The angular momentum of gas in protogalaxies – II. The impact of pre-heating     

Frank C. van den Bosch  Tom Abel  Lars Hernquist 《Monthly notices of the Royal Astronomical Society》2003,346(1):177-185

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A high-resolution rotation curve of NGC 6822: a test-case for cold dark matter     

D. T. F. Weldrake  W. J. G. de Blok †  F. Walter 《Monthly notices of the Royal Astronomical Society》2003,340(1):12-28

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Stellar populations in local star-forming galaxies – II. Recent star formation properties and stellar masses     

P. G. Pérez-González  A. Gil de Paz †  J. Zamorano  J. Gallego  A. Alonso-Herrero  A. Aragón-Salamanca 《Monthly notices of the Royal Astronomical Society》2003,338(2):525-543

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Infrared stellar populations in the central parts of the Milky Way galaxy     

Jacco Th. van Loon  G. F. Gilmore  A. Omont  J. A. D. L. Blommaert  I. S. Glass  M. Messineo  F. Schuller  M. Schultheis  I. Yamamura  H. S. Zhao 《Monthly notices of the Royal Astronomical Society》2003,338(4):857-879

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Period changes in the pulsating extreme helium stars V652 Her and BX Cir     

D. Kilkenny  L. A. Crause  F. van Wyk 《Monthly notices of the Royal Astronomical Society》2005,361(2):559-564

A new statement of the eigenvalue problem of studying small perturbations in arbitrary integrable self-gravitating systems is presented. An example of such a system, a 2D stellar disc, is considered in detail. The theory, based on the general equation for disc eigenmodes, reveals mechanisms for the formation and growth of global galactic structures. This new point of view specifies the limits of the unified theory of bar-like and spiral modes that was based on the assumption that global galactic structures could be understood in terms of low-frequency disc modes.  相似文献   

The Preboreal climate reversal and a subsequent solar‐forced climate shift     

J. van der Plicht  B. van Geel  S. J. P. Bohncke  J. A. A. Bos  M. Blaauw  A. O. M. Speranza  R. Muscheler  S. Bjrck 《第四纪科学杂志》2004,19(3):263-269

Accurate chronologies are essential for linking palaeoclimate archives. Carbon‐14 wiggle‐match dating was used to produce an accurate chronology for part of an early Holocene peat sequence from the Borchert (The Netherlands). Following the Younger Dryas–Preboreal transition, two climatic shifts could be inferred. Around 11 400 cal. yr BP the expansion of birch (Betula) forest was interrupted by a dry continental phase with dominantly open grassland vegetation, coeval with the PBO (Preboreal Oscillation), as observed in the GRIP ice core. At 11 250 cal. yr BP a sudden shift to a humid climate occurred. This second change appears to be contemporaneous with: (i) a sharp increase of atmospheric ¹⁴C; (ii) a temporary decline of atmospheric CO₂; and (iii) an increase in the GRIP ¹⁰Be flux. The close correspondence with excursions of cosmogenic nuclides points to a decline in solar activity, which may have forced the changes in climate and vegetation at around 11 250 cal. yr BP. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

Mono- and dicyclic unsaturated triterpenoid hydrocarbons in sediments from Lake Masoko (Tanzania) widely extend the botryococcene family     

Romain de Mesmay  Pierre Metzger  Vincent Grossi  Sylvie Derenne 《Organic Geochemistry》2008,39(7):879-893

A mixture of C₃₃–C₃₇ botryococcenes and partially reduced derivatives was isolated from ca. 32,000 year old sediment from Lake Masoko, a freshwater crater lake in the Rungwe Range area (Tanzania). Botryococcenes and derivatives accounted for 246 μg/g dry sediment and for >92% of the hydrocarbon fraction; 1D and 2D nuclear magnetic resonance spectroscopy (NMR) and mass spectrometry allowed the structure of the dominant botryococcene (43% of hydrocarbon fraction) to be established, after purification using high performance liquid chromatography (HPLC). The compound is a novel tetraunsaturated dicyclic C₃₄ botryococcene and is named C₃₄ masokocene. Overall, the structures of six other novel botryococcenes and four partially reduced derivatives were tentatively assigned. The structures of the new biomarkers, three dicyclic C₃₄–C₃₆ botryococcenes (or masokocenes) and seven monocyclic C₃₄–C₃₇ analogues are discussed along with their biosynthetic relationship. The high abundance of such polyunsaturated compounds preserved in 32,000 year old sediment from the lake indicates an aquatic ecosystem dominated at the time by the green alga Botryococcus braunii, as well very good preservation of the organic matter.  相似文献   

Paleosol at the Archean—Proterozoic contact in NW India revisited: Evidence for oxidizing conditions during paleo-weathering?     

Manoj K. Pandit  Helga de Wall  Narendra K. Chauhan 《Journal of Earth System Science》2008,117(3):201-209

A number of fine-grained sericite bearing pelitic, schistose lithologies occur along the Archean (Banded Gneiss Complex)-Proterozoic (Aravalli Supergroup) contact (APC) in the Udaipur valley in NW Indian craton. These Al-rich lithologies (subsequently metamorphosed) have been described as ‘paleosols’, developed over a 3.3 Ga old Archean gneissic basement and are overlain by Paleoproterozoic Aravalli quartzite. The paleosol was developed between 2.5 and 2.1, coincident with the globally recognized Great Oxidation Event (GOE). In previous studies these paleosol sections were interpreted to have developed under reducing environment, however, the finding of a ‘ferricrete’ zone in the upper part of Tulsi Namla section (east of Udaipur) during the present study (in addition to earlier reported lithologies) has led to an alternative suggestion of oxygen-rich conditions during paleosol development. The Tulsi Namla paleosol section shows all the features characteristic of a complete paleosol section described from other Archean cratons. The paleosol includes sericite schist with kyanite as the prevalent Al-silicate in the lower part of profile while chloritoid and Fe-oxides typify the Fe-rich upper part. Alumina has remained immobile during the weathering process while Fe and Mn show a decrease in the lower part of the section and an abrupt rise in the upper part, in the ferricrete zone. The field and geochemical data indicate that the Tulsi Namla section is an in situ weathering profile and at least the upper part shows evidence of oxidizing conditions.  相似文献   

Urban effects of Chennai on sea breeze induced convection and precipitation   总被引：2，自引：0，他引：2  

Matthew Simpson  Sethu Raman  R. Suresh  U. C. Mohanty 《Journal of Earth System Science》2008,117(6):897-909

Doppler radar derived wind speed and direction profiles showed a well developed sea breeze circulation over the Chennai, India region on 28 June, 2003. Rainfall totals in excess of 100 mm resulted from convection along the sea breeze front. Inland propagation of the sea breeze front was observed in radar reflectivity imagery. High-resolution MM5 simulations were used to investigate the influence of Chennai urban land use on sea breeze initiated convection and precipitation. A comparison of observed and simulated 10m wind speed and direction over Chennai showed that the model was able to simulate the timing and strength of the sea breeze. Urban effects are shown to increase the near surface air temperature over Chennai by 3.0K during the early morning hours. The larger surface temperature gradient along the coast due to urban effects increased onshore flow by 4.0m s⁻¹. Model sensitivity study revealed that precipitation totals were enhanced by 25mm over a large region 150 km west of Chennai due to urban effects. Deficiency in model physics related to night-time forecasts are addressed.  相似文献   

Late Quaternary Bison diminution on the Great Plains of North America: evaluating the role of human hunting versus climate change     

Matthew E. Hill Jr.  Matthew G. Hill  Christopher C. Widga   《Quaternary Science Reviews》2008,27(17-18):1752-1771

Body size changes of Bison and mortality age structure data document the effects of climate-driven environmental change and human hunting pressure on large mammals in North America. Morphometric and mortality data are drawn from 58 archaeological and 9 paleontological localities dating between 37,000 and 250 calBP. Proxy information on body size is based on measurements recorded on 901 adult calcanei (os calcis) and 1026 humeri. In addition, published mortality profiles from 24 archaeological faunal assemblages spanning the last 14,000 years were used to approximate the age structure of bison populations. These results suggest that dramatic diminution in bison body size occurred in several short bursts, rather than a continuous gradual decline. These periods of rapid size reduction correlate with times of ecological reorganization, when aridity-driven changes in grasslands decreased forage quality and availability. Mortality age data indicate that the decrease in body size occurred in a context where there was no evidence for a progressively severe juvenile bias in bison populations. Overall, it appears that the changes in body size were a reaction to environmental conditions rather than the result of human predation pressure.  相似文献