明显的正相关关系,而且与指示成壤作用强度的频率磁化率百分含量呈现出显著的负相关关系,暗示了阶段Ⅱ的磁化率变化可能受到"风速论"和"还原性成壤"模式的共同影响.本文拓宽了对新疆地区黄土岩石磁学特征及其磁化率变化机制的深入理解,也为利用磁化率恢复新疆及中亚地区全新世以来的古气候变化历史提供了新的线索.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号