三种裸子植物化石木的研究     

杨家驹  齐国凡  徐瑞瑚  杨礼茂 《国土资源》1996,(4)

本文研究了我国长江北岸湖北省武汉市新洲县阳逻镇北部晚第三纪的三种裸子植物化石木。它们是水松ＧｌｙｐｔｏｓｔｒｏｂｕｓｐｅｎｓｉｌｉｓＫ．Ｋｏｃｈ，武汉柏本ＣｕｐｒｅｓｓｕｓｗｕｈａｎｅｎｓｉｓＪ．ＪＹａｎｇｓｐ．ｎｏｖ．和武汉刺柏ＪｕｎｉｐｅｒｕｓｗｕｈａｎｅｎｓｉｓＪ．Ｊ．Ｙａｎｇｓｐ．ｎｏｖ，这些化石本的发现，给研究我国裸子植物提供了良好的依据．此外，对１９９０年发表于《林业科学》（２６卷４期）上的湖北柏木和河南松化石木也给了拉丁种名，它们分别是ＣｕｐｒｅｓｓｕｓｈｕｂｅｉｓｎｓｉｓＪ．Ｊ．Ｙａｎｇｓｐｎｏｖ．和ｐｉｎｕｓｈｅｎａｎｅｎｓｉｓＪ．Ｊ．Ｙａｎｇｓｐ．ｎｏｖ．  相似文献   

辽宁北部地区下白垩统木化石及其古气候意义     

丁秋红  李晓海  李文博  张志斌  陈树旺  张健  孙守亮  张渝金 《地质与资源》2022,31(1):13-20,12

对辽宁北部地区下白垩统沙海组木化石进行的木材解剖学及系统分类学研究显示,木化石均为硅化保存的松柏类裸子植物次生木质部,分别属于Protocupressinoxylon、Protopiceoxylon和Xenoxylon等3个分类群.当前木化石新材料生长轮清晰,早晚材比率在0.84～64之间,年平均敏感度在0.23～0....  相似文献   

Repeated high flows drive morphological change in rivers in recently deglaciated catchments     

Lawrence J. B. Eagle  Jonathan L. Carrivick  Alexander M. Milner  Lee E. Brown  Megan J. Klaar 《地球表面变化过程与地形》2021,46(7):1294-1310

Climate change is decreasing glacier cover and increasing the frequency and magnitude of precipitation-driven high flows and floods in many regions of the world. Precipitation may become the dominant water source for river systems in recently deglaciated catchments, with major rainfall events driving significant changes in river channel morphology. Few studies, however, have examined river channel response to repeated precipitation-driven high flows. In this study, we measured the geomorphological condition of four low-order rivers in recently deglaciated catchments (70–210 years ice free) before and after a series of repeated precipitation-driven high flows during summer 2014. High flows drove substantial initial morphological change, with up to 75% change in baseflow channel planform position and active channel form change from pre- to post-high flow. Post-high flow years were associated with increased instream wood and geomorphological complexity at all but the youngest river. Channel changes were part of an active relaxation stage at all rivers, where channels continued to migrate, and complexity varied through time. Overall, these measurements permit us to propose a conceptual model of the role of geomorphologically effective high flows in the context of paraglacial adjustment theory. Specifically, we suggest that older rivers in recently deglaciated catchments can undergo a short-term (<10 years) increase in the rate of geomorphological development as a result of the recruitment of instream wood and channel migration during and following repeated precipitation-driven high flows. Enhancing our knowledge of these geomorphological and paraglacial processes in response to high flows is important for the effective management of riverine water and ecosystem resources in rapidly changing environments.  相似文献   

中国古建筑简化分析研究   总被引：1，自引：1，他引：0  

高大峰  曹鹏男  丁新建 《地震工程与工程振动》2011,31(2):175-181

为研究中国古建筑的固有特性和抗震性能,根据西安城南门箭楼复原工程资料,采用半刚性弹簧单元来模拟实体结构的榫卯、斗拱和柱础连接,建立箭楼大木作构架的局部和整体两种有限元模型.采用Block Lanczos方法,分别进行模态分析,得出模型的前10阶频率和振型;采用瞬态分析方法,在El Centro波、Taft波和兰州人工波...  相似文献   

Genesis of the channel iron deposits (CID) of the Pilbara region,Western Australia     

R. C. Morris  E. R. Ramanaidou 《Australian Journal of Earth Sciences》2013,60(5):733-756

Miocene fluvial goethite/hematite channel iron deposits (CID) are part of the Cenozoic Detritals 2 (CzD2), of the Western Australian Pilbara region. They range from gravelly mudstones through granular rocks to intraformational pebble, cobble and rare boulder conglomerates, as infill in numerous meandering palaeochannels in a mature surface that includes Precambrian granitoids, volcanics, metasediments, BIF and ferruginous Palaeogene valley fill. In the Hamersley Province of the Pilbara, the consolidated fine gravels and subordinate interbedded conglomerates, with their leached equivalents, are a major source of export iron ore. This granular ore typically comprises pedogenically derived pelletoids comprising hematite nuclei and goethite cortices (ooids and lesser pisoids), with abundant coarser goethitised wood/charcoal fragments and goethitic peloids, minor clay, and generally minimal porous goethitic matrix, with late-stage episodic solution and partial infill by secondary goethite, silica and siderite (now oxidised) in places. Clay horizons and non-ore polymictic basal and marginal conglomerates are also present. The accretionary pedogenic pelletoids were mostly derived from stripping of a mature ferruginous but apparently well-vegetated surface, developed in the Early to Middle Miocene on a wide variety of susceptible rock types including BIF, basic intrusives and sediments. This deep ferruginisation effectively destroyed most remnants of the original rock textures producing a unique surface, very different to those that produced the underlying CzD1 (Palaeogene) and the overlying CzD3 (Pliocene – Quaternary). The peloids were derived both intraformationally from fragmentation and reworking of desiccated goethite-rich muds, and from the regolith. Tiny wood/charcoal fragments replaced in soil by goethite, and dehydrated to hematite, formed nuclei for many pelletoids. Additionally, abundant small (≤10 mm) fragments of wood/charcoal, now goethite, were probably replaced in situ within the consolidating CID. This profusion of fossil wood, both as pelletoid nuclei and as discrete fragments, suggests major episodic wild fires in heavily vegetated catchments, a point supported by the abundance of kenomagnetite – maghemite developed from goethite in the pelletoids, but less commonly in the peloids. The matrix to the heterogeneous colluvial and intraformational components is essentially goethite, primarily derived from modified chemically precipitated iron hydroxyoxides, resulting from leaching of iron-rich soils in an organic environment, together with goethitic soil-derived alluvial material. Major variations in the granular ore CID after deposition have resulted from intermittent groundwater flow in the channels causing dissolution and reprecipitation of goethite and silica, particularly in the basal CID zones, with surface weathering of eroded exposures playing a role in masking some of these effects. However, significant variations in rock types in both the general CID and the granular ore CID have also resulted from the effects of varied provenance.  相似文献   

Wood dynamics in upland streams under different disturbance regimes     

Leonora King  Marwan A. Hassan  Xiaohua Wei  Leif Burge  Xiaoyong Chen 《地球表面变化过程与地形》2013,38(11):1197-1209

This paper presents reach scale large wood (LW) budgets of 12 upland streams in the Okanagan Basin of British Columbia. The study included 100 m long reaches at three wildfire sites and three undisturbed sites in the Interior Douglas‐fir (IDF) biogeoclimatic zone, and three recent Mountain Pine Beetle (MPB) infestation sites and three undisturbed sites in the Montane Spruce (MS) zone. Detailed information on wood recruitment, output and storage were obtained from repeated annual surveys. Recruitment from the riparian zone was found to dominate wood inputs, with fluvial import of secondary importance. In undisturbed streams, wood exhumation was found to be of tertiary importance, but was not observed in disturbed streams. Relative wood length was found to be a strong predictor of wood stability, with wood length to channel width and wood diameter to channel depth ratios of 1:1 forming an approximate maximum threshold of wood mobility. Volumetric decomposition was, on average, a third of the value of fluvial export, and the average residence time of wood in the channels was 20 years. In undisturbed reaches, wood storage indicated a slow depletion of wood from the channels. In the disturbed reaches, wildfire was found to significantly increase annual wood recruitment by more than an order of magnitude over undisturbed or control streams. MPB had not significantly increased LW recruitment, but is expected to increase over the coming decades. Storage rates at the disturbed streams indicated a net accumulation of wood over the study period. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

A component approach for the hysteretic behaviour of connections in cross‐laminated wooden structures     

Giovanni Rinaldin  Claudio Amadio  Massimo Fragiacomo 《地震工程与结构动力学》2013,42(13):2023-2042

In this paper, a numerical model to estimate the dissipative capacity and describe the cyclic response of cross‐laminated (X‐lam) timber buildings is presented. The connections between panels and to the foundation (metal hold‐downs and angle brackets, and screwed connectors) are modelled with nonlinear hysteretic multispring elements taking into account the strength interaction between different degrees of freedom according to a predefined domain. The timber components (solid X‐lam floors and wall panels) are modelled using elastic shell elements. By calibration on experimental cyclic tests carried out on each degree of freedom, important features of timber connection behaviour such as post‐peak strength, pinching and stiffness degradation can all be considered. In addition, the effect of friction at the interface between panels and with foundation can be taken into account. These springs have been implemented as external subroutines in a widespread software package such as Abaqus. By comparison with the experimental results of cyclic tests carried out on single X‐lam walls, coupled X‐lam walls and a single‐storey X‐lam building, the accuracy of the proposed model is demonstrated. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

Organic carbon export in the form of wood during an extreme tropical storm,Upper Rio Chagres,Panama     

Ellen Wohl  Fred L. Ogden 《地球表面变化过程与地形》2013,38(12):1407-1416

Widespread, intense rainfall over the Upper Rio Chagres watershed (414 km²) in central Panama during December 2010 triggered numerous landslides that introduced large numbers of trees to the river network. We flew by helicopter along the mainstem Upper Chagres and the adjacent margins of Lake Alhajuela, into which the Upper Chagres flows, in February and June 2011. We used low‐elevation video photography from these flights to tally the number of wood pieces stored along the lake margin and within the channel, and the number of landslides reaching the mainstem. We used these tallies with ground‐verified estimates of average wood piece size and landslide surface area, and assumptions about wood density, carbon content, and aboveground biomass, to develop a first‐order estimate of carbon export in the form of wood from the Upper Chagres following the 2010 storms. Based on the wood tally, we estimate 9 · 6 to 16 Mg C/km² export, and from the landslide tally we estimate 24 Mg C/km². We believe the landslide tally provides a more accurate minimum estimate of carbon export from the Upper Chagres during the December 2010 storms. These values are an order of magnitude higher than limited data for average or background rates of wood‐based carbon export from other catchments, but two orders of magnitude lower than wood‐based carbon export during extreme storms in Taiwan. The findings suggest that duration of flood flow above a threshold for mobilizing wood within the channel network exerts a more important control on wood export from the Upper Chagres than magnitude of flood peak. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

Geomorphic and stream flow influences on large wood dynamics and displacement lengths in high gradient mountain streams (Chile)          下载免费PDF全文

Andrés Iroumé  Virginia Ruiz‐Villanueva  Luca Mao  Guillermo Barrientos  Markus Stoffel  Gastón Vergara 《水文研究》2018,32(17):2636-2653

Understanding large wood (LW; ≥1 m long and ≥10 cm in diameter) dynamics in rivers is critical for many disciplines including those assessing flood hazard and risk. However, our understanding of wood entrainment and deposition is still limited, mainly because of the lack of long‐term monitoring of wood‐related processes. The dataset presented here was obtained from more than 8 years of monitoring of 1,264 tagged wood pieces placed in 4 low‐order streams of the Chilean mountain ranges and was used to further our understanding of key factors controlling LW dynamics. We show that LW displacement lengths were longer during periods when peak‐flow water depths (H_max) exceeded the bankfull stage (H_Bk) than in periods with H_max ≤ H_Bk and that these differences were significantly higher for smaller wood pieces. LW length and length relative to channel dimensions were the main factors governing LW entrainment; LW displacement lengths were inversely related to the ratio of piece length to H_15% (i.e., the level above which the flow remains for 15% of the time) and to the ratio of H_15% to bankfull width. Unrooted logs and LW pieces located at the bankfull stage travelled significantly longer distances than logs with attached rootwads and those located in other positions within the bankfull channel. A few large logjams were broken during the period of observation, and in all occasions, LW from these broken logjams did not travel over longer distances than other pieces of LW moved in the same periods and in the same stream segments. Most importantly, our work reveals that LW dynamics tend to be concentrated within a few reaches in each stream and that reaches exhibiting high wood dynamics (extensive entrainment, deposition, or repositioning of LW) are significantly wider and less steep than less dynamic reaches.  相似文献   

Uniform hazard versus uniform risk bases for performance‐based earthquake engineering of light‐frame wood construction     

Yue Li  Yuejun Yin  Bruce R. Ellingwood  William M. Bulleit 《地震工程与结构动力学》2010,39(11):1199-1217

This paper investigates the implications of designing for uniform hazard versus uniform risk for light‐frame wood residential construction subjected to earthquakes in the United States. Using simple structural models of one‐story residences with typical lateral force‐resisting systems (shear walls) found in buildings in western, eastern and central regions of the United States as illustrations, the seismic demands are determined using nonlinear dynamic time‐history analyses, whereas the collapse capacities are determined using incremental dynamic analyses. The probabilities of collapse, conditioned on the occurrence of the maximum considered earthquakes and design earthquakes stipulated in ASCE Standard 7‐05, and the collapse margins of these typical residential structures are compared for typical construction practices in different regions in the United States. The calculated collapse inter‐story drifts are compared with the limits stipulated in FEMA 356/ASCE Standard 41‐06 and observed in the recent experimental testing. The results of this study provide insights into residential building risk assessment and the relation between building seismic performance implied by the current earthquake‐resistant design and construction practices and performance levels in performance‐based engineering of light‐frame wood construction being considered by the SEI/ASCE committee on reliability‐based design of wood structures. Further code developments are necessary to achieve the goal of uniform risk in earthquake‐resistant residential construction. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献