detachment faulting mantle serpentinization and serpentinite mud volcanism beneath the porcupine basin southwest of ireland the porcupine basin southwest of ireland provides an opportunity to study the symmetry of rifting at stretching factors approaching continental breakup profiles across the basin image a bright reflection that appears to represent a detachment fault and may in part be a decollement at the top of partially serpentinized mantle although overall the basin appears symmetric the consistent westward structural dip of the detachment implies that at high stretching factors extension was asymmetric farther south the porcupine median high appears in cross section to be a triangular construction overlying tilted fault blocks and onlapped by postrift sediment despite no evidence for synrift magmatism this high has previously been interpreted as a basaltic structure however it may represent a serpentinite mud volcano or diapir we suggest that such structures produce the serpentinite breccias found within the rifted continent ocean transition of nonvolcanic margins

greenland glacial history and local geodynamic consequences summary space time reconstructions of the continental ice sheets that existed at last glacial maximum lgm have previously been produced using two entirely independent methodologies one based upon the use of theoretical models of ice sheet accumulation and flow and one based upon the geophysical inversion of relative sea level rsl histories from previously ice covered regions the analyses described in this paper demonstrate the significant advantages that derive from the simultaneous application of both methods to the particular case of greenland we thereby show that the ice 4g reconstruction of the glaciation history of this region from lgm to present which was based upon the geophysical inversion of rsl data alone was reasonably accurate in the peripheral regions where rsl data were available but inaccurate in the interior of the ice sheet which was unconstrained by such information we test the new model of greenland glacial history determined by the simultaneous application of the constraints that derive from ice sheet modelling and the geophysical inversion of rsl data by employing recently published geodetic inferences of mass balance over the entire interior region of the ice sheet and of gps measurements of vertical crustal motion these observations which were not employed to constrain the ice sheet reconstruction provide significant support for the new glacial history for greenland that our analyses have led us to infer

establishment of modern circulation pattern at c 6000 cal a bp in disko bugt central west greenland opening of the vaigat strait variations in the holocene circulation of the west greenland current wgc in the disko bugt region have been reconstructed from a suite of sediment cores palaeoceanographic proxies include magnetic susceptibility ms and x ray fluorescence titanium counts which document a major shift in circulation at c 6000 cal a bp before this date sediments in southern disko bugt were characterized by high terrigeneous and basaltic input suggesting widespread influence of meltwater plumes our data show that the wgc re circulated in the southern disko bugt area because a potential northern pathway the narrow vaigat strait was blocked by icebergs that calved from marine outlet glaciers in eastern disko bugt sediments in southern disko bugt deposited after c 6000 cal a bp have significantly lower terrestrial and basaltic sediment input which coincides with minimum holocene ice sheet extent the reduced meltwater and iceberg discharge to the embayment caused the vaigat strait to become free of blocking icebergs and terrigenous input was partly diverted to the outer shelf thus the modern circulation pattern of the wgc was established in the disko bugt region through the opening of the vaigat strait c 6000 cal a bp

seafloor geomorphology and glacimarine sedimentation associated with fast flowing ice sheet outlet glaciers in disko bay west greenland fast flowing outlet glaciers currently drain the greenland ice sheet gis delivering ice meltwater and debris to the fjords around greenland although such glaciers strongly affect the ice sheet s mass balance their glacimarine processes and associated products are still poorly understood this study provides a detailed analysis of lithological and geophysical data from disko bay and the vaigat strait in central west greenland disko bay is strongly influenced by jakobshavn isbrae greenland s fastest flowing glacier which currently drains 7 of the ice sheet streamlined glacial landforms record the former flow of an expanded jakobshavn isbrae and adjacent gis outlets through disko bay and the vaigat strait towards the continental shelf thirteen vibrocores contain a complex set of lithofacies including diamict stratified mud interbedded mud and sand and bioturbated mud deposited by 1 suspension settling from meltwater plumes and the water column 2 sediment gravity flows and 3 iceberg rafting and ploughing the importance of meltwater related processes to glacimarine sedimentation in west greenland fjords and bays is emphasised by the abundance of mud preserved in the cores radiocarbon dates constrain the position of the ice margin during deglaciation and suggest that jakobshavn isbrae had retreated into central disko bay before 10 6 cal ka bp and to beyond isfjeldsbanken by 7 6 7 1 cal ka bp sediment accumulation rates were up to 1 7 cm a 1 for ice proximal glacimarine mud and 0 007 0 05 cm a 1 for overlying distal sediments in addition to elucidating the deglacial retreat history of jakobshavn isbrae our findings show that the glacimarine sedimentary processes in west greenland are similar to those in east greenland and that variability in such processes is more a function of time and glacier proximity than of geographic location and associated climatic regime

modeling of store gletscher s calving dynamics west greenland in response to ocean thermal forcing glacier front dynamics is an important control on greenland s ice mass balance warmer ocean waters trigger ice front retreats of marine terminating glaciers and the corresponding loss in resistive stress leads to glacier acceleration and thinning here we present an approach to quantify the sensitivity and vulnerability of marine terminating glaciers to ocean induced melt we develop a plan view model of store gletscher that includes a level set based moving boundary capability a parameterized ocean induced melt and a calving law with complete and precise land and fjord topographies to model the response of the glacier to increased melt we find that the glacier is stabilized by a sill at its terminus the glacier is dislodged from the sill when ocean induced melt quadruples at which point the glacier retreats irreversibly for 27 km into a reverse bed the model suggests that ice ocean interactions are the triggering mechanism of glacier retreat but the bed controls its magnitude

deep structure of the ocean continent transition in the southern iberia abyssal plain from seismic refraction profiles ocean drilling program legs 149 and 173 transect we present a wide angle seismic refraction study of an 80 40 km region of the southern iberia abyssal plain south of galicia bank an intersecting grid of two e w and four n s wide angle reflection refraction profiles is used to define variations of the basement velocity structure within this unusually wide ocean continent transition oct these structures can be systematically linked to variations in acoustic basement morphology and to results from ocean drilling program odp boreholes lateral changes in the velocity structure of the basement occur abruptly over distances of 20 km where complex variations may be found thinned upper continental crust 2 5 km thick with velocities of 5 0 6 6 km s is limited to a series of n s fault blocks immediately south of galicia bank this crust is underlain by a high velocity layer 7 3 7 9 km s of weakly serpentinized i e 0 25 peridotite which exists throughout the eastern part of the survey area basement within the oct appears to consist dominantly of a broad region of exposed upper mantle that has been serpentinized heterogeneously both vertically and horizontally in the southeast sector of our survey where basement topography deepens and becomes subdued continental fault blocks are absent instead basement contains an upper layer of more pervasively serpentinized i e 25 45 peridotite that is 2 km thick this layer is characterized by low velocity at the top of basement 4 2 km s that increases rapidly with depth and it probably corresponds to a seismically unreflective layer previously identified in reflection profiles to the south of our survey in the western section of our survey beneath a series of elevated basement ridges velocities are reduced within both the upper basement layer 3 5 6 0 km s and lower layer 6 4 7 5 km s these changes suggest that both upper and lower layers have become more highly serpentinized with values of 60 100 in the upper layer and 25 45 in the lower layer probably during the last stages of rifting and immediately before formation of oceanic crust a normal or slow spreading oceanic crustal structure is not found within the survey region thus it appears that the onset of seafloor spreading occurs in the region west of the peridotite ridge sampled at odp site 897 and east of the j magnetic anomaly

quantitative reconstruction of holocene sea ice and sea surface temperature off west greenland from the first regional diatom data set holocene oceanographic conditions in disko bay west greenland were reconstructed from high resolution diatom records derived from two marine sediment cores a modern data set composed of 35 dated surface sediment samples collected along the west greenland coast accompanied by remote sensing data was used to develop a diatom transfer function to reconstruct april sea ice concentration sic supported by july sea surface temperature sst in the area our quantitative reconstruction shows that oceanographic changes recorded throughout the last 11 000 years reflect seasonal interplay between spring april sic and summer july sst conditions our records show clear correlation with climate patterns identified from ice core data from gisp2 and agassiz renland for the early to middle holocene the early holocene deglaciation of western greenland ice sheet was characterized in disko bay by initial strong centennial scale fluctuations in april sic with amplitude of over 40 followed by high april sic and july sst these conditions correspond to a general warming of the climate in the northern hemisphere a decrease in april sic and july sst was recorded during the holocene thermal optimum reflecting more stable spring summer conditions in disko bay during the late holocene high april sic characterized the medieval climate anomaly while high july sst prevailed during the little ice age supporting previously identified antiphase relationship between surface waters in west greenland and climate in nw europe this antiphase pattern might reflect seasonal variations in regional oceanographic conditions and large scale fluctuations within the north atlantic oscillation and atlantic meridional overturning circulation

lithospheric extension and magmatism in the porcupine basin west of ireland the porcupine seabight basin is a north south trending extensional sedimentary basin situated on the continental shelf west of ireland it is filled with sedimentary rocks which range in age from devonian to present day a large amount of geological and geophysical data is now available for this basin mainly as a result of oil industry exploration in this paper information from 23 well logs and from regional seismic reflection surveys is used to determine the lithospheric stretching history of the basin subsidence analyses indicate that there was one main phase of stretching which began in late liassic times 180 ma ending in earliest cretaceous 145 ma stretching factors at the northern end of the basin are relatively small 1 1 1 7 but increase rapidly southwards along the axis of the main porcupine basin in the southern part of the region the seabight basin is greater than 6 the simplest way to accommodate this rapid variation in stretching is by clockwise rotation of the porcupine ridge through approximately 20 away from the irish shelf subsidence derived stretching values are in reasonable agreement with those determined previously from crustal thicknesses based on gravity models and deep seismic data normal incidence and wide angle the existence location and inferred age of the porcupine median volcanic ridge are also consistent with the subsidence derived stretching factors after intermittent paleocene igneous activity an anomalous increase in the rate of subsidence occurred in the eocene between 55 and 42 ma there is little evidence that this rapid increase is caused by lithospheric stretching and we conclude that it must be associated in some unknown way with melt generated by development of the iceland hotspot

the recession of the inland ice margin during the holocene climatic optimum in the jakobshavn isfjord area of west greenland recent subsurface mapping of parts of the greenland inland ice margin in the region of jakobshavn isbrae indicates that the fjord system in the period of at least 2700 4700 calendar yr b p was more ice free than at present and that the front of the glacier was at least 15 km behind the present position the 14c datings of subfossils brought to the present ice margin fit with the climatic records from ice cores and confirm the favourable conditions for greenland s first settlers the sarqaq people who arrived in the region about 4000 yr ago to find hunting grounds 10 20 larger than the present

early holocene history of the west greenland ice sheet and the gh 8 2 event abstract the margin of the greenland ice sheet retreated rapidly during the first few thousand years of the holocene during this period of relative warmth known as the holocene thermal maximum ice core records identify a significant short lived cooling event at approximately 8 4 8 0 ka cal yr bp the gh 8 2 event associated with a 5 7 c fall in mean annual air temperature over the centre of the ice sheet in this paper we constrain the history of the ice sheet margin in disko bugt west greenland and that of a major ice stream jakobshavns isbrae during the early holocene which incorporates the interval of the gh 8 2 event our work is based on a new relative sea level curve and minimum age estimates for the timing of deglaciation from two field sites combined with a review of previously published research from the study area we identify important differences in the chronology of ice margin recession during the early holocene most noticeably the margin of jakobshavns isbrae retreated well inland of the adjacent ice sheet at this time we conclude that the early holocene fjord stade moraines in disko bugt do not record a uniform ice sheet margin response to the gh 8 2 event rather these moraines are diachronous and formed between c 10 8 ka cal yr bp their age varying as a function of the interplay between topography and ice sheet ice stream dynamics we hypothesise that one cause for the lack of an identifiable response to the gh 8 2 event is because topographic controls dominated ice sheet behaviour at this time in lowland areas any increase in ice sheet mass balance was probably associated with an increase in calving rather than any major advance of a grounded ice sheet margin

present day jakobshavn isbrae west greenland close to the holocene minimum extent abstract marine terminating glaciers evolve on millenial timescales in response to changes in oceanic and atmospheric conditions however the relative role of oceanic and atmospheric drivers remains uncertain the evolution of marine terminating glaciers under the warmer than present holocene climate optimum climate can give important insights into the dynamics of ice streams as the climate evolves the early holocene evolution of jakobshavn isbrae from the last glacial maximum extent up to 8 2 ka bp is well constrained by geomorphological studies in the area however the holocene minimum extent of the glacier is unknown here we use a high resolution regional ice sheet model to study the retreat and readvance of jakobshavn isbrae from the mid holocene to the little ice age this model of jakobshavn isbrae accurately tracks the terrestrial ice margin and agrees with available estimates of marine grounding line evolution we find that the holocene minimum extent of both the terrestrial ice margin and the grounding line reached at 6 5 ka bp is close to the present day extent of the glacier we also find that the glacier is currently located close to a tipping point from beyond which readvance would require a longer and more significant cooling than the little ice age we assess the importance of the ocean forcing in explaining the holocene evolution of jakobshavn and find that cooling within the fjord during the mid holocene is critical for the glacier to readvance this finding emphasizes the role of ocean forcing when trying to understand the millenial scale evolution of marine terminating glaciers

holocene thinning of the greenland ice sheet the nature of the response of the greenland ice sheet gis to climate change is a fundamental question in climate science but the details remain unclear during the holocene climate optimum 6 000 to 9 000 years ago for example many locations in the northern hemisphere showed unusual warmth yet changes in greenland appeared inconsistent vinther et al now standardize the previously disparate seeming ice core records of climate history on the gis during the holocene and find consistent evidence for a stronger climate optimum than was previously recognized together with elevation reduction and marginal ice sheet thinning the response of the greenland ice sheet gis to changes in climate remains uncertain during the holocene climatic optimum an unusually warm period from about 9 000 to 6 000 years ago changes in greenland appear inconsistent with the rest of the northern hemisphere here the greenland holocene temperature history and the evolution of gis surface elevation are extracted from four gis locations the results may resolve this disparity on entering an era of global warming the stability of the greenland ice sheet gis is an important concern1 especially in the light of new evidence of rapidly changing flow and melt conditions at the gis margins2 studying the response of the gis to past climatic change may help to advance our understanding of gis dynamics the previous interpretation of evidence from stable isotopes 18o in water from gis ice cores was that holocene climate variability on the gis differed spatially3 and that a consistent holocene climate optimum the unusually warm period from about 9 000 to 6 000 years ago found in many northern latitude palaeoclimate records4 did not exist here we extract both the greenland holocene temperature history and the evolution of gis surface elevation at four gis locations we achieve this by comparing 18o from gis ice cores3 5 with 18o from ice cores from small marginal icecaps contrary to the earlier interpretation of 18o evidence from ice cores3 6 our new temperature history reveals a pronounced holocene climatic optimum in greenland coinciding with maximum thinning near the gis margins our 18o based results are corroborated by the air content of ice cores a proxy for surface elevation7 state of the art ice sheet models are generally found to be underestimating the extent and changes in gis elevation and area our findings may help to improve the ability of models to reproduce the gis response to holocene climate

large scale ice flow over a viscous basal sediment theory and application to ice stream b antarctica recent seismic studies of ice stream b antarctica and field analysis of mid latitude glacial deposits suggest that deformable basal sediments e g water saturated till are important in determining ice sheet flow if the ratio of till viscosity to effective ice viscosity is small vertical shear associated with horizontal flow is confined to the deforming bed alone ice flow over a deformable bed is thus akin to that of floating ice shelves because ice shelves flow over inviscid seawater for some antarctic ice streams and possibly for portions of the late wisconsin ice sheets in north america and eurasia basal drag associated with deforming basal sediment does not induce significant vertical gradients of horizontal velocity instead basal drag affects the flow as if it were a horizontal body force balanced by longitudinal and transverse deviatoric stress gradients comparison of the observed flow of ice stream b to finite element simulations incorporating a viscous basal till suggest that a simple till theology is sufficient to explain the current velocity profile these simulations also highlight the importance of horizontal deviatoric stress in regions where driving stress and basal stress do not locally balance while bed deformation is critical to ice stream existence sensitivity tests suggest that ice shelf back pressure is still a crucial control affecting ice stream response to atmospheric and oceanic climate

bedmachine v3 complete bed topography and ocean bathymetry mapping of greenland from multibeam echo sounding combined with mass conservation greenland s bed topography is a primary control on ice flow grounding line migration calving dynamics and subglacial drainage moreover fjord bathymetry regulates the penetration of warm atlantic water aw that rapidly melts and undercuts greenland s marine terminating glaciers here we present a new compilation of greenland bed topography that assimilates seafloor bathymetry and ice thickness data through a mass conservation approach a new 150 m horizontal resolution bed topography bathymetric map of greenland is constructed with seamless transitions at the ice ocean interface yielding major improvements over previous data sets particularly in the marine terminating sectors of northwest and southeast greenland our map reveals that the total sea level potential of the greenland ice sheet is 7 42 0 05 m which is 7 cm greater than previous estimates furthermore it explains recent calving front response of numerous outlet glaciers and reveals new pathways by which aw can access glaciers with marine based basins thereby highlighting sectors of greenland that are most vulnerable to future oceanic forcing

an improved mass budget for the greenland ice sheet extensive ice thickness surveys by nasa s operation icebridge enable over a decade of ice discharge measurements at high precision for the majority of greenland s marine terminating outlet glaciers prompting a reassessment of the temporal and spatial distribution of glacier change annualmeasurements for 178 outlet glaciers reveal that despite widespread acceleration only 15 glaciers accounted for 77 of the 739 29 gt of ice lost due to acceleration since 2000 and four accounted for 50 among the top sources of loss are several glaciers that have received little scientific attention the relative contribution of ice discharge to total loss decreased from 58 before 2005 to 32 between 2009 and 2012 as such 84 of the increase in mass loss after 2009 was due to increased surface runoff these observations support recentmodel projections that surface mass balance rather than ice dynamics will dominate the ice sheet s contribution to 21st century sea level rise

interruption of two decades of jakobshavn isbrae acceleration and thinning as regional ocean cools jakobshavn isbrae has been the single largest source of mass loss from the greenland ice sheet over the last 20 years during that time it has been retreating accelerating and thinning here we use airborne altimetry and satellite imagery to show that since 2016 jakobshavn has been re advancing slowing and thickening we link these changes to concurrent cooling of ocean waters in disko bay that spill over into ilulissat icefjord ocean temperatures in the bay s upper 250 m have cooled to levels not seen since the mid 1980s observations and modelling trace the origins of this cooling to anomalous wintertime heat loss in the boundary current that circulates around the southern half of greenland longer time series of ocean temperature subglacial discharge and glacier variability strongly suggest that ocean induced melting at the front has continued to influence glacier dynamics after the disintegration of its floating tongue in 2003 we conclude that projections of jakobshavn s future contribution to sea level rise that are based on glacier geometry are insufficient and that accounting for external forcing is indispensable

bed elevation of jakobshavn isbrae west greenland from high resolution airborne gravity and other data jakobshavn isbrae west greenland which holds a 0 6 m sea level volume equivalent has been speeding up and retreating since the late 1990s interpretation of its retreat has been hindered by difficulties in measuring its ice thickness with airborne radar depth sounders here we employ high resolution helicopter borne gravity data from 2012 to reconstruct its bed elevation within 50 km of the ocean margin using a three dimensional inversion constrained by fjord bathymetry data offshore and a mass conservation algorithm inland we find the glacier trough to be asymmetric and several 100 m deeper than estimated previously in the lower part from 1996 to 2016 the grounding line migrated at 0 6 km yr from 700 m to 1100 m depth upstream the bed drops to 1600 m over 10 km then slowly climbs to 1200 m depth in 40 km jakobshavn isbrae will continue to retreat along a retrograde slope for decades to come

understanding controls on rapid ice stream retreat during the last deglaciation of marguerite bay antarctica using a numerical model using a one dimensional numerical model of ice stream flow with robust grounding line dynamics we explore controls on paleo ice stream retreat in marguerite bay antarctica during the last deglaciation landforms on the continental shelf constrain the numerical model and suggest retreat was rapid but punctuated by a series of slowdowns we investigate the sensitivity of ice stream retreat to changes in subglacial and lateral topography and to forcing processes including sea level rise enhanced melting beneath an ice shelf atmospheric warming and ice shelf debuttressing our experiments consistently reproduce punctuated retreat on a bed that deepens inland with retreat rate slowdowns controlled by narrowings in the topography sensitivity experiments indicate that the magnitudes of change required for individual forcing mechanisms to initiate retreat are unrealistically high but that thresholds are reduced when processes act in combination the ice stream is however most sensitive to ocean warming and associated ice shelf melting and retreat was most likely in response to external forcing that endured throughout the period of retreat rather than to a single triggering event timescales of retreat are further controlled by the delivery of ice from upstream of the grounding line due to the influence of topography modeled retreat patterns are insensitive to the temporal pattern of forcing evolution we therefore suggest that despite regionally similar forcing mechanisms landscape controls significant contrasts in retreat behavior between adjacent but topographically distinct catchments patterns of ice stream retreat in the past present and future should therefore be expected to vary significantly

the mechanisms behind jakobshavn isbrae s acceleration and mass loss a 3 d thermomechanical model study the mechanisms causing widespread flow acceleration of jakobshavn isbrae west greenland remain unclear despite an abundance of observations and modeling studies here we simulate the glacier s evolution from 1985 to 2016 using a three dimensional thermomechanical ice flow model the model captures the timing and 90 of the observed changes by forcing the calving front basal drag in the trough is low and lateral drag balances the ice stream s driving stress the calving front position is the dominant control on changes of jakobshavn isbrae since the ice viscosity in the shear margins instantaneously drops in response to the stress perturbation caused by calving front retreat which allows for widespread flow acceleration gradual shear margin warming contributes 5 to 10 to the total acceleration our simulations suggest that the glacier will contribute to eustatic sea level rise at a rate comparable to or higher than at present

insight into the nature of the ocean continent transition off west iberia from a deep multichannel seismic reflection profile a high quality multichannel seismic reflection profile across the west iberia margin confirms the existence of a 130 km wide transition zone between tilted continental fault blocks and unequivocal oceanic crust the zone is characterized by a 1 0 2 5 km thick seismically unreflective upper basement layer lying over reflective basement that contains major landward dipping reflectors that mostly truncate against the base of the upper layer and penetrate to depths of 10 19 km below the basement surface and minor seaward dipping reflectors moho reflections are absent in this zone we suggest that the unreflective layer represents tectonically exposed upper mantle that has undergone extensive serpentinization enhanced by vigorous seawater circulation about 40 km farther north the transition zone narrows to 35 km and the unreflective layer is less continuous and widespread

constraints on the greenland ice sheet since the last glacial maximum from sea level observations and glacial rebound models abstract geomorphological descriptions of changes in the extent of the greenland ice sheet gis have been combined with glacial isostatic adjustment models to reproduce the sea level history of greenland since the last glacial maximum lgm the contribution to past sea level change around greenland due to ice load changes outside of that region has been considerable 10 s of meters while still contributing a rise of several mm yr 1 today the isostatic contribution to relative sea level around greenland from changes in the gis is found by iteratively perturbing preliminary ice models with different lgm extents and deglaciation starting times the resulting first order model that provides the best agreement between observed and predicted sea level contributes 3 1 and 1 9 m water equivalent of additional ice relative to present day ice volumes at the lgm and younger dryas respectively the gis in most areas does not appear to have extended far onto the continental shelf the exceptions being southern most southwest greenland and northern east greenland as well as at the coalescence of the northwest greenland and innuitian ice sheets changes in ice thickness since the lgm were 500 m along the present day outer coast and 1500 m along some parts of the present day ice margin the observed mid to late holocene fall in sea level to below the present day level and the subsequent transgression seen in some areas implies that the gis retreated behind the present day margin by distances of the order of 40 km before readvancing

evidence of marine ice cliff instability in pine island bay from iceberg keel plough marks marine ice cliff instability mici processes could accelerate future retreat of the antarctic ice sheet if ice shelves that buttress grounding lines more than 800 metres below sea level are lost the present day grounding zones of the pine island and thwaites glaciers in west antarctica need to retreat only short distances before they reach extensive retrograde slopes when grounding zones of glaciers retreat onto such slopes theoretical considerations and modelling results indicate that the retreat becomes unstable marine ice sheet instability and thus accelerates it is thought that mici is triggered when this retreat produces ice cliffs above the water line with heights approaching about 90 metres however observational evidence confirming the action of mici has not previously been reported here we present observational evidence that rapid deglacial ice sheet retreat into pine island bay proceeded in a similar manner to that simulated in a recent modelling study driven by mici iceberg keel plough marks on the sea floor provide geological evidence of past and present iceberg morphology keel depth and drift direction from the planform shape and cross sectional morphologies of iceberg keel plough marks we find that iceberg calving during the most recent deglaciation was not characterized by small numbers of large tabular icebergs as is observed today which would produce wide flat based plough marks or toothcomb like multi keeled plough marks instead it was characterized by large numbers of smaller icebergs with v shaped keels geological evidence of the form and water depth distribution of the plough marks indicates calving margin thicknesses equivalent to the threshold that is predicted to trigger ice cliff structural collapse as a result of mici we infer rapid and sustained ice sheet retreat driven by mici commencing around 12 300 years ago and terminating before about 11 200 years ago which produced large numbers of icebergs smaller than the typical tabular icebergs produced today our findings demonstrate the effective operation of mici in the past and highlight its potential contribution to accelerated future retreat of the antarctic ice sheet

holocene evolution of the western greenland ice sheet assessing geophysical ice sheet models with geological reconstructions of ice margin change abstract geophysical ice sheet models are used to predict future ice sheet dimensions and in turn these projections help estimate the magnitude of eustatic sea level rise before models can confidently predict ice sheet behavior they must be validated by being able to duplicate the geological record of ice sheet change here we review geological records of greenland ice sheet gris change with emphasis on the warmer than present middle holocene and compare these records to published studies that numerically simulate gris behavior through the holocene geological records are concentrated in west and southwest greenland which are also the regions where the gris margin likely experienced the greatest distance of inland retreat during the middle holocene several records spanning from melville bugt to jakobshavn isfjord in western greenland indicate the gris achieved its minimum extent between 5 and 3 ka and farther south in the kangerlussuaq region new data presented here indicate the ice margin reached its minimum extent between 4 2 and 1 8 ka in the narsarsuaq region in southern greenland the gris likely achieved its minimum configuration between 7 and 4 ka we highlight key similarities and discrepancies between these reconstructions and model results and finally we suggest that despite some degree of inland retreat the west and southwest gris margin remained relatively stable and close to its current position through the holocene thermal maximum

evolution of nonvolcanic rifted margins new results from the conjugate margins of the labrador sea the crust across each margin of the labrador sea a conjugate rift margin pair consists of three well defined zones thinned continental crust 70 80 km wide transitional crust and oceanic crust the transition zone is characterized by a low velocity upper crust 4 5 km s underlain by a 6 4 7 7 km s layer we propose that the lower layer is serpentinized upper mantle it is less likely to be gabbroic igneous lower crust the low velocity upper crust may be either an oceanic basaltic layer or very thin 2 km continental crust results from this first combined seismic reflection and refraction study of a conjugate margin pair support very slow initial continental rifting 1 mm yr on average resulting in an asymmetric crustal thinning profile with breakup occurring against the west greenland shelf this asymmetry may be an important feature of rifting at nonvolcanic margins and may be related to strain hardening of the cooling and thinning lithosphere or to small scale convection in the lower lithosphere and asthenosphere at the sides of the rift the thin crust in the transition zone implies that the amount of melt generated in the upwelling mantle is negligible consistent with slow spreading or rifting the transition from this slow formational period to normal oceanic accretion processes is marked by a sharp decrease in basement depth signaling an increase to normal oceanic crustal thickness

a 100 yr record of ocean temperature control on the stability of jakobshavn isbrae west greenland an understanding of the interaction between ice sheet dynamics and forcing mechanisms such as oceanic and atmospheric circulation is important because of the potential contribution of these processes to constraining models that seek to predict future rates of sea level change here we report new benthic foraminiferal data from disko bugt west greenland showing a close correlation between subsurface ocean temperature changes and the ice margin position of the glacier jakobshavn isbrae over the past 100 yr in particular our faunal data show that warm ocean currents entered a bay disko bugt during the retreat phases of jakobshavn isbrae from a d 1920 to 1950 and since 1998 we also show a link between west greenland ocean temperature and the atlantic multidecadal oscillation a key climate indicator in the north atlantic ocean the close coupling between the oceans and the cryosphere identified here should be assessed in future projections of sea level change

ice flow in greenland for the international polar year 2008 2009 a digital representation of ice surface velocity is essential for a variety of glaciological geologic and geophysical analyses and modeling here we present a new reference comprehensive high resolution digital mosaic of ice motion in greenland assembled from satellite radar interferometry data acquired during the international polar year 2008 to 2009 by the envisat advanced synthetic aperture radar asar the advanced land observation system alos s phase array l band sar palsar and the radarsat 1 sar that covers 99 of the ice sheet in area the best mapping performance is obtained using alos palsar data due to higher levels of temporal coherence at the l band frequency but c band frequency sar data are less affected by the ionosphere the ice motion map reveals various flow regimes ranging from patterned enhanced flow into a few large glaciers in the cold low precipitation areas of north greenland to diffuse enhanced flow into numerous narrow fast moving glaciers in the warmer high precipitation sectors of northwest and southeast greenland we find that the 100 fastest glaciers v 800m yr drain 66 of the ice sheet in area marine terminating glaciers drain 88 of greenland and basal sliding motion dominates internal deformation over more than 50 of the ice sheet this view of ice sheet motion provides significant new constraints on ice flow modeling copyright 2012 by the american geophysical union

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

surface and sub surface multi proxy reconstruction of middle to late holocene palaeoceanographic changes in disko bugt west greenland we present new surface water proxy records of meltwater production alkenone derived relative sea surface temperature diatom alkenones and sea ice diatoms changes from the disko bugt area off central west greenland we combine these new surface water reconstructions with published proxy records benthic foraminifera bottom water proxy dinocyst assemblages surface water proxy along with atmospheric temperature from greenland ice core and greenland lake records this multi proxy approach allows us to reconstruct centennial scale middle to late holocene palaeoenvironmental evolution of disko bugt and the western greenland coastal region with more detail than previously available combining surface and bottom water proxies identifies the coupling between ocean circulation west greenland current conditions the atmosphere and the greenland ice sheet centennial to millennial scale changes in the wider north atlantic region were accompanied by variations in the west greenland current wgc during periods of relatively warm wgc increased surface air temperature over western greenland led to ice sheet retreat and significant meltwater flux in contrast during periods of cold wgc atmospheric cooling resulted in glacier advances we also identify potential linkages between the palaeoceanography of the disko bugt region and key changes in the history of human occupation cooler oceanographic conditions at 3 5 ka bp support the view that the saqqaq culture left disko bugt due to deteriorating climatic conditions the cause of the disappearance of the dorset culture is unclear but the new data presented here indicate that it may be linked to a significant increase in meltwater flux which caused cold and unstable coastal conditions at ca 2 ka bp the subsequent settlement of the norse occurred at the same time as climatic amelioration during the medieval climate anomaly and their disappearance may be related to harsher conditions at the beginning of the little ice age

ice stream stability on a reverse bed slope marine ice streams whose beds deepen inland are thought to be inherently unstable numerical modelling of the maguerite bay ice stream retreat in west antarctica since the last glacial maximum suggests that an ice stream can stabilize on an inland sloping bed owing to increased lateral drag where the ice stream narrows marine based ice streams whose beds deepen inland are thought to be inherently unstable1 2 3 this instability is of particular concern because significant portions of the marine based west antarctic and greenland ice sheets are losing mass and their retreat could contribute significantly to future sea level rise4 5 6 7 however the present understanding of ice stream stability is limited by observational records that are too short to resolve multi decadal to millennial scale behaviour or to validate numerical models8 here we present a dynamic numerical simulation of antarctic ice stream retreat since the last glacial maximum lgm constrained by geophysical data whose behaviour is consistent with the geomorphological record we find that retreat of marguerite bay ice stream following the lgm was highly nonlinear and was interrupted by stabilizations on a reverse sloping bed where theory predicts rapid unstable retreat we demonstrate that these transient stabilizations were caused by enhanced lateral drag as the ice stream narrowed we conclude that as well as bed topography ice stream width and long term retreat history are crucial for understanding decadal to centennial scale ice stream behaviour and marine ice sheet vulnerability

dependence of century scale projections of the greenland ice sheet on its thermal regime observations show that the greenland ice sheet has been losing mass at an increasing rate over the past few decades which makes it a major contributor to sea level rise here we use a threedimensional higher order ice flow model adaptive mesh refinement and inverse methods to accurately reproduce the present day ice flow of the greenland ice sheet we investigate the effect of the ice thermal regime on 1 basal sliding inversion and 2 projections over the next 100 years we show that steady state temperatures based on present day conditions allow a reasonable representation of the thermal regime and that both basal conditions and century scale projections are weakly sensitive to small changes in the initial temperature field compared with changes in atmospheric conditions or basal sliding we conclude that although more englacial temperature measurements should be acquired to validate the models and a better estimation of geothermal heat flux is needed it is reasonable to use steady state temperature profiles for short term projections as external forcings remain the main drivers of the changes occurring in greenland 2014 publishing technology

using proglacial threshold lakes to constrain fluctuations of the jakobshavn isbrae ice margin western greenland during the holocene abstract the future response of the greenland ice sheet gis and its potential contribution to sea level rise are uncertain rapid changes of greenland s outlet glaciers over the past decade have made it difficult to extrapolate ice sheet change into the future this significant short term variability highlights the need for longer term geologic e g holocene records of ice margin fluctuations however a major challenge with reconstructing the gis during the holocene stems from it having been smaller than it is at present thus traditional glacial geologic approaches are not suitable we use radiocarbon dated sediment sequences from seven proglacial threshold lakes spanning 50 km of the western gis margin near jakobshavn isbrae to constrain the timing of early holocene deglaciation the duration that this sector of the western gis was smaller than its present configuration and the timing of its advance during neoglaciation our reconstructions suggest deglaciation 7300 cal yr bp minimum ice extent 6000 5000 cal yr bp and smaller than present ice configuration until at least 2300 cal yr bp for the ice margin south of jakobshavn isbrae and until 400 cal yr bp for the ice margin north of jakobshavn isbrae one relatively large proglacial lake that became briefly ice free during the middle holocene lies in a catchment that likely extends 10s of km inland beneath the gis suggesting significant middle holocene retreat of this portion of the gis the overall pattern of ice sheet change is inconsistent with existing ice sheet model reconstructions for this region but is consistent with numerous paleoclimate proxy and relative sea level data these continuous lacustrine records corroborate but provide closer age control than existing non continuous records of radiocarbon dated reworked bivalves from historical moraines in the region reconstructing ice margin change from proglacial threshold lakes is one of few approaches with the potential to constrain smaller than present ice sheet extent

a 2000 year record of ocean influence on jakobshavn isbrae calving activity based on marine sediment cores the greenland ice sheet has experienced significant mass loss in recent years a substantial component of this is attributable to the retreat of marine terminating outlet glaciers which lose mass

undercrusting by serpentinite beneath rifted margins the west galicia passive margin spain is bounded by a continuous belt of serpentinized peridotite here we present the results of petrological studies of the peridotite 39ar 40ar dating of high temperature ductile shearing of a dioritic dyke crossing the peridotite and regional seismic stratigraphy which demonstrate that the uplift ductile shearing and pervasive serpentinization of mantle rocks occurred at early cretaceous time during the rifting stage of the margin the serpentinite layer resting on fresh peridotite is probably several kilometres thick at present seismic data and gravity data cannot distinguish serpentinite from crustal material but our results from the galicia margin indicate that the seismic crust beneath rifted margin may include mantle material transformed into serpentinite by syn rift hydrothermal activity in some cases this undercrusting process might explain the young age and mobility of the moho beneath stretched continental basement

characteristics of meltwater export from jakobshavn isbrae and ilulissat icefjord jakobshavn isbrae which terminates in ilulissat icefjord has undergone rapid retreat and is currently the largest contributor to ice sheet mass loss among greenland s marine terminating glaciers accelerating mass loss is increasing fresh water discharge to the ocean which can feed back on ice melt impact marine ecosystems and potentially modify regional and larger scale ocean circulation here we present hydrographic observations including inert geochemical tracers that allow the first quantitative description of the glacially modified waters exported from the jakobshavn icefjord system observations within the fjord suggest a deep reaching overturning cell driven by glacial buoyancy forcing modified waters containing submarine meltwater up to 2 5 0 12 subglacial discharge up to 6 0 37 and large portions of entrained ocean waters are seen to exit the fjord and flow north the exported meltwaters form a buoyant coastal gravity current reaching to 100 m depth and extending 10 km offshore

marine ice sheet dynamics hysteresis and neutral equilibrium 1 the stability of marine ice sheets and outlet glaciers is mostly controlled by the dynamics of their grounding line i e where the bottom contact of the ice changes from bedrock or till to ocean water the last report of the intergovernmental panel on climate change has clearly underlined the poor ability of models to capture the dynamics of outlet glaciers here we present computations of grounding line dynamics on the basis of numerical solutions of the full stokes equations for ice velocity coupled with the evolution of the air ice and sea ice free interfaces the grounding line position is determined by solving the contact problem between the ice and a rigid bedrock using the finite element code elmer results of the simulations show that marine ice sheets are unstable on upsloping beds and undergo hysteresis under perturbation of ice viscosity confirming conclusions from boundary layer theory the present approach also indicates that a 2 d unconfined marine ice sheet sliding over a downsloping bedrock does not exhibit neutral equilibrium it is shown that mesh resolution around the grounding line is a crucial issue a very fine grid size 100 m spacing is needed in order to achieve consistent results

an extensive and dynamic ice sheet on the west greenland shelf during the last glacial cycle considerable uncertainty surrounds the extent and timing of the advance and retreat of the greenland ice sheet gis on the continental shelf bordering baffin bay during the last glacial cycle here we use marine geophysical and geological data to show that fast flowing ice sheet outlets including the ancestral jakobshavn isbrae expanded several hundred kilometers to the shelf edge during the last glaciation ca 20 ka retreat of these outlets was asynchronous initial retreat from the shelf edge was underway by 14 880 calibrated cal yr b p in uummannaq trough radiocarbon dates from the adjacent disko trough and adjoining trough mouth fan imply later deglaciation of jakobshavn isbrae and significantly an extensive readvance and rapid retreat of this outlet during the younger dryas stadial yd this is notable because it is the first evidence of a major advance of the gis during the yd on the west greenland shelf although the short duration suggests that it may have been out of phase with yd temperatures

mariana blueschist mud volcanism implications for conditions within the subduction zone several recently discovered active mud volcanoes on the nonaccretionary mariana convergent plate margin are erupting slab derived fluids serpentine mud and metamorphosed rocks from depths of as great as 25 km blueschist materials from the metamorphosed subducted plate are contained in the muds pore fluids indicate a depth dependence for decarbonation the mud volcanoes record in situ conditions along the decollement including pressure and temperature conditions and physical properties within the subduction zone similar mud flow material occurs worldwide as sedimentary serpentinite deposits in accreted fragments of former convergent margins making this kind of mud volcanism a more important phenomenon in convergent margins than previously recognized

early holocene palaeoceanography and deglacial chronology of disko bugt west greenland two new cores from disko bugt a large marine embayment in west greenland are used to assess the deglacial chronology and palaeoceanography of the area during the early holocene deglaciation of the main part of disko bugt is directly dated for the first time to a minimum of ca 10 2 ka cal bp by this time the jakobshavn isbrae ice stream had retreated to the eastern margins of disko bugt initial deglaciation took place before the relatively warm west greenland current wgc was initiated a strengthening in the wgc is identified from ca 9 2 ka cal bp accompanied by retreat of the ice stream to a shallow bank immediately west of the present day jakobshavn isfjord while in this position the calving and melting ice stream produced large volumes of sediment and meltwater the large volumes of meltwater prevented the relatively warm waters of the wgc reaching the eastern margins of disko bugt the ice stream remained grounded on this bank for over a thousand years until ca 7 9 ka cal bp when reduced sedimentation rates and an increase in boreal fauna point to its retreat inside the present day fjord system

sensitivity of grounding line dynamics to the choice of the friction law basal slip accounts for a large part of the flow of ice streams draining ice from antarctica and greenland into the ocean therefore an appropriate representation of basal slip in ice flow models is a prerequisite for accurate sea level rise projections various friction laws have been proposed to describe basal slip in models here we compare the influence on grounding line gl dynamics of four friction laws the traditional weertman law and three effective pressure dependent laws namely the schoof tsai and budd laws it turns out that even when they are tuned to a common initial reference state the weertman budd and schoof laws lead to thoroughly different steady state positions although the schoof and tsai laws lead to much the same result in particular under certain circumstances it is possible to obtain a steady gl located on a reverse slope area using the weertman law furthermore the predicted transient evolution of the gl as well as the projected contributions to sea level rise over a 100 year time horizon vary significantly depending on the friction law we conclude on the importance of choosing an appropriate law for reliable sea level rise projections and emphasise the need for a coupling between ice flow models and physically based subglacial hydrological models

the lithosphere below the rockall trough wide angle seismic evidence for extensive serpentinisation abstract the longer range data from wide angle seismic reflection experiments along an axial and transverse profile determined the seismic properties of the shallow mantle lithosphere beneath the rockall trough in the north atlantic two subcrustal p and s wave reflections are observed the first defines a layer 3 10 km thick below the moho where p wave velocities vary from 7 5 to 7 8 km s v p v s ratios increase within this layer from 1 80 in the north to 1 83 in the south of the basin along the axial profile the second reflection from approximately 34 km depth identifies a layer 15 20 km thick with a vp velocity of approximately 8 1 km s and a v p v s ratio of 1 73 these values are typical for normal mantle peridotites both p and s wave velocities and v p v s ratios constrain the possible composition of the first layer which is interpreted as a zone of partially serpentinised peridotites below the moho about 15 volume alteration of the parent mantle peridotite is required to produce the observed seismic properties this degree of alteration accounts for a systematic deficit in total tectonic subsidence when compared with that predicted from the variation in bulk crustal stretching along the axis of the basin syntectonic cooling occurred during differential lithospheric stretching as the upper to mid crust became more extended over a narrower region than the mantle lithosphere this served to rheologically couple the lower crust to the mantle near the final stage of deformation as the primary brittle ductile transition zone in the crust migrated downwards into the mantle lithosphere the resultant fracturing generated the permeability necessary to facilitate the seawater circulation which hydrated the cold mantle

sea level changes at the lgm from ice dynamic reconstructions of the greenland and antarctic ice sheets during the glacial cycles abstract new experiments were performed with three dimensional thermomechanical models of the greenland and antarctic ice sheets to simulate their behaviour during the glacial cycles to reconstruct their thickness and extent at the last glacial maximum lgm and to estimate their glacio eustatic contribution to the global sea level stand the calculations used improved ice dynamic and isostatic treatments updated datasets on higher grid resolutions and refined climatic treatments based on newly calibrated transfer functions between ice core records and climatic perturbations results are discussed from a reference run with standard parameters that is compared with available glacial geological observations and from a series of sensitivity experiments focusing on isostatic adjustment thermomechanical coupling climatic forcing mass balance changes and basal melting rates and viscosity changes of antarctic ice shelves for the antarctic ice sheet we find that volume changes are closely linked with grounding line changes of the west antarctic ice sheet at the lgm the grounding line extended close to the continental shelf break almost everywhere ice over central east antarctica was generally thinner than today and varied mainly in accordance with accumulation fluctuations for the greenland ice sheet melting is important only during interglacial periods and the most sensitive period concerns the size of the ice sheet during the eemian at the lgm the greenland ice sheet extended beyond the present coastline to cover at least the inner continental shelf and thinned by up to several hundred meters in central areas for a plausible range of parameters the experiments indicate that at the time of maximum sea level depression 21 kyr bp the antarctic ice sheet contributed 14 18 m to the sea level lowering and the greenland ice sheet 2 3 m significantly less than the older climap reconstructions whereas both ice sheets were at 21 kyr bp close to their maximum extent the experiments also indicate that their maximum volumes were reached only by 16 5 kyr bp greenland and 10 kyr bp antarctica equal to an additional sea level lowering of respectively 0 4 and 3 7 m holocene retreat was essentially complete by 5 kyr bp in greenland but is found to still continue today in west antarctica before reversing to growth during the next millenium the models were found to reproduce gross features of the ice sheet s history since the lgm in reasonably good agreement with available glacial geological data although observational control on ice thickness changes remains very limited

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

rate of serpentinization in seafloor environments the permeability of partially serpentinized peridotites has been measured near room temperature and found to be in the range of 10 11 to 10 12 darcys or 10 7 to 10 8 cm2 sec 1 comparable to shaley materials serpentinites act as semipermeable membranes to salt ions in ocean crust and deep groundwater environments serpentinization must be a continuous process controlled only by water access to ultramafic rocks through their cover given this it will generate high local strains and stresses with episodic cracking the serpentinization process may also contribute to heat flow and ocean floor topography

deglaciation of a major palaeo ice stream in disko trough west greenland recent work has confirmed that grounded ice reached the shelf break in central west greenland during the last glacial maximum lgm here we use a combination of marine sediment core data including glacimarine lithofacies and ird proxy records and geomorphological and acoustic facies evidence to examine the nature of and controls on the retreat of a major outlet of the western sector of the greenland ice sheet gris across the shelf retreat of this outlet which contained the ancestral jakobshavns isbrae ice stream from the outer shelf in disko trough was rapid and progressed predominantly through iceberg calving however minor pauses in retreat tens of years occurred on the middle shelf at a trough narrowing forming subtle grounding zone wedges by 12 1 cal kyr bp ice had retreated to a basalt escarpment and shallow banks on the inner continental shelf where it was pinned and stabilised for at least 100 years during this time the ice margin appears to have formed a calving bay over the trough and melting became an important mechanism of ice mass loss fine grained sediments muds were deposited alternately with ird rich sediments diamictons forming a characteristic deglacial lithofacies that may be related to seasonal climatic cycles high influxes of subglacial meltwater emanating from the nearby ice margins deposited muddy sediments during the warmer summer months whereas winters were dominated by iceberg calving leading to the deposition of the diamictons this is the first example of this glacimarine lithofacies from a continental shelf setting and we suggest that the calving bay configuration of the ice margin plus the switching between calving and melting as ablation mechanisms facilitated its deposition by channelling meltwater and icebergs through the inner trough the occurrence of a major stillstand on the inner shelf in disko trough demonstrates that the ice dynamical response to local topography was a crucial control on the behaviour of a major outlet in this sector of the gris during retreat

the s reflector west of galicia spain evidence from prestack depth migration for detachment faulting during continental breakup the mechanism of continental extension and breakup and the role of detachment faults in these processes are currently the subject of intense debate one possible detachment fault is the s reflector imaged as an undulating in time locally discontinuous reflection on existing seismic profiles west of iberia here we present new images in depth of the four margin normal profiles across the west galicia rifted margin where the s reflector was originally defined and is best imaged it is shown that faults bounding wedge shaped units of late synrift sediments which hence were active during rifting immediately prior to breakup appear to detach at shallow levels onto the s reflector s itself appears as a continuous locally domal feature and does not generally appear offset the waveform of s is compatible with a reflection from a single interface such as a sharp tectonic boundary the depth sections show that s was active at 1 3 km below the seafloor during final rifting s is interpreted as a brittle detachment fault which controlled the final breakup of the continent west of galicia furthermore the data provide constraints on the sense of shear of s analogy with detachment terranes the present synrift and structural dips of s and the identification of a breakaway to s imply that s accommodated top to the west shear toward the east of the profiles s becomes more complex possibly because of different phases of detachment faulting and the development of both incisement and excisement structures from the geometry of wedge shaped sedimentary units deposited during faulting above s it also appears that s was active at an angle of 20 or less and hence may be considered a genuine low angle normal fault

a model of greenland ice sheet deglaciation constrained by observations of relative sea level and ice extent an ice sheet model was constrained to reconstruct the evolution of the greenland ice sheet gris from the last glacial maximum lgm to present to improve our understanding of its response to climate change the study involved applying a glaciological model in series with a glacial isostatic adjustment and relative sea level rsl model the model reconstruction builds upon the work of simpson et al 2009 through four main extensions 1 a larger constraint database consisting of rsl and ice extent data model improvements to the 2 climate and 3 sea level forcing components 4 accounting for uncertainties in non greenland ice the research was conducted primarily to address data model misfits and to quantify inherent model uncertainties with the earth structure and non greenland ice our new model termed huy3 fits the majority of observations and is characterised by a number of defining features during the lgm the ice sheet had an excess of 4 7 m ice equivalent sea level iesl which reached a maximum volume of 5 1 m iesl at 16 5 cal ka bp modelled retreat of ice from the continental shelf progressed at different rates and timings in different sectors southwest and southeast greenland began to retreat from the continental shelf by 16 to 14 cal ka bp thus responding in part to the bolling allerod warm event c 14 5 cal ka bp subsequently ice at the southern tip of greenland readvanced during the younger dryas cold event in northern greenland the ice retreated rapidly from the continental shelf upon the climatic recovery out of the younger dryas to present day conditions upon entering the holocene 11 7 cal ka bp the ice sheet soon became land based during the holocene thermal maximum htm 9 5 cal ka bp air temperatures across greenland were marginally higher than those at present and the gris margin retreated inland of its present day southwest position by 40 60 km at 4 cal ka bp which produced a deficit volume of 0 16 m iesl relative to present in response to the htm warmth our optimal model reconstruction lost mass at a maximum centennial rate of c 103 4 gt yr our results suggest that remaining data model discrepancies are affiliated with missing physics and sub grid processes of the glaciological model uncertainties in the climate forcing lateral earth structure and non greenland ice particularly the north american component finally applying the huy3 greenland reconstruction with our optimal earth model we generate present day uplift rates across greenland due to past changes in the ocean and ice loads with explicit error bars due to uncertainties in the earth structure present day uplift rates due to past changes are spatially variable and range from 3 5 to 7 mm a including earth model uncertainty

calibrating a glaciological model of the greenland ice sheet from the last glacial maximum to present day using field observations of relative sea level and ice extent we constrain a three dimensional thermomechanical model of greenland ice sheet gris evolution from the last glacial maximum lgm 21 ka bp to the present day using primarily observations of relative sea level rsl as well as field data on past ice extent our new model huy2 fits a majority of the observations and is characterised by a number of key features i the ice sheet had an excess volume relative to present of 4 1 m ice equivalent sea level at the lgm which increased to reach a maximum value of 4 6 m at 16 5 ka bp ii retreat from the continental shelf was not continuous around the entire margin as there was a younger dryas readvance in some areas the final episode of marine retreat was rapid and relatively late c 12 ka bp leaving the ice sheet land based by 10 ka bp iii in response to the holocene thermal maximum htm the ice margin retreated behind its present day position by up to 80 km in the southwest 20 km in the south and 80 km in a small area of the northeast as a result of this retreat the modelled ice sheet reaches a minimum extent between 5 and 4 ka bp which corresponds to a deficit volume relative to present of 0 17 m ice equivalent sea level our results suggest that remaining discrepancies between the model and the observations are likely associated with non greenland ice load differences between modelled and observed present day ice elevation around the margin lateral variations in earth structure and or the pattern of ice margin retreat

continued evolution of jakobshavn isbrae following its rapid speedup 1 several new data sets reveal that thinning and speedup of jakobshavn isbrae continue following its recent rapid increase in speed as its floating ice tongue disintegrated the present speedup rate of 5 a 1 over much of the fast moving region appears to be a diffusive response to the initial much larger speedup near the front there is strong seasonality in speed over much of the fast flowing main trunk that shows a good inverse correlation with the seasonally varying length of a short typically 6 km floating ice tongue this modulation of speed with ice front position supports the hypothesis that the major speedup was caused by loss of the larger floating ice tongue from 1998 to 2003 analysis of image time series suggests that the transient winter ice tongue is formed when sea ice bonds glacier ice in the fjord to produce a nearly rigid mass that almost entirely suppresses calving major calving only resumes in late winter when much of this ice clears from the fjord the collapse of the ice tongue in the late 1990s followed almost immediately after a sharp decline in winter sea ice concentration in disko bay this decline may have extended the length of the calving season for several consecutive years leading to the ice tongue s collapse

the effect of a holocene climatic optimum on the evolution of the greenland ice sheet during the last 10 kyr the holocene climatic optimum was a period 8 5 kyr ago when annual mean surface temperatures in greenland were 2 3 c warmer than present day values however this warming left little imprint on commonly used temperature proxies often used to derive the climate forcing for simulations of the past evolution of the greenland ice sheet in this study we investigate the evolution of the greenland ice sheet through the holocene when forced by different proxy derived temperature histories from ice core records focusing on the effect of sustained higher surface temperatures during the early holocene we find that the ice sheet retreats to a minimum volume of 0 15 1 2 m sea level equivalent smaller than present in the early or mid holocene when forcing an ice sheet model with temperature reconstructions that contain a climatic optimum and that the ice sheet has continued to recover from this minimum up to present day reconstructions without a warm climatic optimum in the early holocene result in smaller ice losses continuing throughout the last 10 kyr for all the simulated ice sheet histories the ice sheet is approaching a steady state at the end of the 20th century

rapid retreat of a scandinavian marine outlet glacier in response to warming at the last glacial termination abstract marine outlet glaciers on greenland are retreating yet it is unclear if the recent fast retreat will persist and how atmosphere and ocean warming will impact future retreat we show how a marine outlet glacier in hardangerfjorden retreated rapidly in response to the abrupt warming following the younger dryas cold period approximately 11 600 years before present this almost 1000 m deep fjord with several sills at 300 500 m depth hosted a 175 km long outlet glacier at the western rim of the scandinavian ice sheet we use a dynamic ice flow model constrained by well dated terminal and lateral moraines to simulate the reconstructed 500 year retreat of hardangerfjorden glacier the model includes an idealized oceanic and atmospheric forcing based on reconstructions but excludes the surface mass balance elevation feedback our simulations show a highly episodic retreat driven by surface melt and warming fjord waters paced by the fjord bathymetry warming air and ocean temperatures by 4 5 c during the period of retreat result in a 125 km retreat of hardangerfjorden glacier in 500 years retreat rates throughout the deglaciation vary by an order of magnitude from 50 to 2500 m a 1 generally close to 200 m a 1 punctuated by brief events of swift retreat exceeding 500 m a 1 each event lasting a few decades we show that the fastest retreat rates occur in regions of the bed with the largest retrograde slopes ice shelf length and fjord water depth is less important our results have implications for modern glacial fjord settings similar to hardangerfjorden where high retreat rates have been observed our findings imply that increasing air temperatures and warming subsurface waters in greenland fjords will continue to drive extensive retreat of marine outlet glaciers however the recent high retreat rates are not expected to be sustained for longer than a few decades due to constraints by the fjord bathymetry

local glaciation in west greenland linked to north atlantic ocean circulation during the holocene recent observations indicate that ice ocean interaction drives much of the recent increase in mass loss from the greenland ice sheet however the role of ocean forcing in driving past glacier change is poorly understood to extend the observational record and our understanding of the ocean cryosphere link we used a multi proxy approach that combines new data from proglacial lake sediments 14 c dated in situ moss that recently emerged from beneath cold based ice caps and 10 be ages to reconstruct centennial scale records of mountain glacier activity for the past 10 k y in west greenland proglacial lake sediment records and 14 c dating of moss indicate the onset of neoglaciation in west greenland at ca 5 ka with substantial snowline lowering and glacier expansion at ca 3 7 ka followed by additional ice expansion phases at ca 2 9 ca 1 7 and ca 1 4 ka and during the little ice age we find that widespread glacier growth at ca 3 7 ka in west greenland coincides with marked cooling and reduced strength of the west greenland current in disko bugt the transition to cooler ocean conditions at ca 3 7 ka identified in disko bugt is registered by marine proxy data farther afield in east greenland and on the northwestern icelandic shelf implying large scale paleoceanographic changes across the north atlantic during this interval the similarity between glacier change on west greenland and multiple marine and terrestrial records across the north atlantic suggests that glaciers are strongly influenced by changes in ocean circulation and consequently implies that the ocean cryosphere teleconnection is a persistent feature of the arctic system

seasonal to decadal scale variations in the surface velocity of jakobshavn isbrae greenland observation and model based analysis 1 using new data we build upon the nearly two decade long record of observations from jakobshavn isbrae to investigate the processes driving its dynamic evolution while winter flow speed has not increased substantially over the last three winters there remains a strong seasonal variation in flow speed that coincides with a cycle of summer thinning and winter thickening we relate changes in glacier speed to geometry through variations in basal traction and horizontal stresses using ice flow models constrained by satellite and airborne observations these results suggest that the bed provides little flow resistance along the main trough within about 20 km of the terminus while the loss of buttressing from the retreat of grounded and floating ice likely contributed to the initial speedup other processes are of comparable significance at seasonal to decadal time scales from analysis of the models we hypothesize that thinning induced change in basal effective pressure is the dominant process influencing near terminus behavior while diffusive processes drive the upstream response the apparent need for the terminus to thin to near flotation before it can calve may limit the rate at which retreat occurs our analysis of the processes controlling the speed suggests little potential for further large acceleration thinning and elevated speeds may continue at rates similar to present however putting the glacier on course to retreat to the head of its deep trough in about a century at which point it likely would stabilize with a thinner terminus

the source fault of the 1983 nihonkai chubu earthquake revealed by seismic imaging abstract from 2011 to 2012 we conducted marine seismic surveys using a multichannel seismic reflection mcs system and ocean bottom seismometers obss in and around the hypocentral region of the 1983 nihonkai chubu earthquake the survey covered the area from the japan coast of the japan sea to the yamato basin and the japan basin we investigated the relationship between the nihonkai chubu earthquake and the crustal structure obtained from seismic reflection imaging and the p wave velocity structure near the hypocentral region the focus in the present paper is on seismic data that were acquired using both the mcs system and obss based on the results the crustal structure of the japan sea around the hypocentral region of the 1983 nihonkai chubu earthquake is divided into three types island arc crust thick oceanic crust and oceanic crust the nihonkai chubu earthquake occurred near the boundary between the thick oceanic crust and the island arc crust there is a strong possibility that the east dipping reflector detected in the crust was the source fault of the nihonkai chubu earthquake in addition the location of anticlines formed by the east dipping reverse fault occurring in the boundary of the crustal structure associated with the source fault of the nihonkai chubu earthquake was confirmed in the range of latitude from 39 20 to 41 15 n although they were not found south of this segment there is also a concentrated area of reverse faults and folds approximately 100 km west of the source region of the nihonkai chubu earthquake this area is located near the boundary between the thick oceanic crust and the oceanic crust

intermittent thinning of jakobshavn isbrae west greenland since the little ice age rapid thinning and velocity increase on major greenland outlet glaciers during the last two decades may indicate that these glaciers became unstable as a consequence of the jakobshavn effect hughes 1986 with terminus retreat leading to increased discharge from the interior and consequent further thinning and retreat to assess whether recent trends deviate from longer term behavior we measured glacier surface elevations and terminus positions for jakobshavn isbrae west greenland using historical photographs acquired in 1944 1953 1959 1964 and 1985 these results were combined with data from historical records aerial photographs ground surveys airborne laser altimetry and field mapping of lateral moraines and trimlines to reconstruct the history of changes since the little ice age lia we identified three periods of rapid thinning since the lia 1902 13 1930 59 and 1999 present during the first half of the 20th century the calving front appears to have been grounded and it started to float during the late 1940s the south and north tributaries exhibit different behavior for example the north tributary was thinning between 1959 and 1985 during a period when the calving front was stationary and the south tributary was in balance the record of intermittent thinning combined with changes in ice marginal extent and position of the calving front together with changes in velocity imply that the behavior of the lower parts of this glacier represents a complex ice dynamical response to local climate forcings and interactions with drainage from the interior

generating melt during lithospheric extension pure shear vs simple shear the uniform stretching model has been used to calculate the volume and composition of melt generated by adiabatic decompression during extension of the continental lithosphere the consequences of this approach for melt generation by litbospheric simple shear were investigated the results show that given an initially planar detachment fault it is extremely difficult to generate melt from the asthenosphere under either normal or elevated asthenospheric potential temperatures our conclusion is independent of the initial dip of the detachment fault and also holds when the initial dip in the lithospheric mantle is double that in the crust in the north sea lithospheric simple shear fails to account for the existence and location of magmatism these conclusions may also apply more generally to passive margin development

response of a land terminating sector of the western greenland ice sheet to early holocene climate change evidence from 10 be dating in the sondre isortoq region abstract the prevalence of land terminating greenland ice sheet margins is expected to increase as tidewater glaciers retreat onto land yet few studies have characterized the sensitivity of these slow moving margins to climatic variability the response of land terminating ice sheet sectors to climate forcing can be assessed by examining records of paleo ice margin positions preserved in ice sheet moraine systems in western greenland the extensive fjord stade moraine system was deposited by minor readvances or stillstands of the greenland ice sheet margin during early holocene net recession here we combine new moraine mapping and cosmogenic 10be exposure dating to constrain the timing of fjord stade moraine deposition in the sondre isortoq region we find that the fjord stade moraines are composed of a western stage and an eastern stage which we constrain to 9 7 0 7 ka n 7 1 sd and 9 0 0 3 ka n 7 1 sd respectively synchronous deposition of the fjord stade moraine complex across at least 350 km of western greenland implies a widespread response of the western greenland ice sheet to the 9 3 ka event furthermore these new moraine ages may correlate with laurentide ice sheet sourced freshwater pulses into the north atlantic ocean we also show that the response of the western greenland ice sheet to early holocene freshwater forcing was not restricted to fast flowing marine terminating outlet glaciers the land terminating margin in the sondre isortoq region halted or reversed its pattern of retreat in response to climatic change