分类： 地球科学 >> 地质学 提交时间： 2017-08-24 合作期刊: 《古脊椎动物学报》
摘要： The diets and environments of fossil rhinocerotoids from the Linxia Basin, Gansu, China, ranging in age from 25 to 2.5 Ma, were reconstructed based on bulk and serial carbon (C) and oxygen (O) isotope analyses of tooth enamel. The results support many previous hypotheses inferred from dentition and cranial and limb morphology and offer new insight on the paleoecology of some genera. The isotopic results support the following previous hypotheses: the Late Oligocene rhino Paraceratherium inhabited a forested environment, and the coexisting rhino Allacerops lived in a relatively open habitat and had a less specialized diet; the Middle Miocene Hispanotherium grazed in open territory, whereas the contemporaneous Alicornops had a more generalized diet in a forested environment; and the Late Miocene rhino Parelasmotherium grazed in an open steppe habitat. The isotope data indicate that the rhinos Acerorhinus and Dicerorhinus inhabited open steppe environments, inconsistent with previous inferences that these two rhinos dwelled in forested environments. The isotopic results are not conclusive concerning the habitat of Iranotherium, but support previous hypotheses that this rhino was a specialized C3 grazer. The results also suggest that Chilotherium was a forest-dweller throughout much of the Late Miocene, but occupied a more open environment by the end of the Late Miocene. Additionally, the results are consistent with previous hypotheses that the Pliocene rhino Shansirhinus and the Pleistocene rhino Coelodonta were grazers in open habitats. Finally, the C isotope data support that all rhinos in this study were pure C3 feeders, confirming that C4 grasses were not an important component of the plant biomass in the Linxia Basin from 25 to 2.5 Ma. 根据牙齿釉质的全样和系列碳、氧同位素分析，重建了甘肃临夏盆地25~2.5Ma期间犀牛的食性和环境。其结果支持先前根据牙齿和头骨-肢骨形态做出的一些推断，并对部分属的古生态提出了新的看法。同位素结果支持如下的推断：晚渐新世的巨犀Paraceratherium生活于森林环境，而与其共生的异角犀Allacerops栖息地相对开阔，食性特化程度低；中中新世的西班牙犀Hispanotherium在开阔领地上取食草本植物，而同时代的奇角犀Alicornops在森林环境中具有更特化的食性；晚中新世的副板齿犀Parelasmotherium在开阔的稀树草原上取食草本植物。与先前认为无鼻角犀Acerorhinus和额鼻角犀Dicerorhinus生活于森林化环境的推断不同，数据指示这两种犀牛也生活于开阔的稀树草原环境。同位素结果未能得出伊朗犀Iranotherium的确切生活环境，但支持先前认为它特化为C3草本植物取食者的推断。研究结果也显示大唇犀Chilotherium在晚中新世的大多数时间内是森林生活者，但其在晚中新世末期占据了更开阔的环境。此外，分析结果与先前的推断一致，即上新世的山西犀Shansirhinus和更新世的披毛犀Coelodonta是开阔生活环境中的食草者。最后，碳同位素数据支持在此项研究中的所有犀牛都是纯C3取食者，确认C4植物不是临夏盆地从25Ma到2.5Ma期间植被的重要成分。
分类： 地球科学 >> 空间物理学 提交时间： 2016-05-12
摘要： The spatial and temporal invariance in the spectra of energetic particles in gradual solar events is reproduced in simulations. Based on a numerical solution of the focused transport equation, we obtain the intensity time profiles of solar energetic particles (SEPs) accelerated by an interplanetary shock in three-dimensional interplanetary space. The shock is treated as a moving source of energetic particles with a distribution function. The time profiles of particle fluxes. with different energies are calculated in the ecliptic at 1 AU. According to our model, we find that shock acceleration strength, parallel diffusion, and adiabatic cooling are the main factors in forming the spatial invariance in SEP spectra, and perpendicular diffusion is a secondary factor. In addition, the temporal invariance in SEP spectra is mainly due to the effects of adiabatic cooling. Furthermore, a spectra invariant region, which agrees with observations but is different from. the one suggested by Reames et al. is proposed based on our simulations.
分类： 地球科学 >> 空间物理学 提交时间： 2016-05-12
摘要： In this work, a gradual solar energetic particle (SEP) event observed by multi-spacecraft has been simulated. The time profiles of SEP fluxes accelerated by an interplanetary shock in the three-dimensional interplanetary space are obtained by solving numerically the Fokker-Planck focused transport equation. The interplanetary shock is modeled as a moving source of energetic particles. By fitting the 1979 March 01 SEP fluxes observed by Helios 1, Helios 2, and IMP 8 with our simulations, we obtain the best parameters for the shock acceleration efficiency model. And we also find that the particle perpendicular diffusion coefficient with the level of similar to 1%-3% of parallel diffusion coefficient at 1 AU should be included. The reservoir phenomenon is reproduced in the simulations, and the longitudinal gradient of SEP fluxes in the decay phase, which is observed by three spacecraft at different locations, is more sensitive to the shock acceleration efficiency parameters than that is to the perpendicular diffusion coefficient.
分类： 地球科学 >> 空间物理学 提交时间： 2016-05-12
摘要： This paper investigates the onset time of solar energetic particle (SEP) events with numerical simulations and analyzes the accuracy of the velocity dispersion analysis (VDA) method. Using a three-dimensional focused transport model, we calculate the fluxes of protons observed in the ecliptic at 1 AU in the energy range between 10 MeV and 80 MeV. In particular, three models are used to describe different SEP sources produced by flare or coronal shock, and the effects of particle perpendicular diffusion in the interplanetary space are also studied. We have the following findings. When the observer is disconnected from the source, the effects of perpendicular diffusion in the interplanetary space and particles propagating in the solar atmosphere have a significant influence on the VDA results. As a result, although the VDA method is valid with impulsive source duration, low background, and weak scattering in the interplanetary space or fast diffusion in the solar atmosphere, the method is not valid with gradual source duration, high background, or strong scattering.
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