Heavy mineral weathering under acidic soil conditions

The weathering of heavy minerals in acid soil profiles (pH 4–5) has been studied in terms of their relative mineral proportions and by using the surface etching features of apatite and hornblende grains. An increasing stability order of: apatite, titanite, hornblende, garnet, epidote, zircon is found in the 45–63 μm coarse-silt fraction of soils developed on tills in southwestern Sweden. In glaciofluvial deposits, sorting processes during deposition have largely determined the variations in heavy mineral content of soils, and only the dissolution of apatite is significant. Hornblende etching is more extensive in soils with low hornblende content, indicating that the release of cations relative to the amount of hornblende is greater in such soils.     

Electrochemical characterization of pyrrhotite reactivity under simulated weathering conditions

A previously reported methodology applied to evaluate the factors affecting the reactivity pyrite and pyritic samples under simulated weathering conditions is now utilized to analyze pyrrhotite reactivity at different environmental alteration stages. The methodology is based on the comparison of the voltammetric responses of the mineral obtained through the alteration process; changes in the electrochemical behavior are associated with changes in the surface state of the mineral and finally are related with changes of the mineral reactivity. The results of an initial study of the electrochemical behavior of pyrrothite before alteration suggest that its alteration involves the formation of 3 surface layers (in agreement with previous reports): (1) in immediate contact with pyrrhotite corresponding to a metal-deficient sulfide; (2) an intermediate layer corresponding to elemental S, and; (3) the most external layer, consisting of precipitates of Fe oxy-hydroxides, like goethite. The pyrrhotite reactivity seems to be controlled by the formation of oxidation product layers that coat and passivate the pyrrhotite surface, where the elemental S layer has most significance. The results confirm the advantage of incorporating cyclic voltammetry as an auxiliary method for acid rock drainage prediction, due to its demonstrated capacity to describe the factors that influence sulfide mineral reactivity which are not evaluated by other predictive techniques.     

Basalt weathering and trace elements migration in the boreal Arctic zone

Chemical weathering of basalts in the Putorana Plateau, Central Siberia, has been studied by combining chemical and mineralogical analysis of solids (rocks, soils, river sediments and suspended matter) and fluid solution chemistry in order to quantify CO₂ consumption and to assess the major factors controlling basic rock weathering under permafrost-dominated taiga climate. The chemical status of  40 major and trace elements (TE) in pristine boreal rivers and interstitial solutions of permafrost soils has been investigated using in-field ultrafiltration procedure. This revealed strong relationships between concentration of TE and that of major inorganic components of colloids, i.e., Fe or Al. Decomposing plant litter and permafrost thawing are considered important sources of most major and trace elements in Arctic rivers during summertime.     

Mineralogy and geochemical behaviour during weathering of greenstone belt under tropical dry conditions in the extreme North Cameroon (Central Africa)

Mineralogy, major, trace and rare earth elements of a weathering profile developed on tertiary greenstone belt in the extreme North Cameroon are reported. The aim of which was to investigate mineralogical evolution and element mobilization and redistribution during weathering under dry tropical climate. The weathering profile consists of four main horizons: (1) a spheroidal weathering zone constituted by a corestone–shell complex, (2) a C horizon, (3) a Bw horizon and an Ah horizon. The results indicate that nontronite, a Fe-rich smectite, is the exclusive clay mineral formed in the exfoliated shells and the C horizon. It is associated with kaolinite in the upper horizons. The coexistence of these two clay minerals induced a decrease of CEC and pH which becomes neutral. The weathering index (WI) values reveal that weathering becomes more and more intensive from the corestone up to Bw horizon, which is the most weathered horizon in the weathering profile. Mass balance calculations, using Th as immobile element, indicate that Ti is quite mobile and that Al and Fe are relatively enriched at the bottom and strongly leached at the top of the profile. Alkalis and alkaline earth elements are strong leached through out the profile, except Ca which displays similar trend as Al and Fe. The same goes for LILE (Cs, Sr), TTE (Cr, Co, Ni) and HSFE (Y, Nb, Hf). In opposite, REE are depleted at the bottom and enriched in the upper horizons, with more enrichment for LREE than for HREE. It appears that weathering of greenstone belt causes a fractionation of HREE and induces a concentration of LREEs. Ce and Eu anomalies display opposite behaviour.     

Forecasting and prospecting criteria of Au-bearing weathering mantles in southern Siberia,Russia

The geological and genetic model of gold concentration at different stages of weathering has been developed on the basis of long-standing geological and geochemical research of Au-bearing weathering mantles in the main ore fields of Siberia. This model makes it possible to substantiate forecasting and prospecting criteria of gold deposits hosted in linear karst zones of weathering, to delineate prospective areas in southern Siberia, and to estimate their resource potential.     

The behaviour of consolidated volcanic tuffs: weathering mechanisms under simulated laboratory conditions

Five volcanic tuffs ranging from dacitic tuffs of Hungary to rhyolite, phonolite and basaltic tuffs of Germany were consolidated under laboratory conditions. Prior to consolidation an anti-hygro, a hydrous consolidant, which reduces the swelling ability of clay minerals, was applied. The three consolidants, a silicic acid ester (SAE), an elastic silicic acid ester (eSAE) and an acrylate resin (PMMA) were applied on test specimens under vacuum. Petrographic characterisation (polarizing microscopy, XRD, SEM) provided data for fabric analyses and the mineral composition of the tuffs. Changes in fabric, effective porosity, density, tensile strength, ultrasonic wave velocity were evaluated after the treatment. Weathering simulation tests such as hygric dilatation and thermal dilatation aimed to prove the effectiveness of consolidation and the durability of consolidated tuff samples. More than 500 samples were analysed. The tests showed that SAE caused the highest increase in indirect tensile strength. The water absorption and the pore size distribution of the tuffs were modified by consolidation. The PMMA reduced the water absorption the most, whereas SAE modified it the least. All the tested consolidants increased the thermal dilatation of the tuffs. The changes in hygric dilatation were not uniform: for most tuffs SAE increased and PMMA decreased the hygric dilatation, although the clay-rich Habichtswald tuff showed the opposite trend. The changes in hygric and thermal behaviour of consolidated tuff require special care when specific consolidants are chosen. These products modify the physical properties of consolidated tuffs and change the behaviour of weathering.     

Resistance of quartz silt to isotopic exchange under burial and intense weathering conditions

Oxygen isotopic compositions of quartz from silty sediments buried to 5400 m depth from two wells in the Gulf of Mexico each showed δ5¹⁸O variations of less than 1.7%. Clay diagenesis has been reported within this depth. The observed variations in the quartz do not appear to be primarly diagenetic effects, but rather are mainly depositional features reflecting variations in the sources of the sediments. Sources may be influenced by the variation of distance from the shore at different depths in a given sampling location and by sediment production by continental glaciations. Stability of the oxygen isotopic composition of quartz in the 10–20 μm size range under long-time humid, temperate weathering conditions was studied by analysis of saprolites formed from Pennsylvanian to Precambrian crystalline rocks. In four of the five cases, the 10–20 μm fraction was found to have only 0.1–0.5%. greater δ¹⁸O than the corresponding 20–50 μm fraction. This increase may be attributable either to a slight oxygen isotopic exchange with ambient ground waters or original differences within the rock since the saprolites were sufficiently coherent to make an influx of extraneous detrital silt unlikely. The amount of oxygen isotopic exchange in silt size quartz over periods of many million years of shallow burial or weathering appears to be small enough to permit the use of the oxygen isotopic ratio of quartz in tracing the origin of eolian and fluvial additions of minerals to continental soils and pelagic sediments.     

Coastal permafrost landscape development since the Late Pleistocene in the western Laptev Sea,Siberia

Winterfeld, M., Schirrmeister, L., Grigoriev, M. N., Kunitsky, V. V., Andreev, A., Murray, A. & Overduin, P. P. 2011: Coastal permafrost landscape development since the Late Pleistocene in the western Laptev Sea, Siberia. Boreas, 10.1111/j.1502‐3885.2011.00203.x. ISSN 0300‐9483. The palaeoenvironmental development of the western Laptev Sea is understood primarily from investigations of exposed cliffs and surface sediment cores from the shelf. In 2005, a core transect was drilled between the Taymyr Peninsula and the Lena Delta, an area that was part of the westernmost region of the non‐glaciated Beringian landmass during the late Quaternary. The transect of five cores, one terrestrial and four marine, taken near Cape Mamontov Klyk reached 12 km offshore and 77 m below sea level. A multiproxy approach combined cryolithological, sedimentological, geochronological (¹⁴C‐AMS, OSL on quartz, IR‐OSL on feldspars) and palaeoecological (pollen, diatoms) methods. Our interpretation of the proxies focuses on landscape history and the transition of terrestrial into subsea permafrost. Marine interglacial deposits overlain by relict terrestrial permafrost within the same offshore core were encountered in the western Laptev Sea. Moreover, the marine interglacial deposits lay unexpectedly deep at 64 m below modern sea level 12 km from the current coastline, while no marine deposits were encountered onshore. This implies that the position of the Eemian coastline presumably was similar to today's. The landscape reconstruction suggests Eemian coastal lagoons and thermokarst lakes, followed by Early to Middle Weichselian fluvially dominated terrestrial deposition. During the Late Weichselian, this fluvial landscape was transformed into a poorly drained accumulation plain, characterized by widespread and broad ice‐wedge polygons. Finally, the shelf plain was flooded by the sea during the Holocene, resulting in the inundation and degradation of terrestrial permafrost and its transformation into subsea permafrost.     

不同地表条件下青藏公路对多年冻土的热影响差异研究北大核心CSCD

在多年冻土区,道路工程会对周边的多年冻土产生热影响,但不同地表条件下的多年冻土对道路热影响的反馈差异尚不完全清楚。本研究基于青藏公路沿线两处监测场地的多年冻土监测数据,研究了不同地表条件下青藏公路对多年冻土的热影响差异。结果表明,青藏公路对多年冻土的热影响因地表条件的不同而存在差异。与植被覆盖率较高的监测场地相比,在植被覆盖率较低的监测场地,其多年冻土年平均地温更高、多年冻土活动层厚度更大,且青藏公路对多年冻土的水平热影响范围也相对更大。此外,在植被覆盖率较低的监测场地最靠近坡脚的位置处,由于地表条件的不同,其浅层土壤更易受到外界扰动,导致该位置浅层土壤与外界的热交换特征迥异于其他监测位置,这可能也是导致两处监测场地多年冻土的热状态存在差异的原因。目前,青藏工程走廊内各线性工程密布,工程间的相互影响及其与多年冻土间的关系已成为必须考虑的问题。本研究工作对于青藏高原多年冻土区工程走廊内线性工程之间的合理间距设定,以及即将建设的青藏高速公路双向路基间的合理距离设计都可提供参考,以达到减少工程热扰动,保障工程安全运营的目的。     

Late Quaternary ice-rich permafrost sequences as a paleoenvironmental archive for the Laptev Sea Region in northern Siberia

Ice-rich permafrost sequences with large polygonal ice wedges represent excellent archives for paleoenvironmental reconstruction. Such deposits contain numerous well-preserved records (ground ice, paleosols, peat beds, different types of fossils), which permit characterization of environmental conditions during a clearly defined period of the past 60 ka. Based on field investigations carried out within framework of the German-Russian project "Laptev Sea System 2000" on the Bykovsky Peninsula (SE of the Lena Delta) results from cryolithological studies, sedimentological analyses, as well as new radiocarbon data are presented. For the first time it is shown that the Ice Complex accumulated without significant interruptions from approximately 60 k.y. B.P. until the end of the Pleistocene. Geochemical data (total organic carbon, C/N, '¹³C_org) and the mass-specific magnetic susceptibility clearly show changing environmental conditions from stadial to interstadial times in the Late Pleistocene and during the transition to the Holocene. These results permit us to reconstruct the development of an Arctic periglacial landscape in the coastal lowland during Ice Complex formation in the Late Quaternary. This evolution coincided generally with the global environmental trend up to marine isotope stage 4.     

Time scale and conditions of weathering under tropical climate: Study of the Amazon basin with U-series

The Rio Solimões/Amazonas (Amazon River) and its major tributaries have been analyzed for U-series nuclides. ²³⁸U-²³⁴U-²³⁰Th-²²⁶Ra disequilibria have been measured in the dissolved (<0.2 μm) and suspended loads (>0.2 μm) as well as bed sands. U-series disequilibria are closely related to major and trace element compositions and therefore reflect elemental fractionation during chemical weathering. Moreover, while the dissolved load records present-day weathering, suspended particles integrate the erosion history over much longer time scales (>100 ka). Lowland rivers are characterized by long time scales of chemical erosion (?100 ka) resulting in a high weathering intensity. Moreover, exchange between suspended particles and the dissolved load may explain the U-series signature for these rivers. By combining U-series and Pb isotopes in suspended particles, we show that erosion in the Rio Madeira basin occurred as a multi-step process, whereby the pristine continental crust was eroded several hundreds of Ma ago to produce sediments that have then been integrated in the Cordillera by crustal shortening and are currently eroded. In contrast, recent erosion of a pristine crust is more likely for the Rio Solimões/Amazonas (<10 ka). The suspended particles of the rivers draining the Andes (Solimões/Amazonas, Madeira) suggest time scales of weathering ranging between 4 and 20 ka. This indicates that suspended particles transported by those rivers are not stored for long periods in the Andean foreland basin and the tropical plain. The sediments delivered to the ocean have resided only a few ka in the Amazon basin (6.3 ± 1 ka for the Rio Amazonas at Óbidos). Nevertheless, a large fraction of the sediments coming out from the Andes are trapped in the foreland basin and may never reach the ocean. Erosion in the Andes is not operating in steady state. U-series systematics shows unambiguously that rivers are exporting a lot more sediments than predicted by steady-state erosion and that is a consequence of soil destruction greater than production. By relating this observation to the short time scales of weathering inferred for the Andes (a few ka), it appears that the erosion regime has been recently perturbed, resulting in high denudation rates. A possible explanation would be the increase in precipitation less than 5 ka proposed by recent paleoclimatic studies. Our results indicate that erosion responds rapidly to high-frequency climatic fluctuations.     

Flood Basalt from Mont Tourmente in the Central Kerguelen Archipelago: the Change from Transitional to Alkalic Basalt at ~25 Ma

The surface of the Cenozoic Kerguelen Archipelago, constructedon the Kerguelen Plateau in the southern Indian Ocean, is dominantlyflood basalt. With the objective of understanding the Cenozoichistory of the Kerguelen mantle plume, the age and geochemicalcharacteristics of this flood basalt province are being determinedby studying stratigraphic sections of basalt flows at severallocations. Sections from the NW, north–central, east andSE parts of the archipelago have been studied. Here we reportresults for a 597 m succession of lavas from Mont Tourmentefrom the Plateau Central, a region of the archipelago that hasnot been studied in detail. Mont Tourmente lavas range from     

Fluvial and permafrost history of the lower Lena River,north-eastern Siberia,over late Quaternary time

Arctic warming and permafrost thaw visibly expose changes in the landscape of the Lena River delta, the largest Arctic delta. Determining the past and modern river regime of thick deltaic deposits shaping the Lena River mouth in north-eastern Siberia is critical for understanding the history of delta formation and carbon sequestration. Using a 65 m long sediment core from the delta apex a set of sedimentological techniques is applied to aid in reconstructing the Lena River history. The analysis includes: (i) grain-size measurements and the determination of the bedload composition; (ii) X-ray fluorescence, X-ray diffractometry, and magnetic susceptibility measurements and heavy mineral analysis for tracking mineral change; (iii) pH, electrical conductivity, ionic concentrations, and the δ¹⁸O and δD stable isotope composition from ground ice for reconstructing permafrost formation. In addition; (iv) total and dissolved organic carbon is assessed. Chronology is based on; (vi) radiocarbon dating of organic material (accelerator mass spectrometry and conventional) and is complemented by two infrared – optically stimulated luminescence dates. The record stretches back approximately to Marine Isotope Stage 7. It holds periods from traction, over saltation, to suspension load sedimentation. Minerogenic signals do not indicate provenance change over time. They rather reflect the change from high energy to a lower energy regime after Last Glacial Maximum time parallel to the fining-up grain-size trend. A prominent minimum in the ground ice stable isotope record at early Holocene highlights that a river arm migration and an associated refreeze of the underlying river talik has altered the isotopic composition at that time. Fluvial re-routing might be explained by internal dynamics in the Lena River lowland or due to a tectonic movement, since the study area is placed in a zone of seismic activity. At the southern Laptev Sea margin, onshore continental compressional patterns are bordering offshore extensional normal faults.     

Petrogenesis of the Flood-Basalt Sequence at Noril'sk,North Central Siberia

The 3500-m-thick sequence of volcanic rocks at Noril'sk, formed during a brief interval (～1 m.y.) at the Permian/Triassic time boundary (～251 Ma), represents the earliest part of the ～6500-m-thick sequence presently ascribed to the Siberian flood-basalt province. It is composed of picritic and basaltic lavas of both low-Ti and high-Ti parentage. Extensive geological, geochemical, and isotopic study of the lava sequence and related intrusions allows detailed reconstruction of its petrogenesis. Various crustal-related processes-fractionation, crustal contamination, sulfide separation, and magma mixing-participated in the formation of the lavas. The geochemical and isotopic characteristics indicative of these processes, as well as mantle-related signatures of lava compositions, are discussed. Based on these characteristics, detailed interpretations of lava genesis and evolution throughout the Noril'sk sequence are presented. Eight varieties of lavas are recognized to be primitive, similar in composition to primary mantle melts; they varied from low-Mg basalts to olivine tholeiites or picrites, with normal tholeiites predominating.

The primitive lavas are subdivided into four groups (magma types) on the basis of trace-element ratios (principally, Gd/Yb, Th/U, La/Yb, Ta/La, Ti/Sc, and V/Yb) and isotopic data. Three of the groups include both basaltic and picritic primitive lavas (with low-Mg basalts present in one of them), whereas the fourth group is represented exclusively by tholeiites. Distinctions among the groups cannot be related to degree of melting, and isotopic data indicate that none of the magma types could have formed by mixing or contamination of other types. Apparently, only differences in source composition and/or depth of melting can explain the magmatic diversity.

This multitude of primitive magma types may be explained by melting in different layers of the upper mantle, which is complexly layered beneath Siberia to depths of 270 km. Moreover, no clear boundary between lithosphere and asthenosphere is evident in the deep seismic profile. A large-scale event is necessary to account for melting in different parts of the upper mantle and formation of the great volume of the Siberian flood basalts in ～1 m.y. Extension, caused by ascent of a mantle plume, would provide a reasonable explanation, but no plume-related uplift is documented in north-central Siberia prior to, or during, eruption of the volcanic sequence.     

上边界条件对多年冻土地温场数值模拟结果的影响分析

以玛多地区多年冻土为背景,建立多年冻土地温场的数值计算模型,以不同的方式考虑近 60 a 来的气温变化构成不同的上边界条件,通过模型计算分析不同上边界条件下的不同时期温度场、未来冻土退化特征.结果表明：在上边界条件中采用气象站实测近60 a波动温度值和采用近60 a平均恒定值时,浅层冻土地温差异明显,且越浅层地温与越近时间的上边界条件相关.预测未来100 a冻土地温变化趋势发现,相同升温速率和升温初始温度条件下,上边界采用实测60 a波动温度值对冻土退化过程影响较小;升温初始温度值提高到与趋势线衔接后,冻土退化起始时间从约第45年提前到约第20年;60 a实测温度和升温初始温度值均提高到与其初始温度场上边界条件衔接后,冻土退化起始时间从约第20年提前到约第15年;冻土退化从开始到完全退化经历时间为25 a左右.     

边界条件对多年冻土路基热稳定性的影响分析

多年冻土区的年平均气温是影响冻土路基边界条件的重要因素. 在附面层原理的基础上,考虑采用带有相变的控制方程和数值方法,以相同尺度的路基模型为前提,选取不同的年平均气温为影响因素,对青藏工程走廊公路路基的人为冻土上限和年平均地温进行了研究. 结果表明：公路路基下年平均地温随着年平均气温的升高而升高,人为冻土上限随着年平均气温的升高而显著下降. 在年平均气温为-7.16 ℃时,路基修筑50 a后其年平均地温为-3.61 ℃,其人为冻土上限为-0.97 m;年平均气温为-3.21 ℃的条件下,路基修筑50 a后其年平均地温仅为-0.1 ℃,其人为冻土上限也降至-13.11 m. 因此,可以看出：在未来气候持续变暖的背景下,现有处于稳定状态的冻土路基将逐渐变得不稳定.     

Thermal and settlement analyses under a riverbank over permafrost

Thermal regime evolution and settlement behavior of a riverbank over ice-rich permafrost in 20 years were estimated by numerical simulation and layerwise summation calculation. The results show that the permafrost under the riverbank will undergo a thawing process at a dramatically rate under the conditions of practical geology and construction structure, which is responsible for a remarkable settlement. The cases on different conditions of concrete hydration, backfilled soil temperature, seepage and mean annual ground temperature were also discussed, suggesting that these factors have the capability in improving the riverbank performance, among them seepage may be of the most importance.