排序方式： 共有54条查询结果，搜索用时 15 毫秒
51.
52.
K. Gajewski André E. Viau Michael Sawada David E. Atkinson Philippe Fines 《Climatic change》2006,78(2-4):341-361
Interpreting the postglacial climate history of the European continent using pollen data has proven difficult due in part to human modification of the landscape. Separating climate from human-caused changes in the vegetation requires a strategy for determining times of change across the entire region. We quantified transitions in the vegetation across Europe during the past 12,000 years using a mixture model approach on two datasets: radiocarbon dates from pollen diagrams and zone boundaries from selected reference sites. Major transitions in the vegetation, as recorded in pollen diagrams, appear synchronous across the continent. These transitions were also synchronous with those identified in North America pollen diagrams and major environmental changes recorded in North Atlantic marine records and Greenland ice cores. This synchronicity suggests that the major vegetation transitions in Europe during the Holocene and late glacial were primarily caused by large-scale atmospheric circulation change. These climate changes may have caused some of the cultural, political and migration changes in European societies during the Holocene. 相似文献
53.
Structure refinement was carried out on a nonstoichiometric clinopyroxene grown under the ambient pressure from a glass with
composition of 23%(mol)Di+53%Es+24%An. The degree of nonstoichiometry in this crystal is significantly larger than those of
clinopyroxenes reported previously, which were found in high pressure products. The refinement gave the empirical chemical
formula (Ca0.742Fe0.087)(Mg0.016Al0.888Fe0.075)(Al0.500Si1.500)O6 for the crystal and showed that vacancies are located mainly at M2 sites. Despite replacement of Si by Al, the crystal has
a smaller cell volume than diopside primarily owing to a significant amount of Mg at M1 being replaced by Al.
Received: 25 June 1997 / Revised, accepted: 6 September 1997 相似文献
54.
Forbidden reflections of some pyroxenes described as having a lower symmetry than Pbca or C2/c have been examined, mainly using the detailed ψ-scanning method of the four-circle automated diffractometer. All reflections violating the systematic absences of Pbca orthopyroxene, C2/c spodumene, P2/n omphacite and P21/c pigeonite were found to be due to the Umweganregung process of multiple diffraction; the Umweganregung peaks observed for the ψ azimuth were indexed in terms of a four-circle geometry. Thus, the space groups of orthopyroxene, α spodumene, low omphacite and low pigeonite were confirmed to be Pbca, C2/c, P2/n and P21/c, respectively. 相似文献