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191.
NICK EYLES THOMAS MEULENDYK 《Boreas: An International Journal of Quaternary Research》2008,37(2):226-233
Ice scouring of lake and sea-floor substrates by the keels of drifting ice masses is a common geological process in modern northern lakes and continental shelves, and was widespread during the Pleistocene. Nonetheless, the importance of scouring as a geological process is not yet matched by many sedimentological studies of scour structures exposed in outcrop. This article presents an integrated study combining outcrop sedimentology and subsurface ground-penetrating radar (GPR) data from a relict late Pleistocene ice-scoured glacial lake floor now preserved below beach sediments in Ontario, Canada. Scours occur along a regressive sequence boundary where deep-water muddy facies are abruptly overlain by shallow-water sands resulting from an abrupt drop in water levels. This has allowed the keels of drifting ice masses to scour into muds. Three-dimensional data gained from the GPR survey show that scours are as much as 2.5 m deep and 7 m wide; they have berms of displaced sediment and are oriented parallel to the former shoreline. Scoured shoreface sediments that fill scours show abundant liquefaction structures, indicating substrate dewatering during repeated scouring events similar to that recently reported in the modern Beaufort Sea in Canada's far north. Marked changes in water depths are typical of glacially influenced lakes and seas, creating opportunities for drifting ice to scour into offshore muddy cohesive facies and be preserved. The data presented here may aid identification in ancient successions elsewhere. 相似文献
192.
THOMAS B. KELLOGG 《Boreas: An International Journal of Quaternary Research》1986,15(4):331-343
Previous interpretations of Labrador Sea and Baffin Bay sediment cores were hampered by failure to recognize that the presence of small (62–149 μm) specimens of 'subpolar' planktic foraminifera in high-latitude marine sediments is primarily a function of the geochemistry of the water column and/or sediments rather than an indicator of environmental conditions in overlying surface waters. Assuming this rationale is correct, foraminiferal data from core HU75–42 indicate that surface conditions in the Labrador Sea were characterized by polar waters, with probable year-round sea-ice cover, throughout most of the period from isotope stage 5a to Termination I. The single exception to this sustained cold history for the eastern Labrador Sea was a transient pulse that apparently brought relatively warm, subpolar waters to the eastern Labrador Sea for a short (probably < 600 years) interval at the isotope stage 5a/4 transition. 相似文献
193.
ALBERTO V. REYES GRANT D. ZAZULA SVETLANA KUZMINA THOMAS A. AGER DUANE G. FROESE 《第四纪科学杂志》2011,26(3):345-352
Last interglacial sediments in unglaciated Alaska and Yukon (eastern Beringia) are commonly identified by palaeoecological indicators and stratigraphic position ~2–5 m above the regionally prominent Old Crow tephra (124 ± 10 ka). We demonstrate that this approach can yield erroneous age assignments using data from a new exposure at the Palisades, a site in interior Alaska with numerous exposures of last interglacial sediments. Tephrochronology, stratigraphy, plant macrofossils, pollen and fossil insects from a prominent wood‐rich organic silt unit are all consistent with a last interglacial age assignment. However, six 14C dates on plant and insect macrofossils from the organic silt range from non‐finite to 4.0 14C ka BP, indicating that the organic silt instead represents a Holocene deposit with a mixed‐age assemblage of organic material. In contrast, wood samples from presumed last interglacial organic‐rich sediments elsewhere at the Palisades, in a similar stratigraphic position with respect to Old Crow tephra, yield non‐finite 14C ages. Given that local permafrost thaw since the last interglaciation may facilitate reworking of older sediments into new stratigraphic positions, minimum constraining ages based on 14C dating or other methods should supplement age assignments for last interglacial sediments in eastern Beringia that are based on palaeoecology and stratigraphic association with Old Crow tephra. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
194.
Small hypabyssal intrusives of biotite pyroxenite and biotiteperidotite are described from part of theGardar alkaline province.The intrusives, composed essentially of diopside, olivine, phlogopiteand Fe-Ti oxides (and numerous accessory minerals) are inferredto have crystallized from silica-undersaturated ultramafic magmasof similar bulk composition. Crystallization occurred at anestimated depth of 34 km. Despite relatively rapid consolidation(probably attendant on a partial devolatilization) producingtypically fine-grained and sometimes porphyritic rocks, small-scaledifferentiation nevertheless occurred. The differentiates, asveins, pegmatoidal patches and roofing-facies tend to be olivine-poorphlogopite+ferriandiopside+ Fe-Ti oxide rocks, in which perovskite,andradite, apatite and sphene are significant components. Pyroxenecompositions in the ultramafites are consistent with the thesisthat the rocks formed from soda-deficient magmas in stronglyoxidizing conditions. Relative potassium enrichment (averageK2O: Na2O wt. % = 1.6) is expressed modally in the ubiquitouspresence of phlogopite and normatively by an average of 6.9%or and/or Ic. Despite compositional affinity with certain olivinemelilitites, melilite is absent, a fact that may be attributableto the comparatively high hydrostatic pressure (c. 1 kb) pertainingduring crystallization. Petrographic and geochemical evidencesuggests that the ultramafic magmas were residua after fractionationof forsteritic olivine from more magnesian magmas of kimberlitetype. It is proposed that the latter may in turn have been residuafrom eclogite fractionation of Fe-Ti rich primary magmas inthe upper mantle. 相似文献
195.
THOMAS H. BURBINE 《Meteoritics & planetary science》1998,33(2):253-258
Abstract— I review the dynamical and compositional evidence for possibly linking CM chondrites and asteroids having G-class taxonomic designations. Three G asteroids have been identified through previous theoretical studies as being likely meteorite source bodies due to their locations near resonances. Two of these objects, 19 Fortuna and 13 Egeria, have spectral properties that are consistent with such a linkage with CM chondrites. Fortuna has a similar strength 0.7 μm absorption feature and near-infrared spectral slope to CM chondrites but a weaker ultraviolet feature. Egeria also has the characteristic 0.7 μm feature of CM chondrite spectra but does not match as well in the near-infrared. However, since the 0.7 μm feature is apparent in the spectra of approximately one-half of measured C-type asteroids, no definitive statement about any linkages can be made. Ceres is spectrally different from known meteorites in the 3 μm wavelength region and cannot be convincingly linked with any meteorite group. 相似文献
196.
197.
The major element chemical compositions of lava from four eruptionson the east rift zone of Kilauea between August 1968 and October1971 reflect three petrologic processes:
- Production of chemically distinct batches of magma in the mantle.
- Separation of olivine, augite, and plagioclase from liquidduringflow in the rift-zone conduits.
- Mixing of differentmagmas during ascent to the surface.
198.
THOMAS A. WIKLE 《Geographical review》2002,92(1):45-62
ABSTRACT. Since the early 1980s the growing popularity of cellular communication has wrought dramatic landscape changes on the American scene through an invasion of thousands of cellular telephone towers. Objections raised to new tower construction by local residents, interest groups, and regulatory boards range from visual impacts to perceived health risks. This essay traces the origins of wireless telephony, its proliferation across the United States, and the visual impacts associated with tower construction. Three stages in the geographical expansion of wireless networks are identified. 相似文献
199.
Impact of the Arctic Fox Alopex lagopus on nesting success of geese in southeast Svalbard, 1989 总被引:1,自引:0,他引:1
Goose populations, with special emphasis on Light-bellied Brent Geese Branta bernicla hrota , were censured in Tusenøyane and Tjuvfjorden, southeast Svalbard, July-August 1989. A total of 425 non-breeding Brent Geese, 210 Barnacle Geese Branta leucopsis and 421 Pink-footed Geese Anser bra-chyrhynchus and 11, 2 and 3 families of the three species, respectively, were counted. Brent Geese attempted to nest on 6 of the 20 islands surveyed, and were successful on four. Barnacle Geese attempted to nest on three islands and were successful on two. Many islands known to be traditional nesting sites were entirely void of geese. Pink-footed Geese were only seen in Tjuvfjorden. The breeding failure on the islands was linked to the presence of the Arctic Fox Alopex lagopus which probably caused the geese to completely give up the attempt to nest. On one island the fox had apparently arrived after nest initiation and ravaged 45 Brent Goose and Barnacle Goose nests. It is likely that the foxes stranded on the islands during ice break-up. As a consequence of the presence of foxes on most of Tusenøyane, more than half of the potential breeding segment of the Svalbard Brent Goose population failed to nest in the summer of 1989. 相似文献