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1.
C. S. M. Turney K. Van Den Burg S. Wastegrd S. M. Davies N. J. Whitehouse J. R. Pilcher C. Callaghan 《第四纪科学杂志》2006,21(4):335-345
High‐precision correlation of palaeoclimatic and palaeoenvironmental records is crucial for testing hypotheses of synchronous change. Although radiocarbon is the traditional method for dating late Quaternary sedimentary sequences, particularly during the last glacial–interglacial transition (LGIT; 15–9 ka), there are inherent problems with the method, particularly during periods of climate change which are often accompanied by major perturbations in atmospheric radiocarbon content. An alternative method is the use of tephras that act as time‐parallel marker horizons. Within Europe, numerous volcanic centres are known to have erupted during the LGIT, providing considerable potential for high‐precision correlation independent of past radiocarbon fluctuations. Here we report the first identification of the Vedde Ash and Askja Tephra in Ireland, significantly extending the known provenance of these events. We have also identified two new horizons (the Roddans Port Tephras A and B) and tentatively recognise an additional horizon from Vallensgård Mose (Denmark) that provide crucial additional chronological control for the LGIT. Two phases of the Laacher See Tephra (LST) are reported, the lower Laacher See Tephra (LLST) and probably the C2 phase of the Middle Laacher See Tephra (MLST‐C2) indicating a more northeasterly distribution of this fan than reported previously. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
2.
The Palisades Site is an extensive silt-loam bluff complex on the central Yukon River preserving a nearly continuous record of the last 2 myr. Volcanic ash deposits present include the Old Crow (OCt; 140,000 yr), Sheep Creek (SCt; 190,000 yr), PA (2.02 myr), EC (ca. 2 myr), and Mining Camp (ca. 2 myr) tephras. Two new tephras, PAL and PAU, are geochemically similar to the PA and EC tephras and appear to be comagmatic. The PA tephra occurs in ice-wedge casts and solifluction deposits, marking the oldest occurrence of permafrost in central Alaska. Three buried forest horizons are present in association with dated tephras. The uppermost forest bed occurs immediately above the OCt; the middle forest horizon occurs below the SCt. The lowest forest bed occurs between the EC and the PA tephras, and correlates with the Dawson Cut Forest Bed. Plant taxa in all three peats are common elements of moist taiga forest found in lowlands of central Alaska today. Large mammal fossils are all from common late Pleistocene taxa. Those recovered in situ came from a single horizon radiocarbon dated to ca. 27,000 14C yr B.P. The incongruous small mammal assemblage in that horizon reflects a diverse landscape with both wet and mesic environments. 相似文献
3.
Investigations of Lateglacial to Early Holocene lake sediments from the Nahe palaeolake (northern Germany) provided a high-resolution palynological record. To increase the temporal resolution of the record a targeted search for cryptotephra was carried out on the basis of pollen stratigraphy. Three cryptotephra horizons were detected and geochemically identified as G10ka series tephra (a Saksunarvatn Ash), Vedde Ash and Laacher See Tephra. Here we present the first geochemically confirmed finding of the ash from the Laacher See Eruption in Schleswig-Holstein—extending the so far detected fallout fan of the eruption further to the north-west. These finds enable direct stratigraphical correlations and underline the potential of the site for further investigations. 相似文献
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A vertebrate burrow-bearing layer of late Pleistocene age is commonly found at many Paleolithic archaeological sites in Korea. The burrows are straight to slightly curved in horizontal (plan) view and gently inclined in lateral (sectional) view. They are interpreted as having been produced by rodent-like mammals based on their size and architecture. The significance of such burrow-bearing layers as a characteristic stratigraphic marker unit is demonstrated by high burrow abundance, consistent stratigraphic position, lack of stratigraphic recurrence at these sites, and widespread geographic extent. Three dating methods, tephrochronology, radiocarbon, and OSL dating, were used to infer the age of these burrow-bearing layers. The dating results indicate that they were formed between ca. 40,000 and 25,000 yr (MIS 3−2), and this suggests that this layer can be used as a stratigraphic time-marker in late Pleistocene paleosol sequences for this region. 相似文献
6.
Marie Auclair Michel Lamothe France Lagroix Subir K. Banerjee 《Quaternary Geochronology》2007,2(1-4):34-38
Infrared stimulated luminescence (IRSL) properties of the Old Crow tephra and bracketing loess from the Halfway House site in Central Alaska are investigated in order to test newly developed techniques, including SAR and recently proposed fading corrections. Loess samples investigated show a standard growth of luminescence with regenerative dose while the tephra sample is less sensitive by an order of magnitude and saturates at lower dose. The growth curves obtained using multiple-aliquots regeneration (MAR) saturate at a higher value than those with the single-aliquot regeneration (SAR) protocol. Fading rate determinations for these samples are shown to be imprecise and no noticeable difference was observed between loess and tephra materials. Anomalous fading corrections using an average g value of 5% are applied to the natural test dose signal intensity using the dose rate correction (DRC) method. IRSL ages obtained for loess are in agreement with the expected age while the tephra age is lower than expected, suggesting the measured fading rate is underestimated for this material. 相似文献
7.
This paper demonstrates the use of tephrochronology in dating the earliest archaeological evidence for the settlement of Iceland. This island was one of the last places on Earth settled by people and there are conflicting ideas about the pace and scale of initial colonisation. Three tephra layers, the Landnám (‘land-taking’) tephra layer (A.D. 877 ± 1), the Eldgjá tephra (A.D. 939) and the recently dated V-Sv tephra (A.D. 938 ± 6) can be found at 58% of 253 securely-dated early settlement sites across the country. The presence of the tephras permits both a countrywide comparison, and a classification of these settlement sites into pre-Landnám, Landnám and post-Landnám. The data summarised here for the first time indicate that it will be possible to reconstruct the tempo and development of the colonisation process in decadal resolution by more systematically utilising the dating potential of tephrochronology. 相似文献
8.
A 5 mm thick tephra layer has been identified in the lacustrine sediments of Moon Lake in the Arxan-Chaihe volcanic field (ACVF) in Greater Khingan Mountains (NE China). The visible tephra layer is clearly revealed as a distinct peak in magnetic susceptibility measurements. The tephra layer consists mainly of brown vesicular glass shards and minor amounts of plagioclase, olivine and clinopyroxene. Major and minor element analysis has been carried out on the glass shards and plagioclase minerals. Glass shards show low concentrations of K2O, similar to the eruptive products derived from post-Miocene volcanoes of the ACVF. The plagioclase phenocrysts in both lava and tephra from ACVF, and in the tephra recorded in Moon Lake are labradorites. During the Late Pleistocene to Holocene, there were also extensive explosive eruptions in the nearby Nuominhe volcanic field (NVF). Volcanic rocks from the ACVF are easily distinguished from those derived from the NVF, having distinctly different K2O concentrations. This compositional variation is likely the result of different magmatic processes operating in the ACVF and NVF. Radiocarbon dating on organic materials from the lacustrine sediments dates the tephra layer to ca. 14,200 cal yrs BP, which implies that it was generated by a previously unknown Late Pleistocene explosive eruption in the ACVF. These results, for the first time, give a direct tephra record in this area, and suggest that identification of further tephra and/or cryptotephra in local sedimentary basins such as crater lakes of scoria cones and maars will be significant for dating the Late Pleistocene to Holocene volcanic eruptions and will help to establish a detailed record of the volcanic activity in the ACVF. The newly discovered tephra layer also provides a dated tephrochronological marker layer, which will in future studies provide a means to synchronise local sedimentary records of the climatically variable Late Glacial. 相似文献
9.
Middle and late Pleistocene glaciations in the Campo Felice Basin (central Apennines, Italy) 总被引：1，自引：0，他引：1
The present paper refers to research conducted in the tectonic-karst depression of Campo Felice in the central Apennines (Italy), in which glacial, alluvial and lacustrine sediments have been preserved. Stratigraphic interpretations of sediments underlying the Campo Felice Plain are based on evidence obtained from nine continuous-core boreholes. The boreholes reach a depth of 120 m and provide evidence of five sedimentation cycles separated by erosion surfaces. Each cycle is interpreted as an initial response to a mainly warm stage, characterized by low-energy alluvial and colluvial deposition, pedogenesis, and limited episodes of marsh formation. In turn, a mainly cold stage follows during which a lake formed, and glaciers developed and expanded, leading to deposition of glacial and fluvioglacial deposits. The chronological framework is established by eleven accelerator mass spectrometer 14C ages and three 39Ar-40Ar ages on leucites from interbedded tephra layers. These age determinations indicate five glacial advances that respectively occurred during marine oxygen isotope stages 2, 3-4, 6, 10 and 14. 相似文献
10.
Katla volcano,Iceland: magma composition,dynamics and eruption frequency as recorded by Holocene tephra layers 总被引：1，自引：0，他引：1
Bergrún Arna Óladóttir Olgeir Sigmarsson Gudrun Larsen Thor Thordarson 《Bulletin of Volcanology》2008,70(4):475-493
The Katla volcano in Iceland is characterized by subglacial explosive eruptions of Fe–Ti basalt composition. Although the
nature and products of historical Katla eruptions (i.e. over the last 1,100 years) at the volcano is well-documented, the
long term evolution of Katla’s volcanic activity and magma production is less well known. A study of the tephra stratigraphy
from a composite soil section to the east of the volcano has been undertaken with emphasis on the prehistoric deposits. The
section records ∼8,400 years of explosive activity at Katla volcano and includes 208 tephra layers of which 126 samples were
analysed for major-element composition. The age of individual Katla layers was calculated using soil accumulation rates (SAR)
derived from soil thicknesses between 14C-dated marker tephra layers. Temporal variations in major-element compositions of the basaltic tephra divide the ∼8,400-year
record into eight intervals with durations of 510–1,750 years. Concentrations of incompatible elements (e.g. K2O) in individual intervals reveal changes that are characterized as constant, irregular, and increasing. These variations
in incompatible elements correlate with changes in other major-element concentrations and suggest that the magmatic evolution
of the basalts beneath Katla is primarily controlled by fractional crystallisation. In addition, binary mixing between a basaltic
component and a silicic melt is inferred for several tephra layers of intermediate composition. Small to moderate eruptions
of silicic tephra (SILK) occur throughout the Holocene. However, these events do not appear to exhibit strong influence on
the magmatic evolution of the basalts. Nevertheless, peaks in the frequency of basaltic and silicic eruptions are contemporaneous.
The observed pattern of change in tephra composition within individual time intervals suggests different conditions in the
plumbing system beneath Katla volcano. At present, the cause of change of the magma plumbing system is not clear, but might
be related to eruptions of eight known Holocene lavas around the volcano. Two cycles are observed throughout the Holocene,
each involving three stages of plumbing system evolution. A cycle begins with an interval characterized by simple plumbing
system, as indicated by uniform major element compositions. This is followed by an interval of sill and dyke system, as depicted
by irregular temporal variations in major element compositions. This stage eventually leads to a formation of a magma chamber,
represented by an interval with increasing concentrations of incompatible elements with time. The eruption frequency within
the cycle increases from the stage of a simple plumbing system to the sill and dyke complex stage and then drops again during
magma chamber stage. In accordance with this model, Katla volcano is at present in the first interval (i.e. simple plumbing
system) of the third cycle because the activity in historical time has been characterized by uniform magma composition and
relatively low eruption frequency. 相似文献