early holocene climate oscillations recorded in three greenland ice cores a new ice core chronology for the greenland dye 3 grip and ngrip ice cores has been constructed making it possible to compare the 18 o and accumulation signals recorded in the three cores on an almost annual scale throughout the holocene we here introduce the new time scale and investigate 18 o and accumulation anoma lies that are common to the three cores in the early holocene 7 9 11 7 ka before present three time periods with significant and synchronous anomalies in the 18 o and accumulation signals stand out the well known 8 2 ka event an event of shorter duration but of almost similar amplitude around 9 3 ka before present and the preboreal oscillation during the first centuries of the holocene for each of these sections we present a 18 o anomaly curve and a common accumulation signal that represents regional changes in the accumulation rate over the greenland ice cap

a new early holocene cryptotephra from northwest scotland clarification of the temporal relationships amongst records of environmental change is dependent on accurate timescales event markers such as tephra layers are extremely important for constraining chronologies and providing tie points in this report we present evidence of a previously unknown early holocene icelandic cryptotephra from a lake in northern scotland the an druim tephra the calibrated radiocarbon age of 9560 cal yr bp for this new cryptotephra makes it an important addition to the suite of cryptotephras now recorded from the last glacial termination and early holocene in northwest europe in addition we report evidence in support of a younger borrobol tephra from lateglacial sediments of allerod age copyright 2005 john wiley sons ltd

identification and significance of a visible basalt rich vedde ash layer in a late glacial sequence on the isle of skye inner hebrides scotland this paper reports the discovery of a visible tephra horizon of late glacial age from the site of loch ashik in the isle of skye the inner hebrides scotland although the tephra shards have a bimodal geochemical composition identical to that of the vedde ash a well known marker horizon within late glacial sequences the horizon at ashik is dominated by basaltic shards and devitrified tephra shards giving the layer its characteristic black colour only rhyolitic shards have previously been reported from vedde ash horizons in the british isles this new evidence raises some important questions about the factors that govern the distribution and accumulation of basaltic tephra and about the methods used to detect ash shards in basins distal to centres of volcanic activity copyright 2001 john wiley sons ltd

ash layers from iceland in the greenland grip ice core correlated with oceanic and land sediments four previously known ash layers ash zones i and ii saksunarvatn and the settlement layer all originating in iceland have been identified in the central greenland ice core grip this correlation of the ash between the different environments is achieved by comparison of the chemical composition of glass shards from the ash this establishes and confirms detailed correlations between the different types of depositional records and the absolute dating of the younger part of the ice core by counting annual layers dates the eruptions accurately a precise connection with dates obtained by14c beyond the range of dendrochronology is established which provides an excellent confirmation of230th 234u dates from corals four additional icelandic ash layers have also been identified in the core but not yet correlated with known ash deposits

the chronology of palaeoenvironmental changes during the last glacial holocene transition towards an event stratigraphy for the british isles the overall aim of the tigger iib project is to increase our understanding of the manner and rates by which ecosystems responded to climate changes during the last glacial holocene transition success in this venture requires better constrained palaeoenvironmental reconstructions than have been achieved thus far and the tigger project focused in particular on three main aims 1 off setting the limitations of conventional radiocarbon dating in order to provide a more secure chronology of events 2 increasing the resolution and precision of palaeoclimatic reconstructions 3 widening the scope of site specific palaeoecological investigations in this paper we focus on the first of these strategies and describe the progress made in developing a more coherent timescale for the climate history of the lateglacial period this has been achieved by using a number of independent methods including calibration of ams radiocarbon dates obtained from terrestrial plant macrofossils mcr estimates of summer temperatures based on coleopteran records analysis of stable carbon isotope ratios in terrestrial plant macrofossils and tephrochronology following bjorck et al 9s 1998 recommendations we integrate the new results to construct a provisional event stratigraphy for the last glacial holocene transition in the british isles which is based on a sequence of features that are believed to be time parallel this approach is considered to provide a more coherent framework for direct comparison of the palaeoenvironmental evidence from britain with that from elsewhere

consistently large marine reservoir ages in the norwegian sea during the last deglaciation with the exception of the gs 1 younger dryas coldperiodmarine reservoir ages for the last deglaciation in the north atlantic norwegian sea are generally assumed to have been around 400 500 radiocarbon years in magnitude earth planet sci lett 126 1994 275 radiocarbon 37 1995 53 quat res 52 1999 104 nature 412 2001 724 by comparing the climate records obtained from the grip ice core nature 359 1992 311 j quat sci 13 4 1998 283 andfrom north atlantic norwegian sea cores quat res 52 1999 104 geology 23 12 1995 1059 nature 356 1991 757 nature 356 1992 757 paleoceanography 3 1 1988 1 nature 343 1990 612 earth planet sci lett 126 1994 275 with radiocarbon dated european continental records we show that marine reservoir ages in the norwegian sea were of the order of 1000 14 c yr including large uncertainties this approach rests on the reasonable assumption that climate changes throughout the ne atlantic andeurope were more or less synchronous at the centennial scale fairly large variations in reservoir ages over time may have been causedby changing atmospheric 14 c content the results indicate that detailed land sea correlations for the north atlantic during the last deglaciation are not feasible using radiocarbon dating alone r 2003 elsevier science ltd all rights reserved

source of ash zone 1 in the north atlantic geochemical evidence shows that the silicic component of the widespread ash zone 1 in the north atlantic is derived from a major ignimbrite forming eruption which occurred at the katla caldera in southern iceland during the transition from glacial to interglacial conditions in younger dryas time both trace and major element evidence of the rhyolitic products excludes the oraefajokull volcano as a source the high ti basaltic component in the marine ash zone can also be attributed to contemporaneous eruption in the katla volcanic complex dispersal of tephra from this event is primarily attributed to the generation of co ignimbrite ash columns in the atmosphere with ash fallout on both sea ice and on the ocean floor north and east of iceland owing to the changing ocean circulation characteristics of the glacial regime including suppression of the irminger current and a stronger north atlantic current tephra was rafted on sea ice south into the central north atlantic and deposited as dispersed ash zone 1 sediments south of iceland also show evidence of the formation of ash turbidites generated either by the entrance of pyroclastic flows into the sea or during discharge of jokulhlaups or glacier bursts from this subglacial eruption

geochemistry of holocene cryptotephras from the north iceland shelf md99 2269 intercalibration with radiocarbon and palaeomagnetic chronostratigraphies tephra grain counting morphology and geochemistry in the 150 1000 m size fraction were used to identify 22 individual tephra horizons and one visible tephra layer saksunarvatn from postglacial sediment of the north iceland shelf core md99 2269 66 37 53 n 20 51 16 w water depth 365 m length 2533 cm from the reykjafjardarall basin is constrained at the base by the vedde tephra c 12 000 cal yr bp geochemical analyses and stratigraphical position of the 22 tephra horizons were used to identify the following cryptotephras kol 1372 ad 1372 hekla 1104 ad 1104 snaefellsnes 1 c 1790 cal yr bp hekla 3 c 2970 cal yr bp hekla 4 c 4230 cal yr bp tv 5 c 6860 cal yr bp and suduroy c 8070 cal yr bp a tephra of the same geochemistry as kol 1372 was found near the base of the core and is here named kol3 2269 11 000 cal yr bp eleven additional cryptotephras were geochemically typed to their respective icelandic volcanic centres of origin and their ages constrained four teph

seven tephra isochrones in scotland this paper reports the presence and geochemical characteristics of seven separate tephras in scotland that fell in ad 1510 and about 450 bp 2100 bp 3600 bp 3830 bp 5600 bp and around 6000 bp the results come from 14 peat bogs spread throughout the highlands and islands where 26 tephra deposits have been characterized using over 400 grain discrete electron probe micro analysis firstly tephras are identified and defined by major element characteristics at a reference site secondly tephras are linked between sites on the basis of geochemistry and dated informal names based on british sites are suggested for tephras not yet linked to precise sources in iceland despite some apparent variation in colour all analysed grains in these seven deposits are silicic or intermediate in composition sio2 57 this phenomenon does not appear to be an artifact of preservation for two reasons first small 40 m basic glass shards in peat bogs in northern iceland are found to have undergone minimal alte

the chronology of the last termination a review of radiocarbon dated high resolution terrestrial stratigraphies abstract the last termination is traditionally subdivided into the oldest dryas bolling older dryas allerod younger dryas and preboreal chronozones this paper briefly summarizes the history of the late weichselian chronostratigraphic system and gives an overview of recent developments in radiocarbon dating terrestrial records between 15 000 and 10 000 14 c years bp problems inherent in the traditional chronostratigraphy are discussed in the light of new and more advanced dating possibilities including accelerator mass spectrometry ams measurements on terrestrial macrofossils new results obtained using ams 14 c dated varve sequences are compared with each other and with the marine u th coral record in addition the importance of time synchronous marker horizons as a tool for long distance correlation between different archives is illustrated

towards a holocene tephrochronology for sweden geochemistry and correlation with the north atlantic tephra stratigraphy 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

evidence for the occurrence of vedde ash in sweden radiocarbon and calendar age estimates a tephra layer of rhyolitic composition has been recorded in sediments from lake madtjarn southwestern sweden geochemical analyses have shown that the tephra is identical to the rhyolitic component of the middle younger dryas vedde ash a series of ams radiocarbon measurements places the radiocarbon age of the tephra within a 14c plateau at 10 400 10 300 14c y bp based on a linear younger dryas sedimentation rate and assumptions about the apparent synchroneity of changes in lake sediments tree rings and ice core records the calendar year age of the tephra concentrations is estimated at 12 045 11 975 yr bp which accords well with the age of the equivalent tephra in the grip core 1998 john wiley sons ltd

rhyolitic and dacitic component of the askja 1875 tephra in southern and central finland first step towards a finnish tephrochronology tephra from icelandic volcanic eruptions is frequently dispersed to northern europe but so far tephra has not been detected in finnish sedimentary records we report the first finding of a cryptot

magnetic characterisation and correlation of a younger dryas tephra in north atlantic marine sediments a technique for identifying non visible basaltic tephra rich horizons of younger dryas yd greenland stadial gs 1 age in northeast atlantic sediments using rapid non destructive magnetic measurements is presented three high resolution marine sediment cores have been studied in an e w transect across the hebridean margin st kilda basin md95 2007 barra fan md95 2006 and rockall trough md04 2822 magnetic susceptibilities and remanent magnetisations were measured at contiguous 1 cm resolution on bulk sediments in all three cores an interval with higher proportions of hard magnetic minerals coincides with a clearly defined peak in basaltic tephra shard 250 m counts which can be constrained to the early part of the yd gs1 based on faunal climate proxies electron microprobe analyses of the magnetically distinct basaltic tephra interval in all three cores displays the same major element geochemistry as published for the vedde basaltic i tab 1 i e sourced from the icelandic volcano katla the identification of transitional alkalic basaltic tephras within marine sediments could potentially be facilitated by magnetic analysis as a useful chronostratigraphic screening tool copyright 2009 john wiley sons ltd

constraining the age of lateglacial and early holocene pollen zones and tephra horizons in southern sweden with bayesian probability methods the sediment sequence from hasseldala 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 hasseldalen tephra and the 10 ka askja tephra twenty eight high resolution 14 c 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 strati graphic 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 hasseldalen 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 hasseldala 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

selective concentration and enumeration of tephra shards from lake sediment cores this paper describes a technique for the enumeration of tephra shards from lake sediment cores whereby approximately 85 of unwanted sediment fractions organic matter biogenic silica carbonates are removed by selective stepwise chemical attack the order of 100 samples can be prepared simultaneously in a modest sized water bath and the recovery rate of the technique is greater than 90 the procedure is appli cable to all lake sediment types

identification and definition of primary and reworked tephra in late glacial and holocene marine shelf sediments off north iceland nine tephra layers in marine sediment cores md99 2271 and md99 2275 from the north icelandic shelf spanning the late glacial and the holocene have been investigated to evaluate the effectiveness of methods to detect tephra layers in marine environments to pinpoint the stratigraphic level of the time signal the tephra layers provide and to discriminate between primary and reworked tephra layers in a marine environment these nine tephra layers are the borrobol like tephra vedde ash askja s tephra saksunarvatn ash and hekla 5 hekla 4 hekla 3 hekla 1104 and v1477 tephras the methods used were visual inspection magnetic susceptibility x ray photography mineralogical counts grain size and morphological measurements and microprobe analysis the results demonstrate that grain size measurements and mineralogical counts are the most effective methods to detect tephra layers in this environment revealing all nine tephra layers in question definition of the tephra layers revealed a 2 3 cm diffuse upper boundary in eight of the nine tephra layers and 2 3 cm diffuse lower boundary in two tephra layers using a multi parameter approach the stratigraphic position of a tephra layer was determined where the rate of change of the parameters tested was the greatest compared with background values below the tephra the first attempt to use grain morphology to distinguish between primary and reworked tephra in a marine environment suggests that this method can be effective in verifying whether a tephra layer is primary or reworked morphological measurements and microprobe analyses in combination with other methods can be used to identify primary tephra layers securely the study shows that there is a need to apply a combination of methods to detect define the time signal and discriminate between primary and reworked tephra in marine environments copyright 2011 john wiley sons ltd

the dynamic rupture process of the 2001 geiyo japan earthquake 1 the 2001 geiyo earthquake mw 6 7 occurred as a normal fault event within the young subducting philippine sea plate we investigate a dynamic rupture process of the earthquake and estimate the dynamic parameters firstly the space time distribution of slip of the earthquake i e the kinematic model was investigated by inverting the near field strong ground motion data and teleseismic waveform data secondly we reconstruct a dynamic model in constraint with the above kinematic model the distribution of dc is also estimated from the inverted slip rate time functions mikumo et al 2003 and found to be less than 70 cm we found that dynamic parameters are very heterogeneous there exist two asperities the peak stress drop on the first asperity is about 20 mpa and 10 mpa on the second one very low or negative stress drop zone is found between two asperities the strength excess and fracture energy distribution are also heterogeneous

linking the north atlantic to central europe a high resolution holocene tephrochronological record from northern germany 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 torfajokull 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

synchronized terrestrial atmospheric deglacial records around the north atlantic on the basis of synchronization of three carbon 14 c 14 dated lacustrine sequences from sweden with tree ring and ice core records the absolute age of the younger dryas preboreal climatic shift was determined to be 11 450 to 11 390 80 years before the present a 150 year long cooling in the early preboreal associated with rising delta 14 c values is evident in all records and indicates an ocean ventilation change this cooling is similar to earlier deglacial coolings and box model calculations suggest that they all may have been the result of increased freshwater forcing that inhibited the strength of the north atlantic heat conveyor although the younger dryas may have begun as an anomalous meltwater event

the use of microtephra horizons to correlate late glacial lake sediment successions in scotland evidence is presented to show that two measurable concentrations of microtephra particles can be detected in deposits of late devensian late glacial age in three sites in scotland one layer is attributed to the vedde ash a marker horizon within the younger dryas chronozone the second is a new tephra reported for the first time which we name the borrobol tephra this occurs consistently near the base of the late glacial interstadial organic sediments at each site and is thought to date to around 12 5 14c ka bp geochemical determinations using an electron microprobe confirm the identification of the vedde ash suggest the borrobol tephra to have an icelandic origin and demonstrate the consistency of the geochemical signals at all three sites 1997 john wiley sons ltd

irregular glacial interstadials recorded in a new greenland ice core the greenland ice sheet offers the most favourable conditions in the northern hemisphere for obtaining high resolution continuous time series of climate related parameters profiles of 18o 16o ratio along three previous deep greenland ice cores1 3 seemed to reveal irregular but well defined episodes of relatively mild climate conditions interstadials during the mid and late parts of the last glaciation but there has been some doubt as to whether the shifts in oxygen isotope ratio were genuine representations of changes in climate rather than artefacts due to disturbed stratification here we present results from a new deep ice core drilled at the summit of the greenland ice sheet where the depositional environ ment and the flow pattern of the ice are close to ideal for core recovery and analysis the results reproduce the previous findings to such a degree that the existence of the interstadial episodes can no longer be in doubt according to a preliminary timescale based on stratigraphic studies the interstadials lasted from 500 to 2 000 years and their irregular occurrence suggests complexity in the behaviour of the north atlantic ocean circulation

was the 12 1 ka icelandic vedde ash one of a kind abstract the vedde ash is the most important volcanic event marker layer for the correlation of late quaternary palaeoenvironmental archives in europe and the north atlantic first defined from its type site localities near alesund western norway the vedde ash has now been traced across much of northern and central europe into northwest russia within north atlantic marine sediments and into the greenland ice cores the vedde ash is thought to derive from an eruption of the katla volcano in iceland that occurred midway through the younger dryas greenland stadial 1 gs 1 12 1 ka bp visible and cryptotephra deposits of the vedde ash have been found in numerous stratified sites with robust chronologies which allow its age to be constrained and its dispersal to be mapped the eruption must have been highly explosive however few proximal outcrops have been confirmed and this crucial ash layer remains almost exclusively distally described and characterised using major element glass compositions the widespread distribution stratigraphic associations and consistent major element glass chemistry have led the quaternary tephrochronological community to see the vedde ash as a robust and unique chronological marker layer for the last glacial to interglacial transition 10 18 ka bp here we present new glass analyses of the vedde ash from multiple sites around the dispersal area using a full suite of compositional analysis including for the first time single grain trace element data these data demonstrate the strong compositional coherence of vedde ash deposits however comparison with major minor and trace element compositional data from several other distally described icelandic tephras reveals that both before and after the younger dryas chronozone there were eruptions that generated widespread tephra layers that have comparable glass shard compositions to the vedde ash this implies that these numerous events not only hail from the same volcanic system but that the melts share similar crystallisation trends and mixing patterns prior to eruption it seems therefore that composition alone is insufficient for the correlation of some widespread tephra layers good stratigraphic information and or robust dating control are also essential

a younger dryas ash bed in western norway and its possible correlations with tephra in cores from the norwegian sea and the north atlantic abstract a bed of volcanic ash up to 23 cm thick is found in lacustrine and marine sediments in western norway it is formally mamed the vedde ash bed and its age is approximately 10 600 yr b p i e mid younger dryas the bed consits of pure glass having a bimodal basaltic and rhyolitic somposition the geochemistry of the glass shards suggests an icelandic source by means of stratigraphic position and geochemistry the ash is correlated with ash zones found in cores from the continental shelf the norwegian sea and the north atlatic

a late younger dryas early holocene tephrostratigraphy for fosen central norway a number of rapid climate oscillations occur during the lateglacial early holocene 15 8 ka bp period and a well developed tephrostratigraphy in association with these oscillations increases the po

three new distal tephras in sediments spanning the last glacial interglacial transition in scotland three new microtephras are reported from a number of lake sites from the inner hebrides and scottish mainland one occurs stratigrapically in the middle of greenland interstadial 1 gi 1 and has been named the penifiler tephra it is rhyolitic and possesses a geochemical signature that is very similar to that of the borrobol tephra which also occurs in three of the sequences reported here but which lies close to the lower boundary of gi 1 the second occurs stratigraphically in the early holocene below the saksunarvatn ash and is named the ashik tephra this tephra is geochemically bimodal with a rhyolitic component comparable to the an druim tephra that occurs later in the holocene and a basaltic component which is similar to the saksunarvatn ash a third tephra occurs stratigraphically above the saksunarvatn ash and is provisionally named the breakish tephra the consistent inter site correlation demonstrated for these new tephras at several sites enhances the regional tephrostratigraphic framework and increases the potential for correlating palaeoenvironmental events during gi 1 and the early holocene however the occurrence of multiple tephras with similar geochemistry in close stratigraphic and temporal proximity has implications for the rigour with which tephrostratigraphic investigations must be performed copyright 2006 john wiley sons ltd

the north atlantic atmosphere sea surface 14c gradient during the younger dryas climatic event abstract we attempt to quantify the 14c difference between the atmosphere and the north atlantic surface during a prominent climatic period of the last deglaciation the younger dryas event yd our working hypothesis is that the north atlantic may have experienced a measurable change in 14c reservoir age due to large changes of the polar front position and variations in the mode and rate of north atlantic deep water nadw production we dated contemporaneous samples of terrestrial plant remains and sea surface carbonates in order to evaluate the past atmosphere sea surface 14c gradient we selected terrestrial vegetal macrofossils and planktonic foraminifera neogloboquadrina pachyderma left coiling mixed with the same volcanic tephra the vedde ash bed which occurred during the yd and which can be recognized in north european lake sediments and north atlantic deep sea sediments based on ams ages from two norwegian sites we obtained about 10 300 yr bp for the atmospheric 14c age of the volcanic eruption foraminifera from four north atlantic deep sea cores selected for their high sedimentation rates 10 cm kyr 1 were dated by ams 21 samples for each core the raw 14c ages assigned to the ash layer peak is significantly older than the 14c age obtained on land part of this discrepancy is due to bioturbation which is shown by numerical modelling nevertheless after correction of a bioturbation bias the mean 14c age obtained on the planktonic foraminifera is still about 11 000 11 100 yr bp the atmosphere sea surface 14c difference was roughly 700 800 yr during the yd whereas today it is 400 500 yr a reduced advection of surface waters to the north atlantic and the presence of sea ice are identified as potential causes of the high 14c reservoir age during the yd

towards a european tephrochronological framework for termination 1 and the early holocene the record of deposition of tephras in europe and the north atlantic during the period 18 5 8 0 14 c ka bp the last termination and early holocene is reviewed altogether 34 tephras originating from four main volcanic provinces iceland the eifel district the massif central and italy have been identi ed so far in geological sequences spanning this time interval most of the records have been based until very recently on observations of visible layers of tephras here we report on the potential for extending the areas over which some of the tephras can be traced by the search for layers of micro tephra which are not visible to the naked eye and on the use of geochemical methods to correlate them with known tephra horizons this approach has greatly extended the area in northern europe over which the vedde ash can be traced the same potential exists in southern europe which is demonstrated for the rst time by the discovery of a distinct layer of micro tephra of the neapolitan yellow tu in a site in the northern apennines in italy far to the north of the occurrences of visible records of this tephra the paper closes by considering the potential for developing a robust european tephrostratigraphy to underpin the chronology of records of the last termination and early holocene thereby promoting a better understanding of the nature timing and environmental e ects of the abrupt climatic changes that characterized this period

the marine 14c age of the vedde ash bed along the west coast of norway articulated molluscs sea urchins and barnacle fragments close to the vedde ash bed in a shallow marine deposit on the west coast of norway have been 14c dated the weighted mean of four dates from a sediment slice 8 cm thick centred on the vedde ash bed is 10920 24 14c yr bp the most accurate 14c age of the vedde ash from terrestrial plant macrofossils is 10310 50 yr bp the difference is the 14c reservoir age for coastal water at the west coast of norway during the mid younger dryas and equals 610 55 yr this is 230 yr older than the reservoir age for the bolling allerod and for the present day in this area the result supports earlier conclusions of a higher reservoir age for the younger dryas in the north atlantic and nordic seas although our reservoir age of 610 55 yr is a few hundred years younger this suggests that the 14c reservoir age at vedde ash time may increase from coastal water towards the open north atlantic and nordic seas copyright 2001 john wiley sons ltd

an ash fall within the loch lomond stadial cores recovered from the witch ground basin central north sea and the northern rockall trough near the wyville thomson ridge have been found to contain volcanic glass shards these have been correlated with the vedde ash bed of western norway which has an age of 10 600 f 60yr bp and with north atlantic ash zone 1 this is the first time that this important chrono stratigraphic marker has been identified on the uk continental shelf and it is suggested that it might also be present in northern scotland if so it would be a useful tool in the correlation of terrestrial lacustrine and marine sequences of the loch lomond stadial journal of quaternary science

a new and less destructive laboratory procedure for the physical separation of distal glass tephra shards from sediments abstract tephrochronology a key tool in the correlation of quaternary sequences relies on the extraction of tephra shards from sediments for visual identification and high precision geochemical comparison a prerequisite for the reliable correlation of tephra layers is that the geochemical composition of glass shards remains unaltered by natural processes e g chemical exchange in the sedimentary environment and or by laboratory analytical procedures however natural glasses particularly when in the form of small shards with a high surface to volume ratio are prone to chemical alteration in both acidic and basic environments current techniques for the extraction of distal tephra from sediments involve the cleaning of samples in precisely such environments and at elevated temperatures the acid phase of the cleaning process risks alteration of the geochemical signature of the shards while the basic phase leads to considerable sample loss through dissolution of the silica network here we illustrate the degree of alteration and loss to which distal tephras may be prone and introduce a less destructive procedure for their extraction this method is based on stepped heavy liquid flotation and which results in samples of sufficient quality for analysis while preserving their geochemical integrity in trials this method out performed chemical extraction procedures in terms of the number of shards recovered and has resulted in the detection of new tephra layers with low shard concentrations the implications of this study are highly significant because i the current database of distal tephra records and their corresponding geochemical signatures may require refinement and ii the record of distal tephras may be incomplete due to sample loss induced by corrosive laboratory procedures it is therefore vital that less corrosive laboratory procedures are developed to make the detection and classification of distal glass tephra more secure

the tephrochronology of iceland and the north atlantic region during the middle and late quaternary a review 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

new radiocarbon dates for the vedde ash and the saksunarvatn ash from western norway abstract the vedde ash bed mid younger dryas and the saksunarvatn ash early holocene are important regional stratigraphic event markers in the north atlantic the norwegian sea and the adjacent land area it is thus essential to date them as precisely as possible the occurrence of the saksunarvatn ash is reported for the first time from western norway and both tephras are dated precisely by ams analyses of terrestrial plant material and lake sediment at krakenes the vedde ash has been previously dated at sites in western norway to about 10 600 yr b p it is obvious in the younger dryas sediments at krakenes and its identity is confirmed geochemically the mean of four ams dates of samples ofsalix herbacealeaves adjacent to the tephra is 10 310 50 yr b p the saksunarvatn ash is not visible in the early holocene lake sediment at krakenes after removal of organic material and diatoms the identity of the tephra particles was confirmed geochemically and their stratigraphic concentration was estimated from curve matching of a series of seven ams dates of terrestrial plant macrofossils and whole sediment the radiocarbon age of the ash is 8930 9060 yr b p corresponding to an age of 9930 10 010 cal yr b p 7980 8060 cal yr b c

a new type of silicate liquid immiscibility in peralkaline nepheline syenites lujavrites of the ilimaussaq complex south greenland the 1 15 ga old ilimaussaq intrusive complex in south greenland shows an extensive fractionation trend from alkaline augite syenite to various varieties of strongly peralkaline agpaitic nepheline syenites lujavrites the lujavrites are rocks with highly fluidal textures and are interpreted to have compositions close to melt compositions with the possible exception of significant loss of volatiles they crystallized between 800 and 450 c at 1 kbar and the different textural and mineralogical varieties exhibit variable but extreme enrichment in incompatible elements such as u th nb li be ree zr or rb dykes of one variety of these extreme agpaites exhibit spectacular features of macroscopically visible liquid immiscibility between an fe mn nb ree rich and a na al rich melt the latter forms light greyish pillow like structures of up to some centimetres size comparable to the ocelli known from lamprophyres in a dark fine grained matrix which is one of the most nb and ree rich melts known up to 1 2 wt nb2o5 and up to 6 6 wt reetot 2o3 unmixed between 500 and 600 c at 1 kbar both former melt phases crystallized assemblages involving arfvedsonite k feldspar steenstrupine and a variety of rare trace minerals presumably the na al rich melt additionally crystallized nepheline which was completely converted to analcime in an auto hydration process during cooling thisoccurrence offers the possibility to study the effect of compositional changes in melts and their bearing on immiscibility in great detail because the two immiscible melt phases are macroscopically visible can easily be separated from each other and a large part of the fractionation trend towards the solvus is recorded in the various lujavritic rock types of the ilimaussaq complex

estimating the 3d shape of volcanic ash to better understand sedimentation processes and improve atmospheric dispersion modelling abstract the sedimentation rate of volcanic ash through the atmosphere influences its travel distance with important implications for aviation and health the fall velocity of a particle depends on its size and density but also shape and volcanic ash is not spherical to capture the sedimentation of ash atmospheric dispersion models use empirical drag equations calibrated using geometric shape descriptors however particle shape data are scarce and there is no standard method of shape measurement in addition shape measurements are not always available during an eruption when dispersion models are used operationally to forecast ash hazard we assess the variability in the shape of volcanic ash from icelandic eruptions using x ray computed tomography to consider how good different drag equations and shape descriptors are at representing the sedimentation of volcanic ash we compare calculated fall velocities to measured fall velocities of volcanic ash in air in a settling column we then suggest the best drag equations and shape descriptors for use in atmospheric dispersion models we find that shape dependent drag equations produce more accurate results than a spherical approximation however accurate drag calculations based on the shape descriptor sphericity which is a function of surface area require the imaging resolution to be within the range of 102 105 voxels per particle where a voxel is a volumetric pixel as surface area is sensitive to imaging resolution we suggest that the large scale form of the particle impacts sedimentation more than small scale surface roughness shape descriptors based on ratios between principal axis lengths are more practical as they are less variable among particle size classes and much less sensitive to imaging resolution finally we use particle shape data from this study and literature sources to make recommendations on default values for use with atmospheric dispersion models where no shape data are available

tephrochronology of last termination sequences in europe a protocol for improved analytical precision and robust correlation procedures a joint scotav intimate proposal the precise sequence of events during the last termination 18 000 9000 ka 14 cy r bp and the extent to which major environmental changes were synchronous are difficult to estab lish 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 there fore 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 pro pose 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