Late Pleistocene genesis of the middle Yarlung Zhangbo Valley,southern Tibet (China), as deduced by sedimentological and luminescence data

Knowledge on valley formation and palaeohydrology of the Yarlung Zhangbo (named Brahmaputra south of the Himalayas) is still in an early stage. Research was conducted in the middle valley reach around Gonggar in order to investigate the sediment properties, age and palaeoenvironmental implication of a widespread aggradational valley terrace. This heavily dissected terrace, lying c. 25 m above the present floodplain, consists of topping aeolian and colluvial deposits, intermediate lacustrine fines and basal fluvial sands. Sedimentological properties of the lacustrine layers suggest a cold-climate depositional environment. Geochemical data refer to a common provenance of all sediments investigated from a regional catchment area, implying a local provenance of topping loesses and sands by aeolian sorting of nearby fluvial deposits. OSL dates, representing the first luminescence dating effort partly applied to non-aeolian sediments in this valley, cluster closely in the interval 11.7 ± 0.8–17.4 ± 1.4 ka. During the Late Pleistocene (until c. 16 ka) the investigated middle valley reach around Gonggar was occupied by a lake attaining an extension of at least 100 km.     

Assimilation of ice concentration in a coupled ice–ocean model,using the Ensemble Kalman filter

An implementation of the Ensemble Kalman filter (EnKF) with a coupled ice–ocean model is presented. The model system consists of a dynamic–thermodynamic ice model using the elastic-viscous-plastic (EVP) rheology coupled with the HYbrid Coordinate Ocean Model (HYCOM). The observed variable is ice concentration from passive microwave sensor data (SSM/I). The assimilation of ice concentration has the desired effect of reducing the difference between observations and model. Comparison of the assimilation experiment with a free-run experiment shows that there are large differences, especially in summer. In winter the differences are relatively small, partly because the atmospheric forcing used to run the model depends upon SSM/I data. The assimilation has the strongest impact close to the ice edge, where it ensures a correct location of the ice edge throughout the simulation. An inspection of the model ensemble statistics reveals that the error estimates of the model are too small in winter, partly a result of too low model ice-concentration variance in the central ice pack. It is found that the ensemble covariance between ice concentration and sea-surface temperature in the same grid cell is of the same sign (negative) throughout the year. The ensemble covariance between ice concentration and salinity is more dependent upon the physical mechanisms involved, with ice transport and freeze/melt giving different signs of the covariances. The ice-transport and ice-melt mechanisms also impact the ice-concentration variance and the covariance between ice concentration and ice thickness. The ensemble statistics show a high degree of complexity, which to some extent merits the use of computationally expensive assimilation methods, such as the Ensemble Kalman filter. The present study focuses on the assimilation of ice concentration, but it is understood that assimilation of other datasets, such as sea-surface temperature, would be beneficial.Responsible Editor: Jin-Song von Storch     

Configuration,history and impact of the Norwegian Channel Ice Stream

The Norwegian Channel between Skagerrak, in the southeast, and the continental margin of the northern North Sea, in the northwest, is the result of processes related to repeated ice stream activity through the last 1.1 m yr. In such periods the Skagerrak Trough (700 m deep) has acted as a confluence area for glacial ice from southeastern Norway, southern Sweden and parts of the Baltic. Possibly related to the threshold in the Norwegian Channel off Jæren (250 m deep), the ice stream, on a number of occasions over the last 400 ka, inundated the coastal lowlands and left an imprint of NW‐oriented ice directional features (drumlins, stone orientations in tills and striations). Marine interstadial sediments found up to 200 m a.s.l. on Jæren have been suggested to reflect glacial isostasy related to the Norwegian Channel Ice Stream (NCIS). In the channel itself, the ice stream activity is evidenced by mega‐scale glacial lineations on till surfaces. As a result of subsidence, the most complete sedimentary records of early phases of the NCIS are preserved close to the continental margin in the North Sea Fan region. The strongest evidence for ice stream erosion during the last glacial phase is found in the Skagerrak. On the continental slope the ice stream activity is evidenced by the large North Sea Fan, which is mainly a result of deposition of glacial‐fed debris flows. Northwards of the North Sea Fan, rapid deposition of meltwater plume deposits, possibly related to the NCIS, is detected as far north as the Vøring Plateau. The NCIS system offers a unique possibility to study ice stream related processes and the impact the ice stream development had on open ocean sedimentation and circulation.     

Analytic approach to auroral electron transport and energy degradation

The interaction of a beam of auroral electrons with the atmosphere is described by the linear transport equation, encompassing discrete energy loss, multiple scattering and secondary electrons. A solution to the transport equation provides the electron intensity as a function of altitude, pitch angle (with respect to the geomagnetic field) and energy. A multi-stream (discrete ordinate) approximation to the transport equation is developed. An analytic solution is obtained in this approximation. The computational scheme obtained by combining the present transport code with the energy degradation method of Swartz (1979) conserves energy identically. The theory provides a framework within which angular distributions can be easily calculated and interpreted. Thus, a detailed study of the angular distributions of “non-absorbed” electrons (i.e., electrons that have lost just a small fraction of their incident energy) reveals a systematic variation with incident angle and energy, and with penetration depth. The present approach also gives simple yet accurate solutions in low order multi-stream approximations. The accuracy of the four-stream approximation is generally within a few per cent, whereas two-stream results for backscattered mean intensities and fluxes are accurate to within 10–15%.     

The Meteoritical Bulletin,No. 104

Meteoritical Bulletin 104 contains 2279 meteorites including 12 falls (Annama, Cartersville, Creston, Diepenveen, Famenin, Izarzar, Nkayi, Porangaba, San Juan de Ocotán, Trâpe?ng Rôno?s, Xinglongquan, ?d’ár nad Sázavou), with 1847 ordinary chondrites, 138 carbonaceous chondrites, 128 HED achondrites, 38 lunar meteorites, 24 ureilites, 22 Martian meteorites, 19 iron meteorites, 17 primitive achondrites, 14 enstatite chondrites, 10 mesosiderites, 9 Rumuruti chondrites, 5 pallasites, 4 ungrouped achondrites, 2 enstatite achondrites, 1 ungrouped chondrite, and 1 Kakangari chondrite, and with 996 from Antarctica, 790 from Africa, 337 from Asia, 111 from South America, 30 from North America, 11 from Oceania, and 4 from Europe. Note: 1 meteorite from Russia was counted as European.     

Dynamics of carbohydrates in the Norwegian Sea inferred from monthly profiles collected during 3 years at 66°N, 2°E

Water samples were collected monthly for 3 years at 66°N, 2°E in the Norwegian Sea, 250 nautical miles off the Norwegian coast. Concentrations of mono- and polysaccharides were measured with the 2,4,6-tripyridyl-s-triazine (TPTZ) spectroscopic method. Total dissolved carbohydrates varied from 3.4 to 28.2 μM C of all samples and the ratio of carbohydrate to dissolved organic C (DOC) varied from an average of 14% at 0–25 m depth to 11% at 800–2000 m depth. This indicates that dissolved carbohydrates were a significant constituent of DOC in the Norwegian Sea. Polysaccharides varied from 0.4 to 21.5 μM C and monosaccharides from 0.7 to 11.7 μM C at all depths. The level of monosaccharides was relatively constant at 2.8–3.2 μM C below the euphotic zone, whereas polysaccharides showed more varying concentrations. Dissolved carbohydrates accumulated during the productive season, reaching maximum concentrations during summer although interannual differences were observed. A significant positive correlation between Chl a and soluble carbohydrate was found in one growing season with nutrient analyses. Average values for total carbohydrates were highest in the surface – 0 to 25 m – with 13.3 μM C and decreased to 8.4 μM C at 800–2000 m depth. The ratio of monosaccharides to polysaccharides exhibited a marked seasonal variation, increased from January to a maximum in June of 1.1, and declined to 0.5 in July.     

Sandstone diagenesis and porosity modification during basin evolution

The properties of sandstones as potential reservoirs and shales as source rocks depend on primary facies relationships and diagenesis. Porostiy loss due to mechanical compation and pressure solution is essentially a function of grain parameters (sorting, packing and composition) and net overburden stress. The porosity loss can be predicted to a certain extent. The importance of secondary porosity caused by dissolution of framework grains and cements has been fully recognized. The discussion has focused on the processes causing such dissolution and to what extent it can cause net increase in porosity.The most critical factor in clastic diagenesis is the nature of porewater flow and the degree of mass transfer taking place as a result of this. In the North Sea reservoir rocks, petrographic and geochemical evidence suggest that most of the leaching of feldspar and mica resulting in the formation of kaolinite occurred early during fresh wather flushing. Recent calculations indicate that »acids« derived from source rocks are inadequate to explain the secondary porosity observed in reservoir rocks. Mathematical modelling suggests that thermal convection is of limited importance in sedimentary basins, except where there are high lateral changes in geothermal gradients. Evidence from porewater geochemistry suggests that porewaters in sedimentary basins are often stratified or compartmentalized in a way which is inconsistent with large scale convection or compactional flow, making it necessary to assume that diagenetic reactions are relatively isochemical during deeper burial.A better understanding of the diagenetic reactions will help us to improve our predictions about porosity/depth relations, pore size, and pore geometry distribution in reservoir rocks.Porosity depth trends from offshore Norway and published data from other basins are discussed. Empirical linear best fit lines are found to illustrate the relationship quite well for depths between one and five km. Within a specific region, the linear porosity gradient is a function of mineral composition and of temperature and pressure gradients. Primary porosity tends to be best preserved in sandstones with high proportions of stable grains (e. g. in quartz arenites) down to about 3 or 4 km. At greater depth, porosity loss is accelerated due to increased pressure solution. Secondary and primary porosity adjacent to feldspar grains then tends to be selectively preserved relative to primary pores between quartz grains.

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Zusammenfassung Die bestimmenden Faktoren von Sandsteinen als mögliche Speichergesteine und von Schiefern als Bildungsort von Kohlenwasserstoffen liegen in den Verhältnissen der primären Fazieausbildungen der Gesteine und deren Diagenesegeschichte. Korn-Parameter (Sortierung, Pakkungsdichte und Zusammensetzung) und Netto-Auflast steuern mechanische Kompaktion und Drucklösung. Diese sind für Porositätsreduzierungen verantwortlich, die bis zu einem bestimmten Grad abgeschätzt werden können. Der Einfluß von Lösungserscheinungen für die Bildung sekundären Porenraums, der hier definiert werden konnte und die Vorgänge, die Lösungsphasen iniziieren, bilden den zentralen Punkt der Diskussion.Größte Bedeutung in der Diagenese klastischer Sedimente kommt dem Charakter der Grundwasserbewegungen und deren Eigenschaft im Bereich des Stofftransportes zu. Daten der Petrographie und der Geochemie ergeben für die Speichergesteine der Nordsee, daß der Hauptanteil der Lösung von Feldspat und Muskovit, ein Vorgang, der zur Bildung von Kaolinit führte, frühdiadenetisch unter Einfluß von Süßwasser stattfand. Aufgrund neuerer Kalkulationen kann die Bedeutung saurer Lösungen, die man von dem Bildungsgestein hergeleitet, für die sekundäre Porosität der Speichergesteine ausgeschlossen werden. Mathematische Berechnungen räumen dem Einfluß thermaler Konvektion in sedimentären Becken meist geringe Bedeutung zu, es sei denn es gibt große laterale Unterschiede des geothermischen Gradienten. Die geochemische Analyse des Porenwassers ergibt vertikale und horizontale Zonierungen im Chemismus des Wassers. Diese Tatsache widerspricht der These einer großräumischen Konvektion, da in diesem Fall die Diagenese-Prozesse unter Bedeckung bei relativ einheitlicher chemischer Zusammensetzung ablaufen würden.Ein besseres Verständnis der diagenetischen Vorgänge wird die Abhängigkeit des Verhältnisses von Porosität zur Tiefe, der Verteilung des Hohlraumvolumens und der Hohlraumgeometrie in Speichergesteinen erleichtern.Diskutiert werden Zusammenhänge von Porosität und Tiefe, die Daten an Stellen vor der Küste Norwegens und anderen Becken entnommen wurden. Für Tiefen zwischen einem und fünf Kilometern scheinen empirische, lineare Geraden am besten die Verhältnisse zu verdeutlichen. Regional begrenzt kann der lineare Porositätsgradient als eine Funktion der Gradienten von Mineralzusammensetzung, Temperatur und Druck beschrieben werden. Primäre Porosität ist am besten in Sandsteinen mit einem hohen Anteil an stabilen Komponenten (z. B. Quatzarenite) bis zu einer Tiefe von drei oder vier Kilometern erhalten.Drucklösungserscheinungen sind für die zunehmende Abnahme der Porosität in größeren Tiefen verantwortlich. Dabei scheinen sekundäre und intergranulare Porenräume zwischen Feldspatkörnern relativ zu primären Porenräumen zwischen Quartzkörnern stabiler zu sein.

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Résumé Les facteurs qui caractérisent des grès en tant que réservoirs potentiels d'hydrocarbures et des shales en tant que roches-mères sont les relations facielles primaires et la diagenèse. La perte de porosité due à la compaction mécanique et aux impressionnements («pressure-solution») est fonction essentiellement des paramètres des grains (classement, ordonnance et composition) et des contraintes dues à la surcharge. La perte de porosité peut être prévue dans une certaine mesure. On connaît l'importance de la porosité secondaire provoquée par la dissolution des grains et des ciments. La discussion est centrée sur les processus responsables de cette dissolution et sur leur incidence dans l'accroissement net de la porosité.Le facteur le plus critique dans la diagenèse des élastiques est la nature du flux d'eau intersticielle et le degré de transfert de matière qui en résulte. Dans les roches-magasins de la Mer du Nord, les observations pétrographiques et géochimiques montrent que la plus grande partie de l'altération en kaolin du feldspath et du mica s'est produite lors du lessivage par de l'eau douce. Selon des calculs récents, les «acides» dérivés des roches-mères ne peuvent expliquer la porosité secondaire des roches-magasins. La modélisation mathématique suggère que la convection thermique ne joue qu'un rôle subordonné dans un bassin de sédimentation, sauf aux endroits de forte variation latérale du gradient géothermique. L'étude géochimique des eaux intersticielles montre que dans les bassins sédimentaires, elles sont souvent stratifiées ou compartimentées d'une manière qui est incompatible avec une convection à grande échelle car un tel phénomène impliquerait des réactions diagénétiques relativement isochimiques.Une meilleure compréhension des réactions diagénétiques doit nous aider à améliorer nos prévisions relatives à la relation porosité/profondeur, et à la répartition de la taille et de la forme des pores dans les roches-magasins.Une discussion est présentée à propos de la relation porosité/profondeur, à partir des données recueillies off-shore en Norvège et de données publiées provenant d'autres bassins. Il apparaît qu'entre 0 et 5 km de profondeur, des courbes empiriques linéaires rendent le mieux compte de ces relations. Dans une région donnée, le gradient linéaire de porosité est fonction de la composition minéralogique et des gradients de pression et de température. La porosité primaire tend à être bien préservée dans les grès riches en grains stables (p. ex. dans les arénites quartziques) jusqu'à une profondeur de 3 à 4 km. Plus bas, la perte de porosité s'accélère en raison des phénomènes d'impressionnement. Dans ces conditions profondes, les pores primaires et secondaires adjacents aux grains de feldspath semblent relativement plus stables que les pores primaires situés entre les grains de quartz.

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海冰盐度影响下冰水热力过程的试验研究

北极多年海冰的渡夏与冰脊形成的初始阶段均属于海冰升温的热力学过程。在卤水作用下,海冰在升温过程中需要融化内部的固态冰以达到温盐平衡。为研究海冰升温过程中盐度对宏观热力学性质的影响,本文设计了冰块试样在水中的一维浸没试验。将具有相同初始温度 （-32℃）、不同盐度 （0 ppt, 2.65 ppt, 12 ppt） 的冰试样浸至 0.2 ℃的淡水环境中,并测试试样内部温度场与厚度变化。试验结果表明,高盐度海冰的最终冰厚增加量超过 31%,而淡水冰则仅为 15%。海冰内部卤水的活跃改变了海冰的宏观比热并大幅度提高了其潜在的内能变化量。同时,将内能变化的实测结果与 Schwerdfeger 模型理论计算结果进行了对比。分析得出,真实环境中在海冰卤水迁移引起的盐度不均匀分布与卤水外排使得 Schwerdfeger 模型无法准确描述海冰升温过程中热力学性质。此外,通过对新生冰进行冰晶测试中所观测的柱状结构及晶粒尺寸特征也进一步验证了试验结果的可靠性。     

自然对流条件下冰水界面换热系数的试验研究

在冰脊的固结过程中,由于接触面积与温差的大幅提升,冰水之间的换热强度显著增强。本文通过浸没试验对自然对流条件下冰水间的换热系数进行了研究。在试验过程中,对试样内部的温度分布与体积变化分别用温度梯度测试系统与数字图像进行测量。为研究初始条件对换热系数的影响,分别采用不同初始温度与厚度的试样在瞬态热传导的环境下进行测试。试验结果表明,换热系数与表面温差呈指数增长,且在本文试验条件下的变化区间为0.3~175 W/（m2·K）。试样的初始温度及厚度并不是影响换热系数的直接因素,而其根本因素为流-固界面的边界层状态。在自然对流状态下流体的驱动条件是热胀效应,即当边界层存在温度差时,虽然外界并不存在扰动流体状态的因素,但由于液体自身温差引起的密度差进而驱动流体运动并影响了换热系数。随着边界层温度梯度的增加,边界层的影响区域扩大,从而导致了较高的换热系数。