wind generated eddy characteristics in the lee of the island of hawaii 1 weekly satellite sea surface height ssh anomaly data are used to clarify the mesoscale eddy characteristics in the lee of the island of hawaii the largest island in the hawaiian island chain the lee eddy variability can be separated into two geographical regions in the immediate lee southwest of hawaii region e eddy signals have a predominant 60 day period and a short life span whereas in the region along 19 n west of 160 w region w the eddy variability is dominated by 100 day signals and extends over a broad region by applying a linear ekman pumping model forced by the weekly quikscat wind data we find that the observed 60 day eddy signals originate in the southwest corner of hawaii and are induced by the local 60 day wind stress curl variability associated with the blocking of the trade wind by the island of hawaii the relationship between the wind forcing and the observed ssh signals demonstrates the role of the ocean as an integrator that responds more effectively to the low frequency synoptic atmospheric forcing 60 days than to the higher frequency forcing 30 days since the large amplitude 60 day ssh anomalies take 1 2 weeks to fully develop it is possible that real time observed wind stress data can be used for the prediction of these anomalies in contrast to the wind induced 60 day eddy signals in the lee of the island of hawaii the 100 day eddy signals in region w are likely generated by the instability of the sheared north equatorial current and hawaii lee countercurrent

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

light δ13c events during deglaciation of the east greenland continental shelf attributed to methane release from gas hydrates we have documented high resolution stable isotope records from three marine cores on the east greenland shelf these records exhibit three rapid light 13c events 3 to 7 in benthic and planktic foraminifera during deglaciation that are spatially and temporally transgressive the light 13c events are associated with light planktic 18o indicative of meltwater in kangerlussuaq trough the first 13c event occurs at initial deglaciation 13 94 14 0 cal ka and the second occurs at 12 85 12 9 cal ka a younger event at 10 3 9 3 cal ka is recorded near the mouth of nansen fjord during final deglaciation the hypothesized mechanism for the light 13c is expulsion of methane from gas hydrates in the seafloor from pressure release during ice sheet retreat

iceberg discharges into the north atlantic on millennial time scales during the last glaciation high resolution studies of north atlantic deep sea cores demonstrate that prominent increases in iceberg calving recurred at intervals of 2000 to 3000 years much more frequently than the 7000 to 10 000 year pacing of massive ice discharges associated with heinrich events the calving cycles correlate with warm cold oscillations called dansgaard oeschger events in greenland ice cores each cycle records synchronous discharges of ice from different sources and the cycles are decoupled from sea surface temperatures these findings point to a mechanism operating within the atmosphere that caused rapid oscillations in air temperatures above greenland and in calving from more than one ice sheet

marine evidence for the last glacial advance across eastern hudson strait eastern canadian arctic data from accelerator mass spectrometer radiocarbon dated sediment cores and huntec high resolution seismic profiles were used to investigate the age and origin of the sediments in the eastern basin of hudson strait the data indicate that the ice contact and glacial marine sediments on the basin flanks and much of the upper sequence in the deep floor of the basin were produced during the noble inlet advance 8 9 to 8 4 ka the last northward expansion of the labrador dome on to southeastern baffin island on the northern flank of eastern basin one sequence of ice contact sediments and glacial marine deposits overlies bedrock the glacial marine sediments are transitional upslope to ice contact sediments and form at least two successive ice sheet grounding zones the earliest abundance peaks of benthic foramininfera in glacial marine sediments date ca 8 6 and 8 4 ka and correlate to sediments near the base of the 58 m thick glacial marine section in the deepest part of eastern basin this correlation suggests that noble inlet ice was grounded throughout eastern basin during the early part of its advance in later stages the thinning ice produced grounding zones on the basin flanks while glacial marine sediments were deposited in the deep basin 1998 john wiley sons ltd

high resolution study of icelandic tephras in the kangerlussuaq trough southeast greenland during the last deglaciation tephra abundance data and geochemistry in late glacial and holocene sediments on the east greenland shelf are presented two well known tephras were identified from electron microprobe analysis of tephra shards picked from ash peaks in the cores these are the vedde ash and saksunarvatn ash which probably were deposited on the shelf after transport on drifting ice the radiocarbon dates marine reservoir corrected by 550 yr that constrain the timing of deposition of the tephra layers compare well with the terrestrial and ice core ages of the tephras without requiring additional reservoir correction to align them with the known tephra ages several prominent tephra layers with a composition of ash zone 2 tephra punctuate the deglacial sediments these tephra peaks coincide with significant light stable isotope events signifying glacial meltwater and fine grained sediments poor in ice rafted detritus we interpret the ash zone 2 tephra peaks as sediment released from the greenland ice sheet during strong melting pulses of the deglaciation copyright 2002 john wiley sons ltd

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

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

deglacial changes in ocean circulation from an extended radiocarbon calibration temporal variations in the atmospheric concentration of radiocarbon sometimes result in radiocarbon based age estimates of biogenic material that do not agree with true calendar age this problem is particularly severe beyond the limit of the high resolution radiocarbon calibration based on tree ring data which stretches back only to1 2 about 11 8 kyr before present bp near the termination of the younger dryas cold period if a wide range of palaeoclimate records are to be exploited for better understanding the rates and patterns of environmental change during the last deglaciation extending the well calibrated radiocarbon timescale back further in time is crucial several studies attempting such an extension using uranium thorium dated corals3 4 5 and laminae counts in varved sediments6 7 8 9 show conflicting results here we use radiocarbon data from varved sediments in the cariaco basin in the southern caribbean sea to construct an accurate and continuous radiocarbon calibration for the period 9 to 14 5 kyr bp nearly 3 000 years beyond the tree ring based calibration a simple model compared to the calculated atmospheric radiocarbon concentration and palaeoclimate data from the same sediment core suggests that north atlantic deep water formation shut down during the younger dryas period but was gradually replaced by an alternative mode of convection possibly via the formation of north atlantic intermediate water

a 9000 year old ash bed on the faroe islands abstract radiocarbon datings of the saksunarvatn ash bed on the faroe islands indicate an age of 9000 9100 yr b p the saksunarvatn ash bed differs geochemically from both ash zone 1 in the north atlantic and the vedde ash bed of norway and the norwegian sea all mentioned ashes are assumed to originate from iceland the vedde ash has been dated at 10 600 60 yr b p consequently the saksunarvatn and vedde ash beds provide an opportunity for precise dating of events around the pleistocene holocene boundary in marine cores especially from the region where the two plumes overlap

melt evolution in subarc mantle evidence from heating experiments on spinel hosted melt inclusions in peridotite xenoliths from the andesitic avacha volcano kamchatka russia glass bearing inclusions hosted by cr spinel in harzburgite xenoliths from avacha are grouped based on homogenization temperatures and daughter minerals into high t 1 200 c opx cpx intermediate 900 1 100 c cpx amph and low t 900 c amph and are commonly accompanied by larger melt pockets unlike previous work on unheated inclusions and interstitial glass in xenoliths from kamchatka the homogenized glass compositions in this study are not affected by low pressure melt fractionation during transport and cooling or by interaction with host magma primary melt compositions constrained for each inclusion type differ in major and trace element abundances and were formed by different events but all are silica saturated ca rich and k poor with enrichments in lree sr rb and ba and negative nb anomalies these melts are inferred to have been formed with participation of fluids produced by dehydration of slab materials the high t inclusions trapped liquids produced by ancient high degree fluid induced melting in the mantle wedge the low t inclusions are related to percolation of low t melts or hydrous fluids in arc mantle lithosphere melt pockets arise from localized heating and fluid assisted melting induced by rising magmas shortly before the entrapment of the xenoliths the high t melt inclusions in avacha xenoliths are unique in preserving evidence of ancient high t melting events in arc mantle whereas the published data appear to characterize pre eruption enrichment events

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

volcanic ash from icelandic 57 300 yr bp eruption found in gisp2 greenland ice core we have found volcanic ash in wisconsinan ice from the gisp2 ice core retrieved from the summit location in central greenland the ash consists predominantly of large rhyolitic glass shards similar to the ones found in wisconsinan dye 3 ice from southern greenland we give the results of our analysis of the ash and present evidence that correlates this ash with the z2 ash layer found in atlantic sediment cores with the dye 3 ash with ash recently found in the grip ice core from central greenland and with ash associated with the icelandic thorsmork ignimbrite the mineralogy of the ash is very similar to that of ash from the icelandic tindfjallajokull volcano that is also present in the thorsmork ignimbrite and we suggest that this is the source volcano tindfjallajokull has also been suggested as the source of the z2 ash based on the excellent dating of the gisp2 ice core we determined that the eruptions that produced the gisp2 ash occured 57 300 1 700 yr bp which agrees extremely well with the most recent dating of 57 500 1 300 yr bp of the z2 ash based on the specmap chronology this provides us with a reliable revision of the dye 3 ice core dating at a depth of 1950 m this ice was originally dated at 70 000 yr bp

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

calibration of stable isotopic data an enriched δ18o standard used for source gas mixing detection and correction we present empirically based calibrations of our measurements made on a finnigan mat252 equipped with a kiel device to vienna pee dee belemnite using an enriched 18o standard calibrations include corrections for biases caused by the differences in isotopic composition of carbonate standards measured on the two parallel extraction lines of the kiel device and for decreases in the isotopic difference between the reference and sample gas caused by mixing in the source after correcting for these biases the precision of 2200 nbs19 analyses 10 300 g is 0 07 for 18o and 0 03 for 13c we have shared our standard enriched in 18o with 18 laboratories engaged in paleoceanographic research producing the first large scale interlaboratory calibrations for this community using correction procedures reported here water mass reconstructions using data produced on multiple mass spectrometers may now be possible with a precision approaching the level necessary to reconstruct temperature salinity and density variability in the deep ocean

coupled response of the late glacial climatic shifts of northwest europe reflected in greenland ice cores evidence from the northern north sea the climatic regimes of the land areas adjacent to the north sea are controlled by the influx of atlantic water north of the british isles and into the north sea a high resolution record from the norwegian channel off western norway covering the past 15 ka 14 c shows that prior to the younger dryas there were three periods of sea surface conditions there similar to those of the present interspersed with arctic conditions these changes corresponding to an increase of approx 5 c in summer sea surface temperatures took place on the scale of

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

chronology of late holocene climatic events in the northern north atlantic based on ams 14c dates and tephra markers from the volcano hekla iceland a combination of ams14c dating and tephrochronology has been used to date late holocene oceanographic events in a 335 cm marine record covering about 4600 cal yr with sedimentation rates exceeding 80 cm 1000 yr 1 the core site is located 50 km offshore on the northern icelandic shelf tephra markers from iceland serve to correlate the marine and terrestrial records especially notable is the presence of three geochemically correlated tephra markers from the icelandic volcano hekla hekla 4 hekla 3 and hekla 1104 benthic and planktonic foraminiferal abundance and distribution as well as the petrography of the sand fraction of the muddy shelf sediments are used as palaeoceanographic proxies the foraminiferal assemblages reflect a general cooling trend during the last 4600 yr a marked drop in sea surface temperatures is registered at about 3000 cal yr bp corresponding to the level of the hekla 3 tephra there is faunal indication of temperature amelioration during the medieval warm period and a cooling again during the little ice age periods of ice rafting events are indicated by ice rafted debris ird concentrations e g at around 3000 cal yr bp and during the little ice age the former event occurred just prior to the deposition of the hekla 3 tephra marker the largest holocene hekla eruption a correlation with terrestrial climatic events in iceland is presented a standard marine reservoir correction of 400 14c yr appears to be reasonable at least during periods with high influence of water masses from the irminger current on the northern icelandic shelf an increase to ca 530 14c yr may have occurred however when water masses derived from the east greenland current were dominant in the area copyright 2000 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

millennial scale iceberg discharges in the irminger basin during the last glacial period relationship with the heinrich events and environmental settings high resolution records of coarse lithic content and oxygen isotope have been obtained in a piston core from the irminger basin the last glacial period is characterized by numerous periods of increased iceberg discharges originating partly from iceland and corresponding to millennial scale instabilities of the coastal ice sheets and ice shelves in the nordic area a comparison with midlatitude sediment cores shows that ice rafted material corresponding to the heinrich events was deposited synchronously from 40 to 60 n there are thus two oscillating systems every 5 10 kyr massive iceberg armadas are released from large continental ice caps whereas more frequent instabilities of the coastal ice sheets in the high latitude regions occur every 1 2 3 8 kyr at the time of the heinrich events the synchroneity of the response from all the northern hemisphere ice sheets attests the existence of strong interactions between the two systems

long range transport of volcanic ash to the greenland ice sheet betzer et al 1 reported the surprising discovery of giant 75 m mineral particles transported more than 10 000 km from their source we have found volcanic ash containing glass shards as large as 300 m in a section of wisconsinian ice from the dye 3 core in greenland which is similarly unexpected and raises interesting questions concerning the long range aeolian transport of particulate matter as the volcanic ash correlates well with similar ash in deep sea sediment cores from the atlantic it also allows us to date the ice with some confidence

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

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

integrating the intimate records using tephrochronology rising to the challenge little has challenged our understanding of climate change more so than the abruptness with which large scale shifts in temperature occurred during the late quaternary the causal mechanisms driving

synchronous radiocarbon and climate shifts during the last deglaciation radiocarbon data from the cariaco basin provide calibration of the carbon 14 time scale across the period of deglaciation 15 000 to 10 000 years ago with resolution available previously only from holocene tree rings reconstructed changes in atmospheric carbon 14 are larger than previously thought with the largest change occurring simultaneously with the sudden climatic cooling of the younger dryas event carbon 14 and published beryllium 10 data together suggest that concurrent climate and carbon 14 changes were predominantly the result of abrupt shifts in deep ocean ventilation