共查询到20条相似文献，搜索用时 46 毫秒
1.
Two pedogenic models for paleoclimatic records of magnetic susceptibility from Chinese and Siberian loess 总被引：3，自引：0，他引：3
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
2.
风成黄土是陆地上分布最广泛的沉积物之一,记载了各种古气候演化信息.目前巴基斯坦的黄土研究甚少,磁化率与气候对应的变化机制研究尚未开展.本文对位于巴基斯坦印度河平原Bahawalpur地区新发现的黄土-古土壤剖面进行系统的岩石磁学研究,结合粒度和漫反射光谱(DRS)数据,讨论巴基斯坦黄土的磁化率变化机制.实验结果显示:Bahawalpur(BH)剖面黄土层主要的载磁矿物为磁铁矿,同时含有少量磁赤铁矿和针铁矿,磁性颗粒以原生的MD和PSD颗粒为主.相对于黄土层,古土壤层则是以针铁矿为主,含有顺磁性矿物和少量磁铁矿.BH剖面磁化率与成土作用关系和中国黄土高原典型剖面相反,磁化率的变化可能存在一个阈值12.8×10-8 m3·kg-1,在阈值之上,强磁性矿物(磁铁矿、磁赤铁矿)占主导;阈值之下,以弱磁性矿物(主要是针铁矿)为主,这种磁性矿物的转变可能导致磁化率降低.本文可为今后利用磁化率解读该地区地层蕴含的古气候信息提供新线索. 相似文献
3.
本研究选择川西高原、天山和西伯利亚Kurtak剖面的黄土古土壤样品及亚热带非风成样品进行CBD处理,系统测量并对比处理前后的磁学参数,包括低频磁化率、频率磁化率、非磁滞剩磁、饱和等温剩磁、剩磁矫顽力和热磁曲线(J-T曲线),分析处理前后磁性矿物种类、含量和磁畴的变化.结果表明,CBD方法对于磁性矿物的溶解并无明显的选择性,在温度与反应时间一定的条件下,磁性矿物的溶出量主要受控于其粒径分布.CBD方法可以非常有效地去除具有更大比表面积的细粒(< 1 μm)磁性矿物,同时溶解粗粒(>1 μm)磁性矿物外缘,使其粒径变细.CBD处理后磁化率变化存在多种可能,对于成土作用较强的古土壤,CBD方法可以较为准确地提取成土成因的磁性信息;而干旱和过度湿润条件下的风积黄土,不宜使用CBD方法区分原生与次生磁性矿物. 相似文献
4.
Jana Richter-Krautz Mandy Hofmann Johannes Zieger Ulf Linnemann Arno Kleber 《地球表面变化过程与地形》2021,46(5):968-989
U–Pb dating of detrital zircons (DZ) in Quaternary slope deposits (cover beds) and examination of the surface structure of single zircon grains may enable reconstruction of the provenance of their aeolian components and help to reconstruct sedimentary palaeo-transportation cascades. We distinguish several layers of slope deposits (cover beds) using a palaeosol-supported sequence-stratigraphic approach. In a pilot study, we demonstrate the usability of using end-member-modelled analyses of grain sizes to identify the aeolian matter, multi-dimensional scaling and density functions (probability density functions (PDF) and Kernel density estimations (KDE)) of the U–Pb data, and the grain morphology of DZ, and show that the age and appearance variation of DZ in the Great Basin differ remarkably from those of the Colorado Plateau. All samples contain aeolian matter. Density functions of the DZ ages show similarities within each of the two regions. The Great Basin samples are dominated by late Cretaceous and Paleogene zircons, which are assumed to derive from volcanism. In contrast, the Colorado Plateau samples are almost free of these contributions. Presumably, the difference is due to palaeolakes, which were frequent in the Great Basin only, whose sediments contain a similar spectrum of DZ ages. Zircon morphology indicates that most grains have a history of aeolian transportation, whereas the grains assumed to be of volcanic origin often do not show similar transportation marks; this indicates that their transportation was dominated by the eruptive process but fewer steps of a transportation cascade. Multidimensional scaling of age data defines clusters of samples with similar DZ ages and – compared with published ages from possible sources – allows first interpretations of provenance. Quaternary aeolian sediments have not been studied in, and the applied mix of methods has not been applied to, the southwestern USA before. We demonstrate their feasibility for this study area. 相似文献
5.
QIANG Xiaoke AN Zhisheng LI Huamei CHANG Hong & SONG Yougui . SKLLQG Institute of Earth Environment Chinese Academy of Sciences Xi’an China . Guangzhou Institute of Geochemistry Chinese Academy of Sciences Guangzhou China 《中国科学D辑(英文版)》2005,48(8):1234-1245
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
6.
Regional grain size variations in aeolian sediments along the transition between Tibetan highlands and north‐western Chinese deserts – the influence of geomorphological settings on aeolian transport pathways 
下载免费PDF全文
下载免费PDF全文 Veit Nottebaum Frank Lehmkuhl Georg Stauch Kai Hartmann Bernd Wünnemann Stefan Schimpf Huayu Lu 《地球表面变化过程与地形》2014,39(14):1960-1978
Sediment distribution is investigated applying grain size analysis to 279 surface samples from the transitional zone between high mountains (Qilian Shan) and their arid forelands (Hexi Corridor) in north‐western China. Six main sediment types were classified. Medium scale (103 m) geomorphological setting is carefully considered as it may play an important role concerning sediment supply and availability. A tripartite distribution of sedimentological landscape units along the mountain to foreland transition is evident. Aeolian sediments (e.g. loess and dune sands) are widespread. They are used to identify aeolian transport pathways. The mU/fS‐ratio (5–11 µm/48–70 µm) among primary loess opposes the two grain size fractions being most sensitive to varying accumulation conditions. The first fraction is attributed to long‐distance transport in high suspension clouds whereas the latter represents local transport in saltation mode. The ratio shows strong correlation with elevation (R2 = 0.77). Thus, it indicates a relatively higher far‐traveled dust supply in mountainous areas (>3000 m above sea level [a.s.l.]) compared to the foreland. The contribution of westerlies to high mountain loess deposits is considered likely. Hereby, the influence of the geomorphological setting on grain size composition of aeolian sediments becomes apparent: the contribution from distant dust sources is ubiquitous in the study area. However, the far‐distance contribution may be reduced by the availability of fine sand provided in low topography settings. Plain foreland areas support fine sand deflation from supplying river beds, allowing the formation of sandy loess in foreland areas and intramontane basins. In contrast, high mountain topography inhibits strong sand deflation into loess deposits. Eastern parts of the Hexi Corridor show higher aeolian sand occurrence. In contrast, the western parts are dominated by gravel gobi surfaces. This is attributed to higher sand supply in eastern parts provided by the Badain Jaran Desert and fluvial storages as sand sources. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
7.
浅议中国黄土磁化率的物理意义 总被引：5，自引：0，他引：5
中国的陆相黄土地层连续地记录了第四纪以来的古气候变化。基于中国黄土磁化率曲线与深海氧同位素记录具有可比性这一事实，磁化率作为一种气候替代性指标在黄土研究中得到广泛的应用。但是，磁化率的物理意义仍不清楚。本文作者对陕西榆林蔡家沟剖面末次间冰期以来的风成沉积作了研究，指出黄土、古土壤的磁化率差异不仅与前人提出的碳酸盐淋失、空隙度变化、“就地”新生成磁性矿物等作用有关，而且还可能与有机质有重要关系。有机 相似文献
8.
Qinghai Lake, on the northeastern Qinghai-Tibetan Plateau, is the largest extant closed-basin lake in China, and has been the subject of numerous palaeoclimatological and palaeoenvironmental studies. In this study, 32 samples of aeolian sand, loess and palaeosol at six sites, and 1 sample of shoreline deposits underlying aeolian deposits were dated using optically stimulated luminescence (OSL). Where available, OSL ages are in agreement with previously published 14C ages. Our dating results, in combination with previous published ages on aeolian deposits showed that: (1) The oldest aeolian deposits around Qinghai Lake are in excess of 165 ka. (2) Aeolian deposition then began at ∼14 ka in the Qinghai Lake area. Periods of palaeosol formation occurred at ∼16.9 ka, ∼12.2–11 ka, ∼10–9 ka, ∼5.2–4 ka, and ∼3.9–0.7 ka. (3) The accumulation intervals of palaeosols are generally consistent with drilling-core-based environmental change proxies, indicating that palaeosols were formed during wet periods with higher vegetation cover. (4) A depositional hiatus period of ∼40–50 ka exists between the surface mantle aeolian deposits and underlying gravel deposits. (5) Lake levels during the Holocene did not exceed 3205.2 m elevation (11.8 m above recent lake level of April, 2010). 相似文献
9.
自从大荔人化石被发现以来,其遗址剖面便成为研究热点.先前的研究主要集中在地层的对比划分与头盖骨年龄的推断方面,然而对于载磁矿物的鉴定及其古气候含义方面的研究却相对较少.鉴于此,本文运用热磁分析、饱和等温剩磁和剩磁矫顽力谱分析、磁滞回线分析、频率磁化率分析、热退磁分析等方法,对大荔人遗址剖面进行了系统的岩石磁学性质研究,鉴定出其主要载磁矿物为磁铁矿,赤铁矿,磁赤铁矿,磁畴状态主要是似单畴.磁铁矿,赤铁矿是样品中特征剩磁的携带者;主要起源于成土作用的超顺磁颗粒和新生成的亚铁磁性矿物,是古土壤样品磁化率增强的主要贡献者;古土壤中软磁性矿物的含量高于黄土.样品中磁赤铁矿的含量并不高.黄土-古土壤样品的频率磁化率曲线,古里雅冰芯氧同位素、细微粒浓度曲线,岐山五里铺剖面有机质含量曲线在古气候记录方面具有一致性,都展示出至少从MIS5以来,气候从冰期到间冰期的变化是渐变的,反之则表现了突变特征.上述岩石磁学研究丰富了大荔剖面的磁学领域研究内容,为相关课题的深入研究提供了依据. 相似文献
10.
TianShui Yang Masayuki Hyodo ZhenYu Yang ShiHong Zhang Toshiaki Mishima HuaiChun Wu HaiYan Li Yi Li XingAn Shi Kan Wang YiMing Ma 《中国科学:地球科学(英文版)》2014,57(8):1929-1943
Paleomagnetic records of the Gauss-Matuyama reversal were obtained from two loess sections at Baoji on the Chinese Loess Plateau. Stepwise thermal demagnetization shows two obvious magnetization components. A low-temperature component isolated between 100 and 200–250°C is close to the present geomagnetic field direction, and a high-temperature component isolated above 200–250°C reveals clearly normal, reversed, and transitional polarities. Magnetostratigraphic results of both sections indicated that the Gauss-Matuyama reversal consists of a high-frequency polarity fluctuation zone, but the characteristic remanent magnetization directions during the reversal are clearly inconsistent. Rock magnetic experiments demonstrated that for all the specimens with normal, reversed, and transitional polarities magnetite and hematite are the main magnetic carriers. Anisotropy of magnetic susceptibility indicates that the studied loess sediments have a primary sedimentary fabric. Based on virtual geomagnetic pole latitudes, the Gauss-Matuyama reversal records in the two sections are accompanied by 14 short-lived geomagnetic episodes (15 rapid polarity swings) and 12 short-lived geomagnetic episodes (13 rapid polarity swings), respectively. Our new records, together with previous ones from lacustrine, marine, and aeolian deposits, suggest that high-frequency polarity swings coexist with the Gauss-Matuyama reversal, and that the Gauss-Matuyama reversal may have taken more than 11 kyr to complete. However, we need more detailed analyses of sections across polarity swings during reversals as well as more high-resolution reversal records to understand geomagnetic behavior and inconsistent characteristic remanent magnetization directions during polarity reversals. 相似文献
11.
12.
Signs of the evolutionary process are well preserved in the aeolian deposits of desert margins and enable reconstruction of desert expansion and contraction. The sand content (percentage >63 µm) of loess in the loess–desert transitional zone was used as a proxy indicator for determining the proximity of the desert margin. According to the dominant wind direction, generally NW–SE in the northwest Chinese Loess Plateau, a transect was projected to observe spatial changes in sedimentological characteristics of aeolian deposits during the Last Glacial Period. The transect consists of 12 loess sections, starting at Shapotou in the transitional region between the Loess Plateau and the Tengger Desert and ending at Tianshui, which is located at the north foot of the Western Qinling Mountains in the southernmost part of the Loess Plateau. It shows the properties of temporal and spatial distribution of the Malan loess, with reduced sand content from northwest to southeast. A linear correlation model of sand content and distance was obtained, which was similar to the east of the Chinese Loess Plateau. According to this model, and on the basis of optically stimulated luminescence (OSL) dating from Binggou section, a detailed and quantificational expansion and contraction for the Tengger Desert was established. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
13.
Quantitative measurements on the paleo-weathering intensity of the loess-soil sequences and implication on paleomonsoon 总被引：2，自引：0，他引：2
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
14.
The quartz Al centre has been used in pioneering studies for ESR dating of sediments. Acceptable age estimates could be obtained for a range of deposits using this centre after estimating the residual level. To inspect the feasibility and reliability of ESR dating of windblown sediments, six loess samples from the loess–paleosol sequences of Luochuan profile on central Loess Plateau, China, were dated by ESR with regeneration method using the quartz Al centre measured at 115 K. The samples were exposed to sunlight for 430 h. Only about 25% signal intensity was bleached. According to our preliminary results, ESR age estimates increased with the depth along the loess profile, however, the ESR age of each sample is only about a half of the reference age of the corresponding strata. For example, the ESR age of a sample from the top of L8 loess near B/M boundary (a known age of 780 ka) yields an age of only 385 ka. It seems that ESR dating using quartz Al centre with dose regeneration protocol may have the potential for dating of loess and other aeolian deposits, but the studies on the nature of the quartz Al centre and experiment protocols have to be studied further. 相似文献
15.
16.
对甘肃西山坪遗址剖面样品进行磁化率、频率磁化率、热磁分析、非磁滞剩磁和等温剩磁等一系列岩石磁学实验测试,分析环境磁学参数特征变化,试图探讨西山坪遗址研究意义,为今后深入研究该沉积序列蕴含的古环境记录提供了岩石磁学基础.结果表明：西山坪遗址剖面沉积物中磁性矿物主要以低矫顽力的磁铁矿和磁赤铁矿为主,高矫顽力磁性矿物赤铁矿含量极其有限,越向剖面表层,强磁性矿物含量逐渐增加.磁性矿物颗粒表现出黄土的典型特征,以单畴、准单畴为主,含有一定量的超顺磁颗粒,颗粒度越向剖面表层越细.从物源上看,遗址沉积物主要来自近源的黄土风尘堆积,主要是风成堆积,并混合大量人类活动信息,给今后综合研究岩石磁学与人类活动特征的关系有一定借鉴意义. 相似文献
17.
Loess deposits distributed in southeastern China play an important role for paleoclimate reconstruction of the subtropical regions. These loess-paleosol deposits are mainly spread within the middle and lower reaches of the Yangtze River as well as in the drainage area of the Huai River. The ages of loess paleosol sequences that are distributed along the Huai River are not well constrained. In this study, the standard single-aliquot regenerative dose (SAR) optically stimulated luminescence (OSL) protocol and two elevated temperature post infrared-infrared stimulated luminescence SAR protocols (pIRIR225 and pIRIR290) were applied on 4–11 μm quartz and polymineral fine grains, respectively, in order to obtain the first numerical luminescence chronology for a loess-paleosol sequence in northern Jiangsu Province. Our results show a good agreement between quartz SAR-OSL and polymineral pIRIR ages up to ~70 ka. These findings confirm that Xiashu loess accumulated during the Last Glaciation. For samples older than this, the ages increasingly deviate with depth. Fine quartz ages beyond 70 ka are interpreted as underestimates, as previous studies reported that quartz ages >70 ka from various sedimentary origins worldwide may underestimate even if they pass rejection criteria and dose recovery tests. On the other hand, the pIRIR ages are most likely overestimating the true depositional ages as indicated by the results of dose recovery tests, where a 30–60% overestimation of the recovered dose is reported for values larger than ~400 Gy. The overestimation of pIRIR protocols was also confirmed by the results obtained when large beta doses were added on top of the natural accrued dose. Moreover, our dating results suggest that L1/S1 transition (corresponding to MIS 5/4 boundary) occurred much higher in the stratigraphic sequence than may have been interpreted from the magnetic susceptibility enhancement. This inconsistency can be assigned to invalidity of magnetic susceptibility as a chronostratigraphical proxy due to ferrimagnetic minerals dissolution or transformation during paedogenesis processes in this humid subtropical region in the southeastern China. 相似文献
18.
19.
The geographical location of Qaidam Basin corresponds to N36°05’-38°10’ and E90°10’-98°05’. The thickness of the Quaternary
deposits in the basin reaches 2000-3000 m. This paper deals mainly with the aspect of paleoclimate change of the basin. The
approach used for the study consisted of (1) establishing the paleoclimate curve of the basin according to the climate indices,
including the spore-pollen statistics, carbonate content and δ18O, obtained from the samples of the 760 m continuously-cored exploration holes sited in the basin, (2) comparing the paleoclimate
curve with the natural gamma-logs of the same exploration holes, and (3) determining the paleoclimatic changes in the last
2.85 Ma on the basis of detailed geochronological calculations. These results are extremely important for comparison with
the δ18O-based climate curve derived from the ODP659 in the Atlantic Ocean and with the cycles of paleosol in the Chinese loess. 相似文献
20.
Based on the study of palaeo-environmental evolution in the shelves of the Eastern China Seas, the concept of “shelf desertization”
in the late stage of Upper Pleistocene is defined; the environmental background and evolutionary process of shelf desertization
are analysed. Study on the records of subbottom profiling and the data of core samples from shelf areas revealed that during
low sea-level stages, the sedimentary environment in the exposed shelf plains was dominated by aeolian depositional process
under cold and dry climatic conditions, i.e. under the action of strong winter-monsoon winds. Parts of the exposed marine
strata were disintegrated, and aeolian sand dunes were formed on the disintegrated marine deposits, from which the finer sediment
grains were blown away by wind and deposited in the downwind areas to form the derivative loess deposits. Thus a desertization
environmental system was formed in the exposed shelf plains of the Eastern China Seas. 相似文献