4,422 research outputs found
Model experiments of soil erosion by V.T.O.L. aircraft downwash impingement
No full textAbstractThe paper describes research currently being carried out on the processes involved in the initiation of erosion of soil by the downwash from V.T.O.L. aircraft engines. Apparatus for simulating jet downwash is described together with the results of preliminary tests on erosion behaviour of moist sands. Pore air pressures generated in the sand by the downwash have been measured and shown to be in excess of the surface static pressures beyond a certain radius from the stagnation point.RésuméL'article décrit les recherches effectuées actuellement sur le phénomène de début d'érosion provoqué par les turbulences sous les avions à décollage vertical. L'appareil de simulation est décrit ainsi que le résultat des essais préliminaires sur le processus d'érosion des sables humides. La pression dans les pores du sable provoquée par la turbulence a été mesurée et est apparue supérieure à la pression statique de surface, au-delà d'une certaine distance du point de stationnement.ZusammenfassungDer Aufsatz beschreibt Forschungsarbeiten, die kürzlich über die Vorgänge durchgeführt wurden, welche mit de Einleitung der Erosion von Boden durch Abwärtsstrahl von senkrecht startenden Luftfahrtzeugen mit Strahltriebwerken zusammenhängen. Ein Apparat zur Simulierung des Abtriebsstrahls von Strahltriebwerken wird zusammen mit den Ergebnissen von früheren Versuchen über das Erosionsverhalten von feuchten Sanden beschrieben. Der Druck in den Lurtporen, der im Sand bei dem Abwärtsstrahl erzeugt wird, wurde gemessen und es wurde gezeigt, daßerheblich höher ist, als der statische Druck an der Bodenberfläche außerhalb eines bestimmten Radius von dem mittleren Staupunkt
Evidence of Quaternary climatic variations in a sequence of loess and related deposits at Birch Creek, Alaska: implications for the stage 5 climatic chronology
No full textA 45 m outcrop of Quaternary sediments on Birch Creek, near Circle, Alaska, reveals a record of fluctuating environmental conditions that probably spans several glacial-interglacial cycles. From base to top the deposits are forested floodplain (warm), colluvium with ice wedges (cold), forest soil (warm), loess (cold), paleosol containing Old Crow Tephra (OCt) (cool-to-warm), loess (cold), lacustrine (very warm), loess (cold), and modern forest soil (warm). Resolution of the paleoclimatic history associated with the OCt event is critical to understanding the nature of stage 5 in the western North American Arctic. Application of recent age estimates for the OCt tephra (ca. 140,000 yr BP) to the Birch Creek section would indicate that either (i) the tephra/paleosol dates from the 6/5.5 transition, a strongly developed glacial interval occurred within stage 5, and the overlying very warm interval occurred in 5.3 or 5.1, or (ii) the tephra was deposited during a non-Milankovitcha warming event late in stage 6. A paleoclimate chronology provides analternative interpretation, (iii), in which the tephra/paleosol corresponds to stage 6 or even stage 7, the overlying loess to stage 6, and the lake sediments to all or part of stage 5, but the OCt is older than 140,000 yr BP. Chronologies (ii) and (iii) imply a very warm beginning to stage 5, consistent with paleoclimate model simulations and data from other regions
Coming home to mother
No full textGift of Dr. Mary Jane Esplen.Piano vocal [instrumentation]We love to think of years ago [first line]We are coming home to Mother [first line of chorus]A flat [key]Piano [tempo]House, families, birds, photograph of M.E. Mollins [illustration]Popular song [form/genre]Publisher's advertisement on inside front [note]Mediatoon by G.A. Boyton [note
Holocene paleoclimate data from the Arctic: testing models of global climate change
No full textTo evaluate the spatial variability of Arctic climate change during the present interglacial, CAPE Project Members compiled well-dated terrestrial, marine, and ice-core paleoenvironmental records spanning the past 10-12 thousand years (ka). Six tundra biomes of increasing summer temperature requirements were defined based on regionally coherent pollen assemblages. Using a rule-based approach, pollen spectra were converted to tundra, forest/tundra, or forest biomes ranked by their average growing season requirements. Marine sea-surface reconstructions were based on proxy data following a similar rule-based approach. From these data-based reconstructions, departures in summer temperatures from modern normals were calculated in 1 ka time slices through the Holocene. To test predictive models, data-based summer temperature reconstructions were compared with general circulation model (GCM) simulations for 10 ka and 6 ka ago. Paleodata and model results both show that warming occurred earlier across Beringia and Asia relative to lands adjacent to the North Atlantic, and that Late Holocene cooling was most apparent in the North Atlantic region. However, the GCM over-predicts the magnitude of Mid-Holocene warming over northern Asia and underestimates the intensification of the North Atlantic drift in the early Holocene. Strong spatial variability in environmental response during the Holocene, despite symmetric (insolation) forcing, suggests that any future changes, whether caused by anthropogenic or natural factors, are unlikely to result in a uniform change across the Arctic, adding additional complexity to forecasts of global impacts
Tundra and boreal forest of interior Alaska during terminal MIS 6 and MIS 5e
No full textTwo sites within the boreal forest of interiorAlaska shed light on the climate and vegetation of terminalmarine isotope stage (MIS) 6 (ca. 140–130 kyr ago) andMIS 5e (125–116 kyr ago). The Birch Creek and Koyukuklocalities are river-cut exposures with sediments datingfrom the penultimate glaciation (at least) to the present.Plant macrofossils, pollen, and beetles were analyzed atthese sites. Terminal MIS 6-aged samples indicate a coolerthan modern climate and the presence of shrub tundra.During MIS 5e, boreal forest grew at the sites and temperatureswere similar to modern times. However, theforest may also have been more mesic than today, asindicated by relatively abundant ferns. Winters may havebeen warmer than today, as suggested by beetle-basedclimatic reconstructions as well as the presence of twoextralimital taxa that today live in regions where wintertemperatures are up to 15 ?C warmer than at the sitelocalitie
Holocene climate change and carbon cycling in a groundwater-fed,boreal forest lake: Dune Lake, Alaska
No full textInfluence of microclimate and species interactions on the composition of plant and invertebrate communities in alpine northern Norway
No full textWe assessed the effects of both biotic processes and abiotic factors on the community composition of vascular plant species and invertebrates at a site in northern Norway. Plant species were assigned to functional (woody versus herbaceous) and biogeographic (boreal versus alpine) groups. Invertebrate species were classified as either herbivore or predator. When species interactions and effects of the abiotic environment were partitioned, boreal species appeared to influence the distribution of alpine species and woody species the distribution of herbaceous species. Analysis of partial correlations indicated that facilitation was the dominant mode of interaction between the two pairs of plant groups. Among abiotic factors, the thermal environment probably influenced all components of the plant and invertebrate communities, except for predatory invertebrates, and wind appeared important in determining the composition of woody and alpine components of the plant community but not the herbaceous component. The composition of the boreal component of the plant community apparently influenced the composition of all invertebrate communities, except for predatory invertebrates. The composition of the woody component of the plant community influenced the composition of both herbivore and predator communities. The alpine plant-community composition influenced predatory invertebrate community composition. Woody plant community composition influenced the composition of both herbivore and predator communities. Our analytic approach, based on two kinds of structural equation models (d-separation and pathanalysis), provides a useful method for identifying the biotic as well as abiotic factors that influence community structure
Interglacial extension of the boreal forest limit in the Noatak Valley, Northwest Alaska: evidence from an exhumed river-cut bluff and debris apron
No full textNumerous exposures of Pleistocene sediments occur in the Noatak basin, which extends for 130 km along the Noatak River in northwestern Alaska. Nk-37, an extensive bluff exposure near the west end of the basin, contains a record of at least three glacial advances separated by interglacial and interstadial deposits. An ancient river-cut bluff and associated debris apron is exposed in profile through the central part of Nk-37. The debris apron contains a rich biotic record and represents part of an interglaciation that is probably assignable to marine-isotope stage 5. Pollen spectra from the lower part of the debris apron closely resemble modern samples taken from the Noatak floodplain in spruce gallery forest, and macrofossils of spruce are also present at this level. Fossil bark beetles and carpenter ants occur higher in the debris apron. Mutual Climatic Range (MCR) estimates from the fossil beetles suggest temperatures similar to or warmer than today. Together, these fossils indicate the presence of an interglacial spruce forest in the western part of the Noatak Basin, which lies about 80 km upstream of the modern limit of spruce forest
- …
