the quaternary lancaster sound trough mouth fan nw baffin bay the development of the lancaster sound trough mouth fan tmf and glacial history in arctic canada were studied using a high resolution seismic profile across the entire fan and two piston cores stacked tills separated by erosion surfaces on the shelf pass seaward through till deltas into thick transparent glacigenic debris flow gdf deposits on the slope separated by thin well stratified glaciomarine layers an age model was built by ties to the ocean drilling program site 645 the deepest gdf on the seismic profile was indicative of the onset of shelf crossing glaciation in the early pleistocene the transition of the growth of lancaster sound tmf from an aggradational sequence unit m to an aggradational progradational sequence unit f occurred at the middle pleistocene transition in glacial cyclicity in the most recent glacial cycle gdf sheets were deposited during heinrich events 4 and 2 according to the correlation of the main detrital carbonate beds in two piston cores the outmost till wedge reflects the maximum advance of the grounding glacier far seaward of previously proposed last glacial maximum ice extent copyright 2011 john wiley sons ltd

till beneath ice stream b 4 a coupled ice till flow model a nonsteady model of an ice stream flowing on deforming till shows that the system responds rapidly and in a stable manner to reasonable marginal perturbations for the model we require one dimensional flow and continuity of ice and till and we assume linear viscous till rheology and balance between the driving stress for ice flow and the resistive stress at the bed this allows us to write coupled equations for the time rate of change of the ice thickness and the till thickness in terms of these thicknesses and the till viscosity an analytic steady state solution for ice stream b west antarctica shows that till viscosity decreases slowly downstream probably in response to decreasing effective pressure downstream nonsteady numerical experiments with fixed ice thicknesses at the ends show that a marginal perturbation causes a wave of adjustment to travel the length of the ice stream in about 50 years with a new steady state in about 200 years changes in till thickness tend to moderate perturbations in the ice and thus stabilize the system

late holocene terrestrial glacial history of miki and i c jacobsen fjords east greenland we present a late holocene glacial and environmental history of three valleys within miki and i c jacobsen fjords east greenland 68 n the fjords have no direct connection with the main ice sheet and are presently glacierized by a series of local ice caps and glaciers little sediment has been deposited within the study area but bedrock is distinctly glacially striated which suggests that extensive valley glaciers reached the present coastline in the past moraines are the prominent morphological features including distinct end and lateral moraines most of the moraines are related to the existing glaciers in the area apparent moraines extend as far as 2 to 2 5 km in front of present day glaciers based on morphological and stratigraphical studies of the sediments in these valleys five glacier advances during the late holocene are implied stabilization of the oldest moraines is lichenometrically dated to c 1450 to 1650 years ago younger moraines in both miki and i c jacobsen fjords are lichenometrically dated to c 1050 1100 years ago c 750 950 years ago and c 600 700 years ago the youngest moraines containing measurable lichens stabilized c 300 400 years ago the timing of this moraine formation correlates to marine records from previous research on the inner part of miki fjord where apparent intensification in sediment stratification is related to a glacier advance into the fjord c 290 years ago this glacial advance may correlate to the little ice age cooling observed in many records from the north atlantic region our glacial record from miki and i c jacobsen fjords also shows good correlation with temperature proxies within the grip ice core in greenland and foraminifera records from nansen fjord east greenland less pronounced correlation is observed with the icelandic sea ice index and records of glacier fluctuation in iceland during the last 1000 years these results suggest that widespread climatic cooling was the direct cause of many of the late holocene glacier variations documented in this study although the glacial advance at c 1050 1100 years ago coincides with the so called medieval warm period and could reflect the response of glaciers to increased precipitation within the area

seismic measurements reveal a saturated porous layer beneath an active antarctic ice stream seismic reflection studies recently conducted on ice stream b part of the marine ice sheet of west antarctica show a metres thick layer immediately beneath the ice in which both compressional p and shear s wave speeds are very low these low wave speeds imply that the material in the layer is highly porous and is saturated with water at a high pore pressure from this and from arguments presented in an accompanying paper1 to the effect that the layer would be too weak to support the shear stress exerted by the overlying ice we conclude that the layer is deforming and that the ice stream probably moves principally by such deformation

authigenic 10be 9be ratios and 10be fluxes 230thxs normalized in central baffin bay sediments during the last glacial cycle paleoenvironmental implications abstract authigenic 10 be 9 be ratios and 10 be fluxes reconstructed using the 230 th xs normalization proxies of the cosmogenic radionuclide 10 be production rate in the atmosphere have been measured in a sedimentary core from baffin bay north atlantic spanning the last 136 ka bp the normalization applied on the exchangeable authigenic 10 be concentrations using the authigenic 9 be isotope and 230 th xs methods yield equivalent results strongly correlated with sedimentological parameters grain size and mineralogy lower authigenic beryllium be concentrations and 10 be 9 be ratios are associated with coarse grained carbonate rich layers while higher authigenic be values are related to fine grained felspar rich sediments this variability is due to i sediment composition control over beryllium scavenging efficiency and ii glacial history that contributed to modify the 10 be concentration in baffin bay by input and boundary scavenging condition changes most paleo denudation rates inferred from the 10 be 9 be ratio vary weakly around 220 76 tons km 2 yr 1 0 09 0 03 mm yr 1 corresponding to relatively steady weathering fluxes over the last glacial cycle except for six brief intervals characterized by sharp increases of the denudation rate these intervals are related to ice surging episodes coeval with heinrich events and the last deglaciation period an average freshwater flux of 180 6 km 3 yr 1 0 006 sv consistent with recent models has been calculated in order to sustain glacially derived 10 be inputs into baffin bay it is concluded that in such environments the authigenic 10 be measured mainly depends on climatic effects related to the glacial dynamics which masks the 10 be production variation modulated by geomagnetic field changes altogether these results challenge the simple interpretation of 10 be concentration variation as a proxy of interglacial glacial interstadial stadial cycles in arctic and sub arctic regions they rather suggest the effect of higher frequency paleoclimatic changes and local glacial dynamics on 10 be signature

late wisconsinan glaciation of amund and ellef ringnes islands nunavut evidence for the configuration dynamics and deglacial chronology of the northwest sector of the innuitian ice sheet geomorphic and chronologic evidence from amund and ellef ringnes islands documents the configuration dynamics and collapse of the northwest sector of the innuitian ice sheet these data record the inundation of the ringnes islands by northwestward flowing ice from divides spanning the alpine and lowland sectors of the innuitian ice sheet ice flow indicators and granite dispersal along eastern amund ringnes island suggest massey sound was filled by an ice stream discharging coalescent alpine and lowland ice from norwegian bay in contrast the interior of amund ringnes island was overridden by predominantly non erosive granite free ice from a divide in the lowland sector of the ice sheet glacial landforms on ellef ringnes island record coverage by largely non erosive ice but it remains uncertain whether these features relate to northward flowing lowland ice or a cold based local ice cap deglaciation of the ringnes islands commenced 10 000 14c years ago deglacial dates between 9 7 and 9 2 ka bp rec

variations in melt layer frequency in the gisp2 ice core implications for holocene summer temperatures in central greenland the rare melt features in the gisp2 central greenland deep ice core have decreased in frequency over the most recent 7000 years calibration of this change in melt frequency against modern spatial variation of melt frequency and temperature in central greenland and against modern temporal variability of temperatures in central greenland indicates that mean mid summer temperatures have cooled over the most recent 7000 years probably by slightly more than l c if variability of summer temperatures has not changed comparison to grip isotopic records from central greenland and to the melt record from the agassiz ice cap arctic canada suggests some seasonal and regional coherence for this cooling signal as well as for a cold event about 8000 8500 bp

last glacial maximum and deglaciation of devon island arctic canada support for an innuitian ice sheet abstract the extent and origin of quarternary glaciers in the queen elizabeth islands of arctic canada especially during the last glacial maximum lgm has been debated for well over a century no consensus has yet emerged and the spectrum of interpretations within the last decade is the broadest ever the glacial geology of devon island strongly supports the late wisconsinan innuitian ice sheet hypothesis during lgm the southeastern part of the queen elizabeth islands was covered by an ice sheet flowing from a centre or a divide centrally located in the archipelago another ice divide extended from the centre across devon island flow from the innuitian centre and convergent flow from devon island and probably also from cornwallis and bathurst islands sustained an ice stream in wellington channel ice was locally advancing to its lgm limit about 23 ka bp recession in the vicinity of the present coastline was underway by 10 ka bp and final ice remnants west of the present devon ice cap vanished about 8 ka bp the island bears the same two part landscape zonation as do other islands and peninsulas in the region these reflect a cold based core and warm based periphery at lgm

the physiography of high arctic cross shelf troughs abstract a comprehensive inventory of high arctic cross shelf troughs is compiled from international bathymetric chart of the arctic ocean ibcao bathymetric data v 3 0 the location of 75 cross shelf troughs is presented alongside a synthesis of their key physiographic characteristics and available glacial geological evidence the troughs are interpreted to have been occupied intermittently and eroded by marine terminating ice streams that traversed the shelf during at least one and often many quaternary full glacial periods considerable variation in cross shelf trough physiography exists in the high arctic trough lengths range between 35 and 1400 km widths from 12 to 260 km and maximum depths from 200 to 1000 m the longest cross shelf troughs extend through inter island channels on the beaufort sea queen elizabeth islands and barents kara sea margins the gradient of the upper slope beyond high arctic glacial troughs which ranges between 0 3 and 13 is shown to have a negative relationship with palaeo ice stream drainage basin area and trough length glacial sedimentary depocentres or trough mouth fans tmfs are inferred on the basis of bathymetric and where available seismic evidence to exist beyond the majority of high arctic cross shelf troughs evidence of shelf progradation or tmf development is absent from the slope beyond several high arctic troughs probably as a result of limited sediment supply to the margin and or sediment by passing of the upper slope on the south greenland continental margin it is likely that steep slopes prevented the development of significant glacial sedimentary depocentres beyond cross shelf troughs or that glacigenic debris has been removed from the upper slope by submarine slope failure

seismic reflection evidence for a deep subglacial trough beneath jakobshavns isbrae west greenland seismic reflection methods were used to determine the ice thickness and basal topography of jakobshavns isbrae a large fast moving ice stream outlet glacier in west greenland a method of data analysis was developed which involves the pointwise migration of data from a linear seismic array and a single explosive source the method yields the depth horizontal position and slope of the basal reflector a deep u shaped subglacial trough was found beneath the entire length of the well defined ice stream the trough is incised up to 1500 m into bedrock and its base lies 1200 1500 m below sea level for at least 70km inland center line ice thickness along most of the channel is about 2500 m or about 2 5 times that of the surrounding ice sheet this prominent bedrock trough was not apparent in existing radio echo sounding data reflection coefficients indicate that much of the basal interface is probably underlain by compacted non deforming sediment the large ice thickness coupled with relatively steep surface slopes leads to high basal shear stresses 200 300 kpa along the ice stream the large shear stresses and lack of a deformable bed imply that internal deformation plays a dominant role in the dynamics of jakobshavns isbrae

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

late wisconsinan glaciation of southern eureka sound evidence for extensive innuitian ice in the canadian high arctic during the last glacial maximum abstract southern eureka sound was originally proposed as the centre of an innuitian ice sheet in the canadian high arctic at the last glacial maximum lgm based largely on the pattern of holocene emergence this paper focuses on the glacial geological evidence for such an ice sheet in the region granite dispersal trains and ice moulded bedrock record regional westward flow of warm based ice into eureka sound from se ellesmere island regional ice was coalescent with local ice domes on inter fiord peninsulas marine limit in the form of raised deltas beaches and washing limits formed during deglaciation of the regional ice throughout southern eureka sound marine limit dates 9 2 ka bp indicating that ice commenced retreat during the early holocene ice divides were located along the highlands of central ellesmere and axel heiberg islands from which ice inundated eureka sound flowing north and south along the channel regional radiocarbon dates on marine limit show that deglaciation occurred in two steps initial break up and radial retreat of ice from eureka sound to the inner fiords was rapid and preceded stabilisation along adjacent coastlines and at fiord heads two step deglaciation is also reflected in differences in glacial geomorphology between the inner and outer parts of many fiords a prominent belt of fiord head glaciogenic landforms long proposed to mark the last glacial limit is re interpreted to record initial stabilisation of ice margins due predominantly to bathymetric control

intrusion of ice into porous media by regelation a mechanism of sediment entrainment by glaciers unlithified sediment at glacier beds should be entrained in ice as a result of melting above individual grains and refreezing below grains regelation to test this hypothesis a device that simulates subglacial conditions was used to push ice downward through idealized and natural porous media in contact with a flat bed ice regelated toward the bed at speeds proportional to the gradient in ice pressure across the thickness of particles ice temperatures responded predictably to changes in driving stress and demonstrated that the ice particle mixtures were at or very near the pressure melting temperature a theory of pore ice motion by regelation predicted the measured speed usually to within a factor of two although at low driving stresses the speed was less than the predicted value by as much as a factor of five the low speeds were probably caused by imperfect temperature control that resulted either in heat loss from the ice particle mixtures or in slightly subfreezing pore ice temperatures that were not detected by thermistors ice should intrude the pores of underlying sediment to a steady depth at which the downward regelation speed equals the rate of basal melting departures from the steady state should cause release of debris from ice or further entrainment an approximate calculation of the steady intrusion depth yields values comparable to typical thicknesses of debris in basal ice a few centimeters to more than a meter intrusion of sediment by regelation is consistent with the isotopic composition of debris bearing ice if melting exceeds refreezing

increased water storage at ice stream onsets a critical mechanism the interdependence of rapid ice flow surface topography and the spatial distribution of subglacial water are examined by linking existing theories the motivation is to investigate whether the acceleration of an ice stream tributary contains a positive feedback that encourages the retention of subglacial water that leads to faster flow periodically varying surface and bed topographies are related through a linear ice flow perturbation theory for various values of mean surface slope perturbation amplitude and basal sliding speeds the topographic variations lead to a periodic variation in hydraulic potential that is used to infer the tendency for subglacial water to be retained in local hydraulic potential minima if water retention leads to enhanced basal sliding a positive feedback loop is closed that could explain the transition from slower tributary flow to faster streaming flow and the sustained downstream acceleration along the tributary ice stream system a sensitivity study illustrates that the same range of topographic wavelengths most effectively transmitted from the bed to the surface also strongly influences the behavior of subglacial water a lubrication index is defined to qualitatively measure the heterogeneity of the subglacial hydrologic system application of this index to field data shows that the transition from tributary to ice stream closely agrees with the location where subglacial water may be first stored

soil consolidation behavior assessed by seismic velocity measurements large gravity platforms are often used as alternatives to the more conventional pilesupported structures in hydrocarbon exploitation a gravity structure as opposed to the piled structure is sitting on the sea floor by virtue of its weight and base width as such it poses considerable problems for the site investigation engineer one such problem is the calculation of the settlement of the structure and its time history these depend upon the permeability and compressibility of the soil and its drainage conditions the required data are usually obtained by sampling for subsequent laboratory testing the collection of an undisturbed sample is beset by problems so that the consolidation behavior of the foundation material can only be inadequately assessed by laboratory testing however a series of laboratory consolidation experiments during which seismic velocities have been measured on the sample as consolidation proceeds shows that it is possible to reconstruct the stress strain and time dependent curves from the seismic data once the initial void ratio and permeability of the soil are known this leads the way to an in situ technique for predicting settlement using a combination of geophysical techniques electrical resistivity and seismic velocities to obtain the required engineering properties

sediment thicknesses and holocene glacial marine sedimentation rates in three east greenland fjords ca 68 n we compared measured and estimated sediment budgets in heavily glaciated fjords in east greenland mass balance calculations and regional glacio climatic conditions suggest that the sediment flux to the seafloor in kangerdlugssuaq and nansen fjords should be dominated by iceberg rafting and not by the rain out of suspended particulates in meltwater as the glacier calving flux is estimated at 15 and 2 km 3 yr compared to meltwater volumes of 4 4 and 1 7 km 3 yr respectively gravity cores in the three fjords indicate that the uppermost 1 2 5 m of sediment consists of diamictons or fine grained laminated muds ams radiocarbon dates on calcareous foramininfera or shells 16 total indicate sedimentation rates of 110 to 340 cm ka within the fjords over the last 1 ka and 10 20 cm ka during the holecene on the inner and middle shelf annual sediment discharge is around 0 67 times 10 6 tonnes yr within the kangerdlugssuaq fjord and trough system which translates into an average basin wide rate

quaternary glacial and marine environmental history of northwest greenland a review and reappraisal new information on middle and late quaternary deposits of the thule region northwest greenland necessitates the revision of the chronostratigraphy of the area and allows fuller understanding of the marine environmental changes in ba n bay the stratigraphic record is interpreted as the product of three marine events saunders qarmat and nuna in decreasing age during which marine sedimentation occurred on coastal areas now land and three or four glacial events of decreasing age and ice cover agpat narsaarsuk and kap abernathy wolstenholme fjord to which the marine events were glacio isostatically related subarctic atlantic water reached this northern part of ba n bay during all three marine events but the warmest conditions warmer than the present occurred in the qarmat event correlated with the deep sea oxygen isotope stage 5e the question of late stage 5 events is considered the nuna event dates to latest stage 2 and the holocene whilst the saunders event is of uncertain age but at least stage 6 a number of glacial events with advances confined to the fjords are dated to stage 2 wolstenholme fjord and late stage 6 narsaarsuk and an intermediate advance kap abernathy is undated in the range of stage 2 5 extension of glaciation to the shelf edge is dated to stage 6 agpat event 1999 elsevier science ltd all rights reserved

till beneath ice stream b 2 structure and continuity during the 1984 1985 antarctic field season 8 3 km of high resolution seismic reflection data were collected in order to image a thin till layer beneath ice stream b west antarctica two parallel seismic reflection lines were oriented transverse to ice flow these data show that the till layer varies in thickness but is continuous over almost the entire length of the profiles with an average thickness of 6 5 m the upper surface of the till layer is smooth but the lower boundary is fluted parallel to flow these till filled flutes are as much as 13 m deep and 1000 m across nowhere on the profiles can any feature be discerned to penetrate more than a few meters into the ice from the bed reflection events from lithified sediments of unknown type are observed to be truncated by the till in an angular unconformity

till beneath ice stream b 3 till deformation evidence and implications most of the velocity of ice stream b near the upstream b camp upb west antarctica appears to arise from deformation of a seismically detected subglacial till layer that averages 6 m thick available evidence indicates that the entire thickness of this till layer is deforming and is eroding subjacent bedrock into flutes parallel to ice flow and hundreds of meters across the resulting till flux beneath upb is equivalent to an average erosion rate of about 0 4 mm yr 1 in the catchment area and suggests that till deltas tens of kilometers long have been deposited at the grounding line during the holocene such deltas should be characterized by partial ice till decoupling across a water film and by a small ice air surface slope they may have been discovered by recent geophysical work

the deforming bed characteristics of a stratified till assemblage in north east anglia uk investigating controls on sediment rheology and strain signatures the glacial coastal exposures of north norfolk are a type site for subglacial glaciotectonic deforming bed sediments this investigation of the lower stratified diamict within the north sea drift at west runton reveals two distinct lamina types type 1 laminae are the product of primary extensional glaciotectonism with ductile intergranular pervasive shear predominating over brittle shear type 2 laminae also exhibit structures that can be attributed to ductile intergranular pervasive shear and brittle shear but the lateral continuity of type 2 laminae and the presence of dropstone like structures supports a primary subaqueous origin with secondary subglacial deformation when coupled with micromorphological analysis these findings show that ductile viscous creep mechanisms control sedimentary architecture and that shear stratification in particular has the potential to affect the rheological properties of the sediment pile and the hydraulic routing of basal water ultimately influencing critical effective pressure fluctuations and the thresholds controlling the subglacial drainage system

characterizing an unstable mountain slope using shallow 2d and 3d seismic tomography as transport routes and population centers in mountainous areas expand risks associated with rockfalls and rockslides grow at an alarming rate as a consequence there is an urgent need to delineate mountain slopes susceptible to catastrophic collapse in a safe and noninvasive manner for this purpose we have developed a 3d tomographic seismic refraction technique and applied it to an unstable alpine mountain slope a significant segment of which is moving at 0 01 0 02 m year toward the adjacent valley floor first arrivals recorded across an extensive region of the exposed gneissic rock mass have extraordinarily low apparent velocities at short 0 2 m to long 100 m shot receiver offsets inversion of the first arrival traveltimes produces a 3d tomogram that reveals the presence of a huge volume of very low quality rock with ultralow to very low p wave velocities of 500 2700 m s these values are astonishingly low compared to the average horizontal p wave velocity of 5400 m s determined from labora

sprites and possible mesospheric effects images of sprites have been recorded at 1 ms resolution revealing several new sprite properties sprites appear to occur in a highly structured mesosphere and we suggest that the cause of some of this structure is the sprite activity itself evidence is seen in events where a subsequent nearby sprite appears to re activate the volume of a previous sprite in sprites where tendrils and branches develop away from the normally observed vertical direction and in beads that lasts much longer that the parent sprite sprites can be large with horizontal widths of more than 40 km and can extend from the clouds up to the lower ionosphere thus affecting a large volume of the atmosphere the total horizontal area of sprites during one storm over nebraska was a significant fraction of the area covered by the associated thunderstorm raising the possibility of larger scale measurable mesospheric effects

georods the development of a four element geophone for improved seismic imaging of glaciers and ice sheets active seismic imaging of glaciers and ice sheets is important for constraining inputs to climate models such as englacial ice fabric and the nature of the basal interface however acquiring high quality seismic data is time consuming and resource intensive using traditional single element geophones requires ideal weather conditions e g light winds and excellent source coupling in addition deploying and retrieving these geophones is slow and cumbersome we have developed a four element georod that enhances signal levels by 20 30 db in a variety of conditions including blowing snow and poorly coupled source detonations the long slender design of these georods makes them easy to deploy and retrieve allowing researchers to acquire greater line kilometers of seismic data during field campaigns that are commonly time constrained

present day kinematics of the plate boundary zone between africa and europe from the azores to the aegean anticlockwise rotation of africa relative to europe around an euler pole in the eastern atlantic causes plate convergence in the central mediterranean towards n20 w at rate considered a priori to be 5 10 mm yr 1 however previous summaries of the present day deformation sense in the central mediterranean are inconsistent with observations in southern italy recent investigations of italy and adjacent parts of north africa enable local deformation sense observed to be reconciled with sense predicted from relative plate motions suggested angular velocity of the african plate relative to europe is 0 07 myr 1 around a pole near 21 n 21 w this predicts 2 6 mm yr 1 maximum extension rate towards n64 e near the azores consistent with the minimum local accretion of new oceanic crust in the past few millions of years and 4 3 mm yr 1 plate convergence towards n20 w in the central mediterranean at 13 e further east the northern margin of the african plate can be regarded as the gargano seismic zone in the central adriatic sea the northern adriatic sea is a microplate rotating anticlockwise relative to europe at 0 3 myr 1 around a pole in northwestern italy causing northeastward shortening in yugoslavia and up to 4 mm yr 1 extension in central and northern italy the african plate is deforming internally in tunisia and northern libya extending obliquely in direction e10 s at rate up to 5 5 mm yr 1 the resultant velocity relative to europe in the ionian and southern adriatic seas 5 mm yr 1 towards n50 e explains the observed relative velocity across southern italy kinematic consistency requires present day shortening rate east of calabria associated with subduction on the tyrrhenian sea benioff zone and tyrrhenian sea extension rate to be both zero or equal to within 1 mm yr 1

history of subduction and back arc extension in the central mediterranean summary geological and geophysical constraints to reconstruct the evolution of the central mediterranean subduction zone are presented geological observations such as upper plate stratigraphy hp lt metamorphic assemblages foredeep trench stratigraphy arc volcanism and the back arc extension process are used to define the infant stage of the subduction zone and its latest back arc phase based on this data set the time dependence of the amount of subducted material in comparison with the tomographic images of the upper mantle along two cross sections from the northern apennines and from calabria to the gulf of lyon can be derived further the reconstruction is used to unravel the main evolutionary trends of the subduction process results of this analysis indicate that 1 subduction in the central mediterranean is as old as 80 myr 2 the slab descended slowly into the mantle during the first 20 30 myr subduction speeds were probably less than 1 cm year x1 3 subduction accelerated afterwards producing arc volcanism and back arc extension and 4 the slab reached the 660 km transition zone after 60 70 myr this time dependent scenario where a slow initiation is followed by a roughly exponential increase in the subduction speed can be modelled by equating the viscous dissipation per unit length due to the bending of oceanic lithosphere to the rate of change of potential energy by slab pull finally the third stage is controlled by the interaction between the slab and the 660 km transition zone in the southern region this results in an important re shaping of the slab and intermittent pulses of back arc extension in the northern region the decrease in the trench retreat can be explained by the entrance of light continental material at the trench

changes in west antarctic ice stream velocities observation and analysis 1 we have produced a map of velocity covering much of the siple coast ice streams the map confirms earlier estimates of deceleration on whillans ice stream comparison with bed elevation data indicates that subglacial topography and the location of consolidated sediment play a strong role in determining the location of the tributaries feeding the ice streams force balance estimates based on these data indicate that the tributaries have beds nearly an order of magnitude stronger than those beneath many of the ice streams we have used a theoretical analysis to examine the controls on fast flow this analysis suggests that ice plains very wide ice streams are inherently unstable this instability may be responsible for the current deceleration on the ice plain of whillans ice stream and the shutdown of ice stream c 150 years ago thinning induced reductions in driving stress may also explain some of the observed deceleration particularly in upstream areas the active portions of ice stream c coincide well with the areas where we estimate that melt should be taking place current topography and inferences of large thickening following a shutdown suggest the upstream migration of a stagnation front that initiated at the ice plain uncertainty remains about the basal conditions on ice stream d while the basal resistance on ice stream e is large enough to ensure basal melting

gps velocity and strain fields in sicily and southern calabria italy updated geodetic constraints on tectonic block interaction in the central mediterranean 1 we present an improved rendition of the geodetic velocity and strain fields in sicily and southern calabria obtained through the analysis of 18 years of gps observations from continuous and survey station networks the dense spatial coverage of geodetic data provides precise quantitative estimates of previously established first order active kinematic features including i a narrow east west elongated belt of contraction 1 1 5 mm yr extending offshore northern sicily from ustica to stromboli across the aeolian islands ii a narrow east west trending contractional belt located along the northern rim of the hyblean plateau in southern sicily with shortening at up to 4 4 mm yr iii right motion 3 6 mm yr on the aeolian tindari letojanni fault atlf system a main shear zone extending from the aeolian islands to the ionian coast of sicily with significant transpression and transtension partitioned between discrete sectors of the fault iv transtension 1 mm yr across the sicily channel between sicily and north africa we use geodetic observations coupled to geological constraints to better elucidate the interplay of crustal blocks revealed in the investigated area in particular we focus on the atlf which forms the primary boundary between the sicilian and calabrian blocks the atlf juxtaposes north south contraction between sicily and the tyrrhenian block with northwest southeast extension in northeastern sicily and calabria contraction between sicily and tyrrhenian blocks probably arises from the main europe nubia convergence although sicily has a component of lateral motion away from nubia we found that convergence is not restricted to the northern offshore as commonly believed but is widely accommodated between the frontal belt and the northern rim of the hyblean foreland in southern sicily geodetic data also indicate that active right shear on the atlf occurs to the southeast of the mapped fault array in northern sicily suggesting the fault cuts through till the ionian coast of the island the small geodetic divergence between the hyblean and apulian blocks rimming on both sides the calabria block and subjacent ionian slab coupled with marine geophysical evidences in the ionian sea lends credit to the proposed deep root of the atlf and to a fragmentation of the ionian domain

development of the topography controlled upper odra ice lobe scandinavian ice sheet in the fore mountain area of southern poland during the saalian glaciation abstract during the saalian drenthe glaciation the upper odra ice lobe constituted the most southern part of the scandinavian ice sheet in central europe the lobe was situated in an area with varied topography in the foreland of a mountain range the ice lobe had a complex form because it was subdivided into several smaller sublobes the primary factors that controlled the lobe were investigated it was found that the ice lobe development depended largely on the topography but the lithology of the ice sheet substratum was also highly important the topography controlled the main movement direction of the ice and caused a concentration of the ice flow in the axial part of the silesian lowland between subglacial morphological barriers which thereby formed a corridor of relatively fast flowing ice which was probably streaming and which supplied the lobe the topography also determined the ice flow within the lobe which strongly matched the valley system the presence of sediments with relatively low permeability at the base of the lobe the quaternary deposits are underlain by neogene clays caused high pore water pressure in the subglacial zone and triggered the basal movement of the ice sheet which promoted a rapid flow of the ice and favoured the development of the lobe the basal tills deposited from the active and passive ice and extant till patches without a fluvioglacial cover atop some parts of the studied area indicate spatially varying ice flow dynamics especially in the later stage of glaciation certain parts of the lobe underwent stagnation while in other parts the ice was probably still active the varied behaviour of the ice within the lobe was determined primarily by the topography and lithology of the substratum which produced spatially varied marginal compression and basal friction a model of the development of the lobe is proposed

forward modeling of ice topography on mars to infer basal shear stress conditions 1 understanding the history of ice caps on mars could reveal important information about martian geologic and climatic history to do this an ice reconstruction model is needed that operates over complex topography and can be constrained with a limited number of free parameters in this study we developed a threshold sliding model for ice cap morphology based on the classic model of nye later incorporated into the models of reeh and colleagues we have updated the nye reeh model with a new numerical algorithm although the model was originally developed to model perfectly plastic deformation it is applicable to any ice body that deforms when a threshold basal shear stress is exceeded the model requires three inputs a digital elevation model of bed topography a mask grid that defines the position of the ice terminus and a function defining the threshold basal shear stress to test the robustness of the model the morphology of the greenland ice sheet is reconstructed using an empirical equation between threshold basal shear stress and ice surface slope the model is then used to reconstruct the morphology of ice draping impact craters on the margins of the south polar layered deposits using an inferred constant basal shear stress of 0 6 bar for the majority of the examples this inferred basal shear stress value is almost 1 3 of the average basal shear stress calculated for the greenland ice sheet what causes this lower basal shear stress value on mars is unclear but could involve the strain weakening behavior of ice

new perspectives on paleoglaciology abstract paleoglaciology deals with glaciation cycles of the quaternary ice age it combines the dynamics of present day ice sheets deduced from glaciology with the history of former ice sheets deduced from glacial geology cosmogenic dating now makes a detailed chronology of a cycle possible radiocarbon dating has provided a detailed chronology for termination of the last cycle but cannot date how the cycle began here we identify the central challenge in applying paleoglaciology to quaternary glaciation cycles in general and initiation of these cycles in particular the challenge is the role of thickening sea ice in the arctic ocean possibly becoming thick ice shelves floating over deep arctic basins and high marine ice sheets grounded on shallow arctic continental shelves in that case an arctic ice sheet existed that was larger and less stable than the antarctic ice sheet is now or in the past two approaches to this problem are presented as part 1 and part 2 in part 1 the height of these former ice sheets is linked primarily to the strength of ice bed coupling which is deduced from glacial geology it provides snapshots of ice sheets that give ice elevations independent of the past history and largely independent of ice surface conditions temperatures and the mass balance in particular in part 2 heights of former ice sheets depend on both their past history particularly initial conditions and changing surface conditions during the glaciation cycle this provides a motion picture of ice sheets during the full cycle the challenge is to establish a mind meld of these two approaches they allow new perspectives on paleoglaciology

reconstruction of the glacial ice covers of greenland and the canadian arctic islands by three dimensional perfectly plastic ice sheet modelling a three dimensional perfectly plastic ice sheet model developed for determining the surface elevations and the flow pattern of an ice sheet with given bottom topography and ice margin positions is applied to the reconstruction of the glacial ice covers of greenland and the canadian arctic islands in the northern regions two different reconstructions have been performed with ice margins along the present 600 and 200 m sea depth contours respectively in central greenland the ice margin is considered to be at the outermost ice margin deposits on the coastal shelf to the west and at the present 200 m sea depth contour to the east the main conclusions to be drawn from the reconstructions are 1 the flow pattern of the glacial ice cover of greenland shows a great resemblance to the present one the central ice divide being displaced less than 50 km from its present position and being no more than 200 m higher than today 2 the main ice divide of the ice sheet covering the canadian arctic islands the innuitian ice sheet was located over the highlands of eastern ellesmere island with local domes positioned over the present ice caps indicating that even the deep ice of wisconsin age in these ice caps is of local origin this is also the case for the devon island ice cap 3 even in the not very likely case of a rather extensive glacial ice cover in north west greenland the ice flow pattern upstream of the camp century deep drill site would not have changed radically compared to the present flow pattern thus it is concluded that even advanced ice margins in late wisconsin time could at most have resulted in an elevation of the deposition site of the late wisconsin ice at camp century 600 m higher than at present the consequences of this conclusion are discussed

ice volume and subglacial topography for western canadian glaciers from mass balance fields thinning rates and a bed stress model abstracta method is described to estimate the thickness of glacier ice using information derived from the measured ice extent surface topography surface mass balance and rate of thinning or thickening of the ice column shear stress beneath an ice column is assumed to be simply related to ice thickness and surface slope as for an inclined slab but this calculation is cast as a linear optimization problem so that a smoothness regularization can be applied assignment of bed stress is based on the flow law for ice and a mass balance calculation but must be preceded by delineation of the ice flow drainage basin validation of the method is accomplished by comparing thickness estimates to the known thickness generated by a numerical ice dynamics model once validated the method is used to estimate the subglacial topography for all glaciers in western canada that lie south of 60 n adding the present ice volume of each glacier gives the estimated total volume as 2320 km3 equivalent to 5 8 mm of sea leve

seismic noise interferometry reveals transverse drainage configuration beneath the surging bering glacier subglacial drainage systems are known to critically control ice flows but their spatial configuration and temporal evolution are poorly constrained due to inaccessibility here we report a 12 year long monitoring of the drainage underneath bering glacier alaska by correlating ambient noise recorded at two seismic stations on the sides of the glacier we find that the seismic surface waves traveling across bering glacier slowed down by 1 2 during its latest 2008 2011 surge likely due to the switch of the subglacial drainage from a channelized system to a distributed system in contrast to current models the relative amplitude of velocity reductions for rayleigh and love waves requires the distributed drainage to be highly anisotropic and aligned perpendicular to the ice flow direction we infer that the subglacial water flow is mainly through a network of transverse basal crevasses during surges and thus can sustain the high water pressure and ice flow speed

englacial seismic reflectivity imaging crystal orientation fabric in west antarctica abrupt changes in crystal orientation fabric cof and therefore viscosity are observed near the base of the ice sheet throughout west antarctica we report on active source seismic observations from wais divide mid stream and downstream on thwaites glacier and the onset region of bindschadler ice stream these data reveal a prevalence of englacial seismic reflectivity in the bottom quarter of the ice sheet the observed seismic reflectivity is complex but largely bed conformable with long spatial wavelength features observed in the flow direction and short wavelength features observed across flow a correspondence of englacial structures with bed features is also observed we determine the origin of the reflectivity to be abrupt changes in the cof of ice based on the following 1 observations of englacial reflectivity are consistent with current knowledge of cof within ice sheets 2 englacial reflectivity caused by cof contrasts requires the simplest genesis especially at ice divides and 3 amplitude analysis shows that the observed englacial reflectivity can be explained by contrasts in seismic velocity due to cof changes we note that the downstream increase in the quantity and complexity of observations indicates that direct observations of cof at ice divides likely underestimate the role that fabric plays in ice sheet dynamics

characteristics of the sticky spot of kamb ice stream west antarctica amplitude analysis of reflection seismic data reveals the presence of highly variable bed conditions under the main sticky spot and adjacent regions of the kamb ice stream kis formerly ice stream c the sticky spot which is a zone of bed that imparts high basal resistance to ice flow is situated on a local topographic high composed of consolidated sediments or sedimentary rock any meltwater draining from upglacier along the base of the ice is routed around the sticky spot the ice over the sticky spot includes in at least some places a seismically detectable basal layer containing a low concentration of debris which locally thickens to 40 m over a topographic low in the bed the ice contact basal material ranges from dilated and highly porous to more compacted and stiff and perhaps locally frozen the softer material is preferentially in topographic lows but there is not a one to one correspondence between basal character and basal topography we speculate that the 40 m thick frozen on debris layer formed by glaciohydraulic supercooling of lake drainage events along a basal channel during the former active phase of the ice stream we also speculate that loss of lubricating water perhaps from piracy upstream contributed to the slowdown of the ice stream with drag from the sticky spot playing an important role and with the basal heterogeneity greatly increasing after the slowdown of the ice stream

recognition of earthquake induced damage in the abakainon necropolis ne sicily results from geomorphological geophysical and numerical analyses seismotectonic activity and slope instability are a permanent threat in the archaeological site of abakainon and in the nearby village of tripi in ne sicily in recent times signs of an ancient earthquake have been identified in the necropolis of abakainon which dating was ascertained to the first century ad earthquake the site is located on a slope of peloritani mts along the tindari fault line and contains evidence for earthquake induced landslide including fallen columns and blocks horizontal shift and counter slope tilting of the tomb basements in this paper we used an integrated geomorphological and geophysical analysis to constrain the landslide the research was directed to the acquisition of deep geological data for the reconstruction of slope process and the thickness of mobilized materials the applied geophysical techniques included seismic refraction tomography and electrical resistivity tomography the surveys were performed to delineate the sliding surface and to assess approximately the thickness of mobilized materials the geophysical and geomorphologic data confirmed the presence of different overlapped landslides in the studied area moreover a numerical simulation of the slope under seismic loads supports the hypothesis of a mobilization of the landslide mass in case of strong earthquakes pga 0 3 g however numerical results highlight that the main cause of destruction for the abakainon necropolis is the amplification of the seismic waves occasionally accompanied by surficial sliding

measurements of past ice sheet elevations in interior west antarctica a lateral moraine band on mount waesche a volcanic nunatak in marie byrd land provides estimates of past ice sheet surface elevations in west antarctica helium 3 and chlorine 36 surface exposure ages indicate that the proximal part of the moraine up to 45 meters above the present ice surface was deposited about 10 000 years ago substantially later than the maximum ice extent in the ross embayment the upper distal part of the moraine may record multiple earlier ice sheet high stands a nonequilibrium ice sheet model predicts a delay of several thousand years in maximum ice levels at mount waesche relative to the maximum ice extent in the ross sea the glacial geologic evidence coupled with the ice sheet model indicates that the contribution of the ross sea sector of the west antarctic ice sheet to holocene sea level rise was only about 3 meters these results eliminate west antarctic ice as the principle source of the large meltwater pulse during the early holocene

an extended perfect plasticity method for estimating ice thickness along the flow line of mountain glaciers 1 direct measurement of the thickness of mountain glaciers is difficult over large areas yet knowledge of the thickness is essential for calculating their volumes and future evolution we develop a new method for estimating the ice thickness along glacier flow lines using the perfect plasticity rheological assumption that relates the thickness and surface slope to a yield stress previous studies have used this assumption with the shallow ice approximation to estimate the ice thickness but the standard approach neglects the effect of side drag on glacier stress balance our method addresses this shortcoming and extends the standard method by accounting for the side drag via the glacier width besides the assumed yield stress the inputs for our method are the outline and surface topography of the glacier surface velocity and mass balance data are unnecessary we validated the extended method on five glaciers in northwest china where thickness data are available from radio echo soundings finding that it can reproduce measured thicknesses with a mean absolute error of 11 8 like the standard method moreover for long glacier tongues confined to flow between parallel valley sides this method is found to give more accurate thickness estimates than does the standard method with a mean absolute error of as low as 5 3 sensitivity analysis shows that the estimated ice thickness depends strongly on yield stress and surface slope and less strongly on glacier width because this method is physically more realistic than the standard method and its inputs are easily derivable from remote sensing observations it has the potential to be used for processing large glacier data sets

landform and sediment imprints of fast glacier flow in the southwest laurentide ice sheet evidence for former fast glacier flow ice streaming in the southwest laurentide ice sheet is identified on the basis of regional glacial geomorphology and sedimentology highlighting the depositional processes associated with the margin of a terrestrial terminating ice stream preliminary mapping from a digital elevation model of alberta identifies corridors of smoothed topography and corridor parallel streamlined landforms megaflutes to mega lineations that display high levels of spatial coherency ridges that lie transverse to the dominant streamlining patterns are interpreted as a series of minor recessional push moraines b thrust block moraines or composite ridges hill hole pairs constructed during readvances surges and c overridden moraines cupola hills apparently of thrust origin together these landforms demarcate the beds and margins of former fast ice flow trunks or ice streams that terminated as lobate forms localised cross cutting and or misalignment of flow sets indicates temporal separation and the overprinting of ice streams lobes the fast flow tracks are separated by areas of interlobate or inter stream terrain in which moraines have been constructed at the margins of neighbouring competing ice streams outlet glaciers this inter stream terrain was covered by more sluggish non streaming ice during full glacial conditions thin tills at the centres of the fast flow corridors in many places unconformably overlying stratified sediments suggest that widespread till deformation may have been subordinate to basal sliding in driving fast ice flow but the general thickening of tills towards the lobate terminal margins of ice streams outlet glaciers is consistent with subglacial deformation theory in this area of relatively low relief we speculate that fast glacier flow or streaming was highly dynamic and transitory sometimes with fast flowing trunks topographically fixed in their onset zones and with the terminus migrating laterally the occurrence of minor push moraines and flutings and associated landforms because of their similarity to modern active temperate glacial landsystems are interpreted as indicative of ice lobe marginal oscillations possibly in response to seasonal climatic forcing in locations where meltwater was more effectively drained from the glacier bed further north the occurrence of surging glacier landsystems suggests that persistent fast glacier flow gave way to more transitory surging possibly in response to the decreasing size of ice reservoir areas in dispersal centres and also locally facilitated by ice bed decoupling and drawdown initiated by the development of ice dammed lakes copyright 2008 john wiley sons ltd

subglacial seismic reflection strategies when source amplitude and medium attenuation are poorly known seismic reflection techniques are a powerful way to probe physical properties of subglacial strata inversion of seismic data for physical properties may be hampered however by lack of knowledge of the source amplitude as well as lack of knowledge of the compressional and shear attenuation in the ice new methods are described to measure the source signature that require no a priori knowledge of the ice attenuation profile another new method is described to obtain the angular dependence of the subglacial bed reflection coefficient that is relatively insensitive to knowledge of the ice attenuation finally a correction is provided to a long standing error in the literature regarding measurement of the bed normal incidence reflection coefficient

archaeological evidence for a possible first century ad earthquake in the necropolis of abakainon ne sicily abstract the greek necropolis of abakainon ne sicily southern italy was destroyed suddenly some time after the 2nd century bc in order to identify the destruction cause of the necropolis we analyzed geological geomorphological data and site stratigraphy evidence on the site suggests that the observed collapse and deformation may be consistent with a seismic event optically stimulated luminescence dating constrains the age of the collapse to the 1st century ad probably during the time span 14 37 ad when other nearby sites were also damaged by an earthquake as reported by historical sources this study contributes to a better characterization of earthquakes in the area during the first millennium ad and improves knowledge on historical seismicity in ne sicily

wisconsinan and pre wisconsinan ice thicknesses on ellesmere island canada inferences from ice cores study of two cores through an ice cap on northern ellesmere island suggests 1 during the wisconsin glaciation the ice near the crest was never more than about 200 m thicker than at present 2 in a preceding glaciation the ice was thicker than this and the local ice divide near the boreholes was eliminated 3 early in the sangamon interglacial basal melting occurred in a deep bedrock valley south of the borehole site and 4 the change in 18o at the wisconsinan holocene transition is 11 of which at least 8 results from climatic warming

autochthonous block fields in southern norway implications for the geometry thickness and isostatic loading of the late weichselian scandinavian ice sheet the consistent geographical and altitudinal distribution of autochthonous block fields mantle of bedrock weathered in situ and trimlines in southern norway suggests a multi domed and asymmetric late weichselian ice sheet low gradient ice sheet profiles in the southern baltic region in the north sea and along the outer fjord areas of southern norway are best explained by movement of ice on a bed of deforming sediment although water lubricated sliding or a combination of the two may not be excluded the ice thickness distribution of the late weichselian scandinavian ice sheet is not in correspondence with the modern uplift pattern of fennoscandia early holocene crustal rebound was apparently determined by an exponential glacio isostatic rise later however crustal movements appear to have been dominated by large scale tectonic uplift of the fennoscandian shield centred on the gulf of bothnia the region of maximum lithosphere thickness

dynamics of the north american ice sheet complex during its inception and build up to the last glacial maximum the north american ice sheet complex played a major role in global sea level fluctuations during the late quaternary but our knowledge of its dynamics is based mostly on its demise from the last glacial maximum lgm a period characterised by non linear behaviour in the form of punctuated ice margin recession episodic ice streaming and major shifts in the location of ice divides in comparison knowledge of the pre lgm ice complex is poorly constrained largely because of the fragmentary nature of the evidence relating to ice sheet build up in this paper we explore the inception and growth of ice 120 20 ka using a glacial systems model which has been calibrated against a large and diverse set of data relating to the deglacial interval we make use of calibration data prior to the lgm but its scarcity introduces greater uncertainty which is partly alleviated by our large ensemble analysis results suggest that following the last interglaciation oxygen isotope stage ois 5e the ice complex initiated over the north eastern canadian arctic and in the cordillera within a few thousand years it then underwent rapid growth to an ois 5 maximum at 110 ka 5d and covered 70 of the area occupied by the lgm ice cover although only 30 by volume an ois 5 minimum is modelled at 80 ka 5a before a second phase of rapid growth at the start of ois 4 which culminated in a large ice complex at 65 ka almost as large as at the lgm subsequent deglaciation was rapid maximum modelled sea level contribution of 16 cm per century and resulted in an ois 3 minimum between ca 55 60 ka thereafter the ice complex grew towards its lgm configuration interrupted by several phases of successively less significant mass loss our results support and extend previous inferences based on geological evidence and reinforce the notion of a highly dynamic pre lgm ice complex e g with episodes of 10 s m of eustatic sea level equivalent in 50 but even the thin ois 5 ice sheets exhibit fast flow features several 1000 m a 1 in major topographic troughs notwithstanding the severe limitations imposed by the use of the shallow ice approximation we note that most fast flow features generated prior to the lgm correspond to the location of known ice streams during deglaciation i e in major topographic troughs and over soft sediments at the southern and western margins moreover the modelled flux of these ice streams sensu lato appears to be non linearly scaled to ice sheet volume i e there is no evidence that decay phases were associated with significantly increased ice stream activity this hypothesis requires testing using a model with higher order physics and future modelling would also benefit from additional pre lgm constraints e g dated ice free margin positions to help reduce and quantify uncertainties

history of the greenland ice sheet paleoclimatic insights abstract paleoclimatic records show that the greenland ice sheet consistently has lost mass in response to warming and grown in response to cooling such changes have occurred even at times of slow or zero sea level change so changing sea level cannot have been the cause of at least some of the ice sheet changes in contrast there are no documented major ice sheet changes that occurred independent of temperature changes moreover snowfall has increased when the climate warmed but the ice sheet lost mass nonetheless increased accumulation in the ice sheet s center has not been sufficient to counteract increased melting and flow near the edges most documented forcings and ice sheet responses spanned periods of several thousand years but limited data also show rapid response to rapid forcings in particular regions near the ice margin have responded within decades however major changes of central regions of the ice sheet are thought to require centuries to millennia the paleoclimatic record does not yet strongly constrain how rapidly a major shrinkage or nearly complete loss of the ice sheet could occur the evidence suggests nearly total ice sheet loss may result from warming of more than a few degrees above mean 20th century values but this threshold is poorly defined perhaps as little as 2 c or more than 7 c paleoclimatic records are sufficiently sketchy that the ice sheet may have grown temporarily in response to warming or changes may have been induced by factors other than temperature without having been recorded

characterization of heterogeneous near surface materials by joint 2d inversion of dc resistivity and seismic data 1 we have developed a robust 2d joint inversion scheme incorporating the new concept of cross gradients of electrical resistivity and seismic velocity as constraints so as to investigate more precisely the resistivity velocity relationships in complex near surface environments the results of joint inversion of dc resistivity and seismic traveltime data from collocated experiments suggest that one can distinguish between different types or facies of unconsolidated and consolidated materials refining a previously proposed resistivity velocity interrelationship derived from separate inversions of the respective data sets a consistent interpretive structural model can be obtained from the joint inversion models

steady thermomechanical flow along two dimensional flow lines in large grounded ice sheets a thermomechanical flow model has been developed in order to perform simultaneous calculations of the surface elevation and the distribution of stresses velocities and temperatures along two dimensional flow lines on large grounded ice sheets in order to solve the complex system of full thermomechanical coupled equations the following approximations have been made a coordinate scaling giving a lead order reduced model and an iteration procedure which decouples the energy balance equation from the rest of the equations glen s flow law is used as the constitutive relationship between stresses and deformation in the model and it is found that the longitudinal deviatoric stresses have a significant role in the lead order equations at the ice divide the solution shows that the velocity stress and temperature distributions change rapidly here the basal temperature increases creating a hot spot the surface strain rates increase 50 and the horizontal velocity profile is concave near the base with an inflection point away from the ice divide basal temperature increases and in the example presented here the basal temperature reaches the pressure melting point after 2 3 of the lateral extent and basal sliding begins to influence the flow in the ablation region it is seen that the heat produced by internal deformation is of the same order of magnitude as the geothermal heat flux from the bedrock

rupture speed dependence on initial stress profiles insights from glacier and laboratory stick slip a r t i c l e i n f o a b s t r a c t slow slip events are now well established in fault and glacier systems though the processes controlling slow rupture remain poorly understood the whillans ice plain provides a window into these processes through bi daily stick slip seismic events that displace an ice mass over 100 km long with a variety of rupture speeds observed at a single location we compare the glacier events with laboratory experiments that have analogous loading conditions both systems exhibit average rupture velocities that increase systematically with the pre rupture stresses with local rupture velocities exhibiting large variability that correlates well with local interfacial stresses the slip events in both cases are not time predictable but clearly slip predictable local pre stress may control rupture behavior in a range of frictional failure events including earthquakes 2014 the authors published by elsevier b v this is an open access article under the cc by nc nd license http creativecommons org licenses by nc nd 3 0

till beneath ice stream b 1 properties derived from seismic travel times seismic experiments conducted on ice stream b part of the marine ice sheet of west antarctica show a meters thick layer immediately beneath the 1000 m thick ice a seismic experiment consisting of wide angle reflection profiling along a line parallel to ice stream flow was conducted to determine the properties of this layer inversion of seismic travel times yields a compressional wave speed of less than 1700 m s 1 and a shear wave speed less than 160 m s 1 for the layer these very low wave speeds imply that the material in the layer is highly porous and is saturated with water at a high pore pressure based on wave speeds in other saturated unconsolidated sediments we believe that a porosity substantially greater than 0 32 probably around 0 4 and an excess of overburden pressure over pore pressure of only 50 kpa 0 5 bar characterize the layer at this location

the greenland ice sheet margin a mine of ice for paleo environmental studies abstract stable isotope analysis of surface ice samples from three locations on the margin of the greenland ice sheet have provided 18 o records dating back into the wisconsinan the samples were collected in profiles transverse to the ice margin on a location about 40 km northeast of jakobshavn west greenland and at two locations in warming land north greenland the holocene wisconsinan transition as detected by a large aburpt shift was found between 470 and more than 850 m from the ice edge the magnitude of the shift was about 8 and 12 in the west and north greenland records respectively the west greenland record which is the more detailed documents that some variations in the wisconsinan are apparently preserved in the ice margin isotopic records by linking locations in the accumulation and ablation zones with the same observed value the surface ice of the ablation zone can be dated back to at least the wisconsinan holocene transition if the velocity of the ice motion is measured or estimated by ice flow modelling this means that dated records of ice constituents gas composition chemistry content of microparticles micro meteorites etc can be established by mining the easily accessible surface ice of the ablation zone

deformation of till beneath ice stream b west antarctica the behaviour and possible instability of the west antarctic ice sheet depend fundamentally on the dynamics of the large ice streams which drain it model calculations show that most ice stream velocity arises at the bed1 2 and radar sounding has shown the bed to be wet3 but the basal boundary condition is not well understood seismic evidence from the upstream b camp upb on the siple coast of west antarctica4 shows that the ice stream there rests on a layer of unconsolidated sediment averaging 5 or 6 m thick in which the water pressure is only 50 kpa less than the overburden pressure because this thin layer occurs well inland beneath an active ice sheet and rests on a surface showing flutes4 characteristic of glacial erosion5 we presume that it is glacial till we propose here that deformation within the till is the primary mechanism by which the ice stream moves and we discuss implications of this hypothesis

study of streamers in gradient density air table top modeling of red sprites red sprites are large scale weakly ionized non equilibrium electrical discharges that occur high above thunderstorm clouds spanning the altitude range 50 kilometers to 90 kilometers above the earth s surface their streamer like nature has been pointed out by a number of groups streamer models used for the description of sprites are usually verified experimentally however sprites develop in a highly non uniform air where density changes by a factor of 2 7 every 7 2 km whereas streamer studies have been performed at different but uniform densities in this paper we present the results of the first attempt to simulate sprites in laboratory by using streamer discharges in a gradient density air the results of this paper were presented in 2009 fall agu meeting the purpose of the experiments is to obtain data that could be used for validation of numerical and analytical models the first results of a numerical study of red sprites streamers in a gradient density atmosphere were recently published by luque and ebert 2010

the influence of permeability on compressional wave velocity in marine sediments to investigate the effect of permeability on the propagation of seismo acoustic waves through marine sediments a theoretical model based on biot s equations is established which relates the compressional wave velocity measured at a fixed frequency to computed velocities at zero and infinite frequencies in terms of sediment porosity and permeability the model is examined experimentally in a standard soil mechanics consolidation test itself dependent among other things on sediment porosity and permeability which has been modified to include measurements of compressional wave velocity at l mhz and shear wave velocity at 5 khz this test allows the elastic modulus of the sediment frame to be assessed under different load conditions simultaneous with the velocity determinations from a number of tests on different samples five samples are chosen to typify the range of sediment sizes the results show that the difference between the measured velocity at l mhz and the model derived velocity at zero frequency increases with increasing particle size from clays to fine sand with decreasing porosity and with increasing permeability for sediments coarser than fine sand the simple model breaks down possibly because of the dominance of scattering diffraction effects at the high frequency of the experiment within this limitation themore model seems satisfactory to offer a capability of predicting the permeability of a sea floor sediment to an order of magnitude by the in situ measurement of seismic velocities over a wide range of frequencies the prediction process requires a good in situ determination of sediment porosity such as that offered by electrical formation factor measurements less

basal flow conditions at the northeastern margin of the laurentide ice sheet lancaster sound a major moraine system developed by outlet glaciers of the laurentide ice sheet on coastal mountains of bylot island provides a basis for reconstructing paleoglacier surfaces and estimating basal shear stresses within the ice the system named eclipse moraine lies between elevations of 100 and 500 m asl and can be traced almost continuously over distances of 50 70 km where the glaciers flowed inland within valleys the outline of the moraine on valley walls is used to portray the profile of the glacier along the valley median if ideal plastic flow is assumed ice thicknesses and surface slopes derived from the reconstructed profiles indicate yield stresses of about 125 46 kpa reconstructed profiles of the main outlet glaciers entering baffin bay from lancaster sound and pond inlet indicate lesser yield stresses of about 55 80 kpa respectively within the channels estimates are 35 60 kpa if isostatic depression is assumed calculations indicate mat basal shear stresses in the channels were near th

multidisciplinary study of the tindari fault sicily italy separating ongoing contractional and extensional compartments along the active africa eurasia convergent boundary abstract the africa eurasia convergence in sicily and southern calabria is currently expressed by two different tectonic and geodynamic domains the western region governed by a roughly n s compression generated by a continental collision the eastern one controlled by a nw se extension related to the south east directed expansion of the calabro peloritan arc the different deformation pattern of these two domains is accommodated by a right lateral shear zone aeolian tindari letojanni fault system which from the ionian sea north of mt etna extends across the peloritani chain to the aeolian islands in this work we study the evidence of active tectonics characterizing this shear zone through the analysis of seismic and geodetic data acquired by the ingv networks in the last 15 years the study is completed by structural and morphological surveys carried out between capo tindari and the watershed of the chain the results allowed defining a clear structural picture depicting the tectonic interferences between the two different geodynamic domains the results indicate that besides the regional n130 e horizontal extensional stress field another one ne sw oriented is active in the investigated area both tension axes are mutually independent and have been active up to the present at different times the coexistence of these different active horizontal extensions is the result of complex interactions between several induced stresses 1 the regional extension nw se related to the slab rollback and back arc extension 2 the strong uplift of the chain 3 the accommodation between compressional and extensional tectonic regimes along the aeolian tindari letojanni faults through a sse nnw right lateral transtensional displacement in these conditions the greater and recurring uplift activity is not able to induce a radial extensional dynamics but under the directing action of the shear system it can only act on the regional extension nw se and produce the second system of extension ne sw

the supply of subglacial meltwater to the grounding line of the siple coast west antarctica recent satellite studies have shown that active subglacial lakes exist under the antarctic ice streams and persist almost to their grounding lines when the lowest lying lakes flood the water crosses the grounding line and enters the sub ice shelf cavity modeling results suggest that this additional freshwater influx may significantly enhance melting at the ice shelf base we examine the spatial and temporal variability in subglacial water supply to the grounding lines of the siple coast ice streams by combining estimates for lake volume change derived from ice cloud and land elevation satellite icesat data with a model for subglacial water transport our results suggest that subglacial outflow tends to concentrate towards six embayments in the siple coast grounding line although mean grounding line outflow is 60m3 s 1 for the entire siple coast maximum local grounding line outflow may temporarily exceed 300 m3 s 1 during the synchronized flooding of multiple lakes in a hydrologic basin variability in subglacial outflow due to subglacial lake drainage may account for a substantial portion of the observed variability in freshwater flux out of the ross ice shelf cavity the temporal variability in grounding line outflow results in a net reduction in long term average melt rate but temporary peak melting rates may exceed the long term average by a factor of three

evidence of seismic deformation of the paved floor of the decumanus at tindari ne sicily summary most of the ancient town of tindari ne sicily was settled on a plateau the most surficial layer of which was made of unconsolidated material ongoing excavations at the archaeological site at tindari uncovered a large portion of the decumanus which suffered deformations preliminarily assigned to coseismic effects an analysis of the local dynamic response through the simulation of strong seismic shaking to the bedrock and modelling of spectral ratios of the bedrock soft soil was carried out to verify the susceptibility of superficial terrains of the promontory to coseismic deformations to perform this simulation the finite element method fem was used four accelerometric recordings of three earthquakes of medium high magnitude recorded on rocky sites were chosen to simulate the seismic shaking using a constitutive law for the materials composing the promontory layers both of linear elastic type and of elastoplastic type the analysis of the linear elastic field allowed the definition of the frequencies for which the spectral ratios of the accelerations recorded the highest amplifications in particular the frequency range 31 5 37 2 hz can be combined with deformation of the paved floor of the decumanus the analysis in the elastoplastic field highlighted the zones of promontory more susceptible to suffer plasticization process the results show that the topmost layer of the decumanus is the most susceptible to suffer plasticization therefore the performed analysis lends greater support to the hypothesis that the deformations were produced by seismic shaking

unusual rates and patterns of holocene emergence ellesmere island arctic canada substantial postglacial emergence up to 150 m characterizes the canadian high arctic following the last glacial maximum across northern ellesmere island rapid emergence is not observed during the early stages of deglaciation as it is in other parts of arctic canada rather the curves show an interval of slow emergence c 1 m per century during initial ice retreat this is attributed to a slow rate of glacial rebound that was countered by a similar rate of eustatic sea level rise normally isobases drawn on postglacial shorelines define broad cells of uplift that mimic centres of former maximum ice thickness however isobases on the 8 ka bp shoreline surveyed throughout northern ellesmere island indicate more complex patterns of crustal uplift that seem in congruent with reported ice thicknesses on west central ellesmere island the isobases 110 150 m asl trace a plunging ridge aligned with geological structure across an area considered to have had minimal ice cover during the last glacial maximum the ridge may record an important structural influence associated with the geologically youthful sverdrup basin collectively this evidence cautions against the view that postglacial emergence has a universally predictable signature solely portraying former ice loads

tectonics and seismicity of the tindari fault system southern italy crustal deformations at the transition between ongoing contractional and extensional domains located above the edge of a subducting slab 1 the tindari fault system southern tyrrhenian sea italy is a regional zone of brittle deformation located at the transition between ongoing contractional and extensional crustal compartments and lying above the western edge of a narrow subducting slab onshore structural data an offshore seismic reflection profile and earthquake data are analyzed to constrain the present geometry of the tindari fault system and its tectonic evolution since neogene including the present seismicity results show that this zone of deformation consists of a broad nnw trending system of faults including sets of right lateral left lateral and extensional faults as well as early strike slip faults reworked under late extension earthquakes and other neotectonic data provide evidence that the tindari fault system is still active in the central and northern sectors and mostly accommodates extensional or right lateral transtensional displacements on a diffuse array of faults from these data a multiphase tectonic history is inferred including an early phase as a right lateral strike slip fault and a late extensional reworking under the influence of the subduction related processes which have led to the formation of the tyrrhenian back arc basin within the present regional geodynamic context the tindari fault system is interpreted as an ongoing accommodation zone between the adjacent contractional and extensional crustal compartments these tectonic compartments relating to the complex processes of plate convergence occurring in the region the tindari fault system might also be included in an incipient oblique extensional transfer zone linking the ongoing contractional belts in the calabrian ionian and southern tyrrhenian compartments

telescopic imaging of sprites telescopic images of sprites show a wide variety of generally vertical but also slanted fine structure including branching tree like shapes and well defined but isolated columns with transverse spatial scales ranging from tens of meters to a few hundred meters at 60 85 km altitude simultaneous analysis of radio atmospheric and lightning data indicates that specific columnar regions are selectively excited by successive discharges

antarctic ice streams a review an ice stream is a part of an inland ice sheet that flows rapidly through the surrounding ice the distinction between an ice sheet and an outlet glacier is clear in principle but muddy in practice many antarctic glaciers are intermediate in character the ross ice streams which flow through the west antarctic inland ice into the ross ice shelf are distinct in character differing even from other ice streams in the marine ice sheet of west antarctica their surface elevation profiles are low their bed slopes are low and smooth and their driving stresses diminish monotonically downglacier in transverse profile they are broader in relation to ice thickness and exhibit shallower subglacial troughs than other ice streams many models for the fast sliding of glaciers have been applied to the ross ice streams most have included in some form a reduction in basal drag resulting from a lesser effective than glaciostatic pressure at the bed most models are only semi physical at best so need to be tuned by fitting to assumed parameters at some place or places in the real ice sheet the recent discovery of a very small effective pressure beneath one ice stream consequently has led to some gross errors in the velocities predicted by the models the difficulty may be resolved if it is true as recent experiments suggest that ice stream b and by extrapolation other ross ice streams as well slide on a deforming bed that absorbs most or all of the differential motion between the ice and the bedrock