quiescent explosive transitions during dome forming volcanic eruptions using seismicity to probe the volcanic processes leading to the 29 july 2008 vulcanian explosion of soufriere hills volcano montserrat

absolute timing of sulfide and gold mineralization a comparison of re os molybdenite and ar ar mica methods from the tintina gold belt alaska new re os molybdenite dates from two lode gold deposits of the tintina gold belt alaska provide direct timing constraints for sulfide and gold mineralization at fort knox the re os molybdenite date is identical to the u pb zircon age for the host intrusion supporting an intrusive related origin for the deposit however 40ar 39ar dates from hydrothermal and igneous mica are considerably younger at the pogo deposit re os molybdenite dates are also much older than 40ar 39ar dates from hydrothermal mica but dissimilar to the age of local granites these age relationships indicate that the re os molybdenite method records the timing of sulfide and gold mineralization whereas much younger 40ar 39ar dates are affected by post ore thermal events slow cooling and or systemic analytical effects the results of this study complement a growing body of evidence to indicate that the re os chronometer in molybdenite can be an accurate and robust tool for establishing timing relations in ore systems

formation and palaeozoic evolution of the gorny altai altai mongolia suture zone south siberia zircon u pb constraints on the igneous record abstract the altai sayan fold belt epitomises how phanerozoic island arc continental crust contributed to the growth of the palaeo eurasian continent its formation is dominated by the closure of the palaeo asian ocean pao when island arc systems and gondwana derived terranes accreted to the siberian margin relics of the pao related tectonic units and associated granitoids occur along the ophiolitic charysh terekta ulagan sayan suture ctuss between gorny altai and altai mongolia zircon la icp ms u pb dating of this igneous record constrains the multi stage geodynamic pao evolution primitive kuznetsk altai island arc crust formed at the siberian margin during the ediacaran early cambrian 525 555 ma this island arc matured during the middle late cambrian 510 ma and was consumed by pao subduction in the late cambrian early ordovician 480 490 ma forming the siberian early caledonian accretion collision belt south of the ctuss granitic magmatism occurred within gondwana derived altai mongolia during the middle late ordovician 450 470 ma palaeo kazakhstan assembly and during a silurian early devonian 400 425 ma andean type collision middle late devonian 360 395 ma granitoids were emplaced as a result of the collision of altai mongolia with siberia during final pangaea amalgamation the suture was strike slip reactivated associated magmatism 295 ma the youngest 220 255 ma sampled granitoids originated in an intra plate setting

a multivariate mixing model for identifying sediment source from magnetic measurements a sequential method for quantitative identification of sediment source components based on magnetic measurements has been developed and tested for sediments from the rhode river maryland simulated mixing tests and multiple regression were employed to establish numerical relationships between source component proportions and the magnetic measurements of mixtures on the basis of these multivariate mixing models source components of three estuarine sediment cores were estimated by linear programming the results strongly support the previous studies on this catchment which indicated a dramatic change in sediment source some 150 to 200 yr ago quantitative calculations are more useful and informative than purely qualitative descriptions

compositional variability of ice rafted debris in heinrich layers 1 and 2 on the northwest european continental slope identified by environmental magnetic analyses the composition of ice rafted debris ird within a sediment core from the european continental slope core omex 2k 49 5 n 13 26 w has been examined using environmental magnetic analyses the data demonstrate compositional variability of the ird within heinrich layers 2 h2 and 1 h1 and these differences are most readily explained by changes in the contribution of different ird sources to the core site some ird within the main heinrich layers show magnetic signatures that are similar to ird derived from the laurentide ice sheet found in cores from within the main north atlantic ird belt in contrast other ird rich layers both prior to and within the main heinrich layers demonstrate different magnetic behaviour suggesting a contribution from a non laurentide sourced ird most likely derived from ice streams discharging from northeast atlantic ice sheets such as the british and fennoscandian ice sheets these data are consistent with published compositional data from the same core and given the rapid highly sensitive and non destructive nature of the method suggest that environmental magnetic analysis has considerable potential for characterising ird materials within heinrich layers for the purposes of defining provenance

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

real time seismic amplitude measurement rsam a volcano monitoring and prediction tool seismicity is one of the most commonly monitored phenomena used to determine the state of a volcano and for the prediction of volcanic eruptions although several real time earthquake detection and data acquisition systems exist few continuously measure seismic amplitude in circumstances where individual events are difficult to recognize or where volcanic tremor is prevalent analog seismic records provide a quick visual overview of activity however continuous rapid quantitative analysis to define the intensity of seismic activity for the purpose of predicing volcanic eruptions is not always possible because of clipping that results from the limited dynamic range of analog recorders at the cascades volcano observatory an inexpensive 8 bit analog to digital system controlled by a laptop computer is used to provide 1 min average amplitude information from eight telemetered seismic stations the absolute voltage level for each station is digitized averaged and appended in near real time to a data file on a multiuser computer system raw realtime seismic amplitude measurement rsam data or transformed rsam data are then plotted on a common time base with other available volcano monitoring information such as tilt changes in earthquake activity associated with dome building episodes weather and instrumental difficulties are recognized as distinct patterns in the rsam data set rsam data for domebuilding episodes gradually develop into exponential increases that terminate just before the time of magma extrusion mount st helens crater earthquakes show up as isolated spikes on amplitude plots for crater seismic stations but seldom for more distant stations weather related noise shows up as low level long term disturbances on all seismic stations regardless of distance from the volcano implemented in mid 1985 the rsam system has proved valuable in providing up to date information on seismic activity for three mount st helens eruptive episodes from 1985 to 1986 may 1985 may 1986 and october 1986 tiltmeter data the only other telemetered geophysical information that was available for the three dome building episodes is compared to rsam data to show that the increase in rsam data was related to the transport of magma to the surface thus if tiltmeter data is not available rsam data can be used to predict future magmatic eruptions at mount st helens we also recognize the limitations of rsam data two examples of rsam data associated with phreatic or shallow phreatomagmatic explosions were not preceded by the same increases in rsam data or changes in tilt associated with the three dome building eruptions

rock magnetic criteria for the detection of biogenic magnetite abstract we report results on the magnetic properties of magnetites produced by magnetotactic and dissimilatory iron reducing bacteria magnetotactic bacterial mtb strains ms1 mv1 and mv2 and dissimilatory iron reducing bacterium strain gs 15 grown in pure cultures were used in this study our results suggest that a combination of room temperature coercivity analysis and low temperature remanence measurements provides a characteristic magnetic signature for intact chains of single domain sd particles of magnetite from mtbs the most useful magnetic property measurements include 1 acquisition and demagnetization of isothermal remanent magnetization irm using static pulse and alternating fields 2 acquisition of anhysteretic remanent magnetization arm and 3 thermal dependence of low temperature 20 k saturation irm after cooling in zero field zfc or in a 2 5 t field fc from 300 k however potentially the most diagnostic magnetic parameter for magnetosome chain identification in bulk sediment samples is related to the difference between low temperature zero field and field cooled sirms on warming through the verwey transition t 100 k intact chains of unoxidized magnetite magnetosomes have ratios of fc zfc greater than 2 where the parameter is a measure of the amount of remanence lost by warming through the verwey transition disruption of the chain structure or conversion of the magnetosomes to maghemite reduces the fc zfc ratio to around 1 similar to values observed for some inorganic magnetite maghemite greigite and gs 15 particles numerical simulations of fc zfc ratios for simple binary mixtures of magnetosome chains and inorganic magnetic fractions suggest that the fc zfc parameter can be a sensitive indicator of biogenic magnetite in the form of intact chains of magnetite magnetosomes and can be a useful magnetic technique for identifying them in whole sediment samples the strength of our approach lies in the comparative ease and rapidity with which magnetic measurements can be made compared to techniques such as electron microscopy

seismicity accompanying the 1999 eruptive episode at telica volcano nicaragua abstract telica volcano nicaragua is a persistently restless basaltic andesite stratovolcano located in the central american volcanic front a high rate of low frequency seismic events lfs has been recorded at telica since the installation of a single vertical component 1 hz seismic sensor teln near its summit in 1993 due to the high rate of lfs at telica traditional methods of forecasting volcanic activity based on increases in the overall rate of seismicity are not applicable therefore an understanding of the nature of precursory changes in telica s seismicity is necessary to forecast future volcanic activity in may 1999 a nine month eruptive episode started at telica consisting of phreatic to phreatomagmatic explosions here we analyse over 29 000 seismic events recorded during a fifteen month period of seismicity bracketing this eruptive episode in an attempt to retrospectively identify precursory changes in seismicity seismic event rates between january 1999 and march 2000 show a reduction in the lf event rate three months before the onset of eruptive activity closely followed by a short lived swarm of high frequency hf 5 hz events after a three month data gap a second reduction in the lf event rate started in august 1999 directly following eruptive activity in august and approximately two months before a series of explosions in october 1999 this reduction in the lf event rate was closely followed by a short lived swarm of hf events that was coincident with the onset of numerous 22 short lived but populous lf multiplets a further reduction in event rate for both lfs and hfs is evident in the months between the october 1999 explosions and explosions on the 29th of december 1999 we suggest that these changes in seismicity reflect a transition from open system degassing to closed system degassing at telica and could signify a change in the volcanic system preceding future episodes of phreatic to phreatomagmatic activity at telica and similar persistently restless volcanic systems worldwide we note that these signals are for phreatic to phreatomagmatic activity and thus may not pertain to magmatic volcanism or to other persistently restless volcanoes prior to their magmatic activity

binge purge oscillations of the laurentide ice sheet as a cause of the north atlantic s heinrich events ice rafted debris in sediment cores from the north atlantic suggests that the laurentide ice sheet lis periodically disgorged icebergs in brief but violent episodes which occurred approximately every 7 000 years here i propose that heinrich events i e what these episodes are called were caused by free oscillations in the flow of the laurentide ice sheet which arose because the floor of hudson bay and hudson strait is covered with soft unconsolidated sediment that forms a slippery lubricant when thawed the proposed heinrich event cycle has two phases the growth phase occurs when the sediment is frozen and the lis is stranded immobile on a rigid bed the volume of the lis slowly grows during this phase at a rate dictated by snow accumulation the purge phase occurs when the basal sediment thaws and a basally lubricated discharge pathway i e an ice stream such as those which occur in west antarctica today developes through hudson strait the volume of the lis rapidly equilibrates to the reduced basal friction during this phase by dumping icebergs into the labrador sea the periodicity t k sl2g 2 7000 years of the proposed heinrich event cycle is a function of the thermal conductivity and diffusivity of ice k and respectively the atmospheric sea level temperature sl in degrees celsius and the excess geothermal heat flux defined by g g k where is the atmospheric lapse rate and g is the geothermal heat flux agreement between the predicted t and the apparent periodicity implied by the marine record is the main virtue of the free oscillation mechanism i propose an alternative mechanism in which heinrich events are forced by periodic variations in external climate is implausible because periodic atmospheric temperature perturbations are strongly attenuated with depth in an ice sheet

persistent solar influence on north atlantic climate during the holocene surface winds and surface ocean hydrography in the subpolar north atlantic appear to have been influenced by variations in solar output through the entire holocene the evidence comes from a close correlation between inferred changes in production rates of the cosmogenic nuclides carbon 14 and beryllium 10 and centennial to millennial time scale changes in proxies of drift ice measured in deep sea sediment cores a solar forcing mechanism therefore may underlie at least the holocene segment of the north atlantic9s 1500 year cycle the surface hydrographic changes may have affected production of north atlantic deep water potentially providing an additional mechanism for amplifying the solar signals and transmitting them globally

the influence of co2 on the solubility of quartz in single phase hydrothermal fluids implications for the formation of stockwork veins in porphyry copper deposits

paragenesis and re os molybdenite age of the cambrian ak sug porphyry cu au mo deposit tyva republic russian federation the ak sug porphyry cu au mo deposit is situated within the altai sayan region that forms part of the tuva mongol arc of the central asian orogenic belt the ak sug intrusive complex is made up of a series of nested intrusions including diorite feldspar porphyry quartz feldspar porphyry tonalite i and tonalite ii cu au mo mineralization occurs mainly within quartz carbonate k feldspar sulfide veins and later sericite sulfide veins and alteration in the northern zone sulfide zoning consists of a central bornite core flanked by chalcopyrite and pyrite chalcopyrite zones with a pyrite zone on the hanging wall side re os molybdenite dating of an early stage quartz carbonate k feldspar sulfide vein and a late stage molybdenite only vein gives ages of 517 3 3 and 517 4 3 ma respectively this indicates an early cambrian age for mineralization at ak sug making it one of the oldest porphyry copper deposits in the central asian orogenic belt published whole rock geochemical data indicate that intrusive rocks at ak sug and other cambrian to early ordovician intrusive complexes in the tuva mongol arc evolved by hornblende titanite fractionation due to high magmatic water contents leading to adakite like compositions with high sr y and flat to listric shaped rare earth element patterns that characterize many fertile arc magmas

grain size dependence of the magnetic behavior of pyrrhotite during its low temperature transition at 34 k the grain size dependent behavior of pyrrhotite during a magnetic lt transition at 34 k is suitable for granulometric purposes cycling down to liquid helium temperature suppresses soft md jrs more than hard sd jrs so that it may be effective in removing soft secondary nrm components the domain structure of large pyrrhotite grains changes from md at room temperature to sd or psd below the lt transition

stability and instability of quiescently active volcanoes the case of masaya nicaragua quiescently active volcanoes are enigmatic due to their restlessness but lack of eruptive activity i present a model of coupled conduit convection and foam accumulation to explain degassing behavior of masaya an active volcano in nicaragua that is currently emitting large amounts of gas but not erupting gas rich magma is transported through a conduit 2 6 m in radius and then released into a shallow reservoir the magma is degassed in the reservoir and forms a foam 1 3 m thick at the top with bubbles 20 60 m in diameter the foam layer is stable because the input of gas into the foam is balanced by gas release through a vent to the surface if the foam layer is destabilized the volcano can erupt explosively the most likely cause of foam destabilization is a large injection of volatile rich magma from deeper levels into the shallow conduit reservoir system thereby increasing magma fluxes and gas fluxes exsolving large gas bubbles and reducing surface tension of the magma

multidisciplinary observations of the 2011 explosive eruption of telica volcano nicaragua implications for the dynamics of low explosivity ash eruptions we present multidisciplinary observations of the march june 2011 vei 2 eruptive episode of the basalticandesite telica volcano nicaragua which allow for a comprehensive study of the eruption mechanics of low explosivity eruptions at persistently active volcanoes the observations are from a dense network of seismic and gps instrumentation augmented by visual observations of the eruptive episode geochemical and petrologic analysis of eruptive products plume so2 measurements and temperature measurements of fumaroles inside and outside the active vent the 2011 eruptive episode was telica s most explosive since 1999 and consisted of numerous vulcanian explosions with maximum column heights of 1 5 2 km above the crater rim depositing a low volume of dominantly hydrothermally altered ash based on observed variations in seismicity temperature and so2 flux the lack of deformation of the edifice the non juvenile origin of and predominance of accretionary lapilli in the ash we propose that temporary sealing of the hydrothermal system between similar to 0 5 and 2 km depth allowed pressure to build up prior to vulcanian explosions making this a phreatic eruptive episode

ice sheet variability around the north atlantic ocean during the last deglaciation millennial scale variability in the flux of ice rafted detritus to north atlantic sediments during the last glacial period has been interpreted to reflect a climate forced increase in the discharge of icebergs from ice sheet margins surrounding the northern north atlantic ocean 1 but the relationship between ice sheet variability and climate change is not clear as both the sources of ice rafted detritus and the ice marginal processes are varied and complex 2 4 terrestrial records are helpful in unravelling this complexity because they can demonstrate the scale of ice sheet oscillations and whether the ice sheet or sector was advancing or retreating with respect to climate change here we constrain the age and anatomy of a prominent readvance of the british ice sheet in the northern irish sea region at similar to 14 14 c kyr bp similar to 16 4 calendar kyr bp the analysis indicates that the british ice sheet participated in an iceberg discharge episode known as heinrich event 1 comparison with other terrestrial and marine ice sheet records suggests that the dynamic collapse of the laurentide ice sheet beginning at 14 6 15 0 c 14 kyr bp1 4 similar to 17 2 17 6 calendar kyr bp 5 initiated varied responses from other ice sheet margins around the northern north atlantic region these observations support the argument that the release of icebergs and meltwater during heinrich event 1 disrupted the north atlantic thermohaline circulation 6 8 leading to a delay or reversal of deglaciation of the northern hemisphere and at least as far south as 40 degrees s for two to three thousand years 5 9 10 suggesting a climate forcing and response similar to that of the ensuing younger dryas cold snap 11 12

modelling magnetization data using simplex two new approaches to modelling magnetization data are described the first approach models the magnetic properties of natural materials in terms of mixtures of possible source materials the second approach models the magnetic properties of natural materials in terms of mixtures of magnetite and haematite crystals of varying concentrations and grain sizes in the second approach the magnetic characteristics of magnetite and haematite and their variation with grain size are described by bicubic spline functions the methods are applied to remanence and susceptibility measurements on a suite of lake sediment particle size fractions and on a set of samples from a soil profile

architecture and mineral deposit settings of the altaid orogenic collage a revised model abstract the altaids are an orogenic collage of neoproterozoic paleozoic rocks located in the center of eurasia this collage consists of only three oroclinally bent neoproterozoic early paleozoic magmatic arcs kipchak tuva mongol and mugodzhar rudny altai separated by sutures of their former backarc basins which were stitched by new generations of overlapping magmatic arcs in addition the altaids host accreted fragments of the neoproterozoic to early paleozoic oceanic island chains and neoproterozoic to cenozoic plume related magmatic rocks superimposed on the accreted fragments all these assemblages host important many world class late proterozoic to early mesozoic gold copper molybdenum lead zinc nickel and other deposits of various types in the late proterozoic during breakup of the supercontinent rodinia the kipchak and tuva mongol magmatic arcs were rifted off eastern europe siberia and laurentia to produce oceanic backarc basins in the late ordovician the siberian craton began its clockwise rotation with respect to eastern europe and this coincides with the beginning of formation of the mugodzhar rudny altai arc behind the kipchak arc these earlier arcs produced mostly cu pb zn vms deposits although some important intrusion related orogenic au deposits formed during arc arc collision events in the middle cambrian and late ordovician the clockwise rotation of siberia continued through the paleozoic until the early permian producing several episodes of oroclinal bending strike slip duplication and reorganization of the magmatic arcs to produce the overlapping kazakh mongol and zharma saur valerianov beltau kurama arcs that welded the extinct kipchak and tuva mongol arcs this resulted in amalgamation of the western portion of the altaid orogenic collage in the late paleozoic its eastern portion amalgamated only in the early mesozoic and was overlapped by the transbaikal magmatic arc which developed in response to subduction of the oceanic crust of the paleo pacific ocean several world class cu mo porphyry cu pb zn vms and intrusion related au mineral camps which formed in the altaids at this stage coincided with the episodes of plate reorganization and oroclinal bending of magmatic arcs major pb zn and cu sedimentary rock hosted deposits of kazakhstan and central asia formed in backarc rifts which developed on the earlier amalgamated fragments major orogenic gold deposits are intrusion related deposits often occurring within black shale bearing sutured backarc basins with oceanic crust after amalgamation of the western altaids this part of the collage and adjacent cratons were affected by the siberian superplume which ascended at the permian triassic transition this plume related magmatism produced various deposits such as famous ni cu pge deposits of norilsk in the northwest of the siberian craton in the early mesozoic the eastern altaids were oroclinally bent together with the overlapping transbaikal magmatic arc in response to the northward migration and anti clockwise rotation of the north china craton the following collision of the eastern portion of the altaid collage with the siberian craton formed the mongol okhotsk suture zone which still links the accretionary wedges of central mongolia and circum pacific belts in the late mesozoic a system of continent scale conjugate northwest trending and northeast trending strike slip faults developed in response to the southward propagation of the siberian craton with subsequent post mineral offset of some metallogenic belts for as much as 70 400 km possibly in response to spreading in the canadian basin india asia collision rejuvenated some of these faults and generated a system of impact rifts

patterns of ice rafted detritus in the glacial north atlantic 40 55 n the observation by heinrich 1988 that during the last glacial period much of the input of ice rafted detritus to the north atlantic sediments may have occurred as a succession of catastrophic events rekindled interest on the history of the northern ice sheets over the last glacial period in this paper we present a rapid method to study the distribution of these events both in space and time using whole core low field magnetic susceptibility we report on approximately 20 cores covering the last 150 to 250 kyr well defined patterns of ice rafted detritus appear during periods of large continental ice sheet extent although these are not always associated within their maxima most of the events may be traced across the north atlantic ocean for the six most recent heinrich layers hl two distinct patterns exist hl1 hl2 hl4 hl5 are distributed along the northern boundary of the glacial polar front over most of the north atlantic between 40 and 50 n hl3 is more restricted to the central and eastern part of the northern atlantic the nd sr isotopic composition of the material constituting different heinrich events indicates the different provenance of the two patterns hl3 has a typical scandinavia arctic icelandic young crust signature and the others have a large component of northern quebec and northern west greenland old crust material these isotopic results obtained on core su 9008 from the north american basin are in agreement with the study by jantschik and huon 1992 who used k ar dating of silt and clay size fractions of an eastern basin core me 68 89 these data confirm the large spatial scale of these events and the enormous amount of ice rafted detritus they represent

environmental magnetism a practical guide j walden f oldfield j and smith eds technical guide no 6 quaternary research association london 1999 243 pp price 17 00 9 00 to qra members isbn 0 907780 42 3

comment on catastrophic ice shelf breakup as the source of heinrich event icebergs by c l hulbe et al hulbe et al 2004 argue that the original binge purge model of their coauthor macayeal 1993a 1993b is not appropriate for heinrich events but that a new ice shelf collapse mechanism may work we believe that thenew collapse mechanism disagrees with important dataand that macayeal s original model remains viable afterappropriate modifications we have enjoyed a fascinatingdiscussion with hulbe et al on this topic which isstimulating our thinking and research and we presentsome of the arguments here for a wider audience

assessing the likelihood and magnitude of volcanic explosions based on seismic quiescence abstract volcanic eruptions are generally forecast based on strong increases in monitoring parameters such as seismicity or gas emissions above a relatively low background level e g voight 1988 sparks 2003 because of this forecasting individual explosions during an ongoing eruption or at persistently restless volcanoes is difficult as seismicity gas emissions and other indicators of unrest are already in a heightened state therefore identification of short term precursors to individual explosions at volcanoes already in heightened states of unrest and an understanding of explosion trigger mechanisms is important for the reduction of volcanic risk worldwide seismic and visual observations at telica volcano nicaragua demonstrate that a episodes of seismic quiescence reliably preceded explosions during an eruption in may 2011 and b the duration of precursory quiescence and the energy released in the ensuing explosion were strongly correlated precursory seismic quiescence is interpreted as the result of sealing of shallow gas pathways leading to pressure accumulation and eventual catastrophic failure of the system culminating in an explosion longer periods of sealing and pressurization lead to greater energy release in the ensuing explosion near real time observations of seismic quiescence at restless or erupting volcanoes can thus be useful for both timely eruption warnings and for forecasting the energy of impending explosions

asynchronous deposition of ice rafted layers in the nordic seas and north atlantic ocean instabilities in ice stream flow within the north american laurentide ice sheet leading to the periodic release of armadas of icebergs into the north atlantic ocean over the past 60 000 years have produced extensive layers of coarse grained iceberg rafted debris heinrich layers in north atlantic sediments1 2 correlation of these layers with iceberg discharge events from the ice sheets on greenland iceland and scandinavia suggested in previous studies for some heinrich layers and in some areas3 4 5 would imply that ice sheet instability had been synchronous across the north atlantic presumably in response to a common environmental cause here we show a lack of widespread systematic correlations both between ice rafted debris layers in different sediment cores from the nordic seas and between the nordic layers and the north atlantic heinrich layers this suggests that the full glacial nordic ice sheets did not exhibit unstable behaviour coincident with iceberg discharge from the vast hudson bay drainage basin of the laurentide ice sheet6 7 off the hudson strait significant ice sheet discharge of melt water is indicated by size sorted sandy and muddy turbidite sediments different from the poorly sorted debris flows which dominate sedimentation on the margins of the nordic seas8 9 10 together these results suggest that the dynamics of quaternary ice sheets surrounding the nordic seas were different from the outlet glacier draining the hudson bay basin and they provide evidence against a common circum north atlantic mechanism driving the discharge of icebergs

glaciation and sea level change for ireland and the irish sea since late devensian midlandian time the sea level change around the coast of ireland and the irish sea for the past 20 000 years is primarily the combined result of the glacio isostatic adjustment of the crust to the removal of ice over the british isles and the total eustatic change from the global ice sheets however the isostatic effects due to the removal of ice from northern europe and north america and the addition of meltwater into the oceans also make a significant contribution predictions of sea level change based on glacio hydro isostatic models are compared with observations to constrain the ice volume over ireland at the time of the last glacial maximum and the maximum ice height appears to have been of the order of 600 m the models predict well the spatial variability in sea level change observed across the region for holocene and lateglacial time with levels above present being predicted only for northeastern ireland and north of about morecambe bay the models do not support suggestions that lateglacial levels along the east coast of ireland or the coast of wales were 50 150 m above their present levels consistent models that would produce such large lateglacial highstands are incompatible with all other sea level and glacial evidence for the british isles palaeobathymetry and palaeoshoreline reconstructions for the irish sea indicates that a tenuous landbridge between britain and ireland developed only across the celtic sea between about 18000 and 14000 years bp

stable and unstable phases of elevated seismic activity at the persistently restless telica volcano nicaragua abstract telica volcano nicaragua is a persistently restless volcano with daily seismicity rates that can vary by orders of magnitude without apparent connection to eruptive activity low frequency lf events are dominant and peaks in seismicity rate show little correlation with eruptive episodes presenting a challenge for seismic monitoring and eruption forecasting a short period seismic station teln has been operated on telica s summit since 1993 and in 2010 the installation of a six station broadband seismic and eleven station continuous gps network the tesand network was completed to document in detail the seismic characteristics of a persistently restless volcano between our study period of november 2009 and may 2013 over 400 000 events were detected at the tesand summit station tbtn with daily event rates ranging from 5 to 1400 we present spectral analyses and classifications of 200 000 events recorded by the tesand network between april 2010 and march 2013 and earthquake locations for a sub set of events between july 2010 and february 2012 in 2011 telica erupted in a series of phreatic vulcanian explosions six months before the 2011 eruption we observe a sudden decrease in lf events concurrent with a swarm of high frequency hf events followed by a decline in overall event rates which reached a minimum at the eruption onset we observe repeated periods of high and low seismicity rates and suggest these changes in seismicity represent repeated transitions between open system and closed system degassing we suggest that these short and long term transitions between open to closed system degassing form part of a long term pattern of stable vs unstable phases at telica stable phases are characterised by steady high rate seismicity and represent stable open system degassing whereas unstable phases are characterised by highly variable seismicity rates and represent repeated transitions from open to closed system degassing where the system is unable to sustain steady open system degassing these observations have implications for seismic monitoring at persistently restless volcanoes as the recognition of unstable seismic phases may indicate the open system degassing process cannot be sustained and explosions are likely

millennial scale depositional cycles related to british ice sheet variability and north atlantic paleocirculation since 45 kyr b p barra fan u k margin lithology lithic petrology planktonic foraminiferal abundances and clastic grain sizes have been determined in a 30 m long core recovered from the barra fan off northwest scotland the record extends back to around 45 kyr b p with sedimentation rates ranging between 50 and 200 cm kyr the abundance of ice rafted debris indicates 16 glacimarine events including temporal equivalents to heinrich events 1 4 enhanced concentrations of basaltic material derived from the british tertiary province suggest that the glacimarine sediments record variations in a glacial source on the hebrides shelf margin glacimarine zones are separated by silty intervals with high planktonic foraminifera concentrations that reflect an interstadial circulation regime in the rockall trough the results suggest that the last british ice sheet fluctuated with a periodicity of 2000 3000 years in common with the dansgaard oeschger climate cycle

seismic characterization of the fall 2007 eruptive sequence at bezymianny volcano russia we examine an eruptive sequence in late 2007 at bezymianny volcano to characterize the magmatic plumbing system and eruption related seismicity earthquake locations reveal seismicity below and offset to the north of the volcano along a tectonic fault based on historical seismicity the magma chamber is postulated to have a top at about 6 km depth minor dome explosions large sub plinian eruptions and dome collapses are analyzed using an automated event classification scheme low frequency tremor interpreted as gas escape and low frequency earthquakes are a dominant proportion of the energy released we also examine multiplet earthquakes whose behavior during the study period changed significantly and systematically before the largest eruption demonstrating the potential of tracking multiplets to assess changing conditions with the conduit

further evaluation of the re os geochronometer in organic rich sedimentary rocks a test of hydrocarbon maturation effects in the exshaw formation western canada sedimentary basin abstract re os isotopic analyses of a single organic rich sedimentary rock unit ors of known depositional age and at three levels of regional hydrocarbon maturity show that hydrocarbon maturation does not affect the ability the 187re 187os chronometer to yield a depositional age for such rocks we present re os isotope analyses from the late devonian exshaw formation in the subsurface of the western canada sedimentary basin alberta and obtain a re os isochron age of 358 10 ma 2 model 3 1 666 10 11 a 1 for samples ranging from hydrocarbon immature to overmature this age is within uncertainty of the established absolute age for the exshaw formation hydrocarbon immature and mature plus overmature samples show no significant age differences if regressed individually indicating that hydrocarbon maturation did not greatly disturb the re os isotope system in the exshaw formation as such we propose that the re os geochronometer may be used as a reliable tool for measuring the depositional ages of ors regardless of their level of hydrocarbon maturity we find that minimizing natural variation in the initial 187os 188os ratio is more important than avoiding hydrocarbon maturation in obtaining precise re os ages in particular the exshaw formation appears to contain a nonhydrogenous component of unradiogenic os in addition to the hydrogenous os load a subset of exshaw formation samples with 5 total organic carbon toc which should best reflect the hydrogenous os load alone yields a very well fitted isochron having a depositional age of 358 9 ma 2 1 666 10 11 a 1 with an initial 187os 188os ratio of 0 59 0 05 model 3 mean square of weighted deviates mswd 1 8 the initial 187os 188os ratio of this regression may provide an estimate of the os isotopic composition of local seawater at the time of deposition

understanding the timing of eruption end using a machine learning approach to classification of seismic time series abstract the timing and processes that govern the end of volcanic eruptions are not yet fully understood and there currently exists no systematic definition for the end of a volcanic eruption currently end of eruption is established either by generic criteria typically 90 days after the end of visual signals of eruption or criteria specific to a given volcano we explore the application of supervised machine learning classification methods support vector machine logistic regression random forest and gaussian process classifiers and define a decisiveness index d to evaluate the consistency of the classifications obtained by these models we apply these methods to seismic time series from two volcanoes chosen because they display contrasting styles of eruption telica nicaragua and nevado del ruiz colombia we find that for both volcanic systems the end date we obtain by classification of seismic data is 2 4 months later than end dates defined by the last occurrence of visual eruption such as ash emission this finding is in agreement with previous general definitions of eruption end and is consistent across models our classifications have a higher correspondence of eruptive activity with visual activity than with database records of eruption start and end we analyze the relative importance of the different features of seismic activity used in our models e g peak event amplitude daily event counts and find little consistency between the two volcanic systems in terms of the most important features which determine whether activity is eruptive or non eruptive these initial results look promising and our approach may offer a robust tool to help determine when an eruption has ended in the absence of visual confirmation

magnetic susceptibility variations in upper pleistocene deep sea sediments of the ne atlantic implications for ice rafting and paleocirculation at the last glacial maximum magnetic susceptibility ms variations are used to intercorrelate 17 upper pleistocene sediment cores taken from the ne atlantic between 40 degrees and 60 degrees n the ms based correlation depends on regionally consistent patterns of variation in the deposition of ice rafted detritus ird in response to pleistocene glaciations and especially to high frequency ice rafting episodes referred to in recent studies as heinrich events the sedimentological and rock magnetic basis for the apparent relationship between the ms signal and ird content of ne atlantic sediments is examined by 1 comparing the ms profiles of selected cores with their records of coarse fraction 150 mu m lithic fragment abundance and neogloboquadrina pachyderma sin percentages and 2 normalizing ms by expressing it both on a carbonate free basis and as a quotient with anhysteretic remanent magnetization a parameter sensitive to magnetic mineral grain size variations these comparisons show that variations in bulk sediment ms are only partly driven by simple carbonate dilution productivity and dissolution effects changes in both the concentration and grain size of magnetic minerals within the lithogenic noncarbonate fraction also impose a significant influence on bulk ms values in particular horizons rich in ird are associated with significant increases in the relative proportion of coarse grained multidomain ferrimagnetic particles in the sediment this is because ice rafting in contrast to most other mechanisms capable of transporting detrital magnetic minerals to pelagic environments has a high potential for delivering large ferrimagnetic grains as components of sand sized polycrystalline lithic fragments this fundamental linkage between the ird content and ms signal of ne atlantic sediments is used to reconstruct the patterns of variation in ird deposition and by inference surface currents of the last glacial maximum lgm similar to 18 19 ka relative to the present day ne atlantic using the time slice mapping approach developed by the climap project group our lgm holocene ms ratio map based on sample pairs from over 80 deep sea cores confirms that there was a weak cyclonic gyre north of the polar front in the lgm north atlantic the gyre comprised a sluggish warm current in the ne atlantic flowing north between latitudes 47 degrees and 62 degrees n partly fed by subtropical waters from south of the polar front and carrying large numbers of icebergs derived from several sources most of which melted between latitudes 45 degrees and 52 degrees n the warm current probably continued its flow into the iceland basin where it fed into a south flowing current which transported melting icebergs from iceland and scandinavia along the western flank of the reykjanes ridge

the origin of volcano tectonic earthquake swarms volcano tectonic vt seismicity is commonly recorded prior to and during eruptions vt seismicity may reflect stresses induced by dike propagation as indicated by propagating hypocenters and fault plane solutions fps reflecting regional stresses or stresses induced by dike inflation indicated by randomly distributed hypocenters and fps with pressure axes rotated 90 to regional maximum compression although numerical models of dike induced stresses indicate that both regimes should occur during dike emplacement this has not yet been observed according to published seismic data instead 90 rotated fps are observed in some cases while propagating hypocenters mark dike formation in other cases we suggest that differences in the seismic expression of upper crustal magma migration may result from differences in the regional tectonic setting and in the nature of magma wall rock interactions ultimately it may be possible to use information contained in vt seismicity to forecast changes in a volcano s behavior by establishing the characteristic stress field response for a given volcano or through a deeper understanding of the complex relationships between vt seismicity local crustal stresses and the physical mechanisms of magma migration

sedimentology and micropalaeontology of glacimarine sediments from the central and southwestern celtic sea thin discontinuous glacigenic sediments occur at or close to the sea bed as far south as 49 n in the celtic sea the northern samples facies a are clast rich overconsolidated lodgement tills or proximal glacimarine sediments containing sparse reworked microfaunas the southern samples facies b are distal glacimarine plastic silty clays containing abundant cold water microfaunas both these facies are correlated with the late devensian scilly till enabling a quantitative reconstruction of ice thicknesses grounding line sea level and shoreline elevations in the celtic sea at 19 000 years bp the ice advance terminated in marine waters towards the shelf edge break and is likely to have constituted a thin lobate surge over deformable marine sediments 1990 the geological society

a new source process for evolving repetitious earthquakes at ngauruhoe volcano new zealand abstract since early 2005 ngauruhoe volcano has produced repeating low frequency earthquakes with evolving waveforms and spectral features which become progressively enriched in higher frequency energy during the period 2005 to 2009 with the trend reversing after that time the earthquakes also show a seasonal cycle since january 2006 with peak numbers of events occurring in the spring and summer period and lower numbers of events at other times we explain these patterns by the excitation of a shallow two phase water gas or water steam cavity having temporal variations in volume fraction of bubbles such variations in two phase systems are known to produce a large range of acoustic velocities 2 300 m s and corresponding changes in impedance contrast we suggest that an increasing bubble volume fraction is caused by progressive heating of melt water in the resonant cavity system which in turn promotes the scattering excitation of higher frequencies explaining both spectral shift and seasonal dependence we have conducted a constrained waveform inversion and grid search for moment position and source geometry for the onset of two example earthquakes occurring 17 and 19 january 2008 a time when events showed a frequency enrichment episode occurring over a period of a few days the inversion and associated error analysis in conjunction with an earthquake phase analysis show that the two earthquakes represent an excitation of a single source position and geometry the observed spectral changes from a stationary earthquake source and geometry suggest that an evolution in both near source resonance and scattering is occurring over periods from days to months

temporal variations of repeating low frequency volcanic earthquakes at ngauruhoe volcano new zealand abstract a persistent swarm of micro earthquakes was observed at ngauruhoe volcano new zealand from 2005 to 2010 marking the first unrest episode at the volcano since 1975 detailed waveform and spectral analysis of the continuous seismic record revealed eight earthquake families f1 f8 within a sequence of 8561 low frequency earthquakes an analysis of stacked waveforms of each family allowed the families to be linked systematically into four super families that may indicate four different mechanisms and or source locations or geometries the first super family mostly occurred in 2005 and showed stable waveforms and no significant evolution the second super family was dominant over a two year period starting in mid 2006 and displayed a slow evolution with progressively higher frequency the third super family emerged from early 2008 with intermittent occurrence and high seismicity and progressed to higher peak frequency in 2010 the fourth super family was mainly observed from late 2008 to mid 2009 and had the highest peak frequency in 2009 the observed evolutionary pattern may be driven by a modest multi year input of gas and heat from a deeper relict magmatic zone into the shallow volcano hydrothermal system the interplay of gas from below and seasonal meltwater from above may have modulated the seismic activity and may relate to repeated over pressurization and failure of constricted evolving pathways within a pervasive crack system

understanding magmatic processes at telica volcano nicaragua crystal size distribution and textural analysis telica volcano in nicaragua currently exhibits persistent activity with continuous seismicity and degassing yet it has not produced lava flows since 1529 to provide insight into magma chamber processes including replenishment and crystallization crystal size distribution csd profiles of plagioclase feldspar phenocrysts were determined for quaternary telica basalts and basaltic andesites textural analysis of 14 highly crystalline lavas 20 vol phenocrysts indicates that the samples are dominated by sieve textured plagioclase feldspar phenocrysts whose origin requires thermochemical disequilibrium within the magmatic system the csd curves display an inverse relationship between phenocryst length and population density concave up patterns observed for the telica lava samples can be represented by linear segments that define two crystal populations a steeply sloping segment for small crystals 1 5 mm in length the two crystal populations may be explained by magma replenishment and a mixing model in which a mafic magma is introduced to a stable chamber that is petrologically and geochemically evolving residence times calculated using the defined linear segments of the csd curves suggest these magmatic processes occur over timescales on the order of decades to centuries the crystal size distribution and textural analysis advocate for the current persistent activity as being consistent throughout telica s historic and prehistoric eruptive periods and driven by replenishment of mafic magma

reworked nannofossils in the north atlantic ocean and subpolar basins implications for heinrich events and ocean circulation the age and geographic distribution of reworked nannofossils in surface sediments from the north atlantic ocean suggest that their major source is northwest europe the concentration of reworked nannofossils in down core sediments indicates discharge of icebergs from the british and fennoscandian ice sheets throughout the last glacial interval during heinrich intervals 1 and 2 an increased number of icebergs from these sources reached the northeastern labrador sea these results suggest that the origin of heinrich intervals 1 and 2 involved both north american and european ice sheets

determination of large scale velocity structure of the crust and upper mantle in the vicinity of pavlof volcano alaska pavlof volcano is a 2715 m high stratovolcano located at latitude 55 4 n and longitude 161 9 w near the western end of the alaska peninsula in this study the large scale lateral perturbations to a vertical reference velocity structure in the vicinity of pavlof volcano are determined using modified versions of standard three dimensional inversion techniques we use as the primary data set p wave arrival times and residuals from shallow 40 km deep regional earthquakes located by the shumagin islands seismic network relative residuals mainly from refracted e g pn arrivals allow us to constrain models for a laterally varying crustal velocity structure several strong ray path dependent patterns are seen in the residuals the most pronounced is that rays passing through the crust beneath the volcano are delayed positive residuals by up to 1 0 s while those rays traveling subparallel to and behind the volcanic axis on the retroarc bering sea side arrive as much as 0 5 s early negative residuals the corresponding results of a formal velocity inversion show the presence of a lowvelocity body beneath the volcanic arc that coincides geographically with emmons caldera 10 km sw of pavlof computed velocity values are about 12 14 lower than in the surrounding crust and are comparable to values found at other volcanic areas the exact shape and position of the low velocity body and its suitability for geothermal power development have not yet been sufficiently determined for such an assessment a much denser station distribution is required

a re os study of sulfide minerals from the bagdad porphyry cu mo deposit northern arizona usa rhenium and osmium isotopes in sulfide minerals from the bagdad porphyry cu mo deposit have been used to determine timing of mineralization and the source of osmium and by inference ore metals molybdenite chalcopyrite and pyrite were analyzed mainly from the quartz monzonite and porphyritic quartz monzonite units which are characterized by moderate to strong potassic alteration secondary biotite and k feldspar rhenium concentrations in molybdenite are between 330 and 642 ppm four re os analyses of two molybdenite samples from the quartz monzonite and porphyritic quartz monzonite yield a weighted average age of 71 8 0 2 ma 2 analyses of a third sample from a molybdenite vein in precambrian rocks outside of the main ore zone yield a weighted average age of 75 9 0 2 ma 2 and provide evidence of two separate mineralization episodes chalcopyrite samples contain 6 to 12 ppt os and 1 7 to 4 1 ppb re 187os 188os initial ratios are between 0 1 and 0 8 pyrite samples have osmium and rhenium concentrations varying in the range 8 17 ppt and 3 9 6 8 ppb respectively analyses from these pyrite samples yield an eight point isochron with an age of 77 15 ma 2 and an initial 187os 188os ratio of 2 1 0 8 mswd 0 90 the results presented here add to the growing body of work indicating that porphyry type mineralization is produced by long term multiple episodes of magmatism and associated mineralization the data also support the hypothesis that a significant part of the metals and magmas may have a crustal source as has been suggested for other copper deposits and districts in arizona

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

mechanisms of unrest and eruption at persistently restless volcanoes insights from the 2015 eruption of telica volcano nicaragua many of earth s volcanoes experience well defined states of quiescence and unrest with unrest occasionally culminating in eruption some volcanoes however experience an unusually protracted i e decades long period of noneruptive unrest and are thus categorized as persistently restless volcanoes prvs the processes that drive persistently restless volcanism are poorly understood as our knowledge of prvs is currently based on a small number of case studies here we examine multidisciplinary observations of the 2015 eruptive episode at telica volcano nicaragua in the context of its long term behavior we suggest that the latter phases of the 2015 eruption were ultimately driven by destabilization of its shallow magma reservoir based on previous geodetic seismic studies of telica geirsson et al 2014 https doi org 10 1016 j jvolgeores 2013 11 009 rodgers et al 2013 https doi org 10 1016 j jvolgeores 2013 08 010 and 2015 https doi org 10 1016 j jvolgeores 2014 11 012 and on multiparameter observations at telica over a 7 year period we propose that three distinct states of unrest occur at telica over decadal timescales a stable open state involving steady conduit convection and two distinct unstable states that may lead to eruptions in the weak sealing state phreatic explosions result from steady conduit convection underlying a weak seal in the destabilized state destabilization of the top of the convecting magma in the conduit leads to rapid accumulation of high pressures leading to strong impulsive phreatomagmatic explosions our observations and interpretations suggest that continuous seismic ground based deformation gas emission and thermal monitoring and interpretation of these data within a paradigm of sustained conduit convection modulated by episodes of sealing and destabilization of shallow magma reservoirs may allow robust forecasting of eruption potential energy and duration at telica and similar prvs worldwide

geodynamics and metallogeny of the central eurasian porphyry and related epithermal mineral systems a review abstract major porphyry cu au and cu mo deposits are distributed across almost 5000 km across central eurasia from the urals mountains in russia in the west to inner mongolia in north eastern china these deposits were formed during multiple magmatic episodes from the ordovician to the jurassic they are associated with magmatic arcs within the extensive subduction accretion complex of the altaid and transbaikal mongolian orogenic collages that developed from the late neoproterozoic through the palaeozoic to the jurassic intracratonic extension the arcs formed predominantly on the palaeo tethys ocean margin of the proto asian continent but also within two back arc basins the development of the collages commenced when slivers of an older proterozoic subduction complex were rifted from an existing cratonic mass and accreted to the palaeo tethys ocean margin of the combined eastern europe and siberian cratons subduction of the palaeo tethys ocean beneath the karakum and altai tarim microcontinents and the associated back arc basin produced the overlapping late neoproterozoic to early palaeozoic tuva mongol and kipchak magmatic arcs contemporaneous intra oceanic subduction within the back arc basin from the late ordovician produced the parallel urals zharma magmatic arc and separated the main khanty mansi back arc basin from the inboard sakmara marginal sea by the late devonian the tuva mongol and kipchak arcs had amalgamated to form the kazakh mongol arc by the mid palaeozoic the two principal cratonic elements the siberian and eastern european cratons had begun to rotate relative to each other drawing in the two sets of parallel arcs to form the kazakh orocline between the two cratons during the late devonian to early carboniferous the palaeo pacific ocean began subducting below the siberian craton to form the sayan transbaikal arc which expanded by the permian to become the selanga gobi khanka arc by the middle to late permian as the kazakh orocline continued to develop both the sakmara and khanty mansi back arc basins were closed and the collage of cratons and arcs were sutured by accretionary complexes during the permian and triassic the north china craton approached and docked with the continent closing the mongol okhotsk sea an embayment on the palaeo pacific margin to form the mongolian orocline subduction and arc building activity on the palaeo pacific ocean margin continued to the mid mesozoic as the indosinian and yanshanian orogens significant porphyry cu au mo and au cu deposits were formed during the ordovician in the kipchak arc e g bozshakol cu au in kazakhstan and taldy bulak porphyry cu au in kyrgyzstan silurian to devonian in the kazakh mongol arc e g nurkazgan cu au in kazakhstan and taldy bulak levoberezhny au in kyrgyzstan devonian in the urals zharma arc e g yubileinoe au cu in russia devonian in the kazakh mongol arc e g oyu tolgoi cu au and tsagaan suvarga cu au in mongolia carboniferous in the kazakh mongol arc e g kharmagtai au cu in mongolia tuwu yandong cu au in xinjiang china koksai cu au kounrad cu au and the aktogai group of cu au deposits in kazakhstan carboniferous in the valerianov beltau kurama arc e g kal makyr dalnee cu au in uzbekistan benqala cu au in kazakhstan late carboniferous to permian in the selanga gobi khanka arc e g duobaoshan cu au in inner mongolia china triassic in the selanga gobi khanka arc and jurassic in the selanga gobi khanka arc e g wunugetushan cu mo and jiguanshan mo in inner mongolia china in addition to the tectonic geologic and metallogenic setting and distribution of porphyry cu au mo mineralisation within central eurasia the setting geology alteration and mineralisation at each of the deposits listed above is described and summarised in table 1

evidence for an additional heinrich event between h5 and h6 in the labrador sea 1 an additional heinrich ice rafting event is identified between heinrich events 5 and 6 in eight cores from the labrador sea and the northwest atlantic ocean it is characterized by sediment rich in detrital carbonate 40 caco3 with high concentration of floating dropstones high coarse fraction 150 m content and has a sharp contact with the underlying but grades into the overlying hemipelagic sediment it also shows lighter 18onpl values indicating freshening due to iceberg rafting and or meltwater discharge this event is correlated with dansgaard oeschger event 14 and interpreted as an additional heinrich event h5a the thickness of h5a in the labrador sea reaches up to 220 cm this additional heinrich event has also been reported in cores ps2644 and so82 5 from the northern north atlantic with the recognition of h5a the temporal spacing between heinrich events 1 to 6 becomes more uniform 7 ka

evidence for massive discharges of icebergs into the north atlantic ocean during the last glacial period sediments in the north atlantic ocean contain a series of layers that are rich in ice rafted debris and unusually poor in foraminifera1 here we present evidence that the most recent six of these heinrich layers deposited between 14 000 and 70 000 years ago record marked decreases in sea surface temperature and salinity decreases in the flux of planktonic foraminifera to the sediments and short lived massive discharges of icebergs originating in eastern canada the path of the icebergs clearly marked by the presence of ice rafted detrital carbonate can be traced for more than 3 000 km a remarkable distance attesting to extreme cooling of surface waters and enormous amounts of drifting ice the cause of these extreme events is puzzling they may reflect repeated rapid advances of the laurentide ice sheet perhaps associated with reductions in air temperatures yet temperature records from greenland ice cores appear to exhibit only a weak corresponding signal moreover the 5 10 000 yr intervals between the events are inconsistent with milankovitch orbital periodicities raising the question of what the ultimate cause of the postulated cooling may have been

earthquake classification location and error analysis in a volcanic environment implications for the magmatic system of the 1989 1990 eruptions at redoubt volcano alaska abstract determination of the precise locations of seismic events associated with the 1989 1990 eruptions of redoubt volcano posed a number of problems including poorly known crustal velocities a sparse station distribution and an abundance of events with emergent phase onsets in addition the high relief of the volcano could not be incorporated into the hypoellipse earthquake location algorithm this algorithm was modified to allow hypocenters to be located above the elevation of the seismic stations the velocity model was calibrated on the basis of a posteruptive seismic survey in which four chemical explosions were recorded by eight stations of the permanent network supplemented with 20 temporary seismographs deployed on and around the volcanic edifice the model consists of a stack of homogeneous horizontal layers setting the top of the model at the summit allows events to be located anywhere within the volcanic edifice detailed analysis of hypocentral errors shows that the long period lp events constituting the vigorous 23 hour swarm that preceded the initial eruption on december 14 could have originated from a point 1 4 km below the crater floor a similar analysis of lp events in the swarm preceding the major eruption on january 2 shows they also could have originated from a point the location of which is shifted 0 8 km northwest and 0 7 km deeper than the source of the initial swarm we suggest this shift in lp activity reflects a northward jump in the pathway for magmatic gases caused by the sealing of the initial pathway by magma extrusion during the last half of december volcano tectonic vt earthquakes did not occur until after the initial 23 hour long swarm they began slowly just below the lp source and their rate of occurrence increased after the eruption of 01 52 ast on december 15 when they shifted to depths of 6 to 10 km after january 2 the vt activity migrated gradually northward this migration suggests northward propagating withdrawal of magma from a plexus of dikes and or sills located in the 6 to 10 km depth range precise relocations of selected events prior to january 2 clearly resolve a narrow steeply dipping pencil shaped concentration of activity in the depth range of 1 7 km which illuminates the conduit along which magma was transported to the surface a third event type named hybrid which blends the characteristics of both vt and lp events originates just below the lp source and may reflect brittle failure along a zone intersecting a fluid filled crack the distribution of hybrid events is elongated 0 2 0 4 km in an east west direction this distribution may offer constraints on the orientation and size of the fluid filled crack inferred to be the source of the lp events

the origin of heinrich layers evidence from h2 for european precursor events recent well dated isotopic sr nd fingerprinting of heinrich layer ice rafted detritus ird on the european margin indicates supply from european ice sheets as precursors to laurentide ice sheet lis supply f e grousset et al geology 28 2000 123 126 h snoeckx et al mar geol 158 1999 197 208 these precursorevents lead lis input by up to 1 5 ka f e grousset et al geology 28 2000 123 126 and have been interpreted to indicate lis collapse during heinrichevents stimulated by events originating on the european side of the atlantic f e grousset et al geology 28 2000 123 126 such phasing of ird supply from different sources within heinrich layers therefore has implications for the origin and mechanics of heinrichevents we present evidence here that the ird comprising heinrich layer 2 h2 20 21 14c ka bp on the european continental margin contains detrital campanian upper chalk deriving from bedrock sources eroded on the celtic shelf by the british ice sheet bis in addition to lithic material sourced from the lis high resolution radiocarbon chronology indicates chalk grain deposition as discrete pulses both before and coincident with supply of lis sourced detritus the specificity of the chalk fingerprint to the bis enables a 700 1000 yr lag between the bis and lis events to be identified this phasing indicates a more rapid response of the outlet lobes draining the smaller bis than those draining the lis and implicates external climatic forcing of heinrichevents it is unlikely that this precursorevent represents ird event 18 the recently identified 1 2 ka ird cycle event which immediately precedes h2 because the lag between precursor and main event is here less than 1 5 ka and because such pervasive periodicity is not apparent in european continental margin ird records the later synchroneity between the bis and lis input in h2 identifies glacio eustatic sea level rise associated with lis discharges as a possible feedback mechanism causing destabilisation of ice streams elsewhere during heinrichevents

sub surface structures and collapse mechanisms of summit pit craters abstract summit pit craters are found in many types of volcanoes and are generally thought to be the product of collapse into an underpressured reservoir caused by magma withdrawal we investigate the mechanisms and structures associated with summit pit crater formation by scaled analogue experiments and make comparisons with natural examples models use a sand plaster mixture as analogue rock over a cylinder of silicone simulating an underpressured magma reservoir experiments are carried out using different roof aspect ratios roof thickness roof width of 0 2 2 they reveal two basic collapse mechanisms dependant on the roof aspect ratio one occurs at low aspect ratios 1 as illustrated by aspect ratios of 0 2 and 1 outward dipping reverse faults initiated at the silicone margins propagates through the entire roof thickness and cause subsidence of a coherent block collapse along the reverse faults is accommodated by marginal flexure of the block and tension fractures at the surface aspect ratio of 0 2 or by the creation of inward dipping normal faults delimiting a terrace aspect ratio of 1 at an aspect ratio of 1 overhanging pit walls are the surface expressions of the reverse faults experiments at high aspect ratio 1 2 reveal a second mechanism in this case collapse occurs by stopping which propagates upwards by a complex pattern of both reverse faults and tension fractures the initial underground collapse is restricted to a zone above the reservoir and creates a cavity with a stable roof above it an intermediate mechanism occurs at aspect ratios of 1 1 1 2 in this case stopping leads to the formation of a cavity with a thin and unstable roof which collapses suddenly the newly formed depression then exhibits overhanging walls surface morphology and structure of natural examples such as the summit pit craters at masaya volcano nicaragua have many of the features created in the models indicating that the internal structural geometry of experiments can be applied to real examples in particular the surface area and depth of the underpressured reservoir can be roughly estimated we present a morphological analysis of summit pit craters at volcanoes such as kilimanjaro tanzania san cristobal telica and masaya nicaragua and ubinas peru and indicate a likely type of subsidence and possible position of the former magma reservoir responsible for collapse in each case

sustained long period seismicity at shishaldin volcano alaska abstract from september 1999 through april 2004 shishaldin volcano aleutian islands alaska exhibited a continuous and extremely high level of background seismicity this activity consisted of many hundreds to thousands of long period lp 1 2 hz earthquakes per day recorded by a 6 station monitoring network around shishaldin the lp events originate beneath the summit at shallow depths 0 3 km volcano tectonic events and tremor have rarely been observed in the summit region such a high rate of lp events with no eruption suggests that a steady state process has been occurring ever since shishaldin last erupted in april may 1999 following the eruption the only other signs of volcanic unrest have been occasional weak thermal anomalies and an omnipresent puffing volcanic plume the lp waveforms are nearly identical for time spans of days to months but vary over longer time scales the observations imply that the spatially close source processes are repeating stable and non destructive event sizes vary but the rate of occurrence remains roughly constant the events range from magnitude 0 1 to 1 8 with most events having magnitudes

high sr y arc magmas and porphyry cu mo au deposits just add water porphyry cu mo au deposits are commonly associated with intermediate composition arc related igneous rocks with high sr y and la yb ratios and such rocks are equated by some researchers with melts derived from subducted oceanic crust undergoing eclogite facies metamorphism adakites however similar geochemical characteristics are readily developed in normal asthenospheric mantle wedge derived magmas by fractionation of amphibole and or garnet and or by interaction with crustal materials during ascent through the upper plate lithosphere while there is widespread evidence for such fractionation and contamination processes in porphyry magmas there is little direct evidence for an origin by slab melting the enhanced fertility of arc magmas relates primarily to their high water content which is required for the formation of magmatic hydrothermal systems upon shallow crustal emplacement magmatic water contents 4 wt percent h 2 o cause abundant amphibole garnet fractionation but suppression of plagioclase crystallization at deep crustal levels resulting in increasing sr y and la yb ratios with differentiation commonly into adakitic ranges but not reflecting slab melting additional factors that affect arc magma fertility are relatively high oxidation states and sulfur content but these are secondary to the requirement for sufficient water thus arc magmatic suites with high sr y ratios and evidence for the presence of hydrous phenocryst phases hornblende and or biotite are indeed prospective for porphyry cu mo au deposits but only because these parameters indicate high magmatic water content