hydraulics of subglacial supercooling theory and simulations for clear water flows 1 glaciohydraulic supercooling is a mechanism for accreting ice and sediment to the base of glaciers we extend existing models by reworking the spring hutter model for subglacial water flow in tubular conduits to allow for distributed water sheets our goal is to determine diagnostic features of supercooling relative to controlling variables results focus on along path water flow under time varying conditions with attention to ice accretion in and along subglacial overdeepenings we contrast simulations with constant recharge to diurnally varying recharge and expose behavior that cannot be inferred from simple models for example locations of simulated ice accretion differ from those found for steady state models even though total ice accretion remains comparable to field estimates downstream accretion influences upstream effective pressures that then modify the hydraulic gradient that drives water flow this modified gradient tends to inhibit additional accretion by increasing velocity and heat production via viscous dissipation during diurnal cycles accretion varies considerably in daytime viscous dissipation dominates the heat balance and ice melts in morning and evening when flow is rising or falling viscous dissipation is lower and accretion can proceed over nighttime the largest temperature depressions occur in the subglacial system but water flux is lowest and accretion rates are negligible we conclude by inferring that overdeepened glaciers with only clear water flows evolve toward stronger supercooling regimes rather than toward a dynamic equilibrium stabilizing feedbacks are unlikely to occur through glacier hydrology alone and other processes such as erosion sedimentation and sliding must play an important role

devils hole nevada δ18o record extended to the mid holocene abstract the mid to late pleistocene devils hole 18 o record has been extended from 60 000 to 4500 yr ago the new 18 o time series in conjunction with the one previously published is shown to be a proxy of pacific ocean sea surface temperature sst off the coast of california during marine oxygen isotope stages mis 2 and 6 the devil hole and sst time series exhibit a steady warming that began 5000 to 10 000 yr prior to the last and penultimate deglaciations several possible proximate causes for this early warming are evaluated the magnitude of the peak 18 o or sst during the last interglacial lig is significantly greater 1 per mill and 2 to 3 c respectively than the peak value of these parameters for the holocene in contrast benthic 18 o records of ice volume show only a few tenths per mill difference in the peak value for these interglacials statistical analysis provides an estimate of the large shared information variation between the devils hole and eastern pacific sst time series from 41 to 2 n and enforces the concept of a common forcing among all of these records the extended devils hole record adds to evidence of the importance of uplands bordering the eastern pacific as a source of archives for reconstructing pacific climate variability

seismic stratigraphic record of the amundsen sea embayment shelf from pre glacial to recent times evidence for a dynamic west antarctic ice sheet studies of the sedimentary architecture and characteristics of the antarctic continental margin provide clues about past ice sheet advance retreat cycles and help improve constraints for paleo ice dynamic models since early glacial periods a first seismostratigraphic analysis of the amundsen sea embayment shelf and slope of west antarctica reveals insights into the structural architecture of the continental margin and shows stages of sediment deposition erosion and transport reflecting the history from pre glacial times to early glaciation and to the late pleistocene glacial interglacial cycles the shelf geometry consists of a large pre and syn rift basin in the middle shelf region between basement cropping out on the inner shelf and buried basement ridge and highs on the outer shelf a subordinate basin within the large basin on the mid shelf may be associated with motion along an early west antarctic rift system branch at least 4 km of pre glacial strata have been eroded from the present inner shelf and coastal hinterland by glacial processes six major sedimentary units ass 1 to ass 6 separated by five major erosional unconformities ass u1 to ass u5 are distinguished from bottom to top unconformity ass u4 results from a major truncational event by glacial advance to the middle and outer shelf which was followed by several episodes of glacial advance and retreat as observed from smaller scale truncational unconformities within the units above ass u4 some of the eroded sediments were deposited as a progradional wedge that extends the outer shelf by 25 to 65 km oceanward of the pre glacial shelf break we compare the observed seismic characteristics with those of other antarctic shelf sequences and assign an early cretaceous age to bottom sedimentary unit ass 1 a late cretaceous to oligocene age to unit ass 2 an early to mid miocene age to unit ass 3 a mid miocene age to unit ass 4 a late miocene to early pliocene age to unit ass 5 and a pliocene to pleistocene age to the top unit ass 6 buried grounding zone wedges in the upper part of unit ass 5 on the outer shelf suggest pronounced warming phases and ice sheet retreats during the early pliocene as observed for the ross sea shelf and predicted by paleo ice sheet models our data also reveal that on the middle and outer shelf the flow path of the pine island thwaites paleo ice stream system has remained stationary in the central pine island trough since the earliest glacial advances which is different from the ross sea shelf where glacial troughs shifted more dynamically this study and its stratigraphic constraints will serve as a basis for future drilling operations required for an improved understanding of processes and mechanisms leading to change in the west antarctic ice sheet such as the contemporary thinning and grounding line retreat in the amundsen sea drainage sector

real and apparent changes in sediment deposition rates through time 1 field measurements show that estimated sediment deposition rate decreases as a power law function of the measurement interval this apparent decrease in sediment deposition has been attributed to completeness of the sedimentary record the effect arises because of incorporation of longer hiatuses in deposition as averaging time is increased we demonstrate that a heavy tailed distribution of periods of nondeposition hiatuses produces this phenomenon and that observed accumulation rate decreases as t 1 over multiple orders of magnitude where 0 1 is the parameter describing the tail of the distribution of quiescent period length by using continuous time random walks and limit theory we can estimate the actual average deposition rate from observations of the surface location over time if geologic and geometric constraints place an upper limit on the length of hiatuses then average accumulation rates approach a constant value at very long times our model suggests an alternative explanation for the apparent increase in global sediment accumulation rates over the last 5 million years

improved spatial delineation of streambed properties and water fluxes using distributed temperature sensing a new method was developed for analysing and delineating streambed water fluxes flow conditions and hydraulic properties using coiled fibre optic distributed temperature sensing or closely spaced discrete temperature sensors this method allows for a thorough treatment of the spatial information embedded in temperature data by creating a matrix visualization of all possible sensor pairs application of the method to a 5 day field dataset reveals the complexity of shallow streambed thermal regimes to understand how velocity estimates are affected by violations of assumptions of one dimensional saturated homogeneous flow and to aid in the interpretation of field observations the method was also applied to temperature data generated by numerical models of common field conditions horizontal layering presence of lateral flow and variable streambed saturation the results show that each condition creates a distinct signature visible in the triangular matrices the matrices are used to perform a comparison of the behaviour of one dimensional analytical heat tracing models the results show that the amplitude ratio based method of velocity calculation leads to the most reliable estimates the minimum sensor spacing required to obtain reliable velocity estimates with discrete sensors is also investigated using field data the developed method will aid future heat tracing studies by providing a technique for visualizing and comparing results from fibre optic distributed temperature sensing installations and testing the robustness of analytical heat tracing models

radiocarbon constraints on antarctic peninsula ice sheet retreat following the last glacial maximum lgm abstract we present marine sedimentologic and radiocarbon data for the timing of retreat of the largely marine based antarctic peninsula ice sheet since the last glacial maximum lgm our findings indicate minimum estimates of deglaciation between 18 000 and 9000 calibrated years before present cal yr bp roughly in phase with the northern hemisphere deglaciation and eustatic sea level rise our findings show this retreat occurred progressively from the outer middle and inner continental shelf regions as well as progressively from the north to the south retreat initiated on the outer shelf of the northern peninsula by 18 000 cal yr bp and continued southward by 14 000 cal yr bp on the outer shelf of marguerite bay several thousand years earlier than estimated by numeric models while individual cores yield estimates of glacial retreat that may vary up to 1100 years we note steps in the data occur at 14 000 and possibly 11 000 cal yr bp coincidental to rapidly rising eustatic sea level including the well documented melt water pulses mwp 1a and 1b these data support the hypothesis that rapidly rising sea level is associated with marine ice sheet destabilization although additional dates are necessary to substantiate this finding this study highlights problems with radiocarbon dating acid insoluble organic aio matter in proximal lateglacial sediments as well as the need for more accurate dating techniques

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

comment on reconciliation of the devils hole climate record with orbital forcing moseley et al reports 8 january 2016 p 165 postulate an increase in dissolved thorium isotope 230 th with depth below the water table as the explanation for the differing ages of termination ii flow of geothermal water through the devils hole caverns precludes this explanation deposition of younger secondary calcite into the initial porosity of the calcite comprising their cores is a plausible alternate explanation

a subglacial hydrologic drainage hypothesis for silt sorting and deposition during retreat in pine island bay late holocene sediment deposits in pine island bay west antarctica are hypothesized to be linked to intensive meltwater drainage during the retreat of the paleo pine island ice stream after the last glacial maximum the uppermost sediment units show an abrupt transition from ice proximal debris to a draped silt during the late holocene which is interpreted to coincide with rapid deglaciation the small scale and fine sorting of the upper unit could be attributed to origins in subglacial meltwater however the thickness and deposition rate for this unit imply punctuated rather than continuous deposition this combined with the deposit s location seaward of large bedrock basins has led to the interpretation of this unit as the result of subglacial lake outbursts in these basins however the fine scale sorting of the silt unit is problematic for this energetic interpretation which should mobilize and deposit a wider range of sediment sizes to resolve this discrepancy we present an alternative mechanism in which the silt was sorted by a distributed subglacial water system stored in bedrock basins far inland of the grounding line and subsequently eroded at higher flow speeds during retreat we demonstrate that this mechanism is physically plausible given the subglacial conditions during the late holocene we hypothesize that similar silt units observed elsewhere in antarctica downstream of bedrock basins could be the result of the same mechanism

extent and dynamics of the west antarctic ice sheet on the outer continental shelf of pine island bay during the last glaciation abstract swath bathymetry and topas sub bottom profiler acoustic data reveal the presence of a major cross shelf bathymetric trough containing streamlined subglacial bedforms formed in soft till that extends to the continental shelf edge of outer pine island bay amundsen sea west antarctica the new data indicate that the west antarctic ice sheet wais extended to the shelf edge of pine island bay during the last glaciation and was drained by a grounded fast flowing palaeo ice stream through this outer shelf trough topography and subglacial geology are likely to have exerted a strong control on the development and location of palaeo ice streams in pine island bay topas records show that mega scale glacial lineations are formed in an acoustically transparent sediment layer interpreted to be soft till and are inferred to be the product of subglacial sediment deformation beneath the palaeo ice stream the flat to irregular nature of the basal reflector of the acoustically transparent layer of soft till implies that a combination of groove ploughing of the substrate and subglacial sediment deformation were important processes beneath the palaeo ice stream in the outer trough of pine island bay as well as palaeo ice streams elsewhere in antarctica therefore the formation of the soft till layer and possibly associated subglacial bedforms are likely to be a function of these palaeo ice stream processes in a regional context the wais extended to the continental shelf edge of the bellingshausen amundsen and ross seas and was drained by a number of palaeo ice streams during the last glaciation

interpreting seasonal convective mixing in devils hole death valley national park from temperature profiles observed by fiber optic distributed temperature sensing 1 devils hole a groundwater filled fracture in the carbonate aquifer of the southern nevada mojave desert represents a unique ecohydrological setting as home to the only extant population of cyprinodon diabolis the endangered devils hole pupfish using water column temperatures collected with a fiber optic distributed temperature sensor dts during four field campaigns in 2009 evidence of deep circulation and nutrient export are for the first time documented the dts was deployed to measure vertical temperature profiles in the system and the raw data returned were postprocessed to refine the calibration beyond the precision of the instrument s native calibration routines calibrated temperature data serve as a tracer for water movement and reveal a seasonal pattern of convective mixing that is supported by numerical simulations of the system the periodic presence of divers in the water is considered and their impacts on the temperature profiles are examined and found to be minimal the seasonal mixing cycle may deplete the pupfish s food supplies when nutrients are at their scarcest the spatial and temporal scales of the dts observations make it possible to observe temperature gradients on the order of 0 001 c m 1 revealing phenomena that would have been lost in instrument noise and uncertainty

grounding line retreat of the west antarctic ice sheet from inner pine island bay ice loss from the marine based potentially unstable west antarctic ice sheet wais contributes to current sea level rise and may raise sea level by 3 3 m or even 5 m in the future over the past few decades glaciers draining the wais into the amundsen sea embayment ase have shown accelerated ice flow rapid thinning and fast retreat of the grounding line gl however the long term context of this ice loss is poorly constrained limiting our ability to accurately predict future wais behavior here we present a new chronology for wais retreat from the inner continental shelf of the eastern ase based on radiocarbon dates from three marine sediment cores the ages document a retreat of the gl to within 100 km of its modern position before ca 10 000 calibrated cal yr b p this early deglaciation is consistent with ages for gl retreat from the western ase our new data demonstrate that in contrast to the ross sea wais retreat from the ase shelf was largely complete by the start of the holocene our results further suggest either slow gl retreat from the inner ase shelf throughout the holocene or that any episodes of fast gl retreat must have been short lived thus today s rapid retreat may be exceptional during the holocene and may originate in recent changes in regional climate ocean circulation or ice sheet dynamics

flow and retreat of the late quaternary pine island thwaites palaeo ice stream west antarctica multibeam swath bathymetry and sub bottom profiler data are used to establish constraints on the flow and retreat history of a major palaeo ice stream that carried the combined discharge from the parts of the west antarctic ice sheet now occupied by the pine island and thwaites glacier basins sets of highly elongated bedforms show that at the last glacial maximum the route of the pine island thwaites palaeo ice stream arced north northeast following a prominent cross shelf trough in this area the grounding line advanced to within similar to 68 km of and probably reached the shelf edge minimum ice thickness is estimated at 715 m on the outer shelf and we estimate a minimum ice discharge of similar to 108 km 3 yr 1 assuming velocities similar to today s pine island glacier similar to 2 5 km yr 1 additional bed forms observed in a trough northwest of pine island bay likely formed via diachronous ice flows across the outer shelf and demonstrate switching ice stream behavior the style of ice retreat is also evident in five grounding zone wedges which suggest episodic deglaciation characterized by halts in grounding line migration up trough stillstands occurred in association with changes in ice bed gradient and phases of inferred rapid retreat correlate to higher bed slopes supporting theoretical studies that show bed geometry as a control on ice margin recession however estimates that individual wedges could have formed within several centuries still imply a relatively rapid overall retreat our findings show that the ice stream channeled a substantial fraction of west antarctica s discharge in the past just as the pine island and thwaites glaciers do today

glacial marine sedimentation and quaternary glacial history of marguerite bay antarctic peninsula abstract marguerite bay situated between the subpolar glacial regime of the northern antarctic peninsula and the polar glacial regime of west antarctica is ideally located to test various models of glacial and glacial marine sedimentation and specific scenarios of late wisconsin ice sheet expansion piston cores and single channel seismic reflection data were collected during the deep freeze 85 and 86 expeditions to determine the late quaternary history of the area seismic data in the bay show a rugged seafloor with numerous deep troughs and a very thin layer of sediment over crystalline basement or older sediments glacial erosion is important in modifying existing features although the ultimate repository of the eroded material is not known it is not found within the bay the piston cores are topped by diatomaceous muds which are underlain by terrigenous muds and muddy gravels that imply deposition beneath an ice shelf basal tills were penetrated at three sites reflecting deposition by a grounded marine ice sheet a reconstruction of the glacial history of marguerite bay since the last glacial maximum shows grounded ice filling the bay in late wisconsin time rising sea level caused an uncoupling of the ice sheet and slow retreat of an ice shelf throughout the holocene

post lgm deglaciation in pine island bay west antarctica abstract to date understanding of ice sheet retreat within pine island bay pib following the last glacial maximum lgm was based on seven radiocarbon dates and only fragmentary seafloor geomorphic evidence during the austral summer 2009 2010 restricted sea ice cover allowed for the collection of 27 sediment cores from the outer pib trough region combining these cores with data from prior cruises over 133 cores have been used to conduct a detailed sedimentological facies analysis these results augmented by 23 new radiocarbon dates are used to reconstruct the post lgm deglacial history of pib our results record a clear retreat stratigraphy in pib composed of from top to base terrigenous sandy silt distal glacimarine pebbly sandy mud ice proximal glacimarine and till initial retreat from the outer continental shelf began shortly after the lgm and before 16 4 k cal yr bp as a likely response to rising sea level bedforms in outer pib document episodic retreat in the form of back stepping grounding zone wedges and are associated with proximal glacimarine sediments a sub ice shelf facies is observed in central pib and spans 12 3 10 6 k cal yr bp it is possible that widespread impingement of warm water onto the continental shelf caused an abrupt and widespread change from sub ice shelf sedimentation to distal glacimarine sedimentation dominated by widespread dispersal of terrigenous silt between 7 8 and 7 0 k cal yr bp the final phase of retreat ended before 1 3 k cal yr bp when the grounding line migrated to a location near the current ice margin

evolution of subglacial overdeepenings in response to sediment redistribution and glaciohydraulic supercooling received 16 august 2012 revised 18 december 2012 accepted 15 january 2013 1 glaciers erode bedrock rapidly but evacuation of sediments requires efficient subglacial drainage networks if glaciers erode more rapidly than evacuation proceeds a protective subglacial till layer can form to armor the bed where glaciers cross overdeepenings local closed depressions the bed slope opposes the ice surface and lowers the hydraulic potential gradient that drives water flow here we present results of a dynamic distributed model of coupled basal water flow and sediment transport to show how overdeepenings evolve over the course of a melt season we use steady state calculations as well as numerical simulations to understand how alluvial bed erosion alters overdeepenings numerical results from a modified form of the spring hutter equations show behaviors that cannot be inferred from either local or steady state calculations in general opposition of surface and bed slopes lessens sediment transport regardless of ice accretion from glaciohydraulic supercooling drainage efficiency strongly affects erosion and deposition rates results show characteristic behaviors of flow through overdeepenings such as overpressured water systems and accretion rates compatible with field measurements simulations that start with overdeepened glacier configurations progress out of a freezing regime where glaciohydraulic supercooling occurs this progression indicates that glacier hydrology is more strongly affected by erosion and deposition than by freezing from glaciohydraulic supercooling we discuss how this outcome affects glacier erosion and sediment transport under modern and past ice sheets

ice sheet retreat dynamics inferred from glacial morphology of the central pine island bay trough west antarctica abstract pine island glacier drains portions of the west antarctic ice sheet into the amundsen sea during the last glacial maximum the glacier extended nearly 500 km from its present location onto the outer continental shelf unusually restricted sea ice cover during the austral summer of 2010 allowed for a systematic multibeam swath bathymetric and chirp sonar survey of the mid shelf section of pine island trough the mapped glacial landforms reveal new information about the paleo pine island ice stream s dynamic retreat from the mid shelf area and confirm previous suggestion of a retreat in distinct steps the periods of grounding line stability during the overall retreat phase are marked by sediment accumulations i e grounding zone wedges these wedges are here mapped in sufficient detail to characterize spatial dimensions and estimate the volume of deposited sediment considering a range of sediment flux rates from the paleo pine island ice stream we estimate that the largest and most clearly defined grounding zone wedge located at about 73 s in the surveyed area took between 600 and 2000 years to form the ice stream retreated landward of this wedge before 12 3 cal ka bp the swath bathymetric imagery of landforms in pine island trough includes glacial features that suggest that retreat between periods of grounding line stability may be associated with episodes of ice shelf break up the depths of grounding line wedges decrease in a landward direction from 740 to 670 m and record elevation of the grounding line as it stepped landward in all the grounding line elevation varied by only 80 m over a distance of just over 100 km implying a low ice sheet profile during retreat finally we revisited seismic reflection profile nb9902 acquired along pine island trough in 1999 in combination with the newly acquired swath bathymetric imagery from 2010 together these data show that the ice stream paused during its retreat to form grounding zone wedges at an area in central pine island trough where a high in dipping bedrock strata exists and the glacial trough is narrow forming a bathymetric bottle neck

water sheet outburst floods from the laurentide ice sheet field evidence and a theoretical model support the concept that during wisconsinan glaciation subglacial water sheet outburst floods issued from a large subglacial lake located in the hudson bay ba

a subglacial water flow model for west antarctica the current generation of continental scale ice sheet models cannot successfully reproduce the complex ice water sediment interactions of the west antarctic ice sheet wais in a physically meaningful manner the potential of a thin film based subglacial water flow model for incorporation into a continental scale coupled ice water flow model of the wais is evaluated in this paper the subglacial water flow model is applied to the ross sea sector of the wais in both a steady state and time dependent form to derive the equilibrium water depth for the present day configuration the potential for coupling the model to an ice flow model is then demonstrated using a variable sliding parameter that is a function of the subglacial water depth a coupled ice water flow model using the parameterization tested in this paper could have the potential for reproducing the surface elevation velocity and thermal regime of the wais successfully these requirements are crucial in modelling the evolution of the wais and must be addressed before reliable continental scale predictive models can be utilized

the international bathymetric chart of the southern ocean ibcso version 1 0 a new bathymetric compilation covering circum antarctic waters the international bathymetric chart of the southern ocean ibcso version 1 0 is a new digital bathymetric model dbm portraying the seafloor of the circum antarctic waters south of 60 degrees s

marine ice sheet collapse potentially under way for the thwaites glacier basin west antarctica resting atop a deep marine basin the west antarctic ice sheet has long been considered prone to instability using a numerical model we investigated the sensitivity of thwaites glacier to ocean melt and whether its unstable retreat is already under way our model reproduces observed losses when forced with ocean melt comparable to estimates simulated losses are moderate 1 mm per year of sea level rise collapse in the different simulations within the range of 200 to 900 years

devils hole paleotemperatures and implications for oxygen isotope equilibrium fractionation subaqueous calcite in devils hole nevada was growing continuously from slightly super saturated groundwater providing a 570 kyr long o18 paleoclimate record due to its very slow growth it has been assumed to have grown under conditions of isotopic equilibrium however its holocene o18 value is 1 5 higher than predicted by laboratory precipitation based oxygen isotope thermometer calibrations the suggestion that devils hole calcite anchors the isotope thermometer to more 18o enriched values has stirred a debate as to which paleothermometer calibration is relevant for paleoclimate and casts doubt on the validity of o18 based paleotemperatures we used clumped isotopes to test the assumptions of the devils hole alternative 18o thermometer carbonate clumped isotopes are a temperature proxy that measures the abundance of 13c18o bonds in caco3 this proxy is independent of the water composition and therefore gives independent estimates of temperatures when calcite forms at thermodynamic and isotopic equilibrium we find that devils hole water paleotemperatures were constant at 30 6 2 6 c between 27 and 180 ka similarly to the modern groundwater temperature of 32 8 34 3 c the proximity of the devils hole clumped isotope data to values expected based on modern groundwater temperatures supports the notion that devils hole calcite grew under equilibrium conditions therefore the commonly used laboratory based o18 temperature calibrations should be reconsidered the constant water temperature over the glacial interglacial cycles indicates that the long devils hole o18 record reflects only variations in the groundwater o18 values and as such represents a valuable archive of paleoclimate and isotope paleohydrology

the influence of pretreatment chemistry on the radiocarbon dating of campanian ignimbrite aged charcoal from kostenki 14 russia the presence of an independently dated marker in an archaeological site offers rare opportunities for assessing the reliability of radiocarbon dates especially when these are close to the age limit of the technique two different pretreatment protocols routine aba and more rigorous abox sc were employed in the chemical preparation of the same charcoal sample from a layer closely associated to the campanian ignimbrite tephra at the russian palaeolithic site of kostenki 14 markina gora the aba treated fraction gave an age of 33 14c ka bp comparable to a previous determination from the same layer whereas the abox sc produced an older age of 35 14c ka bp this is the first radiocarbon determination of an archaeological sample to provide an age consistent with the calendar age for the ci tephra marker

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

direct observations of evolving subglacial drainage beneath the greenland ice sheet simultaneous observations of moulins and boreholes in western greenland show that water delivery to the base of the ice sheet via moulins affects short term ice velocity fluctuations but not late season ice velocity decelerations increased meltwater delivery to the base of the greenland ice sheet will increase the ice sheet velocity accelerating its inevitable rush towards the ocean and subsequent sea level rise or will it debate on this topic is at the forefront of cryospheric research but has been hampered by the lack of simultaneous observations of hydraulic head in moulins vertical shafts that deliver water to the base of the ice sheet and boreholes which monitor basal water pressure lauren andrews and colleagues now provide these observations from a small region in western greenland and reveal that water delivery by moulins into channelized basal flow is indeed linked to short term fluctuations in ice velocity late season decelerations in ice velocity however seem to be controlled by changes in unchannelized flow rather than any shifts in the moulin system seasonal acceleration of the greenland ice sheet is influenced by the dynamic response of the subglacial hydrologic system to variability in meltwater delivery to the bed1 2 via crevasses and moulins vertical conduits connecting supraglacial water to the bed of the ice sheet as the melt season progresses the subglacial hydrologic system drains supraglacial meltwater more efficiently1 2 3 4 decreasing basal water pressure4 and moderating the ice velocity response to surface melting1 2 however limited direct observations of subglacial water pressure4 5 6 7 mean that the spatiotemporal evolution of the subglacial hydrologic system remains poorly understood here we show that ice velocity is well correlated with moulin hydraulic head but is out of phase with that of nearby 0 3 2 kilometres away boreholes indicating that moulins connect to an efficient channelized component of the subglacial hydrologic system which exerts the primary control on diurnal and multi day changes in ice velocity our simultaneous measurements of moulin and borehole hydraulic head and ice velocity in the paakitsoq region of western greenland show that decreasing trends in ice velocity during the latter part of the melt season cannot be explained by changes in the ability of moulin connected channels to convey supraglacial melt instead these observations suggest that decreasing late season ice velocity may be caused by changes in connectivity in unchannelized regions of the subglacial hydrologic system understanding this spatiotemporal variability in subglacial pressures is increasingly important because melt season dynamics affect ice velocity beyond the conclusion of the melt season8 9 10

outburst flooding and the initiation of ice stream surges in response to climatic cooling a hypothesis abstract outburst flooding from subglacial reservoirs and associated surging of ice streams are caused by processes following climatic cooling that produce ice shelf grounding on proglacial sills trapping subglacial water according to an hypothesis presented here glaciers often advance into proglacial water bodies cooling may allow ice shelf formation and advance causing grounding on a proglacial sill trapping subglacial water ice shelf freeze on to the sill and development of a local reversal in ice air surface slope over the sill then are likely allowing ice thickening and water overpressurization if basal thawing then occurs as is likely an outburst flood and a surge may be triggered these processes may have been involved in heinrich events generation of antarctic subglacial lakes and meltwater scouring of some regions

reconstruction of the west antarctic ice sheet in pine island bay during the last glacial maximum and its subsequent retreat history abstract marine geological and geophysical studies were conducted to determine the late quaternary glacial history of the west antarctic ice sheet in pine island bay antarctica the data set includes sea floor imagery in the form of multibeam swath bathymetry and deep tow side scan sonar surveys distribution of sedimentary deposits and basal surface conditions are revealed through intermediate resolution seismic data and chirp 3 5 khz sub bottom profiles sedimentary deposits were sampled with piston cores and calcareous material was dated using ams radiocarbon techniques results from the study indicate the ice sheet was grounded on the outer shelf during its maximum extent sometime in the past thick ice flowing slowly and resting on crystalline bedrock was influenced by significant amounts of subglacial meltwater on the inner shelf thinner but more rapidly flowing ice characterized the seaward extent of the ice sheet which was grounded on a sedimentary substrate deglaciation occurred in two phases beginning soon after the last glacial maximum the outer most extension of the ice sheet retreated gradually pausing on a bathymetric high on the middle shelf by 16 000 14c years ago between 16 000 and 12 000 14c years ago ice sheet retreat resumed and may have proceeded rapidly to reach a present day position by 10 000 14c years ago

drainage through subglacial water sheets 1 subglacial drainage plays an important role in controlling coupling between glacial ice and underlying bed here we study the flow of water in thin macroporous sheets between ice and bed previous work shows that small perturbations in depth of a nearly parallel sided water film grow unstably because these areas have enhanced viscous dissipation that leads to enhanced melting of an ice roof we argue that in the presence of bed protrusions bridging a water sheet downward motion of the ice roof can stabilize this sheet stability results when the rate of roof closure increases faster with water depth than the rate of viscous dissipation we therefore modify existing theory to include protrusions that partially support the overlying glacier differences in the pressure on protrusions relative to water pressure drive roof closure the mechanisms of both regelation and creep normal to the bed close the overlying ice roof and decrease the ice bed gap in order to account for multiple protrusion sizes along the bed for instance resulting from an assortment of various sized sediment grains we incorporate a method of partitioning overburden pressure among different protrusion size classes and the water sheet partitioning is dependent on the amount of ice protrusion contact and therefore water depth this method allows prediction of roof closure rates we then investigate stable steady sheet configurations for reasonable parameter choices and find that these steady states can occur for modest water depths at very low effective pressures as is appropriate for ice streams moreover we find that multiple steady sheet thicknesses exist raising the possibility of switches between low and high hydraulic conductivity regimes for the subglacial water system

reconstruction of ice sheet changes in the antarctic peninsula since the last glacial maximum this paper compiles and reviews marine and terrestrial data constraining the dimensions and configuration of the antarctic peninsula ice sheet apis from the last glacial maximum lgm through deglaciation to the present day these data are used to reconstruct grounding line retreat in 5 ka time steps from 25 ka bp to present glacial landforms and subglacial tills on the eastern and western antarctic peninsula ap shelf indicate that the apis was grounded to the outer shelf shelf edge at the lgm and contained a series of fast flowing ice streams that drained along cross shelf bathymetric troughs the ice sheet was grounded at the shelf edge until 20 cal ka bp chronological control on retreat is provided by radiocarbon dates on glacimarine sediments from the shelf troughs and on lacustrine and terrestrial organic remains as well as cosmogenic nuclide dates on erratics and ice moulded bedrock retreat in the east was underway by about 18 cal ka bp the earliest dates on recession in the west are from bransfield basin where recession was underway by 17 5 cal ka bp ice streams were active during deglaciation at least until the ice sheet had pulled back to the mid shelf the timing of initial retreat decreased progressively southwards along the western ap shelf the large ice stream in marguerite trough may have remained grounded at the shelf edge until about 14 cal ka bp although terrestrial cosmogenic nuclide ages indicate that thinning had commenced by 18 ka bp between 15 and 10 cal ka bp the apis underwent significant recession along the western ap margin although retreat between individual troughs was asynchronous ice in marguerite trough may have still been grounded on the mid shelf at 10 cal ka bp in the larsen a region the transition from grounded to floating ice was established by 10 7 10 6 cal ka bp the apis had retreated towards its present configuration in the western ap by the mid holocene but on the eastern peninsula may have approached its present configuration several thousand years earlier by the start of the holocene mid to late holocene retreat was diachronous with stillstands re advances and changes in ice shelf configuration being recorded in most places subglacial topography exerted a major control on grounding line retreat with grounding zone wedges and thus by inference slow downs or stillstands in the retreat of the grounding line occurring in some cases on reverse bed slopes

three dimensional dense distributed temperature sensing for measuring layered thermohaline systems distributed temperature sensing has proven a useful technique for geoscientists to obtain spatially distributed temperature data when studies require high resolution temperature data in three spatial dimensions current practices to enhance the spatial resolution do not suffice for example double diffusive phenomena induce sharp and small scale temperature patterns in water bodies subject to thermohaline gradients this article presents a novel approach for a 3 d dense distributed temperature sensing setup the design of which can be customized to the required spatial resolution in each dimension temperature is measured along fiber optic cables that can be arranged as needed in this case we built a dense cage of very thin 1 6 mm cables to ensure that interference with flow patterns was minimal application in water bodies with double diffusion induced sharp temperature gradients shows that the setup is well able to capture small scale temperature patterns and even detects small unsuspected seeps and potential salt fingers however the potential effect of the setup on the flow patterns requires further study

meltwater intensive glacial retreat in polar environments and investigation of associated sediments example from pine island bay west antarctica abstract modern pine island and thwaites glaciers which both drain into pine island bay are among the fastest changing portions of the cryosphere and the least stable ice streams in antarctica here we show that the uppermost sediment unit in pine island bay was deposited from a meltwater plume a plumite during the late stages of ice sheet retreat 7 8 6 k cal yr bp and argue that this deposit records episodes of meltwater intensive sedimentation the plumite is a hydraulically sorted glacially sourced draping deposit that overlies proximal glacimarine sediments and thickens towards the modern grounding line the uppermost sediment unit is interpreted as a product of non steady state processes in which low background sedimentation in large bedrock carved basins alternates with episodic purging of sediment laden water from these basins the inner part of pine island bay contains several basins that are linked by channels with a storage capacity on the order of 70 km 3 of stagnant water and significant sediment storage capacity purging of these basins is caused by changes in hydraulic potential and glacial reorganization the sediment mobilized by these processes is found here to total 120 km 3 this study demonstrates that episodes of meltwater intensive sedimentation in pine island bay occurred at least three times in the holocene the most recent episode coincides with rapid retreat of the grounding line in historical time and has an order of magnitude greater flux relative to the entire unit we note that the final phase of ice stream retreat in marguerite bay was marked by a similar sedimentary event and suggest that the modern thwaites glacier is poised for an analogous meltwater intensive phase of retreat