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321.
322.
K.D. Wake-Dyster M.J. Sexton D.W. Johnstone C. Wright D.M. Finlayson 《Geophysical Journal International》1987,89(1):423-430
Summary. In 1984, the Australian Bureau of Mineral Resources and the Geological Survey of Queensland recorded a regional seismic reflection profile of over 800 km length from the eastern part of the Eromanga Basin to the Beenleigh Block east of the Clarence Moreton Basin. A relatively transparent upper crustal basement with an underlying, more reflective lower crust is characteristic of much of the region. Prominent westerly dipping reflectors occur well below the sediments of the eastern margin of the Clarence Moreton Basin and the adjacent Beenleigh Block, and provide some of the most interesting features of the entire survey. A wide angle reflection/refraction survey of 192 km length and an expanding reflection spread of 25 km length were recorded across the Nebine Ridge. The only clear deep reflectors are interpreted as P-to-SV or SV-to-P converted reflections from a mid-crustal boundary at a depth of about 17 km. The combined Nebine Ridge data provide well-constrained P and S wave velocity models of the upper crust, and suggest a crustal structure quite different from that beneath the adjacent Mesozoic basins. 相似文献
323.
Potential implications of differential preservation of testate amoeba shells for paleoenvironmental reconstruction in peatlands 总被引:1,自引:0,他引:1
Edward A. D. Mitchell Richard J. Payne Mariusz Lamentowicz 《Journal of Paleolimnology》2008,40(2):603-618
Testate amoebae are now commonly used in paleoenvironmental studies but little is known of their taphonomy. There is some
experimental evidence for differential preservation of some testate amoeba shell types over others, but it is unclear what,
if any impact this has on palaeoenvironmental reconstruction. To investigate this issue we looked at palaeoecological evidence
for the preservation of different shell types. We then investigated the possible impact of selective preservation on quantitative
palaeoenvironmental inference. We first used existing palaeoecological data sets to assess the vertical patterns of relative
abundance in four testate amoeba shell types: (1) shells made of secreted biosilica plates (idiosomes, e.g. Euglypha), (2) idiosomes with thick organic coating (Assulina), (3) proteinaceous shells (e.g. Hyalosphenia), (4) shells built from recycled organic or mineral particles (xenosomes) (e.g. Difflugia, Centropyxis). In three diagrams a clear pattern of decay was only observed for the idiosome type. In order to assess the implications
of differential preservation of testate amoeba taxa for paleoenvironmental reconstruction we then carried out simulations
using three existing transfer functions and a wide range of scenarios, downweighting different test categories to represent
the impact of selective test decomposition. Simulation results showed that downweighting generally reduced overall model performance.
However downweighting a shell type only produced a consistent directional bias in inferred water table depth where that shell
type is both dominant and shows a clear preference along the ecological gradient. Applying a scenario derived from previous
experimental work did not lead to significant difference in inferred water table. Our results show that differential shell
preservation has little impact on paleohydrological reconstruction from Sphagnum-dominated peatlands. By contrast, for the minerotrophic peatlands data-set loss of idiosome tests leads to consistent underestimation
of water table depth. However there are few studies from fens and it is possible that idiosome tests are not always dominant,
and/or that differential decomposition is less marked than in Sphagnum peatlands. Further work is clearly needed to assess the potential of testate amoebae for paleoecological studies of minerotrophic
peatlands. 相似文献
324.
Summary This paper concerns the use of airborne or ground-based Doppler radars to observe precipitating systems over complex orography.
As nearly all of the previous experiments involving Doppler radars were conducted over flat surfaces over the continents or
the oceans, new techniques are needed firstly to separate ground clutter from meteorological signal and, in the case of airborne
Doppler observations, to deduce navigational errors. Secondly, it is necessary to take the atmospheric circulation induced
by orography into account in the three-dimensional wind field analysis. Variational techniques are presented to solve these
problems.
The proposed methods are tested with simulated ground-based and airborne Doppler radar observations for analytic flows over
analytic terrains and for numerically simulated wind and reflectivity fields for the Brig event (22 September 1993) of heavy
precipitation over the southern flank of the Alps (Cosma and Richard, 1998), and with actual airborne Doppler data relative
to weak snow showers over the Rocky Mountains on 12 March 1995.
Received March 22, 1999/Revised June 1, 1999 相似文献
325.
326.
The question of what lies ahead is of particular concern for Latin Americanists. The last decade has witnessed a serious erosion of both the popularity of their specialty, and an equally troublesome reduction in employment opportunities. This paper uses Association of American Geographers (AAG) data bases to document the age-gender structure of contemporary Latin Americanist geographers, and projects likely compositional changes through the end of the century. 相似文献
327.
328.
Geocentre motion from the DORIS space system and laser data to the Lageos satellites: comparison with surface loading data 总被引:3,自引:0,他引:3
F. Bouillé A. Cazenave J. M. Lemoine J. F. Crétaux 《Geophysical Journal International》2000,143(1):71-82
Surface mass redistribution within the Earth system, especially in the atmosphere, oceans, continents and ice sheets, causes the position of the centre of mass to vary in a reference frame attached to the solid Earth. Space techniques are now precise enough to measure the centre of mass motion. Here we present a determination of the centre of mass coordinates at regular monthly intervals using DORIS data on SPOT‐2, SPOT‐3 and Topex–Poseidon (1993–1997) and laser data on Lageos‐1 and Lageos‐2 (1993–1996). The amplitude and phase of the space‐geodesy‐derived annual cycle for each coordinate are further compared to estimates based on surface mass redistribution at the Earth surface derived from various climatic data sources: surface pressure, soil moisture, snow depth and ocean mass variations. 相似文献
329.
J. Christopher Brown Lisa Rausch Verônica Gronau Luz 《The Professional geographer》2013,65(2):249-259
Brazilian agricultural census data at the municipal level are used to develop and map a simple index of staple food versus nonstaple food agriculture for Brazil over time (1996–2006). The results show spatial variation in the direction and degree of the shift toward or away from staple food cropping across Brazil. The index is presented as an important methodological step toward a systematic geographic understanding of crop share changes surrounding food versus fuel and other nonfood crop production. 相似文献
330.
Mats P. Björkman Elke Morgner Robert G. Björk Elisabeth J. Cooper Bo Elberling & Leif Klemedtsson 《Polar research》2010,29(1):75-84
Recent climate change predictions suggest altered patterns of winter precipitation across the Arctic. It has been suggested that the presence, timing and quantity of snow all affect microbial activity, thus influencing CO2 production in soil. In this study annual and seasonal emissions of CO2 were estimated in High-Arctic Adventdalen, Svalbard, and sub-Arctic Latnjajaure, Sweden, using a new trace gas-based method to track real-time diffusion rates through the snow. Summer measurements from snow-free soils were made using a chamber-based method. Measurements were obtained from different snow regimes in order to evaluate the effect of snow depth on winter CO2 effluxes. Total annual emissions of CO2 from the sub-Arctic site (0.662–1.487 kg CO2 m–2 yr–1 ) were found to be more than double the emissions from the High-Arctic site (0.369–0.591 kg CO2 m–2 yr–1 ). There were no significant differences in winter effluxes between snow regimes or vegetation types, indicating that spatial variability in winter soil CO2 effluxes are not directly linked to snow cover thickness or soil temperatures. Total winter emissions (0.004–0.248 kg CO2 m–2 ) were found to be in the lower range of those previously described in the literature. Winter emissions varied in their contribution to total annual production between 1 and 18%. Artificial snow drifts shortened the snow-free period by 2 weeks and decreased the annual CO2 emission by up to 20%. This study suggests that future shifts in vegetation zones may increase soil respiration from Arctic tundra regions. 相似文献