青藏高原多年冻土区天然气水合物形成条件模拟研究

基于野外气体地球化学调查研究,以及前人有关冻土表层温度、冻土层内地温梯度、冻土层下地温梯度等的资料,对青藏高原多年冻土区天然气水合物的形成条件开展了模拟研究. 结果显示：研究区冻土条件能够满足天然气水合物形成的基本要求;气体组成、冻土特征（如冻土厚度或冻土表层温度、冻土层内地温梯度、冻土层下地温梯度等）是影响研究区天然气水合物稳定带厚度的最重要因素,其在不同点位上的差异性可能导致天然气水合物分布的不均匀性的主要原因;研究区最可能的天然气水合物为甲烷与重烃（乙烷和丙烷）的混合气体型天然气水合物;在天然气水合物分布的区域,其产出的上临界点深度在几十至一百多米间,下临界点深度在几百至近一千米间,厚度可达到几百米. 与Canadian Mallik三角洲多年冻土区相比,青藏高原多年冻土区除了冻土厚度小些外,其他条件,如冻土层内地温梯度、冻土层下地温梯度、气体组成等条件较为相近,具有一定的可比性,预示着良好的天然气水合物潜力.     

The influence of freeze–thaw cycles of active soil layer on surface runoff in a permafrost watershed

As a result of global warming, the discharges from rivers in permafrost regions have varied significantly. However, its mechanism remains unclear. One of possible factors is active soil freeze–thaw cycle, which may influence surface runoff in the variation of permafrost water cycle processes. In this study, a typical permafrost watershed in the Qinghai-Tibet plateau was selected, its hydrological processes were monitored from 2004 to 2007, and the effects of the freezing and thawing depth of the soil active layer on runoff processes were assessed. The runoff modulus, runoff coefficient, direct runoff ratio, recession gradient and their seasonal variations were estimated and analyzed. The active soil dynamics and water budget were analyzed to prove the features of the surface runoff and the influences of active soil freeze–thaw processes. The primary factors influencing surface runoff processes during different seasons were analyzed by Principal Component Analysis (PCA) and statistical regression methods. The results showed that the high runoff coefficient and low direct runoff ratio were the main characteristics during the spring flood period (May–June) and during the autumn recession period (September). The runoff modulus and its year-to-year variability were the greatest in the summer flood period. The direct runoff ratio decreased from 0.43 in May to 0.29 in September, with the exception of the highest ratio, which occurred during the summer recession period (July). The active soil thawing in the upper layer of depth of 60 cm had contributed to increase in discharge, but the increase in thawing depth deeper than 60 cm led to a decrease in surface runoff and slowness in the recession process. Precipitation played a small role in the spring flood runoff and the autumn runoff. The soil active layer freeze–thaw variation, which affected seasonal soil water dynamic and water budget and reformed seasonal runoff characteristics, along with vegetation cover changes, is considered the potential major factor in control of the hydrological processes in the permafrost region.     

Analysis and prevention measures for typical geological disasters formation and mechanisms within permafrost zone of Greater Khingan Range

The types, factors and formation conditions of typical geological disasters within the permafrost zone in the Greater Khingan Range area of Heilongjiang province were analysed in the paper. The formation mechanism behind geological disasters was illustrated, and the development trend, prevention programs and effects of the typical disaster on communities, agricultural lands and landscapes were reviewed.     

Hydrogeochemistry of the permafrost zone in the central part of the Yakutian diamond-bearing province,Russia

The hydrogeochemical characteristics and zonation of the permafrost zone in the central part of the Yakutian diamond-bearing province formed mainly under the influence of cryogenic processes that occurred in the late Cenozoic. On the basis of field and experimental studies, conclusions were formulated that the perennial cryogenesis of the sedimentary cover in this region resulted in increasing the concentration of carbonate and sulfate compounds in groundwaters as well as increasing the concentration of easily soluble compounds (sodium, magnesium, and calcium chlorides).

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Resumen Las características hidrogeoquímicas y la zonación del permafrost existente en el centro de la provincia de Yakutia, rica en diamantes, se formaron fundamentalmente bajo la influencia de procesos criogénicos durante el tramo final del Cenozoico. Con base en estudios de campo y experimentales, se ha concluido que la criogénesis perenne de la cubierta sedimentaria de la región ha provocado un aumento de la concentración de compuestos de carbonato y sulfato en las aguas subterráneas, además de incrementar la concentración de compuestos solubles, como el sodio, magnesio y cloruros de calcio.

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Résumé Les caractéristiques hydrogéochimiques et la zonation de la région de permafrost dans la partie centrale de la province diamantifère de Yakoutie ont été acquises principalement sous l'influence des processus cryogéniques qui existent depuis la fin du Tertiaire. Sur la base d'études expérimentales et de terrain, on conclut que la cryogenèse permanente de la couverture sédimentaire de cette région a provoqué un accroissement de la concentration des composés carbonatés et sulfatés dans les eaux souterraines, en même temps que celui de composés facilement solubles, comme les chlorures de sodium, de magnésium et de calcium.

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An isotopic and geochemical study of snowmelt runoff in a small arctic watershed

‘Old’ water contributions to snowmelt runoff in a stream can be defined as water which was stored in the catchment prior to the start of the runoff event in question. We used mass balance techniques for natural oxygen-18 and several chemical parameters (electrical conductivity, Ca and Mg) to estimate the magnitude and timing of ‘old’ water contributions to snowmelt runoff in the Apex River watershed during the 1983 field season. The Apex River catchment is located in the southern part of Baffin Island, Northwest Territories, Canada. The watershed has an area of 60 km², it is in the zone of continuous permafrost, and the geology is dominantly Precambrian gneiss with sporadic, thin, glacial overburden. The isotopic data indicate that for the snowmelt season of 1983 (third highest peak discharge of 11 years of record), approximately 50 per cent of the peak stream discharge consisted of ‘old’ water. Our data also suggest that about 60 per cent of the entire 1983 hydrograph was ‘old’ water. The chemical parameters give old water contributions which are at least 10 per cent less than the isotopically-derived estimate, but they are consistent with the isotopic estimate during peak flow.     

Geocryological characteristics of the upper permafrost in a tundra-forest transition of the Indigirka River Valley,Russia

Understanding geocryological characteristics of frozen sediment, such as cryostratigraphy, ice content, and stable isotope ratio of ground ice, is essential to predicting consequences of projected permafrost thaw in response to global warming. These characteristics determine thermokarst extent and controls hydrological regime—and hence vegetation growth—especially in areas of high latitude; it also yields knowledge about the history of changes in the hydrological regime. To obtain these fundamental data, we sampled and analyzed unfrozen and frozen surficial sediments from 18 boreholes down to 1–2.3 m depth at five sites near Chokurdakh, Russia. Profiles of volumetric ice content in upper permafrost excluding wedge ice volume showed large variation, ranging from 40 to 96%, with an average of 75%. This large amount of ground ice was in the form of ice lenses or veins forming well-developed cryostructures, mainly due to freezing of frost-susceptible sediment under water-saturated condition. Our analysis of geocryological characteristics in frozen ground including ice content, cryostratigraphy, soil mechanical characteristics, organic matter content and components, and water stable isotope ratio provided information to reconstruct terrestrial paleo-environments and to estimate the influence of recent maximum thaw depth, microtopography, and flooding upon permafrost development in permafrost regions of NE Russia.     

Recent air temperature changes in the permafrost landscapes of northeastern Eurasia

In the last two decades, climatic change has resulted in increased cryogenic activity in northeastern Eurasia, with adverse consequences for landscapes and socio-economic systems in the permafrost zone.The main purpose of this study was to assess the recent phases of warming, starting with Arctic warming. We performed a spatiotemporal analysis of climatic conditions during phases of maximum warming (i.e., 1935–1945, 1988–1995, and 2005–2009) in northeastern Eurasia and compared the magnitude of warming and its effect on permafrost among these critical periods. Our observations of permafrost landscape dynamics confirmed that the last two warming phases have played major roles in changing the environment.Data analysis has revealed regional patterns in the intensity of warming. Areas south of 60–62° latitude experienced no rise in air temperature during the Arctic warming period (1935–1945), whereas during 1988–1995, the center of warming shifted to the south of northeastern Eurasia. The last phase of warming (2005–2009) was characterized by maximum values of mean annual air temperature and the thawing index, and a decrease in the freezing index throughout northeastern Eurasia.     

Enigmatic fractures in Pliocene sands of Suffolk,U.K: hydrofracked permafrost with later warmer-climate calcrete fills?

Two types of fracture occur in the Pliocene (Red Crag and Coralline Crag) shelly sands of south-east Suffolk. Post-lithification, planar fractures of tectonic origin are confined to the more lithified Coralline Crag. More irregular fractures containing calcretized sediment-fills occur in both the Coralline and the more friable Red Crags, and are of less certain origin. The calcrete-filled fractures formed when the Red Crag sands were overpressured (with the fracture-filling sediment injected upward). Calcretes are associated with roots (rhizoliths) and are confined to fracture fills, despite the high porosity and permeability of the host sands. This confinement remains unexplained. Root penetration and calcrete formation may have occurred when the host sediments were frozen — but this would imply the fissure fills remained open because of upward water flow. This possibility intensifies a problem of finding a source for the overpressured water. If, alternatively, roots and accompanying calcretes were introduced after permafrost conditions had disappeared, their precise confinement to the fracture fills (with calcrete carbonates disappearing within only a few pore diameters into the host sediment) remains inexplicable. If both fractures and their calcrete fills formed during non-permafrost conditions, both the origin of the fractures and their precise filling by calcrete are problematic.     

Thermal-mechanical Influences and Environmental Effects of Expressway Construction on the Qinghai-Tibet Permafrost Engineering Corridor

The Qinghai-Tibet Engineering Corridor (QTEC) is a strategic passage between Tibet and central China. In the past half century, several major projects have been built in this narrow corridor with vulnerable geoenvironment. Along with a new round of economic development of the Tibet autonomous region, some major linear projects including expressway, double-tracking railway, high voltage power transmission line have been incorporated into the national development planning within the corridor, and especially the expressway construction is imminent now. In the QTEC, permafrost is a controlling factor of geological environment, which will impose great restrictions on engineering construction. In turn, engineering construction will induce significant effects on permafrost geological environment. With more and more linear infrastructures crowding into the QTEC, the thermal interaction among permafrost engineering and engineering disturbance on permafrost thermal-mechanical regime will be more significant. With respect to this issue, a research program focusing on thermal-mechanical influences and environmental effects of expressway construction on the Qinghai-Tibet permafrost engineering corridor was started and supported by the National Natural Science Foundation of China. In this paper, the research significance, key scientific issues, main research contents and goals of the program are introduced so as to provide some references for related researchers and engineers.     

Elastic-wave velocity characterization of gas hydrate-bearing fractured reservoirs in a permafrost area of the Qilian Mountain,Northwest China

There are two types of gas hydrate-bearing reservoirs in the permafrost area of Qilian Mountain. Most of the gas hydrates occur mainly in the fractured mudstone reservoirs and rarely in the pores of the sandstone reservoirs. In this study, for the acoustic velocity characterization of the fractured gas hydrate reservoirs of the Qilian Mountain permafrost area, some mudstone core samples were collected for physical rock experiments, such as the acoustic experiment and the porosity and permeability experiment. An acoustic velocity numerical simulation of gas hydrate reservoirs was performed according to the Biot theory and the differential effective medium theory, with the conditions of multiple gas hydrate occurrence models, including the suspension model, the semi-cementation model and the cementation model, and considering both infinite and penny-shaped cracks. Fracture porosity was added to the core samples that only contain matrix porosity. With fracture porosity ranging from 0.01% to 5%, the variation laws between acoustic velocity with fractured porosity and hydrate saturation are obtained: (1) In the case of an infinite crack, if the fractured porosity is 0.01%–1%, the P-wave velocity decreases rapidly in the case of the three occurrence models. If the fractured porosity is higher than 1%, the acoustic velocity decreases gradually. If the crack shape is a penny-shaped crack, the P-wave velocity decreases almost linearly with increasing fracture porosity. (2) If the hydrate occurrence model is the suspension model, the P-wave velocity increases slightly with increasing hydrate saturation. If the occurrence model is the semi-cementation model or the cementation model, when the gas hydrate saturation of the infinite crack ranges from 0 to 80%, the acoustic velocity increases approximately linearly, whereas when the gas hydrate saturation ranges from 80% to 100%, the velocity increases rapidly. If the crack is a penny-shaped crack, the velocity increases almost linearly with increasing gas hydrate saturation from 0 to 100%. (3) It is found that the fractured gas hydrate reservoirs of the Qilian Mountain permafrost area contain both penny-shaped and infinite cracks, of which the infinite crack is the main crack shape. The gas hydrate occurrence in the Qilian Mountain permafrost area mainly follows the suspension model. This has significance for the seismic exploration and log evaluation of gas hydrate-bearing fractured reservoirs in the permafrost area of the Qilian Mountain in studying the acoustic velocity characterization, the crack shapes and occurrence models of gas hydrate reservoirs in the study area.