Leachate Lithium Characteristics of Loess-paleosol Sequences on the Chinese Loess Plateau and their Paleoclimatic Significance

The geochemical components of the leachate from loess-paleosol deposits can provide information about climaterelated post-depositional processes.For example,leachate lithium([Li]_(leachate))is a potential paleoclimate proxy because lithium is a typical lithophile element that is readily adsorbed by clay minerals during weathering and pedogenesis,and thus stratigraphic variations in[Li]leachatecan reflect these processes.We investigated the[Li]leachatevalues of two loess-paleosols profiles(the Luochuan and Weinan sections),on a north-south climatic gradient on the Chinese Loess Plateau.Independent paleoclimate information was provided by measurements of magnetic susceptibility,grain size,Rb/Sr ratios,and clay mineral content.During the last glacial-interglacial period,[Li]leachateincreased from 0.39 to 1.97μg/g at Luochuan and from 0.67 to 2.45μg/g at Weinan,mainly due to increasing pedogenesis.Based on these results we developed a conceptual model to explain the variations in[Li]leachate,Li^(+)within loess layers is mainly derived from dust input and the decomposition of primary minerals,influenced by the East Asian winter monsoon,while in paleosol layers Li is mainly derived from clay mineral adsorption during pedogenic processes,influenced by the East Asian summer monsoon.

Red Ratings for Loess-Paleosol Sequences on China’s Loess Plateau and Their Paleo-Climatic Implications

Comparisons of red ratings (RR) with Fe_d, Fe_d/Fet, clay content, andmagnetic susceptibility (x) of two loess-paleosol sequences at Luochuan and Lingtai on China's LoessPlateau were conducted to study the possible relationship between RR and pedogenic degrees of thetwo loess-paleosol sequences, and to discuss whether the RR could become new paleo-climaticindicators. Results showed that the RR of the two loess-paleosol sequences had positive, highlysignificant (P < 0.01) correlations with: 1) citrate-bicarbonate-dithionite (CBD) extracted iron(Fe_d), 2) ratios of CBD extracted iron to total iron (Fe_d/Fet), 3) clay (< 2 mum), and 4) magneticsusceptibility (x). This suggested that the RR of these loess-paleosol sequences could indicatedegreesof loess weathering and pedogenesis and were potential paleo-climatic proxies. The strongcorrelations of RR to Fe_d and x also implied that during pedogenic processes, pedogenic hematite inloess and paleosols were closely related to the amount of total secondary iron oxides and pedogenicferrimagnetic minerals (predominantly maghemite).

A preliminary study of the quantitative couplingbetween loess-paleosol sequences and summermon soon changes

The Loess Plateau-Northeastern Tibetan Plateau in China is famous for its deep andcontmuous distributed loess, in which soils (developed from loess) vary gradually with differentbiological climates changing from subhumid to arid, which provide a natural proving ground forstUd}' the coupling of loess-paleosol and climatic changes quantitatively. Thirty-two sections weresampled (collected) for measuring chemical element contents, clay content (and other items) and 60sections were collected for carbonate analysis to establish transfer functions between pedogenesisand climate. Finally, we estimated the paleoclAnate (P/T) in the region with transfer functions,It isrevealed that the variation of paleocldriate reflected by transfer functions accorded closely with theclimatic pattern resulted from recent work. Specifically, it was about 140 ha BP that summermonsoon reached Lanzhou or further, paleosol S1 (equaling to MIS 5 in deep sea records) developedstrongly in a large scale, even touching upon northeanstern Tibetan Plateau. And, the estimated P/Tinformation also indicated the cold and humid period during 50-30 ka BP, mean annual Precipitationwas about 400-500 nun in Angutan recorded by mega-interstadial paleosol Sm.

Magnetostratigraphy of a Loess-Paleosol Sequence from Higher Terrace of the Daduhe River in the Eastern Margin of the Tibetan Plateau and Its Geological Significance

The eolian deposits distributed in the river valleys in the eastern margin of the Tibetan Plateau （TP） are very useful in neotectonic and paleoclimatic studies. Firstly, the climate in the eastern margin of the TP is mainly controlled by the Indian summer monsoon, and detailed studies on the loess-paleosol sequences in this region can provide valuable terrestrial evidence of past changes in the Indian summer monsoon. Secondly, the river terraces in the eastern margin of the TP are considered to be a sensitive recorder of neotectonism to reflect the timing and amplitude of the TP uplift.

Variability of Carbonate Pedofeatures in a Loess-Paleosol Sequence and their Use for Paleoreconstructions

Carbonate pedofeatures were studied in details in a loess-paleosol pedocomplex near Kursk, in the central part of the European Plain. The soils studied included a modern Chernozem and five Pleistocene paleosols. Carbonate morphologies and distributions were described in the field. Various morphologies were sampled for further studies including micromorphology in thin sections and submicroscopic studies under a scanning electron microscope. The complex approach of investigation at macro-, micro-, and submicroscopic levels found that most of carbonate pedofeatures are secondary and multiphase. Many of them are related to cryogenic features. The general trend was of decreasing carbonate content in the older paleosols except for increased content in the 1St and 3rd paleosols. Most of carbonate morphologies are similar in the modern soil and paleosols, but pseudomycelium was found only in the modern Chernozem, while soft masses and impregnations are associated with the paleosols. Detailed study on carbonate attributes revealed their specific features in the modern soil and in paleosols. Veins have dissolution features under the modern environment and seem to reprecipitate in form of pseudomycelium. Coatings in paleosols at a depth of the Ist paleosol include specific fine tubes. Powdery soft masses and impregnations in the paleosols are strongly related to cryogenic fissures and paleopermafrost level. Under SEM they consist of tiny crystals, much smaller than in regular soft masses. Several types of hard concretions were identified：rounded, irregular and platy. Small concretions in the modern Chernozem have similar morphology and similar 14C-age as the rounded concretions of the upper paleosols. They are expected to represent the same generation of concretions. Platy concretions fill the bottom part of the large fissures. The 14C-age of hard concretions varies from 3310±80 yrs. B.P in the Ist paleosol up to 20 400 yrs. B.P. in the 3^rd. As a result we found the reflection of several wet-dry and cold-warm stages in carbonate morphologies and distributions. High variability of carbonates at macro-, micro- and submicroscopic levels indicates several generations and multiphase formation of carbonate pedofeatures in the loess-paleosol pedocomplex. Carbonate pedofeatures in the examined pedocomplex are more informative for the study of late Pleistocene and Holocene climatic cycles rather than characterization of a single paleosol.

Mineral Composition of Loess-Paleosol Samplesfrom the Loess Plateau of China and Its Environmental Significance

34 samples of loess-paleosol from the Luochuan and kifeng sections in the Loess Plateau, north-ern China were separated into sand, silt and clay fractions and analyzed for their mineral composi -tions. The results indicate that there is almost no difference between loess and paleosol in mineralcomposition. Major mineral species are quartz, mica, feIdspar and chlorite, accounhng for about88-92% of the total, other minerals are kandite, smectite, vermiculite and a few heavy minerals. Thecalcite, magnetite and hematite were not taken into consideration because of their removal in the pro-cess of sample preparation. The main difference with respect to the mineral composition of samplescollected from different sections and different stratigraphic leveIs lies in the amount and grain sise ofminerals hosted. Comparisons between the Luochuan section and the kifeng section, between paleosoland loess and between the upper part and the lower part of some paleosol layers show that the for-mers contain less feldspar but more mica and vermiculite and are finer in grain sise, indicahng theco-occurrence of both biochemical weathering process responsible for mineral change and physicalweathering process leading to grain-stie change during the soiI-forming processes. This result favourssuch an explanation of the soi1-forming mechanism that loess deposition and paleosol developmentoccurred synchronously, though the rate of soil formation was greater than that of loess deposition,thus leading to soil development.

黄土-古土壤互层对土壤水分运移及土体微结构的影响

黄土地区地质灾害问题的发生大多与水在黄土中的入渗有关,而马兰黄土-古土壤互层结构对土壤水分入渗规律的影响显著。为揭示古土壤阻滞作用下黄土水分运移规律及其对黄土体微结构的影响,为黄土地区工程实践提供理论基础,该研究以陕西省泾阳县南塬的黄土为研究对象,采用土柱模型进行水分入渗试验,研究黄土-古土壤互层条件下土壤水分运移规律。在此基础上,通过微结构测试、分形维数和概率熵等指标的计算,分析黄土-古土壤互层条件下土壤水分运移对黄土微结构的影响。结果表明:古土壤层的透水性弱,湿润锋抵达黄土与古土壤界面处产生瞬态滞水,且随着入渗强度增加滞水时间增加;古土壤层影响下黄土与古土壤界面处的滞水会导致孔隙结构相互连通,孔隙空间平均增加4.13%,孔隙方向概率熵平均减少0.029,分形维数平均减小0.076,即古土壤层的阻水作用使得界面处黄土的孔隙空间增大,孔隙排列有序,孔隙形态规则。研究结果为黄土地区的工程建设和生态环境保护提供科学支撑。

黄土高原古气候变化定量重建的新进展

中国黄土-红黏土沉积是可与深海沉积媲美的陆相沉积载体,记录了晚新生代东亚大陆气候环境变化历史。基于中国黄土的多种理化指标,重建了黄土高原地区构造-千年尺度东亚季风变化历史,为探讨区域与全球气候的联系提供了关键证据。近年来,黄土高原古气候变化研究逐步从定性描述拓展到定量重建,本文旨在回顾基于中国黄土定量重建古温度和古降雨变化取得的重要进展。首先,梳理了古气候要素定量重建的指标和方法,古温度重建指标包括植硅体、碳酸盐耦合同位素、微生物脂类代用指标等;古降水变化敏感指标包括磁化率、白云石/方解石含量、生物微钙体Sr/Ca比值、有机碳同位素以及^(10)Be等。然后,汇总了典型黄土剖面定量重建的古气候变化序列,分别从构造、轨道及千年时间尺度上探讨了古温度和古降雨的变化特征。结果表明,基于生标重建的不同时间跨度的土壤古温度变化序列,在冰期—间冰期尺度上的波动特征基本一致,但在冰盛期—冰消期时段出现了增温超前现象,说明陆地植被对土壤温度变化有重要调制作用。然而,不同指标重建的降水变化幅度差异较大,主导周期也存在差异,说明定量重建降水变化仍有较大挑战。最后,简要总结了黄土高原古气候定量重建存在的问题,明确了区分温度和降水季节性变化的重要性,指出加强地质记录与模拟结果的对比同化,将有助深化对多尺度季风变化动力学的理解。

陕西渭北晚更新世黄土-古土壤结合水膜厚度的试验研究

为深入剖析黄土与古土壤工程性质的影响因素,以陕西渭北晚更新世黄土与古土壤为研究对象,基于核磁共振、比表面积测试并结合理论分析对结合水膜厚度进行研究。试验结果显示:黄土与古土壤的结合水膜厚度存在明显差异,黄土的结合水膜厚度明显大于古土壤,原因在于古土壤孔隙体积及孔径小于黄土,而古土壤中高价阳离子含量大于黄土,同时古土壤颗粒表面分布铁锰质薄膜,其亲水性较弱。分析结合水膜与黄土湿陷性、回弹变形等工程特性间的关系,发现黄土-古土壤的湿陷性随结合水膜厚度的增大而增强,随结合水膜厚度的减小而减弱,结合水膜厚度与卸荷变形系数呈正相关关系。综合分析说明,高价阳离子含量、颗粒表面特性,及孔隙性的不同造成了黄土与古土壤结合水膜厚度的差异,从而导致黄土与古土壤的湿陷性等工程性质有显著区别。

古土壤层间富水对黄土场地湿陷性的影响

黄土地层中往往交替分布着多层红色古土壤层,由于其硬度相对较高,遇水不易湿陷,对黄土地层湿陷量的测量结果有很大的影响。然而,对于古土壤对湿陷量的控制作用目前还缺乏相关研究,导致对湿陷地层湿陷量的科学取舍缺乏理论依据。为此该文统计了黄土高原区的浸水试验结果,分析了不同区域湿陷量室内外差异特征,并以西安地区两个试验场地的大型浸水试验为研究对象,进行了不同试验条件下土体中水分的扩散、含水率的变化、土压力变化以及累计湿陷量等测量工作。最终结果表明,古土壤层的存在阻碍湿陷进程,在阻止水分下渗的同时,阻碍深部地层湿陷量传递至地表,使得湿陷量实测值与计算值之间的差异与古土壤层数呈现正相关的趋势。该文将为普遍存在古土壤的黄土地层自重湿陷机理的研究提供参考。

黄土-古土壤饱和渗透性与孔隙分布特征关系研究

由于沉积环境的差异,古土壤较上覆黄土致密,其饱和渗透系数应低于黄土,但试验结果却显示二者的饱和渗透系数相近。为揭示这一现象的机理,在陕西泾阳南黄土塬开挖33m的探井,沿井壁按1m间距取黄土和古土壤原状试样,进行变水头渗透试验,测定试样的饱和渗透系数。同时用压汞试验(mercury intrusion porosimetry,MIP)及扫描电镜(scanning electron microscope,SEM)测试分别获取试样的孔隙分布曲线和微观结构图像,以分析黄土-古土壤饱和渗透性与孔隙分布特征的关系。结果表明:(1)黄土-古土壤地层的饱和渗透系数整体上沿深度方向规律性减小,但相邻黄土和古土壤层的饱和渗透系数无明显差异;(2)MIP及SEM测试结果表明,黄土结构均匀、孔隙大小较为一致,而古土壤具有不均匀的团块-裂隙结构,虽然团块内部较黄土致密,但团块间存在裂隙;(3)饱和渗透系数的大小取决于透水孔隙的体积分数,其中黄土的透水孔隙主要为较大孔隙(孔径>2μm),而古土壤的透水孔隙主要为团块间的微裂隙,虽然二者渗透系数相近,但渗透机理完全不同。为研究黄土与古土壤的孔隙分布特性和解决黄土区工程建设中的问题提供了理论依据。

不同纬度黄土-古土壤赤铁矿含量差异及其气候指示意义——基于漫反射光谱和色度方法

在全球变暖的背景下,北极和第三极(青藏高原)的增温幅度高于世界上其他地区,其增温幅度和机制并不明晰,因此有必要开展不同时间尺度、不同气候记录载体的研究。利用赤铁矿研究末次间冰期-末次冰期的增温效应,使用漫反射光谱和色度方法量化俄罗斯西伯利亚、中国黄土高原西峰和中国川西高原甘孜3个沉积连续的黄土-古土壤序列中的赤铁矿,揭示其蕴含的气候信息。研究结果显示:(1)漫反射光谱和色度方法相结合是定量赤铁矿的有效方法。(2)在大空间尺度中,赤铁矿主要受控于温度而非降水。(3)在末次间冰期和末次冰期旋回时期,较高纬度气温变化幅度大于较低纬度气温变化幅度。不同纬度的气温“轨道尺度较差”可能是轨道尺度太阳辐射差异和年内太阳辐射差异的综合体现。

Comparison of paleotemperature reconstructions using microbial tetraether thermometers of the Chinese loess-paleosol sequence for the past 350000 years

The recently proposed global and local calibrations for the mean annual air temperature(MAT) reconstruction on the basis of 5-and 6-methyl brGDGTs have rarely been applied to the Chinese LPS yet, leaving the applicability of these calibrations unclear. Here, we used the improved chromatography method to analyze 198 loess-paleosol samples from the Weinan section in the southern CLP for the past 350 kyr. The 6-methyl brGDGTs comprise a major proportion of total brGDGTs, pointing to alkaline conditions for most soil samples from the Weinan LPS. The decoupled profile variation of MBT′6ME and MBT′_(5ME) suggests their response to different enviromental factors, possibly soil pH(or soil moisture) and temperature, respectively. This discrimination further corraborates that temperature and monsoonal precipitation were not in phase during the last four deglaciations on the CLP. Temperature estimates for the six calibrations tested show similar trends but remarkably differ in amplitudes. The soil moisture appears to affect the global MBT′/CBT calibration, the global MATmr calibration(a calibration based on the multiple linear regression) and the Chinese local SSM(Stepwise Selection Method) calibration, resulting in a significant underestimation of late Holocene temperature. In contrast, the dry climate has no effect on the global MBT′5ME calibration. Of the six calibrations,only the Chinese local SSM calibration and global MATmr calibration produce temperature variation amplitude over the past 350 ka that is consistent with other independent proxy data. The recently proposed local calibrations based on 5-and 6-methyl brGDGTs for North China yield the amplitude of temperature changes that is much larger than the results determined by other approaches.All the six calibrations have their own weakness in the MAT reconstruction, due partly to the inconsistency between the brGDGT distribution in the Weinan LPS and the modern soils used to establish these calibrations.

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 (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.

Paleo-environmental study on the growth of magnetotactic bacteria and precipitation of magnetosomes in Chinese loess-paleosol sequences

405 samples were collected from L5-S5-L6 in consideration of obvious variations in susceptibility of the geological sections, which are section Xifeng in Gansu Province and section Duanjiapo in Shaanxi Province for study of magnetotactic bacteria (MB) and magnetosomes (MS) in Chinese loess-paleosol sequences. MB in each sample were observed by TEM after being cultured under 8-18℃, room temperature (RT), 25℃, 26℃ and 30℃ conditions. In general, MB are distributed widely in loess-paleosol sequences, fewer in loess layers with predomination of vibriod in shape. However, there are more MB in paleosol layers with morphological varieties such as roddish, vibriod and occasionally approximately coccus. The magnetosomes (MS) in MB of paleosol are usually arranged in chains along the cells. It was also found that MB growth and MS formation are associated with the environment in which MB live. It can be inferred from the distributions of MB and MS that the paleoclimates fluctuated during the formation of

Transformation relationship among different magnetic minerals within loess-paleosol sediments of the Chinese Loess Plateau

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 evo- lution 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.

Modeling and simulation of dynamic carbonate deposition in a loess-paleosol sequence

A compartment model of dynamic deposition processes has been developed and testedin the Potou loess section of China. There are four components in the model : carbonate input ofeolian dust, carbonate output of erosion, carbonate chemical equilibrium and soil water balance. Aclimate stochastic model software (CLIMAK) generates the daily weather data. The processes ofeolian dust deposition, erosion, and pedogenesis simulate the complex process of carbonatedeposition in a loess-paleosol sequence. We consider both climatic change and soil-forming factors.The results indicate that since the Late Pleistocene in Potou section, the mean carbonate fluxesfrom precipitation, eolian dust and erosion are 0.14g/cm^2/kg, 1.45g/cm^2/ka and 0.23g/cm^2/karespectively. Of this, 91.2% of the carbonate input is from eolian dust.

Geochemical characterization of loess-paleosol sequences:Comparison between the upper reaches of the Hanjiang and Weihe river valleys,China

This paper aims to compare the geochemical characteristics of loess-paleosol sequences in the upper reaches of the Hanjiang and Weihe river valleyswhich are located in the semi-humid temperate zone and humid subtropical zonerespectively. The Mituosi（MTS） profile in the upper reaches of the Hanjiang River valley and the Yaohecun（YHC） profile in the Weihe River valley were selected for this comparative research. The stratigraphic characteristicscompositionchemical weathering intensityleaching rates of Ca and Namobility of major elementsand transport features of Na and Fe were analyzed with respect to depth and compared between the two profiles. This study reached the following conclusions.（1） The composition of the loess-paleosol sequences in two regions are quite similar to the average composition of the upper continental crust（UCC）indicating that the loess in the two regions came from multiple sources and was mixed well. Thereforethe loess in the two regions is considered aeolian loess.（2） Compared with the loess-paleosol sequence in the Weihe River valleythe loess-paleosol sequence in the upper reaches of the Hanjiang River valley features a darker color; a higher chemical index of alteration（CIA） value; higher leaching rates of Na and Ca; higher migration ratio（relative to K） of AlSiMgand Na; and lower migration ratio of Fe and Ca. This evidence indicates that the loess-paleosol sequence in the humid subtropical environment experienced stronger chemical weathering intensity than the loess-paleosol sequence in the semi-humid temperate zone.（3） Both the YHC profile and MTS profile record a period of climate deterioration at 6000–5000 a BP. The period punctuated the mid-Holocene Climatic Optimum（8500–3100 a BP） in the study area.

淮河支流汝河全新世古洪水沉积序列粒度指示的地表过程

【目的和方法】为了深入理解淮河流域全新世环境与地表过程演变,通过对淮河流域详细的野外调查,在汝河上游诸市乡段一级河流阶地前沿发现了夹有多层古洪水滞流沉积层的全新世黄土土壤沉积物地层剖面。对该剖面进行详细观察测量,地层划分和分层采样,在实验室进行多种指标分析研究。在前期沉积学性质研究和年代测定的基础上,应用端元分析方法进行粒度成分解析,深入分析古洪水沉积物序列的物源及其沉积学特征变化规律。【结果】采用非参数法从沉积序列中解析出具有不同物源和沉积动力学意义的4个端元组分。各土壤沉积物地层单元中除了现代洪水沉积层和河漫滩相细沙层外,其余层次都显示出多端元组分混合叠加的特点。【结论】结合野外宏观特征及其理化性质综合分析,可知其端元1组分代表西北季风沙尘暴远源风成沉积物及其风化成壤改造的产物,端元2组分代表了东北风吹扬黄泛平原松散沉积物沙尘暴的近源沉积物,端元3组分代表了河流洪水悬移质泥沙在高水位滞流环境条件下的沉积物,端元4组分为河流在河槽之内水动力作用下河漫滩冲积物。表明通过对全样粒度进行端元解析,能够更清晰地反映出汝河流域多元化的沉积动力特点。

淮河流域颍河上游河谷沉积物物源分析

通过详细的野外考察,在淮河流域颍河上游的河谷地段见到具有典型代表性的完整沉积剖面,进一步对其进行野外宏观分层描述并每隔2 cm进行高分辨率采样,带回实验室进行分析。采用粒度端元分析手段,分析了地层剖面的物源问题。分析结果表明:本区黄土物源和黄土高原典型黄土有所不同,区域东北风作用下近源的风沙流沉积物占了相当份额;依据粒度端元分析,沉积动力组分主要由3部分组成,分别代表了风化成壤改造作用下的产物、东亚冬季风作用下从西北荒漠搬运而来的粉尘物质以及东北风作用下从近源搬运而来的黄泛沉积物;在不同的时期,沉积作用动力强弱有明显的变化,表明了全新世以来区域季风气候对全球气候变化的响应过程。研究对淮河流域颍河上游局部河谷沉积物物源组成特点进行了量化分析,这对于深入理解淮河上游沉积物物源及全新世以来其季风作用下的地表过程演变特征具有重要的科学意义。