variations in alaska tidewater glacier frontal ablation 1985 2013 our incomplete knowledge of the proportion of mass loss due to frontal ablation the sum of ice loss through calving and submarine melt from tidewater glaciers outside of the greenland and antarctic ice sheets has been cited as a major hindrance to accurate predictions of global sea level rise we present a 28 year record 1985 2013 of frontal ablation for 27 alaska tidewater glaciers representing 96 of the total tidewater glacier area in the region calculated from satellite derived ice velocities and modeled estimates of glacier ice thickness we account for cross sectional ice thickness variation long term thickness changes mass lost between an upstream fluxgate and the terminus and mass change due to changes in terminus position the total mean rate of frontal ablation for these 27 glaciers over the period 1985 2013 is 15 11 3 63gta 1 two glaciers hubbard and columbia account for approximately 50 of these losses the regional total ablation has decreased at a rate of 0 14gta 1 over this time period likely due to the slowing and thinning of many of the glaciers in the study area frontal ablation constitutes only 4 of the total annual regional ablation but roughly 20 of net mass loss comparing several commonly used approximations in the calculation of frontal ablation we find that neglecting cross sectional thickness variations severely underestimates frontal ablation

modification of sediment geochemistry by the hydrocarbon seep tubeworm lamellibrachia luymesi a combined empirical and modeling approach the sulfide oxidizing symbiotic tubeworm lamellibrachia luymesi is a dominant member of deep sea hydrocarbon seep ecosystems on the gulf of mexico seafloor this tubeworm forms large aggregations that can live for centuries and provide habitat for an assortment of associated fauna previous studies have suggested that persistence of these tubeworms for such long time periods is contingent upon their ability to supply sediments with sulfate to examine this hypothesis we characterized the tubeworm s geochemical environment using pore water peepers and compared the measured depth profiles with those predicted by a sulfur diffusion reaction supply model we found a large range of sulfide concentrations in the tubeworm habitat indicating that this species can live under conditions of both high and low sulfide availability in sediments rich in hydrocarbons we found compelling evidence that tubeworms enhance microbial sulfide production likely through a combination of sulfide uptake and sulfate release through their root like structures buried in the sediment our in situ empirical data combined with the results of the geochemical model corroborate previous physiological studies that indicate that tubeworms release between 70 and 90 of the sulfate produced during sulfide oxidation by their symbionts across their roots into the surrounding sediment in sediments low in hydrocarbon content sulfide production is hydrocarbon limited rather than sulfate limited and our model predicts that tubeworm growth could be limited by low sulfide availability

debris bed friction of hard bedded glaciers 1 field measurements of debris bed friction on a smooth rock tablet at the bed of engabreen a hard bedded temperate glacier in northern norway indicated that basal ice containing 10 debris by volume exerted local shear traction of up to 500 kpa the corresponding bulk friction coefficient between the dirty basal ice and the tablet was between 0 05 and 0 08 a model of friction in which nonrotating spherical rock particles are held in frictional contact with the bed by bed normal ice flow can account for these measurements if the power law exponent for ice flowing past large clasts is 1 a small exponent n 2 is likely because stresses in ice are small and flow is transient numerical calculations of the bed normal drag force on a sphere in contact with a flat bed using n 1 show that this force can reach values several hundred times that on a sphere isolated from the bed thus drastically increasing frictional resistance various estimates of basal friction are obtained from this model for example the shear traction at the bed of a glacier sliding at 20 m a 1 with a geothermally induced melt rate of 0 006 m a 1 and an effective pressure of 300 kpa can exceed 100 kpa debris bed friction can therefore be a major component of sliding resistance contradicting the common assumption that debris bed friction is negligible

greenland flow variability from ice sheet wide velocity mapping using radarsat synthetic aperture radar data we have mapped the flow velocity over much of the greenland ice sheet for the winters of 2000 01 and 2005 06 these maps provide a detailed view of the ice sheet flow including that of the hundreds of glaciers draining the interior the focused patterns of flow at the coast suggest a strong influence of bedrock topography differences between our two maps confirm numerous early observations of accelerated outlet glacier flow as well as revealing previously unrecognized changes the overall pattern is one of speed up accompanied by terminus retreat but there are also several instances of surge behavior and a few cases of glacier slowdown comprehensive mappings such as these at regular intervals provide an important new observational capability for understanding ice sheet variability

basal dynamics of kronebreen a fast flowing tidewater glacier in svalbard non local spatio temporal response to water input we evaluate the variability in basal friction for kronebreen svalbard a fast flowing tidewater glacier we invert 3 years 2013 15 of surface velocities at high temporal resolution generally 11 days to estimate the changing basal properties of the glacier our results suggest that sliding behaviour of kronebreen within a year is primarily influenced by changes in water input patterns during the meltwater season and basal friction is highly variable from a year to another at present models usually employ parameterisations to encompass the complex physics of glacier sliding by mathematically simulate their net effect for such ice masses with strong seasonal variations of surface melt the spatio temporal patterns of basal friction imply that it is neither possible nor appropriate to use a parameterisation for bed friction that is fixed in space and or time at least in a timescale of a few years basal sliding may not only be governed by local processes such as basal topography or summer melt but also be mediated by factors that vary over a larger distance and over a longer time period such as subglacial hydrology organisation ice thickness changes or calving front geometry

the anaerobic oxidation of methane and sulfate reduction in sediments from gulf of mexico cold seeps we determined the geochemical characteristics of sediments and measured rates of the anaerobic oxidation of methane aom and sulfate reduction sr in samples collected near thermogenic structure ii gas hydrate mounds and in areas lacking hydrates along the continental slope in the gulf of mexico we used radiotracer c 14 and s 35 techniques to determine rates of aom and sr over depth in sediment cores abundant mats of white and orange beggiatoa spp were common in areas of active seepage and these sediments were enriched in hydrogen sulfide and methane in cores collected from areas without beggiatoa or hydrate concentrations of redox metabolites showed little variation over depth and these sites were inferred to be areas of low seepage integrated aom rates were low in beggiatoa free cores 0 05 mmol m 2 day 1 and averaged 2 8 4 6 mmol m 2 day 1 in seep cores that contained beggiatoa or gas hydrate integrated sr rates were also low in beggiatoa free cores 1 mmol m 2 day 1 and averaged 54 94 mmol m 2 day 1 in cores with beggiatoa or hydrate rates of sr generally exceeded rates of aom and the two processes were loosely coupled suggesting that the majority of sr at gulf of mexico hydrocarbon seep sites is likely fueled by the oxidation of other organic matter possibly other hydrocarbons and oil rather than by aom

persistent flow acceleration within the interior of the greenland ice sheet we present surface velocity measurements from a high elevation site located 140 km from the western margin of the greenland ice sheet and 50 km into its accumulation area annual velocity increased each year from 51 78 0 01 m yr 1 in 2009 to 52 92 0 01 m yr 1 in 2012 a net increase of 2 2 these data also reveal a strong seasonal velocity cycle of up to 8 1 above the winter mean driven by seasonal melt and supraglacial lake drainage sole et al 2013 recently argued that ice motion in the ablation area is mediated by reduced winter flow following the development of efficient subglacial drainage during warmer faster summers our data extend this analysis and reveal a year on year increase in annual velocity above the equilibrium line altitude where despite surface melt increasing it is still sufficiently low to hinder the development of efficient drainage under thick ice

distinct patterns of seasonal greenland glacier velocity predicting greenland ice sheet mass loss due to ice dynamics requires a complete understanding of spatiotemporal velocity fluctuations and related control mechanisms we present a 5 year record of seasonal velocity measurements for 55 marine terminating glaciers distributed around the ice sheet margin along with ice front position and runoff data sets for each glacier among glaciers with substantial speed variations we find three distinct seasonal velocity patterns one pattern indicates relatively high glacier sensitivity to ice front position the other two patterns are more prevalent and appear to be meltwater controlled these patterns reveal differences in which some subglacial systems likely transition seasonally from inefficient distributed hydrologic networks to efficient channelized drainage while others do not the difference may be determined by meltwater availability which in some regions may be influenced by perennial firn aquifers our results highlight the need to understand subglacial meltwater availability on an ice sheet wide scale to predict future dynamic changes

development of late paleozoic volcanic arcs in the canadian cordillera an example from the klinkit group northern british columbia and southern yukon the late paleozoic volcanic rocks of the northern canadian cordillera lying between ancestral north america to the east and the accreted terranes of the omineca belt to the west record early arc and rift magmatism along the paleo pacific margin of the north american craton the mississippian to permian volcano sedimentary klinkit group extends discontinuously over 250 km in northern british columbia and southern yukon the two stratotype areas are as follows 1 in the englishman range southern yukon the english creek limestone is conformably overlain by the volcano sedimentary mount mccleary formation lower clastic member alkali basalt member and volcaniclastic member and 2 in the stikine ranges northern british columbia the screw creek limestone is conformably overlain by the volcano sedimentary butsih formation volcaniclastic member and upper clastic member the calc alkali nature of the basaltic volcaniclastic members of the klinkit group indicates a volcanic arc setting la yb n 2 77

deforming beds viscous and plastic scales of deformation small scale field and laboratory observations of sub glacial sediment deformation suggest that failure is the predominant method of sediment deformation while large scale modelling studies using a viscous model of till deformation have been reasonably successful in predicting the geological consequences of ice sheet action this suggests that the cross over scale between viscous and plastic deformation occurs at a much larger scale than the grain grain interaction scale previously thought to be appropriate to deforming till sediment deformation and the drainage of sub glacial areas are considered at different scales and how these interact to produce failure and viscous behaviour is discussed a fluctuation length scale is proposed whose horizontal dimensions are of the order of the depth of the base of deformation 1 10 m on this length scale the glacier deforming bed system is highly variable in time as well as in space till deformation occurring at this length scale is plastic happening though failure events three other specific scales are proposed whose different properties depend on the relative spatial variability of static and seepage pressures and topography the key theoretical problem which has yet to be solved is how multiple small scale failure events combine into a viscous type flow this is discussed in the context of self organised criticality it is argued that the length scales appropriate for some drainage features are not appreciably larger than the scale for plastic failure events implying that modelling sub glacial drainage channels by balancing erosion rates against viscous closure of the channel is misconceived the outlines of an alternative model involving percolation theory are presented certain key questions which can only be settled through observations by glaciologists and glacial geologists are discussed it is suggested that the difference between lodgement tills and deformation tills reflects the rate of motion of till by deformation and that lodgement from the ice is unimportant

greenland ice sheet motion coupled with daily melting in late summer 1 we use ground based and satellite observations to detect large diurnal and longer period variations in the flow of the greenland ice sheet gris during late summer that are strongly coupled with changes in its surface hydrology the diurnal signals are associated with periodic changes in surface melting and the longer period signals are associated with the episodic drainage of supra glacial lakes ice velocity doubles around 2 hours after peak daily melting and returns approximately to wintertime levels around 12 hours afterwards demonstrating an intimate link between the surface and basal hydrology during late summer the ice sheet accelerates by 35 per positive degree day of melting the observed link between surface melting and enhanced flow is typical of alpine glaciers which may provide an appropriate analogue for the evolution of the gris in a warming climate

potential sea level rise from antarctic ice sheet instability constrained by observations large parts of the antarctic ice sheet lying on bedrock below sea level may be vulnerable to marine ice sheet instability misi a self sustaining retreat of the grounding line triggered by oceanic or atmospheric changes there is growing evidence that misi may be underway throughout the amundsen sea embayment ase which contains ice equivalent to more than a metre of global sea level rise if triggered in other regions the centennial to millennial contribution could be several metres physically plausible projections are challenging numerical models with sufficient spatial resolution to simulate grounding line processes have been too computationally expensive to generate large ensembles for uncertainty assessment and lower resolution model projections rely on parameterizations that are only loosely constrained by present day changes here we project that the antarctic ice sheet will contribute up to 30 cm sea level equivalent by 2100 and 72 cm by 2200 95 quantiles where the ase dominates our process based statistical approach gives skewed and complex probability distributions single mode 10 cm at 2100 two modes 49 cm and 6 cm at 2200 the dependence of sliding on basal friction is a key unknown nonlinear relationships favour higher contributions results are conditional on assessments of misi risk on the basis of projected triggers under the climate scenario a1b ref 9 although sensitivity to these is limited by theoretical and topographical constraints on the rate and extent of ice loss we find that contributions are restricted by a combination of these constraints calibration with success in simulating observed ase losses and low assessed risk in some basins our assessment suggests that upper bound estimates from low resolution models and physical arguments up to a metre by 2100 and around one and a half by 2200 are implausible under current understanding of physical mechanisms and potential triggers

the sylvester allochthon upper paleozoic marginal basin and island arc terranes in northern british columbia the sylvester allochthon is a composite klippe of upper paleozoic ophiolitic island arc and pericratonic assemblages which rests directly on the cassiar terrane a displaced sliver of ancestral north america each tectonic assemblage occurs at a distinct and consistent structural level within the allochthon they are assigned respectively to the slide mountain harper ranch and yukon tanana terranes the sylvester allochthon provides a view of the structural relationships between these terranes prior to early cretaceous early tertiary strike slip dismemberment as well as possible sedimentological links to late paleozoic north america slide mountain terrane assemblages designated divisions i and ii form the lowest structural panels chert quartz sandstones are interbedded with lower mississippian deep water sediments in division i and ocean floor basalts and deep water sediments in division ii they are similar in age and character to sandstones in the autochthonous earn group division ii as

oceanic forcing of ice sheet retreat west antarctica and more ocean ice interactions have exerted primary control on the antarctic ice sheet and parts of the greenland ice sheet and will continue to do so in the near future especially through melting of ice shelves and calving cliffs retreat in response to increasing marine melting typically exhibits threshold behavior with little change for forcing below the threshold but a rapid possibly delayed shift to a reduced state once the threshold is exceeded for thwaites glacier west antarctica the threshold may already have been exceeded although rapid change may be delayed by centuries and the reduced state will likely involve loss of most of the west antarctic ice sheet causing 3 m of sea level rise because of shortcomings in physical understanding and available data uncertainty persists about this threshold and the subsequent rate of change although sea level histories and physical understanding allow the possibility that ice sheet response could be quite fast no strong constraints are yet available on

accretionary tectonics of the north american cordillera continental geology stands on the threshold of a change that is likely to be as fundamental as plate tectonic theory was for marine geology ongoing seismic reflection investigations into the deep crustal structure of north america are verifying that orogenic zones are underlain by low angle faults of regional extent brown et al 1981 the growing body of regional field relations is likewise delineating numerous orogenic sutures that bound discrete crustal fragments paleomagnetic and paleobiogeographic studies are revealing major latitudinal shifts and rotations within and between suture bounded fragments particularly within the north american cordillera such interdisciplinary studies are leading to a consensus that the cordillera has been built by progressive tectonic addition of crustal fragments along the continent edge in mesozoic and early cenozoic time such crustal growth is referred to as accretionary tectonics in this paper we review some of the important concepts in accretionary tectonics discuss the nature of the materials accreted between central alaska and southern california in jurassic and cretaceous time and consider the general relations between cordilleran accretion and the movement of lithospheric plates the concept of continents growing by peripheral accretion through geologic time has long been a topic of great interest with the advent of plate tectonics a number of different mechanisms for crustal accretion have arisen along with mechanisms for crustal attrition accretion mechanisms include the growth of imbricated sedimentary prisms along inner trench walls slicing off of submarine topographic irregularities within subducting plates and collision of continents and volcanic arcs by ocean basin closure tectonic attrition mechanisms include rifting transform faulting and strike slip or underthrust removal of inner trench wall materials coincident with or in place of accretionary prism growth growth of intraorogenic ocean basins by seafloor spreading is an additional important mechanism for creating accretionary materials as well as displacing crustal fragments an important implication of plate kinematic theory is the likelihood for accretionary and attritionary mechanics to operate in series both in time and space along continental margins since attrition by nature leaves little material evidence of having operated one of the major problems confronting cordilleran geologists lies in the recognition of such attrition within the ancient record particularly when interspersed with accretionary events the spectrum of accretion and attrition mechanisms viewed at cm yr 1 plate transport rates over time scales of 100 m y leads one to suspect a highly mobile history for continental margin orogens the serial arrangement of subducting transform and rifting links along the modern cordillera plate juncture system and both serial and parallel arrangements in the western pacific systems show the complex interplay of such mechanisms through space similar arrangements overprinted through time are suggested by the rock assemblages and structural patterns within the cordillera which presently resemble a collage of crustal fragments davis et al 1978 recognition of the structural state of this collage geologic field mapping and geophysical investigations will bring about a new level of understanding in the growth of continental crust and the reading of stratigraphic records within the fragments and future palinspastic restorations will lead to a new level of understanding in paleogeography and earth history the first problem to be considered is the recognition of native north american crust from exotic fragments that have been accreted to its edge

shear resistance and continuity of subglacial till hydrology rules the field observations of g s boulton stimulated widespread interest in deformable beds shear resistance of till in its critical state is insensitive to strain rate and increases linearly with effective pressure during unsteady deformation pseudo viscous shear resistance can be caused by dilation of consolidated tills and resultant pore pressure decline this effect is probably uncommon however because susceptible tills of low hydraulic diffusivity are also those least likely to consolidate significantly during effective pressure transients stick slip motion at whillans ice stream antarctica indicates that its basal till must weaken during rapid slip and strengthen during longer periods of slower slip recurrence intervals for rapid slip episodes there 6 18 hours indicate that till strength variations if driven by changes in pore pressure either related or unrelated to basal freezing are focused in the uppermost several centimeters of the bed ploughing of grains at the bed surface and associated excess pore pressures in adjacent till can account for rate weakening during rapid slip with pore pressure decay causing strengthening between slip episodes by promoting shallow sluggish subglacial water flow and low effective pressure soft beds may help sustain themselves by slowing their own transport soft bed shear resistance kinematics and continuity are problems rooted in subglacial hydrology

modelling distributed and channelized subglacial drainage the spacing of channels models are proposed for channelized and distributed flow of meltwater at the base of an ice sheet the volumes of both channel and distributed systems evolve according to a competition between processes that open drainage space e g sliding over bedrock melting of the ice and processes that close it e g viscous creep of the ice due to a positive effective pressure channels are generally predicted to have lower water pressure and therefore capture water from the surrounding regions of distributed flow there is a natural length scale associated with the distributed system that determines the width of the bed from which water can be drawn into a channel it is suggested that this determines the spacing between major channels and that this may be reflected in the spacing of eskers a more permeable distributed system results in more widely spaced and therefore larger channels calculations of the flow into the head of a channel reveal that there is a critical discharge necessary for it to form and provide a criterion for where channels can exist

modelling the long term mass balance and firn evolution of glaciers around kongsfjorden svalbard we analyse the long term 1961 2012 distributed surface mass balance and firn evolution of the kongsvegen and holtedahlfonna glacier systems in northwestern svalbard we couple a surface energy balance model to a firn model with forcing provided from regional climate model output in situ observational data are used to calibrate model parameters and validate the output the simulated area averaged surface mass balance for 1961 2012 is slightly positive 0 08 m w e a 1 which only fractionally compensates for mass loss by calving refreezing of percolating water in spring summer 0 13 m w e a 1 and stored water in fall winter 0 18 m w e a 1 provides a buffer for runoff internal accumulation i e refreezing below the previous year s summer surface in the accumulation zone peaks up to 0 22 m w e a 1 and is unaccounted for by stake observations superimposed ice formation in the lower accumulation zone ranges as high as 0 25 m w e a 1 a comparison of the periods 1961 99 and 2000 12 reveals 21 higher annual melt rates since 2000 and a 31 increase in runoff which can only in part be ascribed to recent warmer and drier conditions in response to firn line retreat both albedo lowering snow ice albedo feedback and lower refreezing rates refreezing feedback further amplified runoff

a tutorial on the use of control methods in ice sheet modeling control methods are recommended as an efficient means to estimate undetermined physical parameters and boundary conditions and in so doing to improve the fidelity of a given ice sheet flow model to specific ice sheet velocity observations to accomplish this task the underlying dynamics of the model are inverted this permits model tuning adjustments to be represented explicitly in terms of model observation misfit

characterization of seasonal glacial seismicity from a single station on ice record at holtedahlfonna svalbard glacial seismicity provides important insights into glacier dynamic processes we study the temporal distribution of cryogenic seismic signals icequakes at holtedahlfonna svalbard between april

scale independence of till rheology representation of till rheology in glaciological models of ice motion over deformable sediments has until now focused largely on two end member cases 1 linear or mildly non linear viscous rheology and 2 nearly plastic rheology most laboratory and in situ experiments support the latter model hindmarsh 1997 and fowler 2002 2003 proposed that experimental results represent the behavior of small till samples characteristic length scales of 0 1 to 1 m but that till behaves viscously over length scales that are relevant to determination of ice flow rates in glaciers and ice sheets 1 km or more observations of short speed up events on the ice plain of whillans ice stream west antarctica provide an opportunity to compare the in situ rheology of this till integrated over 10 100 km with the rheology of till from beneath the same ice stream determined on small laboratory samples and in local borehole experiments this comparison indicates that the rheology of the subglacial till beneath whillans ice stream is independent of scale

a laboratory study of particle ploughing and pore pressure feedback a velocity weakening mechanism for soft glacier beds if basal water discharge and pressure are sufficiently high a soft bedded glacier will slip over its bed by ploughing the process in which particles that span the ice bed interface are dragged across the bed surface results of laboratory experiments indicate that resistance to ploughing can decrease with increasing ploughing velocity velocity weakening during ploughing at various velocities 15 400 m a 1 till was compacted in front of idealized particles causing pore pressures there that were orders of magnitude higher than the ambient value this excess pore pressure locally weakened the till in shear thereby decreasing ploughing resistance by a factor of 3 0 6 6 with a six fold increase in ploughing velocity characteristic timescales of pore pressure diffusion and compaction down glacier from ploughing particles depend on till diffusivity ploughing velocity and sizes of ploughing particles these timescales accurately predict the ranges of these variables over which excess pore pressure and velocity weakening occurred existing ploughing models do not account for velocity weakening a new ploughing model with no adjustable parameters predicts ploughing resistance to no worse than 38 but requires that excess pore pressures be measured velocity weakening by this mechanism may affect fast glacier flow sediment transport by bed deformation and basal seismicity

a modeling study of the effect of runoff variability on the effective pressure beneath russell glacier west greenland basal sliding is a main control on glacier flow primarily driven by water pressure at the glacier base the ongoing increase in surface melting of the greenland ice sheet warrants an examination of its impact on basal water pressure and in turn on basal sliding here we examine the case of russell glacier in west greenland where an extensive set of observations has been collected these observations suggest that the recent increase in melt has had an equivocal impact on the annual velocity with stable flow on the lower part of the drainage basin but accelerated flow above the equilibrium line altitude ela these distinct behaviors have been attributed to different evolutions of the subglacial draining system during and after the melt season here we use a high resolution subglacial hydrological model forced by reconstructed surface runoff for the period 2008 to 2012 to investigate the cause of these distinct behaviors we find that the increase in meltwater production at low elevation yields a more efficient drainage system compatible with the observed stagnation of the mean annual flow below the ela at higher elevation the model indicates that the drainage system is mostly inefficient and is therefore strongly sensitive to an increase in meltwater availability which is consistent with the observed increase in ice velocity

ridge segmentation and the magnetic structure of the southwest indian ridge at 50 30 e 55 30 e and 66 20 e implications for magmatic processes at ultraslow spreading centers 1 the aim of this paper is to investigate the relationships between the segmentation and the magnetic structure of the ultraslow spreading southwest indian ridge contrary to faster spreading ridges magnetization usually decreases from high values along the neovolcanic axis to low values in the nontransform discontinuities there is a direct correlation between the deepening of the axial valley and the decrease of the magnetization from the neovolcanic axis toward the deepest parts of the axial discontinuities we suggest that less frequent eruptions as the distance from the segment center and the length of these discontinuities increase result in thinner extrusive lavas and thus control the along axis magnetization variations by thinning the magnetic source layer a unique segment centered at 50 28 e shows a marked low magnetization anomaly at its center similarly to the segments of the slow spreading mid atlantic ridge we suggest that in this segment both the mantle temperature and the magmatic activity are high enough for the lavas not to be highly fractionated a higher rate of melt production to the west of gallieni transform fault may have created some form of reservoir where mixing of melts occurs and where crystalline fractionation is low producing low magnetization lavas to the east magma chambers may be smaller with cooler mantle temperatures resulting in restricted mixing and significant fractionation which may lead to relatively high intensity magnetization lavas finally we propose that serpentinization of peridotites has no significant contribution to the variation of the magnetization along the axial valley off axis in thin crust areas upper mantle rocks may become progressively more altered as distance from the axis increases the strong faulting and alteration of a thin basaltic cap and underlying upper mantle rocks can produce the disappearance of the magnetic reversal pattern and the increase of the magnetization which is observed along the traces of the largest amagmatic discontinuities by contrast in thicker crust areas the upper mantle rocks are shielded from the alteration and the serpentinization process may be delayed resulting as on the mid atlantic ridge in slightly more positive magnetization values along the traces of axial discontinuities regardless of polarity

the basal stress distribution of ice stream e antarctica inferred by control methods the irregular spatial distribution and velocity independence of basal friction derived from landsat measured surface velocity suggests that ice stream flow is not controlled by the properties of a deformable basal till alone rigid bedrock substrata may contact the base of the ice stream in small 100 km2 areas where the velocity field displays strong vorticity and where the ice stream surface appears rumpled in landsat images

seasonal speedup along the western flank of the greenland ice sheet it has been widely hypothesized that a warmer climate in greenland would increase the volume of lubricating surface meltwater reaching the ice bedrock interface accelerating ice flow and increasing mass loss we have assembled a data set that provides a synoptic scale view spanning ice sheet to outlet glacier flow with which to evaluate this hypothesis on the ice sheet these data reveal summer speedups 50 to 100 consistent with but somewhat larger than earlier observations the relative speedup of outlet glaciers however is far smaller 15 furthermore the dominant seasonal influence on jakobshavn isbrae s flow is the calving front s annual advance and retreat with other effects producing outlet glacier speedups an order of magnitude larger seasonal melt s influence on ice flow is likely confined to those regions dominated by ice sheet flow

estimating the long term calving flux of kronebreen svalbard from geodetic elevation changes and mass balance modelling this study independently quantifies geodetic elevation change and models surface mass balance to solve the continuity equation the approach is tested on two dynamically different glaciers kongsvegen and kronebreen in northwest svalbard through two time epochs 1966 1990 95 i and 1990 95 2007 ii on kongsvegen a dynamically inactive glacier the residual term represents an error associated with determining elevation changes and surface mass balance it is apparent that centerline mass balance estimates are not representative of the entire glacier which we relate to center line accumulation being larger than the elevation bin average on kronebreen a fast flowing and actively calving glacier a significant part of the residual is identified with the long term calving flux for both glaciers the cumulative surface mass balance remained close to zero during the first epoch but became increasingly negative in the second epoch the long term calving flux of kronebreen is estimated to be 0 14 0 03 km3 w e a 1 during epoch i and 0 20 0 05 km3 w e a 1 in epoch ii

alpine subglacial hydrology abstract the size and shape of subglacial flow pathways exert a strong influence over the pressure and speed at which meltwaters are routed through glaciers these characteristics in turn influence glacier sliding velocity and the quantity and quality of bulk meltwaters discharging from glaciers in this paper we summarise four empirical research approaches that have been developed to improve our understanding of the subglacial hydrology of predominantly temperate based alpine glaciers investigations of bulk meltwater discharge and chemistry tracer studies proglacial bedrock investigations and borehole investigations as a result of linking these studies to conceptual or numerical models three arborescent and four non arborescent or distributed subglacial drainage configurations are described these drainage networks may be neither spatially nor temporally persistent but may compete with and replace each other on a seasonal basis a qualitative model of the temporal and spatial evolution of a typical alpine subglacial drainage system is advanced on the basis of research conducted at haut glacier d arolla switzerland

extrusion of high pressure cache creek rocks into the triassic stikinia quesnellia arc of the canadian cordillera implications for terrane analysis of ancient orogens and palaeogeography

stratigraphy and structure of the schoonover sequence northeastern nevada implications for paleozoic plate margin tectonics radiolarian biostratigraphy and detailed geologic mapping have been used to resolve the complex structure and stratigraphy of part of the golconda allochthon in the independence mountains nevada here the schoonover sequence is latest devonian to early permian in age spanning the time interval from the emplacement of the roberts mountains allochthon onto the shelf in the earliest mississippian antler orogeny to the inception of the sonoma orogeny the history of the schoonover basin is tied to that of the adjacent shelf in nevada and provides important insights into the upper paleozoic paleogeographic framework of the continental margin in the schoonover sequence latest devonian famennian chert overlies basaltic and andesitic greenstone and is in turn overlain by early mississippian kinderhookian chert interbedded with tuff and volcaniclastic rocks derived from an island arc the kinderhookian volcaniclastic rocks grade upward with no obvious depositional break into siliciclastic turbidites and pebbly mudstones that contain debris derived primarily from erosion of the antler orogenic belt although in some beds volcanic rock fragments and greenschist grade metasedimentary clasts are also common the age of underlying kinderhookian and interbedded and overlying osagian to meramecian chert sequences indicates that the siliciclastic turbidites are synchronous with the early deposits of the autochthonous foredeep basin of the antler orogenic belt the dual or composite source terranes represented by the schoonover siliciclastic rocks place the basin between an arc and the antler orogen on the edge of the shelf meramecian age basalt flows in the schoonover are coeval with subsidence and basaltic volcanism in autochthonous shelf sequences in northern nevada suggesting that a rifting event may have occasioned the end of antler age compression in nevada the onset of limestone turbidite deposition in the schoonover corresponds to the reestablishment of carbonate shelf conditions on the continental margin in latest mississippian earliest pennsylvanian time the late devonian earliest mississippian part of the schoonover depositional history has not yet been documented elsewhere in the golconda allochthon but late mississippian to permian rocks in the schoonover sequence are analogous to those of the havallah sequence suggesting deposition in the same basin thrust faults in the schoonover repeat the late devonian to permian section detaching the basinal sequence from its depositional basement thrust faults extend along strike for at least 10 km but thrust plates are the stratigraphic relations documented in the schoonover sequence are compatible with a back arc thrusting model for the formation of both the roberts mountains and golconda allochthons but they are more difficult to reconcile with models that interpret the allochthons as accretionary prisms developed in front of farther traveled arcs that collided with a passive margin in a broader context the stratigraphy of upper paleozoic allochthonous rocks in nevada records short lived episodes of crustal shortening along the continental margin separated by longer episodes of extensional and or transcurrent tectonics this data suggest that the western united states was a southwest pacific style active margin at least as far back as the devonian

paleogeographic implications of permian tethyan corals from the klamath mountains california the discovery of a new species of late permian coral belonging to the waagenophyllid family a family formerly considered diagnostic of the tethyan faunal realm occurs within limestone lenses interbedded with volcaniclastic strata of the redding section eastern klamath mountains and provides new constraints on permian biogeography the coherent permian succession in the redding section is now known to contain coral species of both tethyan and north american affinity and therefore along with other terranes of the mccloud belt this island arc succession represents a transitional biogeographic province thus the influence of the permian north american faunal realm was not everywhere isolated from that of the tethyan realm the same species of waagenophyllid coral also has been found along with a typical tethyan fusulinid assemblage in a seamount derived olistostromal limestone block which was incorporated into the eastern hayfork subduction complex these two occurrences suggest that atolls provided a network of shallow water tropical environments which allowed the migration of tethyan forms into the eastern paleo pacific ocean in rare instances individual species belonging to families formerly considered diagnostic of tethyan fauna migrated even farther eastward and colonized terranes of the mccloud faunal belt these relations limit the longitudinal displacement of terranes with respect to the continental margin required by the presence of exotic tethyan fauna

late paleozoic and mesozoic terrane interactions in north central british columbia five clearly defined terranes comprising from northeast to southwest ancestral north america slide mountain quesnellia cache creek and stikinia are the dominant tectonic elements of north central british columbia stratigraphic sedimentological plutonic metamorphic and structural data show that the slide mountain terrane evolved as a subduction accretion and island arc complex during permian time sedimentological data hint at the demise of the slide mountain and cache creek oceanic environments in the permian or early triassic and late triassic respectively subduction led to the development of volcanic plutonic island arcs on stikinia quesnellia and locally on the cache creek terrane in late triassic to middle jurassic time marked inter and intra terrane contraction in the middle jurassic resulted in the south westward thrusting of the cache creek terrane onto stikinia the subsequent development of the bowser basin on stikinia and possible coeval culmination of the emplacement of que

basal shear stress of the ross ice streams from control method inversions 1 we used control method inversions to determine the basal shear stress beneath the ross ice streams where new high resolution velocity data sets have recently become available the inversion algorithm was adapted from an earlier viscous bed algorithm to allow solution for the basal shear stress corresponding to a weak plastic bed we performed several experiments using synthetic data to determine the quality of the inversions these experiments indicate that with high quality surface elevation data e g errors 5 m the inversions are relatively robust with respect to errors in ice flow velocity and bed topography the inversions are consistent with seismic and borehole observations and indicate that the ross ice streams lie atop a bed that is nearly everywhere weak in contrast the tributaries feeding these ice streams overlie alternating patches of strong and weak bed

paleomagnetism of the late paleozoic slide mountain terrane northern and central british columbia paleomagnetic samples were collected from mid pennsylvanian to lower permian red argillaceous cherts at two localities of the slide mountain terrane 18 sites from the sylvester allochthon in northern british columbia and 11 sites from sliding mountain in central british columbia a secondary component of natural remanent magnetization in the sylvester samples yields a paleomagnetic pole that can be brought into coincidence with the jurassic portion of the north american apparent polar wander path by inferring vertical axis rotation during obduction of the allochthon both localities yield a characteristic component chrm with unblocking temperatures from 650 to 680 c after structural correction for bedding tilt all inclinations of chrm are negative consistent with magnetization during a reversed polarity interval in the northern hemisphere site mean chrm directions show consistent inclinations but distinct stratigraphic groupings of declinations inclination only statistics indicate that the chrm p

syn accretionary multistage assembly of an early jurassic alaskan type intrusion in the canadian cordillera u pb and 40ar 39ar geochronology of the turnagain ultramafic mafic intrusive complex yukon tanana terrane the magmatic evolution of the early jurassic turnagain alaskan type intrusion is inextricably linked to terrane accretionary events at the north american continental margin we have calibrated the

isotope fractionation by sulfate reducing natural populations and the isotopic composition of sulfide in marine sediments isotope fractionations during sulfate reduction by natural bacterial populations were measured in seven different marine sediments and compared with the isotopic composition of solid phase sulfides in the same sediments the measured fractionations during sulfate reduction could explain only between 41 and 85 of the 34s depletion in the sedimentary sulfides this result directly demonstrates that the depletion of 34s in solid sulfides is an expression of the combined activity of sulfate reducing organisms with additional fractionations accumulated during the oxidative part of the sulfur cycle the only known process to significantly augment the fractionations created during sulfate reduction is the microbial disproportionation of the intermediate sulfur compounds elemental sulfur thiosulfate and sulfite in a simple model we show how important each of these disproportionation processes could be in generating the isotopic composition of sedimentary sulfides the sulfate reduction rates in the sediments studied varied by a factor of 1000 and did not correlate with the fractionation during sulfate reduction by contrast a correlation was observed between sulfate reduction rate and the extent of 34s depletion into sedimentary sulfides the most 34s depleted sulfides found in sediments supporting the lowest rates of sulfate reduction thus the fractionations imposed during the disproportionating processes are better expressed in sediments with low sulfate reduction rates the direct phototrophic oxidation of sulfide to sulfate with minimal fractionation may have been important in the sediments that had a high sulfate reduction rate

evidence for excess pore water pressure generated in subglacial sediment implications for clast ploughing 1 pore water pressure in front of an object pulled through subglacial sediment and water pressure at the base of a borehole were simultaneously recorded at unteraargletscher switzerland to investigate the generation of excess pore pressures downglacier from ploughing clasts analysis of the strongly correlated pressure records revealed that the amplitude of the pore pressure fluctuations was larger than that of the corresponding borehole pressure fluctuations guided by high temporal resolution measurements of glacier surface motion from a different year this finding suggests that temporal changes in glacier speed lead to changes in the degree of excess pore pressure in front of the ploughing object the generation of excess pore water pressure has the potential to weaken the sediment downglacier from ploughing clasts instead of these clasts providing the roughness at the ice bed interface to resist glacier motion the ploughing process may in fact help to decouple the ice from the bed reducing basal resistance

major dextral transcurrent displacements along the northern rocky mountain trench and related lineaments in north central british columbia the northern rocky mountain trench and a number of other prominent lineaments along and east of the eastern margin of the intermontane belt mark faults along which dextral transcurrent movements have been dominant offsets of shelf to off shelf facies boundaries in lower paleozoic rocks indicate a cumulative displacement of at least 750 km and probably 900 km within the system of faults related to those in the northern rocky mountain and the tintina trenches farther west another system of faults appears to offset plutons and stratigraphic assemblages along the eastern margin of the intermontane belt by as much as 300 km these faults including the kutcho and the pinchi connect in part with the teslin suture zone in yukon territory and probably with the fraser river straight creek fault zone in southern british columbia although dextral transcurrent faulting may have taken place between the middle jurassic and early cenozoic the most convincing evidence points to middle cretaceous and particularly to early cenozoic eocene displacements the eocene movements were temporally related to plutonism volcanism lamprophyre dike emplacement high heat flow sedimentation in grabens and rapid uplift of northwesterly trending elongate ranges climactic episodes of granite emplacement particularly in and near the northern omineca crystalline belt at 100 m y 70 m y and 50 m y ago may have been facilitated by changes from dominantly compressional to dominantly transcurrent and related tensional strain

winter motion mediates dynamic response of the greenland ice sheet to warmer summers 1 we present ice velocities from a land terminating transect extending 115 km into the western greenland ice sheet during three contrasting melt years 2009 2011 to determine whether enhanced melting accelerates dynamic mass loss we find no significant correlation between surface melt and annual ice flow there is however a positive correlation between melt and summer ice displacement but a negative correlation with winter displacement this response is consistent with hydro dynamic coupling enhanced summer ice flow results from longer periods of increasing surface melting and greater duration ice surface to bed connections while reduced winter motion is explicable by drainage of high basal water pressure regions by larger more extensive subglacial channels despite mean interannual surface melt variability of up to 70 mean annual ice velocities changed by 7 5 increased summer melting thereby preconditions the ice bed interface for reduced winter motion resulting in limited dynamic sensitivity to interannual variations in surface melting

seasonal speedup of a greenland marine terminating outlet glacier forced by surface melt induced changes in subglacial hydrology we present subdaily ice flow measurements at four gps sites between 36 and 72 km from the margin of a marine terminating greenland outlet glacier spanning the 2009 melt season our data show that 35 km from the margin seasonal and shorter time scale ice flow variations are controlled by surface melt induced changes in subglacial hydrology following the onset of melting at each site ice motion increased above background for up to 2 months with resultant up glacier migration of both the onset and peak of acceleration later in our survey ice flow at all sites decreased to below background multiple 1 to 15 day speedups increased ice motion by up to 40 above background these events were typically accompanied by uplift and coincided with enhanced surface melt or lake drainage our results indicate that the subglacial drainage system evolved through the season with efficient drainage extending to at least 48 km inland during the melt season while we can explain our observations with reference to evolution of the glacier drainage system the net effect of the summer speed variations on annual motion is small 1 this in part is because the speedups are compensated for by slowdowns beneath background associated with the establishment of an efficient subglacial drainage system in addition the speedups are less pronounced in comparison to land terminating systems our results reveal similarities between the inland ice flow response of greenland marine and land terminating outlet glaciers

modeling channelized and distributed subglacial drainage in two dimensions 1 we present a two dimensional glacier drainage system model glads that couples distributed and channelized subglacial water flow distributed flow occurs through linked cavities that are represented as a continuous water sheet of variable thickness channelized flow occurs through rothlisberger channels that can form on any of the edges of a prescribed unstructured network of potential channels water storage is accounted for in an englacial aquifer and in moulins which also act as point sources of water to the subglacial system solutions are presented for a synthetic topography designed to mimic an ice sheet margin for low discharge all the flow is accommodated in the sheet whereas for sufficiently high discharge the model exhibits a channelization instability which leads to the formation of a self organized channel system the random orientation of the network edges allows the channel system geometry to be relatively unbiased in contrast to previous structured grid based models under steady conditions the model supports the classical view of the subglacial drainage system with low pressure regions forming around the channels under diurnally varying input water flows in and out of the channels and a rather complex spatiotemporal pattern of water pressures is predicted we explore the effects of parameter variations on the channel system topology and mean effective pressure the model is then applied to a mountain glacier and forced with meltwater calculated by a temperature index model the results are broadly consistent with our current understanding of the glacier drainage system and demonstrate the applicability of the model to real settings

formation of tubular carbonate conduits at athina mud volcano eastern mediterranean sea abstract tubular carbonate conduits tcc represent the termination of fluid plumbing systems in environments of hydrocarbon seepage and play a relevant role in the discharge of methane from sub seafloor sediments to the water column however the biogeochemical reactions and biological activities involved in their formation are not fully understood to address this tcc samples were collected with a remotely operated vehicle from the seabed on the sw flank of the athina mud volcano in the eastern mediterranean sea petrographic mineralogical stable carbon and oxygen isotope and lipid biomarker analyses were performed to elucidate the formation processes of the tubular carbonates clotted and fibrous aragonite form the internal lining of the cavities while the outer portion of the tubes is formed by micritic mg calcite cementing hemipelagic sediment 13c depleted mg calcite and aragonite as low as 14 4 v pdb and lipid biomarkers archaeol 89 8 v pdb indicate that carbonate precipitation was influenced by sulfate dependent anaerobic oxidation of methane aom aom locally enhances aragonite precipitation thereby facilitating early lithification of the conduits within the mud volcano sediments the size and morphology of the tcc comparable with the buried portion of tubeworm colonies found in the proximity of the sampling site however our results suggest that tcc likely formed by the action of burrowing organism rather than being mineralizations of the tubeworm colonies this study provides new insights into the interpretation and understanding of tcc highlighting the role of macrofaunal activity in the formation of migration pathways for hydrocarbon rich fluids on the flank of a mud volcano

geological and geochemical evidence for variable magmatism and tectonics in the southern canadian cordillera paleozoic to jurassic suites greenwood southern british columbia the paleozoic and early mesozoic rocks of the greenwood mining camp in southern british columbia are a part of the quesnel terrane in the eastern part of the intermontane belt of the canadian cordillera upper paleozoic rocks include the knob hill group composed of oceanic tholeiitic basalts with la yb n 0 4 1 2 associated with deep ocean sedimentary rocks and serpentinites the attwood group that comprises island arc tholeiites with la yb n 1 4 and positive end values clastic sedimentary rocks and limestones and a unit of oceanic gabbros with la yb n 0 5 these lithologically defined units occur as tectonically emplaced slivers of oceanic crust probably produced during the closure of the slide mountain basin during the permian they are unconformably overlain by middle triassic calc alkaline volcanic and sedimentary rocks of the brooklyn group the brooklyn group volcanic rocks have characteristics of mature island arc rocks including la yb n 2 5 4 5 and positive end values the paleozoic r

surface melt induced acceleration of greenland ice sheet flow ice flow at a location in the equilibrium zone of the west central greenland ice sheet accelerates above the midwinter average rate during periods of summer melting the near coincidence of the ice acceleration with the duration of surface melting followed by deceleration after the melting ceases indicates that glacial sliding is enhanced by rapid migration of surface meltwater to the ice bedrock interface interannual variations in the ice acceleration are correlated with variations in the intensity of the surface melting with larger increases accompanying higher amounts of summer melting the indicated coupling between surface melting and ice sheet flow provides a mechanism for rapid large scale dynamic responses of ice sheets to climate warming

blueschist and eclogite in mylonitic allochthons ross river and watson lake areas southeastern yukon two new occurrences of eclogite associated with mylonitic rocks are documented the first near ross river is in a quartz rich metasedimentary host that includes glaucophane schist the host forms part of a belt of high pressure rocks more than 50 km long and several kilometres wide between ross river and faro in the second occurrence north of watson lake eclogite is associated with mafic and ultramafic rocks occurring in a klippe more than 100 km2 in area that rests on unmetamorphosed triassic rocks of the north american miogeocline geothermobarometry shows that the eclogites were metamorphosed at 10 15 kb 1 kb 100 mpa between 470 and 750 c these peak conditions are comparable to those for other eclogites in the yukon and adjacent alaska the age of metamorphism and mylonitization is triassic or older the high pressure rocks occur at the leading edge of the most inboard accreted terrane along its boundary with north american miogeoclinal strata over a length of several hundred kilometres t

assimilation of surface velocities acquired between 1996 and 2010 to constrain the form of the basal friction law under pine island glacier in ice sheet models slip conditions at the base between the ice and the bed are parameterized by a friction law the most common relation has two poorly constrained parameters c and m the basal slipperiness coefficient c depends on local unobserved quantities and is routinely inferred using inverse methods while model results have shown that transient responses to external forcing are highly sensitive to the stress exponent m no consensus value has emerged with values commonly used ranging from 1 to depending on the slip processes by assimilation of pine island glacier surface velocities from 1996 to 2010 we show that observed accelerations are best reproduced with m 5 we conclude that basal motion in much of the fast flowing region is governed by plastic deformation of the underlying sediments this implies that the glacier bed in this area can not deliver resistive stresses higher than today making the drainage basin potentially more sensitive to dynamical perturbations than predicted with models using standard values m 1 or 3

decadal slowdown of a land terminating sector of the greenland ice sheet despite warming ice flow along land terminating margins of the greenland ice sheet gis varies considerably in response to fluctuating inputs of surface meltwater to the bed of the ice sheet such inputs lubricate the ice bed interface transiently speeding up the flow of ice greater melting results in faster ice motion during summer but slower motion over the subsequent winter owing to the evolution of an efficient drainage system that enables water to drain from regions of the ice sheet bed that have a high basal water pressure however the impact of hydrodynamic coupling on ice motion over decadal timescales remains poorly constrained here we show that annual ice motion across an 8 000 km 2 land terminating region of the west gis margin extending to 1 100 m above sea level was 12 slower in 2007 14 compared with 1985 94 despite a 50 increase in surface meltwater production our findings suggest that over these three decades hydrodynamic coupling in this section of the ablation zone resulted in a net slowdown of ice motion not a speed up as previously postulated increases in meltwater production from projected climate warming may therefore further reduce the motion of land terminating margins of the gis our findings suggest that these sectors of the ice sheet are more resilient to the dynamic impacts of enhanced meltwater production than previously thought

lipid biomarker patterns reflect different formation environments of mussel and tubeworm dominated seep carbonates from the gulf of mexico atwater valley and green canyon abstract mussels and tubeworms thriving at many methane seeps typically live in symbiosis with chemosynthetic chiefly methanotrophic or thiotrophic bacteria it has been shown that the activities of chemosymbiotic animals can result in large differences in the sedimentary environments of their habitats here we put forward the concept that such environmental variability can be archived in the lipid biomarker inventories of authigenic carbonates forming in different locally confined environments at seeps mussel beds and tubeworm bushes in this case to test this hypothesis lipid biomarker patterns of carbonates from mussel and tubeworm environments from two seep sites atwater valley 340 and green canyon 852 of the gulf of mexico were analyzed previous work revealed stronger carbon isotope fractionation between the methane source and biomarkers of anaerobic methane oxidizing archaea 2 anme 2 sulfate reducing desulfosarcina desulfococcus dss consortia than for anme 1 dss consortia both performing anaerobic oxidation of methane aom similar 13cmethane values were found at the mussel and tubeworm sites from the same seeps and the local microbial consortia also appear to be largely similar based on the observed aom biomarker inventories yet a large average offset of 32 between the 13c values of molecular fossils of sulfate reducing bacteria srb involved in aom was observed with lower values typifying tubeworm carbonates than mussel carbonates this pattern is interpreted to reflect local effects on isotope fractionation caused by the chemosymbiotic metazoans at mussel and tubeworm dominated sites at tubeworm dominated sites the excess sulfate produced by thiotrophic symbionts of tubeworms and pumped down into the sediment results in persistent production of aom derived bicarbonate and the enrichment of 12c in sub surface sediments interestingly tubeworm carbonates also contain high amounts of non isoprenoidal dialkyl glycerol diethers dages with extreme 13c depletions representing compounds that derived from non dss cluster srb most likely 13c depleted aom derived organic intermediates were used as carbon sources by the dage producing non dss cluster srb possibly performing organoclastic sulfate reduction our study identifies significant variation in biomarker patterns between mussel and tubeworm carbonates at two seep sites in the gulf of mexico such variation allows to characterize different habitats at seeps which are shaped by the interaction of chemosymbiotic seep metazoans and their symbionts with the local environment metazoan community composition apparently controls geobiological interaction in seep ecosystems to a large degree which may allow tracing of the effects of chemosymbiosis into the rock record

a mass conservation approach for mapping glacier ice thickness the traditional method for interpolating ice thickness data from airborne radar sounding surveys onto regular grids is to employ geostatistical techniques such as kriging while this approach provides continuous maps of ice thickness it generates products that are not consistent with ice flow dynamics and are impractical for high resolution ice flow simulations here we present a novel approach that combines sparse ice thickness data collected by airborne radar sounding profilers with high resolution swath mapping of ice velocity derived from satellite synthetic aperture interferometry to obtain a high resolution map of ice thickness that conserves mass and minimizes the departure from observations we apply this approach to the case of nioghalvfjerdsfjorden 79north glacier a major outlet glacier in northeast greenland that has been relatively stable in recent decades the results show that our mass conserving method removes the anomalies in mass flux divergence yields interpolated data that are within about 5 of the original data and produces thickness maps that are directly usable in high spatial resolution high order ice flow models we discuss the application of this method to the broad and detailed radar surveys of ice sheet and glacier thickness copyright 2011 by the american geophysical union

dispersed remnants of a northeast pacific fringing arc upper paleozoic terranes of permian mccloud faunal affinity western u s two fragmentary subparallel belts of terranes within the western north american cordillera contain upper paleozoic rocks and are characterized by contrasting lithotectonic assemblages and contrasting permian faunal affinity these two belts are a volcanic arc related successions of permian mccloud faunal affinity mccloud belt and b subduction related accretionary complexes of permian tethy an faunal affinity cache creek belt this paper supports the hypothesis that the fragmentary terranes of the mccloud belt once constituted parts of a northeast pacific fringing arc system and in constrast to some earlier interpretations concludes that the volcanic arc evolved above an eastward dipping subduction zone the absolute distance between this arc and western north america during the late paleozoic cannot be constrained however there is little evidence to suggest closure of a major 10 km ocean basin or protracted periods of westward dipping subduction parts of devonian to permian volcanic island arc sequences of the western u s cordillera are represented in the northern sierra eastern klamath bilk creek grindstone and chilliwack terranes these scattered volcanic arc remnants share several fundamental characteristics 1 the sequences were constructed across continental affinity basement assemblages 2 they underwent similar tectonic evolution during late paleozoic time such as coeval pulses in volcanism and related depositional histories 3 they contain early permian mccloud type fauna of distinctive biogeographic affinity 4 mccloud belt terranes are spatially and possibly genetically related to westward lying accretionary complexes of the cache creek belt which contain fragments of upper triassic blueschist and permian limestone blocks bearing tethyan permian fusulinids and corals based on the presence of distinctive early permian mccloud fauna the island arc remnants discussed in this paper are referred to as the mccloud belt the similarities between mccloud belt terranes suggest that they were once parts of a long lived late paleozoic volcanic arc complex the stratigraphy of these terranes records the tectonic evolution of the volcanic arc volcanism within the paleozoic arc began as early as the early devonian and was succeeded or accompanied by carbonate and epiclastic sedimentation during the middle and late devonian an influx of detritus derived from sedimentary and metasedimentary rocks during the late devonian and mississippian records the uplift and erosion of arc basement sequences an event that was at least locally accompanied by ongoing volcanism a pulse in volcanism during the pennsylvanian and coeval uplift led to the establishment of shallow marine and deltaic volcanogenic and carbonate sedimentation and locally subaerial exposure which persisted into the early early permian elsewhere during the early permian shallow water carbonate deposition was established with only minor influx of volcanogenic debris during the late permian and locally as early as late early permian a major pulse in volcanism occurred and is now represented by voluminous sequences of volcanogenic debris and lavas most of these sequences are overlain by upper triassic carbonate platformal sequences arc volcanism was recorded at least locally until the middle jurassic paleomagnetic data are used to constrain a schematic pre cretaceous reconstruction of these volcanic arc sequences and their associated subduction assemblages the inferred paleogeography of island arc sequences and the adjacent accretionary terranes suggests that volcanism occurred above an east dipping subduction zone with a long lived paleogeographic relation to north america the current structural position of these terranes is largely the result of mesozoic and locally younger deformation

mid paleozoic initiation of the northern cordilleran marginal backarc basin geologic geochemical and neodymium isotope evidence from the oldest mafic magmatic rocks in the yukon tanana terrane finlayson lake district southeast yukon canada the fire lake formation of the yukon tanana terrane in the finlayson lake region yukon canada consists primarily of late devonian ca 365 360 ma mafic metavolcanic rocks and smaller volumes of mafic and ultramafic subvolcanic metamorphosed intrusions in this paper field geochemical and nd isotope attributes of these rocks are presented in an attempt to understand their tectonic setting the magmatic processes involved in their formation and their roles in cordil leran crustal growth the mafic rocks of the fire lake formation exhibit a wide diversity of geochemical signatures and are classified into seven chemically defined suites 1 back arc basin basalt 2 enriched mid oceanic ridge basalt e morb 3 oceanic island basalt oib 4 th rich oib 5 boninite 6 island arc tholeiite and 7 light rare earth element lree enriched island arc tholeiite the diversity of geochemical signatures is interpreted to represent variable mixtures of asthenospheric morb type mantle subarc mantle wedge and lithospheric oib type mantle with or without elemental contributions from the subducted slab and or continental crust these suites of rocks are also associated with fine grained basinal sedimentary facies variations in metavolcanic and metasedimentary unit thickness extensional synvolcanic faults and apparent extensional fault controlled emplacement of mafic intrusive rocks and hydrothermal volcanic hosted massive sulfide mineralization the suites also exhibit a broad spatial distribution those with arc signatures nb thmn 1 mn normalized to primitive mantle values are located primarily in the western parts of the formation and suites with nonarc signatures nb thmn 1 are located primarily within the eastern parts of the formation collectively these geologic and geochemical attributes are interpreted to stem from the transition from arc magmatism to the initiation of an extensional backarc basinal environment associated with an east dipping subduction zone the initiation of backarc basin magmatism recorded in the fire lake formation was part of a much larger late devonian backarc basinal system forming along the western edge of the margin of north america the fire lake formation is interpreted to represent 1 the commencement of yukon tanana arc rifting and separation from the north american cratonic margin and 2 the initiation of a marginal backarc basin now the slide mountain terrane inboard of the yukon tanana arc system this tectonic evolution likely occurred either as a result of slab rollback toward the west within the convergent margin responsible for yukon tanana arc activity or as a result of the propagation of the slide mountain backarc basin spreading ridges into the yukon tanana arc system this yukon tanana arc rifting episode was also broadly coincident with rifting and hydrothermal activity within rocks of the north american craton the geochemical and isotopic signatures of magmatic rocks in the fire lake formation have some features similar to intraoceanic arc rocks e g boninites island arc tholei ites and many have juvenile nd isotope signatures i e nd t 0 most have nd t 5 suggesting that the pericratonic terranes of the northern cordillera have a significant juvenile component if this is the case throughout the yukon tanana terrane then the pericratonic terranes may have contributed much more juvenile material to cordil leran crustal growth in the phanerozoic than has previously been considered

nina creek group and lay range assemblage north central british columbia remnants of late paleozoic oceanic and arc terranes in north central british columbia a belt of upper paleozoic volcanic and sedimentary rocks lies between mesozoic arc rocks of quesnellia and ancestral north america these rocks belong to two distinct terranes the nina creek group of the slide mountain terrane and the lay range assemblage of the quesnel terrane the nina creek group is composed of mississippian to late permian argillite chert and mid ocean ridge tholeiitic basalt formed in an ocean floor setting the sedimentary and volcanic rocks the mount howell and pillow ridge successions respectively form discrete generally coeval sequences interpreted as facies equivalents that have been interleaved by thrusting the entire assemblage has been faulted against the cassiar terrane of the north american miogeocline west of the nina creek group is the lay range assemblage correlated with the harper ranch subterrane of quesnellia it includes a lower division of mississippian to early pennsylvanian sedimentary and volcanic rocks some with con

allochthonous permian micro and macrofauna kamloops area british columbia the first waagenophyllid coral to be described from the western hemisphere has been found in permian rocks of the harper ranch group near kamloops southern british columbia it is parawentzelella miyagiella johnstonae sp nov associated with the tabulate coral multithecopora larushi sp nov numerous brachiopods and other fossils the containing rocks are considered to be allochthonous originating in the western part of the paleopacific tethyan ocean or in an island arc complex farther to the north the outcrop yielding the fossils appears to be a stratigraphic misfit both in age and origin fusulinids from adjacent areas are of early permian wolfcampian age but those associated with the fossils indicate a late permian guadalupian age these conflicting dates cannot at present be reconciled the tethyan origin of the fauna also contrasts with that of surrounding rocks which are non tethyan

stable isotope signatures and methane use by new zealand cold seep benthos the carbon isotopic composition of seep faunal tissue represents a time integrated view of the interaction between biology and the biogeochemical gradients within the environment here we provide an initial description of carbon and nitrogen stable isotope signatures of dominant symbiont bearing megafauna and heterotrophic mega and macrofauna from 10 methane seep sites on the continental margin of the north island of new zealand 662 1201 m water depth isotopic signatures suggest that sulfide oxidation supports symbiont bearing taxa including solemyid and vesicomyid bivalves and methanotrophic symbionts are present in the seep mussel bathymodiolus sp multiple species of frenulata siboglinidae are present and have a range of isotopic values that are indicative of both thiotroph and methanotroph based nutrition isotopic composition of the tubeworm lamellibrachia sp varied by 23 3 among individuals although there was no consistent difference among sites variation in methane use by heterotrophic fauna appears to reflect the availability of hard vs soft substrate macrofauna on hard substrates had high 13c signatures reflecting consumption of photosynthetic derived organic matter two unique biotic assemblages were discovered to be fueled largely by methane a hard substrate multi phyla sponge associated community inhabiting the sponge pseudosuberites sp and a soft sediment assemblage dominated by ampharetid polychaetes isotope signatures yield estimates of 38 100 and 6 100 methane derived carbon in sponge associates and ampharetid bed macrofauna respectively these estimates are comparable to those made for deeper methane seeps at the florida escarpment 3290 m and kodiak alaska seeps 4445 m the overall high use of methane as a carbon source by both symbiont bearing and heterotrophic fauna suggests that new zealand methane seeps are an ideal model system to study the interaction among metazoans bacteria archaea and their resulting effect on methane cycles

permian longitudes of wrangellia stikinia and eastern klamath terranes based on coral biogeography trend surface analysis of biogeographic data and probabilistic estimates of diversity and similarity provide a new approach to understanding the origin of north american suspect terranes results from permian corals indicate that 1 the eastern boundary of the tethyan coral province was about 2000 km west of the north american craton 2 eastern klamath and stikinia were close to one another and were up to 6700 km west of north america during the early permian 3 wrangellia was situated to the southeast of stikinia as much as 5000 km west of the craton 4 the late permian location of the eastern klamath terrane was 3000 5800 km west of the craton and between 11 and 21 n and 5 the eastern klamath terrane moved westward during the permian possibly as a result of back arc spreading

rate weakening drag during glacier sliding accurately specifying the relationship between basal drag on a hard rough glacier bed and sliding speed is a long standing and central challenge in glaciology drag on a rigid bed consisting of steps with linear treads inclined upglacier a good idealization for the bedrock morphology of some hard bedded glaciers has been considered in sliding theories but never studied empirically balancing forces parallel to step treads indicates that drag should be independent of sliding speed and cavity size and set by the limit equilibrium condition sometimes called iken s bound in this study we used a large ring shear device to slide ice at its pressure melting temperature across a stepped bed over a range of steady sliding speeds 29 348 m yr 1 and under a steady effective pressure 500 kpa contrary to expectation drag decreased 42 with increasing sliding speed and cavity size experimental deviations from theory cannot explain this decrease in drag with increasing sliding speed i e rate weakening we suggest that stress bridging in ice between ice bed contact zones and cavities causes stress gradients that require viscous deformation of ice to sustain stress equilibrium so that contact zones can be at shear stresses below limit equilibrium values a parameter linearly dependent on sliding speed that scales the extent of ice deformation to areas of ice bed contact allows the experimental drag relationship to be fitted with a simple sliding model rate weakening drag has now been observed for two contrasting bed morphologies stepped and sinusoidal highlighting the need to consider such behavior in glacier flow models

the relationship between subglacial water pressure and velocity of findelengletscher switzerland during its advance and retreat findelengletscher switzerland advanced about 250 m between 1979 and 1985 and retreated thereafter subglacial water pressure surface velocity and surface strain rate were determined at several sites the measurements were made early in the melt seasons of 1980 1982 1985 and 1994 and in the autumn of 1983 and the winter of 1984 changes of surface geometry were assessed from aerial photographs the estimated basal shear stress changed little between 1982 and 1994 nevertheless large changes in the relationship of subglacial water pressure and surface velocity were observed which cannot be reconciled with the most commonly used sliding law unless it is modified substantially consideration of possible reasons indicales that a change in the subglacial drainage system occurred probably involving a change in the degree of cavity interconnection isolated cavities damp the variations in sliding velocity that normally result from changes in water pressure because the pressure in isolated cavities decreases as the sliding speed increases in contrast by transmitting water pressure fluctuations to a larger area of the bed interconnected cavities amplify the effect of water pressure fluctuations on sliding speed thus we suggest that an observed decrease in velocity for a given water pressure between 1982 and 1994 was a consequence of a decrease in the interconnectedness of the subglacial cavity system

initial results of comparing cold seep carbonates from mussel and tubeworm associated environments at atwater valley lease block 340 northern gulf of mexico abstract chemosymbiotic macrofauna such as mussels and tubeworms and authigenic carbonates are typical of many hydrocarbon seeps to address whether mussels and tubeworms could impact the sediment geochemistry of their habitat where authigenic carbonates are precipitated a comparative study of petrographic and geochemical features of the authigenic carbonates from mussel and tubeworm associated environments at hydrocarbon seeps in atwater valley lease area block 340 at340 of the gulf of mexico was undertaken both mussel and tubeworm associated carbonates are dominated by high magnesium calcite hmc and aragonite and two tubeworm associated carbonate samples have minor amounts of dolomite the 13c values of all carbonates are low ranging from 60 8 to 35 5 pdb although there is much overlap surprisingly the 13c values of mussel associated carbonates are generally higher than those of tubeworm associated carbonates 51 8 vs 54 8 for an average of over 60 subsamples it is suggested that 1 carbon isotopic vital effect of seep mussels and tubeworms 2 fluid physical pumping of mussels and 3 release of sulfate by tubeworm roots may be responsible for the relatively lower 13c values of tubeworm associated carbonates it has been suggested that the heterogeneities in mineralogy and stable carbon isotope geochemistry of the seep carbonates may be attributed to the activity of macrofauna mussels and tubeworms and associated microbes our observations also suggest that at at340 the geochemical evolution of seep macrofauna is from a mussel dominated environment to a mixed mussel tubeworm environment and finally to a mostly tubeworm dominated environment this evolution is controlled mainly by the habitat e g hydrocarbon seep flux

a general theory of glacier surges we present the first general theory of glacier surging that includes both temperate and polythermal glacier surges based on coupled mass and enthalpy budgets enthalpy in the form of thermal energy and water is gained at the glacier bed from geothermal heating plus frictional heating expenditure of potential energy as a consequence of ice flow enthalpy losses occur by conduction and loss of meltwater from the system because enthalpy directly impacts flow speeds mass and enthalpy budgets must simultaneously balance if a glacier is to maintain a steady flow if not glaciers undergo out of phase mass and enthalpy cycles manifest as quiescent and surge phases we illustrate the theory using a lumped element model which parameterizes key thermodynamic and hydrological processes including surface to bed drainage and distributed and channelized drainage systems model output exhibits many of the observed characteristics of polythermal and temperate glacier surges including the association of surging behaviour with particular combinations of climate precipitation temperature geometry length slope and bed properties hydraulic conductivity enthalpy balance theory explains a broad spectrum of observed surging behaviour in a single framework and offers an answer to the wider question of why the majority of glaciers do not surge

u pb zircon monazite and sphene ages for granitic orthogneiss of the barkerville terrane east central british columbia granitic orthogneiss forms an important component of the barkerville terrane of southeastern british columbia rb sr whole rock ages for the orthogneisses are ambiguous and range from late proterozoic to mid paleozoic with large associated errors u pb dating of zircon monazite and sphene has been employed in an attempt to establish precise crystallization ages for two of the orthogneiss bodies u pb systematics for zircons from both bodies show the combined effects of inheritance of zircon cores and postcrystallization pb loss this complexity precludes a precise estimate of the age of emplacement of the granitic protoliths of the gneiss the data do however constrain possible emplacement ages for the bodies to between 335 and 375 ma late devonian mid mississippian a u pb age of 174 4 ma for metamorphic sphene from one of the orthogneiss bodies is interpreted as dating the end of the second phase of deformation in the area two nearly concordant u pb ages of 114 and 117 ma for monazite from th

hydrocarbon seepage during the messinian salinity crisis in the tertiary piedmont basin nw italy abstract seep carbonate deposits of messinian age have been recently found in the tertiary piedmont basin nw italy these carbonates are preserved as blocks within a chaotic unit emplaced during the messinian salinity crisis msc they show negative 13 c values from 27 to 15 vpdb that indicate the involvement of hydrocarbon rich fluids in their genesis three types of carbonates are recognised i vuggy carbonates ii lucina carbonates and iii tubeworm carbonates vuggy carbonates are characterised by carbonate pseudomorphs after gypsum and probably formed during the first stage of the msc they are the product of a complex diagenesis influenced by both hypersalinity and seepage of hydrocarbon rich fluids these rocks lack chemosymbiotic assemblages reflecting their formation under extreme environmental conditions inhospitable for most metazoans in contrast lucina and tubeworm carbonates are characterised by chemosymbiotic macrofauna represented respectively by lucina bivalves and putative vestimentiferan tubeworms the latter have not commonly been documented in ancient seep carbonates and have never been reported from the messinian sediments of the piedmont basin both lucina and tubeworm carbonates are interpreted as the product of hydrocarbon seepage during the second msc stage these two types of carbonates formed under less severe conditions than the vuggy carbonates allowing the survival of seep dwelling metazoans during the second msc stage the seafloor was probably characterised by an irregular topography and a thin bottom layer of dense anoxic brines produced by the dissolution of gypsum it is suggested that vestimentiferan worms were able to thrive on morphologic highs with the posterior part of tubes just below the oxic anoxic interface but the anterior part projecting into oxic water the infaunal lucina bivalves were only able to live at seeps with an overlying oxic water column the studied carbonate deposits show features reflecting the uncommon interaction of hydrocarbon rich seep fluids and sulphate enriched waters the latter resulting from both evaporation and dissolution of gypsum and allow to reconstruct the evolution of a seepage system during the msc

le terrane de slide mountain cordilleres canadiennes une lithosphere oceanique marquee par des points chauds the slide mountain terrane consists of devonian to permian siliceous and detrital sediments in which are interbedded basalts and dolerites locally ultramafic cumulates intrude these sediments the slide mountain terrane is considered to represent a back arc basin related to the quesnellia paleozoic arc terrane however the slide mountain mafic volcanic rocks exposed in central british colombia do not exhibit features of back arc basin basalts babb but those of mid oceanic ridge morb and oceanic island oib basalts the n morb type volcanic rocks are characterized by light rare earth element lree depleted patterns la nb ratios ranging between 1 and 2 moreover their nd and pb isotopic compositions suggest that they derived from a depleted mantle source the within plate basalts differ from those of morb affinity by lree enriched patterns higher tio2 nb ta and th abundances lower end values and correlatively higher isotopic pb ratios the nd and pb isotopic compositions of the ultramafic c

melt induced speed up of greenland ice sheet offset by efficient subglacial drainage fluctuations in surface melting are known to affect the speed of glaciers and ice sheets but their impact on the greenland ice sheet in a warming climate remains uncertain although some studies suggest that greater melting produces greater ice sheet acceleration others have identified a long term decrease in greenland s flow despite increased melting here we use satellite observations of ice motion recorded in a land terminating sector of southwest greenland to investigate the manner in which ice flow develops during years of markedly different melting although peak rates of ice speed up are positively correlated with the degree of melting mean summer flow rates are not because glacier slowdown occurs on average when a critical run off threshold of about 1 4 centimetres a day is exceeded in contrast to the first half of summer when flow is similar in all years speed up during the latter half is 62 16 per cent less in warmer years consequently in warmer years the period of fast ice flow is three times shorter and overall summer ice flow is slower this behaviour is at odds with that expected from basal lubrication alone instead it mirrors that of mountain glaciers where melt induced acceleration of flow ceases during years of high melting once subglacial drainage becomes efficient a model of ice sheet flow that captures switching between cavity and channel drainage modes is consistent with the run off threshold fast flow periods and later summer speeds we have observed simulations of the greenland ice sheet flow under climate warming scenarios should account for the dynamic evolution of subglacial drainage a simple model of basal lubrication alone misses key aspects of the ice sheet s response to climate warming

interannual variability of glacier basal pressure from a 20 year record basal pressure has been recorded at the svartisen subglacial laboratory northern norway for 20 years and is measured by load cells installed at the ice rock interface under 200 m of glacier ice synchronous pressure variations between load cells are investigated as evidence of stress redistribution and hydrological bed connectivity a running pearson correlation is used to study the temporal variation in the response of several sensors by studying the nature of this correlation as well as the correlation between sensor pairs it is possible to investigate the evolution of the degree of synchronous response and to some extent basal connectivity at the glacier bed persistent seasonal variations associated with the melt season are observed throughout the measurement period indicating dependence on surface hydrological forcing overlying this pattern specific years with longer periods of positive and negative correlation of pressure between sensors are presented to show contrasting interannual variability in basal pressure an anticorrelated connectivity is associated with a local increase in the rate of daily subglacial discharge and is caused by load transfer or passive cavity opening stable weather appears to enhance connectivity of the sensors which is attributed to the development of a persistent drainage system and stress redistribution

structure of the havallah sequence golconda allochthon nevada evidence for prolonged evolution in an accretionary prism the golconda allochthon of northern and central nevada contains the havallah sequence and correlative units of latest devonian to early late permian age the havallah sequence is dominated by radiolarian ribbon chert and argillite associated with variable but generally subordinate siliciclastic cal carenitic and volcaniclastic turbidites and slump deposits cherts of all ages except pennsylvanian locally rest depositionally on tholeiitic basalts some were altered and mineralized by ridge type hydrothermal systems suggesting deposition in an ocean basin containing active spreading center s for much of the upper paleozoic the havallah sequence was thrust eastward along the basal golconda thrust onto coeval sediments of the western north american shelf during the latest paleozoic earliest mesozoic sonoma orogeny detailed studies of the havallah sequence in the sonoma and tobin ranges and in battle mountain indicate the presence of complex diagenetic and structural fabrics that developed prior to the obduction of the allochthon a large number of thrusts composite thrusts and shear zones slice the havallah into numerous tectonic packets of contrasting lithology and or internal structural style internal structures in chert packets include bedding parallel and bedding normal solution cleavage solution boudins three or more sets of east verging folds of variable geometry and features associated with high pore fluid pressures such as crack seal fractures dilation breccias clastic dikes and sills and clastic intrusions along thrust surfaces the fabrics suggest a general progressive evolution from deformation characterized by bedding normal compression and slight east west extension d1 to deformation dominated by bedding parallel east directed shear d2 local chert packets may have suffered alternating episodes of high and low pore fluid pressure the degree of development and the styles and orientations of various structural elements vary sometimes radically from packet to packet the pre golconda thrust fabrics can be modeled as the result of imbrication and deformation of successive batches of ocean floor sediments into the toe of an accretionary prism in front of an east facing arc relatively undeformed permian calcarenite units interpreted as being trench slope deposits suggest that this prism was well developed by permian time coarse slump deposits containing chert clasts with pre depositional structural fabrics suggest a prism that was active long enough to recycle previously tectonized cherts if we are correct the classical sonoma orogeny d3 marked the culmination of a protracted structural evolution that may have spanned much of the upper paleozoic

late paleozoic conodonts from ribbon chert delineate imbricate thrusts within the antler formation of the slide mountain terrane central british columbia late devonian early carboniferous to permian conodonts from the antler formation of slide mountain group near barkerville british columbia were extracted from 23 of 28 chert samples collected from a nearly continuous section on sliding mountain the conodonts delineate three possibly four thrust sheets within the section from bottom to top the ages recorded in the sheets consist of i early permian ii pennsylvanian to early permian iii early mississippian to early permian and iv late devonian or early mississippian in sheet iii 100 m y of chert and basalt stratigraphy the entire known age span of the antler formation is recorded in less than 300 m prior to thrust imbrication the antler formation must have been very thin and possibly covered a large area within individual thrust sheets the stratigraphic features and their continuity are preserved

superimposed quesnel late paleozoic jurassic and yukon tanana devonian mississippian arc assemblages cassiar mountains northern british columbia field u pb and igneous petrochemical evidence allochthons in the cassiar mountains of northern british columbia contain assemblages belonging to two distinct canadian cordilleran terranes yukon tanana ytt and quesnellia these assemblages of pre late devonian devonian mississippian pennsylvanian permian and early jurassic age occur in intrusive and depositional as well as structural contact with each other the allochthons are gently dipping thrust panels interrupted by the mid cretaceous cassiar batholith a key element for correlation across the batholith is the mississippian and older pericratonic dorsey complex new devonian mississippian u pb ages for deformed plutons within it document an igneous suite like those in type yukon tanana exposures farther north other characteristics of the dorsey complex that ally it with ytt are orthoquartzites and grits and amphibolite bodies with transitional mid ocean ridge basalt morb to ocean island basalt oib petrochemical signatures unconformities deformed clasts in the late paleozoic sequences and a shared mid permian intrusive suite show that later arcs onlapped the mid paleozoic and older ytt assemblage the early jurassic intrusive suite cuts all major contacts and fabrics except the terrane bounding fault between the slide mountain and combined ytt quesnel terranes it represents a northern continuation of a plutonic belt that extends the length of the mesozoic quesnel magmatic arc these relationships carry important implications for cordilleran terrane history and the tectonic evolution of the north american margin at least some of the major terranes were not unrelated entities prior to their accretion to the continent but a system of superimposed and interconnected arcs that developed over a protracted time interval with complex and evolving paleogeographic configurations much like the modern western pacific province

water flow through temperate glaciers understanding water movement through a glacier is fundamental to several critical issues in glaci ology including glacier dynamics glacier induced floods and the prediction of runoff from glacierized drainage basins to this end we have synthesized a conceptual model of water movement through a temper ate glacier from the surface to the outlet stream pro cesses that regulate the rate and distribution of water input at the glacier surface and that regulate water movement from the surface to the bed play important but commonly neglected roles in glacier hydrology where a glacier is covered by a layer of porous perme able firn the accumulation zone the flux of water to the glacier interior varies slowly because the firn tempo rarily stores water and thereby smooths out variations in the supply rate in the firn free ablation zone in con trast the flux of water into the glacier depends directly on the rate of surface melt or rainfall and therefore varies greatly in time water moves from the surface to the bed through an upward branching arborescent net work consisting of both steeply inclined conduits formed by the enlargement of intergranular veins and gently inclined conduits spawned by water flow along the bottoms of near surface fractures crevasses engla cial drainage conduits deliver water to the glacier bed at a limited number of points probably a long distance downglacier of where water enters the glacier englacial conduits supplied from the accumulation zone are quasi steady state features that convey the slowly varying water flux delivered via the firn their size adjusts so that they are usually full of water and flow is pressurized in contrast water flow in englacial conduits supplied from the ablation area is pressurized only near times of peak daily flow or during rainstorms flow is otherwise in an open channel configuration the subglacial drainage system typically consists of several elements that are distinct both morphologically and hydrologically an up glacier branching arborescent network of channels in cised into the basal ice conveys water rapidly much of the water flux to the bed probably enters directly into the arborescent channel network which covers only a small fraction of the glacier bed more extensive spatially is a nonarborescent network which commonly includes cav ities gaps between the glacier sole and bed channels incised into the bed and a layer of permeable sediment the nonarborescent network conveys water slowly and is usually poorly connected to the arborescent system the arborescent channel network largely collapses during winter but reforms in the spring as the first flush of meltwater to the bed destabilizes the cavities within the nonarborescent network the volume of water stored by a glacier varies diurnally and seasonally small temper ate alpine glaciers seem to attain a maximum seasonal water storage of 200 mm of water averaged over the area of the glacier bed with daily fluctuations of as much as 20 30 mm the likely storage capacity of subglacial cavities is insufficient to account for estimated stored water volumes so most water storage may actually occur englacially stored water may also be released abruptly and catastrophically in the form of outburst floods

borehole water level variations and the structure of the subglacial hydrological system of haut glacier d arolla valais switzerland late summer subglacial water pressures have been measured in a dense array of boreholes in the ablation area of haut glacier d arolla switzerland interpolated surfaces of minimum diurnal water pressure and diurnal water pressure variation suggest the presence of a subglacial channel within a more widespread distributed drainage system the channel flows along the centre of a variable pressure axis vpa some tens of metres wide that is characterized by low minimum diurnal water pressures frequently atmospheric and high diurnal water pressure variations these characteristics are transitional over a lateral distance of c 70 m to higher and more stable subglacial water pressures in the adjacent distributed system water pressure variations recorded in boreholes located close to the centre of the vpa reflect the delivery of surface derived meltwater to the glacier bed and result in a diurnally reversing transverse hydraulic gradient that drives water out from the channel into the distributed system during the afternoon and back to the channel overnight subglacial observations suggest that such flow occurs through a vertically confined sediment layer borehole turbidity records indicate that the resulting diurnal water flows are responsible for the mobilization and transport of fine debris in suspension analysis of the propagation velocity and amplitude attenuation of the diurnal pressure waves suggests that the hydraulic conductivity of the sediment layer decreases exponentially with distance from the channel falling from c 10 4 m s 1 at the channel boundary to c 10 7 ms 1 70m away these apparent hydraulic conductivities are consistent with darcian flow through clean sand and typical glacial till respectively we suggest that fine material is systematically flushed from basal sediments located adjacent to large melt season drainage channels beneath warm based glaciers this process may have important implications for patterns of glacier erosion hydrochemistry and dynamics

large and rapid melt induced velocity changes in the ablation zone of the greenland ice sheet continuous global positioning system observations reveal rapid and large ice velocity fluctuations in the western ablation zone of the greenland ice sheet within days ice velocity reacts to increased meltwater production and increases by a factor of 4 such a response is much stronger and much faster than previously reported over a longer period of 17 years annual ice velocities have decreased slightly which suggests that the englacial hydraulic system adjusts constantly to the variable meltwater input which results in a more or less constant ice flux over the years the positive feedback mechanism between melt rate and ice velocity appears to be a seasonal process that may have only a limited effect on the response of the ice sheet to climate warming over the next decades

geology and juxtaposition history of the yukon tanana slide mountain and cassiar terranes in the glenlyon area of central yukon in central yukon the pericratonic yukon tanana terrane yt is juxtaposed with the cassiar terrane ct parautochthonous north america along the tummel fault zone tfz a 3 4 km wide northwest trending belt comprising imbricate fault slices of slide mountain terrane sm greenstone chert serpentinite and synorogenic clastic rocks north east of the tfz the ct comprises paleozoic metapelitic rocks marble and amphibolite of continental margin affinity to the southwest the yt consists of a pre late devonian metasedimentary complex overlain and intruded by mississippian clastic volcanic and plutonic successions of continental arc affinity in the tfz middle to late permian ocean floor basalt of the sm shows evidence of crustal contamination suggesting deposition at the edge of a marginal ocean basin deformation features in the tfz include early ductile fabrics overprinted by younger brittle structures triassic synorogenic clastic rocks in the tfz and at the base of a klippen above the yt suggest that terrane imbrication began shortly after the early triassic 40 ar 39 ar mica ages from the region suggest cooling of the yt sm and part of ct below 300 c by early jurassic time pervasive brittle structures in the ragged lake klippe which roots into the tfz indicate brittle thrusting of the sm over the ct in post triassic time early cretaceous plutons intrude the ct glenlyon batholith and the tfz leucogabbro and impose a contact aureole that extends westward into the yt steep brittle structures that deformed the tfz also affect in part the glenlyon batholith but do not significantly offset its contact aureole consequently little displacement can have occurred along the tfz after early cretaceous time

trace element geochemistry of metabasaltic rocks from the yukon tanana upland and implications for the origin of tectonic assemblages in east central alaska we present major and trace element geochemical data for 27 amphibolites and six greenstones from three structural packages in the yukon tanana upland of east central alaska the lake george assem

seasonal evolution of subglacial drainage and acceleration in a greenland outlet glacier an acceleration of ice mass loss has been observed near the margin of the greenland ice sheet partly as a result of faster ice motion observations by gps receivers reveal high seasonal variability in ice motion with summer motion up to 220 higher than winter background levels the greenland ice sheet contains enough water to raise sea levels by 7 m however its present mass balance and future contribution to sea level rise is poorly understood1 accelerated mass loss has been observed near the ice sheet margin partly as a result of faster ice motion2 3 4 surface melt waters can reach the base of the ice sheet and enhance basal ice motion5 6 however the response of ice motion to seasonal variations in meltwater supply is poorly constrained both in space and time here we present ice motion data obtained with global positioning system receivers located along a 35 km transect at the western margin of the greenland ice sheet throughout a summer melt season our measurements reveal substantial increases in ice velocity during summer up to 220 above winter background values these speed up events migrate up the glacier over the course of the summer the relationship between melt and ice motion varies both at each site throughout the melt season and between sites we suggest that these patterns can be explained by the seasonal evolution of the subglacial drainage system similar to hydraulic forcing mechanisms for ice dynamics that have been observed at smaller glaciers

upper triassic takla group volcanic rocks stikine terrane north central british columbia geochemistry petrogenesis and tectonic implications the upper triassic takla group volcano sedimentary assemblage is part of the stikine terrane of the intermontane belt in the canadian cordillera and covers an area of more than 30 000 km2 in a belt up to 50 km wide and more than 800 km long in the mcconnell creek area of north central british columbia the assemblage consists of plagioclase clinopyroxene phyric dominantly basaltic to andesitic flows and pyroclastic rocks interlayered with volcanogenic sedimentary rocks compositionally the volcanic rocks are intermediate between tholeiitic and calc alkaline their mantle normalized trace element patterns are characterized by a moderate large ion lithophile element enrichment and nb and ti depletion suggesting that magmatism occurred in a volcanic arc setting flat heavy rare earth element chondrite normalized patterns with la yb n ratios from 2 to 4 5 suggest that the parent magma was produced by mantle melting in the spinel stability field the low sr isotopic ratios 87sr 86sri approximately equa

plastic bed beneath hofsjokull ice cap central iceland and the sensitivity of ice flow to surface meltwater flux author s minchew b simons m bjornsson h palsson f morlighem m seroussi h larour e hensley s abstract the author s 2016 the mechanical properties of glacier beds play a fundamental role in regulating the sensitivity of glaciers to environmental forcing across a wide range of timescales glaciers are commonly underlain by deformable till whose mechanical properties and influence on ice flow are not well understood but are critical for reliable projections of future glacier states using synoptic scale observations of glacier motion in different seasons to constrain numerical ice flow models we study the mechanics of the bed beneath hofsjokull a land terminating ice cap in central iceland our results indicate that the bed deforms plastically and weakens following incipient summertime surface melt combining the inferred basal shear traction fields with a coulomb plastic bed model we estimate the spatially distributed effective basal water pressure and show that changes in basal water pressure and glacier accelerations are non local and non linear these results motivate an idealized physical model relating mean basal water pressure and basal slip rate wherein the sensitivity of glacier flow to changes in basal water pressure is inversely related to the ice surface slope

contribution of antarctica to past and future sea level rise polar temperatures over the last several million years have at times been slightly warmer than today yet global mean sea level has been 6 9 metres higher as recently as the last interglacial 130 000 to 115 000 years ago and possibly higher during the pliocene epoch about three million years ago in both cases the antarctic ice sheet has been implicated as the primary contributor hinting at its future vulnerability here we use a model coupling ice sheet and climate dynamics including previously underappreciated processes linking atmospheric warming with hydrofracturing of buttressing ice shelves and structural collapse of marine terminating ice cliffs that is calibrated against pliocene and last interglacial sea level estimates and applied to future greenhouse gas emission scenarios antarctica has the potential to contribute more than a metre of sea level rise by 2100 and more than 15 metres by 2500 if emissions continue unabated in this case atmospheric warming will soon become the dominant driver of ice loss but prolonged ocean warming will delay its recovery for thousands of years