Models on Snowball Earth and Cambrian explosion: A synopsis   总被引：5，自引：5，他引：0  

S. Maruyama  M. Santosh 《Gondwana Research》2008,14(1-2):22

During the late Proterozoic from 1000 to 542 Ma, the Earth is thought to have been frozen at least during two times: in the Sturtian (715–680 Ma) and in the Marinoan (680–635 Ma) global glaciations. Following the Marinoan Snowball Earth, large multi-cellular animals of the Ediacara fauna flourished as a prelude to the Phanerozoic world. Here we summarize the most popular models on the cause and cessation of Snowball Earth. Episodic decrease of greenhouse gas occurs through the effect of erosion and weathering promoted by either mountain building or by an increase in the coastlines during the break-up of supercontinents. Effects on the globe caused by true polar wander, eruption of voluminous flood basalts, or dramatic reduction in planetary obliquity can also lead to ice ages and mass extinction. A radically revised concept based on Earth's magnetic intensity has also been proposed, which explains the true polar wander through a quasi-polar dynamo model. The ‘switch-on’ and ‘switch-off’ of the Earth's strong dynamo can lead to the onset and disappearance of the Snowball Earth. The galactic model infers that gamma ray burst associated with starburst creates huge amounts of clouds which would cut off sun rays and freeze the Earth.The Snowball Earth event is considered to have exerted a significant control on the subsequent revolutionary changes in the evolution of life forms. Although according to the biological clock, extensive re-organisation of genome is thought to have been completed by around 900 Ma, the evolution of modern life in Cambrian occurred only after the geochemical bridge was in place with elevated oxygen and nutrient levels in lakes that developed within continental rifts where the hydrothermal system in the granitic basement created the chemical environment enriched in Ca²⁺, Fe²⁺, V, Mo, HCO₃, phosphate and other elements required for building the skeleton and bone of the first modern animals. With cosmic radiation exerting a significant control on the mutation, the Neoproterozoic Earth history illustrates the possible link from Galaxy to the genome level.  相似文献   

Magnetic fields in galaxy halos and their importance     

Ulrich Klein 《地球物理与天体物理流体动力学》2013,107(1-3):27-34

Abstract

The observational facts about magnetic fields in galactic halos are reviewed. The existence, origin and significance of poloidal field components are described. Observational evidence that magnetic fields channel winds from active galactic centres is discussed. Field strengths adduced from the radio polarizations of edge-on galaxies are given. Priorities for future research are suggested.  相似文献   

Non-local effects in the mean-field disc dynamo I. An asymptotic expansion     

Vladimir Priklonsky  Anvar Shukurov  Dmitry Sokoloff  Andrew Soward 《地球物理与天体物理流体动力学》2013,107(1-2):97-114

Abstract

We reconsider thin-disc global asymptotics for kinematic, axisymmetric mean-field dynamos with vacuum boundary conditions. Non-local terms arising from a small but finite radial field component at the disc surface are consistently taken into account for quadrupole modes. As in earlier approaches, the solution splits into a local part describing the field distribution along the vertical direction and a radial part describing the radial (global) variation of the eigenfunction. However, the radial part of the eigenfunction is now governed by an integro-differential equation whose kernel has a weak (logarithmic) singularity. The integral term arises from non-local interactions of magnetic fields at different radii through vacuum outside the disc. The non-local interaction can have a stronger effect on the solution than the local radial diffusion in a thin disc, however the effect of the integral term is still qualitatively similar to magnetic diffusion.  相似文献   

Assessing the ‘usability’ of climate change research for decision-making: A case study of the Canadian International Polar Year     

James D. Ford  Maggie Knight  Tristan Pearce 《Global Environmental Change》2013,23(5):1317-1326

The creation of ‘usable science’ is widely promoted by many environmental change focused research programs. Few studies however, have examined the relationship between research conducted as part of such programs and the decision-making outcomes that the work is supposed to advance, and is constrained by limited methodological development on how to empirically assess the ‘usability’ of science. Herein, this paper develops a conceptual model and assessment rubric to quantitatively and systematically evaluate the usability of climate change research for informing decision-making. We focus on the process through which data is collected, analyzed and reported and examine the extent to which key principles of usable science are integrated into project design, using grant proposals as our data source. The approach is applied to analyze climate change research conducted as part of the International Polar Year in Canada, with 23 projects identified as having explicit goals to inform decision-making.While the creation of usable science was promoted by funded projects in the International Polar Year, this was not generally reflected in research design: fewer than half determined objectives with input of decision makers, decision context was not widely considered, and knowledge users were not widely reported to be engaged in assessing the quality of data or in resolving conflict in evidence. The importance of science communication was widely emphasized, although only 8/23 projects discussed tailoring specific results for end user needs. Thus while International Polar Year research has made significant advances in understanding the human dimensions of Arctic climate change, key attributes necessary for determining success in linking science to decision-making (pertinence, quality, timeliness) were not captured by many projects. Integrating these attributes into research design from the outset is essential for creating usable science, and needs to be at the forefront of future research programs which aim to advance societal outcomes. The framework for assessing usability here, while developed and tested in an Arctic climate change context, has broader applicability in the general environmental change field.  相似文献   

The galactic dynamo: Axisymmetric and non-axisymmetric modes     

Yulia Krasheninnikova  Alexander Ruzmaikin  Dmitry Sokoloff  Anvar Shukurov 《地球物理与天体物理流体动力学》2013,107(1-3):131-146

Abstract

We discuss recent developments in the theory of large-scale magnetic structures in spiral galaxies. In addition to a review of galactic dynamo models developed for axisymmetric disks of variable thickness, we consider the possibility of dominance of non-axisymmetric magnetic modes in disks with weak deviations from axial symmetry. Difficulties of straightforward numerical simulation of galactic dynamos are discussed and asymptotic solutions of the dynamo equations relevant for galactic conditions are considered. Theoretical results are compared with observational data.  相似文献   

Ultrahigh energy nuclei in the turbulent Galactic magnetic field     

G. Giacinti  M. KachelrießD.V. Semikoz  G. Sigl 《Astroparticle Physics》2011,35(4):192-200

In this work we study how the turbulent component of the Galactic magnetic field (GMF) affects the propagation of ultrahigh energy heavy nuclei. We investigate first how the images of individual sources and of the supergalactic plane depend on the properties of the turbulent GMF. Then we present a quantitative study of the impact of the turbulent field on (de-)magnification of source fluxes, due to magnetic lensing effects. We also show that it is impossible to explain the Pierre Auger data assuming that all ultrahigh energy nuclei are coming from Cen A, even in the most favorable case of a strong, extended turbulent field in the Galactic halo.  相似文献   

我国极地科考事业发展与第26次南极科考成果   总被引：1，自引：0，他引：1  

吴荣荣  何剑锋  王建国 《上海国土资源》2010,31(3):83-90

我国极地科考事业走过了26年光辉历程,实现了跨越式发展,取得了大量具有国际领先水平的极地考察与研究成果,在国际极地事务中发挥着越来越重要的作用,有效争取和维护了我国在极地领域的权益。中国第26次南极科考自2009年10月11日启程,于2010年4月10日凯旋回沪,＂雪龙号＂极地科考船载249名队员,历时182天,圆满完成了59项科学调查与21项后勤保障任务,为国际极地年和中国行动计划提交了出色答卷。  相似文献   

银河系厚盘恒星的年龄-金属度关系     

赵君亮 《天文学进展》2005,23(4):346-354

对太阳邻域内银河系盘星、特别是厚盘恒星年龄-金属度关系的研究进展作了较为详细的回顾和评述；说明目前对盘星是否确实存在年龄-金属度关系还存在争议，而厚盘恒星可能存在这种关系。同时指出，要得出明晰的结论，在观测资料的基础上取得合理的恒星样本至关重要。  相似文献   

Stability of axial orbits in galactic potentials     

Cinzia Belmonte  Dino Boccaletti  Giuseppe Pucacco 《Celestial Mechanics and Dynamical Astronomy》2006,95(1-4):101-116

We investigate the dynamics in a galactic potential with two reflection symmetries. The phase-space structure of the real system is approximated with a resonant detuned normal form constructed with the method based on the Lie transform. Attention is focused on the stability properties of the axial periodic orbits that play an important role in galactic models. Using energy and ellipticity as parameters, we find analytical expressions of bifurcations and compare them with numerical results available in the literature.  相似文献   

Radial velocities andDDO, BV photometry of Henry Draper G5-M stars near the North Galactic Pole     

K. M. Yoss  R. F. Griffin 《Journal of Astrophysics and Astronomy》1997,18(2-3):161-227

Radial velocities are given for some 900 stars within 15‡ of the North Galactic Pole, including almost all such stars classified G5 or later in theHenry Draper Catalogue. Luminosities, two-dimensional spectral classes, composition indices, and distances are derived for the majority of the sample throughDDO andBV photometry. More than half of the stars are classified as G5-K5 giants: they show a clear relationship between composition and velocity dispersion for the two Galactic componentsV andW, and a less well-defined trend forU. Four abundance groups exhibit characteristics which imply association with, respectively, the thick disk, old thin disk, young thin disk, and Roman’s “4150” group. The sample is contained within l kpc of the Galactic plane, and no trends with distance are evident.  相似文献