两类El Niño型对西北太平洋季风槽及热带气旋生成的可能影响

通过对1948~2015年不同El Ni?o事件下西北太平洋季风槽变化和热带气旋(tropical cyclone,TC)生成进行分析,初步探讨了不同El Ni?o型事件对季风槽及其对TC的可能影响。分析结果表明,较东太平洋增暖(eastern Pacific warming,EPW)年,中太平洋增暖(central Pacific warming,CPW)年季风槽偏弱,位置相对偏西、偏北。在CPW年,中(西和东)太平洋海温增暖(降低)引起了从中到西太平洋热带地区的西风异常和中太平洋地区上升运动及对流活动加强,使得季风槽加强东伸,同时西太平洋副高偏弱、偏北,季风槽向北推进;而在EPW年,赤道东(西)太平洋海温增暖(降低)使得赤道地区西风异常显著加强东扩,异常Walker环流的上升支东移至东太平洋,季风活动加强,副高偏强、偏南,这使得季风槽较CPW年相比更强、更偏东。利于TC生成的大尺度环境因子随季风槽强度和位置的变化而发生改变,在CPW年,低层气旋性涡度、高层辐散、高的中层相对湿度以及低垂直风切变区随着季风槽向北移动;而在EPW年,这些因子随季风槽向南、向东偏移。这些大尺度环境因子的变化使得西北太平洋TC生成的位置在CPW年比EPW年更加偏北、偏西。     

Ages and Hf isotopes of detrital zircons from the Permian strata in the Bengbatu area (Inner Mongolia) and tectonic implications

The Central Asian Orogenic Belt (CAOB) was built up through protracted accretion and collision of a variety of terranes/micro-continents during Neoproterozoic–Mesozoic time. To understand potential links among Paleozoic subduction and accretionary processes that were operative during the development of the southeastern CAOB, we conducted a combined U-Pb and Hf-isotope analysis of detrital zircons from previously defined Devonian, Carboniferous and Early Permian strata in the Bengbatu area, Inner Mongolia. Detrital zircons from (meta-) sandstones in these strata commonly yield major Paleozoic age populations at ca. 300–261 Ma, 351–300 Ma and 517–419 Ma, and also give several Precambrian ages that range from 2687 Ma to 544 Ma. The youngest ages redefine the deposition of all these strata to be in the Middle Permian (Wordian–Capitanian) or later, much younger than previously considered. These ages, coupled with regional magmatic records, support an interpretation of most surrounding areas as possible detritus sources, including the Mongolian arcs to the north, the Northern Accretionary Orogen to the south, and the intervening Erenhot–Hegenshan Ophiolite Belt. Zircons with magmatic ages of ca. 500–350 Ma and ca. 300–261 Ma display a large range of ε_Hf(t) values (?13.97 to +15.31), whereas ca. 350–300 Ma zircons are dominated by positive ε_Hf(t) values (+0.14 to +16.00). These results support the occurrence of two significant shifts of the zircon ε_Hf(t) values, which has tectonic implications for the understanding of the Carboniferous–Permian evolution of the southeastern CAOB. A marked shift from mixed to positive zircon ε_Hf(t) values at 350–330 Ma likely manifests the incipient opening of the Hegenshan Ocean, due to the slab rollback of the subducting Paleo-Asian Oceanic lithosphere. Another shift from positive to mixed zircon ε_Hf(t) values at ca. 300 Ma likely corresponds to a tectonic switch from syn-orogenic subduction-related to post-orogenic extensional setting, genetically related to the tectonic collapse of a formerly overthickened crust.     

The coral communities of the Islas Marias archipelago,Mexico: structure and biogeographic relevance to the Eastern Pacific

Various authors have suggested that the Islas Marias archipelago, Mexico, may play a significant biogeographic role in the dispersal of Indo Pacific coral species into the Eastern Pacific; however, the coral communities of this archipelago have received scarce attention to date. Here, we first addressed coral community structure across the islands and, by employing ordination analysis, minimum spanning tree and particle‐tracking experiments, used this information to evaluate the relevance of the archipelago for coral dispersal. Twenty‐four coral communities were recorded in the archipelago. Coral cover varied significantly among islands: Maria Cleofas had large values (38.5%), intermediate values were observed for Maria Madre (26.5%) and Maria Magdalena (22.84%), and relatively low values were recorded for San Juanito (18.5%). Coral communities mainly consisted of Pocillopora (57.3%) and Porites (25.5%) species, while species of Pavona (16%) and Psammocora (0.6%) made relatively minor contributions. Thirteen stony coral species were identified in the archipelago; of these Psammocora profundacella and Pavona duerdeni represent new records. Ordination analysis, minimum spanning tree and particle‐tracking experiments suggested similar connecting paths in the studied area; in general, the Islas Marias stands as a route for coral dispersal of Indo Pacific species into the Northeastern Pacific. In a regional context, the Islas Marias has three major biogeographic implications to coral dispersal: (i) the archipelago stands as a major stepping stone for the transport of species and individuals among the Revillagigedo archipelago, the Gulf of California and the tropical Mexican Pacific; (ii) the Islas Marias may play a seminal role in maintaining the genetic connectivity between southern and northern coral populations along the Mexican Pacific and (iii) because of its relatively pristine status and low levels of human impact, the archipelago may potentially serve as a source of coral propagules for ecosystem recovery in the Gulf of California and along the Pacific coast of the Mexican mainland following natural and/or human induced perturbations.     

Holocene climate change in the Central Tibetan Plateau inferred by lacustrine sediment geochemical records

Multi-proxies of lacustrine sediments, such as total carbon (TC), total organic carbon (TOC), total inorganic carbon (TIC), total nitrogen (TN), total sulfur (TS), hydrogen index (HI), oxygen index (OI) and stable carbon isotopic composition of organic matter (δ 13Corg), were analyzed using a 7.3 m core from Zigê Tangco. The source of the organic matter in the sediment was mainly from autochthonous phytoplankton, therefore the significances of proxies can be interpreted as that high TOC, TOC/TS, HI and δ13Corg values, low TC, TIC values corresponded to warm and wet climatic condition, and vice versa. The process of climatic development in the Zigê Tangco region was hence recovered. During the early and Mid-Holocene, the climate was warm and wet and intensive cold events occurred during the periods of 8600 to 8400 cal a BP and 7400 to 7000 cal a BP. In the second half of Holocene, the climate became cold and dry gradually. The palaeoclimatic process during Holocene in Zigê Tangco region matched well with that in Co Ngoin region which is ca 40 km to the south-east. Therefore this palaeoclimatic process represents the Holocene climatic feature in the Central Tibetan Plateau which has the same pattern in the Northern Tibetan Plateau, but the time and duration of some climatic events might be different. We can conclude that in Holocene solar insolation controlled the climatic pattern on the central Tibetan Plateau.     

Tangled up in folds: tectonic significance of superimposed folding at the core of the Central Iberian curve (West Iberia)

The amalgamation of Pangea during the Carboniferous produced a winding mountain belt: the Variscan orogen of West Europe. In the Iberian Peninsula, this tortuous geometry is dominated by two major structures: the Cantabrian Orocline, to the north, and the Central Iberian curve (CIC) to the south. Here, we perform a detailed structural analysis of an area within the core of the CIC. This core was intensively deformed resulting in a corrugated superimposed folding pattern. We have identified three different phases of deformation that can be linked to regional Variscan deformation phases. The main collisional event produced upright to moderately inclined cylindrical folds with an associated axial planar cleavage. These folds were subsequently folded during extensional collapse, in which a second fold system with subhorizontal axes and an intense subhorizontal cleavage formed. Finally, during the formation of the Cantabrian Orocline, a third folding event refolded the two previous fold systems. This later phase formed upright open folds with fold axis trending 100° to 130°, a crenulation cleavage and brittle–ductile transcurrent conjugated shearing. Our results show that the first and last deformation phases are close to coaxial, which does not allow the CIC to be formed as a product of vertical axis rotations, i.e. an orocline. The origin of the curvature in Central Iberia, if a single process, had to be coeval or previous to the first deformation phase.     

中亚造山带东段古元古代结晶基底:来自中蒙边境花岗岩和沉积地层的证据

中亚造山带东段多个地块内鲜有古老结晶基底物质报道,严重制约了我们对其早期属性的认识。本研究在内蒙古北部与蒙古国接壤的乌力吉特敖包地区,发现了被中下泥盆统泥鳅河组不整合覆盖的古元古代细粒二长花岗岩和沉积地层(乌兰敖包组)。对二长花岗岩开展的LA-ICM-MS锆石U-Pb定年显示其结晶年龄为(1686±10) Ma,说明形成于古元古代。乌力吉特敖包二长花岗岩高钾钙碱性,过铝质(A/CNK=1.08～1.11),且含有大量白云母,属S-型花岗岩。不同于常见的显生宙以来的S-型花岗岩,乌力吉特敖包古元古代花岗岩具有正的εHf(t)值(+2.9～+6.7),但εHf(t)值远低于1.7 Ga地壳演化趋势线,且锆石原位Hf同位素二阶段模式年龄为2.0～2.3 Ga,因此其应该来源于古老变泥质岩部分熔融。乌力吉特敖包古元古代花岗岩形成于陆陆同碰撞的构造环境。在古元古代乌兰敖包组变质泥岩获得了一个显著的碎屑锆石峰值年龄(1698 Ma),且该地层被元古宙花岗岩侵入。本研究发现的乌力吉特敖包古元古代末期花岗岩和乌兰敖包组沉积地层说明:中亚造山带东缘各地块内存在古老的结晶基底物质。这为我们认识这些地块早期演化历史提供了重要地质证据。     

Changes in Geotechnical Properties of Sediments from the Central Indian Basin Induced by Disturbance Experiment

During the Indian Deep-sea Environment Experiment (INDEX) conducted in the Central Indian Basin to simulate nodule mining activity, the sediments were physically disturbed, lifted from the seafloor, and then redeposited to study the effects of sediment redistribution on geological, chemical and biological characteristics of benthic environment. The first monitoring cruise, 44 months after the experiment, was part of long-term observations for restoration of conditions. This study describes the effects of the experiment on geotechnical properties of sediments measured in predisturbance, postdisturbance and monitoring phases. To compare the effects, sediment cores were collected from the same locations during the three phases. Siliceous, fine-grained sediments from the study area showed change in geotechnical properties induced due to the disturbance. Marginal increase in natural water content and significant reduction in undrained shear strength at the 0–5 cm sediment layer of cores from the tow zone during postdisturbance was observed. However, during the monitoring phase, an increase in shear strength and reduction in water content was noticed, which might indicate that the sediments are gradually acquiring predisturbance characteristics. The study also revealed that the meiofaunal density has a positive correlation with the water content, but a negative relationship with the shear strength of these siliceous sediments. Specific gravity of solids and porosity showed marginal change, whereas wet density remained unchanged.     

藏北羌塘中部冈玛日——桃形错蓝片岩的发现

位于藏北羌塘中部的冈玛日-桃形错地区首次发现含典型蓝闪石的蓝片岩。蓝片岩是由蓝闪石、铁蓝闪石、青铝闪石、多硅白云母、阳起石、黝帘石、绿帘石、方解石、榍石、金红石等矿物组成,变质温压条件为400℃±和7kbar±,属于高压变质作用。这是一条高压变质带,推测它可能向东与滇西的澜沧江高压变质带相连。冈玛日-桃形错地区含典型蓝闪石的蓝片岩的发现对羌塘陆块的重新认识以及冈瓦纳大陆与劳亚大陆的板块边界及晚古生代古特提斯洋的闭合,具有重要的大地构造意义。     

Sensitivity of seismic hazard evaluations to uncertainties determined from seismic source characterization

The sensitivity and overall uncertainty in peak ground acceleration (PGA)estimates have been calculated for the city of Tabriz, northwestern Iran byusing a specific randomized blocks design. Eight seismic hazard models andparameters with randomly selected uncertainties at two levels have beenconsidered and then a linear model between predicted PGA at a givenprobability level and the uncertainties has been performed. The inputmodels and parameters are those related to the attenuation, magnituderupture-length and recurrence relationships with their uncertainties.Application of this procedure to the studied area indicates that effects ofthe simultaneous variation of all eight input models and parameters on thesensitivity of the seismic hazard can be investigated with a decreasingnumber of computations for all possible combinations at a fixed annualprobability. The results show that the choice of a mathematical model ofthe source mechanism, attenuation relationships and the definition ofseismic parameters are most critical in estimating the sensitivity of seismichazard evaluation, in particular at low levels of probability of exceedance.The overall uncertainty in the expected PGA for an annual probability of0.0021 (10% exceedence in 50 yr) is expressed by a coefficient ofvariation (CV) of about 34% at 68% confidence level for a distance ofabout 5km from the field of the major faults. The CV will decrease withincreasing site-source distance and remains constant, CV = 15%, fordistances larger than 15 km. Finally, treating alternative models on theoverall uncertainty are investigated by additional outliers in input decision.     

基于生态足迹的中亚区域生态安全评价

利用生态足迹理论与方法,运用生态足迹压力指数模型,对中亚不同区域的生态安全进行了评价。分别测算中亚四国的人均与区域总生态足迹、生态承载力、生态赤字和生态足迹压力指数。最后按照生态足迹压力指数来评价中亚各国生态安全状态。评价结果如下:(1)吉尔吉斯坦14年平均生态赤字为0.0900ha/capita;平均生态足迹压力指数为0.9388,处于稍不安全状态。(2)土库曼斯坦的多年平均生态赤字为0.3303 ha/capita;平均生态足迹压力指数为1.1327;哈萨克斯坦的多年平均生态赤字为0.8379 ha/capita,平均生态足迹压力指数为1.2320,都处于较不安全状态 。(3)乌兹别克斯坦的区域总承载力由1994年的28807119ha下降到2005年的28527138ha,而区域总生态足迹有所增加,即从44598092ha增加到51761368ha;多年平均生态赤字为0.8181 ha/capita;平均生态足迹压力指数为1.7540,处于相对很不安全状态。根据计算结果,中亚4国的多年平均生态安全都处于不安全状态,但不安全程度有所差别,吉尔吉斯坦的生态安全处于相对较好状态,其次为土库曼斯坦,哈萨克斯坦;乌兹别克斯坦的生态安全相对其他中亚国家而言处于相对很不安全状态。