ocean circulation ice shelf and sea ice interactions explain dansgaard oeschger cycles the last glacial interval experienced abrupt climatic changes called dansgaard oeschger do events these events manifest themselves as rapid increases followed by slow decreases of oxygen isotope ratios in greenland ice core records despite promising advances a comprehensive theory of the do cycles with their repeated ups and downs of isotope ratios is still lacking here based on earlier hypotheses we introduce a dynamical model that explains the do variability by rapid retreat and slow regrowth of thick ice shelves and thin sea ice in conjunction with changing subsurface water temperatures due to insulation by the ice cover our model successfully reproduces observed features of the records such as the sawtooth shape of the do cycles waiting times between do events across the last glacial and the shifted antiphase relationship between greenland and antarctic ice cores our results show that these features can be obtained via internal feedbacks alone warming subsurface waters could have also contributed to the triggering of heinrich events our model thus offers a unified framework for explaining major features of multimillennial climate variability during glacial intervals

rapid submarine melting of the calving faces of west greenland glaciers widespread glacier acceleration has been observed in greenland in the past few years oceanographic observations taken in summer 2008 show that ocean waters melted a substantial fraction of ice along the calving fronts of three west greenland glaciers indicating that submarine melting has a profound influence on grounding line stability widespread glacier acceleration has been observed in greenland in the past few years1 2 3 4 associated with the thinning of the lower reaches of the glaciers as they terminate in the ocean5 6 7 these glaciers thin both at the surface from warm air temperatures and along their submerged faces in contact with warm ocean waters8 little is known about the rates of submarine melting9 10 11 and how they may affect glacier dynamics here we present measurements of ocean currents temperature and salinity near the calving fronts of the eqip sermia kangilerngata sermia sermeq kujatdleq and sermeq avangnardleq glaciers in central west greenland as well as ice front bathymetry and geographical positions we calculate water mass and heat budgets that reveal summer submarine melt rates ranging from 0 7 0 2 to 3 9 0 8 m d 1 these rates of submarine melting are two orders of magnitude larger than surface melt rates but comparable to rates of iceberg discharge we conclude that ocean waters melt a considerable but highly variable fraction of the calving fronts of glaciers before they disintegrate into icebergs and suggest that submarine melting must have a profound influence on grounding line stability and ice flow dynamics

origin and consequences of cyclic ice rafting in the northeast atlantic ocean during the past 130 000 years deep sea sediment cores recovered from the northeast atlantic ocean were examined in order to elucidate the influence of the earth s orbital parameters on major ice rafting analyses of coarse grained ice rafted debris and planktonic foraminifers revealed a strong reaction to the precession signal since 130 000 yr b p dropstone layers have been deposited each half period of a precessional cycle 11 000 1000 yr ice rafting occurs during times of winter minimum summer maximum insolation and summer minimum winter maximum insolation in the first case high summer insolation forces meltwater discharge from the ice sheets into the polar seas which subsequently enhances formation of sea ice during the winter in the second case growth of continental ice enhances iceberg production which also leads to a salinity reduction of surface seawater both situations result in a southward penetration of polar water thus the marine record of dropstones documents ice rafting not only during weichselian stades but also during cold events within interstades the regularity of ice rafting yields a useful framework to calibrate and elucidate climatic changes not only in the region of the north atlantic ocean but also in remote areas such as the pacific ocean and the antarctic

catastrophic ice shelf breakup as the source of heinrich event icebergs 1 heinrich layers of the glacial north atlantic record abrupt widespread iceberg rafting of detrital carbonate and other lithic material at the extreme cold culminations of bond climate cycles both internal glaciologic and external climate forcings have been proposed here we suggest an explanation for the iceberg release that encompasses external climate forcing on the basis of a new glaciological process recently witnessed along the antarctic peninsula rapid disintegrations of fringing ice shelves induced by climate controlled meltwater infilling of surface crevasses we postulate that peripheral ice shelves formed along the eastern canadian seaboard during extreme cold conditions would be vulnerable to sudden climate driven disintegration during any climate amelioration ice shelf disintegration then would be the source of heinrich event icebergs

binge purge oscillations of the laurentide ice sheet as a cause of the north atlantic s heinrich events ice rafted debris in sediment cores from the north atlantic suggests that the laurentide ice sheet lis periodically disgorged icebergs in brief but violent episodes which occurred approximately every 7 000 years here i propose that heinrich events i e what these episodes are called were caused by free oscillations in the flow of the laurentide ice sheet which arose because the floor of hudson bay and hudson strait is covered with soft unconsolidated sediment that forms a slippery lubricant when thawed the proposed heinrich event cycle has two phases the growth phase occurs when the sediment is frozen and the lis is stranded immobile on a rigid bed the volume of the lis slowly grows during this phase at a rate dictated by snow accumulation the purge phase occurs when the basal sediment thaws and a basally lubricated discharge pathway i e an ice stream such as those which occur in west antarctica today developes through hudson strait the volume of the lis rapidly equilibrates to the reduced basal friction during this phase by dumping icebergs into the labrador sea the periodicity t k sl2g 2 7000 years of the proposed heinrich event cycle is a function of the thermal conductivity and diffusivity of ice k and respectively the atmospheric sea level temperature sl in degrees celsius and the excess geothermal heat flux defined by g g k where is the atmospheric lapse rate and g is the geothermal heat flux agreement between the predicted t and the apparent periodicity implied by the marine record is the main virtue of the free oscillation mechanism i propose an alternative mechanism in which heinrich events are forced by periodic variations in external climate is implausible because periodic atmospheric temperature perturbations are strongly attenuated with depth in an ice sheet

ice shelf collapse from subsurface warming as a trigger for heinrich events episodic iceberg discharge events from the hudson strait ice stream hsis of the laurentide ice sheet referred to as heinrich events are commonly attributed to internal ice sheet instabilities but their systematic occurrence at the culmination of a large reduction in the atlantic meridional overturning circulation amoc indicates a climate control we report mg ca data on benthic foraminifera from an intermediate depth site in the northwest atlantic and results from a climate model simulation that reveal basin wide subsurface warming at the same time as large reductions in the amoc with temperature increasing by approximately 2 c over a 1 2 kyr interval prior to a heinrich event in simulations with an ocean model coupled to a thermodynamically active ice shelf the increase in subsurface temperature increases basal melt rate under an ice shelf fronting the hsis by a factor of approximately 6 by analogy with recent observations in antarctica the resulting ice shelf loss and attendant hsis acceleration would produce a heinrich event

global ocean heat content in the last interglacial the last interglacial 129 116 thousand years ago ka represents one of the warmest climate intervals of the past 800 000 years and the most recent time when sea level was metres higher than today however the timing and magnitude of the peak warmth varies between reconstructions and the relative importance of individual sources that contribute to the elevated sea level mass gain versus seawater expansion during the last interglacial remains uncertain here we present the first mean ocean temperature record for this interval from noble gas measurements in ice cores and constrain the thermal expansion contribution to sea level mean ocean temperature reached its maximum value of 1 1 0 3 c warmer than modern values at the end of the penultimate deglaciation at 129 ka which resulted in 0 7 0 3 m of thermosteric sea level rise relative to present level however this maximum in ocean heat content was a transient feature mean ocean temperature decreased in the first several thousand years of the interglacial and achieved a stable comparable to modern value by 127 ka the synchroneity of the peak in mean ocean temperature with proxy records of abrupt transitions in the oceanic and atmospheric circulation suggests that the mean ocean temperature maximum is related to the accumulation of heat in the ocean interior during the preceding period of reduced overturning circulation rapid oceanic and atmospheric circulation shifts led to a transient peak in the mean temperature of the ocean at the start of the last interglacial according to noble gas isotope records from an antarctic ice core

the sensitivity of northern hemisphere ice sheets to atmospheric forcing during the last glacial cycle using pmip3 models the evolution of northern hemisphere ice sheets through the last glacial cycle is simulated with the glacial index method by using the climate forcing from one general circulation model cosmos by comparing the simulated results to geological reconstructions we first show that the modelled climate is capable of capturing the main features of the ice sheet evolution however large deviations exist likely due to the absence of nonlinear interactions between ice sheet and other climate components the model uncertainties of the climate forcing are examined using the output from nine climate models from the paleoclimate modelling intercomparison project phase iii the results show a large variability in simulated ice sheets between the different models we find that the ice sheet extent pattern resembles summer surface air temperature pattern at the last glacial maximum confirming the dominant role of surface ablation process for high latitude northern hemisphere ice sheets this study shows the importance of the upper boundary condition for ice sheet modelling and implies that careful constraints on climate output is essential for simulating realistic glacial northern hemisphere ice sheets

potential of the solid earth response for limiting long term west antarctic ice sheet retreat in a warming climate abstract we employ a coupled model for ice sheet dynamics and maxwell viscoelastic solid earth dynamics including a gravitationally consistent description of sea level with this model we study the influence of the solid earth on the future evolution of the west antarctic ice sheet wais starting from steady state conditions close to the present day configuration of the antarctic ice sheet we apply different atmospheric and oceanic forcings and solid earth rheologies in order to analyse the retreat of the wais climate forcing is the primary control on the occurrence of wais collapse for moderate climate forcing and weak solid earth rheologies however we find that the relative sea level rsl fall associated with the viscoelastic solid earth response due to unloading by wais retreat limits the retreat to the amundsen sea embayment on time scales of several millennia whereas stiffer earth structures yield a collapse under these conditions under stronger climate forcing weak earth structures associated with the west antarctic rift system produce a delay of up to 5000 years in comparison to stiffer antarctic average solid earth rheologies furthermore we find that sea level rise from an assumed fast deglaciation of the greenland ice sheet induces wais collapse in the presence of higher asthenosphere viscosities in cases when the climatic forcing is too weak to force wais collapse alone

phasing of millennial scale climate variability in the pacific and atlantic oceans new radiocarbon and sedimentological results from the gulf of alaska document recurrent millennial scale episodes of reorganized pacific ocean ventilation synchronous with rapid cordilleran ice sheet discharge indicating close coupling of ice ocean dynamics spanning the past 42 000 years ventilation of the intermediate depth north pacific tracks strength of the asian monsoon supporting a role for moisture and heat transport from low latitudes in north pacific paleoclimate changes in 14c age of intermediate waters are in phase with peaks in cordilleran ice rafted debris delivery and both consistently precede ice discharge events from the laurentide ice sheet known as heinrich events this timing precludes an atlantic trigger for cordilleran ice sheet retreat and instead implicates the pacific as an early part of a cascade of dynamic climate events with global impact

iceberg discharges into the north atlantic on millennial time scales during the last glaciation high resolution studies of north atlantic deep sea cores demonstrate that prominent increases in iceberg calving recurred at intervals of 2000 to 3000 years much more frequently than the 7000 to 10 000 year pacing of massive ice discharges associated with heinrich events the calving cycles correlate with warm cold oscillations called dansgaard oeschger events in greenland ice cores each cycle records synchronous discharges of ice from different sources and the cycles are decoupled from sea surface temperatures these findings point to a mechanism operating within the atmosphere that caused rapid oscillations in air temperatures above greenland and in calving from more than one ice sheet

frozen fringe explains sediment freeze on during heinrich events abstract anomalous coarse grained sediment layers beneath the north atlantic likely originated from sediment freeze on to the base of ice sheets during the last glacial period these layers represent periods of extreme ice discharge called heinrich events and are variously attributed to ice stream flow instability ice shelf collapse or enhanced terminus melting due to ocean warming in this paper we study the processes controlling how sediment freezes on to the base of ice streams and predict the volume of sediment conveyed by icebergs during a heinrich event the local thickness of frozen sediment is sensitive to the heat flux at the ice bed interface and the water pressure both of which also contribute to the controls on basal friction as the basal water pressure increases both the frozen sediment thickness and the basal friction decrease the sediment discharged during a heinrich event must have frozen on to the ice during the inter heinrich period as the heinrich event proceeds the frozen sediment melts off the base of the ice stream indicating that the thickness of sediments deposits in the north atlantic may not reliably constrain heinrich event duration choosing reasonable parameters corresponding to the hudson strait ice stream our model of sediment freeze on and discharge is consistent with observational estimates of heinrich event sediment discharge volume

the subglacial mosaic of the laurentide ice sheet a study of the interior region of southwestern hudson bay abstract reconstructions of past ice flow provide useful insights into the long term behaviour of past ice sheets and help to understand how glaciated landscapes are shaped here we present reconstruction of a 10 phase ice flow history from southwestern hudson bay in northeastern manitoba canada a dynamic region situated between two major ice dispersal centres of the laurentide ice sheet we utilize a diverse geologic dataset including 1900 field based erosional indicators 12 streamlined landform flowsets esker and meltwater corridor orientations 103 till fabrics analyses and 1344 till clast lithology counts our reconstruction suggests that both pre mis 2 and mis 2 glaciations followed similar growth patterns where ice advanced into study area from ice centered to the east probably in northern quebec followed by a switch in ice flow direction indicating flow from the keewatin ice centre to the northwest and north the cause for this switch in ice flow orientation is uncertain but the youngest switch may relate to retreat of ice during mis 3 that then left space for keewatin sourced ice to advance over the study area while modelling experiments indicate widespread cold based conditions in the study area during the last glacial cycle uniformly relict landscapes are not common instead the glaciated landscape is palimpsest and commonly fragmented forming a subglacial bed mosaic of erosional and depositional assemblages that record both shifting ice flow direction through time and shifting subglacial conditions each assemblage formed or modified during times of dynamic warm based ice and later preserved under conditions below or close to the pressure melting point slow and sluggish or cold based

strain accumulation and rotation in western oregon and southwestern washington 1 velocities of 75 geodetic monuments in western oregon and southwestern washington extending from the coast to more than 300 km inland have been determined from gps surveys over the interval 1992 2000 the average standard deviation in each of the horizontal velocity components is 1 mm yr 1 the observed velocity field is approximated by a combination of rigid rotation euler vector relative to interior north america 43 40 n 0 14 119 33 w 0 28 and 0 822 0 057 myr 1 clockwise quoted uncertainties are standard deviations uniform regional strain rate eee 7 4 1 8 een 3 4 1 0 and enn 5 0 0 8 nstrain yr 1 extension reckoned positive and a dislocation model representing subduction of the juan de fuca plate beneath north america subduction south of 44 5 n was represented by a 40 km wide locked thrust and subduction north of 44 5 n by a 75 km wide locked thrust

abrupt north atlantic circulation changes in response to gradual co2 forcing in a glacial climate state glacial climate is marked by abrupt millennial scale climate changes known as dansgaard oeschger cycles the most pronounced stadial coolings heinrich events are associated with massive iceberg discharges to the north atlantic these events have been linked to variations in the strength of the atlantic meridional overturning circulation however the factors that lead to abrupt transitions between strong and weak circulation regimes remain unclear here we show that in a fully coupled atmosphere ocean model gradual changes in atmospheric co2 concentrations can trigger abrupt climate changes associated with a regime of bi stability of the atlantic meridional overturning circulation under intermediate glacial conditions we find that changes in atmospheric co2 concentrations alter the transport of atmospheric moisture across central america which modulates the freshwater budget of the north atlantic and hence deep water formation in our simulations a change in atmospheric co2 levels of about 15 ppmv comparable to variations during dansgaard oeschger cycles containing heinrich events is sufficient to cause transitions between a weak stadial and a strong interstadial circulation mode because changes in the atlantic meridional overturning circulation are thought to alter atmospheric co2 levels we infer that atmospheric co2 may serve as a negative feedback to transitions between strong and weak circulation modes

new kinematic models for pacific north america motion from 3 ma to present i evidence for steady motion and biases in the nuvel 1a model we use velocities derived from 2 4 5 years of continuous gps observations at 21 sites on the pacific and north american plates along with a subset of the nuvel 1a data to examine the steadiness of pacific north america motion since 3 16 ma the transfer of baja california to the pacific plate and the magnitude of biases in the nuvel 1a estimate of pacific north america motion we find that pacific north america motion has remained steady since 3 16 ma but at rates significantly faster than predicted by nuvel 1a in the vicinity of baja california our gps derived model and recent seafloor spreading rates in the southern gulf of california both indicate that the nuvel 1a model underestimates pacific north america rates by 4 2 mm yr 1 steady pacific north america motion since 3 16 myr and increasing seafloor spreading rates since 3 58 myr in the gulf of california imply that pacific north america motion was partitioned between seafloor spreading in the gulf of california and decelerating slip along faults in or offshore from the baja peninsula

dansgaard oeschger events is there a signal off the hudson strait ice stream abstract it is generally accepted that the dramatic heinrich h events were principally sourced from the major hudson strait drainage basin of the laurentide ice sheet but it is not known whether the higher frequency dansgaard oeschger d o events have an imprint on sediments deposited during the inter h intervals in ice proximal slope sediments this paper examines this question by an analysis of data from cores on the se baffin slope because the proxies for heinrich events are so massive in scale we removed the major h events h 1 h 2 and h 4 from the records so as to better examine the structure of data in the intervening hemipelagic sediments the period of interest extends between 15 and 40 cal ka interstadial events 2 8 resolution varies with the proxy to between 210 and 700 yrs per sample in hu87033 009 from the lower slope a total of twelve variables for example 18o mass magnetic susceptibility calcite dolomite ratio number of ird 2 mm show a variety of multi millennial oscillations although not all of the same amplitude nor frequency several parameters show little correlation with the gisp2 18o record however a comparison of cie sediment color l from hu87033 009 and hu97048 007 from the upper slope with the gisp2 oxygen isotope record suggests that a substantial correlation exists between the records such changes might be associated with either detrital carbonate content or foraminiferal concentrations analysis suggests however that the first possibility is correct hence suggestive of a glaciological response of the hudson strait ice stream with d o events in the nw labrador sea the sediment records from around h 3 time 30 cal ka show excursions which might be correlative within this event but its amplitude is not unusual compared to events both before and after this reaffirms the argument that h 3 represents a different glaciological response a significant issue discussed in this paper is the inherent ambiguity of correlating records whose errors in the time domain approach that of a dominating periodicity in this case ca 1 5 kyr

north atlantic warming and the retreat of greenland s outlet glaciers mass loss from the greenland ice sheet quadrupled over the past two decades contributing a quarter of the observed global sea level rise increased submarine melting is thought to have triggered the retreat of greenland s outlet glaciers which is partly responsible for the ice loss however the chain of events and physical processes remain elusive recent evidence suggests that an anomalous inflow of subtropical waters driven by atmospheric changes multidecadal natural ocean variability and a long term increase in the north atlantic s upper ocean heat content since the 1950s all contributed to a warming of the subpolar north atlantic this led in conjunction with increased runoff to enhanced submarine glacier melting future climate projections raise the potential for continued increases in warming and ice mass loss with implications for sea level and climate

consistent fluctuations in intermediate water temperature off the coast of greenland and norway during dansgaard oeschger events abstract rapid warmings epitomize the dansgaard oeschger events that are recorded in greenland ice cores and imprinted in ocean sediment cores while the abrupt climate changes appear connected to perturbations in sea ice and ocean circulation it is unclear how the water masses within the nordic seas responded and were influenced by the inflowing atlantic water in the absence or presence of sea ice high resolution reconstructions of benthic mg ca together with stable isotopes of carbon and oxygen 13c and 18o show a recurring warming 2 5 0 5 c occurring consistently at the inflow and outflow of the nordic seas at intermediate depths down to 1500 m during greenland stadials 9 6 using idealized numerical simulations with an eddy resolving ocean model we investigate the impact of an isolating sea ice cover and freshwater lid in the nordic seas with the presence of an extensive sea ice cover the warm atlantic water entering the nordic seas in the east retains its heat as it exits in the west the depth of the recirculating warm atlantic water increases when including an external freshwater source at the surface of the nordic seas these findings support the view that cold stadials are accompanied by pervasive intermediate water warming across the nordic seas given the current rates of arctic sea ice loss these results provide a potential mechanism for water column destabilization and inception of abrupt climate change

reconstruction of sea surface temperature salinity and sea ice cover in the northern north atlantic during the last glacial maximum based on dinocyst assemblages past sea surface conditions over the northern north atlantic during the last glacial maximum were examined from the study of 61 deep sea cores the last glacial maximum time slice studied here corresponds to an interval between heinrich layers h2 and h1 and spanningabout 20 16 ka on a 14c time scale transfer functions based on dinocyst assemblages were used to reconstruct sea surface temperature salinity and sea ice cover the results illustrate extensive sea ice cover along the eastern canadian margins and sea ice spreading only during winter over most of the northern north atlantic on the whole much colder winter prevailed despite relatively mild conditions in august 10 15 c at most offshore sites thus suggesting a larger seasonal contrast of temperatures than today lower salinity than at present is reconstructed especially along the eastern canadian and scandinavian margins likely because of meltwater supply from the surrounding ice sheets these reconstructions contrast with those established by climap on the basis of planktonic foraminifera these differences are discussed with reference to the stratigraphical frame of the last glacial maximum which was not the coldest phase of the last glacial stage the respective significance of dinocyst and foraminifer records is also examined in terms of the thermohaline characteristics of surface waters and the vertical structure of upper water masses which was apparently much more stratified than at present in the northern north atlantic thus preventing deep water formation

fast computation of a viscoelastic deformable earth model for ice sheet simulations the model used by lingle and clark 1985 to approximate the deformation of the earth under a single ice stream is adapted to the purposes of continent scale ice sheet simulation the model combines a layered elastic spherical earth farrell 1972 with a viscous half space overlain by an elastic plate lithosphere cathles 1975 for the half space model we identify a new mathematical formulation essentially a time dependent partial differential equation which generalizes and improves upon the standard elastic plate lithosphere with relaxing asthenosphere model widely used in ice sheet simulation the new formulation allows a significantly faster numerical strategy a spectral collocation method based directly on the fast fourier transform we verify this method by comparing to an integral formula for a disk load we also demonstrate that the magnitudes of numerical errors made in approximating coupled ice flow earth deformation systems are significantly smaller than pairwise differences between several earth models our implementation of the lingle and clark 1985 model offers important features of spherical layered self gravitating viscoelastic earth models without the computational expense

tectonic deformation in western washington from continuous gps measurements the analysis of 3 years of continuous data from 7 permanent gps stations along the western washington section of the cascadia subduction zone indicates that the direction of the observed horizontal velocities with respect to station drao nominally representing the stable north american continent is roughly parallel to the relative plate convergence direction of the juan de fuca and north america plates and that their magnitude decreases away from the trench most of this deformation can be attributed to inter seismic strain accumulation due to the locking of the thrust interface when the dislocation model predictions are subtracted from the observed velocities there is evidence for an additional n s oriented contraction at a rate of 4 mm yr over a distance of 250 km this signal presumably represents a more long term deformation pattern than the periodic accumulation and release of elastic strain connected with subduction earthquakes and is most likely related to the occurrence of shallow earthquakes in western washington that are characterized by predominantly n s oriented maximum principal stress

an inverse dislocation model of surface deformation in western washington abstract active horizontal and vertical deformation in western washington is modeled using an inverse dislocation model the modeling technique uses published horizontal crustal velocities derived from campaign and continuous gps data and uplift rates calculated from repeated historical leveling surveys the plate interface is represented by both screw and edge dislocations divided into segments the rate of slip deficit accumulation on each segment is estimated by inverting the surface deformation data using a linear least squares procedure the model predicts the data well with an overall rms residual of 0 85 mm year the horizontal residual vectors show no discernable pattern and no significant uniform regional strain and rigid rotations can be identified the estimated slip deficit accumulation rates suggest that the plate interface is locked in a broad region beneath the coastline and possibly partially locked further downdip the maximum slip deficit rate was estimated at 32 1 1 9 mm year or approximately 91 of the plate convergence rate the partially locked zone appears to be accumulating slip deficit at 8 2 1 9 mm year

a new inference of mantle viscosity based upon joint inversion of convection and glacial isostatic adjustment data we present new profiles of mantle viscosity derived on the basis of non linear occam style joint inversions of an extensive set of data associated with mantle convection and glacial isostatic adjustment gia the convection related observables include satellite derived free air gravity harmonics the geodetically inferred excess ellipticity of the cmb and tectonic plate motions the gia constraints involve two classes of observables previously shown to be relatively insensitive to errors in the late pleistocene ice history the so called fennoscandian relaxation spectrum frs and a set of site specific decay times determined from the postglacial sea level history in hudson bay and sweden the inverted viscosity profiles show a significant three orders of magnitude increase from the upper mantle mean value of f4 10 20 pa s to a high viscosity 10 23 pa s peak at

sea level as a stabilizing factor for marine ice sheet grounding lines climate change could potentially destabilize marine ice sheets which would affect projections of future sea level rise1 2 3 4 specifically an instability mechanism5 6 7 8 has been predicted for marine ice sheets such as the west antarctic ice sheet that rest on reversed bed slopes whereby ice sheet thinning or rising sea level leads to irreversible retreat of the grounding line however existing analyses of this instability mechanism have not accounted for deformational and gravitational effects that lead to a sea level fall at the margin of a rapidly shrinking ice sheet9 10 11 here we present a suite of predictions of gravitationally self consistent sea level change following grounding line migration our predictions vary the initial ice sheet size and also consider the contribution to sea level change from various subregions of the simulated ice sheet using these results we revisit a canonical analysis of marine ice sheet stability5 and demonstrate that gravity and deformation induced sea level changes local to the grounding line contribute a stabilizing influence on ice sheets grounded on reversed bed slopes we conclude that accurate treatments of sea level change should be incorporated into analyses of past and future marine ice sheet dynamics

large scale changes in greenland outlet glacier dynamics triggered at the terminus the recent dramatic retreat of many outlet glaciers of the greenland ice sheet has raised concerns over greenland s contribution to future sea level rise simulations with a numerical ice flow model indicate that the recent rates of mass loss in greenland s outlet glaciers are transient and should not be extrapolated into the future

early deglaciation of the british irish ice sheet on the atlantic shelf northwest of ireland driven by glacioisostatic depression and high relative sea level understanding the triggers and pace of marine based ice sheet decay is critical for constraining the future mass loss and dynamic behaviour of marine based sectors of the large polar ice sheets in greenland and antarctica numerical models which seek to predict this behaviour need to be calibrated against data from both contemporary and palaeo ice sheets and the latter requires accurate reconstruction of former ice sheet extent dynamics and timing marine geophysics sediment cores benthic foraminiferal assemblages and radiocarbon dating are used to reconstruct the extent of the last british irish ice sheet biis and the timing and style of its retreat on the atlantic shelf northwest of ireland shelf edge moraines and subglacial till recovered in cores from the outer continental shelf are dated to younger than 26 3 ka cal bp and indicate an extensive ice sheet at the last glacial maximum lgm that was grounded to the shelf edge nested arcuate moraines record the subsequent episodic retreat of the ice sheet across the shelf lithofacies and associated foraminiferal assemblages demonstrate that this retreat occurred in a glacimarine environment as a grounded tidewater margin and that high relative sea level and cold waters prevailed during retreat radiocarbon dates indicate that the timing of initial ice sheet retreat from the shelf edge occurred in the interval between 26 3 and 24 8 ka cal bp during the period of minimum global eustatic sea level and that the ice sheet had retreated to the mid shelf by 24 8 ka cal bp the donegal bay moraine a large moraine at the mouth of donegal bay records a major stillstand and readvance of the ice sheet during deglaciation between 20 2 and 17 9 ka cal bp estimated retreat rates of 5 5 35 m a 1 across the shelf demonstrate that retreat was slow it is noteworthy that retreat was initiated in the absence of ocean warming and when eustatic sea level was at a minimum the sea level rise that initiated deglaciation from the shelf edge therefore is inferred to have been a product of local glacio isostatic crustal depression rather than external forcing this demonstrates that marine based sectors of ice sheets can trigger their own demise internally through glacio isostatic adjustment and it provides an explanation for the early retreat of the biis on the atlantic shelf during the global lgm glgm

north atlantic subsurface temperature response controlled by effective freshwater input in heinrich events abstract the northern north atlantic nna subsurface temperature in response to the slowdown of the atlantic meridional overturning circulation amoc is crucial for ice sheet calving and recovery of the amoc in heinrich events paleoclimate proxies and modeling studies suggest that the nna subsurface exhibits a robust warming during heinrich 1 but with a less clear response during younger dryas yd the mechanism for the potentially different subsurface responses has remained not well understood previous studies show different nna subsurface response depending on hosing at different locations here by examining a suite of water hosing experiments with different hosing regions and intensities we show that regardless of the hosing location nna subsurface temperature response is determined by the effective freshening over the nna deep convective regions through the competition between the warming associated with the suppressed vertical mixing and the cooling associated with the weakened amoc heat transport a weak effective freshening favors advective effect and in turn cooling while a strong effective freshening strengthens the mixing effect and leads to a warming our results suggest that a cooling may occur in the nna subsurface during yd

ocean subsurface warming as a mechanism for coupling dansgaard oeschger climate cycles and ice rafting events 1 heinrich events have been attributed to surging of the laurentide ice sheet every 7 thousand years or so during glacial time these massive ice rafting events only occurred during cold phases of millennial scale dansgaard oeschger climate cycles other observed ice rafting events sourced from ice streams at various locations around the northern north atlantic occurred during all dansgaard oeschger cold phases and led heinrich events when the latter took place here it is suggested that ocean subsurface warming in the northern north atlantic during the cold phases may provide the key to explaining these climate ice rafting phasings such warming would lead to ice shelf melting and breakup without buttressing by ice shelves ice streams may surge leading to increased iceberg production this interpretation is supported by results of a simplified coupled climate model and by available sediment and ice sheet data

interseismic uplift rates for western oregon and along strike variation in locking on the cascadia subduction zone 1 we quantify the spatial pattern of uplift rate in western oregon and northernmost california using tidal and leveling records to better understand the pattern of interseismic locking on the cascadia subduction zone we extend relative sea level time series of the six primary noaa tide gauges to include all observations from 1925 to 2006 previously unidentified bench mark instability biases portions of tidal records by 1 6 mm a 1 before correction we determine precise relative uplift rates at the six tidal sites with an adjustment that includes rates of differenced time series as additional constraints our analysis of national geodetic survey leveling data between tide gauges corrects errors in 1941 leveling and 184 secondary ties double the number of highest quality uplift rate estimates relative uplift rates from leveling are adjusted to the tidal rates accounting for uncertainties in both data types tidal and leveling uplift rates agree within error for all but one of the coastal segments where we infer systematic leveling error affects the 1988 line uplift rates are made absolute using an interval and location specific geocentric sea level rise rate of 2 3 0 2 mm a 1 total propagated one sigma errors for the absolute uplift rates of bench marks are 0 4 mm a 1 along strike changes in uplift rate near 45 n and 42 8 n require two distinct changes in locking depth as inferred from elastic dislocation modeling the along strike changes in locking on this portion of the cascadia subduction zone interface correlate to the western and southern extent of the mafic siletzia block in the fore arc

iceberg discharges of the last glacial period driven by oceanic circulation changes proxy data reveal the existence of episodes of increased deposition of ice rafted detritus in the north atlantic ocean during the last glacial period interpreted as massive iceberg discharges from the laurentide ice sheet although these have long been attributed to self sustained ice sheet oscillations growing evidence of the crucial role that the ocean plays both for past and future behavior of the cryosphere suggests a climatic control of these ice surges here we present simulations of the last glacial period carried out with a hybrid ice sheet ice shelf model forced by an oceanic warming index derived from proxy data that accounts for the impact of past ocean circulation changes on ocean temperatures the model generates a time series of iceberg discharge that closely agrees with ice rafted debris records over the past 80 ka indicating that oceanic circulation variations were responsible for the enigmatic ice purges of the last ice age

depth of seismic coupling along subduction zones underthrusting at subduction zones can cause large earthquakes at shallow depths but it is always accommodated by aseismic deformation below a certain depth the maximum depth of the seismically coupled zone or seismogenic zone is a transition from unstable to stable sliding along the plate interface we have determined the depth of this stability transition for the circurn pacific subduction zones of honshu kuriles kamchatka aleutians alaska mexico and chile these subduction zones have experienced great interplate earthquakes and the aftershock regions are well located depth estimates of interplate events that are located at the downdip edge of the aftershock regions are used to determine the maximum depth of seismic coupling for an average p wave velocity of 6 7 km s 1 above the plate interface we find that for most subduction zones the stability transition occurs at 40 5 km depth there are however several exceptions at the hokkaido trench junction where the japan trench and the kurile trench intersect seismic coupling is deep and extends down to 52 55 km deep coupling was also found in the coquimbo region in central chile the mexico subduction zone has shallow coupling the transition occurs at 20 30 km depth previous studies of micro earthquakes in honshu hokkaido the aleutians and alaska show that earthquakes within the upper plate extend no deeper than the downdip edge of the coupled zone that we find given our measurements of seismic coupling depth we then explore the mechanism that may determine coupling depth the concept of critical temperature has been used to explain the depth of seismic coupling in other tectonic environments thus we first test whether a critical temperature can explain our results temperatures at the plate interface are dependent on many variables but two that are poorly determined are shear stress and radiogenic heat generation shear stress has been constrained by inversion of heat flow data assuming a crustal radiogenic heat production rate of 3 1 exp z 8 5 wm 3 and a constant coefficient of friction we find two critical temperatures of about 400 c and 550 c the lower critical temperature may be characteristic of regions with a relatively thick continental crust and the higher temperature of regions with a relatively thin continental crust on the other hand one single critical temperature of about 250 c can explain the coupling depths if shear stresses are constant with depth

constraints on accumulated strain near the ets zone along cascadia abstract current national seismic hazard models for cascadia use the zone of episodic tremor and slip ets to denote the lower boundary of the seismogenic zone recent numerical models have suggested that an appreciable amount of long term strain may accumulate at the depth of ets and questions this assumption we use uplift rates from leveling campaigns spanning approximately 50 70 yrs in washington and oregon to investigate the amount of potential long term locking near the ets zone we evaluate the potential for deeper locking in cascadia by exploring a range of locking parameters along the subduction zone including the ets zone of the four east west leveling profiles studied three show a reduction in the misfit when secondary locking near the ets zone is included however the reduction in misfit values is only statistically significant for one profile this would suggest that models including a small amount of secondary locking are broadly indistinguishable from models without any secondary locking if secondary locking is considered the leveling data allow for locking up to 20 of the plate rate near the updip edge of the ets zone these results are consistent with but less resolved by gps observations

reconciling records of ice streaming and ice margin retreat to produce a palaeogeographic reconstruction of the deglaciation of the laurentide ice sheet abstract this paper reconstructs the deglaciation of the laurentide ice sheet lis including the innuitian ice sheet from the last glacial maximum lgm with a particular focus on the spatial and temporal variations in ice streaming and the associated changes in flow patterns and ice divides we build on a recent inventory of laurentide ice streams and use an existing ice margin chronology to produce the first detailed transient reconstruction of the ice stream drainage network in the lis which we depict in a series of palaeogeographic maps results show that the drainage network at the lgm was similar to modern day antarctica the majority of the ice streams were marine terminating and topographically controlled and many of these continued to function late into the deglaciation until the ice sheet lost its marine margin ice streams with a terrestrial ice margin in the west and south were more transient and ice flow directions changed with the build up peak phase and collapse of the cordilleran laurentide ice saddle the south eastern marine margin in atlantic canada started to retreat relatively early and some of the ice streams in this region switched off at or shortly after the lgm in contrast the ice streams draining towards the north western and north eastern marine margins in the beaufort sea and in baffin bay appear to have remained stable throughout most of the late glacial and some of them continued to function until after the younger dryas yd the yd influenced the dynamics of the deglaciation but there remains uncertainty about the response of the ice sheet in several sectors we tentatively ascribe the switching on of some major ice streams during this period e g m clintock channel ice stream at the north west margin but for other large ice streams whose timing partially overlaps with the yd the drivers are less clear and ice dynamical processes rather than effects of climate and surface mass balance are viewed as more likely drivers retreat rates markedly increased after the yd and the ice sheet became limited to the canadian shield this hard bed substrate brought a change in the character of ice streaming which became less frequent but generated much broader terrestrial ice streams the final collapse of the ice sheet saw a series of small ephemeral ice streams that resulted from the rapidly changing ice sheet geometry in and around hudson bay our reconstruction indicates that the lis underwent a transition from a topographically controlled ice drainage network at the lgm to an ice drainage network characterised by less frequent broad ice streams during the later stages of deglaciation these deglacial ice streams are mostly interpreted as a reaction to localised ice dynamical forcing flotation and calving of the ice front in glacial lakes and transgressing sea basal de coupling due to large amount of meltwater reaching the bed debuttressing due to rapid changes in ice sheet geometry rather than as conveyors of excess mass from the accumulation area of the ice sheet at an ice sheet scale the ice stream drainage network became less widespread and less efficient with the decreasing size of the deglaciating ice sheet the final elimination of which was mostly driven by surface melt

heinrich events triggered by ocean forcing and modulated by isostatic adjustment heinrich events episodes of massive iceberg discharge from the laurentide ice sheet into the north atlantic ocean are triggered by the incursion of warm ocean waters destabilizing the calving front heinrich events are the great belching of armadas of icebergs from ice sheets such as the laurentide ice sheet into the north atlantic but despite decades of research and numerous proposals the mechanism that triggers heinrich events remains intensely debated jeremy bassis et al present new modelling evidence showing that heinrich events are produced by a surprisingly simple mechanism the incursion of warm ocean waters destabilizes calving fronts leading to the abrupt release of ice critically the isostatic rebound of the ocean bed elevates the calving tongue cutting off the marine interface and producing the characteristic sharpness of heinrich events during the last glacial period the laurentide ice sheet sporadically discharged huge numbers of icebergs through the hudson strait into the north atlantic ocean leaving behind distinct layers of ice rafted debris in the ocean sediments1 2 3 perplexingly these massive discharge events heinrich events occurred during the cold portion of millennial scale climate oscillations called dansgaard oeschger cycles2 4 this is in contrast to the expectation that ice sheets expand in colder climates and shrink in warmer climates here we use an ice sheet model to show that the magnitude and timing of heinrich events can be explained by the same processes that drive the retreat of modern marine terminating glaciers in our model subsurface ocean warming associated with variations in the overturning circulation increases underwater melt along the calving face triggering rapid margin retreat and increased iceberg discharge on millennial timescales isostatic adjustment causes the bed to uplift isolating the terminus from subsurface warming and allowing the ice sheet to advance again until at its most advanced position it is poised for another heinrich event this mechanism not only explains the timing and magnitude of observed heinrich events but also suggests that ice sheets in contact with warming oceans may be vulnerable to catastrophic collapse even with little atmospheric warming

bounds on the calving cliff height of marine terminating glaciers increased calving and rapid retreat of glaciers can contribute significantly to sea level rise but the processes controlling glacier retreat remain poorly understood we seek to improve our understanding of calving by investigating the stress field controlling tensile and shear failure using a 2d full stokes finite element model using idealized rectangular geometries we find that when rapidly sliding glaciers thin to near buoyancy full thickness tensile failure occurs similar to observations motivating height above buoyancy calving laws in contrast when glaciers are frozen to their beds basal crevasse penetration is suppressed and calving is minimal we also find shear stresses are largest when glaciers are thickest together the tensile and shear failure criteria map out a stable envelope in an ice thickness water depth diagram the upper and lower bounds on cliff height can be incorporated into numerical ice sheet models as boundary conditions thus bracketing the magnitude of calving rates in marine terminating glaciers

geologic evidence for earthquakes during the past 2000 years along the copalis river southern coastal washington evidence for several prehistoric earthquakes is present in deposits less than 2000 years old that crop out at the copalis river estuary 47 07 2 n 124 09 7 w the deposits show that land subsided decimeters into the intertidal zone on as many as three occasions and that sand vented while the land underwent little or no subsidence on another occasion the evidence for earthquake induced subsidence consists of buried marsh and forest soils that evince sudden estuarine submergence attended on at least one occasion by a landward directed surge of sandy water this combination of submergence and surge implies subsidence and tsunami exposed evidence for subsidence and tsunami along the copalis river is strong for a time within a few decades of 300 years ago moderate for a time 1400 1900 years ago and weak subsidence or lacking tsunami for an intermediate time also about 1400 1900 years ago the history of sudden submergence and tsunami in the past 2000 years at the copalis river estuary resembles that shown by buried soils farther south in coastal washington at grays harbor and willapa bay the history can be explained most simply by great mw of 8 or 9 thrust earthquakes at the boundary between the juan de fuca and north america plates with one or two earthquakes occurring 1400 1900 years ago and another close to 300 years ago the evidence for earthquake induced venting along the copalis river consists of sand intrusions and vented sand volcanoes the sand rose through muddy estuarine deposits at least 3 m thick entraining these deposits in fragments as much as 0 3 m long intrusion occurred at one or more times in the past 2000 years and at least some of the venting occurred 900 1300 years ago the sand bodies do not imply artesian flow of flood pressured groundwater because the sand vented forcefully and infrequently through a lowland that probably lacked natural levees the earthquake implied by the venting of sand 900 1300 years ago could have come from a source below above or at the boundary between the juan de fuca and north america plates if from the subducted juan de fuca plate the implied earthquake was farther west or larger than the largest earthquakes within that plate during the past 100 years evidence permitting a source within the north america plate includes a fault scarp an uplifted tideflat and landslides near puget sound dating from the interval 700 1700 years ago a plate boundary source would explain buried soils whose estuarine submergence and burial imply coseismic subsidence for parts of coastal washington in the interval 600 1300 years ago however stratigraphy shows that little or no coseismic subsidence occurred along the copalis river during the venting 900 1300 years ago if a plate boundary earthquake struck coastal washington 900 1300 years ago the earthquake produced a different distribution of land level change than did the plate boundary earthquakes inferred for 1400 1900 and 300 years ago

abrupt uplift within the past 1700 years at southern puget sound washington shorelines rose as much as 7 meters along southern puget sound and hood canal between 500 and 1700 years ago evidence for this uplift consists of elevated wave cut shore platforms near seattle and emerged peat covered tidal flats as much as 60 kilometers to the southwest the uplift was too rapid for waves to leave intermediate shorelines on even the best preserved platform the tidal flats also emerged abruptly they changed into freshwater swamps and meadows without first becoming tidal marshes where uplift was greatest it adjoined an inferred fault that crosses puget sound at seattle and it probably accompanied reverse slip on that fault 1000 to 1100 years ago the uplift and probable fault slip show that the crust of the north america plate contains potential sources of damaging earthquakes in the puget sound region

geomagnetic paleointensity and environmental record from labrador sea core md95 2024 global marine sediment and ice core chronostratigraphy for the last 110 kyr piston core md95 2024 from the labrador rise provides a continuous record of rapidly deposited detrital layers denoting laurentide ice sheet lis instability the core also provides a high resolution record of geomagnetic paleointensity that is consistent with but at higher temporal resolution than previous labrador sea records correlation to the greenland summit ice cores grip gisp2 is achieved by assuming that labrador sea detrital layers correspond to cold stadials in the ice cores this allows a gisp2 official chronology to be placed on md95 2024 which is consistent with the inverse correlation between paleointensity and the flux of cosmogenic isotopes 10 be and 36 cl in greenland ice cores synchronous millennial scale variability observed from the md95 2024 paleointensity

confidence intervals for motion and deformation of the juan de fuca plate finite rotation parameters for 17 isochrons from anomalies 1 4a for the juan de fuca pacific plate pair are reported comparison of confidence intervals for plate rotations derived by different techniques indicates that a single plate misfit criterion significantly underestimates the confidence region defining the confidence interval by a threshold level of misfit determined by an f statistic gives similar results to the technique of bootstrap resampling for internally consistent data bootstrap resampling offers far greater robustness in the case of inconsistent data precision of about 1 km in the direction normal to isochrons is available for the younger anomalies this precision allows confident recognition of 2 5 km of eastward deformation of the pacific plate 50 100 km south of the active sovanco transform slight deformation of the northern juan de fuca plate adjacent to the nootka fault zone is also apparent east of the northern gorda ridge younger 3 ma anomalies do not show measurable deformation near the blanco fracture zone but older anomalies are up to 30 km east of their position predicted by rigid behavior of the juan de fuca plate reconstruction of the overlap geometry of past propagators shows no evidence that the extent of overlap has ever exceeded 70 km even with offset of 75 km in one case and 140 km in another new techniques for testing for intervals of constant plate motion require at least two changes in instantaneous motion at 99 confidence a clockwise change in motion direction at about 5 ma and an increase in the gradient of spreading rate since 2 ma two other changes in gradient since 5 ma are required at 95 confidence

crustal structure along the west flank of the cascades western washington knowledge of the crustal structure of the washington cascades and adjacent puget lowland is important to both earthquake hazards studies and geologic studies of the evolution of this tectonically active region we present a model for crustal velocity structure derived from analysis of seismic refraction wide angle reflection data collected in 1991 in western washington the 280 km long north south transect skirts the west flank of the cascades as it crosses three tectonic provinces including the northwest cascades thrust system nwcs the puget lowland and the volcanic arc of the southern cascades within the nwcs upper crustal velocities range from 4 2 to 5 7 km s 1 and are consistent with the presence of a diverse suite of mesozoic and paleozoic metasediments and metavolcanics in the upper 2 3 km of the puget lowland velocities drop to 1 7 3 5 km s 1 and reflect the occurrence of oligocene to recent sediments within the basin in the southern washington cascades upper crustal velocities range from 4 0 to 5 5 km s 1 and are consistent with a large volume of tertiary sediments and volcanics a sharp change in velocity gradient at 5 10 km marks the division between the upper and middle crust from approximately 10 to 35 km depth the velocity field is characterized by a velocity increase from 6 0 to 7 2 km s 1 these high velocities do not support the presence of marine sedimentary rocks at depths of 10 20 km beneath the cascades as previously proposed on the basis of magnetotelluric data crustal thickness ranges from 42 to 47 km along the profile the lowermost crust consists of a 2 to 8 km thick transitional layer with velocities of 7 3 7 4 km s 1 the upper mantle velocity appears to be an unusually low 7 6 7 8 km s 1 when compared to velocity models from other regions this model most closely resembles those found in active continental arcs distinct seismicity patterns can be associated with individual tectonic provinces along the seismic transect in the nwcs and puget lowland most of the seismicity occurs below the base of the upper crust as defined by a seismic boundary at 5 10 km depth and continues to 20 30 km depth the region of transition between the nwcs and the puget lowland appears as a gap in seismicity with notably less seismic activity north of the boundary between the two earthquakes within the cascades are generally shallower 0 20 km and are dominated by events associated with the rainier seismic zone

palaeoclimate science pulsating ice sheet during the last ice age huge numbers of icebergs were episodically discharged from an ice sheet that covered north america numerical modelling suggests that these events resulted from a conceptually simple feedback cycle see letter p 332 heinrich events are the great belching of armadas of icebergs from ice sheets such as the laurentide ice sheet into the north atlantic but despite decades of research and numerous proposals the mechanism that triggers heinrich events remains intensely debated jeremy bassis et al present new modelling evidence showing that heinrich events are produced by a surprisingly simple mechanism the incursion of warm ocean waters destabilizes calving fronts leading to the abrupt release of ice critically the isostatic rebound of the ocean bed elevates the calving tongue cutting off the marine interface and producing the characteristic sharpness of heinrich events

anatomy of heinrich layer 1 and its role in the last deglaciation this research used data acquired at the xrf core scanner lab at the marum center for marine environmental sciences university of bremen germany this research used samples provided by the international ocean discovery program iodp funding for this research was provided by the uk natural environmental research council nerc to hodell the nerc radiocarbon facility supported two radiocarbon dates and wally broecker generously supported the remainder with funding from the comer family foundation research by rodriguez tovar and dorador was financed by project cgl2015 66835 p b m acknowledges support from the csic ramon y cajal postdoctoral programme ryc 2013 14073 j f e would like to acknowledge funding under erc advanced grant 320750 nanopaleomagnetism

consistently dated records from the greenland grip gisp2 and ngrip ice cores for the past 104 ka reveal regional millennial scale δ18o gradients with possible heinrich event imprint abstract we present a synchronization of the ngrip grip and gisp2 ice cores onto a master chronology extending back to 104 ka before present providing a consistent chronological framework for these three greenland records the synchronization aligns distinct peaks in volcanic proxy records and other impurity records chemo stratigraphic matching and assumes that these layers of elevated impurity content represent the same instantaneous event in the past at all three sites more than 900 marker horizons between the three cores have been identified and our matching is independently confirmed by 24 new and previously identified volcanic ash tephra tie points using the reference horizons we transfer the widely used greenland ice core chronology gicc05modelext to the two summit cores grip and gisp2 furthermore we provide gas chronologies for the summit cores that are consistent with the gicc05modelext timescale by utilizing both existing and new gas data ch4 concentration and 15n of n2 we infer that the accumulation contrast between the stadial and interstadial phases of the glacial period was 10 greater at summit compared to at ngrip the 18o temperature proxy records from ngrip grip and gisp2 are generally very similar and display synchronous behaviour at climate transitions the 18o differences between summit and ngrip however changed slowly over the last glacial interglacial cycle and also underwent abrupt millennial to centennial scale variations we suggest that this observed latitudinal 18o gradient in greenland during the glacial period is the result of 1 relatively higher degree of precipitation with a pacific signature at ngrip 2 increased summer bias in precipitation at summit and 3 enhanced rayleigh distillation due to an increased source to site distance and a potentially larger source to site temperature gradient we propose that these processes are governed by changes in the north american ice sheet nais volume and north atlantic sea ice extent and or sea surface temperatures sst on orbital timescales and that changing sea ice extent and ssts are the driving mechanisms on shorter timescales finally we observe that maxima in the summit ngrip 18o difference are roughly coincident with prominent heinrich events this suggests that the climatic reorganization that takes place during stadials with heinrich events possibly driven by a southward expansion of sea ice and low ssts in the north atlantic are recorded in the ice core records

a new constraint on the size of heinrich events from an iceberg sediment model abstract heinrich layers anomalously thick layers of ice borne sediment in the north atlantic ocean have long been associated with abrupt climate changes in glacial times however there is still no consensus on either the exact amount of ice needed to transport this sediment or how such a large volume of ice could be produced using an iceberg model that includes sediment we simulate the delivery of sediment to the north atlantic during such an event our model assumes that sediment is uniformly distributed within the ice with a concentration of 4 unlike sediment models which assume that the sediment lies in a single layer this model can carry sediment all the way from the western to the eastern north atlantic we use a variety of different estimates for the total volume of ice released to model the sediment layer thickness and we show that to best fit the observations 60 10 4 km 3 with a plausible range of 30 120 10 4 km 3 of ice needs to be released this is equivalent to a 0 04 sv 10 6 m 3 s 1 with a plausible range of 0 02 0 08 sv release of freshwater over the 500 yr of a typical heinrich event this is a smaller flux of water than is required to show a significant impact on the global climate in most current state of the art gcms

acceleration of jakobshavn isbrae triggered by warm subsurface ocean waters observations over past decades show a sudden switch of jakobshavn isbrae a large outlet glacier feeding a deep ocean fjord on greenland s west coast from slow thickening to rapid thinning in 1997 this switch is associated with a doubling in glacier velocity hydrographic data show a concurrent sudden increase in subsurface ocean temperatures along the entire west coast of greenland suggesting that the changes in jakobshavn isbrae were triggered by the arrival of relatively warm water originating from the irminger sea observations over the past decades show a rapid acceleration of several outlet glaciers in greenland and antarctica1 one of the largest changes is a sudden switch of jakobshavn isbrae a large outlet glacier feeding a deep ocean fjord on greenland s west coast from slow thickening to rapid thinning2 in 1997 associated with a doubling in glacier velocity3 suggested explanations for the speed up of jakobshavn isbrae include increased lubrication of the ice bedrock interface as more meltwater has drained to the glacier bed during recent warmer summers4 and weakening and break up of the floating ice tongue that buttressed the glacier5 here we present hydrographic data that show a sudden increase in subsurface ocean temperature in 1997 along the entire west coast of greenland suggesting that the changes in jakobshavn isbrae were instead triggered by the arrival of relatively warm water originating from the irminger sea near iceland we trace these oceanic changes back to changes in the atmospheric circulation in the north atlantic region we conclude that the prediction of future rapid dynamic responses of other outlet glaciers to climate change will require an improved understanding of the effect of changes in regional ocean and atmosphere circulation on the delivery of warm subsurface waters to the periphery of the ice sheets

continental slope sedimentation adjacent to an ice margin iii the upper labrador slope abstract the upper labrador slope is the key area for sediment transfer from the northeastern margin of the pleistocene laurentide ice sheets lis to the deep basin it is subdivided from north to south into 8 sectors based on relief differences and echo character on 750 line km of continuous sleeve gun and 3 5 khz seismic profiles high to moderate relief sectors 2 and 4 and in part 6 and 8 are seismically transparent and well stratified with continuous individual high amplitude reflections and deep penetration low relief sectors 1 3 5 and 7 show strong bottom reflections and with the exception of sector 1 low seismic penetration poor stratification and low amplitude if any subbottom reflections the lateral distribution of these alternating high to moderate and low relief sectors reflects fundamental differences in the sediment transfer mechanisms through outlets from the lis onto the slope low relief slope sectors represent debrite and turbidite slopes and are located in front of ice outlets on the slope and adjacent regions to the north they originate from mass wasting on the upper slope of glacial detritus with a significant coarse component originally delivered as englacial or subglacial material and deposited in end moraines or as bedload by subglacial or supraglacial rivers high to moderate relief sectors originate from fall out of suspended sediment from turbid surface plumes tsp and preferentially occur off the southern half of outlets and south of major outlets as exemplified by sector 2 south of the hudson strait outlet this asymmetry is caused by the south flowing labrador current which entrains the buoyantly rising turbid meltwater plumes at the glacier front the present high relief topography which shows a dendritic pattern of upslope canyon branching is the result of retrograde headward canyon erosion by mass wasting processes of an originally much smoother mud blanket surface the sediment is remobilized by slumping and entrained in debris flows and turbidity currents the occurrence of tsp deposits on the high to moderate relief upper slope sectors requires a summer sea surface not frozen over during major parts of the pleistocene including glacial maxima tps deposition on the upper slope ice rafting and a plethora of ice margin depositional phenomena many of which give rise to mass wasting and mass flow phenomena are the main features that make high latitude continental slopes adjacent to continental ice sheets different from their lower latitude counterparts

strain accumulation in western washington the juan de fuca plate is subducted beneath the north american plate off the coast of washington at a rate of about 40 mm yr n68 e the average principal strain rates extension reckoned positive measured in northwestern washington are as follows olympic peninsula 25 km south of port angeles from 1982 through 1990 e 1 0 011 0 027 strain yr n31 e 6 6 and e 2 0 092 0 028 strain yr n59 w 6 6 and near seattle from 1972 through 1985 e 1 0 027 0 019 strain yr n22 e 6 4 and e 2 0 036 0 013 strain yr n68 w 6 4 both strain measurements are consistent with uniaxial contraction in the direction of plate convergence uplift rates inferred from tide gage recordings are about 4 mm yr on the pacific coast and near 0 mm yr farther inland near seattle these deformation rates are consistent with a model of the cascadia subduction zone in which the plate interface beneath the continental slope and outer continental shelf is locked but free to slip farther landward the limited downdip extent of the locked segment of the plate interface is consistent with a shallow depth 20 km of the isotherm 450 c that defines the brittle ductile transition small thrust events diagnostic of seismic subduction should then occur only offshore and at shallow depths the principal strain rates measured from 1972 through 1983 in the back arc region near richland washington are e 1 0 016 0 013 strain yr n03 w 34 and e 2 0 024 0 013 strain yr n87 e 34

three dimensional dislocation model for great earthquakes of the cascadia subduction zone there have been no historical cascadia great subduction thrust earthquakes but there is good recent evidence that very large earthquakes have occurred in the past and that strain is building up toward a future great event geodetic measurements in the coastal region from northern california to southern british columbia show vertical and horizontal deformation as expected for the strain accumulation of a locked thrust fault the segment of the subduction thrust that is locked and may rupture in future great events has previously been estimated through two dimensional 2 d elastic dislocation modeling of interseismic deformation geodetic data in this study a general 3 d dislocation model for thrust faults has been developed that accommodates curved fault geometry and nonuniform interseismic locking or coseismic rupture the model is based on die surface deformation due to shear faulting in an elastic half space the 3 d model of the cascadia subduction zone calculates the surface deformation for a locked zone or a rupture zone of variable width along the margin the bend in the margin trend and subducting slab end effects are included there is a downdip transition zone between interseismic completely locked and free slip portions of the fault or between coseismic full rupture and no displacement an initial 3 d model based upon 2 d dislocation models and upon thermal constraints was adjusted to optimize the fit of the predicted interseismic surface deformation to current deformation geodetic data the best fit model has the thrust locked along the whole margin with an average locked zone width of 60 km and a transition zone width of 60 km the two zones lie mainly offshore beneath the continental shelf and slope the locked and transition zone widths vary smoothly along the margin being greater off northern washington where the thrust dip is shallow and narrower off central oregon assuming that the locked plus transition zones approximate the maximum coseismic rupture area these widths permit amw 9 earthquake

a dislocation model of strain accumulation and release at a subduction zone strain accumulation and release at a subduction zone are attributed to stick slip on the main thrust zone and steady aseismic slip on the remainder of the plate interface this process can be described as a superposition of steady state subduction and a repetitive cycle of slip on the main thrust zone consisting of steady normal slip at the plate convergence rate plus occasional thrust events that recover the accumulated normal slip because steady state subduction does not contribute to the deformation at the free surface deformation observed there is completely equivalent to that produced by the slip cycle alone the response to that slip is simply the response of a particular earth model to embedded dislocations for a purely elastic earth model the deformation cycle consists of a coseismic offset followed by a linear in time recovery to the initial value during the interval between earthquakes for an elastic viscoelastic earth model elastic lithosphere over a viscoelastic asthenosphere the postearthquake recovery is not linear in time records of local uplift as a function of time indicate that the long term postseimic recovery is approximately linear suggesting that elastic earth models are adequate to describe the deformation cycle however the deformation predicted for amore simple elastic half space earth does not reproduce the deformation observed along the subduction zones in japna at all well if stick slip is restricted to the main thrust zone as recognized earlier by shimazaki seno and kato the uplift profiles could be explained if stick slips were postulated to extend along the plate interface beyond the main thrust zone to a depth of perhaps 100 km but independent evidence suggests that stick slip at such depths if unlikely less

contemporary vertical crustal deformation in cascadia abstract data from 29 tide gauges and 113 pairs of first and second order leveling lines are analyzed to determine the pattern of vertical deformation in the pacific northwest of the united states the data span nearly 100 years and represent the interseismic elastic deformation related to the great earthquake cycle uplift rates calculated from leveling surveys are adjusted to a model surface in the tidal reference frame using a robust weighted linear least square technique rapid uplift occurs in two distinct broad regions along the coast separated by a narrow zone of slow subsidence vertical deformation rates range from 4 mm year of uplift on southern vancouver island to 2 mm year of subsidence in northern coastal oregon the deformation pattern is consistent with the results of previous studies and subduction models

a new view into the cascadia subduction zone and volcanic arc implications for earthquake hazards along the washington margin in light of suggestions that the cascadia subduction margin may pose a significant seismic hazard for the highly populated pacific northwest region of the united states the u s geological survey usgs the research center for marine geosciences geomar and university collaborators collected and interpreted a 530 km long wide angle onshore offshore seismic transect across the subduction zone and volcanic arc to study the major structures that contribute to seismogenic deformation we observed 1 an increase in the dip of the juan de fuca slab from 2 7 to 12 where it encounters a 20 km thick block of the siletz terrane or other accreted oceanic crust 2 a distinct transition from siletz crust into cascade arc crust that coincides with the mount st helens seismic zone supporting the idea that the mafic siletz block focuses seismic deformation at its edges and 3 a crustal root 35 45 km deep beneath the cascade range with thinner crust 30 35 km east of the volcanic arc beneath the columbia plateau flood basalt province from the measured crustal structure and subduction geometry we identify two zones that may concentrate future seismic activity 1 a broad because of the shallow dip possibly locked part of the interplate contact that extends from 25 km depth beneath the coastline to perhaps as far west as the deformation front 120 km offshore and 2 a crustal zone at the eastern boundary between the siletz terrane and the cascade range

environmental change and human activity in the northeastern part of the north china plain during early mis 2 abstract marine isotope stage 2 mis 2 the coldest and the longest glacial during the last glacial period was an important interval for the development and evolution of microlithic cultures the zhaocun site located in the northeastern part of the north china plain is not only the first late paleolithic site discovered in the southern piedmont of the yan mountains but also an important representative of late pleistocene strata and fossil fauna in north china however there have been few studies of the upper cultural layer at the site which have focused on the development of paleolithic culture especially the microlithic technology in the context of environmental changes moreover no absolute age determinations are available here we present a study of the paleoenvironmental record of the zhibuchang profile at the zhaocun site during the interval of 27 6 23 6 cal kyr bp we conducted a stratigraphic study based on pollen and grain size analyses of 78 samples from the profile and obtained 7 ams 14c dates during 27 6 26 6 cal kyr bp the regional vegetation was relatively diverse meadow steppe dominated by artemisia and asteraceae the climate although colder and drier than today was the warmest record within the studied profile and was relatively favorable for human beings from 26 6 cal kyr bp with the onset of the last glacial maximum the climate became colder and more variable with two extreme climatic events occurred at 26 2 25 5 cal kyr bp and 24 3 23 6 cal kyr bp respectively a cold wet event from 26 2 to 25 5 cal kyr bp led to a drastic decline in local biodiversity which would have resulted in food shortages for human beings in addition previous areas available for human activities were gradually flooded due to the rising river level forcing the relocation of human beings and resulting in the termination of the cultural layer a cold dry event from 24 3 to 23 6 cal kyr bp which may correspond to north atlantic heinrich event 2 caused a further deterioration of the environment for human beings and may be a major cause of the absence of microlithic cultural remains on the northeastern part of the north china plain during late mis 2

three dimensional model of vertical deformation at the southern cascadia subduction zone western united states recent studies indicate that there have been several episodes of coseismic subsidence along the coasts of central and southern oregon and northern california during the past several thousand years a three dimensional dislocation model is used to predict the amount of coseismic subsidence produced by different magnitude earthquakes the magnitude of coseismic subsidence is equal to the interseismic uplift in an elastic earth model the model is constrained mainly by interseismic uplift data from repeated leveling surveys in western oregon over the past 50 60 yr the results suggest that earthquakes with moment magnitude of 8 5 are required to produce the coseismic subsidence observed the model also suggests that larger earthquakes mw 9 0 would produce coseismic subsidence in areas where none is observed in the geologic record

diverse calving patterns linked to glacier geometry iceberg calving implicated in the retreat of ice shelves is a complex process constrained by few observations numerical simulations suggest that the pattern of iceberg calving is controlled by the geometry of the glacier and that regions of greenland and antarctica may be particularly vulnerable to catastrophic calving driven retreat

current deformation and the width of the seismogenic zone of the northern cascadia subduction thrust evidence has been obtained for the accumulation of elastic strain across the northern cascadia subduction zone that may be released in a future very large subduction thrust earthquake vertical and horizontal strain rates across the southern vancouver island region have been determined through 1 long term trends in tide gauge data 2 changes in repeated accurate leveling surveys 3 changes in repeated high accuracy gravity profiles and 4 horizontal shortening observed in repeated precise positioning surveys the outer coast is uplifting at a rate of a few millimeters per year decreasing landward and shortening is occurring across the 100 km wide coastal region at a rate of about 0 1 microstrain per year mm km 1yr 1 the results are compared with the distribution of strain accumulation predicted from elastic dislocation and viscoelastic models for a subduction thrust fault the location of the fault as used in the models is well defined by multichannel seismic reflection and other geophysical data most of the observed current deformation can be explained by interseismic strain accumulation associated with the subduction thrust of southern vancouver island and northern washington provided the locked portion is restricted to a 60 km wide band offshore beneath the continental shelf and slope this conclusion also results from modeling the coseismic subsidence on the outer coast of vancouver island about 300 years ago deduced from paleoseismicity data the unusually narrow downdip extent of the subduction thrust seismogenic zone that extends little if at all beneath the coast is a consequence of high temperatures associated with the young age of the subducted oceanic lithosphere and the thick blanket of insulating sediments the high temperatures limit brittle seismogenic behavior downdip to where the thrust fault is at a depth of less than 15 km the distance from the seismic portion of the megathrust limits the estimated ground motion at the major centers of vancouver and victoria from this source the narrow width may also limit the earthquake size however events of magnitude well over 8 are possible

geodetic evidence for active landward tilting of the oregon and washington coastal ranges nine leveling routes that extend inland from near the coast of oregon and washington indicate consistent landward tilting of the 600 km long coastal ranges at about 3 10 8 rad yr 1 analysis of tide gauge observations indicates tilting with roughly the same magnitude and sense as the leveling measurements these short term tilt rates 10 80 yr are comparable with longer term rates 100 000 yr deduced from tilted marine terraces contemporary landward tilting is interpreted to result from continuing subduction of the juan de fuca plate beneath north america the similarity between long and short term deformation suggests that present day subduction is occurring via aseismic creep as hypothesized by ando and balazs 1979 however recent measurements of horizontal deformation in northwest washington which have been interpreted to indicate elastic strain accumulation appear to be incompatible with aseismic subduction additional releveling observations along the oregon routes last observed in 1941 and further studies of late pleistocene deformation along the coastal region of oregon and washington could help determine present day tectonics and assess the risk of large thrust earthquakes in this area

a stratigraphic framework for abrupt climatic changes during the last glacial period based on three synchronized greenland ice core records refining and extending the intimate event stratigraphy due to their outstanding resolution and well constrained chronologies greenland ice core records provide a master record of past climatic changes throughout the last interglacial glacial cycle in the north atlantic region as part of the intimate integration of ice core marine and terrestrial records project protocols have been proposed to ensure consistent and robust correlation between different records of past climate a key element of these protocols has been the formal definition and ordinal numbering of the sequence of greenland stadials gs and greenland interstadials gi within the most recent glacial period the gs and gi periods are the greenland expressions of the characteristic dansgaard oeschger events that represent cold and warm phases of the north atlantic region respectively we present here a more detailed and extended gs gi template for the whole of the last glacial period it is based on a synchronization of the ngrip grip and gisp2 ice core records that allows the parallel analysis of all three records on a common time scale the boundaries of the gs and gi periods are defined based on a combination of stable oxygen isotope ratios of the ice 18o reflecting mainly local temperature and calcium ion concentrations reflecting mainly atmospheric dust loading measured in the ice the data not only resolve the well known sequence of dansgaard oeschger events that were first defined and numbered in the ice core records more than two decades ago but also better resolve a number of short lived climatic oscillations some defined here for the first time using this revised scheme we propose a consistent approach for discriminating and naming all the significant abrupt climatic events of the last glacial period that are represented in the greenland ice records the final product constitutes an extended and better resolved greenland stratotype sequence against which other proxy records can be compared and correlated it also provides a more secure basis for investigating the dynamics and fundamental causes of these climatic perturbations

the updip and downdip limits to great subduction earthquakes thermal and structural models of cascadia south alaska sw japan and chile this study examines thermal and structural controls of the updip and downdip rupture limits of great subduction thrust earthquakes data on past great earthquake seismic limits have been compiled for four continental subduction zones cascadia sw japan south alaska and chile these limits have then been compared to the predictions of several models for what constrains great earthquake rupture temperatures on the subduction thrusts have been estimated by finite element numerical models the landward limits of the observed updip aseismic zones correspond to the position where the thrust temperature reaches about 100 c that is depths of about 2 to 10 km for the subduction zones studied this temperature agrees with the dehydration of stable sliding smectite clays to illite chlorite the temperatures in this region are controlled mainly by the thickness of sediment on the incoming crust and by the crustal age and thus heat flow the downdip limits correspond to the depth on the thrust where either 1 the temperature reaches about 350 c which corresponds to thermally activated stable sliding behavior for crustal rocks with a transition to 450 c or 2 about 40 km depth if 350 c is reached at greater depth depths of about 40 km approximately correspond to the intersection of the thrust with the continental forearc moho and this downdip limit may be a consequence of stable sliding serpentinite or talc and other hydrated forearc mantle rocks the primary temperature controls on the downdip region are the age of the subducting oceanic plate and the thrust dip profile secondary control comes from the thickness of incoming sediment the convergence rate and the radioactive heat generation in the overlying forearc the 100 c updip limit occurs near the trench for young subducting plates with a thick sediment section such as cascadia 6 8 ma and up to 80 km landward for older oceanic crust such as south alaska 50 ma the 350 c downdip thermal limit is applicable for young oceanic plates e g cascadia and nankai whereas the forearc mantle limit applies for older plates e g south alaska and chile except near the chile rise for the margins studied that have experienced great earthquakes there is generally good agreement between the postulated thermal and moho limits and the rupture or seismogenic zone as defined by the distribution of aftershocks and by waveform tsunami and dislocation modeling the downdip limit of the interseismic locked zone from dislocation modeling is also in agreement with these limits

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

the effects of sliding velocity on the frictional and physical properties of heated fault gouge the frictional properties of a crushed granite gouge and of gouges rich in montmorillonite illite and serpentine minerals have been investigated at temperatures as high as 600 c confining pressures as high as 2 5 kbar a pore pressure of 30 bar and sliding velocities of 4 8 and 4 8 10 2 m sec the gouges showed nearly identical strength behaviors at the two sliding velocities all four gouges however showed a greater tendency to stick slip movement and somewhat higher stress drops in the experiments at 4 8 10 2 m sec varying the sliding velocity also had an effect on the mineral assemblages and deformation textures developed in the heated gouges the principal mineralogical difference was that at 400 c and 1 kbar confining pressure a serpentine breakdown reaction occurred in the experiments at 4 8 10 2 m sec but not in those at 4 8 m sec the textures developed in the gouge layers were in part functions of the gouge type and the temperature but changes in the sliding velocity affected among other features the degree of mineral deformation and the orientation of some fractures

ice streams of the late wisconsin cordilleran ice sheet in western north america abstract the late wisconsin cordilleran ice sheet cis of western north america is thought to have reached its maximum extent 2 5 106 km2 as late at c 14 5 ka most 80 of the ice sheet s bed consists of high mountains but its core zone sited on plateaux of the intermontane belt of british columbia and coterminous parts of the usa shows broad swaths of subglacially streamlined rock and sediment broad scale mapping from new digital imagery data identifies three subglacial bed types 1 hard beds of variably streamlined bedrock 2 drumlinized soft beds of deformation till reworked from antecedent sediment and 3 mixed beds of variably streamlined bedrock protruding through drumlinized sediment drumlins on soft beds appear to be erosional features cut into till and antecedent sediments and identify the catchment areas of paleo ice streams expressed downglacier as flow sets of megascale glacial lineations msgls grooved and cloned drumlins appear to record the transition from drumlins to msgls the location of paleo ice streams reflects topographic funneling of ice from plateau surfaces through outlet valleys and a soft bed that sustained fast flow rock cut msgls are also present locally on the floors of outlet valleys cis disintegrated in

present day vertical deformation of the cascadia margin pacific northwest united states we estimate present day uplift rates along the cascadia subduction zone in california oregon and washington in the pacific northwest united states by utilizing repeated leveling surveys and tide gauge records these two independent data sets give similar profiles for latitudinal variation of contemporary uplift rates along the coast uplift rates are extended inland through east west leveling lines that connect the north south line along the coast to the north south line along the inland valleys just west of the cascades the results are summarized as a contour map of present day uplift rates for the western pacific northwest we find that rates of present day uplift vary latitudinally along the coast and inland valleys as well as longitudinally along transects connecting the coast to the inland valleys long term tidal records of neah bay astoria and crescent city indicate uplift of land relative to sea level of 1 6 0 2 0 0 0 2 and 0 9 0 2 mm yr respectively 1 standard error unlike previous estimates of relative sea level change at astoria we adjust for discharge effects of the columbia river including human management influences after approximating an absolute framework by using 1 8 0 1 mm yr to compensate for global sea level rise results indicate that much of the western pacific northwest is rising at rates between 0 and 5 mm yr the most rapid uplift rates are near the coast particularly near the olympic peninsula the mouth of the columbia river cape blanco and cape mendocino two axes of uplift are identified one trends northeast from the southwest oregon coast and the other trends south southeasterly from the olympic peninsula to the columbia river the puget sound vicinity and a small east west region from the north central oregon coast to the inland willamette valley are subsiding at rates up to 1 mm yr we interpret the overall pattern of rapid present day uplift to be generated by interseismic strain accumulation in the subduction zone this interseismic elastic strain accumulation implies significant seismic hazard

source characteristics of hypothetical subduction earthquakes in the northwestern united states historic earthquake sequences on subduction zones that are similar to the cascadia subduction zone are used to hypothesize the nature of shallow subduction earthquakes that might occur in the northwestern united states based on systematic comparisons of several physical characteristics including physiography and seismicity subduction zones that are deemed most similar to the cascadia subduction zone are those in southern chile southwestern japan and colombia these zones have all experienced very large earthquake sequences and if the cascadia subduction zone is also capable of storing elastic strain energy along its greater than 1000 km length then earthquakes of very large size m w 8 1 2 must be considered circumstantial evidence is presented that suggests but does not prove that large subduction earthquakes along the cascadia subduction zone may have an average repeat time of 400 to 500 yr

the effect of second phase particles on stable grain size in regionally metamorphosed polyphase calcite marbles samples of the calcite rich shelburne marble collected at the pfizer quarry in adams massachusetts show an order of magnitude variation in grain size calcite grain size ranges from 94 to 1101 m because these calcite marbles share the same pressure temperature and strain histories some other factor must be responsible for the grain size variation grain size appears to be controlled by the concentration of impurity or second phase particles large calcite grain size occurs where the volume fraction of second phase particles is low and grain size decreases as second phase volume fraction increases the relationship between calcite grain size d second phase grain size d and second phase volume fraction f can be described by the power law d d 1 4 f 0 36 a result that is consistent with models based upon short term hours or days laboratory experiments with metals and ceramics and computer simulations of grain growth grain growth appears to be greatly restricted by as little as a few per cent of second phase particles with a transition from highly restricted to almost unrestricted grain growth occurring at 5 volume of second phase particles these results indicate that second phase particles exercise an important control on grain size and can effectively inhibit grain growth in metamorphic rocks the behaviour of second phases in short term laboratory experiments may closely approximate the behaviour of second phases in grain growth lasting several orders of magnitude longer in the metamorphic environment

heinrich events massive late pleistocene detritus layers of the north atlantic and their global climate imprint 1 millennial climate oscillations of the glacial interval are interrupted by extreme events the so called heinrich events of the north atlantic their near global footprint is a testament to coherent interactions among earth s atmosphere oceans and cryosphere on millennial timescales heinrich detritus appears to have been derived from the region around hudson strait it was deposited over approximately 500 250 years several mechanisms have been proposed for the origin of the layers binge purge cycle of the laurentide ice sheet jokulhlaup activity from a hudson bay lake and an ice shelf buildup collapse fed by hudson strait to determine the origin of the heinrich events i recommend 1 further studies of the timing and duration of the events 2 further sedimentology study near the hudson strait and 3 greater spatial and temporal resolution studies of the layers as well as their precursory intervals studies of previous glacial intervals may also provide important constraints

contribution of antarctica to past and future sea level rise polar temperatures over the last several million years have at times been slightly warmer than today yet global mean sea level has been 6 9 metres higher as recently as the last interglacial 130 000 to 115 000 years ago and possibly higher during the pliocene epoch about three million years ago in both cases the antarctic ice sheet has been implicated as the primary contributor hinting at its future vulnerability here we use a model coupling ice sheet and climate dynamics including previously underappreciated processes linking atmospheric warming with hydrofracturing of buttressing ice shelves and structural collapse of marine terminating ice cliffs that is calibrated against pliocene and last interglacial sea level estimates and applied to future greenhouse gas emission scenarios antarctica has the potential to contribute more than a metre of sea level rise by 2100 and more than 15 metres by 2500 if emissions continue unabated in this case atmospheric warming will soon become the dominant driver of ice loss but prolonged ocean warming will delay its recovery for thousands of years