A conceptual model for the longitudinal distribution of wood in mountain streams

Wood load, channel parameters and valley parameters were surveyed in 50 contiguous stream segments each 25 m in length along 12 streams in the Colorado Front Range. Length and diameter of each piece of wood were measured, and the orientation of each piece was tallied as a ramp, buried, bridge or unattached. These data were then used to evaluate longitudinal patterns of wood distribution in forested headwater streams of the Colorado Front Range, and potential channel‐, valley‐ and watershed‐scale controls on these patterns. We hypothesized that (i) wood load decreases downstream, (ii) wood is non‐randomly distributed at channel lengths of tens to hundreds of meters as a result of the presence of wood jams and (iii) the proportion of wood clustered into jams increases with drainage area as a result of downstream increases in relative capacity of a stream to transport wood introduced from the adjacent riparian zone and valley bottom. Results indicate a progressive downstream decrease in wood load within channels, and correlations between wood load and drainage area, elevation, channel width, bed gradient and total stream power. Results support the first and second hypotheses, but are inconclusive with respect to the third hypothesis. Wood is non‐randomly distributed at lengths of tens to hundreds of meters, but the proportion of pieces in jams reaches a maximum at intermediate downstream distances within the study area. We use these results to propose a conceptual model illustrating downstream trends in wood within streams of the Colorado Front Range. Copyright © 2009 John Wiley & Sons, Ltd.     

Grain-size distribution characteristics of red sandy sediments in Dongjiang River valley, southern Nanling Mountains, during the MIS2 stage

Layer LJ3 of Linjiang stratigraphic section in Dongjiang River valley in the south of the Nanling Mountains is a set of red sandy sediments.Measured by thermoluminescence(TL) dating,it was found to be formed in MIS2-9,500 ± 800 yr to 19,600 ± 1,800 yr B.P.After analy-sis of the grain sizes of the 16 samples(LJ3-100 to LJ3-85) in this layer,it was discovered that(1) The contents of each grain group in dif-ferent samples are similar.(2) The values of Md,Mz,,Sk,and Kg vary from LJ3-100 to LJ3-85 in a narrow range.(3) The segments of each sample in the accumulative curves extend parallel with similar slopes.All the three aspects reveal the Aeolian characteristics of Layer LJ3.Therefore,it is thought that Layer LJ3 consists of red sandy sediments formed in MIS2 in the south of Nanling Mountain,which reflects the arid climate at that time.     

SENSITIVITY OF AQUATIC ECOSYSTEMS TO CLIMATIC AND ANTHROPOGENIC CHANGES: THE BASIN AND RANGE,AMERICAN SOUTHWEST AND MEXICO

Variability and unpredictability are characteristics of the aquatic ecosystems, hydrological patterns and climate of the largely dryland region that encompasses the Basin and Range, American Southwest and western Mexico. Neither hydrological nor climatological models for the region are sufficiently developed to describe the magnitude or direction of change in response to increased carbon dioxide; thus, an attempt to predict specific responses of aquatic ecosystems is premature. Instead, we focus on the sensitivity of rivers, streams, springs, wetlands, reservoirs, and lakes of the region to potential changes in climate, especially those inducing a change in hydrological patterns such as amount, timing and predictability of stream flow. The major sensitivities of aquatic ecosystems are their permanence and even existence in the face of potential reduced net basin supply of water, stability of geomorphological structure and riparian ecotones with alterations in disturbance regimes, and water quality changes resulting from a modified water balance. In all of these respects, aquatic ecosystems of the region are also sensitive to the extensive modifications imposed by human use of water resources, which underscores the difficulty of separating this type of anthropogenic change from climate change. We advocate a focus in future research on reconstruction and analysis of past climates and associated ecosystem characteristics, long-term studies to discriminate directional change vs. year to year variability (including evidence of aquatic ecosystem responses or sensitivity to extremes), and studies of ecosystems affected by human activity. © 1997 John Wiley & Sons, Ltd.     

大兴安岭北段新林地区晚石炭世花岗岩的岩石成因及地质意义

大兴安岭北段新林地区晚古生代花岗岩主要出露在大乌苏和富西里附近,岩性主要为二长花岗岩,另有少量花岗闪长岩。对其中二长花岗岩样品进行LA-ICP-MS锆石U-Pb测年表明,大乌苏和富西里岩体侵位年龄分别为（303.7±2.2）和（300.5±0.5）Ma,均为晚石炭世岩浆活动的产物。花岗岩具有富硅（w（SiO₂）为66.77%~75.85%）、富碱（w（Na₂O+K₂O）为7.41%~8.69%）、高铝（w（Al₂O₃）为12.90%~16.22%）,低MgO、CaO、TiO₂的特点,属于钙碱性系列;铝饱和指数（A/CNK）为1.06~1.44,为过铝质岩石;镜下未见原生白云母、堇青石、石榴石等富铝矿物,不同于富铝的S型花岗岩;而w（P₂O₅）与w（SiO₂）负相关,呈I型花岗岩特征;富集LREE和Ba、Rb、K等大离子亲石元素,亏损Nb、Ta、Ti等高场强元素,与后造山I型花岗岩特征相似,应形成于拉张的构造环境。花岗岩的⁸⁷Sr/⁸⁶Sr为0.712 938、¹⁴³Nd/¹⁴⁴Nd为0.512 386,（⁸⁷Sr/⁸⁶Sr）_i值为0.704 4,ε_Nd（t）值为-1.09,T_DM2=1 172 Ma,源区物质主要为中-新元古代从亏损地幔增生的地壳物质。结合区域研究成果,大兴安岭新林地区晚石炭世岩浆侵位活动与额尔古纳-兴安地块和松嫩地块碰撞拼合后岩石圈伸展环境有关。     

南岭早侏罗世稀有金属成矿作用研究——以闽西南大坪花岗斑岩为例

华南地区是我国重要的稀有金属矿产区,绝大部分具经济规模的稀有金属矿床均与高演化的富Li-F花岗岩有成因联系。大坪花岗斑岩位于南岭构造带最东缘福建省永定地区,与区内Nb-Ta矿床形成有关。该岩体SIMS和LA-ICP-MS锆石U-Pb定年结果分别为186.7±1.2Ma和190.7±1.1Ma,是华南少见的早侏罗世(200~180Ma)侵入的花岗岩侵入体,也是华南最早报道的早侏罗世稀有金属成矿事件。大坪花岗斑岩具高钾低镁、准铝质到弱过铝质的特征,属于高钾钙碱性花岗岩,并显示A型花岗岩的地球化学特征,如富硅(72.81%~76.44%)、高10000×Ga/Al比值、高FeO^T/MgO和高的Zr+Nb+Ce+Y含量、亏损高场强元素和Eu负异常明显等。全岩体系低的Zr/Hf、Nb/Ta比值,指示岩浆具有较高的分异演化程度,Nb 2 O 5和Ta 2 O 5均含量达到了花岗岩型稀有金属矿床的工业品位。花岗斑岩中锆石Hf-O同位素分析结果显示,其具有比较亏损的Hf同位素与比较均一的O同为素组成(εHf(t)=-2.4~3.4,δ^18 O=6.0‰~6.6‰)。结合微量元素地球化学特征,大坪花岗斑岩源区主要来源于软流圈地幔,并有约20%~30%壳源岩浆的加入,在成岩过程中发生了显著的分离结晶。晚期富氟的流体出熔并向上迁移可能对于Nb和Ta的再次富集与分异具有重要作用。大坪花岗斑岩与闽西南地区同时期的火山岩,如藩坑组双峰式火山岩,在空间上可与前人提出的“南岭山脉早侏罗世发育的东西向裂谷岩浆岩带(OIB型玄武岩、辉长岩和A型花岗岩组合)”相对应,是该裂谷带向东的延伸。     

张广才岭地块早侏罗世晚期花岗闪长岩及其闪长质包体的岩石成因及构造意义

对松嫩-张广才岭地块早侏罗世晚期北长岗花岗闪长岩及其暗色包体开展了详细的岩石学,地球化学及年代学研究。结果表明,北长岗花岗闪长岩中含有大量闪长质暗色微粒包体,包体具有典型的岩浆结构,并发育针状磷灰石,同时斜长石斑晶从核部到边部显示复杂的成分和结构不平衡。寄主岩与包体均为准铝质,高钾钙碱性或高钾钙碱性-钙碱性过渡岩石,两者具有基本一致的稀土元素及微量元素组成,同时都具有正的εNd(t)和锆石εHf(t)值。主微量元素-SiO2协变图中,包体成分的两端分别向寄主花岗闪长岩及该区时代略早的基性岩延伸,三者具有良好的线性关系。以上特征表明北长岗花岗闪长岩中暗色包体为岩浆混合成因,且其原始岩浆可能为受到俯冲沉积物熔流体交代的软流圈地幔。寄主花岗闪长岩原岩主要为中-新元古代新生基性下地壳,并有少量亏损地幔物质的加入。寄主岩和暗色微粒包体具有基本一致的锆石U-Pb年龄(分别为173±2.0 Ma和173±1.5Ma),显示岩浆混合作用发生在早侏罗世晚期。区域构造演化显示,早侏罗世(～185 Ma),张广才岭地块处于古太平洋西-北西向俯冲到亚洲大陆之下形成的弧后伸展环境,北长岗花岗闪长岩及其暗色包体的形成暗示早侏罗世晚期(～173Ma)依然存在幔源岩浆活动,该区仍然处于弧后伸展环境,幔源物质的底侵诱发下地壳部分熔融形成研究区长英质岩浆,同时也说明该区早侏罗世晚期仍然存在地壳增生,且增生方式为垂向增生。     

1972-2016年东天山哈尔里克山地区冰川物质平衡研究

新疆哈密地区是世界上极端干旱的地区之一,依托高峻的哈尔里克山发育的现代冰川成为维系本区绿洲生存与发展的固体水库。对哈尔里克山地区冰川的监测与研究可获得本区冰川对全球气候变化的响应,也为哈密地区的水资源评估、生态建设与绿洲经济的可持续发展提供基础数据与理论支持。基于地形图、SRTM DEM和资源三号立体像对等数据,采用大地测量法对哈尔里克山地区自地形图成图以来44年间冰川物质平衡进行了研究。结果表明：1972-2016年间冰川平均表面高程下降了（9.15±0.98）m,冰川物质平衡为（-0.18±0.02）m·a^-1 w.e.,冰量损失显著。其中,1972-1999年间,冰川物质平衡为（-0.04±0.01）m·a^-1 w.e.,冰量损失较缓慢;1999-2016年间,冰川物质平衡增至（-0.36±0.05）m·a^-1 w.e.,冰量损失加剧。此外,南北坡的冰川物质平衡空间差异性明显。1972-1999年间,北坡冰川物质平衡为（-0.05±0.01）m·a^-1 w.e.,南坡冰川物质平衡为（-0.02±0.01）m·a^-1 w.e.,北坡冰川物质损失略大于南坡;1999-2016年间,北坡冰川物质平衡为（-0.27±0.04）m·a^-1 w.e.,南坡冰川物质平衡为（-0.45±0.05）m·a^-1 w.e.,南坡冰川损失量远大于北坡。结合哈密与伊吾气象观测资料分析可知：冰川的物质平衡与气温变化呈现出很强的相关性,研究区的降水量自20世纪50年代以来呈增加趋势,但温度变化对冰川物质平衡的影响大于略微增加的降水的影响。1999年后,南坡冬夏季气温的增长幅度均超过北坡,这是南坡冰量损失加剧的主因。     

大兴安岭海拉尔地区北秋叶山岩体的锆石U-Pb年龄及其地质意义

大兴安岭中生代花岗岩成因和构造背景的确定对讨论东北地区中生代构造-岩浆演化具有重要意义。笔者对大兴安岭中北段海拉尔盆地北缘的北秋叶山石英闪长岩岩体进行了锆石LA-ICP-MSU-Pb定年研究。结果显示:锆石具有清晰的生长振荡环带,其Th/U比值为0.64~1.16,属于典型的岩浆成因锆石;获得的206Pb/238U加权平均年龄为(227.6±1.2)Ma(n=25,MSWD=1.08),代表了北秋叶山岩体的结晶年龄,岩体的时代为晚三叠世,该岩体的形成可能与该区西侧的蒙古—鄂霍茨克洋演化相关。     

对“燕山早期花岗岩基印支期侵位的岩浆动力学证据及构造意义: 基于南岭8个岩体侵位年龄计算结果”的质疑

     基于构造学、岩石学和矿床学的地质事实,本文认为南岭地区燕山早期同造山花岗岩的岩浆形成、侵位和结晶在一 个很短的时间段内完成,其时间可以用锆石U-Pb年龄代表。因此, 章邦桐等（2014） 一文的结论应该是不成立的。     

Luminescence dating without sand lenses: An application of OSL to coarse-grained alluvial fan deposits of the Lost River Range,Idaho, USA

Optically stimulated luminescence (OSL) dating is increasingly used to estimate the age of fluvial deposits. A significant limitation, however, has been that conventional techniques of sampling and dose rate estimation are suitable only for thick (>60 cm) layers consisting of sand size or finer grains. Application of OSL dating to deposits lacking such layers remains a significant challenge. Alluvial fans along the western front of the Lost River Range in east-central Idaho, USA are one example. Deposits are typically pebble to cobble sheetflood gravels with a sandy matrix but thin to absent sand lenses. As a result, the majority of samples for this project were collected by excavating matrix material from gravelly deposits under light-safe tarps or at night. To examine the contributions of different grain-size fractions to calculated dose-rates, multiple grain-size fractions were analyzed using ICP–MS, high resolution gamma spectrometry and XRF. Dose rates from bulk sediment samples were 0.4–40% (mean of 18%) lower than dose-rate estimates from the sand-size fractions alone, illustrating the importance of representative sampling for dose rate determination. We attribute the difference to the low dose-rate contribution from radio-nuclide poor carbonate pebbles and cobbles that occur disproportionately in clast sizes larger than sand. Where possible, dose rates were based on bulk sediment samples since they integrate the dose-rate contribution from all grain sizes. Equivalent dose distributions showed little evidence for partial bleaching. However, many samples had significant kurtosis and/or overdispersion, possibly due to grain-size related microdosimetry effects, accumulation of pedogenic carbonate or post-depositional sediment mixing. Our OSL age estimates range from 4 to 120 ka, preserve stratigraphic and geomorphic order, and show good agreement with independent ages from tephra correlation and U-series dating of pedogenic carbonate. Furthermore, multiple samples from the same deposit produced ages in good agreement. This study demonstrates that with modified sampling methods and careful consideration of the dose rate, OSL dating can be successfully applied to coarse-grained deposits of climatic and tectonic significance that may be difficult to date by other methods.