initial tectonothermal evolution within the scandinavian caledonide accretionary prism constraints from40ar 39ar mineral ages within the seve nappe complex sarek mountains sweden in the caledonide orogen of northern sweden the seve nappe complex is dominated by rift facies sedimentary and mafic rocks derived from the late proterozoic baltoscandian miogeocline and offshore continent iapetus transition metamorphic breaks and structural inversions characterize the nappe complex within the sarek mountains the sarektjakka nappe is composed of c 600 ma old dolerites with subordinate screens of sedimentary rocks these lithological elements preserve parageneses which record contact metamorphism at shallow crustal levels the sarektjakka nappe is situated between eclogite bearing nappes mikka and tsakkok nappes which underwent high p metamorphism at c 500 ma during westward subduction of the baltoscandian margin 40ar 39ar mineral ages of c 520 500 ma are recorded by hornblende within variably foliated amphibolite derived from mafic dyke protoliths within the sarektjakka nappe plateau ages of 500 ma are displayed by muscovite within the basal thrust of the nappe and are consistent with metamorphic evidence which indicates that the nappe escaped crustal depression as a result of detachment at an early stage of subduction cooling ages recorded by hornblende from variably retrogressed eclogites in the entire region are in the range of c 510 490 ma and suggest that imbrication of the subducting miogeocline was followed by differential exhumation of the various imbricate sheets hornblende cooling ages of 470 460 ma are recorded from massive dyke protoliths within the sarektjakka nappe these are similar to ages reported from the seve nappe complex in the central scandinavian caledonides probably these date imbrication and uplift related to early ordovician arrival of outboard terranes e g island arc sequences represented by structurally lower horizons of the koli nappes metamorphic contrasts and the distinct grouping of mineral cooling ages suggest that the various seve structural units are themselves internally imbricated and were individually tectonically uplifted through argon closure temperatures during assembly of the seve nappe complex the cooling ages of 520 500 ma recorded within seve terranes and along terrane boundaries of the sarek mountains provide evidence of significant accretionary activity in the northern scandinavian caledonides in the late cambrian early ordovician

variations in basal conditions on rutford ice stream west antarctica seismic reflection data from two lines on rutford ice stream are presented and are compared with data already published from a third line on the ice stream the amplitude and phase of the reflections have been used to investigate the properties of the sub ice material multiple reflections on long record length data allowed calibration of the reflection coefficient at the ice bed interface and determination of the acoustic impedance of the bed material the characteristics of the bed material vary both along and across the ice stream the average acoustic impedance of the bed material across the glacier at the upstream line is 3 88 x 106 kg m 2s 1 this decreases to 3 19 x 106kg m 2s 1 52 km further downstream these values are within the range which is typical of soft sediments using acoustic impedance as an indicator of subglacial porosity some areas of the ice stream bed are interpreted as dilatant water saturated sediments undergoing pervasive deformation in other areas the bed is not deforming and basal sliding may be a more important process the proportion of the ice stream width over which bed deformation occurs increases downstream

c o h s fluids and granitic magma how s partitions and modifies co2 concentrations of fluid saturated felsic melt at 200 mpa hydrothermal volatile solubility and partitioning experiments were conducted with fluid saturated haplogranitic melt h2o co2 and s in an internally heated pressure vessel at 900 c and 200 mpa three additional experiments were conducted with iron bearing melt the run product glasses were analyzed by electron microprobe ftir and sims and they contain 0 12 wt s 0 097 wt co2 and 6 4 wt h2o apparent values of log f o2 for the experiments at run conditions were computed from the s6 s6 s2 ratio of the glasses and they range from nno 0 4 to nno 1 4 the c o h s fluid compositions at run conditions were computed by mass balance and they contained 22 99 mol h2o 0 78 mol co2 0 12 mol s and 3 wt alkalis eight s free experiments were conducted to determine the h2o and co2 concentrations of melt and fluid compositions and to compare them with prior experimental results for c o h fluid saturated rhyolite melt and the agreement is excellent sulfur partitions very strongly in favor of fluid in all experiments and the presence of s modifies the fluid compositions and hence the co2 solubilities in coexisting felsic melt the square of the mole fraction of h2o in melt increases in a linear fashion from 0 05 to 0 25 with the h2o concentration of the fluid the mole fraction of co2 in melt increases linearly from 0 0003 to 0 0045 with the co2 concentration of c o h s fluids interestingly the co2 concentration in melts involving relatively reduced runs log f o2 nno 0 3 that contain 2 5 7 mol s in the fluid decreases significantly with increasing s in the system this response to the changing fluid composition causes the h2o and co2 solubility curve for c o h s fluid saturated haplogranitic melts at 200 mpa to shift to values near that modeled for c o h fluid saturated s free rhyolite melt at 150 mpa the concentration of s in haplogranitic melt increases in a linear fashion with increasing s in c o h s fluids but these data show significant dispersion that likely reflects the strong influence of f o2 on s speciation in melt and fluid importantly the partitioning of s between fluid and melt does not vary with the h2o h2o co2 ratio of the fluid the fluid melt partition coefficients for h2o co2 and s and the atomic c s ratios of the run product fluids are virtually identical to thermodynamic constraints on volatile partitioning and the h s and c contents of pre eruptive magmatic fluids and volcanic gases for subduction related magmatic systems thus confirming our experiments are relevant to natural eruptive systems

modis based mosaic of antarctica moa data sets continent wide surface morphology and snow grain size abstract we present digital image mosaics of surface morphology and optical snow grain size for the antarctic continent and surrounding islands assembled from 260 moderate resolution imaging spectroradiometer modis images the products are derived from modis band 1 red light 647 nm and band 2 infrared 857 nm orbit swath data acquired during the 2003 2004 austral summer multiple images of all areas are combined in a data cumulation scheme to improve spatial resolution and increase radiometric content of the mosaics the component images were de striped geo registered and re sampled to the projection grid using the modis swath to grid toolbox ms2gt software a 125 m ground equivalent polar stereographic projection was used identical to previous radar image mosaics and similar to several other continent wide data sets geo location accuracy of the final mosaics is better than 125 m swath acquisition times were limited to the range 05 00 13 30 ut to provide broad but uniform mosaic illumination regions of cloud cover blowing snow and intense forward scattering were masked prior to compositing the moa snow grain size data set image provides the first continent wide semi quantitative map of mean summer snow firn and blue ice grain size optical grain size radius was determined using a normalized difference index derived from modis band 1 and band 2 rad values atmospheric correction adjustments for directional reflectance variation and snow grain size estimation were accomplished by a look up table generated by a combination of published models of snow spectral reflectance and atmosphere radiative transfer validation of the snow grain size image is provided by in situ spectra of snow covered sea ice made in october 2003 we use the new data sets to determine major ice shelf areas and shelf edge coastline and ice grounding line vector files and determine areas of selected blue ice regions these data sets are available for download at http nsidc org data moa

spatial and temporal evolution of pine island glacier thinning 1995 2006 we use ers 2 and envisat satellite radar altimetry to examine spatial and temporal changes in the rate of thinning of the pine island glacier west antarctica during the period 1995 to 2006 we show that the pattern of thinning has both accelerated and spread inland to encompass tributaries flowing into the central trunk of the glacier within the 5 400 km 2 central trunk the average rate of volume loss quadrupled from 2 6 0 3 km 3 yr 1 in 1995 to 10 1 0 3 km 3 yr 1 in 2006 the region of lightly grounded ice at the glacier terminus is extending upstream and the changes inland are consistent with the effects of a prolonged disturbance to the ice flow such as the effects of ocean driven melting if the acceleration continues at its present rate the main trunk of pig will be afloat within some 100 years six times sooner than anticipated citation wingham d j d w wallis and a shepherd 2009 spatial and temporal evolution of pine island glacier thinning 1995 2006 geophys res lett 36 l17501 doi 10 1029 2009gl039126

surface elevation change and mass balance of icelandic ice caps derived from swath mode cryosat 2 altimetry we apply swath processing to cryosat 2 interferometric mode data acquired over the icelandic ice caps to generate maps of rates of surface elevation change at 0 5 km postings this high resolution mapping reveals complex surface elevation changes in the region related to climate ice dynamics and subglacial geothermal and magmatic processes we estimate rates of volume and mass change independently for the six major icelandic ice caps 90 of iceland s permanent ice cover for five glaciological years between october 2010 and september 2015 annual mass balance is highly variable during the 2014 2015 glaciological year the vatnajokull ice cap 70 of the glaciated area experienced positive mass balance for the first time since 1992 1993 our results indicate that between glaciological years 2010 2011and 2014 2015 icelandic ice caps have lost 5 8 0 7 gt a 1 on average 40 less than the preceding 15 years contributing 0 016 0 002 mm a 1 to sea level rise

pine island glacier antarctica a study of the catchment using interferometric synthetic aperture radar measurements and radar altimetry we have mapped the catchment region of pine island glacier antarctica using ers 1 and ers 2 radar altimetry and synthetic aperture radar sar interferometry the radar altimeter data were converted to a digital elevation model dem using an algorithm we developed that corrects for slopes and grids the surface elevation estimates in a single step we used the dem and precise postprocessed orbital ephemerides to generate terrain corrected and geolocated complex radar imagery interferometric pairs on both ascending and descending orbits corrected by means of the dem yielded components of ice surface motion in the radar look direction where possible we combined velocity components from ascending and descending tracks to produce vector velocities elsewhere the velocity was calculated by assuming the direction of motion is down the surface slope the velocity map reveals a system of tributaries that channel ice from the basin like catchment area into the fast flowing downstream trunk of pine island glacier none of the tributaries shows a clear onset region instead the ice speed gradually increases along with a gradual decrease in driving stress along each tributary we combined the velocity data with sparse information on ice thicknesses and snow accumulation rates to calculate an approximate mass balance we find that the region is in balance within an uncertainty of 30 of the total mass throughput

a gabbro to eclogite transition on flemsoy sunnmore western norway the migmatitic gneisses on the nordoyane islands western norway enclose abundant inclusions of coronitic gabbros and different types of country rock eclogites gabbros on flemsoy show a prograde step wise transition to eclogite via a transitional stage in which garnet and orthopyroxene form coronas between igneous olivine and sodic plagioclase relics after igneous plagioclase cumulates eclogitization proceeds by a complex set of continuous and discontinuous reactions opx and spinel form as transient phases but are consumed in various omphacite forming reactions during eclogite recrystallization relict igneous clinopyroxene shows a continuous increase in jd content while garnet becomes less calcic and more homogeneous from the corona stage to recrystallized eclogite the reactions are strongly diffusion controlled and have been enhanced locally by oxidation and the presence of a fluid phase and deformation while low strain areas of the gabbros remained coronitic whole rock chemistry mineral parageneses and calculated p t conditions are similar for the gabbroeclogites and two types of country rock eclogites some of the layered country rock eclogites appear to have been derived from the coronite bodies p estimates are in the order 15 20 kbar while kgarcpxd values suggest maximum t values of 750 60 c for the eclogite metamorphism in the area

the role of subglacial water in ice sheet mass balance subglacial water can significantly affect the velocity of ice streams and outlet glaciers of ice sheets depending on the geometry and capacity of the subglacial hydrologic system increased surface melting in greenland over the coming decades may influence the ice sheet s mass balance furthermore subglacial lakes in antarctica can modulate ice velocities and act as nucleation points for new fast flowing ice streams in the coming decades significant changes in the polar regions will increase the contribution of ice sheets to global sea level rise under the ice streams and outlet glaciers that deliver ice to the oceans water and deformable wet sediments lubricate the base facilitating fast ice flow the influence of subglacial water on fast ice flow depends on the geometry and capacity of the subglacial hydrologic system water moving rapidly through a well connected system of conduits or channels will have little impact on ice sheet velocities but water injected into a spatially dispersed subglacial system may reduce the effective pressure at the base of the ice sheet and thereby trigger increased ice sheet velocities in greenland the form of the subglacial hydrologic system encountered by increasing surface melt water will determine the influence of changing atmospheric conditions on ice sheet mass balance in antarctica subglacial lakes have the capacity to both modulate velocities in ice streams and outlet glaciers and provide nucleation points for new fast ice flow tributaries climate models of ice sheet responses to global change remain incomplete without a parameterization of subglacial hydrodynamics and ice dynamics

increased ice losses from antarctica detected by cryosat 2 we use 3 years of cryosat 2 radar altimeter data to develop the first comprehensive assessment of antarctic ice sheet elevation change this new data set provides near continuous 96 coverage of the entire continent extending to within 215 km of the south pole and leading to a fivefold increase in the sampling of coastal regions where the vast majority of all ice losses occur between 2010 and 2013 west antarctica east antarctica and the antarctic peninsula changed in mass by 134 27 3 36 and 23 18 gt yr 1 respectively in west antarctica signals of imbalance are present in areas that were poorly surveyed by past missions contributing additional losses that bring altimeter observations closer to estimates based on other geodetic techniques however the average rate of ice thinning in west antarctica has also continued to rise and mass losses from this sector are now 31 greater than over the period 2005 2010

temperature dependence of sulfide and sulfate solubility in olivine saturated basaltic magmas abstract the sulfur concentration at pyrrhotite and anhydrite saturation in primitive hydrous basaltic melt of the 2001 2002 eruption of mt etna was determined at 200 mpa t 1050 1250 c and at log fo2 from fmq to fmq 2 2 fmq is fayalite magnetite quartz oxygen buffer at 1050 c au sample containers were used a double capsule technique using a single crystal olivine sample container closed with an olivine piston embedded in a sealed au80pd20 capsule was developed to perform experiments in s bearing hydrous basaltic systems at t 1050 c pyrrhotite is found to be a stable phase coexisting with melt at fmq fmq 0 3 whereas anhydrite is stable at fmq 1 4 fmq 2 2 the s concentration in the melt increases almost linearly from 0 12 0 01 to 0 39 0 02 wt s at fes saturation and from 0 74 0 01 to 1 08 0 04 wt s at anhydrite saturation with t ranging from 1050 1250 c the relationships between s concentration at pyrrhotite and or anhydrite saturation mgo content of the olivine saturated melt t and log fo2 observed in this study and from previous data are used to develop an empirical model for estimating the magmatic t and fo2 from the s and mgo concentrations of h2o bearing olivine saturated basaltic melts the model can also be used to determine maximum s concentrations if fo2 and mgo content of the melt are known the application of the model to compositions of melt inclusions in olivines from mt etna indicates that the most primitive magmas trapped in inclusions might have been stored at log fo2 slightly higher than fmq 1 and at t 1100 1150 c whereas more evolved melts could have been trapped at t 1100 c these values are in a good agreement with the estimates obtained by other independent methods reported in the literature

anhydrite solubility in differentiated arc magmas abstract the solubility of anhydrite in differentiated arc magmas was experimentally studied at 200 mpa and 800 1000 c over a range of oxygen fugacities from 0 5 log units above the ni nio buffer to the hematite magnetite buffer anhydrite is stable only at oxidizing conditions fo2 re reo2 whereas sulfides only form under reducing conditions the solubility of anhydrite in the melt ultimately regulates the amount of sulfur available to partition between melt and fluid phase during the eruption at oxidizing conditions the solubility product of anhydrite increases with temperature nbo t and melt water content we provide a new calibration of the anhydrite solubility product ksp xcao xso3 which reproduces all available experimental data with greatly improved accuracy ln k sp anhydrite 8 95 146 5 nbo t 2 696 10 4 t k 19 72 nbo t 10 4 t k 0 409 h 2 o wt in this equation the molar fractions xcao and xso3 in the melt as well as the number of non bridging oxygen atoms per tetrahedron nbo t are calculated on an anhydrous basis h2o refers to the melt water content t is temperature in kelvin we apply our model to estimate the sulfur yield of some recent volcanic eruptions and we show that the sulfur yield of the 1991 mt pinatubo dacite eruption was unusually large because only a small fraction of the sulfur was locked up in anhydrite in general high sulfur yields are expected when anhydrite solubility in the melt is high i e for somewhat depolymerized melts for rhyolitic systems most of the available sulfur will be locked up in anhydrite so that even very large eruptions may only have a small effect on global surface temperatures our model therefore allows improved predictions of the environmental impact of explosive volcanic eruptions

deep glacial troughs and stabilizing ridges unveiled beneath the margins of the antarctic ice sheet the antarctic ice sheet has been losing mass over past decades through the accelerated flow of its glaciers conditioned by ocean temperature and bed topography glaciers retreating along retrograde slopes that is the bed elevation drops in the inland direction are potentially unstable while subglacial ridges slow down the glacial retreat despite major advances in the mapping of subglacial bed topography significant sectors of antarctica remain poorly resolved and critical spatial details are missing here we present a novel high resolution and physically based description of antarctic bed topography using mass conservation our results reveal previously unknown basal features with major implications for glacier response to climate change for example glaciers flowing across the transantarctic mountains are protected by broad stabilizing ridges conversely in the marine basin of wilkes land east antarctica we find retrograde slopes along ninnis and denman glaciers with stabilizing slopes beneath moscow university totten and lambert glacier system despite corrections in bed elevation of up to 1 km for the latter this transformative description of bed topography redefines the high and lower risk sectors for rapid sea level rise from antarctica it will also significantly impact model projections of sea level rise from antarctica in the coming centuries a high resolution update of antarctic bed topography using mass conservation reveals broad stabilizing ridges for glaciers flowing across the transantarctic mountains and stabilizing slopes beneath moscow university totten and lambert glacier system

increased flow speed on a large east antarctic outlet glacier caused by subglacial floods large ice streams and outlet glaciers drain greenland and antarctica an observed acceleration of ice velocity in one of these outlet glaciers byrd glacier east antarctica coincides with a large water discharge from two subglacial lakes allowing direct attribution of the change in glacier dynamics to the water drainage network beneath the ice

forecasting etna eruptions by real time observation of volcanic gas composition it is generally accepted but not experimentally proven that a quantitative prediction of volcanic eruptions is possible from the evaluation of volcanic gas data by discussing the results of two years of real time observation of h2o co2 and so2 in volcanic gases from mount etna volcano we unambiguously demonstrate that increasing co2 so2 ratios can allow detection of the pre eruptive degassing of rising magmas quantitative modeling by the use of a saturation model allows us to relate the pre eruptive increases of the co2 so2 ratio to the refilling of etna s shallow conduits with co2 rich deep reservoir magmas leading to pressurization and triggering of eruption the advent of real time observations of h2o co2 and so2 combined with well constrained models of degassing represents a step forward in eruption forecasting

continent wide estimates of antarctic strain rates from landsat 8 derived velocity grids strain rates are fundamental measures of ice flow and are used in a wide variety of glaciological applications including investigations of bed properties calculations of basal mass balance on ice shelves and constraints on ice rheological models however despite their extensive application strain rates are calculated using a variety of methods and length scales and the details are often not specified in this study we compare the results of nominal and logarithmic strain rate calculations based on a satellite derived velocity field of the antarctic ice sheet generated from landsat 8 satellite data our comparison highlights the differences between the two common approaches in the glaciological literature we evaluate the errors introduced by each approach and their impacts on the results we also demonstrate the importance of choosing and specifying a length scale over which strain rate calculations are made which can strongly influence other derived quantities such as basal mass balance on ice shelves finally we present strain rate data products calculated using an approximate viscous length scale with satellite observations of ice velocity for the antarctic continent

fluid melt rock interaction in mafic eclogites and coesite bearing metasediments constraints on volatile recycling during subduction the question as to whether the metasomatizing slab agent needed to form island arc magmas is an aqueous fluid or a hydrous melt requires a knowledge of fluid melt rock interaction during high pressure low temperature metamorphism in the western alps typical anhydrous mafic eclogites monviso massif and coesite bearing metasediments dora maira massif have experienced a prograde p t path characteristic of a mature cold subduction zone microstructural petrologic geochemical and fluid inclusion studies show that fluid flow was limited during eclogite and high grade blueschist facies metamorphism most of the fluids were driven off the rocks at pressures lower than 1 and 1 6 gpa respectively with increasing pressure and temperature the remaining fluid phase was released during crystal plastic flow processes or devolatilisation reactions but retain the host rocks to form veins partial melts or dense hydrous silicates tectonic erosion at modern convergent margins delivers large amounts of terrigenous sediments kmash system in subduction zones in contrast to the basalt and peridotite systems the kmash system is characterized by a wide range of pressure sensitive reactions which allow the reconstruction of dehydration hydration depths in the subducted slab independently of the thermal structure of subduction zones terrigenous sediments will begin to melt at pressures 2 gpa the melt phase can absorb tremendeous quantities of water and represent major sinks for devolatilisation fluids dense hydrous silicates will form in any subducted sediments at all depths these silicates can store significant amount of water in their structure in the subducted oceanic crust and upper mantle heterogeneous distribution of fluids inherited from hydrothermal alteration at mid ocean ridges will cause contemporaneous dehydration and hydration reactions on a local scale rather than large scale fluid infiltration in the overlying mantle wedge the situation where a cold competent and fluid rich subducted slab is bounded above by a hot relatively weak and anhydrous material is likely to act against buoyant rise of devolatilisation fluids by forcing the fluids to flow downwards at sub arc depths 50 10 to 80150 km and for any geothermal gradient continuous internal buffering of volatile activities will retain the fluid phase in the slab this is in contrast with shallow sub forearc depths 20 to 50 10 km where tectonic vein arrays localized thrust faults serpentine diapirs mud volcanoes and boninite magmas attest to the release of large volume of fluids into the overriding mantle wedge

structural development and petrofabrics of eclogite facies shear zones bergen arcs western norway implications for deep crustal deformational processes caledonian eclogite facies shear zones developed from grenvillian garnet granulite facies anorthosites and gabbros in the bergen arcs of western norway allow direct investigation of the relations between macroscopic structures and crystallographic preferred orientation cpo in lower continental crust field relations on the island of holsnoy show that the eclogites formed locally from granulite facies rocks by progressive development of 1 eclogite adjacent to fractures 2 eclogite in discrete shear zones 2 m thick 3 eclogite breccia consisting of 80 well foliated eclogite that wraps around rotated granulite blocks and 4 anastomosing subparallel eclogite facies shear zones 30 100 m thick continuous over distances 1 km within the granulite terrane these shear zones deformed under eclogite facies conditions at an estimated temperature of 670 50 c and a minimum pressure of 1460 mpa which corresponds to depths of 55 km in the continental crust detailed investigation of the major shear zones shows the development of a strong foliation defined by the shape preferred orientation of omphacite and by alternating segregations of omphacite garnet rich and kyanite zoisite rich layers a consistent lineation throughout the shear zones is defined by elongate aggregates of garnet and omphacite the cpo of omphacite determined from five axis universal stage measurements shows a strong b axis maximum normal to foliation and a c axis girdle within the foliation plane with weak maxima parallel to the lineation direction these patterns are consistent with deformation of omphacite by slip parallel to 001 and suggest glide along 010 the lineation and cpo data reveal a consistent sense of shear zone movement although the displacement was small localized faulting of high grade rocks accompanied by fluid infiltration can be an important mode of failure in the lower continental crust field relations show that granulite facies rocks can exist in a metastable state under eclogite facies conditions and imply that the lower crust can host differing metamorphic facies at the same depth deformation of granulite and partial conversion to eclogite such as is exposed on holsnoy island may be an orogenic scale process in the lowermost crust of collisional orogens

sulfur bearing magmatic accessory minerals almost all magmas contain sulfide or sulfate bearing phases in most natural samples the sulfur bearing phase is a sulfide which typically is pyrrhotite fe1 x s or pyrite fes2 although chalcopyrite cufes2 pentlandite fe ni 9s8 sphalerite zns or molybdenite mos2 may be present as well sulfate minerals are rare at magmatic conditions and anhydrite caso4 is the most common other magmatic so4 bearing minerals include the sodalite group minerals hauyne simplified formula na ca 4 8 al6si6 o s 24 so4 cl 1 2 scapolite minerals silvialite ca na 4al6si6o24 so4 co3 and s bearing apatite ca5 po4 3 f cl oh barite baso4 has been mentioned in rare cases marchev 1991 sulfur bearing minerals usually constitute a negligible fraction of the mineral assemblage in magmatic rocks and thus can be classified as accessory minerals the crystallization of sulfide sulfate and s bearing minerals strongly depends on melt composition temperature and pressure and the s speciation in melt which in turn is strongly dependent on the prevailing oxygen fugacity baker and moretti 2011 this volume wilke et al 2011 this volume irrespectively of the low abundance of s bearing minerals the evolution of sulfur in magmas may be evaluated from the occurrence of sulfides and sulfates these minerals are critical for estimating the activity of various sulfur bearing species in magmas and can be used to constrain the oxygen fugacity and the s concentration in the melt e g pre eruptive sulfur concentration in melts the presence of either sulfide or sulfate in silicate melt indicates that the predominant dissolved sulfur species in the melt are s2 or s6 respectively typically one of these species is dominant but there are conditions where s2

the basal roughness of pine island glacier west antarctica we assess basal roughness beneath pine island glacier pig west antarctica based on a recent airborne radio echo sounding dataset we identify a clear relationship between faster ice flow and decreased basal roughness in significant parts of pig the central portion and two of its tributaries are particularly smooth but the majority of the tributaries feeding the main trunk are rougher we interpret the presence of a smooth bed as being a consequence of the deposition of marine sediments following disappearance of the west antarctic ice sheet in the pliocene or pleistocene and conversely a lack of marine sedimentation where the bed is rough importantly we also identify a patchy distribution of marine sediments and thus a bed over which the controls on flow vary while there is a notable correspondence between ice velocity and bed roughness we do not assume a direct causal relationship but find that an indirect one is likely where low basal roughness results in low basal resistance to flow a lower driving stress is required to produce the flux required to achieve mass balance this in turn means that the surface in that area will be lower than surrounding areas with a rougher bed and this will tend to draw flow into the area with low bed roughness since our studies shows that bed roughness beneath the tributaries of the trunk varies substantially there is a strong likelihood that these tributaries will differ in the rate at which they transmit current velocity changes on the main trunk into the interior of the glacier basin

revisiting complete miscibility between silicate melts and hydrous fluids and the extreme enrichment of some elements in the supercritical state consequences for the formation of pegmatites and ore deposits abstract using water and major and trace element data obtained from melt inclusions primarily in pegmatite quartz we have shown in this and previous papers that melt melt fluid immiscibility is widespread and an important process during the generation of granitic pegmatites furthermore we have shown that the formation of pegmatites actually begins in the supercritical melt fluid stage in this study we extend previous work and in particular explore the behavior of 10 different elements be f p s cl as rb sn cs and ta in the supercritical range from preliminary studies we suggest that other elements such as b na k zn nb and sb will behave similarly near the solvus crest of a generalized pseudo binary melt water system this provides important evidence that pegmatite forming processes have already begun at high temperatures in the range of 850 750 c and that may require a re think on the partitioning behavior of metals in late stage residual melts exsolved from granitic magmas the existence of this parental supercritical silicate melt fluid although derived from granitic magmas imposes significant constraints of its own so drawing conclusions about pegmatite forming processes from data gained from slightly modified granite compositions becomes highly problematic we propose that the extremely high concentration of some elements over a small but well defined water concentration range symmetrically distributed around the critical point of the pseudo binary solvus must be derived from the unusual properties of the near critical or supercritical phases these unusual properties include extremely low density viscosity and surface tension and high diffusivity reactivity and mobility these are exactly the qualities that make such supercritical fluids an excellent medium for the formations of pegmatites by reactions with the existing matrix melt crystal mush and for the extreme enrichment of some ore forming elements highly soluble in alkali and water rich supercritical fluids a very important point in mineral crystallization is the non uniform local enrichment of rare and economically significant elements at specific locations in pegmatites granites and other rocks given the properties of the supercritical phase identified from melt inclusions the process of ostwald ripening can operate very effectively the driving force for ostwald ripening arises because the concentration of solute in the vicinity of small crystals or clusters is greater than and in the vicinity of larger crystals or clusters less than the average supersaturation the solute in the supercritical fluid therefore flows from the small crystals to the larger crystals and permits the growth of very large crystals at the expense of smaller ones this is favored by the extraordinary properties of supercritical fluids given the low surface tension the supercritical fluid can move also through grain boundaries and small channels or dislocation pipes aiding incompatible element transport additionally their consequences for the formation of some important granite related ore deposits are briefly outlined

sub catchment melt and long term stability of ice stream d west antarctica 1 the apparent long term persistence and short term variability of the siple coast ice streams of west antarctica are tied to regional thermal conditions and local basal lubrication numerical simulations indicate that the flux of latent heat in a throughgoing hydrologic system fed by melt beneath thick inland ice maintains the lubrication of fast moving modern and ice age ice streams despite their tendency to freeze to the bed and would allow additional thinning and grounding line retreat

evidence for subglacial water transport in the west antarctic ice sheet through three dimensional satellite radar interferometry 1 radarsat data from the 1997 antarctic mapping mission are used interferometrically to solve for the 3 dimensional surface ice motion in the interior of the west antarctic ice sheet wais an area of 125 km2 in a tributary of the kamb ice stream slumped vertically downwards by up to 50 cm between september 26 and october 18 1997 areas in the bindschadler ice stream also exhibited comparable upward and downward surface displacements as the uplift and subsidence features correspond to sites at which the basal water apparently experiences a hydraulic potential well we suggest transient movement of pockets of subglacial water as the most likely cause for the vertical surface displacements these results and related lidar observations imply that imaging the change in ice surface elevation can help reveal the key role of water in the difficult to observe subglacial environment and its important influence on ice dynamics

rapid discharge connects antarctic subglacial lakes a hidden subglacial world consisting of more than 140 lakes lies beneath the antarctic ice sheet it is widely assumed that these lakes are a static ecosystem cut off from each other and the rest of the world far from it according to a 16 month study of surface elevation changes in the region changes indicating a rapid discharge of water from one lake to another were detected and it is possible that the entire subglacial drainage basin is flushed through periodically by massive water transfers observation of surface elevation changes over 16 months in an area of the east antarctic ice sheet overlying subglacial lakes suggests a rapid discharge of one lake to at least two other lakes involving a water volume of 1 8 km3 in conflict with previous expectations that subglacial lakes have long residence times and slow circulations the existence of many subglacial lakes1 provides clear evidence for the widespread presence of water beneath the east antarctic ice sheet but the hydrology beneath this ice mass is poorly understood2 such knowledge is critical to understanding ice flow basal water transfer to the ice margin glacial landform development and subglacial lake habitats here we present ice sheet surface elevation changes in central east antarctica that we interpret to represent rapid discharge from a subglacial lake our observations indicate that during a period of 16 months 1 8 km3 of water was transferred over 290 km to at least two other subglacial lakes while viscous deformation of the ice roof above may moderate discharge the intrinsic instability of such a system3 suggests that discharge events are a common mode of basal drainage4 if large lakes such as lake vostok or lake concordia1 are pressurizing it is possible that substantial discharges could reach the coast5 6 our observations conflict with expectations that subglacial lakes have long residence times and slow circulations2 7 8 and we suggest that entire subglacial drainage basins may be flushed periodically the rapid transfer of water between lakes would result in large scale solute and microbe relocation and drainage system contamination from in situ exploration is therefore a distinct risk

ice sheet elevations from across track processing of airborne interferometric radar altimetry 1 interferometric radar altimeters ira s use dual receive antennas to overcome one of the spatial limitations of pulse limited altimeters in a conventional ira measurement the range and across track direction of a scatterer are determined using the phase difference between the antennas we demonstrate a method of determining multiple elevation points across a swath orthogonal to the instrument ground track in regions of steep terrain such as ice sheet margins we use data from an airborne ira a prototype of the cryosat 2 instrument and compare the results to simultaneous airborne laser scanner als observations this application results in a 75 fold increase in measurement density compared to conventional radar altimetry along a 2 5 km ground track the rms departure between the ira and als derived measurements was 1 67 m based on our result although our approach is limited to areas of relatively steep slope a 25 to 75 fold increase in elevation measurements could be achieved in coastal regions of antarctica and greenland with similar processing of cryosat 2 data

an inventory of active subglacial lakes in antarctica detected by icesat 2003 2008 through the detection of surface deformation in response to water movement recent satellite studies have demonstrated the existence of subglacial lakes in antarctica that fill and drain on timescales of months to years these studies however were confined to specific regions of the ice sheet here we present the first comprehensive study of these active lakes for the antarctic ice sheet north of 868 s based on 4 5 years 2003 08 of nasa s ice cloud and land elevation satellite icesat laser altimeter data our analysis has detected 124 lakes that were active during this period and we estimate volume changes for each lake the icesat detected lakes are prevalent in coastal antarctica and are present under most of the largest ice stream catchments lakes sometimes appear to transfer water from one to another but also often exchange water with distributed sources undetectable by icesat suggesting that the lakes may provide water to or withdraw water from the hydrologic systems that lubricate glacier flow thus these reservoirs may contribute pulses of water to produce rapid temporal changes in glacier speeds but also may withdraw water at other times to slow flow

basal mechanics of ice stream b west antarctica 2 undrained plastic bed model based on the results of our studies of the physical conditions beneath ice stream b we formulate a new analytical ice stream model the undrained plastic bed model henceforth the upb model mathematically the upb model is represented by a non linear system of four coupled equations which express the relationships among ice sliding velocity till strength water storage in till and basal melt rate we examine this system of equations for conditions of ice stream stability over short timescales that permit holding ice stream geometry constant less than hundreds of years temporal variability is introduced into the upb model only by the direct dependence of till void ratio changes e t on the basal melting rate m r since till strength b e and ice stream velocity u b b change as long as till void ratio varies the first condition for ice stream stability is that of constant till water storage 0 the second condition for ice stream stability arises from the feedback between ice stream velocity till strength and the basal melting rate which depends on shear heating m r u b b this is the weak till condition which requires that in a steady state till strength is a small fraction of the gravitational driving stress b n 1 1 d the salient feature of the upb model is its ability to produce two thermo mechanically controlled equilibrium states one with a strong bed and slow ice velocities ice sheet mode and one with a weak bed and fast ice velocities ice stream mode this bimodality of basal conditions is consistent with the available observations of subglacial conditions beneath slow and fast moving ice in west antarctica basal conditions that do not correspond to these two steady states may occur transiently during switches between the two stable modes the upb model demonstrates that ice streams may be prone to thermally triggered instabilities during which small perturbations in the basal thermal energy balance grow leading to generation or elimination of the basal conditions which cause ice streaming

derivation of 500 ma eclogites from the passive margin of baltica and a note on the tectonometamorphic heterogeneity of eclogite bearing crust several recent plate reconstructions of the iapetus ocean describe the margins of baltica as passive until silurian collision with laurentia yet there is a variety of evidence to suggest that the accretion of the scandinavian caledonides began by latest cambrian early ordovician subduction and imbrication of the passive continental margin one such evidence is provided by eclogites occurring in the seve nappe complex previous work by others dated the high pressure metamorphism at 503 14 ma sm nd garnet omphacite age and the uplift through the c 500 c isotherm at 491 8 ma 40 ar 39 ar hornblende plateau ages the protolith dolerites of the eclogites have been correlated with iapetan rift facies dolerites of the baltoscandian margin if valid such a correlation implies early caledonian destruction of the margin and thus modification of those plate reconstructions which require passive margins around baltica in latest cambrian early ordovician time this paper provides a substantially improved basis for the concept that the protoliths of eclogites and their host rocks derived from baltoscandian rift basins the chemical similarity between coronitic dolerites and dolerites of the rift basins pertains not only to element concentrations and variations but also to the specific t morb signature shared by the two groups the variation of psammitic and pelitic schists graphitic schists calc silicate gneisses and marbles of the eclogite host rocks equates with sequences of sandstones siltstones shales black shale quartzite dolomite and limestones of baltoscandian palaeobasins at the same time the paper calls attention to the remarkable preservation of structural and metamorphic contrasts within the eclogite bearing thrust sheets of the seve nappe complex such disequilibrium is generally ascribed to the kinetics of localized deformation and fluid infiltration into dry crust this paper presents evidence that disequilibrium is found also within inferred subducted sedimentary complexes which are generally assumed to be pervasively flushed by fluids preservation of sedimentary volcanic and magmatic structures and fabrics and of both undeformed dolerite dykes and eclogitized dykes demonstrates that neither deformation nor high pressure metamorphism were pervasive

active lakes of recovery ice stream east antarctica a bedrock controlled subglacial hydrological system a connected system of active subglacial lakes was revealed beneath recovery ice stream east antarctica by icesat laser altimetry here we combine repeat track analysis of icesat 2003 09 operation icebridge laser altimetry and radio echo sounding 2011 and 2012 and modis image differencing 2009 2011 to learn more about the lake activity history the surface and bedrock topographic setting of the lakes and the constraints on water flow through the system we extend the lake activity time series to 2012 for the three lower lakes and capture twomajor lake drainages one lake underwent a large deflation between 2009 and 2011 while another lake which had been continuously filling between 2003 and 2010 started to drain after 2011 most of the active lakes are located in a 1000 km long bedrock trough under the main trunk of recovery ice stream whose base is 1500 2000 m below present day sea level the hydrologic system beneath recovery ice stream is controlled by this unusually pronounced bedrock topography in contrast to most antarctic systems studied to date which are controlled by the ice surface topography hydrologic connections among the lakes appear to be direct and responsive and we reproduce the lake activity using a simple subglacial water model we discuss potential causes of non steady hydrologic behavior in major antarctic catchments

an active subglacial water system in west antarctica mapped from space satellite laser altimeter elevation profiles from 2003 to 2006 collected over the lower parts of whillans and mercer ice streams west antarctica reveal 14 regions of temporally varying elevation which we interpret as the surface expression of subglacial water movement vertical motion and spatial extent of two of the largest regions are confirmed by satellite image differencing a major previously unknown subglacial lake near the grounding line of whillans ice stream is observed to drain 2 0 cubic kilometers of water into the ocean over 3 years while elsewhere a similar volume of water is being stored subglacially these observations reveal a wide spread dynamic subglacial water system that may exert an important control on ice flow and mass balance

modeling the solubility of sulfur in magmas a 50 year old geochemical challenge there are myriad reasons why we wish to understand the behavior of sulfur in magmatic systems reasons that vary from pure intellectual curiosity to possible impacts on society and its resources since ancient times sulfur has been associated with volcanic activity and the role of sulfur in the formation of ore deposits has long been recognized because of the necessity of metal ores for our modern life style e g barnes 1979 naldrett 1989 simon and ripley 2011 this volume recently the mechanisms and quantities of sulfur freed from natural magmas have become an important environmental issue due to their potential effects on global climate change for example the average annual volcanic so2 emission rate of 7 5 to 10 5 teragrams tg per year halmer et al 2002 may contribute 10 of the global atmospheric sulfur input halmer et al 2002 smith et al 2004 and individual eruptive episodes can rapidly contribute gigantic sulfur loads to the atmosphere 100 s to 1000 s of tg depending on the scale of the eruption self 2006 such sulfur emissions can produce potentially catastrophic local and global changes e g fedele et al 2003 ward 2009 courtillot and rennes 2003 correlated the timing of flood basalts with extinction events in earth s history and hypothesized a causal relation part of the kill mechanism responsible for extinction may be volcanically derived sulfur creating anoxic oceans and another part of the mechanism may be climatic changes brought about by sulfur injection into the atmosphere ward 2009 indeed erwin 2006 advocates that sulfur released from the eruption of the siberian flood basalts played a role in the end permian extinction in light of the evidence that volcanic degassing is a significant source of sulfur to the atmosphere stoiber et al 1987 symonds et al 1994 andres

ice flow of the antarctic ice sheet we present a reference comprehensive high resolution digital mosaic of ice motion in antarctica assembled from multiple satellite interferometric synthetic aperture radar data acquired during the international polar year 2007 to 2009 the data reveal widespread patterned enhanced flow with tributary glaciers reaching hundreds to thousands of kilometers inland over the entire continent this view of ice sheet motion emphasizes the importance of basal slip dominated tributary flow over deformation dominated ice sheet flow redefines our understanding of ice sheet dynamics and has far reaching implications for the reconstruction and prediction of ice sheet evolution

connected subglacial lake activity on lower mercer and whillans ice streams west antarctica 2003 2008 we examine patterns of localized surface elevation change in lower mercer and whillans ice streams west antarctica which we interpret as subglacial water movement through a system of lakes and channels we detect and measure the lake activity using repeat track laser altimetry from icesat and image differencing from modis image pairs a hydrostatic potential map for the region shows that the lakes are distributed across three distinct hydrologic regimes our analysis shows that within these regimes some of the subglacial lakes appear to be linked with drainage events in one reservoir causing filling and follow on drainage in adjacent lakes we also observe changes near ice raft a in lower whillans ice stream and interpret them as evidence of subglacial water and other changes at the bed the study provides quantitative information about the properties of this complex subglacial hydrologic system and a relatively unstudied component of ice sheet mass balance subglacial drainage across the grounding line

cambrian palaeomagnetic data from baltica implications for true polar wander and cambrian palaeogeography a reliable early cambrian c 535 ma and a preliminary late cambrian c 500 ma palaeomagnetic pole from baltica sweden overlap within uncertainty and they are also broadly compatible with vendian c 583 ma palaeomagnetic data apparent polar wander for baltica amounts to less than 25 between 583 and 500 ma and therefore negates recent speculations that the earth tipped 90 during the early cambrian true polar wander throughout vendian and cambrian times baltica lay at southerly latitudes c 30 60 s baltica was geographically inverted and present day northern baltica faced the nw margin of gondwana which covered the south pole laurentia eastern baltica and laurentia west gondwana were separated by the iapetus ocean while the aegir sea separated western baltica from the taimyr region of siberia during the cambrian baltica probably moved eastward along the gondwana margin and by c 515 520 ma subduction in the aegir sea was initiated a major event is recognized in late cambrian or early ordovician times c 500 478 ma when baltica must have undergone a 55 counter clockwise rotation in c 22 million years 3 ma we relate this to the early caledonian finnmarkian orogeny which involved arc continent collision following subduction

sediment deposition at the modern grounding zone of whillans ice stream west antarctica 1 much of the threshold behavior of marine ice sheets is thought to result from processes occurring at the grounding zone where the ice sheet transitions into the ice shelf at short timescales decades to centuries grounding zone behavior is likely to be influenced by ongoing sediment deposition which can stabilize the grounding zone position we present a ground based geophysical study across the modern grounding zone of whillans ice stream within an embayment setting where subglacial meltwater drains we image sedimentary deposits however we do not observe the classic asymmetric grounding zone wedge deposits which are found in the paleorecord and are thought to stabilize the grounding zone position at an adjacent peninsula we image the grounding zone and an ice shelf pinning point and again observe no wedge deposits

basal mechanics of ice stream b west antarctica 1 till mechanics data from laboratory geotechnical tests on till recovered from beneath ice stream b west antarctica at the upstream b camp hereinafter the upb till show that failure strength of this till is strongly dependent on effective stress but is practically independent of strain and strain rate these data support use of a coulomb plastic rheology in modeling of ice stream behavior and subglacial till deformation our testing program combined triaxial ring shear and confined uniaxial tests to investigate till strength and compressibility results show that the upb till follows closely coulomb s equation in which shear strength is a linear function of normal effective stress apparent cohesion near zero and internal friction angle equal to 24 till compressibility is best described by a logarithmic function that relates void ratio to normal effective stress in general the behavior of the upb till is consistent with other experimental evidence regarding mechanical behavior of granular materials based on our laboratory results we formulate the compressible coulomb plastic till model in which there are three interrelated primary state variables shear strength void ratio and normal effective stress this model is used in the second part of our study to simulate response of subglacial till to realistic effective stress forcings these simulations demonstrate that the model is capable of reproducing fundamental aspects of subglacial till kinematics 1 occurrence of tilt rate oscillations and negative tilt rates in tiltmeter records and 2 distributed till deformation to depths of 0 1 1 0 m beneath the ice base our laboratory and modeling results substantiate application of the compressible coulomb plastic model in simulations of the motion of ice stream b over its weak till bed

bed conditions of pine island glacier west antarctica although 90 of antarctica s discharge occurs via its fast flowing ice streams our ability to project future ice sheet response has been limited by poor observational constraints on the ice bed conditions used in numerical models to determine basal slip we have helped address this observational deficit by acquiring and analyzing a series of seismic reflection profiles to determine basal conditions beneath the main trunk and tributaries of pine island glacier pig west antarctica seismic profiles indicate large scale sedimentary deposits combined with seismic reflection images measured acoustic impedance values indicate relatively uniform bed conditions directly beneath the main trunk and tributaries comprising a widespread reworked sediment layer with a dilated sediment lid of minimum thickness 1 5 0 4 m beneath a slow moving intertributary region a discrete low porosity sediment layer of 7 3 m thickness is imaged despite considerable basal topography seismic observations indicate that a till layer at the ice base is ubiquitous beneath pig which requires a highly mobile sediment body to maintain an abundant supply these results are compatible with existing ice sheet models used to invert for basal shear stress existing basal conditions upstream will not inhibit further rapid retreat of pig if the high friction region currently restraining flow directly upstream of the grounding line is breached however small changes in the pressure regime at the bed as a result of stress reorganization following retreat may result in a less readily deformable bed and conditions which are less likely to maintain high ice flow rates

sm nd dating of seve eclogites norrbotten sweden evidence for early caledonian 505 ma subduction the seve nappes consist of long transported thrust sheets belonging to the upper allochthon within the scandinavian caledonides eclogites from two different megalenses within the seve nappe complex in norrbotten northern sweden have been dated with the sm nd method the two eclogite bearing megalenses have been subjected to different pressure and temperature conditions during the high pressure metamorphism garnet omphacite whole rock from ts2 within the tsakkok lens give an age of 505 18 ma 2 i 0 512231 0 000024 mswd 0 10 garnet and whole rock from ts5 and whole rock ts3 also fall on the above isochron the composite isochron gives an age of 503 18 ma 2 these results are interpreted to date the peak of the high pressure metamorphism 500 630 c and 12 15 kbars for the tsakkok lens the eclogites in the vaimok lens at grapesvarre were subjected to higher pressure and temperature conditions and more extensive reequilibration during the early cooling stages retrograde breakdown reactions accompanied by retrograde zoning of relict garnet seems to be associated with disturbance of the sm nd isotopic systems in contrast a sample with unzoned garnet and well preserved high pressure mineralogy from grapesvarre gives a sm nd garnet omphacite age of 503 14 ma 2 i 0 512010 0 000038 2 the ages for the seve eclogites are significantly older than the sm nd eclogite dates from the western gneiss region of norway wgr suggesting the existence of at least two eclogite forming events in the caledonide orogen the younger event has been related to the main continent continent collision stage of the caledonian orogeny while the older event that led to the production of the norrbotten eclogites must have taken place several hundred kilometres to the north and in a different tectonic setting more oceanward to the wgr it appears that the older event ca 505 ma was restricted to the subduction of dyke intruded sedimentary cover rocks which are thought to represent the rifted edge of the baltic continent

coronitic reactions and ductile shear zones in eclogitised ophiolite metagabbro western alps north italy abstract the objective of this work is to study the most significant static and syn tectonic transformations which accompanied the eclogite metamorphism 450 50 c 12 15 kbar of an ophiolite metagabbro from the western alps in partly undeformed volumes abundant igneous mineral relics occur and the eclogite metamorphism produced coronitic or incomplete pseudomorphic reactions the disequilibrium or local equilibrium conditions implied by these features suggest that slow diffusion and interface kinetics were the rate limiting factors which prevented a large scale eclogite equilibration the undeformed volumes are bounded by syn eclogite ductile shear zones where the magmatic minerals and textures are obliterated and a new fabric foliation develops the enlargement of equilibrium domains observed in the shear zones indicates that a close approach to eclogite equilibration of these metagabbros was not attained without the catalytic effect of pervasive deformation the bulk rock chemistry also played an important role in determining the kind of eclogite assemblage in particular only sheared gabbros rich in fe can produce bimineralic eclogites

kamb ice stream flow history and surge potential a basal zone tens of meters thick of debris laden ice was observed in kamb ice stream west antarctica using a video camera lowered into boreholes made by hot water drilling the debris content varies sometimes abruptly forming a sequence of layers that reflect the complex history of fast ice flow and bed interaction in most parts the concentration of debris is low a few percent by weight with particles often mud clots dispersed in a matrix of clear ice the nature of the debris distribution can be interpreted in terms of specific time intervals in the history of fast motion of kamb ice stream including processes leading up to the termination of its streaming behavior and possible reactivation

englacial seismic reflectivity imaging crystal orientation fabric in west antarctica abrupt changes in crystal orientation fabric cof and therefore viscosity are observed near the base of the ice sheet throughout west antarctica we report on active source seismic observations from wais divide mid stream and downstream on thwaites glacier and the onset region of bindschadler ice stream these data reveal a prevalence of englacial seismic reflectivity in the bottom quarter of the ice sheet the observed seismic reflectivity is complex but largely bed conformable with long spatial wavelength features observed in the flow direction and short wavelength features observed across flow a correspondence of englacial structures with bed features is also observed we determine the origin of the reflectivity to be abrupt changes in the cof of ice based on the following 1 observations of englacial reflectivity are consistent with current knowledge of cof within ice sheets 2 englacial reflectivity caused by cof contrasts requires the simplest genesis especially at ice divides and 3 amplitude analysis shows that the observed englacial reflectivity can be explained by contrasts in seismic velocity due to cof changes we note that the downstream increase in the quantity and complexity of observations indicates that direct observations of cof at ice divides likely underestimate the role that fabric plays in ice sheet dynamics

basal sliding of ice stream b west antarctica a tethered stake apparatus is used to measure basal sliding in a borehole on ice stream b west antarctica about 300 km upstream east from its grounding line near the head of the ross ice shelf a metal stake emplaced at the top of a layer of unfrozen till underlying the ice is connected by a tether line to a metering unit that measures the tether line as it is pulled out from the borehole by the stake as a result of basal sliding the measured sliding motion includes any actual slip across the ice till interface and may include in addition a possible contribution from shear deformation of till within about 3 cm of the interface this 3 cm figure follows from a qualitative model of the movements of the stake in the course of the experiment based on features of the record of apparent sliding alternative but less likely models would increase the figure from 3 cm to 10 cm or 25 cm in any case it is small compared to the seismically inferred till thickness of 9 m measured apparent sliding averages 69 of the total motion of 1 2 m d 1 over 26 days of observation if a 3 5 day period of slow apparent sliding 8 of the total motion is included in the average the occurrence of the slow period raises the possibility that the sliding motion switches back and forth between c 80 and c 8 of the total motion on a time scale of a few days however it is likely that the period of slow apparent sliding represents instead a period when the stake got caught on the ice sole if the slow period is therefore omitted the indicated average basal sliding rate is 83 of the total motion in either case basal sliding predominates as the cause of the rapid ice stream motion in the last 2 days of observation the average apparent sliding rate reached 1 17 m d 1 essentially 100 of the motion of the ice stream if till deformation contributes significantly to the ice stream motion the contribution is concentrated in a shear zone 3 cm to possibly 25 cm thick at the top of the 9 m thick till layer these observations if applicable to the west antarctic ice sheet in general pose complications in modeling the rapid ice streaming motion

shear deformation and eclogite formation within granulite facies anorthosites of the bergen arcs western norway the anorthosites of the bergen arcs contain two distinct metamorphic mineral assemblages which are stable under different p t conditions a granulite facies assemblage which equilibrated at t 800 900 c p 10 kbar is defined by the minerals plagioclase al rich diopside cpx i garnet gnt i scapolite orthopyroxene dark brown hornblende this assemblage is linked by corona structures to a primary magmatic mineralogy where coexistence of olivine and plagioclase limits the maximum crystallization pressure of the anorthosites to 8 kbar at 1200 c the anorthosites are transected by a series of fine grained shear zones in which the granulite facies assemblage is replaced by an eclogite mineralogy defined by omphacite cpx ii jd40 50 garnet kyanite epidote clinozoisite phengite quartz amphibole with plagioclase sometimes remaining in the marginal parts mineral compositions indicate equilibration conditions for the eclogite paragenesis of t 700 800 c p 16 19 kbar a series of textural and mineralogical changes which can be observed as the strain increases into the shear zones demonstrates that the eclogite mineralogy is formed at the expense of the granulite facies minerals during the early stages of shear zone development the eclogite mineralogy defines a strong mylonitic fabric which again is truncated by centimetres thick secondary shear zones containing clasts of both granulite and eclogite minerals these latter fine grained ultramylonites may be connected to uplift of this deep seated complex the eclogite shear zones are highly oblique to the granulite structures indicating that the eclogite shear zones formed under a new stress regime rb sr dating shows that the granulite facies assemblage formed in sveconorwegian time or earlier while minerals from shear zones define ages around 400 ma indicating a caledonian formation of the eclogites this work demonstrates the critical role of deformation in triggering metamorphic reactions whether the shear stress also was important in stabilizing the eclogite assemblages is not clear from the available data

solubility of anhydrite caso4 in nacl h2o solutions at high pressures and temperatures applications to fluid rock interaction anhydrite solubility in h2o nacl solutions was measured at 6 14 kbar 600 800 c and nacl mole fractions xnacl of 0 0 3 in piston cylinder apparatus solubilities were determined by weight changes of natural anhydrite in perforated pt envelopes confined with fluid in larger pt capsules in initially pure h2o at 10 kbar and 800 c caso4 concentration is low 0 03molal though much larger than at the same temperature and 1 kbar hematite buffered experiments showed slightly lower solubilities than unbuffered runs caso4 solubility increases enormously with nacl activity at 800 c and 10 kbar and xnacl of 0 3 caso4 molality is 200 times higher than with pure h2o whereas caso4 solubility in pure h2o decreases with rising t at low t and p the high p results show that anhydrite solubility increases with t at constant p at all xnacl investigated the effects of salinity and temperature are so great at 10 kbar that critical mixing between sulfate rich hydrosaline melts and aqueous salt solutions is probable at 900 c at xnacl 0 3 recent experimental evidence that volatile laden magmas crystallizing in the deep crust may evolve concentrated salt solutions could in light of the present work have important implications regarding such diverse processes as mount pinatubo type s rich volcanism high fo2 regional metamorphism and emplacement of porphyry cu mo ore bodies where anhydrite hematite alteration and fluid inclusions reveal the action of very oxidized saline solutions rich in sulfur

observations beneath pine island glacier in west antarctica and implications for its retreat thinning ice in west antarctica is currently contributing about 10 of the observed rise in global sea level observations obtained from an autonomous underwater vehicle operating beneath pine island glacier west antarctica reveal that the glacier was recently grounded on a transverse ridge in the sea floor but now warm sea water flows through the widening gap above the ridge thinning ice in west antarctica resulting from acceleration in the flow of outlet glaciers is at present contributing about 10 of the observed rise in global sea level1 pine island glacier in particular has shown nearly continuous acceleration2 3 and thinning4 5 throughout the short observational record the floating ice shelf that forms where the glacier reaches the coast has been thinning rapidly6 driven by changes in ocean heat transport beneath it as a result the line that separates grounded and floating ice has retreated inland7 these events have been postulated as the cause for the inland thinning and acceleration8 9 here we report evidence gathered by an autonomous underwater vehicle operating beneath the ice shelf that pine island glacier was recently grounded on a transverse ridge in the sea floor warm sea water now flows through a widening gap above the submarine ridge rapidly melting the thick ice of the newly formed upstream half of the ice shelf the present evolution of pine island glacier is thus part of a longer term trend that has moved the downstream limit of grounded ice inland by 30 km into water that is 300 m deeper than over the ridge crest the pace and ultimate extent of such potentially unstable retreat10 are central to the debate over the possibility of widespread ice sheet collapse triggered by climate change11 12

subglacial seismic reflection strategies when source amplitude and medium attenuation are poorly known seismic reflection techniques are a powerful way to probe physical properties of subglacial strata inversion of seismic data for physical properties may be hampered however by lack of knowledge of the source amplitude as well as lack of knowledge of the compressional and shear attenuation in the ice new methods are described to measure the source signature that require no a priori knowledge of the ice attenuation profile another new method is described to obtain the angular dependence of the subglacial bed reflection coefficient that is relatively insensitive to knowledge of the ice attenuation finally a correction is provided to a long standing error in the literature regarding measurement of the bed normal incidence reflection coefficient

anhydrite an important sulfur binder limiting the climatic impact of subaerial volcanic eruptions gaseous sulfur compounds are critical climate active volatile components released by volcanoes volcanic eruptions can emit massive amounts of h 2 s and so 2 into the atmosphere which react with oxygen and h 2 o to sulfuric acid formation of h 2 so 4 aerosols which have on a geological timescale short term residence times of month or years may have global climatic impact the phase stability of sulfur bearing minerals such as anhydrite in erupting magmas may be a key controlling factor limiting so 2 emission during subaerial volcanic activity huang and keppler january 2015 issue of american mineralogist

incomplete high p t metamorphic transitions within the kvamsoy pyroxenite complex west norway a case study of disequilibrium the kvamsoy pyroxenite complex consists of olivine websterite olivine gabbro and leucogabbro norite which have been subjected to regional high p t hpt metamorphism the metamorphism has resulted in a range of disequilibrium textures with the development of coronas and pseudomorphism of the igneous phases reactions between felsic and mafic mineral domains have been controlled by variable and selective diffusion of elements resulting in a variety of local plagioclase breakdown reactions and in significant compositional variations for the product garnet restricted diffusion favours transient reaction stages with garnet spinel corundum zoisite after calcic plagioclase in olivine gabbro and olivine websterite and garnet spinel kyanite quartz sodic plagioclase in leucogabbro norite complete hpt reaction has produced garnet pyroxenite which consists of garnet diopside hornblende zoisite in gabbroic rocks while amphibolitization continued during the cooling and uplift history grt ky pl qtz geobarometry suggests pressures in the range 13 16 kbar for t 750 c comparable with the regional eclogite forming metamorphism

primary igneous anhydrite progress since its recognition in the 1982 el chichon trachyandesite abstract primary igneous anhydrite was first identified in 1982 el chichon pumices analysis of the sulfur budget for the eruption provided compelling evidence that the pre eruptive magma contained a significant gas phase at 7 km depth in order to account for the excess gas release of 5 9 million tons of so2 to the stratosphere by the eruption primary igneous anhydrite and a larger excess gas release of 20 million tons of so2 were noted for the significantly larger eruption of mount pinatubo in 1991 for which a separate gas phase at 7 9 km depth was also required by the sulfur budget pumices from both eruptions have mineral assemblages dominated by plagioclase and hornblende with minor biotite and show evidence for co nucleation and mutual inclusions of anhydrite and apatite both magmas were also very water rich and highly oxidized with oxygen fugacities 1 log unit above the synthetic ni nio buffer furthering the similarities between these two eruptions ion microprobe analyses of sulfur isotopic compositions of anhydrites in pumices from el chichon and mount pinatubo both showed that individual crystals are isotopically homogeneous but inter crystalline variations in 34s are well beyond analytical error this study reviews the history of thought regarding primary igneous anhydrite ten volcanic and plutonic analogues to the el chichon and mount pinatubo magmas are discussed all were similarly water rich and highly oxidized magmas hornblende is the dominant mafic phenocryst in all but one plutonic example in that case so much early anhydrite precipitated that no calcium remained to form hornblende biotite is very abundant in that exceptional plutonic case and is present in six of the other examples with primary igneous anhydrite sphene is known as an indicator mineral of high magmatic oxygen fugacity it was present in the 1982 el chichon pumices and in 6 of the other anhydrite bearing samples hornblende and biotite phenocrysts are also characteristic of the intermediate to felsic plutonic rocks that typically host porphyry cu deposits such magmas are also known to be water rich and highly oxidized a close spatial and petrogenetic connection exists between magmas with primary igneous anhydrite and magmatic hydrothermal ore deposits

thirteen years of subglacial lake activity in antarctica from multi mission satellite altimetry the ability to detect the surface expression of moving water beneath the antarctic ice sheet by satellite has revealed a dynamic basal environment with implications for regional ice dynamics grounding line stability and fluxes of freshwater and nutrients to the southern ocean knowledge of subglacial activity on timescales important for near term prediction of ice sheet fluctuations decadal to century is limited by the short observational record of nasa s ice cloud and land elevation satellite icesat laser altimetry mission used to generate the last continent wide survey 2003 08 here we use synthetic aperture radar interferometric mode data from esa s cryosat 2 radar altimetry mission 2010 present which samples 45 of the icesat derived subglacial lakes to extend their time series to the end of 2016 the extended time series show that there have been surface height changes at 20 of the 45 lakes since 2008 indicating that some of these features are persistent and potentially cyclic while other features show negligible changes suggesting these may be transient or nonhydrological features continued monitoring of active lakes for both height and velocity changes as well as developing methods for identifying additional lakes is critical to quantifying the full distribution of active subglacial lakes in antarctica

the oxidation state of sulfur in magmatic fluids abstract sulfur compounds in volcanic gases are responsible for the global cooling after explosive eruptions and they probably controlled the early evolution of the earth s atmosphere we have therefore studied the oxidation state of sulfur in aqueous fluids under the pressure and temperature conditions and oxygen fugacities typical for magma chambers 0 5 3 kbar 650 950 c ni nio to re reo 2 buffer conditions sulfur speciation was determined by raman spectroscopy of quenched fluids trapped as inclusions in quartz our results show that sulfur in hydrothermal fluids and volcanic gases is much more oxidized than previously thought and in particular some explosive eruptions may release a significant fraction of sulfur as so 3 or its hydrated forms in the pressure range from 500 to 2000 bar the equilibrium constant k 1 of the reaction 2h 2 s 3o 2 2so 2 2h 2 o in aqueous fluids can be described by lnk 1 57 1 7 1 173 480 7592 t 1 where t is temperature in kelvin the equilibrium constant k 2 for the reaction so 2 o 2 so 3 in aqueous fluids where so 3 may include hydrated forms such as h 2 so 4 was found to be strongly pressure dependent with lnk 2 5 2 5 7 19 243 5993 t 1 at 1500 bar lnk 2 11 1 1 3 25 383 1371 t 1 at 2000 bar and lnk 2 22 1 2 2 37 082 2248 t 1 at 2500 bar our data imply that volcanoes may directly inject hexavalent sulfur in the form of h 2 so 4 into the atmosphere not only on earth but possibly also on venus and on mars when it was still tectonically active remote measurements from satellites may have underestimated the sulfur yield of some recent eruptions moreover the mechanisms of the interaction of volcanic gases with the stratosphere need to be reconsidered

subglacial sediments as a control on the onset and location of two siple coast ice streams west antarctica laterally continuous subglacial sediments are a necessary component for ice streaming in the modern onset regions of the ice streams draining the siple coast of west antarctica on the basis of new seismic data combined with previous results we present geophysical results from seismic reflection and refraction experiments in the upper reaches of ice streams c and d that highlight continuous sedimentary basins within and upstream of the current onset regions of both ice streams with streaming ice overlying these sedimentary packages the subglacial environment changes from no sediment to discontinuous sediment to continuous sediment cover along a longitudinal profile from the ice sheet to tributary c1b along this same profile we observe a speedup of ice flow and then full development of the ice stream tributary ice stream d flows above a thick sedimentary package with an uppermost low seismic velocity zone indicative of soft till and the upglacier and lateral extensions of ice stream d are tightly constrained by the extent of continuous sediments the inland termination of these sediments suggests that future migration of high velocity low shear stress ice flow in these regions appears unlikely

history of lower pine island glacier west antarctica from landsat imagery a 28 year record of lower pine island glacier west antarctica constructed from 15 landsat images shows changes at the terminus grounding zone and both margins the north margin has expanded 5 km into the adjacent ice shelf in a sustained event that was underway in 1973 and may have begun in 1957 between 1991 and 1997 this expansion ceased and a new region of rifting was created associated with an ice rise on the glacier s floating tongue changes in the topography of a nearby ice rise are used to deduce an upper bound estimate of a 134 m thinning of the adjacent ice shelf on the south margin widening was limited to 1 km over the observation period and is seen propagating downstream in an intermediate dated image new areas of crevassing are also evident in the grounding zone of the glacier ice loss by the calving of large tabular bergs vastly exceeds mass loss by calving of many small bergs these observations are consistent with reported changes of the 1990s and indicate that changes in the flow of pine island glacier have occurred over the full period of satellite observations

dynamics of active subglacial lakes in recovery ice stream recovery ice stream has a substantial number of active subglacial lakes that are observed with satellite altimetry to grow and drain over multiple years these lakes store and release water that could be important for controlling the velocity of the ice stream we apply a subglacial hydrology model to analyze lake growth and drainage characteristics together with the simultaneous development of the ice stream hydrological network our outputs produce a good match between modeled lake location and those identified using satellite altimetry for many of the lakes the modeled subglacial system demonstrates development of pressure waves that initiate at the ice stream neck and transit to within 100 km of the terminus these waves alter the hydraulic potential of the ice stream and encourage growth and drainage of the subglacial lakes lake drainage can cause large r channels to develop between basal overdeepenings that persist for multiple years the pressure waves along with lake growth and drainage rates do not identically repeat over multiple years due to basal network development this suggests that the subglacial hydrology of recovery ice stream is influenced by regional drainage development on the scale of hundreds of kilometers rather than local conditions over tens of kilometers

deformation of till beneath ice stream b west antarctica the behaviour and possible instability of the west antarctic ice sheet depend fundamentally on the dynamics of the large ice streams which drain it model calculations show that most ice stream velocity arises at the bed1 2 and radar sounding has shown the bed to be wet3 but the basal boundary condition is not well understood seismic evidence from the upstream b camp upb on the siple coast of west antarctica4 shows that the ice stream there rests on a layer of unconsolidated sediment averaging 5 or 6 m thick in which the water pressure is only 50 kpa less than the overburden pressure because this thin layer occurs well inland beneath an active ice sheet and rests on a surface showing flutes4 characteristic of glacial erosion5 we presume that it is glacial till we propose here that deformation within the till is the primary mechanism by which the ice stream moves and we discuss implications of this hypothesis

till beneath ice stream b 1 properties derived from seismic travel times seismic experiments conducted on ice stream b part of the marine ice sheet of west antarctica show a meters thick layer immediately beneath the 1000 m thick ice a seismic experiment consisting of wide angle reflection profiling along a line parallel to ice stream flow was conducted to determine the properties of this layer inversion of seismic travel times yields a compressional wave speed of less than 1700 m s 1 and a shear wave speed less than 160 m s 1 for the layer these very low wave speeds imply that the material in the layer is highly porous and is saturated with water at a high pore pressure based on wave speeds in other saturated unconsolidated sediments we believe that a porosity substantially greater than 0 32 probably around 0 4 and an excess of overburden pressure over pore pressure of only 50 kpa 0 5 bar characterize the layer at this location

tubular folds and sheath folds definitions and conceptual models for their development with examples from the grapesvare area northern sweden abstract two parameters are used in the classification of highly non cylindrical folds the hinge line angle and the ratio of the length of the cone axis to that of the long axis of the cross section of the structure at the position where the hinge angle was measured if in a non cylindrical fold a cross section can be selected so that the hinge angle w measured where the cross section cuts the cone is less than 90 and the length of the cone axis x is more than a quarter of the length of the long axis y of the cross section the fold is a sheath fold a tight sheath fold with w x y 1 can be termed a tubular fold a tubular fold is thus an extreme type of sheath fold while sheath folds with w 20 but x y 0 25 but tubular folds from the grapesvare area comply to the definition given above and they occur in a structural setting that is consistent with the models described

the stability and composition of sulfate melts in arc magmas the stability field and composition of immiscible sulfate melts in equilibrium with silicate magmas has been determined using experiments over a range of crustal pressures allowing an assessment of their possible role in transporting sulfur into the sub volcanic arc and porphyry copper deposit systems experimental starting materials were based on natural trachyandesite and trachydacite compositions with 3 5 7 wt h2o and 3 5 5 5 wt sulfur added to produce large analyzable amounts of sulfate phases conditions ranged over 800 1200 c 0 2 1gpa and o2 nno 2 5 sulfate melts formed at temperatures above 1000 c at 0 75 and 1 gpa and above 900 c at 0 2 gpa suggesting some pressure dependence on their stability at temperatures below 1100 c sulfate melts and anhydrite crystals commonly coexist sulfate melts quenched to an intergrowth that was difficult to prepare for analysis however the composition was approximated by epma and further constrained by mass balance calculations sulfate melts were dominated by cao and so3 but also contained in order of decreasing abundance na2o k2o mgo feo cl and p2o5 chlorine showed a particular preference for the sulfate melt relative to the coexisting silicate melt and calculated partition coefficients for sulfate silicate melts were 5 13 at 1200 c 0 75 1 gpa experimental data show that in the absence of an exsolved hydrous fluid phase sulfate melts can form in natural arc magmas at near liquidus temperatures 1000 c assuming that magmas are oxidized and contain sufficient sulfur 2000 3000 ppm s these results suggest that sulfate melt could be an important component in transporting sulfur as well as chlorine to shallow levels in the crust for hydrous magmas under a specific range of conditions both the non quenchable and water soluble nature of sulfate melts and anhydrite make them difficult to identify unless trapped as mineral inclusions similar to the wormy anhydrite trapped in high temperature amphiboles from yanacocha peru

evidence for abundant subglacial meltwater beneath the paleo ice sheet in pine island bay antarctica marine geological and geophysical data collected from the continental shelf in pine island bay antarctica reveal a complex paleo subglacial drainage system controlled by bedrock topography and subglacial meltwater discharge significant amounts of freely flowing meltwater existed beneath former ice sheets in pine island bay subglacial drainage is characterized by descriptions of glacial landforms imaged on the sea floor and sedimentary deposits collected in piston cores bedrock geology is characterized using seismic data large scale landforms on the shelf include channels and cavities incised into impermeable crystalline bedrock there is a transition from randomly oriented channels on the inner shelf to a dendritic pattern of elongate channels on the middle shelf on the outer shelf a change in basal conditions occurs where sedimentary deposits bury crystalline bedrock no evidence for flowing meltwater exists on sedimentary substrates instead meltwater formed a the ice sediment contact was incorporated into the sediments contributing to development of a deforming lied which was sampled in piston cores characterization of subglacial meltwater processes that occurred in the past may aid in understanding the role meltwater plays in stability of the west antarctic ice sheet today

rapid soft bed sliding of the puget glacial lobe the puget lobe of the late pleis inferred to be lodgment till based on the tocene cordilleran ice sheet advanced into presence of oriented stones oriented stria western washington over a thick sequence of tions on stones overconsolidation and unlithified sediments the basal drag due to deformation features sliding over this surface was calculated by 1 this paper investigates the rapid motion of idealizing the sediment bed as a rigid planar the puget lobe over its bed of deformable sedi surface scattered with roughness elements cor ment studies of large vanished glaciers com responding to individual sedimentary particles plement basal motion studies of modern glaci and 2 taking the minimum reconstructed slid ers in which the glacier geometry and velocity ing velocity to be 500 m yr 1 the calculated are well constrained but the characteristics of basal drag minimized by choices of parameters the bed can only be inferred from observations is 400 kpa much higher than the reconstructed over very limited areas kamb et al 1985 gravitational driving stress of 40 kpa indi seismic measurements blankenship et al 1986 cating that a rigid bed and low water pressure or modelling lingle and brown 1987 obser are not consistent with the glacierfs rapid vations on glacier beds recently exposed by motion high subglacial water pressure aver glacial retreat can provide information about aging 90 of the ice overburden 10 mpa is conditions during a somewhat greater time span inferred from overconsolidation of subglacial but are restricted to the marginal environ clays the occurrence and deformation of ment in addition inferences about the bed of water deposited sediments within the till and modern glaciers are limited by the short dura the requirement to drain large quantities of tion and seasonality of the observations a water from the interface suggest that water notable benefit of studying the beds of pieis pressure reached 99 of the ice overburden tocene glaciers is that they record basal ploughing of till by ice entrained clasts per conditions for intervals spanning entire glaci vasive shearing and finally ice bed separa ations and accessible far from the glacier tion become possible as water pressure margins increases at the low inferred effective nor after a brief summary of data available from mal stress the basal drag is reduced from the previous reconstructions of the puget lobe the rigid bed case by water layers which decouple basal conditions of the lobe are explored along the smallest particles from the glacier sole two complementary lines we first present cal and ploughing which reduces the resistance culations of basal drag due to glacier sliding offered by the largest particles the limited for different models of till bed roughness that shear strain observed in much of the till indicate that rapid sliding requires very low implies pervasive shearing did not contribute effective stresses and extensive ice bed significantly to ice motion basal motion was decoupling or ploughing second we summarize confined to the ice bed interface or to dis geological and geotechnical data that indicate tinct faults within the substrate

pillow lavas as protoliths for eclogites evidence from a late precambrian cambrian continental margin seve nappes scandinavian caledonides pillow lavas containing an eclogitic mineral assemblage locally with glaucophane crossite are described from the tsakkok lens within the seve nappes north central scandinavian caledonides critical primary relationships to the host metasedimentary rocks which are dominated by marble and quartz garnet phengite schist are preserved it is argued that these clastic rocks and the ongoing igneous activity in the tsakkok lens are late precambrian to cambrian in age and that these rocks were deposited emplaced along the outermost part of a continental margin related to the continent baltica

acid rain and ozone depletion from pulsed siberian traps magmatism the siberian traps flood basalts have been invoked as a trigger for the catastrophic end permian mass extinction widespread aberrant plant remains across the permian triassic boundary provide evidence that atmospheric stress contributed to the collapse in terrestrial diversity we used detailed estimates of magmatic degassing from the siberian traps to complete the first three dimensional global climate modeling of atmospheric chemistry during eruption of a large igneous province our results show that both strongly acidic rain and global ozone collapse are possible transient consequences of episodic pyroclastic volcanism and heating of volatile rich siberian country rocks we suggest that in conjunction with abrupt warming from greenhouse gas emissions these repeated rapidly applied atmospheric stresses directly linked siberian magmatism to end permian ecological failure on land our comprehensive modeling supplies the first picture of the global distribution and severity of acid rain and ozone depletion providing testable predictions for the geography of end permian environmental proxies

evidence from ice shelves for channelized meltwater flow beneath the antarctic ice sheet subglacial meltwater channels beneath the antarctic ice sheet have been reported but the nature and distribution of these meltwater pathways are unclear remote sensing observations reveal persistent channelized features beneath the filchner ronne ice shelf in west antarctica suggesting widespread channelized flow driven by melting

stagnation of ice stream c west antarctica by water piracy the dynamic behavior of the west antarctic ice sheet is of interest because of the possibility that it may change and cause rapid sea level rise attention is focused on the fast moving and rapidly responding ice streams that drain the ice sheet one of these ice stream c largely stopped about a century ago and some models for this shutdown postulate negative feedbacks that would tend to stabilize the ice sheet here new data are presented indicating that the slowdown of the ice stream is restricted to its lower part and occurred because of loss of lubrication on localized sticky spots at the bed of the ice stream the increased friction probably arises from a topographic accident of the glacier bed that has directed lubricating water to the neighboring ice stream b together with slow drawdown of the ice sheet rather than from any general stabilizing feedbacks

anhydrite stability and the effect of ca on the behavior of sulfur in felsic magmas the distribution of sulfur in the system na2o k2o cao al2o3 sio2 h2o s was investigated at 2 kbar 750 950 c and oxygen fugacities ranging from the co coo to the re reo2 buffer anhydrite caso4 crystallized in all the experiments conducted at oxidizing conditions ni nio 0 5 to 1 and above under otherwise equal conditions an inverse relationship between sulfur and cao concentration was observed in melts coexisting with anhydrite with the solubility product k cao so3 being constant where cao and so3 are the molar fraction of cao and so3 in the quenched glasses this suggests that anhydrite dissociates upon dissolution in the melt according to caso4 anhydrite ca2 melt so42 melt the solubility product strongly depends on temperature with lnk 28 573 917 t 11 26 0 80 this corresponds to an enthalpy of dissolution of h r 237 5 7 6 kj mol under reducing conditions co coo and ni nio buffer cao has no effect on the fluid melt partition coefficient of sulfur d sfluid melt at 850 c 2 kbar partition coefficients were 519 30 at the ni nio buffer and 516 11 at the co coo buffer for cao contents in the melt up to 1 wt these data are virtually identical to those measured in the cao free haplogranite system under reducing conditions however under more oxidizing conditions the fluid melt partition coefficient of sulfur appeared to have increased somewhat in the presence of cao this increase may however also be related to the fact that the final melt compositions in these runs were distinctly peraluminous our data show that calcium has no effect on the degassing of sulfur at reducing conditions but it greatly reduces the amount of sulfur available for rapid degassing under oxidizing conditions by stabilizing anhydrite

onset of seafloor spreading in the iapetus ocean at 608 ma precise age of the sarek dyke swarm northern swedish caledonides abstract the sarek dyke swarm sds crops out in the sarektjakka nappe sn of the seve kalak superterrane in the northern swedish caledonides the sn has two main components 1 a 4 5 km thick succession of rift related sedimentary rocks which is intruded by 2 a suite of tholeiitic dykes the sds constituting 70 80 of the nappe the nappe was deformed during caledonian thrusting but dykes and sedimentary rocks in the interior of the eastern parts of the sn are preserved in a pristine state the tholeiitic dykes of the sds commonly occur in sheeted dyke complexes and up to 11 successive generations can be identified from crosscutting relations the sn represents the fossil continent ocean transition between the baltic craton and the iapetus ocean marking the initiation of seafloor spreading bubble shaped pods and veinlets of diorite are present in the sds sheeted dyke complexes the pods are absent in the oldest dykes but the younger a dyke the more frequent the pods the diorite pods are the equivalent of gabbro pegmatites and both cogenetic and coeval with the dykes the rapid successive emplacement of tholeiitic magma raised the ambient temperature in the dyke complex so that crystallization in the youngest dykes mimicked similar processes in gabbro plutons six zircon fractions from the diorite pods including two single grains were analysed geochronologically by the u pb thermal ionization mass spectrometry method the data yield a linear array of points that are 0 4 0 8 normally discordant indicating a crystallization age of 608 1 ma 207pb 206pb 607 9 0 7 ma mswd 0 33 this age is inferred to date the onset of seafloor spreading in the iapetus ocean along the baltoscandian margin

flow speed within the antarctic ice sheet and its controls inferred from satellite observations accurate dynamical models of the antarctic ice sheet with carefully specified initial conditions and well calibrated rheological parameters are needed to forecast global sea level by adapting an inverse method previously used in electric impedance tomography we infer present day flow speeds within the ice sheet this inversion uses satellite observations of surface velocity snow accumulation rate and rate of change of surface elevation to estimate the basal drag coefficient and an ice stiffness parameter that influences viscosity we represent interior ice motion using a vertically integrated approximation to incompressible stokes flow this model represents vertical shearing within the ice and membrane stresses caused by horizontal stretching and shearing combining observations and model we recover marked geographical variations in the basal drag coefficient relative changes in basal shear stress are smaller no simple sliding law adequately represents basal shear stress as a function of sliding speed low basal shear stress predominates in central east antarctica where thick insulating ice allows liquid water at the base to lubricate sliding higher shear stress occurs in coastal east antarctica where a frozen bed is more likely examining thwaites glacier in more detail shows that the slowest sliding often coincides with elevated basal topography differences between our results and a similar adjoint based inversion suggest that inversion or regularization methods can influence recovered parameters for slow sliding and finer scales on broader scales we recover a similar pattern of low basal drag underneath major ice streams and extensive regions in east antarctica that move by basal sliding

basal conditions on rutford ice stream west antarctica from seismic observations a seismic reflection profile perpendicular to the ice flow direction has been acquired on rutford ice stream antarctica an interpretation of both the amplitude and phase of the seismic reflections from the ice bed interface has been made to investigate the properties of the sub ice material the interpretation assumes that acoustic impedance can be used to imply subglacial sediment porosity multiple reflections from the ice bed interface on a number of seismic wide angle lines allowed a calibration of the reflection coefficient at the bed this enabled the acoustic impedance of the bed material to be calculated from the seismic reflection data mean acoustic impedance in the different sections of the bed ranges from 2 70 106 kg m 2 s 1 to 4 31 106 kg m 2 s 1 almost three quarters of the ice stream bed at this site appears to be saturated deforming sediments the rest of the bed is probably also saturated sediments but they are not deforming to any significant degree ice flow could include a combination of subglacial deformation and basal sliding localized regions which support disproportionately high amounts of basal shear stress may also occur from the seismic data it is not possible to apportion relative amounts of restraint to these different processes there does not appear to be any correlation between the different sections identified on the ice stream bed with either satellite images or nearby surface velocity data a feature which is interpreted as a subglacial drumlin is seen on the seismic section

causes of sudden short term changes in ice stream surface elevation 1 recent satellite borne observations of antarctica s ice streams show sudden spatially confined surface elevation changes that are interpreted as caused by subglacial water movement using a numerical model of idealized ice stream flow coupled to various simple treatments of subglacial bed conditions we demonstrate that ice stream flow dynamics significantly modulates the surface elevation expression of processes taking place at the ice stream bed this modulation means that observed surface elevation changes do not directly translate to basal elevation changes e g inflation or deflation of subglacial water pockets of equal magnitude and shape thus subglacial water volume change is not directly proportional to the area integral of surface elevation changes model results show that ambiguities in interpretation of surface elevation changes can be overcome with additional measurements such as of surface velocity change and through development of methodology designed to understand transfer of basal change to surface change

early ordovician u pb metamorphic ages of the eclogite bearing seve nappes northern scandinavian caledonides eclogite grade metamorphism of the seve nappe complex snc in norrbotten sweden records the attempted subduction of the baltic continental margin during the early palaeozoic evolution of the iapetus ocean metamorphic titanite sampled from several calcsilicate gneisses of the snc in norrbotten occurs as part of a prograde eclogite facies metamorphic mineral assemblage and yields concordant to nearly concordant u pb ages of 500 475 ma later structural disruption of these rocks occurred during the siluro devonian scandian phase of the caledonide orogeny but the u pb systematics show no evidence of a second generation metamorphic or recrystallized of titanite or of post early ordovician disturbance through pb loss hence the u pb ages are believed to record the time of prograde mineral growth during eclogite facies metamorphism of the snc these results support earlier sm nd and 40ar 39ar studies indicating an early ordovician metamorphic age for the eclogitic norrbotten snc and confirm the early ordovician destruction of at least this segment of the palaeozoic passive margin of baltica these results indicate that the snc in the northern scandinavian caledonides was subducted and metamorphosed to high grade some 50 70 myr prior to the high grade metamorphism of the snc in the central scandinavian caledonides this result requires significantly different early palaeozoic tectonic histories for rocks mapped as snc in the northern caledonides and those in the central caledonides despite a seemingly similar tectonostratigraphic position and broadly similar high grade metamorphism

triggers for the formation of porphyry ore deposits in magmatic arcs porphyry ore deposits supply much of the copper molybdenum gold and silver used by humans a review of the main processes that trigger porphyry ore formation suggests that sulphide saturation of the magmas that supply the metals could be the overriding mechanism that helps control the temporal and spatial distribution of the ore deposits