排序方式： 共有7条查询结果，搜索用时 15 毫秒
1
1.
Lutz Nasdala Wolfgang Hofmeister Nicholas Norberg James M. Martinson Fernando Corfu Wolfgang Dörr Sandra L. Kamo Allen K. Kennedy Andreas Kronz Peter W. Reiners Dirk Frei Jan Kosler Yusheng Wan Jens Götze Tobias Häger Alfred Kröner John W. Valley 《Geostandards and Geoanalytical Research》2008,32(3):247-265
We introduce and propose zircon M257 as a future reference material for the determination of zircon U‐Pb ages by means of secondary ion mass spectrometry. This light brownish, flawless, cut gemstone specimen from Sri Lanka weighed 5.14 g (25.7 carats). Zircon M257 has TIMS‐determined, mean isotopic ratios (2s uncertainties) of 0.09100 ± 0.00003 for 206pb/238U and 0.7392 ± 0.0003 for 207pb/235U. Its 206pb/238U age is 561.3 ± 0.3 Ma (unweighted mean, uncertainty quoted at the 95% confidence level); the U‐Pb system is concordant within uncertainty of decay constants. Zircon M257 contains ～ 840 μg g?1 U (Th/U ～ 0.27). The material exhibits remarkably low heterogeneity, with a virtual absence of any internal textures even in cathodoluminescence images. The uniform, moderate degree of radiation damage (estimated from the expansion of unit‐cell parameters, broadening of Raman spectral parameters and density) corresponds well, within the “Sri Lankan trends”, with actinide concentrations, U‐Pb age, and the calculated alpha fluence of 1.66 × 1018 g?1. This, and a (U+Th)/He age of 419 ± 9 Ma (2s), enables us to exclude any unusual thermal history or heat treatment, which could potentially have affected the retention of radiogenic Pb. The oxygen isotope ratio of this zircon is 13.9%o VSMOW suggesting a metamorphic genesis in a marble or calc‐silicate skarn. 相似文献
2.
3.
Tom L. Martinson 《The Professional geographer》1980,32(4):471-477
Much science-fiction literature is based upon the creation and exploration of alternative environments. Stanley G. Weinbaum's short story, “Parasite Planet,”introduces an unusual environment that may be wrong in the light of recent evidence on Venus but still stimulates the geographical imagination. 相似文献
4.
Glenn R. Gay Hubert H. Gay William H. Gay Holly A. Martinson Robert H. Meade John A. Moody 《地球表面变化过程与地形》1998,23(7):651-662
Over a period of several decades, gullies have been observed in various stages of forming, growing and completing the cutoff of meander necks in Powder River. During one episode of overbank flow, water flowing over the down-stream bank of the neck forms a headcut. The headcut migrates up-valley, forming a gully in its wake, until it has traversed the entire neck, cutting off the meander. The river then follows the course of the gully, which is subsequently enlarged as the river develops its new channel. The complete process usually requires several episodes of high water: in only one of the five cases described herein was a meander cutoff initiated and completed during a single large flood. © 1998 John Wiley & Sons, Ltd. 相似文献
5.
Mountain ecosystems have been projected to experience faster rates of warming than surrounding lowlands. These changes in climatic conditions could have significant impacts on high-altitude Andean environments, affecting the quality and magnitude of their economic and environmental services. Even though long-term data in these regions are limited, it is important to identify any discernible long-term trends in local climatic conditions. Time series of several variables were analyzed to detect statistically significant long-term linear trends that occurred over recent years in a páramo ecosystem of the Colombian Central Andes. Records included cloud characteristics, sunshine, rainfall, minimum and maximum temperatures, diurnal temperature range, and relative humidity. Conditions of atmospheric stability were also explored. Total sunshine exhibited decreasing trends ranging from ?3.7 to ?8.5% per decade at altitudes around the pluviometric optimum. The strongest changes in sunshine occurred during the December-January-February season. Mean relative humidity observed at altitudes around and below this threshold showed increasing trends of +0.6 to +0.7% per decade. Annual rainfall and mean relative humidity above the optimum showed decreasing trends ranging from ?7 to ?11% per decade and from ?1.5 to ?3.6% per decade, respectively. Minimum temperatures on the coldest days and maximum temperatures on the warmest days exhibited increasing trends at all altitudes ranging from +0.1 to +0.6, and from +0.2 to +1.1°C per decade, respectively. Increases in minimum and maximum temperatures at higher altitudes were significantly greater than those observed in average at lower altitudes. The strongest changes in minimum temperatures, particularly, occurred during the December–January–February and June–July–August dry seasons. All these changes suggest that atmospheric conditions in the area are shifting from statically unstable conditions to conditionally unstable or statically stable conditions. Observed historical trends indicate that climate impacts and other human activities have stressed these unique and fragile environments. 相似文献
6.
The Arctic as a trigger for glacial terminations 总被引：1，自引：0，他引：1
We propose a hypothesis to explain the very abrupt terminations that end most of the glacial episodes. During the last glaciation,
the buildup and southerly expansion of large continental ice-sheets in the Northern Hemisphere and extensive cover of sea
ice in the N. Pacific and the N. Atlantic imposed a much more zonal climatic circulation system than exists today. We hypothesize
that this, in combination with the frigid (dry) polar air led to a significant decrease in freshwater runoff into the Arctic
Ocean. In addition the freshwater contribution of the fresher Pacific water was completely eliminated by the emergence of
the Bering Strait (sill depth 50 m). As the Arctic freshwater input was depleted, regions of the Arctic Ocean lost surface
stability and eventually overturned, bringing warmer deep water to the surface where it melted the overlying sea ice. This
upwelled water was quickly cooled and sank as newly formed deep water. For sustained overturn events, such as might have occurred
during the peak of very large glacial periods (i.e. the last glacial maximum), the voluminous deep water formed would eventually
overflow into the Nordic Seas and North Atlantic necessitating an equally voluminous rate of return flow of warmer surface
waters from the North Atlantic thus breaking down the Arctic's zonal isolation, melting the expansive NA sea ice cover and
initiating oceanic heating of the atmosphere over the ice-sheets bordering the NA. We suggest that the combined effect of
these overturn-induced events in concert with a Milankovitch warming cycle, was sufficient to drive the system to a termination.
We elaborate on this proposed sequence of events, using the model for the formation of the Weddell Sea polynya as proposed
by Martinson et al. (1981) and various, albeit sparse, data sets from the circum-Arctic region to apply and evaluate this
hypothesis to the problem of glacial terminations. 相似文献
7.
Pozdnyakov Sh. R. Ivanova E. V. Guzeva A. V. Shalunova E. P. Martinson K. D. Tikhonova D. A. 《Water Resources》2020,47(4):599-607
Water Resources - The concentration of microplastic particles (with size from 100 µm to 5 mm) and their chemistry were studied in water, subsoils and sediments in the coastal area of the Neva... 相似文献
1