Quantitative measurements on the paleo-weathering intensity of the loess-soil sequences and implication on paleomonsoon

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.     

Impacts of post-depositional processes on rapid monsoon signals recorded by the last glacial loess deposits of northern China

Rapid East Asian Monsoon oscillations recorded by Chinese loess are thought to be dynamically linked to north Atlantic climate. However, few efforts have been made to assess the effects of post-depositional processes (e.g., surface mixing and pedogenesis) on loess paleoclimatic records. Here a detailed optically stimulated luminescence dating of a thick loess sequence from the western Loess Plateau is presented, offering a reliable chronology for last glacial deposits. Magnetic susceptibility and mean grain size records from three loess–paleosol sequences along a northwest–southeast transect are investigated to evaluate impacts of post-depositional processes on these loess-based proxy records. Our results indicate that: (1) loess sequences developed within the flat tableland of the central and western Loess Plateau are nearly continuous during the last glaciation; and (2) post-depositional processes have distinct impacts on rapid monsoon signals recorded in loess sequences from different regions. In the central Loess Plateau, rapid monsoon signals have been attenuated to various degrees depending on the sedimentation rate and pedogenic intensity. In the northwestern Loess Plateau, however, due to high sedimentation rate and relatively weak pedogenesis, high-resolution grain size oscillations reliably record rapid monsoon changes and can be well correlated to rapid climate changes recorded in the Greenland ice core and Hulu cave stalagmite.     

Paleoclimatic significance of grain size of loess-palaeosol deposit in Chinese Loess Plateau

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.     

Grain-size records at ODP Site 1146 from the northern South China Sea: Implications on the East Asian monsoon evolution since 20 Ma

273 samples from Ocean Drilling Program (ODP) Site 1146 in the northern South China Sea (SCS) were analyzed for grain-size distributions using grain-size class vs. standard deviation method and end-member modeling algorithm (EMMA) in order to investigate the evolution of the East Asian monsoon since about 20 Ma. 10–19 μm/1.3–2.4 μm, the ratio of two grain-size populations with the highest variability through time was used to indicate East Asian winter monsoon intensity relative to summer monsoon. The mass accumulation rate of the coarsest end member EM1 (eolian), resulting from EMMA, can be used as a proxy of winter monsoon strength and Asian inland aridity, and the ratio of EM1/(EM2 EM3) as a proxy of winter monsoon intensity relative to summer monsoon. The combined proxies show that a profound enhancement of East Asian winter monsoon strength and winter monsoon intensity relative to summer monsoon occurred at about 8 Ma, and it is possible that the summer monsoon simultaneously intensified with winter monsoon at 3 Ma. Our results are well consistent with the previous studies in loess, eolian deposion in the Pacifc, radiolarians and planktonic foraminifera in the SCS. The phased uplift of the Himalaya-Tibetan Plateau may have played a significant role in strengthening the Asian monsoon at 8 Ma and 3 Ma.     

OSL chronology of loess deposits in East China and its implications for East Asian monsoon history

Controversy exists regarding the chronology of loess in the Nanjing area in Jiangsu Province and the Jiujiang area in Jiangxi Province, East China. The chronology is of special interest, because the age of the initial accumulation for loess in these areas indicates that dust accumulation has been extended from the Loess Plateau in North China to East China at that time. This implies a threshold in the evolution of the East Asian monsoon, with the strengthened winter monsoon transporting aeolian dust further to the south. In this study nine luminescence samples were collected from the Jiujiang area and four samples from the Nanjing area. Quartz grains of 38–63 μm were isolated and the single-aliquot regenerative-dose (SAR) protocol was employed for D_e determination. For all samples from the Nanjing area, during the SAR measurement cycles the test-dose sensitivity was dose dependent and dropped significantly when the regeneration dose reached >300 Gy, while it increased linearly when the regeneration dose value ranged from 25 to 300 Gy. The reason for this is not yet known. The OSL chronology and its implications for paleoenvironmental change are discussed. It is concluded that: (1) loess deposition in the Jiujiang area started in the early Last Glaciation; and (2) the first loess layer in the Nanjing area was deposited during the last Glaciation.     

Late quaternary monsoon patterns on the Loess Plateau of China

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.     

Rock magnetic properties of a loess-paleosol couple along an N-S transect in the Chinese Loess Plateau

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 (χ_h) 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 χ_L of paleosol unit S8 in Yichuan is highest, and the χ_L of Duanjiapo is lower than that in Yichuan section, suggesting that there is not correlation between the χ_L and the degree of pedogenesis in loess-paleosols. (iii) With the increasing of χ_L, 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.     

我国黄土的微结构类型与震陷区域划分

对比我国中西部成土年代相近的浅层风成黄土（不考虑地形地貌）,针对不同成土环境下微观结构及其震陷性大小,初步将中西部黄土的微结构类型划分为五类：Ⅰ,近砂源快速降砂微结构;Ⅱ,强降尘弱成壤微结构;Ⅲ,冷干慢速降尘弱成壤微结构;Ⅳ,中湿成壤微结构;Ⅵ,温湿成壤微结构.微结构类型对于震陷性等工程性质分析有借鉴作用,具体体现在颗粒大小、粒径分布、排列方式,颗粒之间的胶结程度、接触方式等.通过分析黄土地区气候变化引起的地域性微观结构类型差异,区域性微观结构的气候形成机制来研究黄土的致灾特征,进一步获得黄土地区土层微结构的地域性差别导致的震陷变形强弱信息.这对于黄土建筑工程场地的设计以及提出针对性的防护措施有重要意义.     

Detrital and pedogenic magnetic mineral phases in the loess/palaeosol sequence at Lingtai (Central Chinese Loess Plateau)

A detailed rock magnetic investigation of loess/palaeosol samples from the section at Lingtai on the central Chinese Loess Plateau (CLP) is presented. Thermal demagnetisation of isothermal remanent magnetisation (IRM) and Curie temperature measurements suggest the presence of magnetite, maghemite and hematite as remanence carrying components. Bulk and grain size fractionated samples have been analysed using coercivity spectra of remanence acquisition/demagnetisation curves, which identify four main remanence carriers in different grain size fractions of loesses and palaeosols. A linear source mixing model quantifies the contribution of the four components which have been experimentally derived as dominating endmembers in specific grain size fractions. Up to two thirds of the total IRM of the palaeosols are due to slightly oxidised pedogenic magnetite. Two detrital components dominate up to 90% of the IRM of the loess samples and are ascribed to maghemite of different oxidation degree. Detrital hematite is present in all samples and contributes up to 10% of the IRM. The iron content of the grain size fractions gives evidence that iron in pedogenically grown remanence carriers does not originate from the detrital iron oxides, but rather from iron-bearing clays and mafic silicates. The contribution of pedogenic magnetite to the bulk IRM increases with the increasing degree of pedogenesis, which depends in turn on climate change.     

Carbon isotopic composition of modern soil and paleosol as a response to vegetation change on the Chinese Loess Plateau

The relationship between the carbon isotopic composition of paleosols and pale-ovegetation on the Loess Plateau is still unclear. One of the main reasons is that we are short of knowledge about the characteristics of the carbon isotopic composition of modern soil in this area. A preliminary investigation of the carbon isotopic compositions of the modern soil and the loess/paleosol sequence on the Loess Plateau shows that the carbon isotopic composition of modern soil is consistent with the distribution of modern plants on the Loess Plateau, where the ecosystem is dominated by a mixture of C4 and C3 plants. Comparing the δ13C values of modern soil and loess-paleosol sequences from the Xunyi profile, we conclude that C3 plants dominated the landscape during loess sediment stages, while C4 plants expanded during paleosol stages.     

Mineral composition and particle size distribution of river sediment and loess in the middle and lower Yellow River

Hyperconcentrated flows often occur in the middle and lower Yellow River(MLYR)and its tributaries,within which the main sediment source originates from the Loess Plateau of China due to serious water erosion.Little is known about the properties of river sediment that is transported by hyperconcentrated flows,particularly with respect to the mineral composition and size distribution.Samples of sediment and loess were collected in the northern,middle,and southern Loess Plateau and the mainstream and tributaries of the MLYR.A total of 18 loess samples and 24 river sediment samples were analyzed to determine their sediment size distribution and mineral composition.The bottom loess samples reflected the original sedimentary features of the Loess Plateau,and the median particle size reduced,and the clay content increased from the north to the south of the study region.The surface loess has been weathered under the action of wind and rainfall,and the clay particle content in the surface loess samples was higher than that in the undisturbed bottom loess.Erosion of the surface soil due to rainfall and surface runoff means that fine particles(mostly clay)have been washed away.The median diameter of surface loess particles was a little larger than that of the bottom loess particles where water erosion dominates.The particle size became coarser with increasing distance from the estuary in the MLYR,which reflects depositional sorting in the river channel.Significant logarithmic relations were found between the median diameter of the sediment particles and the i)non-clay mineral content and ii)clay mineral content.Thus,clay and non-clay mineral compositions can be conveniently estimated from the particle size distribution.     

The behavior of specific sediment yield in different grain size fractions in the tributaries of the middle Yellow River as influenced by eolian and fluvial processes

Based on data from 35 stations on the tributaries of the Yellow River, annual specific sediment yield (Y_s) in eight grain size fractions has been related to basin‐averaged annual sand–dust storm days (D_ss) and annual precipitation (P_m) to reveal the influence of eolian and fluvial processes on specific sediment yield in different grain size fractions. The results show that Y_s in fine grain size fractions has the highest values in the areas dominated by the coupled wind–water process. From these areas to those dominated by the eolian process or to those dominated by the fluvial process, Y_s tends to decrease. For relatively coarse grain size fractions, Y_s has monotonic variation, i.e. with the increase in D_ss or the decrease in P_m, Y_s increases. This indicates that the sediment producing behavior for fine sediments is different from that for relatively coarse sediments. The results all show that Y_s for relatively coarse sediments depends on the eolian process more than on the fluvial process, and the coarser the sediment fractions the stronger the dependence of the Y_s on the eolian process. The Y_s–D_ss and Y_s–P_m curves for fine grain size fractions show some peaks and the fitted straight lines for Y_s–D_ss and Y_s–P_m relationships for relatively coarse grain size fractions show some breaks. Almost all these break points may be regarded as thresholds. These thresholds are all located in the areas dominated by the coupled wind–water process, indicating that these areas are sensitive for erosion and sediment production, to which more attention should be given for the purpose of erosion and sediment control. A number of regression equations were established, based which the effect of rainfall, sand–dust storms and surface material grain size on specific sediment yield can be assessed. Copyright © 2007 John Wiley & Sons, Ltd.     

High resolution OSL and post-IR IRSL dating of the last interglacial–glacial cycle at the Sanbahuo loess site (northeastern China)

Northeastern China is located in the East Asian monsoon region; it is sensitive to both high and low latitude global climate systems. Loess deposits in the region have considerable potential as sensitive archives of past climate changes. However, research into loess deposition and climate change in this region is restricted by the lack of independent age control. In this study, coarse-grained quartz SAR OSL and K-feldspar post-IR infrared (IR) stimulated luminescence (post-IR IRSL; pIRIR₂₉₀) methods have been used to date the Sanbahuo loess site in northeastern China. The quartz OSL characteristics are satisfactory. The measured pIRIR₂₉₀ D_e's do not vary significantly with IR stimulation temperatures between 50 °C and 260 °C; a first IR stimulation temperature of 200 °C was adopted. Dose recovery tests were performed by adding different laboratory doses to both laboratory bleached (300 h SOL2) samples and natural samples; the results are satisfactory up to ∼800 Gy. Resulting quartz OSL and feldspar pIRIR₂₉₀ ages are in good agreement at least back to ∼44 ka; beyond this feldspar pIRIR₂₉₀ ages are older. The feldspar ages are consistent with the expected age of the S1 palaeosol (MIS 5). There appears to have been a period of fast loess deposition at ∼62 ka, perhaps indicative of winter monsoon intensification with a very cold and dry climate that lead to a serious desertification of dunefields in northeastern China.     

中更新世以来青藏高原RH孔和洛川黄土记录的比较研究

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。     

OSL chronostratigraphy of a loess-palaeosol sequence in Saxony/Germany using quartz of different grain sizes

Luminescence dating is one of the leading techniques to establish chronologies for loess-palaeosol sequences and has been successfully applied to different minerals and grain size fractions. Using optical stimulated luminescence (OSL) from quartz, we present for the first time a high resolution chronology for the loess section Ostrau in Saxony/Germany. We compare OSL ages derived from two different grain size fractions, coarse (90–200 μm) and the fine grain (4–11 μm) separates. Our results show that the loess section is divided into two parts, separated by a hiatus. OSL samples from the upper part of the loess section show equivalent doses of D_e < 100 Gy. D_e values >180 Gy are observed for the lower part of the loess section. The coarse and fine grain ages agree and also fit to the litho- and pedostratigraphy for the upper part of the profile. For the lower part of the profile the coarse grained quartz OSL is in saturation. The fine grained quartz OSL is not saturated but it appears that the fine grain OSL ages underestimate the sedimentation age. Approaches to explain the D_e differences between the grain size fractions are presented (e.g. post-depositional translocation, dosimetry). A modified SAR protocol for the fine grain fraction produced ages that are in good agreement with expected ages based on litho- and pedostratigraphy. Although further investigations are needed, our results show the suitability of the Saxonian loess belt for OSL dating.     

The petrogenesis of komatiites and related rocks as evidence for a layered upper mantle

Although the CaO/Al₂O₃ ratio of komatiites has been regarded as one of the distinguishing features of these rocks, a comparison of various komatiite and oceanic tholeiite analyses suggests that there is a continuum of ratios between the two. The extremely high MgO values of peridotitic komatiites suggest that they are the result of high degrees of partial melting of the mantle, leaving a harzburgitic residuum depleted in CaO and Al₂O₃, and hence preserving in the melt the original CaO/Al₂O₃ ratio of the parental material. Available chemical models of the mantle have CaO/Al₂O₃ ratios too low to explain the origin of komatiite by such a process. Shallow-level melting of a layered mantle in which clinopyroxene content decreases and garnet content increases with depth, may explain the chemistry of komatiites and related ultrabasic lavas.     

A sedimentological proxy indicator linking changes in loess and deserts in the Quaternary

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)     

Holocene dust accumulation and the formation of polycyclic cinnamon soils (luvisols) in the Chinese Loess Plateau

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.     

Eolian evidence from the Chinese Loess Plateau: the onset of the Late Cenozoic Great Glaciation in the Northern Hemisphere and Qinghai-Xizang Plateau uplift forcing

On the basis of a newly-constructed record of magnetic susceptibility (SUS) and the depositional rate change of eolian loess-red clay sequences in the last 7.2 Ma BP from the hea Plateau, together with a cornperison of a record of °¹⁸O values from the equatorial East Pacific Ocean and eolian Quartz flux variations fmm the North Pacific Ocean, the evolutiomuy process of the Late Cenozoic Great Glaciation in the Northern Hemisphere can be divided into three stages: the arrival stage around 7.2–3.4 Ma BP, the initial stage at about 3.4—2.6 Ma BP, and the Great Ice Age since 2.6 Ma BP. The evolution of the East Asian monsoon is characterized by paid winter and summer monsoons, and it is basically composed of the initial stage of weak winter and summer monsoons, the transitional stage of simultaneous increase in intensity of winter and summer monsoons, and the prevailing stage of strong winter and week summer monsoons, or weak winter and strong summer monsoons. The Late Cenowic global tectonic uplift, paaicdarly the Qinghai-Xizang Plateau uplift and the associated CO₂ concentration variation, controls the dng processes of the onset of Great Glaciation and the long-term changes of East Asian monsoom climate in the Northern Hemisphere to a large extent. The accelerating uplift of the Qinghai-Xizang Plateau between 3.4 and 2.6 Ma BP provided an important driving force to global climiatic change. Project supported by the foundation of Chinese Academy of Sciences (Grant No. KZ951-A1-402), the State Science and Technology Committee (Grant No. 95-pre-40)and the Chinese Nature Science Foundation (Grant No. 49672140)     

The system enstatite-pyrope at high pressures and temperatures and the mineralogy of the earth's mantle

In a diamond-anvil pressure cell coupled with laser heating, the system enstatite (MgSiO₃)-pyrope (3 MgSiO₃ · Al₂O₃) has been studied in the pressure region between about 100 and 300 kbar at about 1000°C using glass starting materials. The high-pressure phase behavior of the intermediate compositions of the system contrasts greatly with that of the two end-members. Differences between MgSiO₃ and 95% MgSiO₃ · 5% Al₂O₃ are especially remarkable. The phase assemblages β-Mg₂SiO₄ + stishovite and γ-Mg₂SiO₄ (spinel) + stishovite displayed by MgSiO₃ were not observed in 95% MgSiO₃ · 5% Al₂O₃, and the garnet phase, which was observed in 95% MgSiO₃ · 5% Al₂O₃ at high pressure, was not detected in MgSiO₃. These results suggest that the high-pressure phase transformations found in pure MgSiO₃ would be inhibited under mantle conditions by the presence even of small amounts of Al₂O₃ (?4% by weight). On the other hand, pyrope displays a wide stability field, finally transforming at 240–250 kbar directly to an ilmenite-type modification of the same stoichiometry. The two-phase region, within which orthopyroxene and garnet solid solutions coexist, is very broad. The structure of the earth's mantle is discussed in terms of the phase transformations to be expected in a simple mixture of 90% MgSiO₃ · 10% Al₂O₃ and Mg₂SiO₄. The seismic discontinuity at a depth of 400 km in the earth's mantle is probably due entirely to the olivine → β-phase transition in Mg₂SiO₄, with the progressive solution of pyroxene in garnet (displayed in 90% MgSiO₃ · 10% Al₂O₃) occurring at shallower depths. The inferred discontinuity at 650 km is due to the combination of the phase changes spinel → perovskite + rocksalt in Mg₂SiO₄ and garnet → ilmenite in 90% MgSiO₃ · 10% Al₂O₃. The 650-km discontinuity is thus characterized by an increase in the primary coordination of silicon from 4 to 6. A further discontinuity in the density and seismic wave velocities at greater depth associated with the ilmenite-perovskite phase transformation in 90% MgSiO₃ · 10% Al₂O₃ is expected.