Tracing marine cryptotephras in the North Atlantic during the last glacial period: Improving the North Atlantic marine tephrostratigraphic framework
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Currently the Late-glacial and Holocene marine tephrochronology on the shelf around Iceland comprises 130 tephra layers from 30 sediment cores ranging in age from 15,000 years cal. BP to AD 1947. A vast majority of the cores and tephra layers are from the North Iceland shelf. Much fewer tephra layers have been found on the South and West Iceland shelf. The early Holocene Saksunarvatn ash and Vedde Ash are the only tephra layers identified on all investigated shelf areas. For the last 15,000 years correlated tephra layers from the shelf sediments around Iceland to their terrestrial counterparts both in Iceland and overseas are 40 of which 26 are terrestrially dated tephra markers. Thirty correlations are within the last 7050 years. The terrestrially dated tephra markers found on the shelf have been used to constrain past environmental variability in the region, as well as marine reservoir age. The marine tephra stratigraphy on the North Iceland shelf has revealed variations in volcanic activity in Iceland further back in time than terrestrial records in Iceland. The numerous tephra layers identified in the sediments on the shelf demonstrate the potential of marine tephrochronology for dating purposes, land-sea correlation, marine reservoir estimations and reconstruction of past volcanic activity of Icelandic volcanoes.
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The purpose of this symposium is two-fold: first, to review recent developments in tephrochronological studies in various districts of Japan, and second, to verify the impact of tephrogenic explosive eruptions on human beings and their environment.Several problems concerning the Japanese tephrochronology are discussed, for example: (1) standardization of petrographic characterizations for the identification of tephra layers, (2) identification of pre-Late Quaternary widespread tephra layers, (3) revision and refinement of several radiometric ages for specific tephra layers, (4) application of the tephrochronology established on land to the deep sea chronology, (5) long distance correlation of terraces and their sediments using widespread tephra layers, (6) establishing the relationship between tephrogenic eruptions and climatic change, and (7) examining the process and mechanism of the impact of explosive eruptions on human beings.
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Cryptotephrochronology, the use of hidden, diminutive volcanic ash layers to date sediments, has rarely been applied outside western Europe but has the potential to improve the tephrochronology of other regions of the world. Here we present the first comprehensive cryptotephra study in Alaska. Cores were extracted from five peatland sites, with cryptotephras located by ashing and microscopy and their glass geochemistry examined using electron probe microanalysis. Glass geochemical data from nine tephras were compared between sites and with data from previous Alaskan tephra studies. One tephra present in all the cores is believed to represent a previously unidentified eruption of Mt. Churchill and is named here as the ‘Lena tephra’. A mid-Holocene tephra in one site is very similar to Aniakchak tephra and most likely represents a previously unidentified Aniakchak eruption, ca. 5300–5030 cal yr BP. Other tephras are from the late Holocene White River eruption, a mid-Holocene Mt. Churchill eruption, and possibly eruptions of Redoubt and Augustine volcanoes. These results show the potential of cryptotephras to expand the geographic limits of tephrochronology and demonstrate that Mt. Churchill has been more active in the Holocene than previously appreciated. This finding may necessitate reassessment of volcanic hazards in the region.
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Tephrochronology is the use of volcanic ash layers to provide a chronological framework that can be applied in a range of disciplines, from volcanology to archaeology. Tephra layers form isochrons that mark a moment in time (the time of deposition). Reliable dates (radiometric, historical or ice core) can be “transferred” to any location where a particular tephra isochron is found, providing an independent check of other dating methods. Tephra isochrons can also connect the different locations where they are found and, in particular, archaeological sites with the wider paleolandscape. The identification of tephra layers invisible to the naked eye (cryptotephrochronology) has expanded the reach of tephrochronology to more distal areas downwind of the major volcanically active regions, enabling global correlations and connections.
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Regional tephrochronological frameworks serve as dating and correlating tools that enable critical assessment of the validity of single cryptotephra findings. In this study, our aim was to construct an outline for a Finnish Holocene tephra framework by investigating 12 peatlands and one lake site for presence of cryptotephra. As a result, glass shards from 19 individual tephras were geochemically characterized and correlated to their source volcanoes in Iceland and Alaska. Fifteen of these tephras were identified in Finland for the first time. The oldest identified cryptotephra in the Finnish environmental records is the 7 ka Hekla 5 tephra, and the youngest one is the Askja 1875. The identification of two Alaskan tephras (White River Ash eastern lobe and Aniakchak tephra) in Finland demonstrates an opportunity for intercontinental tephra correlations. The first Finnish Holocene tephrochronology presented here reveals a great potential for using tephrochronology as a dating method in Finland and is expected to aid in future cryptotephra research in the region. Additionally, the cryptotephra findings in this study help to refine the dispersal areas of several Holocene tephras.
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Tephrochronology, the reconstruction of past volcanic ash deposition, provides a valuable method for dating sediments and determining long-term volcanic history. Tephra layers are highly numerous in Alaska, but knowledge of their occurrence and distribution is incomplete. This study expands the regional tephrochronology for the Kenai Peninsula of southcentral Alaska by investigating the tephrostratigraphy of two peatland sites. We located seven visible tephras and seven microtephras and investigated the particle size and geochemistry of the visible tephras. Radiocarbon dates were used to estimate the timescale of each core. Geochemical comparison showed that the visible tephras originated from late Holocene eruptions of Augustine, Crater Peak–Mt. Spurr, and Hayes volcanoes. Some of the tephras had been documented previously, and these new findings expand their known range. Others represent eruptions not previously reported, including a Crater Peak–Mt. Spurr eruption around 430 cal. BP. The results provide new tephra data for the region, illustrate the spatial heterogeneity of tephra deposition, and show the potential of microtephras for expanding the regional tephra record.
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The tephrochronology of Iceland and the North Atlantic region is reviewed in order to construct a unified framework for the last 400 kyr BP. Nearly all of the tephra layers described are also characterised geochemically. A number of new tephra layers are analysed for the first time for their geochemical signature and a number of pre-Holocene tephra layers have been given an informal denotation. The tephrostratigraphy of Ash Zone II is highlighted. Where possible the rhyolitic tephra layers found outside Iceland have been correlated to known Icelandic tephra layers or to the volcanic source area. The application of tephra fallout in various depositional environments is described and discussed. Copyright © 2000 John Wiley & Sons, Ltd.
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