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Tephra Layers (tephra + layer)
Selected AbstractsHigh-resolution stratigraphy of the northernmost concentric raised bog in Europe: Sellevollmyra, Andøya, northern NorwayBOREAS, Issue 3 2007KARL-DAG VORREN From the Sellevollmyra bog at Andøya, northern Norway, a 440-cm long peat core covering the last c. 7000 calendar years was examined for humification, loss-on-ignition, microfossils, macrofossils and tephra. The age model was based on a Bayesian wiggle-match of 35 14C dates and two historically anchored tephra layers. Based on changes in lithology and biostratigraphical climate proxies, several climatic changes were identified (periods of the most fundamental changes in italics): 6410,6380, 6230,6050, 5730,5640, 5470,5430, 5340,5310, 5270,5100, 4790,4710, 4890,4820, 4380,4320, 4220,4120, 4000,3810, 3610,3580, 3370,3340 (regionally 2850,2750; in Sellevollmyra a hiatus between 2960,2520), 2330,2220, 1950, 1530,1450, 1150,840, 730? and c. 600? cal. yr BP. Most of these climate changes are known from other investigations of different palaeoclimate proxies in northern and middle Europe. Some volcanic eruptions seemingly coincide with vegetation changes recorded in the peat, e.g. about 5760 cal. yr BP; however, the known climatic deterioration at the time of the Hekla-4 tephra layer started some decades before the eruption event. [source] The geochemical characterization and correlation of Late Holocene tephra layers at Ambra Crater and Kuk Swamp, Papua New GuineaGEOLOGICAL JOURNAL, Issue 5 2009Sarah E. Coulter Abstract Major element geochemical analyses of volcanic glass shards from visible, macroscopic tephra layers in Papua New Guinea were used to test field-based correlations between local Late Holocene sequences. Previously, synchronization of sediment records at archaeological and palaeoecological sites across the Papua New Guinea highlands had been based largely on the physical properties and stratigraphic relationships of visible volcanic layers. The geochemical analysis of tephra-derived glass demonstrates miscorrelations and enables more robust tephrostratigraphic links to be made between a globally significant archaeological site, Kuk Swamp, and a nearby volcanic cone, Ambra Crater. The results indicate that field-based correlations of tephras can be problematic, especially given variable post-depositional changes in colour and texture of relatively thin tephra layers in different depositional environments. Additionally, the findings call into question some previous tephrochronological associations at archaeological and palaeoecological sites in the Papua New Guinea highlands, and demonstrate the necessity of grain-discrete geochemical analysis of glass shards as a basis for future tephrochronological studies in this region. Copyright © 2009 John Wiley & Sons, Ltd. [source] Detection of diluted marine tertiary tephra by electron spin resonance and magnetic measurementsGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 2 2003B. Ananou SUMMARY Oligocene sediments from ODP leg 115 (South Indian Ocean), Site 709 and Site 711, have been investigated using electron spin resonance (ESR) and magnetic susceptibility measurements, to detect volcanic tephra layers of supposed Ethiopian traps origin. The results obtained at room temperature, without separating the volcanic material from the bulk sediment, show that the ash-content strongly influences the lineshape and intensity of the ESR signal. As a result, the ESR alone, can be used as a powerful tool for characterizing the diluted ash-content of marine sediments. We have also found a strong similarity between the ESR spectra of the same tephra layers from the two sites. [source] Variations of magnetic susceptibility and fine quartz accumulation rate in Daisen loam over the past 200 000 years: Interaction between winter and summer monsoons in south-west JapanISLAND ARC, Issue 2 2001Jun-Ichi Kimura Abstract A loam section near Daisen volcano, South-west Japan, has been examined for low-field magnetic susceptibility (MS) and fine quartz accumulation rate. Fission track dating of tephra layers interbedded in the deposit shows that the loam age ranges from about 200 ka to the Present. The MS was measured for both bulk sample and the < 63 ,m fine fraction. Fine quartz contents in the < 63 ,m fraction were also determined using acid-alkali digestions and recalculated to derive fine quartz accumulation rate (Rqz). Grain size analysis was then carried out on the separated fine quartz. Low-field MS varies from low frequency magnetic suspectibility (,(LF)) 5 to 100 (× 10,6 m3/kg) for bulk samples and from 1 to 30 for fine fractions. The fine fraction ,(LF) variation correlated with Chinese loess MS stratigraphy, which indicated changes in pedogenic enhancement of the MS and is reflected by summer monsoon intensity. The Rqz are high in cool climate stages, with volumes between 0.2 and 0.4 (× 10,2 kg/m2 per yr), whereas in warm stages the rate falls to about 0.1. These values compare well with those reported from the Hokkaido and Kanto areas, suggesting the fine quartz originates from tropospheric dust. The strong winter monsoons during glacial stages alternated with weak summer monsoons as a result of a southward shift of the jet stream. In interglacials, summer monsoons were stronger. Seasonal alternating monsoons appear to have operated in South-west Japan through the past 200 000 years. [source] Late-glacial and Holocene palaeovegetation zonal reconstruction for central and north-central North AmericaJOURNAL OF BIOGEOGRAPHY, Issue 6 2005W. L Strong Abstract Aim, The purpose of this study is to develop palaeovegetation zonation models for central and north-central North America, based on late-Quaternary and Holocene pollen stratigraphic data (n = 246 sites). A secondary purpose was to evaluate an hypothesis (Strong & Hills, 2003) to explain the disjunct distribution of species in western Alberta. Location, Hudson Bay-Lake Michigan to the Rocky Mountains region, north of 36° N to the Arctic Ocean (c. 70° N). Methods, Pollen profiles spanning 40 years of palaeoecological research in North America were extracted from published and unpublished archival sources. Individual profiles were subdivided into 1000-year increments based on the assumption of a constant sedimentation rate between stratigraphic dates (e.g. surface sediments, radiocarbon 14C dates, tephra layers). The pollen composition among profiles was standardized to 54 commonly recognized taxa, with percentage composition within each stratigraphic sample prorated to 100% prior to analysis. Near-surface sediments from these profiles were included as analogues of modern vegetation. Cluster analysis was used as a guide to the classification of 2356 temporal stratigraphic samples, which resulted in the recognition of 16 pollen groups. These groups were summarized in terms of their pollen composition, mapped, and used in combination with terrain information and an ecological knowledge of the study area to construct six physiognomically-based palaeovegetation zonation models at 2000-year intervals from 14,000 to 4000 yr bp (radiocarbon years before present). Results, The 14,000 yr bp model placed Boreal and Cordilleran Forests proximal to the southern glacial front, whereas Arctic tundra dominated the Yukon Territory,Alaska ice-free zone. Pollen and macrofossil evidence suggests that this Boreal Forest zone contained a mixture of coniferous and deciduous tree species. Grassland was postulated immediately south of the forest zone, with its northern extreme near 49° N latitude in the Alberta,Montana border area. Separation of the Laurentide and Cordilleran glacial fronts about 12,000 yr bp initiated the northward advance of Boreal Forests into western Canada. By the end of the Hypsithermal at about 6000 yr bp, Boreal Forests occurred near the Arctic Ocean, and Grassland and Aspen Parkland zones may have extended to 54° N and 59° N latitude in Alberta, respectively. Between 6000 and 4000 yr bp, a 5° and 1° latitudinal southward shift of the northern Boreal Forest and Grassland/Aspen Parkland boundaries occurred, respectively, near their contemporary positions with corresponding expansions of the Subarctic and Arctic zones. Modern Canadian Cordilleran Forests along the eastern slopes of the Rocky Mountains were interpreted as originating from the north-central Montana,south-western Alberta area. Jack pine (Pinus banksiana Lamb.), a common Boreal Forest species, appears to have entered central Canada via the north side of Lake Superior after 11,000 yr bp. Main conclusions, Modern vegetation in central Canada evolved from biomes located in the northern USA during the late-Quaternary. The Boreal Forest biome contained the same arboreal taxa as the modern vegetation, except it lacked jack pine. The proposed regional palaeovegetation models support the hypothesis of Strong & Hills (2003), but new independent palaeoecological data will be needed for a proper evaluation. [source] Late Quaternary terrestrial tephrochronology of Iceland,frequency of explosive eruptions, type and volume of tephra deposits,JOURNAL OF QUATERNARY SCIENCE, Issue 2 2008Gudrún Larsen Abstract Close to 100 silicic tephra layers have been identified in Icelandic terrestrial soils of Holocene age. The majority of these tephras were erupted at the Hekla, Torfajökull, Öræfajökull, Askja, Snæfellsjökull, Eyjafjallajökull and Katla central volcanoes. By far the most active is Hekla with close to 50 identified silicic tephra layers in ,8000 years, showing an inverse relationship between eruption frequency and volume of erupted tephra. Volumes of uncompacted silicic tephra layers range from <0.01,km3 to >10,km3, with nearly 50% of known tephra volumes lying between 0.1 and 0.5,km3. Seven closely spaced Hekla eruptions in the period 950,550 BC with identical major element composition illustrate the need of stratigraphic control in short and long distance correlations. A 2600 year old tephra geochemically indistinguishable from the Borrobol tephra is a reminder that no tephra is geochemically unique. The major producers of basaltic tephra are the volcanic systems that are partly covered by ice, or partly lying within areas of high groundwater or extending offshore, i.e. the Grímsvötn, Katla, Veidivötn-Bárdarbunga, Reykjanes, Kverkfjöll and Vestmannaeyjar volcanic systems. The best known eruption record is that of the Katla system with over 170 identified basaltic tephra layers and more than 300 estimated in 8400 years. The Grímsvötn system is currently the most active volcanic system with over 70 eruptions during the last 1100 years. Volumes of uncompacted basaltic tephra layers range from <0.01,km3 to >20,km3, the majority of known tephra volumes lying between 0.1 and 1,km3. The most voluminous basaltic tephra deposit, the ,10,200,yr old Saksunarvatn tephra, may however represent more than one eruption on the Grímsvötn system. Deposition of approximately 800 basaltic tephra layers during the last 9000 14C years is estimated but many of those erupted from volcanoes within ice caps have not been preserved. Copyright © 2007 John Wiley & Sons, Ltd. [source] Constraining the age of Lateglacial and early Holocene pollen zones and tephra horizons in southern Sweden with Bayesian probability methods,JOURNAL OF QUATERNARY SCIENCE, Issue 4 2006B. Wohlfarth Abstract The sediment sequence from Hässeldala port in southeastern Sweden provides a unique Lateglacial/early Holocene record that contains five different tephra layers. Three of these have been geochemically identified as the Borrobol Tephra, the Hässeldalen Tephra and the 10-ka Askja Tephra. Twenty-eight high-resolution 14C measurements have been obtained and three different age models based on Bayesian statistics are employed to provide age estimates for the five different tephra layers. The chrono- and pollen stratigraphic framework supports the stratigraphic position of the Borrobol Tephra as found in Sweden at the very end of the Older Dryas pollen zone and provides the first age estimates for the Askja and Hässeldalen tephras. Our results, however, highlight the limitations that arise in attempting to establish a robust, chronologically independent lacustrine sequence that can be correlated in great detail to ice core or marine records. Radiocarbon samples are prone to error and sedimentation rates in lake basins may vary considerably due to a number of factors. Any type of valid and ,realistic' age model, therefore, has to take these limitations into account and needs to include this information in its prior assumptions. As a result, the age ranges for the specific horizons at Hässeldala port are large and calendar year estimates differ according to the assumptions of the age-model. Not only do these results provide a cautionary note for over-dependence on one age-model for the derivation of age estimates for specific horizons, but they also demonstrate that precise correlations to other palaeoarchives to detect leads or lags is problematic. Given the uncertainties associated with establishing age,depth models for sedimentary sequences spanning the Lateglacial period, however, this exercise employing Bayesian probability methods represents the best possible approach and provides the most statistically significant age estimates for the pollen zone boundaries and tephra horizons. Copyright © 2006 John Wiley & Sons, Ltd. [source] Towards a Holocene tephrochronology for Sweden: geochemistry and correlation with the North Atlantic tephra stratigraphyJOURNAL OF QUATERNARY SCIENCE, Issue 2 2004Jane Boygle Abstract Northern Europe has been affected by fallout of tephra from volcanic eruptions in Iceland throughout at least the Late-glacial and Holocene. Microscopic volcanic ash horizons found in the British Isles, northern Germany and Scandinavia (e.g. Hekla 4) now provide important regional isochrons. Over recent years analytical techniques such as electron probe microanalysis (EPMA) have helped identify these distal tephra layers by glass geochemistry. Early tephra work in Sweden used refractive indices and bracketing radiocarbon dates for identification, resulting in tentative correlation. This paper presents geochemical data for Icelandic Holocene tephra in Sweden. Several sites in central Sweden have four distinct mid-Holocene tephra layers. Correlation with the Icelandic record indicates that the following tephra layers are present: Hekla 4 (ca. 3830,yr,BP), Kebister tephra (ca. 3600,yr,BP), Hekla 3 (ca. 2880,yr,BP) and Askja ad 1875. Preliminary analyses suggest that more, previously unidentified, tephra layers are present in low concentrations in the region, indicating potential for expanding the Swedish tephrochronology for palaeoecological and palaeoenvironmental work in northern Europe. Copyright © 2004 John Wiley & Sons, Ltd. [source] Tephrochronology of last termination sequences in Europe: a protocol for improved analytical precision and robust correlation procedures (a joint SCOTAV,INTIMATE proposal)JOURNAL OF QUATERNARY SCIENCE, Issue 2 2004Chris S. M. Turney Abstract The precise sequence of events during the Last Termination (18,000,9000,ka 14C,yr BP), and the extent to which major environmental changes were synchronous, are difficult to establish using the radiocarbon method alone because of serious distortions of the radiocarbon time-scale, as well as the influences of site-specific errors that can affect the materials dated. Attention has therefore turned to other methods that can provide independent tests of the chronology and correlation of events during the Last Termination. With emphasis on European sequences, we summarise here the potential of tephrostratigraphy and tephrochronology to fulfil this role. Recent advances in the detection and analysis of ,hidden' tephra layers (cryptotephra) indicate that some tephras of Last Termination age are much more widespread in Europe than appreciated hitherto, and a number of new tephra deposits have also been identified. There is much potential for developing an integrated tephrochronological framework for Europe, which can help to underpin the overall chronology of events during the Last Termination. For that potential to be realised, however, there needs to be a more systematic and robust analysis of tephra layers than has been the practice in the past. We propose a protocol for improving analytical and reporting procedures, as well as the establishment of a centralised data base of the results, which will provide an important geochronological tool to support a diverse range of stratigraphical studies, including opportunities to reassess volcanic hazards. Although aimed primarily at Europe, the protocol proposed here is of equal relevance to other regions and periods of interest. Copyright © 2004 John Wiley & Sons, Ltd. [source] Linking the North Atlantic to central Europe: a high-resolution Holocene tephrochronological record from northern GermanyJOURNAL OF QUATERNARY SCIENCE, Issue 1 2002Christel van den Bogaard Abstract A high-resolution Holocene tephrochronology for northern Germany has been established based on systematic tephrostratigraphical analysis of three peat bogs. Microscopic volcanic ash layers have been traced and characterised petrographically and by the chemical composition of the glass shards. At least 37 ash horizons representing 16 different explosive volcanic eruptions have been identified and many can be correlated between the three sites, up to 100 km apart. The tephra layers can be related to Icelandic volcanic sources and some correlated to the eruptions of Askja 1875, Hekla 3, Hekla Selsund, Hekla 4 and Hekla 5, as well as to unspecified eruptions of Icelandic volcanic systems, e.g. Torfajökull. The source volcanoes for some tephra layers remain unidentified. Some tephra layers were known previously from the North Atlantic region (e.g. Sluggan, Glen Garry), others have not been recorded previously in the literature (e.g. microlite tephra). This study provides the first comprehensive Holocene tephrostratigraphical record for northern Germany, complementing the North Atlantic tephrostratigraphical dating framework, effectively extending it into central Europe. The study shows that Icelandic ash layers are even more widespread than hitherto thought. Copyright © 2002 John Wiley & Sons, Ltd. [source] Geochemistry, dispersal, volumes and chronology of Holocene silicic tephra layers from the Katla volcanic system, IcelandJOURNAL OF QUATERNARY SCIENCE, Issue 2 2001Gudrún Larsen Abstract At least 12 silicic tephra layers (SILK tephras) erupted between ca. 6600 and ca. 1675 yr BP from the Katla volcanic system, have been identified in southern Iceland. In addition to providing significant new knowledge on the Holocene volcanism of the Katla system which typically produces basaltic tephra, the SILK tephras form distinct and precise isochronous marker horizons in a climatically sensitive location close to both the atmospheric and marine polar fronts. With one exception the SILK tephras have a narrow compositional range, with SiO2 between 63 and 67%. Geochemically they are indistinguishable from ocean transported pumice found on beaches in the North Atlantic region, although they differ significantly from the silicic component of the North Atlantic Ash Zone One (NAAZO). Volumes of airborne SILK tephra range from 0.05 to 0.3 km3. We present new isopach maps of the six largest layers and demonstrate that they originate within the Katla caldera. The apparently stable magma system conditions that produced the SILK tephras may have been established as a consequence of the eruption of the silicic component of NAAZO (ca. 10.3 ka) and disrupted by another large-scale event, the tenth century ad Eldgjá eruption (ca. 1 ka). Despite the current long repose, silicic activity of this type may occur again in the future, presenting hitherto unknown hazards. Copyright © 2001 John Wiley & Sons, Ltd. [source] Short-term environmental changes in Lake Morenito (41°S, 71°W, Patagonia, Argentina) from the analysis of sub-fossil chironomidsAQUATIC CONSERVATION: MARINE AND FRESHWATER ECOSYSTEMS, Issue 1 2005Julieta Massaferro Abstract 1.A short sediment core from Lake Morenito was studied to assess the impact of environmental changes on chironomid communities occurring during the last ca 100 yr. 2.Lake Morenito (41°S, 71°W) is located 20 km west of the city of Bariloche, in northern Patagonia, Argentina. Before 1960, this lake was a branch of Lake Moreno; by that time, an artificial dam closed the system, establishing the new lake. Another human disturbance that took place during the time span of the core was the introduction of salmonids to the area ca 1910. 3.The most important natural events that occurred in the area during the last 100 yr were related to volcanic episodes. One of them, occured in Chile in 1960 affecting the Argentinian side, coincided with the dam's construction. 4.Changes in the chironomid community were recorded by studying the sub-fossil remains (the chitinized head capsule of the larvae) present in the sedimentary sequence. The results show that volcanic tephra layers deposited along the core led to a sharp instantaneous drop in the diversity and abundance of chironomid assemblages. Human activities are also associated with a change in chironomid community composition. 5.Chironomus reached its maximum abundance values in 1910 and 1960. The organic matter content also increased at the same time. The increase of Chironomus after 1910 is clearly related to an increase in the trophic status of the lake. However, owing to the synchronicity of events in 1960, i.e. the volcanic event and the dam's construction, it is difficult to establish whether the change in the chironomid assemblage was in response to an increase in trophic enrichment, to natural disturbance, or both. Copyright © 2004 John Wiley & Sons, Ltd. [source] High-resolution stratigraphy of the northernmost concentric raised bog in Europe: Sellevollmyra, Andøya, northern NorwayBOREAS, Issue 3 2007KARL-DAG VORREN From the Sellevollmyra bog at Andøya, northern Norway, a 440-cm long peat core covering the last c. 7000 calendar years was examined for humification, loss-on-ignition, microfossils, macrofossils and tephra. The age model was based on a Bayesian wiggle-match of 35 14C dates and two historically anchored tephra layers. Based on changes in lithology and biostratigraphical climate proxies, several climatic changes were identified (periods of the most fundamental changes in italics): 6410,6380, 6230,6050, 5730,5640, 5470,5430, 5340,5310, 5270,5100, 4790,4710, 4890,4820, 4380,4320, 4220,4120, 4000,3810, 3610,3580, 3370,3340 (regionally 2850,2750; in Sellevollmyra a hiatus between 2960,2520), 2330,2220, 1950, 1530,1450, 1150,840, 730? and c. 600? cal. yr BP. Most of these climate changes are known from other investigations of different palaeoclimate proxies in northern and middle Europe. Some volcanic eruptions seemingly coincide with vegetation changes recorded in the peat, e.g. about 5760 cal. yr BP; however, the known climatic deterioration at the time of the Hekla-4 tephra layer started some decades before the eruption event. [source] Detection of Lateglacial distal tephra layers in the NetherlandsBOREAS, Issue 2 2005SIWAN M. DAVIES Davies, S. M., Hoek, W. Z., Bohncke, S. J. P., Lowe, J. J., Pyne O'Donnell, S. & Turney, C. S. M. 2005 (May): Detection of Lateglacial distal tephra layers in the Netherlands. Boreas, Vol. 34, pp. 123,135. Oslo. ISSN 0300,9483. Three distal tephra layers or cryptotephras have been detected within a sedimentary sequence from the Netherlands that spans the last glacial-interglacial transition. Geochemical analyses identify one as the Vedde Ash, which represents the southernmost discovery of this mid-Younger Dryas tephra so far. This tephra was found as a distinct horizon in three different cores sampled within the basin. The remaining two tephras have not been geochemically ,fingerprinted', partly due to low concentrations and uneven distributions of shards within the sequences sampled. Nevertheless, there is the potential for tracing these tephra layers throughout the Netherlands and into other parts of continental Europe. Accordingly, the possibilities for precise correlation of Dutch palaeoenvironmental records with other continental, marine and ice-core records from the North Atlantic region are highlighted. [source] |