a vertically integrated treatment of ice stream and ice shelf thermodynamics the extremely small vertical shear in ice stream and ice shelf flow simplifies the equations which govern their thermodynamic evolution complemented by the widely used shallow shelf approximation used to simplify the ice flow momentum balance a vertically integrated formulation of heat transfer presented here reduces the dimensionality of the thermodynamic problem from three to two plan view dimensions and thus significantly reduces the computational cost of treating ice stream and ice shelf thermodynamics in models for realistic conditions errors in ice stiffness parameter ice thickness and speed caused by the vertically integrated treatment of heat transfer are less than 5 of magnitudes of these values compared to the standard three dimensional thermomechanical computations in addition for the specific case of ice shelves with strong bottom melting the governing equation describing evolution of the vertically integrated ice stiffness parameter is derived which further reduces computational cost the presented error analysis and formulations of ice stream and ice shelf thermodynamics in terms of the vertically integrated temperature allow the thermodynamic effects on ice deformation to be easily incorporated into studies that traditionally disregard them

antarctic ice sheet melting in the southeast pacific the first oceanographic measurements across a deep channel beneath the calving front of pine island glacier reveal a sub ice circulation driven by basal melting of 10 12 m yr 1 a salt box model described here gives a melt rate similar to that of ice balance and numerical models 5 50 times higher than averages for the george vi and ross ice shelves melting is fueled by relatively warm circumpolar deep water that floods the deep floor of the amundsen and bellingshausen sea continental shelves reaching the deep draft of this floating glacier a revised melt rate for ice shelves in the southeast pacific sector raises circumpolar ice shelf melting to 756 gt yr 1 given prior estimates of surface accumulation and iceberg calving this suggests that the antarctic ice sheet is currently losing mass to the ocean

modeling the formation and deposition of frazil ice beneath filchner ronne ice shelf large areas of the filchner ronne ice shelf are underlain by layers of marine ice which form when supercooled seawater circulating beneath the ice shelf freezes the freezing process initially produces a suspension of disc shaped frazil ice crystals and these are subs quently deposited onto the ice shelf base in areas where the flow of water is slack enough this has been modeled assuming that the freezing takes place within buoyant plumes of ice shelf water ascending the ice shelf base from source regions near the grounding lines of the major inlet glaciers the deposition of the majority of the suspended frazil ice is found to occur in spatially discrete bursts where peak rates of accumulation at the ice shelf base exceed 1 m yr 1 of solid ice there is a good correlation between the location of the zones of crystal deposition and the position of the upstream limits of the marine ice layers the high rates of localized accumulation account for the rapid buildup observed in the layer thickness which then gradually declines as the marine ice is carried downstream with the flow of the ice shelf model results also suggest an origin for the ice platelets observed at depth in the water column near the filchner ice shelf

shear margins in glaciers and ice sheets analytical and numerical techniques are used to examine the flow response of a sloped slab of power law fluid power n subjected to basal boundary conditions that vary spatially across the flow direction as for example near an ice stream margin with planar basal topography the primary assumption is that basal shear stress is proportional to the basal speed time a spatially variable slip resistance the ratio of mean basal speed to the speed originating from shearing through the thickness denoted as r gives a measure of how slippery the bed is the principal conclusion is that a localized disturbance in slip resistance affects the basal stress and speed in a zone spread over a greater width of the flow in units of ice thickness h the spatial scale of spreading is proportional to a single dimensionless number r n r n 1 1 n 1 derived from n and r the consequence for a shear zone above a sharp jump in slip resistance is that the shearing is spread out over a boundary layer with a width proportional to r n for an ice stream caused by a band of low slip resistance with a half width of wh the margins influence velocity and stress in the central part of the band depending on r n in comparison to w three regimes can be identified which for n 3 are quantified as follows low r defined as r 3 1w for which the boundary layers from both sides bridge across the full flow width and the driving stress in the center is supported almost entirely by side drag intermediate r for which the driving stress in the center is supported by a combination of basal and side drag shear zones that are narrower than predicted on the basis of this theory r 3 would require localized softening of the ice to explain the concentration of deformation at a shorter scale

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

carbonate platforms of passive extensional continental margins types characteristics and evolution abstract carbonate platforms of extensional margins may be grouped into several major categories homoclinal ramps have gentle slopes into deep water may have skeletal or ooid pellet sand shoal complexes that grade without break in slope into deep ramp nodular limestone and then into pelagic hemipelagic basin facies they generally lack significant slump and sediment gravity flow deposits in the deeper water facies distally steepened ramps differ from the above in having a marked increase in slope at the seaward edge of the deep ramp and abundant slumps slope breccias and turbidites however clasts of shallow platform margin facies are generally absent from breccias rimmed shelves have linear trends of shelf edge lime sands and reefs a marked increase in slope into deep water and foreslope and slope sands breccias with clasts of platform margin rocks and turbidites grading seaward into basin hemipelagic pelagic muds they may be divided into accretionary bypass and erosional margins isolated platforms are broad flat topped shallow platforms surrounded by deeper water few hundred meters to 4 km deep many are bypass margins but accretionary and erosional margins also occur finally incipiently to completely drowned or open platforms may develop by rapid submergence of ramps shelves or isolated platforms shallow platform margin facies are shifted landward and the earlier shallow water platform is covered with transgressive lags and deeper water blankets of hemipelagic or pelagic facies or open marine whole fossil wackestone the carbonate prisms commonly develop above a basal clastic phase ramps develop early and evolve into rimmed shelves however with drowning rimmed shelves may evolve up into ramps rimmed shelves may also develop into ramps where miogeoclinal faces prograde into filling marginal basins carbonate miogeocline deposition terminates when the shelf becomes a prograding clastic shelf or where it becomes part of a collisional orogen the various platform types may be recognized from continental margin sequences ranging from proterozoic to holocene in age

topographically mediated ice stream subglacial drainage networks satellite laser altimetry reveals short timescale changes in antarctic ice sheet surface elevation that are suggested to be driven by subglacial water transport and storage here details of the interaction between the dynamics of ice stream flow subglacial water system and bed elevation relief are examined in the context of idealized heterogeneous bed geometries using a two way coupled model of ice and subglacial water flow we show that basal topography controls the temporal and spatial variability of the sub ice stream hydraulic system the orientation and characteristic dimensions of the topographic undulations determine the morphology connected subglacial ponds or channel like subglacial water features and timescales of the sub ice stream drainage system the short term several years to decades variability of the simulated coupled ice stream subglacial water system suggests that the short term surface variations detected in remote sensing observations may be indicative of a rapidly evolving subglacial water system our simulations also show that interaction between ice flow and the highly dynamic subglacial water system has a strong effect on effective stress in the ice large effective stress magnitudes arise over areas where the basal traction is characterized by strong spatial gradients that is transitions from high to low basal traction or vise versa these transitions migrate on multiyear timescales and thus cause large effective stress variability on the same temporal scales

physical conditions at the base of a fast moving antarctic ice stream boreholes drilled to the bottom of ice stream b in the west antarctic ice sheet reveal that the base of the ice stream is at the melting point and the basal water pressure is within about 1 6 bars of the ice overburden pressure these conditions allow the rapid ice streaming motion to occur by basal sliding or by shear deformation of unconsolidated sediments that underlie the ice in a layer at least 2 meters thick the mechanics of ice streaming plays a role in the response of the ice sheet to climatic change

on radiating waves generated from barotropic shear instability of a western boundary current abstract results from barotropic stability analyses of strong westerly jets generally tend to conclude that those waves that can extract energy from the shear of the jet are largely trapped by the jet therefore the available shear energy of the flow cannot be transmitted by propagating waves away from the strong zone we show here that the situation is different if the jet is in the meridional instead of the zonal direction in this case unstable waves are generated that are able to propagate energy eastward even in the presence of a realistic dissipation it is then possible for the shear energy of a western boundary current to be transported large distances into the interior of the ocean

ice stream basal conditions from block wise surface data inversion and simple regression models of ice stream flow application to bindschadler ice stream 1 widespread basal conditions controlling ice stream flows are still beyond the scope of direct observation thus knowledge of their magnitudes and variabilities comes from inversion of surface measurements ice velocities surface elevations and thicknesses we present a new approach to implement a widely accepted inverse method on regular 10 10 km blocks smaller than the whole domain to enhance the spatial resolution of calculated basal conditions inverted basal friction coefficients and calculated shear stress have sharp transitions and large variations in small areas overall the obtained basal shear stress is very small in regions of fast flowing ice the results of the inversion along with the surface variables are used to construct two simple regression models of bindschadler ice stream former ice stream d that reproduce 96 of observed velocity variations while highly idealized these regression models are sufficiently informative to be considered as parameterizations for ice stream flow in large scale ice sheet models which lack the spatial and temporal resolution necessary to simulate ice stream dynamics in detail

carbonate apron models alternatives to the submarine fan model for paleoenvironmental analysis and hydrocarbon exploration sediment gravity flow deposition along the deep water flanks of carbonate platforms typically does not produce submarine fans rather wedge shaped carbonate aprons develop parallel to the adjacent shelf slope break the major difference between submarine fans and carbonate aprons is a point source with channelized sedimentation on fans versus a line source with sheet flow sedimentation on aprons two types of carbonate aprons may develop along relatively gentle 4 platform margin slopes aprons form immediately adjacent to the shallow water platform and are referred to as carbonate slope aprons along relatively steep 4 15 platform margin slopes redeposited limestones accumulate in a base of slope setting by passing an upper slope via a multitude of small submarine canyons and are referred to as carbonate base of slope aprons both apron types are further subdivided into inner and outer facies belts inner apron sediments consist of thick mud supported conglomerates and megabreccias facies f as well as thick coarse grained turbidites facies a interbedded with subordinate amounts of fine grained peri platform ooze facies g outer apron sediments consist of thinner grain supported conglomerates and turbidites facies a as well as classical turbidites facies c with recognizable bouma divisions interbedded with approximately equal proportions of peri platform ooze facies g seaward aprons grade laterally into basinal facies of thin base cut out carbonate turbidites facies d that are subordinate to peri platform oozes facies g carbonate base of slope aprons grade shelfward into an upper slope facies of fine grained peri platform ooze facies g cut by numerous small canyons that are filled with coarse debris as well as intraformational truncation surfaces which result from submarine sliding in contrast slope aprons grade shelfward immediately into shoal water platform margin facies without an intervening by pass slope the two carbonate apron models presented here offer alternatives to the submarine fan model for paleoenvironmental analysis and hydrocarbon exploration for mass transported carbonate facies

sedimentary and tectonic evolution of the eastern hyblean plateau southeastern sicily during late cretaceous to quaternary time abstract two palaeogeographic domains have been distinguished in the cretaceous quaternary sequences which crop out in the hyblean region of southeastern sicily in the northeast shallow water algal bryozoan calcarenites of oligocene miocene age overlie either cretaceous rudist limestones or volcanics further to the northeast the stratigraphic gap increases as the miocene limestone thins and in the a g i p well catania 10 plio pleistocene sediments and volcanics directly overlie upper jurassic rocks in the southwestern part of the hyblean plateau pelagic sediments occupy the cretaceous miocene interval they were deposited in a shallow basin dipping gently towards the southwest the transition between the two zones is marked by much evidence of synsedimentary tectonic activity such as widespread slumping and the development of megabreccias composed of cretaceous and eocene blocks the palaeogeographic picture is one of carbonate reefs and volcanic seamounts developing in the upper cretaceous they formed platforms fault bounded to the west and southwest from which were derived the detrital sediments found in the basin and occasional spectacular gravity slides in present day terms the structural high probably corresponds to the northeastern margin of the hyblean plateau and in part to the siracusa syracuse malta escarpment the pattern is little changed in the upper miocene with volcanoclastics of phreatomagmatic origin intercalated with coral bioherms overlying algal bryozoan calcarenites upper tortonian lower messinian limestones with a restricted fauna indicate a shallow water environment which grades up into continental conditions in the eastern hyblean plateau the tertiary sequence is capped by pliocene submarine to subaerial mafic lava flows lower pleistocene sediments consist of biocalcarenites and clays which fill deep graben delimited by plio pleistocene faults the inshore continuation of the malta syracuse escarpment system of faults from a tectonic standpoint the hyblean region a gently deformed segment of the african continental margin acted as a foreland during tertiary times a cretaceous eocene tectonic phase was responsible for the palaeogeographic pattern which existed during most of the cenozoic the present pattern is the result of three main tectonic phases following the regional upper miocene uplift of early middle pliocene plio pleistocene and middle pleistocene recent age which influenced the tectonic evolution of the present ionian coast and the malta syracuse escarpment

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

optical televiewer based identification and characterization of material facies associated with an antarctic ice shelf rift we have drilled 13 boreholes within and around a through cutting rift on the unofficially named roi baudouin ice shelf east antarctica logging by optical televiewer optv combined with core inspection has resulted in the identification and characterization of several material facies outside the rift optv imaged annual layering indicates 150 years of accumulation over the 66 m length of one of the boreholes luminosity analysis of this image also reveals the presence of numerous dark melt layers as well as a systematic decrease in background luminosity interpreted in terms of a progressive increase in light transmission during firnification we identify four material facies within the rift snow granular ice marine ice and unconsolidated platelets we interpret the granular ice facies as snow that has been saturated by percolating sea water and the underlying marine ice as compacted buoyant platelets that have adhered to the rift base core sections reveal the presence of tubular channels within the marine ice indicating that it is macroporous and permeable to sea water the lower boundary of this facies merges into a mushy layer of unconsolidated platelets that were successfully imaged by optv revealing irregular sub horizontal layering similar to that reported previously on the basis of directional borehole video

modeling the effects of frazil ice crystals on the dynamics and thermodynamics of ice shelf water plumes the seawater that comes into contact with the base of a floating ice shelf is modified as a result of the phase changes that occur melting is prevalent in the deepest parts of the subice cavity and this drives a buoyant flow of ice shelf water isw along the sloping ice shelf base the ascent of the isw toward the surface of the ocean causes supercooling because the freezing point rises with the falling pressure and this induces a change from melting to freezing assuming that seed crystals exist the isw now fulfills the condition for the rapid growth of disc shaped frazil ice crystals which may subsequently settle upward out of suspension under the action of gravity a simple numerical model of these processes has been developed based on the theory of inclined plumes the isw is treated as a turbulent particle laden gravity current ascending a reactive boundary and containing a suspended crystal load which evolves in response to the supercooling of the water and the inverted sedimentation of the crystals the frazil ice has two important effects on the behavior of the isw plume because the generation of crystals through out the plume provides such a large area over which phase changes can occur the conversion of supercooling into ice happens much more readily than is possible through turbulent transfer of heat and mass at the ice shelf base a suspended crystal load reduces the bulk density of the isw so that a growing suspension causes the plume to accelerate while deposition of crystals onto the ice shelf effects a deceleration there are positive feedbacks in that an acceleration of the plume induces more rapid crystal growth while a deceleration allows suspended crystals to settle out more easily

late tortonian lower messinian miocene palaeogeography of se sicily information from two new formations of the sorting group abstract a thick volcanoclastic biohermal and lagoonal sequence of strata succeeds the siracusa limestone member monti climiti formation of se sicily this overlying sequence may be divided into two new formations on the basis of lithostratigraphy petrology and palaeontology the older formation here named the carlentini formation is predominantly a thick volcanoclastic sequence but with two intervening carbonate horizons faunas are rare within the volcanic strata but may be abundant and associated with corals and patch reefs within the two carbonate levels the younger formation here named the monte carrubba formation is almost exclusively composed of oolitic grainstones and lagoonal wackestones peritidal carbonates are developed in the northeast of the area faunas are frequently of low diversity euryhaline mollusc associations are dominant although pecten vigolenesis occurs in the lower beds collectively the new formations span the late tortonian lower messinian time interval and may be correlated with the calcare di base of the main sicilian basin to the west the palaeoenvironmental reconstruction suggests that both formations were deposited around the sw margins to a low island or landmass lying to the north of the hyblean region

gravity slide and resedimentation processes in a miocene carbonate ramp hyblean platcau southeastern sicily abstract more than twenty sharp based channel form structures occur in the miocene palazzolo formation carbonates within a distally steepened se sicilian miocene carbonate ramp contemporaneous biota indicate deposition in water depths of 70 150 m fully exposed structures commonly exceed one kilometre along palacostrike their bases are concordant with the host strata but these steepen to 15 in proximity of their margins internal bedding planes generally are concordant with the basal truncation surface the lithologies of the channel fills are similar to the host sediment there is little evidence that these structures are erosional since they contain neither breccias nor turbidites rather they are interpreted as the product of gravitational sliding involving thin detached sheets of consolidated ramp carbonates the resulting irregular topography was draped by redeposited fine grained carbonate sediments once healed and further oversteepened renewed gravity sliding occurred this resulted in a 250 m thick slide dominated succession in a zone 4 km wide located between the outer ramp apron comprised of coralline algal and foraminifera rich sediments and basin margin pelagic marls this phenomenon although unusual in shallow water settings is well known from modern continental slope environments sliding was probably triggered by seismicity in the region associated with two known mesozoic basement faults an alternative hypothesis involving pressure gradient build up along distally steepened ramp zones during storm surges is dismissed as a cause although this mechanism may have facilitated resedimentation of seafloor sediments over the slide zone

parameter sensitivity studies for the ice flow of the ross ice shelf antarctica 1 the diagnostic dynamic thermodynamic ice shelf model finite element shallow shelf approximation code fessacode is applied to the ross ice shelf we simulate the present ice flow which results from the ice thickness distribution the inflow at the grounding line and the surface and bottom temperatures and compare results with measured flow velocities our reference simulation reproduces the general flow pattern and the magnitudes of the flow velocities reasonably well the ice flow is found to be very sensitive to the flow enhancement factor the ice thickness and the ice temperature but robust against inflow velocities from ice streams glaciers and ice rises the ice rises roosevelt island and crary ice rise stabilize the ice shelf by significantly decreasing the flow velocities for the entire ice shelf area the ice shelf is susceptible to global warming in that a 2 c surface warming entails an increase of the flow velocities by a factor 1 25 whereas a 10 c warming leads to an increase by a factor 3 1

basal channels on ice shelves 1 recent surveys of floating ice shelves associated with pine island glacier antarctica and petermann glacier greenland indicate that there are channels incised upward into their bottoms that may serve as the conduits of meltwater outflow from the sub ice shelf cavity the formation of the channels their evolution over time and their impact on ice shelf flow are investigated using a fully coupled ice shelf sub ice shelf ocean model the model simulations suggest that channels may form spontaneously in response to meltwater plume flow initiated at the grounding line if there are relatively high melt rates and if there is transverse to ice flow variability in ice shelf thickness typical channels formed in the simulations have a width of about 1 3 km and a vertical relief of about 100 200 m melt rates and sea water transport in the channels are significantly higher than on the smooth flat ice bottom between the channels the melt channels develop through melting deformation and advection with ice shelf flow simulations suggest that both steady state and cyclic state solutions are possible depending on conditions along the lateral ice shelf boundaries this peculiar dynamics of the system has strong implications on the interpretation of observations the richness of channel morphology and evolution seen in this study suggests that further observations and theoretical analysis are imperative for understanding ice shelf behavior in warm oceanic conditions

phosphorites hardgrounds and syndepositional solution subsidence a palaeoenvironmental model from the miocene of the maltese islands abstract two major phosphorite conglomerate horizons occur throughout the central mediterranean maltese islands within the lower middle miocene globigerina limestone formation the lower conglomerate lies directly upon a hardground and both conglomerates include lithoclasts of hardground material variation in clast size and bed thickness indicates a source area to the west and north of the present day islands of malta and gozo particularly thick phosphorite conglomerates occur in solution subsidence structures in western gozo being developed as a result of synsedimentary seafloor collapse these structures indicate that phosphorite clasts were deposited during several discrete pulses of turbulence punctuated by periods of non depositional hardground formation or normal marine pelagic sedimentation geochemical analysis of the contained phosphorite clasts within the conglomerates shows that p 2 o 5 generally is concentrated around the outer margins of the pebbles and contributes up to 17 by weight of some samples some 654 580 m 3 and 837 870 m 3 of phosphorite bearing strata are contained within the xlendi and qawra subsidence structures respectively the maltese islands represent part of a miocene carbonate shelf whose margin lay nearby to the west and which was subject to periodic erosion by strong currents transporting phosphorite pebbles eastwards to their present sites these currents further contributed phosphorus to semi autochthonous phosphorite developments located within the present maltese area the maltese phosphorites are related to other phosphorite areas in sicily a major province of miocene phosphogenesis is proposed to account for these occurrences it extends along the western margins of the malta ragusa rise

syndepositional tectonics affecting cenozoic and mesozoic deposition in the malta and se sicily areas central mediterranean and their bearing on mesozoic reservoir development in the n malta offshore region abstract tertiary sedimentation patterns within the maltese islands and se sicily pelagian block of the central mediterranean reflect subtle depositional control by the concurrent evolution of the malta graben lying to the south west and reactivation of regionally important mesozoic fault systems the visible structural style of the maltese islands is the product of malta graben development and commenced some 24 million years ago however north south oriented structures identified from maltese tertiary facies strike patterns can be closely related to reactivated mesozoic faults in se sicily here synsedimentary tectonogenesis is recognized in both mesozoic and cenozoic strata mesozoic synsedimentary faulting was initiated during a former tectonic regime when the pelagian block african plate lay at the southern side of a wide tethys ocean reactivation of these fractures during the tertiary is believed to be related to subsequent african european plate collision the unsuccessful oil search in maltese territorial waters results from the mistaken significance attached to the young malta graben related tectonic pattern of the maltese islands potential traps are more likely to be associated with a cryptic though formerly dominant mesozoic tectonic pattern the hypothesis presented predicts hydrocarbon prospects in horst traps immediately to the north of the malta shelf these can be expected to show many of the characteristics of the gela and ragusa fields

consistent generation of ice streams via thermo viscous instabilities modulated by membrane stresses accurate computation of ice stream location and dynamics is a key aspiration for theoretical glaciology ice sheet models with thermo viscous coupling have been shown to exhibit stream like instabilities using shallow ice approximation mechanics but the location and width of these streams depends on the numerical implementation and are not unique we present results from thermo viscously coupled ice sheet models incorporating membrane stresses spontaneous generation of fast flowing linear features still occurs under certain parameter regimes with computed stream widths between 20 km to 100 km comparable with observations these features are maintained as the grid size is decreased the thermo viscous feedback mechanism that generates ice streams under the shallow ice approximation still operates now selecting a unique stream size computations of thermo viscous ice flows should include membrane stresses when the bed is approximately flat e g parts of antarctica and former ice sheets of the northern hemisphere previous calculations of spontaneous ice stream generation using the shallow ice approximation should be reassessed citation hindmarsh r c a 2009 consistent generation of ice streams via thermo viscous instabilities modulated by membrane stresses geophys res lett 36 l06502 doi 10 1029 2008gl036877

sedimentology and palaeoenvironment of the southeast sicilian tertiary platform carbonates abstract a thick succession of shelf and shelf slope carbonates is developed in southeastern sicily sedimentological aspects of the oligocene to tortonian sequence including distribution and palaeoenvironmental factors are considered a new formation and four new members are proposed in the upper part of the succession the region comprises a western half typically exhibiting the principal deeper water planktonic facies including marls and an eastern area displaying typically inner shelf carbonates and an attenuated succession coralline algal limestones and calcarenites are more typical of this latter region but there are also some volcanic intercalations an attempt is made to relate the hyblean succession to that of the maltese islands which lie at the southern end of this tertiary carbonate platform

thermally driven migration of ice stream shear margins ice stream shear margins are the lateral boundaries of narrow fast flowing bands of ice within an ice sheet we develop a theory for the migration of shear margins over time driven by viscous dissipation of heat within the ice focusing on widening of the ice stream the location of the margin is modelled as a transition from a cold to a temperate ice sheet bed and simultaneously as the transition from no slip to free slip at the same location the temperature field in the ice is affected by intense shear heating as well as by the migration velocity of the margin i e by the widening rate of the ice stream if migration is too fast there is little time for the ice to warm up and the margin remains cold causing the bed to freeze this suppresses widening conversely if the migration speed is too slow the ice in the margin warms up causing the bed on the far side of the cold temperate transition to reach the melting point and migration to speed up using a wiener hopf method we show that for a given far field shear stress geothermal heat flux and ice geometry there is a single migration velocity that balances the two effects and permits widening at a steady rate this velocity increases with the far field lateral shear stress imposed by the ice stream which controls shear heating in the margin our results also indicate that i a region of temperate ice must form in the margin and that ii lateral advection of ice may play a significant role in controlling migration speeds

basal mechanics of ice stream b west antarctica 1 till mechanics data from laboratory geotechnical tests on till recovered from beneath ice stream b west antarctica at the upstream b camp hereinafter the upb till show that failure strength of this till is strongly dependent on effective stress but is practically independent of strain and strain rate these data support use of a coulomb plastic rheology in modeling of ice stream behavior and subglacial till deformation our testing program combined triaxial ring shear and confined uniaxial tests to investigate till strength and compressibility results show that the upb till follows closely coulomb s equation in which shear strength is a linear function of normal effective stress apparent cohesion near zero and internal friction angle equal to 24 till compressibility is best described by a logarithmic function that relates void ratio to normal effective stress in general the behavior of the upb till is consistent with other experimental evidence regarding mechanical behavior of granular materials based on our laboratory results we formulate the compressible coulomb plastic till model in which there are three interrelated primary state variables shear strength void ratio and normal effective stress this model is used in the second part of our study to simulate response of subglacial till to realistic effective stress forcings these simulations demonstrate that the model is capable of reproducing fundamental aspects of subglacial till kinematics 1 occurrence of tilt rate oscillations and negative tilt rates in tiltmeter records and 2 distributed till deformation to depths of 0 1 1 0 m beneath the ice base our laboratory and modeling results substantiate application of the compressible coulomb plastic model in simulations of the motion of ice stream b over its weak till bed

modeling thermodynamic ice ocean interactions at the base of an ice shelf abstract models of ocean circulation beneath ice shelves are driven primarily by the heat and freshwater fluxes that are associated with phase changes at the ice ocean boundary their behavior is therefore closely linked to the mathematical description of the interaction between ice and ocean that is included in the code an hierarchy of formulations that could be used to describe this interaction is presented the main difference between them is the treatment of turbulent transfer within the oceanic boundary layer the computed response to various levels of thermal driving and turbulent agitation in the mixed layer is discussed as is the effect of various treatments of the conductive heat flux into the ice shelf the performance of the different formulations that have been used in models of sub ice shelf circulation is assessed in comparison with observations of the turbulent heat flux beneath sea ice formulations that include an explicit parameterization of the oceanic boundary layer give results that

eocene seismicity in the pyrenees from megaturbidites of the south pyrenean basin spain abstract the triggering mechanism for emplacement of nine calcareous megaturbidites mt in the clastic turbidite eocene basin of the pyrenees is examined the thickness of each mt is 10 to 200 m and their volume may reach 200 km 3 their mobilization is thought to result from seismic activity related to thrust faults along the northern border of the basin we attempt to define the seismic characteristics for a volume of 100 km 3 the dimensions of the slope failures are estimated to be 100 5 10 0 1 0 2 km and related to a fault rupture length of about 50 100 km and liquefaction of basal clayey layers given these two constraints superficial earthquakes of m 7 would be required to initiate landslides

the effects of transverse bed topography variations in ice flow models 1 a suite of ice flow models is subject to a performance test designed to investigate treatment of flow over variable basal topography using transfer functions developed by gudmundsson 2003 and numerical models of various complexity this study demonstrates that a widely used flowband model has strong limitations associated with its underlying assumptions and thus should be applied only in specific geometrical settings its performance can be significantly improved by using the laterally averaged basal topography instead of centerline topography in applications where spatial variability of flow fields is important hybrid ice flow models can be a viable alternative to flowband models in addition analysis of horizontal distributions of the various ice flow characteristics e g surface elevation velocity and horizontal stress components shows that field observations that focus on assessing these parameters only along the centerline of ice flow could be misleading these results also suggest that such spatial variability needs to be taken into account when designing field surveys of ice flow

ice shelf water plume flow beneath filchner ronne ice shelf antarctica 1 a two dimensional plume model is used to study the interaction between filchner ronne ice shelf antarctica and its underlying ocean cavity ice shelf water isw plumes are initiated by the freshwater released from a melting ice shelf and if they rise may become supercooled and deposit marine ice due to the pressure increase in the in situ freezing temperature the aim of this modeling study is to determine the origin of the thick accretions of marine ice at the base of filchner ronne ice shelf and thus improve our understanding of isw flow paths the model domain is defined from measurements of ice shelf draft and from this isw the model is able to predict plumes that exit the cavity in the correct locations the modeled plumes also produce basal freezing rates that account for measured marine ice thicknesses in the western part of ronne ice shelf we find that the freezing rate and plume properties are significantly influenced by the confluence of plumes from different meltwater sources we are less successful in matching observations of marine ice under the rest of filchner ronne ice shelf which we attribute primarily to this model s neglect of circulations in the ocean outside the plume

gordo megabed a possible seismite in a tortonian submarine fan tabernas basin province almeria southeast spain abstract the mainly tortonian sediments of the tabernas basin contain the elements of a well developed small submarine fan synsedimentary unconformities and randomly intercalated megabeds in both basin plain and fan lobe deposits indicate tectonic activity during fan growth one of the megabeds the gordo megabed overlies dislocated and deformed fan lobe and basin plain deposits the megabed with a thickness of up to 40 m has a sheet like geometry and consists of four superimposed sedimentary units unit i is a matrix supported conglomerate and contains reefal limestone debris metamorphic rocks and mainly at its top large slabs of ripped up fan sediments up to 60 m across the irregular surface of the unit is filled in by unit ii a cobble pebble conglomerate this conglomerate is covered by unit iii a sandstone layer of 6 8 m thickness which grades into unit iv a normal graded to massive mudstone the extra basinal content of the megabed is conservatively estimated at 0 9 km3 its total volume at 6 km3 from sedimentary structures and textures it is inferred that unit i represents a deposit from a debris flow while unit ii consists of a complex mixture of deposits of debris flow and turbulent flow and units iii and iv are deposits from a turbulent flow the megabed is interpreted to originate from a single catastrophic collapse along the basin margin because of its exceptional volume the sheet like geometry and the random position within the fan sequence the megabed is referred to as a seismite the depositional history succeeding a tectonic triggering is envisioned as follows the collapse of the basin border evolved through stages of sliding and slumping into a debris flow body transformation of the debris flow produced turbulent flow it is tentatively assumed that the debris flow outraced its turbidite

the effects of basal melting on the present flow of the ross ice shelf antarctica on utilise un modele mixte aux elements finis et differences finies de l ecoulement et du transfert de chaleur avec deux hypotheses fusion nulle partout et fusion equilibrant tout epaississement a grande echelle

on the mechanics of ice stream shear margins we investigate the mechanics of ice stream shear margins based on the assumption that the underlying bed behaves plastically sliding is assumed to occur if a prescribed locally defined yield stress is attained while no sliding is assumed possible if basal shear stress is lower than the yield stress mathematically the ice flow problem takes the form of a contact problem in which the zones of sliding are part of the solution and cannot be prescribed arbitrarily simplistic assumptions about the location of till failure or about mechanical conditions at the bed predict stress singularities at the margins which lead to corresponding singularities in the basal melt rate the ice flow problem is solved using a complex variable method and an associated quasi static thermal problem is also solved using a green s function high stress concentrations which coincide with high rates of strain heating are found on the ice stream side of the margins where basal melting is also greatest our results further indicate that a temperate zone may form over time above the bed in the margins these findings differ from earlier studies based on dif ferent sliding laws suggesting a high sensitivity of margin behaviour to basal conditions

dynamic patterns of ice stream flow in a 3 d higher order ice sheet model with plastic bed and simplified hydrology 1 predicting ice sheet mass balance is challenging because of the complex flow of ice streams to address this issue we have coupled a three dimensional higher order ice sheet model to a basal processes model where subglacial till has a plastic rheology and evolving yield stress the model was tested for its sensitivity to regional water availability first with an assumed undrained bed the ice stream oscillates between active and stagnant phases solely as a result of thermodynamic feedbacks occurring at the ice till interface however the velocity amplitude decreases over time as insufficient basal meltwater causes the ice stream to gradually thicken and enter a slow flowing ice sheet mode second we assume that the till is able to assimilate water from a hypothetical regional hydrological system this leads to significantly different long term behavior as a continuously oscillating ice stream mode is maintained the extra water incorporated in the till leads to higher velocities triggering stronger thermodynamic feedbacks between the ice and till layer results also suggest that fast flowing ice streams may be modulated by till properties as a result of the duration of thermal conditions during the preceding stagnant phase similarly till properties beneath stagnant ice streams are influenced by basal conditions during the preceding fast flow phase our findings support the inference that ice streams are strongly influenced by the presence of a regional hydrological system underscoring the need to accurately describe the coupling between ice dynamics basal conditions and regional subglacial hydrology in ice sheet models

dynamics of the siple coast ice streams west antarctica results from a thermomechanical ice sheet model most of the ice leaving the antarctic ice sheet is discharged to the sea by ice streams an understanding of the dynamics of these ice streams is therefore crucial if we are to predict the future evolution of the ice sheet and its effect on global sea level the behavior of ice streams a to e of the siple coast west antarctica has attracted much attention with evidence of recent stagnation and numerous relict ice flow features here i present results from a three dimensional thermodynamic ice sheet model of the west antarctic ice sheet wais the model generates variability in the flow of the siple coast ice streams reminiscent of that mentioned above this is caused by competition between several preferred ice flow pathways in the area individual ice streams interact through changes in catchment area size a reduction in catchment area reduces the flow of ice entering a stream and can cause stagnation through the associated reduction in deformational heating

textural characteristics and impurity content of meteoric and marine ice in the ronne ice shelf antarctica the texture and physical properties of an ice core recovered to 215 m depth from the ronne ice shelf antarctica have been studied with regard to formation and transformation of the ice at a depth of 152 8 m a sharp discontinuity marks the transition between meteoric ice accumulated from above and marine ice accreted from below as testified by electrolytical conductivity and stable isotope measurements as well as geophysical field surveys automated image analysis of thin sections indicates that the decrease in grain boundary density and the increase in grain cross sectional area with depth is commensurate with though not necessarily caused bythermodynamically driven grain growth down to 120m depth corresponding to a vertical strain of roughly 65 as computed with a simple temperature history particle path model the observed increase of grain boundary density i e a decrease of grain size with age in the marine ice is in part explained by the thermal history of this layer sediment inclusions at the top of the marine ice layer affect the observed grain boundary density profile by inhibiting grain growth and dynamic recrystallization this may allow some conclusions on the role of temperature particulate inclusions stress and strain rate in controlling the grain size evolution of deforming ice supplementing earlier laboratory experiments conducted at much shorter time scales salinities 0 026 parts per thousand brine volumes 0 09 0 2 parts per thousand and solid salt concentrations have been computed from electrolytical conductivity measurements mean of 51 0 x 10 6 s cm 1 for the marine ice an assessment of salt incorporation and desalination rates shows that these low salinities can at present only be explained by a unique densification mechanism of under water ice crystals at the base of the ice shelf

thermal effects on the location of ice stream margins west antarctic ice streams move rapidly over beds that are thawed by heat generated by the motion intervening zones of slow moving ice are probably freezing at their bases where heat generation is absent heat is also generated in the shear zone between the slow and fast moving ice through modeling of coupled heat and mass flow we show that the position of the boundary between freezing and melting at the bed is unstable for an otherwise morphologically uniform bed above a threshold speed in the ice stream heat generation in the shear zone allows melting beneath the slow ice and possible widening of the stream below this threshold heat generation at the edge of the thawed zone is insufficient to maintain melting leading to freezing and narrowing of the stream the threshold speed appears to be a function of the temperature at the base of slow ice relative to the melting point and is also highly sensitive to the inflow of colder ice from the slow zone for environmental conditions typical of west antarctica the threshold speed is about 100 m yr 1 in the absence of inflow and increases to typical ice stream speeds 500 m yr 1 for inflow speeds of a few meters per year thus the ice streams can be in a delicate thermal balance with the potential of melting outward or freezing inward rapidly moving ice streams could expand unless blocked by bed properties that would prevent fast motion even when there is melting at the bed

anatomy of a modern open ocean carbonate slope northern little bahama bank the open ocean carbonate slope north of little bahama bank consists of a relatively steep 4 upper slope between water depths of 200 and 900 m and a more gentle 1 2 lower slope between depths of 900 and 1300 m the upper slope is dissected by numerous small submarine canyons 50 150 m in relief that act as a line source for the downslope transport of coarse grained carbonate debris the lower slope is devoid of any well defined canyons but does contain numerous small 1 5 m hummocks of uncertain origin and numerous larger 5 40 m patchily distributed ahermatypic coral mounds sediments along the upper slope have prograded seaward during the cenozoic as a slope front fill seismic facies of fine grained peri platform ooze surface sediments show lateral gradation of both grain size and carbonate mineralogy with the fine fraction derived largely from the adjacent shallow water platform near surface sedimentary facies along the upper slope display a gradual downslope decrease in the degree of submarine cementation from well lithified hardgrounds to patchily cemented nodular ooze to unlithified peri platform ooze controlled by lateral variations in diagenetic potential and or winnowing by bottom currents submarine cementation stabilizes the upper part of the slope allowing upbuilding of the platform margin and controls the distribution of submarine slides as well as the headward extent of submarine canyons where unlithified sediments are heavily bioturbated and are locally undergoing dolomitization upper slope sediments are also conditioned eustatically resulting in vertical cyclic sequences of diagenetically unstable aragonite and magnesian calcite rich and stable calcite rich carbonates that may explain the well bedded nature of ancient peri platform ooze sequences lower slope sediments have prograded seaward during the cenozoic as a chaotic fill seismic facies of coarse grained carbonate turbidites and debris flow deposits with subordinate amounts of peri platform ooze coarse clasts are internally derived from fine grained upper slope sediments via incipient cementation submarine sliding and the generation of sediment gravity flows gravity flows bypass the upper slope via a multitude of canyons and are deposited along the lower slope as a wedge shaped apron of debris parallel to the adjacent shelf edge consisting of a complex spatial arrangement of localized turbidites and debris flow deposits a proximal apron facies of thick mud supported debris flow deposits plus thick coarse grained ta turbidites grades seaward into a distal apron facies of thinner grain supported debris flow deposits and thinner finer grained ta b turbidites with increasing proportions of peri platform ooze both the geomorphology and sedimentary facies relationships of the carbonate apron north of little bahama bank differ significantly from the classic submarine fan model as such a carbonate apron model offers an alternative to the fan model for palaeoenvironmental analysis of ancient open ocean carbonate slope sequences

modelling west antarctic ice sheet growth and collapse through the past five million years changes in earth s orbit are known to influence climate shifts from cold glacials to warm interglacials how the vast west antarctic ice sheet responds to these fluctuations is uncertain but because its collapse could raise sea levels by about 5 metres of great interest naish et al have analysed the and 1b ocean sediment core extracted from beneath the ross ice shelf as part of the andrill drilling project and find evidence that the ice sheet collapsed periodically during the early pliocene 3 5 million years ago when atmospheric co2 levels were similar to or slightly higher than today s the pattern of collapse suggests an influence of approximately 40 000 year cycles in the tilt of earth s rotational axis obliquity also in this issue of nature in a numerical modelling study focused on the past 5 million years in antarctica david pollard and robert deconto combine ice sheet land supported and ice shelf water supported modelling approaches to simulate the movement of the grounding line the border between land and sea ice their results show that over the past 5 million years the west antarctic ice sheet transitioned between full intermediate and collapsed states in just a few thousand years this means that the ice sheet is likely to disintegrate if ocean temperatures in the area rise by 5 c if the west antarctic ice sheet wais melted sea levels would rise by about 5 m such changes are thought to have occurred in the past but could not be simulated by models pollard and deconto combine ice sheet with ice shelf modelling and show that over the past 5 million years the wais transitioned among full intermediate and collapsed states in only a few thousand years suggesting possible disintegration of the wais if ocean temperatures in the area rise by 5 c the west antarctic ice sheet wais with ice volume equivalent to 5 m of sea level1 has long been considered capable of past and future catastrophic collapse2 3 4 today the ice sheet is fringed by vulnerable floating ice shelves that buttress the fast flow of inland ice streams grounding lines are several hundred metres below sea level and the bed deepens upstream raising the prospect of runaway retreat3 5 projections of future wais behaviour have been hampered by limited understanding of past variations and their underlying forcing mechanisms6 7 its variation since the last glacial maximum is best known with grounding lines advancing to the continental shelf edges around 15 kyr ago before retreating to near modern locations by 3 kyr ago8 prior collapses during the warmth of the early pliocene epoch9 and some pleistocene interglacials have been suggested indirectly from records of sea level and deep sea core isotopes and by the discovery of open ocean diatoms in subglacial sediments10 until now11 however little direct evidence of such behaviour has been available here we use a combined ice sheet ice shelf model12 capable of high resolution nesting with a new treatment of grounding line dynamics and ice shelf buttressing5 to simulate antarctic ice sheet variations over the past five million years modelled wais variations range from full glacial extents with grounding lines near the continental shelf break intermediate states similar to modern and brief but dramatic retreats leaving only small isolated ice caps on west antarctic islands transitions between glacial intermediate and collapsed states are relatively rapid taking one to several thousand years our simulation is in good agreement with a new sediment record andrill and 1b recovered from the western ross sea11 indicating a long term trend from more frequently collapsed to more glaciated states dominant 40 kyr cyclicity in the pliocene and major retreats at marine isotope stage 31 1 07 myr ago and other super interglacials

sedimentology and palaeoecology of a miocene coralline algal biostrome from the maltese islands abstract a large coralline algal biostrome 20 km 5 km 16 m thick is well exposed in western malta and eastern gozo the biostrome is composed of limestones largely constructed by in situ coralline algae or transported algal debris some sediments are dominated by rhodoliths and others by crustose coralline frameworks six facies are distinguished crustose algal marl crustose algal wackestone rhodolith pavement algal branch packstone crustose pavement and algal crust packstone each facies is characterised by different rhodolith morphologies and coralline crusts which are constructed mainly bymesophyllum commune the coralline morphogenesis is related to the palaeoenvironmental conditions the biostrome is thought to have developed from a series of relic sand ridges to form a low relief platform of moderate depths containing a mosaic of sedimentary facies the sequence is affected by growth faulting which divides the biostrome into eight units

dynamics of thermally induced ice streams simulated with a higher order flow model we use a new discretization technique to solve the higher order thermomechanically coupled equations of glacier evolution we find that under radially symmetric continuum equations small perturbations in symmetry due to the discretization are sufficient to produce the initiation of nonsymmetric thermomechanical instabilities which we interpret as ice streams in good agreement with previous studieswhich have indicated a similar instability we find that the inclusion of membrane stresses regularizes the size of predicted streams eliminating the ill posedness evident in previous investigations of ice stream generation through thermomechanical instability ice streams exhibit strongly irregular periodicity which is influenced by neighboring ice streams and the synoptic state of the ice stream ice streams are not always the same size but instead appear to follow a temperature dependent distribution of widths that is robust to grid refinement the morphology of the predicted ice streams corresponds reasonably well to extant ice streams in physically similar environments

alternative ice shelf equilibria determined by ocean environment 1 dynamic and thermodynamic regimes of ice shelves experiencing weak 1 m year 1 to strong 10 m year 1 basal melting in cold bottom temperature close to the in situ freezing point and warm oceans bottom temperature more than half of a degree warmer than the in situ freezing point are investigated using a 1 d coupled ice ocean model complemented with a newly derived analytic expression for the steady state temperature distribution in ice shelves this expression suggests the existence of a basal thermal boundary layer with thickness inversely proportional to the basal melt rate model simulations show that ice shelves afloat in warm ocean waters have significantly colder internal ice temperatures than those that float in cold waters our results indicate that in steady states the mass balance of ice shelves experiencing strong and weak melting is controlled by different processes in ice shelves with strong melting it is a balance between ice advection and basal melting and in ice shelves with weak melting it is a balance between ice advection and deformation sensitivity simulations show that ice shelves in cold and warm oceans respond differently to increase of the ocean heat content ice shelves in cold waters are more sensitive to warming of the ocean bottom waters while ice shelves in warm waters are more sensitive to shallowing of the depth of the thermocline

a new numerical model of coupled inland ice sheet ice stream and ice shelf flow and its application to the west antarctic ice sheet we have developed a dynamic thermodynamic finite element numerical model that couples inland ice sheet ice stream and ice shelf dynamics this new model stands apart from other whole ice sheet models in its explicit treatment of ice stream flow additionally the model accounts for both horizontal and vertical advection and diffusion of temperature in the flowing ice in present day simulations of the west antarctic ice sheet wais modeled ice velocity agrees well with observed ice flow in particular the model reproduces the pattern of speed variation across ice streams although the continuous downstream speed up of ice flow cannot be reproduced without concurrent downstream variation in basal friction model thermodynamics evaluated qualitatively by model prediction of the spatial distribution of basal melting and quantitatively by comparison with ice temperature measured in boreholes at several locations are sound in particular the model reproduces the broad pattern of frozen bed inter ice stream ridges and melted bed ice streams model initialization for long time simulations is somewhat limited by computation time and by a thermodynamic feedback at sites of large viscous heating that can be a problem in heat balance only model initializations methods for averting those initialization problems are discussed the new model can accommodate a variety of boundary conditions such as various bed rheologies and is well suited to investigate the origin and evolution of wais ice streams within the context of the whole ice sheet system

melting and freezing beneath the ross ice streams antarctica we have estimated temperature gradients and melt rates at the bottom of the ice streams in west antarctica measured velocities were used to include the effects of horizontal advection and strain heating in the temperature model and to determine shear heating at the bed our modeled temperatures agree well with measured temperatures from boreholes in regions of steady flow we find that ice stream tributaries and the inland ice account for about 87 of the total melt generated beneath the ross ice streams and their catchments our estimates indicate that the ice plains of whillans ice stream and ice stream c even when active have large areas subject to basal freezing confirming earlier estimates that import of water from upstream is necessary to sustain motion the relatively low melt rates on whillans ice stream are consistent with observations of deceleration over the last few decades and suggest a shutdown may take place in the future possibly within this century while there are pockets of basal freezing beneath ice streams d and e there are larger areas of basal melt that produce enough melt to more than offset the freezing which is consistent with inferences of relatively steady flow for these ice streams over the last millennium

century scale discharge stagnation and reactivation of the ross ice streams west antarctica 1 flow features on the surface of the ross ice shelf west antarctica record two episodes of ice stream stagnation and reactivation within the last 1000 years we document these events using maps of streaklines emerging from individual ice streams made using visible band imagery together with numerical models of ice shelf flow forward model experiments demonstrate that only a limited set of discharge scenarios could have produced the current streakline configuration according to our analysis whillans ice stream ceased rapid flow about 850 calendar years ago and restarted about 400 years later and macayeal ice stream either stopped or slowed significantly between 800 and 700 years ago restarting about 150 years later until now ice stream scenarios emphasized runaway retreat or stagnation on millennial timescales here we identify a new scenario century scale stagnation and reactivation cycles as well as lateral communication with adjacent ice streams through thickness changes on lightly grounded ice plains this introduces uncertainty into predictions for future sea level withdrawals by the west antarctic ice sheet which are based in part on recent slowing of whillans ice stream and the stagnant condition of kamb ice stream