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1.
NIELS NIELSEN 《Boreas: An International Journal of Quaternary Research》1992,21(2):159-168
A large-scale boulder beach close to a tidal glacier was examined at Eqip Sermia, Disko Bugt, West Greenland, in 1989. Photographs from 1912 and 1929 show an advance of the glacier of more than 1.5 km beyond its present location. Lateral and terminal moraines were formed in the sea, and subaerial parts and their positions can be detected from the old photographs. Today the outermost part of this moraine system has disappeared totally, except for about 1 km of the lateral moraine. The distal 300 m of the still existing moraine apparently has been displaced and transformed into a shape that, in plan view and cross-section, resembles a barrier spit. The material of the boulder beach consists mainly of coarse clasts with boulders of 1 m to more than 1.5 m in diameter. Distributions of clast sizes and sediment structures on the barrier surface also suggest wave and overwash dynamics as being the responsible agents. Located in the inner part of a fiord system, the fetches are restricted and thus normal waves are very small. Large waves generated by glacier calving, and/or sea-ice action, are therefore the only processes that can explain the geomorphology and clast distribution of this coastal feature. 相似文献
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3.
Palynology in a polar desert, eastern North Greenland 总被引：2，自引：0，他引：2
SVEND FUNDER NIELS ABRAHAMSEN 《Boreas: An International Journal of Quaternary Research》1988,17(2):195-207
A pollen diagram from a lake in a polar desert in eastern North Greenland records the regional vegetation history back to c. 7,000 years calBP (6,000 years convBP) in this extreme environment, which presents the coldest thermal regime where vascular plants can grow. The diagram shows that polar desert developed from sparse high arctic tundra at c. 4,300 years calBP (3,900 years convBP), owing to reduced summer heat. Also adjacent parts of high arctic Greenland, Canada and Svalbard suffered environmental decline, and polar deserts - presently restricted to a narrow fringe of land at the shores of the Arctic Ocean - were even more restricted before this time. Like other arctic vegetation types, polar desert is highly sensitive to summer temperatures, and its southern limit coincides with the isotherm for mean July temperatures of 3.5C. A comparison with the Northwest European ice-age pollen record shows no evidence of summers as cold as those now prevailing in the extreme north, and the results support the contention that the present Arctic and the ice-age mid-latitude environments are not identical. 相似文献
4.
NIELS RAMEIL ADRIAN IMMENHAUSER GEORG WARRLICH HEIKO HILLGÄRTNER HENK J. DROSTE 《Sedimentology》2010,57(3):883-911
Lithocodium aggregatum and Bacinella irregularis are now extinct, shallow marine life forms of unknown taxonomic origin. Forming part of the tropical platform biota during much of the Mesozoic, these organisms experienced bloom periods and temporarily replaced rudist–coral assemblages during parts of the Early Aptian. Within the limitations of time resolution, this ‘out‐of‐balance’ facies is coeval with the Oceanic Anoxic Event 1a‐related black shale deposition in oceanic basins but the triggering factors remain poorly understood. Here, a platform‐wide comparison of Lithocodium–Bacinella geobodies and morphotypes from the Sultanate of Oman is presented and placed in its environmental, bathymetric and physiographic context. Lithocodium–Bacinella geobodies reach from kilometre‐scale ‘superstructures’ to delicate centimetre‐sized growth forms. Clearly, scale matters and care must be taken when drawing conclusions based on spatially limited observational data. Whilst the factors that cause Lithocodium–Bacinella expansion should probably be considered in a global context, regional to local factors affected growth patterns in a more predictable manner. Here, the unresolved taxonomic relationship remains the main obstacle in any attempt to unravel the response of Lithocodium–Bacinella to specific or interlinked environmental parameters as different organisms respond differently to changing environment. Acknowledging these limitations, the following tentative patterns are observed: (i) Lithocodium–Bacinella tolerated a wide range of hydrodynamic levels and responded to differences in energy level or physiographic settings (margin, intrashelf basin, inner platform) by obtaining characteristic growth forms. (ii) Lithocodium–Bacinella favoured low‐sediment input but had the ability to react to higher sedimentation rates by enhanced upward growth; a feature perhaps pointing to a phototrophic metabolism. Circumstantial evidence for continuous growth within the upper‐sediment column is debated. (iii) The availability of accommodation space had a direct influence on the maximum size of geobodies formed. (iv) Fluctuating nutrient levels and sea water alkalinity may have affected the growth potential of Lithocodium–Bacinella. Understanding the relationship between Lithocodium–Bacinella morphogenesis on a wide range of scales and local environmental parameters allows for better prediction of the spatial distribution of reservoir properties and also results in an improved interpretation of palaeoenvironments. This study might represent a useful first step in this direction. 相似文献
5.
ÁRNÝ E. SVEINBJÖRNSOTTIR JÓN EIRÍKSSON ÁSLAUG GEIRSDÓTTIR JAN HEINEMEIER NIELS RUD 《Boreas: An International Journal of Quaternary Research》1993,22(2):147-157
An exhaustive 14 C dating programme of molluscs from the Fossvogur sediments in Reykjavik. Iceland is presented. For the first time all the fossiliferous units of the sediments are dated. The results confirm earlier conclusions of a widespread occurrence of marine sediments of Allerød age in Reykjavik. The set of dates from the Fossvogur sediments shows a narrow 14 C age distribution (standard deviation of ±235 years) of molluscs from all localities and from successive marine units in vertical sections. The weighted mean conventional 14 C age is 11,400 BP. Assuming a reservoir effect of 400 years. this corresponds to a reservoir-corrected age of I1,000 BP. i.e. the Allerød- Younger Dryas transition for the sampled units, These new 14 C dates from Fossvogur confirm the need for a revision of the Upper Pleistocene chronology of the Reykjavik region. They also have a bearing on the Late Weichselian record of glacier readvances and sea-level changes in the area. The dates suggest that the marine units in Fossvogur accumulated within a restricted time-span of a few hundred years. The sediments in Fossvogur are of volcaniclastic origin and are extremely lithified, indicating local geothermal activity soon after their deposition. This may explain anomalously high D/L amino acid ratios measured in molluscs from the Fossvogur sediments. δ13 C and δ18 O results suggest that temperatures may have ranged up to 60°C. 相似文献
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NIELS STENTOFT 《Sedimentology》1994,41(3):585-604
Bottom sediments from the sea floor west of Barbados between depths of 110 and 324 m are composed of nodular or crusted carbonate deposits. Individual biogenic sediment grains and the cemented aggregates, nodules and crusts are usually more or less altered by bioerosion and may support one or more generations of encrusting organisms. On the basis of component analysis of the topmost part of the bottom sediments it is possible to recognize three facies: (1) a proximal slope facies down to a depth of about 140 m, rich in mollusc fragments, benthic foraminifera and bryozoans; (2) an upper distal slope facies between about 140 m and about 215 m, rich in benthic foraminifera, molluses and crustaceans; and (3) a lower distal slope facies from about 215 m to at least 300 m, dominated by molluscs, especially pelecypods, with subordinate scleractinians and tubes of the polychaete Lygdamus asteriformis. The appearance and quantitative importance of the cemented aggregates is also related to these facles. In the proximal slope facies, only relatively few irregular and very porous nodules are found, whereas in the lower distal slope facies, aggregates are very common. Most aggregates are crust-like with a smooth upper surface and a more or less irregular, knobby lower surface. The crusts are massive compared with the nodules of the proximal slope facies and, unlike the latter, the lower surfaces and walls of larger cavities are usually coated with Fe and/or Mn oxides. In the upper distal slope facies a gradual transition between the two types of aggregates is found. Petrographical and morphological evidence, together with carbon and oxygen isotopic data, indicates that the nodules and crusts were formed in situ by submarine lithification processes. Radiocarbon dating of two bulk samples suggests that the cementation took place during late Pleistocene and/or early Holocene. 相似文献
7.
Regular spacing of drainage outlets from linear mountain belts 总被引：1，自引：0，他引：1
NIELS Hovius 《Basin Research》1996,8(1):29-44
Abstract Straight sections of many actively uplifting mountain belts have simple patterns of drainage, transverse to their main structural trend. Streams rising near or beyond the topographic ridgepole of these sections are spaced at seemingly regular intervals. To test whether this regularity exists, morphometric aspects of drainage networks were measured in 11 mountain belts. The spacing of drainage basins can be expressed using a spacing ratio, which in effect is the ratio of the length and the width of the catchments under consideration. Average spacing ratios for most linear mountain belts are within a narrow range of values between 1.91 and 2.23. A linear relationship exists between the spacing of catchment outlets and the distance between the main divide and the front of the mountain belt in which they have developed. The Nepalese Himalaya form an exception to this regular pattern. In this mountain belt drainage is blocked and diverted by structures that have developed in relation to the Main Boundary Thrust. Structural complications cause drainage patterns to become less regular, introducing important non trans verse components. The linear relationship between spacing of catchment outlets and half-width of the mountain belt may be expressed in an equation of the same general form as Hack's law of stream length and drainage basin area. It seems likely that the mechanism underlying Hack's law also explains the consistent regularity of drainage spacing in active mountain belts. However, no generally accepted explanation for Hack's law has been offered. The narrow range of spacing ratios found for drainage networks in active orogens may represent an optimal catchment geometry that embodies a ‘most probable state’ in the uplift-erosion system of a linear mountain belt. The linear relationship between the half-width of a mountain belt and spacing of catchment outlets has profound implications for the modelling of erosion of orogenic topography, and for the formation and filling of foreland basins. 相似文献
8.
OLE BENNIKE KAREN LUISE KNUDSEN NIELS ABRAHAMSEN JENS BÖCHER HOLGER CREMER BERND WAGNER 《Boreas: An International Journal of Quaternary Research》2010,39(3):603-619
Bennike, O., Knudsen, K. L., Abrahamsen, N., Böcher, J., Cremer, H. & Wagner, B. 2010: Early Pleistocene sediments on Store Koldewey, northeast Greenland. Boreas, Vol. 39, pp. 603–619. 10.1111/j.1502‐3885.2010.00147.x. ISSN 0300‐9483. Marine Quaternary deposits, here named the Store Koldewey Formation, are found at ～120 m above sea level in northeast Greenland (76°N). The sequence is referred to the Olduvai normal polarity subchron at 1.95–1.78 Ma BP based on palaeomagnetic studies (palaeomagnetically reversed), amino acid epimerization ratios and evidence from marine and non‐marine fossils. The sediments and the fauna show that the sequence was deposited on a mid or inner shelf, and some elements of the marine mollusc and foraminiferal assemblages indicate water temperatures between ?1 and +1 °C and seasonal sea ice cover during deposition. Mean summer air temperatures were around 6 °C higher than at present, as demonstrated by the occurrence of southern extralimital terrestrial species. Well‐preserved remains of land plants indicate that the adjacent land area was dominated by sub‐arctic forest‐tundra with the trees Larix and Betula, shrubs, herbs and mosses. Most of the species represented as fossils have recent circumpolar geographical ranges. An extinct brachiopod species and an extinct gastropod species have been found, but the other macrofossils are referred to extant species. The brachiopod is erected as a new genus and species, Laugekochiana groenlandica. Correlation of the Koldewey Formation with the Île de France Formation farther to the north is suggested. Member A of the Kap København Formation in North Greenland is referred to the Late Pliocene, whereas Member B of the Kap København Formation is suggested to be slightly older than the Store Koldewey Formation. 相似文献
9.
OSKARSSON NIELS; SIGVALDASON GUDMUNDUR E.; STEINTHORSSON SIGURDUR 《Journal of Petrology》1982,23(1):28-74
The article describes the petrochemical evolution of oceanicrocks in terms of plate tectonics with special reference toIceland. The compositional variation along the rift zone isrelated to different production rates of mantle-derived olivinetholeiite of invariant composition which is added to the crustfrom below and modified by mixing with anatectic melts in thecrust and concomitant crystal fractionation. The kinematic processes of crustal accretion cause rocks depositedin the rift zone to subside towards higher temperatures wherethey suffer hydration and progressive metamorphism before becominga part of the stable crustal plate. Rocks deposited near therift-centre assume the highest metamorphic grade (greatest depth)while rocks deposited at the rift-margins follow a shallow pathbefore being carried towards lower temperatures in the stableplate. The material transport through stationary metamorphiczones produces the layering of the oceanic crust. As the hydrated rocks cross their solidus isotherm, silicicmagma is formed by incongruent partial melting. The meltingcontinues until rocks crossing the boundary between the amphiboliteand granulite fades are finally dehydrated by the break-downof amphibole. This reaction boundary defines the surface ofthe upper mantle. The segregation and retention of crust-derived magmas withinthe rift zone results in chemical fractionation in the oceaniccrust, for its lower sections are depleted in elements enteringthe early melt fractions, which are silicic and enriched inthe dispersed elements. The last melt-increments from the samesubsiding pile are ne-normative basalts. The rift-zone rocks are shown to be mantle-derived olivine tholeiitemodified by minor amounts of crustal rhyolite and nephelinebasalt, while volcanism outside the rift zone is dominated bythe crust-derived magmas themselves. All mixtures undergo furthermineralogical evolution towards invariant compositions in thebasalt system, resembling the olivine tholeiite, quartz tholeiite,and nepheline basalt of synthetic systems. The dispersed-elementgeochemistry of the oceanic rocks is but slightly modified bycrystal fractionation, and reflects the mixing ratios of theolivine tholeiite and the different crustal magmas. The geochemistry of radiogenic isotopes is controlled by continuousprocesses of crustal fractionation separating mother and daughterelements. The oxygen-isotope geochemistry can be referred tomagma mixing, for rhyolites formed by anatexis in the hydratedcrust are enriched in light oxygen relative to the mantle-derivedmelts. 相似文献
10.
NIELS B
IE CHRISTENSEN 《Geophysical Prospecting》1990,38(5):545-568
In the linear digital filter theory for calculation of Hankel transforms it is possible to find explicit series expansions for the filter coefficients. A method is presented for optimizing the Hankel filters calculated in this way. For a certain desired accuracy of computation, the sampling density and filter length are minimized by choosing the parameters determining the filter characteristics according to the analytical properties of the input function. A new approach to the calculation of the filter coefficients has been developed for these optimized filters. The length of the filters may be further reduced by introducing a shift in the sampling scheme. 相似文献