The Geochemical Features and Evolution of Laterite in the Karst Areas of Guizhou Province

Guizhou is one of the dominating karst regions,where laterite is widespread,in China.Seventy-two laterite samples were taken from twelve laterite sections in the karst areas of Guizhou Province,which are possessed of typical subtropical karst geomphological and ecological environmental features and have evolved completely from dolostones and limestones.In terms of the major,trace and REE deta for thd samples this paper discusses the geochemical characteristics of laterite in the karst areas with an attempt to disclose the geochemical process and evolutionary rule of laterite formation.There have been involved three important pedogenetic geochemical precesses in the formation of laterite in the karst areas:(1)enrichment of silicon and aluminum and depletion of calcium and magnesium;(2)enrichment of iron and manganese;and(3) enrichment of aluminum and depletion of silicon.During the formation and evolution of laterite,obvious enrichment and differentiation of trace elements such as Cu,Pb,Zn,Ni,Co,Cr,Pb,F,Cl,and As can be observed,but for the rare-earth elements,their enrichment is remarkable against a weak differentiation.The REE distribution patterns in the laterite are similar to those of its parental carbonate rocks.     

Snowball morphology and SEM analysis of pedogenic gypsum, southern New Mexico, U.S.A.

Late Quaternary soils in southern New Mexico support previous work that suggested pedogenic gypsum accumulates in successive stages as a function of time, similar to pedogenic carbonate. However, gypsum also occurs as small (0·5–1 mm), powdery spheres that we term snowball morphology. The snowball morphology represents one of the initial stages of pedogenic gypsum development. Scanning electron microscope analyses indicate that soil microorganisms and organic material may play a crucial role in the development of this morphology. However, no trends were found in the crystal habits of gypsum suggesting the snowball morphology forms in a dynamic environment.     

Influence of Quaternary paleoclimate change on the permeability of the loess–paleosol sequence in the Loess Plateau,northern China

Permeability differences in multi-cycle loess–paleosol aeolian sediments, which are still poorly understood, have the potential to significantly improve our understanding of climatic change during the glacial–interglacial periods of the Quaternary. In this study, the permeability of a well-preserved and continuous loess–paleosol sequence in the South Jingyang Plateau was investigated. Weathering intensity was inferred using a series of climate proxies including grain-size distribution, magnetic susceptibility and mineralogy. The results of laboratory tests showed that the average saturated hydraulic conductivity of loess layers is higher than that of paleosol layers. Also, clear differences between loess and paleosol were found in terms of depth variations of the vertical and horizontal saturated hydraulic conductivities. Differences in loess–paleosol were also found for other proxies for pedogenic weathering [i.e. clay content, sand content, Kd value (ratio of coarse silt to clay), magnetic susceptibility, dolomite content and the ratios of hornblende/illite and hornblende/chlorite]. Our results showed a high permeability of loess layers associated with weak pedogenic weathering during cold/dry paleoclimatic conditions in glacial stages. On the contrary, paleosol layers developed in a warm/humid climate during the interglacial stages experienced strong pedogenic weathering that resulted in lower permeability. Based on these results, we construct a connection between Quaternary climate change theory and the modern hydrological system. This provides a scientific basis for investigating the distribution and pollution of groundwater resources in the local region. © 2020 John Wiley & Sons, Ltd.     

汉江上游谷地全新世风成黄土及其成壤改造特征

对汉江上游郧县-郧西段一级阶地物质上堆积的黄土状覆盖层的剖面构型、粒度、矿物组合、元素分布、微形态及形成时代进行了研究,并与渭河谷地全新世黄土-古土壤序列对比。结果表明,这些“黄土状覆盖层”是晚更新世以来沙尘暴在区内连续堆积的结果,具有TS-L₀-S₀-L_t-L₁-AD剖面构型;这些风积物在不同时期受到了不同程度的成壤改造作用,晚更新世受到弱成壤改造→全新世中期受到强烈成壤改造→全新世晚期较弱成壤改造。汉江上游地区在晚更新世末期(11500 a BP),气候比较冷湿,沙尘暴频繁出现,形成较厚的黄土层L₁;全新世初期(11500-8500 a BP),气候从冷湿向暖湿方向转变,沙尘暴活动减弱,形成了过渡黄土层位L_t;全新世中期(8500-3000 a BP),气候达到全新世最为暖湿的阶段,沙尘暴沉积物受到强烈成壤改造而形成了古土壤S₀;全新世晚期(3000 a BP以来),气候暖湿程度降低,越过秦岭的沙尘暴又明显增强,形成了近代黄土L₀。     

Fine‐grained meandering systems of the Lower Permian Clear Fork Formation of north‐central Texas,USA: Lateral and oblique accretion on an arid plain

Facies models that adequately represent the diverse range of fine‐grained fluvial systems are currently lacking from the literature. In this paper, the spectrum of these systems on the arid plains of western equatorial Pangea is explored, as well as the source and nature of the fine‐grained sediments. Eight fluvial elements in the Early Permian Clear Fork Formation of north‐central Texas represent channel systems up to 7 m deep with coarse basal deposits, three types of lateral‐accretion deposits and sandstone sheets, with laminated, disrupted and massive mudstones laid down in abandoned channels and on floodplains. The three fine‐grained fluvial styles represent a continuum between two end‐members: sustained lateral accretion of bedload composed of quartzose sediments and mud aggregates on point bars, and oblique accretion of suspended sediment on steep accretionary benches and banks with limited lateral migration. This spectrum is controlled, in part, by grain size and the proportion of suspended to bedload sediments. The presence of rarely documented swept ripples on exhumed accretion surfaces is attributed to rapid decline in water levels and downstream re‐entry of overbank floodwaters into the channel. Rill casts, roots and disrupted mudstones low down in channel bodies indicate periods of near‐dryness. Laterally extensive sheet sandstones were formed by episodic flows in broad, sandbed channels. The fluvial sediments were primarily intrabasinally sourced with extrabasinal sediments brought in during major floods from upland source areas or reworked from local storage in the basin, representing a supply limited system. The upward change in cement composition from mainly calcite and ankerite to dolomite and gypsum with minor celestine implies increasingly saline groundwater and progressive aridification, supporting Late Palaeozoic palaeoclimatic models. By integrating petrographic data with sedimentology, a plethora of information about ancient landscapes and climate is provided, allowing a fuller comparison between the Clear Fork Formation and modern dryland alluvial plains.     

Facies and palaeosol analysis in a progradational distributive fluvial system from the Campanian–Maastrichtian Bauru Group,Brazil

The Upper Cretaceous Bauru Group in south‐east Brazil consists of alluvial strata whose characteristics and distribution indicate a fluvial system developed in a semi‐arid to arid climate. Sections exposed within a 90 000 km² study area in Minas Gerais State (in south‐eastern Brazil) were evaluated using facies and palaeosol analysis to formulate depositional and pedogenic models that may account for geomorphic and climate features. From east to west, the study succession records a gradual decrease in grain size, an increase in the width/thickness ratio in channels, a decrease in the lateral and vertical connectivity of channel deposits, and an increase in overbank deposits. The fluvial architecture indicates a braided channel belt, ephemeral ribbon–channels, and an unconfined fluvial facies from east to west in the study area. The lateral and vertical distribution of facies, stratigraphic architecture and palaeocurrent data suggest proximal, medial and distal portions of a progradational distributive fluvial system. The sedimentary dynamics were marked by the building and abandonment of channels related to processes of aggradation, vegetation growth and palaeosol generation. Macromorphological and micromorphological analyses have identified pedological and mineralogical features that indicate an arid to semi‐arid climate with a provenance from the north‐eastern part of the basin (Alto Paranaiba Uplift). From the proximal to the distal portions of the distributive fluvial system, the palaeosol development is different. In the proximal portion, the palaeosols are absent or poorly developed, allowing a possible general comparison with the present soil order: Inceptisols and Aridisols. In the medial portion of the fluvial system, the palaeosols are well‐developed and characterized by Bt, Btk, C and Ck horizons (Alfisols, Aridisols, Inceptisols and Entisols). Poorly drained to well‐drained palaeosols from the base to the top in the distal plain (Aridisols and Inceptisols) are associated with geomorphic and hydromorphic changes in the fluvial system due to progradational evolution. The genetic relationship between the fluvial architecture and the palaeosols enhances the understanding that the sedimentation and pedogenesis that occurs in different portions of the distributive fluvial system are related to the tectonic and climatic evolution of the basin.     

试论黄土高原的生态环境与“土壤水库”──重塑黄土地的理论依据

值今西部开发如火如荼之际, 黄土高原生态环境的重建又在全国生态环境建设总体 布局中被列为最严峻的问题来对待。可见谋求一个为各家共识的治黄对策已刻不容缓。笔者 愿就５０年来的研究所得, 对黄土高原的形成及其演变规律和“土壤水库”确是其中关键等问题 的机理作一比较系统的介绍, 以广求共识和加强各家协作共事, 从而使“顺天时, 得地利, 促人 和”的“黄土高原国土整治２８字方略”能够顺利有序地实施。一个山川秀美、举世无双的黄土 高原可望在下个世纪初重现于中国大陆。     

湖泊沉积物C4植物含量重建方法的讨论——以新生代柴达木盆地大红沟剖面为例

当前气候变暖是一个全球面临的重大问题,它对人类赖以生存的植被生态系统造成的影响已经在全球各地逐步显现出来。为了深入了解植物生态系统对环境和气候变化的响应机制,我们需要更好地借鉴地质历史时期气候环境和植物协同演化的重要事件。C₄植物作为陆地生产力较强的植物,在植物生态演化中占举足轻重的地位。目前研究表明C₄植物可能最晚起源于始新世-渐新世之交,但从它早期起源到随后在生态系统中的大规模扩张时间间隔长达20多个百万年。是什么因素导致了C₄植物的起源和扩张是一个悬而未决的重要问题,需要开展大量的调查研究来评估和重建C₄植物在过去生态系统中的相对生物量变化。重建C₄植物的含量目前主要的方法是建立在C₃/C₄植物碳同位素和植物内部结构形态差异基础之上。最常运用的研究材料包括(古)土壤有机质、成壤碳酸盐、陆地食草动物体组织、沉积物生物标志物、孢粉、植硅体等。这些方法在重建现代以及地质历史时期C₄植物相对生物量变化的研究中发挥了重要作用,但同时也存在很多无法避免的问题。本文介绍了C₄植物起源和扩张机制的主流观点以及不同研究材料的碳同位素所推算C₄植物生物量的基本原理,并以柴达木盆地大红沟剖面为例,针对新生代湖泊沉积物中陆生高等植物的长链正构烷烃特征和单体烃碳同位素的研究结果,详细讨论C₄植物含量重建的方法与缺点,为探讨C₄植物起源、演化及控制因素提供参考。我们通过分析前人研究的大红沟剖面长链正构烷烃及单体烃 δ¹³C_alk 值特征,推测在30~24 Ma、20~17 Ma和13~7 Ma期间δ¹³C_alk值显示相对正偏的原因,可能是干旱和C₄植物在当地生态系统中出现的双重因素叠加造成的。但这一推断还需要借助于新的研究方法,即单颗粒孢粉碳同位素的方法来提供C₄植物的确凿证据。     

Role of latitudinal shift and climate change in evolution of red and yellow palaeosols of the Himalayas: Implications for early Oligocene seasonality and Mid-Miocene enhanced precipitation

Cenozoic atmospheric circulation, climatic changes, sedimentation and weathering over the Indian sub-continent were mainly influenced by the northward drift of the Indian Plate, the shrinking Paratethys, India-Asia collision and the rise of the Himalayas. This study is aimed at exploring the fluvial sedimentary record of the north-west part of the Himalayan Foreland Basin to interpret weathering and pedogenesis during early Oligocene to Mid-Miocene time. Palaeopedological investigation of a 3.1 km thick succession from Kangra sub-basin of the Himalayan Foreland Basin shows that the lower 2 km part of the succession is characterized by the red (10R hue) and the upper 1.1 km part of the succession by the yellow (2.5Y hue) palaeosols with varying intensity of weathering and pedogenesis. The association of sedimentary rocks and pedogenic expression in palaeosols indicate four (Type-A to Type-D) pedofacies in the entire Oligocene–Miocene succession. The pedofacies are defined by a decrease in the intensity of palaeopedogenic development from strongly-developed palaeopedofeatures in Type-A, moderately-developed palaeopedofeatures in Type-B, weakly-developed palaeopedofeatures in Type-C and to the only incipient stage of palaeopedogenesis in Type-D pedofacies. The palaeolatitudinal shift during the convergence of the Indian Plate played a major role in weathering and palaeopedogenesis with the inception of seasonality during the early Oligocene, which is demonstrated by the formation of the red palaeosols with pedogenic CaCO₃ and vertic features in tropical conditions. The transition to yellow palaeosols at about 20 Ma is marked by increased humidity, rapid aggradation, pronounced uplift and enhanced erosion of the hinterland. These yellow palaeosols are characterized by the abundance of weakly-developed Bw and Bss horizons, pure clay pedofeatures and absence of any pedogenic CaCO₃ during short pedogenic intervals in subtropical conditions.     

Dissolved organic carbon in a eutrophic lake; dynamics,biodegradability and origin

The seasonal and spatial changes in dissolved organic carbon (DOC) in Lake Kasumigaura, a shallow, eutrophic lake, were analyzed and the lability of DOC was tested by long-term incubations. There was a nearly 1 mgCl^–1 downstream increase in refractory DOC in the lake; at the center it fluctuated little seasonally. The characteristic UV-absorbance: DOC ratios were determined for samples from the influent rivers (pedogenic: used interchangeably with allochthonous) and outdoor experimental ponds (autochthonous) during incubations. These ratios were then used to calculate the proportion of total measured lake water DOC in each of four components: pedogenic-refractory (PR), pedogenic-labile (PL), autochthonous-refractory (AR) and autochthonous-labile (AL). PR was uniform (around 1.5 mgCl^–1) or diminished very slightly over time. AR increased from nearly zero at the station closest to an influent river to 1 mgCl^–1 at the lake center. PL declined downstream from 0.3 mgCl^–1 to zero. AL was virtually constant at 0.8 mgCl^–1 except at the station closest to the influent river. The constancy of the UV-absorbance: DOC ratio during the biodegradation process was confirmed for Lake Kasumigaura; hence a two-component model (pedogenic-autochthonous) could be applied here without consideration of DOC lability. However, this assumption is not always met for other water bodies, and therefore it should be checked before applying a two-component model elsewhere.