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211.
Jaco Weinstock Beth Shapiro Alfredo Prieto Juan Carlos Marín Benito A. González M. Thomas P. Gilbert Eske Willerslev 《Quaternary Science Reviews》2009,28(15-16):1369-1373
We present the results of a palaeogenetic analysis of two Late Pleistocene camelids originating in southern Chile. Our analysis of two mitochondrial DNA fragments (control region and cytochrome b gene) reveals that these specimens do not belong to an extinct taxon, but rather to extant vicuña (Vicugna vicugna), whose modern distribution is restricted to the extreme elevations of the Andes, more than 3500 km to the north of where these specimens originated. Our results also suggest fossil specimens from Patagonia that are currently assigned to the extinct taxon Lama gracilis, may actually belong to V. vicugna, implying a continuous distribution of the latter from the southern tip of South America to the Andes during the Final Pleistocene. The haplotypes of both specimens are not present in modern populations, suggesting a loss of genetic diversity concomitant with the contraction of the vicuña geographical distribution during the Final Pleistocene or early Holocene. 相似文献
212.
Adeline A. J. Wall Michel Magny Edward A. D. Mitchell Boris Vannière Daniel Gilbert 《第四纪科学杂志》2010,25(6):945-956
We tested the response of lacustrine testate amoebae (thecamoebians) to climate and environmental changes for the Lateglacial–Holocene transition. The palaeoenvironmental history of the study site (Lake Lautrey, Jura Mountains, eastern France) was previously established based on high‐resolution multi‐proxy studies of the same core. The present study is characterised by a high taxonomic resolution (54 taxa), inclusion of small species (down to 25 µm) and high total counts (>500 individuals per sample on average). Changes in the composition of testate amoeba assemblages (dominant species and assemblage structure), as well as in the accumulation rate (tests cm?2 a?1), corresponded to major climatic phases (i.e. Oldest Dryas, Bølling–Allerød Interstadial, Younger Dryas, Preboreal) as well as changes in organic matter inputs. Furthermore, decreases in the accumulation rate characterised minor short‐lived cooling events, such as Older Dryas event or Gerzensee oscillation. However, the Preboreal oscillation, which was well registered by other proxies at Lake Lautrey, could not be recognised in the testate amoeba record. This work demonstrates that lacustrine testate amoebae can be used for palaeoclimatic and palaeoecological reconstructions. Nevertheless, a better understanding of the relation between climate, organic matter and lacustrine testate amoebae requires further high‐resolution studies based on multi‐proxy approaches and the development of appropriate modern analogues. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
213.
ALEXIS Alain LE BRAS Gilbert Researcher Laboratoire de Genie Civil de Names Saint-Nazaire IUT de Saint-Nazaire Saint Nazaire France PHD Student Laboratoire de Genie Civil de Names Saint-Nazaire IUT de Saint-Nazaire Saint Nazaire France Tel. 《国际泥沙研究》2000,(3)
I INTRODUCTIONHigh water coment soils are often at the centre of delicate industrial and environmental problems,particularly concerning the behaviour of port or estuary mud, and deposits at sea.In soil mechanics, an understanding of the behaviour of soils is based upon the concept of effectivestress proposed by Terzaghi (1936) and defined by the difference betWeen the total stress and the porepressure. Theoretical studies (Aleds et al., 1992) highlight the effective stress as the main co… 相似文献
214.
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216.
Gilbert Feraud Ishiro Kaneoka Claude Jean Allègre 《Earth and Planetary Science Letters》1980,46(2):275-286
We have dated by the K/Ar method the oldest volcanic formations known in the Azores archipelago. The results show a development of this oceanic island system from 5.5 m.y. to the present day.The age pattern is not compatible with a simple migration over a small fixed hot spot, but, plotting tectonic directions which control the volcanism versus age and distance to the Mid-Atlantic Ridge, a simple relationship seems to appear showing an anisotropic but fixed distribution of stress orientation at least for the last 0.7 m.y. 相似文献
217.
218.
Morten Rasch Niels Nielsen Christian Christiansen Thomas Balstrøm Robert Gilbert Joseph Desloges 《Geografisk tidskrift / udgivet af Bestyrelsen for Det Kongelige danske geografiske selskab》2013,113(2):1-11
Measurements of discharge and suspended sediment and organic matter yield from nine different drainage basins on the island Disko in central West Greenland were carried out in the period 19–30 July 1997. A series of landscape properties (glacier cover, altitude, slope, aspect and vegetation cover) were measured for each drainage basin from a digital elevation model and a satellite image. Principal Component Analysis and regression statistics have been used to examine which landscape parameters dominate the measured discharges and yields. It is concluded that differences in suspended sediment and organic matter yield between the drainage basins can be explained by the measured morphometric properties while differences in water run-off can not. Glacier cover percentage and percent area above 800 m a.s.l. were the most important parameters influencing suspended sediment and organic matter yield in the drainage basins on Disko during the study period. 相似文献
219.
This article considers magnetic field generation by a fluid flow in a system referred to as the Archontis dynamo: a steady nonlinear MHD state is driven by a prescribed body force. The field and flow become almost equal and dissipation is concentrated in cigar-like structures centred on straight-line separatrices. Numerical scaling laws for energy and dissipation are given that extend previous calculations to smaller diffusivities. The symmetries of the dynamo are set out, together with their implications for the structure of field and flow along the separatrices. The scaling of the cigar-like dissipative regions, as the square root of the diffusivities, is explained by approximations near the separatrices. Rigorous results on the existence and smoothness of solutions to the steady, forced MHD equations are given. 相似文献
220.
Rebecca M. Briant Martin R. Bates Gilbert D. Marshall Jean-Luc Schwenninger Francis F. Wenban-Smith 《Proceedings of the Geologists' Association. Geologists' Association》2012,123(3):438-449
River terrace sequences are important frameworks for archaeological evidence and as such it is important to produce robust correlations between what are often fragmentary remnants of ancient terraces. This paper examines both conceptual and practical issues related to such correlations, using a case study from the eastern part of the former Solent River system near Southampton, England. In this region two recent terrace schemes have been constructed using different data to describe the terrace deposits: one based mainly on terrace surfaces; the other on gravel thicknesses, often not recording the terrace surface itself. The utility of each of these types of data in terrace correlation is discussed in relation to the complexity of the record, the probability of post-depositional alteration of surface sediments and comparison of straight-line projections with modern river long profiles. Correlation using age estimates is also discussed, in relation to optically stimulated luminescence dating of sand lenses within terrace gravels in this region during the PASHCC project. It is concluded that the need for replication at single sites means that this approach has limited use for correlative purposes, although dating of sediments is important for understanding wider landscape evolution and patterns of human occupation. 相似文献