Paleolimnological investigations of anthropogenic environmental change in Lake Tanganyika: VI. Geochemical indicators

Geochemical signals of bulk sedimentary organic matter from three cores from Lake Tanganyika provided information about both internal processes and terrestrial inputs to the lake. Indications of land use change were detected in the geochemical records of the watersheds, and the timing of these changes was consistent with historical records of population demographics. While C:N ratios suggested that the distance from shore might be important in influencing the relative amount of allochthonous vs. autochthonous material, all cores were dominated by autochthonous organic matter. In general, nitrogen isotopes were more positive at disturbed sites, indicating inputs of enriched soil nitrate that was subsequently taken up by phytoplankton. In contrast, carbon isotopes did not reflect land use patterns, and a post-1950s decline in carbon isotope ratios found in all cores may indicate a lake-wide decrease in productivity. These interpretations were consistent with pollen and climate records.     

羌塘雁石坪中侏罗统雀莫错组地层及沉积环境

本文详细描述了雁石坪附近一段地层剖面,并根据地层的岩石组合特征将其归入雀莫错组;依据古生物化石特征,将其时代划为侏罗纪早巴通期。沉积环境分析表明,雁石坪剖面为一套海侵的半咸水潮坪相沉积。     

Late Paleozoic volcanic rocks of the Intra-Sudetic Basin, Bohemian Massif: petrological and geochemical characteristics

Late Paleozoic volcanic rocks in the Intra-Sudetic Basin of the Bohemian Massif in the Czech Republic can be subdivided into two series: (I) a minor bimodal trachyandesite-rhyolite series of Upper Carboniferous age with initial ⁸⁷Sr/⁸⁶Sr of ca. 0.710 and ε_Nd values of −6.1 also characteristic of volcanics of the near Krkonoše Piedmont Basin (0.707 and −6.0, Ulrych et al., 2003) and (II) a major differentiated basaltic trachyandesite-trachyandesite-trachyte-rhyolite series of Lower Permian age with lower initial ⁸⁷Sr/⁸⁶Sr of ca. 0.705-0.708 and ε_Nd values ranging from −2.7 to −3.4/−4.1/. The newly recognized volcanic rocks of trachytic composition indicate that the rocks were formed by magmatic differentiation of similar parental melts rather than constituting a bimodal mafic-felsic sequence from different sources. Both series are generally of subalkaline affinity and calc-alkaline character with some tholeiitic tint (FeO/MgO vs. SiO₂, presence of orthopyroxene). The magmatic activity occurred in cycles in a layered chamber, each starting primarily with felsic volcanics and ending with mafic ones. The mafic rocks represent mantle-melt(s) overprinted by crust during assimilation-fractional crystallization. The Sr-Nd isotopic data confirm a significant crustal component in the volcanic rocks that may have been inherited from the upper mantle source and/or from assimilation of older crust during magmatic underplating and shallow-level melt fractionation.     

Determining the equivalent permeability tensor for fractured rock masses using a stochastic REV approach: Method and application to the field data from Sellafield,UK

A numerical procedure to determine the equivalent permeability tensor of a fractured rock is presented, using a stochastic REV (Representative Elementary Volume) concept that uses multiple realizations of stochastic DFN (Discrete Fracture Network) models. Ten square DFN models are generated using the Monte Carlo simulations of the fracture system based on the data obtained from a site characterization program at Sellafield, Cumbria, UK. Smaller models with varying sizes of from 0.25 m×0.25 m to 10 m×10 m are extracted from the generated DFN models and are used as two-dimensional geometrical models for calculation of equivalent permeability tensor. The DFN models are also rotated in 30º intervals to evaluate the tensor characteristics of calculated directional permeability. Results show that the variance of the calculated permeability values decreases significantly as the side lengths of the DFN models increase, which justifies the existence of a REV. The REV side length found in this analysis is about 5 m and 8 m with 20% and 10% acceptable variations, respectively. The calculated directional permeability values at the REV size have tensor characteristic that is confirmed by a close approximation of an ellipse in a polar plot of the reciprocal of square roots of the directional permeability.

塔里木盆地晚泥盆世-早石炭世东河砂岩沉积相

东河砂岩是塔里木盆地主要的勘探目的层和产油层之一。经过多年艰苦勘探和多学科综合研究,现已认识到东河砂岩为晚泥盆世晚期至早石炭世早期海平面上升背景下沉积的一套海侵底砂（砾）岩,在盆地范围内是一个明显的穿时沉积体。东河砂岩以滨浅海相陆源碎屑沉积占优势,局部发育海陆过渡相陆源碎屑沉积。根据盆地内100多口井资料的岩石学特征、沉积特征以及其它指相标志的综合分析,在东河砂岩中识别出滨岸、河口湾、辫状河三角洲以及冲积扇等不同的沉积相类型,其中高能碎屑滨岸相最重要,分布最广。东河砂岩沉积早期和中期,盆地主要为高能碎屑滨岸相和碎屑陆棚相沉积,末期海侵达到最大,盆地内为碳酸盐台地沉积,而轮南及盆地北部一带受物源区的影响,出现混积滨岸和碎屑滨岸沉积。东河砂岩明显具有填平补齐的沉积特征,其砂体厚度在缓坡处减薄、陡坡处增厚、遇孤岛减薄或尖灭,这些沉积特征为形成东河砂岩非构造圈闭奠定了良好的地质基础。满加尔、轮南、塔中、玛扎塔格以及草湖地区都有可能找到该类油气藏（田）,展示了东河砂岩非构造圈闭的良好勘探前景。     

Syntectonic emplacement of the Middle Jurassic Concón Mafic Dike Swarm, Coastal Range, central Chile (33° S)

The Concón Mafic Dike Swarm (CMDS) consists of basaltic to andesitic dikes emplaced into deformed Late Paleozoic granitoids during the development of the Jurassic arc of central Chile. The dikes are divided into an early group of thick dikes (5–12 m) and a late group of thin dikes (0.5–3 m). Two new amphibole ⁴⁰Ar/³⁹Ar dates obtained from undeformed and deformed dikes, constrain the age of emplacement and deformation of the CMDS between 163 and 157 Ma. Based on radiometric ages, field observations, AMS studies and petrographic data, we conclude that the emplacement of the CMDS was syntectonic with the Jurassic arc extension and associated with sinistral displacements along the NW-trending structures that host the CMDS. The common occurrence of already deformed and rotated xenoliths in the dikes indicates that deformation in the granitoids started previously.The early thick dikes and country rocks appear to have been remagnetized during the exhumation of deep-seated coastal rocks in the Early Cretaceous (around 100 Ma). The remanent magnetization in late thin dikes is mainly retained by small amounts of low-Ti magnetite at high temperature and pyrrhotite at low temperature. The magnetization in these dikes appears to be primary in origin. Paleomagnetic results from the thin dikes also indicate that the whole area was tilted  23° to the NNW during cooling of the CMDS.The NNW–SSE extension vectors deduced from the paleomagnetic data and internal fabric of dikes are different with respect to extension direction deduced for the Middle–Late Jurassic of northern Chile, pointing to major heterogeneities along the margin of the overriding plate during the Mesozoic or differences in the mechanisms driving extension during such period.     

鄂尔多斯盆地东北缘中侏罗统延安组植物群与古气候分析

鄂尔多斯盆地东北缘中侏罗统延安组为一套以河流滨湖相沉积为主的中粒中细粒长石砂岩、粉砂岩(泥质粉砂岩)、黑色页岩、粘土岩夹煤层。其中产大量的植物化石,计20属39种。经分析认为,这个植物群是一个属于我国北方区以Coniopteris-Phoenicopsis为代表的中侏罗世早期植物群,植物群组合反映鄂尔多斯盆地东北缘中侏罗世为偏潮湿的暖温带气候。     

冬小麦晚霜冻害时空分布与多时间尺度变化规律分析

冬小麦晚霜冻害主要发生在中国的黄淮区域,河南占黄淮区域面积较大,是中国小麦的主要产地,又是晚霜冻害的重发区,因此文章以河南为例,从构成晚霜冻害的最低温度和小麦发育期两个因素出发,提出了晚霜冻害指数构建方法,使晚霜冻害指标定量化,并从多年数据库中计算出逐年晚霜冻害发生强度和发生天数.由于小麦发育期观测资料序列较短,影响了大尺度小麦晚霜冻害变化规律的分析,因而提出了冬小麦发育期资料推算方法;对计算出的近50年晚霜冻害资料进行EOF分析和Morlet小波分析,以发现晚霜冻害的时空分布与多时间尺度变化规律.结果表明:冬小麦晚霜冻害发生频率较高,高发区频率超过40%,且在20世纪70-90年代有增多的趋势;发生强度和天数变化趋势基本相似,具有发生强度较重区对应着发生天数较多区的趋势,且前者对小麦产量的影响大于后者;晚霜冻害发生强度的重发区分布在豫东南和豫西南,轻发区主要分布在淮河以南及豫西部分地区,其中第1模态分布型存在16年、第2模态分布型存在准2年的多时间尺度变化规律;发生天数的多发区主要分布在豫北北部和豫西南边界,少发区在豫东北及豫西南中部,其中第1模态分布型存在着准4年、第2模态分布型存在准3年和准6-7年的周期.     

Quaternary uplift of northern England

Upland flats, attributable to erosion, have long been recognised in the landscape of the Lake District region of NW England, at altitudes of up to ~ 800 m O.D. Extrapolation using uplift rates derived from dated Pleistocene sites (karstic caves and other features) in the adjacent Pennine uplands suggests that if this succession of flats formed close to sea-level they date from the Middle Pliocene onwards, indicating a subsequent time-averaged uplift rate of almost 0.3 mm a^− 1. Numerical modelling indicates that erosion of surrounding areas at a typical rate of 0.2 mm a^− 1 since 3.1 Ma could have caused this uplift, as well as constraining the local effective viscosity of the lower crust as ~ 4 × 10¹⁸ Pa s and the typical local Moho temperature as ~ 650 °C. It is thus feasible that most of the topography of northern England has developed since the Middle Pliocene, as a consequence of coupling between erosion and the resulting induced flow in the lower continental crust. The much faster vertical crustal motions indicated in this part of northern England, compared with SE England, are thus mainly a consequence of much greater mobility of the lower crust in the north, due to its younger thermal age and the heating effect of radioactive Palaeozoic granites. Uplift of this magnitude, which has previously gone unrecognised, may have affected post-Pliocene global climate.     

Late Pleistocene and Holocene environments in the Nile basin

Owing to the very gently sloping nature of the flood plain in the lower White Nile valley, which is underlain by a former lake-bed, the depositional record in that area is unusually well preserved. In Egypt and along the Blue Nile phases of erosion have destroyed segments of the sedimentary record, but the White Nile sequence is a good proxy for both the main Nile and the Blue Nile. During the last 15 ka, at least, times of high flow in the Blue Nile and main Nile were synchronous with those in the White Nile.Not all the White Nile flood deposits have been preserved but calibrated radiocarbon dates obtained on fossil freshwater and amphibious Pila shells and fish bones indicate that White Nile levels were high around 14.7–13.1 ka, 9.7–9.0 ka, 7.9–7.6 ka, 6.3 ka and 3.2–2.8 ka. The Blue Nile record is more fragmentary and that of the main Nile even more so except for the Holocene Nile delta. Calibrated radiocarbon ages for high Blue Nile flows indicate very high flood levels towards 13.9–13.2 ka, 8.6 ka, 7.7 ka and 6.3 ka.Incision by the Blue Nile and main Nile has caused progressive incision in the White Nile amounting to at least 4 m since the terminal Pleistocene  15 ka ago and at least 2 m over the past 9 ka. The Blue Nile seems to have cut down at least 10 m since  15 ka and at least 4 m since 9 ka. The time-transgressive and relatively late inception of plant domestication in the Nile valley may partly reflect this history of incision. Nile incision would propagate upstream into the White Nile valley, draining previously swampy areas along the valley floor, which would then become accessible to cultivation.