low pressure granulite facies metamorphism in the larsemann hills area east antarctica petrology and tectonic implications for the evolution of the prydz bay area thermobarometric studies on various granulite facies areas along the prydz bay coast east antarctica 73 79 e 68 70 s show that at around 1100 ma during a late proterozoic orogeny the rocks of the larsemann hills suffered a lower pressure metamorphic peak than the surrounding areas along the prydz bay coast the rocks affected by this event include parts of the vestfold hills block plus all of the rauer group the larsemann hills and the munro kerr mountains the dykes in the south west corner of the vestfold hills were recrystallized during this event with little deformation at temperatures not quite as high as in the areas further south west 650 c 6 5 kbar collerson et al 1983 the rauer group was metamorphosed at 800 c and 7 5 kbar harley 1987a the larsemann hills at 750 c and 4 5 kbar and the munro kerr mountains probably at around 850 c and 5 kbar retrograde equilibration in the different areas occurred during decompression to about 10 km depth in all areas followed by isobaric cooling at this depth this paper shows that the peak metamorphism in the larsemann hills occurred at a pressure which is too low to have been the consequence of thermal relaxation of overthickened crust with normal mantle heat flow although other areas in prydz bay were metamorphosed at sufficiently high pressures so that their decompression paths are not inconsistent with a continental collision model the inferred pre metamorphic peak histories and the requirement of consistency with the larsemann hills make it unlikely that collision followed by erosion driven decompression is an appropriate model we suggest that the thermal regime of the crust in the larsemann hills region was controlled by a perturbation in the asthenosphere with magma invasion of the crust we suggest that the 500 ma event represented in prydz bay by granitic outcrops at landing bluff and by several k ar ages from the larsemann hills area was responsible for the final excavation of the terrane

10 be in ice at vostok antarctica during the last climatic cycle we have previously reported accelerator mass spectrometer measurements of the cosmogenic isotope 10be half life 1 5 myr in an 906 m ice core recovered at dome c antarctica1 in addition to an enhanced concentration during a period of reduced solar activity maunder minimum ad 1645 1715 2 we found substantially increased concentrations during that part of the core corresponding to the last ice age both effects have been confirmed and extended in greenland ice cores3 4 the recovery of a 2 083 m ice core at vostok antarctica5 together with an extended oxygen isotope chronology reported elsewhere in this issue6 permits an extension of 10be studies over the whole of the last climatic cycle 125 kyr we report here such measurements which show an excellent correlation with the oxygen isotope and thus presumably climate record over the whole of this period the results imply that precipitation rates in the antarctic during the last interglacial were similar to those of the holocene but were roughly halved during the last glaciation

reconstruction of millennial changes in dust emission transport and regional sea ice coverage using the deep epica ice cores from the atlantic and indian ocean sector of antarctica continuous sea salt and mineral dust aerosol records have been studied on the two epica european project for ice coring in antarctica deep ice cores the joint use of these records from opposite sides of the east antarctic plateau allows for an estimate of changes in dust transport and emission intensity as well as for the identification of regional differences in the sea salt aerosol source the mineral dust flux records at both sites show a strong coherency over the last 150 kyr related to dust emission changes in the glacial patagonian dust source with three times higher dust fluxes in the atlantic compared to the indian ocean sector of the southern ocean so using a simple conceptual transport model this indicates that transport can explain only 40 of the atmospheric dust concentration changes in antarctica while factor 5 10 changes occurred accordingly the main cause for the strong glacial dust flux changes in antarctica must lie in environmental changes in patagonia dust emissions hence environmental conditions in patagonia were very similar during the last two glacials and interglacials respectively despite 2 4 c warmer temperatures recorded in antarctica during the penultimate interglacial than today 2 3 times higher sea salt fluxes found in both ice cores in the glacial compared to the holocene are difficult to reconcile with a largely unchanged transport intensity and the distant open ocean source the substantial glacial enhancements in sea salt aerosol fluxes can be readily explained assuming sea ice formation as the main sea salt aerosol source with a significantly larger expansion of summer sea ice in the weddell sea than in the indian ocean sector during the penultimate interglacial our sea salt records point to a 50 reduction of winter sea ice coverage compared to the holocene both in the indian and atlantic ocean sector of the so however from 20 to 80 ka before present sea salt fluxes show only very subdued millennial changes despite pronounced temperature fluctuations likely due to the large distance of the sea ice salt source to our drill sites

a cellular model of river meandering river meandering has been successfully modelled using vector based methods but these can not simulate multiple or braided channels conversely cellular braided river models fail to replicate meandering this paper describes a new method to simulate river meandering within a cellular model caesar a novel technique for determining bend radius of curvature on a cell by cell basis is described that importantly allows regional information on bend curvature to be transferred to local points this local curvature is then used to drive meandering and lateral erosion through two methods key difficulties are identified including the deposition of material on point bars and cut off development but the method illustrates how meandering can be integrated within a cellular framework this demonstrates the potential to produce a single model that can incorporate both meandering and braiding copyright 2006 john wiley sons ltd

metamorphism and granite genesis in the hidaka metamorphic belt hokkaido japan the main zone of the hidaka metamorphic belt is an uplifted crustal section of island arc type the crust was formed during early tertiary time as a result of collision between two arc trench systems of cretaceous age the crustal metamorphic sequence is divided into four metamorphic zones i iv in which zone iv is in the granulite facies a detailed study of the evolution of the hidaka belt based on a revised p t t analysis of the metamorphic rocks notably a newly found staurolite bearing granulite confirms a prograde isobaric heating path after a supposed event of tectonic thickening of accretionary sedimentary and oceanic crustal rocks during the peak metamorphic event c 53 ma the regional geothermal gradient attained 33 40 c km 1 and the highest p t condition obtained from the lowest part of the granulite unit is 830 c 7 kbar in this part xh2o of gt opx cd gneiss is about 0 15 and that of gt cd bt gneiss is 0 4 the p t xh2o condition of the granulite unit is well within a field where fluid present partial melting of pelitic and greywacke metamorphic rocks takes place this is in harmony with the restitic nature of the gt opx cd gneiss in the lowest part of the granulite unit the possibility that partial melting took place in the main zone is significant for the genesis of the peraluminous s type granitic rocks within it the s type granitic rocks in this zone are opx gt bt tonalite in the granulite zone gt cd bt tonalite in the amphibolite zone and cd bt mus tonalite in the bt mus gneiss zone the mineralogical and chemical nature of these strongly peraluminous tonalitic rocks permit them to be regarded as having been derived from s type granitic magma generated by crustal anatexis of pelitic metamorphic rocks in deeper crust

an ice core indicator of antarctic sea ice production 1 the sea ice surface not open water is the dominant source of sea salt to aerosol and ice cores in coastal antarctica here we show that it may also form the dominant source for central antarctica we can then explain higher concentrations in the winter and last glacial maximum lgm as being due to increased sea ice production this suggests that ice core sea salt can indicate at least the timing of changes in antarctic sea ice production the pattern of sea salt in ice cores is consistent with marine evidence about sea ice changes in the holocene and lgm sea salt shows no change at the initial co2 increase out of the last glacial making it unlikely this was primarily due to changing sea ice cover the sea salt record should not be treated as an indicator of meridional transport

high resolution ammonium ice core record covering a complete glacial interglacial cycle high resolution ammonium measurements were performed along the greenland ice core program grip deep ice core covering a complete climatic cycle no overall anthropogenic increase is observed over the last 300 years however springtime concentrations have roughly doubled since 1950 biomass burning is estimated to be a major source for ammonia emissions for preindustrial times it contributes between 10 to 40 to the total ammonium deposited on the central greenland ice sheet during the holocene no correlation is found between the ammonium summer concentrations recorded over the last 100 years and the area burned in northern north america which is considered to be the main source area for ammonium deposited on the central greenland ice sheet this suggests that the meteorological factor is predominant for the pattern of ammonium spikes observed in the ice core if unchanged meteorological conditions are assumed for the holocene as indicated by the 18o ice record a decreasing biomass burning activity toward present time can be derived from the ammonium ice record soil and vegetation emissions are responsible for the ammonium background concentrations in the ice the record therefore may be used to trace back the biomass history of the north american continent a pronounced decreasing trend in background ammonium is found during the holocene reflecting decreasing temperature and therefore lower nh3 emissions in the source region variations in the ammonium concentration during the glacial age are discussed in terms of changes in transport and deposition mechanisms and changes in source strength which can be related to the extent of the laurentide ice sheet the data suggest that the laurentide ice sheet was built up immediately after the last interglacial and went through several large fluctuations during the last ice age

contrasting seismic reflectivity of the lower crust and uppermost mantle between ne japan and sw japan as illustrated by petrophysical analyses of mafic and ultramafic xenoliths 1 petrophysical analyses of mafic and ultramafic xenoliths reveal large contrasts in acoustic impedance between the lower crustal rocks and uppermost mantle rocks both beneath ichinomegata crater in ne japan and beneath oki dogo island in sw japan in contrast to minor contrasts in acoustic impedance both among the lower crustal rocks and among the uppermost mantle rocks beneath ichinomegata crater relatively large contrasts in acoustic impedance are present beneath oki dogo island between granulite and olivine gabbro in the lower crustal rocks as well as between plagioclase bearing wehrlite and other cumulus peridotites in the uppermost mantle rocks based on the existing petrologic models of the lower crust and uppermost mantle beneath ichinomegata crater and oki dogo island we then constructed model structures of the lower crust and uppermost mantle beneath these two areas composed of units with randomly arranged 100 m thick foliation parallel horizontal layers of xenolith samples and used these model structures to examine the seismic reflectivity beneath these two areas the results suggest the presence of strong seismic reflectors between the lower crust and the uppermost mantle in both areas although no other recognizable seismic reflector is expected beneath ichinomegata crater many seismic reflectors due to the above contrasts in acoustic impedance are expected both in the lower crust and in the uppermost mantle beneath oki dogo island thus a remarkable contrast in seismic reflectivity in the lower crust and uppermost mantle is likely present between these two areas as well as between ne japan and sw japan

the hidaka shear zone hokkaido japan genesis during a right lateral strike slip movement the mesozoic structure of hokkaido which is characterized by a wide spread ophiolite sequence unconformably overlain by a cretaceous flysch sequence is cut by a north south trending metamorphic belt the hidaka belt this contribution aims to demonstrate that the hidaka belt is a wide right lateral strike slip shear zone the movement is contemporaneous with the metamorphism and the magmatism of oligo miocene time

crustal structure and tectonics of the hidaka collision zone hokkaido japan revealed by vibroseis seismic reflection and gravity surveys abstract this study is the first integrated geological and geophysical investigation of the hidaka collision zone in southern central hokkaido japan which shows complex collision tectonics with a westward vergence the hidaka collision zone consists of the idon nappu belt ib the poroshiri ophiolite belt pob and the hidaka metamorphic belt hmb with the hidaka belt from west to east the pob metamorphosed ophiolites is overthrust by the hmb steeply eastward dipping palaeo arc crust along the hidaka main thrust hmt and in turn thrusts over the idon nappu belt melanges along the hidaka western thrust hwt seismic reflection and gravity surveys along a 20 km long traverse across the southern hidaka mountains revealed hitherto unknown crustal structures of the collision zone such as listric thrusts back thrusts frontal thrust and fold structures and duplex structures the main findings are as follows 1 the hmt which dips steeply at the surface is a listric fault dipping gently at a depth of 7 km beneath the eastern end of the hmb and cutting across the lithological boundaries and schistosity of the hidaka metamorphic rocks 2 a second reflector is detected 1 km below the hmt reflector the intervening part between these two reflectors is inferred to be the pob which is only little exposed at the surface this inference is supported by the high positive bouguer anomalies along the hidaka mountains 3 the shallow portion of the ib at the front of the collision zone has a number of nne dipping reflectors indicative of imbricated fold and thrust structures 4 subhorizontal reflectors at a depth of 14 km are recognized intermittently at both sides of the seismic profile these reflectors may correspond to the velocity boundary 5 9 6 6 km s previously obtained from seismic refraction profiling in the northern hidaka mountains 5 these crustal structures as well as the back thrust found in the eastern end of the traverse represent characteristics of collisional tectonics resulting from the two collisional events since the early tertiary

impurity sources of f cl no3 and so4 2 in greenland and antarctic precipitation five polar impurity sources are proposed anthropogenic no3 and so42 in recent greenland but not antarctic snow marine aerosols the dominant cl source volcanic eruptions which produce detectable but variable peaks in so42 f and or cl concentrations biogenic sulfurous gases from soil and coastal waters the dominant source for so42 in nonvolcanic strata an unidentified source of background nitrate which is more active in the northern hemisphere and which displays a summer concentration maximum solar activity and supernova events do not modulate no3 concentrations in greenland snow

interpreting ancient ice in a shallow ice core from the south yamato antarctica blue ice area using flow modeling and compositional matching to deep ice cores 1 we explore methods of dating a 101 m ice core from a bare ice ablation area in the yamato mountains dronning maud land east antarctica there are two unknowns the age of the ice at the surface and the age spanned by the core the ice crystal growth rate was used to estimate the age span of the core at about 5 kyr co2 ch4 and n2o data on the core were compared with well dated records from deep cores leading to two plausible matches both within isotope stage c detailed comparison of high resolution dep records from this core and the dome fuji core support the 55 61 kyr bp fit best oxygen isotope values in the core were then used to constrain the source elevation of the snow in the core and hence the velocities in a simple flow line model we inverted the ice core surface age age span and origin site and their confidence intervals in a sensitivity study of flow model parameter space the flow line model predicts good matches to the core by reducing glacial flow rates to 70 of present day accumulation rates by 45 with 10 confidence intervals the modeled surface age for the whole meteorite field yields maximum surface ages of about 90 kyr which is consistent with known but poorly constrained tephra dating meteorite terrestrial ages and the frequency of meteorite discoveries this approach can be used quite generally to link deep ice cores to surface outcrops on blue ice fields

electrical resistivity sounding of the east antarctic ice sheet electrical resistivity soundings using a schlumberger array have been carried out at dome c east antarctica 74 39 s 124 10 e elevation 3400 m to sound the entire 3500 m depth of the ice sheet changes in density and temperature are largely separated in the ice at dome c so activation energies for both firn and ice could be determined we find an activation energy of 0 25 ev in both solid ice and firn between 15 c and 58 c a common value of the activation energy points to a single transport regime in which the charge carriers and conduction paths are the same in firn and ice to evaluate the variation of resistivity with density we have considered five dielectric mixture models that fit the available data on the high frequency dielectric constant of firn only looyenga s equation fits the field data for dc resistivity in the upper 900 m of the ice sheet where impurity concentrations are known from core samples we find no correlation between resistivities and the concentrations of salts or acids instead we find it likely that resistivities are correlated with the crystal size hence with the holocene wisconsin boundary in the ice column a pronounced increase in resistivity to a value comparable with that in temperate glacier ice occurs deep within the ice sheet we attribute this to a large increase in the size and irregularity of the ice crystals which destroys the continuity of the impurity shells surrounding the ice crystals that we believe supply the conduction paths shabtaie and bentley 1994a high resistivity does not imply removal of the impurities from the system moderate concentrations of impurities can be accommodated by locating them in disconnected domains

decompressional coronas and symplectites in granulites of the musgrave complex central australia abstract in granulite facies metapelitic rocks in the musgrave complex central australia reaction between s1 garnet and sillimanite involves the development in s2 of both garnet cordierite hercynitic spinel biotite and hercynitic spinel cordierite sillimanite biotite the s2 assemblages occur either in coronas and symplectites mainly around garnet or in rocks in which s2 is more strongly developed as recrystallized assemblages ignoring the presence of biotite and ilmenite the mineral textures can be accounted for qualitatively by a consideration of the model system feo mgo al2o3 sio2 fmas the textural relationships accord with decompression accompanying the change from s1 to s2 however since biotite and ilmenite are involved in the assemblages the parageneses are better accounted for in terms of equilibria in the expanded model system k2o feo mgo al2o3 sio2 h2 tio2 fe2o3 kfmashto i e afm tio2 fe2o3 the coronas reflect the tectonic unroofing of at least part of the musgrave complex from peak s1 conditions of about 8 kbar to s2 conditions of about 4 kbar

tertiary deep crustal ultrametamorphism in the hidaka metamorphic belt northern japan the main zone of the hidaka metamorphic belt is an imbricate stack of crustal material derived from an island arc in which a sequence of units with increasing metamorphic grade from low to high structural levels is exposed the basal part of the metamorphic sequence underwent granulite facies metamorphism with peak p t conditions of 7kbar 870 c in this zone pelitic granulite includes leucosomes which consist mainly of orthopyroxene plagioclase quartz to test whether the leucosome was derived by partial melting of the surrounding pelite melting experiments of the pelitic granulite were carried out for water saturated and dry systems at 7 kbar and 850 c the chemical composition of the leucosome produced during these runs shows a peraluminous s type tonalitic affinity and is located very close to the tie line between the average melts produced in water saturated systems and the average composition of the residual orthopyroxene plagioclase this therefore suggests that the lecosome in pelitic granulite was formed by incipient anatexis at close to the highest p t condition of the main zone the age of the crustal anatexis is determined by the rb sr whole rock isochron method for garnet cordierite biotite gneiss host rock garnet orthopyroxene cordierite gneiss restite and s type tonalite melt this gives an age of 56 0 ma with an initial 87sr 86sr ratio of 0 705711 the s type tonalite magmas that form large intrusive masses in the main zone were probably generated by crustal anatexis in deeper parts of the crust at the same time late palaeocene

seasonality of sulfur species dimethyl sulfide sulfate and methanesulfonate in antarctica inland versus coastal regions 1 to gain a better understanding of sulfate and methanesulfonate ms signals recorded in central antarctic ice cores in terms of past atmospheric changes an atmospheric year round study of these aerosols was performed in 2006 at the concordia station 75 s 123 e located on the high antarctic plateau in addition a year round study of dimethyl sulfide dms the gaseous precursor of sulfur aerosol was conducted in 2007 the dms mixing ratio remains below 1 pptv from october to january and exhibits a maximum of 10 pptv during the first half of winter from april to july surprisingly the well marked maximum of sulfur aerosol recorded in january at coastal antarctic sites is observed at concordia for sulfate but not for ms which peaks before and after sulfate in november and march respectively this first study of dms and of its by oxidation aerosol species conducted at inland antarctica points out the complex coupling between transport and photochemistry of sulfur species over antarctica the findings highlight the complexity of the link between ms ice core records extracted at high antarctic plateau sites and dms emissions from the southern ocean

morphological modelling of rivers with erodible banks a bank erosion mechanism and provisions to account for the associated planform changes and input of bank erosion products are added to a two dimensional depth averaged model of river morphology the model is applied to a reach of the meandering gravel bed river oh e eger in the former state of czechoslovakia the agreement with observations is poor but this can be ascribed to shortcomings in the flow and bed topography submodels rather than to shortcomings in the bank erosion submodel better results are expected when a three dimensional flow model equations for sediment mixtures and a bank accretion mechanism are included this inclusion will have to be preceded by fundamental research on bank accretion mechanisms and on hiding and exposure effects in the relationship for the influence of gravity pull on sediment transport direction 1998 john wiley sons ltd

the occurrence of zinc in antarctic ancient ice and recent snow concentrations of zinc zn have been measured in various sections of the dome c and vostok deep antarctic ice cores whose ages range from 3850 to 155 000 years bp and in several large size surface antarctic snow blocks collected in adelie land and at the geographic south pole all the samples were mechanically decontaminated and detailed outside inside variation profiles were drawn for most of them then allowing us to clearly establish the accuracy of the data obtained from the analysis of the most central parts of each individual core section or snow block natural zn concentrations are found to have strongly varied in antarctic ice during the past 155 000 years the highest values up to about 100 pg zn g being observed during the last glacial maximum and possibly during the end of the next to last ice age wind blown dust from crustal rock and soil appears to be the main natural source of zn during the glacial periods especially the last glacial maximum zn concentrations in present day antarctic snow from central east antarctica about 5 pg zn g are found to be comparable with those in holocene ice several thousand years old which evidences that the antarctic tropospheric cell is still little affected by anthropogenic zn

acidity decline in antarctic ice cores during the little ice age linked to changes in atmospheric nitrate and sea salt concentrations acidity is an important chemical variable that impacts atmospheric and snowpack chemistry here we describe composite time series and the spatial pattern of acidity concentration acy h hco3 during the last 2000 years across the dronning maud land region of the east antarctic plateau using measurements in seven ice cores coregistered measurements of the major ion species show that sulfuric acid h2so4 nitric acid hno3 and hydrochloric acid hcl determine greater than 98 of the acidity value the latter also described as excess chloride excl is shown mostly to be derived from postdepositional diffusion of chloride with little net gain or loss from the snowpack a strong inverse linear relationship between nitrate concentration and inverse accumulation rate provides evidence of spatially homogenous fresh snow concentrations and reemission rates of nitrate from the snowpack across the study area a decline in acidity during the little ice age lia 1500 1900 common era is observed and is linked to declines in hno3 and excl during that time the nitrate decline is found to correlate well with published methane isotope data from antarctica 13ch4 indicating that it is caused by a decline in biomass burning the decrease in excl concentration during the lia is well correlated to published sea surface temperature reconstructions in the atlantic ocean which suggests increased sea salt aerosol production associated with greater sea ice extent

holocene volcanic history as recorded in the sulfate stratigraphy of the european project for ice coring in antarctica dome c edc96 ice core 1 a detailed history of holocene volcanism was reconstructed using the sulfate record of the european project for ice coring in antarctica dome c edc96 ice core this first complete holocene volcanic record from an antarctic core provides a reliable database to compare with long records from antarctic and greenland ice cores a threshold method based on statistical treatment of the lognormal sulfate flux distribution was used to differentiate volcanic sulfate spikes from sulfate background concentrations ninety six eruptions were identified in the edc96 ice core during the holocene with a mean of 7 9 events per millennium the frequency distribution events per millennium showed that the last 2000 years were a period of enhanced volcanic activity edc96 volcanic signatures for the last millennium are in good agreement with those recorded in other antarctic ice cores for older periods comparison is in some cases less reliable mainly because of dating uncertainties sulfate depositional fluxes of individual volcanic events vary greatly among the different cores a volcanic flux normalization volcanic flux tambora flux ratio was used to evaluate the relative intensity of the same event recorded at different sites in the last millennium normalized flux variability for the same event showed the highest value in the 1100 1500 ad period this pattern could mirror changes in regional transport linked to climatic variations such as slight warming stages in the southern hemisphere southern hemisphere medieval warming like period

numerical simulation of effects of sediment supply on bedrock channel morphology abstractnatural bedrock rivers exhibit diverse erosional morphologies although the formation of alternate bars on bedrock has been noted in previous studies the influence of these alternate bars on bedrock erosion has not been clarified in this study the authors propose a model for bedrock alluvial channels that reproduces both bar formation and erosional morphology in addition the authors report on numerical simulations to evaluate the influence of sediment supply on the state of the bed at and over the bedrock surface the numerical results illustrate the formation of different morphologies for different supply rates when the sediment supply rate is close to transport capacity mixed alluvial bedrock alternate bars form these bars are analogous to purely alluvial alternate bars a meandering thread of alluvial material migrates downstream over a uniformly eroding bedrock surface when the sediment supply rate is well below capacity however multiple incisional troughs grooves form on the bedr

river width adjustment i processes and mechanisms in 1993 a task committee tc of the asce was established to study the hydraulics bank mechanics and modeling of width adjustment in alluvial channels the work of the tc in reviewing width adjustment processes and mechanisms is reported in this paper a companion paper presents the findings of the tc with regard to width adjustment modeling this paper first establishes the geomorphic context within which width adjustments occur and it demonstrates that width adjustment may take place over a wide range of scales in time and space in the past engineering analyses of channel width have tended to concentrate on prediction of the equilibrium width for stable channels most commonly the regime extremal hypothesis and rational mechanistic approaches are used and these are reviewed herein more recently attention has switched to channels that are adjusting their morphology either due to natural instability or in response to changes in watershed land use river regulation or channel engineering chara

potential and limitations of marine and ice core sea ice proxies an example from the indian ocean sector diatom assemblages in marine cores and sea salt deposition fluxes in ice cores have been used as sea ice proxies in the southern hemisphere here a marine and an ice core proxy record for the indian ocean covering the last two glacial cycles are compared in order to illustrate their potential and limitations the marine core was extracted in a location completely ice free under present day conditions and therefore was unable to record changes to the recent sea ice extent similarly no sea ice was recorded at that location during the previous interglacial period during the last glacial period however the site was seasonally covered by sea ice and the diatom assemblages allowed an estimation of average seasonal sea ice presence the ice core data originated from the east antarctic plateau the marine sodium present in the ice core was used as a proxy of the sea ice coverage and on average a larger sea ice surface led to an increased sea salt aerosol flux seen e g at the last glacial inception however the response of the sea salt flux to increasing sea ice extent diminished during peak glacial conditions when only minimal variability was recorded in the ice core record a first order approximation is used to take this non linear response of the ice core sea ice proxy into account based on the ice core proxy record sea ice extent was reduced considerably during the warm episodes of the previous two interglacial periods compared to modern sea ice extent in particular during the peak warmth of the last interglacial the ice core proxy also showed a very strong precessional variability pronounced spectral peak at 23 ka period over the past 240 ka the advantage of combining the two proxy records lies in the complementary nature of their response while the ice core proxy showed limited sensitivity during full glacial conditions the marine proxy recorded the seasonal sea ice coverage once the sea ice retreated south of the location of the marine core the ice core proxy responded to changes in sea ice extent a composite of marine and ice core records may thus provide a data basis for a more detailed reconstruction of sea ice around antarctica over the last few glacial interglacial cycles

location of cation impurities in ngrip deep ice revealed by cryo cell uv laser ablation icpms in polar ice sheets the average grain size varies with depth ice grain size increases due to several factors including ice temperature and impurity content which in turn varies with climate the effect of impurities on grain growth is thought to be crucial but has never been observed experimentally using a methodology recently developed at royal holloway university of london in situ chemical analysis of frozen ice at sub ppm concentrations with unprecedented spatial resolution 150 m is achievable using ultraviolet laser ablation inductively coupled plasma mass spectrometry uv la icpms featuring a two volume cryo la cell following surface cleaning with a custom built vice equipped with a ceramic blade ngrip ice slabs 86 ka before ad 2000 have been analysed using a series of one dimensional profiles and two dimensional maps of laser spots at a resolution of 200 300 m results demonstrate that cation impurities are not uniformly distributed in ice layers and show significant variations in concentration on a sub millimetre scale furthermore a different pattern of elemental distribution between clear ice and layers enriched in impurities cloudy bands has been identified while concentration differences for cloudy bands are not resolvable between boundaries and inner grain domains within clear ice grain boundaries and junctions are significantly up to 100 times impurity enriched relative to corresponding grain interiors

the rates of sea salt sulfatization in the atmosphere and surface snow of inland antarctica 1 most of the aerosol particles present in the surface snow and ice of inland antarctica come from primary sea salt sodium chloride and marine biological activity methansulfonic and sulfuric acids melted water from surface snow firn and holocene ice contains mainly sodium chloride and sulfate ions although it is well known that sea salt aerosols react rapidly with sulfuric acid a process known as sulfatization it is not known when this process takes place in this research we undertake to measure the proportion of sea salt aerosols that undergo sulfatization in the atmosphere and surface snow as opposed to deeper ice in order to understand the suitability of sea salt aerosols as a proxy for past climates in deep ice cores we directly measure the sulfatization rates in recently fallen snow 0 4 m in depth collected at the dome fuji station using x ray dispersion spectroscopy to determine the constituent elements of soluble particles and computing the molar ratios of sodium chloride and sodium sulfate we estimate that about 90 of the initial sea salt aerosols sulfatize as they are taken up by precipitation over dome fuji or in the snowpack within one year after being deposited on the ice sheet

southern ocean sea ice extent productivity and iron flux over the past eight glacial cycles sea ice and dust flux increased greatly in the southern ocean during the last glacial period palaeorecords provide contradictory evidence about marine productivity in this region but beyond one glacial cycle data were sparse here we present continuous chemical proxy data spanning the last eight glacial cycles 740 000 years from the dome c antarctic ice core these data constrain winter sea ice extent in the indian ocean southern ocean biogenic productivity and patagonian climatic conditions we found that maximum sea ice extent is closely tied to antarctic temperature on multi millennial timescales but less so on shorter timescales biological dimethylsulphide emissions south of the polar front seem to have changed little with climate suggesting that sulphur compounds were not active in climate regulation we observe large glacial interglacial contrasts in iron deposition which we infer reflects strongly changing patagonian conditions during glacial terminations changes in patagonia apparently preceded sea ice reduction indicating that multiple mechanisms may be responsible for different phases of co2 increase during glacial terminations we observe no changes in internal climatic feedbacks that could have caused the change in amplitude of antarctic temperature variations observed 440 000 years ago

universal bank erosion coefficient for meandering rivers an equation for bank erosion and thus channel migration rate is derived theoretically from the equation of sediment continuity the relation so derived lends credence to the hypothesis that bank

garnet orthopyroxene plagioclase quartz barometry refinement and application to the english river subprovince and the minnesota river valley recent experimental theoretical and thermodynamic studies permit better calibration of two reactions for geobarometry grossular pyrope quartz anorthite enstatite grossular almandine quartz anorthite ferrosilite if both reactions are applied using the same thermodynamic data and activity models they should yield the same pressure for a given garnet pyroxene plagioclase quartz assemblage application to a variety of high grade terrains generally yielded excellent results however poor results have been obtained for high fe rocks which can be traced to errors in activity models for garnet and or pyroxene either a two site ideal mixing model for orthopyroxene cf wood and banno 1973 underestimates enstatite activity for high fe orthopyroxenes or the ganguly and saxena 1984 model overestimates pyrope activity in low mg garnets

distribution of radiogenic heat generation in the arc s crust of the hokkaido island japan radiogenic heat generation was measured for rocks collected in the vertical section of the crust of the arc exposed in the hokkaido island japan measured heat generation value of metasediments is nearly constant of 1 0 w m and independent of the metamorphic grade using the measured heat generation value together with the published heat generation data of a granitic body emplaced in this section vertical profile of heat generation in the crust is constructed the estimated heat generation profile shows steep decrease with depth but is not lower than a value of 1 0 w m in the upper crust in the lower crust metabasites are dominant and heat generation value is one order of magnitude lower than that in the upper crust surface heat flow derived from the crustal heat generation is estimated to be 25 mw m when the crustal heat generation is considered temperature at the bottom of the crust estimated from surface heat flow becomes 300 c lower this temperature difference corresponds to the viscosity variation of more than three orders of magnitude which is important for understanding lithosphere dynamics

sea salt as an ice core proxy for past sea ice extent a process based model study sea ice is a reflection of and a feedback on the earth s climate we explore here using a global atmospheric chemistry transport model the use of sea salt in antarctic ice cores to obtain continuous long term regionally integrated records of past sea ice extent synchronous with ice core records of climate the model includes the production transport and deposition of sea salt aerosol from the open ocean and blowing snow on sea ice under current climate conditions we find that meteorology not sea ice extent is the dominant control on the atmospheric concentration of sea salt reaching coastal and continental antarctic sites on interannual timescales however through a series of idealized sensitivity experiments we demonstrate that sea salt has potential as a proxy for larger changes in sea ice extent e g glacial interglacial treating much of the sea ice under glacial conditions as a source of salty blowing snow we demonstrate that the increase in sea ice extent alone without changing the meteorology could drive for instance a 68 increase in atmospheric sea salt concentration at the site of the dome c ice core which exhibits an approximate twofold glacial increase in sea salt flux we also show how the sensitivity of this potential proxy decreases toward glacial sea ice extent the basis of an explanation previously proposed for the lag observed between changes in sea salt flux and d an ice core proxy for air temperature at glacial terminations the data thereby permit simultaneous changes in sea ice extent and climate

a simplified 2d model for meander migration with physically based bank evolution abstract the rate of migration calculated by numerical models of river meandering is commonly based on a method that relates the rate of migration to near bank excess velocity multiplied by a dimensionless coefficient notwithstanding its simplicity since the early 1980s this method has provided important insight into the long term evolution of meander planforms through theoretical exercises its use in practice has not been as successful because the complexity of the physical processes responsible for bank retreat the heterogeneity in floodplain soils and the presence of vegetation make the calibration of the dimensionless coefficient rather challenging this paper presents a new approach that calculates rates of meander migration using physically based streambank erosion formulations the university of illinois rvr meander model which simulates meandering river flow and bed morphodynamics is integrated with algorithms for streambank erosion of the us department of agriculture channel evolution computer model concepts the performance of the proposed approach is compared to that of the more simple classic method through the application to several test cases for idealized and natural planform geometry the advantages and limitations of the approach are discussed focusing on simulated planform pattern the impact of soil spatial heterogeneity the relative importance of the different processes controlling bank erosion hydraulic erosion cantilever and planar failure the requirements for obtaining stable migration patterns centerline filtering and interpolation of bank physical properties and the capability of predicting the planform evolution of natural rivers over engineering time scales i e 50 to 100 years the applications show that the improved physically based method of bank retreat is required to capture the complex long term migration patterns of natural channels which cannot be merely predicted from hydrodynamics only

size resolved chemical composition of aerosol emitted by erebus volcano antarctica 1 persistent open vent degassing of erebus volcano antarctica is a significant point source of gases and aerosol to the austral polar troposphere we report here on the chemical composition and size distribution of the erebus aerosol focusing on the water soluble fraction the aerosol was sampled at the rim of the active crater using a cascade impactor which collected and sized particles in 14 size bins from 10 to 0 01 m the soluble fraction of the erebus aerosol is distinct from other volcanic sources in several respects it is dominated by chloride bearing particles over 30 of total mass and has an unusually high cl so42 molar ratio of 3 5 coarse particles contribute little to the total mass of the soluble fraction elevated concentrations of f cl br and so42 are found in a narrow particle size fraction of 0 1 0 25 m the detection of particulate br reinforces our understanding of the potential for quiescent volcanic emissions to deplete tropospheric ozone the small aerosol size reflects the low atmospheric temperature and humidity which inhibit particle growth halide alkali metal salts na k cl f appear to be the most abundant species in the aerosol the concentration of pb is high compared to other volcanoes its exsolution may be promoted by the high abundance of halogens in erebus magma despite the previously reported high nox content in the plume we did not detect significant quantities of nitrate in the near vent aerosol our findings emphasize the potential regional significance of emissions from erebus for understanding the antarctic atmospheric composition and glaciochemical records

dust and sea salt variability in central east antarctica dome c over the last 45 kyrs and its implications for southern high latitude climate a detailed record of non sea salt calcium a proxy for dust and sea salt sodium a proxy for sea salt covering the last 45 kyr is presented it shows that in the first part of the transition from the last glacial period to the holocene 18 15 kyr bp the changes in dust flux mainly reflect changes at the dust source namely vegetation cover and local climate the changes in the later part of the transition 12 11 kyr bp are similar in extent to the changes seen in sea salt and most likely reflect a reorganization of the atmospheric circulation during the last glacial period considerable variation of dust but not of sea salt is observed pointing to climatic changes in patagonia the main dust source for dome c a comparison of the glacial records from dome c and taylor dome suggests that similar influences controlled aerosol input at both sites during this period

soluble impurities in the byrd station ice core antarctica their origin and sources polar ice cores contain long and detailed records of paleoatmospheric composition paleoclimate and their evolution through time by making a complete ionic balance in melted samples of ice it is possible to evaluate the origin identification of chemical compounds and sources continental marine volcanic extraterrestrial of soluble impurities contained in these ice cores once these are known an attempt can be made to try and explain the cause of any observed changes in this paper we present results of analyses of the major soluble impurities h na nh4 k so4 no3 cl made on bulk samples selected at approximately 50 m intervals down the length of an ice core from byrd station antarctica 80 01 s 119 31 w 1530 m most samples are in ionic balance and it is therefore possible to suggest with which compounds the ionic impurities are linked these compounds include an important contribution from both sea salts nacl and na2so4 and strong acids approximately equal amounts of hno3 and h2so4 the cl na mole ratio is found to be quite stable throughout the core and hovers about the expected bulk sea water ratio in general the marine derived components at byrd station decreased by a factor of 2 between the late glacial maximum and holocene while the gas derived acid components decreased by a factor of only 1 5 some possible causes of these variations are suggested

comparison of holocene electrical records from dome c and vostok antarctica as part of the european project for ice coring in antarctica a new deep ice core is being drilled at dome c two electrical methods have been used on the core drilled so far a new design of electrical conductivity method ecm instrument and a traditional dielectric profiler the two profiles are very similar in both peaks and background consistent with acidity being the dominant influence on both in this part of the record the dome c records have been compared to ecm records from vostok and a tentative match has been made between them this suggests a long term average ratio of accumulation rate of 1 36 between this dome c core and vostok and that the dome c core analyzed so far to 358 m probably includes the very end of the glacial holocene transition

fe mg mixing in cordierite constraints from natural data and implications for cordierite garnet geothermometry in granulites from chemicaldata on 108 pairs of naturally occurring garnet and cordierite the interchange energy of fe mg mixing in cordierite wo and the free energy change of the exchange equilibrium feralrsiro t t zmgraiosiro y mgralrsiro yzferalosiro in the temperature range of 700 850 c have been extracted using standard methods of linear regression analysis the input thermochemical parameters for multicomponent camg fe mn garnet solid solutions are based on the updated and revised values after ganguly and saxena 1984 e g wr 2500 cal w w 3000 cal calculated on the basis of one third formula unit on the basis of the derived values at 710 760 800 and 850 c fe mg mixing in cordierite has been approximated to a symmetric regular solution the fe mg substitution shows positive deviation from ideality in cordierite with tzi 1 9 34 4 54kj mol of divalent cation the values derived for the free energy of mixing at xi d xi ds 0 5 are 3 26 kj at 700 c and 4 39 kl at 900 c the ag value in joules for the above equilibrium can be expressed as 15079 8 552 incorporating these parameters the geothermometric expression for the exchange equilibrium is

year round records of sea salt gaseous and particulate inorganic bromine in the atmospheric boundary layer at coastal dumont d urville and central concordia east antarctic sites multiple year round records of bulk and size segregated compositions of aerosol were obtained at the coastal dumont d urville ddu and inland concordia sites located in east antarctica they document the sea salt aerosol load and composition including for the first time in antarctica the bromide depletion of sea salt aerosol relative to sodium with respect to seawater in parallel measurements of bromide trapped in mist chambers and denuder tubes were done to investigate the concentrations of gaseous inorganic bromine species these data are compared to simulations done with an off line chemistry transport model coupled with a full tropospheric bromine chemistry scheme and a process based sea salt production module that includes both sea ice sourced and open ocean sourced aerosol emissions observed and simulated sea salt concentrations sometime differ by up to a factor of 2 to 3 particularly at ddu possibly due to local wind pattern in spite of these discrepancies both at coastal and inland antarctica the dominance of sea ice related processes with respect to open ocean emissions for the sea salt aerosol load in winter is confirmed for summer observations and simulations point out sea salt as the main source of gaseous inorganic bromine species investigations of bromide in snow pit samples do not support the importance of snowpack bromine emissions over the antarctic plateau to evaluate the overall importance of the bromine chemistry over east antarctica bro simulations were also discussed with respect data derived from gome 2 satellite observations over antarctica

prograde and anatectic reactions in the deep arc crust exposed in the hidaka metamorphic belt hokkaido japan abstract the hidaka metamorphic belt is a tilted crustal assembly of a magmatic arc exposing two third of a total thickness of the arc crust the igneous activities and metamorphism took place during the eocene in a tectonically stacked and thickened accretionary prism of the latest cretaceous to earliest tertiary age the lower part of the exposed section consists mainly of mafic rocks with thin intercalations of pelitic and psammitic rocks metamorphosed under amphibolite and granulite and granulite facies while the upper part consists almost of pelitic and psammitic rocks metamorphosed under amphibolite and greenschist facies which are gradually changed to very weakly metamorphosed sedimentary rocks toward the top of the section reexamination of prograde metamorphism of pelitic granulite facies rocks confirmed a path of increasing temperature with a constant pressure or with slight decrease of pressure associated with moderate shearing norite and diorite veining as a part of mafic intrusives were followed to give rise to the peak metamorphism associating incipient anatexis of pelitic and psammitic gneisses in the basal part of the exposed section due to the anatexis small leucocratic patches or viens are formed with orthopyroxene and cordierite as residual phases cutting strong foliations of biotite with garnet and cordierite porphyroblasts the mafic intrusives played an important role in the metamorphic evolution in the deep part of the hidaka crust

limited dechlorination of sea salt aerosols during the last glacial period evidence from the european project for ice coring in antarctica epica dome c ice core 1 chloride cl and sodium na in ice cores originate mainly from sea salt and one would thus expect the cl na ratio to reflect the seawater ratio however at dome c a low accumulation site in east antarctica this is not the case in present day snow instead a cl excess relative to na is observed in surface snow and within a few meters depth the cl concentration decreases and the cl na ratio becomes significantly lower than the seawater ratio aerosol studies at coastal antarctic sites have shown that the reaction of sea salt aerosols with nitric and sulphuric acid leads to the formation of hcl that eventually escapes the sea salt aerosol the observed decrease in cl concentrations in the uppermost snow layers is due to reemission of hcl from the snow postdepositional loss of hcl depends among other factors on the accumulation rate at the site with lower accumulation rates leading to larger losses during the last glacial maximum lgm the cl na ratio is relatively stable and close to the seawater ratio despite the even lower accumulation rate during that time the likely explanation for this conflicting observation is that high levels of dust neutralized nitric and sulphuric acids during the lgm which in turn reduced the formation of hcl from sea salt aerosol with less or no hcl formed postdepositional loss would be prevented keeping the cl na ratio close to that of sea water

low pressure granulite facies metapelitic assemblages and corona textures from macrobertson land east antarctica the importance of fe2o3and tio2in accounting for spinel bearing assemblages low pressure granulite facies metasedimentary gneisses exposed in macrobertson land east antarctica include hercynitic spinel bearing metapelitic gneisses peak metamorphic mineral assemblages include spinel rutile ilmenite sillimanite garnet spinel ilmenite sillimanite garnet cordierite ortho pyroxene magnetite ilmenite garnet spinel cordierite biotite ilmenite and orthopyroxene cordierite biotite each with quartz k feldspar and melt the presence of garnet biotite and cordierite orthopyroxene bearing assemblages implies crossing tie lines in afm projection for the k2o feo mgo al2o3 sio2 h2o kfmash system this apparent contradiction and the presence of spinel rutile and ilmenite in the assemblages is acounted for by using the kfmash tio2 o2 system i e afm tio2 fe2o3 we derive a petrogenetic grid for this system applicable to low pressure granulite facies metamorphic conditions retrograde assemblages are interpreted from corona textures on hercynitic spinel and fe ti oxides the relative positions of the peak and retrograde metamorphic assemblages on the petrogenetic grid suggest that corona development occurred during essentially isobaric cooling

coastal antarctic aerosol and snowfall chemistry aerosol samples have been collected at three stations on the coast of antarctica at dumont d urville station samples have been collected for 4 years at halley station for 2 years and at neumayer station for 12 years fresh snow samples have also been collected at the three sites at halley daily surface snow samples were taken as well as firn cores covering the sampling period meteorological including upper air data are available at the three sites in the subsequent papers of this special section the data from the three stations have been combined in order to make general statements about the chemistry of aerosol in coastal antarctica about the differences between sectors of antarctica and about the relationship between air concentrations and the chemistry that is eventually seen in ice cores this paper summarizes the samples collected and the sampling methods used at the three sites it also highlights the main role of each paper in this special section

numerical simulation of large scale bed load particle tracer advection dispersion in rivers with free bars asymptotic characteristics of the transport of bed load tracer particles in rivers have been described by advection dispersion equations here we perform numerical simulations designed to study the role of free bars and more specifically single row alternate bars on streamwise tracer particle dispersion in treating the conservation of tracer particle mass we use two alternative formulations for the exner equation of sediment mass conservation the flux based formulation in which bed elevation varies with the divergence of the bed load transport rate and the entrainment based formulation in which bed elevation changes with the net deposition rate under the condition of no net bed aggradation degradation a 1 d flux based deterministic model that does not describe free bars yields no streamwise dispersion the entrainment based 1 d formulation on the other hand models stochasticity via the probability density function pdf of particle step length and as a result does show tracer dispersion when the formulation is generalized to 2 d to include free alternate bars however both models yield almost identical asymptotic advection dispersion characteristics in which streamwise dispersion is dominated by randomness inherent in free bar morphodynamics this randomness can result in a heavy tailed pdf of waiting time in addition migrating bars may constrain the travel distance through temporary burial causing a thin tailed pdf of travel distance the superdiffusive character of streamwise particle dispersion predicted by the model is attributable to the interaction of these two effects

evidence of sea ice source in aerosol sulfate loading and size distribution in the canadian high arctic from isotopic analysis the influence of frost flowers and seawater brine on ion chemistry in snow snowpack ice cores and aerosols is detected when a lower sulfate to sodium ratio than in seawater is present in polar regions this evidence can be masked when large amounts of non sea salt sulfate are present from other sources such as biogenic and anthropogenic sulfate frost flower 34s values were measured for the first time in frost flower sulfates and did not differ significantly from the sea salt 34s values of 21 a method using stable isotopes is introduced to determine the limit of contributions from sea salt and sea ice sources including frost flowers and brine on sulfate concentrations in aerosol samples from alert canada knowledge of the range of values of 34snss and the so4 na ratio found in sea ice sources i e frost flowers is used to quantitatively constrain the contributions from frost flowers and sea salt in the arctic aerosol mass during the onset of winter in 2007 and 2008 allowing for quantification of non sea salt sulfate amounts during times when frost flowers are present frost flower and or brine influence was found predominantly in the coarse mode aerosols 0 95 m this method to determine the contributions from sea salt and sea ice sources can be carried over to future studies with snow and ice cores

temporal variations and sources of elements in the south pole atmosphere 1 nonenriched and moderately enriched elements high volume particle samples were collected from 1979 to 1983 from the south pole atmosphere and analyzed by nuclear methods the new results are combined with similar studies at the south pole there is no trend of concentrations during 12 years of sampling but concentrations of elements fluctuate through seasonal cycles concentrations of elements associated with crustal dust and sea salt showed opposite seasonal variations concentrations of most crustal elements are low during winters but double during summers concentration of marine elements are low in the summers and high in winters enrichment factors of some crustal elements such as ba la ce yb and hf are higher in the winter than summer indicating the presence of a second crustal component with a different chemical composition in the south pole atmosphere a similar difference in the crustal enrichment factor of co is due to the influence of volcanic or anthropogenic sources sulfate seasalt meteoritic particles volatiles and two different types of crustal particles are found to be components of the south polar aerosols the relative contribution of each component to total aerosol mass is as follows crustal 4 0 in summer 2 5 in winter marine elements 16 6 in summer 78 6 in winter sulfate 77 5 in summer 18 0 in winter meteoritic particles 0 1 in summer 0 1 in winter volatiles 0 9 in summer 0 5 in winter crustal component ii 0 8 in summer 0 7 in winter

seasonal variations of glaciochemical isotopic and stratigraphic properties in siple dome antarctica surface snow six snow pit records recovered from siple dome west antarctica during 1994 are used to study seasonal variations in chemical major ion and h 202 isotopic deuterium and physical stratigraphic properties during the 1988 94 period comparison of d measurements and satellite derived brightness temperature for the siple dome area suggests that most seasonal sd maxima occur within 4 weeks of each 1 january several other chemical species h 2o2 non sea salt nss so 4 2 methanesulfonic acid and no3 show coeval peaks with sd together providing an accurate method for identifying summer accumulation sea salt derived species generally peak during winter spring but episodic input is noted throughout some years no reliable seasonal signal is identified in species with continental sources nssca 2 nss mg 2 nh 4 or nsscl visible strata such as large depth hoar layers 5 cm are associated with summer accumulation and its metamorphosis but smaller hoar layers and crusts are more difficult to interpret a multi parameter approach is found to provide the most accurate dating of these snow pit records and is used to determine annual layer thicknesses at each site significant spatial accumulation variability exists on an annual basis but mean accumulation in the sampled 10 km 2 grid for the 1988 94 period is fairly uniform

frost flowers implications for tropospheric chemistry and ice core interpretation 1 this paper discusses the chemical composition of frost flowers and their accompanying slush layers and the evidence for their role as a salt source in processes important to atmospheric chemistry and ice core interpretation analysis of antarctic frost flowers shows that they are highly saline and fractionated in sea salt ions with sulfate being depleted strongly relative to sodium because frost flowers give a bright return on satellite scatterometer images the times and places of their formation can be identified when winds blow towards an aerosol sampling station from areas identified by the scatterometer as covered with flowers the collected aerosol is also depleted in sulfate because the flowers have a large salinity bromide concentrations are elevated in frost flowers relative to seawater with their high surface area it is possible that bromine is released to the atmosphere from frost flowers with consequent implications for tropospheric ozone depletion the finding that quantities of fractionated sea salt are available at the sea ice interface in the winter months and may be transported inland as aerosol also has implications for the interpretation of ice core records analysis of one near coastal core shows that the majority of the sodium comes from a fractionated source rather than from open water hitherto strong sea salt signals in ice cores have been attributed to increased open water and more efficient transport inland perhaps due to stormier weather at least in coastal regions however these signals may be related instead to the increased formation of sea ice and frost flowers

compositions of dust and sea salts in the dome c and dome fuji ice cores from last glacial maximum to early holocene based on ice sublimation and single particle measurements we analyzed the chemical compositions of dust and sea salt particles in the epica dome c edc ice core during 26 7 kyr bp using an ice sublimation technique and compared the results with existing

magmatic sulfide formation by reduction of oxidized arc basalt the hidaka metamorphic belt in southeastern hokkaido japan provides insights into how magmatic sulfide deposits may form through magma mixing deep within arcs here a near complete cross section of arc crust is exposed with large mafic igneous complexes preserved at deeper levels magmatic sulfide mineralization occurs within the opirarukaomappu gabbroic complex ogc which preserves a record of crustal contamination of mafic magmas via assimilation and magma mixing involving introduction of crust derived tonalite assimilation fractional crystallization modelling suggests that the gabbro evolved through a combination of approximately 10 wt mixing and 14 wt fractional crystallization magmatic sulfides and associated gabbros diorites and tonalites at this locality contain graphite with carbon isotope signatures consistent with derivation from the surrounding partially melted carbonaceous shales this indicates that crust derived carbon was added to the mafic magma through the magma mixing and assimilation process sulfur isotope data suggest that sulfur was also added from crustal sources during assimilation and magma mixing the relationships observed in the ogc suggest that intrusion of basalt into a segment of deep arc crust drove partial melting of carbonaceous metamorphic rocks producing graphite bearing felsic magmas with high reducing potential redox budget modelling shows that mixing of only small proportions of these magmas is sufficient to lower the oxidation state of oxidized basaltic magmas enough to induce sulfide saturation and consequent exsolution of immiscible sulfide melt magmatic sulfide deposits are likely to form by this reduction induced sulfide saturation mechanism deep within other arcs where magma mixing is thought to be common

micro investigation of epica dome c bottom ice evidence of long term in situ processes involving acid salt interactions mineral dust and organic matter abstract the epica dome c ice core edc reached a final depth of 3260 m at a maximum height of about 15 m above the ice bedrock interface in december 2004 we present here data gained from a detailed investigation of selected samples of the deeper part of the core located below 3200 m and referred to as bottom ice this part of the core has been poorly investigated so far mainly because there are significant challenges in interpreting paleo records that were very likely modified by long term in situ processes our study combines high resolution ion chromatography high resolution synchrotron x ray micro fluorescence micro xrf scanning and transmission electron microscopy our aim was to identify the long term physico chemical processes at work close to the bedrock to determine how they have altered the initial registers and ultimately to extract information on the very ancient antarctic environment the ubiquitous presence of nanometer iron oxide crystals at the surface of wind borne dust aggregates containing also large amount of organic matter raises the possibility that the consolidation of windborne dust clusters formed during ice recrystallization could be related to microbial iron reduction and thus to the progressive reactivation of dormant bacterial activity in warming ice inclusions of size and number density increasing with depth observed in the 12 last meters 3248 3260 m contain liquid and solid species among them marine biogenic acids numerous wind borne dust aggregates and clusters of large reversible calcium carbonate particles precipitated once the inclusion was formed and often covered by secondary gypsum the refreezing of slush lenses is discussed as a potential cause of the formation of such heterogeneous and complex mixtures in addition to the very fine micrometer size minerals windborne from extra antarctic continental sources and often accreted in large aggregates single medium size particles a few to ca 20 m and among them organic debris are commonly encountered their size surface shape and mineralogy suggest that aerosol transport from antarctic ice free areas played a significant role at the time edc bottom ice was formed concentrations and concentration ratios of biogenic sulfur species also advocate for the strengthening of peri antarctic meteorological patterns that favor the inland penetration of disturbed flow carrying local material very large well preserved mineral particles several tens of micrometers in diameter and biotope relics in deeper ice close to 3260 m likely come from the sub glacial environment

temperatures of granulite facies metamorphism constraints from experimental phase equilibria and thermobarometry corrected for retrograde exchange this study assesses temperatures of formation of common granulites by combining experimental constraints on the p t stability of granulite facies mineral associations with a garnet orthopyroxene grt opx thermobarometry scheme based on al solubility in opx corrected for late fe mg exchange we applied this scheme to 414 granulites of mafic intermediate and aluminous bulk compositions our findings suggest that granulites are much hotter than traditionally assumed and that the p t conditions of the amphibolite granulite transition portrayed in current petrology textbooks are significant underestimates by over 100 c for aluminous and intermediate granulites mean corrected temperatures based on our method are 890 17 and 841 11 c respectively uncertainties reported as 95 confidence limits on the mean consistent with minimum temperatures for orthopyroxene production by fluid absent partial melting in these bulk compositions in contrast mean temperatures based on grt opx fe mg exchange equilibria using the same thermodynamic data are 732 22 and 723 11 c respectively well below the minimum temperatures for opx stability for mafic granulites the mean corrected temperature using our method is 816 12 c similar to the mean temperature of 793 13 c from fe mg exchange reasons for the differences between the mafic granulites and aluminous intermediate granulites are unclear but may be due to the lower al concentrations in opx in the mafic rocks and possible deficiencies in the thermodynamic modelling of these low concentrations we discuss a number of well known granulite terrains in the context of our findings including the adirondacks the acadian granulites of new england the incipient charnockites of southern india and sri lanka and the kerala khondalite belt our findings carry implications for thermotectonic models of granulite formation a computer program to perform our thermobarometry calculations rclc is available from the journal of petrology website at http www petrology oupjournals org or from the authors at http www geo ucalgary ca pattison drm pattison rclc htm

a review of sea ice proxy information from polar ice cores sea ice plays an important role in earth s climate system the lack of direct indications of past sea ice coverage however means that there is limited knowledge of the sensitivity and rate at which sea ice dynamics are involved in amplifying climate changes as such there is a need to develop new proxy records for reconstructing past sea ice conditions here we review the advances that have been made in using chemical tracers preserved in ice cores to determine past changes in sea ice cover around antarctica ice core records of sea salt concentration show promise for revealing patterns of sea ice extent particularly over glacial interglacial time scales in the coldest climates however the sea salt signal appears to lose sensitivity and further work is required to determine how this proxy can be developed into a quantitative sea ice indicator methane sulphonic acid msa in near coastal ice cores has been used to reconstruct quantified changes and interannual variability in sea ice extent over shorter time scales spanning the last 160 years and has potential to be extended to produce records of antarctic sea ice changes throughout the holocene however the msa ice core proxy also requires careful site assessment and interpretation alongside other palaeoclimate indicators to ensure reconstructions are not biased by non sea ice factors and we summarise some recommended strategies for the further development of sea ice histories from ice core msa for both proxies the limited information about the production and transfer of chemical markers from the sea ice zone to the antarctic ice sheets remains an issue that requires further multidisciplinary study despite some exploratory and statistical work the application of either proxy as an indicator of sea ice change in the arctic also remains largely unknown as information about these new ice core proxies builds so too does the potential to develop a more comprehensive understanding of past changes in sea ice and its role in both long and short term climate changes

a 2680 year volcanic record from the dt 401 east antarctic ice core 1 volcanic signals recorded in the antarctic and greenland ice cores can provide useful information on past explosive volcanism and its impact in this study we carried out a continuous sulfate analysis of a 102 65 m east antarctic ice core dt 401 dated as 2682 years and identified 36 extensive volcanic eruption signals using cole dai s method which gives an average of 1 4 eruptions per century consistent with the results from the plateau remote pr b ice core when the record is divided into three parts the latest millennium 1999 1000 a d the middle millennium 999 1 a d and the earliest 682 years 0 a d to 682 b c it is found that there were more volcanic eruptions that occurred during the latest millennium 19 eruptions than during the middle millennium 10 eruptions of the record and that the intensities of the eruptions in the latest millennium are markedly larger than those in the middle one there were only seven events recorded in the earliest 682 years but their intensities were greater and nearly half of the eruptions had a similar intensity to tambora s 1815 a d which differs from the pr b record it is also found that volcanism and its average accumulation rate were lower during the little ice age than during the medieval warm period comparison of volcanic records between dt 401 and other antarctica ice cores pr b dome c dt 263 and byrd show that in the east antarctica area with its lower accumulation rates postdepositional effects may play an important role in the deposition of the sulfate

glacio chemical study spanning the past 2 kyr on three ice cores from dronning maud land antarctica 2 seasonally resolved chemical records the ammonium calcium and sodium concentrations from three intermediate depth ice cores drilled in the area of dronning maud land east antarctica have been investigated since all measurements were performed by a high resolution continuous flow analysis system for the first time seasonal signals of chemical trace species could be obtained from the interior of central antarctica over a period of approximately 2 millennia although the elevation as well as the accumulation rate differ between the drilling sites similar values were obtained by comparing mean concentrations spanning the last 900 years however a distinct lack of intersite correlation was found on decadal timescales despite a noticeable accumulation change apparent in one core no significant concentration change of all three species has occurred all the measured ions show clear seasonal signals over the whole records while the sea salt related component sodium peaks simultaneously with calcium the maximum ammonium concentration occurs in the snow with a time lag of 2 months after the sea salt peak more than 60 of the calcium concentration can be attributed to an ocean source elevated sodium concentrations were found within this millennium compared to mean values of the whole records but the spatially varying shape of the increase suggests that a possible climatic signal is biased by local deposition effects

twentieth century sea ice variability in the weddell sea and its effect on moisture transport evidence from a coastal east antarctic ice core record high resolution study of deuterium excess d excess sea salt sodium ss na and methane sulfonic acid msa in an ice core from coastal dronning maud land cdml east antarctica revealed the

mylonitic deformation of gabbro in the lower crust a case study from the pankenushi gabbro in the hidaka metamorphic belt of central hokkaido japan abstract the mylonitization of the pankenushi gabbro in the hidaka metamorphic belt of central hokkaido japan occurred along its western margin at 600 mpa and 660 700 c through dynamic recrystallization of plagioclase and a retrograde reaction from granulite facies to amphibolite facies orthopyroxene clinopyroxene plagioclase h2o hornblende quartz the reaction produced a fine grained 100 m polymineralic aggregate composed of orthopyroxene clinopyroxene quartz hornblende biotite and ilmenite into which strain is localized the dynamic recrystallization of plagioclase occurred by grain boundary migration and produced a monomineralic aggregate of grains whose crystallographic orientations are mostly unrelated to those of porphyroclasts the monomineralic plagioclase aggregates and the fine grained polymineralic aggregates are interlayered and define the mylonitic foliation while the latter is also mixed into the former by grain boundary sliding to form a rather homogeneous polymineralic matrix in ultramylonites however in both mylonite and ultramylonite plagioclase aggregates form a stress supporting framework and therefore controlled the rock rheology crystal plastic deformation of pyroxenes and plagioclase with dominant 100 001 and 001 1 2 1 1 0 slip systems respectively produced distinct shape and crystallographic preferred orientations of pyroxene porphyroclasts and dynamically recrystallized plagioclase grains in both mylonite and ultramylonite euhedral to subhedral growth of hornblende in pyroxene porphyroclast tails during the reaction and its subsequent rigid rotation in the fine grained polymineralic aggregate or matrix produced clear shape and crystallographic preferred orientations of hornblende grains in both mylonite and ultramylonite in contrast the dominant grain boundary sliding of pyroxene and quartz grains in the fine grained polymineralic aggregate of the mylonite resulted in their very weak shape and crystallographic preferred orientations in the fine grained polymineralic matrix of the ultramylonite however pyroxene and quartz grains became scattered and isolated in the plagioclase aggregate so that they were crystal plastically deformed leading to stronger shape and crystallographic preferred orientations than those seen in the mylonite

examining the physical meaning of the bank erosion coefficient used in meander migration modeling widely used models of meander evolution relate migration rate to vertically averaged near bank velocity through the use of a coefficient of bank erosion e in applications to floodplain management problems e is typically determined through calibration to historical planform changes and thus its physical meaning remains unclear this study attempts to clarify the extent to which e depends on measurable physical characteristics of the channel boundary materials using data from the sacramento river california usa bend average values of e were calculated from measured long term migration rates and computed near bank velocities in the field unvegetated bank material resistance to fluvial shear k was measured for four cohesive and noncohesive bank types using a jet test device at a small set of bends for which both e and k were obtained we discovered that variability in k explains much of the variability in e the form of this relationship suggests that when modeling long term meander migration of large rivers e depends largely on bank material properties this finding opens up the possibility that e may be estimated directly from field data enabling prediction of meander migration rates for systems where historical data are unavailable or controlling conditions have changed another implication is that vegetation plays a limited role in affecting long term meander migration rates of large rivers like the sacramento river these hypotheses require further testing with data sets from other large rivers

flow and bed profile in meandering sand silt rivers mathematical models for defining three dimensional flow and bed topography in sinuous channels with suspendable bed material are presented it is shown theoretically that the secondary flow in sinuous channels shows a reduction in its magnitude below that of a uniformly curved channel and also displays a phase lag relative to the channel plan form the model for bed topography is derived by considering the sediment balance for bed load and suspended load the transport rate and direction all of which are governed by the three dimensional flow a laboratory test supports the model the present study has made it possible to predict the large scale bed topography of meandering sand silt rivers including such features as the magnitude and the location of local scour and point bar deposits

holocene electrical and chemical measurements from the epica dome c ice core the comparison between electric electric conductivity measurement ecm and dielectric profiling dep and chemical sulphate and chloride depth profiles along the first 400 m of the epica dome c ice core revealed a very good fit especially for peaks related to volcanic emissions from the comparison between these profiles a dominant contribution of sulphuric acid to the ionic balance of antarctic ice for the holocene was confirmed a progressive increase with depth was observed for chloride concentrations showing a change of relative contribution between sulphate and chloride a higher increase of chloride was evident between 270 and 360 m depth probably due to a change in source or transport processes or to an increase of the annual snow accumulation rate the dep ecm and sulphate ice signatures of tambora ad 1816 and el chichon ad 1259 eruptions are described in detail a characteristic peak series due to hcl deposition was identified at 103 109 m depth from the ecm dep and chloride profiles

simulating river meandering processes using stochastic bank erosion coefficient this study first compares the first order analytical solutions for flow field by ikeda et al 1981 and johanesson and parker 1989b ikeda et al s 1981 linear model of bank erosion was implemented to predict the rate of bank erosion in which the bank erosion coefficient is treated as a stochastic variable that varies with physical properties of the bank e g cohesiveness stratigraphy vegetation density the developed model was used to predict the evolution of meandering planforms then the modeling results were analyzed and compared to the observed data because the migration of meandering channels consists of downstream translation lateral expansion and downstream or upstream rotations several measures are formulated to determine which of the resulting planform is closest to the experimental measured one results from the deterministic model highly depend on the calibrated erosion coefficient because field measurements are always limited the stochastic model yielded more realistic predictions of meandering planform evolutions because the coefficient of bank erosion is a random variable the meandering planform evolution is a stochastic process that can only be accurately predicted by a stochastic model

river and floodplain process simulation for subsurface characterization subsurface heterogeneity is characterized with a method based on facies delineation and process modeling a geologic process model is formulated to simulate the development of four facies channel fill levee splay and floodplain which commonly form in a meandering river and its floodplain the model accounts for meandering cutoff flooding and crevasse splay processes and links these formational processes to the statistical properties of the resulting facies the model is utilized in a case study to simulate the horizontal pattern of deposition and facies distributions of the stratton field part of the middle frio formation of southeast texas using known or estimated relationships and historical information for the stratton field region the simulated distributions of longitudinal and transverse facies dimensions and corresponding transition probabilities among facies are compared to those determined from sampling an observed section of the field yielding similar results though some differences similarities and differences are related to the model processes and sampling procedures model sensitivity is investigated by changing selected model variables and analyzing the change in the facies distributions the application of the geologic process model to the stratton field demonstrates the viability of linking process understanding and modeling of complex meandering stream processes to characterization of subsurface heterogeneity

holocene late pleistocene climatic ice core records from qinghai tibetan plateau three ice cores to bedrock from the dunde ice cap on the north central qinghai tibetan plateau of china provide a detailed record of holocene and wisconsin wurm late glacial stage lgs climate changes in the subtropics the records reveal that lgs conditions were apparently colder wetter and dustier than holocene conditions the lgs part of the cores is characterized by more negative 18 o ratios increased dust content decreased soluble aerosol concentrations and reduced ice crystal sizes than the holocene part these changes occurred rapidly 10 000 years ago in addition the last 60 years were apparently one of the warmest periods in the entire record equalling levels of the holocene maximum between 6000 and 8000 years ago

atmospheric decadal variability from high resolution dome c ice core records of aerosol constituents beyond the last interglacial along the epica dome c ice core we measured concentrations of different water soluble aerosol constituents and deduced total depositional flux records here we present high resolution sodium calcium ammonium and nitrate data covering the last 173 000 years the first three of these species are passive tracers and reveal source and long range transport changes whereas nitrate is deposited reversibly however it can be used to check isotope derived accumulation rate estimates a prerequisite to calculate total depositional fluxes during the last two transitions from glacial to interglacial periods changes in the total depositional flux differ strongly for different aerosol species the largest changes are observed in the terrestrial aerosol proxy non sea salt calcium only moderate changes occur in the marine sea salt indicator sodium while ammonium a proxy for marine bioproductivity remains rather constant in agreement with previous studies we find that only considerable glacial interglacial changes at both the terrestrial and the marine sea salt aerosol source can explain the observed pronounced changes the unprecedented high resolution of our data allows for the first time the examination of decadal variability back to the penultimate glacial period on the one hand we find occasional fast shifts occurring within a few years here we present such an event in the calcium record from the penultimate glacial period on the other hand we examine variation coefficients and pairwise correlation coefficients both determined in 200 year windows they generally reveal only moderate changes during glacial periods slightly lower variation coefficients are found concurrent with slightly higher correlation coefficients which points to a more uniform and stronger coupled atmospheric long range transport of the different aerosol species to the east antarctic plateau and less influence of cyclonic activities during cold periods the opposite is observed for interglacial periods with probably even reinforced importance of cyclonic influences during the last interglacial period the marine isotope stage 5 5 this period reveals no evidence for abrupt climatic changes in any of the species however the marine sea salt aerosol indicator sodium shows a distinct minimum followed by a pronounced increase this pattern is explained by significantly reduced sea ice production in the indian southern ocean sector which is believed to be the dominant source of sodium deposited in dome c during warm periods

atmospheric methane sulfonate and non sea salt sulfate records at the european project for ice coring in antarctica epica deep drilling site in dronning maud land antarctica abstract during three summer campaigns in january february 2000 2001 and 2002 the ionic compo sition of the aerosol at the epica deep drilling site at kohnen station was measured in daily resolution in 2000 and 2002 we observed mean std non sea salt sulfate nss so42 con centrations of 353 100 ng m 3 and 320 250 ng m 3 as well as methane sulfonate ms con centrations of 59 36 ng m 3 and 74 80 ng m 3 respectively for the summer campaign in 2001 significantly lower nss so42 and ms levels of 164 150 ng m 3 and 19 12 ng m 3 respectively were typical the mean ms nss so42 ratio ranged from about 0 1 to 0 2 ms and nss so42 concentrations and their variability were roughly comparable to coastal stations at summer supported by air mass back trajectory analyses this finding documented an effi cient long range transport to kohnen via the free troposphere ms nss so42 ratios exhibited a strong dependence on the ms concentration with systematically higher ratios at higher ms concentrations a peculiarity which is also evident in a firn core drilled at this site

year round record of size segregated aerosol composition in central antarctica concordia station implications for the degree of fractionation of sea salt particles the origin of sea salt aerosol that reaches the high antarctic plateau and is trapped in snow and ice cores remains still unclear in particular the respective role of emissions from the open ocean versus those from the sea ice surface is not yet quantified to progress on this question the composition of bulk and size segregated aerosol was studied in 2006 at the concordia station 75 s 123 e located on the high antarctic plateau a depletion of sulfate relative to sodium with respect to the seawater composition is observed on sea salt aerosol reaching concordia from april to september that suggests that in winter when the sea salt atmospheric load reaches a maximum emissions from the sea ice surface significantly contribute to the sea salt budget of inland antarctica citation jourdain b s preunkert o cerri h castebrunet r udisti and m legrand 2008 year round record of size segregated aerosol composition in central antarctica concordia station implications for the degree of fractionation of sea salt particles

seasonal variation of fractionated sea salt particles on the antarctic coast 1 aerosol sampling was conducted at syowa station antarctica coastal station in 2004 2006 so42 depletion by mirabilite precipitation was identified from april through november the fractionated sea salt particles were distributed in ultrafine coarse modes molar ratios of mg2 na and k na were higher than in bulk seawater ratio during winter spring the mg2 na ratio in aerosols greatly exceeded the upper limit in the case only with mirabilite precipitation the temperature dependence of mg2 na ratio strongly suggested that higher ratios of mg2 na and k na were associated with sea salt fractionation by precipitation of mirabilite at 9 c hydrohalite at ca 23 c and other salts such as ikaite at ca 5 c and gypsum at ca 22 c during winter spring mg salts with lower deliquescence relative humidity can be enriched gradually in the fractionated sea salt particles results suggests that sea salt fractionation can alter aerosol hygroscopicity and atmospheric chemistry in polar regions

antarctic aerosol and snowfall chemistry implications for deep antarctic ice core chemistry in this paper we first summarise major findings of recent atmospheric studies of nitrogen and sulphur species present in the boundary layer of coastal antarctic regions we then discuss the implications of such atmospheric data for the interpretation of nitrate ammonium methanesulphonate and sulphate records in deep ice cores extracted from central antarctica in terms of past atmospheric chemistry changes

chemical compositions of solid particles present in the greenland neem ice core over the last 110 000 years this study reports the chemical composition of particles present along greenland s north greenland eemian ice drilling neem ice core back to 110 000 years before present insoluble and soluble particles larger than 0 45 m were extracted from the ice core by ice sublimation and their chemical composition was analyzed using scanning electron microscope and energy dispersive x ray spectroscopy and micro raman spectroscopy we show that the dominant insoluble components are silicates whereas nacl na so caso and caco represent major soluble salts for the first time particles of camg co and ca no 4h o are identified in a greenland ice core the chemical speciation of salts varies with past climatic conditions whereas the fraction of na salts nacl na so exceeds that of ca salts caso caco during the holocene 0 6 11 7 kyr b p the two fractions are similar during the bolling allerod period 12 9 14 6 kyr b p during cold climate such as over the younger dryas 12 0 12 6 kyr b p and the last glacial maximum 15 0 26 9 kyr b p the fraction of ca salts exceeds that of na salts showing that the most abundant ion generally controls the salt budget in each period high resolution analyses reveal changing particle compositions those in holocene ice show seasonal changes and those in lgm ice show a difference between cloudy bands and clear layers which again can be largely explained by the availability of ionic components in the atmospheric aerosol body of air masses reaching greenland

changes in environment over the last 800 000 years from chemical analysis of the epica dome c ice core the epica ice core from dome c extends 3259 m in depth and encompasses 800 ka of datable and sequential ice numerous chemical species have been measured along the length of the cores here we concentrate on interpreting the main low resolution patterns of major ions we extend the published record for non sea salt calcium sea salt sodium and non sea salt sulfate flux to 800 ka the non sea salt calcium record confirms that terrestrial dust originating from south america closely mirrored antarctic climate both at orbital and millennial timescales a major cause of the main trends is most likely climate in southern south america which could be sensitive to subtle changes in atmospheric circulation sea salt sodium also follows temperature but with a threshold at low temperature we re examine the use of sodium as a sea ice proxy concluding that it is probably reflecting extent with high salt concentrations reflecting larger ice extents with this interpretation the sodium flux record indicates low ice extent operating as an amplifier in warm interglacials non sea salt sulfate flux is almost constant along the core confirming the lack of change in marine productivity for sulfur producing organisms in the areas of the southern ocean contributing to the flux at dome c for the first time we also present long records of reversible species such as nitrate and chloride and show that the pattern of post depositional losses described for shallower ice is maintained in older ice it appears possible to use these concentrations to constrain snow accumulation rates in interglacial ice at this site and the results suggest a possible correction to accumulation rates in one early interglacial taken together the chemistry records offer a number of constraints on the way the earth system combined to give the major climate fluctuations of the late quaternary period

continental biogenic species in the greenland ice core project ice core tracing back the biomass history of the north american continent ammonium nitrate and organic acid records from the greenland ice core project deep ice core are discussed all species have a continental biogenic source that is situated predominantly on the north american continent for species deposited in summit central greenland the record therefore can be used to trace back the biomass history of the north american continent difficulty in the interpretation of these records arises from their unknown transfer behavior in a more alkaline atmosphere which characterizes glacial time periods compared to interglacial stadials this may have implications not only for weak acids such as formate and acetate but also possibly for the transport and incorporation of hno3 into aerosols whereas ammonium is probably not affected by the alkalinity change of the atmosphere our approach is to compare samples with similar h concentrations throughout the record from the records we infer several significant fluctuations in the extent of the laurentide ice sheet during the last glaciation we find evidence against the occurrence of an extensive younger dryas event in north america