Paleoclimatic significance of mineral magnetic properties of loess sediments in northeastern Qinghai-Tibetan Plateau

Loess deposit in the northeastern Qinghai-Tibetan Plateau offers an excellent record of climate and environment changes in this region.We have conducted multiparameter mineral magnetic analyses of smaples of loess deposits from the Dongchuan,Lalakou and Panzishan sections.The methods used include magnetic susceptibility,fre-quency-dependent suceptibility,temperature-dependent susceptibility,isothermal remanent magnetization acquisibility.Maghemite and hematite is also present in the loess layers and paleosol horizons.The higher concentration of maghemite in paleosols suggests that the formation of maghemite occurred during in situ pedogenesis,which plays an important role in the enhancement of the magnetic susceptibitily.Similar to that in the Chinese Loess Plateau lying to the east of the Liupan Mountains,magnetic granulometry in the studied loess and paleosols is predominantly pseudo-single-domain(PSD). However,the mean grain-size of the ferrimagnetic minerals in loess is evidently coarser (larger PSD and multiomain-like) than in paleosols (mainly PSD).The magnetic susceptibility of loess and paleosols is positively correlated with the content of ultrafine magnet-ite/maghemite grains,and hence with the intensity of in situ pedogenesis.Therefore,the enhanced magnetic susceptibility in the studied loess-paleosol sequences can be interpreted as being due to climatically induced in situ pedogenesis.     

Mid-Late Quaternary loess-paleosol sequence in Lantian’s Yushan,China: An environmental magnetism approach and its paleoclimatic significance

The application of rock magnetism methods to investigating the variations of magnetic minerals in the sediments is an important approach to the reestablishment of paleoclimate evolution. Thus we performed fine magnetic measurements on the loess-paleosol sequence (from L15 upwards to S5, in which L is short for Loess, S is short for Paleosol, the same hereinafter) of Yushan stratigraphic section, which is on the southeastern margin of Chinese Loess Plateau, in Lantian County of China’s Shaanxi Province, and the thickness of which is ca. 40 m. Our study shows that the primary magnetic carriers of loess and paleosol in this section are magnetite, maghemite, hematite and goethite. Thermomagnetic analyses on the samples of representative horizons show that the higher pedogenesis degree of the sediments, the smaller variations of magnetization there will be before and after heating, probably related to the pedogenic alteration of loess sediments. Analyses of several magnetic parameters show a significant discrepancy between the paleoclimatic conditions recorded in the strata from the loess unit L15 upward to the paleosol unit S5 in the study area and those recorded in the relative strata of other sections on the Chinese Loess Plateau, and those recorded in marine sediments, indicating the great impact of regional geological background. Similarly, the rapid and intensive change recorded in the segment from L15 to S9-1, and the significant difference between the paleoclimate evolutions of the two periods before and after the change (from L15 to S9-1, and from L9 to the base of S6) indicate the strong alteration of magnetic carriers in the study area as a result of the alternations of summer and winter monsoons in East Asia.     

Rock-magnetic investigation of Siberia loess and its implication

Multiple-rock magnetic investigations conducted on the loess-paleosol sequences at Kurtak in Southwestern Siberia reveal that the mass-normalized low-field magnetic susceptibility profiles reflect changes in lithology between relatively unweathered primary loess of glacial periods and the interglacial paleosols. Maxima in susceptibility values correspond with the least-weathered loess horizons, and minima with the humic horizons of soils. Frequency-dependent susceptibility of the loess-paleosol sequences at Kurtak is very low and practically uniform, indicating the dominance of non-SP ferrimagnetic minerals and negligible pedogenesis. The history of temperature-dependence of susceptibility (TDS) and stepwise acquisition of the isothermal remanent magnetization (SIRM) have confirmed that magnetite is predominant magnetic mineral, and only few maghemite and probably hematite are present within the studied section. Anisotropy of the magnetic susceptibility (AMS) can be used to monitor tilt and disturbance of the sedimentary layers, and also to provide information about the paleo-transport direction for Siberia loess.     

Pedogenesis affecting the Matuyama-Brunhes polarity transition recorded in Chinese loess?

A detailed record of the Matuyama-Brunhes (M-B) transition has been obtained from the loess unit 8 (L8) at Duanjiapo (34.2°N, 109.2°E),Shannxi Province of China. An investigation of the rock magnetic properties using hysteresis loops, thermomagnetic analyses identifies pseudo- single domain magnetite as the main carrier of the remanence, with a small contribution from maghemite and hematite. The paleo-direction records obtained reveal: (ⅰ) The M-B transition was recorded in the middle and lower part of L8, and comprises of five fast reversals. (ⅱ) The duration of the M-B polarity transition related to the directional change is about 4800 a. (ⅲ) The virtual geomagnetic pole (VGP) path during the transition is confined over Africa, peaked 90° away from the sampling site, in contrast with the results obtained from the Weinan loess section. The different VGPs are probably attributed to the pedogenesis.     

The magnetic properties of Serbian loess and its environmental significance

This paper reports a loess-paleosol sequence located in the Danube River basin,Serbia,which formed at least since the latest part of the early Pleistocene,and before the paleomagnetic B/M boundary.Various magnetic parameters of the Serbian V-L1-V-S4 loess-paleosol sequence have been measured and analyzed in the Titel Loess Plateau.These parameters show a very similar magnetic behavior compared with that of the Chinese loess.There is a general positive relationship between magnetic susceptibility() and pedogenesis.The main contributors to are the magnetic grains of SP(superparamagnetic) and SD(single domain) magnetic domains,while MD(multi domain) contributes only a small percentage.The difference in between loess and paleosol mainly is caused by pedogenesis.The very fine magnetic minerals increase gradually with increasing soil development(from loess to soil),and they lead to higher.The thermomagnetic curves show thatmagnetic minerals in the loess layers are magnetite and maghemite,both providing a major contribution to.In contrast the paleosol layers mainly are composed of magnetite,with almost no or a very small amount of maghemite,as implied by a reversible thermomagnetic behavior.This indicates that pedogenic conditions during V-S3 and V-S4 strong soil development have resulted in maghemite that is no longer stable,and has been resolved or converted to other stable phase minerals.This likely indicates that soil moisture during V-S3 and V-S4 development exceeded a critical condition of maghemite stability.     

Environmental magnetism and paleoclimatic interpretation of the Sanmenxia loess-paleosol sequence in the southeastern extremity of the Chinese Loess Plateau

This is a report of multi-parameter mineral magnetic results of the Sanmenxia loess-pa-leosol couplets in the SE extremity of the Chinese Loess Plateau (CLP) over the interval of L1- L13 spanning the last 1 Ma. These results show that mass-specific magnetic susceptibility (X), XARM and SIRM and their ratios exhibit parallel stratigraphic variations. Since superparamagnetic (SP) particles have significant contribution to X but no contribution to remanence, e.g., ARM and SIRM, the strong linear correlations between X and remanence (e.g., ARM and SIRM) suggest that the pedogenesis-produced magnetic grains are predominantly in small single domain range. Consequently, the significant contribution of SP particles to the bulk X as supposed in previous studies has been obviously overestimated. XARM/X data in Sanmenxia and median grain size records of typical loess sections from the hinterland of the CLP display a strong resemblance, implying that XARM/X may be regarded as a reliable indicator for monitoring the variations of ferrimagnetic grain sizes. The results of S-ratio and HIRM ((SIRM IRM-0.3T)/2) measurements indicate that there is significant enhancement of both soft magnetite/maghemite and hard hematite/goethite in paleosols associated with pedogenesis.     

Development of a pediment on western slopes of Liupan Mountain related to the Neotectonic Uplift

The late Cenozoic uplift of Qinghai-Xizang Plateau plays an important role in adjacent geomorphic evolution and depositional environments. Liupan Mountain, located at the northeast margin of Qinghai-Xizang Plateau, has different landforms on the east and west sides. Recently, a pediment, which lies above the highest terrace of the Yellow River at the northeast edge of Longzhong Basin, has been described. The paleomagnetic measurements indicate that the pediment had developed at least 1.8 MaBP and its age is in accordance with that of the peneplains near Lanzhou and Linxia, in the southwest of Longzhong Basin. The results suggest that the Longzhong Basin lying to the west of Liupan Mountain had been subject to erosion and developed an extensive pediment; this pediment is named the Gansu Period Peneplain. It also indicates that Qinghai-Xizang Plateau and Liupan Mountain were uplifted strongly at that time, which caused not only the end of pediplanation but also Yellow River appearance and loess accumulation.     

Age and provenance of loess in West Qinling

Magnetic polarity stratigraphy of loess in West Qinling (Mts.) demonstrates that loess began to deposit in West Qinling at ca. 800 kaBP. Its material may come from the nearby Tibetan Plateau to the west. This suggests that a sharp change in air circulation and surface environment on and around the Tibetan Plateau occurred at that time.     

Susceptibility variations of multiple origins of loess from the Ily Basin (NW China)

The magnetic susceptibility of loess from the Ily Basin,northwestern China shows maximum values in S0 paleosols but minimum values in other paleosols,the mechanism of which has been well debated.In this work,systematic magnetic measurements were made on a representative section from Neleke county.The results show that the loess horizons(L1,L2 and L3) have multi-domain magnetite grains of aeolian origin,S0 is characterized by production of pedogenetic ultrafine-grained ferrimagnetic minerals,and the other paleosols(S1,S2,and S3) are characterized by the formation of nonferrimagnetic minerals associated with waterlogging.The correlation between the low concentration of ferrimagnets,high paramagnetic content,high magnetic coercivity remanence,fine ferrimagnetic grain size and intensified pedogenesis suggest two competing processes of pedogenetic enhancement and pedogenetic depletion in the lower paleosols.Pedogenetic depletion dominates and is responsible for the low susceptibility.Changes in magnetic grain size distribution occur during pedogenetic depletion.The susceptibility variations are of multiple origins in the loess of the Ily Basin.Pedogenetic enhancement,pedogenetic depletion,and allochthonous input of magnetic minerals should all be taken into account to explain the variations of magnetic parameters.     

咸阳市区晚更新世黄土微结构特征及其环境意义

目的揭示咸阳市区晚更新世黄土的微结构特征,探讨黄土微结构特征与其形成环境的关系,为区域环境提供科学依据。方法应用粒度分析和扫描电子显微镜对咸阳市区晚更新世黄土．古土壤系列的微结构进行分析研究。结果粒度小于0．005 mm的黏粒级含量在黄土与古土壤中差异最大,S1古土壤高达39．7％,而L1黄土仅24％;S1古土壤黏粒中大量铁质黏粒胶膜呈流胶状出现并有次生伊利石的产出,结构密实;L1黄土微骨架颗粒相互支架,黏粒胶结物含量较少,胶结程度差,土质疏松。结论咸阳市区晚更新世S1古土壤形成在湿热的森林环境,L1黄土形成于干冷的草原环境,S0黑垆土形成于凉湿气候环境。     

末次间冰期陇西黄土记录与气候不稳定性问题的初步研究

陕西黄土高原黄土记录的分辩率较高，Ｓ１古土壤厚达８．５ｍ，主要由两个古土壤组和其间黄土层组成。临夏塬堡剖面上可分为５个古土壤层，主要黄土层记录了末次间冰期内的较干冷高粉尘堆积时期。     

S2以来下蜀黄土沉积序列磁化率记录与深海氧同位素记录的对比

对镇江下蜀黄土剖面进行了系统的磁化率测试,并同深海氧同位素记录进行了对比。结果表明,S2以来下蜀黄土磁化率记录与深海氧同位素记录存在良好的对应关系。首次在下蜀黄土记录中发现了相当于深海氧同位素5a,5b,5c,5d和5e亚阶段的波动特征;下蜀黄土S2磁化率记录则可与深海氧同位素7阶段进行细节上的详细对比。对比揭示,最近20多万年以来,下蜀黄土堆积区对全球古气候变化有着积极的响应;同时,下蜀黄土记录的古气候变化在某些阶段又具有明显的区域特征,可能反映的是全球气候变化效应与东亚古季风气候影响的叠加作用。镇江下蜀黄土剖面能获得精度较高的全球变化信息与其特定的地理位置、纬度位置和地貌条件有关。     

Problems of the magnetic susceptibility signature as the proxy of the summer monsoon intensity in the Chinese Loess Plateau

The magnetic susceptibility in the Chinese loess is considered to have recorded the history of pedogenesis or land-surface stability, and has thus been used as an acute proxy of the summer monsoon intensity. With regard to pedogenesis, most authors failed to appreciate the complexity of weathering processes. Specifically, not acknowledged are local redox conditions, carbonate concentration and leaching. Pre-depositional pedogenesis may be another factor undermining the legitimacy of using the susceptibility as an acute proxy of the summer monsoon intensity. These factors may not necessarily reflect the degree of pedogenesis although they have definitely affected the susceptibility signature. Some contributors to the susceptibility are demonstrated to be inorganic iron and other organically complexed iron. The organic iron associated with decomposed plant litter is proven to be the most important contributor to the susceptibility in modern loess. But, magnetotactic bacteria are believed to be the major generators of the ultrafine ferromagnetic particles responsible for the susceptibility signature in the Quaternary loess and paleosols. The factors affecting susceptibility signature may well be site- and time-dependent even within the Chinese Loess Plateau. Therefore, a more comprehensive model and a more comprehensive approach are needed to address the relationship between the paleoclimate and the proxy.     

Recent rapid shortening of crust across the Tianshan Mts. and relative motion of tectonic blocks in the north and south

Based on the multiple-epoch Global Positioning System observations during a period from 1992 to 1999, we document directly a rapid crustal shortening of ～20 mm/a across the western Tianshan Mts. (76°E), in contrast to a 4 mm/a convergent rate across the eastern Tianshan Mts. (87°E)and the north-south convergence across the mountain belt descends laterally from west to east. The direction of current crustal movement inferred by GPS sites along the southern flank of the Tianshan Mts. is approximately perpendicular to the easterly-trending mountain belt, indicating that the Tarim Basin thrust almost rightly into the Tianshan Mts. The Tarim Basin accommodates nearly no or a minor, if any, crustal deformation and rotates clockwise, as a rigid body in a whole, at a rate of 0.64°/Ma around a Euler pole at 95.7°E, 40.3°N (Anxi, Gansu) with respect to the stable Siberia. The relative motion between the Kazakh platform and the Dzungarian Basin is quite apparent. The Dzungar should be regarded as an independent active block from the view of the Asia tectonic settings.     

秦岭北麓黄土沉积环境

秦岭北麓黄土最早形成于2.45Ma.区内各地质时代的黄土层中几乎都发现了大量哺乳动物化石和人类化石.黄土颜色深,颗粒细,粘化重;古土壤色鲜艳,层数多,底部钙质结核常联结成钙板;黄土中的原生碳酸钙几乎全部被淋溶,光性方位粘土较显著.这些说明本区黄土物质沉积后经受了比较强烈的淋滤作用和成土作用,反映出第四纪期间秦岭北麓黄土沉积环境要比黄土高原西北部温暖而湿润。     

Magnetic properties of surface soils across the southern Tarim Basin and their relationship with climate and source materials

The magnetic susceptibility (χ) of surface soils on the Chinese Loess Plateau (CLP) has strong positive correlation with mean annual precipitation.It is widely accepted that ultrafine magnetite/maghemite grains produced during pedogenesis are responsible for the enhancement of χ,and loess χ on the CLP has been widely used as a proxy for the intensity of the East Asian summer monsoon.However,few works have investigated mechanisms for the enhancement of χ in the case of surface aeolian sediments in westerly-dominated inland China,north of the Tibetan Plateau.Here detailed rock magnetism and grain size studies of 49 surface samples taken across the southern Tarim Basin for different rainfall/temperature/altitude conditions are presented.The results show that samples taken from desert have the lowest χ values and that χ of loessic sand and loess samples decreases with increasing altitude.In addition,the rock magnetism studies suggest that the magnetic properties of surface soils across the southern Tarim Basin are mainly controlled by the concentration of coarse aeolian ferrimagnetic minerals.The contribution of ultrafine pedogenically produced magnetic grains to χ is very limited.     

Variation of Zr/Rb ratios on the Loess Plateau of Central China during the last 130000 years and its implications for winter monsoon

To find a new index of winter monsoon without effects of chemical weathering and pedogenesis is an important task in loess research. Chemical analysis of five grain size fractions (i.e. >32 μm, 32–20 μm, 20–8 μm, 8–2 μm and <2 μm) show that Zr is enriched in coarser grain size, while Rb is concentrated in finer fractions. An excellent correlation occurs between Zr/Rb ratios and mean grain size in loess units (L₁LL₁ and L₁LL₂) from Huanxian and Xifeng sections, reflecting that Zr/Rb ratios increase when grain size is coarser. Due to immobility of the two elements during the postdepositional process, Zr/Rb ratios can eliminate the effect of weathering and pedogenesis and serve as an excellent proxy on winter monsoon variation. The Zr/Rb ratio curves of the Huanxian, Xifeng and Luochuan sections during the last 130000 years show that the winter monsoon strength was very weak during MIS 3, which corresponded to the intensity during MIS 5.     

Geomagnetic anomalies recorded in L9 of the Songjiadian loess section in southeastern Chinese Loess Plateau

Detailed magnetostratigraphic and rock magnetic investigations on L8-S12 of the Songjiadian loess section in the Sanmenxia area,southernmost margin of the Chinese Loess Plateau were conducted in this study.Matuyama/Brunhes (M/B) boundary occurred at the bottom of the loess unit L8.The top and bottom boundaries of the Jaramillo polarity subchron are found in the middle of L10 and the bottom of L12,respectively.Magnetic fabric of the loess layers maintains the original depositional features and the recorded r...     

中国风尘沉积环境磁学研究现状

中国黄土-古土壤-红黏土序列完整记录了7.5 Ma以来东亚季风演化历史。黄土-古土壤的低频磁化率已成为东亚夏季风演化的代用指标之一,并得到广泛的应用和全球对比。虽然古土壤磁化率增强的成土观点已得到广泛认可,但磁化率与成壤强度的相关性还不是十分清楚,磁化率应用方面仍存在问题。磁化率与成壤强度、生物化学作用和古气候关系尚在研究中。     

Effects of low-temperature oxidation on natural remanent magnetization of Chinese loess

This study systematically investigates changes in both compositions and grain-sizes of magnetic minerals in the Chinese loess/paleosol samples (Yichuan, China) during thermal demagnetization processes. Between 100 and 200℃, (1) H_c and H_cr significantly decrease while M_rs and M_s remain stable; (2) concentration of the multi-domain (MD) grains increases while that of superparamagnetic (SP) grains decreases; and (3) there exists the abnormal behavior of the thermal demagnetization in the natural remanent magnetization (NRM). In addition, the corresponding changes in all these parameters are gradually muted with the increase of the pedogenesis degree. The results indicate that the observed alteration probably relates to reducing processes caused by the burning of the organic matter in samples. Before the thermal treatment, the MD grains in loess samples had been partially altered in nature by low-temperature oxidization (LTO). This CRM can be sufficiently attenuated or even removed by the reducing process between 100 and 200℃ during the thermal treatment and has no apparent harmful effects on the characteristic remanent magnetization (ChRM) between 300 and 500℃.