thickness and extent of the subglacial till layer beneath an antarctic paleo ice stream fast flowing ice streams and outlet glaciers currently account for as much as 90 of the discharge from the antarctic and greenland ice sheets although the deformation of subglacial material has been proposed as the mechanism for this rapid motion such sediment is usually hidden under several kilometers of ice marine geophysical records have allowed reconstruction of the three dimensional thickness of the sedimentary bed beneath a large antarctic paleo ice stream for the first time fast flow is indicated by streamlined seafloor lineations that form the surface of a layer of low shear strength unsorted sediment averaging 4 6 m thick rapid motion of the paleo ice stream was a result of subglacial deformation within this layer

past ice sheet flow east of svalbard inferred from streamlined subglacial landforms the pattern of late weichselian ca 20 ka ice flow in the northern barents sea is not well known due mainly to a lack of marine data east of svalbard several years with little summer sea ice have allowed acquisition of swath bathymetric imagery of well preserved subglacial landforms characterizing late weichselian ice flow directions over 150 000 km2 of the northwestern barents sea we show that a major ice dome was located on easternmost spitsbergen or southern hinlopen strait at least 500 km west of its previously inferred position in the northern barents sea this dome controlled the regional flow pattern ice flowed eastward around kong karls land into franz victoria trough and north through hinlopen strait an ice dome west of kong karls land is required to explain the observed ice flow pattern but does not preclude an additional ice dome to the southeast discrepancies with earlier ice sheet reconstructions reflect the lack of previous seafloor observations with evidence limited mainly to past ice loading and postglacial rebound the new pattern of ice flow directions shows predominantly eastward rather than northward flow with franz victoria trough a major drainage pathway with a full glacial balance flux of 40 km3 yr 1

high resolution landform assemblage along a buried glacio erosive surface in the sw barents sea revealed by p cable 3d seismic data abstract the quaternary sedimentary record in the arctic captures a diverse and evolving range of landscapes reflecting climate changes here we study the geological landform assemblage of the upper regional unconformity uru in the sw barents sea the aims are i to characterize buried geological landforms on a meter scale resolution ii to understand their link with underlying structures and iii to reconstruct paleo ice sheet dynamics and configurations the data consist of a high resolution three dimensional 3d p cable seismic cube with an extent of c 200 km2 and an inline separation of 6 m dominant frequencies of c 150 hz allow to image landforms at uru with a vertical resolution of 1 5 m and a horizontal resolution of 3 6 m we conduct detailed horizon picking and seismic attribute analysis of the buried uru horizon we identified four sets of mega scale glacial lineations and shear band ridges located to the west of a shear margin moraine other characteristic features include hill hole pairs transverse ridges rhombohedral ridges and depressions iceberg ploughmarks and pockmarks polygonal faults below uru and deeper faults have a strong effect on the location of structures observed on uru bedrock packages deformed down to 30 m below uru and up to 5 m high transverse ridges at uru are imprints of glacio tectonic activity deformed strata below uru indicate normal faulting superimposed by glaciotectonic deformation the four sets of mega scale glacial lineations indicate four streaming events with thawed glacial beds with shear band ridges forming in the shearing zone during one of these streaming events hill hole pairs and rhombohedral ridges are frozen bed features which indicate a polythermal regime at the base of the barents sea ice sheet during multiple streaming phases this study therefore shows that paleo ice streams have been temporarily frozen to the ground in the sw barents sea and that landforms evidencing this freezing are associated with underlying faults

deglaciation of a marine based ice sheet late weichselian palaeo ice dynamics and retreat in the southern barents sea reconstructed from onshore and offshore glacial geomorphology abstract marine and terrestrial datasets swath bathymetry 2d and 3d seismic and remote sensing data are used to map the megascale glacial geomorphology of the southern barents sea seafloor northern fennoscandia and the kola peninsula giving new insights into the dynamics and behaviour of the barents sea ice sheet during retreat the glacial geomorphology indicates clear spatial and temporal variations in ice dynamics with evidence for both active ice streaming and frozen bed conditions at the maximum and during deglaciation a five stage reconstruction of the late weichselian deglaciation is proposed at the late weichselian maximum ice covered the whole of the barents sea shelf and the configuration and dynamics of the ice sheet were controlled by the bjornoyrenna bear island trough ice stream this was fed by source areas in the central and north eastern barents sea and in the fennoscandian mainland deglaciation was initiated along the western continental margin available dating results show that this occurred after 19 cal ka bp the bjornoyrenna ice stream retreated rapidly ice margin retreat rates of 275 m a 1 but was periodically grounded as evidenced by a well developed series of grounding zone wedges this was followed by an advance of two short lived ice streams from the eastern sector of the ice sheet into the south western barents sea these ice streams advanced and retreated very rapidly with no evidence for ice margin standstills correlation to dated cores suggests an age of around 16 000 cal ka bp for this readvance the period of operation of these two ice streams was coincident with maximum ice extent along the eastern margin in russia the western barents sea ice margin then retreated onshore with deglaciation continuing at a reduced rate the initiation of deglaciation coincided with rising global eustatic sea level acceleration of calving due to grounding line floatation rapidly opened up a major calving bay in bjornoyrenna encouraging rapid loss of the marine based ice the retreat rate stabilised only once the ice sheet retreated onshore and calving ceased we note that the observed pattern of rapid retreat with high variability in ice streaming for the marine based barents sea ice sheet is similar to that of the marine based sectors of the late weichselian british laurentide and west antarctic ice sheets

ice stream dynamics of the sw barents sea revealed by high resolution 3d seismic imaging of glacial deposits in the hoop area abstract recent developments in seismic acquisition systems and seismic data visualization have contributed to improve the imaging of seismic geomorphologies over a broad range of topics this study focuses on a new high resolution p cable 3d seismic cube located in the hoop area in the sw barents sea the scientific motivations of the study are 1 to document glacial landforms on a meter scale 2 to study the link of these landforms with the subsurface and 3 to understand ice stream dynamics by detailed interpretation of main horizons including the seabed intra glacial horizons and upper regional unconformity uru the horizons are assessed using the concept of seismic geomorphology and compared with interpretations derived from conventional 3d seismic and state of the art multibeam echosounder technologies to discuss the benefits and limitations of the different geophysical technologies a well defined intra glacial seismic reflection is imaged in the eastern part of the p cable cube this reflection has an overall ne trending ridge shaped morphology which is up to 50 m high and 5 10 km wide and interpreted as a shear margin moraine the top of the moraine displays well defined iceberg ploughmarks and clear slope failure events along its eastern flank the seabed geomorphology which has been directly shaped by glacial processes related to multiple phases of ice streaming appears to be influenced by the buried shear margin moraine mega scale glacial lineations msgls indicative of fast flowing ice streams characterize the seabed west of the moraine infilled msgls and different sets of iceberg ploughmarks with large berms dominate the seabed geomorphology above the moraine the seabed east of the moraine characterized by a smooth morphology and few pockmarks is inferred to reflect more slowly moving ice not resolved by other geophysical techniques seismic attribute maps of p cable 3d allowed the identification of smaller ploughmarks and corrugated ridges related to paleo tides eroded by the larger iceberg ploughmarks these small ploughmarks are suggested to be linked to an earlier rapid disintegration of the barents sea ice stream the new high resolution 3d seismic data have led to the development of a revised ice stream model of the barents sea linking the glacial deposits to different ice stream events

insights into the emplacement dynamics of volcanic landslides from high resolution 3d seismic data acquired offshore montserrat lesser antilles we present results from the first three dimensional 3d marine seismic dataset ever collected over volcanic landslide deposits acquired offshore of the soufriere hills volcano on the island of montserrat in the lesser antilles the 3d data enable detailed analysis of various features in and around these mass wasting deposits such as surface deformation fabrics the distribution and size of transported blocks change of emplacement direction and erosion into seafloor strata deformational features preserved on the surface of the most recent debris avalanche deposit deposit 1 reveal evidence for spatially variant deceleration as the mass failure came to rest on the seafloor block distributions suggest that the failure spread out very rapidly with no tendency to develop longitudinal ridges an older volcanic flank collapse deposit deposit 2 appears to be intrinsically related to large scale secondary failure of seafloor sediments we observe pronounced erosion directly down slope of a prominent headwall where translational sliding of well stratified sediments was initiated deep reaching faults controlled the form and location of the headwall and stratigraphic relationships suggest that sliding was concurrent with volcanic flank collapse emplacement we also identified a very different mass wasting unit between deposit 1 and deposit 2 that was likely emplaced as a series of particle laden mass flows derived from pyroclastic flows much like the recent since 1995 phase of deposition offshore montserrat but at a much larger scale this study highlights the power of 3d seismic data in understanding landslide emplacement processes offshore of volcanic islands highlights 3d seismic data show new detail of volcanic landslide deposits offshore montserrat volcanic flank collapse material has been diverted around seafloor topographic highs this bending during emplacement has caused pronounced erosion into seafloor strata erosion can destabilize seafloor slopes which then fail as translational slides block distributions and surface deformation give insight into debris avalanche style

ice stream shear margin moraines the distribution and pattern of subglacial bedforms provides valuable insights into the configuration and behaviour of former ice sheets and recent work has begun to address the landform assemblages of palaeo ice streams in this paper we focus on subglacially formed ice stream shear margin moraines these landforms are thought to form in the shear zone between fast flowing stream ice and slow moving sheet ice using satellite imagery and aerial photographs we identify four ice stream shear margin moraines associated with the former m clintock channel palaeo ice stream victoria island arctic canada the moraines range in length from 11 to 22 km maintain fairly constant widths of around 500 m and range in height from 10 to 50 m above the surrounding terrain they are composed of carbonate drift of a similar composition as the ice stream bedforms and have been laid down irrespective of local and regional topography two of the moraines display a lateral offset and are thought to reflect minor migrations of the ice stream margin we explore their possible mode of formation and suggest that they occur when erosion at the ice stream margin provides a surplus of sediment which is smeared out in the downstream direction the identification of other palaeo ice stream shear margin moraines has probably been hampered by their interrupted continuity which can only be appreciated at the large scale it is hoped that the descriptions presented in this paper will help to identify these landforms and increased detection of palaeo ice streams will reveal other ice stream shear margin moraines copyright 2002 john wiley sons ltd

geological record of ice shelf break up and grounding line retreat pine island bay west antarctica the catastrophic break ups of the floating larsen a and b ice shelves antarctica in 1995 and 2002 and associated acceleration of glaciers that flowed into these ice shelves were among the most dramatic glaciological events observed in historical time this raises a question about the larger west antarctic ice shelves do these shelves with their much greater glacial discharge have a history of collapse here we describe features from the seafloor in pine island bay west antarctica which we interpret as having been formed during a massive ice shelf break up and associated grounding line retreat this evidence exists in the form of seafloor landforms that we argue were produced daily as a consequence of tidally influenced motion of mega icebergs maintained upright in an iceberg armada produced from the disintegrating ice shelf and retreating grounding line the break up occurred prior to ca 12 ka and was likely a response to rapid sea level rise or ocean warming at that time

geochemical and paleomagnetic evidence for the occurrence of missing sapropels in eastern mediterranean sediments the cyclic occurrence of sapropels appears to be associated with maxima in the 65 n summer insolation target curve studies of the most recent sapropel have revealed extensive oxidation of organic matter or even the complete removal of this unit geochemical and magnetic signals allow for the detection of such a missing sapropel in older sediments these signals are altered by ongoing diagenetic processes an important parameter to reveal missing sapropels is ba elevated concentrations of ba are always found in visible sapropels and even after oxidation of part of the sapropel the initial ba profile remains visible we use this tool to trace down missing sapropels in kc01b a 37 m kullenberg core from the calabrian ridge eastern mediterranean at least 8 of 11 sampled intervals around insolation maxima contain evidence of a completely oxidized sapropel ba trace metal distributions and magnetic parameters are similar to those observed around visible sapropels

the norwegian greenland sea continental margins morphology and late quaternary sedimentary processes and environment the continental margins surrounding the norwegian greenland sea are to a large degree shaped by processes during the late quaternary the paper gives an overview of the morphology and the processes responsible for the formation of three main groups of morphological features slides trough mouth fans and channels several large late quaternary slides have been identified on the eastern norwegian greenland sea continental margin the origin of the slides may be due to high sedimentation rates leading to a build up of excess pore water pressure perhaps with additional pressure caused by gas bubbles triggering might have been prompted by earthquakes or by decomposition of gas hydrates trough mouth fans tmf are fans at the mouths of transverse troughs on presently or formerly glaciated continental shelves in the norwegian greenland sea seven tmfs have been identified varying in area from 2700 km2 to 215 000 km2 the trough mouth fans are depocentres of sediments which have accumulated in front of ice streams draining the large northwest european ice sheets the sediments deposited at the shelf break upper slope by the ice stream were remobilized and transported downslope mostly as debris flows the trough mouth fans hold the potential for giving information about the various ice streams feeding them with regard to velocity and ice discharge two large deep sea channel systems have been observed along the norwegian continental margin the lofoten basin channel and the inbis channel along the east greenland margin several channel systems have been identified the deep sea channels may have been formed by dense water originating from cooling sea ice formation and brine rejection close to the glacier margin or they may originate from small slides on the upper slope transforming into debris flows and turbidity currents

seafloor geomorphology of the sw barents sea and its glaci dynamic implications abstract three dimensional 3d reflection seismic imaging techniques provide a unique means of identifying large scale submarine geomorphic features produced by former ice sheets we have used such techniques to image the geomorphology of the sw barents sea but using a dense grid of 2d seismic covering a 240 000 2 km large study area supplemented with 3d seismic from selected areas the study area is located at the convergence of ice flow from the former fennoscandian barents sea and svalbard ice sheets the morphology of the major cross shelf troughs is characterized by elongated ridge groove features interpreted to be glacial bedforms produced by fast flowing grounded glaciers mega scale glacial lineations have been grouped into distinct flow sets related to three specific ice flow events these events document the last glacial maximum lgm and two readvances during deglaciation of specific interest is a detailed and well preserved set of seafloor imprints in the bjornoyrenna trough which acted as the main pathway for ice streams draining the surrounding ice sheets the geomorphic features indicate that bjornoyrenna hosted six separate ice stream lobes that diverged fan like at their margins but were not all active simultaneously an end moraine zone 68 km wide up to 85 m high and 300 km long filled the whole width of this major glacial trough as a result of high sediment flux within sub ice stream deformable beds the imprint of this ice stream system can be followed on our seabed images for 500 km however detailed geomorphologic elements demonstrate that these imprints reflect time transgressive behaviour and do not indicate the length or width of single ice streams the seismic derived seafloor geomorphology demonstrates that the barents sea ice behaved in a much more dynamic way during the last glaciation than previously assumed with major shifts between ice originating from the fennoscandian spitsbergen and barents sea ice sheets

deglaciation of the central barents sea abstract the marine based barents sea ice sheet covered the polar continental shelf north of norway and western russia during the last glacial maximum initial ice sheet retreat along the western margin is well established while the retreat pattern in the interior parts of the ice sheet remains poorly known here we present new geological data from the central barents sea the results are based on analysis of several marine geophysical datasets including geomorphological mapping of multibeam swath bathymetry data and analysis of seismic and acoustic stratigraphy the new results provide insights into the configuration and dynamics of the ice sheet during its retreat across the central barents sea in particular they show clear changes in the location of the main ice divides and domes with ice flow becoming gradually more topographically controlled as deglaciation progressed major troughs were characterised by episodic retreat and reoccurring cycles of fast and slow ice flow sometimes leading to stagnation and ice shelf formation meanwhile adjacent bank areas were covered by comparatively slowly retreating ice although evidence of streaming ice is also seen locally

seismic geomorphology an overview seismic geomorphology the extraction of geomorphic insights using predominantly 3d seismic data is a rapidly evolving discipline that facilitates the study of the subsurface using plan view images a variety of analytical techniques is employed to image and visualize depositional elements and other geologically signi cant features this volume presents key technical papers presented at a recent research conference seismic geomorphology conference february 10 11 2005 co sponsored by the society for sedimentary geology and the geological society london these papers cover a broad range of topics from detailed depositional element analysis to big picture regional issues from lithology prediction to diagenetic modi cation of the stratigraphic section this discipline is only in its early stages of development and will henceforth expand rapidly in response to the growing availability to researchers of high quality 3d seismic data

the isotopic composition of radiogenic helium and its use to study groundwater movement in confined aquifers abstract the 3 he 4 he ratio for radiogenic he is shown to be primarily determined by the abundance of li and neutron absorbing elements in the host formations and is almost independent of the radioelement abundance the 3 he 4 he ratios have been computed for various li contents in rock matrices with average abundances of other elements the 3 he 4 he ratio is specific for each formation and may therefore be used to study crustal diffusion of he it is shown however that both the 3he and 4he contents of continental basements are likely to exceed those of the mantle so that diffusion of mantle he through continental crust is not likely to occur changes in the 3 he 4 he ratio for dissolved he have potential for studying groundwater movement and mixing processes atmospheric he which was dissolved at recharge is progressively diluted by radiogenic he with an isotopic ratio characteristic of the aquifer as the groundwater migrates down dip mixing proportions for formation he and atmospheric he in aquifers for which there is admixture of fracture borne water and interstitially stored water can also be estimated from isotopic measurements these processes are discussed in relation to two aquifers in eastern england

a groove ploughing theory for the production of mega scale glacial lineations and implications for ice stream mechanics mega scale glacial lineations msgls are longitudinally aligned corrugations ridge groove structures 6 100 km long in sediment produced subglacially they are indicators of fast flow and a common signature of ice stream beds we develop a qualitative theory that accounts for their formation and use numerical modelling and observations of ice stream beds to provide supporting evidence ice in contact with a rough scale of 10 10 3 m bedrock surface will mimic the form of the bed because of flow acceleration and convergence in ice stream onset zones the ice base roughness elements experience transverse strain transforming them from irregular bumps into longitudinally aligned keels of ice protruding downwards where such keels slide across a soft sedimentary bed they plough through the sediments carving elongate grooves and deforming material up into intervening ridges this explains msgls and has important implications for ice stream mechanics groove ploughing provides the means to acquire new lubricating sediment and to transport large volumes of it downstream keels may provide basal drag in the force budget of ice streams thereby playing a role in flow regulation and stability we speculate that groove ploughing permits significant ice stream widening thus facilitating high magnitude ice discharge

ice melange dynamics and implications for terminus stability jakobshavn isbrae greenland 1 we used time lapse imagery seismic and audio recordings iceberg and glacier velocities ocean wave measurements and simple theoretical considerations to investigate the interactions between jakobshavn isbrae and its proglacial ice melange the melange behaves as a weak granular ice shelf whose rheology varies seasonally sea ice growth in winter stiffens the melange matrix by binding iceberg clasts together ultimately preventing the calving of full glacier thickness icebergs the dominant style of calving and enabling a several kilometer terminus advance each summer the melange weakens and the terminus retreats the melange remains strong enough however to be largely unaffected by ocean currents except during calving events and to influence the timing and sequence of calving events furthermore motion of the melange is highly episodic between calving events including the entire winter it is pushed down fjord by the advancing terminus at 40 m d 1 whereas during calving events it can move in excess of 50 103 m d 1 for more than 10 min by influencing the timing of calving events the melange contributes to the glacier s several kilometer seasonal advance and retreat the associated geometric changes of the terminus area affect glacier flow furthermore a force balance analysis shows that large scale calving is only possible from a terminus that is near floatation especially in the presence of a resistive ice melange the net annual retreat of the glacier is therefore limited by its proximity to floatation potentially providing a physical mechanism for a previously described near floatation criterion for calving

reconstructing the retreat dynamics of the bjornoyrenna ice stream based on new 3d seismic data from the central barents sea the stability of contemporary ice sheets is influenced by the discharge from ice streams corridors of fast flowing ice bounded by ice flowing an order of magnitude slower reconstructions of palaeo ice stream dynamics contribute to our understanding of ice stream sensitivity to the ocean climate system and can aid in the numerical modelling and prediction of future changes in contemporary ice sheets here we use 3d seismic data covering 13 000 km2 in the central bjornoyrenna bear island trough barents sea to investigate the record of ice streaming preserved on the seafloor and on a buried palaeo seafloor surface the unusually broad coverage and high resolution of the dataset as well as its location in the central area of the trough enables improved reconstruction of dynamics of the former bjornoyrenna ice stream in terms of number of streaming events their trajectory and their relative age sequence during deglaciation our results reveal major changes in the configuration and flow dynamics of the ice stream with up to 10 flow switching events identified for the first time we also document ice streaming sourced from the eastern barents sea around the time of the lgm this high degree of flow variability is suggested to have resulted from climate driven changes in ice sheet geometry and ice divide migration and variations in topography that influenced calving at the ice stream terminus

cenozoic erosion and sedimentation in the western barents sea abstract seismic reflection data and well logs are used to interpret cenozoic sedimentary environments in the south western barents sea using volumetric calculations and compaction trend analysis the cenozoic erosion and sedimentation rates have been quantified marine environments ranging from neritic to bathyal water depths dominated the southern barents sea during the late palaeocene at the palaeocene eocene boundary a southwards progradation commenced along the uplifted and eroded loppa and stappen highs during the early eocene to mid miocene sedimentation continued in the intracratonic basins as well as along the newly established continent ocean boundary in the embryonic northern norwegian greenland sea substantial erosion occurred on the westernmost margin particularly on the stappen high during this time interval in the mid miocene during a period of relative lowstand of sea level a system of lowstand fans was deposited west of the stappen high later a pronounced progradation of the shelf edge commenced during the mid miocene to late pliocene the shelf edge prograded between 20 and 40 km during the mid pliocene to early pleistocene the barents sea and adjacent land areas were uplifted a fluvial glaciofluvial drainage system developed on the continental shelf resulting in pronounced erosion there and very high sedimentation rates on the continental margin during the last 0 8 ma several large glaciations on the barents sea shelf proper have resulted in overdeepened troughs and high sedimentation rates at the continental shelfbreak and on the continental slope the overall cenozoic erosion varies spatially as well as temporally in the svalbard area close to 3 km of rocks have been eroded since the oligocene the same amount or more was eroded from the stappen high during the palaeocene and early neogene in the south western areas during the eocene mid miocene erosion occurred along the margin in addition the loppa high and adjacent areas served as sediment source areas during the late pliocene and pleistoncene the average erosion in the south western barents sea amounted to about 600 m erosion was greater in the south east than to the west and north and was more extensive where the former glacial ice streams had flowed

mantle fluids in the karakoram fault helium isotope evidence abstract the karakoram fault kkf is the 1000 km long strike slip fault separating the western himalaya from the tibetan plateau from geologic and geodetic data the kkf is argued either to be a lithospheric scale fault with hundreds of km of offset at several cm a or to be almost inactive with cumulative offset of only a few tens of kilometers and to be just the upper crustal localization of distributed deformation at depth here we show 3he 4he ratios in geothermal springs along a 500 km segment of the kkf are 3 100 times the normal ratio in continental crust providing unequivocal evidence that a component of these hydrologic systems is derived from tectonically active mantle mantle enrichment is absent along the indus yarlung suture zone isz just 35 km southwest of the kkf suggesting that the mantle fluids flow only within the kkf within the last few ma the kkf must have accessed tectonically active tibetan mantle northeast of the mantle suture which we therefore locate vertically beneath the kkf very close to the surface trace of the isz hence in southwestern tibet indian crust may not now be underthrusting substantially north of the isz even though miocene underthrusting may have placed indian crust north of the isz in the lower half of the tibetan plateau crust this is in significant contrast to central and eastern tibet where underthrust indian material not only forms the lower half of the tibetan crust but is also currently underthrusting for 200 km north of the isz our new constraint on kkf penetration to the mantle allows an improved description of the continuously evolving karakoram fault as a tectonically significant yet perhaps geologically ephemeral lithospheric structure

grounding line proximal sediment characteristics at a marine based late stage ice stream margin this is the peer reviewed version of the following article ruther d c winsborrow m andreassen k forwick m 2017 grounding line proximal sediment characteristics at a marine based late stage ice stream margin journal of quaternary science 32 4 463 474 https doi org 10 1002 jqs 2939 which has been published in final form at https doi org 10 1002 jqs 2939 this article may be used for non commercial purposes in accordance with wiley terms and conditions for use of self archived versions

cenozoic margin development and erosion of the barents sea core evidence from southwest of bjornoya abstract five stratigraphic boreholes at the western barents sea margin reveal an evolution from a palaeocene normal shelf setting to a complex late cenozoic offshore marginal marine system influenced by mass flows and finally a glacially influenced shelf environment palaeocene epeiric sea sedimentation was sourced by erosion at the stappen high earliest eocene extension associated with volcanism was followed by rapid subsidence of the vestbakken volcanic province which became filled by sediments from an uplifted stappen high by the middle eocene siliciclastic shelf sediments were deposited and the infill had reached a balance with subsidence tectonic movements at this time may have caused local uplift which restricted the ventilation of local basins west of the stappen high but marine conditions existed in the area until early oligocene times when subaerial exposure was caused by tectonic uplift and probably also a fall in eustatic sea level pronounced oligocene and pliocene unconformities reflect major events of uplift and erosion at the margin and abundant fossil reworking indicates erosion of the uplifted cretaceous and tertiary strata the oligocene erosion had its associated depocentre to the south and west of the study area and was less important regionally than the pliocene pleistocene phase the pliocene uplift first led to local erosion along the margin subsequent subsidence may initially have had a thermal component this was followed by extensive downwarping under the load of largescale deposition sourced by regional erosion in the barents shelf region an eocene volcanic phase was probably related to the initiation of seafloor spreading isotopically dated volcaniclastic intervals in one of the cores give evidence of a late pliocene volcanic phase this event was associated with local tectonic movements which may be part of a regional uplift in the western northwestern part of the barents sea glaciations caused by the neogene climatic deterioration became enhanced by uplift and this led to extensive regional glacial erosion the main depocentre was offset to the south by ice flow curving around the uplifted area

evolution of subglacial bedforms along a paleo ice stream antarctic peninsula continental shelf geophysical data from the antarctic peninsula continental shelf reveal streamlined subglacial bedforms in a cross shelf trough bedforms exhibit progressive elongation with distance along the trough and record flow of a paleo ice stream from the antarctic peninsula ice sheet during the last glacial maximum downflow evolution of the bedforms indicates increasing flow velocities as the ice stream traversed the shelf this in turn is related to a transition from crystalline bedrock on the inner shelf to a soft sedimentary substrate on the outer shelf although streaming flow operated across both substrates the highest flow velocities occurred over the soft bed spatial variation in the inferred nature of fast flow from sliding to subglacial sediment deformation and or ploughing was also lithologically controlled these data highlight the control of subglacial geology on ice stream dynamics in the geological record and demonstrate a direct relationship between the formation of streamlined subglacial bedforms and paleo ice streams

the vestfjorden traenadjupet palaeo ice stream drainage system mid norwegian continental shelf abstract an ice stream in the vestfjorden traenadjupet depressions off northern norway transported glacial debris to the shelf edge and into the deep sea during the last glaciation our interpretation is based mainly on regional and detailed bathymetry and 2d and 3d seismic data this palaeo ice stream was approximately 400 km long and covered an area of 20 000 km 2 including a complex system of tributary fjords and valleys in nordland county the drainage basin had an area of ca 150 000 km 2 within the fennoscandian ice sheet fis during the last glacial maximum lgm the location of the palaeo ice stream prevented large ice masses from reaching the montainous areas of lofoten and vesteralen a local ice dome was established in these areas the palaeo ice stream eroded extensively into both mesozoic and cenozoic sedimentary rocks and quaternary sediments a pattern of well developed subglacial sedimentary bedforms was produced e g mega scale lineations drumlins ice stream shear margin moraines are located on both sides of traenadjupet defining the width of the palaeo ice stream 90 km at the shelf edge these moraines are commonly 10 20 m high a few kilometres wide and reach several tens of kilometres in length a large recessional moraine the tennholmen ridge has been mapped in vestfjorden it is 80 m high 20 km wide and 60 km long we have interpreted this ridge as a grounding line moraine formed during a halt or a readvance of the ice stream during the last deglaciation the large transport of glacial debris towards the grounding line occurred during this stage northeast of the tennholmen ridge several smaller moraine ridges were developed at still stands during deglaciation

submarine glacial landforms and rates of ice stream collapse the rate of deglacial ice sheet retreat across polar continental shelves and possible ice stream collapse and sea level rise has been much debated high resolution imagery of seafloor morphology is available for many polar shelves and fjords the rapidity of ice retreat is inferred from diagnostic assemblages of submarine landforms produced at ice stream sedimentary beds these landforms exposed by ice retreat across high latitude shelves demonstrate that deglaciation occurs in three main ways rapidly by flotation and breakup episodically by still stands and or grounding events punctuating rapid retreat or by slower retreat of grounded ice submarine landform assemblages imply through the presence of grounding zone wedges overprinting mega scale glacial lineations on many polar shelves that ice stream retreat is more often episodic than catastrophic these observations provide a robust test of the ability of numerical models to predict the varied response of ice sheet basins to environmental changes

ice stream retreat dynamics inferred from an assemblage of landforms in the northern barents sea abstract palaeo records of rapid ice stream retreat are important as they can be used to identify the processes involved and inform investigations of present day ice masses here we document a previously unknown retreat stage in northernmost bjornoyrenna bear island trough in the northern barents sea representing the youngest stage in a stepwise retreat of the bjornoyrenna ice stream we present a descriptive landsystem model for retreat of a marine based ice stream which provides new insights into the glacial dynamics of the episodic retreat stages this model captures the landforms produced during a cycle of 1 fast ice stream flow 2 intense calving of large icebergs locked in a dense matrix of smaller icebergs probably from a collapsed ice shelf 3 ice stream stagnation 4 ice stream floating off forming an ice shelf as the grounding line retreats and eventually 5 ice shelf disintegration we hypothesize that the presented ice stream retreat model reflects glacial surging the formation of corrugated furrows in the study area is consistent with interpretation of similar features in pine island bay west antarctica which are interpreted to have been formed during a massive ice shelf break up and associated grounding line retreat two directions of past ice flow are indicated from streamlined glacial landforms in the study area ice flow from north north east is indicated for the last glacial maximum and an early phase of deglaciation whereas a shift to flow from north north west from an ice dome located over hinlopenstretet svalbard is indicated for the latest deglaciation phases of the bjornoyrenna ice stream

flow dynamics and till genesis associated with a marine based antarctic palaeo ice stream geophysical and geological data indicate that during the last glacial cycle a palaeo ice stream drained the antarctic peninsula ice sheet apis through marguerite bay to the edge of the continental shelf via a bathymetric trough marguerite trough mega scale glacial lineations msgl in outer marguerite trough are formed in an acoustically transparent sediment unit consisting of a soft deformation till the association of msgl with deformation till indicates that bed deformation occurred beneath the ice stream and the thickness of the subglacial deforming layer was on the order of several metres msgl within outer marguerite trough are related to a pre existing thick sequence of soft sediments on the outer shelf and are inferred to have formed by a combination of subglacial sediment deformation by attenuation from a point source and groove ploughing morphologically the msgl differ in several respects from those documented elsewhere and this variation suggests a polygenetic origin for this subglacial landform an initial advance of the apis across the marguerite bay continental shelf at the lgm deposited a stiff subglacial till following this advance the ice margin within marguerite trough retreated 70 100 km before stabilising and depositing a grounding zone wedge an ice stream then developed in the trough and extended to the shelf edge forming a soft deformation till development of the ice stream may have been a glacio dynamic response to regional deglaciation subsequent retreat of the marguerite trough ice stream was underway by 13 490 uncorrected 14c years before present based on the geophysical and core data ice stream retreat appears to have been rapid and it contrasts with reconstructions of palaeo ice streams from other locations in antarctica this implies marked regional variations in antarctic ice stream dynamics during late quaternary deglaciation

buried iceberg ploughmarks in the early quaternary sediments of the central north sea a two million year record of glacial influence from 3d seismic data abstract buried linear to curvilinear depressions interpreted as iceberg ploughmarks were identified through most of the 2 ma long and about 700 m thick early quaternary sedimentary record in two 3d seismic cubes from the central north sea basin 56 58 n 2 3 e the mean width of 402 features measured at 6 time slices in cube c08 was between 49 and 63 m and mean length was 2 5 to 3 7 km mapping the

aqueous and isotope geochemistry of mineral springs along the southern margin of the tibetan plateau implications for fluid sources and regional degassing of co2 springs issuing from different faults and shear zones along the crest of the himalayas tap three different levels of crust beneath the tibetan plateau from structurally highest to lowest these are the tingri graben the south tibetan detachment system stds and the ama drime massif adm the aqueous chemistry reflects water rock interactions along faults and is consistent with mapped rock types major ion chemistry and calculated temperatures indicate that spring waters have circulated to greater depths along the n s trending faults that bound the tingri graben and ama drime detachment add compared to the stds suggesting that these structures penetrate to greater depths springs have excess co2 n2 he and ch4 compared to meteoric water values implying addition from crustal sources the 3he 4he ratios range from 0 018 to 0 063 ra and are consistent with a crustal source for he the 13c values of dissolved inorganic carbon dic and co2 gas range from 5 5 to 3 8 and 13 1 to 0 3 versus peedee belemnite respectively sources of carbon are evaluated by calculating isotopic trajectories associated with near surface effervescence of co2 positive 13c values of the tingri graben and stds springs are consistent with decarbonation of marine carbonates as the source of co2 negative values for the add springs overlap with mantle values but are best explained by metamorphic devolatilization of reduced sedimentary carbon the 15n values of n2 range from 2 2 to 2 1 versus air and are explained by mixtures of air derived nitrogen metamorphic devolatilization of sedimentary nitrogen and nitrogen from near surface biogenic processes co2 flux is estimated by scaling from individual springs 105 mol a 1 per spring to extensional structures across the southern limit of the tibetan plateau and likely contributes between 108 and 1011 mol a 1 up to 10 to the global carbon budget

the tectonized central peak of the mjolnir impact crater barents sea abstract the mjolnir structure sw barents sea is one of the best preserved marine impact craters on earth after impact on the paleo seafloor about 142 ma ago this crater experienced an atypical deformation of its central peak which is now elevated 435 m above the crater rims here we investigate the effect of far field tectonic stresses on the central peak uplift based on interpretation of new high resolution p cable and conventional seismic reflection data nearby wells provided stratigraphic control on the interpreted horizons the reconstruction of the crater sedimentary infill supports a subdued original central peak relief with a 5 km wide gentle mound 15 m below the rim our interpretation shows that subvertical outward dipping impact induced faults were reactivated by uplifted segments of the central peak up to 500 m above the platform level during one or several contractional episodes we postulate that post albian tectonic compressional events triggered the structural uplift of the mjolnir central peak differential compaction previously seen as the main deformation process may have increased the original central peak height by only 10 m the mobilization of impact shattered rocks by tectonic compression provides a new and robust explanation for the structural rise of mjolnir s central peak

repeated fluid expulsion through sub seabed chimneys offshore norway in response to glacial cycles abstract focused fluid flow through sub seabed sediments is a common phenomenon on continental margins worldwide however the governing controls and timing of this fluid release have been difficult to understand in particular for fluid flow features buried beneath sub surface sediments a link between fluid flow activity ensuing pockmark formation and the last glacial maximum has been hypothesized on the formerly glaciated norwegian margin new high resolution p cable 3d seismic data from the nyegga area on the mid norwegian margin reveal at least two more periods of fluid expulsion from sub seabed sediments the 3d seismic data show depositional patterns within chimney features expressed as truncations of seismic horizons against the flanks of the chimneys the truncations are interpreted by analogy with present day observations as evidence for buried carbonate mounds and or sediment wash out during formation of pockmarks in the past the truncations are hence an indicator and a chronological marker for fluid expulsion in the past the classification of chimneys results in three major groups 1 chimneys that have been formed and consecutively reactivated one or two times during the last 200 kyr and that have a fluid flow expression at the present seafloor 2 chimneys that are approx 125 160 ka bp old without any associated fluid flow expression at the present day seafloor 3 chimneys with no stratigraphical evidence for reactivation formed after the last glacial maximum 18 25 ka bp the observations suggest that each activity period was likely related to the last stages of maximum glaciations in the region the emplacement of thick sequences of glacigenic debris flow deposits during these maximum stages most likely caused rapid increase in overpressure and subsequently the formation of focused fluid flow features piercing through sediments of the naust formation at nyegga

the seismic architecture and geometry of grounding zone wedges formed at the marine margins of past ice sheets grounding zone wedges form where halts of decades to centuries during ice sheet retreat across polar shelves allow sediment buildup at the grounding zone through delivery of deforming basal debris from fast flowing ice streams thirty grounding zone wedges were identified on two dimensional seismic reflection profiles across the continental shelves of ne and nw greenland grounding zone wedges close to the seafloor are probably of late weichselian age others are on paleoshelves buried beneath thick prograding glacial sediments several greenland shelf grounding zone wedges occur on topographic highs that provide stabilizing pinning points in relatively shallow water grounding zone wedges are asymmetric in the ice flow direction with steeper ice distal sides typical grounding zone wedges are approximately 5 20 km long and 50 100 m thick with a lateral width of several tens of kilometers forty grounding zone wedges from the greenland norwegian and antarctica margins show considerable variability about these values grounding zone wedge dimensions are controlled by sediment flux the duration of halts in ice retreat subice cavity shape and ice stream width low angle ice shelf cavity roofs immediately beyond the grounding zone probably restrict vertical accommodation space preventing formation of high amplitude ridges grounding zone wedges are mainly transparent or chaotic on seismic profiles probably resulting from delivery of diamictic debris offlapping reflectors represent progradation into an ice roofed cavity reflector truncations of grounding zone wedge bases indicate erosion during initiation channels are present within some grounding zone wedges meltwater flow is under high pressure and v shaped incisions suggest high energy flow channels are not ubiquitous within greenland grounding zone wedges and may represent nonsteady meltwater release perhaps through lake drainage where ice sheet mass loss is dominated by meltwater runoff grounding zone wedges probably contain more sorted sediment than those from greenland simple models of grounding zone wedge architecture are presented grounding zone wedges indicate episodic rather than catastrophic ice sheet retreat and are a mechanism for ice shelf stabilization because wedge growth counteracts collapse induced by ice sheet thinning and sea level rise

spatial variations in heat at the base of the antarctic ice sheet from analysis of the thermal regime above subglacial lakes antarctic subglacial lakes provide an important boundary condition for thermal analysis of the ice sheet in that the basal ice temperature over lakes may be assumed to be at the pressure melting point we have used a one dimensional vertical heat transfer equation to determine theoretical temperature values for the ice sheet base above 77 subglacial lakes identified from airborne radio echo sounding data covering 50 of antarctica variations in our temperature results to below the pressure melting temperature over lakes are due to either our estimate of the geothermal heat flux or a neglect of heat derived from a internal ice deformation and b basal sliding in the thermal model our results indicate that when the geothermal heat flux is set at 54 mw m 2 the ice sheet base above 70 of the known antarctic subglacial lakes is calculated to be at the pressure melting value these lakes are located mainly around dome c ridge b and vostok station for the ice sheet above subglacial lakes located hundreds of kilometres from the ice divide using the same thermal model loss of heat due to vertical advection is calculated to be relatively high in such regions if the ice sheet base is at the pressure melting point heat lost due to vertical advection must be supplemented by heat from other sources for the three lakes beneath terre adelie and george v land for instance the basal thermal gradient calculated to produce pressure melting at the ice sheet base is equivalent to 1 5 2 times the value obtained when 54 mw m 2 of geothermal heat is used as the sole basal thermal component we suggest that as distance from the ice divide increases so too does the amount of heat due to internal ice deformation and basal sliding moreover by considering the ice sheet basal thermal characteristics above subglacial lakes which lie on the same ice flowline we demonstrate empirically that the heat due to these horizontal ice motion terms varies pseudo exponentially with distance from the ice divide the location along a flowline where a rapid increase in the basal heat gradient is calculated may correspond to the onset of large scale basal sliding

flow of mantle fluids through the ductile lower crust helium isotope trends heat and mass are injected into the shallow crust when mantle fluids are able to flow through the ductile lower crust minimum 3he 4he ratios in surface fluids from the northern basin and range province western north america increase systematically from low crustal values in the east to high mantle values in the west a regional trend that correlates with the rates of active crustal deformation the highest ratios occur where the extension and shear strain rates are greatest the correspondence of helium isotope ratios and active transtensional deformation indicates a deformation enhanced permeability and that mantle fluids can penetrate the ductile lithosphere even in regions where there is no substantial magmatism superimposed on the regional trend are local high 3he 4he anomalies indicating hidden magmatic activity and or deep fluid production with locally enhanced permeability identifying zones with high resource potential particularly for geothermal energy development

seismic architecture and sedimentology of a major grounding zone system deposited by the bjornoyrenna ice stream during late weichselian deglaciation abstract a 280 km wide sediment wedge in outer bjornoyrenna bear island trough south western barents sea has been investigated using 2d and 3d seismic data sediment gravity cores as well as regional swath and large scale bathymetry data the bathymetry data indicate a division into an up to 35 m high frontal wedge with large depressions and an upstream part characterized by mega scale glacial lineations msgl from seismic sections increasing erosion is demonstrated for the upstream part coinciding with the location of msgl whether the latter are depositional features postdating an extensive erosional event or formed by erosion remains inconclusive based on the distinct morphology and internal structures we infer that the system was deposited during a rapid readvance whereby the ice front pushed and bulldozed predominantly soft diluted proglacial sediments analyses in the eastern part of the sediment system reveal the existence of imbricated thrust sheets in the frontal part of the wedge this is suggested to imply upstream erosion of sedimentary rock and incorporation of thrusted blocks into the moraine forming a composite ridge locally we argue that observed large scale depressions are dead ice features in the marine environment it is envisioned that intense englacial thrusting may have developed into a decollement as the cold glacier snout got overrun by ice masses from the interior thereby enabling the inclusion of slabs of ice in the push moraine mass radiocarbon dates indicate that the sediment wedge was deposited around 17 090 cal yrs bp 14 530 14 c yrs bp and that the ice front probably remained stable until 16 580 cal yrs bp 13 835 14 c yrs bp

signature of ice streaming in bjornoyrenna polar north atlantic through the pleistocene and implications for ice stream dynamics the geomorphology of palaeo ice stream beds and the internal structure of underlying tills can provide important information about the subglacial conditions during periods of fast flow and quiescence this paper presents observations from three dimensional seismic data revealing the geomorphology of buried beds of the bjornoyrenna bear island trough ice stream the main drainage outlet of the former barents sea ice sheet repeated changes in ice dynamics are inferred from the observed successions of geomorphic features megablocks aligned in long chains parallel to inferred ice stream flowlines and forming dipping plates that are thrust one on top of another are taken as evidence for conditions of compressive ice flow mega scale glacial lineations msgl and pull apart of underlying sediment blocks suggest extensional flow the observed pattern of megablocks and rafts overprinted by msgl indicates a change in ice dynamics from a compressional to an extensional flow regime till stiffening due to subglacial freezing is the favoured mechanism for creating switches in sub ice stream conditions the observed pattern of geomorphic features indicates that periods of slowdown or quiescence were commonly followed by reactivation and fast flow during several glaciations suggesting that this may be a common behaviour of marine ice streams

late cenozoic erosion of the high latitude southwestern barents sea shelf revisited the southwestern barents sea has experienced profound erosion during the last 2 7 m y that has resulted in the development of a characteristic glacial morphology of the continental shelf and deposition of a several kilometer thick sediment fan along the western margin prograding into the deep sea in the period from ca 2 7 to 1 5 ma proglacial processes including fluvial and glaciofluvial erosion dominated for this period the total average erosion of the shelf was 170 230 m the average erosion rate was 0 15 0 2 mm yr and the average sedimentation rates on the fan were 16 22 cm k y subglacial erosion affected an area of 575 000 km 2 during the period from ca 1 5 to 0 7 ma total average erosion is estimated at 330 420 m for this interval and the average erosion rate was 0 4 0 5 mm yr average sedimentation rates were 50 64 cm k y during the last 0 7 m y glacial erosion mainly has occurred beneath fast flowing paleo ice streams topographically confined to troughs 200 000 km 2 the total average erosion is estimated at 440 530 m average erosion rate is 0 6 0 8 mm yr and average sedimentation rate on the continental slope is 18 22 cm k y the amount of erosion was mainly determined by the duration of the glaciations and the location velocity and basal properties of the ice streams in total glacial erosion of the troughs has been relatively high throughout the last 2 7 m y at 1000 1100 m for the banks erosion is inferred to have increased from ca 2 7 ma to a peak between 1 5 and 0 7 ma subsequently little erosion occurred in these areas which implies a total of 500 650 m of erosion compared with other high latitude areas our rates are among the highest so far reported this comparison also demonstrates that there have been large variations in the rate of sediment delivery to the glaciated continental margins

toward one meter resolution in 3d seismic high resolution 3d hr3d seismic data are important for hydrocarbon exploration of shallow reservoirs site characterization and geohazard assessments the goal of this contribution is to identify and quantify the parameters to increase the resolution of hr3d seismic data to meter scale the main acquisition parameters controlling the resolution of the collected data are the spectrum of the seismic source source receiver offset range and trace density an evolution to one meter scale resolution of 3d seismic will rely on combining a reproducible seismic source with high frequencies up to at least 600 hz a high uniform trace density of more than 4 million traces per square kilometer and an offset range shorter than approximately 200 m the resulting 3d seismic data volume will reach meter scale resolution for water and target depths of less than 600 m the proposed hr3d system will be suitable for 3d and 4d characterization of seabed properties and shallow stratigraphy the identification of geohazards and hydrocarbon leakage and monitoring the environmental impact of offshore activities the p cable 3d system is an excellent starting point for achieving one meter scale resolution due to its flexible and tight meter scale shot and receiver spacing

high resolution 3d seismic study of pockmarks and shallow fluid flow systems at the snohvit hydrocarbon field in the sw barents sea accepted manuscript version published version available at https doi org 10 1016 j margeo 2018 06 012 accepted manuscript version licensed cc by nc nd 4 0

carbon release from submarine seeps at the costa rica fore arc implications for the volatile cycle at the central america convergent margin 1 we report total dissolved inorganic carbon dic abundances and isotope ratios as well as helium isotope ratios 3he 4he of cold seep fluids sampled at the costa rica fore arc in order to evaluate the extent of carbon loss from the submarine segment of the central america convergent margin seep fluids were collected over a 12 month period at mound 11 mound 12 and jaco scar using copper tubing attached to submarine flux meters operating in continuous pumping mode the fluids show minimum 3he 4he ratios of 1 3 ra where ra is air 3he 4he consistent with a small but discernable contribution of mantle derived helium at mound 11 13c co2 values between 23 9 and 11 6 indicate that dic is predominantly derived from deep methanogenesis and is carried to the surface by fluids derived from sediments of the subducting slab in contrast at mound 12 most of the ascending dissolved methane is oxidized due to lower flow rates giving extremely low 13c co2 values ranging from 68 2 to 60 3 we estimate that the carbon flux co2 plus methane through submarine fluid venting at the outer fore arc is 8 0 105 g c km 1 yr 1 which is virtually negligible compared to the total sedimentary carbon input to the margin and the output at the volcanic front unless there is a significant but hitherto unidentified carbon flux at the inner fore arc the implication is that most of the carbon being subducted in costa rica must be transferred to the deeper mantle i e beyond the depth of arc magma generation

icebergs rework shelf sediments to 500 m off antarctica icebergs and sea ice rework the sediments of high latitude shelves producing modern diamicts ice keel turbates unrelated to glacial proximity off antarctica sidescan sonar data indicate the presence of ice gouge features formed by the physical interaction between ice keels and the sea bed these are recognized as incisions a few metres deep and tens of metres wide in water depths up to 500 m on the submarine bank tops and slopes off wilkes land and in the weddell sea subcircular depressions 30 to 150 m in diameter a washboard pattern and hummocky bed features also represent iceberg resting sites the freshness of sea bed morphology nearby holocene sediment ponding and active hydraulic sedimentary processes indicate that the sea floor is being reworked by iceberg keels tabular iceberg drafts in excess of 330 m have been measured and modeling studies suggest that nontabular iceberg drafts of 500 m are possible we conclude that a modern ice keel turbate deposit in the form of a poorly stratified diamicton is probably widespread on that part 54 of the antarctic shelf less than 500 m deep

the effects of volatile recycling degassing and crustal contamination on the helium and carbon geochemistry of hydrothermal fluids from the southern volcanic zone of chile abstract we report new helium and carbon isotope 3he 4he 13c and relative abundance co2 3he characteristics of a suite of hydrothermal gases and fluids fumaroles hot springs geothermal wells from 18 localities in the central southern volcanic zone csvz of chile the csvz is characterized by a wide range of 3he 4he ratios from 1 50 to 6 47 ra where ra air 3he 4he 13c co2 values from 2 9 to 17 7 vs pdb and co2 3he ratios which vary over 5 orders of magnitude 3 1 105 to 2 3 1011 one hydrothermal locality aguas calientes has combined he co2 characteristics remarkably similar to other arc related systems worldwide implying that the underlying subduction zone complex and mantle wedge supplies volatiles to the volcanic front with little or no modification en route to the surface the mechanism controlling helium isotope ratios of other hydrothermal systems appears to be mixing between mantle derived helium and a radiogenic component derived from 4he rich country rock the variable he co2 elemental relationships and 13c co2 values at these localities are consistent with gas separation gas samples or temperature dependent calcite precipitation water samples in shallow level hydrothermal systems both processes result in co2 loss which exacerbates the effects of contamination by crustal gases whereas the aguas calientes locality is useful for understanding the role of the underlying mantle wedge and subducting slab in supplying volatiles to the andean volcanic front the value of the majority of hydrothermal samples in the present study lies with discerning the potentially complicating effects of degassing and or crustal contamination on the resulting he co2 record

archean blocks and their boundaries in the north china craton lithological geochemical structural and p t path constraints and tectonic evolution an examination of lithological geochemical geochronological structural and metamorphic p t path data suggests that the basement of the north china craton can be divided into eastern and western blocks separated by major crustal boundaries that roughly correspond with the limits of a 300 km wide zone called the trans north china orogen the eastern block consists predominantly of late archean domiform tonalitic trondhjemitic granodioritic ttg batholiths surrounded by anastomosing networks and linear belts of open to tight synforms of minor volcanic and sedimentary rocks metamorphosed from greenschist to granulite facies at 2 5 ga with anticlockwise p t paths some early to middle archean rocks are locally present in the eastern block but their tectonic history is unclear due to reworking by the 2 5 ga tectonothermal event the western block has a late archean assemblage structural style and metamorphic history similar to that of the eastern block but it differs in the absence of early to middle archean assemblages and in being overlain by and interleaved with paleoproterozoic khondalites which were affected by a 1 8 ga metamorphic event involving clockwise p t paths a mantle plume model is proposed for the formation and evolution of late archean basement rocks in the eastern and western blocks based on a combination of extensive exposure of ttg gneisses affinities of mafic rocks to continental tholeiitic basalts presence of voluminous komatiitic rocks dominant diaprism related domiform structures anticlockwise p t paths and a short time span from the primary emplacement of ttg and ultramafic to mafic rocks until the onset of regional metamorphism between the two blocks is the trans north china orogen which is bounded by two major fault systems and is composed of late archean to paleoproterozoic ttg gneisses and granitoids interleaved with abundant sedimentary and volcanic rocks that are geochemically interpreted as having developed in magmatic arc and intra arc basin environments these rocks underwent multiple phases of compressional deformation and peak high pressure metamorphism followed by rapid exhumation during the late paleoproterozoic at 1 8 ga as a result of collision between the eastern and western blocks resulting in the amalgamation of the north china craton

glacial geology of outer bjornoyrenna southwestern barents sea the upper cenozoic glacigenic succession in outer bjornoyrenna bear island trough southwestern barents sea is investigated utilizing data from six shallow boreholes 143 m below sea bed and seismic profiling overdeepened troughs in the lowermost part of a large plio pleistocene wedge at the shelf margin are interpreted as glacial erosion forms this suggests that the overlying succession which basinward attains a thickness of several km was deposited during a period with glaciations large scale possible sand ridges in the stratigraphic lower part of this succession are confined to two north south trending zones parallel to the present shelf break they probably formed in a continental shelf coastal environment with exceptionally high sediment supply and fluctuating relative sea level related to glaciations a core from a channel fill in the lower part of the succession consists of possibly glaciofluvial gravelly sand intercalated with till most of the other cores are dominated by middle and upper pleistocene muddy massive partly fissile diamictons interpreted to be mainly tills undrained shear strength of these cores ranges from about 20 kn m 2 to more than 300 kn m 2 and both underconsolidated intervals compared to the present burial depth are found an underconsolidated interval comprising eemian interglacial biourbated marine sediments overlain by laminated weichselian glaciomarine sediments on the outher shelf is covered by 50 100 m of muddy diamicton this diamicton is suggested to be mainly till deposited by a late weichselian ice sheet which was grounded to more than 500 m below present sea level earlier weichselian ice expansions did not reach the outer shelf based on the stratigraphy in the shallow cores and seismic interpretation we suggest that there have been five major expansions of grounded ice to the shelf break since ca 440 ka the present study suggests that glacial erosion was a major process in the late cenozoic denudation of the barents sea and in large areas perhaps as much as 200 250 m of preexisting strata could have been eroded by glaciers since about 440 ka

a ploughing model for the origin of weak tills beneath ice streams a qualitative treatment abstract glaciological studies of west antarctic ice streams have shown that weak sub ice stream tills provide the basal lubrication that makes fast ice streaming possible under low driving stresses given the significant current interest in time dependent ice stream behavior there is a clear need for a conceptual model of weak sub ice stream tills that treats in a simple but physically correct way the coupling between evolution of till properties and ice stream dynamics as a possible alternative to the previous viscous bed model we propose a ploughing model that is consistent with the experimentally determined coulomb plastic rheology of sub ice stream till in the ploughing model the till is a several meters thick layer of sedimentary material that is disturbed and transported by ploughing that occurs during sliding of a bumpy ice base the thickness of the till layer is determined in the ploughing model by the amplitude of the largest roughness elements ice keels there is no direct proof for the existence of ice bumps and ice keels beneath the modern west antarctic ice streams but bedforms e g megalineations and bundle structures left behind by pleistocene ice streams strongly support our assumption that an ice stream base is irregular generation of new till material occurs when ice keels protrude through the existing till layer and erode the top of the sub till preglacial sediments based on a single tethered stake measurement of engelhardt and kamb j glaciol 44 1998 223 made at the upb camp ice stream b west antarctica fig 1 we estimate that the till flux due to sliding with ploughing is there 3 yr 1 per meter width to balance the estimated till flux in the upb area substrata erosion by ice keels would have to take place at a high but not unreasonable non dimensional rate of 4 assuming 1 contact area in the case of the west antarctic ice streams erosion of sub till materials by ice keels may be particularly fast and unimpeded because these ice streams are overriding unlithified preglacial tertiary sediments the most significant implication of the proposed ploughing model is that it permits treating basal resistance to ice motion as being velocity independent plastic till rheology while allowing subglacial transport of till as in the viscous bed model models of ice streams with a plastic bed exhibit a greater potential for unstable behavior than models of ice streams with viscous beds

mantle volatiles in spring gases in the basin and range province on the west of beijing china constraints from helium and carbon isotopes abstract the mantle degassing observed at the earth surface demonstrates both a provenance of fluids in the mantle and a pathway to the surface quantities of this process are discovered on the plate boundaries where there are plenty of active volcanoes and active faults releasing plenty of mantle volatiles however in intraplate tectonic settings without obvious mantle plume the work for mantle degassing observed in spring gasses seems comparatively limited we selected the basin and range province on the west of beijing an area in the inner part of north china craton to discuss the mantle degassing based on the helium and carbon isotopes of spring gasses and the previous works on seismic tomography and fault slip rate the spring gas helium and carbon co2 isotopes indicate the mixture of crustal and mantle materials the helium ratios reported as rc ra air corrected 3he 4he ratio ra 1 4 10 6 ra is the air ratio vary in the range of 0 33 2 08 the calculated mantle helium contributes 4 26 of helium in spring gasses and the remaining is generated in the crust by radiogenic decay of u th series with tiny air mixture co2 acquires analytical 13cv pdb values in the range from 20 3 to 10 2 affected by carbonate precipitation the unaffected values are calculated to be 8 5 5 1 by temperature dependent isotope fractionation indicating the mixture of mantle and crustal limestone materials the mantle volatiles are possibly generated in the upwelling asthenosphere in that the 3he 4he ratio corresponds well with the negative anomaly of p wave velocity at the depth of 70 km imaged by seismic tomography the 3he 4he ratio also correlates with time averaged fault slip rate suggesting higher slip rate renders more permeable mantle vent these consequences help to construct a conceptual model for intraplate mantle degassing that the mantle volatiles generate in the upwelling asthenosphere and uprise through faults and fractures whose permeabilities are controlled by slip rates

subglacial roughness of the former barents sea ice sheet 1 the roughness of a glacier bed has high importance for the estimation of the sliding velocity and can also provide valuable insights into the dynamics and history of ice sheets depending on scale measurement of basal properties in present day ice sheets is restricted to ground penetrating radar and seismics with surveys retrieving relatively coarse data sets deglaciated areas like the barents sea can be surveyed by shipborne 2 d and 3 d seismics and multibeam sonar and provide the possibility of studying the basal roughness of former ice sheets and ice streams with high resolution here for the first time we quantify the subglacial roughness of the former barents sea ice sheet by estimating the spectral roughness of the basal topography we also make deductions about the past flow directions by investigating how the roughness varies along a 2 d line as the orientation of the line changes lastly we investigate how the estimated basal roughness is affected by the resolution of the basal topography data set by comparing the spectral roughness along a cross section using various sampling intervals we find that the roughness typically varies on a similar scale as for other previously marine inundated areas in west antarctica with subglacial troughs having very low roughness consistent with fast ice flow and high rates of basal erosion the resolution of the data set seems to be of minor importance when comparing roughness indices calculated with a fixed profile length a strong dependence on track orientation is shown for all wavelengths with profiles having higher roughness across former flow directions than along them

lateral shear moraines and lateral marginal moraines of palaeo ice streams abstract an understanding of the nature of sedimentation at ice stream lateral margins is important in reconstructing the dynamics of former ice sheets and modelling the mechanisms by which sediment is transported beneath contemporary ice streams theories of the formation of ice stream lateral moraines islms have hitherto been based on a relatively limited number of terrestrial and marine examples here an inventory of islms is compiled from available studies together with independent analysis of seismic reflection and bathymetric datasets the locations and dimensions of 70 islms alongside a synthesis of their key architectural and geomorphic characteristics are presented two different types of islms are identified type 1 islms are up to 3 5 km wide and 60 m thick they maintain a constant width thickness and cross sectional shape along their length type 1 islms are interpreted and referred to as ice stream lateral shear moraines that form subglacially in the shear zone between ice streams and slower flowing regions of an ice sheet in contrast type 2 islms are up to 50 km wide and 300 m thick they are only identified close to the shelf break in the marine environment type 2 islms exhibit an increase in width and thickness along their length and their distal slopes become steeper in a seaward direction they contain internal dipping reflections that indicate sediment progradation away from the former ice stream type 2 islms are interpreted and referred to as ice stream lateral marginal moraines that were formed at the lateral boundary between ice streams and seafloor terrain that was free of grounded ice we suggest that using bathymetric images and acoustic profiles it is possible to differentiate between ice stream lateral shear moraines and lateral marginal moraines in the geological record this distinction is important for understanding the mechanisms of sediment transfer beneath ice streams and for making inferences about the conditions that existed beyond the lateral ice stream margin at the time of lateral moraine formation slow moving ice beyond lateral shear moraines and ocean water rather than ice beyond lateral marginal moraines

a conceptual model of the deglaciation of atlantic canada abstract we present a conceptual model for the deglaciation of atlantic canada in which a role is played by ice streams the ice margin lay close to the edge of the continental shelf at the last glacial maximum ice streams occupying shelf troughs comprised 1 a major ice stream in the laurentian channel 2 secondary streams in the bay of fundy gulf of maine trinity trough and notre dame channel and 3 lesser ice streams elsewhere early retreat in the northeast notre dame bay and in the southwest bay of fundy gulf of maine was mainly by calving along embayments by 18 ka radiocarbon years a calving embayment had opened on emerald basin scotian shelf retreat along channels marooned ice on intervening banks a major calving episode beginning just before 14 ka removed ice from the gulf of st lawrence triggering re advances down secondary troughs isolation of a newfoundland ice cap produced radial drainage via fiords and more extensive flow into the st george s bay lowlands on the west coast by 13 ka most ice was on land and was ablating by melting rather than calving shelf ice caps persisted until 11 ka

mega scale glacial lineations and cross cutting ice flow landforms landsat images reveal a previously unsuspected large scale pattern of streamlining within drift that is assumed to reflect former phases of ice flow such a glacial grain can be regarded as a landform assemblage comprised of a number of components drumlins and megaflutes form part of the pattern but in addition there are two previously undocumented ice moulded landform elements streamlined lineations of much greater proportions referred to as mega scale glacial lineations and a distinctive cross cutting topology within the grain the ice moulded landform assemblage is described and illustrated with reference to examples from canada possible modes of genesis of such landforms are discussed and their glaciological implications outlined the discovery of this pattern indicates the pervasive nature of subglacial deformation of sediment and demands a radical re interpretation of ice sheet dynamics

helium isotope variations in turkey relationship to tectonics volcanism and recent seismic activities the distribution of helium isotope ratios in the various tectonic provinces of turkey is examined through a synthesis of previously published data and the results of a recent survey along the north anatolian fault zone nafz following the catastrophic 1999 earthquakes the r ra values where r sample 3he 4he and ra air 3he 4he cover a wide range from 0 05 to 7 87 and the mantle derived helium is clearly identified in most locations the mantle derived component is high 50 of total he in a regions of central and eastern anatolia both of which are associated with historically active volcanoes and b the seismically active west to central segment of nafz while the mantle contribution reaches a maximum at nemrut volcano in eastern anatolia a region of dominantly compressional tectonics and with moderate enthalpy geothermal fields it is relatively low 50 in the western anatolian extensional province even though this region has the highest geothermal potential the average 3he enthalpy ratios estimated for the different provinces suggest lithospheric stretching and rise of the geotherm as the major mechanism of high heat flow yet limited intrusive activity in western anatolia is suggested as the reason for the comparatively low he additions and consequently low 1 7 106 atoms j 3he enthalpy ratios a more extensive magmatic activity appears to be responsible for the greater input of both heat and helium in eastern and central anatolia with intra province 3he enthalpy variations from 3 1 106 atoms j in eastern to 0 25 106 atoms j in central anatolia reflecting the ageing of hydro magmatic systems the relatively high 3he concentrations in low enthalpy waters of northern anatolia 2 8 106 atoms j are particularly significant since there is no evidence of volcanic activity associated with the strike slip motion along the seismically active segment of nafz continuous monitoring of he isotope compositions along nafz should lead to a better understanding of this apparent anomaly as well as the relationship if any between he isotope variations and seismic activities in this region

massive blow out craters formed by hydrate controlled methane expulsion from the arctic seafloor widespread methane release from thawing arctic gas hydrates is a major concern yet the processes sources and fluxes involved remain unconstrained we present geophysical data documenting a cluster of kilometer wide craters and mounds from the barents sea floor associated with large scale methane expulsion combined with ice sheet gas hydrate modeling our results indicate that during glaciation natural gas migrated from underlying hydrocarbon reservoirs and was sequestered extensively as subglacial gas hydrates upon ice sheet retreat methane from this hydrate reservoir concentrated in massive mounds before being abruptly released to form craters we propose that these processes were likely widespread across past glaciated petroleum provinces and that they also provide an analog for the potential future destabilization of subglacial gas hydrate reservoirs beneath contemporary ice sheets

trough mouth fans palaeoclimate and ice sheet monitors abstract trough mouth fans are fans at the mouth of transverse troughs channels on glaciated continental shelves on the northwest european glaciated continental margin eight trough mouth fans varying in size between 2700 and 215 000 km 2 have been identified the trough mouth fans are depocentres dominated by debris flows accumulated in front of ice streams draining the former large northwest european ice sheets the debris flow units are separated by hemmipelagic interglacial interstadial sediments it is inferred that the number of debris flow units record the number of shelfbreak positions of the ice sheet margins the number found varies between three in the south and eight in the north typical trough mouth fans and related debris flow units seem to have been formed later than the early mid pleistocene thus the north european ice sheets did not form ice streams extending to the shelfbreak in any appreciable length before the mid pleistocene besides being loci for sediment deposition the trough mouth fans were also the main sites of fresh water supply to the ocean in the form of icebergs during the mid late pleistocene ice ages