u th ages obtained by mass spectrometry in corals from barbados sea level during the past 130 000 years the study of the sea level record during the last glacial cycle has primarily proceeded indirectly by means of oxygen isotope measurements on foraminifera from deep sea sediments1 2 the direct approach of dating sea level indicators stagnated during the past decade mainly because the samples required to complete our knowledge of the past glaciations are below the present day sea level3 7 using the 14c ages of acropora palmata samples collected by drilling offshore the island of barbados fairbanks8 presented the first detailed chronology for the last deglaciation this radiocarbon chronology is limited to the past 30 kyr because of the short half life of 14c 5 730 yr we must therefore rely on other dating methods to obtain information for the whole last glacial cycle during the past four years 230th 234u dating of corals by thermal ionization mass spectrometry has been shown to be significantly more precise and accurate than the classical counting method9 14 we have used this technique to measure u th ages in coral samples from the barbados collection and from subaerially exposed outcrops see also ref 15 here we present results bearing on the sea level record for the past 130 000 years we conclude that the last deglaciation started 3 000 years earlier than previously thought and confirm that there were two surges in melt water8 at about 11 kyr and 14 kyr bp before present

opening of the fram strait gateway a review of plate tectonic constraints abstract we have revised the regional crustal structure oceanic age distribution and conjugate margin segmentation in and around the lena trough the oceanic part of the fram strait between the norwegian greenland sea and the eurasia basin arctic ocean the lena trough started to open after eurasia greenland relative plate motions changed from right lateral shear to oblique divergence at chron 13 times 33 3 ma earliest oligocene a new bouguer gravity map supported by existing seismic data and aeromagnetic profiles has been applied to interpret the continent ocean transition and the influence of eocene shear structures on the timing of breakup and initial seafloor spreading assuming that the onset of deep water exchange depended on the formation of a narrow oceanic corridor the gateway formed during early miocene times 20 15 ma however if the initial lena trough was blocked by terrigenous sediments or was insufficiently subsided to allow for deep water circulation the gateway probably formed with the first well developed magnetic seafloor spreading anomaly around chron 5 times 9 8 ma late miocene paleoceanographic changes at odp site 909 northern hovgard ridge are consistent with both hypotheses of gateway formation we cannot rule out that a minor gateway formed across stretched continental crust prior to the onset of seafloor spreading in the lena trough the gravity seismic and magnetic observations question the prevailing hypotheses on the yermak plateau and the morris jesup rise as eocene oceanic plateaus and the hovgard ridge as a microcontinent

deglaciation of a marine based ice sheet late weichselian palaeo ice dynamics and retreat in the southern barents sea reconstructed from onshore and offshore glacial geomorphology abstract marine and terrestrial datasets swath bathymetry 2d and 3d seismic and remote sensing data are used to map the megascale glacial geomorphology of the southern barents sea seafloor northern fennoscandia and the kola peninsula giving new insights into the dynamics and behaviour of the barents sea ice sheet during retreat the glacial geomorphology indicates clear spatial and temporal variations in ice dynamics with evidence for both active ice streaming and frozen bed conditions at the maximum and during deglaciation a five stage reconstruction of the late weichselian deglaciation is proposed at the late weichselian maximum ice covered the whole of the barents sea shelf and the configuration and dynamics of the ice sheet were controlled by the bjornoyrenna bear island trough ice stream this was fed by source areas in the central and north eastern barents sea and in the fennoscandian mainland deglaciation was initiated along the western continental margin available dating results show that this occurred after 19 cal ka bp the bjornoyrenna ice stream retreated rapidly ice margin retreat rates of 275 m a 1 but was periodically grounded as evidenced by a well developed series of grounding zone wedges this was followed by an advance of two short lived ice streams from the eastern sector of the ice sheet into the south western barents sea these ice streams advanced and retreated very rapidly with no evidence for ice margin standstills correlation to dated cores suggests an age of around 16 000 cal ka bp for this readvance the period of operation of these two ice streams was coincident with maximum ice extent along the eastern margin in russia the western barents sea ice margin then retreated onshore with deglaciation continuing at a reduced rate the initiation of deglaciation coincided with rising global eustatic sea level acceleration of calving due to grounding line floatation rapidly opened up a major calving bay in bjornoyrenna encouraging rapid loss of the marine based ice the retreat rate stabilised only once the ice sheet retreated onshore and calving ceased we note that the observed pattern of rapid retreat with high variability in ice streaming for the marine based barents sea ice sheet is similar to that of the marine based sectors of the late weichselian british laurentide and west antarctic ice sheets

rapid flooding of the sunda shelf a late glacial sea level record the increase in sea level from the last glacial maximum has been derived from a siliciclastic system on the tectonically stable sunda shelf in southeast asia the time from 21 to 14 thousand calendar years before the present has been poorly covered in other records the record generally confirms sea level reconstructions from coral reefs the rise of sea level during meltwater pulse 1a was as much as 16 meters within 300 years 14 6 to 14 3 thousand years ago

submarine landforms and the behavior of a surging ice cap since the last glacial maximum the open marine setting of eastern austfonna svalbard abstract multi beam swath bathymetry data and topographic parametric sonar topas 3 5 khz sub bottom acoustic profiles were acquired from the inlet of hartogbutka an open marine setting offshore of austfonna northeastern nordaustlandet svalbard the largest ice cap in the eurasian arctic using these marine geophysical data the seafloor morphological and sedimentary evidence of former ice extent and dynamics of the three drainage basins basins 3 4 and 5 adjacent to hartogbukta were examined submarine landforms and landform assemblages that provide evidence of past ice dynamics are described and interpreted in the context of previous studies of surge affected submarine environments elsewhere in svalbard a sequence of four distinct ice flow episodes in hartogbukta from the late weichselian deglaciation to the present is identified to explain the morphological evolution of the seafloor a full glacial ice sheet extended over hartogbukta during the late weichselian flowing ese at intervals throughout the holocene the drainage basins adjacent to hartogbukta probably underwent repeated surges most recently basins 3 and 4 probably surged as one cohesive ice mass in the late 1800s and basin 5 most likely underwent a less extensive surge in the 20th century these ice flow events are interpreted in the context of the glaciology and known glacial history of austfonna and the nw barents sea the 200 km long ice cliffs of eastern austfonna provide an unusual and important analog for open marine deposition that would have been much more prevalent under full glacial conditions where ice sheets in many parts of the arctic advanced from fjord systems onto the adjacent continental shelves

polyphase zircon in ultrahigh temperature granulites rogaland sw norway constraints for pb diffusion in zircon shrimp u pb ages have been obtained for zircon in granitic gneisses from the aureole of the rogaland anorthosite norite intrusive complex both from the ultrahigh temperature uht 900 c pigeonite in zone and from outside the hypersthene in isograd magmatic and metamorphic segments of composite zircon were characterised on the basis of electron backscattered electron and cathodoluminescence images plus trace element analysis a sample from outside the uht zone has magmatic cores with an age of 1034 7 ma 2 n 8 and 1052 5 ma 1 n 1 overgrown by m1 metamorphic rims giving ages between 1020 7 and 1007 5 ma in contrast samples from the uht zone exhibit four major age groups 1 magmatic cores yielding ages over 1500 ma 2 magmatic cores giving ages of 1034 13 ma 2 n 4 and 1056 10 ma 1 n 1 3 metamorphic overgrowths ranging in age between 1017 6 ma and 992 7 ma 1 corresponding to the regional m1 sveconorwegian granulite facies metamorphism and 4 overgrowths corresponding to m2 uht contact metamorphism giving values of 922 14 ma 2 n 6 recrystallized areas in zircon from both areas define a further age group at 974 13 ma 2 n 4 this study presents the first evidence from rogaland for new growth of zircon resulting from uht contact metamorphism more importantly it shows the survival of magmatic and regional metamorphic zircon relics in rocks that experienced a thermal overprint of c 950 c for at least 1 myr magmatic and different metamorphic zones in the same zircon are sharply bounded and preserve original crystallization age information a result inconsistent with some experimental data on pb diffusion in zircon which predict measurable pb diffusion under such conditions the implication is that resetting of zircon ages by diffusion during m2 was negligible in these dry granulite facies rocks imaging and th u y systematics indicate that the main processes affecting zircon were dissolution reprecipitation in a closed system and solid state recrystallization during and soon after m1

mobilization of radiogenic pb in zircon revealed by ion imaging implications for early earth geochronology zircon is arguably the most commonly used geochronometer but the reliability of ages obtained requires a full understanding of processes that might compromise the integrity of its u pb systematics here we present the results of a multifaceted ion microprobe study of zircon grains from the napier complex east antarctica a region affected by pervasive high temperature metamorphism at 2 5 ga and from which previous zircon geochronological interpretations have been problematic both u pb spot analysis 15 m and high spatial resolution 2 m scanning ion imaging of pb isotopes have been applied in an attempt to quantify the effects of metamorphism spot analyses spread along concordia yielding 207 pb 206 pb ages from 2 5 ga to 3 9 ga with the oldest grains reversely discordant ion images of uranogenic pb reveal a surprising micrometer scale patchy distribution that is unrelated to crystal morphology or damage the 207 pb 206 pb ratios within these subdomains correspond to apparent zircon ages as old as 4 2 ga these are interpreted as artifacts of ancient redistribution of radiogenic pb a process that can generate meaningless ages and are not relicts of ancient including hadean zircon scanning ion imaging thus facilitates identification of unsupported radiogenic pb and enables testing of the validity of old ages from zircon known to have a long and complicated history

keel depths of modern antarctic icebergs and implications for sea floor scouring in the geological record abstract icebergs affect the geological record through the scouring or ploughing action of their keels reworking sediments where they contact the sea floor satellite radar altimeter measurements of the surface elevation of the marine margins of the antarctic ice sheet are inverted to produce ice thickness values these data are assumed to represent the keel depths of newly calved icebergs there are three peaks in the frequency distribution of antarctic iceberg keel depths a about 140 200 m in thickness associated with smaller ice shelves fringing antarctica e g wilkins george vi and brunt ice shelves b about 250 300 m from large ice shelves e g ross ronne amery and c approximately 500 600 m from fast flowing ice sheet outlet glaciers e g pine island and dibble glaciers together with the filchner ice shelf controls on iceberg keel depth are ice thickness at the grounding line creep thinning and the rate of melting freezing at the floating ice shelf base with distance to the calving margin the distribution of sea floor scours produced by iceberg grounding is dependent on the changing form and flow of the glaciers and ice sheets from which icebergs are derived and on berg drift tracks and melt rates sea floor evidence for icebergs with particularly deep keels has sometimes been interpreted to indicate the presence of extensive former ice shelves however the largest modern antarctic ice shelves do not produce large numbers of icebergs thicker than about 350 m implying that sea floor scours found in deeper water are unlikely to be an indicator of extensive past ice shelf development iceberg scours found at water depths in excess of about 500 m in the geological record are instead probably indicative of either a the former presence of fast flowing ice sheet outlet glaciers b ice shelves fed from major interior basins where lateral spreading is constrained topographically or c a pulse of large icebergs released during major deglaciation or ice sheet collapse

the intercept is a poor estimate of a calibrated radiocarbon age intercept based methods of generating a point estimate of a calibrated radiocarbon date are very popular but exhibit undesirable behaviour they are highly sensitive to the mean of the radiocarbon date and to adjustments of the calibration curve other methods give more stable results the weighted average of the probability distribution function is recommended as the best central point estimate but more consideration should be given to using the full probability distribution rather than a point estimate in developing age depth models

zircon u pb strain chronometry reveals deep impact triggered flow abstract large 100 km meteorite impact cratering events play important roles in surface and biosphere evolution however their potential for widespread ductile modification of the lithosphere has been difficult to assess due partly to our inability to isotopically age correlate deep mineral fabrics with surface records we have integrated benchmark u pb zircon dating methods id tims shrimp with new microstructural techniques ebsd xrd to demonstrate that crystal plastic deformation can cause rapid out diffusion of radiogenic pb and accompanying trace element alteration in crystalline zircon we have used this phenomenon to directly date fabric in archean zircons and xenoliths of the lower crust of south africa at 2023 15 million years coeval with the 2020 3 million year old vredefort cratering event at surface with extent 20 000 km 2 our findings indicate that regional exogenic fabrics similar to high temperature tectonic fabrics exist in ancient crust moreover our results establish that crystal plastic deformation in the lithosphere can now be directly dated and linked to planetary evolution by zircon u pb strain chronometry

relict sea floor ploughmarks record deep keeled antarctic icebergs to 45 s on the argentine margin abstract this paper reports ploughmarks on the argentine continental margin up to 44 50 s the most northerly morphological evidence of giant icebergs in the southern atlantic more than 2500 ploughmarks up to 32 km long 685 m wide and more than 20 m deep have been observed at modern water depths up to 646 m taking into account these iceberg sizes and ocean current directions which appear to have remained similar from the last full glacial to the present interglacial the icebergs that produced the ploughmarks are interpreted to be calved from fast flowing ice streams draining huge basins of the west antarctic ice sheet and the western antarctic peninsula during the last glacial maximum lgm about 20 000 years ago ploughmarks at the deepest water depths correspond to icebergs more than 500 m thick and at least 2 km wide given a sea level rise of 120 m since the lgm the icebergs after having drifted northeast in the antarctic circumpolar current were transported northward by the falklands malvinas current a total distance of 2000 to 4000 km the waters north of the falkland islands in particular were probably several degrees colder than today to prevent rapid iceberg melting and deterioration

ice stream retreat dynamics inferred from an assemblage of landforms in the northern barents sea abstract palaeo records of rapid ice stream retreat are important as they can be used to identify the processes involved and inform investigations of present day ice masses here we document a previously unknown retreat stage in northernmost bjornoyrenna bear island trough in the northern barents sea representing the youngest stage in a stepwise retreat of the bjornoyrenna ice stream we present a descriptive landsystem model for retreat of a marine based ice stream which provides new insights into the glacial dynamics of the episodic retreat stages this model captures the landforms produced during a cycle of 1 fast ice stream flow 2 intense calving of large icebergs locked in a dense matrix of smaller icebergs probably from a collapsed ice shelf 3 ice stream stagnation 4 ice stream floating off forming an ice shelf as the grounding line retreats and eventually 5 ice shelf disintegration we hypothesize that the presented ice stream retreat model reflects glacial surging the formation of corrugated furrows in the study area is consistent with interpretation of similar features in pine island bay west antarctica which are interpreted to have been formed during a massive ice shelf break up and associated grounding line retreat two directions of past ice flow are indicated from streamlined glacial landforms in the study area ice flow from north north east is indicated for the last glacial maximum and an early phase of deglaciation whereas a shift to flow from north north west from an ice dome located over hinlopenstretet svalbard is indicated for the latest deglaciation phases of the bjornoyrenna ice stream

diffusion in zircon despite its low abundance in most rocks zircon is extraordinarily useful in interpreting crustal histories the importance of u th pb isotopic dating of zircon is well and long established davis et al this volume parrish et al this volume zircon also tends to incorporate trace elements useful as geochemical tracers such as the ree y and hf a number of characteristics of zircon encourage the preservation of internal isotopic and chemical variations often on extremely fine scale which provide valuable insight into thermal histories and past geochemical environments the relative insolubility of zircon in crustal melts and fluids as well as its general resistance to chemical and physical breakdown often result in the existence of several generations of geochemical information in a single zircon grain the fact that this information is so frequently retained as evidenced through backscattered electron or cathodoluminescence imaging that often reveal fine scale zoning down to the sub micron scale has long suggested that diffusion of most elements is quite sluggish in zircon in this chapter we present an overview of the findings to date from laboratory measurements of diffusion of cations and oxygen in zircon because of its importance as a geochronometer attempts have been made to measure diffusion especially of pb for over 30 years but only in the last decade or so have profiling techniques with adequate depth resolution been employed in these studies resulting in a plethora of new diffusion data these findings have important implications for isotopic dating interpretation of stable isotope ratios closure temperatures and formation and preservation of primary chemical composition and zoning in zircon efforts have been made for some time to quantify and characterize diffusion in zircon most notably of pb in deference to its significance in interpreting pb isotopic signatures and refining understanding of thermal histories as is evident from

buried iceberg ploughmarks in the early quaternary sediments of the central north sea a two million year record of glacial influence from 3d seismic data abstract buried linear to curvilinear depressions interpreted as iceberg ploughmarks were identified through most of the 2 ma long and about 700 m thick early quaternary sedimentary record in two 3d seismic cubes from the central north sea basin 56 58 n 2 3 e the mean width of 402 features measured at 6 time slices in cube c08 was between 49 and 63 m and mean length was 2 5 to 3 7 km mapping the

brine formation in relation to climate changes and ice retreat during the last 15 000 years in storfjorden svalbard 76 78 n storfjorden svalbard is an area of intense brine formation the brine is cold dense rich in oxygen and co2 and has reduced ph storfjorden is unique because it contains well preserved agglutinated foraminifera dating back to the beginning of the last deglaciation we have investigated the distribution of calcareous and agglutinated benthic foraminifera benthic oxygen and carbon isotopes calcium carbonate total organic carbon and ice rafted debris in five cores from storfjorden comprising the holocene and the deglaciation the purpose of the study is to reconstruct brine formation in the past under different climate scenarios the data indicate that in storfjorden the ratio of agglutinated to calcareous benthic foraminifera can be taken as a measure of the strength of brine formation the foraminiferal data which are supported by stable isotopes degree of fragmentation and geochemical parameters signify that brine formation intensified during cold periods and weakened during warm periods during the deglaciation increased brine flow coincides with the older dryas the intra allerod cold period and the younger dryas brine formation increased from circa 8200 years b p reaching periodic maxima during the last 4000 years b p in response to the unstable climate maximum brine production correlates with the dark ages cold period circa 1500 1100 years b p and the little ice age circa 600 100 years b p lower production correlates with the roman warm period circa 2500 2000 years b p and the medieval warm period circa 1000 700 years b p

identification and preservation of landforms diagnostic of past ice sheet activity on continental shelves from three dimensional seismic evidence ice ages have occurred a number of times in earth9s history and are important in understanding changes in long term climate however it is difficult to demonstrate the presence of ice sheets in the ancient record because their sedimentary products can resemble those from nonglacial processes e g debris flows diagnostic large scale glacial landforms produced beneath ice sheets and preserved on continental shelves after deglaciation can establish a glacial origin three dimensional seismic evidence from the 2 7 ma naust formation norwegian margin illustrates several glacial landforms that are also commonly occurring features on modern high latitude shelves 1 streamlined mega scale lineations produced by fast flowing ice streams 2 ploughmarks formed by iceberg keels and 3 regularly spaced transverse ridges or push moraines formed during ice retreat we have found for example buried ice keel ploughmarks on a paleoshelf dating to ca 2 ma norwegian margin seismic stratigraphy data show that ice advanced and retreated many times and some paleoshelf surfaces were preserved such paleosurfaces must survive ocean basin destruction for glacial landforms to be useful in identification of ancient depositional environments

three dimensional seismic data from the barents sea margin reveal evidence of past ice streams and their dynamics three dimensional seismic data from the southern flank of the outer bear island trough at the western barents sea margin show former ice stream activity the interpretation of such activity is based on megascale lineations and long chains of megablocks and rafts buried in thick till units between glacially eroded horizons in several units the sediment blocks and rafts are aligned in chains up to 2 km wide and 50 km long individual megablocks have an areal extent of up to 2 2 km 2 and their internal structure is well preserved providing information about the processes involved in their incorporation into the ice subsequent detachment and partial disintegration previous discussions of sub ice stream processes have focused on ductile deformation of weak sediments and molding of subglacial landforms this work also demonstrates the importance of brittle sediment deformation in these settings adding to the understanding of ice streams as agents of glacial erosion transport and sedimentation

high resolution sequence stratigraphy of clastic shelves iv high latitude settings abstract high frequency clastic shelf sequences deposited in high latitude settings display marked differences in term of facies and stratigraphic architecture with respect to their lower latitude counterparts this is due to the presence of ice which 1 leads to the accumulation of glacigenic and glacimarine deposits 2 provides an additional control on accommodation and 3 determines the position of the shoreline transgressions and regressions in glaciated settings are controlled respectively by the retreat and advance of the ice shoreline i e the water ice contact irrespective of relative sea level changes once the ice retreats across the land the traditional land shoreline is exposed and the control on sequence architecture is exerted by the interplay between relative sea level changes and sediment supply as in low and middle latitude settings a general model that includes both glacial and non glacial climatic regimes is provided by this paper in this frame the classic sequence stratigraphic model represents one ice free end member which is opposed to an ice permanent end member between these end members sequences may accumulate in part under ice free conditions and in part under conditions dominated by ice on the shelf the main implication of this is that the classic sequence stratigraphic model may be viewed as only a possible scenario in the stratigraphic record rather than the rule

iceberg scouring in scoresby sund and on the east greenland continental shelf modern and relict scours produced by iceberg keels have been reported from a number of high latitude continental shelves in the northern hemisphere and antarctica but observations are largely absent from the east greenland shelf about 18 km3 yr 1 of icebergs are calved into the scoresby sund fjord system east greenland these icebergs have submarine keels with drafts up to 550 m modern iceberg contacts with the sea floor are inferred from calculated keel depths and stationary icebergs identified on sequential landsat imagery acoustic profiles parasound 3 5 khz of the sea floor show that scouring is a significant process in water depths 550 m iceberg scouring intensity varies inversely with water depth the most intense scouring occurs at depths of 300 400 m iceberg scours were observed on over 25 000 km2 of the east greenland shelf at 69 72 n and 75 n scours are being actively produced on the modern shelf but relict scours probably dating back to the late weichselian are also present the rate of iceberg production from greenland ice sheet outlet glaciers and iceberg drift tracks on the shelf suggests that iceberg scouring is important over a significant proportion of the 500 000 km2 area above the shelf break around greenland

atom probe tomography analysis of the reference zircon gj 1 an interlaboratory study abstract in recent years atom probe tomography apt has been increasingly used to study minerals and in particular the mineral zircon zircon zrsio4 is ideally suited for geochronology by utilising the u th pb isotope systems and trace element compositions are also widely used to constrain petrogenetic processes however while standard geoanalytical techniques provide information at micrometer scale lengths the unique combination of chemical isotopic sensitivity and spatial resolution of apt allows compositional and textural measurements at the nanoscale this interlaboratory study aims to define the reproducibility of apt data across research facilities and assess the role of different aspects of the atom probe workflow on reproducibility this is essential to allow correct evaluation of apt results and full utilization of this emerging technique within the geoscience community in this study nine samples from the same homogeneous gj 1 87 zircon reference grain were sent to nine apt institutes in germany the uk usa canada and australia after preparing the sample out of a selectioned slab each institute conducted three different rounds of apt analyses using i unconstrained analysis parameters ii pre defined analysis parameters and iii interpreting and quantifying a provided dataset data such as the measured elemental composition acquisition parameters or mass spectrum peak identifications were recorded and analyzed we observe a significant variation in the measured composition across this interlaboratory study as well as the number of trace elements identified these differences are thought to directly result from the user s choice of atom probe data analysis parameters the type of instrument does not seem to be a critical factor consequently comparison of absolute trace element concentrations on zircon using apt between laboratories is only valid if the same workflow has been ensured

buried iceberg keel scouring on the southern spitsbergenbanken nw barents sea abstract parasound 3 5 khz subbottom echosounder profiles acquired on the southern spitsbergenbanken nw barents sea show iceberg keel scouring features which are buried by sediment that accumulated during the post last glacial maximum lgm sea level rise four acoustic units units 1 to 4 in stratigraphic order were differentiated based on the characterization of their acoustic facies and reflection surfaces unit 1 shows a chaotic internal structure and is interpreted as a glacial till whereas the laminated units 2 to 4 accumulated by sediment settling from suspension clouds and bottom currents during the last deglaciation phase the top of unit 2 was frequently incised by iceberg keels resulting in up to 12 m deep ploughmarks which were later filled and buried by unit 3 and 4 sediments three main paleo evironmental changes controlled the evolution of the facies succession 1 the major shift from till formation unit 1 below grounded ice to the accumulation of laminated sediments unit 2 which are inferred to reflect ice lifting and meltwater release 2 iceberg keel scouring after sedimentation of unit 2 3 the probable abrupt termination of iceberg keel scouring related to the glacio eustatic sea level rise a linkage between these episodes of changes and short lasting phases of rapid post lgm sea level rise such as meltwater pulses is inferred although further studies are needed to better understand the temporal and genetic relationships between the sedimentary events recognized in the barents sea and climate changes

sea floor scour at the mouth of hudson strait by deep keeled icebergs from the laurentide ice sheet physical evidence on the dimensions of icebergs released from the mouth of hudson strait into the northwest atlantic during the last heinrich event h 0 is presented side scan sonar imagery shows scour marks up to 700 m wide and longer than 28 km these scour marks were carved by gigantic icebergs megabergs with keel drafts possibly as great as 660 m capable of scouring trenches 20 to 25 m deep into the seabed these icebergs were likely calved from the grounding margin of a thick possibly as thick as 640 m rapidly flowing glacial margin during the h 0 heinrich event 11 ka bp along with the relatively few megabergs released were large numbers of smaller icebergs that calved from the ice margin at the same time and were also produced from break up of the megabergs scouring of the seabed by the large and smaller icebergs happened at the same time with megabergs scouring only in the deep waters of hatton basin and the smaller bergs scouring only on the eastern shallow margin of the basin at the continental shelf break

late quaternary ice flow in a west greenland fjord and cross shelf trough system submarine landforms from rink isbrae to uummannaq shelf and slope sea floor landforms and acoustic stratigraphic records allow interpretation of the past form and flow of a westward draining ice stream of the greenland ice sheet rink isbrae the late pliocene pleistocene glacial package is several hundred metres thick and down laps onto an upper miocene horizon several acoustic facies are mapped from sub bottom profiler records of the 400 km long uummannaq fjord shelf slope system an acoustically stratified facies covers much of the fjord and trough floor interpreted as glacimarine sediment from rain out of fine grained debris in turbid meltwater beneath this facies is a semi transparent deformation till unit which includes buried streamlined landforms landform distribution in the uummannaq system is used to reconstruct past ice extent and flow directions the presence of streamlined landforms mega scale glacial lineations drumlins crag and tails shows that an ice stream advanced through the fjord system to fill uummannaq trough reaching the shelf edge at the last glacial maximum beyond the trough there is a major fan built mainly of glacigenic debris flows turbidity current channels were not observed on uummannaq fan contrasting with well developed channels on disko fan 300 km to the south ice retreat had begun by 14 8 cal ka ago grounding zone wedges gzw in uummannaq trough imply that retreat was episodic punctuated by several still stands ice retreat between gzws may have been relatively rapid there is little sedimentary evidence for still stands in the inner fjords except for a major moraine ridge marking a little ice age maximum position on the shallow banks either side of uummannaq trough iceberg ploughing has reworked any morphological evidence of earlier ice sheet activity

deformation related microstructures in magmatic zircon and implications for diffusion an undeformed glomeroporphyritic andesite from the sunda arc of java indonesia contains zoned plagioclase and amphibole glomerocrysts in a fine grained groundmass and records a complex history of adcumulate formation and subsequent magmatic disaggregation a suite of xenocrystic zircon records proterozoic and archaean dates whilst a discrete population of zoned euhedral igneous zircon yields a shrimp u pb crystallisation age of 9 3 0 2 ma quantitative microstructural analysis of zircon by electron backscatter diffraction ebsd shows no deformation in the inherited xenocrysts but intragrain orientation variations of up to 30 in 80 of the young zircon population these variations are typically accommodated by both progressive crystallographic bending and discrete low angle boundaries that overprint compositional growth zoning dispersion of crystallographic orientations are dominantly by rotation about an axis parallel to the zircon c axis 001 which is coincident with the dominant orientation of misorientation axes of adjacent analysis points in ebsd maps less common misorientation axes account for minor components of crystallographic dispersion these observations are consistent with zircon deformation by dislocation creep and the formation of tilt and twist boundaries associated with the operation of 100 and 010 slip systems the restriction of deformation microstructures to large glomerocrysts and the young magmatic zircon population and the absence of deformation within the host igneous rock and inherited zircon grains indicate that zircon deformation took place within a low melt fraction 5 melt mid lower crustal cumulate prior to fragmentation during magmatic disaggregation and entrainment of xenocrystic zircons during magmatic decompression tectonic stresses within the compressional sunda arc at the time of magmatism are considered to be the probable driver for low strain deformation of the cumulate in the late stages of initial crystallisation these results provide the first evidence of crystal plastic dislocation creep in zircon associated with magmatic crystallisation and indicate that the development of crystal plastic microstructures in zircon is not restricted to high strain rocks such microstructures have previously been shown to enhance bulk diffusion of trace elements u th and ree in zircon the development of deformation microstructures and therefore multiple diffusion pathways in zircon in the magmatic environment has significant implications for the interpretation of geochemical data from igneous zircon and the trace element budgets of melts due to the potential enhancement of bulk diffusion and dissolution rates

planar microstructures in zircon from paleo seismic zones pseudotachylytes resulted from frictional melts associated with ultramylonites in high grade metapelitic rocks from the ivrea verbano zone in the southern alps northern italy were studied with focus on the deformation microstructures in zircon the aims were to investigate the characteristics of zircon deformation in seismic zones and to recognize specific microstructures generated in zircon during earthquakes which could be useful for mineral dating of paleo seismic events helps to understand how seismic energy is released at depth and interacts with metamorphic processes the interior of polished zircon grains ranging from 30 to 150 m in length were investigated with optical microscope and scanning electron microscope sem techniques including secondary electron se backscattered electron bse forward scattered electron fse cathodoluminescence cl imaging and crystallographic orientation mapping by electron backscatter diffraction analysis ebsd grains were studied in situ and as separated fractions embedded in epoxy disks among different cataclastic and crystal plastic deformation microstructures in zircon we identified characteristic planar deformation bands pdbs planar fractures pfs and curviplanar fractures cfs planar deformation bands in zircon are crystallographically controlled planar lattice volumes with misorientation from the host grain which varies from 0 4 to 2 7 pdbs are usually parallel to 100 crystallographic planes have width from 0 3 to 1 m and average spacing of 5 m in 2d sections planar deformation bands appear as contrast lamellae in orientation contrast images and in ebsd maps and in rare cases can be observed with the optical microscope pdbs form in specifically oriented grains due to high differential stresses high temperatures and high strain rates generated in seismically active environment and or due to shearing in the vicinity of frictional melts discovered structures represent a result of crystal plastic deformation of zircon grains with operating dislocations having 010 glide system and misorientation axis therefore they can be classified as a new type 4 lattice distortion pattern according to the existing classification for zircon piazolo et al 2012 kovaleva et al 2014 we have demonstrated that formation of planar fractures in zircon takes place not only during impacts but also in seismically active zones we observe at least two cases of formation of pfs with 100 orientation 1 as a result of evolution of pdbs and 2 as micro cleavage this study demonstrates that planar microstructures in terrestrial zircon do not exclusively form during impact events but also as a result of seismic events at depth due to unusually high differential stress strain rate and temperature according to the new findings pdbs in zircon from the deep crust are supposed to represent newly recognized evidence of seismicity

trace element homogeneity from micron to atomic scale implication for the suitability of the zircon gj 1 as a trace element reference material the quality of a chemical reference material relies on the fact that the composition of the material is homogeneous across all scales a series of different techniques have been used to evaluate the trace element homogeneity of the gj 1 reference zircon from the micron to atomic scale cathodoluminescence imaging was conducted along with quantitative crystallographic orientation analysis and trace element analysis using laser ablation inductively coupled plasma mass spectrometry la icpms the nanometre scale homogeneity was evaluated by analysing five mineral tips using atom probe tomography which provides atomic scale three dimensional chemical reconstructions with unprecedented spatial resolution results show that the gj 1 reference zircon is homogeneous at all scales both structurally and chemically crystallographic orientation data confirms that this gem quality zircon has no detectable internal crystallographic orientation changes such as crystal plastic deformation features or cracks no mineral inclusions were found atom probe tomography shows that there is a lack of any chemical clustering or other modes of spatially defined elemental accumulation or depletion for the most abundant trace elements such as y yb and hf this finding is supported by la icpms data revealing homogeneity within the analytical precision trace elements of significant abundance include p yb y u and hf with contents of 30 6 65 2 238 5 284 14 and 6681 57 ppm respectively hence the gj 1 zircon used as a reference zircon for upb and hf isotopic studies is also a suitable zircon reference material for trace element analyses

late quaternary ice flow and sediment delivery through hinlopen trough northern svalbard margin submarine landforms and depositional fan abstract the morphology and distribution of submarine landforms in hinlopen trough northern svalbard margin and the upper continental slope beyond are investigated using swath bathymetric data together with side scan sonar acoustic profiler records and sediment cores sun illuminated images reveal a number of landform assemblages on the sea floor of hinlopen trough and confirm that this depression was occupied by an ice stream during the late weichselian glaciation around 20 ka before present the geomorphology of the inner shelf is characterised by bedrock drumlins and ice sculpted bedrock there is a middle shelf transition from a rock bed to an unconsolidated sedimentary bed that contains highly attenuated drumlins and megaflutes formed within a dark grey matrix supported diamict interpreted as subglacial till outer shelf landforms are mainly iceberg ploughmarks the geomorphological imprint identified from hinlopen trough is similar to that of many former ice streams the trough is dominated by elongate landforms orientated parallel to the inferred flow direction and is lacking in transverse features indicative of inter ice stream locations a characteristic down flow progression in landform elongation is also observed suggesting an increase in ice velocity across the continental shelf the lack of grounding zone features imaged from swath bathymetric data implies that deglaciation was probably rapid within the trough a large depositional fan exists on the upper continental slope beyond the trough characterised by debris flow deposits relating to the down slope transport of glacigenic sediment order of magnitude calculations of the volume of glacigenic sediment on the continental slope indicate that the ice stream provided high rates of debris delivery of around 5 7 5 m ka this suggests that hinlopen trough ice stream represented a major route for the transfer of ice and debris to the hinlopen fan on the northern continental margin of svalbard during the late weichselian glaciation

petrogenesis of the colville igneous complex northeast washington implications for eocene tectonics in the northern u s cordillera eocene igneous complexes of the u s and canadian cordillera have long been thought to represent a broad and somewhat discontinuous subduction related magmatic arc extending from northern british columbia to wyoming this model has become increasingly untenable as knowledge of the structural and tectonic settings of these rocks has expanded new major trace and rare earth element and isotope data presented here virtually preclude the interpretation of a contemporaneous subduction related source for mantle derived magmas in the colville igneous complex of northeast washington state we suggest an alternative model whereby post laramide orogenic collapse resulted in partial melting of a mid crustal source followed by partial melting of a distinctly different lower crustal source the magma from which mixed with magma derived from a lithospheric mantle source the subduction signature and the calc alkalic nature of the magmas was inherited from previous proterozoic subduction events

hadean age for a post magma ocean zircon confirmed by atom probe tomography the only physical evidence from the earliest phases of earth s evolution comes from zircons ancient mineral grains that can be dated using the u th pb geochronometer 1 oxygen isotope ratios from such zircons have been used to infer when the hydrosphere and conditions habitable to life were established 2 3 chemical homogenization of earth s crust and the existence of a magma ocean have not been dated directly but must have occurred earlier 4 however the accuracy of the u pbzirconagescanplausiblybebiasedbypoorlyunderstood processes of intracrystalline pb mobility 5 7 here we use atomprobe tomography 8 to identify and map individual atoms in the oldest concordant grain from earth a 4 4 gyr old hadean zircon with a high temperature overgrowth that formed about 1gyrafterthemineral score isolatednanoclusters measuring about 10nm and spaced 10 50nm apart are enriched in incompatible elements including radiogenic pb with unusually high 207 pb 206 pbratios wedemonstratethatthelengthscales of these clusters make u pb age biasing impossible and that they formed during the later reheating event our tomography data thereby confirm that any mixing event of the silicate earth must have occurred before 4 4gyr ago consistent with

aligned glaciotectonic rafts on the central barents sea seafloor revealing extensive glacitectonic erosion during the last deglaciation 1 erosion rates on glaciated continental shelves are remarkably high especially within ice stream troughs although glaciotectonic erosion may have considerably contributed to enhanced glacial erosion of these landscapes entrainment mechanisms of glaciotectonically emplaced megablocks and rafts remain little understood here we report a northeast southwest trending chain of over 1300 glacial rafts observed on the seafloor in bjornoyrenna a paleo ice stream trough with particularly high erosion rates the landform assemblage adjacent to the glacial rafts includes streamlined bedforms and crevasse squeeze ridges and may represent a lateral shear zone where a paleo ice stream met cold stagnant ice during the last deglaciation individual glacial rafts along the chain are up to 30 m high tend to be elongated with lengths and widths on the order of tens of meters and are oriented from northeast southwest to north south the north southern rearrangement of rafts may reflect a compressional strain regime along the ice stream margin