an inverse modeling study in fram strait part i dynamics and circulation abstract in order to reconstruct the circulation in the northern greenland sea between 77 n and 81 n and the exchanges with the arctic ocean through fram strait a variational inverse model is applied to the density field observed in summer 1984 during the mizex 84 experiment an estimate of the three dimensional large scale pressure field is obtained in which the solution is decomposed into a limited number of vertical modes and the mode amplitudes are described by piece wise polynomials on a finite element grid the solution should be consistent with a frictional depth integrated vorticity balance and with the density data the global model parameters are tuned to ensure agreement between the retrieved geostrophic velocity and independent currentmeter data in a companion paper schlichtholz and houssais 1999b the same method but without dynamical constraint is applied to the same hydrographic dataset to perform a detailed water mass analysis and to estimate individual water mass transports a comprehensive picture of the summer geostrophic circulation in fram strait is obtained in which northward recirculations in the east greenland current egc and various recirculations from the west spitsbergen current wsc to the egc are identified it is suggested that the branch of the wsc following the upper western slope of the yermak plateau turns westward beyond 81 n and recirculates southward along the lower slope then merging with a westward recirculating branch south of 79 n at 79 n a southward net transport of 6 5 sv is found in the egc which combined with a northward net transport of only 1 5 sv in the wsc results in a fairly large outflow of 5 sv from the arctic ocean to the greenland sea the inverse solutions show that in summer the local induction of vorticity by the wind stress curl or by meridional advection of planetary vorticity should be small so that in the egc and in the wsc the vorticity balance is mainly achieved between the bottom pressure torque and dissipation of vorticity through bottom friction a substantial barotropic flow associated with along slope potential energy gradients is indeed identified on both sides of the strait

mudwave activity and current controlled sedimentation in powell basin northern weddell sea antarctica an area of mudwave development has been identified from the northwestern powell basin northern weddell sea antarctica the wave field is located near the base of the continental slope in water depths of 2800 3100 m along the pathway of antarctic bottom water aabw flow from the weddell sea the waves are presently active migrating up a low angle 0 6 slope to the northwest they are typically 20 50 m in amplitude and 2 4 km in wavelength and cover an area greater than 1000 km2 swath mapping and high resolution multi channel seismic reflection profiling across the area show that the waves developed in association with basin floor channels and strongly current influenced sediments acoustically the sediments can be divided into three units the uppermost unit c is 0 5 s twt two way time thick with highly continuous parallel reflectors displaying well developed waveforms mimicking those at the seabed and towards the southeast a basin floor channel unit b is up to 0 8 s twt thick with strong and continuous subhorizontal reflectors in the mid upper section of b are developed buried sinusoidal wave forms and a base of slope depression or moat and a sediment drift which is infilled and buried the reflector pattern suggesting sediment supply under current control alongslope interpreted as contouritic sediments a lower transparent unit a is 1 1 5 s twt thick with occasional reflectors the original construction of the waves may have been by turbidity currents predominantly supplied from the basin floor channels present day deposition across the waves is maintained by fine grained sediment as a result of the lateral transfer of distal turbidites from the basin floor channels by bottom currents it is suggested that the initiation of current influenced sedimentation was closely linked to the onset of aabw flow during the early miocene following the separation of the south orkney microcontinent and the opening of the powell basin during the late oligocene

intensification of northern hemisphere glaciations in the circum atlantic region 3 5 2 4 ma ice rafted detritus evidence abstract records of ice rafted detritus ird from ocean drilling program sites 644 and 907 in the nordic seas and deep sea drilling program sites 610 and 607 in the north atlantic ocean are used to determine the growth and history of northern hemisphere glaciations spanning the interval from 3 6 to 2 4 ma the records document that the initiation of large scale glaciation in the circum atlantic region occurred stepwise between 3 5 and 2 4 ma with greenland leading the other areas the first major pulse of ird occurs at 3 3 ma suggesting a distinct expansion of the greenland ice sheet progressive increases in ird occur from 3 0 ma with a synchronous ice sheet development between the greenland scandinavian and north american regions starting around 2 72 2 75 ma the regional records of ird correspond to major trends in the oxygen isotope record suggesting a tight connection between the record of global ice volume and evidence for expanding glaciers in the northern hemisphere it appears as if widespread ird distribution occurs when the global ice volume surpasses certain thresholds similar to the late quaternary situation

the skaergaard layered series part vi excluded trace elements in contrast to the smooth trends of major elements and mineral principal minerals of the skaergaard layered series my purpose here is to examine the corresponding excluded compositions the excluded trace elements in the skaergaard layered series have an irregular distribution that does not conform to the elements and their relations to progressive differentiation of the skaergaard magma the principal conclusion that normal trends of rayleigh type fractionation their concentrations are about constant or even decline through the lower and middle will emerge is that the trends of enrichment of these elements differ markedly from those of the major elezones before increasing sharply to reach maximum concentrations 100 200 m above the sandwich horizon as in the case of ments possibly because their distribution was affected by a build up and eventual exsolution of volatile comincluded elements the relative concentrations of excluded elements in coexisting phases deviate widely from those predicted by exponents in the magma it will also be shown that the observed partition coefficients differ widely from those perimentally determined partition coefficients under presumed magmatic conditions this is seen most clearly in the immiscible predicted from experimental studies several earlier studies addressed various aspects of the melanogranophyres and conjugate ferrogabbros although the major elements conform to the experimentally determined relations for skaergaard trace elements haskin and his co workers haskin haskin 1968 paster et al 1974 were the first immiscible liquids the trace elements do not they follow a totally independent trend the abrupt increase in the concentrations of to attempt to reconcile the distribution of excluded trace elements with earlier interpretations of trace element excluded elements in the upper part of the intrusion could plausibly be attributed to an addition of new magma or to a density inversion behavior during crystallization and differentiation of the layered series other previous work has been sumthat resulted in upward migration of a late liquid or fluid but these possibilities are inconsistent with the compositional and spatial marized by wager brown 1968 and more recently by mcbirney 1996 1998a relations of the upper parts of the intrusion although a late residual liquid certainly migrated upward the most likely explanation for the structural form of the intrusion and its principal units illustrated in simplified form in fig 1 have already the observed distribution of excluded elements is that the partition coefficients were altered by volatile components which gradually been described at length mcbirney 1989 1995 1996 boudreau mcbirney 1997 mcbirney nicolas 1997 increased during the early stages of crystallization then began to exsolve near the top of the middle zone sonnenthal mcbirney 1998 and need not be repeated apart from minor differences noted in the appendix the distribution and selection of samples as well as the methods by which they have been analyzed are essentially the same as those described in part v of this

seismicity and active tectonic processes in the ultra slow spreading lena trough arctic ocean summary with its remote location in the ice covered fram strait lena trough is a poorly known segment of the global mid ocean ridge system it is a prominent member of the ultra slow spreading mid ocean ridges but its spreading mechanisms are not well understood we relocalized teleseismically recorded earthquakes from the past five decades to identify tectonic processes in lena trough and the adjacent spitsbergen fracture zone fz during two cruises with rv polarstern in 2008 and 2009 we deployed seismic arrays on ice floes to record the local seismicity of lena trough we could identify and localize microseismic events which we assume to be present in the entire rift valley in contrast our relocalization of teleseismically recorded earthquakes shows an asymmetric epicentre distribution along lena trough with earthquakes occurring predominately along the western valley flanks of lena trough in 2009 february march several high magnitude earthquakes peaking in an mb 6 6 event occurred in an outside corner setting of the spitsbergen fz this is the strongest earthquake which has ever been recorded in fram strait and its location at the outside corner high of the ultra slow spreading ridge is exceptional comparing the seismicity with the magnetic anomalies and high resolution multibeam bathymetry we divide lena trough in a symmetrically spreading northern part and an asymmetrically spreading southern part south of the south lena fz we propose that a complex interaction between the former de geer megashear zone which separated greenland from svalbard starting at late mesozoic early cenozoic times and the developing rift in the southern lena trough resulted an increasing eastward dislocation towards the spitsbergen fz between older spreading axes and the recent active spreading axis which we believe to be located west of the bathymetric rift valley flanks in a wide extensional plain

the yermak plateau in the arctic ocean in the light of reflection seismic data implication for its tectonic and sedimentary evolution summary the separation of northeast greenland and svalbard is the result of large scale strike slip movements during cenozoic times geological evidence for these movements can be found onshore both on north greenland and svalbard however the role of the submarine yermak plateau yp in this process is unclear the compilation of available multichannel reflection and wide angle seismic data give new insights into the sedimentary and crustal structure and evolution of the plateau the flat surface of the present day plateau is a quite young feature up to 2 km of cenozoic sediments cover a rough basement which show similarities to the rough topography and strike of geological structures of spitsbergen island in some basins more than 4 km of sedimentary rocks could be mapped the most pronounced structure is the sverdrup bank which appears to be part of a larger crustal block p wave velocities of about 4 5 km s 1 derived from sonobuoy data indicate that its uppermost part is most probably composed of sedimentary or volcanic rocks we have made a correlation of previously defined seismic units across the yp to outline the history of sediment deposition in the area the existing graben structures on the plateau might have provided early shallow pathways for water exchange between the arctic and the atlantic oceans a chaotic sedimentary apron east of the sverdrup bank and bright reflections near the mosby seamount interpreted as magmatic sills suggest tectonic and magmatic events during the miocene

abiotic methane from ultraslow spreading ridges can charge arctic gas hydrates biotic gas generation from the degradation of organic carbon in marine sediments supplies and maintains gas hydrates throughout the world s oceans in nascent ultraslow spreading ocean basins methane generation can also be abiotic occurring during the high temperature 200 c serpentinization of ultramafic rocks here we report on the evolution of a growing arctic gas and gas hydrate charged sediment drift on oceanic crust in eastern fram strait a tectonically controlled deep water gateway between the subpolar north atlantic and arctic oceans ultraslow spreading ridges between northwest svalbard and northeast greenland permit the sustained interaction of a mid ocean ridge transform fault and developing sediment drift on both young 10 ma oceanic crust since the late miocene geophysical data image the gas charged drift and crustal structure and constrain the timing of a major 30 km lateral displacement of the drift across the molloy transform fault we describe the buildup of a 2 m y long lived gas hydrate and free gas charged drift system on young oceanic crust that may be fed and maintained by a dominantly abiotic methane source ultraslow spreading sedimented ridge flanks represent a previously unrecognized carbon reservoir for abiotic methane that could supply and maintain deep water methane hydrate systems throughout the arctic

glacial geological implications of overconsolidated sediments on the lomonosov ridge and yermak plateau with the coupled use of multibeam swath bathymetry high resolution subbottom profiling and sediment coring from icebreakers in the arctic ocean there is a growing awareness of the prevalence of quaternary ice grounding events on many of the topographic highs found in present water depths of 1000 m in some regions such as the lomonosov ridge and yermak plateau overconsolidated sediments sampled through either drilling or coring are found beneath seismically imaged unconformities of glacigenic origin however there exists no comprehensive analysis of the geotechnical properties of these sediments or how their inferred stress state may be related to different glacigenic processes or types of ice loading here we combine geophysical stratigraphic and geotechnical measurements from the lomonosov ridge and yermak plateau and discuss the glacial geological implications of overconsolidated sediments the degree of overconsolidation determined from measurements of porosity and shear strength is shown to result from consolidation and or deformation below grounded ice and with the exception of a single region on the lomonosov ridge cannot be explained by erosion of overlying sediments we demonstrate that the amount and depth of porosity loss associated with a middle quaternary w790e950 thousand years ago e ka grounding on the yermak plateau is compatible with sediment consolidation under an ice sheet or ice rise conversely geotechnical properties of sediments from beneath late quaternary ice groundings in both regions independently dated to marine isotope stage mis 6 indicate a more transient event commensurate with a passing tabular iceberg calved from an ice shelf

effects of ice coverage and ice rafted material on sedimentation in the fram strait as little is known about pelagic sedimentation processes in arctic environments1 the interpretation of biological and chemical processes as well as the reconstruction of ancient conditions including those in the glacial north atlantic is difficult here we provide sediment trap results which show that the position of the sea ice boundary significantly influences the particle flux the seasonal variability of the particle flux differed markedly in the various sediment trap sites in fram strait depending on the behaviour of the sea ice under complete ice cover sedimentation is very low whereas maximum sedimentation is found at the ice margin the highest particle flux observed showing a large lithogenic component was observed at the ice edge where the water was warmer 2 c we find that high biogenic opal fluxes are characteristic of the summer ice margin indicating that the sedimentary record of opal fluxes may allow the position of ice margins in the past to be reconstructed

basement structure of the north western yermak plateau 1 the separation of northeast greenland and svalbard was achieved by large strike slip movements in cenozoic times evidence for these movements can be found onshore both on north greenland and svalbard however the role of the yermak plateau in this process is quite speculative new multichannel seismic 10 km spacing and aeromagnetic data 7 5 km spacing across the north western part of the plateau show that the acoustic basement has a similar strike direction to that of the geological units onshore svalbard a prominent fault zone separates these most likely continental structures in the west from a more n s extended transitional crustal block in the eastern part of the plateau this part of the plateau is characterized by strong magnetic anomalies at least indicating highly intruded and stretched continental or even oceanic crust however the seismic data show that the plateau like bathymetry is quite young during most of its cenozoic history the yermak plateau had a rough topography similar to the topography onshore svalbard thus the paleo bathymetry might have played an important role for the water exchange between the arctic ocean and the north atlantic prior to the opening of the fram strait which is today the main pathway for the deep water masses

the fram slide off svalbard a submarine landslide on a low sedimentation rate glacial continental margin submarine slope failures are a widespread hazardous phenomenon on continental margins the prevailing opinion links large submarine landslides along the glaciated nw european continental margins to overpressure generated by the alternation of rapidly deposited glacigenic and hemipelagic material here we report a newly discovered large landslide complex off nw svalbard it differs from all known large slides off nw europe as the available data rule out that this slope failure resulted from rapid glacigenic deposition this suggests that processes such as contour currents tectonic faulting and overpressure build up related to the gas hydrate system must be considered for hazard assessment supplementary material supplementary data are available at http www geolsoc org uk sup18803

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

moisture supply for northern ice sheet growth during the last glacial maximum during the last ice age the barents sea ice sheet began to grow 22 kyr ago1 only 8 kyr before it began to disintegrate2 this implies that the ice must have grown very rapidly from the coast to the edge of the continental shelf such rapid growth of a large ice sheet requires significant amounts of moisture3 but the origin of this moisture has been unclear particularly as the climap climate reconstruction suggests4 5 that the greenland iceland norwegian gin seas were perennially ice covered during this period here we present data from deep sea sediment cores from the fram strait which suggest that relatively warm water from the north atlantic ocean was advected into the gin seas in two short term events 27 22 5 and 19 5 14 5 kyr ago we suggest that the resulting seasonally ice free waters were an important regional moisture source for the barents sea ice sheet and that the gin seas played a much more active role in climate during the last glaciation than has previously been supposed

the plio pleistocene glaciation of the barents sea svalbard region a new model based on revised chronostratigraphy based on a revised chronostratigraphy and compilation of borehole data from the barents sea continental margin a coherent glaciation model is proposed for the barents sea ice sheet over the past 3 5 million years ma three phases of ice growth are suggested 1 the initial build up phase covering mountainous regions and reaching the coastline shelf edge in the northern barents sea during short term glacial intensification is concomitant with the onset of the northern hemisphere glaciation 3 6 2 4 ma 2 a transitional growth phase 2 4 1 0 ma during which the ice sheet expanded towards the southern barents sea and reached the northwestern kara sea this is inferred from step wise decrease of siberian river supplied smectite rich sediments likely caused by ice sheet blockade and possibly reduced sea ice formation in the kara sea as well as glacigenic wedge growth along the northwestern barents sea margin hampering entrainment and transport of sea ice sediments to the arctic atlantic gateway 3 finally large scale glaciation in the barents sea occurred after 1 ma with repeated advances to the shelf edge the timing is inferred from ice grounding on the yermak plateau at about 0 95 ma and higher frequencies of gravity driven mass movements along the western barents sea margin associated with expansive glacial growth

modelling late cenozoic isostatic elevation changes in the barents sea and their implications for oceanic and climatic regimes preliminary results late cenozoic isostatic changes in the elevation of the barents sea are simulated using a numerical model isopach maps of the deposits off present day bear island and storfjorden troughs made earlier are used to calculate the thickness of sediment cover removed from the respective drainages basins at various time intervals during the last 2 3 ma results indicate that barents sea was subaerially exposed at 2 3 ma and major parts of it became submarine after 1 ma barents sea today receives around 40 of the warm and saline north atlantic waters flowing into the scotland greenland ridge and about half of the atlantic water entering the arctic ocean it thus has an important role to play in the present day ocean circulation pattern in the polar north atlantic region and water mass transformations that take place in the greenland iceland norwegian sea and the arctic ocean the effects of an uplifted barents sea on the oceanic regime and the arctic sea ice cover under the present day forcings fields are studied using the miami isopycnic coordinate ocean model preliminary results indicate that a subaerial barents sea causes an increased input of warm atlantic waters into the arctic ocean through the fram strait which results in warming of the atlantic water masses in the arctic ocean followed by a reduction in the sea ice cover the obtained findings can be used to explain the apparent discrepancy in the late cenozoic record of the sub arctic and arctic regions whereby fennoscandia iceland and greenland are envisaged to have been covered by major ice sheets during late pliocene whereas high arctic areas such as svalbard and ne greenland were apparently free of any major ice r 2002 elsevier science ltd all rights reserved

an arctic ocean ice shelf during mis 6 constrained by new geophysical and geological data the hypothesis of floating ice shelves covering the arctic ocean during glacial periods was developed in the 1970s in its most extreme form this theory involved a 1000 m thick continuous ice shelf covering the arctic ocean during quaternary glacial maxima including the last glacial maximum lgm while recent observations clearly demonstrate deep ice grounding events in the central arctic ocean the ice shelf hypothesis has been difficult to evaluate due to a lack of information from key areas with severe sea ice conditions here we present new data from previously inaccessible unmapped areas that constrain the spatial extent and timing of marine ice sheets during past glacials these data include multibeam swath bathymetry and subbottom profiles portraying glaciogenic features on the chukchi borderland southern lomonosov ridge north of greenland morris jesup rise and yermak plateau sediment cores from the mapped areas provide age constraints on the glaciogenic features combining these new geophysical and geological data with earlier results suggests that an especially extensive marine ice sheet complex including an ice shelf existed in the amerasian arctic ocean during marine isotope stage mis 6 from a conceptual oceanographic model we speculate that the cold halocline of the polar surface water may have extended to deeper water depths during mis 6 inhibiting the warm atlantic water from reaching the amerasian arctic ocean and thus creating favorable conditions for ice shelf development the hypothesis of a continuous 1000 m thick ice shelf is rejected because our mapping results show that several areas in the central arctic ocean substantially shallower than 1000 m water depth are free from glacial influence on the seafloor

the impact of refraction correction on leveling interpretations in southern california recent investigations show that the refraction corrections in leveling are large but can be adequately modeled the corrections are proportional to the temperature differences and the square of the sight lengths changes in sight length with time have resulted in apparent crustal movements when non refraction corrected data were used applying refraction corrections to leveling data in southern california significantly alters the size and configuration of the southern california uplift uplift at palmdale and gorman lebec relative to los angeles bm v32 during the 1953 1978 time period is no greater than 10 cm the bulk of the motion about 6 cm occurs as localized motion along the san gabriel fault during the 1953 1964 time period this motion along the san gabriel fault and uplift of 8 cm directly north of the san fernando fault between 1955 and 1968 may represent precursors to the 1971 san fernando earthquake

differentiation and compaction in the skaergaard intrusion igneous differentiation processes are constrained from bulk compositions densities and mineral modes of 116 cumulate gabbro samples in a new reference profile through the layered series of the skaergaard intrusion east greenland the stratigraphic distribution of p u and rb in cumulates and residual magma modeled by rayleigh fractionation constrains the final porosities or trapped liquid contents to 30 52 in lza troctolites decreasing to 4 12 at the top of lzb olivine gabbros and remaining low 1 13 4 middle dot 6 on average in the oxide gabbros of lzc mz and uz local variations in trapped liquid content are associated with modal layering leucocratic low density rocks have higher proportions of trapped melt than adjacent melanocratic high density rocks these observations are explained by varying degrees of compaction compaction was most important after the onset of fe ti oxide crystallization because of the high densities of the crystal matrix here computed rates of compaction exceed the rate of crystal accumulation in layers that are metres to a few tens of metres thick in the basal section lza the crystal pile was too thin and the density of the crystal matrix too low to drive the rate of compaction above the high rate of crystal accumulation promoted by initial cooling through the intrusion floor in the overlying section lzb the efficiency of compaction gradually increased as a result of thickening of the crystal pile and lowering of the rate of crystal accumulation the modeling constrains the p2o5 content of the residual magma to 1 middle dot 7 wt at the level of apatite in suggesting that the magma contained 49 wt sio2 and followed a trend of iron enrichment compaction of the uppermost metres to tens of metres of crystal mush at the top of the cumulate pile was an efficient means of differentiation and resulted in layers with variable final porosities and trace element contents depending on the mineralogy and density of the crystal matrix

a geophysical study of the northern svalbard continental margin summary in the summer of 1999 the first systematic seismic profiles were acquired across the northern svalbard continental margin east of 15 e approximately 1470 km of multi channel seismic reflection data as well as sonobuoy wide angle data were collected up to 82 n with few exceptions the signals imaged the whole sedimentary cover down to the acoustic basement the uppermost sedimentary deposits of the inner shelf yield p wave velocities of 2 km s 1 and higher indicating erosion and compaction due to a former ice load the inner shelf east of hinlopen strait has only a thin veneer of over consolidated sediments above the acoustic basement beneath the outer shelf up to 3 5 km of sedimentary deposits cover the down faulted acoustic basement the continental slope is heavily eroded due to bottom current activity and slumping at about 30 e the morphology of the continental slope has a smooth appearance shelf progradation only in the vicinity of glacial troughs crossing the shelf associated with submarine fans indicates main sediment transport by ice streams during former glacial periods the maximum sedimentary thickness in the sophia basin is more than 9 km and the nansen basin has a sediment thickness of 4 5 km close to the margin gravity modelling along the seismic profiles was performed to constrain the position of the continent ocean transition existing sedimentary thickness and structural maps were extended over the area investigated the new data provide no evidence for the presence of former extensive subaerial volcanic sequences seaward dipping reflectors which would have been emplaced during the break up along the margin thus we consider this part of the margin as non volcanic

trace element constraints on the differentiation and crystal mush solidification in the skaergaard intrusion greenland new major and trace element analyses of plagioclase and clinopyroxene in gabbros from the skaergaard intrusion are used to understand the mechanisms of crystallization in the main magma body and in the crystal mush crystal cores show a continuous chemostratigraphic evolution in the layered series with compatible elements e g cr and ni in clinopyroxene being progressively depleted from the bottom up whereas incompatible elements e g sr ba ree in plagioclase and clinopyroxene become progressively enriched we performed numerical models and showed that these trends can be explained by a simple process of fractional crystallization except for the upper 20 of the intrusion where plagioclase and clinopyroxene trace element compositions depart from fractional crystallization trends at this stage of magma chamber solidification fractional crystallization becomes less efficient and is replaced by a major proportion of in situ equilibrium crystallization trace elements also show significant variations in interstitial overgrowths on plagioclase and clinopyroxene cumulus crystals they result from crystallization of the interstitial liquid in the liquid crystal mush however incompatible elements and especially ree show a degree of enrichment in plagioclase rims 20 ppm ce that strongly exceeds the highest concentrations observed in plagioclase cores up to 4 ppm ce at the top of the layered series such a strong enrichment is difficult to reconcile with a simple process of fractional crystallization of the interstitial liquid in the crystal mush but may be related to the development of silicate liquid immiscibility in the crystal mush or to delayed nucleation of apatite after the crystallization of plagioclase and clinopyroxene overgrowths diffusive re equilibration during a period of 0 1 0 25 myr significantly changed the original zoning profiles ce flux from clinopyroxene into the plagioclase lattice of adjacent crystals could also have contributed to the strong ce enrichment observed in plagioclase rims

a dinoflagellate cyst record for the last 0 7 ma from the rockall plateau northeast atlantic ocean organic walled dinoflagellate cyst analysis of samples previously used to establish a high resolution oxygen and carbon isotope stratigraphy for the last 0 7 ma in the northeastern atlantic is detailed data from hole 552a leg 81 of the deep sea drilling project drilled on the western flank of the rockall plateau reveal peaks of dinoflagellate cyst recovery relatable with isotope stages 1 11 13 and 17 cysts most prersistently present include the oceanic impagidinium species nematosphaeropsb labyrinthea ostenfeld reid and the ubiquitous operculodinium centrocarpum deflandre cookson wall the results offer a correlation between the oceanic oxygen isotope stratigraphy and the dinoflagellate cyst record the potential for further correlation with the extensive quaternary marine sequences of the continental shelf around the british isles is explored the dinoflagellate cyst signal is an important factor in understanding oceanographic changes both at the surface and at depth

holocene paleoceanography and glacial history of the west spitsbergen area euro arctic margin two sediment cores from the west spitsbergen area euro arctic margin md99 2304 and md99 2305 have been investigated for paleoceanographic proxies including benthic and planktonic foraminifera benthic foraminiferal stable isotopes and ice rafted debris core md99 2304 is located on the upper continental margin reflecting variations in the influx of atlantic water in the west spitsbergen current core md99 2305 is located in van mijenfjord picturing variations in tidewater glacier activity as well as fjordocean circulation changes surface water warmer than today was present on the margin as soon as the van mijenfjord was deglaciated by 11 200 cal years bp relatively warm water invaded the fjord bottom almost immediately after the deglaciation a relatively warm early holocene was followedby an abrupt cooling at 8800 cal years bp on the continental margin another cooling in the fjord record 8000 4000 cal years bp is documented by an increase in ice rafted debris and an increase in benthic foraminiferal d 18 o the ird record indicates that central spitsbergen never was completely deglaciated during the holocene relatively cool andstable cond itions similar to the present were establishedabout 4000 cal years bp

trapped intercumulus liquid in the main zone of the eastern bushveld complex south africa lateral variations in the amount of trapped intercumulus liquid in the main zone of the eastern bushveld complex are constrained by new sr isotopic whole rock and mineralogical data from three profiles that are separated laterally by ca 100 km and represent thicknesses of 551 1 127 m of main zone gabbronorites below the pyroxenite marker an analysis of the an contents 100 ca ca na of plagioclase cores within the thornhill north roossenekal and stoffberg south profiles show similar systematic up section variations from an66 to an59 in contrast both the an contents of bulk plagioclase separates and the mg numbers 100 mg mg fet of orthopyroxenes show pronounced lateral variations from thornhill an67 61 mg 67 61 through roossenekal an64 58 mg 64 55 to stoffberg an59 55 mg 59 50 these mineralogical variations are interpreted to be the result of reaction between cumulus minerals and an increasing amount of trapped liquid from north to south modelling of the trapped liquid shift of orthopyroxene compositions suggests that the amount of trapped liquid in the cumulates increased from near 0 at thornhill through 10 30 at roossenekal to 30 45 at stoffberg a two to eightfold southward increase in whole rock concentrations of p ti y zr and ba is consistent with the trapped liquid model however the 14 fold increase in rb from thornhill to stoffberg is too great to be accounted for by trapped liquid alone but can be explained by local assimilation of partial melts of the country rock constant initial 87sr 86sr isotopic ratios of main zone plagioclase separates 0 7081 0 7085 in all the three profiles do not preclude assimilation of adjacent basaltic to rhyolitic country rock with initial ratios between 0 6924 0 7096 the southward increase in the amount of trapped liquid is ascribed to an increased cooling rate by enhanced heat loss and partial assimilation of country rock xenoliths in the distal cumulate sequence at stoffberg

late quaternary contourites and glaciomarine sedimentation in the fram strait two sites in the eastern fram strait the vestnesa ridge and the yermak plateau have been surveyed and sampled providing a depositional record over the last glacial interglacial cycle the fram strait is the only deep water connection from the arctic ocean to the north atlantic and contains a marine sediment record of both high latitude thermohaline flow and ice sheet interaction on the vestnesa ridge the western svalbard margin a sediment drift was identified in 1226 m of water gravity and multicores from the crest of the drift recovered turbidites and contourites 14c dating indicates an age range of 8287 to 26 900 years bp early holocene to late weichselian the yermak plateau is characterized by slope sediments in 961 m of water gravity and multicores recovered contourites and hemipelagites 14c ages were between 8615 and 46 437 years bp early holocene to mid weichselian downcore dinoflagellate cyst analyses from both sites provide a record of changing surface water conditions since the mid weichselian suggesting variable sea ice extent productivity and polynyas present even during the last glacial maximum four layers of ice rafted debris were also identified and correlated within the cores these events occurred ca at 9 24 to 25 26 to 27 and 43 ka asynchronous with heinrich layers in the wider north east atlantic and here interpreted as reflecting instability in the svalbard barents ice sheet and the northward advection of warm atlantic water during the late weichselian the activity of the ancestral west spitsbergen current is interpreted using mean sortable silt records from the cores on the vestnesa ridge drift the modern mass accumulation rate calculated using excess 210pb is 0 076 g cm 2 year 1 on the yermak plateau slope the modern mass accumulation rate is 0 053 g cm 2 year 1

vertical geodesy without slope dependent errors a proposed hydrostatic pressure level using water at 4 c abstract bilham r 1983 vertical geodesy without slope dependent errors a proposed hydrostatic pressure level using water at 4 c in p vyskocil m a wassef and r green editors recent crustal movements 1982 tectonophysics 97 337 349 conventional vertical geodesy using spirit levelling has two fundamental shortcomings it is expensive to perform and it is associated with systematic errors that are temperature and slope dependent a hydrostatic level is described that in theory can more than double the speed of 1st order levelling and provide a measurement precision of better than 1 cm per vertical kilometer of traverse in the favoured design digital quartz pressure transducers are used to measure the pressure at each end of a 100 200 m long flexible tube containing water at its maximum density the entire instrument is maintained at 4 1 c by truck mounted machinery at the leading end of the hydrostatic level the digitally recorded measurements are expected to provide a single set up resolution better than 0 1 mm over a vertical range of 4 10 m a random error of 0 3 mm km of horizontal traverse and a vertical precision of the order of one part in 105 independent of weather conditions

seabed erosion on the lomonosov ridge central arctic ocean a tale of deep draft icebergs in the eurasia basin and the influence of atlantic water inflow on iceberg motion 1 the submarine lomonosov ridge which bisects the arctic ocean is covered by 450 m of hemipelagic sediments high resolution chirp sonar and side scan data have documented erosion of a section of the ridge here we present multichannel seismic and chirp sonar data which show removal of unconsolidated sediments along the eurasia basin side of the lomonosov ridge where the central ridge is shallower than 1000 m the incomplete erosion of the hemipelagic drape and probable topographic control of ice motion suggest that the deepest draft glacier ice which reached the central arctic ocean from the eurasia basin was in the form of armadas of large icebergs embedded in sea ice rather than a single continuous floating ice shelf massive rapid discharge of glacier ice was probably required to produce the deepest draft 800 m icebergs eastward and northward drift trajectories from a likely iceberg source area on the kara barents sea margin would reflect the relative strength of atlantic inflow through the fram strait however only icebergs with drafts exceeding bathymetric thresholds on the lomonosov ridge would leave an erosive imprint on the seabed advection of atlantic water may have been relatively strong at times of discharge from the saalian ice sheet icebergs exiting the arctic ocean caused massive erosion and redeposition of the hemipelagic sediments on top of yermak plateau

paleoceanography of the last interglacial glacial cycle in the polar north atlantic abstract using to its position close to the sites of waxing and waning of the major northern hemisphere ice sheets during the late quaternary climatic cycles the polar north atlantic plays a key role in driving global change therefore analyzing the present day sedimentation processes and reconstructing the late quaternary paleoceanography in this region has been a focus of major scientific interest through the last fifteen years this paper aims to combine the vast amount of new results about modern sediment fluxes surface sediment distribution and the paleoceanographic record and to present a comprehensive overview of the paleoceanography in the polar north atlantic through the last 200 000 years which is based on the basinwide analysis of carbonate and ice rafted detritus records during the last fifteen years the climap paleoceanographic view of the glacial polar north atlantic as an almost permanently isolated sea covered by heavy sea ice throughout the year has been shifted to a much more dynamic view of the environmental conditions a meridional current system similar to the present day conditions secured the exchange of water ice and heat between the polar north atlantic and the alantic and arctic oceans although affected by variations in strength and intensity this meridional current pattern resulted in an almost permanent presence of at least some seasonally ice free areas with all the consequences for e g marine life and deep water formation in addition the development of the continental ice sheets namely the fennoscandian and the barents sea ice sheets is closely related to this dynamic circulation pattern during the so called nordway events the pronounced inflow of temperate waters from the south provided moisture for the growth of the ice sheets the most prominent of these events stage 6 stage 4 and substage 3 1 2 ended in major glaciations reflected in the terrestrial sequences and in the deep sea ird records

surface and deep ocean interactions during the cold climate event 8200 years ago evidence from a north atlantic deep sea sediment core reveals that the largest climatic perturbation in our present interglacial the 8200 year event is marked by two distinct cooling events in the subpolar north atlantic at 8490 and 8290 years ago an associated reduction in deep flow speed provides evidence of a significant change to a major downwelling limb of the atlantic meridional overturning circulation the existence of a distinct surface freshening signal during these events strongly suggests that the sequenced surface and deep ocean changes were forced by pulsed meltwater outbursts from a multistep final drainage of the proglacial lakes associated with the decaying laurentide ice sheet margin

asynchronous deposition of ice rafted layers in the nordic seas and north atlantic ocean instabilities in ice stream flow within the north american laurentide ice sheet leading to the periodic release of armadas of icebergs into the north atlantic ocean over the past 60 000 years have produced extensive layers of coarse grained iceberg rafted debris heinrich layers in north atlantic sediments1 2 correlation of these layers with iceberg discharge events from the ice sheets on greenland iceland and scandinavia suggested in previous studies for some heinrich layers and in some areas3 4 5 would imply that ice sheet instability had been synchronous across the north atlantic presumably in response to a common environmental cause here we show a lack of widespread systematic correlations both between ice rafted debris layers in different sediment cores from the nordic seas and between the nordic layers and the north atlantic heinrich layers this suggests that the full glacial nordic ice sheets did not exhibit unstable behaviour coincident with iceberg discharge from the vast hudson bay drainage basin of the laurentide ice sheet6 7 off the hudson strait significant ice sheet discharge of melt water is indicated by size sorted sandy and muddy turbidite sediments different from the poorly sorted debris flows which dominate sedimentation on the margins of the nordic seas8 9 10 together these results suggest that the dynamics of quaternary ice sheets surrounding the nordic seas were different from the outlet glacier draining the hudson bay basin and they provide evidence against a common circum north atlantic mechanism driving the discharge of icebergs

changes in current patterns in the fram strait at the pliocene pleistocene boundary seismic reflection profiles from the northwestern and central part of the fram strait show thick packages of drift type sediments mainly along the western yermak plateau flank but also in the central flat part of the fram strait north of 80 5 n a large scale field of sediment waves along the yermak plateau rise separates a western lower from an eastern upper drift body these drift bodies were deposited by bottom currents most likely the northbound yermak branch of the west spitsbergen current but we cannot rule out that the western drift body may also have been influenced by southbound bottom currents a stratigraphic boundary is clearly visible within the drift bodies and even more pronounced within the sediment waves separating a lower package of waves migrating upslope at low angle 5 from an upper package with significantly increased wave crest migration 16 5 this stratigraphic boundary could be tracked along the seismic network and corresponds to the lithostratigraphic boundary between units ia and ib at odp leg 151 site 911 that was dated to 2 7 ma the increase in wave crest migration angle indicates a shift towards higher sedimentation rates at 2 7 ma which corresponds to the intensification of the northern hemisphere glaciation with a major expansion of the greenland scandinavian northern barents sea and north american ice sheets the subaerially exposed barents shelf and the expansion of the northern barents sea ice sheet as well as svalbard are likely sources for enhanced erosion and enhanced fluvial input along the pathway of the west spitsbergen current resulting in higher sedimentation rates in the fram strait

rare earth element abundances in apatite in the bushveld complex a consequence of the trapped liquid shift effect reply the concentrations of the rare earth elements rees in apatite from a 600 m thick sequence toward the top of the bushveld complex south africa have previously been published they show two ranges of compositions in a lower section and an upper section the data were interpreted in terms of liquid immiscibility the evidence for this model is reassessed and an alternative explanation offered that involves the trapped liquid shift effect in this process the true cumulus phase compositions reequilibrated with trapped liquid as it solidified and ree abundances in apatite were variably increased the extent of the change depends on the relative proportions of cumulus apatite and trapped liquid calculations show that if there was 5 apatite and 10 trapped liquid as found in the lower section the ree abundance in the final apatite would have only increased marginally relative to the original cumulus composition however in the upper section where there was only 2 apatite and 25 trapped liquid the ree abundances were increased by a factor of 3 most of the rees are essentially incompatible in the other cumulus phases except apatite and the other phases have little influence on the final concentrations of rees in apatite however for eu which partitions into plagioclase this mineral acts as a major buffering influence and relatively less eu than the other rees entered apatite during the solidification process as a result the calculated final apatite in the upper section developed a large negative eu anomaly as observed in the actual mineral analyses the concept of large scale liquid immiscibility in the upper part of the bushveld complex based on rees in apatite is challenged and an interpretation involving the trapped liquid shift effect is considered more plausible

major and trace element composition of skaergaard plagioclase geochemical evidence for changes in magma dynamics during the final stage of crystallization of the skaergaard intrusion plagioclase separates from the layered series ls upper border series ubs and marginal border series mbs of the skaergaard intrusion were analyzed to examine major and trace element variations in general plagioclase from the ls ubs and mbs show similar trends in major elements vs crystallization sio2 na2o and k2o progressively increase and cao and mgo progressively decrease with fractionation no abrupt changes in the trends of major components of skaergaard plagioclase during the differentiation of the intrusion are observed trace elements in plagioclase reflect changes in the skaergaard magma and changes in plagioclase distribution coefficients with differentiation sr ga and probably ba are included elements in skaergaard plagioclase but were excluded from the other cumulus phases and as a result systematically increased in the magma and plagioclase during differentiation be cs hf rb ta u and zr and the transition metals co cr cu ni sc v and zn were excluded elements in skaergaard plagioclase and remained low in plagioclase during differentiation changes in the abundances of these elements in plagioclase during differentiation reflect changes in their abundance in the magma with the exception of the lower zone which is enriched in the light rare earth elements rare earth elements in ls plagioclase in general increase with differentiation of the skaergaard intrusion but decrease dramatically at the uza uzb boundary where abundant apatite first appears rare earth elements in ubs plagioclase followed a similar trend to ls plagioclase except during the initial and final stages of differentiation ubs plagioclase is much more enriched in rare earth elements during the final 20 of crystallization except for eu which is similar in plagioclase from the two series the observed trends suggest that the floor and roof sequences became isolated from each other and that the floor sequence may have been more reducing and the roof sequence more oxidizing during the final 20 of crystallization as the skaergaard magma ceased convection or convected as isolated cells during the final stages of differentiation volatile elements may have accumulated in the ubs magma resulting in an increase in o2 and a decrease in eu sm in ubs plagioclase the observed trends of rare earth elements in plagioclase from the ls and ubs fit well with theoretical calculations that assume closed system crystallization and would be difficult to reconcile with any model requiring significant discharge of magma from the chamber during the final 20 of crystallization the enrichment of light rare earth elements in plagioclase suggests that the lower part of the intrusion re equilibrated with a late light rare earth element rich fluid or melt the recharge model proposed by earlier workers to explain anomalous sr and nd isotopes appears unlikely in light of the two to fourfold enrichment of light rare earth elements in these samples

north pacific seasonality and the glaciation of north america 2 7 million years ago in the context of gradual cenozoic cooling the timing of the onset of significant northern hemisphere glaciation 2 7 million years ago is consistent with milankovitch s orbital theory which posited that ice sheets grow when polar summertime insolation and temperature are low however the role of moisture supply in the initiation of large northern hemisphere ice sheets has remained unclear the subarctic pacific ocean represents a significant source of water vapour to boreal north america but it has been largely overlooked in efforts to explain northern hemisphere glaciation here we present alkenone unsaturation ratios and diatom oxygen isotope ratios from a sediment core in the western subarctic pacific ocean indicating that 2 7 million years ago late summer sea surface temperatures in this ocean region rose in response to an increase in stratification at the same time winter sea surface temperatures cooled winter floating ice became more abundant and global climate descended into glacial conditions we suggest that the observed summer warming extended into the autumn providing water vapour to northern north america where it precipitated and accumulated as snow and thus allowed the initiation of northern hemisphere glaciation

quantitative textural analysis of packings of elongate crystals the spatial distribution of grains in a solidifying igneous rock controls the physical properties of the crystal mush and in turn is controlled by the rate of crystal growth and accumulation a predominant non spherical habit for igneous minerals brings into question the use of spherical particles in reference packings used for quantification of spatial distribution furthermore variations of crystal clustering ordering with length scale require spatial statistics which take into account the distribution of particles beyond nearest neighbours using random close packings of spherocylinders we demonstrate the importance of aspect ratio for the aggregation index usually known as r and show that packings of spherical particles have more structure than packings of rods the spatial distribution functions demonstrate that the plagioclase grains in the colonnade from the holyoke basalt are clustered on a length scale of 0 5 mm understanding the controls on grain spatial distribution in igneous rocks will depend on the application of these techniques to well understood environments

holocene periodicity in north atlantic climate and deep ocean flow south of iceland climate fluctuations during the past millennium are relatively well documented1 on a longer timescale there is growing evidence of millennial scale variability of holocene climate at periodicities of 2 500 and 950 years possibly caused by changes in solar flux 2 3 and 1 500 years maybe related to an internal oscillation of the climate system 4 5 6 but the involvement of deep water masses in these holocene climate changes has yet to be established here we use sediment grain size data from the iceland basin to reconstruct past changes in the speed of deep water flow the study site is under the influence of iceland scotland overflow water isow the flow of which is an important component of the thermohaline circulation that modulates european climate flow changes coincide with some known climate events the little ice age and the mediaeval warm period and extend over the entire holocene epoch with aquasi periodicity of 1 500 years the grain size data indicate afaster isow flow when the climate of northern europe is warmer however a second mode of operation is observed in the early holocene when warm climate intervals are associated with slower isow flow at that time the melting remnant of land based glacial age ice may have provided a sufficient source of fresh water to the ocean to reduce isow flow south of iceland

the effect of trapped liquid crystallization on cumulus mineral compositions in layered intrusions a series of calculations have been carried out to evaluate the effect on cumulus mineral compositions of solidification of trapped intercumulus liquid in orthocumulates the calculation assumes local equilibrium between phases and that the system remains chemically closed during crystallization of the trapped liquid the latter assumption is held to be valid on a scale of tens to hundreds of centimeters it is not necessary to know the composition of the trapped liquid as the calculation only requires an estimate of feo content and trapping temperature the change in composition of a mineral from that of the initially precipitated cumulus crystals to the final composition after complete solidification is termed the trapped liquid shift its magnitude depends on the modal proportions of cumulus phases and the initial porosity and is only weakly dependent on initial phase compositions trapped liquid shifts are significant when compared with mineral composition changes occurring during fractional crystallization crystallization of 30 trapped liquid gives rise to shifts of up to 10 mol percent in mg number of olivine or pyroxene the size of the shift becomes greater when the initial cumulus assemblage has a lower total feo mgo content and vice versa as a result of the relationship between trapped liquid shift and cumulus mode mineral composition variations and trends may be generated in sequences of cumulates which originally had constant compositions of cumulus minerals for example in a cyclic unit grading from a pyroxenitic base to an anorthositic top crystallization of a uniform proportion of trapped liquid will result in an apparent iron enrichment trend from bottom to top of the cycle as has been observed in the upper critical zone of the bushveld complex

dating and palaeoenvironmental reconstruction of the sediments around the miocene pliocene boundary in yermak plateau odp hole 911a using marine palynology abstract the late neogene evolution of the arctic to subarctic region is poorly understood due to few available records and poor age control at the margin of the arctic ocean yermak plateau ocean drilling program odp hole 911a is strategically located for establishing a stratigraphic framework for the arctic here we present dinoflagellate cyst and acritarch data from 24 stratigraphic levels in the lower part 474 26 505 64 metres below the seafloor of odp hole 911a the marine palynomorphs indicate a latest miocene to earliest pliocene age between 5 8 and 5 0 ma for the base of the hole based on the co occurrence of the dinoflagellate cyst barssidinium evangelineae and acritarch lavradosphaera crista our age estimate can possibly be further refined to 5 0 5 33 ma based on the presence of achomosphaera andalousiensis suttonensis which apparently has a range restricted to the pliocene an age close to the miocene pliocene boundary agrees with the planktonic foraminifer data together with recently available magnetostratigraphic data the base of the hole is likely to be placed at 5 2 ma this new chronostratigraphy is a first step towards a better understanding of the late neogene palaeoenvironment for the yermak plateau and also for the wider arctic to the subarctic region the terrestrial and fresh water palynomorphs were most likely redistributed and or displaced from the shelf towards deeper parts of the basin during contourite deposition under the influence of the west spitsbergen current the in situ marine dinoflagellate cyst assemblage contains a mixture of cool water and thermophilic taxa indicating sea ice free cool temperate and warmer than present conditions at the yermak plateau rivers were likely the source for the freshwater influence warmer than present conditions may have occurred due to northward warm water transport via the west spitsbergen current but also the absence of sea ice and consequent lower albedo could have played a role

high resolution geophysical observations of the yermak plateau and northern svalbard margin implications for ice sheet grounding and deep keeled icebergs high resolution geophysical evidence on the seafloor morphology and acoustic stratigraphy of the yermak plateau and northern svalbard margin between 79 20 and 81 30 n and 5 and 22 e is presented

the westward turning branch of the west spitsbergen current a salinity temperature survey of fram strait in september 1985 extending to north of 81 n demonstrated the western branch of the west spitsbergen current flowing northwest and then west mainly south of 81 n there also was a southwestward recirculation from the eastern branch of the west spitsbergen current baroclinic flow of warm water northward through 81 n was negligible or zero some doubt is cast on past flow diagrams based on core analysis which show the western branch of the west spitsbergen current coursing northward to 83 n one of a probable line of mesoscale eddies is described in some detail

arctic ocean deep sea record of northern eurasian ice sheet history the sediment composition of deep sea cores from the central arctic ocean the fram strait and the yermak plateau was analyzed for several parameters to reconstruct the history of marine paleoenvironment and terrestrial glaciation in the last 200 000 years layers with high amounts of coarse terrigenous ice rafted debris ird and often high contents of smectite were deposited during extensive glaciations in northern eurasia when ice sheets reached the northern continental margins of the barents and kara seas and discharged icebergs into the arctic ocean intercalated layers with relatively low ird and smectite contents but abundant planktic foraminifers in the coarse fraction were deposited during periods of atlantic water inflow to the arctic ocean and seasonally open waters leads in a sea ice cover with only few icebergs in the arctic ocean high ird contents in the sediments reflect the presence of ice sheets on the kara and barents seas shelves and the hinterland during the entire oxygen isotope stage 6 ca 190 130 ka in substage 5b ca 90 80 ka at the stage boundary 5 4 around 75 ka and in late stage 4 early stage 3 ca 65 50 ka these results are in excellent correlation with those from recent field work in northern scandinavia european russia siberia and on the shelves relatively low amounts of ird in central arctic ocean sediments from the late weichselian glacial maximum ca 24 18 ka correlate well with the recent reconstruction of a very limited eastern ice sheet extension during this time oxygen and carbon isotope records of planktic foraminifers from the analyzed sediment cores show a number of prominent excursions which can be interpreted as evidence for freshwater events in the arctic ocean the synchroneity of freshwater events and ird input suggests a common source strongest events were associated with deglaciations of the barents and kara seas after the ice sheets had blocked the outflow of large rivers for several millennia the outflow of freshwater from large ice dammed lakes occurred at ca 130 80 75 and 52 ka freshwater events in the central arctic ocean during the last deglaciation ca 18 ka were relatively small compared to the previous events this indicates that during most of the late weichselian glacial maximum a river outflow from northern siberia to the arctic ocean was possible atlantic water inflow to the arctic ocean and seasonally open waters in the ice leads occurred during the interglacials ofoxygen isotope stage 1 and substage 5e during several interstadials stage 3 substages 5a and 5c and to a lesser degree within stadials and glacials stages 2 4 and 6 with the exception ofthe interglacials these periods were times ofstrong ice growth on the continents as revealed by terrestrial data the coincidence suggests that open waters in the arctic ocean and the nordic seas were an important moisture source in addition to more southerly sources which fostered the growth of ice sheets on northern eurasia

timing and significance of glacially influenced mass wasting in the submarine channels of the greenland basin the most extensive systems of submarine channels on the continental margins of the norwegian greenland sea occur in the greenland basin 72 75jn geophysical records show that these channels are up to 100 m deep 4 km wide and extend for about 300 km from the middle continental slope off northeast greenland to the abyssal depths of the basin mass wasting deposits in the form of debris flows and turbidity currents as well as hemipelagic sediments dominate sediment cores recovered from the channels radiocarbon dates indicate that debris flow and turbidity current activity in the channels had ceased by 13 000 years bp and sedimentation rates show a corresponding order of magnitude decrease after this time masswasting in the submarine channels of the greenland basin therefore relates to full glacial and deglacial conditions this implies a more extensive late weichselian ice sheet in northeast greenland than traditionally thought the ice sheet extended onto the outer continental shelf and may have reached the shelf edge delivering debris and sediment laden meltwater onto the upper slope channel formation is most likely the product of turbidity current activity during successive glaciations of the northeast greenland continental shelf holocene sediments from the channel systems are predominantly of hemipelagic origin and formed in a low energy ice distal environment the low gradient 1j channels of the greenland basin are therefore a glacially influenced sedimentary system contrasts in slope morphology and sediment architecture between the greenland basin and trough mouth fans elsewhere in the norwegian greenland sea are not the result of variations in slope gradient rather these contrasts reflect the interplay between bedrock lithology of the continental shelf rates of sediment delivery sediment texture meltwater volume and relative size of ice sheet drainage basins

arctic warming through the fram strait oceanic heat transport from 3 years of measurements we present estimates of volume and heat transport through fram strait for the period 1997 to 2000 from data of moored instruments full depth volume transports at 78 55 n were in the order of 10 sv both northwards and southwards with an annual mean net transport between 2 and 4 sv to the south the temperature of the northward flow of atlantic water had a strong seasonality with a minimum in winter nevertheless the northward heat transport was highest in winter caused by the winter maximum of northward volume transport during the three years of observation the heat transport in the west spitsbergen current increased from 28 to 46 tw as a result of both increased speed and temperature in contrast to the west spitsbergen current the volume and heat transport of east greenland current remained fairly constant an integration over a subsection of the east greenland current showed similar values of volume transport to that obtained by measurements in the 1980s foldvik et al 1988 the southward heat transport through modified atlantic water weakened slightly between 1997 and 1999 despite increased temperatures

the early miocene onset of a ventilated circulation regime in the arctic ocean deep water formation in the northern north atlantic ocean and the arctic ocean is a key driver of the global thermohaline circulation and hence also of global climate1 deciphering the history of the circulation regime in the arctic ocean has long been prevented by the lack of data from cores of cenozoic sediments from the arctic s deep sea floor similarly the timing of the opening of a connection between the northern north atlantic and the arctic ocean permitting deep water exchange has been poorly constrained this situation changed when the first drill cores were recovered from the central arctic ocean2 here we use these cores to show that the transition from poorly oxygenated to fully oxygenated ventilated conditions in the arctic ocean occurred during the later part of early miocene times we attribute this pronounced change in ventilation regime to the opening of the fram strait a palaeo geographic and palaeo bathymetric reconstruction of the arctic ocean together with a physical oceanographic analysis of the evolving strait and sill conditions in the fram strait suggests that the arctic ocean went from an oxygen poor lake stage to a transitional estuarine sea phase with variable ventilation and finally to the fully ventilated ocean phase 17 5 myr ago the timing of this palaeo oceanographic change coincides with the onset of the middle miocene climatic optimum3 although it remains unclear if there is a causal relationship between these two events

sediment deposition in the northern basins of the north atlantic and characteristic variations in shelf sedimentation along the east greenland margin abstract new seismic data off east greenland were acquired in the summer of 2002 between 77 n and 81 n north of the greenland fracture zone the data were combined with results from the greenland basin and odp site 909 and indicate a pronounced middle miocene unconformity within the deep sea basins between 72 n and 81 n seismic unit na 1 consists of sediments older than middle miocene age and unit na 2 contains sediments younger than the middle miocene classification of a thinly bedded succession in the molloy basin resulted in a subdivision into four units unit i unit ii unit iiia and unit iiib a comparison of volume estimations and sediment thickness maps between 72 n and 81 n indicates differences in sediment accumulation in the greenland boreas and molloy basins important controls on the variation of accumulation included different opening times of the basins as well as tectonic conditions and varying sources of sediment transport due to prominent basement structures and the varying reflection character of the sediments along the entire east greenland margin we defined an age model of shelf sediments on the basis of similar sediment deposit geometry and known results from other regions the seismic sequences on the shelf up to an age of middle miocene are divided into three sub units along the east greenland margin middle miocene middle late miocene su 3 middle late miocene pleistocene su 2 pleistocene su 1 the differences in the geometry of the sequences show more ice stream related sedimentation between 72 n and 77 n and more ice sheet related sedimentation north of 78 n the region south of 68 n is dominated by more aggradational sedimentary strata so that a glacio fluvial drainage seems the main transport mechanism due to the greenland inland ice borderlines we assume the glaciers between the scoresby sund and 68 n did not reach the shelf break a first comparison of the sediment structure of the northeast greenland margin with the southeast greenland margin made it possible to demonstrate significant differences in sedimentation along this margin

large scale oceanography in fram strait during the 1984 marginal ice zone experiment the pattern of large scale circulation in fram strait during summer 1984 is inferred from hydrographic data and water mass analysis and direct current observations by use of surface and subsurface floats the west spitsbergen current carrying warm and salty atlantic water splits into several branches two flowing northward into the arctic ocean and two turning westward causing a recirculation across the main axis of fram strait the individual flow paths are topographically controlled suggesting a strong barotropic component of the circulation

palaeoclimate warmth and glaciation the gradual expansion of northern hemisphere glaciation about 3 6 million years ago followed a period of prolonged warmth an analysis of ancient sea surface temperatures fuels the debate surrounding the roles of atmospheric carbon dioxide and global circulation in the cooling

hydrostatic levels in precision geodesy and crustal deformation measurement previous attempts to apply the principles of hydrostatic leveling to precision geodesy have been limited by the uniformity of the fluid density attainable in field environments this is largely due to the effects of temperature variations in the fluid tube we have overcome this difficulty by using water maintained near its maximum density at 3 98 c inside a counterflow heat exchanger to limit the variations in density to less than 1 ppm we have demonstrated the feasibility of this method with a 14 m prototype level and have used a computer model of the system to demonstrate the theoretical performance of instruments up to 1 km long it appears possible for example to construct a fluid tube 200 m long 12 5 cm in diameter and weighing 3 8 kg m that would provide uniform density to 1 ppm in field environments between 40 and 50 c in our attempt to use the 14 m prototype pressure transfer level we were unable to exploit the density stability that we had achieved due to inadequacies in available pressure gauges

clay mineral sedimentation in high northern latitude deep sea basins since the middle miocene odp leg 151 naag high resolution clay mineralogical investigations and their comparison with other sedimentological data from odp sites 908 and 909 central fram strait were used to reconstruct the paleoclimate and paleoceanography in the high northern latitudes since the middle miocene ice rafting has probably occurred since 15 ma the comparison of sand sized components and clay mineral distribution demonstrate that both were not delivered by the same transport process the input of the clay fraction is related to transport through sea ice and or oceanic currents a provenance change at 11 2 ma is indicated by variations within clay mineral distribution and increased accumulation rates this is interpreted as a result of an increase in water mass exchange through the fram strait decreases of the smectite to illite and chlorite ratio at site 909 suggest a middle miocene cooling phase between 14 8 and 14 6 ma and a further cooling phase between 10 and 9 ma the intensification of glaciation in the northern hemisphere is documented by an increase of illite and chlorite from 3 4 to 3 3 ma which is synchronous to the onset of major northern hemisphere glaciation as indicated by oxygen isotope data

sea level and deep sea temperature variability over the past 5 3 million years ice volume and hence sea level and deep sea temperature are key measures of global climate change sea level has been documented using several independent methods over the past 0 5 million years myr older periods however lack such independent validation all existing records are related to deep sea oxygen isotope 18o data that are influenced by processes unrelated to sea level for deep sea temperature only one continuous high resolution mg ca based record exists with related sea level estimates spanning the past 1 5 myr here we present a novel sea level reconstruction with associated estimates of deep sea temperature which independently validates the previous 0 1 5 myr reconstruction and extends it back to 5 3 myr ago we find that deep sea temperature and sea level generally decreased through time but distinctly out of synchrony which is remarkable given the importance of ice albedo feedbacks on the radiative forcing of climate in particular we observe a large temporal offset during the onset of plio pleistocene ice ages between a marked cooling step at 2 73 myr ago and the first major glaciation at 2 15 myr ago last we tentatively infer that ice sheets may have grown largest during glacials with more modest reductions in deep sea temperature a novel approach to the estimation of sea level and deep sea temperature has been used to determine these quantities over the past 5 3 million years this approach based on oxygen isotope records from the eastern mediterranean shows that temperature and sea level histories are broadly correlated but also show intriguing temporal offsets most records of past sea level variation and associated changes in ocean temperature rely on variations in oxygen isotopes recorded in deep sea marine microorganisms these signals unfortunately are confounded by other influences making it difficult to extract a clear climate related signal eelco rohling and colleagues now present a novel approach to estimating sea level and deep sea temperature variations for the past 5 3 million years the record based on oxygen isotope variations from the eastern mediterranean avoids the issues of the deep sea approach and generally corresponds to an independent record of the past 1 6 million years as expected temperature and sea level are broadly correlated but also show intriguing temporal discontinuities as at the initiation of plio pleistocene ice ages when a major sea level drop lagged a sharp temperature decline by several hundred thousand years

modeling incompatible trace element abundances in plagioclase in the skaergaard intrusion using the trapped liquid shift effect incompatible trace element abundances in minerals and whole rocks from layered intrusions have been used to model the fractionation processes and evolving liquid compositions many such models assume that the analyzed concentration in a mineral represents that of the mineral when it first crystallized however overgrowth from residual liquid and subsequent diffusive equilibration can result in significant changes to the bulk mineral compositions the more incompatible the element the more dramatic the subsequent changes the proportion of that residual liquid relative to the cumulus minerals is the most important parameter in determining the magnitude of this effect trapped liquid shift effect calculations involving ba and la contents in plagioclase quantitatively demonstrate this effect for ba and la partition coefficients of 0 4 and 0 04 50 trapped liquid in a sample can result in two and sevenfold increases respectively in concentration between original and final bulk mineral compositions different cumulus assemblages also have a major effect on final compositions we use examples of the concentrations of ba and la in plagioclase from the skaergaard intrusion from previous publications to illustrate the importance of this effect specifically the la content of bulk plagioclase steadily decreases upward from the lower zone to upper zone c and ba in plagioclase shows no increase from the lower zone to the top of the middle zone such results are not explicable by fractionation processes but can be modeled by the trapped liquid shift effect assuming the well established evidence for upward decrease in trapped liquid proportion through these zones

contourites in the fram strait abstract multichannel seismic reflection profiles from the fram strait show many features that can be attributed to bottom current influence on the sediments by comparison with well known contourite sequences elsewhere a greater than 2 km thick contourite drift with a smooth internal reflection pattern is observed northwest of svalbard the currents here have probably been controlled by local topography we interpret sequences with abundant diffractions in the broader basins to be current related probably deposited during periods of intensified circulation the continental slopes on the svalbard side have fairly continuous sequences whereas the younger sequences on the greenland lower slope onlaps the lowermost one this can be attributed to differences in current regime sediment supply or margin subsidence the contourites of the lower slope in the northeastern part of the strait about 1000 m water depth can be explained by long term influence of the west spitsbergen current but current influenced patterns down to 3000 m water depth are not compatible with the weak deep water circulation observed at present the oldest contourites west of svalbard are estimated to be of late miocene to pliocene age between 10 and 3 ma based on correlation to the dsdp site 344 and inferred basement ages near the present plate boundary hiatuses may be present beneath the contourites the onset of currents may be related to opening of the gateway between greenland and svalbard to climatic cooling and to palaeogeographic changes outside the fram strait contourite deposition has continued through the late neogene and quaternary but vertical changes in the pattern suggest that current intensities have decreased through time

trace element partitioning between apatite and silicate melts we present new experimental apatite melt trace element partition coefficients for a large number of trace elements cs rb ba la ce pr sm gd lu y sr zr hf nb ta u pb and th the experiments were conducted at pressures of 1 0 gpa and temperatures of 1250 c the rare earth elements la ce pr sm gd and lu y and sr are compatible in apatite whereas the larger lithophile elements cs rb and ba are strongly incompatible other trace elements such as u th and pb have partition coefficients close to unity

deep pleistocene iceberg plowmarks on the yermak plateau sidescan and 3 5 khz evidence for thick calving ice fronts and a possible marine ice sheet in the arctic ocean the southern yermak plateau northwest of spitsbergen was investigated with 11 12 khz sidescan sonar and 3 5 khz profiler to test whether thick pleistocene ice streams ever calved into the arctic ocean and or whether a continuous arctic ice sheet existed in either case the ice would have grounded on the plateau crest on its way out of the arctic several distinct sets of relict iceberg plowmarks were discovered at present water depths from less than 450 m to at least 850 m perhaps recording the deepest iceberg keels known the plateau crest 510 530 m is nearly bare of plowmarks perhaps because of beveling by an ice rise formed of grounding shelf ice this implies continuous 400 600 m thick grounded ice consistent with but not proving the postulated existence of an arctic ice sheet most plowmarks are probably late pleistocene products of the calving of barents kara ice sheets into the arctic ocean plowmarks in 450 850 m deep water require unusually deep ice sheet grounding lines e g svyataya anna trough and or drat increasing capsizing events even if sea levels were 100 50 m lower than today at water depths of 800 to 1800 m the sidescan texture and topography suggest bottom currents descending the northeast slope of the plateau

a seismic study along the east greenland margin from 72 n to 77 n summary around 4370 km of new seismic reflection data collected along the east greenland margin between 71 30 n and 77 n in 2003 provide a first detailed view of the sediment distribution and tectonic features along the east greenland margin after processing and converting the data to depth we correlated odp site 913 stratigraphy into the new seismic network unit gb 2 shows the greatest glacial sediment deposits beneath the east greenland continental shelf this unit is characterized by the beginning of prograding sequences and has according to our stratigraphic correlation a middle miocene age it might have been caused by rapid changes in sea level and or glacial erosion by an early ice sheet or glaciers along the coast a basement high presumably a 360 km long basement structure at 77 n 74 54 n prevents continuous sediment transport from the shelf into the deep sea area in times before 15 myr the origin of this prominent structure remains speculative since no rock sample from this structure is available seaward dipping reflectors at the eastern flank of this structure strongly support that it is a volcanic construction and is most likely emplaced on continental or transitional crust the compilation of sediment thickness provide an insight into the regional sediment distribution in the greenland basin an average sediment thickness of 1 km is observed the north bordering boreas basin has a sediment thickness of 1 8 km close to the greenland fracture zone gfz

evidence for glaciation in the northern hemisphere back to 44 ma from ice rafted debris in the greenland sea the widely accepted age estimate for the onset of glaciation in the northern hemisphere ranges between 2 and 15 million years ago ma however recent studies indicate the date for glacial onset may be significantly older we report the presence of ice rafted debris ird in 44 to 30 ma sediments from the greenland sea evidence for glaciation in the north atlantic during the middle eocene to early oligocene detailed sedimentological evidence indicates that glaciers extended to sea level in the region allowing icebergs to be produced ird may have been sourced from tidewater glaciers small ice caps and or a continental ice sheet

ice sheet grounding and iceberg plow marks on the northern and central yermak plateau revealed by geophysical data abstract we present new evidence for a grounded ice sheet and subsequent erosion by large fields of coherent icebergs for the central and northern yermak plateau 80 6 n to 82 2 n sediment echosounder and swath bathymetry data were combined with seismic reflection profiles and reveal at least three different glacial events marked by erosional unconformities i an erosional unconformity was observed at 70 90 m below seafloor down to depths of more than 850 m present water depth extending to 82 n the erosional unconformity is overlain by an acoustically chaotic layer of 50 m thickness interpreted as a diamicton originating from a grounded ice sheet the erosional unconformity and the overlying diamicton can be correlated to the overconsolidated sediments found at odp site 910 at a sediment depth between 19 and 70 95 m the oldest sediments just above the overconsolidated sediments are of late early pleistocene age mis19 20 and provide a minimum age for the grounding event ii parallel to sub parallel mega scale lineations are observed on large parts of the plateau west and northeast of the sverdrup bank at water depths between 725 and 850 m these lineations are mainly oriented nne ssw and were quite likely formed by the keels of deep draft mega scale tabular icebergs entrapped in a coherent mass of icebergs and sea ice the lineations are of late middle pleistocene age iii smaller scale curvilinear plow marks were found in the southernmost part of our study area at water depths between 640 and 775 m these were possibly caused by single icebergs and are of late pleistocene age iceberg scours are also found on three basement heights on the yermak plateau these however cannot be assigned to specific events they might as well originate from additional glacial phases the western at 850 m water depth and eastern at 1000 1200 m water depth flanks of the yermak plateau are relatively featureless and indicate the maximum depth of a grounded ice sheet and of iceberg armadas probably entrapped in sea ice

pliocene warmth polar amplification and stepped pleistocene cooling recorded in ne arctic russia understanding the evolution of arctic polar climate from the protracted warmth of the middle pliocene into the earliest glacial cycles in the northern hemisphere has been hindered by the lack of co

earthquakes and sea level space and terrestrial metrology on a changing planet the complacency with which man inhabits continent ocean margins or plate boundaries is based on the assumption that sea level rise and periodic seismicity are acceptable risks the forecast of future sea level or future seismicity is essential for informed assessment of these risks the historical data from which forecasts may be derived are in each case of limited antiquity and uneven spatial distribution developments in space geodesy and crustal deformation metrology in the past two decades offer the means to acquire a homogeneous global data set for monitoring relative vertical and horizontal motions of the earth s surface to within several millimeters these new tools are reviewed in terms of their application to forecasting trends in sea level rise and forecasting damaging earthquakes

effects of arctic freshwater forcing on thermohaline circulation during the pleistocene in this paper we make use of calcium carbonate bearing sediment sequences in the arctic atlantic gateway to produce a standard reference climate record for the arctic ocean we present a continuous and exceptionally well dated marine sediment record documenting the strong imprint of arctic freshwater pulses on earth s climate system throughout the past 0 8 m y planktic foraminiferal oxygen and carbon isotope data reveal that freshwater plumes released from collapsing circum arctic ice sheets and ice dammed lakes were advected to the arctic ocean gateway on sub milankovitch time scales given the close correspondence of arctic freshwater pulses and strength of north atlantic deep water formation as inferred from carbon isotope data we conclude that freshening of the arctic ocean influenced thermohaline circulation more frequently than previously recognized our data show that it is critical for coupled atmosphere ice ocean general circulation models aimed at unraveling climate forcing factors during the pleistocene to consider arctic freshwater as a crucial forcing factor

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

late pliocene pleistocene development of the barents sea ice sheet the late pliocene pleistocene paleoenvironment has been reconstructed based on three dimensional seismic data from the southwestern barents sea continental margin during the late pliocene early pleistocene continental slope sediments were predominantly deposited from meltwater overflows and underflows the seismic stratigraphy of the early middle pleistocene shows both glaciomarine sediment input from channelized meltwater discharge and the first indications of large debris flow deposits on the continental slope originating from input of subglacial deformation till eroded and transported by ice streams during the middle late pleistocene large debris flows dominated the slope succession from the above results we infer the following evolution in the barents sea 1 a temperate barents sea ice sheet with channelized meltwater flow developed during the late pliocene early pleistocene 2 alternating glacial periods of ice with channelized meltwater flow and the first periods of ice including ice streams characterized the early and middle pleistocene and 3 more polar ice conditions and a barents sea ice sheet that mainly included large ice streams with little or no channelized meltwater flow occurred in the middle and late pleistocene

the earliest history of the skaergaard magma chamber a textural and geochemical study of the cambridge drill core ct and mh were supported by a royal society international joint project grant and by the carlsberg foundation ct was also supported by the danish council of independent research and the danish national research foundation the project was supported by the natural environment research council grant number ne f020325 1 on acknowledges support from magdalene college university of cambridge and the von humboldt foundation germany university of hannover

sortable silt and fine sediment size composition slicing parameters for palaeocurrent speed and palaeoceanography fine sediment size 63 ixm is best measured by a sedimentation technique which records the whole size distribution repeated size measurement with intermediate steps of removal of components by dissolution allows inference of the size distribution of the removed component as well as the residue in this way the size of the biogenic and lithogenic noncarbonate fractions can be determined observations of many size distributions suggest a minimum in grain size frequency curves at 8 to 10 ixm the dynamics of sediment erosion deposition and aggregate breakup suggest that fine sediment behavior is dominantly cohesive below 10 1xm grain size and noncohesive above that size thus silt coarser than 10 ixm displays size sorting in response to hydrodynamlc processes and its properties may be used to infer current speed silt that is f mer than 10 ixm behaves in the same way as clay 2 xm useful parameters of the distribution are the 10 63 ixm mean size and the percentage 10 63 ixm in the fine fraction we cannot use size distributions to distinguish the nature of the currents therefore to infer water mass advection speeds i e the mean kinetic energy of the flow ku regions of high eddy kinetic energy ke must be avoided at the present such abyssal regions lie under the high surface k e of major current systems gulf stream kuroshio agulhas antarctic circmpolar current and brazil falldand currents in the argenthe basin this is probably a satisfactory guide for the pleistocene with regard to the carbonate subfraction of the size spectrum size modes due to both cocco iths and foramlnlferal fragments can be recognized and analyzed with the boundary between them again at about 10 lm the flux of less than 10 lm carbonate at pelagic sites above the lysocline is another candidate for a productivity indicator

influence of brine formation on arctic ocean circulation over the past 15 million years the early oceanographic history of the arctic ocean is important in regulating and responding to climatic changes however constraints on its oceanographic history preceding the quaternary the past 1 8 myr have become available only recently because of the difficulties associated with obtaining continuous sediment records in such a hostile setting here we use the neodymium isotope compositions of two sediment cores recovered near the north pole to reconstruct over the past approx15 myr the sources contributing to arctic intermediate water a water mass found today at depths of 200 to 1 500 m we interpret high neodymium ratios for the period between 15 and 2 myr ago and for the glacial periods thereafter as indicative of weathering input from the siberian putoranan basalts into the arctic ocean arctic intermediate water was then derived from brine formation in the eurasian shelf regions with only a limited contribution of intermediate water from the north atlantic in contrast the modern circulation pattern with relatively high contributions of north atlantic intermediate water and negligible input from brine formation exhibits low neodymium isotope ratios and is typical for the interglacial periods of the past 2 myr we suggest that changes in climatic conditions and the tectonic setting were responsible for switches between these two modes

switching of a paleo ice stream in northwest svalbard ice streams are the fast flowing zones of ice sheets that can discharge a large flux of ice the glaciated western svalbard margin consists of several cross shelf troughs which are the former ice stream drainage pathways during the pliocene pleistocene glaciations from an integrated analysis of high resolution multibeam swath bathymetric data and several high resolution two dimensional reflection seismic profiles across the western and northwestern svalbard margin we infer the ice stream flow directions and the deposition centres of glacial debris that the ice streams deposited on the outer margin our results show that the northwestern margin of svalbard experienced a switching of a major ice stream based on correlation with the regional seismic stratigraphy as well as the results from odp 911 on yermak plateau and odp 986 farther south on the western margin of spitsbergen off van mijenfjord we find that first a northwestward flowing ice stream developed during initial northern hemispheric cooling starting 2 8 2 6 ma a switch in ice stream flow direction to the present day kongsfjorden cross shelf trough took place during a glaciation at 1 5 ma or probably later during an intensive major glaciation phase known as the mid pleistocene revolution starting at 1 0 ma the seismic and bathymetric data suggest that the switch was abrupt rather than gradual and we attribute it to the reaching of a tipping point when growth of the svalbard ice sheet had reached a critical thickness and the ice sheet could overcome a topographic barrier highlights reflection seismic data reveal two glacial fans at northwest svalbard margin the fans are result of ice stream activities during pliocene pleistocene glaciations based on seismic and bathymetric data we find the flow directions of the ice streams we find a switch in ice stream flow direction the switch resulted as the ice sheet became thick and overcame a topographic barrier