Frontiers and route‐ways from Europe: the Early Middle Palaeolithic of Britain          下载免费PDF全文

Nick Ashton  Claire R. E. Harris  Simon G. Lewis 《第四纪科学杂志》2018,33(2):194-211

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Hot stars in old stellar populations: a continuing need for intermediate ages     

S. C. Trager  Guy Worthey  S. M. Faber  Alan Dressler 《Monthly notices of the Royal Astronomical Society》2005,362(1):2-8

We present numerical investigations into the formation of massive stars from turbulent cores of density structure  ρ∝ r ^−1.5  . The results of five hydrodynamical simulations are described, following the collapse of the core, fragmentation and the formation of small clusters of protostars. We generate two different initial turbulent velocity fields corresponding to power-law spectra   P ∝ k ⁻⁴  and   P ∝ k ^−3.5  , and we apply two different initial core radii. Calculations are included for both completely isothermal collapse, and a non-isothermal equation of state above a critical density  (10⁻¹⁴ g cm⁻³)  . Our calculations reveal the preference of fragmentation over monolithic star formation in turbulent cores. Fragmentation was prevalent in all the isothermal cases. Although disc fragmentation was largely suppressed in the non-isothermal runs due to the small dynamic range between the initial density and the critical density, our results show that some fragmentation still persisted. This is inconsistent with previous suggestions that turbulent cores result in the formation of a single massive star. We conclude that turbulence cannot be measured as an isotropic pressure term.  相似文献   

Sensitivity analysis of a deterministic water temperature model to forest canopy and soil temperature in Catamaran Brook (New Brunswick,Canada)     

Andr St‐Hilaire  Nassir El‐Jabi  Daniel Caissie  Guy Morin 《水文研究》2003,17(10):2033-2047

A coupled deterministic hydrological and water temperature model, CEQUEAU, was modified to include soil temperature and crown closure in its calculation of local advective terms in the heat budget. The modified model was than tested to verify its sensitivity to these modifications. An analysis of the heat budget of a small forested catchment in eastern Canada revealed that the advective term related to interflow plays a significant role in the daily water heat budget, providing on average 28% of the local advective budget (which also includes advective heat terms from surface runoff and groundwater) and nearly 14% of the total heat budget (which includes all radiative terms at the water surface, convection and evaporation, as well as the local advective terms). Relative sensitivity indices (RSIs) were used to verify the impact of the newly introduced parameters and variables. Among them, parameters related to the forest cover (crown closure and leaf area index) have a maximum RSI of ?0·6; i.e. a 100% increase in value produces a 60% decrease in the local advective term. Parameters with the greatest influence are the volume of water contributing to interflow and the amplitude of the net radiative flux at the soil surface, which, if doubled, would double the contribution of the local interflow advective term to the heat budget. Copyright © 2003 John Wiley & Sons, Ltd.  相似文献   

The geochemistry of gem opals as evidence of their origin     

Eloïse Gaillou  Aurlien Delaunay  Benjamin Rondeau  Martine Bouhnik-le-Coz  Emmanuel Fritsch  Guy Cornen  Christophe Monnier 《Ore Geology Reviews》2008,34(1-2):113

Seventy-seven gem opals from ten countries were analyzed by inductively coupled plasma-mass spectrometry (ICP-MS) through a dilution process, in order to establish the nature of the impurities. The results are correlated to the mode of formation and physical properties and are instrumental in establishing the geographical origin of a gem opal. The geochemistry of an opal is shown to be dependant mostly on the host rock, at least for examples from Mexico and Brazil, even if modified by weathering processes. In order of decreasing concentration, the main impurities present are Al, Ca, Fe, K, Na, and Mg (more than 500 ppm). Other noticeable elements in lesser amounts are Ba, followed by Zr, Sr, Rb, U, and Pb. For the first time, geochemistry helps to discriminate some varieties of opals. The Ba content, as well as the chondrite-normalized REE pattern, are the keys to separating sedimentary opals (Ba > 110 ppm, Eu and Ce anomalies) from volcanic opals (Ba < 110 ppm, no Eu or Ce anomaly). The Ca content, and to a lesser extent that of Mg, Al, K and Nb, helps to distinguish gem opals from different volcanic environments. The limited range of concentrations for all elements in precious (play-of-color) compared to common opals, indicates that this variety must have very specific, or more restricted, conditions of formation. We tentatively interpreted the presence of impurities in terms of crystallochemistry, even if opal is a poorly crystallized or amorphous material. The main replacement is the substitution of Si⁴⁺ by Al³⁺ and Fe³⁺. The induced charge imbalance is compensated chiefly by Ca²⁺, Mg²⁺, Mn²⁺, Ba²⁺, K⁺, and Na⁺. In terms of origin of color, greater concentrations of iron induce darker colors (from yellow to “chocolate brown”). This element inhibits luminescence for concentrations above 1000 ppm, whereas already a low content in U (≤ 1 ppm) induces a green luminescence.  相似文献   

Groundwater flow and contaminant transport modelling at an air weapons range     

Geneviève Bordeleau  Richard Martel  Dirk Schäfer  Guy Ampleman  Sonia Thiboutot 《Environmental Geology》2008,55(2):385-396

Numerical modelling was done at the Cold Lake Air Weapons Range, Canada, to test whether the dissolved RDX and nitrate detected in groundwater come from the same sources, and to predict whether contamination poses a threat to the surface water receptors near the site. Military live fire training activities may indeed pose a risk of contamination to groundwater resources, however field investigations on military bases are quite recent, and little information is available on the long-term behaviour of munition residues related contaminants. Very limited information was available about the contaminant source zones, which were assigned based on our knowledge of current training activities. The RDX plume was well represented with the model, but the heterogeneous distribution of nitrate concentrations was more difficult to reproduce. It was nonetheless determined that both contaminants originate from the same areas. According to the model, both contaminants should reach the nearby river, but concentrations in the river should remain very low if the source zone concentration does not change. Finally, the model allowed the recommendation of a new location for the main bombing target, which would offer added protection to the river and the lake into which it flows.  相似文献   

北大西洋气候记录中末次冰消期的年代确定     

Claire Waelbroeck 《海洋地质前沿》2001,17(10)

为了确定控制末次冰消期的机制以及引起冰消期事件的顺序，最重要的是获得应用于大陆和海洋两者的气候记录的时间结构。放射性碳测年已经广泛应用于海洋沉积物的日历测年，但它是建立在表层水的表观年龄 (相对于大气层来说 )保持不变的假设上的。这里我们提出了北大西洋 40° N以北的表层水的表观年龄 (或储层年龄 )在过去 20 ka中的变化证据，在两个岩心中，我们发现了在 Heinrich 1 (15 ka BP)结束的地方，表层水表观年龄比今天大 1230± 600 a和 1940± 750 a，在新仙女木冷期结束的地方比今天大 820± 430 a 和 1010± 340 a。在 Bolling_Allerod暖期，两个老的储层年龄之间，表层水表观年龄可以与目前的值进行比较。我们的研究结果证实冰心年代学和整个冰消期的北大西洋海洋记录是一致的。因此，这表示了 40° N以北的北大西洋海洋碳测年需要作这些明显可变效应的校正。  相似文献   

La marge continentale au large de l'estérel (France) et les mouvements verticaux pliocènes     

Guy Pautot 《Marine Geophysical Researches》1970,1(1):61-84

Sommaire La région étudiée s'étend au large du massif des Maures, du massif de l'Estérel, des chaînes calcaires provençales et de la bordure alpine (Figure 1). Cette partie de la marge continentale médíterranéenne a été étudiée dans l'optique de la géodynamique en appliquant un ensemble de méthodes géophysiques, géologiques, sédimentologiques et géochimiques.La Méditerranée est l'objet de nombreuses études générales et son histoire tectonique est encore très discutée. Les relations entre la zone centrale (zone A) et les massifs terrestres géologiquement bien connus peuvent aider à la compréhension du cadre structural. L'étude très détaillée présentée ici concerne la marge continentale au large de la Côte d'Azur. Une étude bathymétrique préliminaire avec des profils espacés d'un mille marin a permis de définir un plateau continental peu étendu, une pente continentale abrupte (10–20°) avec des blocs continentaux effondrés. De nombreux canyons empruntent des accidents anciens qui ont rejoué.Des profils en série de sismique continue (Sparker et air gun) ont permis de délimiter la zone A qui comporte de nombreux dômes, de suivre des accidents majeurs, et de montrer des phénomènes sédimentaires et de néotectonique très importants (glissements, effondrements). Ces profils ont été calés avec 50 carottages. Un effondrement de plus de 1000 m depuis le Pliocène a été mis en évidence.La structure profonde a été abordée avec une étude de magnétométrie marine. La synthèse des résultats de sismique réfraction, de gravimétrie et de magnétométrie fait apparître un bombement du manteau sous la zone A avec abaissement rapide de la discontinuité de Moho sous la marge, la présence de volcans enfouis au Sud d'Antibes et de grands accidents de direction NS et EW.L'intérêt de ces résultats a semblé suffisamment important pour qu'un point de cette zone soit proposé pour le programme de forage JOIDES en Méditerranée.

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This paper deals with the region off the Maures and Estérel Massifs, the Provence calcareous ranges and the Alpine border (Figure 1). This portion of the Mediterranean continental margin has been investigated with the classical methods used in the geosciences: geophysics, geology, sedimentology, geochemistry.Many general studies are dealing with the Mediterranean and its tectonical history is still controversial. The relationship between the central zone (zone A) and the geologically well-known land massifs could help in understanding the structural framework. The present detailed study concerns the continental margin off Côte d'Azur. A relatively narrow continental shelf, a steep continental slope (10–20°) with collapsed continental blocks are defined by preliminary bathymetric studies with a spacing of one nautical mile. Many submarine canyons follow former faults which have been rejuvenated.A series of seismic reflection profiles (sparker and air-gun) provide limits for zone A (with many domes), allow tracing major faults and display important sedimentary and neotectonic processes (sliding, foundering). These profiles have been scaled with 50 cores. Evidence is brought forward for a local subsidence of over 1000 m. The deep structure has been investigated with marine magnetic methods. The results from seismic refraction, gravity and magnetics show a swell of the mantle beneath zone A, with a steep dipping of the M discontinuity under the margin, buried volcanoes south of Antibes and great faults striking NS or EW.These results seem to be important enough to propose a place in this zone of the Mediterranean as a JOIDES drilling site.

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Maintenant au Centre Océanologique de Bretagne, B.P. 337, 29 Brest, France.  相似文献   

Meadow fragmentation and reproductive output of the SE Asian seagrass Enhalus acoroides     

Jan E. Vermaat  Rene N. Rollon  Cristina Day A. Lacap  Claire Billot  Filipe Alberto  Hildie M.E. Nacorda  Frank Wiegman  Jorge Terrados 《Journal of Sea Research》2004,52(4):201

Flower and fruit production of the abundant, tall, long-lived, dioecious, surface-pollinating seagrass species Enhalus acoroides (L.) Royle were estimated at seven sites in the reef flats off Bolinao (NW Luzon, The Philippines) featuring different fragmentation of the seagrass meadows. Fragmentation of the seagrass meadow was quantified as cover of E. acoroides and all seagrass species present in 20×20 m plots. E. acoroides and overall seagrass cover were correlated positively. The proportion of female flowers of E. acoroides that developed a fruit increased sharply as overall seagrass cover was around 50%. Apparent sex ratio bore no relationship with overall seagrass cover. This threshold-type of relationship suggests that fragmentation of seagrass meadows can have a major effect on the reproductive output of this species. A possible mechanism underlying these results would be a non-linear increase of the efficiency of trapping the surface-dispersed pollen with increasing seagrass canopy density. This provides the first evidence based on real data that fragmentation can affect the population dynamics of seagrass species.  相似文献   

Ostracode carbonate δ18O- and δ13C-signature of hydrological and climatic changes affecting Lake Neuchâtel,Switzerland, since the latest Pleistocene     

Antje Schwalb  Guy S. Lister  Kerry Kelts 《Journal of Paleolimnology》1994,11(1):3-17

Floating and grounded peat plateaus were studied in fens in the Yukon Territory (Canada). The peat deposit may be over 4 m thick and consists of a lower bed of aquatic peat overlain by humic fen peat, mesic fen peat and woody peat. Permafrost in the grounded peat plateaus is older than the 1200 year old White River Ash, whereas permafrost in the floating peat plateau is younger.Peat accumulation rates since 1200 years B.P. were greater in the fens (85–100 cm) than on the surface of the peat plateaus (25–55 cm). Where the peat plateau is free-floating, it will persist until the climate changes, causing the icy core to thaw. Where the peat plateau is frozen to the mineral substrate, it slowly drowns since the fen peat accumulates faster than the woody peat. This drowning results in degradation of the landform independently of the climate. Only degradation of floating peat plateaus can be used to identify climatic changes.This publication is the first paper in a series of papers presented at the session on Past Climatic Change and the Development of Peatlands at the ASLO and SWS Meetings in Edmonton, Canada, May 30–June 3, 1993. Dr. P. Kuhry and Dr S. C. Zoltai are serving as Guest Editors.  相似文献   

Relationship of intertidal surface sediment chlorophyll concentration to hyperspectral reflectance and chlorophyll fluorescence     

Jacco C. Kromkamp  Edward P. Morris  Rodney M. Forster  Claire Honeywill  Scott Hagerthey  David M. Paterson 《Estuaries and Coasts》2006,29(2):183-196

Estimating biomass of microphytobenthos (MPB) on intertidal mud flats is extremely difficult due to their patchy occurrence, especially at the scale of an entire mud flat. We tested two optical approaches that can be applied in situ: spectral reflectance and chlorophyll fluorescence. These two approaches were applied in 4 European estuaries with different sediment characteristics. At each site, paired replicate measurements of hyperspectral reflectance, chlorophyll fluorescence (after 15 min dark adaptation, F_o ¹⁵), sediment water content, and chlorophylla concentrations were taken (including breakdown products: [chla+phaeo]). Sediments were further characterized by grain size and organic content analysis. The spectral signatures of tidal flats dominated by benthic microalgae, mainly diatoms, could be easily distinguished from sites dominated by macrophytes; we present a 3 waveband algorithm that can be used to detect the presence of macrophytes. The normalized difference vegetation index (NDVI) was found to be most strongly correlated to sediment [chla+phaeo], except for the predominantly sandy Sylt stations. F_o ¹⁵ was also significantly correlated to sediment [chla+phaeo] in all but one grid (Sylt grid A). Our results suggest that the functional relationships (i.e., the slopes) between NDVI or fluorescence and [chla+ phaeo] were not significantly different in the muddier grids, although the intercepts could differ significantly, especially for F_o ¹⁵. This suggests a mismatch of the optical depth seen by the reflectometer or fluorometer and the depth sampled for pigment analysis. NDVI appears to be a robust proxy for sediment [chla+phaeo] and can be used to quantify MPB biomass in muddy sediments of mid latitude estuaries.  相似文献