In-phase anomalies in Beryllium-10 production and palaeomagnetic field behaviour during the Iceland Basin geomagnetic excursion     

Mads Faurschou Knudsen  Gideon M. Henderson  Martin Frank  Conall Mac Niocaill  Peter W. Kubik 《Earth and Planetary Science Letters》2008,265(3-4):588-599

Increases in the production rate of cosmogenic radionuclides associated with geomagnetic excursions have been used as global tie-points for correlation between records of past climate from marine and terrestrial archives. We have investigated the relative timing of variations in ¹⁰Be production rate and the corresponding palaeomagnetic signal during one of the largest Pleistocene excursions, the Iceland Basin (IB) event (ca. 190 kyr), as recorded in two marine sediment cores (ODP Sites 1063 and 983) with high sedimentation rates. Variations in ¹⁰Be production rate during the excursion were estimated by use of ²³⁰Th_xs normalized ¹⁰Be deposition rates and authigenic ¹⁰Be/⁹Be. Resulting ¹⁰Be production rates are compared with high-resolution records of geomagnetic field behaviour acquired from the same discrete samples. We find no evidence for a significant lock-in depth of the palaeomagnetic signal in these high sedimentation-rate cores. Apparent lock-in depths in other cores may sometimes be the result of lower sample resolution. Our results also indicate that the period of increased ¹⁰Be production during the IB excursion lasted longer and, most likely, started earlier than the corresponding palaeomagnetic anomaly, in accordance with previous observations that polarity transitions occur after periods of reduced geomagnetic field intensity prior to the transition. The lack of evidence in this study for a significant palaeomagnetic lock-in depth suggests that there is no systematic offset between the ¹⁰Be signal and palaeomagnetic anomalies associated with excursions and reversals, with significance for the global correlation of climate records from different archives.  相似文献   

Vegetation and topographic controls on sediment deposition and storage on gully beds in a degraded mountain area   总被引：1，自引：0，他引：1  

Armando Molina  Gerard Govers  Felipe Cisneros  Veerle Vanacker 《地球表面变化过程与地形》2009,34(6):755-767

Active gully systems developed on highly weathered or loose parent material are an important source of runoff and sediment production in degraded areas. However, a decrease of land pressure may lead to a return of a partial vegetation cover, whereby gully beds are preferred recolonization spots. Although the current knowledge on the role of vegetation on reducing sediment production on slopes is well developed, few studies exist on the significance of restoring sediment transport pathways on the total sediment budget of degraded mountainous catchments. This study in the Ecuadorian Andes evaluates the potential of vegetation to stabilize active gully systems by trapping and retaining eroded sediment in the gully bed, and analyses the significance of vegetation restoration in the gully bed in reducing sediment export from degraded catchments. Field measurements on 138 gully segments located in 13 ephemeral steep gullies with different ground vegetation cover indicate that gully bed vegetation is the most important factor in promoting short‐term (1–15 years) sediment deposition and gully stabilization. In well‐vegetated gully systems ( ≥ 30% of ground vegetation cover), 0.035 m³ m^–1 of sediment is deposited yearly in the gully bed. Almost 50 per cent of the observed variance in sediment deposition volumes can be explained by the mean ground vegetation cover of the gully bed. The presence of vegetation in gully beds gives rise to the formation of vegetated buffer zones, which enhance short‐term sediment trapping even in active gully systems in mountainous environments. Vegetation buffer zones are shown to modify the connectivity of sediment fluxes, as they reduce the transport efficiency of gully systems. First calculations on data on sediment deposition patterns in our study area show that gully bed deposition in response to gully bed revegetation can represent more than 25 per cent of the volume of sediment generated within the catchment. Our findings indicate that relatively small changes in landscape connectivity have the potential to create strong (positive) feedback loops between erosion and vegetation dynamics. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

A new multiscale scheme for computing statistical moments in single phase flow in heterogeneous porous media     

Marcio R. Borges  Márcio A. Murad  Felipe Pereira  Frederico Furtado 《Advances in water resources》2009

In this work we develop a new multiscale procedure to compute numerically the statistical moments of the stochastic variables which govern single phase flow in heterogeneous porous media. The technique explores the properties of the log-normally distributed hydraulic conductivity, characterized by power-law or exponential covariances, which shows invariance in its statistical structure upon a simultaneous change of the scale of observation and strength of heterogeneity. We construct a family of equivalent stochastic hydrodynamic variables satisfying the same flow equations at different scales and strengths of heterogeneity or correlation lengths. Within the new procedure the governing equations are solved in a scaled geology and the numerical results are mapped onto the original medium at coarser scales by a straightforward rescaling. The new procedure is implemented numerically within the Monte Carlo algorithm and also in conjunction with the discretization of the low-order effective equations derived from perturbation analysis. Numerical results obtained by the finite element method show the accuracy of the new procedure to approximated the two first moments of the pressure and velocity along with its potential in reducing drastically the computational cost involved in the numerical modeling of both power-law and exponential covariance functions.  相似文献   

The story of galaxy evolution in full colour     

Richard S Ellis  Roberto G Abraham  Jarle Brinchmann  Felipe Menanteau 《Astronomy& Geophysics》2000,41(2):2.10-2.16

A Holy Grail of modern astronomy is understanding the origin of Edwin Hubble's morphological sequence of galaxy types. What made some collapsing gas clouds turn into elegant spiral systems like our own Milky Way, whereas others became smooth, featureless ellipticals? More fundamentally, does the taxonomic scheme introduced by Hubble in the 1920s have any physical relevance? The Space Telescope that bears Hubble's name is providing answers to these and other questions in the context of modern theories of structure formation.
Richard S Ellis, Roberto G Abraham, Jarle Brinchmann and Felipe Menanteau show how modern telescopes producing colourful images are pointing the way to understanding galaxy evolution.  相似文献   

Quantum Instability of Magnetized Stellar Objects     

R. González Felipe  H. J. Mosquera Cuesta  A. Pérez Martínez  H. Pérez Rojas 《中国天文和天体物理学报》2005,5(4):399-411

The equations of state for degenerate electron and neutron gases are studied in the presence of magnetic fields. After including quantum effects in the investigation of the structural properties of these systems, it is found that some hypermagnetized stars can be unstable according to the criterion of stability of pressures. Highly magnetized white dwarfs should collapse producing a supernova type Ia, while superstrong magnetized neutron stars cannot stand their own magnetic field and must implode, too. A comparison of our results with a set of the available observational data of some compact stars is also presented, and the agreement between this theory and observations is verified.  相似文献   

(Super) cosmological solution for the Kasner model     

A. Macías  O. Obregón 《Astrophysics and Space Science》1988,148(2):297-304

In the theory of supergravity (N=1), the supersymmetric version of general relativity, and for the Kasner cosmological model (Bianchi type I) we find a non-trivial solution (for the metric and spinor-vector) under the most simple assumption =₁₁ + ₂₂; ₁₂+₂₁=0 and for a special choosed gaugeN=1,N ^j=0, ₀=0. This method could be also applied to other cosmological metrics and extended to enlarged Grassmann basis.O. Obregón was partially supported by the Alexander von Humboldt Stiftung.  相似文献   

Late Cenozoic magnetostratigraphy (11 – 0 Ma) of the Dongshanding and Wangjiashan sections in the Longzhong Basin,western China     

Li  Ji-Jun  Fang  Xiao-Min  Van der Voo  Rob  Zhu  Jun-Jie  Mac Niocaill  Conall  Cao  Ji-Xiu  Zhong  Wei  Chen  Huai-Lu  Wang  Jianli  Wang  Jian-Ming  Zhang  Yie-Chun 《Geologie en Mijnbouw》1997,76(1-2):121-134

A paleomagnetic study of the 510-m-thick Wangjiashan section of Late Miocene and Pliocene terrestrial sediments reveals a fairly complete reversal record with ages from 11 to 1.8 Ma. The magnetostratigraphy of the Dongshanding section, located nearby, reveals a partially overlapping reversal record with ages from 2.2 to 0 Ma, and facilitates correlation of the Wangjiashan section with the global polarity time scale. A new stratigraphic division of the Wangjiashan section replaces the name Linxia formation by five new formation names, based on lithologic variation and mammalian fossil finds. The new formations and their magnetostratigraphically determined ages are: Dongshan Formation (c. 1.75–2.6 Ma), Jishi Fm. (c. 2.6–3.6 Ma), Hewangjia Fm. (4.5–6.0 Ma), Liushu Fm. (6.0–7.6 Ma), and Dongxiang Fm. (7.6–c. 12 Ma). The Neogene stratigraphy and fossil mammals suggest that the nearby part of the Tibetan Plateau experienced a persistent denudation during the Late Miocene and Early Pliocene, but that it was uplifted more rapidly at about 3.6 Ma.  相似文献   

3D modelling of hydrothermal alteration associated with VHMS deposits in the Kristineberg area,Skellefte district,northern Sweden     

Riia M. Chmielowski  Nils Jansson  Mac Fjellerad Persson  Pia Fagerström 《Mineralium Deposita》2016,51(1):113-130

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Degradation of rope properties under increasing monotonic load     

Juan Felipe Beltran  Eric B. Williamson 《Ocean Engineering》2005,32(7):826-844

This paper describes the development of a computational model to predict the response of synthetic-fiber ropes under both monotonic and cyclic loads. The model addresses the issue of damage to a rope cross-section and its effect on rope response. Isotropic damage is assumed and is therefore characterized by a scalar quantity known as the damage index. The damage index is used to quantify the amount of deterioration that takes place in a damaged rope throughout its loading history. For the current study, data obtained from static tension tests are used to determine the evolution relationship for the damage index. This relationship is based on the stiffness ratio between experimental data and simulation of rope response without any source of damage and is assumed to be described by an exponential equation. Numerical simulations of damaged rope behavior compare well with experimental results.  相似文献   

Chlorophyll a Biomass of Netplankton in Surface Waters in the Pacific Sector of the Southern Ocean in Austral Summer     

Dong-Hoon Han  Masayuki Mac Takahashi 《Journal of Oceanography》2001,57(5):581-592

Chlorophyll a of total and particles retained on 30 μm mesh plankton net were both determined in surface waters along two cruise tracks ranging from the Subtropical water to the marginal ice zone in the Pacific sector of the Southern Ocean in austral summer. Total surface chlorophyll a in the study area was mostly less than 1 μg chl a 1⁻¹, and showed distributions with no obvious trend associated with different waters masses of the Antarctic and the Subantarctic, although total chlorophyll a concentrations changed greatly within each water mass. Particularly low concentrations of chlorophyll a were detected in the marginal ice zone. Chlorophyll a contained in 30 μm netplankton made up 5∼60% of total chlorophyll a: large near the marginal ice zone and becoming small with travel towards the north. High percentage shares of netplankton chlorophyll a were confirmed even in low total chlorophyll a concentrations in summer in the Southern Ocean. A positive relation was observed between the percentage of 30 μm netplankton and the “average total chlorophyll a”, although there was great scatter. This revised version was published online in August 2006 with corrections to the Cover Date.  相似文献