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1.
An early Holocene caribou antler from northern Ellesmere Island, Canadian Arctic Archipelago 总被引：1，自引：0，他引：1
THOMAS G. STEWART JOHN ENGLAND 《Boreas: An International Journal of Quaternary Research》1986,15(1):25-31
During the last glaciation of northern Ellesmere Island many areas remained ice-free. A caribou antler from deglacial-marine sediments in Clements Markham Inlet dates 8,415 ± 135 B.P. (S-2501). If locally derived it places caribou at the northern limit of their contemporary range at the onset of deglaciation in this area. Immediately to the south, on the Hazen Plateau, ice remained at its limit until c . 8,000 B.P. Therefore, this antler may indicate the presence of caribou during full glacial time. 相似文献
2.
JORIE CLARK A. MARSHALL McCABE CHRISTOPH SCHNABEL PETER U. CLARK STEWART FREEMAN COLIN MADEN SHENG XU 《Boreas: An International Journal of Quaternary Research》2009,38(1):111-118
A well-preserved moraine on the northern coast of County Donegal, Ireland, has played a critical role in our understanding of the glacial history of this sector of the Irish Ice Sheet (IIS). Because of a lack of numerical dating of the moraine, however, previous interpretations of its age and significance to the glacial history of this region have varied widely. Here we report eight in situ cosmogenic 10 Be ages on boulders sampled from the moraine. Two of these ages are outliers, with the remaining six ranging from 18.8±1.0 10 Be kyr to 20.9±1.3 10 Be kyr, with an uncertainty-weighted mean age of 19.4±0.3 10 Be kyr (19.4±1.2 kyr accounting for production rate uncertainty). Our results confirm one previous 10 Be age obtained from the moraine, with the combined data ( n =7) constraining the age of initial deglaciation of the IIS from its LGM position on the continental shelf to be 19.3±0.3 10 Be kyr (19.3±1.2 kyr accounting for production rate uncertainty). These ages are in excellent agreement with calibrated 14 C ages that constrain retreat of the IIS margin from the continental shelf elsewhere in northwestern and western Ireland and the Irish Sea Basin associated with the start of the Cooley Point Interstadial (≥20–≤18.2 cal. kyr BP), suggesting widespread deglaciation of the IIS ∼19.5–20 kyr ago. 相似文献
3.
Abstract It is known that the El Niño Southern Oscillation (ENSO) phenomenon induces marked climate variability across many parts of the world. However, in seeking useful relationships between ENSO and climate, several indices are available. In addition to the choice of index, previous studies assessing ENSO effects have employed a range of different methods to classify periods as El Niño, La Niña or Neutral. It is therefore clear that significant subjectivity exists in the adoption of ENSO classification schemes. In this study, several ENSO classification methods are applied to a range of ENSO indices. Each method-index combination is investigated to determine which provides the strongest relationship with rainfall and runoff in the Williams River catchment, New South Wales, Australia. The results demonstrate substantial differences between the methods and indices. The Multivariate ENSO Index (or MEI) is found to provide the best classification irrespective of method. The potential for forecasting ENSO-related effects on rainfall, runoff and river abstractions is then investigated. A “rise rule” to account for dynamic ENSO trends is also assessed. Strong relationships were found to exist with runoff (rainfall) up to nine (eight) months in advance of the Summer/autumn period. Implications for improved forecasting of potential river abstractions are apparent. 相似文献
4.
BY TIMOTHY F. JOHNSEN HELENA ALEXANDERSON DEREK FABEL STEWART P.H.T. FREEMAN 《Geografiska Annaler: Series A, Physical Geography》2009,91(2):113-120
The overall pattern of deglaciation of the southern part of the Scandinavian Ice Sheet has been considered established, although details of the chronology and ice sheet dynamics are less well known. Even less is known for the south Swedish Upland because the area was deglaciated mostly by stagnation. Within this area lies the conspicuous Vimmerby moraine, for which we have used the terrestrial cosmogenic nuclide (10 Be) exposure dating technique to derive the exposure age of six glacially transported boulders. The six 10 Be cosmogenic ages are internally consistent, ranging from 14.9 ± 1.5 to 12.4 ± 1.3 ka with a mean of 13.6 ±0.9 ka. Adjusting for the effects of surface erosion, snow burial and glacio-isostatic rebound causes the mean age to increase only by c. 6% to c. 14.4± 0.9 ka. The 10 Be derived age for the Vimmerby moraine is in agreement with previous estimates forthe timing of deglaciation based on radiocarbon dating and varve chronology. This result shows promise for further terrestrial cosmogenic nuclide exposure studies in southern Sweden. 相似文献
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6.
ALASTAIR J. STEWART 《Sedimentology》1979,26(1):33-62
The Ringwood evaporite is part of the 900 m.y. old Bitter Springs Formation, a warm-water shallow-marine sequence of stromatolitic dolomite and limestone, microfossiliferous chert, red beds, quartzite, and evaporites. The evaporite at Ringwood comprises two parts: (i) a lower 127 m characterized by brecciated pyritic bituminous dolomite, together with smaller amounts of dolomite-gypsum breccia, friable chloritic dololutite, coarsely crystalline anhydrite, and satin-spar gypsum; and (ii) an upper 133 m which is similar except that bituminous dolomite forms only one bed, and the characteristic rock-type is dolomite-gypsum breccia. The evaporite is overlain by limestone breccia and massive stromatolitic limestone, interpreted as an algal reef. Gypsum is secondary after anhydrite, and the ratio of gypsum to anhydrite increases upwards. The evaporite shows none of the features of a sabkha or desiccated deep ocean basin deposit, and instead is interpreted as the filling of a barred basin which was cut off from the ocean by growth of an algal barrier reef. As circulation became restricted, bituminous dolomite deposited in the lagoon behind the reef, together with pyrite from the destruction by anaerobic bacteria of algal debris derived from the reef. With continued evaporation, brine concentration increased and gypsum precipitated. Occasional dust storms contributed wind-blown clay to the deposit. The barrier reef transgressed diachronously across the evaporite lagoon, and was eventually drowned when normal marine conditions became established. Burial of the evaporite to about 7000 m beneath the succeeding sediments of the Amadeus Basin converted gypsum to anhydrite, and formed chlorite by reaction of clay with dolomite. Late Palaeozoic tectonism folded and brecciated the rocks, and was followed by erosion which eventually exposed the evaporite to ingress of meteoric water. Hydration of anhydrite to gypsum ensued, the reaction becoming less complete with increasing depth from the ground surface. 相似文献
7.
DONA J. STEWART 《Geographical review》2005,95(3):400-424
ABSTRACT. In pursuit of its foreign‐policy goals, the administration of President George W. Bush has attempted a dramatic reshaping of the vision of the Middle East in the American mind. References to the “new” or “greater” Middle East now include countries far outside traditional concepts of the region, including those in West Africa, Southeast Asia, and Central Asia. The administration argues that this region is defined not by cultural (Arab) or religious (Islam) characteristics but by a lack of democracy; hence a grand strategy is needed to execute reform. This article examines current U.S. efforts to achieve reform in the region, the components of the ideological construction of the New Middle East, the perceived role of Iraq, Turkey's potential role as a “model” for the region, and responses from the region to current U.S.‐led reform efforts. 相似文献
8.
The Influence of Viscosity on Fountains in Magma Chambers 总被引：5，自引：4，他引：5
Geological observations suggest that basaltic magmas mix readilybut that rhyolites and basalts can erupt through the same volcanicvent without mixing. The implication is that viscosity may havean important influence on the mixing of magmas. This paper examines the influence of viscosity on magma mixingwhen a magma of low viscosity is injected as a turbulent fountaininto a chamber containing a magma of higher viscosity. Threeseries of experiments were carried out. In the first, the lowviscosity fluid was injected into the tank at a Reynolds number(Re1) of about 1000 to ensure that flow within the fountainwas fully turbulent. Both fluids were maintained at the sametemperature and the viscosity of the host fluid was varied systematically.If the viscosities of the two fluids are similar, turbulencein the fountain leads to extensive mixing and to formation ofa density-stratified layer which collects at the bottom of thetank. However, if the viscosity ratio of the fluids used isgreater than 400, no detectable mixing occurs. Motion withinthe fountain is again fully turbulent but the high momentumof the input fluid is not transmitted to the more viscous hostfluid. Experiments carried out at intermediate viscosity ratiosresulted in intermediate amounts of mixing. Theoretical considerations suggest that the criterion for mixingin turbulent fountains, when a fluid of low viscosity v1 isinjected into a fluid of much higher viscosity v2 is wd>kv2 where w is the flow velocity through an input pipe of diameterd and k is a constant. This relationship is believed to be ageneral one, applying equally to jets and plumes, although thevalues of k may vary with the nature of the flow. The secondseries of experiments was designed to test this relationshipand determine values of k for fountains. The results show that,if wd/v2 > 70, the inflowing fluid mixes with the host fluidas there was no viscosity difference between them. However,if wd/v2 < 7, little or no mixing occurs even if motion withinthe fountain is fully turbulent. In the third series of experiments the temperature of the inputfluid was 70?C above that of the host fluid. This led to heatingofa thin boundary layer of the host fluid, lowering the viscosityof the host fluid adjacent to the fountain and thus allowingadditional mixing between the fluids. This effect was most obviousin the experiment with the highest viscosity ratio (1229) wherea small amount of mixing was detected, compared with no mixingin the equivalent experiment in the first series. The criterion for mixing, wd> kv2 can be applied to naturalmagmas. The results show that if a primitive basaltic magmais injected, as a turbulent fountain or plume, into a chambercontaining fractionated basaltic magma, the two magmas willmix readily. However, if basaltic magma is injected into a chambercontaining granitic melt, little or no mixing occurs. 相似文献
9.
Groundwater silcretes have been recognized recently as major terrestrial silicon sinks and yet their origins are poorly understood. This paper aims to further the understanding of the micro‐fabric, geochemistry and formation of groundwater silcretes, through detailed analyses of silcrete boulders from the South Downs, Sussex, UK. In‐depth petrological investigations of silcrete blocks identified three varieties of silcrete across the study area (saccharoid, hard and pebbly conglomeratic or silcrete breccia), similar to those elsewhere in England. Silcrete fabrics were universally simple and preserved host material structures. Optically continuous quartz overgrowths were the dominant cement and developed on framework grains in the absence of silt‐ and clay‐sized particles. Finer‐grained silica cements occurred in isolated patches and as vein‐ or cap‐like geopetal features. These caps and veins formed through the silicification of illuviated clay‐rich material that entered the host sediment prior to, or in the early stages of, overgrowth formation. Titanium contents were related to the amount of fine‐grained silica and appear to reflect variations in host material chemistry. Subsequent to silicification, the silcretes were altered by at least two phases of ferruginization, characterized by the ingress of iron minerals and partial fabric dissolution or replacement. The study reveals strong similarities in the micromorphology and geochemistry of groundwater silcretes in the study area and those developed in sandy host sediments in neighbouring parts of northwest Europe. Variations that do occur could be explained by differences in the host sediment, geological setting or tectonic history of the respective basins (or sub‐basins), suggesting that there may be a common mechanism for silcrete formation across north‐west Europe. Silcrete development appears to be related to the release of silica accompanying acid leaching of the host material or adjacent strata. In the South Downs, this probably resulted from oxidation of pyrite in the argillaceous and lignitic sediments overlying the host sands. It is envisaged that palaeoenvironmental conditions were of limited importance for silcrete development and that there was no specific ‘era’ of formation, with phased development accompanying landscape evolution through the Neogene into the Pleistocene. 相似文献
10.
Abstract— We present numerical simulations of crater formation under Martian conditions with a single near‐surface icy layer to investigate changes in crater morphology between glacial and interglacial periods. The ice fraction, thickness, and depth to the icy layer are varied to understand the systematic effects on observable crater features. To accurately model impact cratering into ice, a new equation of state table and strength model parameters for H2O are fitted to laboratory data. The presence of an icy layer significantly modifies the cratering mechanics. Observable features demonstrated by the modeling include variations in crater morphometry (depth and rim height) and icy infill of the crater floor during the late stages of crater formation. In addition, an icy layer modifies the velocities, angles, and volumes of ejecta, leading to deviations of ejecta blanket thickness from the predicted power law. The dramatic changes in crater excavation are a result of both the shock impedance and the strength mismatch between layers of icy and rocky materials. Our simulations suggest that many of the unusual features of Martian craters may be explained by the presence of icy layers, including shallow craters with well‐preserved ejecta blankets, icy flow related features, some layered ejecta structures, and crater lakes. Therefore, the cratering record implies that near‐surface icy layers are widespread on Mars. 相似文献