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Reliability of Relative Paleointensity Recorded in chinese loess-Paleosol Sediments

Acta Geologica Sinica(English Edition) 2012年第5期86卷 1276-1288页

作者： YANG Tianshui LI Haiyan WU Huaichun YANG Zhenyu ZHANG Shihong HYODO Masayuki State Key Laboratory of Biogeology and Environmental Geology China University of GeosciencesBeijing 100083China School of the Earth Sciences and Resources China University of GeosciencesBeijing 100083China School of Ocean Sciences and Resources China University of GeosciencesBeijing 100083China Institute of Geomechanics Chinese Academy of Geological SciencesBeijing 100081China Research Center for Inland Seas Kobe UniversityKobe 657-8501Japan

A detailed paleomagnetic and rock-magnetic investigation spanning loess L7 to paleosol S8 has been carried out at the Baoji and Xifeng sections. Results of anisotropy of magnetic susceptibility confirm that the studied loess-paleosol sediments retain primary sedimentary fabrics. Stepwise thermal demagnetization shows that two well-defined magnetization components can be isolated from both loess and paleosol specimens. A low-temperature component, isolated between 100°C and 200°C, is consistent with the present geomagnetic field direction, and a high-temperature component, isolated between 200-300°C and 620-680°C, includes clearly normal and reversed polarities. Isothermal remanent magnetization and thermomagnetic analyses indicate that characteristic remanent magnetization is mainly carried by magnetite and hematite. The Day plot, together with the stratigraphic variations of rock-magnetic parameters, shows that the uniformity of magnetic mineralogy and grain size fulfills the criteria for relative paleointensity （RPI） studies. RPI records have been constructed using natural remanent magnetization （NRM） intensity after thermal demagnetization at 300°C normalized by low-frequency magnetic susceptibility （NRM 300 /χ）. The results show that the RPI record from the Baoji section, where pedogenesis is quite weak, is compatible with the stacked PISO-1500 paleointensity record, suggesting that it might reflect the paleointensity variation of the geomagnetic field. The RPI record from the Xifeng section, where pedogenesis is rather strong, indicates a clear dissimilarity with the stacked PISO-1500 paleointensity record, implying that it does not reflect the paleointensity variation of the geomagnetic field. Our new results show that the NRM 300 /χ from the strongly pedogenetic paleosols does not completely eliminate the pedogenetic （climatic） influence, so it might be unsuitable for a reliable paleointensity study.

关键词： chinese loess pedogenesis relative paleointensity rock magnetism geomagnetic field

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Failure mechanisms and risk mitigation of check dams on the chinese loess Plateau: A case study at the Gutun Gully

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Journal of Mountain Science 2022年第6期19卷 1509-1524页

作者： JIANG Rui-jun ZHANG Mao-sheng FENG Li SUN Ping-ping Department of Geological Engineering Chang’an UniversityXi’anShaanxi 710054China Department of Civil Engineering School of Human Settlements and Civil EngineeringXi’an Jiaotong UniversityXi'an 710049China Key Laboratory for Geo-hazards in loess Area Ministry of Natural ResourcesXi’an Center of China Geological SurveyXi’an 710054China

loess is long-term aeolian dust deposition, characterized by loose structure, concentrated participle distribution and unstable mineral composition, and thus easy to cause extensive collapsibility and have general water sensitivity. To reveal the difference in water sensitivity between naturally intact(NI) loess and mechanically compacted(MC) loess used for the check dam, the transient water release and imbibition method(TRIM) was used to acquire the suction stress–expanded hydraulic characteristic curves for the NI and MC loess and explore possible approaches for formulating the potential of loess water sensitivity. Based on the Local Field of Safety(LFS) associated with slope stability, we constructed a finite element model of a check dam to depict its failure processes under different rainfall scenarios. The results revealed the strong water sensitivity in NI loess, while the MC loess retained a certain water-sensitive potential. This capacity depends on the ‘water sensitivity coefficient’ obtained from the suction-stress characteristic curve,which better presented the deformation potential of the two loess samples at different water content levels. In the context of LFS, we identified two failure patterns in the dam body that were involved in loess water sensitivity under hydromechanical conditions: rainfall erosion-induced shallow mudflow failure, and preferential-infiltration progressive failure. These patterns may provide new insights into dam-breakage mechanisms and potential chain effects of check dams on the chinese loess Plateau from the perspective of soil–water interactions, which is vital for predicting the position and timing of check dam failure, and mitigating risks.

关键词： chinese loess Water sensitivity Suction stress Failure patterns Slope stability Check dam

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High-resolution anisotropy of magnetic susceptibility record in the central chinese loess Plateau and its paleoenvironment implications

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Science China Earth Sciences 2012年第3期55卷 488-494页

作者： LIU WeiMing SUN JiMin Key Laboratory of Cenozoic Geology and Environment Institute of Geology and GeophysicsChinese Academy of SciencesBeijing 100029China Key Laboratory of Mountain Hazards and Earth Surface Process Institute of Mountain Hazards and EnvironmentChinese Academy of SciencesChengdu 610041China

We analyze high-resolution anisotropy of magnetic susceptibility (AMS) of the loess-paleosol successions at Luochuan, central chinese loess Plateau, in order to investigate the AMS characteristics and their climatic implications. Our results indicate a normal sedimentary magnetic fabric for almost of all samples, characterized by minimum susceptibility axes grouped in an almost vertical direction. Magnetic foliation and anisotropy degree show upwards decreasing trend due to decreasing post-depositional compaction. Magnetic lineations show no preferred directions and thus cannot indicate paleowind patterns. AMS parameters at Luochuan are controlled by particle size, pedogenesis, and sedimentary compaction. The high peaks of magnetic foliation and anisotropy degree of L2, L3, L6, L9, and L15 correspond to the coarse particle sizes of these loess beds, indicating the grain-size dependence of AMS.

关键词： chinese loess anisotropy of magnetic susceptibility Quaternary Luochuan

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An integrated study of the grain-size-dependent magnetic mineralogy of the chinese loess/paleosol and its environmental significance

An integrated study of the grain-size-dependent magnetic min...

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中国科学院地质与地球物理研究所2003学术年会

作者： Subir K.Banerjee Michael J.Jackson Institute for Rock Magnetism Department of Geology and GeophysicsUniversity of MinnesotaMinneapolisMinnesotaUSA

To investigate the grain-size-dependent properties of the magnetic minerals in chinese loess/paleosol samples(Touxiangdao,Xining,Qinghai province,China),magnetic extracts were divided into two size fractions by gravitational *** the basis of hysteresis measurements,thermal demagnetization of low-temperature saturation isothermal remanent magnetization,and nonmagnetic studies(SEM and XRD) we identified magnetic phases both in the grain size fractions of the magnetic extracts and in the less magnetic residues to provide more accurate and complete descriptions of all the magnetic components in the bulk natural *** results show that the oxidation degree(nonstoichiometry) of magnetic minerals is strongly affected by both grain size and the paleoclimatic environment in which they were deposited and *** ascending order of the oxidation degree of our samples,we find(1) loess-coarse particles (LC) are multidomain(MD) magnetite with slight oxidation,(2) paleosol-coarse(PC) particles are also MD magnetite but with a higher oxidation degree compared to LC, (3) loess-fine(LF) particles are pseudo-single domain(PSD) magnetite with a high oxidation degree,and(4) paleosol-fine(PF) particles are PSD *** domain (SD) and superparamagnetic(SP) maghemite mainly stay in the *** thermomagnetic analysis of PF(PSD maghemite) revealed that this natural maghemite has a Curie temperature identical to that of magnetite and that the conversion efficiency of transformation from maghemite to hematite is only about 50%after a 700℃heating/ cooling *** new results identify the sources of multicomponent NRM in chinese loess sequences as well as clarify the paleoenviromental and paleoclimatic controls on the remanence *** TERMS:1512 Geomagnetism and Paleomagnetism:Environmental magnetism;1519 Geomagnetism and Paleomagnetism:Magnetic mineralogy and petrology;1540 Geomagnetism and Paleomagnetism:Rock and mineral magnetism

关键词： chinese loess Xining magnetic mineralogy extraction oxidation low-temperature experiments

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

Effects of low-temperature oxidation on natural remanent mag...

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中国科学院地质与地球物理研究所2002学术年会

作者： S.K.Banerjee M.J.Jackson Institute for Rock Magnetism Department of Geology and GeophysicsUniversity of Minnesota

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 *** 100 and 200°,(1) Hc and Hcr significantly decrease while Mrs and Ms 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 *** results indicate that the observed alteration probably relates to reducing processes caused by the burning of the organic matter in *** 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°.

关键词： chemical remanent magnetization low-temperature oxidation Verwey transition natural remanent magnetization chinese loess

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Remagnetization mechanism and a new age model for L9 in chinese loess

Remagnetization mechanism and a new age model for L9 in Chin...

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中国科学院地质与地球物理研究所第11届（2011年度）学术年会

作者： Chunsheng Jin,Qingsong Liu~* State Key Laboratory of Lithospheric Evolution,Institute of Geology and Geophysics,chinese Academy of Sciences,Beijing 100029,PR China

chinese loess during the Quaternary is important for both paleomagnetic and paleoclimatic studies, Although magnetostratigraphy has been used for the initial loess timescale,paleomagnetic records in loess still remain in debates,such as magnetic polarity anomalies in L9,which is a thick and coarse sandy horizon as a typical marker *** better understand the magnetization mechanism of these anomalies, a new age model for L9 was first constructed based on a new correlation between loess and marineδO, which proposed that L9 should be correlated to marine oxygen stage(MIS) 20-24,not 22-24 as previous *** combined grain size and environmental magnetism parameters,an extended climatic optimum corresponding to MIS 21 can be identified in middle of L9.A remagnetization mechanism of viscous remanent magnetization overprinting on characteristic remanent magnetization(ChRM) carriers (mainly large pseudo single domain and multidomain magnetite particles),associated with the chaotic detrital remanent magnetization caused by low efficiency alignment of ChRM carriers along with the ambient field during deposition,is proposed to interpret the magnetic *** study provides new insights into the magnetization mechanism of the chinese loess,and the new age model for L9 helps to accurately refine the finer scale correlation of paleoclimatic records between the chinese loess and marine sediments.

关键词： Remagnetization L9 Coarse-grained magnetite particles Decreased winter monsoon Viscous remanent magnetization overprinting chinese loess

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Geomagnetic episodes of the last 1.2 Myr recorded in chinese loess

Geomagnetic episodes of the last 1.2 Myr recorded in Chinese...

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中国科学院地质与地球物理研究所2002学术年会

作者： Y.X.Pan,~a R.X.Zhu,~a Q.S.Liu,~b B.Guo,1 L.P.Yue,~c and H.N.Wu~c a.Institute of Geology and Geophysics,chinese Academy of Sciences,Beijing China b.Institute for Rock Magnetism,University of Minnesota,Minneapolis,USA c.Department of Geology,Northwest University,Xi’an,China.

The Matuyama-Brunhes(M/B) boundary,termination of Jaramillo normal subchron (J/M) as well as three young geomagnetic excursions has been revealed in the Weinan loess section [Zhu et al.,1994,1998,1999].Recently,we carried out further paleomagnetic directional studies on the same section and recovered the onset of the Jaramillo normal subchron(M/J) and four geomagnetic excursions,two in the Brunhes and two in the late *** fact that all seven excursions found in this section are in loess units and expected excursions in paleosols are not found suggests that short-lived geomagnetic signals could be only recorded in loess units,and may have been fully smoothed or wiped out in paleosol units by strong *** by a new astronomical timescale(HLD00),ages of the geomagnetic excursions and reversal boundaries have been *** tentatively correlated these excursions with previous results,indicating that a large lock-in model may not be correct for the whole loess profile,although some apparently big positive age discrepancies were observed for the main boundaries.

关键词： Geomagnetic excursion chinese loess 1.2 Myr

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Paleoenvironmental significance of the magnetic fabrics in chinese loess-paleosols since the last interglacial (＜130 ka)

Paleoenvironmental significance of the magnetic fabrics in C...

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中国科学院地质与地球物理研究所2004学术年会

作者： Michael J.Jackson Institute for Rock Magnetism and Department of Geology and Geophysics University of MinnesotaMinneapolisMN 55455USA

We compare new anisotropy of magnetic susceptibility(AMS) data with previously-determined paleoclimaticaily sensitive parameters for a loess profile at Lingtai,central loess Plateau,China,to unravel factors that control loess *** lineations are clustered along NE-SW and NW-SE orientations during cold and warm periods, respectively,which probably corresponded to paleowind directions of both winter and summer monsoons. Anomalously steep magnetic foliations coincidentally occur at the paleoclimatic stage boundaries,indicating sediment *** strength appears to be sensitive to millennial-scale rapid paleoclimatic fluctuations. Generally,the winter monsoon produces a lower degree of lineation than the summer ***,by studying lineation subsets for the cold and warm periods,we find that the cold-interval lineation strengths co-vary with grain size over the last～70 *** the last interglacial(about 74-128 ka),the lineation and declination of the susceptibility maximum(Dec-K) are sensitive to rapid fluctuations in winter and summer ***-term trends of magnetic foliation and anisotropy degree replicate grain size variations,with superimposed short-term peaks related to post-depositional compaction,as suggested by *** AMS shape parameter(7) indicates oblate *** new findings will not only improve our understandings of loess AMS,but also expand the possibilities of paleoclimatic studies in the chinese loess.

关键词： AMS chinese loess Lingtai paleowind strength paleowind direction monsoon

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Color reflectance of chinese loess and its implications for climate gradient changes during the last two glacial-interglacial cycles

Color reflectance of Chinese loess and its implications for ...

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中国科学院地质与地球物理研究所2003学术年会

作者： S.L.Yang and Z.L.Ding Institute of Geology and Geophysics,chinese Academy of Sciences,Beijing,China

To examine the relationship between color changes of loess deposits and paleoclimate conditions,a north-south transect consisting of twelve loess sections were studied. Color reflectance was measured on loess-paleosol... 详细信息

关键词： chinese loess Color reflectance Giacial-intergiacial cycles Weathering intensity

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Stacked 2.6-Ma grain size record from the chinese loess based on five sections and correlation with the deep-seaδ18O record

Stacked 2.6-Ma grain size record from the Chinese loess base...

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中国科学院地质与地球物理研究所2002学术年会

作者： Derbyshire E. Centre for Quaternary Research Department of GeographyRoyal HollowayUniversity of London

This study aims to establish a stacked climate record of the Quaternary period from the chinese loess sequence,and to address the forcing mechanisms for the regional climate history of the loess Plateau by correlating the stacked record with a composite O record in deep-sea sediments.A total of 18352 samples were obtained from five loess sections,located at Baoji,Lingtai, Jingchuan,Puxian and Pingliang in the southern and middle loess *** yielded high-resolution grain size ***-section correlation of these shows that,although small depositional hiatuses are present in places within a single section,most parts of the sections display near-continuous dust deposition throughout the *** grain size records were tuned simultaneously to the theoretical variations in obliquity and precession of the Earth’s *** grain size records plotted on their orbital time scales were then averaged to form a stacked loess grain size time series,termed the"Chiloparts"*** resolves most of the orbital time-scale paleoclimate events buried in the loess-soil sequences of the southern and middle loess Plateau,and can be used as a regional archive of the Pleistocene climate history in the loess *** of the"Chiloparts"record with a composite marine O record shows that,for the past 1.8 Ma, the loess-paleosol record can be correlated almost cycle by cycle with the marine *** discrepancies in the climatic events between the two records have also been identified,implying that regional forcing mechanisms may have played a part in the climatic evolution of the chinese loess Plateau.

关键词： chinese loess grain size orbital timescale continental climate record land-sea climate correlation

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