coupled ice shelf ocean modeling and complex grounding line retreat from a seabed ridge recent observations and modeling work have shown a complex mechanical coupling between antarctica s floating ice shelves and the adjacent grounded ice sheet a prime example is pine island glacier west antarctica which has a strong negative mass balance caused by a recent increase in ocean induced melting of its ice shelf the mass loss coincides with the retreat of the grounding line from a seabed ridge on which it was at least partly grounded until the 1970s at present it is unclear what has caused the onset of this retreat and how feedback mechanisms between the ocean and ice shelf geometry have influenced the ice dynamics to address these questions we present the first results from an offline coupling between a state of the art shallow ice flow model with grounding line resolving capabilities and a three dimensional ocean general circulation model with a static implementation of the ice shelf a series of idealized experiments simulate the retreat from a seabed ridge in response to changes in the ocean forcing and we show that the retreat becomes irreversible after 20 years of warm ocean conditions a comparison to experiments with a simple depth dependent melt rate parameterization demonstrates that such parameterizations are unable to capture the details of the retreat process and they overestimate mass loss by more than 40 over a 50 year timescale

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

history and isostatic effects of the last ice sheet in southern british columbia abstract the late wisconsinan cordilleran ice sheet covered british columbia southern yukon territory and parts of alaska washington idaho and montana its major source areas were the high mountain ranges of the canadian cordillera and flow was strongly controlled by topography the last ice sheet began to develop about 30 000 25 000 14c yr ago but it did not achieve its maximum extent until 15 000 14 000 14c yr bp ice sheet growth was interrupted locally by stillstands and retreat ice sheet decay was rapid and was characterized by complex frontal retreat at the periphery accompanied locally by brief readvances and by downwasting and stagnation by 10 000 14c yr bp

assessing the ability of numerical ice sheet models to simulate grounding line migration grounding line migration is a key process affecting the stability of marine ice sheets such as the west antarctic ice sheet wais grounding line motion is often included in numerical models simulating the past and future evolution of the wais however little attention has been paid to the numerical consistency of these models the aim of this paper is to assess the ability of simple versions of existing marine ice sheet models to simulate grounding line migration in particular we investigate the response of the grounding line to external forcing and the sensitivity of the models predictions to their numerics and the mechanical coupling between ice sheet and shelf from the model comparison there is no consensus on how the grounding line should react to changes in boundary conditions a crucial finding is the strong dependency of models using a fixed grid on numerical details such as the horizontal grid size this implies that we should be very wary about grounding line predictions from such models including mechanical coupling at the grounding line does not seem to change the qualitative behavior of the models this suggests that the way the grounding line is treated in marine ice sheet models dominantly determines the grounding line dynamics we find that models that employ a moving grid to explicitly track the grounding line do not share many of the deficiencies of the fixed grid models we conclude that at present no reliable model of the grounding line is available and further model development is urgently needed

turbulent heat flux in the upper ocean under sea ice turbulence data from three arctic drift station experiments demonstrate features of turbulent heat transfer in the oceanic boundary layer time series analysis of several w t records shows that heat and momentum flux occur at nearly the same scales typically by turbulent eddies of the order of 10 20 m in horizontal extent and a few meters in vertical extent probability distribution functions of w t have large skewness and kurtosis where the latter confirms that most of the flux occurs in intermittent events with positive and negative excursions an order of magnitude larger than the mean value an estimate of the eddy heat diffusivity in the outer ekman part of the boundary layer based on measured heat flux and temperature gradient during a diurnal tidal cycle over the yermak plateau slope north of fram strait agrees reasonably well with the eddy viscosity with values as high as 0 15 m2 s 1 an analysis of measurements made near the ice ocean interface at the three stations shows that heat flux increases with both temperature elevation above freezing and with friction velocity at the interface it also reveals a surprising uniformity in parameters describing the heat and mass transfer e g the thickness of the transition sublayer from a modified version of the yaglom kader theory is about 10 cm at all three sites despite nearly a fivefold difference in the under ice roughness z0 which ranges from approximately 2 to 9 cm a much simplified model for heat and mass transfer at the ice ocean interface suggested by the relative uniformity of the heat transfer coefficients at the three sites is outlined

precise measurement of changes in ice shelf thickness by phase sensitive radar to determine basal melt rates 1 we describe a novel technique for measuring the basal melt rate at a point on an ice shelf the method uses a phase sensitive radar to observe the change in thickness of the ice column between an upper reference horizon and the ice shelf base the observed thickness change is a combination of basal melting horizontal ice divergence and firn compaction the latter two components are determined independently by the repeat survey of surface markers and from estimates of surface accumulation rate and density at the depth of the reference horizon the technique has been applied at a site on george vi ice shelf antarctica where a melt rate of 2 78 0 08 m yr 1 was observed over a twelve day period our technique is applicable to almost any ice shelf or floating glacier tongue and its high precision over short time intervals will permit the measurement of seasonal variations in melt rate

fast recession of a west antarctic glacier satellite radar interferometry observations of pine island glacier west antarctica reveal that the glacier hinge line position retreated 1 2 0 3 kilometers per year between 1992 and 1996 which in turn implies that the ice thinned by 3 5 0 9 meters per year the fast recession of pine island glacier predicted to be a possible trigger for the disintegration of the west antarctic ice sheet is attributed to enhanced basal melting of the glacier floating tongue by warm ocean waters

the response of ice shelf basal melting to variations in ocean temperature abstract a three dimensional ocean general circulation model is used to study the response of idealized ice shelves to a series of ocean warming scenarios the model predicts that the total ice shelf basal melt increases quadratically as the ocean offshore of the ice front warms this occurs because the melt rate is proportional to the product of ocean flow speed and temperature in the mixed layer directly beneath the ice shelf both of which are found to increase linearly with ocean warming the behavior of this complex primitive equation model can be described surprisingly well with recourse to an idealized reduced system of equations and it is shown that this system supports a melt rate response to warming that is generally quadratic in nature this study confirms and unifies several previous examinations of the relation between melt rate and ocean temperature but disagrees with other results particularly the claim that a single melt rate sensitivity to warming is universally valid the hypothesized

effect of orbital scale climate cycling and meltwater drainage on ice sheet grounding line migration 1 ice shelf response to climate cycling introduces hysteretic behavior in outlet glaciers with the evolution of meltwater drainage networks enhancing this effect the 2002 collapse of the larsen b ice shelf one of the floating extensions formed from glaciers flowing off the antarctic peninsula into the weddell sea followed the development of widespread poorly drained surface melt ponds during the atypical warmth of the 2001 2002 summer melt season yet for at least the past half century the floating termini of outlet glaciers in greenland did not collapse despite being exposed to summer mean temperatures and melt rates that far exceed those recorded in 2001 2002 on the antarctic peninsula we suggest that this paradox can be explained by the advection of well developed englacial drainage networks within greenland s extensive ablation zone the efficiency of this plumbing system limits meltwater ponding within crevasses and the subsequent ice shelf failure observed on the antarctic peninsula whether progressive or abrupt ice shelf removal leads to near synchronous glacier retreat conversely glaciation of fjords is delayed during ice shelf formation ultimately the relatively rapid loss and gradual establishment of ice shelf buttressing amid climate cycling leads to an asymmetric response in outlet glacier dynamics this asymmetry may have implications for the future of existing and recently calved ice shelves eustatic sea level and the hysteretic behavior of ice sheets

warm ocean is eroding west antarctic ice sheet 1 satellite radar measurements show that ice shelves in pine island bay have thinned by up to 5 5 m yr 1 over the past decade the pattern of shelf thinning mirrors that of their grounded tributaries the pine island thwaites and smith glaciers and ocean currents on average 0 5 c warmer than freezing appear to be the source the synchronised imbalance of the inland glaciers is the result of reduced lateral and basal tractions at their termini and the drawdown of grounded ice shows that antarctica is more sensitive to changing climates than was previously considered

mapping crustal stress and strain in southwest british columbia 1 this paper investigates the orientation and sources of stress in the forearc of the cascadia subduction zone in southwest british columbia using bayesian inversion results from focal mechanism data and comparing results with gps derived short term strain rates the subduction margin in this region includes a change in orientation from n s in washington state to nw se in british columbia over 1000 focal mechanisms from north american crustal earthquakes have been calculated to identify the dominant style of faulting and 600 were inverted to estimate the principal stress orientations and the stress ratio our results indicate the maximum horizontal compressive stress orientation changes with distance to the trench from approximately margin normal along the coast to approximately margin parallel 100 150 km inland from the coast comparing stress orientations with gps data we relate the margin normal stress direction to subduction related strain rates due to the locked interface between the north american and juan de fuca plates just west of vancouver island further from the margin the plates are coupled less strongly and the margin parallel maximum horizontal compressive stress in the north american plate relates to the northward push of the oregon block which is also observed in the horizontal shortening direction of the residual strain rates after the subduction component is removed

ice ocean processes over the continental shelf of the southern weddell sea antarctica a review 1 interactions between the southern ocean and the weddell sea ice shelves are important both to the antarctic ice sheet and to the production of globally significant water masses here we review the interaction between the filchner ronne ice shelf and the shelf sea in which it floats the continental shelf processes leading to the production of weddell sea deep and bottom waters from the original off shelf source waters are discussed and a new view is offered of the initial production of high salinity shelf water data from ship based measurements at the ice front from glaciological methods and from measurements made within the sub ice shelf cavity itself are used to describe the pattern of flows beneath the ice shelf we also consider the variability observed within the cavity from tidal to interannual time scales and finish with a discussion of future research priorities in the region

a zonally averaged ocean model for the thermohaline circulation part i model development and flow dynamics abstract a two dimensional latitude depth ocean model is developed on the basis of the zonally averaged balance equations of mass momentum heat and salt its purpose is to investigate the dynamics and variability of the buoyancy forced thermohaline circulation for the time scales of interest an annually averaged model is selected and the momentum balance is taken to be diagnostic the east west pressure gradient which arises upon zonally averaging the momentum equations is parameterized in terms of the meridional pressure gradient the thermohaline circulation is driven by mixed surface boundary conditions i e temperatures are relaxed to prescribed values while the salt flux is held constant the dynamics of the flow is investigated in hemispheric and global geometries for both short and long time integrations the latter extending over many thousands of years as has been noted by previous investigators it is possible to perturb a steady state such that a diffusively dominated regime results

new empirical relationships among magnitude rupture length rupture width rupture area and surface displacement source parameters for historical earthquakes worldwide are compiled to develop a series of empirical relationships among moment magnitude m surface rupture length subsurface rupture length downdip rupture width rupture area and maximum and average displacement per event the resulting data base is a significant update of previous compilations and includes the additional source parameters of seismic moment moment magnitude subsurface rupture length downdip rupture width and average surface displacement each source parameter is classified as reliable or unreliable based on our evaluation of the accuracy of individual values only the reliable source parameters are used in the final analyses in comparing source parameters we note the following trends 1 generally the length of rupture at the surface is equal to 75 of the subsurface rupture length however the ratio of surface rupture length to subsurface rupture length increases with magnitude 2 the average surface displacement per event is about one half the maximum surface displacement per event and 3 the average subsurface displacement on the fault plane is less than the maximum surface displacement but more than the average surface displacement thus for most earthquakes in this data base slip on the fault plane at seismogenic depths is manifested by similar displacements at the surface log linear regressions between earthquake magnitude and surface rupture length subsurface rupture length and rupture area are especially well correlated showing standard deviations of 0 25 to 0 35 magnitude units most relationships are not statistically different at a 95 significance level as a function of the style of faulting thus we consider the regressions for all slip types to be appropriate for most applications regressions between magnitude and displacement magnitude and rupture width and between displacement and rupture length are less well correlated and have larger standard deviation than regressions between magnitude and length or area the large number of data points in most of these regressions and their statistical stability suggest that they are unlikely to change significantly in response to additional data separating the data according to extensional and compressional tectonic environments neither provides statistically different results nor improves the statistical significance of the regressions regressions for cases in which earthquake magnitude is either the independent or the dependent parameter can be used to estimate maximum earthquake magnitudes both for surface faults and for subsurface seismic sources such as blind faults and to estimate the expected surface displacement along a fault for a given size earthquake

melting of floating ice and sea level rise 1 contrary to popular belief the melting of floating ice in the form of ice shelves icebergs and sea ice may have a non zero impact on sea level this is because the melting process cools and dilutes the oceans on average and unless these opposing effects exactly balance each other there will be a net change in the ocean density we discuss how these subtle effects can be quantified and put bounds on the potential sea level rise associated with melting of the ice masses that are currently afloat in the world s oceans

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

measurements beneath an antarctic ice shelf using an autonomous underwater vehicle the cavities beneath antarctic ice shelves are among the least studied regions of the world ocean yet they are sites of globally important water mass transformations here we report results from a mission beneath fimbul ice shelf of an autonomous underwater vehicle the data reveal a spatially complex oceanographic environment an ice base with widely varying roughness and a cavity periodically exposed to water with a temperature significantly above the surface freezing point the results of this the briefest of glimpses of conditions in this extraordinary environment are already reforming our view of the topographic and oceanographic conditions beneath ice shelves holding out great promises for future missions from similar platforms

characterisation of soils by mineral magnetic measurements processes of iron mineral authigenesis diagenesis and dissolution in interaction with primary inputs of iron minerals act to produce vertical differentiation of soil magnetic properties thus magnetic iron oxides in common with other iron forms may both respond to and reflect soil forming processes investigation of the degree of detectable and persistent magnetic differentiation within different soil materials can provide insights into the nature and direction of distinct sets of soil processes evidence of clear association between magnetic variation and discrete types of soil environment also has applied significance in several areas of environmental study for example the observed magnetic variation can be used to discriminate between individual soils and soil horizons for the purposes of soil surveying sediment tagging and tracing and empirical modelling of source sediment linkages this paper reports the use of mineral magnetic techniques to characterise the iron oxide assemblages within soils of varying type and provenance the effect of different pedogenic regimes have been investigated through measurement of a range of magnetic parameters including magnetic susceptibility frequency dependent susceptibility and anhysteretic and saturation remanences contrary to current pedological thought but in accord with other magnetically based studies e g mullins the presence of magnetite maghemite typically of superparamagnetic single domain grain size within soils has been found to be an extremely widespread phenomenon its contemporary neoformation within the soil environment is inferred however the input of artificially generated ferrimagnetic material is indicated for one of the magnetically enhanced soils examined here and the possibility of extraneous sources of magnetite in topsoils should be considered before pedogenic processes of magnetic enhancement are inferred from the data presented discrimination of industrially derived magnetic particles appears possible on the basis of their very low levels of frequency dependent susceptibility fd low anhysteretic remanence arm and dissociation from the finer clay sized not the formation but the active dissolution of ferrimagnetic minerals is indicated for soils affected by the processes of gleying and podsolisation eluvial horizons of podsolised profiles demonstrate almost total loss of detectable magnetic content variations in the form of the iron reprecipitated in the underlying illuvial layers may reflect differences in the pedological characteristics of these horizons within gleyed waterlogged soils processes of magnetite dissolution appear to be grain size specific preferential reduction of those grains of ultrafine superparamagnetic and fine single domain magnetic grain size is indicated where soil forms a major contributor to catchment sediment loads this has significance in the area of source sediment modelling soil derived from permanently wet or highly leached catchment areas is unlikely to contribute to peaks in the magnetic content of deposited sediments further only those erosional processes that operate at a faster rate than those of magnetic depletion will have the capacity to produce peaks in sediment ferrimagnetic concentrations such as have been observed within the historical magnetic record

thermohaline circulation below the ross ice shelf a consequence of tidally induced vertical mixing and basal melting the warmest water below parts of the ross ice shelf resides in the lowest portion of the water column because of its high salinity vertical mixing caused by tidal stirring can thus induce ablation by lifting the warm but dense water into contact with the ice shelf a numerical tidal simulation indicates that vertically well mixed conditions predominate in the southeastern part of the sub ice shelf cavity where the water column thickness is small basal melting in this region is expected to be between 0 05 and 0 5 m yr and will drive a thermohaline circulation having the following characteristics high salinity shelf water at 1 8oc formed by winter sea ice production in the open ross sea flows along the seabed toward the tidal mixing fronts below the ice shelf and meltwater at 2 2oc produced in the wellmixed region flows out of the sub ice shelf cavity along the ice shelf bottom sensitivity of this ablation process to climatic change is expected to be small because high salinity shelf water is constrained to have the sea surface freezing temperature

intrusion of warm surface water beneath the mcmurdo ice shelf antarctica 1 a 6 month temperature record collected below mcmurdo ice shelf in 2011 2012 shows the temporal and spatial structure of the summertime warm water signal that penetrates beneath the ice shelf the strength and duration of the warm water intrusion suggest an annual melt rate at windless bight of 0 71 m yr a ross sea numerical model demonstrates a seasonal warm water pathway leading from the west side of the ross sea polynya rsp toward mcmurdo sound the warm water enters mcmurdo sound subducts beneath the ice shelf and causes accelerated summer melting temperature data were recorded using distributed temperature sensing fiber optics which gives a vertical temperature profile at a 1 m vertical resolution this study constitutes one of the first successful implementations of this technology in polar regions

coupling of ice shelf melting and buttressing is a key process in ice sheets dynamics 1 increase in ice shelf melting is generally presumed to have triggered recent coastal ice sheet thinning using a full stokes finite element model which includes a proper description of the grounding line dynamics we investigate the impact of melting below ice shelves we argue that the influence of ice shelf melting on the ice sheet dynamics induces a complex response and the first naive view that melting inevitably leads to loss of grounded ice is erroneous we demonstrate that melting acts directly on the magnitude of the buttressing force by modifying both the area experiencing lateral resistance and the ice shelf velocity indicating that the decrease of back stress imposed by the ice shelf is the prevailing cause of inland dynamical thinning we further show that feedback from melting and buttressing forces can lead to nontrivial results as an increase in the average melt rate may lead to inland ice thickening and grounding line advance

ocean circulation and sea ice thinning induced by melting ice shelves in the amundsen sea a 1 12 ocean model configuration of the amundsen sea sector is developed to better understand the circulation induced by ice shelf melt and the impacts on the surrounding ocean and sea ice eighteen sensitivity experiments to drag and heat exchange coefficients at the ice shelf ocean interface are performed the total melt rate simulated in each cavity is function of the thermal stanton number and for a given thermal stanton number melt is slightly higher for lower values of the drag coefficient sub ice shelf melt induces a thermohaline circulation that pumps warm circumpolar deep water into the cavity the related volume flux into a cavity is 100 500 times stronger than the melt volume flux itself ice shelf melt also induces a coastal barotropic current that contributes 45 12 of the total simulated coastal transport due to the presence of warm circumpolar deep waters the melt induced inflow typically brings 4 20 times more heat into the cavities than the latent heat required for melt for currently observed melt rates approximately 6 31 of the heat that enters a cavity with melting potential is actually used to melt ice shelves for increasing sub ice shelf melt rates the transport in the cavity becomes stronger and more heat is pumped from the deep layers to the upper part of the cavity then advected toward the ocean surface in front of the ice shelf therefore more ice shelf melt induces less sea ice volume near the ice sheet margins

efficient flowline simulations of ice shelf ocean interactions sensitivity studies with a fully coupled model abstractthermodynamic flowline and plume models for the ice shelf ocean system simplify the ice and ocean dynamics sufficiently to allow extensive exploration of parameters affecting ice sheet stability while including key physical processes comparison between geophysically and laboratory based treatments of ice ocean interface thermodynamics shows reasonable agreement between calculated melt rates except where steep basal slopes and relatively high ocean temperatures are present results are especially sensitive to the poorly known drag coefficient highlighting the need for additional field experiments to constrain its value these experiments also suggest that if the ice ocean interface near the grounding line is steeper than some threshold further steepening of the slope may drive higher entrainment that limits buoyancy slowing the plume and reducing melting if confirmed this will provide a stabilizing feedback on ice sheets under some circumstances

sea level change and paleogeographic reconstructions southern vancouver island british columbia canada abstract forty eight new and previously published radiocarbon ages constrain deglacial and postglacial sea levels on southern vancouver island british columbia sea level fell rapidly from its high stand of about 75 m elevation just before 14 000 cal bp 12 000 radiocarbon yrs bp to below the present shoreline by 13 200 cal bp 11 400 radiocarbon years bp the sea fell below its present level 1000 years later in the central strait of georgia and 2000 years later in the northern strait of georgia reflecting regional differences in ice sheet retreat and downwasting direct observations only constrain the low stand to be below 11 m and above 40 m analysis of the crustal isostatic depression with equations utilizing exponential decay functions appropriate to the cascadia subduction zone however places the low stand at 30 5 m at about 11 200 cal bp 9800 bp the inferred low stand for southern vancouver island when compared to the sea level curve previously derived for the central strait of georgia to the northwest generates differential isostatic depression that is consistent with the expected crustal response between the two regions morphologic and sub bottom features previously interpreted to indicate a low stand of 50 to 65 m are re evaluated and found to be consistent with a low stand of 30 5 m submarine banks in eastern juan de fuca strait were emergent at the time of the low stand but marine passages persisted between southern vancouver island and the mainland the crustal uplift presently occurring in response to the late pleistocene collapse of the southwestern sector of the cordilleran ice sheet amounts to about 0 1 mm yr the small glacial isostatic adjustment rate is a consequence of low viscosity mantle in this tectonically active region

late quaternary stratigraphy and seafloor geology of eastern juan de fuca strait british columbia and washington abstract a combination of shallow sediment cores and high resolution seismic reflection data were used to identify and map four main surficial geologic units in eastern juan de fuca strait these units comprise bedrock unit 1 till diamict unit 2 glacial marine unit 3 and post glacial sediments unit 4 bedrock crops out only in nearshore environments off vancouver island diamicton forms the numerous morainal and drumlin like banks in the region a series of banks including middle and hein banks and the offshore extension of dungeness spit divide the strait into two major areas based on the dominant surficial sedimentary units post glacial sediments are most common to the east and glacial marine sediments to the west glacial marine sediments are represented by several seismo lithologic subunits that suggest progression from ice proximal to ice distal depositional environments radiocarbon dates indicate these sediments are a product of the last phase of glacier retreat there is relatively little modern sediment input to eastern juan de fuca strait so most post glacial sediments are thought to consist of material reworked from glacial outwash or eroded from shallow bank deposits this conclusion is supported by radiocarbon ages from the post glacial sediments most of which are early holocene or younger also there are sandwave fields sediment drifts and barrier spits that appear to be actively maintained the source of sediment to these features is probably the pre existing deposits that occur in banks and extensive coastal exposures

observation and parameterization of ablation at the base of ronne ice shelf antarctica parameterizations of turbulent transfer through the oceanic boundary layer beneath an ice shelf are tested using direct measurements of basal ablation observations were made in the southwestern part of ronne ice shelf about 500 km from open water the mean basal ablation rate was measured over a month long and a year long period using phase sensitive radar to record the thinning of the ice shelf ocean temperatures were observed within about 25 m of the ice shelf base over the period of the radar observations while the tidally dominated ocean currents were estimated from tidal analysis of collocated current observations from an earlier period ablation rates derived using these ocean data and a number of bulk parameterizations of turbulent transfer within the boundary layer are compared with the direct measurements the ablation rates derived using a parameterization that explicitly includes the impact of ocean currents on the turbulent transfer of heat and salt match the observations to within 40 with suitable tuning of the drag coefficient the mismatch can be reduced below the level of the observational errors equally good agreement can be obtained with two slightly simpler current dependent parameterizations that use constant turbulent transfer coefficients andtheoptimalvaluesforthecoefficientsatthis particularlocationon ronneice shelfaregiven

ocean circulation beneath larsen c ice shelf antarctica from in situ observations 1 hot water drilled access holes were used to obtain oceanographic data from beneath two sites on larsen c ice shelf one in the north and one in the south at both sites the entire water column was colder than the surface freezing point and the temperature salinity characteristics are consistent with a high salinity shelf water source of maximum salinity 34 65 psu at the southern site the 0 08 c thermal driving at the ice base and the 0 2 m s 1 rms water speed resulted in a melt rate of 1 3 0 2 m a 1 as measured over an eight day period when combined with the available ship based data the evidence suggests that the sub ice cavity is flushed only by water at the surface freezing point this implies that the reported decrease in surface elevation of larsen c ice shelf is unlikely to be a result of thinning due to an increasing rate of basal melting

how ice shelf morphology controls basal melting 1 the response of ice shelf basal melting to climate is a function of ocean temperature circulation and mixing in the open ocean and the coupling of this external forcing to the sub ice shelf circulation because slope strongly influences the properties of buoyancy driven flow near the ice shelf base ice shelf morphology plays a critical role in linking external subsurface heat sources to the ice in this paper the slope driven dynamic control of local and area integrated melting rates is examined under a wide range of ocean temperatures and ice shelf shapes with an emphasis on smaller steeper ice shelves a 3 d numerical ocean model is used to simulate the circulation underneath five idealized ice shelves forced with subsurface ocean temperatures ranging from 2 0 c to 1 5 c in the sub ice shelf mixed layer three spatially distinct dynamic regimes are present entrainment of heat occurs predominately under deeper sections of the ice shelf local and area integrated melting rates are most sensitive to changes in slope in this initiation region some entrained heat is advected upslope and used to melt ice in the maintenance region however flow convergence in the outflow region limits heat loss in flatter portions of the ice shelf heat flux to the ice exhibits 1 a spatially nonuniform superlinear dependence on slope and 2 a shape and temperature dependent internally controlled efficiency because the efficiency of heat flux through the mixed layer decreases with increasing ocean temperature numerical simulations diverge from a simple quadratic scaling law

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

suppression of marine ice sheet instability a long standing open question in glaciology concerns the propensity for ice sheets that lie predominantly submerged in the ocean marine ice sheets to destabilise under buoyancy this paper addresses the processes by which a buoyancy driven mechanism for the retreat and ultimate collapse of such ice sheets the marine ice sheet instability is suppressed by lateral stresses acting on its floating component the ice shelf the key results are to demonstrate the transition between a mode of stable easily reversible retreat along a stable steady state branch created by ice shelf buttressing to tipped almost irreversible retreat across a critical parametric threshold the conditions for triggering tipped retreat can be controlled by the calving position and other properties of the ice shelf profile and can be largely independent of basal stress in contrast to principles established from studies of unbuttressed grounding line dynamics the stability and recovery conditions introduced by lateral stresses are analysed by developing a method of constructing grounding line stability bifurcation diagrams which provide a rapid assessment of the steady state positions their natures and the conditions for secondary grounding giving clear visualisations of global stabilisation conditions a further result is to reveal the possibility of a third structural component of a marine ice sheet that lies intermediate to the fully grounded and floating components the region forms an extended grounding area in which the ice sheet lies very close to flotation and there is no clearly distinguished grounding line the formation of this region generates an upsurge in buttressing that provides the most feasible mechanism for reversal of a tipped grounding line the results of this paper provide conceptual insight into the phenomena controlling the stability of the west antarctic ice sheet the collapse of which has the potential to dominate future contributions to global sea level rise

recent dramatic thinning of largest west antarctic ice stream triggered by oceans a growing body of observational data suggests that pine island glacier pig is changing on decadal or shorter timescales these changes may have far reaching consequences for the future of the west antarctic ice sheet wais and global sea levels because of pig s role as the ice sheet s primary drainage portal we test the hypothesis that these changes are triggered by the adjoining ocean specifically we employ an advanced numerical ice flow model to simulate the effects of perturbations at the grounding line on pig s dynamics the speed at which these changes are propagated upstream implies a tight coupling between ice sheet interior and surrounding ocean

rapid bottom melting widespread near antarctic ice sheet grounding lines as continental ice from antarctica reaches the grounding line and begins to float its underside melts into the ocean results obtained with satellite radar interferometry reveal that bottom melt rates experienced by large outlet glaciers near their grounding lines are far higher than generally assumed the melting rate is positively correlated with thermal forcing increasing by 1 meter per year for each 0 1 c rise in ocean temperature where deep water has direct access to grounding lines glaciers and ice shelves are vulnerable to ongoing increases in ocean temperature

effects of basal melting distribution on the retreat of ice shelf grounding lines 1 the stability of marine ice streams depends on the distribution as well as the magnitude of melting beneath the adjacent ice shelf as shown by new model results recent observations of rapid retreat of ice shelf grounding lines in the amundsen sea sector of west antarctica have highlighted the need for understanding how basal melting of ice shelves by warm ocean waters affects ice dynamics and potentially contributes indirectly to sea level rise we apply two ice stream ice shelf flow line models to investigate the effects of varying the spatial distribution of basal melting on grounding line dynamics for experiments with identical average melting we find that retreat increases significantly as melting is concentrated near the grounding line indicating that knowledge of the basal melting distribution is likely necessary for accurate prediction of grounding line migration

the effects of rotation and ice shelf topography on frazil laden ice shelf water plumes a model of the dynamics and thermodynamics of a plume of meltwater at the base of an ice shelf is presented such ice shelf water plumes may become supercooled and deposit marine ice if they rise because of the pressure decrease in the in situ freezing temperature so the model incorporates both melting and freezing at the ice shelf base and a multiple size class model of frazil ice dynamics and deposition the plume is considered in two horizontal dimensions so the influence of coriolis forces is incorporated for the first time it is found that rotation is extremely influential with simulated plumes flowing in near geostrophy because of the low friction at a smooth ice shelf base as a result an ice shelf water plume will only rise and become supercooled and thus deposit marine ice if it is constrained to flow upslope by topography this result agrees with the observed distribution of marine ice under filchner ronne ice shelf antarctica in addition it is found that the model only produces reasonable marine ice formation rates when an accurate ice shelf draft is used implying that the characteristics of real ice shelf water plumes can only be captured using models with both rotation and a realistic topography

observations beneath pine island glacier in west antarctica and implications for its retreat thinning ice in west antarctica is currently contributing about 10 of the observed rise in global sea level observations obtained from an autonomous underwater vehicle operating beneath pine island glacier west antarctica reveal that the glacier was recently grounded on a transverse ridge in the sea floor but now warm sea water flows through the widening gap above the ridge thinning ice in west antarctica resulting from acceleration in the flow of outlet glaciers is at present contributing about 10 of the observed rise in global sea level1 pine island glacier in particular has shown nearly continuous acceleration2 3 and thinning4 5 throughout the short observational record the floating ice shelf that forms where the glacier reaches the coast has been thinning rapidly6 driven by changes in ocean heat transport beneath it as a result the line that separates grounded and floating ice has retreated inland7 these events have been postulated as the cause for the inland thinning and acceleration8 9 here we report evidence gathered by an autonomous underwater vehicle operating beneath the ice shelf that pine island glacier was recently grounded on a transverse ridge in the sea floor warm sea water now flows through a widening gap above the submarine ridge rapidly melting the thick ice of the newly formed upstream half of the ice shelf the present evolution of pine island glacier is thus part of a longer term trend that has moved the downstream limit of grounded ice inland by 30 km into water that is 300 m deeper than over the ridge crest the pace and ultimate extent of such potentially unstable retreat10 are central to the debate over the possibility of widespread ice sheet collapse triggered by climate change11 12

gravity current dynamics and entrainment a process study based on observations in the arkona basin abstract a 19 h time series of dissipation stratification and horizontal velocities has been obtained for a dense gravity current flowing into the arkona basin in the western baltic sea the observations are compared with one dimensional quasi steady theory in which the gravity component in the flow direction is balanced by bottom friction while that in the cross flow direction is balanced by the coriolis force the observations deviate from the theory in that the bottom shear stress is more than 2 times as large as that required to balance the gravity several reasons for this discrepancy are discussed a 1d turbulence model is also compared with the observations profiles of velocity stratification and dissipation rates generally show similar variations with depth as the observations although the observed dissipation rates are somewhat larger than the modeled and the modeled transverse velocities are much larger than the observed subsequently the model is used to investigate the variation of t

gps deformation in a region of high crustal seismicity n cascadia forearc we estimate the rate of crustal deformation in the central and northern cascadia forearc based on a combination of existing global positioning system gps velocity data along the cascadia subduction zone gps strain rates and velocities show that the northwestern washington southwestern british columbia region is currently shortening at 3 3 5 mm yr 31 in a n s direction in good agreement with inference from crustal earthquake statistics on the longterm the shortening rate is 5 6 mm yr 31 providing that the subduction related interseismic loading of the margin is purely elastic compared to the velocity of the oregon forearc with respect to north america v 7m m yr 31 this indicates that most of the forearc motion is accommodated in the puget georgia basin area corresponding to the main concentration of crustal seismicity the difference between the current and long term shortening rates may be taken up during subduction megathrust earthquakes thus these events could produce a sudden increase of n s compression in the puget sound region and could trigger major seattle fault type crustal earthquakes published by elsevier science b v

evidence for late holocene earthquakes on the utsalady point fault northern puget lowland washington trenches across the utsalady point fault in the northern puget lowland of washington reveal evidence of at least one and probably two late holocene earthquakes the teeka and duffers trenches were located along a 1 4 km long 1 to 4 m high northwest trending southwest facing topographic scarp recognized from airborne laser swath mapping glaciomarine drift exposed in the trenches reveals evidence of about 95 to 150 cm of vertical and 200 to 220 cm of left lateral slip in the teeka trench radiocarbon ages from a buried soil a horizon and overlying slope colluvium along with the historical record of earthquakes suggest that this faulting occurred 100 to 400 calendar years b p a d 1550 to 1850 in the duffers trench 370 to 450 cm of vertical separation is accommodated by faulting 210 cm and folding 160 to 240 cm with probable but undetermined amounts of lateral slip stratigraphic relations and radiocarbon ages from buried soil colluvium and fissure fill in the hanging wall suggest the deformation at duffers is most likely from two earthquakes that occurred between 100 to 500 and 1100 to 2200 calendar years b p but deformation during a single earthquake is also possible for the two earthquake hypothesis deformation at teeka trench in the first event involved folding but not faulting regional relations suggest that the earthquake s were m 6 7 and that offshore rupture may have produced tsunamis based on this investigation and related recent studies the maximum recurrence interval for large ground rupturing crustal fault earthquakes in the puget lowland is about 400 to 600 years or less

the southern whidbey island fault an active structure in the puget lowland washington information from seismic reflection profiles outcrops boreholes and potential field surveys is used to interpret the structure and history of the southern whidbey island fault in the puget lowland of western washington this northwest trending fault comprises a broad as wide as 6 11 km steep northeast dipping zone that includes several splays with inferred strike slip reverse and thrust displacement transpressional deformation along the southern whidbey island fault is indicated by along strike variations in structural style and geometry positive flower structure local unconformities out of plane displacements and juxtaposition of correlative sedimentary units with different histories the southern whidbey island fault represents a segment of a boundary between two major crustal blocks the cascade block to the northeast is floored by diverse assemblages of pre tertiary rocks the coast range block to the southwest is floored by lower eocene marine basaltic rocks of the crescent formation the fault probably originated during the early eocene as a dextral strike slip fault along the eastern side of a continental margin rift bending of the fault and transpressional deformation began during the late middle eocene and continues to the present oblique convergence and clockwise rotation along the continental margin are the inferred driving forces for ongoing deformation evidence for quaternary movement on the southern whidbey island fault includes 1 offset and disrupted upper quaternary strata imaged on seismic reflection profiles 2 borehole data that suggests as much as 420 m of structural relief on the tertiary quaternary boundary in the fault zone 3 several meters of displacement along exposed faults in upper quaternary sediments 4 late quaternary folds with limb dips of as much as 9 5 large scale liquefaction features in upper quaternary sediments within the fault zone and 6 minor historical seismicity the southern whidbey island fault should be considered capable of generating large earthquakes m s 7 and represents a potential seismic hazard to residents of the puget lowland

late mesozoic and possible early tertiary accretion in western washington state the helena haystack melange and the darrington devils mountain fault zone the helena haystack melange hh melange and coincident darrington devils mountain fault zone ddmfz in northwestern washington separate two terranes the northwest cascade system nwcs and the western and eastern melange belts wemb the two terranes of paleozoic and mesozoic rocks superficially resemble each other but record considerable differences in structural and metamorphic history the hh melange is a serpentinite matrix melange containing blocks of adjacent terranes but also exotic blocks of schistose metavolcanic rocks and jurassic tonalite and associated amphibolite the hh melange must have formed between early cretaceous and late middle eocene time because it contains tectonic clasts of early cretaceous shuksan greenschist and is overlain by late middle eocene sedimentary and volcanic rocks less certain constraints on its age are a tectonic clast of metarhyolite that yields 90 ma metamorphic ages and the presumption that the melange was emplaced before the outboard olympic terrane arrived at about 50 ma the apparent continuity of the hh melange and the decatur terrane of the san juan islands suggests that the melange is the strongly tectonized equivalent of the fidalgo ophiolite the out crop pattern suggests that the hh melange overlies rocks of the nwcs and it may have formed when the wemb terranes were thrust over rocks of the nwcs much of the exposed belt of the hh melange is overlain by late middle eocene feldspathic sandstone and volcanic rocks of the barlow pass volcanics of vance 1957a which are cut by numerous faults of the ddmfz paralleling the melange the barlow pass volcanics appear to overlie the straight creek fault without large offset but a displaced exotic block of amphibolite with attached early or early middle eocene sandstone in the melange suggests that strike slip movement along the ddmfz was synchronous with movement on the straight creek fault and stretched cobbles in the conglomerates of the barlow pass volcanics suggest post straight creek movement the possible continuation of the ddmfz to the northwest as the san juan and the west coast faults on vancouver island suggests that the structure has had a major role in the emplacement of all the westernmost terranes in the pacific northwest this major suture is strongly bowed to the northeast opposite the great oroclinal bend of the olympic terrane suggesting that the emplacement of that terrane may have deformed a once straighter strike slip zone

structural variation along the devil s mountain fault zone northwestern washington the eastern juan de fuca strait is subject to long term north south oriented shortening the observed deformation is interpreted to result from the northward motion of the oregon block which is being driven north by oblique subduction of the oceanic juan de fuca plate seismic data acquired during the seismic hazards investigation in puget sound survey are used with coincident first arrival tomographic velocities to interpret structural variation along the devil s mountain fault zone in the eastern juan de fuca strait the primary fault of the devil s mountain fault zone developed at the northern boundary of the everett basin during north south oriented tertiary compression interpretation of seismic reflection data suggests that based on their similar geometry including the large magnitude of pre tertiary basement offset the primary fault of the devil s mountain fault west of 122 95 w and the utsalady point fault represent the main fault of the tertiary devil s mountain fault zone the tertiary

holocene faulting in the bellingham forearc basin upper plate deformation at the northern end of the cascadia subduction zone 1 the northern cascadia forearc takes up most of the strain transmitted northward via the oregon coast block from the northward migrating sierra nevada block the north south contractional strain in the forearc manifests in upper plate faults active during the holocene the northern most components of which are faults within the bellingham basin the bellingham basin is the northern of four basins of the actively deforming northern cascadia forearc a set of holocene faults drayton harbor birch bay and sandy point faults occur within the bellingham basin and can be traced from onshore to offshore using a combination of aeromagnetic lineaments paleoseismic investigations and scarps identified using lidar imagery with the recognition of such holocene faults the northernmost margin of the actively deforming cascadia forearc extends 60 km north of the previously recognized limit of holocene forearc deformation although to date no holocene faults are recognized at the northern boundary of the bellingham basin which is 15 km north of the international border there is no compelling tectonic reason to expect that holocene faults are limited to south of the international border

interactions between ice and ocean observed with phase sensitive radar near an antarctic ice shelf grounding line precise measurements of basal melting have been made at a series of 14 sites lying within a few kilometres of the grounding line of the ronne ice shelf antarctica where the ice thickness ranges from 1570 to 1940 m the study was conducted over the course of 1 year and included a detailed survey of the horizontal deformation as well as phase sensitive radar measurements of the vertical displacement of both internal reflecting horizons and the ice shelf base results from the surface survey show that the long term viscous strain rate is modulated at tidal frequencies by probably elastic strains of order 10 5 per metre of tidal elevation the radar measurements show a similar modulation of the long term thinning thickening of the ice shelf with thickness oscillations up to a few centimetres in range the long term trends in ice thickness determined at points moving with the ice shelf flow are consistent with a steady state thickness profile vertical strain rates within the ice shelf were determined from the relative motion of internal reflectors at two sites the observations were sufficient to discern the effect of tidal bending about a neutral surface 60 of the way down the ice column coincident measurements of horizontal and vertical strain imply a poisson s ratio of 0 5 and this combined with the asymmetric bending gives rise to the observed oscillations in thickness at a number of sites the long term viscous strain rates were found to be a linear function of depth for an ice shelf this is an unexpected result it can be attributed to the presence of significant vertical shear stresses set up close to the grounding line where the ice is still adjusting to flotation additional vertical motion arising from firn compaction was observed within the upper layers of the ice shelf the additional motion was consistent with the assumption that firn density is a function only of the time since burial by steady surface accumulation with both spatial and temporal fluctuations in the vertical strain rate accurately quantified it was possible to estimate the vertical motion of the ice shelf base in response differences between the calculated and observed motion of the basal reflector arise because of basal melting derived melt rates at the 14 sites ranged from 0 11 0 31 to 2 51 0 10 ma 1 with a mean of 0 85 m a 1 and a standard deviation of 0 69 m a 1 and showed no signs of significant sub annual temporal variability there was no obvious global correlation with either ice thickness or distance from the grounding line although melt rates tended to decrease downstream along each of the flowlines studied previous estimates of basal melting in this region have been obtained indirectly from an assumption that the ice shelf is locally in equilibrium and have included a broad range of values only those at the lower end of the published range are consistent with the directly measured melt rates reported here

late quaternary deglaciation and sea level history of eastern juan de fuca strait cascadia abstract eastern juan de fuca strait of southwestern british columbia northwestern washington state is near the western and southern terminus of late wisconsinan glaciation in north america seismic reflection profiles bathymetric data sediment core data and 41 new radiocarbon dates provide insight into the history of ice retreat and sea level change in the eastern portion of the strait the retreating ice margin appears to have passed through the area between 14 460 200 and 13 595 145 14c yr glacial marine sediments date between 12 889 50 and 11 110 50 14c yr bp wave eroded unconformities drowned prograded deposits and a partially isolated low stand basin are evidence attributed to low stand sea levels following deglaciation and five new control points on the local eustatic sea level curve are assigned based on radiocarbon dating of these features sea level change was rapid from 75 m immediately after deglaciation to possibly 60 4 m at 9920 60 14c yr bp after this time gradual sea level transgression took place relative sea level reached its approximate present position by 5470 115 14c yr bp since then relative sea level has remained constant suggesting eustatic rise is balanced by crustal uplift

ice sheet model sensitivities to environmental forcing and their use in projecting future sea level the searise project ten ice sheet models are used to study sensitivity of the greenland and antarctic ice sheets to prescribed changes of surface mass balance sub ice shelf melting and basal sliding results exhibit a large range in projected contributions to sea level change in most cases the ice volume above flotation lost is linearly dependent on the strength of the forcing combinations of forcings can be closely approximated by linearly summing the contributions from single forcing experiments suggesting that nonlinear feedbacks are modest our models indicate that greenland is more sensitive than antarctica to likely atmospheric changes in temperature and precipitation while antarctica is more sensitive to increased ice shelf basal melting an experiment approximating the intergovernmental panel on climate change s rcp8 5 scenario produces additional first century contributions to sea level of 22 3 and 8 1 cm from greenland and antarctica respectively with a range among models of 62 and 14 cm respectively by 200 years projections increase to 53 2 and 26 7 cm respectively with ranges of 79 and 43 cm linear interpolation of the sensitivity results closely approximates these projections revealing the relative contributions of the individual forcings on the combined volume change and suggesting that total ice sheet response to complicated forcings over 200 years can be linearized

marine ice sheet dynamics the impacts of ice shelf buttressing marine ice sheets are continent scale glacial masses that lie partially submerged in the ocean as applies to significant regions of antarctica and greenland such ice sheets have the potential to destabilise under a buoyancy driven instability mechanism with considerable implications for future sea level this paper and its companion present a theoretical analysis of marine ice sheet dynamics under the effect of a potentially dominant control of the buttressing force generated by lateral stresses on the downstream floating component of the ice sheet the ice shelf the analysis reveals critical conditions under which ice shelf buttressing suppresses the buoyancy driven collapse of an ice sheet and elucidates the implications of lateral stresses on grounding line control and overall ice sheet structure integrations of a suitably simplified quasi two dimensional model are conducted yielding analytical results that provide a quick assessment of steady state balances for a given ice sheet configuration an analytical balance equation describing the spectrum of marine ice sheet flow regimes spanning zero to strong ice shelf buttressing is developed it is determined that the dynamics across this spectrum exhibits markedly different flow regimes and structural characteristics for sufficient buttressing the grounding line occurs near to where a lateral drag controlled section of the ice shelf meets the bedrock implying an independent control of the grounding line by the ice shelf the role of basal stresses is relegated to controlling only the thickness of the ice sheet upstream of the grounding line with no significant control of the grounding line itself it is further demonstrated that lateral stresses are responsible for inducing additional secondary contacts between the ice shelf and the bedrock downstream of the grounding line resulting in a rich variety of additional steady states these inducements generate a further stabilising mechanism that can fully suppress grounding line retreat and eliminate otherwise irreparable hysteresis effects the results provide a conceptual framework for numerical and observational interpretation of marine ice sheet dynamics and clarifies the manner in which ice shelves can control grounding line positions independently it is thus indicated that a full resolution of the fine details of the flow of ice shelves and the processes controlling their erosion and disintegration is necessary for the confident forecasting of possible ice sheet collapse over the course of the next few centuries

role of small ice shelves in sea level rise 1 recent observations show marked acceleration and thinning of tributary ice streams following shrinkage or loss of a few of the many small ice shelves that ring antarctica and greenland our model experiments which adopt a simplified geometry show that calving from or partial disintegration of a small ice shelf has more effect on tributary ice streams than does loss of a berg of equal length from a larger ice shelf our results which are consistent with the recent observations indicate that even small ice shelves can provide important restraint on tributary flow from ice sheets implying that characterization and modeling of the full range of ice shelves will be required for accurate projection of ice sheet changes and their influence on sea level

ocean heat flux in the central weddell sea during winter abstract seasonal sea ice which plays a pivotal role in air sea interaction in the weddell sea a region of large deep water formation with potential impact on climate depends critically on heat flux from the deep ocean during the austral winter of 1994 an intensive process oriented field program named the antarctic zone flux experiment measured upper ocean turbulent fluxes during two short manned ice drift station experiments near the maud rise seamount region of the weddell sea unmanned data buoys left at the site of the first manned drift provided a season long time series of ice motion mixed layer temperature and salinity plus a truncated high resolution record of temperature within the ice column direct turbulence flux measurements made in the ocean boundary layer during the manned drift stations were extended to the ice ocean interface with a mixing length model and were used to evaluate parameters in bulk expressions for interfacial stress a rossby similarity drag law and ocean to

azimuth dependent amplification of weak and strong ground motions within a fault zone nocera umbra central italy 1 during three moderate magnitude earthquakes occurred in september october 1997 in the central apennines italy accelerations larger than 0 5 g were recorded in the town of nocera umbra 10 to 15 km n nw of the epicenters the accelerograph is sited in a fault zone close to a n30 e tectonic contact six temporary seismological stations installed across the fault recorded 82 aftershocks occurred in two seismogenic zones the colfiorito sellano area s se of the array and the gualdo tadino area to the north the array data reveal large variations in terms of both peak ground motions and spectral amplitudes within the fault zone amplifications show a strong dependence on the source azimuth at the accelerograph site the effects are particularly large for events from s se peak ground motions are a factor of 14 larger than those of a reference site and conventional spectral ratios attain amplitudes as large as 50 at 7 hz along the n30 e direction of motion parallel to the strike of the fault nineteen strong motion accelerograms were then used to compare ground motion properties between weak and strong events up to m 0 1 2 x 10 25 dyn cm a particle motion analysis shows that the directional effect is also present in the strongest motions even though the amplification of peak ground motion decreases when m 0 increases results from stochastic simulations indicate that such a behavior is not due to nonlinearity applying the empirical weak motion transfer functions in a purely linear model the observed peak ground motions of the largest events are fit satisfactorily

radiocarbon age constraints on rates of advance and retreat of the puget lobe of the cordilleran ice sheet during the last glaciation abstract calibrated radiocarbon dates of organic matter below and above till of the last fraser glaciation provide limiting ages that constrain the chronology and duration of the last advance retreat cycle of the puget lobe in the central and southeastern puget lowland seven dates for wood near the top of a thick proglacial delta have a weighted mean age of 17 420 90 cal yr b p which is the closest limiting age for arrival of the glacier near the latitude of seattle a time distance curve constructed along a flowline extending south from southwestern british columbia to the central puget lowland implies an average glacier advance rate of ca 135 m yr the glacier terminus reached its southernmost limit ca 16 950 yr ago and likely remained there for ca 100 yr in the vicinity of seattle where the glacier reached a maximum thickness of 1000 m ice covered the landscape for ca 1020 yr postglacial dates constraining the timing of ice retreat in the central lowland are as old as 16 420 cal yr b p and show that the terminus had retreated to the northern limit of the lowland within three to four centuries after the glacial maximum the average rate of retreat was about twice the rate of advance and was enhanced by rapid calving recession along flowline sectors where the glacier front crossed deep proglacial lakes

study of forest soils on an area of magnetic and geochemical anomaly in north eastern poland abstract the kolno plateau located in ne poland is an area of wide soil geochemical and magnetic anomaly whose origin has not been reliably recognized up to now field measurements of surface magnetic susceptibility as well as 34 vertical topsoil profiles 25 cm were performed in small forests as they were more or less regularly distributed in the whole area of study where the profiles were not disturbed by current cultivation selected topsoil cores of 30 cm were sampled for grain size and chemical heavy metals analysis additionally six deep soil pits were excavated for study of soil morphology and soil type identification the predominant soil type of the study areas was described as rusty soils according to the taxonomy of polish soils 1989 iuss 2006 brunic arenosols such soils have characteristic bwo horizons and developed from sandy and or gravelly material the topsoil magnetic screening exhibited very low values of between 2 and 11 10 5 si units in soil profiles the maximum value as well as the maximum total fe content up to 5250 mg kg 1 was observed mostly in a ap or bwo horizons without magnetic enhancement in the uppermost organic horizon typical for polluted soils the value in the topsoil profile is rather stable at depths between 2 and 20 cm which is commonly observed in arable soils it suggests the former agricultural use of studied soils which are now afforested these values together with the pattern of their vertical distribution in soil profile suggest that the measured values are of geogenic origin these soils did not exhibit any increased concentration of heavy metals max pb 21 zn 19 and cu 6 mg kg 1 the former arable use of rusty soils was also confirmed by morphological observation of the studied profiles where in many cases the ap horizon of relatively large thickness with considerable homogenization and a distinct underneath boundary was distinguished geochemical and magnetic topsoil anomalies in the area of kolno plateau reported in former publications are probably limited to more fertile haplic luvisols used as arable soils in this case the sources of their geochemical and magnetic enrichments are moraine tills occurring in the geological background which are usually enriched in heavy metals connected with clay mineral fractions

the devils mountain fault zone an active cascadia upper plate zone of deformation pacific northwest of north america abstract the devils mountain fault zone dmfz extends east to west from washington state to just south of victoria british columbia in the northern strait of juan de fuca of canada and the usa recently collected geophysical data were used to map this fault zone in detail which show the main fault trace and associated primary and secondary conjugate strands and extensive northeast southwest oriented folding that occurs within a 6 km wide deformation zone the fault zone has been active in the holocene as seen in the offset and disrupted upper quaternary strata seafloor displacement and deformation within sediment cores taken close to the seafloor expression of the faults data suggest that the present dmfz and the re activated leech river fault may be part of the same fault system based on the length and previously estimated slip rates of the fault zone in washington state the dmfz appears to have the potential of producing a strong earthquake perhaps as large as magnitude 7 5 or greater within 2 km of the city of victoria

implications of initial conditions and ice ocean coupling for grounding line evolution abstract ice sheet grounding lines are sensitive to initial conditions and to small perturbations in boundary conditions based on new model results coupling ocean and ice flow to study ice ocean dynamics near ice stream grounding lines we couple an ocean plume model that simulates ice shelf basal melting with a two dimensional isothermal model of ice stream and ice shelf flow the notable results of the coupled model experiments are to reveal grounding line migration sensitivities to i specific aspects of modeling derived and history dependent initial conditions ii to the overall melt magnitude and iii to a positive feedback between focused melting and local slopes of basal ice that is eventually stabilized by buttressing for lengthening ice shelves these interactions can lead to multiple steady states for ice flow over a bed that shallows in the along flow direction and have an important bearing on the effects of bedrock bumps when in the vicinity of bedrock highs grounding lines tend to rapidly advance or retreat towards the basal asperity a significant delay or cessation of de glaciation occurs once the grounding line reaches the leeward side of the bedrock crest however while bedrock bumps can offer stability in the grounding zone minor changes in ocean conditions can easily offset their effect through basal melting feedbacks

initial effects of oceanic warming on a coupled ocean ice shelf ice stream system abstract the initial retreat of ice shelf grounding lines stabilized on seaward sloping beds is influenced by the rheology of these beds according to new model results we apply a fully coupled process model to investigate how the response of an ice stream to increased ocean temperature beneath its ice shelf depends on the assumed form of its basal flow law for the same applied oceanic warming the increase in grounding line flux can be twice as great for an effectively plastic bed as for a linear viscous bed suggesting that improved knowledge of the basal flow law of ice streams is necessary for predicting ice sheet response to climatic forcing

calibrated prediction of pine island glacier retreat during the 21st and 22nd centuries with a coupled flowline model a flowline ice sheet model is coupled to a box model for cavity circulation and configured for the pine island glacier an ensemble of 5000 simulations are carried out from 1900 to 2200 with varying inputs and parameters forced by ocean temperatures predicted by a regional ocean model under the a1b business as usual emissions scenario comparison is made against recent observations to provide a calibrated prediction in the form of a 95 confidence set predictions are for monotonic apart from some small scale fluctuations in a minority of cases retreat of the grounding line over the next 200 yr with huge uncertainty in the rate of retreat full collapse of the main trunk of the pig during the 22nd century remains a possibility

stronger ocean circulation and increased melting under pine island glacier ice shelf the ice shelf buttressing antarctica s pine island glacier has been melting rapidly observations taken between 1994 and 2009 show that meltwater production has increased by about 50 since 1994 as a result of a stronger circulation below the ice shelf

magnetic properties of modern soils and quaternary loessic paleosols paleoclimatic implications the magnetic properties of paleosols developed in quaternary sequences of loess have been used for stratigraphic definition correlation with other terrestrial and deep sea sequences and paleoclimatic paleorainfall reconstruction in some loess paleosol sequences including those of the chinese loess plateau tajikistan and the czech republic maxima in magnetic susceptibility values correspond with the paleosol horizons and minima with the least weathered loess layers in other loess soil sequences including those of siberia alaska and argentina the relationship is completely opposite with susceptibility minima associated with the most developed paleosols to account for these opposite relationships the respective roles of 1 magnetic enhancement and 2 magnetic depletion and or dilution in determining soil magnetic properties are investigated for a range of modern soil types most magnetic enhancement is seen in the upper horizons of well drained cambisols absence or loss of magnetic iron oxides is apparent in acid podsol profiles and waterlogged soils for the cambisol profiles significant correlation is found between susceptibility and organic carbon cation exchange capacity and clay content the mineralogy morphology and grain size of soil magnetic carriers extracted from three modern enhanced soils and a paleosol from the chinese loess plateau are also identified by independent petrographic means microscopy and x ray diffraction of magnetic extracts magnetite and maghemite of ultrafine grain size from 0 4 to 0 001 i m are the major contributors to the magnetically enhanced soils weathering can concentrate detrital magnetic grains especially in the fine silt size fractions of soils the magnetic data from the modern soils indicate that interpretation of paleosol magnetic properties must be done on a site specific basis taking into account the possibilities of pedogenic enhancement pedogenic dilution or depletion and allochthonous inputs of magnetic minerals excessively arid wet or acid soils are unable to form significant amounts of pedogenic ferrimagnets well drained intermittently wet dry soils with reasonable buffering capacity and a substrate source of fe show most magnetic enhancement for those soils which favour magnetic enhancement processes correlation has been found between the maximum value of the pedogenic susceptibility and the annual rainfall the almost unique pedogenic system in the chinese loess plateau where variation in soil forming factors other than climate is reduced to a global minimum allows use of the paleo susceptibility values as proxy paleorainfall values to identify the mechanism of any link between magnetic properties for example susceptibility and climate change the mineralogy and grain size of soil magnetic carriers requires detailed investigation given the prospect of 1 quantitative paleorainfall records from continental loess records 2 predictions of present and future climate change and 3 the value of paleoclimate data for testing of numerical climate models such magnetic investigations can contribute significantly to our understanding of past and future environmental change

interaction between local tectonics and glacial unloading on the pacific margin of canada abstract variations in late quaternary coastal deformation along the pacific margin of canada reflect not only differences in glacial history but also local non glacial tectonics a forebulge developed to the north in the queen charlotte basin qcb from 13 2 until after 9 7 ka bp resulting in exposure of the currently submerged shelf whereas similar exposure was not evident to the south in the georgia basin a semi quantitative effective elastic lithosphere thickness of between 10 and 20 km is estimated for the qcb we propose that this warm relatively thin lithosphere combined with tectonic movement influenced crustal displacement particularly the decoupling of the north american plate along the queen charlotte qc fault which may have allowed the qcb to behave like a hinge in response to glacial unloading this would explain large magnitude crustal tilting 2 1 m km 1 observed adjacent to the qc fault the 50 m of depression observed in the early and mid holocene may imply that the western edge of the north america plate flexed down below present elevation producing a second wave of response to ice loading and unloading

a one dimensional model of ice shelf ocean interaction large scale oceanic circulation beneath antarctic ice shelves is driven by the thenmohaline differences which result from mass and energy exchange at the ice ocean interface dense saline waters are drawn underneath the ice shelves and emerge cooled and diluted as plumes of ice shelf water a simple one dimensional model of this process has been developed in which the ice shelf water plume is treated as a turbulent gravity current initiated at the inland margin by a flow of fresh meltwater emerging from beneath the grounded ice subsequent evolution of the plume as it ascends along an ice shelf base of specified geometry can be simulated the model has been applied to a flow line on ronne ice shelf antarctica to explain the observed distribution and rate of basal melting and freezing calculations indicate that the present mean melt rate of 0 6 m yr 1 would increase to 2 6 m yr 1 if the underlying water were to warm by 0 6 c this would not only lead to significant thinning of the ice shelf but could also cause a profound change in ocean circulation on the open continental shelf

a model for the vertial circulation of the baltic deep water abstract the time dependent vertical circulation of the baltic proper is modeled using a horizontally integrated model of high vertical resolution a seasonal pycnocline model computes the properties of the mixed layer below this an advection vertical diffusion model computes the evolution of the salinity and temperature fields a simple model for an entraining dense bottom current which carries the intruding seawater and drives the vertical advection in the basin is developed and used in the derivation of entrainment velocity we it is shown that e we u where u is the speed of the bottom current may be expressed in the well known empirical constants m0 and cd the hypsographic features of the baltic are accounted for in the model the model is forced using realistic meteorological and hydrological time series the inflow of dense seawater to the baltic with large fluctuations in flow rate and salinity is realistically described and constitutes the upstream boundary condition for the bottom current