| 摘要: |
| 火山灰的研究作为探讨古气候和古环境的一种手段，近十年来引起了国内外学者的广泛关注。气候学和古环境学的研究表明，火山喷发可以导致某一区域内短期的气候变化或者对冰期产生强烈影响。同时，火山灰层的成因机制决定了其空间分布的等时性和广泛性，因此，火山灰的地层学特征和精确定年，具有重要的地层对比意义。目前，全球各地已经开展了许多火山灰年代学与地层学的研究。在中国，尤其是东北地区，虽然有着丰富的火山资源，但是火山灰年代学与地层学的研究工作依然比较匮乏，本文简要回顾了一下中国东北、欧洲、新西兰、日本、俄罗斯等地区的火山灰研究工作，旨在为以后的研究提供一定的参考资料。 |
| 关键词: 火山灰年代学 火山灰地层学 第四纪 |
| DOI：10.7515/JEE201603001 |
| CSTR：32259.14.JEE201603001 |
| 分类号: |
| 基金项目:国家自然科学基金项目（41202260）；Overseas Research Scholarship（UK, 2007?—?2010） |
| 英文基金项目:National Natural Science Foundation of China (41202260); Overseas Research Scholarship (UK, 2007?—?2010) |
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| Quaternary tephrochronological and tephrostratigraphical studies: a brief review |
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ZHAO Hongli, HALL Valerie A, LIU Jiaqi1,2,3
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1. State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an 710061, China;2. School of Geography, Archaeology and Palaeoecology, Queen’s University
Belfast, BT7 1NN UK;3. Key Laboratory of Cenozoic Geology and Environment, Institute of
Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
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| Abstract: |
| Background, aim, and scope As a dating method, Quaternary tephrochronology and tephrostratigraphy have now been employed in many areas of the world. This article presents a brief review of tephra studies in some countries in Europe, New Zealand and the countries around NE China (Japan and Russia). Tephrochronology and tephrostratigraphy are well established in Europe and New Zealand, however only few work has been done in NE China (to date). Since the 1930s, tephrochronological methods have been used extensively to solve problems in Quaternary geology, archaeology, paleopetrology, and paleogeography. A pioneer in the use of tephra layers to establish chronology was Sigurdur Thorarinsson, who began by studying the layers he found in the deposits of his native Iceland. In the early stage of tephra studies, the layers were often mapped with their source volcanoes by observing the colour and petrology of the tephra, as these are characteristics of a particular volcano. Then, the real age of the tephra layers was dated directly with reasonable accuracy using radiometric dating methods such as glass-fission-track, Potassium-Argon (K-Ar), Argon-Argon (39Ar-40Ar), radiocarbon (14C), uranium series, thermo-luminescence and electron spin resonance. Therefore, more and more tephras have been dated indirectly by correlation and now many improved reference sets for indirect dating through tephras from most volcanic centers are accessible in geological databases such as Tephrabase available at http://www.tephrabase.org/. Further information can be added to the Tephrabase in the future, whenever the details are available. Therefore, more and more tephras have been dated indirectly by correlation. Materials and methods Glass shards which were detected in the sediments need to be concentrated and purified in the lab and then made slides for the geochemical analysis. Laboratory procedures for the determination of loss-on-ignition and extraction of glass shards from the sediments could follow the ashing method, and then for the density separation technique. Sometimes, some of the samples were rich in diatoms, whose presence obscured the glass particles during microscopic examination. The diatoms were dissolved in 5% KOH solution heated in the water bath at 90℃ for an hour and shaken after 30 minutes. The samples were then processed, following procedures in the following chapters. Samples were analyzed under polarized light at ×400 magnification using a microscope. Samples for wave-length dispersive electron probe microanalysis (WDS-EPMA) were prepared and performed on a scanning electron microscope. Results Tephra and cryptotephra deposits provide records of volcano eruption histories, and their study thus aids volcanic hazard analysis and mitigation. As well, it is evident that tephra sequences may be more comprehensive at medial to distal locations in sediments, including ice, than at proximal locations, and that the interbedding of the layers from multiple sources at such locations uniquely reveals their complex stratigraphic interrelationships. Discussion Ash can be transported against the prevailing wind flow. Some tephras, such as the one found in the Tasman Sea, have travelled several hundreds of kilometers from source to the north and northwest of North Island by southeast trade winds, as well as having an eastern dispersal in the Pacific Ocean. This distribution occurred at a higher altitude than the westerlies which drive most of the material to the east. Fluvial systems can deposit nearly pure tephra in units exceeding 20 m in thickness at distances up to 250 km from the vent. Therefore, for the stratigraphic purposes many fluvial tephra deposits can be considered almost simultaneous with the fall or flow event. Conclusions Tephrochronology, the characterization and use of volcanic-ash layers as a unique stratigraphic linking, synchronizing, and age- equivalent dating tool has become a globally-practised discipline of immense practical value in a wide range of subjects including Quaternary stratigraphy and geochronology, palaeoenvironmental reconstruction, volcanology, geomorphology, archaeology, human evolution, and palaeoanthropology. Although much more work has been done on tephrochrnology and tephrostratigraphy in New Zealand, Europe and other parts of the world (Japan, Russia etc.), in NE China it still has lots of work could be done in the future. Recommendations and perspectives Tephra research in Europe and New Zealand are advanced the benefit of tephra for effecting high-precision correlations between late Quaternary records has been demonstrated in both the area. These correlations enable the testing of past environmental change where good time control is absent or lacks precision. Northeastern China has a long history of volcanism, so tephra is potentially an important chronological tool in the area for reconstructing past environments. |
| Key words: tephrochoronology tephrostratigraphy Quaternary |
