submarine landforms and shallow acoustic stratigraphy of a 400 km long fjord shelf slope transect kangerlussuaq margin east greenland kangerlussuaq fjord is a relatively uniform steep walled basin whose floor has an almost smooth surface debris is supplied mainly from icebergs from the fast flowing kangerlussuaq glacier sedimentation after iceberg release from multi year sea ice is mainly by rain out of fine grained englacial debris streamlined glacial lineations and drumlins were produced at the sedimentary bed of an ice sheet that expanded into kangerlussuaq trough at the last glacial maximum lgm bedrock channels and crescentic overdeepenings indicate warm based ice and free water beneath parts of the former ice sheet cross cutting iceberg scour marks which characterise outer kangerlussuaq shelf were produced not only during deglaciation but also occasionally through the holocene by deep keeled icebergs from further north in east greenland the outward convex contours of the shelf edge and slope beyond kangerlussuaq trough and debris flows on the slope suggest a glacier influenced high latitude fan the distribution of streamlined subglacial landforms demonstrates that the greenland ice sheet extended throughout kangerlussuaq fjord and reached at least 200 km across the shelf in kangerlussuaq trough at the lgm streamlined landform orientation indicates ice flow from the interior of greenland down the axis of kangerlussuaq trough there is little evidence for discrete sedimentary depocentres in the trough implying that ice probably retreated rapidly from the outer and mid shelf during deglaciation

improvement of tomographic maps by using surface reflected rays abstract the results of the experiment conducted in the northwest atlantic in 1981 have demonstrated the possibilities of acoustic tomography the first maps based only on purely refracted rays showed the evolution of a cold eddy confirmed by direct measurement of temperature and salinity a more complete use of the 1981 dataset with incorporation of surface reflected rays is proposed here the addition of new data reduces the statistical error on the estimation of the sound speed field resolution at levels already well estimated in the earlier computations is improved and individual maps exhibit a better continuity information is now available about the average properties of the upper layers of the ocean which could not be monitored with purely refracted rays

evaluating the information content of tomographic data application to mesoscale observations integrating tomography in the design of future experiments requires knowledge of the information content of this type of data the case of mesoscale observations is examined here the evaluation of the performances of tomography as the only data type available is done by simulating acoustic transmissions in a quasi geostrophic model during a period of 106 geophysical days a linear inversion is then applied to recover the stream function field the linear hypothesis is validated for that particular scale and type of oceanic perturbation as is the robustness to erroneous data the full three dimensional stream function fields are reconstructed information brought by tomography varies according to the mode the first baroclinic mode is well estimated the barotropic mode is partially estimated only one component of the current is felt and higher modes are not resolved the relative vorticity is assessed with horizontal resolution at the scale of the elementary cells defined by the tomographic array it is compared to direct triangular computations the original contribution of tomography is in the simultaneous time and space dimensions complementary to the classical point measurements

high spatial resolution in vertical slice ocean acoustic tomography most studies of ocean acoustic tomography have assumed that little horizontal information is available from the many acoustic multipath travel times observed in a single vertical plane slice between source and receiver moorings there is in fact significant small scale information present in such data sets we examine single vertical slice tomography in spectral terms and show that the acoustic measurements resemble a high pass filter which is more sensitive to small scales shorter than 100 km than to longer scales with the exception of the mean which is well measured the sensitivity extends to scales smaller than 10 km in theory although the level of the ocean energy spectrum is so low at these scales that even small data errors limit the measurement we use analytical calculations supplemented by numerical simulations with realistic data sets to show that accurate reconstructions of the high wave number features are possible out to the limits of the parameterization 9 2 km wavelength when the power spectrum of the ocean features is white or red the total measurement error is 1 ms and multiple receivers are used the ultimate limit of spatial resolution may be smaller still depending on array configuration measurement errors and the shape of the power spectrum

surface freshening of the canada basin 2003 2007 river runoff versus sea ice meltwater 1 the extent of summer arctic sea ice has reduced dramatically in recent years and simultaneously we have observed surface freshening over the canada basin in 2006 and 2007 in order to identify the source of this fresh water either meteoric or sea ice meltwater salinity 18o and alkalinity were analyzed results show that sea ice meltwater increased in the surface water over the central part of the basin in 2006 and 2007 corresponding to the melting of an additional 2 7 m 1 3 m a 1 of sea ice anomalously fresh surface water observed in the southern part in 2007 however was mostly attributed to mackenzie river water extending into the basin interior a source that was mainly absent in the early 2000s comparison with previous data shows that the meltwater component of surface water in the southern part of the canada basin has progressively increased at a mean rate of 0 27 m a 1 since 1987 this can be explained by a reduction of winter sea ice formation rate by 0 45 m or more during the past two decades the runoff component showed larger variability in the southern basin but no obvious temporal trend in the central basin the river runoff component showed an increasing trend of 0 7 m a 1

an improved glacial isostatic adjustment model for the british isles the glacial isostatic adjustment gia of the british isles is of interest due to the constraints that can be provided on key model parameters such as the global meltwater signal local ice sheet history and viscoelastic earth structure a number of recent studies have modelled relative sea level rsl data from this region to constrain model parameters as indicated in these studies the sensitivity of these data to both local and global parameters results in a highly non unique problem this study aims to address this inherent non uniqueness by combining a previously published british irish ice model that is based on the most recent geomorphological data with a new global ice sheet model that provides an accurate prediction of eustatic sea level change in addition constraints from global positioning system gps measurements of present day vertical land motion are considered alongside the entirety of rsl data from both great britain and ireland a model solution is found that provides a high quality fit to both the rsl data and the gps data within the range of earth viscosity values considered the optimal data model fits were achieved with a relatively thin lithosphere 71 km upper mantle viscosities in the range 4 6 1020 pa s and lower mantle viscosities 3 1022 pa s copyright 2011 john wiley sons ltd

millennial scale surface and subsurface paleothermometry from the northeast atlantic 55 8 ka bp we present high resolution records of upper ocean temperatures derived from mg ca ratios of surface dwelling globigerina bulloides and subsurface dwelling neogloboquadrina pachyderma sinistral and the relative abundance of n pachyderma sinistral for the period 55 8 ka bp from ne atlantic sediment core md01 2461 millennial scale temporal variability and longer term trends in these records enable us to develop a detailed picture of past ocean conditions such as a weakening of thermocline intensity from marine isotope stage 3 mis 3 to the last glacial maximum lgm the correspondence of all temperature proxies and convergence of paired oxygen isotope 18o records from both planktonic species implies a breakdown in the thermocline and year round mixing of the upper water column through the lgm perhaps related to decreasing insolation and additional cooling in association with the expansion of the circum north atlantic ice sheets millennial scale divergence in surface and subsurface temperatures and 18o across the last glacial correspond to meltwater release and the development of a strong halocline associated with both heinrich h events and instabilities of the nw european ice sheet during such episodes g bulloides mg ca appears to record ambient even warming summer sea surface temperatures across h events while the other proxies record maximum cooling

growth and decay of a marine terminating sector of the last british irish ice sheet a geomorphological reconstruction the boundary conditions that govern ice sheet dynamics can change significantly with the development of marine margins this paper uses the glacial landscape in western scotland to reconstruct changes in the british irish ice sheet that accompanied the growth and decay of a marine sector over the malin shelf ice advanced from a restricted mountain ice sheet with tidewater margins after 35 ka bp and reached the continental shelf in 7 ka average rate of 30 m a 1 early ice flow had been directed through north south geologically controlled over deepened fjords that were carved during previous restricted glaciations this flow regime was abandoned with development of the malin shelf ice sheet sector ice flow direction switched by 90 degrees and was drawn westwards towards the shelf edge the marine ice sheet phase saw episodes of west east ice divide migration by up to 60 km over west central scotland possibly linked to ice streaming and calving events at the ice sheet margin however permanent and stationary ice divides and zones of cold based ice associated with subglacial topographic highs also characterised the marine glacial stage over western scotland the north channel ice divide remained a constant though migratory feature while the biis occupied the malin shelf it finally collapsed at the end of the killard point stadial when the irish ice sheet began to rapidly decay 16 5 ka bp this permitted the scottish ice sheet to temporarily advance over north east ireland previously identified as the east antrim coastal readvance before it too retreated at rates in the order of 10 2 m a 1 although the imprint of extensive shelf edge ice sheet glaciation exists in the coastal landscape of western scotland the dominant landscape features relate to a restricted marine proximal mountain ice sheet with markedly different flow configurations similar first order geomorphological features relating to restricted glacial conditions are likely to be preserved in subglacial highlands under interior parts of modern ice sheets

a review of ocean acoustic tomography 1987 1990 the techniques and instruments used in acoustic tomography of the oceans are surveyed and results are summarized sections are devoted to theoretical studies and numerical simulations mesoscale mapping vertical slice tomography flow imaging and nonlinear bias series of tomographic experiments analyses to identify global changes and outstanding problems the forward and inverse problems and ocean physics typical data are presented in tables and graphs and a comprehensive bibliography is provided 72 refs

tomographic measurements of the kuroshio extension meander and its associated eddies a 1000 km scale three dimensional 3d acoustic tomography experiment has been conducted in the kuroshio extension region from july to september 1997 the 3d temperature fields show the energetic meander of the kuroshio extension with high time resolution mesoscale eddies associated with the meander with horizontal scale greater than 200 km are resolved in the tomographic inversion these results are consistent with other measurements such as adcp xctd and topex poseidon satellite altimetry variable oceanic structures in a 1000 km domain which are difficult targets for conventional point measurement techniques can be measured well by ocean acoustic tomography with real time data telemetry

ice sheet grounding line dynamics steady states stability and hysteresis 1 the ice sheet ice shelf transition zone plays an important role in controlling marine ice sheet dynamics as it determines the rate at which ice flows out of the grounded part of the ice sheet together with accumulation this outflow is the main control on the mass balance of the grounded sheet in this paper we verify the results of a boundary layer theory for ice flux in the transition zone against numerical solutions that are able to resolve the transition zone very close agreement is obtained and grid refinement in the transition zone is identified as a critical component in obtaining reliable numerical results the boundary layer theory confirms that ice flux through the grounding line in a two dimensional sheet shelf system increases sharply with ice thickness at the grounding line this result is then applied to the large scale dynamics of a marine ice sheet our principal results are that 1 marine ice sheets do not exhibit neutral equilibrium but have well defined discrete equilibrium profiles 2 steady grounding lines cannot be stable on reverse bed slopes and 3 marine ice sheets with overdeepened beds can undergo hysteresis under variations in sea level accumulation rate basal slipperiness and ice viscosity this hysteretic behavior can in principle explain the retreat of the west antarctic ice sheet following the last glacial maximum and may play a role in the dynamics of heinrich events

deep water geomorphology of the glaciated irish margin from high resolution marine geophysical data abstract the continental margin offshore of western ireland offers an opportunity to study the effects of glacial forcing on the morphology and sediment architecture of a mid latitude margin high resolution multibeam bathymetry and backscatter data combined with shallow seismic and tobi deep towed side scan sonar profiles provide the basis for this study and allow a detailed geomorphological interpretation of the northwest irish continental margin several features including submarine mass failures canyon systems and escarpments are identified in the rockall trough for the first time a new physiographic classification of the irish margin is proposed and linked to the impact of glaciations along the margin correlation of the position and dimensions of moraines on the continental shelf with the level of canyon evolution suggests that the sediment and meltwater delivered by the british irish ice sheet played a fundamental role in shaping the margin including the upslope development of some of the canyon systems the glacial influence is also suggested by the variable extent and backscatter signal of sedimentary lobes associated with the canyons these lobes provide an indirect measurement of the amount of glaciogenic sediment delivered by the ice sheet into the rockall trough during the last glacial maximum none of the sedimentary lobes demonstrates notable relief indicating that the amount of glaciogenic sediment delivered by the british irish ice sheet into the rockall trough was limited their southward disappearance suggests a more restricted biis which did not reach the shelf edge south of 54 23 n the various slope styles observed on the irish margin represent snapshots of the progressive stages of slope development for a glacially influenced passive margin and may provide a predictive model for the evolution of other such margins

an oceanic origin for the increase of atmospheric radiocarbon during the younger dryas 1 variations in carbon 14 to carbon 12 ratio in the atmosphere 14catm provide a powerful diagnostic for elucidating the timing and nature of geophysical and anthropological change the atlantic marine archive suggests a rapid 14catm increase of 50 at the onset of the younger dryas yd cold reversal 12 9 11 7 kyr bp which has not yet been satisfactorily explained in terms of magnitude or causal mechanism as either a change in ocean ventilation or production rate using earth system model simulations and comparison of marine based radiocarbon records from different ocean basins we demonstrate that the yd 14catm increase is smaller than suggested by the marine archive this is due to changes in reservoir age predominantly caused by reduced ocean ventilation

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

freshwater influx hydrographic reorganization and the dispersal of ice rafted detritus in the sub polar north atlantic ocean during the last deglaciation a sediment core from the north east north atlantic contains high resolution co registered foraminiferal 18o and ice rafted detritus ird records for the last deglaciation these reveal a distinct ice rafting event that occurred at the time of greenland interstade 1d gi 1d a feature also seen in other high resolution cores from the north atlantic the occurrence of a geographically widespread peak in ird at ice distal sites at a time when increased freshwater flux to the surface ocean is inferred to have caused rapid cooling suggests a mechanistic link between the processes analogous to the younger dryas gs 1 cooling episode the general absence of ird at southern locations at other times during gi 1 when the flux of icebergs from surviving ice sheets to northern locations continued suggests that the gi 1d ird peak represents a time of hyrdrographic reorganization which changed ird dispersal while numerous studies have suggested freshwater flux as a major driver of rapid climate oscillations observed around the north atlantic during the last deglaciation the evidence presented here both supports that mechanism and highlights the potential for rapid and major reorganization of the north atlantic s surface hydrography to explain changes in ird flux independently of ice sheet calving dynamics

terrigenous events and climate history of the sophia basin arctic ocean periods of enhanced terrigenous input to the ocean s basins of the north atlantic have been reported for the last glacial period we present a set of new sediment cores recovered from the sophia basin north of svalbard which exhibit widespread ice rafted debris layers reflecting enhanced terrigenous input throughout the last 200 ka b p their consistent stratigraphic position sedimentological character high sedimentation rate and geochemical characteristic point to synchronously deposited layers which we name terrigenous input events ties owing to their higher densities they generate excellent reflectors for sediment penetrating acoustic devices and prominent acoustic layers in the imagery of sedimentary structures therefore ties can be used for regional acoustic stratigraphy each of the events can be linked to major glacial activity on svalbard however the early weichselian glaciation is not recorded as a tie and in agreement with other work might not have occurred on svalbard as a major glacial advance to the shelf break nonsynchronous timing of western and northern sources on svalbard points against sea level induced iceberg discharge events

submarine geomorphology and glacial history of the sea of the hebrides uk the sea of the hebrides is an island studded region of complex bathymetry on the uk continental shelf west of the scottish mainland an extensive area 2200 km2 of recently collected multibeam bathymetry data combined with seismic reflection profiles reveal this part of the shelf to have been extensively modified by both glacial and modern processes our new geomorphological evidence strongly supports the contention that an ice stream drained ice from western scotland and the inner hebrides towards the barra fan at the continental shelf break at the height of the last glaciation marine isotope stage 2 3 convergent seabed glacial lineations and other subglacially streamlined features eroded in bedrock around the islands of canna and rum preserve the direction of ice sheet movement and strongly suggest the onset of ice streaming in a southwesterly direction on the continental shelf in the sea of the hebrides region this fast flow zone formed part of a larger convergent ice stream system draining much of western scotland and the north of ireland the southern part of which has been postulated already a number of rock basins linear troughs and deeps west and south of muck kept sediment free by the dominant modern tidal regime are interpreted as being at least partly subglacial in origin similarly a large complex of buried deeps between eigg and arisaig are interpreted as an overdeepened glacial drainage network it is suggested that intense bedrock erosion by focused subglacial abrasion and subglacial meltwater discharge over multiple glacial advance and decay cycles led to significant modifications of the pre quaternary bedrock surface other features such as moraines are only found in shallower water typically 50 m west of canna west of rum and south of skye their relative paucity indicates that the modern tidally dominated environment of the area may have removed much of the localised geomorphological evidence of offshore glacial limits within the sea of the hebrides or alternatively that the ice sheet margin was floating and retreated predominantly by calving leaving little geomorphological evidence of its retreat

environmental change in eastern greenland during the last 1300 years evidence from foraminifera and lithofacies in nansen fjord 68 n lithofacies and benthic foraminiferal analyses of two sediment cores bs1191 k13b and k14 from nansen fjord eastern greenland show evidence of changing oceanographic and sea ice conditions from

ice sheet grounding zone wedges gzws on high latitude continental margins abstract grounding zone wedges gzws are asymmetric sedimentary depocentres which form through the rapid accumulation of glacigenic debris along a line source at the grounding zone of marine terminating ice sheets during still stands in ice sheet retreat gzws form largely through the delivery of deforming subglacial sediments the presence of gzws in the geological record indicates an episodic style of ice retreat punctuated by still stands in grounding zone position moraine ridges and ice proximal fans may also build up at the grounding zone during still stands of the ice margin but these require either considerable vertical accommodation space or sediment derived from point sourced subglacial meltwater streams by contrast gzws form mainly where floating ice shelves constrain vertical accommodation space immediately beyond the grounding zone an inventory of gzws is compiled from available studies of bathymetric and acoustic data from high latitude continental margins the locations and dimensions of gzws from the arctic and antarctic alongside a synthesis of their key architectural and geomorphic characteristics are presented gzws are only observed within cross shelf troughs and major fjord systems which are the former locations of ice streams and fast flowing outlet glaciers typical high latitude gzws are less than 15 km in along flow direction and 15 to 100 m thick gzws possess a transparent to chaotic acoustic character which reflects the delivery of diamictic subglacial debris many gzws contain seaward dipping reflections which indicate sediment progradation and wedge growth through continued delivery of basal sediments gzw formation is inferred to require high rates of sediment delivery to a fast flowing ice margin that is relatively stable for probably decades to centuries although the long term stability of the grounding zone is controlled by ice sheet mass balance the precise location of any still stands is influenced strongly by the geometry of the continental shelf the majority of high latitude gzws occur at vertical or lateral pinning points which encourage grounding zone stabilisation through increasing basal and lateral drag and reducing mass flow across the grounding zone

submarine glacial landforms record late pleistocene ice sheet dynamics inner hebrides scotland we use 7000 km2 of high resolution swath bathymetry data to describe and map the submarine glacial geomorphology and reconstruct late pleistocene ice sheet flow configurations and retreat dynamics within the inner hebrides western scotland frequently dominated by outcrops of structurally complex bedrock the seabed also comprises numerous assemblages of well preserved glacigenic landforms typical of grounded ice sheet flow and punctuated ice margin retreat the occurrence and character of the glacially streamlined landforms is controlled in part by the shallow geology and topography however these factors alone cannot account for the location orientation and configuration of the observed landforms we attribute the distribution of these elongate streamlined landforms to the onset zone of the former hebrides ice stream his part of a major ice stream system that drained 5 10 of the last british irish ice sheet biis we suggest this geomorphic signature represents the transition from slow sheet flow to streaming flow as ice accelerated out from an environment characterized by numerous bedrock obstacles e g islands headlands towards the smooth sediment dominated shelf the majority of streamlined landforms associated with the his indicate ice sheet flow to the southwest with regional scale topography clearly playing a major role in governing the configuration of flow during maximal glacial conditions 29 23 ka we infer that the his merged with the north channel malin shelf ice stream to form a composite ice stream system that ultimately reached the continental shelf edge at the barra donegal trough mouth fan taken collectively however the pattern of landforms now preserved at seabed e g convergent flow indicators cross cutting flow sets is more indicative of a thinning ice mass undergoing reorganization during overall ice sheet retreat during latter stages of late weischselian glaciation suites of moraines overprinting the streamlined landforms suggest partial stabilization of the his prior to the ice sheet retreating to more isolated topographically confined troughs and basins retreat from the shelf towards and back into the inner hebrides may have been rapid due the prevalence of overdeepened troughs within the near shore fjord like troughs and deeps basin aligned streamlined landforms indicate the subsequent flow of thinner topographically partitioned ice masses and overprinted moraines record further ice margin retreat potentially along tide water margins this work provides the first geomorphological constraints for this large marine influenced sector of the former biis we also shed new light on the glacial geomorphic record found at the transition from terrestrial to marine continental shelf settings and examine the interplay between substrate geology bed topography bathymetry and grounding line positions relationships which are important for characterizing contemporary marine ice sheet margins

glacimarine sedimentary processes and depositional environments in an embayment fed by west greenland ice streams abstract the extent and behaviour of the central west greenland sector of the greenland ice sheet gis during the last glacial maximum lgm and the following deglaciation remains poorly constrained to investigate deglacial marine environments and sedimentary processes in an area fed by several lgm ice streams we now report findings from a 3 5 khz sub bottom profiler survey acquired in and around disko bay in 2009 sediment thicknesses and volumes in three glaciated areas disko bay southern vaigat and egedesminde dyb are fully constrained for the first time acoustic facies types and distributions were also mapped the largest sediment thicknesses 256 m and widest variation in acoustic character was observed in disko bay adjacent to the mouth of jakobshavns isfjord significant sediment thicknesses were also observed in isolated basins in the egedesminde dyb trough and in a trough in southern vaigat strait up to 110 m given low modern sedimentation rates these thick sediment packages are inferred to record the retreat of glaciers from the middle shelf through disko bay and into both the jakobshavns and torssukatak fjord systems in central west greenland our results highlight similarities in deglacial sedimentary processes across the study area when the margins of retreating glaciers supplied large volumes of sediment to the marine environment from sediment laden meltwater plumes and melting icebergs we estimate the sediment flux for the palaeo jakobshavn ice stream while the ice margin was stable at a fjord entrance sill to be 1 6 3 6 10 7 m 3 a 1 assuming a flux at the upper end of our range for the torssukatak fjord system requires the grounded ice margin to be stable for about 450 years during retreat the sediment fluxes we estimate are within the range of previously calculated fluxes for other arctic and antarctic ice streams and suggest that large volumes of meltwater and sediment were available at the base of the outlet glaciers during deglaciation even during periods of quasi stability retreat of the ice margins into the fjord systems is characterised by a rapid transition to ice distal sedimentation by rainout of iceberg rafted detritus ird and hemipelagic material accumulation rates drop to 2 mm a 1 this study demonstrates the variability in glacimarine processes and environments on high latitude continental margins and highlights the importance of local factors such as bedrock morphology as well as glacio dynamic factors on sedimentation

high resolution evidence for linkages between nw european ice sheet instability and atlantic meridional overturning circulation published studies show that ice rafted debris ird deposition preceding heinrich h events h1 and h2 in the ne atlantic was derived from the nw european ice sheets nweis possibly offering clues about ice sheet sensitivity and stability and the mechanisms that caused periodic collapse of the laurentide ice sheet lis we present detailed lithological and geochemical records including radiogenic isotope fingerprinting of ird deposits from core md01 2461 proximal to the last glacial british ice sheet bis demonstrating persistent instability of the bis with significant destabilisation occurring 1 5 1 9 kyr prior to both h1 and h2 dated at 16 9 and 24 1 kyr bp respectively in the ne atlantic paired mg ca and 18o data from the surface dwelling globigerina bulloides and subsurface dwelling neogloboquadrina pachyderma sinistral are used to determine late glacial variability of temperature salinity and stratification of the upper water column a picture emerges that the bis was in a continuing state of readjustment and never fully reached steady state increased sea surface temperatures appear to have triggered the episode of nweis instability preceding h1 it seems most probable that the so called precursor events were not linked to the h events however if response to a common thermal forcing is considered an increased response time of the lis up to 2 kyr longer than the nweis may be inferred negative salinity excursions of up to 2 6 indicate significant incursions of melt water associated with peaks in nweis instability decreased surface density led to a more stable stratification of the upper water column and is associated with reduced ventilation of intermediate waters recorded in depleted epibenthic 13c cibicidoides wuellerstorfi we suggest that instability and meltwater forcing of the nweis temporarily weakened atlantic meridional overturning circulation allowing transient advance of southern sourced waters to this site prior to h events 1 and 2

subglacial till formation sedimentary characteristics and classification we review the major subglacial till forming processes as presently understood by glacial researchers and define the parameters within which tills are produced and reconcile them with sedimentary end members processes of deformation flow sliding lodgement and ploughing coexist at the base of temperate glacier ice and act to mobilize and transport sediment and deposit it as various end members ranging from glacitectonically folded and faulted stratified material to texturally homogeneous diamicton the dominance of any one subglacial process varies both spatially and temporally giving rise to the possibility that a till or complex till sequence contains a superimposed signature of former transportation deposition at the ice bed interface we recommend that while glacial geologists and geomorphologists should be able to recognize the sedimentary imprints of various subglacial processes genetic fingerprinting of subglacial tills should be less process specific and till classification must reflect the range of products encompassed by the subglacial till production continuum glacial geologists can presently unequivocally identify a glacitectonite rock or sediment that has been deformed by subglacial shearing deformation but retains some of the structural characteristics of the parent material b subglacial traction till sediment deposited by a glacier sole either sliding over and or deforming its bed the sediment having been released directly from the ice by pressure melting and or liberated from the substrate and then disaggregated and completely or largely homogenised by shearing and support the theoretical case for c melt out till sediment released by the melting of stagnant or slowly moving debris rich glacier ice and directly deposited without subsequent transport or deformation because observations on contemporary glaciers reveal that their beds are most likely to be mosaics of deformation and sliding and warm based and cold based conditions the patterns of which change temporally and spatially it is extremely unlikely that subglacial till end members in the geological record will be anything but hybrids produced by the range of processes operative in the subglacial traction zone

extent and retreat history of the barra fan ice stream offshore western scotland and northern ireland during the last glaciation abstract during the last glacial maximum lgm the marine terminating barra fan ice stream bfis a major conduit of the british irish ice sheet biis drained much of western scotland and northwest ireland with ice streaming onto the continental shelf of the malin sea the extent and retreat history of this ice stream across the shelf until now is not well known in particular geochronological constraints on the history of this ice stream have thus far been restricted to deep sea cores or terrestrial cosmogenic nuclide dating onshore with ages across the shelf absent to understand the possible external forcing factors acting on this marine terminating ice stream during retreat improved geochronological constraint on its deglaciation is necessary here we present new geophysical data marine sediment cores and over forty radiocarbon dates to provide important constraints on maximum extent of the bfis as well as the timing and pattern of retreat back across the malin shelf dated moraines and grounding zone wedges gzw seen in seafloor sub bottom profiles provide evidence that the bfis reached the malin shelf edge during the lgm and was at its maximum extent around 26 7 ka bp the presence of two sets of gzws suggests that the style of retreat was episodic the new radiocarbon chronology shows that retreat from the shelf edge was underway by 25 9 ka bp with the majority of the continental shelf ice free by 23 2 ka bp and that glacimarine conditions were present in the sea of hebrides by 20 2 ka bp at the latest collectively these results indicate that the majority of the malin shelf was free of grounded ice by 21 5 20 ka bp which is up to 4000 years earlier than previously reconstructed we attribute this early deglaciation to high relative sea level caused by glacial isostatic depression when the biis reached its maximum extent promoting ice shelf and grounding line instability two deep troughs forming reverse bed slopes aided the continued retreat of the bfis this suggests that local ice loading and bed morphology can be significant controls on the destabilisation of a marine terminating ice stream and can override the influence of ocean and atmospheric temperatures

bottom current controlled sedimentation a synthesis of the contourite problem abstract an overview of the main items concerning deep bottom current controlled deposits is presented these include the definition of contourites their processes of deposition and facies characteristics contourite drift types and seismic patterns the interplay of turbidity and bottom current processes and the correlation between bottom current processes and global changes in climate sea level from this discussion several of the more significant problems are highlighted which we believe should be priority targets for future research 1 identification of contourites especially in ancient series where they are interbedded with turbidites and other facies 2 refinement of the numerous parameters used as tracers of palaeocirculation patterns 3 understanding the interaction between the different types of bottom currents and also between bottom current and other deep water processes 4 distinction in the sedimentary record between the effects of short time scale bottom current variations due to regional causes like benthic storms and geological scale current fluctuations related to climatic or astronomic control 5 understanding the control of glacial interglacial climatic cycles on the nature and rate of contourite deposition