the mjauvotn tephra and other holocene tephra horizons from the faroe islands a link between the icelandic source region the nordic seas and the european continent six different tephra horizons were found in peat and lacustrine sediments on the faroe islands in the north atlantic their geochemistry showed that they were of icelandic origin three of these can be correlated with previously described tephra horizons from the faroe islands the saksunarvatn tephra c 9000 bp the hekla 4 c 3800 bp and hekla selsund c 3600 bp a previously unrecorded basaltic tephra dated to c 5700 5300 bp was found in two lacustrine successions this horizon is named the mjauvotn tephra after mjauvotn lake on the island of streymoy two further tephras were recorded the basaltic phase of the landnam tephra viia c ad 870s and the rhyolitic tjornuvk tephra these were found in sediments deposited shortly after the first phase of human settlement at tjornuvik which is ams dated to ad 675 861 calibrated age the faroe islands are an important link between the volcanic sources on iceland and the established tephrochronological frameworks on the british isles scandinavia and germany their position within a sensitive region of the northeastern branch of the north atlantic drift make the faroe islands ideal for registering climate changes during the holocene the establishment of a modern tephrochronology may provide a tool for more precisely dating and correlating regional climate events in and around the nordic seas

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

time dependent brittle creep in darley dale sandstone 1 the characterization of time dependent brittle rock deformation is fundamental to understanding the long term evolution and dynamics of the earth s crust the chemical influence of pore water promotes time dependent deformation through stress corrosion cracking that allows rocks to deform at stresses far below their short term failure strength here we report results from a study of time dependent brittle creep in water saturated samples of darley dale sandstone initial porosity 13 under triaxial stress conditions results from conventional creep experiments show that axial strain rate is heavily dependent on the applied differential stress a reduction of only 10 in differential stress results in a decrease in strain rate of more than two orders of magnitude however natural sample variability means that multiple experiments must be performed to yield consistent results hence we also demonstrate that the use of stress stepping creep experiments can successfully overcome this issue we have used the stress stepping technique to investigate the influence of confining pressure at effective confining pressures of 10 30 and 50 mpa while maintaining a constant 20 mpa pore fluid pressure our results demonstrate that the stress corrosion process appears to be significantly inhibited at higher effective pressures with the creep strain rate reduced by multiple orders of magnitude the influence of doubling the pore fluid pressure however while maintaining a constant effective confining pressure is shown to influence the rate of stress corrosion within the range expected from sample variability we discuss these results in the context of microstructural analysis acoustic emission hypocenter locations and fits to proposed macroscopic creep laws

composition and origin of ash zones from marine isotope stages 3 and 2 in the north atlantic abstract the composition and distribution of four north atlantic ash zones from the faroe islands margin is presented including the widespread north atlantic ash zone ii naaz ii 52 53 ka bp and three mainly basaltic ash zones here referred to as the faroe marine ash zones fmaz i ca 15 4 14 c ka bp ii ca 23 14 c ka bp also referred to as the fugloyarbanki tephra and iii ca 33 14 c ka bp one core from the south east labrador sea was also analysed naaz ii was found in all investigated cores indicating very large eruption events in the early marine isotope stage mis 3 detailed investigations of naaz ii in high resolution cores show that two separate layers can be distinguished a lower mixed alkalic basalt and rhyolitic layer and an upper predominantly tholeiitic basalt tephra the main component of the basaltic fmaz ii was found in all cores from the faroe island margin and the labrador sea indicating that this tephra is the most widespread tephra from mis 2 in the north atlantic discovered so far we discuss the possibility to use these tephra layers for exact correlations between marine records and land sections such as the greenland ice cores and terrestrial records on the european mainland

stratigraphy and distribution of tephra layers in marine sediment cores from the faeroe islands north atlantic the stratigraphical position of eight discrete tephra layers is investigated in four marine sediment cores from north and south of the faeroe islands the tephras are correlated to the marine oxygen isotope stratigraphy and the interstadial stadial cycles dansgaard oeschger cycles of the greenland ice cores a very prominent volcanic event occurred during interstadial 23 in marine oxygen isotope stage mis 5c correlating with a cold event within this otherwise warm interval two tephra horizons in mis 5a correlate with the beginning and end of interstadial 21 respectively the most widely distributed ash is the well known ash zone 2 from the lower part of mis 3 in the two cores from north of the faeroe islands it consists of two discrete layers a lower one composed of rhyolitic and basaltic tephra and an upper layer composed exclusively of basaltic glass the two tephra falls coincide with the two warm intervals of dansgaard oeschger cycle no 15 a hitherto unknown basaltic tephra was recorded just above heinrich event h4 in the lower part of the warm interstadial 8 and dated to ca 33 14c ka bp another frequently recorded layer termed the fugloyarbanki tephra is likewise basaltic it is dated to 23 1 14c ka bp it was deposited after the peak warm phase of interstadial 3 at the mis 3 2 transition finally a tephra dated ca 15 4 14c ka bp was found in two cores from north of the ridge it was deposited at the beginning of the deglaciation between the last glacial maximum and heinrich event h1

the age and chronostratigraphical significance of north atlantic ash zone ii rhyolitic tephra with the geochemical characteristics of north atlantic ash zone naaz ii are described from the giant piston core md95 2006 from the northeast atlantic ocean shard size distribution indicates that the tephra represent a wind sorted primary air fall deposit but with a mode close to 200 m they are too coarse to be air fall deposits at the core site which lies ca 1000 km from the possible tephra source randomly sampled shards reveal a single geochemical population belonging to the icelandic transitional alkali magma series suggesting that they are unlikely to represent ice rafted debris derived from icelandic icebergs the tephra probably represent air fall deposits transported to the core site by sea ice within the northeast atlantic gyre the naaz ii peak coincides with the rapid climate transition cooling at the end of interstadial 15 which can be assigned an age of 53 260 2660 yr bp from direct correlation with the greenland ice core gisp2 record a comparison of the md95 2006 neogloboquadrina pachyderma sinistral relative abundance and gisp2 18o records relative to the naaz ii isochron suggests that this climatic event was synchronous across the north atlantic by direct correlation of interstadial maxima between the two records the gisp2 time scale is transferred to md95 2006 comparison of corrected and calibrated radiocarbon ages derived from monospecific foraminifers with the gisp2 ages at the same stratigraphical horizon suggest major age differences these probably result from large variations in atmospheric 14c concentration and highlight the significant uncertainties associated with radiocarbon calibration during marine isotope stage 3 copyright 2004 john wiley sons ltd

evidence for the presence of the vedde ash in central europe abstract as part of ongoing research to find distal tephra two lacustrine cores were analysed for the presence of volcanic ash layers not visible to the naked eye soppensee in switzerland and rotmeer in southern germany the laacher see tephra 12 900 ka bp is present as a visible layer in both sites in both cores we found a discrete tephra horizon with similar morphologies in the middle of the biostratigraphic units equivalent to the younger dryas stadial the vitreous components of these two tephra layers are geochemically identical comparison of the geochemical stratigraphical and chronological data from both sites strongly suggest that the tephra can be attributed to the icelandic vedde ash a widely distributed horizon found throughout the north atlantic and northern europe our results indicate that a precise and direct correlation between the greenland ice cores and central european sequences is now possible based on a co located tephra layer this means that there is now the potential to independently test climate synchronicity between greenland and europe as far south as the alps

circulation changes in the faeroe shetland channel correlating with cold events during the last glacial period 58 10 ka a core from the margin of the faeroe shetland channel covering the last glacial period 58 10 ka reveals a very detailed record of oscillations in benthic and planktic foraminifera oxygen isotopes and ice rafted detritus the oscillations correlate almost exactly with the dansgaard oeschger cycles in the greenland ice cores including similar subdivision of the cycles into warmer interstadial and colder stadial sections the remarkably close agreement between the benthic faunas and their 18 o data and the record of the ice cores provides new empirical evidence for a close relationship between the deep ocean circulation and the abrupt climatic changes of the last glacial period

high resolution record of northern hemisphere climate extending into the last interglacial period high resolution record of northern hemisphere climate extending into the last interglacial period

tracing volcanic events in the ngrip ice core and synchronising north atlantic marine records during the last glacial period five basaltic tephra horizons have been identified in the ngrip ice core during the last glacial period geochemical analyses indicate that these tephras are of icelandic origin with one horizon identified as the faroe marine ash zone iii fmaz iii an important marker horizon in several marine records within the faroe islands margin the fmaz iii is a few millimetres thick and is constrained to 38 122 723 yr b2k before year ad 2000 according to the greenland ice core chronology gicc05 this event falls directly at the thermal peak of greenland interstadial 8 gi 8 and a marine ice correlation based on the position of this tephra suggests a close coupling of the marine and atmospheric systems at this time the remaining tephras originate from the eastern volcanic zone with the katla volcanic system pinpointed as the dominant source these are thought to represent previously unknown volcanic events which add considerably to our knowledge of icelandic volcanism during this period independent ages are assigned to these events and together with major element characterisation form the backbone of the tephrochronology framework presented here despite the identification of these tephras this study also demonstrates that the sulphate record may not always act as a suitable proxy for the identification of tephra horizons in ice core records

composition and origin of volcanic ash zones in late quaternary sediments from the reykjanes ridge evidence for ash fallout and ice rafting abstract seven quaternary volcanic ash zones including the well known ash zones az i and az ii have been investigated in five well dated sediment cores from the reykjanes ridge between 59 and 60 n the zones were defined on their geochemical composition morphological variation of the particles and stratigraphic position the ash zones az i and az ii show a mixture of colorless bubble wall shards and brown blocky glasses whereas ash zones vz 1 to 5 are composed of dark to light brown volcanic glasses ranging in shape and texture from blocky nonvesicular shards to highly vesicular pumiceous types the dark brown glass shards of ash zones vz 1 to 5 represent subalkalic and low k subalkalic basaltic series geochemical correlation of the ash zones with possible source areas indicates that they all derived from iceland the colorless shards in az i and az ii are of rhyolitic composition the eastern volcanic zone on iceland is the most important source with abundant volcaniclastics being produced by subaerial subglacial eruptions and secondary erosion of volcanic rocks explosive volcanism on iceland is manifested by marine ash layers az i and az ii most of the volcaniclastic zones vz 1 to 5 coincide chronologically with peak abundances of coarse ice rafted material ird indicating that most of this volcanic glass was incorporated into the icelandic ice sheet and released to the ocean by iceberg calving and subsequent melting however two layers in vz 1 and vz 5 have no correlatives in the ird record suggesting that they were probably transported by local sediment gravity flows from this we must infer that most of the ice rafting episodes bearing volcanic particles as observed in sediments from reykjanes ridge monitor glacier fluctuations on iceland

north atlantic deep sea sedimentation of late quaternary tephra from the iceland hotspot piston cores recovered from the north atlantic were used to study the sedimentation of holocene and pleistocene volcanic ash in the irminger and iceland basins ash zones 1 11 100 yr b p 2 55 000 yr b p and 3 305 000 yr b p were identified from their major element glass composition the silicic and alkalic ash zones 1 and 2 originate from the southeastern volcanic zone of iceland where they correlate with the solheimar ignimbrite from katla volcano and the thorsmork ignimbrite from tindfjallajokull volcano respectively the low alkali composition of silicic ash zone 3 indicates a source from one of the silicic centers in the active rift system ash zones 2 and 3 occur in the irminger basin as dispersed glass shards over a depth interval of several tens of centimeters their compositional and granulometric characteristics reflect an initial fallout on pack ice north of iceland followed by ice rafting sedimentation in the denmark strait prior to bioturbation 18o stratigraphy of foraminifera in the cores indicates that the ash zones were deposited during a cold interval at the time when seas north of iceland were ice covered sedimentary features indicate that turbidity currents were also involved in the dispersal of ash zones 1 and 2 south of iceland the initiation of these gravity currents from the shelf can be attributed to either glacier bursts jokulhlaups carrying tephra or the entrance of pyroclastic flows into the ocean

tephra geochemistry a discussion of some persistent analytical problems distal tephrochronological studies are reliant upon precise and accurate geochemical quantitation this quantitation can involve difficulties not often encountered in more conventional petrology solving these problems is of particular importance in the holocene and late quaternary of northwest europe where numerous tephra layers of icelandic origin have been detected in peat bogs and lake and marine sediments electron probe microanalysis epma has proved the most applicable analytical technique when insufficient quantities of tephra are available for bulk analysis standard epma techniques require fine tuning to the tephra problem as the geochemical instability of glass is a common feature in many previous studies geochemical data of avoidably low quality have been produced and little attention appears to have been directed to modification of epma methods it is suggested that the black box approach to geochemical analysis should be avoided by the consideration of several factors these include

space time precursory features within ground velocities and seismicity in north central italy earthquakes cannot be predicted with ultimate precision so that the progressive reduction of the prediction uncertainty in space and time is an evergreen and challenging task both from the scientific point of view for the intrinsic complexity of seismic phenomenon and for its high societal relevance to this aim algorithms like cn m8 and m8s based on objective recognition of seismicity patterns have been already tested for some decades for intermediate term middle range prediction of strong earthquakes above a pre assigned magnitude threshold here moving from some preliminary ideas we propose an integrated approach to earthquake prediction based on the synergy of high density geodetic observations and seismological information defining a new paradigm for time dependent hazard assessment scenarios through a wider and more refined retrospective analysis duly involving the accuracy analysis of the newly available geodetic results space time precursory features are highlighted within ground velocities and seismicity analyzing the 2016 2017 seismic crisis in central italy and the 2012 emilia sequence overall it is demonstrated that the proper integration of seismological and geodetic information can achieve what here is called intermediate term narrow range earthquake prediction the extent of the alarmed areas identified for the strong earthquakes by earthquake prediction algorithms based on seismicity patterns can be significantly reduced from linear dimensions of a few hundred to a few tens of kilometers leading to an improved more specific implementation of low key preventive actions like those recommended by unesco as early as in 1991

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

volcanic aerosol records and tephrochronology of the summit greenland ice cores the recently collected greenland ice sheet project 2 gisp2 and greenland ice core project ice cores from summit greenland provide lengthy and highly resolved records of the deposition of both the aerosol h2so4 and silicate tephra components of past volcanism both types of data are very beneficial in developing the hemispheric to global chronology of explosive volcanism and evaluating the entire volcanism climate system the continuous time series of volcanic so42 for the last 110 000 years show a strong relationship between periods of increased volcanism and periods of climatic change the greatest number of volcanic so42 signals many of very high magnitude occur during and after the final stages of deglaciation 6000 17 000 years ago possibly reflecting the increased crustal stresses that occur with changing volumes of continental ice sheets and with the subsequent changes in the volume of water in ocean basins sea level change the increase in the number of volcanic so42 signals at 27 000 36 000 and 79 000 85 000 years ago may be related to initial ice sheet growth prior to the glacial maximum and prior to the beginning of the last period of glaciation respectively a comparison of the electrical conductivity of the gisp2 core with that of the volcanic so42 record for the holocene indicates that only about half of the larger volcanic signals are coincident in the two records other volcanic acids besides h2so4 and other so42 sources can complicate the comparisons although the threshold level picked to make such comparisons is especially critical tephra has been found in both cores with a composition similar to that originating from the vatnaoldur eruption that produced the settlement layer in iceland mid a d 870s from the icelandic eruption that produced the saksunarvatn ash 10 300 years ago and from the icelandic eruption s that produced the z2 ash zone in north atlantic marine cores 52 700 years ago the presence of these layers provides absolute time lines for correlation between the two cores and for correlation with proxy records from marine sediment cores and terrestrial deposits containing these same tephras the presence of both rhyolitic and basaltic shards in the z2 ash in thegisp2 core and the composition of the basaltic grains lend support to multiple icelandic sources torfajokull area and katla for the z2 layer deposition of the z2 layer occurs at the beginning of a stadial event further reflecting the possibility of a volcanic triggering by the effects of changing climatic conditions

a new continuous high resolution detection system for sulphate in ice cores sulphate so4 2 is a major ion found in polar ice cores and is related to different aerosol sources and processes explosive volcanic eruptions even far away can cause distinct sulphate peaks in ice core records thus a robust sulphate detection system which is suitable for fieldwork and which enables the measurement of sulphate at high temporal resolution is of great interest in this study we present the adaptation of a new continuous flow analysis system for sulphate that is based on a spectrophotometric method using dimethylsulfonazo iii and an inline reactor column containing barium sulphate particles the method shows a detection limit of 70 ng g 1 and a linear range up to at least 3000 ng g 1 it is simple robust and less prone to interferences compared to the previously used method

correlation and characterisation of individual glass shards from tephra deposits using trace element laser ablation icp ms analyses current status and future potential pearce nicholas denton j s perkins w t westgate j a 2007 correlation and characterisation of individual glass shards from tephra deposits using trace element laser ablation icp ms analyses current status and future potential journal of quaternary science 22 7 pp 721 736 rae2008

volcanic ash layers from the last glacial termination in the ngrip ice core the tephrochronological record of the 1400 1640 m depth 10 000 16 000 calendar ice core years before present of the ngrip ice core has been established by particle screening of selected samples ash was identified in 20 samples correlation with ice marine and terrestrial records from volcanic source regions in the northern hemisphere positively identifies the saksunarvatn ash and the vedde ash ash zone 1 major element chemistry of the remaining identified ash layers mainly points towards an icelandic origin this tephrochronological record provides new important marker horizons for correlating the timing of the climatic changes associated with the last glacial termination within the north atlantic region as well as outlining more details concerning the frequency and composition of volcanic eruptions occurring at this deglaciation copyright 2005 john wiley sons ltd

identification of the fugloyarbanki tephra in the ngrip ice core a key tie point for marine and ice core sequences during the last glacial period a visible tephra horizon in the ngrip ice core has been identified by geochemical analysis as the fugloyarbanki tephra a widespread marker horizon in marine cores from the faroe islands area and the northern north atlantic an age of 26 740 390 yr b2k 1 uncertainty is derived for this tephra according to the new greenland ice core chronology gicc05 based on multi parameter counting of annual layers detection of this tephra for the first time within the ngrip ice core provides a key tie point between marine and ice core records during the transition between mis 3 and 2 identification of this volcanic event within the greenland records demonstrates the future potential of using tephrochronology to precisely correlate palaeoarchives in widely separated localities that span the last glacial period as well as providing a potential method for examining the extent of the radiocarbon marine reservoir effect at this time copyright 2008 john wiley sons ltd

a comparison of the volcanic records over the past 4000 years from the greenland ice core project and dye 3 greenland ice cores since 1980 the electrical conductivity method ecm has been used to infer volcanic acid signals in greenland ice cores the method reveals the great majority of major volcanic eruptions including several known from historic records subsequent ion chromatographic analyses of the acid volcanic layers show the chemical composition i e the concentration of the volcanic acids h2so4 hci and hf plus e g the nonvolcanically derived hno3 while ecm data are available from a large number of shallow depth greenland ice cores covering the past 500 1500 years only the greenland ice core project grip greenland ice sheet project 2 gisp2 and dye 3 deep ice cores exist for a detailed comparative study of volcanic signals in greenland ice cores representing several thousand years comparison of the volcanic signals registered in the grip and gisp2 cores will be presented elsewhere the latter cores were augered 30 km apart and essentially represent the same atmospheric conditions such as temperature snow accumulation and chemical composition of the air here we present a comparison between the major volcanic signals over the past 4000 years in the grip core from central greenland and the dye 3 core from se greenland in order to investigate the depositional differences many of the major signals are detected in both cores but some of the differences in the records can be used to infer the latitudinal band of some eruption sites furthermore the influence of the amount of annual precipitation and glaciological postdepositional processes on the volcanic signals is discussed

a synchronized dating of three greenland ice cores throughout the holocene as part of the effort to create the new greenland ice core chronology 2005 gicc05 a synchronized stratigraphical timescale for the holocene parts of the dye 3 grip and ngrip ice cores is made by using volcanic reference horizons in electri cal conductivity measurements to match the cores the main annual layer counting is carried out on the most suited records only exploit ing that the three ice cores have been drilled at locations with different climatic con ditions and differences in ice flow however supplemental counting on data from all cores has been performed between each set of reference horizons in order to verify the valid ity of the match after the verification the main dating is transferred to all records us ing the volcanic reference horizons as tie points an assessment of the mean annual layer thickness in each core section confirms that the new synchronized dating is consistent for all three cores the data used for the main annual layer counting of the past 7900 years are the dye 3 grip and ngrip stable isotope records as the high accumulation rate at the dye 3 drill site makes the seasonal cycle in the dye 3 stable isotopes very resistant to firn diffusion an effort has been made to extend the dye 3 holocene record the new syn chronized dating relies heavily on this record of 75 000 stable isotope samples the dat ing of the early holocene consists of an already established part of gicc05 for grip and ngrip which has now been transferred to the dye 3 core gicc05 dates the younger dryas termination as defined from deuterium excess to 11 703 b2k 130 years earlier than the previous grip dating

evidence for general instability of past climate from a 250 kyr ice core record recent results1 2 from two ice cores drilled in central greenland have revealed large abrupt climate changes of at least regional extent during the late stages of the last glaciation suggesting that climate in the north atlantic region is able to reorganize itself rapidly perhaps even within a few decades here we present a detailed stable isotope record for the full length of the greenland ice core project summit ice core extending over the past 250 kyr according to a calculated timescale we find that climate instability was not confined to the last glaciation but appears also to have been marked during the last interglacial as explored more fully in a companion paper3 and during the previous saale holstein glacial cycle this is in contrast with the extreme stability of the holocene suggesting that recent climate stability may be the exception rather than the rule the last interglacial seems to have lasted longer than is implied by the deep sea specmap record4 in agreement with other land based observations5 6 we suggest that climate instability in the early part of the last interglacial may have delayed the melting of the saalean ice sheets in america and eurasia perhaps accounting for this discrepancy

distribution sediment magnetism and geochemistry of the saksunarvatn 10 180 60 cal yr bp tephra in marine lake and terrestrial sediments northwest iceland in 1997 seismic surveys in the troughs off northwest and north iceland indicated the presence of a major regional sub bottom reflector that can be traced over large areas of the shelf cores taken in 1997 and later in 1999 on the images v cruise penetrated through the reflector in core md99 2269 in hunafloaall this reflector is shown to be represented by a basaltic tephra with a geochemical signature and radiocarbon age correlative with the north atlantic wide saksunarvatn tephra we trace this tephra throughout northwest iceland in a series of marine and lake cores as well as in terrestrial sediments it forms a layer 1 to 25 cm thick of fine to medium grained basaltic volcanic shards the base of the tephra unit is always sharp but visual inspection and other measurements carbonate and total organic carbon weight indicate a more diffuse upper boundary associated with bioturbation and with sediment reworking off northwest iceland the saksunarvatn tephra has distinct sediment magnetic properties this is evident as a dramatic reduction in magnetic susceptibility an increase in the frequency dependant magnetic susceptibility and hard magnetisation in a 0 1t irm backfield geochemical analyses from 11 sites indicate a tholeiitic basalt composition similar to the geochemistry of a tephra found in the greenland ice core that dates to 10 180 60 cal yr bp and which was correlated with the 9000 14c yr bp saksunarvatn tephra we present accelerator mass spectrometry 14c dates from the marine sites which indicate that the ocean reservoir correction is close to ca 400 yr at 9000 14c yr bp off northwest iceland copyright 2002 john wiley sons ltd

the greenland ice core chronology 2005 15 42 ka part 1 constructing the time scale the greenland ice core chronology 2005 gicc05 is extended back to 42 ka b2k before 2000 ad i e to the end of greenland stadial 11 the chronology is based on independent multi parameter counting of annual layers using comprehensive high resolution measurements available from the north greenland ice core project ngrip these are measurements of visual stratigraphy conductivity of the solid ice electrolytical melt water conductivity and the concentration of na c a 2 s o 4 n o 3 n h 4 a n uncertainty estimate of the time scale is obtained from identification of uncertain annual layers which are counted as 0 570 5 years the sum of the uncertain annual layers the so called maximum counting error of the presented chronology ranges from 4 in the warm interstadial periods to 7 in the cold stadials the annual accumulation rates of the stadials and interstadials are on average one third and half of the present day values respectively and the onset of the greenland interstadials 2 3 and 8 based on 20 year averaged d 18 o

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

using cryptotephras to extend regional tephrochronologies an example from southeast alaska and implications for hazard assessment abstract 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

late quaternary terrestrial tephrochronology of iceland frequency of explosive eruptions type and volume of tephra deposits 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 torfajokull oraefajokull askja snaefellsjokull eyjafjallajokull 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 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 grimsvotn katla veidivotn bardarbunga reykjanes kverkfjoll 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 grimsvotn 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 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 grimsvotn 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