ESTABLISHING LICHENOMETRIC AGES FOR NINETEENTH- AND TWENTIETH-CENTURY GLACIER FLUCTUATIONS ON SOUTH GEORGIA (SOUTH ATLANTIC)     

BY STEPHEN J. ROBERTS  DOMINIC A. HODGSON  SAMANTHA SHELLEY  JESSICA ROYLES  HUW J. GRIFFITHS  TARA J. DEEN  MICHAEL A.S. THORNE 《Geografiska Annaler: Series A, Physical Geography》2010,92(1):125-139

Glaciers in small mountain cirques on South Georgia respond rapidly and sensitively to changes in South Atlantic climate. The timing and rate of their deglaciation can be used to examine the impact that nineteenth- and twentieth-century climate change has had on the glacial dynamics and terrestrial ecosystems of South Georgia. As part of a reconnaissance study in Prince Olav Harbour (POH), South Georgia, we measured the size of lichens ( Rhizocarpon Ram. em Th. Fr. subgenus. Rhizocarpon group) on ice-free moraine ridges around two small mountain cirques. Our aims were twofold: first, to provide age estimates for lichen colonization, and hence, deglaciation of the moraine ridges, and second, to examine the potential of applying lichenometry more widely to provide deglacial age constraints on South Georgia. In the absence of lichen age-size (dating) curves for South Georgia, we use long-term Rhizocarpon lichen growth-rates from recent studies on sub-Antarctic Islands and the western Antarctic Peninsula to calculate likely age estimates. These data suggest ice retreat from the two outermost moraines occurred between the end of the 'Little Ice Age' (post c. 1870) and the early twentieth century on South Georgia. Lichen colonization of the innermost moraines is probably related to glacier retreat during the second half of the twentieth century, which has been linked to a well-defined warming trend since c. 1950. Patterns of possible nineteenth- and twentieth-century glacial retreat identified in POH need to be tested further by establishing species- and site-specific lichen age-size (dating) curves for South Georgia, and by applying lichenometry to other mountain cirques across South Georgia.  相似文献   

LICHENOMETRIC STUDIES ON MORAINES IN THE POLAR URALS     

BY OLGA SOLOMINA  MIKHAIL IVANOV  TOM BRADWELL 《Geografiska Annaler: Series A, Physical Geography》2010,92(1):81-99

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Velocity profiles and incipient motion of frazil particles under ice cover   总被引：1，自引：0，他引：1  

Faye KROL 《国际泥沙研究》2010,25(1):39-51

A series of experiments for the incipient motion of frazil particles under ice cover have been carried out in laboratory under different flow and boundary conditions.Measurements on flow velocities across the measuring cross-section at different water depths have been conducted.Based on these experiments under both ice-covered and open flow conditions,the impacts of solid boundary(such as ice cover and flume sidewall) on the distribution of flow velocity profiles have been discussed.The criteria for the inc...  相似文献   

Surface mass balance and ice flow of the glaciers Austre Lovenbreen and Pedersenbreen,Svalbard,Arctic     

徐明星  闫明  任贾文  艾松涛  康建成  鄂栋臣 《极地研究(英文版)》2010,(2):147-159

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A vortex-tube model of eddies in the inertial range     

Stephen Childress 《地球物理与天体物理流体动力学》2013,107(1-4):29-64

Abstract

A vortex-tube geometry of the cascade of energy to small-scale eddies, in the inertial range of fully-developed turbulence, is proposed. The model is a special case of the beta model of Frisch, Sulem and Nelkin (1978). We require that the cascade conserve the principal invariants of inviscid, incompressible flow, namely volume, topological knottedness, circulation, and, at discrete times marking the termination of steps in the cascade, energy. The process terminates in a finite time, as in any beta model, leaving behind a self-similar network of “inactive” tubes. We associate a self-similar scaling dimension D with the structure, equal to the Hausdorff dimension of the set of “active” tubes at the termination of the cascade. Because circulation Λ plays a key role in the analysis of the cascade, we refer to these vortex-tube geometries as “gamma models”. The viewpoint throughout is entirely deterministic.

We describe two examples of gamma models. In the ring geometry, an eddy is a vortex ring, and the cascade produces “rings upon rings”, so we allow cutting and fusing of tubes while conserving total helicity. In the preferred helical model, no cutting is needed, and the cascade produces an infinite progression of braided “coils upon coils”. We suggest that latter geometry as a candidate for the topology of a singularity of the inviscid limit of a Navier-Stokes flow, when modeled by discrete vortex tubes.

A crucial ingredient of a gamma model, not explicitly present in a beta model, is the possibility of “splitting” a vortex tube into sub-tubes carrying smaller circulation. We suggest a dynamical basis for this process, as an instability of tubes whose cores violate the Rayleigh criterion.

The parameters describing a gamma model are not uniquely determined by our study, but there is a “simplest” helical gamma model, involving minimal splitting and distortion of tubes. The dimension D of the structure is 13/5, with a scale factor Λ = 2^?5/4. This value of D agrees with that suggested by Hentschel and Procaccia (1982), by analogy with established results for certain branched polymers.  相似文献   

Monitoring thickness and volume changes of the Dongkemadi Ice Field on the Qinghai-Tibetan Plateau (1969–2000) using Shuttle Radar Topography Mission and map data     

《International Journal of Digital Earth》2013,6(6):516-532

Abstract

This paper presents the first measurement of multi-decadal thickness and volume changes (1969–2000) of the Dongkemadi Ice Field (DIF) in the Tanggula Mountains, central Qinghai-Tibetan Plateau, China, using multi-source remote sensing data. These include the Shuttle Radar Topography Mission (SRTM) Digital Elevation Model (DEM) acquired in February, 2000, a DEM generated by digitising analogue topographic maps from 1969, and Landsat ETM+ imagery from 2000. Digital glacier outlines and GIS-based processing were used to calculate an elevation difference map to evaluate the relative elevation error of these two DEMs over ice-free areas. This method was also used to identify regions of glacier elevation thinning and thickening corresponding to glacier mass loss and gain. Analysis of 67,520 points on flat grass and rock terrain surrounding the DIF, with a slope less than 25°, showed a mean elevation difference of –0.90 m and a standard deviation of 5.58 m. A thickness change error within ±6 m was estimated. Between 1969 and 2000, 76.51% of the whole DIF area appeared to be thinning while 23.49% showed thickening. The average glacier surface thinning was –12.58 m with a standard deviation of 18.29 m and the estimated volume loss was 1.17 km³. The standard deviation of volume change was 0.0006 km³ over the DIF. A thinning rate up to 0.41±0.194 m a^?1 or 0.038 km³ a^?1 for the volume loss was observed for the whole ice field, which seems to be evidence for the ongoing retreat of glaciers on the Qinghai-Tibetan Plateau. It was found that the spatial thickness change pattern derived from the remote sensing method was consistent with the thickness change results of the Small Dongkemadi Glacier (SDG) from field measurements. The estimated error of the annual thickness change rate was on the order of 5%. The relationship between elevation change and absolute glacier elevation over typical glaciers was also analysed, showing considerable variability. These changes have possibly resulted from increased temperature and decreased precipitation in this region.  相似文献   

GLACIAL GEOMORPHOLOGY OF THE WHITE MOUNTAINS,CALIFORNIA AND NEVADA: ESTABLISHMENT OF A GLACIAL CHRONOLOGY     

Deborah L. Elliott-Fisk 《自然地理学》2013,34(4):299-323

The White Mountains, astride the California-Nevada stateline, are the highest and westernmost of the Great Basin ranges. This range was extensively glaciated during the Quaternary Period. Glacial landforms and scattered erratics were identified in the field using primarily morpho-and lithostratigraphic criteria, and mapped on aerial photographs and topographic maps. Topographic characteristics of the glacial deposits were analyzed using standard statistical procedures. A sequence of glacial deposits was identified in terms of six glacial stages, these glaciations named according to type site, and relative ages inferred. With the exception of perched Stage I (early) deposits along the range crest, reconstructed equilibrium-line altitudes and elevation of the glacier termini increase to the present, with glacier length and inferred size decreasing through time. Preliminary data suggest that weathering and pedogenesis are also progressive, though environmental gradients mask some of these distinctions between deposits. It is hypothesized that the White Mountains have a similar glacial chronology to the adjacent Sierra Nevada, but that with the intensification of the Sierran rainshadow during the Quaternary, the extent of glaciation in the White Mountains apparently decreased through time owing to regional tectonic uplift. This had led to the preservation of a more complete sequence of glacial deposits than in the Sierra Nevada, making the delineation of multiple mid-Quaternary events possible in many valleys. Further radiometric and chronometric dating of these deposits is in progress. [Key words: Glaciation, Quaternary, glacial geomorphology, White Mountains, California, Nevada.  相似文献   

Mapping Ecotone Movements: Holocene Dynamics of the Forest Tension Zone in Central Lower Michigan,Usa     

《自然地理学》2013,34(5):473-490

This paper presents a novel method for detecting and mapping movements of ecotone boundaries, over both time and space, using existing fossil pollen data. The method combines existing statistical techniques, including discriminant analysis and spatial interpolation, to generate statistically robust maps of ecotone boundaries over time. This method was applied to provide a comprehensive vegetative history of the forest tension zone in the Lower Peninsula of Michigan during the last 10,000 years. Pollen data for nine lakes in the Lower Peninsula were analyzed with discriminant analysis and spatially interpolated with inverse distance weighted to track ecotone movements of the forest tension zone. The forest tension zone has been in existence over the last 10,000 years and has been highly variable over space and time, with at least three major latitudinal shifts, spanning 320 km and 6° of latitude, as well as several less pronounced shifts. These shifts occurred primarily in response to millennial-scale oscillations in climate such as the warmer and dryer conditions associated with the mid-Holocene. A slighter shift was evident in association with the cooler and drier conditions of the Little Ice Age (600-100 cal yr BP). This research demonstrates the sensitivity of Great Lakes forest communities to Holocene climate change and suggests a similar sensitivity of future ecotone dynamics in response to modern global warming.  相似文献   

IPCC AR6解读：全球和区域海平面变化的监测和预估     

张通  俞永强  效存德  华莉娟  闫展 《气候变化研究进展》2022,18(1):12-18

IPCC第六次评估报告第一工作组报告第九章综合评估了与海平面相关的最新监测和数值模拟结果,指出目前（2006—2018年）的海平面上升速率处于加速状态（3.7 mm/a),并会在未来持续上升,且呈现不可逆的趋势。其中低排放情景（SSP1-1.9）和高排放情景（SSP5-8.5）下,到2050年,预估全球平均海平面（GMSL）分别上升0.15~0.23 m和0.20~0.30 m;到2100年,预估GMSL分别上升0.28~0.55 m和0.63~1.02 m。南极冰盖不稳定性是影响未来海平面上升预估的最大不确定性来源之一。区域海平面变化是影响沿海极端静水位的重要因素。  相似文献   

Improving the assessment of ICESat water altimetry accuracy accounting for autocorrelation     

Hani Abdallah  Jean-Stéphane Bailly  Nicolas Baghdadi  Nicolas Lemarquand 《ISPRS Journal of Photogrammetry and Remote Sensing》2011,66(6):833-844

Given that water resources are scarce and are strained by competing demands, it has become crucial to develop and improve techniques to observe the temporal and spatial variations in the inland water volume. Due to the lack of data and the heterogeneity of water level stations, remote sensing, and especially altimetry from space, appear as complementary techniques for water level monitoring. In addition to spatial resolution and sampling rates in space or time, one of the most relevant criteria for satellite altimetry on inland water is the accuracy of the elevation data. Here, the accuracy of ICESat LIDAR altimetry product is assessed over the Great Lakes in North America. The accuracy assessment method used in this paper emphasizes on autocorrelation in high temporal frequency ICESat measurements. It also considers uncertainties resulting from both in situ lake level reference data. A probabilistic upscaling process was developed. This process is based on several successive ICESat shots averaged in a spatial transect accounting for autocorrelation between successive shots. The method also applies pre-processing of the ICESat data with saturation correction of ICESat waveforms, spatial filtering to avoid measurement disturbance from the land–water transition effects on waveform saturation and data selection to avoid trends in water elevations across space. Initially this paper analyzes 237 collected ICESat transects, consistent with the available hydrometric ground stations for four of the Great Lakes. By adapting a geostatistical framework, a high frequency autocorrelation between successive shot elevation values was observed and then modeled for 45% of the 237 transects. The modeled autocorrelation was therefore used to estimate water elevations at the transect scale and the resulting uncertainty for the 117 transects without trend. This uncertainty was 8 times greater than the usual computed uncertainty, when no temporal correlation is taken into account. This temporal correlation, corresponding to approximately 11 consecutive ICESat shots, could be linked to low transmitted ICESat GLAS energy and to poor weather conditions. Assuming Gaussian uncertainties for both reference data and ICESat data upscaled at the transect scale, we derived GLAS deviations statistics by averaging the results at station and lake scales. An overall bias of −4.6 cm (underestimation) and an overall standard deviation of 11.6 cm were computed for all lakes. Results demonstrated the relevance of taking autocorrelation into account in satellite data uncertainty assesment.  相似文献