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东亚石炭-二叠纪有孔虫迁移与海水进退规程的相关模式   总被引:1,自引:0,他引:1  
徐炳川 《地球学报》1996,17(Z1):155-163
亚洲东部石炭-二叠纪生物地层,沿北纬42°左右存在一条质变线。此线南北海水进退呈现互为消长的关系,反映了海水南北方向的4次活动过程:早石炭世早与早二叠世,自南而北;中晚石炭世与晚二叠世,自北而南。生物最低层位的变化情况,反映了生物也经历了相应的南北方向的4次迁移过程。这种迁移可能与气候的冷暖变化有关,而海水运动可能源于地球自转速率的不均衡,甚至气候的变化最终也是地球自转不均衡的结果。  相似文献   
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Discoidal sedimentary structures are commonly described in Proterozoic strata, and even more common in Ediacaran to lower Cambrian sedimentary successions. Many abiotic processes are able to produce such circular or discoidal structures in bedding planes, however, their abundance in Ediacaran strata suggests a possible correlation with the evolution and preservation of epibenthic metazoans that emerged at the end of this period. In the South American paleontological record, studies regarding the Ediacaran soft-bodied organisms are meager and restricted to few reports in Brazil, Paraguay, and Argentina. In many cases, such “fossils” were only tentatively characterized in terms of their general morphology and putative taxonomic affinity. Thus, considering the almost absence of work on these enigmatic structures in South America, this paper aims to make a critical analysis on the main occurrences of Ediacaran-Cambrian discoidal structures described in this continent. Based on a detailed review and unpublished data, it was possible to provide a general picture concerning the main paleoenvironmental and sedimentary significance of this structures, as well as on the most promising prospects in terms of the paleontological record of Ediacaran soft-bodied metazoans in South American. In this sense, it was settled that occurrences such as those in the Jaibaras and Itajaí basins should be reassessed in order to establish reliable criteria of biogenicity. In the case of the material from the Sete Lagoas and Tagatiya Guazu formations, it is considered more parsimonious to interpret the discoidal features as resulting from microbial processes. Similarly, the discoidal structures of the Cerro Negro Formation presents a series of internal laminations and textures that resembles those developed by processes of microbial grain binding and trapping suggesting that, at least part of this material, can be related to microbially induced sedimentary structures. Finally, for the ichnologically diversified Puncoviscana and Camaquã basins, two different scenarios were identified. The first presents an ichnological assemblage strongly indicative of lower Paleozoic, and possibly Cambrian affinity. Thus, the discs in association with these traces, should be viewed with caution and interpretations made in light of a Paleozoic context. The second possesses an ichnological association typical of that expected for the Ediacaran-Cambrian transition, and the diversity of discoidal forms can potentially represent imprints of macroorganisms on a microbially bounded substrate, thus deserving a more detailed approach.  相似文献   
3.
90Sr长期被视为最重要的人工放射性核素之一,日本福岛核事故导致包括90Sr在内的大量放射性物质泄漏进入海洋,厂区储水罐中冷却废水至今仍然存在大量90Sr。海洋中90Sr分析方法却繁琐耗时,导致核事故后的90Sr研究较为匮乏,且缺乏系统的认识。本研究在2015?2018年期间测量南海海水和多种海洋生物(马尾藻、海虾、牡蛎、红树林植物、造礁珊瑚)中90Sr的基础上,深入分析核事故后日本周边海域和南海90Sr的比活度水平与环境半衰期。结合文献资料,本研究发现1975?2010年期间日本近岸海水90Sr的环境半衰期为15.4 a,2011年的核事故后日本周边海洋中90Sr比活度显著升高,基于ERICA软件定量计算核事故后90Sr对海洋鱼类的剂量率比核事故前的结果高5个数量级。南海作为福岛核事故后北太平洋环流的下游海域,本文进一步构建1984?2018年期间南海90Sr比活度的历史曲线,发现核事故前后南海90Sr比活度水平没有可识别的变化,进一步定量计算南海90Sr的环境半衰期为26.7 a,发现边缘海和大洋中90Sr和137Cs环境半衰期格局差异与核素(90Sr和137Cs)的源汇过程(河流输入和海洋生物泵)密切相关。鉴于海洋中90Sr分析方法的挑战性,本文发现在10多种海洋生物中造礁珊瑚骨骼几乎拥有最高的90Sr浓集因子(约1 000 L/kg),同时具有较易大量获取、固定附着生长、连续高分辨率记录、前处理简单快速等优点,很可能是海洋中90Sr可靠的指示生物。造礁珊瑚中90Sr研究将有利于揭示人工放射性核素的源汇过程,同时为我国海洋放射性监测方案和相关标准导则的优化和完善提供有益的参考。  相似文献   
4.
Recent geological and paleontological exploration in the Indus basin of Pakistan allowed the discover-ies of numerous remains of non-marine reptiles (titanosaurian sauropod,abelisaurian and noasaurian theropod dinosaurs),and marine reptiles (crocodiles),flying reptiles (pterosaurs),marine and non-marine mammals, fishes,invertebrates,and plants,especially Pakistan is relatively rich in footprints/trackways in the Mesozoic. These vertebrates of Indo-Pakistan are very significant for paleobiogeographic study due to the present-day con-nection of this continent with Asia in Northern Hemisphere,whereas during past (Jurassic and pre-Jurassic)it was connected to the Gondwana.The Mesozoic vertebrates show close affinities with Gondwanan landmasses. The Cenozoic vertebrates show Eurasian affinity and migrated from Indo-Pak subcontinent to Eurasia or vice ver-sa via Paleo Indus River systems along Western Indus Suture,after long journey of about 6 000 km the first col-lision of Indo-Pak subcontinent with Asia occurred at terminal Cretaceous.  相似文献   
5.
Ideas of global glaciations on the Earth repeatedly emerged in geology since the middle of the 19th century, but they all did not withstand the test of time. The hypothesis of snowball Earth that suggests long-lasted continuous glaciations over the entire land and oceans in the Late Riphean and Vendian became a popular topic of discussions worldwide. These glaciations must last continuously 15 million years or more owing to enormous stability of climate on the “White Earth,” and one can expect their cessation only when CO2 concentration in the atmosphere is getting by several orders of magnitude higher in the course of volcanic eruptions. However, many sections of the Late Precambrian glacial deposits evidence repeated alternations of glacial and interglacial events of variable rank and oscillations of glaciers. Consequently, liquid water existed on the Earth, hydrological cycle had not been interrupted, and development of phototrophic phytoplankton, the eukaryotic organisms inclusive, was always in progress. These facts and results of the climate mathematical simulation are inconsistent with the concept of long continuous glaciations over the globe and their consequences. Paleomagnetic data represent main starting point of the snowball Earth hypothesis, although they are to a great extent still of insufficient validity for the Precambrian. Equitable criticism of the hypothesis weak sides turns sometimes into denying all the glacial periods of the Late Proterozoic, when tillites are regarded in majority as deposits of subaqueous slumps and debris flows accumulated on walls of oceanic rifts prograding along splitting lines into the Rodinia continent. Doubtless marks of glaciations in the sedimentary succession are regarded therewith as heterochronous indications of local mountain glaciers on risen shoulders of prograding rifts. Widespread occurrence of the Late Precambrian discrete glacial horizons on the platforms means, however, that glaciations of that time have been associated not always with the rifts and consisted of separate glacial events. Glaciogenic horizons comparable in thickness and structure with those of the Phanerozoic Eonothem are also indicative of discrete glacial periods in the Precambrian. Being confined predominantly to the Late Precambrian succession of rifts, these horizons characterize a high burial potential of these structures, whereas outside them glacial horizons of lesser thickness could be easily subjected to erosion. The hypothesis of snowball Earth is inadequately consistent with well-known facts and needs in additional substantiation. There are also grounds to think that oceans have not been completely covered with ice at the time of Precambrian glaciations.  相似文献   
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