白垩纪四足动物足印的生物地层学、生物年代学与遗迹相

从全球范围来看,白垩纪四足动物的足印多数是非鸟恐龙与鸟类留下的痕迹;少量足印来自翼龙、鳄鱼、龟、哺乳动物和其他四足动物。白垩纪的足迹化石以东亚(尤其是中国和朝鲜)和北美西部的最为人所知。南美(主要是阿根廷和巴西)也有一定数量广泛分布的足迹化石,欧洲、非洲与澳大利亚的白垩纪足迹组合则鲜为人知。以白垩纪四足动物的足印记录为基础,我们对两个全球足印生物年代重新进行了检查。早白垩世生物年代以蜥脚类与鸟脚类的足迹为特征。晚白垩世生物年代中的蜥脚类足迹较少,但是鸭嘴龙、暴龙和角龙的足迹增多了。另外,白垩纪足印化石的记录中记载了许多重要的生物地层学信息,如北美白垩纪中期蜥脚类恐龙的消失,以及白垩纪末恐龙的绝灭。越来越多来自东亚的白垩纪足印记录使我们对更精细的地方性白垩纪足印生物年代学有了初步印象。因此,以地方性四足恐龙(包括鸟类)遗迹属的地层分布为基础,可以识别出三个或四个足印生物年代。种类丰富并具有地方性特色的东亚的白垩纪鸟类动物的遗迹群,可能指示白垩纪时东亚存在着一个独特而繁盛的鸟类动物群。以足印化石为基础的这一假说有待进一步的验证。     

High-pressure phase relations in the system TiO2–ZrO2 to 12?GPa: stability of αPbO2-type srilankite solid solutions of (Ti1?x , Zr x )O2 (0?≤?x?≤?0.6)

Phase relations in the system TiO₂–ZrO₂ were examined in the pressure range of 3.5–12?GPa at 1,800?°C, using multianvil apparatus. At 1,800?°C, TiO₂ rutile transforms to αPbO₂ structure at 10?GPa, and the αPbO₂-type solid solution is stable in compositional range between TiO₂ and about (Ti_0.6, Zr_0.4)O₂ at 3.5–12?GPa. Combination of the present results with the published data at 0–3?GPa demonstrates that continuous solid solution with the αPbO₂-type structure is stable between TiO₂ and (Ti_1?x , Zr _x )O₂ (x?≈?0.6) at 0–12?GPa. This indicates that both the αPbO₂-type TiO₂ and srilankite Ti₂ZrO₆ with the same structure belong to the continuous solid solution system though the two phases have been regarded as different minerals. With increasing ZrO₂ content, lattice parameters of a- and c-axes of the αPbO₂-type solid solution increase, but b-axis is almost constant or slightly decreases. At higher pressure, the αPbO₂-type solid solution dissociates into two phases, αPbO₂-type phase and tetragonal zirconia. Srilankite with more TiO₂-rich composition than Ti₂ZrO₆ might be found in natural rocks derived from the deep upper mantle.     

A case study of Kelvin-Helmholtz vortices on both flanks of the Earth's magnetotail

Kelvin-Helmholtz instability (KHI) is a fundamental fluid dynamical process that develops in a velocity shear layer. It is excited on the tail-flanks of the Earth's magnetosphere where the flowing magnetosheath plasma and the stagnant magnetospheric plasma sit adjacent to each other. This instability is thought to induce vortical structures and play an important role in plasma transport there. While KHI vortices have been detected, the earlier observations were performed only on one flank at a time and questions related to dawn-dusk asymmetry were not addressed. Here, we report a case where KHI vortices grow more or less simultaneously and symmetrically on both flanks, despite all the factors that may have broken the symmetry. Yet, energy distributions of ions in and around the vortices show a remarkable dawn-dusk asymmetry. Our results thus suggest that although the initiation and development of the KHI depend primarily on the macroscopic properties of the flow, the observed enhancement of ion energy transport around the dawn side vortices may be linked to microphysical processes including wave-particle interactions. Possible coupling between macro- and micro-scales, if it is at work, suggests a role for KHI not only within the Earth's magnetosphere (e.g., magnetopause and geomagnetic tail) but also in other regions where shear flows of magnetized plasma play important roles.