does an asymmetric thermohaline ice sheet oscillator drive 100 000 yr glacial cycles a hypothesis is presented that late quaternary 100 000 yr glacial cycles are driven by an asymmetric thermohaline ice sheet oscillator that emerged in the global climate system 650 000 950 000 yr ago perhaps when the main source of northern hemisphere deep water production shifted south from the arctic into the nordic seas it is hypothesised that the asymmetry is due to the increasing difficulty after 950 000 years ago of resetting an interglacial mode of the critical nordic limb of the salinity conveyor once it switches off and an ensuing iceberg flux enters the areas of downwelling a possible reason for both a southward shift and the resulting asymmetry is uplift of the greenland scotland submarine ridge from activity of the iceland mantle plume in this hypothesis an individual 100 000 yr glacial cycle begins when the northernmost limb of the salinity conveyor in the nordic seas is curtailed or even switched off perhaps due to the growing strength of competing antarctic bottom water aabw generated by interglacial recession of the west antarctic ice sheet wais from the west antarctic rift system such recession produces southern marginal seas where dense shelf water can collect and overflow into the abyss when northern ice sheets nucleated by this circulation switch develop marine components that calve icebergs into the nordic seas the salinity conveyor can no longer revert to an interglacial mode from orbital forcing as it did prior to 950 000 yr ago in order to reset an interglacial circulation mode of the conveyor ice sheets must continue to grow for 100 000 years until they capture enough excess volume to produce a gravitational collapse of marine based components so massive that all grounded ice is flushed from north atlantic continental shelves the outburst of icebergs produced by this collapse cripples the glacial mode of overturning in the northern north atlantic once this collapse ends however the nordic seas become nearly free of icebergs for the first time in 100 000 years because of the depletion of adjacent marine based components as a consequence north atlantic salinity increases rapidly switching the conveyor into a vigorous interglacial mode of operation and hence terminating the glacial cycle by lowering sea level the prolonged growth of northern hemisphere ice sheets during each 100 000 yr cycle drives antarctic grounding lines seaward across continental shelves squeezing off the source of densified shelf waters that feed aabw sea level rise and increased basal melting however caused by the subsequent collapse of northern ice sheets and the reintroduction of north atlantic deep water into the southern ocean reverses the process forcing retreat of antarctic grounding lines from their advanced last glacial maximum positions this retreat opens marginal seas for renewed formation of dense shelf water by expanding marginal seas and hence the source of dense shelf water ongoing recession of the wais strengthens aabw during the course of an interglaciation eventually forcing a thermohaline circulation switch in the nordic seas and initiating yet another 100 000 yr glacial cycle the 100 000 yr duration of each cycle is set by two factors inertia is built into the system by the long time required for ice sheets to grow to the excess volume necessary for a marine collapse that resets the salinity conveyor into an interglacial mode eccentricity driven changes in the amplitude of the precession or tropical half precession signal give rise to warming events that trigger such a collapse of excess ice about each 100 000 yr

oxygen isotope analyses and pleistocene ice volumes abstract the oxygen isotope record from fossil foraminifera in deep sea sediments is commonly used as a proxy for global ice volume the linkage between 18 o and ice volume however is probably nonlinear we have developed a simple numerical model of the isotopic response of the oceans to ice volume change the major features it simulates are 1 the changing mean isotopic composition of snow as a function of ice volume colder snow temperatures forced by climate change and higher elevation accumulation areas imply more negative mean 18 o 2 the nonequilibrium isotopic composition of ice sheets the past history of an ice sheet is integrated into its mean isotopic composition which introduces a lag of isotopic ice volume i e the measured 18 o record scaled to ice volume units behind true ice volume 3 selective preservation of isotopically more negative colder higher latitude ice this geographic effect can selectively amplify or dampen the isotopic response to the ice volume signal we illustrate the response of our model to simple hypothetical ice volume transitions of ice growth and ice decay sensitivity tests are illustrated for all model parameters the results suggest that oxygen isotope records reproduce the general patterns of ice volume change fairly accurately the foraminiferal isotope record however may misrepresent the true amplitude of the ice volume signal and lag true ice volume by 1000 to 3000 yr

age and weathering status of granite tors in arctic finland 68 n abstract cold based ice along the fennoscandian glacial ice divide in northern finland allowed preservation of old landscape features because there was little or no movement at the ice land surface contact erosion was at a minimum and relict landscape features such as tors can be found we investigated two such granitic tors located at pyha nattanen 68 07 335 n 27 22 207 e 508 m asl and riestovaara 68 02 613 n 27 09 003 e 390 m asl in subarctic finland at pyha nattanen the sampling sequence included bedrock material and grus taken from within horizontal cracks which are so prevalent at the site as to make the tor resemble a stack of pancakes at riestovaara where the outcrop is more subdued in addition to bedrock and grus samples soil samples were also extracted from a pit dug in an embryonic soil forming on the bedrock surface based on cosmogenic nuclide dating both tors greatly predate recent glaciation the tor at pyha nattanen which is a more prominent landscape feature had longer minimum 10be and 26al apparent exposure ages 89 1 6 7 kyr and 69 6 6 4 kyr respectively than did the tor at riestovaara 55 8 4 5 kyr and 46 5 4 0 kyr respectively both of them are significantly older than the regional last deglaciation age indicating that both sites have survived multiple episodes of glaciation analyses of the 10be and 26al measurements in accordance with marine oxygen isotope records dsdp 607 indicate that the tors have undergone a minimum estimated total exposure burial durations of 997 kyr pyha nattanen and 858 kyr riestovaara and survived at least 14 16 episodes of glaciation weathering as measured by porosity determined with a microprobe was somewhat more advanced in the pyha nattanen granite samples than in the riestovaara granite however with both granites rock porosity did not change to a depth of 4 cm below the rock surface or vary by lichen cover noncovered surfaces indicating that weathering had progressed to a stage where recent lichen growth is overwhelmed by the long weathering history other measures of weathering including total elemental analyses did not detect significant differences among at a site samples perhaps because all samples are highly weathered and we did not have a sample of unweathered granite for comparison all samples rock grus and soil were within the grus weathering range as indicated by chemical weathering indices the soil forming adjacent to the tor at riestovaara exhibited only slight development despite the great apparent age of the landscape the regolith in which the soil is forming must therefore be much younger than the exposed bedrock and most likely represents a post deglaciation accumulation of grus spanning only the last 9 kyr

oxygen isotope calibration of the onset of ice rafting and history of glaciation in the north atlantic region we report here that dsdp site 552a cored with the hydraulic piston corer on the west flank of rockall bank recovered an undisturbed sequence of alternating white deep sea carbonate oozes and dark coloured layers that are rich in glacial debris oxygen isotope analysis of the sequence together with detailed nannofossil and palaeomagnetic stratigraphy shows that the first major horizon of ice rafting occurred at about 2 4 myr and was preceded by a minor pulse of ice rafting at about 2 5 myr the carbon isotope record shows that the site has been bathed by a water mass of similar characteristics to present day north atlantic deep water at least since 3 5 myr

the progressive intensification of northern hemisphere glaciation as seen from the north pacific ocean drilling project odp site 882 50 22 n 167 36 e provides the first high resolution grape density magnetic susceptibility carbonate opal and foraminifera planktonic and benthic stable isotopes records between 3 2 and 2 4 ma in the northwest pacific we observed a dramatic increase in ice rafting debris at site 882 at 2 75 ma which is coeval with that found in the norwegian sea suggesting that the eurasian arctic and northeast asia were significantly glaciated from 2 75 ma onwards prior to 2 75 ma planktonic foraminifera 18o records indicate a warming or freshening trend of 4 c or 2 over 80 ka if this is interpreted as a warm pre glacial pliocene north pacific it may have provided the additional moisture required to initially build up the northern hemisphere continental ice sheet the dramatic drop in sea surface temperatures sst 7 5 c at 2 75 ma ended this suggested period of enhanced sst and thus the proposed moisture pump moreover at 2 79 and 2 73 ma opal mass accumulation rates mar decrease in two steps by five fold and is accompanied by a more gradual long term decrease in caco3 mars evidence from the southern ocean odp site 704 indicates that just prior to 2 6 ma there is a massive increase in opal mars the opposite to what is found in the north pacific this indicates that the intensification of northern hemisphere glaciation was accompanied by a major reorganisation of global oceanic chemical budget possibly caused by changes in deep ocean circulation the initiation of northern hemisphere glaciation occurred in the late miocene with a significant build up of ice on southern greenland however the progressive intensification did not occur until 3 5 3 ma when the greenland ice sheet expanded to include northern greenland following this stage we suggest that the eurasian arctic and northeast asia glaciated at 2 75 ma approximately 100 ka before the glaciation of alaska 2 65 ma and 200 ka before the glaciation of the north east american continent 2 54 ma

topography and ice sheet growth this paper uses a numerical ice sheet model to investigate the role of topography in influencing ice sheet growth the model is applied to the maritime mid latitude uplands of scotland and relies on a series of assumptions about mass balance topography and ice flow it is driven by an imposed pattern of temperature change the model is able to predict effectively the extent and thickness of the loch lomond ice sheet using a palaeotemperature curve based on coleoptera assemblages a series of experiments with a stepped constant july air temperature depression suggests that in scotland a change in excess of 3 0 c is necessary to initiate ice sheet growth that steady state ice caps build up at changes of 3 to 6 5 c and that large ice sheets build up at changes of more than 6 625 c the bifurcation revealed by the last two types of behaviour is the result of topography both the vertical amplitude and the spatial distribution of bedrock basins and ridges are important in determining the pattern rate and extent of ice sheet growth the implication is that topography plays an important role in determining the dynamics of ice sheet growth

the last interglacial glacial cycle in ne fennoscandia a nearly continuous record from sokli finnish lapland abstract boreholes from the sokli basin in finnish lapland have recovered a unique sequence of diamicts interlayered with fine grained sediments that are rich in fossils the diamicts are interpreted as till beds deposited by the fennoscandian ice sheet the fine grained intercalations seem to represent a series of warm stages that include one interglacial and three interstadials the interglacial is characterized by a mixed taiga pollen assemblage and corresponds to a distinct diatom gyttja bed the interstadials show progressively colder floras from open birch forest to arctic forest limit to shrub tundra their sediments comprise gyttja interlayered with wood and sand interstadial 1 sands grading into sandy gyttja interstadial 2 and sands overlain by laminated silt and clay interstadial 3 the interstadial 2 and 3 deposits are underlain by diamict and are interpreted as deglaciation sequences silts with a tundra pollen assemblage separate the interglacial and interstadial 1 deposits luminescence dating evidence indicates that the interglacial bed most probably represents the nw european mainland eemian interglacial marine isotope stage mis 5e the interstadial 1 2 and 3 deposits are tentatively correlated with the early weichselian brorup and odderade interstadials and an interstadial of middle weichselian age respectively mis 5c 5a and 3 respectively the correlation implies that the easternmost part of the ice divide zone of the fennoscandian continental ice sheet was deglaciated during part of mis 3

tracking the sources of icebergs with lead isotopes the provenance of ice rafted debris in heinrich layer 2 the provenance of ice rafted debris ird deposited in the north atlantic before during and after heinrich event 2 has been determined through measuring the lead isotopic composition of single feldspar grains and multiple grain composites from the larger than 150 m size fraction from cores from the eastern and western north atlantic and from the labrador sea single grain analyses are used to identify the specific continental sources of the ird whereas composite samples are used to assess the relative ird contributions from different sources all single grains from heinrich layer 2 h 2 as well as h 2 composites plot along a correlation line on a 207pb 204pb versus 206pb 204pb diagram characteristic of the churchill province of the canadian shield this is yet another strong piece of evidence that this heinrich event was dominated by a massive iceberg discharge of the laurentide ice sheet lobe located over hudson bay in contrast single grains from the ambient glacial sediment above and below h 2 have multiple sources many of them also lie along the correlation line with h 2 grains but many others have pb signatures consistent with derivation from the grenville province and the appalachian range in north america and possibly from scandinavia and greenland composites from the ambient sediment generally lie well to the right of the h 2 reference line in agreement with the results of the single grain analyses the evidence provided by lead isotopes regarding the dominant role played by the hudson bay lobe of the laurentide ice sheet in the development of the heinrich events lends support to the binge purge model advanced by macayeal 1993a b that invokes trapping of geothermal heat by the base of the icecap and subsequent basal melting as the mechanism that triggered the heinrich events

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

some geological implications of average quaternary glacial conditions analyses of quaternary landscape evolution tend to focus on events associated with culminations of glacial and interglacial ages but for most of quaternary time environments were intermediate in character average quaternary glacial conditions based on assessment of the marine isotope record approximated those near the stage 2 stage 1 transition and during substages 5b and 5d isotope stages 3 and 4 and substages 5a d lie within one standard deviation of the mean value under average glacial conditions ice sheets lying over northern north america and europe were much more contracted than their full glacial counterparts and the distribution of mountain glaciers reflected a snowline depression of some 500 m geomorphic processes operating under average quaternary conditions contributed importantly to landscape evolution examples of landscapes that may represent such average conditions include cirques and fluvial deposits of the pacific northwest the fjords and strandflat of western norway and atolls of the tropical oceans by examining the geologic record from the perspective of average conditions rather than those of climatic extremes added insight can be gained regarding the evolution of quaternary landscapes

wavefield polarization in fault zones of the western flank of mt etna observations and fracture orientation modelling ambient noise measurements performed on the western flank of mt etna are analyzed to infer the occurrence of directional amplification effects in fault zones the data were recorded along short 500 m profiles crossing the ragalna fault system ambient noise records were processed to compute the horizontal to vertical noise spectral ratio as a function of frequency and direction of motion wavefield polarization was investigated in the time frequency domain as well peaks of the spectral ratios generally fall in the frequency band 1 0 6 0 hz pointing out directional amplifications that are also confirmed by the results of the time frequency analysis the largest amplification occurring with high angle to the fault strike a variation of the frequency of the spectral peak is observed between the two sides of the fault possibly related to a damage fault asymmetry measurements performed several kilometers away from the fault zone do not show behavior that is as systematic as in the fault zone and this suggests that the observed directional effects can be ascribed to the fault fabric we relate the polarization effect to compliance anisotropy in the fault zone where the presence of predominantly oriented fractures makes the normal component of ground motion larger than the transversal one in order to test the direction and the type of fractures that are expected in the fault zone we modeled the brittle deformation pattern of the investigated fault theoretical results are in good agreement with field observations of the fracture strike

rupture directivity of small earthquakes at parkfield 1 theoretical modeling of strike slip ruptures along a bimaterial interface suggests that earthquakes initiating on the interface will have a preferred rupture direction we test this model with 450 small earthquakes 2 3 exhibiting southeast rupture characteristics some spatial and temporal variability in rupture directivity is also apparent we observe a higher proportion of northwest directivity ruptures following the 2004 m 6 parkfield earthquake which ruptured toward the northwest our results are generally consistent with the preferred southeast rupture directivity model but suggest that directivity is likely due to several contributing factors

ice sheet erosion patterns in valley systems in northern sweden investigated using cosmogenic nuclides erosion patterns associated with glaciation of trunk and hanging valley systems in northern sweden were investigated using cosmogenic nuclide 10be apparent exposure ages and inferred nuclide inheritance sequences of samples taken across valleys known to have been covered repeatedly by the fennoscandian ice sheet revealed two primary patterns of erosion in vavlavagge the exposure age pattern is consistent with 2 m of glacial erosion during the last glacial cycle along the entire profile at ravtasvaggi dievssavaggi and alisvaggi exposure ages in the valley bottom contrast with apparent exposure ages two to four times older on the valley sides the older ages on the valley sides reflect cosmogenic nuclide inheritance due to limited 2 m bedrock erosion of the valley sides during the last glacial cycle the pattern and scale of erosion in these valleys indicates that glacial valley formation is a result of multiple glacial cycles rather than the result of topographic modification during a single glacial cycle initial data comparing hanging valley and trunk valley sites do not show distinct differences in apparent exposure ages slightly older ages for samples from hanging valley bottoms may suggest nuclide inheritance indicating lower erosion than in trunk valley bottoms as would be expected given the marked topographic step between hanging and trunk valleys although quantifying the amount of erosion depends on the assumed cosmogenic nuclide inheritance prior to the onset of erosion the pattern of erosion is independent of this hence the observed pattern of cosmogenic nuclide concentrations provides constraints on spatial patterns of erosion and helps to refine understanding of the timing and extent of landform modification by glaciation copyright 2005 john wiley sons ltd

high resolution relocation of aftershocks of the mw 7 1 darfield new zealand earthquake and implications for fault activity 1 low slip rate regions often represent under recognized hazards and understanding the progression of seismicity when faults in such areas rupture will help us to better understand earthquake rupture patterns the 3 september 2010 utc mw 7 1 darfield earthquake revealed a formerly unrecognized set of faults in the canterbury region of new zealand an area that had previously been mapped as one of the lower hazard areas in the country in this study we analyze the first four months of its aftershock sequence to identify active faults and temporal changes in seismicity along them we jointly invert for three dimensional p wave and s wave velocities and hypocentral locations using data for 2840 aftershocks recorded at 36 temporary and permanent seismic stations within 70 km of the main shock epicenter these relocations delineate eight individual faults active prior to the 22 february 2011 mw 6 3 christchurch earthquake the largest aftershock of the darfield earthquake two of these faults are in the christchurch region one of which corresponds to geodetically determined rupture planes of the christchurch earthquake using focal mechanisms calculated from first motion polarities we find mainly strike slip faulting events with some reverse and normal faulting events as well we compare the orientations of these faults to the prevailing regional stress directions to identify which faults may have been active prior to the darfield earthquake and which may be newly developed

chronology for fluctuations in late pleistocene sierra nevada glaciers and lakes mountain glaciers because of their small size are usually close to equilibrium with the local climate and thus should provide a test of whether temperature oscillations in greenland late in the last glacial period are part of global scale climate variability or are restricted to the north atlantic region correlation of cosmogenic chlorine 36 dates on sierra nevada moraines with a continuous radiocarbon dated sediment record from nearby owens lake shows that sierra nevada glacial advances were associated with heinrich events 5 3 2 and 1

unequal ice sheet erosional impacts across low relief shield terrain in northern fennoscandia abstract much previous work on late cenozoic glacial erosion patterns in bedrock has focussed on mountain areas here we identify varying impacts of ice sheet erosion on the low relief bedrock surface of the fennoscandian shield and examine the geological topographical and glaciological controls on these patterns we combine gis mapping of topographical hydrological and weathering data with field observations we identify and investigate areas with similar geology and general low relief that show different degrees of ice sheet erosional impact despite similar ice cover histories on two transects with a total area of 84 000 km2 across the northern fennoscandian shield we first establish patterns of glacial erosion and then examine why glacially streamlined areas exist adjacent to areas of negligible glacial erosion the northern transect includes two areas of exceptional glacial preservation the parkajoki area in sweden and the so called ice divide zone in finland each of which preserve tors and deep saprolite covers the southern transect overlapping in the northern part with the first transect includes areas of well developed glacial streamlining with bedrock areas stripped of loose material and barely any weathering remnants for both areas we firstly present contrasting indicators for ice sheet erosional impact streamlined and non streamlined inselbergs parallel and dendritic rectangular drainage patterns and the absence and presence of neogene weathering remnants this is followed by an investigation of factors that possibly influence ice sheet erosional impact ice cover history ice cover duration and thickness bedrock type and structure and topography we find that the erosional impact of the fennoscandian ice sheet has varied across the study area distinct zones of ice sheet erosion are identified in which indicators of either low or high erosion coexist in the same parts of the transects no direct impact of rock type on glacial erosion patterns was found but an indirect control appears clear bedrock geology and long term differential weathering and tectonic evolution determined the topography of the pre glacial landscape and these topographic differences subsequently influenced ice sheet dynamics and thereby partly controlled patterns of ice sheet erosion ice cover duration and former ice thickness were not significant controls on glacial erosion patterns extensive preservation of pre glacial relief through low glacial erosion is attributed to the maintenance throughout the pleistocene of divergent flow and frozen bed conditions in the fennoscandian ice sheet in contrast glacial streamlining and strong glacial erosion were caused mainly by acceleration of flow around major obstacles and flow towards major depressions on the ice sheet bed the relatively strong ice sheet erosion towards the gulf of bothnia is the result of a combination of favourable factors bedrock structure and river valleys aligned sub parallel to ice sheet flow and convergent ice flow towards the baltic

can milankovitch orbital variations initiate the growth of ice sheets in a general circulation model the goddard institute for space studies climate model is used to investigate whether the growth of ice sheets could have been initiated by solar insolation variations three different orbital configurations are used corresponding to 116 and 106 kyr b p and a modified insolation field with greater reductions in summer insolation at high northern latitudes the time periods chosen are those in which geophysical evidence suggests ice sheets may have been growing rapidly the reduced summer insolation field characteristic of all the experiments is thought to be a necessary condition for allowing snow to last through the summer and ice sheets to build the results show that the model fails to maintain snow cover through the summer at locations of suspected initiation of the major ice sheets despite the reduced summer and fall insolation when 10 m thick ice was inserted in all locations where continental ice sheets existed during the last glacial maximum the model failed to maintain it as well producing energy and mass imbalances which would remove the ice within 5 years only when the ocean surface temperatures were adjusted to their peak ice age values was the model able to keep any of the additional ice and then only in a very restricted region of northern baffin island the experiments indicate there is a wide discrepancy between the model s response to milankovitch perturbations and the geophysical evidence of ice sheet initiation as the model failed to grow or sustain low altitude ice during the time of high latitude maximum solar radiation reduction 120 110 kyr b p it is unlikely it could have done so at any other time within the last several hundred thousand years if the model results are correct it indicates that the growth of ice occurred in an extremely ablative environment and thus demanded some complicated strategy or else some other climate forcing occurred in addition to the orbital variation influence and co2 reduction which would imply we do not really understand the cause of the ice ages and the milankovitch connection if the model is not nearly sensitive enough to climate forcing it could have implications for projections of future climate change

late weichselian submarine debris flow deposits on the bear island trough mouth fan sedimentary processes on the bear island trough mouth fan during the last glacial were studied using high resolution reflection seismics and gravity cores the fan succession is dominated by large debris flow deposits of up to 24 km in width and 50 m thick debris lobes can be followed over 100 km downslope and the largest cover an area of 1880 km2 the large debris flows were generated when the barents sea ice sheet reached the shelf break glacigenic sediments transported to the grounding line were temporarily stored on the upper slope due to the high sedimentation rate the sediments were unstable and earthquakes oversteepening and or build up of excess pore pressure triggered sediment release generating large debris flows sediments were also eroded and incorporated during the downslope flow debris flows were the most important sediment distributing process due to the relatively high shear strength low flow mobility of the glacigenic diamicton and the low fan gradient the weichselian evolution of the fan was characterized by a relatively low sedimentation rate during the early and mid weichselian the barents sea ice sheet might have reached the shelf break twice during the late weichselian on average one large debris flow comprising 18 km3 of sediments was released between every 35 to 75 years during the last glacial maximum

a time dependent glaciological model of the weichselian ice sheet a time dependent model is used to study the evolution of the weichselian ice sheet the results indicate three stages of the glaciation a mountain centered ice sheet between 110 000 and 80 000 bp followed by a phase with an extension similar to or slightly larger than the younger dryas zone the last and maximum extent was reached about 30 000 20 000 bp and the main deglaciation began about 15 000 bp the model is based on a finite element method solution of the continuity equation the climatic records from the camp century and summit ice cores are used for climate control and the model is calibrated using geological deglaciation evidence in scandinavia the assumption of a general sliding condition for elevations below 100 m with an enhanced sliding zone through the center of the baltic sea and gulf of bothnia yield broad agreement for marginal positions in sweden and finland during a simulated termination

late cenozoic evolution of high gradient trough mouth fans and canyons on the glaciated continental margin offshore troms northern norway paleoclimatic implications and sediment yield trough mouth fans contain information about the evolution of high latitude continental margins including rates of glacial sedimentation and hinterland erosion here the late cenozoic evolution of high gradient trough mouth fans and canyons on the troms margin northern norway is reconstructed paleocanyons were active prior to the quaternary glaciations glaciomarine and glaciofluvial conditions prevailed between ca 2 7 ma and ca 1 5 ma and ice sheets possibly reached the paleo shelf break at least once the minimum average sedimentation rate of this period was 0 22 m k y from ca 1 5 ma to ca 0 7 ma the glaciations intensified and fast flowing ice streams reaching the shelf break were established in the cross shelf troughs subglacial deformation till was deposited at the outer shelf and later reworked by debris flows and turbidity currents the fennoscandian ice sheet started to route much of its ice mass to the north and south of the study area and so the troms margin possibly was a low ice flow sector from this time with a minimum average sedimentation rate of 0 15 m k y during the last ca 0 7 m y ice streams continued to traverse the troughs while sluggish flowing ice prevailed on the banks a minimum average sedimentation rate of 0 14 m k y is estimated for this period the minimum total erosion and erosion rate for the quaternary are 50 140 m and 0 02 0 05 m k y respectively compared with previous studies from other areas this implies up to one order of magnitude variation in average glacial erosion rates along the western sector of the fennoscandian barents sea ice sheets this is interpreted to be due to the size and bedrock composition of the catchment areas and the timing of ice growth and ice sheet dynamics in addition the steep preglacial continental slope promoted high sediment flow velocity for the glacigenic sediments causing much of the debris flows to transform into turbidity currents which efficiently transported sediments across the slope and thereby maintained its steep gradient

patterns of quaternary ice sheet erosion and deposition in fennoscandia and a theoretical framework for explanation it has long been recognised that the formerly glaciated area of fennoscandia shows large spatial differences in thicknesses of quaternary deposits mainly tills and exhibits distinct patterns of glacial scouring and deep linear erosion the reasons for this striking zonation have been elusive and in particular the relative roles of mountain ice sheets mis and full sized fennoscandian ice sheets fis in shaping the landscape surface need clarification on the basis of current advances in our understanding of the climate evolution and basal thermal organisation of ice sheets we perform spatio temporal qualitative modelling of ice sheet extent and migration of erosion and deposition zones through the entire quaternary and proceed to suggest an explanatory model for the current spatial pattern of quaternary deposits and the two different types of erosion zones we use the spatial distribution of fjords and deep non tectonic lakes for delineating zones of deep glacial erosion and relict landscapes as markers for frozen bed conditions on the basis of the amount of exposed bedrock the landscape was classified into a tripartite system of drift thickness thick drift intermediate drift thickness absence of drift scoured zones it is found that a centrally placed central and northern sweden zone of thick drift cannot be explained by deposition under fis style ice sheets but is instead likely to be the combined result of marginal deposition of fluctuating mis style ice sheets primarily during the early and middle quaternary and the inefficiency of later east centered fis style ice sheets in evacuating this drift from underneath their central low velocity and possibly frozen bed areas the western fjord zone of deep glacial erosion formed underneath both mis and fis style ice sheets during the entire quaternary while the eastern lake zone of deep glacial erosion is exclusively related to mis style ice sheets and formed largely during the early and middle quaternary the scouring zones formed under conditions of rapid ice flow towards bathymetrically defined calving margins of fis style ice sheets they likely reflect process patterns of the last two or three fis style ice sheets the three landscape zones differ in their degree of permanence with the deep erosion zones being a long lasting legacy in the landscape more likely to be enhanced than obliterated by subsequent glacial events the thick drift cover zone once established appears to have been surprisingly robust to erosion by subsequent glacial events the scouring zones appear to be the most recent and ephemeral of the three zones with possible major alterations during single glacial events

ice rafted evidence of long term north atlantic circulation abstract the absolute and relative rates of deposition of ice rafted volcanic and terrigenous clastic material were determined for the interval from the base of oxygen isotope stage 5 127 000 yrs b p to the lower part of stage 3 50 000 yrs b p in five subpolar north atlantic deep sea cores during this time interval global climate changed from a full interglaciation to nearly a full glaciation for the sediment derived from the continents dominantly quartz and feldspar the eastward decrease in deposition rates during interglacial intervals and eastward increase during glacial intervals is best explained by a southeastward jump in the locus of iceberg melting during climates colder than that of today a similar model may explain the eastward increase in volcanogenic debris during glacial time intervals however the eastward increase of volcanogenic debris during interglacial stage 5 requires a stronger southeastward component of flow from greenland than is evident in the modern circulation

quaternary glaciations in southern fennoscandia evidence from southwestern norway and the northern north sea region abstract based on records from the northern north sea and southern norway quaternary fennoscandian glaciations are described and discussed maximum glaciation defined as the presence of shelf edge glaciation an ice stream flowing out of the norwegian channel and most likely an ice divide over the central part of fennoscandia sweden gulf of bothnia occurred for the first time in the late cenozoic at ca 1 1 myr the fedje glaciation this glaciation was followed by a ca 600 kyr period with at least two glaciations with ice extension somewhat larger than the younger dryas event in the region the first maximum glaciation of the bruhnes chron was during isotope stage 12 and was followed by repeated maximum glaciations within each of the light isotopic stages until stage 2 during the weichselian the southwestern part of the fennoscandian ice sheet reached maximum position three times sometime between 22 and 28 37 and 50 and 60 and 80 ka limited glaciations probably occurred during stadials between the early and middle weichselian interstadials between 500 ka and 1 1 myr and most likely also prior to 1 1 myr deep norwegian sea ice rafted detritus ird records reflecting contributions from several different ice sheets fennoscandian barents sea iceland and greenland each with large variations in history of ice extent to their respective margins yet to some degree they reflect the glaciation of the northern north sea region with an increasing ird input around 1 1 myr and a relatively low input of north sea components during some cycles around the b m boundary maximum glaciations occurred during insolation minima in the earlier part of the middle pleistocene however not all such minima are reflected by maximum glaciation in southern parts of fennoscandia the relatively poor chronological control and non continuous character of the data on weichselian interstadials stadials precludes a clear relationship to atmospheric changes recorded in greenland ice cores as well as to precession and or obliquity cycles in insolation

cosmogenic 36cl and 10be ages of quaternary glacial and fluvial deposits of the wind river range wyoming we measured cosmogenic 36 cl in 56 samples from boulders on moraines and fluvial terraces in the vicinity of the wind river range wyoming we also measured 10 be in 10 of the same samples most of the 10 be ages were in good agreement with the 36 cl ages indicating that rock surface erosion rates were very low the oldest moraine investigated the type sacagewea ridge site yielded only a limiting minimum age of 232 ka the oldest moraines in the type bull lake complex also could be constrained only to 130 ka the main sequence of type bull lake moraines yielded age distributions indicating deposition within the intervals 130 to 100 ka and 120 to 100 ka the best estimates are closer to the upper limits of these ranges and associated uncertainties are in the range of 10 to 15 these uncertainties could permit deposition in either marine isotope stage 6 or stage 5d we found no evidence of glacial deposits dating to marine isotope stage 4 both bull lake age moraines from fremont lake on the opposite side of the wind river range and boulders on a fluvial terrace above the wind river gave age distributions very similar to that of the second oldest bull lake advance ca 130 to 100 ka the distribution of boulder ages for pinedale moraines at bull lake indicated deposition between 23 and 16 ka nearly identical to the distribution of 10 be ages previously reported for the type pinedale moraines at fremont lake

growth and decay of a marine terminating sector of the last british irish ice sheet a geomorphological reconstruction the boundary conditions that govern ice sheet dynamics can change significantly with the development of marine margins this paper uses the glacial landscape in western scotland to reconstruct changes in the british irish ice sheet that accompanied the growth and decay of a marine sector over the malin shelf ice advanced from a restricted mountain ice sheet with tidewater margins after 35 ka bp and reached the continental shelf in 7 ka average rate of 30 m a 1 early ice flow had been directed through north south geologically controlled over deepened fjords that were carved during previous restricted glaciations this flow regime was abandoned with development of the malin shelf ice sheet sector ice flow direction switched by 90 degrees and was drawn westwards towards the shelf edge the marine ice sheet phase saw episodes of west east ice divide migration by up to 60 km over west central scotland possibly linked to ice streaming and calving events at the ice sheet margin however permanent and stationary ice divides and zones of cold based ice associated with subglacial topographic highs also characterised the marine glacial stage over western scotland the north channel ice divide remained a constant though migratory feature while the biis occupied the malin shelf it finally collapsed at the end of the killard point stadial when the irish ice sheet began to rapidly decay 16 5 ka bp this permitted the scottish ice sheet to temporarily advance over north east ireland previously identified as the east antrim coastal readvance before it too retreated at rates in the order of 10 2 m a 1 although the imprint of extensive shelf edge ice sheet glaciation exists in the coastal landscape of western scotland the dominant landscape features relate to a restricted marine proximal mountain ice sheet with markedly different flow configurations similar first order geomorphological features relating to restricted glacial conditions are likely to be preserved in subglacial highlands under interior parts of modern ice sheets

stable isotope stratigraphy of a late last interglacial speleothem from rana northern norway abstract a stalagmite from rana northern norway dated by the tims uranium series technique yields records of stable oxygen and carbon isotopes covering the period from late marine oxygen isotope substages mis 5e to 5a that is 123 350 to 73 300 yr ago rapid growth 46 m yr between 123 350 and 119 500 yr ago reflects climatic conditions favorable for speleothem growth this period is characterized by century to millennial scale oscillations in both stable isotope records where both the absolute values and the isotope ranges are similar to holocene and older samples from the region from 119 500 to 107 700 yr ago speleothem growth was slow 0 7 m yr and between 107 700 and 73 300 yr ago growth is barely noticeable 0 07 m yr during the period of slow growth the stable isotope records show an overall enrichment trend the transition between rapid and slow growth rate occurring sometime between 119 500 and 107 700 yr ago is believed to reflect the termination of interglacial climate in this region the absence of detritus and corrosion features in the slowly deposited calcite suggests that the valley outside the cave remained sufficiently ice free for speleothem growth to occur until at least 73 300 yr ago

importance of sampling across an assemblage of glacial landforms for interpreting cosmogenic ages of deglaciation deglaciation chronologies for some sectors of former ice sheets are relatively poorly constrained because of the paucity of features or materials traditionally used to constrain the timing of deglaciation in areas without good deglaciation varve chronologies and or without widespread occurrence of material that indicates the start of earliest organic radiocarbon accumulations suitable for radiocarbon dating typically only general patterns and chronologies of deglaciation have been deduced however mid latitude ice sheets that had warm based conditions close to their margins often produced distinctive deglaciation landform assemblages including eskers deltas meltwater channels and aligned lineation systems because these features were formed or significantly altered during the last glaciation boulder or bedrock samples from them have the potential to yield reliable deglaciation ages using terrestrial cosmogenic nuclides tcn for exposure age dating here we present the results of a methodological study designed to examine the consistency of tcn based deglaciation ages from a range of deglaciation landforms at a site in northern norway the strong coherence between exposure ages across several landforms indicates great potential for using tcn techniques on features such as eskers deltas and meltwater channels to enhance the temporal resolution of ice sheet deglaciation chronologies over a range of spatial scales

late cenozoic history of the polar north atlantic results from ocean drilling abstract dsdp odp drilling in the northern north atlantic reveals that so far the onset of northern hemisphere cooling deduced from the occurrence of ice rafted debris ird in deep sea sediments can be traced back to the middle miocene the greenland ice sheet has probably the longest history on the northern hemisphere leg 151 site 909 fram strait recovered a unique continuous sediment record dating from early middle miocene the composition of the coarse fraction indicates that the oldest ird pulses are documented around 14 ma in the fram strait site 909 and around 12 6 ma on the voring plateau leg 104 site 642 in the norwegian greenland sea fram strait site 909 indicates a further stepwise increase of northern hemisphere cooling by serveral ird pulses during late miocene between 10 8 and 8 6 ma around 7 2 6 8 and 6 3 ma which ultimately led to a drastic intensification of northern hemisphere glaciation during plio pleistocene the deep sea sediment records show that plio pleistocene strengthening of ird pulses have been observed since approximately 4 0 ma in the westernmost norwegian greenland sea and labrador sea by contrast strengthening of ird pulses are observed at approximately 3 2 2 7 ma on the eastern side of the norwegian greenland sea including the barents sea margin sediment composition and physical properties of the drilled mostly terrigenous sections reveal that orbital parameters have controlled major aspects of shallow and deep water environments with frequency domains related to 41 ka periods of obliquity dominant in the pre glacial miocene pliocene and early quaternary and to 100 ka periods of eccentricity dominant during the past 600 000 700 000 years the stratigraphy of ice rafted material reveals a frequently and rapidly changing history of the dynamics of various segments in ice sheets around the norwegian greenland sea it is particularly evident from the composition of the ice rafted material that the eurasian ice sheets delivered ird to the eastern part of the norwegian greenland sea whereas greenland source areas are represented in the drill sites of the east greenland continental margin and in the labrador sea the stratigraphic sequences on the yermak plateau suggest that the svalbard ice sheet may have extended far to the north prior to oxygen isope stage 16 but not since then invasions of temperate atlantic waters during interglacials can be observed as short lived and rather irregular events it never reached the area off eastern greenland except during the pliocene interval when tundra floras were able to develop on north eastern greenland and planktonic foraminifers suggesting the presence of temperate water masses in the fram strait as been documented in the drill sites from the yermak plateau

erosion rates in fennoscandia during the past million years abstract the widespread existence of cosmogenic nuclides accumulated in bedrock prior to the last glaciation demonstrates the limited erosional efficacy of the most recent fennoscandian and laurentide ice sheets yet the deeper history of erosion in these landscapes repeatedly blanketed by ice remains essentially unknown here we present the first comprehensive ice sheet wide analysis of cosmogenic 10be data n 953 from the fennoscandian landscape we find 64 of all sampled bedrock surfaces contain 10be inheritance including 85 of blockfields and tors and 50 of ice carved terrain in addition to 27 of ice transported boulders recent ice sheets scoured landscapes well beyond glacial troughs and nuclide inventories reveal a patchy legacy of erosional effectiveness that diminishes at high elevations such that 89 n 55 of bedrock samples retain inheritance above 1600 m we exploit this widespread nuclide inheritance in a markov chain monte carlo based inversion model to estimate long term erosion rates and surface exposure histories from 113 paired 10be 26al bedrock samples nuclide inventories with or without inheritance convey equally important information about the erosional effectiveness of the last ice sheet we define cosmogenic nuclide memory as the residence time of bedrock samples inside the nuclide production window 2 m depth where 80 of the total nuclide production occurs the cosmogenic nuclide memory is set by mean erosion rate and varies from 10 ka for samples eroded 2 m during the last glaciation to 1 ma for the slowest erosion rates we find that mean erosion rates are well constrained compared to the ratio of exposure to burial the inclusion of bedrock erosion in our computations thwarts the capacity to constrain surface exposure history or identify former nunataks from paired 10be 26al data ice carved surfaces reflect diverse erosion histories that are not straightforward to interpret from surficial morphology alone relative to the 10 mm kyr benchmark for polar ice masses we report point based mean erosion rates that vary by more than three orders of magnitude with glacial troughs and areal scour terrain eroding at 1 to 100 mm kyr blockfields at 0 8 16 mm kyr and tors at 0 8 7 7 mm kyr 5th 95th percentiles

seven million years of glaciation in greenland glacial till glaciomarine diamictites and ice rafted detritus found in marine cores collected off the shore of southeast greenland record multiple late cenozoic glaciations beginning in the late miocene distinct rock assemblages and seismic stratigraphic control correlate the diamictites with glaciation of the southeast greenland margin glaciers advanced to the sea during several intervals in the pliocene and pleistocene north atlantic glaciation may have nucleated in southern greenland rather than further north because of the high mountains and the high levels of precipitation in this region

sensitivity of a greenland ice sheet model to ice flow and ablation parameters consequences for the evolution through the last climatic cycle sensitivity experiments are conducted to test the influence of poorly known model parameters on the simulation of the greenland ice sheet by means of a three dimensional numerical model including the mechanical and thermal processes within the ice two types of experiments are performed steady state climatic conditions and simulations over the last climatic cycle with a climatic forcing derived from the grip record the experiments show that the maximum altitude of the ice sheet depends on the ice flow parameters deformation and sliding law coefficients geothermal flux and that it is low when the ice flow is fast on the other hand the maximum altitude is not sensitive to the ablation strength and consequently during the climatic cycle it is driven by changes in accumulation rate the ice sheet extension shows the opposite sensitivity it is barely affected by ice flow velocity and the ice covered area is smaller for large ablation coefficients for colder climates when there is no ablation the ice sheet extension depends on the sea level an interesting result is that the variations with time of the altitude at the ice divide summit do not depend on the parameters we tested the present modelled ice sheets resulting from the climatic cycle experiments are compared with the present measured ice sheet in order to find the set of parameters that gives the best fit between modelled and measured geometry it seems that compared to the parameter set most commonly used higher ablation rate coefficents must be used

new ways of studying ice sheet flow directions and glacial erosion by computer modelling examples from fennoscandia a computer ice sheet model has been used to study regional ice flow directions and glacial erosion of the weichselian ice sheet adopting a new method of presenting modelled ice flow directions ice sheet model results from different time periods during the weichselian were extracted for five regions and presented in rose diagrams when comparing these computer generated results with information on flow directions obtained from current conceptual geological models based on field data large similarities were observed in comparing the ice flow directions as such the similarities were strikingly good in many cases there was also a good agreement in the timing of the events while in some cases a certain flow direction was assigned to a different time period by the ice sheet model than in the current interpretation of geological information nevertheless the overall agreement between the data sets shows that results from ice sheet models can aid in placing geological information into a coarse timeframe and chronostratigraphic context and also fill in time gaps in the glacial geological record where chronological control is sparse ice sheet model results thus constitute a new data set against which glacial geological information can be compared and tested a new quantity basal sliding distance has also been calculated from the ice sheet model results describing the over time accumulated length of ice that has passed over the landscape by basal sliding the results show high basal sliding distance values in sw sweden se norway in skagerrak and along the gulf of bothnia implying relatively large amounts of glacial erosion in these regions on elevated parts of the scandinavian mountain range and on adjacent plains in the east the basal sliding distance values are low implying weaker glacial erosion this compares well with different types of geological and morphological data suggesting that basal sliding distance is a useful entity for studying regional patterns of glacial erosion intensity

glacigenic debris flows on the north sea trough mouth fan during ice stream maxima abstract numerous short sediment cores which penetrate through the glacimarine section and into glacigenic debris flow debrites gdfs on the north sea fan are investigated the sampled gdfs represent the uppermost of several sequences of these stacked bodies many hundreds of metres thick the gdfs occurred over slopes from 0 3 to 0 7 and examples are found from the shelf break at the mouth of the norwegian channel to the norway basin floor 300 km distant the gdf diamicts are structureless and homogeneous in terms of grain size magnetic susceptibility nrm bulk density water carbonate foraminiferal and rock fragment content this applies both from gdf to gdf across the fan and in a proximal distal sense comparison of the investigated lithologic parameters with weichselian till from a borehole in the norwegian channel 200 km up ice shows striking similarity to the gdf debrites ams 14c dates at the gdf glacimarine contact and stratigraphic relationships indicate voluminous deposition before 18 ka continuing possibly uninterrupted until ca 16 ka and a final phase halting just before 15 ka when the norwegian channel became ice free this chronology the comparable lithologic character between till and gdf debrites and a seismic continuity confirms inferences that the gdfs are directly associated with glacial ice at the shelf break the glacial efflux has not been subject to subsequent sorting and the homogeneity of the gdf is thought to reflect thorough mixing in a deforming till layer beneath a fast flowing ice stream the glacimarine section subsequent to gdfs shows no indications of turbidity flow observations are compatible with a slow non disintegrating cohesive plug flow transport mechanism with little sediment water interaction on the upper surface minor distal differentiation might reflect a change in flow process deposition on the fan margins from water borne glacimarine processes simultaneous with gdf deposition is limited

using in situ produced cosmogenic isotopes to estimate rates of landscape evolution a review from the geomorphic perspective the application of in situ produced cosmogenic isotopes to problems in geomorphology has increased rapidly over the past decade at least 57 papers and numerous abstracts have been published since the mid 1980s when the first mass spectrometric measurements of terrestrially produced cosmogenic isotopes were made taken at face value these studies provide quantitative information about rates of landscape evolution and landform age however the significance of calculated erosion rates and exposure ages depends strongly on the models used to interpret isotopic data the validity of assumptions inherent to these models and the geologic surroundings in which the samples were collected this paper attempts to place cosmogenic isotope studies in a geomorphic context by reviewing fundamentals of the method and evaluating the validity of assumptions under which these data have been interpreted at present the establishment of high precision cosmogenically based glacial and alluvial chronologies is stymied by the evolution of geomorphic surfaces the erosion of rock from sampled boulders the potential for isotope inheritance from previous exposure and the uncertainty of isotopic measurements uncertainties in isotope production rates and the observed variability of exposure ages on individual geomorphic surfaces limit the confidence with which cosmogenic ages can be correlated reliably with those obtained by other techniques estimation of erosion rates at single points on the landscape gives useful small scale information extrapolation of these rates over longer time and larger spatial scales is less sure and most likely biased toward lower erosion rates by die inadvertent selection of resistant sample sites however because erosion rates are so poorly constrained at present even estimates to within a factor of 2 may be of significant value to geomorphologists and tectonicists

towards a gis assessment of numerical ice sheet model performance using geomorphological data a major difficulty in assimilating geomorphological information with ice sheet models is the lack of a consistent methodology to systematically compare model output and field data as an initial step in establishing a quantitative comparison methodology automated proximity and conformity analysis apca and automated flow direction analysis afda have been developed to assess the level of correspondence between modelled ice extent and ice marginal features such as end moraines as well as between modelled basal flow directions and palaeo flow direction indicators such as glacial lineations to illustrate the potential of such an approach an ensemble suite of 40 numerical simulations of the fennoscandian ice sheet were compared to end moraines of the last glacial maximum and the younger dryas and to glacial lineations in northern sweden using apca and afda model experiments evaluated in this manner were ranked according to level of correspondence such an approach holds considerable promise for optimizing the parameter space and coherence of ice flow models by automated quantitative assessment of multiple ensemble experiments against a database of geological or glaciological evidence

late cenozoic deep weathering patterns on the fennoscandian shield in northern finland a window on ice sheet bed conditions at the onset of northern hemisphere glaciation abstract the nature of the regolith that existed on the shields of the northern hemisphere at the onset of ice sheet glaciation is poorly constrained in this paper we provide the first detailed account of an exceptionally preserved deeply weathered late neogene landscape in the ice sheet divide zone in northern finland we mine data sets of drilling and pitting records gathered by the geological survey of finland to reconstruct regional preglacial deep weathering patterns within a gis framework using a large geochemical data set we give standardised descriptions of saprolite geochemistry using a variant of the weathering index of parker wip as a proxy to assess the intensity of weathering we also focus on mineral prospects and mines with dense pit and borehole data coverage in order to identify links between geology topography and weathering geology is closely linked to topography on the preglacial shield landscape of northern finland and both factors influence weathering patterns upstanding resistant granulite granite gabbro metabasalt and quartzite rocks were associated with fresh rock outcrops including tors or with thin fines values above 3000 and 4000 beneath valley floors developed along mineralised shear and fracture zones weathering penetrated locally to depths of 50 m and included intensely weathered kaolinitic clays with wip fines values below 1000 late neogene weathering profiles were varied in character tripartite clay gruss saprock profiles occur only in limited areas bipartite gruss saprock profiles were widespread with saprock thicknesses of 10 m weathering profiles included two discontinuities in texture materials and resistance to erosion between saprolite and saprock and between saprock and rock limited core recovery when drilling below the soil base in mixed rocks of the tana belt indicates that weathering locally penetrated deep below upper fresh rock layers such deep seated weathered bands in rock represent a third set of discontinuities incipient weathering and supergene mineralisation also extended to depths of 100 m in mineralised fracture zones the thin weathering crusts found extensively beneath till may represent types of early or middle pleistocene palaeosols we confirm that glacial erosion has been very limited

origin of the middle pleistocene transition by ice sheet erosion of regolith the transition in the middle pleistocene 0 9 ma seen in 18o deep sea core records from relatively low amplitude high frequency 41 kyr to high amplitude low frequency 100 kyr ice volume variations under essentially the same orbital forcing can be attributed to a change from an all soft bedded to a mixed hard soft bedded laurentide ice sheet through glacial erosion of a thick regolith and resulting exposure of unweathered crystalline bedrock a one dimensional ice sheet and bedrock model which includes transport of sediment and ice by subglacial sediment deformation demonstrates that a widespread deforming sediment layer maintains thin ice sheets before the transition which respond linearly to the dominant 23 and 41 kyr orbital forcing progressive removal of the sediment layer eventually causes a transition to thicker ice sheets whose dominant timescale of change 100 kyr reflects nonlinear deglaciation processes in model simulations over the last 3 ma initialized with no ice and a uniform 50 m sediment layer the time series of ice volume and extent agree in several important aspects with the observed records

the use of in situ produced cosmogenic radionuclides in glaciology and glacial geomorphology the usefulness of in situ produced cosmogenic radionuclides in constraining glacial chronologies through exposure age dating has been demonstrated in numerous studies however an understanding of cosmogenic radionuclide techniques and their uncertainties opens up a wide range of other potential applications in glaciology and glacial geomorphology recently developed applications include estimation of spatial and temporal variations in the depth of glacial erosion from cosmogenic radionuclide inheritance which provides important constraints on process based erosion models and burial dating which can provide chronological control for glacial advances and the onset of till deposition an interesting new application currently at the level of theoretical model development concerns the unraveling of complex exposure and burial histories overall in situ produced cosmogenic radionuclide techniques provide a means with which to constrain a wide range of ice sheet and glacial models over time scales of a thousand to a few million years

high resolution modeling of the advance of the younger dryas ice sheet and its climate in scotland abstract ice sheet modeling tightly constrained by empirical studies provides an effective framework to reconstruct past climatic and environmental conditions scotland was severely affected by the abrupt climate change associated with the younger dryas stade during which an extensive ice sheet formed across the west highlands after a period of ice free conditions here a quasi three dimensional time dependent ice flow mass balance model is developed and applied to scotland at 1 km resolution the flow model is based on the driving stress approximation with an additional longitudinal correction term essential at this scale of operation surface mass balance is driven by temperature and precipitation changes and further mass wastage is achieved through an empirically defined calving term the ice dynamics and mass balance components are coupled through the equation for mass continuity which is integrated through time over a finite difference grid which yields the geometric evolution of the ice sheet initial experiments reveal the model to be relatively insensitive to internal parameters but highly sensitive to mass balance furthermore these experiments indicate that scotland is readily susceptible to glaciation with large glaciers building up on the flanks of ben nevis after a temperature depression of 2 5 c under present day precipitation the younger dryas is modeled using a grip temperature series locally adjusted for amplitude and a systematic series of runs enables the isolation of the climate which best matches mapped ice limits this optimum fit configuration requires an annual temperature cooling of 8 c and the introduction of substantial west east and south north precipitation gradients of 40 and 50 respectively to the present day regime under these conditions a series of substantial independent regional ice centers develop in agreement with trimline studies and after 550 years the modeled ice sheet closely resembles the maximum limits as indicated by field mapping however modeled ice continues to expand beyond 550 yr in conflict with the mapped ice limits which suggest a prolonged period of stability this discrepancy may be explained by the onset of extreme aridity ca 400 yr into the stade associated with a southern migration of the polar front leading to a reduction in atmospheric circulation which effectively starved the ice sheet of its moisture source preventing further expansion introduction of an additional 20 reduction in precipitation to the optimum fit regime after 350 yr brings the modeled ice sheet to equilibrium substantiating this conclusion

beryllium 10 dating of the duration and retreat of the last pinedale glacial sequence accurate terrestrial glacial chronologies are needed for comparison with the marine record to establish the dynamics of global climate change during transitions from glacial to interglacial regimes cosmogenic beryllium 10 measurements in the wind river range indicate that the last glacial maximum marine oxygen isotope stage 2 was achieved there by 21 700 700 beryllium 10 years and lasted 5900 years ages of a sequence of recessional moraines and striated bedrock surfaces show that the initial deglaciation was rapid and that the entire glacial system retreated 33 kilometers to the cirque basin by 12 100 500 beryllium 10 years

unraveling complex exposure burial histories of bedrock surfaces under ice sheets by integrating cosmogenic nuclide concentrations with climate proxy records the production accumulation and decay of cosmogenic radionuclides in rock surfaces subjected to episodes of exposure and burial by ice results in nuclide concentrations in present day rock surfaces that can be used to address a variety of questions in glacial geomorphology and quaternary geology of particular importance is the fact that these nuclide concentrations reflect both the timing of initial exposure of the rock surface and the chronology of subsequent exposure burial and erosion episodes for landscapes where geomorphic evidence indicates that little no erosion occurred constraining the timing of initial exposure and the number of phases of exposure and burial that a rock surface has been subjected to is possible using multiple cosmogenic radionuclide concentrations combined with proxies for the timing and duration of periods of ice cover such as ice core or marine isotope records however interpretations based on this approach require determination of an appropriate cutoff value to separate the proxy record into ice free and ice covered conditions and assessment of the sensitivity of the results to different cutoff values we have developed a numerical model to evaluate variations in total exposure and burial durations as a function of different proxy records and cutoff values this program is available at http www missouri edu liyk climateproxycurve zip initial results for sites in west antarctica and northern sweden show that the method provides a quick and robust way to derive best fit cutoff values and chronologies of burial and exposure and small changes in cutoff values can result in significant shifts in results the method described here provides new insight into the interpretation and reliability of multiple nuclide samples this approach also has the potential to provide improved constraints for ice sheet dynamics and landscape evolution and a means to assess the sensitivity of calculated initial exposure dates to assumptions about ice sheet history

provenance of icebergs during heinrich event 3 and the contrast to their sources during other heinrich episodes the sources of ice rafted debris ird from heinrich layer 3 h 3 in the north atlantic were studied by means of pb isotopic analyses of single and multiple grain samples of ice rafted feldspars from core v28 82 49 27 n 22 16 w pb ratios of h 3 grains point to multiple continental sources and are similar to the isotopic composition of the background sediment this similarity as well as previous observations about the age and the sr and nd isotopic composition of the fine fraction of h 3 in cores of the same area are all consistent with the hypothesis that in the eastern north atlantic along the latitude band of maximum ird deposition 40 50 n h 3 is a product of foraminifera dissolution and or low productivity in contrast analyses of composite samples of h 1 h 2 h 4 and h 5 plot in 206pb 204pb versus 206pb 204pb coordinates along the churchill province line supporting the premise that the hudson strait was the major pathway of iceberg drainage during these four events since an advance of the laurentide ice sheet during h 3 is demonstrated by increases in detrital carbonate at the level of h 3 in cores from the western north atlantic the results obtained here suggest that if indeed that advance took place the iceberg discharge melted mostly in the western basin

history of terrigenous sedimentation during the past 10 m y in the north atlantic odp legs 104 and 105 and dsdp leg 81 bulk sediment measurements and analyses of the input of 63 m terrigenous particles have enabled us to reconstruct the variability of sediment flux in the norwegian and the labrador seas as a function of paleoceanographic and paleoclimatic changes in both areas minor amounts of coarse terrigenous particles occur in sediments that were deposited between 9 5 and 7 0 ma the higher amounts are only documented in the norwegian sea around 5 7 5 4 ma variations in bulk sediment accumulation are related to a strengthening of denmark strait overflow which is closely connected with the production of norwegian sea deep water between 8 3 and 7 8 ma 6 9 and 6 1 ma and 5 3 and 4 2 ma in the labrador sea reduced bottom water activity coincided with the highest levels of organic carbon accumulation and an increase in bulk sediment accumulation and may correlate with the latest miocene messinian event in both the labrador sea and the norwegian sea a drastic increase in the amount of coarse terrigenous particles rock fragments and quartz started at around 4 0 ma recording a nearly simultaneous onset of large scale ice rafting this was ultimately caused by a deterioration in the northern hemisphere climate possibly related to the final closure of the isthmus of panama

the carbon isotope composition of atmospheric co 2 in paris one characteristic of air pollution in the urban environment is high co2 concentrations resulting from human activities determining the relative contributions of the different co2 sources can be addressed simply and elegantly by combining isotope and concentration measurements using this approach on atmospheric co2 samples collected in paris its suburbs and the open country provides fairly accurate conclusions our results show that air pollution within the first few metres above ground results basically from binary mixtures among which road traffic is the main contributor and in particular vehicles using unleaded gasoline 90 of the total heating sources which account for 50 of the co2 input below the atmospheric inversion level and vehicles using diesel contribute very little human respiration has a recognisable signature at street level under certain circumstances the combined isotope and concentration analysis provides a sensitive tracer of local variations even detecting the occasional prevalence of human respiration and the onset of actions in which natural gas is burnt it also detects surprising inlets of clean air co2 wise in the very centre of the city

palaeoglaciology of an ice sheet through a glacial cycle the european ice sheet through the weichselian abstract satellite images provide unique means of identifying large scale flow generated lineations produced by former ice sheets they can be interpreted to reconstruct the major elements which make up the integrated large scale structure of ice sheets ice divides ice streams interstream ridges ice shelves calving bays the evolving palaeoglaciological structure of the european ice sheet during its decay from the last glacial maximum lgm is reconstructed by reference to these components and in the context of a new map showing isochrons of retreat during the retreat phase in particular the time dependent dynamic evolution of the ice sheet and the pattern of ice stream development are reconstructed crossing lineations are widespread the older ones are suggested to have formed during molten bed phases of ice sheet growth and preserved by frozen bed conditions during the glacial maximum particularly in areas which lay during deglaciation beneath ice divides and inter ice stream ridges both areas of slow flow and possibly frozen bed conditions four phases of growth a1 to a4 and five phases of decay r1 to r5 are used to describe the major climatically and dynamically determined stages in the evolution of the ice sheet through the last glacial cycle the growth and decay patterns are quite different and associated with major shifts in the ice divide reflecting growth from the fennoscandian mountains and decay away from marine influenced margins these patterns were determined by the locations of nucleation areas spatial patterns of climate and calving at marine margins the prevalence of streaming within the retreating ice sheet suggests that the mean elevation of the ice sheet was lower than predicted from glaciological models which do not include streaming and that this might reconcile glaciological models and earth rheology models which infer paleao ice sheet thickness by inverting sea level data

mid pleistocene cosmogenic minimum age limits for pre wisconsinan glacial surfaces in southwestern minnesota and southern baffin island a multiple nuclide approach abstract paired 10 be and 26 al analyses n 14 indicate that pre wisconsinan glaciated bedrock surfaces near the northern baffin island and southern minnesota paleo margins of the laurentide ice sheet have long and complex histories of cosmic ray exposure including significant periods of partial or complete shielding from cosmic rays using the ratio 26 al 10 be we calculate that striated outcrops of sioux quartzite in southwestern minnesota southern margin were last overrun by ice at least 500 000 years ago weathered bedrock tors on the once glaciated uplands of baffin island northern margin are eroding no faster than 1 1 m myr 1 the equivalent of at least 450 000 years of surface and near surface exposure our data demonstrate that exposure ages and erosion rates calculated from single nuclides can underestimate surface stability dramatically because any intermittent burial and the resultant lowering of nuclide production rates and nuclide abundances will remain undetected

reconstruction of palaeo ice sheets the use of geomorphological data the article discusses the nature of the glacial inversion problem which is defined as the extraction of time slice ice sheet flow patterns from the patchy and partly overprinted landform record present in former ice sheet areas a coherent inversion model for derivation of flow patterns and interior ice sheet configuration from geomorphological data is presented glacial landscapes are classified according to the three criteria of internal age gradients presence or absence of meltwater traces aligned to flow traces and basal condition frozen bed thawed bed inferred from morphology the inversion model uses landscapes classified accordingly spatially delineated into fans as input data relative chronologies at fan intersections are used to sort fans in a relative age stack that can be linked to stratigraphic dating information

preglacial surface remnants and quaternary glacial regimes in northwestern sweden abstract we present a detailed map of the distribution of preglacial surface remnants in the kebnekaise region of northwestern sweden in this mountain area we discern four important large scale geomorphological units each representing a specific set of erosional agents and formative conditions these are i intact preglacial surface remnants characterized by gentle slopes round summits wide shallow valleys and an absence of rock basins ii preglacial surface remnants showing signs of minor glacial erosion and deposition iii glacially scoured surfaces including glacial troughs iv deep fluvial valleys cut into the preglacial surface the pattern of glacial erosion is explained as the result of three specific modes of glaciation known to have existed during the last 120 000 years and inferred to have repeatedly prevailed during the last 2 75 million years cirque glaciation mountain ice sheets and fennoscandian ice sheets a deep ocean oxygen isotope record of foraminifera from the north atlantic dsdp 607 was used to infer the temporal extent of these modes of glaciation during the last 2 75 million years we interpret the preglacial landscape preservation and the pattern of glacial erosion in terms of the configuration the basal thermal regime and the duration of such glaciation events the average subglacial thermal regime of both ice sheet types was frozen on the uplands and melting in the main valleys where outlet glaciers and ice streams formed the pre glacial landscape is best preserved at intermediate elevations low enough not to have been covered by cirque glaciers and apparently high enough not to have experienced melted bed conditions and subglacial erosion during ice sheet overriding events in a narrow high relief zone along the elevation axis interglacial fluvial erosion was morphologically important the absence of glacial erosion on uplands in this zone allowed fluvial erosion to commence on the same locations during each ice free interval in contrast no persistent fluvial valley pattern could develop in zones subjected to repeated glacial scouring and hence derangement of fluvial patterns

ice sheet growth under dirty conditions implications of debris cover for early glaciation advances abstract debris covered glaciers respond differently to any given set of climatic conditions than clean glaciers this difference stems from change in ablation rate caused by a debris cover approaching zero ablation with sufficiently thick protective cover a mechanism not yet considered in the context of ice sheet growth critical to applying the mechanism to ice sheets is supply of debris we postulate that periods after major interglacials offer the best conditions for dirty advances this is because the volume of debris previously deposited and weathered in interglacial time the latter a function of exposure length should be at a maximum advances of dirty ice sheets generate landforms and in positions deviating from clean ice advances under similar climatic conditions hence inferences of both ice sheet properties inferred from such dirty ice advances and the climate conditions related to such advances must be cautious furthermore modeling of past ice sheets must consider the effect of debris cover whenever indications exist for such a cover

fracture related wavefield polarization and seismic anisotropy across the greendale fault we investigate seismic signatures of fracturing in a newly ruptured strike slip fault by determining the wavefield polarization in the new zealand canterbury plains area and across the greendale fault which was responsible for the 3 september 2010 darfield mw 7 1 earthquake previous studies suggested that fractured rocks in fault damage zones cause directional amplification and ground motion polarization in the fracture perpendicular direction as an effect of stiffness anisotropy and cause velocity anisotropy with shear wave velocity larger in the fracture parallel component an array of 14 stations was installed following the darfield earthquake we assess polarization both in the frequency and time domains through the individual station horizontal to vertical spectral ratio and covariance matrix analysis respectively and compare the results to previously reported anisotropy measurements from shear wave splitting stations installed in the canterbury plains have an amplification peak between 0 1 and 0 3 hz for both earthquakes and ambient noise we relate the amplification to the resonance of a considerable thickness c 1 km of soft sediments lying over the metamorphic bedrock analysis of seismic events revealed the existence of another peak in amplification between 2 and 5 hz at two on fault stations which was not visible in the noise analysis in contrast to the lower frequency peak the ones between 2 and 5 hz are more strongly anisotropic attaining amplitudes up to a factor of 4 in the n52 direction to interpret this effect we model the fracture pattern in the fault damage zone produced by the fault kinematics we conclude that the horizontal polarization is orthogonal to extensional fractures which predominate in the shallow layers 2 km expected in n101 direction on the basis of the model thus different fracture distribution at different depths may explain different results for amplification compared to anisotropy we propose polarization amplification analysis as a complementary method to s wave splitting analysis polarization analysis is rapidly computed and robust and it can be applied to either earthquakes or ambient noise recordings giving useful information about the predominant fracture patterns at various depths

simulating the amplification of orbital forcing by ocean feedbacks in the last glaciation according to milankovitch theory the lower summer insolation at high latitudes about 115 000 years ago allowed winter snow to persist throughout summer leading to ice sheet build up and glaciation1 but attempts to simulate the last glaciation using global atmospheric models have failed to produce this outcome when forced by insolation changes only2 3 4 5 these results point towards the importance of feedback effects for example through changes in vegetation or the ocean circulation for the amplification of solar forcing6 7 8 9 here we present a fully coupled ocean atmosphere model of the last glaciation that produces a build up of perennial snow cover at known locations of ice sheets during this period we show that ocean feedbacks lead to a cooling of the high northern latitudes along with an increase in atmospheric moisture transport from the equator to the poles these changes agree with available geological data10 11 12 13 14 15 and together they lead to an increased delivery of snow to high northern latitudes the mechanism we present explains the onset of glaciation which would be amplified by changes in vegetation in response to weak orbital forcing

fennoscandian palaeoglaciology reconstructed using a glacial geological inversion model the evolution of ice sheet configuration and flow pattern in fennoscandia through the last glacial cycle was reconstructed using a glacial geological inversion model i e a theoretical model that formalises the procedure of using the landform record to reconstruct ice sheets the model uses mapped flow traces and deglacial melt water landforms as well as relative chronologies derived from cross cutting striae and till lineations as input data flow trace systems were classified into four types i time transgressive wet bed deglacial fans ii time transgressive frozen bed deglacial fans iii surge fans and iv synchronous non deglacial event fans using relative chronologies and aggregation of fans into glaciologically plausible patterns a series of ice sheet configurations at different time slices was erected a chronology was constructed through correlation with dated stratigraphical records and proxy data reflecting global ice volume geological evidence exists for several discrete ice sheet configurations centred over the scandinavian mountain range during the early weichselian the build up of the main weichselian fennoscandian ice sheet started at approximately 70 ka and our results indicate that it was characterised by an ice sheet with a centre of mass located over southern norway this configuration had a flow pattern which is poorly reproduced by current numerical models of the fennoscandian ice sheet at the last glacial maximum the main ice divide was located overthe gulf of bothnia a major bend in the ice divide was caused by outflow of ice to the northwest over the lowest part of the scandinavian mountain chain widespread areas of preserved pre late weichselian landscapes indicate that the ice sheet had a frozen bed core area which was only partly diminished in size by inward transgressive wet bed zones during the decay phase

a relict landscape in the centre of fennoscandian glaciation cosmogenic radionuclide evidence of tors preserved through multiple glacial cycles the presence of well developed tors boulder fields and weathering mantles in the parkajoki area of northeastern sweden near the centre of fennoscandian glaciation has been used to suggest that these landscapes were preserved during all glacial cycles since ice sheet initiation in the late cainozoic this implies that all successive large scale glaciations must have had frozen bed conditions across this area to allow for subglacial landscape preservation cosmogenic 10be and 26al data from three tors and a meltwater channel in the parkajoki area were used to test this hypothesis of landscape preservation through multiple glacial cycles apparent exposure ages of tor summit bedrock surfaces ranging between 79 and 37 ka in an area deglaciated at 11 ka are consistent with the interpretation of these features as relict landforms that have survived glaciation with little or no erosion single nuclide minimum exposure age data require that the tors have survived at least two complete glacial cycles this estimate is based on i the approximate duration of periods of ice sheet cover versus ice free conditions as deduced from the dsdp 607 marine benthic foraminifer oxygen isotope record in conjunction with ii a record of fennoscandian ice sheet flow traces and hence ice sheet extent and iii noting that cosmogenic nuclides are accumulated only during ice free periods in addition mean cosmogenic 26al 10be concentration ratios from two of the sites indicate a minimum model total history of 605 ka and a maximum erosion rate of 1 6 m ma 1 thus the numerical ages confirm the overall qualitative interpretation of landscape preservation through multiple glacial cycles

evidence for general instability of past climate from a 250 kyr ice core record recent results1 2 from two ice cores drilled in central greenland have revealed large abrupt climate changes of at least regional extent during the late stages of the last glaciation suggesting that climate in the north atlantic region is able to reorganize itself rapidly perhaps even within a few decades here we present a detailed stable isotope record for the full length of the greenland ice core project summit ice core extending over the past 250 kyr according to a calculated timescale we find that climate instability was not confined to the last glaciation but appears also to have been marked during the last interglacial as explored more fully in a companion paper3 and during the previous saale holstein glacial cycle this is in contrast with the extreme stability of the holocene suggesting that recent climate stability may be the exception rather than the rule the last interglacial seems to have lasted longer than is implied by the deep sea specmap record4 in agreement with other land based observations5 6 we suggest that climate instability in the early part of the last interglacial may have delayed the melting of the saalean ice sheets in america and eurasia perhaps accounting for this discrepancy

high resolution paleotemperature proxy record for the last interglaciation based on norwegian speleothems two speleothems from a coastal lowland and an alpine cave site in northern norway grew in isotopic equilibrium during termination ii and marine isotope stage 5 150 000 80 000 yr b p as dated by the u series technique the stable isotope record 18o 13c displays a time resolution down to 20 yr the 18oc signal in the two speleothems appears to be mainly dominated by the meteoric signal i e there is a positive relationship between 18oc and temperature 18oc t 0 the fm 2 couplet a younger dryas type two step structure in termination ii at 132 000 5000 and 129 000 5000 yr is prominent in the speleothem record the record correlates well with details and gross features of the grip ice core and confirms the time scale and the unstable climate proposed for substage 5e the isotopic response in termination ii is delayed relative to the devil s hole record and is in accord with the specmap chronology but cooling at the end of substage 5e seems to be synchronous between devil s hole and the norwegian flowstones

timing and east west correlation of south swedish ice marginal lines during the late weichselian abstract the retreat of the late weichselian ice sheet over the southern part of sweden is marked along the southwest coast by distinct marginal moraine ridges their timing can directly and indirectly be assessed based on a number of radiocarbon dates and pollen stratigraphic investigations on lake sediment sequences adjacent to the ice marginal lines along the southeastern side of the peninsula the ice recession has been reconstructed based on a combination of clay varve chronology pollen and radiocarbon stratigraphy a morphological correlation of ice marginal lines between the west and east coast is problematic since the distinct west coast moraines cannot be followed through the central part of the peninsula towards the east coast this paper is an attempt to reconstruct an age equivalent west east extension of the ice recession lines on the basis of existing data sets for our correlation we use calibrated radiocarbon ages for ice marginal deposits on the west coast and compare these with a partly radiocarbon dated clay varve chronology on the east coast we conclude that the two oldest moraines on the west coast formed at 18 000 16 000 and 15 400 14 500 cal yr bp respectively during the following rapid deglaciation which may have coincided with the beginning of the bolling pollen zone large parts of southernmost sweden became ice free except for higher elevated areas where stagnant ice remained for another 400 500 yr a best guess is that the formation of the next younger ice marginal lines may have occurred at 14 400 14 200 14 200 and 13 400 cal yr bp and during the younger dryas cold event

structure related and stress induced shear wave velocity anisotropy observations from microearthquakes near the calaveras fault in central california we attempt to distinguish between structure related and stress induced shear wave velocity anisotropy in the upper crust using microearthquakes m 2 6 that occurred before and after the tres pinos california earthquake m 5 5 of 26 january 1986 the tres pinos earthquake occurred on the quien sabe fault a northwest southeast trending right lateral strike slip fault approximately 10 km east of the calaveras fault in central california digital three component seismograms are available from 1984 to 1992 from the quien sabe ranch station hqr located only 7 3 km north northeast of the epicenter of the main event we studied polarization directions and delay times using a method that combines cross correlation techniques and particle motion plots to enhance the significance of our interpretations we reduced uncertainties related to the hypocenter location of the earthquakes by source relocation based on joint hypocenter determination and cross correlation in the study area we found two zones with different but stable polarization directions around the quien sabe fault system shear wave polarizations of over 30 microearthquakes clearly indicate that the slow shear wave direction is normal to the northwest southeast trend of this vertical right lateral strike slip fault thus velocity anisotropy appears to be caused by the fault system itself however anisotropy associated with a cluster of microearthquakes located about 4 km to the east is quite different in this zone the fast and slow shear polarization directions are correlative with the maximum and minimum horizontal stress directions respectively thus in this case it appears the crustal velocity anisotropy is controlled by the state of stress in the crust the observation of these two polarization directions at one station suggests that the shallow crust below the station hqr does not influence shear wave polarization directions

cosmogenic nuclide evidence for minimal erosion across two subglacial sliding boundaries of the late glacial fennoscandian ice sheet abstract the existence of sliding and frozen bed areas under ice sheets is significant in understanding basal thermal regimes patterns of erosion and landform development and in constraining boundary conditions for the reconstructions of ice sheets recognition of subglacial boundaries between sliding and frozen bed areas for former ice sheets is typically based on distinct morphological contrasts between areas with glacial landform assemblages and relict areas showing little alteration of pre existing features some of these boundaries especially on continental shield areas however are clearly visible from air photos but have minimal topographic expression understanding the chronology and erosional development of such boundaries is important to provide insight into the pattern and persistence of basal conditions under ice sheets geomorphic evidence and cosmogenic radionuclide concentrations of bedrock outcrops on either side of two sliding boundaries on ultevis and arvestuottar low relief upland plateaus in northern sweden are consistent with negligible erosion in relict landscape frozen bed areas due to the last glaciation but also indicate insignificant erosion in the sliding areas such a pattern and magnitude of landscape modification indicates that sliding was short lived in these areas likely as a transient phase during deglaciation these sites demonstrate that short periods of sliding are in some cases sufficient to produce landscapes that are recognized as glacial from air photos thus regions of sliding identified on shield areas must be viewed as the cumulative total area that has experienced sliding at any time during a glaciation the actual extent of sliding areas during any single ice sheet phase is presumably considerably less than this cumulative total which has important implications for establishing appropriate basal boundary conditions for ice sheet reconstructions

evidence for massive discharges of icebergs into the north atlantic ocean during the last glacial period sediments in the north atlantic ocean contain a series of layers that are rich in ice rafted debris and unusually poor in foraminifera1 here we present evidence that the most recent six of these heinrich layers deposited between 14 000 and 70 000 years ago record marked decreases in sea surface temperature and salinity decreases in the flux of planktonic foraminifera to the sediments and short lived massive discharges of icebergs originating in eastern canada the path of the icebergs clearly marked by the presence of ice rafted detrital carbonate can be traced for more than 3 000 km a remarkable distance attesting to extreme cooling of surface waters and enormous amounts of drifting ice the cause of these extreme events is puzzling they may reflect repeated rapid advances of the laurentide ice sheet perhaps associated with reductions in air temperatures yet temperature records from greenland ice cores appear to exhibit only a weak corresponding signal moreover the 5 10 000 yr intervals between the events are inconsistent with milankovitch orbital periodicities raising the question of what the ultimate cause of the postulated cooling may have been

evolution of the norwegian current and the scandinavian ice sheets during the past 2 6 m y evidence from odp leg 104 biogenic carbonate and terrigenous records abstract records of biogenic and terrigenous components have been obtained from the interval corresponding to the last 2 6 m y of odp sites 643 and 644 in order to reconstruct surface and deep water regimes in the norwegian sea surface water regimes record long lasting moderate glacial conditions during the interval 2 6 1 0 ma small intrusions of atlantic water episodically penetrated into the norwegian sea forming a narrow tongue along the eastern margin which is documented at site 644 the polar front was most probably situated between the site 644 and 643 locations on the outer voring plateau during these time intervals deep water regimes reflect long term persistent corrosive bottom waters most probably due to a weakly undersaturated water column and a low rate of carbonate shell production in surface waters deep water production in the norwegian greenland sea may have operated in a different way e g brine formation during winter sea ice growth bottom waters were oxygenated throughout the entire period and deep water was exchanged persistently with the north atlantic increased glacial interglacial environmental contrasts are documented reflecting a strengthening of the norwegian current and intensified glaciations on the surrounding land masses during the interval 1 0 0 6 ma during this time a major shift in the mode of deep water production occurred the onset of large amplitudes in glacial interglacial environmental conditions with maximum contrasts in surface water regimes different modes of deep water production and intensified exchange with the north atlantic marks the last 0 6 ma a broad development of the norwegian current is observed during peak interglacials while during glacials seasonally variable sea ice cover and iceberg drift dominate surface water conditions

reconstruction of glaciation over the past 6 myr from ice borne deposits in the norwegian sea it is well known that a significant intensification of northern hemisphere glaciation occurred 2 5 myr ago in contrast to the much earlier onset 35 myr of glaciation in antarctica1 3 much less is known about the behaviour of the climate before 2 5 myr and it has remained unclear when sizeable glaciers first started to develop in the northern hemisphere here we deduce the history of high northern latitude glaciation over the past 6 myr from records of ice borne deposits in deep sea sediments of the norwegian sea we find that glaciers large enough to reach sea level were present in the norwegian sea area as early as 5 5 myr three million years before the intensification of glaciation at 2 5 myr fluctuations in ice volume can be deduced from the oxygen isotope record but this provides only a global average which may not reflect the history of ice sheet growth in specific regions from a comparison of the oxygen isotope records with the record of ice rafted material we derive an estimate of the relative contributions of southern and northern hemisphere glaciation to global variations in ice volume

the eem stable isotope record along the grip ice core and its interpretation a 3029 m long deep ice core extending nearly to bedrock has been drilled at the very top of the greenland ice sheet summit by the greenland ice core project grip an international european joint effort organized by the european science foundation the ice core reaches back to 250 000 yr b p according to dating based partly on stratigraphic methods and partly on ice flow modeling a continuous and detailed stable isotope 18o profile along the entire core depicts dramatic temperature changes in greenland through the last two glacial cycles including abrupt climatic shifts during the eem sangamon interglaciation which is elsewhere recorded as a warm and stable period the stratigraphic continuity of the eemian layers has therefore been scrutinized new ice core studies comprising cloudy band observations deconvolution and frequency analyses lead to the conclusion that the climate instability suggested during the eem interglaciation in greenland is likely to be real though no conclusive evidence is available whereas latitudinal displacements of the north atlantic ocean current are considered the immediate cause of the glacial climate instability longitudinal displacements may be the immediate cause of the eemian instability if so the eemian climate changes will be much subdued outside the arctic region and will probably only be recognizable in sedimentary sequences of high sensitivity and temporal resolution

reconstructing the erosion history of glaciated passive margins applications of in situ produced cosmogenic nuclide techniques offshore sediment accumulations provide an intriguing record of the net sediment output resulting from geomorphological evolution of the circum atlantic continental margin since the commencement of neogene glaciation however the onshore record of the timing pattern and amount of bedrock erosion that produced these sediments is comparatively poorly constrained and understood although there are good general models of glaciation history the geomorphology of circum atlantic continental margin mountains as assessed from remote sensing data and field observations includes palimpsest landforms and landscapes that reflect a complex pattern of spatial and temporal variations in the impact of glacial fluvial and periglacial processes perhaps most surprising is that despite having been repeatedly overridden by large ice sheets parts of the landscape appear to be relict with nonglacial morphology this has important implications both for glaciological conditions under ice sheets and for sediment source areas and erosion rates conventional dating and analysis have provided an excellent way to begin unravelling the timing and pattern of erosion landform development and possible landform preservation under ice however testing hypotheses developed from current models and addressing critical unresolved questions requires additional approaches the use of in situ cosmogenic nuclide production in bedrock is a new approach for investigating landscape evolution in mountainous areas with careful interpretation of geomorphological settings cosmogenic nuclides can be used to determine apparent surface exposure age and landscape preservation and constrain erosion depths and duration of burial by ice here we provide a framework for the interpretation of cosmogenic nuclide concentrations in bedrock surfaces of landscapes affected by glacial fluvial and periglacial processes illustrated with examples from the northern swedish mountains this demonstrates potential uses of cosmogenic nuclide techniques and provides a foundation for attempts to improve geomorphologically based reconstructions of relict landscapes to reconstruct and analyse the dynamics of landscape change in glacial times and to define the consequences of different process regimes in terms of erosion patterns sediment transport and the supply of sediments that are deposited offshore

new radiocarbon dates from finnish mammoths indicating large ice free areas in fennoscandia during the middle weichselian new radiocarbon dates from finnish subfossil mammoth material mammuthus sp transported by glacial ice range in age from ca 32000 to ca 22500 yr bp these results suggest that there was a larger ice free area in fennoscandia during the middle weichselian than previously assumed in addition two dates are also presented for bones found in clay with a different transport history copyright 1999 john wiley sons ltd

glacial inception and quaternary mountain glaciations in fennoscandia abstract the aim of this review is twofold first the role of the fennoscandian mountains as a nucleation center for glacial inception is investigated the second objective is to review glacial erosion by mountain glaciers throughout the quaternary all available field data and several modelling experiments show that the fennoscandian mountains seeded the last glaciation since the mountains seem to be of key importance for ice sheet inception it is reasonable to assume a similar inception pattern throughout most of the major pleistocene glaciations this pattern also implies prolonged periods of glacial ice in the fennoscandian mountains during late cenozoic time mountain based glaciers thus have had a long time to act on their substratum resulting in a profound geomorphological impact

on the structure and origin of major glaciation cycles 2 the 100 000 year cycle climate over the past million years has been dominated by glaciation cycles with periods near 23 000 41 000 and 100 000 years in a linear version of the milankovitch theory the two shorter cycles can be explained as responses to insolation cycles driven by precession and obliquity but the 100 000 year radiation cycle arising from eccentricity variation is much too small in amplitude and too late in phase to produce the corresponding climate cycle by direct forcing we present phase observations showing that the geographic progression of local responses over the 100 000 year cycle is similar to the progression in the other two cycles implying that a similar set of internal climatic mechanisms operates in all three but the phase sequence in the 100 000 year cycle requires a source of climatic inertia having a time constant similar to 15 000 years much larger than the other cycles similar to 5 000 years our conceptual model identifies massive northern hemisphere ice sheets as this larger inertial source when these ice sheets forced by precession and obliquity exceed a critical size they cease responding as linear milankovitch slaves and drive atmospheric and oceanic responses that mimic the externally forced responses in our model the coupled system acts as a nonlinear amplifier that is particularly sensitive to eccentricity driven modulations in the 23 000 year sea level cycle during an interval when sea level is forced upward from a major low stand by a milankovitch response acting either alone or in combination with an internally driven higher frequency process ice sheets grounded on continental shelves become unstable mass wasting accelerates and the resulting deglaciation sets the phase of one wave in the train of 100 000 year oscillations whether a glacier or ice sheet influences the climate depends very much on the scale the interesting aspect is that an effect on the local climate can still make an ice mass grow larger and larger thereby gradually increasing its radius of influence

directional topographic site response at tarzana observed in aftershocks of the 1994 northridge california earthquake implications for mainshock motions abstract the northridge earthquake caused 1 78 g acceleration in the east west direction at a site in tarzana california located about 6 km south of the mainshock epicenter the accelerograph was located atop a hill about 15 m high 500 m long and 130 m wide striking about n78 e during the aftershock sequence a temporary array of 21 three component geophones was deployed in six radial lines centered on the accelerograph with an average sensor spacing of 35 m station c00 was located about 2 m from the accelerograph we inverted aftershock spectra to obtain average relative site response at each station as a function of direction of ground motion we identified a 3 2 hz resonance that is a transverse oscillation of the hill a directional topographic effect the top base amplification ratio at 3 2 hz is about 4 5 for horizontal ground motions oriented approximately perpendicular to the long axis of the hill and about 2 for motions parallel to the hill this resonance is seen most strongly within 50 m of c00 other resonant frequencies were also observed a strong lateral variation in attenuation probably associated with a fault caused substantially lower motion at frequencies above 6 hz at the east end of the hill there may be some additional scattered waves associated with the fault zone and seen at both the base and top of the hill causing particle motions not spectral ratios at the top of the hill to be rotated about 20 away from the direction transverse to the hill the resonant frequency but not the amplitude of our observed topographic resonance agrees well with theory even for such a low hill comparisons of our observations with theoretical results indicate that the 3d shape of the hill and its internal structure are important factors affecting its response the strong transverse resonance of the hill does not account for the large east west mainshock motions assuming linear soil response mainshock east west motions at the tarzana accelerograph were amplified by a factor of about 2 or less compared with sites at the base of the hill probable variations in surficial shear wave velocity do not account for the observed differences among mainshock acceleration observed at tarzana and at two different sites within 2 km of tarzana

climate and the initiation of maritime ice sheets the lower latitude extension of polar water during quaternary glaciations has two opposing effects on the mass balances of adjacent man time ice sheets cooler air temperatures reduce ablation and increase the fraction of precipitation falling as snow but also lead to reduced atmospheric moisture content and reduced precipitation the effects of these contrasting processes on ice sheet initiation are investigated using a coupled ice sheet atmospheric moisture model of the loch lomond ice sheet in scotland 1 0000 a a p there is a delicate balance between the degree of cooling required to initiate ice accumulation and that which restricts the flow of moisture over the area at the regional scale of scotland the combined effects of orographic enhancement and the inherent feedback between the rising ice surface elevation and increasing mass balance are dominant and ice sheet growth accelerates comparison of the results from models using different wind directions suggests that south westerly winds were prevalent during the loch lomond glaciation in contrast to the dominant westerly winds of the present day the modelling experi ments demonstrate the sensitivity and complexity of the links between ocean surface cooling and ice sheet growth particularly at the early stages of a glacial cycle

trough mouth fans palaeoclimate and ice sheet monitors abstract trough mouth fans are fans at the mouth of transverse troughs channels on glaciated continental shelves on the northwest european glaciated continental margin eight trough mouth fans varying in size between 2700 and 215 000 km 2 have been identified the trough mouth fans are depocentres dominated by debris flows accumulated in front of ice streams draining the former large northwest european ice sheets the debris flow units are separated by hemmipelagic interglacial interstadial sediments it is inferred that the number of debris flow units record the number of shelfbreak positions of the ice sheet margins the number found varies between three in the south and eight in the north typical trough mouth fans and related debris flow units seem to have been formed later than the early mid pleistocene thus the north european ice sheets did not form ice streams extending to the shelfbreak in any appreciable length before the mid pleistocene besides being loci for sediment deposition the trough mouth fans were also the main sites of fresh water supply to the ocean in the form of icebergs during the mid late pleistocene ice ages

orbital forcing of the inception of the laurentide ice sheet according to the astronomical theory of palaeoclimates the glacial interglacial oscillations of the pleistocene have their origin in the insolation forcing caused by the orbital variations of the earth1 the potential response of the climatic system to the resulting insolation perturbations has been investigated mainly using zonally averaged energy balance models ebm 2 6 atmospheric general circulation models gcm include many essential processes such as the hydrological cycle neglected in ebms but because of their high computational cost can be used only for snapshot reconstructions of climate at certain times in the past we show here that the climatic transition at the end of the last interglacial is a particularly relevant period for gcm studies of the sensitivity of climate to the astronomical variations of insolation we have used a low resolution spectral gcm to simulate two annual cycles with the insolation conditions of 125 000 yr bp and 115 000 yr bp for the 115 kyr simulation we find an annual mean cooling over canada of more than 2 k and increased precipitation we suggest that such a pattern of response to the insolation changes would favour the extension of permanent snow cover over the labrador area and could have triggered formation of the laurentide ice sheet

age dating and the orbital theory of the ice ages development of a high resolution 0 to 300 000 year chronostratigraphy using the concept of orbital tuning a continuous high resolution deep sea chronostratigraphy has been developed spanning the last 300 000 yr the chronology is developed using a stacked oxygen isotope stratigraphy and four different orbital tuning approaches each of which is based upon a different assumption concerning the response of the orbital signal recorded in the data each approach yields a separate chronology the error measured by the standard deviation about the average of these four results which represents the best chronology has an average magnitude of only 2500 yr this small value indicates that the chronology produced is insensitive to the specific orbital tuning technique used excellent convergence between chronologies developed using each of five different paleoclimatological indicators from a single core is also obtained the resultant chronology is also insensitive to the specific indicator used the error associated with each tuning approach is estimated independently and propagated through to the average result the resulting error estimate is independent of that associated with the degree of convergence and has an average magnitude of 3500 yr in excellent agreement with the 2500 yr estimate transfer of the final chronology to the stacked record leads to an estimated error of 1500 yr thus the final chronology has an average error of 5000 yr