Modelling of Ocean Shallow Convection under Fast Ice in the Arctic

Ocean convection in the Antarctic has been studied many times and has been revealed to be responsible for ice-cover reduction. In the Arctic, proof of that phenomenon has not been documented. It is believed that this phenomenon happens on a smaller scale in the Arctic when local circulation of deep warmer water melts and slows ice production. An example of this is the North Water (NOW) polynya in northern Baffin Bay. A polynya is an area of open water in an otherwise ice-covered area. As ice forms under the fast ice near the boundary of the polynya, ocean salts (brine) are ejected from the newly formed ice. This water, which has an increased concentration of salt, sinks and is replaced by warmer water from below, and this slows ice formation. In our study a coupled one-dimensional thermodynamic snow–fast ice model incorporating ocean heat flux input via a shallow convection model was used. Ice thickness was calculated using a thermodynamic model that included a current-induced entrainment model and a convection model to account for brine rejection during ice growth. Atmospheric observations from Grise Fiord and Thule and ocean profiles around the NOW polynya near these sites were used as input to the model. This purely thermodynamic study enables us to obtain ice thickness values that can be compared with qualitative observations. This modelling study compares two sites related to the NOW polynya. The results indicate that the shallow convection model simulates the reduction of fast ice near Thule but not near Grise Fiord.     

Mid–Late Triassic metamorphic event for Changhai meta-sedimentary rocks from the SE Jiao–Liao–Ji Belt,North China Craton: Evidence from monazite U–Th–Pb and muscovite Ar–Ar dating

The precise constraints on the timing of metamorphism of the Changhai metamorphic complex is of great importance considering the prolonged controversial issue of the north margin and the extension of the Sulu–Dabie HP–UHP Belt. While the monazite U–Th–Pb and muscovite ⁴⁰Ar/³⁹Ar techniques are widely accepted as two of the most powerful dating tools for revealing the thermal histories of medium–low grade metamorphic rocks and precisely constraining the timing of metamorphism. The Changhai metamorphic complex at the SE Jiao–Liao–Ji Belt, North China Craton consists of a variety of pelitic schist and Grt–Ky-bearing paragneiss, and minor quartzite and marble. Analyses of mineral inclusions and back-scattered electric (BSE) images of monazites, combined with LA–ICP–MS U–Th–Pb ages for monazites and ⁴⁰Ar/³⁹Ar ages for muscovites, provide evidence of the origin and metamorphic age of the Changhai metamorphic complex. Monazites separates from various Grt–Mus schists and Grt–Ky–St–Mus paragneisses exhibit homogeneous BSE images from cores to rims, and contain inclusion assemblages of Grt + Mus + Qtz ± Ctd ± Ky in schist, and Grt + Ky + St + Mus + Pl + Kfs + Qtz inclusions in paragneiss. These inclusion assemblages are very similar to matrix minerals of host rocks, indicating they are metamorphic rather than inherited or detrital in origin. LA–ICP–MS U–Th–Pb dating reveals that monazites of schist and paragneiss have consistent ²⁰⁶Pb/²³⁸U ages ranging from 228.1 ± 3.8 to 218.2 ± 3.7 Ma. In contrast, muscovites from various schists show slightly older ⁴⁰Ar/³⁹Ar plateau ages of 236.1 ± 1.5 to 230.2 ± 1.2 Ma. These geochronological and petrological data conclude that the pelitic sediments have experienced a metamorphic event at the Mid–Late Triassic (236.1–218.2 Ma) rather than the Paleoproterozoic (1950–1850 Ma), commonly regarded as the Precambrian basement for the Jiao–Liao–Ji Belt. Hence, the Changhai metamorphic complex should be considered as a discrete lithotectonic group.This newly recognized Mid–Late Triassic metamorphic event (236.1–218.2 Ma) for the Changhai metamorphic complex is coeval with the HP–UHP metamorphic event (235–220 Ma) for Sulu–Dabie rocks. This leads us to speculate that the metamorphism of the Changhai complex belt along the SE margin of the North China Craton was genetically related to the Mid–Late Triassic collision of the North China and South China cratons. By the same token, the Sulu–Dabie HP–UHP Belt may have extended through Yantai, and the southern Yellow Sea, and to the southern side of the Changhai metamorphic complex.     

水泥土挡墙压顶梁对基坑变形影响研究

在分析水泥土挡墙变形机理基础上,对压顶梁的作用进行了研究,并深入探讨了压顶梁的有效利用范围和优化措施。工程实例表明,合适的压顶梁是控制挡墙变形的一种有效措施。     

钢板混凝土联肢剪力墙抗震性能试验研究

以轴压比和钢连梁弯剪比为主要参数,设计并制作了3个1/4缩尺的钢板混凝土联肢剪力墙子结构,对试件施加恒定轴压力和水平往复加载.观察了各试件破坏特征,对比了顶层水平荷载-侧移曲线和骨架曲线以及延性系数和等效黏滞阻尼系数等抗震性能指标.结果表明:高轴压比会使整个试件的延性变差;弯剪比越大试件耗能能力越好,弯剪比越小试件刚度...     

外包钢-混凝土组合梁与钢管混凝土柱连接节点的滞回性能研究

为研究外包钢-混凝土组合梁与钢管混凝土柱连接节点的抗震性能,基于外包钢-混凝土组合梁与钢管混凝土柱连接节点低周反复荷载作用下的试验结果和有限元的模拟,分析了两类试验节点的滞回特性,提出外包钢-混凝土组合梁与钢管混凝土柱连接节点的三折线恢复力模型及其特征参数的取值范围,并给出恢复力模型表达式。结果表明,试件具有良好的耗能能力,建立的恢复力模型骨架曲线与试验值接近;有限元与试验所得的滞回曲线及骨架曲线在弹性阶段吻合较好,随着荷载的反复,两者之间的差异逐渐增大。     

不同轴压比RCS梁柱组合件恢复力模型研究

以6个1/2模型RCS梁柱节点拟静力试验为基础,研究不同轴压比下RCS组合件的滞回性能。试验结果表明:梁铰破坏时试件的滞回曲线饱满,耗能能力优于构造破坏;随着轴压比的增大,试件滞回环愈加丰满,初始刚度有所增加,承载力有所增大;随着加载位移的增加,刚度退化速率变慢,且梁铰破坏时随着轴压比增大,刚度退化速率变大。基于试验结果和现有恢复力模型理论,建立的三折线骨架曲线模型与实际试验骨架曲线具有较高的吻合度,能较好的反映轴压比对其滞回特性的影响,可为该RCS梁柱组合件的弹塑性分析及工程应用提供参考。     

基于模型的3D3C采集设计与评价

对目前三维三分量(3D3C)地震勘探数据采集设计的方法进行了系统分析,利用聚焦射束分析技术(CFP),提出了基于模型的3D3C采集设计方法及其评价的参数。从模型出发的设计方法,能够使3D3C的采集结果更加真实可靠。     

Urban re-generations: afterword to special issue on the politics of urban greening in Australian cities

ABSTRACT

‘Urban re-generations' is written as an afterword to the special issue of Australian Geographer on ‘The Politics of Urban Greening in Australian Cities'. The collection prompts a deep questioning of reparative and regenerative work associated with greening, green spaces and green infrastructures. The climate-driven 2019-2020 bushfire crisis and COVID-19 have amplified the visibility of the more-than-human connectivity of our cities and the deep underlying structures of social and environmental inequity underpinning a variety of urban green spaces and agendas. Inspired by the articles in this special issue, the afterword explores how we might call back the grammars and practices of regeneration from their service to the neo-liberal, settler-colonial city and instead nurture reparative de-colonial practices that aid in the collaborative work of re-composing, becoming into better relation with, and working in modes of situated historical and cultural difference, with green and just cities.     

Modeling nonlinear wave–wave interactions with the elliptic mild slope equation

An approach is developed to simulate wave–wave interactions using nonlinear elliptic mild-slope equation in domains where wave reflection, refraction, diffraction and breaking effects must also be considered. This involves the construction of an efficient solution procedure including effective boundary treatment, modification of the nonlinear equation to resolve convergence issues, and validation of the overall approach. For solving the second-order boundary-value problem, the Alternating Direction Implicit (ADI) scheme is employed, and the use of approximate boundary conditions is supplemented, for improved accuracy, with internal wave generation method and dissipative sponge layers. The performance of the nonlinear model is investigated for a range of practical wave conditions involving reflection, diffraction and shoaling in the presence of nonlinear wave–wave interactions. In addition, the transformation of a wave spectrum due to nonlinear shoaling and breaking, and nonlinear resonance inside a rectangular harbor are simulated. Numerical calculations are compared with the results from other relevant nonlinear models and experimental data available in literature. Results show that the approach developed here performs reasonably well, and has thus improved the applicability of this class of wave transformation models.     

Origin of the alkali tonsteins from southwest China: Implications for alkaline magmatism associated with the waning stages of the Emeishan Large Igneous Province

A unique type of Nb–Zr–REE–Ga-enriched alkali tonstein of pyroclastic origin occurs exclusively within the late Permian coal measures of southwest China. The alkali tonsteins are located within the lowest Xuanwei or Longtan formations of Wuchiapingian age, indicating that their age is later than the main episode of Emeishan Large Igneous Province (ELIP) magmatism. The alkali tonsteins have intermediate–felsic Al₂O₃/TiO₂ values (12.6–34.2, mean 22.0), light rare earth element-enriched chondrite-normalised patterns, negative δEu and incompatible element ratios similar to those of ELIP alkaline Nb–Ta-enriched syenites. All available evidence shows that the alkali tonsteins from southwest China originated from coeval ELIP alkaline magmatism. The enrichment of Nb–Zr–REE–Ga in alkali tonsteins is derived from the ELIP alkaline Nb–Ta-enriched volcanic ashes and may represent the last stage of mineralisation associated with the Emeishan mantle plume activity.