potential links between surging ice sheets circulation changes and the dansgaard oeschger cycles in the irminger sea 60 18 kyr surface and deepwater paleoclimate records in irminger sea core so82 5 59on 31 ow and icelandic sea core ps2644 68on 22ow exhibit large fluctuations in thermohaline circulation thc from 60 to 18 calendar kyr b p with a dominant periodicity of 1460 years from 46 to 22 calendar kyr b p matching the dansgaard oeschger d o cycles in the greenland ice sheet project 2 gisp2 temperature record grootes and tuiver 1997 during interstadials summer sea surface temperatures sstsu in the irminger sea averaged to 8oc and sea surface salinities sss averaged to 36 5 recording a strong irminger current and atlantic thc during stadials sstsu dropped to 2o 4oc in phase with sss drops by 1 2 they reveal major meltwater injections along with the east greenland current which turned off the north atlantic deepwater convection and hence the heat advection to the north in harmony with various ocean circulation and ice models on the basis of the ird composition icebergs came from iceland east greenland and perhaps svalbard and other northern ice sheets however the southward drifting icebergs were initially jammed in the denmark strait reaching the irminger sea only with a lag of 155 195 years we also conclude that the abrupt stadial terminations the d o warming events were tied to iceberg melt via abundant seasonal sea ice and brine water formation in the meltwater covered northwestern north atlantic in the 1 1460 year frequency band benthic 15 80 brine water spikes led the temperature maxima above greenland and in the irminger sea by as little as 95 years thus abundant brine formation which was induced by seasonal freezing of large parts of the northwestern atlantic may have finally entrained a current of warm surface water from the subtropics and thereby triggered the sudden reactivation of the thc in summary the internal dynamics of the east greenland ice sheet may have formed the ultimate pacemaker of d o cycles

high spatial resolution electron probe microanalysis of tephras and melt inclusions without beam induced chemical modification wavelength dispersive analysis of tephra grains and melt inclusions by epma has been carried out using a focused beam of 3 m diameter without detected loss of sodium or potassium in standard glasses including anhydrous basalts a slightly hydrated rhyolite and a sodium rich intermediate composition the ability to make analyses without chemical modification is strongly dependent upon current density at the analysis site analysis with narrow beams requires extremely low beam currents that are normally associated with energy dispersive analysis experiments indicate that a value of 0 1 na m2 must not be exceeded at least for moderately hydrated samples if sodium loss is to be avoided high resolution analysis without beam induced analytical artefacts enables fully quantitative analysis of very distal and or highly vesicular tephras and very small melt inclusions without the need to use post analysis corrections this development has enabled high quality analyses from crypto tephra layers that were pre

tracing marine cryptotephras in the north atlantic during the last glacial period protocols for identification characterisation and evaluating depositional controls abstract tephrochronology is increasingly being utilised as a key tool for improving chronological models and correlating disparate palaeoclimatic sequences for many sedimentary environments however there is an increased recognition that a range of processes may impart a delay in deposition and or rework tephra these processes can affect the integrity of tephra deposits as time synchronous markers therefore it is crucial to assess their isochronous nature especially when cryptotephras are investigated in a dynamic marine environment a methodology for the identification and characterisation of marine cryptotephras alongside a protocol for assessing their integrity is outlined this methodology was applied to a wide network of north atlantic marine sequences covering the last glacial period a diverse range of cryptotephra deposits were identified and based on similarities in physical characteristics e g glass shard concentration profiles and geochemical homogeneity heterogeneity indicative of common modes of tephra delivery and post depositional reworking a deposit type classification scheme was defined the presence and dominance of different deposit types within each core allowed an assessment of spatial and temporal controls on tephra deposition and preservation overall isochronous horizons can be identified across a large portion of the north atlantic due to preferential atmospheric dispersal patterns however the variable influence of ice rafting processes and an interplay between the high eruptive frequency of iceland and relatively lower sedimentation rates can also create complex tephrostratigraphies in this sector sites within a wide sector to the south and east of iceland have the greatest potential to be repositories for isochronous horizons that can facilitate the synchronisation of palaeoclimatic records

tephrological implications of beam size sample size effects in electron microprobe analysis of glass shards this paper concerns the potential consequences of varying procedures for the determination of tephra geochemistry by electron microprobe application of electron probe microanalysis to tephrostratigraphical methods has increasingly facilitated the resolution and refinement of quaternary chronology associated with records of proxy environmental or proxy climatic change the geographical range over which tephras are recovered has expanded significantly with the identification and analysis of crypto or hidden tephras in areas far removed from tephra sources these tephras are dominated by glass shards which in many distal environments may be either small in size m or may be highly pumiceous with low glass void ratios and thin 10 m shard walls we demonstrate that reducing the size of the electron beam used to analyse shard geochemistry cannot be used reliably to permit analysis of thin glass walls this approach distorts the geochemical data creating analytical differences that may generate inappropriate tephrogeochemical fingerprints additional distortion of the geochemical fingerprint in the form of hybrid analyses may be encountered in glass fragments containing micron sized crystalline phases such as feldspar copyright 2001 john wiley sons ltd

origin and consequences of cyclic ice rafting in the northeast atlantic ocean during the past 130 000 years deep sea sediment cores recovered from the northeast atlantic ocean were examined in order to elucidate the influence of the earth s orbital parameters on major ice rafting analyses of coarse grained ice rafted debris and planktonic foraminifers revealed a strong reaction to the precession signal since 130 000 yr b p dropstone layers have been deposited each half period of a precessional cycle 11 000 1000 yr ice rafting occurs during times of winter minimum summer maximum insolation and summer minimum winter maximum insolation in the first case high summer insolation forces meltwater discharge from the ice sheets into the polar seas which subsequently enhances formation of sea ice during the winter in the second case growth of continental ice enhances iceberg production which also leads to a salinity reduction of surface seawater both situations result in a southward penetration of polar water thus the marine record of dropstones documents ice rafting not only during weichselian stades but also during cold events within interstades the regularity of ice rafting yields a useful framework to calibrate and elucidate climatic changes not only in the region of the north atlantic ocean but also in remote areas such as the pacific ocean and the antarctic

a tephra lattice for greenland and a reconstruction of volcanic events spanning 25 45 ka b2k tephra layers preserved within the greenland ice cores are crucial for the independent synchronisation of these high resolution records to other palaeoclimatic archives here we present a new and detailed tephrochronological framework for the time period 25 000 45 000 a b2k that brings together results from 4 deep greenland ice cores in total 99 tephra deposits the majority of which are preserved as cryptotephra are described from the ngrip neem grip and dye 3 records the major element signatures of single glass shards within these deposits indicate that 93 are basaltic in composition all originating from iceland specifically 43 originate from grimsvotn 20 are thought to be sourced from the katla volcanic system and 17 show affinity to the kverkfjoll system robust geochemical characterisations independent ages derived from the gicc05 ice core chronology and the stratigraphic positions of these deposits relative to the dansgaard oeschger climate events represent a key framework that provides new information on the frequency and nature of volcanic events in the north atlantic region between gs 3 and gi 12 of particular importance are 19 tephra deposits that lie on the rapid climatic transitions that punctuate the last glacial period this framework of well constrained time synchronous tie lines represents an important step towards the independent synchronisation of marine terrestrial and ice core records from the north atlantic region in order to assess the phasing of rapid climatic changes during the last glacial period

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

stagnant groundwater stored in isolated aquifers implications related to hydraulic calculations and isotopic dating reply abstract not all groundwater flows and traps that host stagnant groundwater may also occur which are similar in nature to the better known traps of oil gas and brine such traps are envisaged as mega inclusions that are hydraulically isolated they are under no hydrostatic head as they are sealed from above and disconnected from recharge but they are pressurized to various degrees by compaction hydraulic gradients based on data obtained from wells that belong to distinct aquifers that are hydraulically isolated from each other are false gradients false hydraulic gradients applied in darcian equations of flow velocity and water age lead to meaningless results e g water ages that are too young criteria to establish hydraulic isolation of trapped groundwater include geological configuration deep aquifer engulfed by impermeable rocks abrupt differences in water properties observed between adjacent wells temperature pressure concentration of solutes water age and high groundwater ages more than 107 years the great artesian basin australia is discussed in some detail in the light of these guidelines revealing a multitude of pressurized isolated aquifers calibration of groundwater dating methods based on hydraulic computations and isotopic methods e g 36cl and 4he is discussed for the great artesian basin with reference to the competing models of dynamic and static groundwater aquifers

crustal growth along a non collisional cratonic margin a lu hf isotopic survey of the eastern cordilleran granitoids of peru abstract an extensive in situ lu hf isotopic study of zircon from the eastern cordilleran batholiths of peru using laser ablation multi collector inductively coupled mass spectrometry la mc icpms reveals a systematic covariance between granitoid magma sources and tectonic regimes which shaped the proto andean margin of central western amazonia since 1 15 ga the hf isotope systematics are characterised by a range in initial 176hf 177hf compositions for a given intrusive event suggesting mixing of material derived from the paleoproterozoic crustal substrate and variable addition of juvenile sources from neoproterozoic to cenozoic time intrusives associated with phases of regional compressive tectonism correspond to mean initial ehf values of 6 73 2 43 1 57 for the ordovician famatinian carboniferous permian and late triassic respectively suggesting a minimum crustal contribution between 40 and 74 by mass the average initial hf systematics from granitoids associated with periods of regional extension such as the middle neoproterozoic permian triassic and cenozoic andean back arc plutonism are consistently shifted towards positive values mean initial ehf 0 7 to 8 0 indicating systematically larger inputs of juvenile magma 25 to 38 of ancient crust by mass in the absence of evidence for significant lateral accretion of exotic crust the time integrated hf record from the central proto andean margin of western amazonia suggests crustal reworking was the dominant process during episodes of arc magmatism implying that most crustal growth took place vertically via crustal underplating of isotopically juvenile mantle derived melts during intervals of crustal extension

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

mid pleistocene silicic tephra beds in the auckland region new zealand their correlation and origins based on the trace element analyses of single glass shards abstract the trace element composition of individual glass shards from twelve distinct mid pleistocene silicic tephra beds from the auckland region new zealand has been determined by laser ablation la icp ms analysis this is the first application of such data to tephra correlation studies in new zealand these analyses support earlier major element based correlations between three surface sections oruarangi and okariha at port waikato and st kentigern s at east auckland and a long core patiki road west auckland the correlations provide a higher resolution record of taupo volcanic zone tvz activity than is preserved in more proximal deposits where burial and erosion obscure much of the volcanic record a combination of major and trace element analysis enables the correlation of three units exposed in the auckland region with their proximal equivalents ignimbrites from the tvz namely deposits associated with 1 the ongatiti ignimbrite 1 21 0 04 ma 2 an un named post ahuroa pre unit e ignimbrite sample ut1274 from the rocky hill section king country and 3 the unit e ignimbrite 1 ma combined major and trace element data show a marked evolution in magma chemistry with the large ongatiti and unit e ignimbrite eruptions occurring after a change in composition caused by replenishment reactivation of the magma chamber changes in magma chemistry between these deposits resulted from a combination of assimilation of metasedimentary country rocks and periods of fractional crystallisation of a plagioclase dominated assemblage trace element data and the wider dispersal of the silicic tephras in the auckland region suggests that they are all sourced from the mangakino volcanic centre mvc in the tvz this study illustrates for the first time in a new zealand context the potential for correlating tephra deposits using single glass shard trace element chemistry

widespread tephra dispersal and ignimbrite emplacement from a subglacial volcano torfajokull iceland the tephra dispersal mechanisms of rhyolitic glaciovolcanic eruptions are little known but can be investigated through the correlation of eruptive products across multiple depositional settings using geochemistry and geochronology we correlate a regionally important pleistocene tephra horizon the rhyolitic component of north atlantic ash zone ii ii rhy 1 and the thorsmork ignimbrite with rhyolitic tuyas at torfajokull volcano iceland the eruption breached an ice mass 400 m thick leading to the widespread dispersal of ii rhy 1 across the north atlantic and the greenland ice sheet locally pyroclastic density currents traveled across the ice surface depositing the variably welded thorsmork ignimbrite beyond the ice margin and 30 km from source the widely dispersed products of this eruption represent a valuable isochronous tie line between terrestrial marine and ice core paleoenvironmental records using the tephra horizon estimates of ice thickness and extent derived from the eruption deposits can be directly linked to the regional climate archive which records the eruption at the onset of greenland stadial 15 2

non synchronous deposition of north atlantic ash zone ii in greenland ice cores and north atlantic and norwegian sea sediments an example of complex glacial stage tephra transport tephra provides regional chronostratigraphical marker horizons that can link different climate archives with highly needed accuracy and precision the results presented in this work exemplify however that the intermittent storage of tephra in ice sheets and during its subsequent iceberg transport especially during glacial stages constitutes a potential source of serious error for the application of tephrochronology to nordic seas and north atlantic sediment archives the peak shard concentration of the rhyolitic component of the north atlantic ash zone ii naaz ii tephra complex often used to correlate marine and ice core records in marine isotope stage mis 3 is shown to lag the eruption event by ca 100 400 years in some north atlantic and norwegian sea cores while still allowing for a correlation of archives on millennial timescales this time delay in deposition is a major obstacle when addressing the lead lag relationship on short timescales years to centuries a precise and accurate determination of lead lag relationships between archives recording different parts of the climate system is crucial in order to test hypotheses about the processes leading to abrupt climate change and to evaluate results from climate models copyright 2011 john wiley sons ltd

30 million years of permian volcanism recorded in the choiyoi igneous province w argentina and their source for younger ash fall deposits in the parana basin shrimp u pb zircon geochronology evidence abstract we present four shrimp u pb zircon ages for the choiyoi igneous province from the san rafael block central western argentina dated samples come from the yacimiento los reyunos formation 281 4 2 5 ma of the cochico group lower choiyoi section andesitic breccias dacitic to rhyolitic ignimbrites and continental conglomerates agua de los burros formation 264 8 2 3 ma and 264 5 3 0 ma and cerro carrizalito formation 251 9 2 7 ma upper choiyoi section rhyolitic ignimbrites and pyroclastic flows spanning the entire permian succession of the choiyoi igneous province a single zircon from the el imperial formation that is overlain unconformably by the choiyoi succession yielded an early permian age 297 2 5 3 ma while the main detrital zircon population indicated an ordovician age 453 7 8 1 ma the new data establishes a more precise permian age artinskian lopingian for the section studied spanning 30 ma of volcanic activity volcanological observations for the choiyoi succession support the occurrence of explosive eruptions of plinian to ultraplinian magnitudes capable of injecting enormous volumes of tephra in the troposphere stratosphere the new shrimp ages indicate contemporaneity between the choyoi succession and the upper part of the parana basin late paleozoic section from the irati up to the rio do rasto formations encompassing about 24 ma geochemical data show a general congruence in compositional and tectonic settings between the volcanics and parana basin permian ash fall derived layers of bentonites thickness and granulometry of ash fall layers broadly fit into the depletion curve versus distance from the remote source vent of ultraplinian eruptions thus we consider that the choiyoi igneous province was the source of ash fall deposits in the upper permian section of the parana basin data presented here allow a more consistent correlation between tectono volcanic permian events along the paleo pacific margin of southwestern gondwana and the geological evolution of neighboring paleozoic foreland basins in south america and africa

late paleozoic to triassic evolution of the gondwana margin evidence from chilean frontal cordilleran batholiths 28 s to 31 s upper paleozoic to triassic chilean granitoids in the andean frontal cordillera between 28 s and 31 s record crustal and mantle conditions at the gondwana margin during the final assembly and initial breakup of the pangea supercontinent this period overlaps the end of paleozoic terrane accretion and precedes andean subduction integration of new trace element and isotopic data with other information on the granitoids and the regional geology leads to a tectonic model that has implications for other parts of the gondwana margin in the model the carboniferous to early permian is a period of oblique convergence associated elqui complex granitoids are diverse those in the guanta and montosa units are predominantly related to subduction processes whereas those in the cochiguas and el volcan units are dominated by melting of the subduction complex and older crust progressive oblique collision of the last pre pangea terrane equis along the margin resulted in crustal thickening associated with shortening deformation of foreland basinal sedimentary rocks and uplift of the elqui complex subsequent gravitational collapse of the inactive slab and lithospheric delamination resulted in the production of large amounts of basalt which intruded and melted the crust producing the post collisional ingaguas complex the los carricitos granitoids formed in thickened crust whereas the chollay el colorado and el leon units formed in thinner crust the ingaguas complex is part of the choiyoi granite rhyolite province whose formation similar to that of other gondwana silicic provinces was probably accentuated by anomalously hot upper mantle associated with the pangea supercontinent

andean forearc dynamics as recorded by detrital zircon from the eocene talara basin northwest peru abstract eocene strata of the talara forearc basin of northwest peru contain detrital zircon dz populations that when integrated with other provenance indicators record paleogeographic shifts in the andean forearc that can be linked to larger scale tectonic drivers such as subduction erosion plate convergence rate and regional accretion events because forearc basins along the western margin of south america are particularly difficult to preserve the new dz results provide rare insight into the trench arc dynamics associated with the andean orogenic cycle the talara basin tapped into both the andean volcanic arc and older basement and the dz populations subsequently reflect long lived plate interactions and progressive crustal growth along the northwestern edge of amazonia gondwana and south america dominant u pb zircon age populations include 36 110 ma andes convergence 215 285 ma western gondwana extension 460 625 ma rodinia extension and western gondwana convergence 950 1250 ma grenville orogeny and 1250 ma amazonia assembly the data corroborate previous geochronology on pre andean basement and support the existence of a famatinian arc 480 ma and carboniferous triassic magmatism along northwestern gondwana pre andean and amazonian dz likely was recycled from paleozoic metasedimentary rocks of the andean cordillera and amotape terrane although the data do not preclude delivery of amazonian detritus directly into the forearc via a large scale westward drainage the andean dz population constrains the timing of arc activity and quiescence with two peaks at 75 ma and 50 ma and a gap at 70 ma linked to accretion of the caribbean oceanic plateau against northwest south america arc volcanism was concurrent with eocene forearc deposition and syndepositional dz is used to revise maximum depositional age for the upper eocene helico member and verdun formation progressive subsidence and arc shut down in the late eocene coincide with arc collision to the north uplift of the amotape block subduction erosion and slab shallowing

stagnant aquifer concept part 1 large scale artesian systems great artesian basin australia artesian systems 1000 2000 m deep occurring in the one and half million km2 great artesian basin gab are described by most researchers as through flow aquifers that are recharged at eastern outcrops in the great dividing range a simplified mathematical model assuming basin wide through flow in two aquifers of jurassic and cretaceous rocks has been applied by previous researchers to calculate hydraulic travel times the latter were described as agreeing with groundwater ages of 105 to 106 years computed from 36cl data assuming part of the observed 36c1 was atmospheric and 4he ages assuming the bulk of observed 4he diffused from the crust an alternative conceptual model is discussed in this paper suggesting that artesian wells drilled into the deep jurassic rocks of the gab are tapping stagnant aquifers that are hydraulically isolated from each other the stagnant aquifers are envisaged as mega fluid inclusions formed during the subsidence of the basin the age of the water in the stagnant aquifers is expected to be high between the age of the host rocks connate water and the age of the subsidence stage at which the aquifer was sealed off by covering sediments and burial to beneath the active base of drainage isotopic ages were calculated to be in this range of very old groundwaters the chlorine 36 data were interpreted as being beyond the dating limit of this method i e over one million years suggesting that the bulk of the observed 36c1 was produced in situ and the 4he ages were computed to be 5 10 million years or more suggesting that all the he was produced in the aquifer rocks alone the groundwater in the artesian systems is concealed and hence the through flow and stagnant aquifer models are both based on interpretation of indirect observations mathematical modeling is not feasible either as too many parameters pertaining to the number of distinct aquifers their rock properties and other components of the groundwater cycle are unknown hence a phenomenological approach is adapted and the through flow and stagnant aquifer models are weighed up in the light of first principles of physics geology hydraulics chemistry and isotopic compositions

optimising the use of marine tephrochronology in the north atlantic a detailed investigation of the faroe marine ash zones ii iii and iv abstract tephrochronology is central to the intimate 1 goals for testing the degree of climatic synchroneity during abrupt climatic events that punctuated the last glacial period since their identification in north atlantic marine sequences the faroe marine ash zone ii fmaz ii fmaz iii and fmaz iv have received considerable attention due to their potential for high precision synchronisation with the greenland ice cores in order to optimise the use of these horizons as isochronous markers a detailed re investigation of their geochemical composition sedimentology and the processes that deposited each ash zone is presented shard concentration profiles geochemical homogeneity and micro sedimentological structures are investigated for each ash zone preserved within core jm11 19pc retrieved from the southeastern norwegian sea on the central north faroe slope this approach allows a thorough assessment of primary ash fall preservation and secondary depositional features and demonstrates its value for assessing depositional integrity in the marine environment results indicate that the fmaz ii and iv are well resolved primary deposits that can be used as isochrons for high precision correlation studies we outline key recommendations for future marine tephra studies and provide a protocol for optimising the application of tephrochronology to meet the intimate synchronisation goals

geodynamic regimes of subduction under an active margin effects of rheological weakening by fluids and melts the dynamics of subduction under an active margin are analysed by using a 2d coupled petrological thermomechanical numerical model of an oceanic continental subduction system the model includes spontaneous slab bending dehydration of the subducted crust aqueous fluid transport partial melting of both crustal and mantle rocks as well as melt extraction processes resulting in magmatic arc crust growth based on our models the following five geodynamic regimes of subduction which may potentially form at active margins are identified i stable subduction with no backarc spreading centre and without plumes in the mantle wedge ii retreating subduction with a focused backarc spreading centre and without plumes iii retreating subduction with distributed intra arc extension and trans lithospheric sedimentary plumes iv advancing subduction with underplating laterally extending sub lithospheric plumes and v stable to advancing subduction with stationary laterally limited sub lithospheric plumes transitions between these different regimes are mainly caused by the concurrence of rheological weakening effects of i aqueous fluids percolating from the subducting slab into the mantle wedge and ii melts propagating from the partially molten areas formed in the mantle wedge towards the surface the aqueous fluids mainly affect the forearc region strong fluid related weakening promotes plate decoupling and reduces subduction drag and thus results in stacking of sediments in the accretion prism in contrast reduced weakening by fluids results in strong coupling of the plates and leads to advancing collision like subduction with enhanced subduction erosion thickening of the overriding plate and large sedimentary plumes in the mantle wedge are the consequences on the other hand melts extracted from the hot regions above the slab rheologically weaken the lithosphere below the arc which thus controls overriding plate extension and shortening strong rheological weakening by melts in combination with weak plate coupling trigger retreating subduction with a pronounced backarc spreading centre also weakening of the continental lithosphere by melts extracted from trans lithospheric sedimentary plumes generate a weak channel through which these structures may be emplaced into subarc crust if there is insufficient melt related weakening plumes cannot ascend but extend horizontally and thus underplate the lithosphere

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

mississippian volcanism in the south central andes new u pb shrimp zircon geochronology and whole rock geochemistry abstract in the northern extension of the famatina and the southern puna nw argentina prominent rhyolitic volcanic rocks traditionally referred to as ordovician are exposed resting on metamorphic basement and covered by thick late paleozoic siliciclastic successions we report new u pb shrimp ages from these rhyolites that show them to be of mississippian 348 342 ma age thus identifying a previously unknown volcanic event in this portion of western gondwana whole rock geochemistry and sr nd isotopic analyses suggest a crustal source for these rocks but with a juvenile input end t between 2 91 and 0 3 and tdm values between 1 09 and 1 1 ga this is different from the early paleozoic magmatism of western argentina where crustal recycling took place without any involvement of mantle material the carboniferous magmatism is compatible with an extensional environment developed along the terra australis accretionary orogen as a result of tectonic switching processes these rhyolites may be related to the coeval mississippian a type granites exposed to the east in the sierras pampeanas confirming the regional character of this magmatism

nd hf o isotopic evidence for subduction induced crustal replacement in ne china abstract convergent plate margins represent an ideal setting to understand the formation and evolution of the earth s crust nevertheless the geodynamic processes involved in crustal evolution remains highly debatable based on a large geochemical dataset of permian cretaceous granitic rocks from the yanbian area in ne china we propose a new geodynamic model involving crustal replacement to decipher crustal evolution during a superimposed multi stage subduction system these phanerozoic granitic rocks from two adjacent zones show contrasting hf nd isotopic variation trends with decreasing age of magmatism whole rock end t and zircon ehf t decrease gradually in the north granite zone ngz but show the opposite trends in the south granite zone sgz zircon 18o shows a slight rise in the ngz but it initially increases from permian to jurassic and then decreases to cretaceous in the sgz the nd hf o isotopic compositions of the cretaceous granitic rocks in both zones converge four crustal components are identified in the source of granites across the yanbian area including 1 juvenile crust 2 subduction accretionary complex 3 new arc crust and 4 precambrian crust the contributions made to the bulk crustal melting source of granites by the juvenile crust component in the ngz and by the precambrian crust in the sgz were both gradually decreased whereas the proportion of the new arc crust increased in both zones pre existing crustal components were extensively eroded destructed diluted and replaced by the new arc crust formed during paleo pacific slab subduction considering the prevalence of these two nd hf o isotopic variation trends in many orogenic belts subduction induced crustal replacement represents an important mechanism for crustal evolution in convergent plate margins

129i and 36cl in dilute hydrocarbon waters marine cosmogenic in situ and anthropogenic sources abstract the long lived halogen radioisotopes 129 i and 36 cl provide valuable information regarding the source of fluids in hydrocarbon systems and in localized areas where infiltration of younger meteoric water has occurred despite the utility of these two isotopes in providing time signatures for fluid end members considerable uncertainty remains regarding the interpretation of intermediate age waters in hydrologic systems these waters are likely the result of the combination of two or more halogen sources at some time in the past each with its own characteristic concentration and isotopic composition in order to unravel the evolution of these intermediate age waters the effect that infiltration of meteoric water has on the isotopic composition of older formation waters is modeled also evaluated is the effect that the timing of dilution has on 129 i and 36 cl signatures observed in the present specifically the hypothesis that halogen isotopic signatures imparted by the mixing of brine and meteoric waters early in the development of a sedimentary basin are quantitatively different from those imparted by the mixing of old brines with recent meteoric waters the modeled results are compared to previously published isotopic data from production wells in the fruitland formation coalbed methane system portions of the basin preserve enough of the original brine source to retain 129 i i ratios roughly recording the age of deposition of organic matter 73 76 ma even where some degree of dilution by meteoric water has occurred however the 129 i signatures of waters that presently contain less than 10 of the original formation water component indicate that mixing has occurred within the past 10 ma consistent with recently published 4 he data both 36 cl concentrations and 36 cl cl ratios show that waters containing more than 10 formation water are all in secular equilibrium with the host formation coals in contrast waters with less than 1 of the original formation water s cl content all have 36 cl cl ratios which are greater than secular equilibrium values indicating the migration of meteoric water into the coal formation more recently than 2 ma although 36 cl signatures in the fruitland formation are not significantly affected by anthropogenic input 129 i i ratios in a subset of samples that retain less than 1 of the initial formation water component suggest input of anthropogenic 129 i during the past 50 a

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

stagnant aquifer concept part 3 stagnant miniaquifers in the stage of formation makhtesh ramon israel abstract observation wells in the arid ramon national geological park were monitored at three sites c t and e each with wells spaced tens of meters apart the distances between the sites being 860 1060 m the following characteristics were observed 1 water level differences of up to a few meters occur between wells that are 20 60 m apart at each site and level differences of over 10 m occur between wells at the three sites 2 the water levels were steady over 2 years of monitoring 3 cl concentrations in the wells vary over three orders of magnitude 0 4 1 8 g 1 1 in site c 1 8 3 7 g 1 1 in site t and 2 5 13 g 1 t1 in site e 4 the relative abundance patterns of the dissolved ions vary remarkably along with the concentration variations from water dominated by ca and so 4 to water dominated by na and cl 5 the chemical composition is steady in each well as indicated by chemical analyses repeated over 6 years and conductivity monitoring over 2 years the observed significant and steady compositional differences are interpreted as indicating the existence of small scale isolated aquifers with no lateral flow between them in spite of the pronounced water level differences flow in fractures is ruled out because of the steady water levels and very slow water level recovery in pumped wells dating strongly confirmed the lack of flow in the investigated shallow system a well situated in the flood plain of nahal ramon revealed a tritium concentration of 8 tu and 14 c concentration of 69 p m c indicating post 1954 water but two wells outside the river bed contained no measurable tritium and 14 c values of 14 and 22 p m c indicating water ages in the order of 10 000 years in 15 additional wells no measurable tritium was determined thus a case of ancient groundwater trapped in shallow aquifers and bounded by efficient hydraulic barriers is demonstrated the geological set up seemed rather homogeneous with no clue to small scale efficient hydrological barriers yet a detailed geological study conveyed in the light of the hydrochemical findings revealed the existence of dykes and small scale structural and lithological controls coinciding with some of the hydraulic discontinuities defined by the water properties

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

tephra horizons identified in the western north atlantic and nordic seas during the last glacial period extending the marine tephra framework abstract geochemically distinct volcanic ash tephra deposits are increasingly acknowledged as a key geochronological tool to synchronize independent paleoclimate archives recent advances in the detection of invisible crypto tephra have led to the ongoing establishment development and integration of regional tephra lattices these frameworks offer an overview of the spatial extent of geochemically characterized tephra from dated eruptions a valuable tool for precise correlation of paleorecords within these areas here we harness cryptotephra analysis to investigate the occurrence of two well known tephra markers from the last glacial period i e fmaz ii 1 26 7 ka b2k and naaz ii ii rhy 1 55 3 ka b2k in marine sediment cores from the nordic irminger and labrador seas in addition we assess the imprint of bioturbation on two of these tephra deposits using computed tomography ct imagery we have successfully identified fmaz ii 1 in the nordic and irminger seas the tephra deposit is a visible deposit in the nordic seas whereas it appears as a single high concentration peak within the fine grained shard size fraction i e 25 80 m in the irminger sea both horizons are primary airfall deposits and this study is the first to identify a fmaz ii 1 deposit of isochronous nature in the irminger sea region in addition we have identified a new tephra horizon in the irminger sea which is stratigraphically associated with fmaz ii 1 and geochemically similar to the known 2 jpc 192 1 population we discuss its potential to serve as a new reference tie point for correlations in the region lastly we have successfully identified naaz ii ii rhy 1 of isochronous nature in both the irminger and labrador sea the layers are interpreted to be deposited by either direct airfall or by sea ice drifting past the sites compared to the existing frameworks which previously mainly focused on sites east of iceland our findings expand the knowledge and utility of the fmaz ii 1 and naaz ii ii rhy 1 horizons

a 200 ka geomagnetic chronostratigraphy for the labrador sea indirect correlation of the sediment record to specmap abstract a chronostratigraphic framework for the last 200 ka based on relative variations of geomagnetic paleointensity has been derived for the labrador sea where 18o stratigraphies are compromised by lack of benthic foraminifera and environmental perturbations of the planktic record relative paleointensity records derived from three labrador sea piston cores are correlated to a recently developed 200 ka paleointensity composite sint 200 y guyodo j p valet earth planet sci lett 143 1996 23 36 the chronology for sint 200 was derived by correlating oxygen isotope stratigraphies from individual paleointensity records to the specmap chronostratigraphy the correlation of sint 200 to labrador sea paleointensity records allows the specmap chronology to be imported into the labrador sea although comparisons of the chronostratigraphy based on paleointensity correlations with those derived from ams 14c dates and 18o data indicate significant discrepancies support for the new chronostratigraphic framework is provided by 1 consistent correlation of labrador sea detrital layers to north atlantic heinrich layers 2 general consistency between planktic 18o values and the specmap reference curve and 3 feasible prediction for the age of ash layer 2 as the new chronostratigraphy is tied to specmap labrador sea sediment cores can now be directly compared with any specmap dated record

norwegian sea tephrostratigraphy of marine isotope stages 4 and 5 prospects and problems for tephrochronology in the north atlantic region abstract this paper presents an account of the tephrostratigraphy of south eastern norwegian sea core md99 2289 for the interval spanning marine isotope stage mis 5 and part of mis 4 from 70 000 to 130 000 years ago tephra deposits from in the order of 100 mainly basaltic volcanic eruptions in iceland and on jan mayen are described in terms of their stratigraphic context and geochemical composition representing the most comprehensive record of iceland and jan mayen volcanism over this time span the most important transport mechanism of tephra to the core site is considered to have been iceberg rafting which is thought to be responsible for a centennial to millennial scale time lag between the volcanic eruption events and the subsequent deposition of tephra in deep sea sediments while this clearly limits the use of some of the tephras as isochronous markers twelve discrete tephra deposits show stratigraphical and geochemical characteristics that suggest them to have been deposited near contemporaneously with their source volcanic events and hence suitable candidates as regional isochrones four of these accords with the geochemical signatures of tephra deposits previously described from the nordic seas region potential problems for the application of tephrochronology highlighted by this work are discussed and recommendations for strategies to overcome them are proposed

devonian magmatism in the accretionary complex of southern chile supposed or potential devonian igneous rocks in the accretionary complex of southern chile were investigated using sensitive high resolution ion microprobe u pb dating of zircon with hf and o isotope analyses of selected grains ages of 384 3 and 382 2 ma are confirmed for two igneous bodies another having been previously dated at 397 1 ma detrital zircon ages in the host rocks some associated with devonian marine fossils indicate maximum possible sedimentation ages of c 330 385 ma devonian ages of 391 10 and 374 3 ma for plutonic rocks at the western edge of the north patagonian massif are somewhat older than those of orthogneisses in the western flank of the andes near chaiten 361 7 and 364 2 ma o and hf isotopes indicate that the devonian intrusions in the accretionary complex crystallized from mantle derived magmas whereas those in the north patagonian massif show a strong crustal influence corresponding to oceanic and continental margin subduction environments of magma genesis respectively devonian zircon provenance in the accretionary complex was from the north patagonian massif and not from the mantle derived intrusions suggesting that the accretionary complex formed an integral part of the gondwana margin during devonian carboniferous times supplementary material description of analytical methods and tables of isotope analytical data are available at http doi org 10 6084 m9 figshare c 2336728

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

dansgaard oeschger cycles interactions between ocean and sea ice intrinsic to the nordic seas dansgaard oeschger d o cycles are the most dramatic frequent and wide reaching abrupt climate changes in the geologic record on greenland d o cycles are characterized by an abrupt warming of 10 5 c from a cold stadial to a warm interstadial phase followed by gradual cooling before a rapid return to stadial conditions the mechanisms responsible for these millennial cycles are not fully understood but are widely thought to involve abrupt changes in atlantic meridional overturning circulation due to freshwater perturbations here we present a new high resolution multiproxy marine sediment core monitoring changes in the warm atlantic inflow to the nordic seas as well as in local sea ice cover and influx of ice rafted debris in contrast to previous studies the freshwater input is found to be coincident with warm interstadials on greenland and has a fennoscandian rather than laurentide source furthermore the data suggest a different thermohaline structure for the nordic seas during cold stadials in which relatively warm atlantic water circulates beneath a fresh surface layer and the presence of sea ice is inferred from benthic oxygen isotopes this implies a delicate balance between the warm subsurface atlantic water and fresh surface layer with the possibility of abrupt changes in sea ice cover and suggests a novel mechanism for the abrupt d o events observed in greenland ice cores

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

geodynamic evolution and tectonostratigraphic terranes of northwestern argentina and northern chile in ordovician time gondwana in the area of northwestern argentina and northern chile had a west facing active margin the evolution of this margin culminated in the ocloyic orogeny at the end of ordovician time this orogeny was caused by the collision of the allochthonous arequipa antofalla terrane with this margin the early paleozoic evolution of northwestern argentina and northern chile contrasts markedly with the accretionary history of central argentina and central chile where the precordillera and chilenia terranes docked in the late ordovician and late devonian periods respectively an inspection of the available stratigraphic and geochronological data on sedimentary volcanic and plutonic units of the southern central andes of northern chile and northwestern argentina reveals a lull in magmatic and metamorphic activity lasting for 100 m y from early silurian to early late carboniferous time this is interpreted as corresponding to a tectonic scenario in which the present andean margin was a passive margin of gondwana this passive margin developed in response to the rifting off of a part of the arequipa antofalla terrane the present location of this block is unknown late carboniferous time marks the renewed onset of subduction initiating the andean plate tectonic setting still prevalent today recently proposed models explain the late ordovician orogeny by the collision of laurentia with western south america during laurentia s clockwise motion around south america and away from its position in the neoproterozoic supercontinent in its present form this hypothesis is difficult to reconcile with the paleozoic tectonostratigraphic evolution of the southern central andean region

tephrochronology and the extended intimate integration of ice core marine and terrestrial records event stratigraphy 8 128 ka b2k the comparison of palaeoclimate records on their own independent timescales is central to the work of the intimate integrating ice core marine and terrestrial records network for the north atlantic region an event stratigraphy has been established from the high precision greenland ice core records and the integrated gicc05 chronology this stratotype provides a palaeoclimate signal to which the timing and nature of palaeoenvironmental change recorded in marine and terrestrial archives can be compared to facilitate this wider comparison without assuming synchroneity of climatic change proxy response intimate has also focussed on the development of tools to achieve this in particular the use of time parallel marker horizons e g tephra layers volcanic ash coupled with the recent temporal extension of the greenland stratotype as part of this special issue we present an updated intimate event stratigraphy highlighting key tephra horizons used for correlation across europe and the north atlantic we discuss the advantages of such an approach and the key challenges for the further integration of terrestrial palaeoenvironmental records with those from ice cores and the marine realm

early permian to late triassic batholiths of the chilean frontal cordillera 28 31 s shrimp u pb zircon ages and lu hf and o isotope systematics abstract one of the major geological units of the main andean range frontal cordillera of north central chile is a group of composite and heterochronous late paleozoic early mesozoic batholiths that extends for 500 km roughly ns along from 26 to 31 s ten new shrimp zircon crystallization ages together with 11 recently published u pb zircon ages by other authors indicate an episodic intrusion history which can be divided in 4 groups mississippian earliest carboniferous 330 326 ma cisuralian earliest permian 301 284 ma latest permian to middle triassic 264 242 ma and late triassic 225 215 ma volcanic rocks in the area span a similar time lu hf and o isotopic systematics in zircon grains from eight of the plutonic rocks indicate the magma source areas have contributed variable amounts of crustal and mantle components zircon 18 o values evolve from crustal values 7 in the earliest permian intrusives to mantle values in the latest permian to upper triassic including evidence for likely hydrothermal alteration of the source 4 zircon ehf values vary in a good linear correlation with the 18 o isotopes from 6 to 0 in rocks older than 270 ma increasing to 2 to 7 from lower to upper triassic 250 to 215 ma the petrogenetic constraints indicated by these values suggest that the influence of magma sources varied with time from predominantly crustal to mantle like in accord with the regional tectonic models the earliest permian rocks were generated in a subduction related magmatic arc which varied towards an extension related environment in the latest permian and triassic

chlorine 36 dating of very old groundwater 1 the great artesian basin australia chlorine 36 has many advantages as a dating tool for very old groundwater these advantages include a suitable half life 3 01 x 10 sup 5 years simple geochemistry conservative behavior in groundwater and a general absence of subsurface sources at levels comparable to the atmospheric input recent advances in tandem accelerator mass spectrometry have permitted the analysis of sup 36 cl at low abundance expected following residence in the subsurface for 10 sup 6 years or more in order to test the suitability of sup 36 cl for dating very old groundwater the sup 36 cl cl ratios of 26 groundwater samples from the great artesian basin of australia have been measured groundwater ages calculated from the sup 36 cl data compare favorably with ages computed independently from hydrodynamic simulations

the relationships between tectonics climate and exhumation in the central andes 18 36 s evidence from low temperature thermochronology abstract the central andes between 18 s and 36 s latitude strike north south for 2000 km along the chilean subduction margin cross several climate zones from hyperarid to humid and exhibit mean elevations in excess of 4000 m a s l here we investigate the relationships between tectonics climate and exhumation by inverting low temperature thermochronological data compiled from the literature 824 ages from 549 samples and new data 238 ages from 146 samples to quantify the exhumation rate history of the central andes since 80 ma our inferred exhumation rates west of the drainage divide and between 18 and 32 s did not exceed 0 25 km ma such low exhumation rates are consistent with low shortening rates and arid conditions in this region local pulses of exhumation occurred only during the eocene as response to active deformation and during the miocene probably as response to uplift of the western andean slope east of the drainage divide between 18 and 28 s the observed exhumation pattern reflects the onset and eastward propagation of deformation here exhumation occurred locally since the middle to late eocene in the eastern cordillera and the altiplano puna and subsequently affected larger parts of these regions and the north western sierra pampeanas during the oligocene in the early miocene 20 ma the interandean zone started exhuming and at 12 10 ma exhumation propagated into the subandean zone enhanced shortening rates and intensified precipitation along the eastern deformation front associated with the onset of the south american monsoon led to increased exhumation rates in the eastern interandean and the subandean zones in the plio pleistocene 0 6 km ma higher pleistocene exhumation rates are also observed in the northern sierra pampeanas 1 5 km ma that can be related to rock uplift along steep reverse faults coupled with high precipitation south of 32 s on the western side exhumation rates in the principal cordillera increased from ca 0 25 km ma in the miocene to rates locally exceeding 2 km ma in the pleistocene whereas the tectonic regime in the southern principal cordillera remained unchanged since the late miocene these higher rates are likely associated with enhanced erosion resulting from intensified pleistocene precipitation and glacial growth in this region reinforced by isostatic rock uplift and active tectonics our study shows that the onset of exhumation correlates mainly with the initiation of horizontal shortening and crustal thickening whereas the magnitude of exhumation is largely set by the amount of precipitation and glacial erosion and by the style of deformation which is controlled by inherited structures and the amount of sediments in the foreland

the greenland ice core chronology 2005 15 42 ka part 2 comparison to other records a new greenland ice core chronology gicc05 based on multi parameter counting of annual layers has been obtained for the last 42 ka here we compare the glacial part of the new time scale which is based entirely on records from the northgrip ice core to existing time scales and reference horizons covering the same period these include the grip and northgrip modelled time scales the meese sowers gisp2 counted time scale the shackleton fairbanks grip time scale sfcp04 based on 14c calibration of a marine core the hulu cave record three volcanic reference horizons and the laschamp geomagnetic excursion event occurring around greenland interstadial 10 gicc05 is generally in good long term agreement with the existing greenland ice core chronologies and with the hulu cave record but on shorter time scales there are significant discrepancies around the last glacial maximum there is a more than 1 ka age difference between gicc05 and sfcp04 and a more than 0 5 ka discrepancy in the same direction between gicc05 and the age of a recently identified tephra layer in the northgrip ice core both sfcp04 and the tephra age are based on 14c dated marine cores and fixed marine reservoir ages for the laschamp event gicc05 agrees with a recent independent dating within the uncertainties

the crustal evolution of south america from a zircon hf isotope perspective hf isotope data of 1100 detrital zircon grains from the palaeozoic south central andean gondwana margin record the complete crustal evolution of south america which was the predominant source the oldest grains with crustal residence ages of 3 8 4 0ga are consistent with complete recycling of existing continental crust around 4ga we confirm three major archaean palaeoproterozoic transamazonian and late mesoproterozoic to early neoproterozoic crust addition phases as well as six igneous phases during proterozoic to palaeozoic time involving mixing of juvenile and crustally reworked material a late mesoproterozoic to early neoproterozoic grenville age igneous belt can be postulated along the palaeo margin of south america this belt was the basement for later magmatic arcs and accreted allochthonous microcontinents as recorded by similar crustal residence ages crustal reworking likely dominated over juvenile addition during the palaeozoic era and proterozoic and archaean zircon was mainly crustally reworked from the eroding thickened ordovician famatinian arc

identification of cryptotephra horizons in a north east atlantic marine record spanning marine isotope stages 4 and 5a 60 000 82 000 a b2k abstract tephrochronology is one of the few techniques that offer considerable but as yet largely unrealised potential for the correlation of north atlantic palaeoclimatic archives spanning the mis 4 and 5a substage period a phase of pronounced global cooling during the initiation of the last glacial period in this investigation two previously unidentified rhyolitic tephra horizons believed to be sourced from iceland were isolated within the giant piston core md04 2822 retrieved from the rockall trough north east atlantic one horizon was deposited at the beginning of greenland stadial 19 70 150 a b2k and the other around the cooling transition at the end of greenland interstadial 20 75 320 a b2k density separation epma and la icp ms techniques were utilised for the identification of these cryptotephra within the fine fraction sediment

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

tracing marine cryptotephras in the north atlantic during the last glacial period improving the north atlantic marine tephrostratigraphic framework tephrochronology is increasingly being recognised as a key tool for the correlation of disparate palaeoclimatic archives underpinning chronological models and facilitating climatically independent comparisons of climate proxies tephra frameworks integrating both distal and proximal tephra occurrences are essential to these investigations providing key details on their spatial distributions geochemical signatures eruptive sources as well as any available chronological and or stratigraphic information frameworks also help to avoid mis correlation of horizons and provide important information on volcanic history here we present a comprehensive chronostratigraphic framework of 14 tephra horizons from north atlantic marine sequences spanning 60 25 cal ka bp horizons previously discovered as visible or coarse grained deposits have been combined with 11 newly recognised volcanic deposits identified through the application of cryptotephra identification and characterisation methods to a wide network of marine sequences their isochronous integrity has been assessed using their physical characteristics all horizons originated from iceland with the vast majority having a basaltic composition sourced from the grimsvotn kverkfjoll hekla vatnafjoll and katla volcanic systems new occurrences improved stratigraphic placements and a refinement of the geochemical signature of the naaz ii are reported and the range of the fmaz iv has been extended in addition several significant geochemical populations that further investigations could show to be isochronous are reported this tephra framework provides the foundation for the correlation and synchronisation of these marine records to the greenland ice cores and european terrestrial records to investigate the phasing rate timing and mechanisms controlling rapid climate changes that characterised the last glacial period

the synchronization of palaeoclimatic events in the north atlantic region during greenland stadial 3 ca 27 5 to 23 3 kyr b2k two high resolution marine sediment cores located 83 km apart in the ne atlantic have been studied md95 2006 barra fan 57 01 82 n 10 03 48 w 2120 m water depth and md04 2822 rockall trough 56 50 54 n 11 22 96 w 2344 m water depth the records are anchored to the ngrip ice core stratigraphy and gicc05 chronology by the presence of geochemically characterized fugloyarbanki tephra and further tested by radiocarbon age control replicated sea surface temperature sst records show evidence for an abrupt and short lived warming within greenland stadial gs 3 to which we tentatively assign an age of ca 25 6 24 8 kyr gicc05 b2k based on these and another marine palaeoclimate record link17 from the faroe shetland channel we propose a new three fold event stratigraphy for gs 3 within the north east atlantic region the recognition of this warming event within gs 3 in ne atlantic sst records demonstrates that such events may not readily be identified within the coldest stadials of the greenland ice cores highlights the need for carefully constructed event stratigraphies independently tested by the use of tephra isochrones and radiocarbon and illustrates pervasive millennial scale climate variability of the north atlantic region dansgaard oeschger d o events is deeply embedded in the dynamics of atlantic meridional overturning circulation amoc

visualizing tephra deposits and sedimentary processes in the marine environment the potential of x ray microtomography localized tephra deposition in marine sequences is the product of many complex primary and secondary depositional processes these can significantly influence the potential applicability of tephra deposits as isochronous marker horizons and current techniques used in isolation may be insufficient to fully unravel these processes here we demonstrate the innovative application of x ray microtomography ct to successfully identify tephra deposits preserved within marine sediments and use these parameters to reconstruct their internal three dimensional structure three dimensional visualizations and animations of tephra dispersal in the sediment permit a more thorough assessment of postdepositional processes revealing a number of complex microsedimentological features that are not revealed by conventional methods these features include bioturbation burrows and horizontally discontinuous tephra packages which have important ramifications for the stratigraphic placement of the isochron in a sedimentary sequence our results demonstrate the potential for utilizing rigorous two and three dimensional microsedimentological analysis of the ichnofabric to enhance and support the use of tephra deposits as isochronous marker horizons and to identify the stratigraphic position that best reflects the primary fallout of ash the application also provides an exceptional insight into the style and rate of sedimentation processes and permits an assessment of the stratigraphic integrity of a tephra deposit we discuss the possibility of applying these ct methods to the identification of cryptotephras within various paleoclimatic sequences and to enhance our understanding of marine sedimentation processes

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

sources and evolution of arc magmas inferred from coupled o and hf isotope systematics of plutonic zircons from the cretaceous separation point suite new zealand abstract coupled o and hf isotopic compositions of zircons from early cretaceous 113 124 ma granitoids of the separation point suite sps new zealand obtained by cathodoluminescence imaging guided micro beam methods sims la icpms are used as a record of evolving magma compositions in a prominent mesozoic arc system eight representative sps samples from individual plutons in the nelson and fiordland regions yield magmatic zircons with initial hf isotope ratios expressed in e hf ranging from 4 to 11 nelson and 5 to 12 fiordland respectively initial hf isotope ratios of zircons are extremely heterogeneous within individual samples with the vast majority of values distinctly less radiogenic than depleted mantle at 120 ma e hf 16 o isotope ratios are likewise variable with 18 o zircon smow values of 2 8 nelson and 0 7 fiordland the within rock variability in both zircon hf and o isotope ratios is testimony to open system processes that operated during magma evolution and zircon crystallisation average 18 o zircon for cores and rims allow constraints to be placed on the o isotopic composition of magmas from which zircon precipitated 18 o magma 4 8 elevated 18 o magma 6 5 require involvement of 18 o enriched supracrustal material weathered continental crust or low t seawater hydrothermally altered oceanic crust while 18 o magma 18 o depleted crustal component whole rock sr isotope nb ta and nd pb systematics are inconsistent with 18 o depleted slab melts 18 o 0 6 as a source component for sps magmas instead low 18 o values suggest incorporation of high t meteoric hydrothermally altered country rocks similar to those of the largs terrane presently exposed in northern fiordland in diagrams of sio 2 versus inferred 18 o magma the most primitive sps samples from the nelson and fiordland regions plot close to the expected composition for primitive arc magmas more evolved granitoids however show strongly divergent trends of o isotope composition as a function of sio 2 suggestive of assimilation coupled to fractional crystallisation afc in contrasting crustal environments emplacement level contamination by local crust is supported by age distributions of inherited zircons which indicate a predominance of palaeozoic and mesozoic zircons in nelson and fiordland granitoids respectively

fallout tuffs of trapper creek idaho a record of miocene explosive volcanism in the snake river plain volcanic province a 900 m thick section of tuffaceous sedimentary rock vitric fallout tuff and ash flow tuff is well exposed along trapper creek in south central idaho this section provides nearly continuous exposure through the fill of the goose creek basin a major north trending miocene extensional basin located along the southern margin of the snake river plain volcanic province srpvp some 51 separate units of vitric fallout tuff are recognized in the trapper creek section petrographic and chemical characteristics of these vitric tuffs indicate that most are from srpvp sources new 40ar 39ar laser fusion dating along with prior isotopic age determinations show that the trapper creek tuffs span the period ca 13 9 8 6 ma chemical correlation indicates that fallout tuffs in the central part of the trapper creek section 12 5 10 0 ma are from sources in the bruneau jarbidge volcanic field of the srpvp centered 100 km west of trapper creek underlying fallout tuffs may have had sources in the owyhee humboldt field of the srpvp centered 200 km west of trapper creek while overlying fallout tuffs interlayered with several ash flow tuffs had a relatively proximal source possibly in the proposed twin falls volcanic field centered 60 km north of trapper creek the trapper creek tuffs provide insight into the characteristics of explosive silicic volcanism within the srpvp during middle late miocene time from ca 13 9 to ca 9 5 ma major eruptions those depositing 1 5 m of fallout tuff were frequent about one event per 200 k y their products display a trend toward the eruption of progressively less evolved higher temperature silicic magma after 12 5 ma this trend to higher temperature eruptions termed the cougar point flare up culminated in the eruption of high temperature 1000 c plagioclase rich magma during the period 10 5 9 5 ma in contrast to these eruptions later 7 0 ma major silicic eruptions within the srpvp were characterized by the lower temperature 850 c of the erupted magma and by the longer intervals about one event per 500 600 k y between eruptions variations in the character of srpvp explosive silicic eruptions may reflect changes in the structure composition or state of stress in the crust beneath the eastward propagating srpvp or perhaps changes in the yellowstone hot spot plume that may drive the srpvp volcanism

cryptotephras the revolution in correlation and precision dating from its icelandic origins in the study of visible tephra horizons tephrochronology took a remarkable step in the late 1980 s with the discovery of a ca 4300 year old microscopic ash layer in a scottish peat bog since then the search for these cryptotephra deposits in distal areas has gone from strength to strength indeed a recent discovery demonstrates how a few fine grained glass shards from an alaskan eruption have been dispersed more than 7000 km to northern europe instantaneous deposition of geochemically distinct volcanic ash over such large geographical areas gives rise to a powerful correlation tool with considerable potential for addressing a range of scientific questions a prerequisite of this work is the establishment of regional tephrochronological frameworks that include well constrained age estimates and robust geochemical signatures for each deposit with distal sites revealing a complex record of previously unknown volcanic events frameworks are regularly revised and it has become apparent that some closely timed eruptions have similar geochemical signatures the search for unique and robust geochemical fingerprints thus hinges on rigorous analysis by electron microprobe and laser ablation inductively coupled plasma mass spectrometry historical developments and significant breakthroughs are presented to chart the revolution in correlation and precision dating over the last 50 years using tephrochronology and cryptotephrochronology

lower carboniferous post orogenic granites in central eastern sierra de velasco sierras pampeanas argentina u pb monazite geochronology geochemistry and sr nd isotopes the central eastern part of the sierra de velasco sierras pampeanas nw argentina is formed by the large huaco 40 30 km and sanagasta 25 15 km granite massifs and the small la chinchilla stock 2 2 km the larger granites intrude into ordovician metagranitoids and crosscut devonian mylonitic shear zones whereas the small stock sharply intrudes into the huaco granite the two voluminous granites are biotitic muscovitic and biotitic porphyritic syeno to monzogranites they contain small and rounded tonalitic and quartz dioritic mafic microgranular enclaves the small stock is an equigranular zinnwaldite and fluorite bearing monzogranite the studied granites are silica rich sio2 70 potassium rich k2o 4 ferroan alkali calcic to slightly calk alkalic and moderately to weakly peraluminous a cnk 1 06 1 18 huaco granite 1 01 1 09 sanagasta granite 1 05 1 06 la chinchilla stock they have moderate to strong enrichments in several lil li rb cs and hfs nb ta y th u elements and low sr ba and eu contents u pb monazite age determinations indicate lower carboniferous crystallization ages 350 358 ma for the huaco granite 352 7 1 4 ma for the sanagasta granite and 344 5 1 4 ma for the la chinchilla stock the larger granites have similar nd values between 2 1 and 4 3 whereas the younger stock has higher nd of 0 6 to 1 4 roughly comparable to the values obtained for the carboniferous san blas granite 1 4 to 1 7 located in the north of the sierra the huaco and sanagasta granites have a mainly crustal source but with some participation of a more primitive possibly mantle derived component the main crustal component can be attributed to ordovician peraluminous metagranitoids the la chinchilla stock derives from a more primitive source suggesting an increase with time in the participation of the primitive component during magma genesis the studied granites were generated during a post orogenic period in a within plate setting possibly as a response to the collapse of the previous famatinian orogen extension of the crust and mantle upwelling they are part of the group of middle devonian lower carboniferous granites of the sierras pampeanas the distribution and u pb ages of these granites suggests a northward arc parallel migration of this mainly post orogenic magmatism with time

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

correlation of late quaternary tephra layers in a long pluvial sequence near summer lake oregon abstract near summer lake in southern oregon 54 tephra beds of late quaternary age are exposed in pluvial lake sediments of lake chewaucan seven of the tephra beds near the top can be correlated with tephra deposits younger than 117 000 yr at mount st helens washington at crater lake oregon and in northwestern nevada in the deposits of pluvial lake lahontan however most of the section at summer lake lies below the correlated units and contains 39 tephra beds older than 117 000 yr major element chemistry of tephra glasses was determined by electron microprobe analysis petrography supports the correlations made from chemical evidence compositions correlated range from 70 to 76 sio 2 the least silicic summer lake glass contained 57 extrapolation of depositional rate suggests that most of the sediments at summer lake are younger than about 335 000 yr but older lake beds containing tephra layers occur at one place the long lacustrine record suggests that lake chewaucan persisted through the last interpluvial stage and that the lake may have dried up at the end of the pleistocene due to diversion of the chewaucan river by relict shore features

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

age extent and climatic significance of the c 3400 bp aniakchak tephra western alaska usa tephra deposits at whitefish lake and cape espenberg on the northernmost seward peninsula of western alaska were derived from the aniakchak caldera on the alaska peninsula more than 1500 km to the south radiocarbon dates on the climactic caldera forming aniakchak eruption and on proximal and distal tephra indicate the aniakchak tephra was deposited about 3435 40 bp 3614 3815 cal bp and forms an isochronous marker horizon across much of western alaska the aniakchak tephra eruption and several other targe eruptions during the seventeenth century bc may have had widespread effects on climate

late paleozoic early triassic magmatism on the western margin of gondwana collahuasi area northern chile abstract the basement in the altiplano high plateau of the andes of northern chile mostly consists of late paleozoic to early triassic felsic igneous rocks collahuasi group that were emplaced and extruded along the western margin of the gondwana supercontinent this igneous suite crops out in the collahuasi area and forms the backbone of most of the high andes from latitude 20 to 22 s rocks of the collahuasi group and correlative formations form an extensive belt of volcanic and subvolcanic rocks throughout the main andes of chile the frontal cordillera of argentina choiyoi group or choiyoi granite rhyolite province and the eastern cordillera of peru thirteen new shrimp u pb zircon ages from the collahuasi area document a bimodal timing for magmatism with a dominant peak at about 300 ma and a less significant one at 244 ma copper mo porphyry mineralization is related to the younger igneous event initial hf isotopic ratios for the 300 ma zircons range from about 2 to 6 indicating that the magmas incorporated components with a significant crustal residence time the 244 ma magmas were derived from a less enriched source with the initial hf values ranging from 2 to 6 suggestive of a mixture with a more depleted component limited whole rock 144nd 143nd and 87sr 86sr isotopic ratios further support the likelihood that the collahuasi group magmatism incorporated significant older crustal components or at least a mixture of crustal sources with more and less evolved isotopic signatures

late paleozoic to jurassic silicic magmatism at the gondwana margin analogy to the middle proterozoic in north america a vast region of upper paleozoic to middle jurassic 300 150 ma silicic magmatic rocks that erupted inboard of the gondwana margin is a possible phanerozoic analogue to the extensive middle proterozoic 1500 1350 ma silicic magmatic province that underlies much of the southern mid continent of north america like the north american rocks the gondwana silicic magmas appear to be melts of crust that formed about 200 300 m y earlier in the north american case this older crust formed and was accreted to the continent during a major period of crustal formation 1700 1900 ma whereas in the gondwana case the crust that melted consisted mainly of magmatic are terranes accreted to the continental margin during the paleozoic in both cases basic to intermediate magmatic rocks are extremely rare and magmatism is less abundant in regions that contain older and previously melted crust the similarities between the north american and gondwana silicic rocks suggest that both suites formed in extensional settings where basaltic magmas ponded at the base of the preheated crust caused extensive crustal melting that inhibited upward passage of the basalts in both cases silicic volcanism occurred after major assembly of a supercontinent by subduction and accretion processes and before breakup of the supercontinent by analogy with the polar wander curves for gondwana the granite rhyolite provinces may have formed during a period of very slow motion of the supercontinents relative to the poles

early carboniferous sub to mid alkaline magmatism in the eastern sierras pampeanas nw argentina a record of crustal growth by the incorporation of mantle derived material in an extensional setting a recently discovered granitic intrusion at cerro la gloria in western sierra de famatina nw argentina is representative of sub to mid alkaline carboniferous magmatism in the region the main rock type consists of microcline quartz and plagioclase with amphibole magnetite ilmenite biotite epidote zircon allanite and sphene as accessory minerals we report a u pb zircon shrimp age for the pluton of 349 3 ma mswd 1 1 i e tournaisian whole rock chemical composition and nd isotope analyses are compatible with an origin by melting of older mafic material in the lower crust endt between 0 58 and 0 46 and tdm values of about 1 1 ga the pluton is intruded by penecontemporaneous to late alkaline mafic dykes that are classified as back arc basalts coeval early carboniferous a type granites occur farther east in the sierras pampeanas probably generated during lithospheric stretching overall the early carboniferous granitic rocks show a west to east mineralogical and isotopic zonation indicating that magma genesis involved a greater contribution of juvenile material of mantle character to the west based on the observed patterns of geochronology geochemistry and field relationships we suggest that a type magma genesis in the eastern sierras pampeanas was linked to an andean type margin where the lithospheric mantle played a role in its generation

quaternary sedimentation and norwegian sea overflow pathways around bill bailey bank northeastern atlantic abstract using high resolution multi channel sleeve gun seismics and tobi side scan sonar and subbottom profiler acoustic data as well as sediment cores and published hydrographic measurements a study has been made of quaternary sedimentation patterns and depositional processes around bill bailey bank bbb in the northeastern atlantic from the western outlet of the faeroe bank fb channel which is the main conduit of norwegian sea overflow water nsow there is a complex pattern of nsow controlled sediment transport pathways extending to areas north of lousy bank lb the acoustic data and sediment cores reveal the existence of a contour current at water depths exceeding 1000 m flowing from the wyville thomson ridge via the southern rise of fb and bbb into the valley between bbb and lb and further northward towards the area north of bbb we conclude that this contour current is related to intermittent nsow overflow of the wyville thomson ridge the maximum bottom flow speed along this secondary nsow flow pathway is inferred to reach about 0 50 m s bedforms observed in the northeastern part of the study area north of bbb suggest a maximum bottom flow speed of locally at least 1 0 m s the maximum thickness 0 5 s twt of the pliocene quaternary seismic sequence at water depths in excess of 1000 m south of bbb is nearly twice that found north of bbb on average higher accumulation rates in the former area can be ascribed to a hydrographic setting favouring sediment accumulation which especially under glacial conditions markedly differed from conditions north of bbb this applies to both bottom and surface circulation linear bulk sedimentation rates north of bbb were largest during deglacial warming when the rates were about three times higher than the average holocene rate 4 5 cm ka increased fluxes during deglaciation reflect transport and depositional conditions associated with reactivation of the nsow current system south of bbb average linear bulk accumulation rates were persistently higher i e in the range 15 23 cm ka during various late pleistocene stages between 30 and 10 ka

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

the tephrochronology of iceland and the north atlantic region during the middle and late quaternary a review the tephrochronology of iceland and the north atlantic region is reviewed in order to construct a unified framework for the last 400 kyr bp nearly all of the tephra layers described are also characterised geochemically a number of new tephra layers are analysed for the first time for their geochemical signature and a number of pre holocene tephra layers have been given an informal denotation the tephrostratigraphy of ash zone ii is highlighted where possible the rhyolitic tephra layers found outside iceland have been correlated to known icelandic tephra layers or to the volcanic source area the application of tephra fallout in various depositional environments is described and discussed copyright 2000 john wiley sons ltd

tectono stratigraphic evolution of the andean orogen between 31 and 37 s chile and western argentina fil charrier reynaldo universidad de chile facultad de ciencias fisicas y matematicas departamento de geologia chile universidad andres bello chile

lithospheric evolution of the pre and early andean convergent margin chile abstract the proto andean and early andean evolution of the southwestern gondwana margin comprises three stages that differ in their magmatic evolution and deformational style the gondwana cycle 330 280 ma the pre andean stage 280 210 ma and the early andean cycle 210 100 ma these stages have been traditionally interpreted as the upper crustal response to changes in the tectonic setting which include cordilleran style continental arc gondwana cycle orogenic collapse and possibly slab break off that led to continental rifting and extensive crustal melting pre andean stage and subsequent subduction re initiation in oceanic arc style context early andean cycle the petrological and geochemical characteristics of carboniferous to jurassic igneous rocks from this region however do not support the described model elemental and sr nd pb isotopic data of 86 samples along with a compilation of 1230 samples from the literature suggest that subduction was the most likely process by which the magmatic record was generated sub alkaline affinities lile enrichment over hfse nb ta troughs porphyritic textures and hornblende and biotite bearing lithologies are present in all studied units whereas isotopes suggest that magma sources are a mixture of depleted mantle and variable contribution from the continental crust even though the aforementioned features are common to all igneous rocks some changes point to a decline in the contribution of crustal lithospheric sources to the magmatism with time thus sio2 lan ybn and 87sr 86srinitial exhibit a systematic decrease from 285 to 150 ma whereas the endinitial parameter increases in the same period these changes were accompanied by the shift from dominant compressional carboniferous early permian to transtentional middle permian jurassic stresses in the upper crust suggesting that the margin went from advancing to retreating due to pangea reorganization and break up following a potential flat slab event slab roll back may have induced extension in the upper crust and lithospheric loss as a consequence of delamination or thermal erosion

geodynamics of late carboniferous early permian forearc in north chile 28 30 29 30 s a large section of the late palaeozoic forearc is exposed along the coastal ranges of north central chile 28 29 30 s this is characterized by three lithotectonic units 1 the punta de choros metamorphic complex basal accretion series composed mostly of micaschists and metabasites 2 the chanaral epimetamorphic complex frontal accretion series formed by metaturbidites and metasediments 3 the llano del chocolate beds forearc basin deposits composed of a sedimentary sequence of clastic sedimentary rocks with minor limestones and acidic volcanic rocks within the basal accretion series two distinctive blocks of garnet bearing schists with amphibolite facies metamorphism have been preserved recording early stages of the subduction system the stratigraphic record and the u pb dating of igneous 291 318 ma and detrital zircons maximal deposition ages between 273 and 292 ma in the forearc basin deposits coupled with 40 ar 39 ar ages for metamorphic rocks 319 280 ma indicate that forearc sedimentation was broadly contemporaneous with metamorphism and exhumation of the basal accretion series the radiometric dating on garnet bearing rocks indicates that the onset of the subduction system took place in the mississippian at a high thermal gradient these conditions were responsible for generation of limited volumes of acidic melt now recognized as acidic domes and tuffs within the forearc basin stratigraphy supplementary material tables of sedimentary facies u pb geochronological analyses and ar ar geochronological data are available at http doi org 10 6084 m9 figshare c 2868307

a continuous 5300 yr holocene cryptotephrostratigraphic record from northern new zealand and implications for tephrochronology and volcanic hazard assessment a continuous 5280 calendar cal yr long cryptotephrostratigraphic record of a peat core from northern new zealand demonstrates that cryptotephra studies can enhance conventional tephra records by extending the known distribution of ash fall and enabling re assessment of volcanic hazards a systematic sampling strategy was used to locate peaks in glass shard concentrations and to determine loci of individual geochemical populations and a palynological method involving spiking samples with lycopodium spores was adapted to facilitate accurate counting of glass shard concentrations using glass shard major element compositions and a core chronology based on eight ams 14c ages and two visible macroscopic tephra layers taupo tephra unit y 1688 1748 cal bp and tuhua tephra 6800 7230 cal bp 2cr age ranges four cryptotephras were correlated with known eruptions whakaipo unit v 2743 2782 cal bp stent unit q 4240 4510 cal bp and unit k 4970 5290 cal bp erupted from taupo volcanic ce