摘要:

AbstractThe Marine Ice Transgression Hypothesis (MITH) was proposed by Denton and Hughes (1981) to supplement the highland origin, windward growth (Flint, 1943) and the instantaneous glacierization (Ives. 1957) hypotheses for initiating glaciation during an ice age. The sequence of transitions from sea ice to fast ice to ice shelves to marine ice domes to an ice sheet is examined more closely, with special attention to how marine ice transgression may have produced the Laurentide Ice Sheet.

ISSN:0435-3676    

DOI:10.1080/04353676.1987.11880211

出版商:Taylor&Francis    

年代:1987

数据来源: Taylor

摘要:

AbstractA dynamic model for the stochastic simulation of wave action on concave-shaped nearshore profiles is presented. The Airy, Stokes and Cnoidal wave theories, together with the appropriate shoaling equations are used in the simulation. Wave heights are adjusted for breaking conditions, and bed load and suspended load are computed with the best available equations.On each iteration, which is approximately seventy minutes of real time, deep water wave heights and periods are parameterized in the model with autocorrelated Rayleigh distributed variates. The tidal range, which changes every twelve iterations, is a normally distributed random variate. Using different initial slopes but identical wave and sediment input values, a FORTRAN IV program is executed for two simulation runs, with each run lasting for 1512 iterations.The results demonstrate that on a short-term basis the configuration of the profile developed for the first run of the simulation is not repeatable for the second simulation run. However, on each simulation run, the cumulative effects of low deepwater steepness values influence the development of a nonbarred configuration, while sustained high steepness values can be correlated with erosion and bar formation. Both barred and nonbarred configurations attain some form of temporary and repeatable equilibrium when sediment transport is not initiated, and when consecutive input values of wave heights and periods remain almost constant.Temporal variations in wave steepness not only give an indication of breaker type, but also reflect changes in the various components of wave velocity, and the magnitude and direction of sediment transport. It is shown that with plunging breakers the bulk of the transported load is deposited just seaward of the breakpoint position, while spilling breakers and higher steepness values generally cause an onshore movement and deposition of material.

ISSN:0435-3676    

DOI:10.1080/04353676.1987.11880212

出版商:Taylor&Francis    

年代:1987

数据来源: Taylor

摘要:

AbstractThe global temperature during the last century has risen about 0,5°C and the Ocean water level about 7–8 cm. The change—which is considered linear with temperature—is thus 15 cm per degree.From rather reliable estimates of the global temperature during past ages and the obtained quantitative relation between eustatic changes and temperature we have estimated the eustatic change during the last thousand years to be of the order of +10 cm. Several earlier estimates of much larger eustatic changes made by geologists and archaeologists in the Stockholm region are discussed and tentative explanations for their differing results are presented based mainly on meteorological, hydrological and oceanographic considerations. Some comments are also given concerning assumed eustatic changes during a time several thousand years ago.The average water level in Lake Mälaren during its transformation from a bay in the Baltic to a secluded lake is depending on the changes of the water level in the Baltic and is thus decreasing with time in relation to a fix point on land.A difference between the average water levels in Mälaren and the Baltic has always existed although it was very small thousand years ago. The difference has since been slowly and smoothly increasing with time up to about 25 cm in the beginning of this century, whereas in individual years the yearly average difference has varied considerably.Large variations in the water levels occur during short periods both in the Baltic and in Mälaren. Such extreme values should not be interpreted as representing secular average values.

ISSN:0435-3676    

DOI:10.1080/04353676.1987.11880213

出版商:Taylor&Francis    

年代:1987

数据来源: Taylor

摘要:

ABSTRACTDuring a week in late Octorber 1986 the formation of ridges of pumice by the movement of a mat of the floating fernSalvinia molestaMITCH was studied at the southern shore of Lake Naivasha, Kenya. The height of the ridges or ramparts was about 0.1 m, the width 0.1–0.5 m and the length c. 100 m. The resemblance to ridges or ramparts formed by floating ice in Arctic and cold-temperature areas in clear. It is assumed that the pumice ramparts in the Lake Naivasha were very ephemeral features.

ISSN:0435-3676    

DOI:10.1080/04353676.1987.11880214

出版商:Taylor&Francis    

年代:1987

数据来源: Taylor

摘要:

AbstractThe slopes of Vestfonna, which is a relatively small (7895 km2) dome-shaped icecap, are in this paper divided into two parts, one above the equilibrium line, where ice layers form in the snow pack and the other below the equilibrium line, where superimposed ice forms and ablation occurs.The main study of the formation of ice layers in the snow pack was carried out on the summit of Vestfonna, at Ahlmann Station (elevation 622 m). Thin ice layers in the uppermost part of the snow formed at the beginning of the melt season, when the snow was still cold and only a small amount of melt water had percolated downwards. The thickness of ice layers was found to increase with depth.Within the zone of superimposed ice measurements were made of crystal sizes in the ice layers. During the formation of these ice layers their grain sizes were recorded. Results of the crystallographic studies indicated that crystals in separate ice layers were small, whereas large crystals were found to form generally during warm summers.

ISSN:0435-3676    

DOI:10.1080/04353676.1987.11880215

出版商:Taylor&Francis    

年代:1987

数据来源: Taylor

摘要:

ABSTRACTA total of 64 subfossil pine samples from the pine tree limit zone or beyond the present limit of pine in Finnish Lapland have been14C dated. Ten new dates from the Enontekiöregion in northwestern Finnish Lapland are reported in the present paper. The majority of the dates from that region falls between approx. 6000 and 4000 B. P., showing that the pine area reached its maximum extent during that period. The dates from Inari and Utsjoki in the northernmost part of Finland are scattered between a few hundred and about 7000 years B.P., indicating that the retreat of the pine limit was a gradual process. Pine reached its maximum distribution during the postglacial climatic optimum, when the outermost occurrences were far beyond its present limit. The slow climatic deterioration since about 5000 B.P. has been the main reason for the decline of northernmost pine forests. The present material does not allow oscillations in the pine limit to be detected, but presumbly some variations did occur within the general long-term cooling trend.

ISSN:0435-3676    

DOI:10.1080/04353676.1987.11880216

出版商:Taylor&Francis    

年代:1987

数据来源: Taylor

摘要:

AbstractThe author discusses the glacial episodes in the Baltic area during the Younger Dryas time in the light of the model of Denton&Hughes presented in the final report of the CLIMAP project. The model implies a main, warm-based ice dome in the Gulf of Bothnia. The following sequence of events and their causes are discussed.1.Bölling-Alleröd deglaciation reached north of the Younger Dryas marginal zone.2.When the receding ice margin passed the Billingen area, South Sweden, the Baltic Ice Lake was lowered to the ocean level.3.At the beginning of the Younger Dryas time the thinning of the Barents Sea ice resulted in an intense outflow of icebergs into the North Atlantic.4.This outflow caused a cooling of the North Atlantic climate and a lowering of the ice dome over the Gulf of Bothnia.5.The lowered Gulf of Bothnia dome began to collapse.6.Advancing lobes from the Gulf of Bothnia dome reached the outermost Younger Dryas limit. They were characterized by a marginal zone with compressive, divergent flow, behind which the flow was extending.7.The climatic deterioration caused expansion of ice spreading from the Scandinavian mountain range.8.The advancing ice in the west blocked the passage between the ocean and the Baltic basin, and the Baltic Ice Lake was redammed.9.The marginal zone broke up as dead-ice, and through it the Baltic Ice Lake was finally drained.10.Because of the lowering of the lake level the ice lobes were grounded and a slower, more continuous recession of their fronts started.11.The changes of water level in the Baltic basin had created basal crevasses in the ice in which debris could form De Geer moraines. Along the crevasses tabular icebergs were broken off.12.In interlobe areas glaciofluvial sediments accumulated in crevasse systems to form broad eskers and deltas.

ISSN:0435-3676    

DOI:10.1080/04353676.1987.11880217

出版商:Taylor&Francis    

年代:1987

数据来源: Taylor

摘要:

AbstractThe Sand Hills moraine of southwest Banks Island is extensively affected by backwearing thermokarst in the form of ground ice slumps. Most are initiated by coastal erosion, with the result that 75% of the land within 100 m of the coastline has been transgressed by slumps. The majority are polycyclic and typically less than 200 m in length. Mean headwall retreat rates are between 8.6 and 11.4 m/a, although a maximum of 15.5 m/a was recorded for one slump in 1983–84. On average, slumps are active for 12–15 years, a substantially shorter time than in the moraine belt of eastern Banks Island (French and Egginton 1973). Surface lowering rates due to backwearing thermokarst in a strip of land within 1 km of the coast, are estimated to be up to two orders of magnitude greater than probable denudation due to slopewash and solifluction processes.

ISSN:0435-3676    

DOI:10.1080/04353676.1987.11880218

出版商:Taylor&Francis    

年代:1987

数据来源: Taylor

摘要:

AbstractThis study was performed to delineate the physical environment responsible for the formation of distinct miniature stone stripes at Haleakala (3055 m) on the island of Maui. The patterned ground occurs above the elevation of 2774 m in the ecologically alpine desert near the summit.The number of 0°C-crossing cycles of daily air temperature at Haleakala is small and comparable to that of the southern coastal states of the US. However, our temperature observations on the summit of Haleakala and related studies by others suggest that the ground of high Hawaiian mountains would experience higher frequency of 0°C-crossing and freeze-thaw cycles than can be expected from the air temperature data.Supply of soil moisture in the form of fog and drizzle and effective radiational cooling were also found vital for the needle ice development at the Haleakala summit.These climatological factors, together with the fact that the distribution of stone stripes is controlled by the presence of a silty sand layer, indicate that the ground features on Haleakala are of periglacial origin and have been produced under the mixture of critical environmental conditions.

ISSN:0435-3676    

DOI:10.1080/04353676.1987.11880219

出版商:Taylor&Francis    

年代:1987

数据来源: Taylor

摘要:

AbstractA relief map of Sweden has been constructed by the Land Survey on the basis of an elevation data base. The relief developed in the Precambrian bedrock is analysed and interpreted with the aid of weathering remnants and outliers of cover rocks of various ages. The following conclusions can be drawn. The different bedrock units have characteristic fracture patterns which reflect old, mainly Precambrian tectonics. The fracture pattern is more or less well expressed in the topography due to differences in the denudational history. In the Early Cambrian most of the basement surface was denuded to an almost level plain with only few residual hills. Cambro-Silurian strata were deposited on this primary planation surface. In Mesozoic time the Palaeozoic cover was stripped from elevated areas in southwestern and northern Sweden, where weathering penetrated deep along old fracture zones creating an uneven weathering front. The resulting undulating hilly land is still a significant component in the topography of southern Sweden (now exhumed from Jurassic and Cretaceous covers) and within large parts of the Norrland Terrain. Other areas, for instance the fissure valley landscape in the Stockholm region, where the sub-Cambrian surface is close to the plateau surfaces between the shallow and narrow valleys, were exhumed much later. The weathered fractures of the Mesozoic etchsurface have been emphasized in the topography of southern Norrland, probably due to unstable tectonic conditions in the Tertiary and later times. As the South Småland Peneplain in southern Sweden large areas in northern Norrland were flattened in the Early Tertiary due to stable tectonic conditions in combination with alternating arid and humid climates prohibiting the evolution of deep saprolites. In other parts of the Norrland Terrain the Mesozoic etchsurface seems to have been preserved, though the causes for preservation is unclear.

ISSN:0435-3676    

DOI:10.1080/04353676.1987.11880220

出版商:Taylor&Francis    

年代:1987

数据来源: Taylor