Response of unconfined turbidity current to deep‐water fold and thrust belt topography: Orthogonal incidence on solitary and segmented folds

Sea‐floor topography of deep‐water folds is widely considered to have a major impact on turbidity currents and their depositional systems, but understanding the flow response to such features was limited mainly to conceptual notions inspired by small‐scale laboratory experiments. High‐resolution three‐dimensional numerical experiments can compensate for the lack of natural‐scale flow observations. The present study combines numerical modelling of thrusts with fault‐propagation folds by Trishear3D software with computational fluid dynamics simulations of a natural‐scale unconfined turbidity current by MassFlow‐3D? software. The study reveals the hydraulic and depositional responses of a turbidity current (ca 50 m thick) to typical topographic features that it might encounter in an orthogonal incidence on a sea‐floor deep‐water fold and thrust belt. The supercritical current (ca 10 m sec^?1) decelerated and thickened due to the hydraulic jump on the fold backlimb counter‐slope, where a reverse overflow formed through current self‐reflection and a reverse underflow was issued by backward squeezing of a dense near‐bed sediment load. The reverse flows were re‐feeding sediment to the parental current, reducing its waning rate and extending its runout. The low‐efficiency current, carrying sand and silt, outran a downslope distance of >17 km with only modest deposition (<0·2 m) beyond the fold. Most of the flow volume diverted sideways along the backlimb to surround the fold and spread further downslope, with some overspill across the fold and another hydraulic jump at the forelimb toe. In the case of a segmented fold, a large part of the flow went downslope through the segment boundary. Preferential deposition (0·2 to 1·8 m) occurred on the fold backlimb and directly upslope, and on the forelimb slope in the case of a smaller fold. The spatial patterns of sand entrapment revealed by the study may serve as guidelines for assessing the influence of substrate folds on turbiditic sedimentation in a basin.     

Comparative genetic structure in two high-dispersal prawn species from the south-west Indian Ocean

The penaeid prawns Fenneropenaeus indicus and Metapenaeus monoceros support shallow-water prawn fisheries in the south-west Indian Ocean. They are sympatric and have similar life histories, including developmental stages that depend on estuarine and marine habitats and a short dispersal duration. Nevertheless, M. monoceros juveniles display a more generalist habitat preference in estuaries and recruit to offshore habitats during a different season than F. indicus. We hypothesised that these differences would affect dispersal patterns, leading to dissimilar geographic genetic structure between the two taxa. Given their short dispersal phase, we also hypothesised that the Mozambique Channel would form a barrier to dispersal between the southeastern African mainland and Madagascar sites. Population differentiation was assessed based on analysis of mitochondrial DNA control-region sequences. Both species displayed high haplotype and low nucleotide diversity. Pairwise ?_ST statistics supported the existence of admixed populations along the African mainland sites for both species, with geographic distance isolating populations at the extremes of the sampled range (Kenya and east coast of South Africa). The Madagascar population differed significantly from African mainland populations. The concordant patterns in population differentiation suggest that F. indicus and M. monoceros can be considered as single African stocks, or fisheries management units.     

Evaluating suitability of a tephra dispersal model as part of a risk assessment framework

In volcanic risk assessment it is necessary to determine the appropriate level of sophistication for a given predictive model within the contexts of multiple sources of uncertainty and coupling between models. A component of volcanic risk assessment for the proposed radioactive waste repository at Yucca Mountain (Nevada, USA) involves prediction of dispersal of contaminated tephra during violent Strombolian eruptions and the subsequent transport of that tephra toward a hypothetical individual via surface processes. We test the suitability of a simplified model for volcanic plume transport and fallout tephra deposition (ASHPLUME) coupled to a surface sediment-transport model (FAR) that calculates the redistribution of tephra, and in light of inherent uncertainties in the system. The study focuses on two simplifying assumptions in the ASHPLUME model: 1) constant eruptive column height and 2) constant wind speed and direction during an eruption. Variations in tephra dispersal resulting from unsteady column height and wind conditions produced variations up to a factor of two in the concentration of tephra in sediment transported to the control population. However, the effects of watershed geometry and terrain, which control local remobilization of tephra, overprint sensitivities to eruption parameters. Because the combination of models used here shows limited sensitivity to the actual details of ash fall, a simple fall model suffices to estimate tephra mass delivered to the hypothetical individual.     

Characterization of shape and terminal velocity of tephra particles erupted during the 2002 eruption of Etna volcano,Italy

In this paper, we present a complete morphological characterization of the ash particles erupted on 18 December 2002 from Etna volcano, Italy. The work is based on the acquisition and processing of bidimensional digital images carried out by scanning electron microscopy (SEM) to obtain shape parameters by image analysis. We measure aspect ratio (AR), form factor (FF), compactness (CC), and rectangularity (RT) of 2065 ash particles with size between 0.026 and 1.122 mm. We evaluate the variation of these parameters as a function of the grain-size. Ash particles with a diameter of <0.125 mm vary from mostly equant to very equant, ash particles between 0.125 and 0.250 mm have an intermediate shape, and particles with diameters >0.250 mm are subelongate. We find that, on average, particles with a diameter of <0.250 mm are subrounded, particles between 0.250 and 0.50 mm are subangular, and particles >0.50 mm are angular. Using this morphological analysis and an empirical relation between the drag coefficient (C _D) and Reynolds number (R _e) of Wilson and Huang (Earth Planet Sci Lett 44:311–324, 1979), we calculate the terminal settling velocities (V _WH). The comparisons between these velocities and those calculated with the formula of Kunii and Levenspiel (Fluidization engineering. Wiley, New York, pp 97, 1969) (V _KL), which considers ash particles as spheres, show that V _KL are in average 1.28 greater than V _WH. Hence, we quantify the systematic error on the spatial distribution of the mass computed around the volcano carried out by tephra dispersal models when the particles are assumed to be spherical.     

鄂尔多斯西南缘前陆盆地沉积物物源分析及其构造意义

前陆盆地充填沉积物中岩屑的分散形式和砂岩成分研究对确定物源区的逆冲事件、构造背景和与沉积物对应的物源层十分有效。通过对鄂尔多斯西南缘安口地区上三叠统沉积剖面系统采样测定发现，沉积物物源为一套变质碎屑岩和变质火山岩，为东祁连褶皱逆冲带中的陈家河组（Ｏ３ｃｈ）和葫芦河组（Ｚ－Ｏ２ｈｌ）。而西南缘千阳县上侏罗统砾岩物源为陇山群（Ｐｔ２ｌｎ）。上三叠统地层的物源分散形式为混合型，而上三叠统和上侏罗统两套地层统一的分散型式为倒序型，它是两期逆冲活动的沉积响应。根据鄂尔多斯西南缘安口地区上三叠统沉积物的ＱｍＦＬｔ端元成分作砂岩三角投影图，确定晚三叠世鄂尔多斯西南缘盆地性质为发育于碰撞造山带之前陆的前陆盆地。其物源为再旋回活动的褶皱逆冲断块     

北京首都机场的人工消雾及大气边界层特征的演变

在首都机场人工消雾作业过程中进行了大气边界层特征的观测,观测结果表明：用喷洒液氮进行人工消雾可以形成小面积降雪从而提高能见度,达到安全飞行的目的．人工消雾作业过程可影响小面积大气湍流特征,使湍流谱略偏离-5／3规律．人工消雾作业期间潜热通量明显大于作业期后．     

黄河入海泥沙扩散与河海相互作用

根据黄河口卫星遥感数据和河口滨海区水文,泥沙实测资料,对河口区洪水的平面扩散和含沙水层特征进行分析,结果表明,在洪水期,河口高高潮到低低潮线之间主河道两侧存在大范围浸流区,片状水流汇入网状汉沟入海,主河槽过流仍很明显,随着流量减小,漫流现象减弱乃至消失,推测口门区主河道的床流量约１０００ｍ＾３／ｓ;ＴＭ１－３合成ＴＭ６温度制图发现,在河口浅水区存在河海相互作用的混合带,混合带宽与河流流量成正比,混     

几种粉末型吸湿性催化剂的试验研究

在云雾中播撒吸湿性催化剂是进行暖云催化降水或消暖雾的重要手段之一,而寻找高效、适宜的吸湿性催化剂仍然是当前人工影响天气领域的重要研究目标。本研究对6种具有吸湿性的粉末型稀土盐催化剂在云室中的消暖雾性能进行了对比试验。试验采用一个43m3的暖云室,在云室中分别进行无催化的空白试验和6种催化剂的消暖雾催化试验,并使用FM-100雾滴谱仪、透光度仪以及温湿度仪等仪器对云室中雾的整个发展过程进行全程观测。通过对比空白试验与催化试验的观测数据,分析了5种吸湿性催化剂在播撒后对暖雾的影响,对它们的消雾效果和催化剂作用机理进行了分析。研究结果表明:6种催化剂中有5种起到了消雾作用,其中氯化铈、硝酸镧、碳酸镧的消雾效果较好,氯化镧、碳酸铈次之,硝酸铈则没有达到消雾目的;催化剂的引入使雾的含水量、雾滴有效直径、雾滴谱等微物理特征发生明显变化,并最终对雾的发展进程造成很大影响,使雾的消散速度加快或延缓。