deposition and imbrication of a 2670 2629 ma supracrustal sequence in the indin lake area southwestern slave province canada new u pb age data from the southwestern slave province demonstrate that units of the indin lake supracrustal belt form an imbricated structural stack the oldest rocks of the belt are undated mafic volcanic flows of the hewitt lake group that are crosscut by a 2670 ma felsic sill itself coeval with mafic through felsic volcanic rocks of the 2668 2671 ma leta arm group the youngest rocks of the belt are 2647 2629 ma turbidites and felsic volcanic rocks of the unconformably overlying chalco lake group tonalite orthogneiss of the adjacent cotterill gneiss complex is 2680 ma suggesting that it does not represent in situ basement to the supracrustal belt intercalation of the older hewitt lake and leta arm groups with the younger chalco lake group is interpreted to result from d1 imbrication and folding between 2629 and 2609 ma the age of a crosscutting tonalite intrusion subsequent d2 folding and regional low pressure metamorphism occurred between 2609 ma and ca 2590 ma d3 normal faulting between the

geochronology at the namew lake ni cu deposit flin flon area manitoba canada a pb pb study of whole rocks and ore minerals pb isotope ratios of whole rock specimens and mineral separates from the gneisses surrounding the namew lake ni cu deposit south of flin flon manitoba are compared with the pb isotope ratios in the ore minerals and with recently reported zircon u pb analyses of samples from the gneisses and the ultramafic host of the ore the pb pb age from the gneisses is 1864 8 ma and there is no evidence for any differences in slopes or initial ratios between individual samples thus there is good agreement between the youngest zircon ages for the gneisses 1862 3 ma and pb pb ages for the same rocks a suite of analyses from a late pegmatite which cuts the gneisses and the ultramafic rock yields a well defined age of 1824 5 ma consistent with the geologic requirement that it be younger than the ultramafic host to the orebody zircon age of 1847 6 ma and similar in age to late intrusive bodies in the flin flon area to the north all these data are in marked contrast to apparent pb pb ages for the sulfp

the isotopic composition of uranium and lead in allende inclusions and meteoritic phosphates we measured the isotopic compositions of u and pb in eight ca al rich inclusions from the allende chondrite in whitlockite from the st severin chondrite and in a whitlockite and a whole rock sample from the angra dos reis achondrite ador the total range in 235 u 238 u in the acid soluble phases of the allende inclusions and meteoritic whitlockites is from 1 137 6 to 1 138 3 and to within the error limits 3 20 indistinguishable from normal terrestrial u the 235 u 238 u ratios for whitlockites from st severin and ador are the same as that in the bulk meteorites and are normal the pb isotopic compositions of the acid soluble phases of the allende coarse grained inclusions except c 1 give a mean 207 pb 206 pb model age of4 559 0 015ae which is consistent with the observation of normal u isotopic composition the pb isotopic compositions of coarse grained fun inclusion c 1 and of two fine grained inclusions from allende indicate that the u pb systems of these samples have evolved differently from the rest of the meteorite the 207 pb 206 pb model ages for the whitlockites and for bulk ador are the same as those for st severin whitlockite 4 551 0 004ae the pb results from st severin indirectly constrain the value of the u isotopic abundance to be within 7 4 of normal both the directly measured235u 238u and the pb data on st severin are in complete disagreement with claims of large 235 u excess reported by other workers on the same meteorite the th u ratios of some acid soluble phases of allende inclusions and of meteoritic whitlockite show varying degrees of enrichment of th relative to u by factors of 2 to 25 relative to the bulk meteorites previous studies have shown that 244 pu 238 u ratios in whitlockite are also fractionated relative to the bulk meteorites the observation of no isotopic effects in u reported here does not support the claims of either fractionation of 247 cm 238 u or of a large amount of live 247 cm at the early stage of formation of the solar system using the present data we obtain a limit of 247 cm 235 u 4 10 3 at the time of meteorite formation or differentiation this implies that the last major r process contribution to the material in the protosolar nebula was 10 8 years prior to 26 al formation and injection

the use of the common pb isotope composition of detrital k feldspar grains as a provenance tool and its application to upper carboniferous paleodrainage northern england k feldspar is a common detrital mineral in sandstones it contains little u or th and hence the common pb isotope composition of unaltered k feldspar is that of the source rock pb isotope variations in igneous and metamorphic crustal rocks define broad spatial patterns that make the pb signature of detrital k feldspar grains a useful provenance tool however it is unclear how robust this signal is and to what extent it can be modified by weathering transport burial diagenesis and or exposure to hydrothermal fluids this study reports an evaluation of the technique using two granite arkose pairs from helmsdale northern scotland and shap northwest england where the composition of the detrital grains can be compared to feldspars in the granite from which they were derived the results of this test have informed a pilot study using the pb isotope composition of detrital k feldspar in upper carboniferous fluvial sedimentary rocks of northern england to constrain the sediment routing pb isotope compositions have been measured using an in situ high resolution single grain technique that can distinguish original composition heterogeneity and subsequent alteration within individual sand grains the results for the granite arkose pairs show that the common pb isotope signature of the detrital k feldspar grains matches the in situ feldspars in the source granites the signal is independent of grain size medium grained sand to granules but alteration along a paleo regolith at helmsdale has resulted in perturbations in pb composition and an increase in the errors on the analyses the application of the technique to arkosic carboniferous serpukhovian bashkirian millstone grit group sandstones in the pennine basin reveals two discrete populations of k feldspars that cannot be distinguished petrographically matching of the compositions to potential sourcelands to the north and northeast of the basin suggest a far traveled 500 km source of k feldspar from the archean paleoproterozoic lewisian rocks of northwest scotland or their extension to the north but also an equally important proximal source emanating from the southern uplands belt of scotland in what would have been the lower part of the hinterland drainage basin of the millstone grit

sustained increase in ice discharge from the amundsen sea embayment west antarctica from 1973 to 2013 we combine measurements of ice velocity from landsat feature tracking and satellite radar interferometry and ice thickness from existing compilations to document 41 years of mass flux from the amundsen sea embayment ase of west antarctica the total ice discharge has increased by 77 since 1973 half of the increase occurred between 2003 and 2009 grounding line ice speeds of pine island glacier stabilized between 2009 and 2013 following a decade of rapid acceleration but that acceleration reached far inland and occurred at a rate faster than predicted by advective processes flow speeds across thwaites glacier increased rapidly after 2006 following a decade of near stability leading to a 33 increase in flux between 2006 and 2013 haynes smith pope and kohler glaciers all accelerated during the entire study period the sustained increase in ice discharge is a possible indicator of the development of a marine ice sheet instability in this part of antarctica key points sustained ase mass flux increase 77 since 1973 thwaites accelerated by 33 during the last 6 years speed changes are pervasive and rapid major implications for ice flow modeling 2014 american geophysical union all rights reserved

petrogenesis of the late cretaceous northern alberta kimberlite province abstract at present 48 late cretaceous ca 70 88 ma kimberlitic pipes have been discovered in three separate areas of the northern alberta the mountain lake cluster the buffalo head hills field and the birch mountains field the regions can be distinguished from one another by their non archetypal kimberlite signature mountain lake or in the case of kimberlite fields primitive buffalo head hills to evolved birch mountains magmatic signatures the dominant process of magmatic differentiation is crystal fractionation and accumulation of olivine which acts as the main criteria to distinguish between primitive and evolved group i type kimberlite fields in the northern alberta this is important from the viewpoint of diamond exploration because the majority about 80 of the more primitive buffalo head hills kimberlites are diamondiferous whereas the more evolved birch mountains pipes are barren of diamonds for the most part petrographically the buffalo head hills samples are distinct from the birch mountains samples in that they contain less carbonate have a smaller modal abundance of late stage minerals such as phlogopite and ilmenite and have a higher amount of fresh coarse macrocrystal 0 5 mm olivine consequently samples from the buffalo head hills have the highest values of mgo cr and ni and have chemistries similar to those of primitive hypabyssal kimberlite in the northwest territories based on whole rock isotopic data the buffalo head hills k6 kimberlite has 87sr 86sr and end values similar to those of south african group i kimberlites whereas the birch mountains legend and phoenix kimberlites have similar end values between 0 and 1 9 but distinctly higher 87sr 86sr values 0 7051 0 7063 the lack of whole rock geochemical overlap between kimberlite and the freshest least contaminated mountain lake south pipe rocks reflects significant mineralogical differences and mountain lake is similar geochemically to olivine alkali basalt and or basanite intra field geochemical variations are also evident the k4 pipe buffalo head hills and xena and kendu pipes birch mountains are characterized by anomalous concentrations of incompatible elements relative to other northern alberta kimberlite pipes including chondrite normalized rare earth element distribution patterns that are less fractionated than the other kimberlite samples from the buffalo head hills and birch mountains the xena pipe has similar major element chemical signatures and high al clinopyroxene similar to or trending towards the mountain lake pipes in addition k4 and kendu have higher 87sr 86sr and lower end than bulk earth and plot in the bottom right quadrant of the nd sr diagram we suggest therefore that the k4 and kendu pipes contain a contribution from old lree enriched low sm nd lithosphere that is absent from the other kimberlites are affected by crustal contamination or both based on xenocryst populations the northern alberta kimberlite province mantle is dominated by carbonate saturated lherzolitic mantle higher levels of melt depletion characterize the buffalo head hills mantle sample despite high diamondiferous to barren pipe ratios in the buffalo head hills pipes mineral indicators of high diamond potential such as g10 garnet diamond inclusion composition chrome spinels and high sodium eclogitic garnet are rare

evolution of the slave craton greenstone belts in the slave craton represent the episodic accumulation of volcanic and sedimentary rocks over as much as 200 m y in the western craton volcanic and sedimentary rocks were deposited between 2 81 and 2 60 ga on 2 9 3 1 ga sialic basement not tectonically juxtaposed with it deposition of 2 6 ga turbidite and conglomerate sequences across the entire slave craton was synchronous with or followed by the main phase of regional deformation and metamorphism at 2 6 2 58 ga and final stabilization of the craton

parentage of archean basement within a paleoproterozoic orogen and implications for on craton diamond preservation slave craton and wopmay orogen northwest canada the wopmay orogen is a paleoproterozoic accretionary belt preserved to the west of the archean slave craton northwest canada reworked archean crystalline basement occurs in the orogen and new bedrock mapping u pb geochronology and sm nd isotopic data further substantiate a slave craton parentage for this basement detrital zircon results from unconformably overlying paleoproterozoic supracrustal rocks also support a slave craton provenance rifting of the slave margin began at ca 2 02 ga with a second rift phase constrained between ca 1 92 and 1 89 ga resulting in thermal weakening of the archean basement and allowing subsequent penetrative deformation during the calderian orogeny ca 1 88 1 85 ga the boundary between the western slave craton and the reworked archean basement in the southern wopmay orogen is interpreted as the rifted cratonic margin which later acted as a rigid backstop during compressional deformation age isotopic characteristics of plutonic phases track the extent and evolu

pb isotopic composition of late archaean granites and the extent of recycling early archaean crust in the slave province northwest canada initial pb isotopic compositions have been determined for potassium feldspar from ca 2 58 to 2 62 ga plutonic rocks in the southern and central slave province of northwestern canada to evaluate the extent of recycling of ancient crust within the province large differences in initial pb compositions were measured which correlate with geographical areas of the province plutons in the east central part of the province have initial compositions only slightly more radiogenic than estimated mantle values 207pb 204pb 14 8 14 9 and were dominantly deruved from juvenile crustal sources in contrast plutons in the point lake and western contwoyto lake areas of the western slave province have radiogenic compositions 207pb 204pb 15 1 15 2 and indicate significant recycling of pre 3 5 ga crust the pb data support previous interpretations based on nd isotopes for a major isotopic boundary in the central part of the province granites from the southern part of the province near yellowknife have intermediate compositions which indicate 1 the age of the protolith to the granitoids in the yellowknife area is younger than at point lake but older than in the eastern slave or 2 the granitoids in the yellowknife area contain a mixture of an older point lake type component and younger crust the absence of pre 3 2 ga crust in the yellowknife area and lack of evidence for pre 2 8 ga inherited zircons in the yellowknife granitoids favour the former possibility evidence for recycling of ancient crustal sources such as the acasta gneiss is limited to a relatively small area of the west central part of the province suggesting that acasta aged or derived crust is not widespread in the province the marked regionality of isotopic composition may reflect a basement in the western part of the province which is itself a collage of crust of different age being younger ca 3 2 2 9 in the south relative to the point lake region 3 9 3 2 ga

lama guanicoe bone collagen stable isotope c and n indicate climatic and ecological variation during holocene in northwest patagonia abstract this paper explores how significant are the ecological and climatic variables to influence the stable isotopes of guanacos lama guanicoe bone collagen carbon and nitrogen stable isotope ratios are assumed as a macro regional average value in west argentina mostly as a baseline to model archaeological human diet if stable isotopes on mammals reflex ecology and climate we need to know how those variables influence mammals bone stable isotope ratio this paper analyses the 13c 12c and 15 n 14n ratio on bone collagen on 122 guanacos from northwest patagonia during the holocene the results confirm significant variation in both isotopes between monte and andean patagonian specimens guanacos from monte shows higher 13c and 15n than those from andean patagonian temporal trends indicate variation through holocene but this variation is not spatially homogeneous in this paper we suggest that medieval climatic anomaly had stronger effect in monte desert than in patagonia desert generating driest and or hottest conditions between 1250 and 600 years bp those variations need to be considered to reconstruct human diet at least during the second part of the holocene

u pb zircon geochronology of the yellowknife volcanic belt nwt canada new constraints on the timing and duration of greenstone belt magmatism volcanic and sedimentary rocks in the slave province collectively termed the yellowknife supergroup have long been considered to represent laterally continuous and correlative stratigraphically coherent homoclinal sequences correlation based on the lithologic analysis of certain distinctive units has been most intensely applied to the yellowknife volcanic belt in the southern slave province u pb zircon data for seven samples collected from within the belt are reported here the distinctiveness of the townsite formation a term applied to three lenticles of rhyodacitic to dacitic tuffs breccias and pillowed flows in comparison to the overlying and underlying tholeiitic basalts kam group has led to its use as a key marker unit for stratigraphic correlations and palinspastic restorations of offsets on proterozoic faults in the area a sample from each of the three segments was collected for u pb zircon geochronology in addition numerous thinner tuffaceous beds from the kam group were also dated

tectonic affinity of nisutlin and anvil assemblage strata from the teslin tectonic zone northern canadian cordillera constraints from neodymium isotope and geochemical evidence we present geochemical and isotopic data for nisutlin assemblage metasedimentary rocks and anvil assemblage greenstones from the teslin tectonic zone of the northern canadian cordillera this study aims to establish the tectonic setting of formation for the sedimentary and basaltic protoliths of these highly deformed and metamorphosed rocks and thereby place constraints on the origin of these enigmatic rocks for which differing tectonic models have been proposed for the nisutlin assemblage metasedimentary rocks the geochemical and isotopic data show that two widely different source regions contributed detritus to the original sediments one source region was felsic upper crustal material with nd isotopic compositions compatible with ultimate derivation from the north american continent depleted mantle model age tdm 2 5 2 8 ga the second source region is deduced to be chemically primitive crust basaltic andesitic with a short crustal residence history 0 9 ga for the anvil assemblage greenstones immobile trace element abundances are dissimilar to within plate oceanic arc and normal mid ocean ridge basalts and similar to calc alkaline basalts in active continental margin settings we interpret the paleosetting for the nisutlin assemblage to be at the outermost margin of the ancestral north american continent in an area which received detrital input from the distal north american craton in the early paleozoic however this area also received detritus from a chemically and isotopically juvenile magmatic arc source a source type not known from paleozoic metasedimentary rocks from the miogeoclinal sequence on the basis of similar nd isotopic relationships recorded elsewhere in deformed paleozoic rocks of the orogen we infer that these geochemical signals reflect tectonic processes of regional extent the trace element geochemistry of the anvil assemblage greenstones does not support a correclation with known paleozoic greenstones of the slide mountain terrane which some tectonic models have advocated

trends in antarctic ice sheet elevation and mass fluctuations in antarctic ice sheet elevation and mass occur over a variety of timescales owing to changes in snowfall and ice flow here we disentangle these signals by combining 25 years of satellite radar altimeter observations and a regional climate model from these measurements patterns of change that are strongly associated with glaciological events emerge while the majority of the ice sheet has remained stable 24 of west antarctica is now in a state of dynamical imbalance thinning of the pine island and thwaites glacier basins reaches 122 m in places and their rates of ice loss are now five times greater than at the start of our survey by partitioning elevation changes into areas of snow and ice variability we estimate that east and west antarctica have contributed 1 1 0 4 and 5 7 0 8 mm to global sea level between 1992 and 2017

west antarctic ice sheet elevations in the ohio range geologic constraints and ice sheet modeling prior to the last highstand abstract knowledge of the timing and extent of past west antarctic ice sheet wais elevation changes provides insight to the wais response to sea level and climate forcing as well as constraints for ice sheet models trimlines and erratics indicate that maximum ice elevation at the ohio range located near the wais divide was 125 m above the present ice surface and was reached at 10 ka however 3 he and 10 be exposure ages of most granitic erratics and supra glacial boulders in adjacent blue ice areas are older than the last highstand and provide constraints on earlier interior wais elevation changes although diffusive loss of 3 he occurs in quartz an excellent correlation between 3 he and 10 be concentrations in samples up to 400 ka is observed in the ohio range demonstrating the utility of 3 he measurements in antarctic quartz the correlation is used to calculate equivalent 10 be exposure ages from the larger 3 he data set the exposure ages occur in distinct clusters 10 20 ka 70 80 ka 130 140 ka and 180 190 ka that we relate to pulses in the emergence of englacial debris in localized blue ice ablation areas a combined ice sheet ice shelf model predicts that wais elevations near the ohio range have varied only 125 m over the last 200 kyr with highstands at 10 ka and 130 ka in good agreement with observations of trimlines and peaks in the exposure age distribution at the ohio range in the model interior wais elevations are controlled by the interaction of accumulation rates and ice dynamics related to ice sheet ocean interactions at the grounding line wais highstands occur early in interglacial periods during intervals of warmer temperatures and high accumulation rates prior to the arrival of a wave of thinning we hypothesize that the correspondence of rising regional wais elevation with debris pulses relates to increased ice debris flux and ablation in the local blue ice areas resulting from the warmer temperatures the occurrence of clasts with exposure ages up to 200 ka on the wais surface indicates elevation changes near the ohio range were limited in extent during stage 5e the geologic observations combined with model results place constraints on wais geometry during stage 5e and limit the wais contribution to the higher sea levels observed during the last interglacial to 3 m

geochemical and nd pb isotopic systematics of late archean granitoids southwestern slave province canada constraints for granitoid origin and crustal isotopic structure new geochemical and nd pb isotopic data for 2 62 2 59 ga granitoids from the southwest slave province are used to determine the source s of granitoid magmas to evaluate the role of pre 2 8 ga basement during this magmatism and to refine the existing nd pb isotopic structure of the western slave province the pb isotopic data require crust older than 3 2 ga as a granitoid protolith whereas the nd isotopic data require input from juvenile crustal material this discrepancy is explained if the granitoid protoliths are mixtures of ancient basement and 2 7 ga juvenile crust in varying proportions specifically granitoids from the southwestern slave province require 10 30 basement whereas granitoids from other parts of the western slave province require 50 incorporation of basement as a protolith may be achieved indirectly by assimilation of basement during juvenile 2 7 ga magmatism or directly during 2 62 2 59 ga magmatism the granitoid isotopic data suggest that indirect basement input was

experimental evidence for high temperature felsic melts formed during basaltic intrusion of the deep crust basaltic intrusion into the continental crust was simulated with melting experiments on layered charges of high alumina olivine tholeiite haot and metapelite at 11 kbar rapid diffusion of cao and na 2 o into the metapelite and h 2 o k 2 o sio 2 and al 2 o 3 into the basalt resulted in spectacular changes in the phase assemblage garnet megacrysts crystallized profusely across the basalt layer cumulate assemblages closely resembled rocks found in underplating localities and glass compositions graded from dacite to rhyolite even at temperatures as high as 1150 c silicic melts can be generated during interaction of basalt with metapelite at temperatures well above the h 2 o saturated granite minimum calculations based on these results and comparison with natural occurrences suggest that high temperature felsic melts with 2 o can form and segregate at rates of 1 cm yr

dehydration melting of tonalites part ii composition of melts and solids dehydration melting of tonalitic compositions phlogopite or biotite plagioclase quartz assemblages is investigated within a temperature range of 700 1000 c and pressure range of 2 15 kbar the solid reaction products in the case of the phlogopite plagioclase an45 quartz starting material are enstatite clinopyroxene and potassium feldspar with amphiboles occurring occasionally at 12 kbar zoisite is observed below 800 c and garnet at 900 c the reaction products of dehydration melting of the biotite ann50 plagioclase an45 quartz assemblage are melt orthopyroxene clinopyroxene amphibole and potassium feldspar at pressures 8 kbar and temperatures below 800 c epidote is also formed almandine rich garnet appears above 10 kbar at temperatures 750 c the composition of melts is granitic to granodioritic hence showing the importance of dehydration melting of tonalites for the formation of granitic melts and granulitic restites at pressure temperature conditions within the continental crust the melt compositions plot close to the cotectic line dividing the liquidus surfaces between quartz and potassium feldspar in the haplogranite system at 5 kbar and ah2o 1 the composition of the melts changes with the composition of the starting material temperature and pressure with increasing temperature the melt becomes enriched in al2o3 and feo mgo potash in the melt is highest just when biotite disappears the amount of cao decreases up to 900 c at 5 kbar whereas at higher temperatures it increases as amphibole clinopyroxene and more an component dissolve in the melt the na2o content of the melt increases slightly with increase in temperature the composition of the melt at temperatures 900 c approaches that of the starting assemblage the melt fraction varies with composition and proportion of hydrous phases in the starting composition as well as temperature and pressure with increasing modal biotite from 20 to 30 wt the melt proportion increases from 19 8 to 22 3 vol 850 c and 5 kbar with increasing temperature from 800 to 950 c at 5 kbar the increase in melt fraction is from 11 to 25 8 vol the effect of pressure on the melt fraction is observed to be relatively small and the melt proportion in the same assemblage decreases at 850 c from 19 8 vol at 5 kbar to 15 3 vol at 15 kbar selected experiments were reversed at 2 and 5 kbar to demonstrate that near equilibrium compositions were obtained in runs of longer duration

age and isotopic relationships among the angrites lewis cliff 86010 and angra dos reis results of a wide ranging isotopic investigation of the unique antarctican angrite lew 86010 lew are presented together with a reassessment of the type angrite angra dos reis ador the principal objectives of this study are to obtain precise radiometric ages initial sr isotopic compositions and to search for the erstwhile presence of the short lived nuclei 146sm and 26al via their daughter products the isotopic compositions of sm u ca and ti were also measured this allows a detailed appraisal to be made of the relations between and the geneology of these two angrites lew proves to be severely contaminated with modern terrestrial pb which is shown to result from terrestrial weathering nevertheless concordant pb pb model ages of pyroxene separates were obtained 2 4 55784 52 ga for lew and 4 55780 42 ga for ador uranium isotopic compositions are normal within error the inferred initial pb isotopic composition is within error of primordial pb as defined by canon diablo troilite because of the extreme upb ratios of both angrites this places an upper limit of 2 ma on the time between volatile element loss of the angrite parent body or its precursor planetesimals and the final crystallization of the angrites as differentiates initial 87sr86sr ratios were found to be indistinguishable in lew ador and the cumulate eucrite moore county the derived initial 87sr86sr for all three meteorites is 0 698970 15 2 a 147sm 143nd isochron for lew was obtained yielding an age of 4 553 34 ga with an initial 143nd144nd of 0 506682 49 2 146sm142nd isotope systematics were also measured anomalies on 142nd arising from extinct 146sm were clearly resolved resulting in an initial 146sm144sm of 0 0071 17 with nd142 2 57 0 62 at the time of isotopic closure both of these sm nd methods imply derivation of lew from a reservoir with chondritic smnd ratio they are also quite consistent with the previously reported systematics for ador overall the age and isotopic similarities between lew and ador are striking it suggests almost simultaneous production on the same asteroid even though recent experimental studies imply that the two are not comagmatic calcium and titanium do not exhibit enrichments in the n rich isotopes in contrast to cais making it unlikely that the angrite parent body is inherently rich in early condensates from the solar nebula no evidence was found for live 26al in lew when combined with the pb pb age and initial 87sr86sr data this firmly excludes 26al being an important heat source in the early solar system heat supplied for for example 1 the differentiation of the angrite and eucrite parent bodies and 2 metamorphism of the chondrite parent bodies must come from some other source the debate over live vs extinct 26al in the earliest solar system remains unresolved by this new data from lew however both the angrite and eucrite parent bodies have extremely low rbsr and high upb ratios compared to the solar nebula therefore the common initial 87sr86sr of 0 698970 can be firmly associated with the u pb angrite age above and the formation of both bodies this enables an absolute chronology of the early solar system to be established based upon 87sr86sr differences the oldest cais are a minimum of 11 4 ma older than the angrites i e 4 569 5 ga the age and isotopic constraints are discussed with respect to current collapse condensation and accretion timescales calculated for the solar nebula

lithofacies analysis and 40ar 39ar geochronology of ice volcano interactions at mt murphy and the crary mountains marie byrd land antarctica

geology and u pb geochronology of the neoarchean snare river terrane tracking evolving tectonic regimes and crustal growth mechanisms u pb zircon crystallization ages determined by isotope dilution thermal ionization mass spectrometry id tims and laser ablation microprobe inductively coupled plasma mass spectrometry lam icp ms for 13 intrusive units in the neoarchean snare river terrane srt provide tight constraints on the timing of crust formation and orogenic evolution seven metaluminous plutons were emplaced over 80 ma from ca 2674 to 2589 ma whereas six peraluminous bodies were emplaced in a 15 ma interval from ca 2598 to 2585 ma a detrital zircon study yielded an age spectrum with peaks correlative with known magmatic events in the slave province with the ca 2635 ma age of the youngest detrital zircon population providing a maximum estimate for the onset of sedimentation this age contrasts with evidence for pre 2635 ma sedimentation elsewhere in the srt indicating that sedimentation was protracted and diachronous evolution of the srt can be subdivided into four stages i 2674 2635 ma formation of a metal

four decades of antarctic ice sheet mass balance from 1979 2017 we use updated drainage inventory ice thickness and ice velocity data to calculate the grounding line ice discharge of 176 basins draining the antarctic ice sheet from 1979 to 2017 we compare the results with a surface mass balance model to deduce the ice sheet mass balance the total mass loss increased from 40 9 gt y in 1979 1990 to 50 14 gt y in 1989 2000 166 18 gt y in 1999 2009 and 252 26 gt y in 2009 2017 in 2009 2017 the mass loss was dominated by the amundsen bellingshausen sea sectors in west antarctica 159 8 gt y wilkes land in east antarctica 51 13 gt y and west and northeast peninsula 42 5 gt y the contribution to sea level rise from antarctica averaged 3 6 0 5 mm per decade with a cumulative 14 0 2 0 mm since 1979 including 6 9 0 6 mm from west antarctica 4 4 0 9 mm from east antarctica and 2 5 0 4 mm from the peninsula i e east antarctica is a major participant in the mass loss during the entire period the mass loss concentrated in areas closest to warm salty subsurface circumpolar deep water cdw that is consistent with enhanced polar westerlies pushing cdw toward antarctica to melt its floating ice shelves destabilize the glaciers and raise sea level

the slave province a distinct archean craton the slave province is a relatively small late archean craton that exhibits distinctive rock associations and structures by comparison with the much larger superior province differences are evident in i the abundance of sedimentary versus volcanic rocks and of felsic versus mafic volcanic rocks ii the greater evidence for sialic basement iii the higher proportion of more evolved potassium rich granite iv the type setting and timing of gold and base metal mineralization and v a regional zonation of gold deposits that seems to be a unique feature of the slave province contrasts in structure are also significant the large scale linear belts and boundaries sutures that characterize the superior province have no obvious counterpart in the slave province despite some similarities with other archean cratons the distinct features of the slave province are important for they imply that no single archean craton should be used to develop a universal paradigm for the genesis and tectonic evol

variable relationship between accumulation and temperature in west antarctica for the past 31 000 years the antarctic contribution to sea level is a balance between ice loss along the margin and accumulation in the interior accumulation records for the past few decades are noisy and show inconsistent relationships with temperature we investigate the relationship between accumulation and temperature for the past 31 ka using high resolution records from the west antarctic ice sheet wais divide ice core in west antarctica although the glacial interglacial increases result in high correlation and moderate sensitivity for the full record the relationship shows considerable variability through time with high correlation and high sensitivity for the 0 8 ka period but no correlation for the 8 15 ka period this contrasts with a general circulation model simulation which shows homogeneous sensitivities between temperature and accumulation across the entire time period these results suggest that variations in atmospheric circulation are an important driver of antarctic accumulation but they are not adequately captured in model simulations model based projections of future antarctic accumulation and its impact on sea level should be treated with caution

the last glaciation of bear peninsula central amundsen sea embayment of antarctica constraints on timing and duration revealed by in situ cosmogenic 14c and 10be dating abstract ice streams in the pine island thwaites region of west antarctica currently dominate contributions to sea level rise from the antarctic ice sheet predictions of future ice mass loss from this area rely on physical models that are validated with geological constraints on past extent thickness and timing of ice cover however terrestrial records of ice sheet history from the region remain sparse resulting in significant model uncertainties we report glacial geological evidence for the duration and timing of the last glaciation of hunt bluff in the central amundsen sea embayment a multi nuclide approach was used measuring cosmogenic 10be and in situ 14c in bedrock surfaces and a perched erratic cobble bedrock 10be ages 118 144 ka reflect multiple periods of exposure and ice cover not continuous exposure since the last interglacial as had previously been hypothesized in situ 14c dating suggests that the last glaciation of hunt bluff did not start until 21 1 5 8 ka probably during the last glacial maximum and finished by 9 6 0 9 ka at the same time as ice sheet retreat from the continental shelf was complete thickening of ice at hunt bluff most likely post dated the maximum extent of grounded ice on the outer continental shelf flow re organisation provides a possible explanation for this with the date for onset of ice cover at hunt bluff providing a minimum age for the timing of convergence of the dotson and getz tributaries to form a single palaeo ice stream this is the first time that timing of onset of ice cover has been constrained in the amundsen sea embayment

west antarctic ice sheet retreat driven by holocene warm water incursions glaciological and oceanographic observations coupled with numerical models show that warm circumpolar deep water cdw incursions onto the west antarctic continental shelf cause melting of the undersides of floating ice shelves because these ice shelves buttress glaciers feeding into them their ocean induced thinning is driving antarctic ice sheet retreat today here we present a multi proxy data based reconstruction of variability in cdw inflow to the amundsen sea sector the most vulnerable part of the west antarctic ice sheet during the holocene epoch from 11 7 thousand years ago to the present the chemical compositions of foraminifer shells and benthic foraminifer assemblages in marine sediments indicate that enhanced cdw upwelling controlled by the latitudinal position of the southern hemisphere westerly winds forced deglaciation of this sector from at least 10 400 years ago until 7 500 years ago when an ice shelf collapse may have caused rapid ice sheet thinning further upstream and since the 1940s these results increase confidence in the predictive capability of current ice sheet models

pb isotopic compositions of archean komatiites and sulfides abstract pb isotopic compositions were measured in komatiites and other volcanic rocks from several archean greenstone belts and in magmatic and exhalative sulfides associated with these volcanics thin komatiite flows generally yielded pb pb isochron ages consistent with other geochronological or geological information but thicker basaltic flows gave ages that are too young these results suggest that the thin komatiite flows were hydrothermally altered during eruption and that this event is recorded by the pb pb age thicker basaltic flows resisted alteration during eruption but were affected by later events their pb pb ages probably correspond to the time of metamorphism and not to the time of emplacement magmatic and exhalative sulfides from the abitibi belt in canada fall on the same trend in the 207 pb 204 pb 206 pb 204 pb diagram as komatiites basalts sedimentary rocks and k feldspars from granites the calculated 1 value is 7 8 0 1 a similar suite of rocks and minerals from the kambalda area in australia has distinctly higher 207 pb 204 pb and a higher 1 of 8 3 nd isotopic data demonstrate that some kambalda rocks are highly contaminated with older continental crust the differences in initial pb isotopic compositions are attributed to the presence or absence and the age of underlying granitoid basement kambalda greenstones are underlain and contaminated by old 3 0 ga granitoid crust but significantly older granitoid basement appears absent from the abitibi belt the pb isotopic compositions of kambalda greenstones are dominated by the crustal component and do not represent the composition of the mantle source of these rocks the abitibi greenstones are not highly influenced by crustal contamination and their pb isotopic compositions may reflect those of archean mantle

neoarchean crustal recycling and mantle metasomatism hf nd pb o isotope evidence from sanukitoids of the fennoscandian shield abstract the growing knowledge of the geological record reveals evidence of crustal recycling in the archean eon in order to assess the role of recycling of crust into the mantle we have analyzed radiogenic hf in zircon as well as whole rock nd and pb isotope compositions of mantle derived granitoids of the sanukitoid series in the neoarchean karelian province fennoscandian shield homogeneous initial hf and nd values suggest that sanukitoids are derived from well mixed source s limited scatter in the initial pb isotope data within intrusions as well as in other radiogenic isotopes point to effective mixing processes in the mantle o isotope data for zircon from karelian sanukitoids indicate a dominant mantle source 18 o 5 5 6 1 with minor contributions from sources with elevated 18 o 7 0 to 8 5 such as sediments and altered upper oceanic crust our results suggest that the parental melts of sanukitoids are mixtures of depleted mantle and recycled crustal components a slab breakoff at the end stage of subduction is a plausible trigger mechanism for sanukitoid magmatism and it explains variable contributions from different 18 o sources

how to evaluate model derived deglaciation chronologies a case study using antarctica abstract we address the evaluation of model derived deglaciation chronologies using observational data the study has been undertaken using the antarctic ice sheet as the focus however the issues addressed and the methods described are applicable to the evaluation of other ice sheet reconstructions within this context we present an initial database of observational data for constraining antarctic ice sheet deglaciation chronologies anticedat the database constrains present day ice sheet configuration relative sea level past ice thickness and grounding line retreat and is made available as a spreadsheet we consider the non trivial translation of an observation to model applicable constraint data through observational error models and data weighting we address the main issues that arise from evaluating modelled reconstructions generated with a glacial systems model that has like all such models inherent structural deficiencies using heterogeneous observational data the evaluation method uses observational error models to quantify model to observational misfits that also incorporate the measurement uncertainties for each data point the data point misfits are adjusted by data weighting and combined to generate a score for the model output as such different chronologies can be evaluated and compared we examine the sensitivity of the score to the different data types and associated weighting using model derived reconstructions in addition suggested reporting requirements are proposed to ensure that maximum value can be extracted from observational data

glacial retreat in the amundsen sea sector west antarctica first cosmogenic evidence from central pine island bay and the kohler range the amundsen sea embayment of west antarctica hosts one of the most rapidly changing sectors of the west antarctic ice sheet with the fastest flowing ice streams in antarctica the region around pine island bay is characterized by rapid ice sheet thinning and grounding line retreat published surface exposure data are limited to a few isolated nunataks making it difficult to assess the long term deglacial history of the area to address this we correlate existing records of lateral ice stream retreat from marine sediment cores with onshore glacial thinning in two key areas of eastern marie byrd land the kohler range and pine island bay our 10be surface exposure ages are the first from the isolated kohler range and show that the nunataks there became ice free between 8 6 and 12 6 ka this implies a minimum long term average thinning rate of 3 3 0 3 cm yr which is one order of magnitude lower than recent rates based on satellite data we also present pre to early holocene 10be surface exposure ages from two islands located approximately 80 km downstream of the pine island glacier ice shelf front to constrain the lateral deglacial history in the pine island bay area this study provides insight into the significance of local ice sheet variations and suggests that the post lgm history in the amundsen sea sector was characterized by glacial thinning as well as lateral retreat in pre to early holocene times

grounding line retreat of pope smith and kohler glaciers west antarctica measured with sentinel 1a radar interferometry data we employ sentinel 1a c band satellite radar interferometry data in terrain observation with progressive scans mode to map the grounding line and ice velocity of pope smith and kohler glaciers in west antarctica for the years 2014 2016 and compare the results with those obtained using earth remote sensing satellites ers 1 2 in 1992 1996 and 2011 we observe an ongoing rapid grounding line retreat of smith at 2 km yr 40 km since 1996 an 11 km retreat of pope 0 5 km yr and a 2 km readvance of kohler since 2011 the variability in glacier retreat is consistent with the distribution of basal slopes i e fast along retrograde beds and slow along prograde beds we find that several pinning points holding dotson and crosson ice shelves disappeared since 1996 due to ice shelf thinning which signal the ongoing weakening of these ice shelves overall the results indicate that ice shelf and glacier retreat in this sector remain unabated

geochemical and nd isotopic constraints for the origin of late archean turbidites from the yellowknife area northwest territories canada abstract a detailed geochemical and isotopic study of late archean turbidites and volcanic rocks from the yellowknife area slave province was undertaken to constrain the nature of exposed crust at the time of 2 6 to 2 7 ga crustal consolidation the nd t values of the volcanic rocks range from 1 7 to 4 4 this variation can be produced by assimilation of pre 2 8 ga basement by a depleted mantle derived magma possibly followed by fractional crystallization the turbidites are typically metamorphosed to greenschist to amphibolite facies and where metamorphosed to greenschist facies different units of bouma sequence can be observed the different units of bouma sequence were sampled and analyzed separately to evaluate the possible differences in geochemical and isotopic signatures the geochemical data presented here is in accord with the previously proposed model that argues for a mixture of 20 mafic intermediate volcanic rocks 55 felsic volcanic rocks and 25 granitic rocks as a source of these turbidites however our revised calculation with the new data presented here argues for 1 to 2 input from an ultramafic source as well as somewhat higher input from mafic intermediate volcanic sources in the upper shale units compared to the lower sand units the nd t values of the turbidites generally are lower in the upper shale units compared to the lower sand units detailed inspection of trace element data suggest that this is not an artifact of rare earth element rich heavy minerals concentrating in the lower sand units of the turbidites but rather is a result of unmixing of detritus with different nd t values during sediment transportation and deposition the upper shale units of the turbidites are isotopically compatible with a derivation mainly from crustally contaminated volcanic rocks similar to those exposed in the yellowknife area the lower sand units contain a higher proportion of westerly derived plutonic rock detritus characterized by higher nd t suggesting that there are area s west of yellowknife not underlain by older 2 8 4 0 ga basement the trace element characteristics of these turbidites i e cr ni la th sc eu eu and gd n yb n are distinct from those of typical post archean turbidites this observation is consistent with the models that predict that the chemical composition of the upper continental crust was slightly different in the archean compared to post archean time

neodymium isotopic evidence for the tectonic assembly of late archean crust in the slave province northwest canada the ca 2 7 2 5 ga slave province is a granitegreenstone terrane comprising deformed sedimentary and subordinate volcanic belts extensively intruded by granitoid rocks the nd isotopic data are reported for 58 samples of supracrustal and granitoid rocks exposed along a 400 km east west transect at 65 n across the structural grain of the province initial nd values reveal distinctly different crustal sources in the eastern compared to the western parts of the province as expected from tectonic assembly of the province through accretion of juvenile crust to older continental crust supracrustal sequences ca 2 71 2 65 ga from the central and eastern parts of the province have positive nd 1 values 0 3 to 3 6 consistent with juvenile sources and formation remote from significantly older crust syn to late deformation ca 2 63 2 60 ga mantle derived diorites and related tonalites type i from the central and eastern parts of the province have similar initial nd values 0 1 to 2 7 in contrast samples from the westernmost plutons which intrude exposed pre 3 1 ga crust have much lower nd 1 values 1 0 to4 6 suggesting contamination of these magmas by older crust the nd 1 values of post deformation granites s s type ii also vary systematically across the province values for granites west of longitude 110 30 w range from 0 2 to 5 3 those to the east range from 0 6 to 3 7 these data suggest mixed crustal sources dominated by mid to early archean material nd 2 6 to 17 at 2 6 ga for the western granitoid rocks and juvenile sources for the eastern granites the nd isotopic data are consistent with the geology of the province in that exposures of mid to early archean crustal rocks predating the principal 2 7 2 5 ga orogenic event are restricted to the western part of the province the asymmetric pattern defined by the nd isotopic data indicates the presence of distinct crustal rocks beneath the slave province similar isotopic variations observed across phanerozoic collisional orogens have been interpreted to reflect tectonic assembly of crust by accretion of juvenile crustal terranes to an older continental margin this process may also have been an important mechanism in the cratonization of the slave province

evolution of an archean basement complex and its autochthonous cover southern slave province canada abstract the sleepy dragon complex sdc is one of several antiformal exposures of a 4 0 2 85 ga basement block underlying the west central region of the archean slave province northwestern canada this basement block the central slave basement complex csbc is overlain by an autochthonous to locally parautochthonous dominantly quartz rich clastic sequence the 2 85 2 80 ga central slave cover group together these units comprised a fundamental building block during neoarchean growth and assembly of the slave craton the well exposed patterson lake morose lake area represents a type locality for study of the csbc central slave cover group and overlying rocks of the yellowknife supergroup field mapping and u pb geochronology employing both thermal ionisation mass spectrometry tims and sensitive high resolution ion microprobe shrimp document a prolonged crustal history oldest intact basement units consist of foliated to gneissic tonalite dated in two places at 2955 12 ma and 2944 9 ma indirect evidence for a ca 3150 ma basement component is obtained both from xenocrystic zircon in tonalite and detrital zircons from the overlying central slave cover group which here consists of the patterson lake formation a granite boulder from this formation is dated at 2934 3 ma and may be locally derived a sheared mafic volcanic unit along the basement cover contact contains 2942 3 ma metamorphic titanite which provides a minimum deposition age this volcanic unit is therefore also a part of the basement complex younger events in the sdc mainly reflect tonalite trondhjemite granodiorite plutonism during construction of the overlying cameron river and beaulieu river greenstone belts of the yellowknife supergroup a unit of k feldspar megacrystic granodiorite is dated at 2726 3 ma and tonalite and granodiorite bodies located toward the centre of the sdc have primary crystallization ages of 2683 3 2 ma 2677 2 ma and 2672 7 6 ma the core of the southern sdc is occupied by a late tectonic to post tectonic granite pluton previously dated at 2586 2 ma an intravolcanic unconformity along the southern margin of the sdc developed in part on 2 7 ga granodiorite is constrained by existing u pb data to have formed sometime between ca 2685 2660 ma an abundance of synvolcanic plutons of similar age and the presence of a 3 km thick mafic volcanic sequence overlying a less dense magmatically active felsic crust suggest that gravitational instability and magmatic diapirism may have played important roles in doming and uplift of the sdc prior to regional d 1 deformation the younger toward centre zonation of plutonic rocks in the sdc is documented in other domal granite gneiss complexes of archean age e g pilbara craton western australia where it is attributed to upward and outward displacement of older crust during repeated magma channeling into a growing mid crustal dome in the west central slave craton such a process might account for the regional presence of local unconformities of apparently similar age the domal basement plutonic complexes would have been further amplified after 2660 ma during the development of regional f 1 and f 2 fold belts

transient simulation of last deglaciation with a new mechanism for bolling allerod warming we conducted the first synchronously coupled atmosphere ocean general circulation model simulation from the last glacial maximum to the bolling allerod ba warming our model reproduces several major features of the deglacial climate evolution suggesting a good agreement in climate sensitivity between the model and observations in particular our model simulates the abrupt ba warming as a transient response of the atlantic meridional overturning circulation amoc to a sudden termination of freshwater discharge to the north atlantic before the ba in contrast to previous mechanisms that invoke amoc multiple equilibrium and southern hemisphere climate forcing we propose that the ba transition is caused by the superposition of climatic responses to the transient co 2 forcing the amoc recovery from heinrich event 1 and an amoc overshoot

geochemistry of the archean yellowknife supergroup the archean yellowknife supergroup slave structural province canada is composed of a thick sequence of supracrustal rocks which differs from most archean greenstone belts in that it contains a large proportion 80 of sedimentary rocks felsic volcanics of the banting formation are characterized by hree depletion without eu anomalies indicating an origin by small degrees of partial melting of a mafic source with minor garnet in the residua granitic rocks include synkinematic granites hree depleted low 87sr86sr i post kinematic granites negative eu anomalies high 87sr86sr i and granitic gneisses with ree patterns similar to the post kinematic granites sedimentary rocks turbidites of the burwash and walsh formations have similar chemical compositions and were derived from 20 mafic intermediate volcanics 55 felsic volcanics and 25 granitic rocks jackson lake formation lithic wackes can be divided into two groups with group a derived from 50 mafic intermediate volcanics and 50 felsic volcanics and group b characterized by hree depletion derived almost exclusively from felsic volcanics ree patterns of yellowknife sedimentary rocks are similar to other archean sedimentary ree patterns although they have higher lanybn these patterns differ significantly from typical post archean sedimentary ree patterns supporting the idea that archean exposed crust had a different composition than the present day exposed crust

the antarctica component of postglacial rebound model ice 6g_c vm5a based on gps positioning exposure age dating of ice thicknesses and relative sea level histories

space geodesy constrains ice age terminal deglaciation the global ice 6g_c vm5a model a new model of the last deglaciation event of the late quaternary ice age is here described and denoted as ice 6g c vm5a it differs from previously published models in this sequence in that it has been explicitly refined by applying all of the available global positioning system gps measurements of vertical motion of the crust that may be brought to bear to constrain the thickness of local ice cover as well as the timing of its removal additional space geodetic constraints have also been applied to specify the reference frame within which the gps data are described the focus of the paper is upon the three main regions of last glacial maximum ice cover namely north america northwestern europe eurasia and antarctica although greenland and the british isles will also be included if peripherally in the discussion in each of the three major regions the model predictions of the time rate of change of the gravitational field are also compared to that being measured by the gravity recovery and climate experiment satellites as an independent means of verifying the improvement of the model achieved by applying the gps constraints several aspects of the global characteristics of this new model are also discussed including the nature of relative sea level history predictions at far field locations in particular the caribbean island of barbados from which especially high quality records of postglacial sea level change are available but which records were not employed in the development of the model although ice 6g c vm5a is a significant improvement insofar as the most recently available gps observations are concerned comparison of model predictions with such far field relative sea level histories enables us to identify a series of additional improvements that should follow from a further stage of model iteration

ice sheet grounding line dynamics steady states stability and hysteresis 1 the ice sheet ice shelf transition zone plays an important role in controlling marine ice sheet dynamics as it determines the rate at which ice flows out of the grounded part of the ice sheet together with accumulation this outflow is the main control on the mass balance of the grounded sheet in this paper we verify the results of a boundary layer theory for ice flux in the transition zone against numerical solutions that are able to resolve the transition zone very close agreement is obtained and grid refinement in the transition zone is identified as a critical component in obtaining reliable numerical results the boundary layer theory confirms that ice flux through the grounding line in a two dimensional sheet shelf system increases sharply with ice thickness at the grounding line this result is then applied to the large scale dynamics of a marine ice sheet our principal results are that 1 marine ice sheets do not exhibit neutral equilibrium but have well defined discrete equilibrium profiles 2 steady grounding lines cannot be stable on reverse bed slopes and 3 marine ice sheets with overdeepened beds can undergo hysteresis under variations in sea level accumulation rate basal slipperiness and ice viscosity this hysteretic behavior can in principle explain the retreat of the west antarctic ice sheet following the last glacial maximum and may play a role in the dynamics of heinrich events

lithosphere development in the slave craton a linked crustal and mantle perspective the late tectonic evolution of the slave craton involves extensive magmatism deformation and high temperature low pressure ht lp metamorphism we argue that the nature of these tectonic events is difficult to reconcile with early pre 2 7 ga development and preservation of a thick tectosphere and suggest that crust mantle coupling and stabilization occurred only late in the orogenic development of the craton the extent and repetitiveness of the tectonic reworking documented within the mesoarchean basement complex of the western slave together with the development of large volume extensional mafic magmatism at 2 7 ga within the basement complex argue against preservation of a widespread thick cool mesoarchean tectosphere beneath the western slave craton prior to neoarchean tectonism broad scale geological and geophysical features of the slave craton including orientation of an early f1 fold belt distribution of ca 2 63 2 62 ga plutonic rocks and the distribution of geochemical petrological and geophysical domains within the mantle lithosphere collectively highlight the importance of an ne sw structural grain to the craton these trends are oblique to the earlier ca 2 7 ga north south trending boundary between mesoarchean and neoarchean crustal domains and are interpreted to represent a younger structural feature imposed during northwest or southeast vergent tectonism at ca 2 64 2 61 ga extensive plutonism in part mantle derived crustal melting and associated ht lp metamorphism argue for widespread mantle heat input to the crust a feature most consistent with thin 100 km lithosphere at that time we propose that the mantle lithosphere developed by tectonic imbrication of one or more slabs subducted beneath the craton at the time of development of the d1 structural grain producing the early 2 63 2 62 ga arc like plutonic rocks subsequent collision external to the present craton boundaries possibly accompanied by partial delamination of some of the underthrust lithosphere produced widespread deformation d2 and granite plutonism throughout the province at 2 6 2 58 ga an implication of this model is that diamond formation in the slave should be neoarchean in age d 2003 elsevier b v all rights reserved

ice flow of the antarctic ice sheet we present a reference comprehensive high resolution digital mosaic of ice motion in antarctica assembled from multiple satellite interferometric synthetic aperture radar data acquired during the international polar year 2007 to 2009 the data reveal widespread patterned enhanced flow with tributary glaciers reaching hundreds to thousands of kilometers inland over the entire continent this view of ice sheet motion emphasizes the importance of basal slip dominated tributary flow over deformation dominated ice sheet flow redefines our understanding of ice sheet dynamics and has far reaching implications for the reconstruction and prediction of ice sheet evolution

ice stream stability on a reverse bed slope marine ice streams whose beds deepen inland are thought to be inherently unstable numerical modelling of the maguerite bay ice stream retreat in west antarctica since the last glacial maximum suggests that an ice stream can stabilize on an inland sloping bed owing to increased lateral drag where the ice stream narrows marine based ice streams whose beds deepen inland are thought to be inherently unstable1 2 3 this instability is of particular concern because significant portions of the marine based west antarctic and greenland ice sheets are losing mass and their retreat could contribute significantly to future sea level rise4 5 6 7 however the present understanding of ice stream stability is limited by observational records that are too short to resolve multi decadal to millennial scale behaviour or to validate numerical models8 here we present a dynamic numerical simulation of antarctic ice stream retreat since the last glacial maximum lgm constrained by geophysical data whose behaviour is consistent with the geomorphological record we find that retreat of marguerite bay ice stream following the lgm was highly nonlinear and was interrupted by stabilizations on a reverse sloping bed where theory predicts rapid unstable retreat we demonstrate that these transient stabilizations were caused by enhanced lateral drag as the ice stream narrowed we conclude that as well as bed topography ice stream width and long term retreat history are crucial for understanding decadal to centennial scale ice stream behaviour and marine ice sheet vulnerability

first exposure ages from the amundsen sea embayment west antarctica the late quaternary context for recent thinning of pine island smith and pope glaciers dramatic changes acceleration thinning and grounding line retreat of major ice streams in the amundsen sea sector of the west antarctic ice sheet wais have been observed during the past two decades but the millennial scale context for these changes is not yet known we present the first surface exposure ages recording thinning of pine island smith and pope glaciers which all drain into the amundsen sea from these we infer progressive thinning of pine island glacier at an average rate of 3 8 0 3 cm yr 1 for at least the past 4 7 k y and of smith and pope glaciers at 2 3 0 2 cm yr 1 over the past 14 5 k y these rates are more than an order of magnitude lower than the similar to 1 6 m yr 1 recorded by satellite altimetry for pine island glacier in the period 1992 1996 similarly low long term rates 2 5 9 ern yr 1 since 10 ka have been reported farther west in the ford ranges marie byrd land but in that area the same rates of thinning continue to the present day our data provide the first evidence that puts into context recent rates of thinning of the wais in the amundsen sea embayment and demonstrates that these are unusually rapid the data also provide much needed constraints for ice sheet models which are the primary tool for predicting the future behavior of the wais and its likely contribution to sea level rise

post lgm deglaciation in pine island bay west antarctica abstract to date understanding of ice sheet retreat within pine island bay pib following the last glacial maximum lgm was based on seven radiocarbon dates and only fragmentary seafloor geomorphic evidence during the austral summer 2009 2010 restricted sea ice cover allowed for the collection of 27 sediment cores from the outer pib trough region combining these cores with data from prior cruises over 133 cores have been used to conduct a detailed sedimentological facies analysis these results augmented by 23 new radiocarbon dates are used to reconstruct the post lgm deglacial history of pib our results record a clear retreat stratigraphy in pib composed of from top to base terrigenous sandy silt distal glacimarine pebbly sandy mud ice proximal glacimarine and till initial retreat from the outer continental shelf began shortly after the lgm and before 16 4 k cal yr bp as a likely response to rising sea level bedforms in outer pib document episodic retreat in the form of back stepping grounding zone wedges and are associated with proximal glacimarine sediments a sub ice shelf facies is observed in central pib and spans 12 3 10 6 k cal yr bp it is possible that widespread impingement of warm water onto the continental shelf caused an abrupt and widespread change from sub ice shelf sedimentation to distal glacimarine sedimentation dominated by widespread dispersal of terrigenous silt between 7 8 and 7 0 k cal yr bp the final phase of retreat ended before 1 3 k cal yr bp when the grounding line migrated to a location near the current ice margin

contributions and unrealized potential contributions of cosmogenic nuclide exposure dating to glacier chronology 1990 2010 abstract this paper reviews the application of cosmogenic nuclide exposure dating to glacier chronology exposure dating of glacial landforms has made an outsize impact on this field because the technique filled an obvious need that had already been recognized by glacial geologists by now hundreds of studies have used cosmogenic nuclide exposure dating to date glacial deposits and in fact it is rare to find a study of glacial geology or glacier chronology or any paleoclimate synthesis that makes use of such studies that does not involve exposure dating these developments have resulted in major contributions to glacier chronology and paleoclimate in particular i reconstructing antarctic ice sheet change ii establishing the chronology of late pleistocene and holocene glacier change in mountain regions where it was previously unknown iii establishing the broad chronological outlines of mountain glaciations prior to the last glacial maximum and iv gaining insight into subglacial erosional processes through the observation that many glaciated surfaces preserve cosmogenic nuclide inventories from long past ice free periods as well as the present one an important potential future contribution will be the application of the large data set of exposure dated glacier chronologies to better understand global and regional climate dynamics during lateglacial and holocene millennial scale climate changes however this contribution cannot be realized without significant progress in two areas i understanding and accounting for geologic processes that cause apparent exposure ages on glacial landforms to differ from the true age of the landform and ii minimizing systematic uncertainties in exposure ages that stem from cosmogenic nuclide production rate estimates and scaling schemes at present there exists an enormous data set of exposure ages on glacial deposits but these data cannot be used to their full potential in paleoclimate syntheses due to an inadequate understanding of geologic scatter and production rate uncertainties the intent of this paper is to highlight this situation and suggest some strategies for realizing this potential

water saturated and undersaturated melting of metaluminous and peraluminous crustal compositions at 10 kb evidence for the origin of silicic magmas in the taupo volcanic zone new zealand and other occurrences

holocene deglaciation of marie byrd land west antarctica surface exposure ages of glacial deposits in the ford ranges of western marie byrd land indicate continuous thinning of the west antarctic ice sheet by more than 700 meters near the coast throughout the past 10 000 years deglaciation lagged the disappearance of ice sheets in the northern hemisphere by thousands of years and may still be under way these results provide further evidence that parts of the west antarctic ice sheet are on a long term trajectory of decline west antarctic melting contributed water to the oceans in the late holocene and may continue to do so in the future

temporal distribution of granitoid plutonic rocks in the archean slave province northwest canadian shield granitoid rocks in the slave province consist of 4 0 2 8 ga granitoid gneisses that predate the 2 71 2 65 ga volcanic and turbiditic rocks of the yellowknife supergroup and 2 70 2 58 ga granitoid plutons that intrude the yellowknife supergroup u pb zircon ages and nd and pb isotopic data indicate that the older granitoids are restricted to the western part of the slave province granitoid gneisses in the eastern slave province previously suspected to predate the yellowknife supergroup are similar in age to the volcanic rocks in this paper the results of a detailed geochronological study of plutonic rocks of the contwoyto lake nose lake area of the northcentral slave province are reviewed and integrated with the available age data base for plutonic rocks elsewhere in the slave province the data indicate that the timing of the later 2 7 ga plutonism is bimodal consisting of two distinct magmatic periods separated by approximately 20 ma of apparent magmatic quiescence the first period was synvolc

geology of mount murphy volcano an 8 m y history of interaction between a rift volcano and the west antarctic ice sheet volcanic rocks glaciogenic sediments and recycled marine microfossils at mount murphy provide evidence for large scale fluctuations in the mass of the west antarctic ice sheet during neogene time mount murphy is a large shield volcano with an atypical structure its basal unit consists of alternating subaquatic and subaerial rock types through a stratigraphic interval of at least 300 m suggesting that ice level changed several times during shield building much larger changes are suggested by an outcrop of glacial lake sediment near the volcano summit 1 300 m above present ice level it contains an assemblage of recycled marine microfossils derived from marine basins in the interior of west antarctica they suggest multiple intervals of near complete deglaciation in west antarctica between about 24 and 3 5 ma the microfossils were probably deposited by meltwater from the ice sheet during a much higher stand of ice in late pliocene time it has long been suspected that the west antarctic ice sheet is unstable and vulnerable to greenhouse warming because it is grounded more than 1 000 m below sea level our results provide field evidence consistent with that proposal more than the west antarctic ice sheet may be involved however the nature of the evidence and timing of events at mount murphy are similar to the record in east antarctica suggesting continent wide synchroneity of the major glacial and interglacial intervals this conflicts with interpretations of marine data which suggest a shorter history for the west antarctic ice sheet and a more stable history for the entire ice sheet

neoarchaean banded iron formation within a 2620 ma turbidite dominated deep water basin slave craton nw canada abstract banded iron formations bifs are iron and silica rich chemical metasedimentary rocks that span much of the archaean and early palaeoproterozoic rock record given that bifs precipitated from ambient seawater they provide an important archive of earth s ancient marine environments within the slave craton occurs one of the largest and well preserved achaean turbidite basins in the world interstratified within the greywacke mudstone turbidites is bif that is commonly associated with ferruginous sediments and locally with a felsic tuff bed field investigations were conducted at four bif locations from west to east across the slave craton these bifs are composed of micro and mesobands of quartz and magnetite with various amounts of metamorphic grunerite ferro actinolite ferro hornblende as well as greenalite and stilpnomelane the latter two are interpreted as retrograde metamorphic phases as the bif was deposited in a deep water turbidite dominated basin s the chemical component was occasionally diluted by clastic detritus as illustrated by highly variable al 2 o 3 content and al 2 o 3 tio 2 and cr ni that are similar to the clastic sediments on a layer by layer basis the iron content in the bif ranges from as low as 4 wt and up to 75 wt fe 2 o 3 whereas the sio 2 content ranges from 20 wt to as high as 94 wt this together with a hyperbolic germanium silicon distribution suggests that continentally sourced silica comprised the background precipitation which was periodically interrupted by pulses of hydrothermally sourced iron high temperature hydrothermal fluids contributed with an average shale normalized positive eu anomaly of 1 92 which indicates input of reduced eu 2 to the ambient seawater the bif shows characteristic aqueous fluid derived rare earths yttrium fractionation pr yb 3 to ce 4 the age and trace element signature of the felsic tuff is similar to arc type volcanic rocks potentially linking the bif and turbidite basin s to the defeat suite plutons ca 2630 2620 ma the slow chemical rainout behind bif deposition presumably took place on the basin plain adjacent to a steep submarine ramp where turbidite currents and slump generated debris flows sourced and triggered by the tectonically active arc terrain episodically delivered clastic sediment

geochemical and nd pb o isotope systematics of granites from the taltson magmatic zone ne alberta implications for early proterozoic tectonics in western laurentia the early proterozoic taltson magmatic zone tmz comprises the southern part of the taltson thelon orogenic belt of northwestern canada the tmz is dominated by 1 96 1 99 ga i type and 1 93 1 95 ga s type suites of granitoids previous workers attributed the formation of the i type granitoids to subduction of oceanic crust beneath the churchill craton in an andean type setting and generation of the s type granitoids to subsequent collision between the churchill craton and the buffalo head terrane to test this hypothesis we carried out a geochemical and isotopic nd pb and o study of granitoids and associated metasedimentary rocks and basement gneisses from the southern part of the tmz both i and s type suites of granitoids are characterized by relatively high sio2 content with the great majority of samples containing more than 64 wt sio2 the two suites also have strongly crustal end t values of 3 8 to 9 8 and an average tdm of 2 8 ga the initial pb isotope composition of the granitoids as determined from leached feldspar mineral separates forms a linear array that is bracketed by the isotopic compositions of basement orthogneisses and metasediments similarly 18o values of the granitoids overlap those of the orthogneisses and metasediments these isotopic data are consistent with derivation of the granitoids from a mixture of metaigneous and metasedimentary source rocks similar to those presently exposed in the tmz importantly the data suggest that both i and s type granitoids of the southern tmz had an exclusively intra crustal origin unlike phanerozoic examples of subduction related magmatism where a significant mantle contribution is easily recognized therefore we argue that the existing andean type model for early tmz magmatism is not viable the tmz granitoids show geochemical and isotopic similarities to the cordilleran interior granitoids of western north america which formed in response to crustal thickening in the distant hinterland of a convergent plate margin we propose that these hinterland granitoids are a more appropriate analogue for tmz magmatism than are the granitoids of plate margins the tectonic implication of this proposal is that the taltson thelon orogenic belt does not mark the location of the plate boundary in western laurentia at 1 9 2 0 ga rather the tectonic setting of the tmz at that time may have been more akin to the present day intra continental mountain belts of central asia e g tian shan mountains where compressional tectonism and crustal thickening are occurring well inboard from a convergent plate margin

widespread rapid grounding line retreat of pine island thwaites smith and kohler glaciers west antarctica from 1992 to 2011 we measure the grounding line retreat of glaciers draining the amundsen sea sector of west antarctica using earth remote sensing ers 1 2 satellite radar interferometry from 1992 to 2011 pine island glacier retreated 31 km at its center with most retreat in 2005 2009 when the glacier ungrounded from its ice plain thwaites glacier retreated 14 km along its fast flow core and 1 to 9 km along the sides haynes glacier retreated 10 km along its flanks smith kohler glaciers retreated the most 35 km along its ice plain and its ice shelf pinning points are vanishing these rapid retreats proceed along regions of retrograde bed elevation mapped at a high spatial resolution using a mass conservation technique that removes residual ambiguities from prior mappings upstream of the 2011 grounding line positions we find no major bed obstacle that would prevent the glaciers from further retreat and draw down the entire basin 2014 american geophysical union all rights reserved

antarctic grounding line mapping from differential satellite radar interferometry 1 the delineation of an ice sheet grounding line i e the transition boundary where ice detaches from the bed and becomes afloat in the ocean is critical to ice sheet mass budget calculations numerical modeling of ice sheet dynamics ice ocean interactions oceanic tides and subglacial environments here we present 15 years of comprehensive high resolution mapping of grounding lines in antarctica using differential satellite synthetic aperture radar interferometry dinsar data from the earth remote sensing satellites 1 2 ers 1 2 radarsat 1 and 2 and the advanced land observing system alos palsar for years 1994 to 2009 dinsar directly measures the vertical motion of floating ice shelves in response to tidal oceanic forcing with millimeter precision at a sample spacing better than 50 m simultaneously over areas several 100 km wide in contrast with earlier methods that detect abrupt changes in surface slope in satellite visible imagery or altimetry data on stagnant and slow moving areas we find that breaks in surface slope are reliable indicators of grounding lines but on most fast moving glaciers and ice streams our dinsar results reveal that prior mappings have positioning errors ranging from a few km to over 100 km a better agreement is found with icesat s data also based on measurements of vertical motion but with a detection noise one order of magnitude larger than with dinsar overall the dinsar mapping of antarctic grounding lines completely redefines the coastline of antarctica

two distinct shortening events during late archean orogeny in the west central slave province northwest territories canada late archean structures in the west central part of the slave province formed during two separate orogenic events evidence for early folding and thrusting in an accretionary prism is confined to a narrow belt along the east margin of an older microcontinent the anton terrane in the west part of the province structures related to this event are overprinted by regional low pressure metamorphism subsequent shortening occurred in a continental arc setting in which folding and faulting was accompanied by calc alkaline magmatism and regional low pressure metamorphism although the entire region was affected the bulk of shortening during the second orogenic event occurred east of the early fold and thrust belt the first orogenic event produced a suture zone between old continental crust to the west and juvenile rocks to the east and during the second orogenic event rocks on either sides of the suture were tectonically underplated and intruded

u pb zircon ages and field relationships of archean basement and proterozoic intrusions south central wopmay orogen nwt implications for tectonic assignments new field and u pb zircon data from the south central wopmay orogen south of 65 n establish crystallization ages of basement and plutonic phases and bring to the forefront questions on the architecture of the orogen the complex and extensive 3300 2575 ma basement domain was derived from the adjacent slave craton paleoproterozoic supracrustal rocks dominated by an extensive pelitic psammitic sequence with basal arenite conglomerate and carbonate unconformably overlie this basement pre to post kinematic paleoproterozoic plutons intrude both basement and overlying strata and crystallized at 1877 1867 1862 and 1858 1850 ma the first pulse of plutonism constrains an early generation of metamorphism and deformation to younger than 1877 ma and is corroborated by metamorphic zircon in archean basement the interval from 1867 to 1862 ma brackets ductile deformation in granite and granodiorite intrusions although corresponding structure in the metasedimentary strata is equivocal post kinematic ca 1

origin and evolution of mid to late archean crust in the hanikahimajuk lake area slave province canada evidence from u pb geochronological geochemical and nd pb isotopic data abstract a geochronological major trace element geochemical and nd pb isotopic study of early to mid archean basement granitoids gneisses 2 68 ga greenstone belt and 2 58 2 62 ga late archean granitoids from the hanikahimajuk lake area of the northern slave province was undertaken to constrain the tectonomagmatic evolution of the north central area of slave province northwest territories canada the basement tonalites are precisely dated at 3378 1 ma by u pb zircon methods and are characterized by steeply fractionated ree patterns with no negative eu anomalies they were likely produced through partial melting of mafic crust at 0 8 gpa the endt of these rocks range from 0 3 to 2 1 implying involvement of even older crustal material in their genesis two different types of 2 68 ga mafic to intermediate volcanic rock are recognized in the hanikahimajuk lake area tholeiitic rocks with no ta nb depletions and calc alkaline rocks with slight negative ta nb depletions all of these rocks show clear evidence for contamination by the 3 4 ga basement tonalite thus precluding the possibility that some of these tholeiitic mafic rocks were produced in a mid oceanic ridge setting from the data presented here we conclude that the tholeiitic rocks were produced in a back arc setting likely similar to that of the japanese arc the calc alkaline rocks may either be a remnant of the volcanic arc or part of the same back arc system however the close proximity with the tholeiitic rocks in the field favours the latter possibility mafic to intermediate rocks with similar geochemical and isotopic characteristics are also found from the yellowknife area of the southwestern slave province and from the indin lake area of the westernmost slave province south of grenville lake area this observation may indicate that the mafic dominated volcanic belts of the western slave province may be a remnant of a single now collapsed and shortened back arc basin the existence of syn volcanic calc alkaline plutonic rocks with similar geochemical and isotopic features as the calc alkaline volcanic rocks supports this possibility the geochemistry of late archean granitoids suggests that they are produced mainly through partial melting of mafic igneous protoliths and clastic sedimentary rocks and possibly tonalitic rocks similarity in the end 2 6 values between these granitoids and the supracrustal rocks in the area is in accord with this model such protoliths are also consistent with the tectonic setting at which the 2 68 ga volcanic rocks were generated because mafic igneous rocks and clastic sedimentary rocks with lesser felsic igneous rocks are the major constituents of a back arc basin

accretion of the archean slave province continental rift models have long been applied to the archean slave province of northwestern canada a reassessment of these models shows them to be incompatible with observed geological relations and suggests that contractional tectonic models may be more appropriate than extensional ones regions composed of different rock suites e g orthogneisses vs mafic volcanics are separated by high strain zones recording large displacements it is proposed that the high strain zones separate four distinct terranes that have been juxtaposed during collisional orogenesis from west to east these include the anton terrane interpreted as an archean microcontinent the sleepy dragon terrane possibly an exhumed more eastern part of the anton terrane the contwoyto terrane a westward verging fold and thrust belt containing tectonic slivers of greenstone volcanics and the hackett river volcanic terrane interpreted as an archean island arc the contwoyto and hackett river terranes represent a paired accretionary prism and island arc system that formed above an east dipping subduction zone these collided with the anton microcontinent producing a basement nappe expressed as the sleepy dragon terrane during the main accretion event within the slave province the whole tectonic assemblage was intruded by late kinematic to postkinematic granitoids

pb isotopes a multi function tool for assessing tectonothermal events and crust mantle recycling at late archaean convergent margins abstract lead isotopes are a powerful tool in studying archaean granitoids and continental growth because 207pb is a sensitive indicator of involvement of lead from old archaean crustal sources this paper aims to demonstrate the significance and promote the use of lead isotopes by reviewing the theory concepts and traditions for interpretations of pb isotope data as well as by introducing a global model based on crust mantle recycling for discriminating late archaean convergent margin settings of granitoids case plots of k feldspar and whole rock data of ca 2 7 ga granitoids from several archaean cratons show that lead isotope systematics when interpreted in the correct geological context can be used for resolving many questions on crustal evolution from magmatic and tectonothermal history of individual intrusions to large scale tectonics and supercontinent correlations

the international bathymetric chart of the southern ocean ibcso version 1 0 a new bathymetric compilation covering circum antarctic waters the international bathymetric chart of the southern ocean ibcso version 1 0 is a new digital bathymetric model dbm portraying the seafloor of the circum antarctic waters south of 60 degrees s

drainage reorganization during breakup of pangea revealed by in situ pb isotopic analysis of detrital k feldspar pb isotopes in detrital k feldspar grains provide a powerful provenance tracer for feldspathic sandstones common pb isotopic compositions show broad hundred kilometer scale regional variation and this signature can survive weathering transport and diagenesis the feldspar pb signature can be measured rapidly using laser ablation multicollector inductively coupled plasma mass spectrometry la mc icp ms and careful targeting can avoid inclusions and altered regions within grains here we combine a new pb domain map for the circum north atlantic with detrital k feldspar pb isotopic data from triassic and jurassic sandstones from basins on the irish atlantic margin the pb isotopic compositions reveal otherwise cryptic feldspar populations that constrain the evolving drainage pattern triassic sandstones originated from distant archean and paleoproterozoic rocks probably in green land labrador and the rockall bank to the nw implying long 500 km transport across a nascent rift system later jurassic sandstones had a composite paleo and mesoproterozoic source in more proximal sources to the north

lead isotopic heterogeneities within alkali feldspars implications for the determination of initial lead isotopic compositions abstract the mobility of uranium during metamorphism and alteration places constraints on the use of lead isotopic compositions as petrogenetic tracers in older rocks the problem of uranium mobility may be partially circumvented by analysis of alkali feldspar which because of its low u pb ratio should have a lead isotopic composition similar to the initial lead isotopic composition of the rock previous leaching and volatilization experiments however have demonstrated that alkali feldspars contain radio genic lead which can be removed preferentially to nonradiogenic lead this paper reports on the results of progressive acid leaching of a single alkali feldspar phenocryst and on albitic perthites separated from the phenocryst this experiment suggests that a number of distinct lead isotopic reservoirs are present within the alkali feldspar including a nonradiogenic reservoir approximating the initial lead isotopic composition of the system a radiogenic reservoir corresponding to albitic exsolution lamellae within the alkali feldspar loosely bound labile lead presumably associated with uranium and thorium occupying fractures and finally a radiogenic component corresponding to unidentified uranium thorium rich phases the commonly observed decrease in radiogenicity with progressive leaching of alkali feldspars is interpreted to be largely the result of preferential leaching of the albitic perthites relative to the host alkali feldspar containing the nonradiogenic reservoir the results of this experiment confirm that initial lead isotopic compositions may be estimated from alkali feldspars through severe acid leaching however this experiment also illustrates the complexity of the lead isotopic systematics of alkali feldspars the determination of the initial lead isotopic composition of an alkali feldspar requires experimental procedures which recognize and compensate for the presence of multiple lead reservoirs within typical alkali feldspars

the bell lake group and anton complex a basement cover sequence beneath the archean yellowknife greenstone belt revealed and implicated in greenstone belt formation u pb zircon and monazite geochronology indicates the presence of a 2 93 ga basement anton complex and 2 8 ga cover sequence bell lake group beneath the 2 70 to 2 72 ga kam group of the yellowknife greenstone belt in the southern slave province the bell lake group comprises remnants of a quartzite rhyolite banded iron formation succession the ages of detrital zircons from the quartzite unit constrain its deposition to be younger than 2 92 and range up to 3 7 ga u pb ages for gneisses beneath the bell lake group are in excess of 2 93 ga and they are locally overlain by the quartzite the contact between the tholeiitic to calc alkaline kam group and the bell lake group is poorly exposed and equivocal approximately 6 km higher in the section the ranney chert a felsic volcaniclastic layer separates 2 70 2 72 ga tholeiitic to calc alkaline upper kam group rocks from a lower tholeiitic section containing sheeted dike complexes zircons from the ranney chert yield pb pb ages ranging from 2 72

chronostratigraphy of the hottah terrane and great bear magmatic zone of wopmay orogen canada and exploration of a terrane translation model the paleoproterozoic hottah terrane is the westernmost exposed bedrock of the canadian shield and a critical component for understanding the evolution of the wopmay orogen thirteen new high precision u pb zircon crystallization ages are presented and support field observations of a volcano plutonic continuum from hottah terrane through to the end of the great bear magmatism from 1950 to 1850 ma the new crystallization ages new geochemical data and newly published detrital zircon u pb data are used to challenge hitherto accepted models for the evolution of the hottah terrane as an exotic arc and microcontinent that arrived over a west dipping subduction zone and collided with the slave craton at ca 1 88 ga although the hottah terrane does have a tectonic history that is distinct from that of the neighbouring slave craton it shares a temporal history with a number of domains to the south and east domains that were tied to the slave craton by ca 1 97 ga it is interpreted herein that hottah terra

structural patterns and tectonic evolution of supracrustal domains in the archean slave province canada a new plate tectonic model accounts for lithological relations and regional structural patterns in late archean supracrustal domains of the slave province multiple dykes and pillowed mafic flows most common in the western part of the province suggest sea floor spreading the mafic volcanics lying in narrow homoclinal belts stratigraphically below more extensive turbidites are viewed as megaxenolithic remnants of oceanic crust preserved on the periphery of granitoid plutons and blocks of sialic crust closure of an oceanic basin was marked by emplacement of the granitoid plutons and coeval felsic volcanics the latter predominating over mafic volcanics in northeastern domains the felsic calc alkaline magmas may have risen from a shallow dipping subduction zone westerly verging folds westerly convex fold arcs and inclinations of later foliations particularly in lower level rocks of higher metamorphic grade are in accord with underthrusting to the east the zone of underthrusting shifted progressi

antarctic ice sheet palaeo thinning rates from vertical transects of cosmogenic exposure ages abstract constraining antarctic ice sheet evolution provides a way to validate numerical ice sheet models that aid predictions of sea level rise in this paper we collate cosmogenic exposure ages from exposed nunataks in antarctica that have been used or have the potential to be used to constrain rates of thinning of the antarctic ice sheets since the last glacial maximum we undertake quality control of the data and adopt a bayesian approach to outlier detection past thinning rates are modelled by monte carlo linear regression analysis we present thinning rates from 23 sites across antarctica the resulting data set is the first antarctic wide collation of past ice sheet thinning rates and provides an empirical starting point for future model data comparisons palaeo thinning rates are spatially variable with high rates appearing to correlate to areas of contemporary rapid changes on centennial timescales past thinning rates are comparable to modern day observations implying that modern day thinning has the potential to persist for centuries in numerous parts of antarctica the onset of abrupt thinning from all sites post dates meltwater pulse 1a suggesting that its source region s are distal to areas where exposure age constraints on ice surface geometry exist

rapid thinning of pine island glacier in the early holocene pine island glacier a major outlet of the west antarctic ice sheet has been undergoing rapid thinning and retreat for the past two decades here we demonstrate using glacial geological and geochronological data that pine island glacier also experienced rapid thinning during the early holocene around 8 000 years ago cosmogenic 10be concentrations in glacially transported rocks show that this thinning was sustained for decades to centuries at an average rate of more than 100 cm yr 1 comparable to contemporary thinning rates the most likely mechanism was a reduction in ice shelf buttressing our findings reveal that pine island glacier has experienced rapid thinning at least once in the past and that once set in motion rapid ice sheet changes in this region can persist for centuries

geology and geochemistry of an archean mafic dike complex in the chan formation basis for a revised plate tectonic model of the yellowknife greenstone belt an archean mafic dike complex in the chan formation at the base of the mafic volcanic section of the yellowknife greenstone belt consists of multiple metagabbro dikes and sills separated by screens of pillowed mafic volcanic rocks which are cut by a younger postvolcanic metadiabase dike swarm field relationships and geochemical characteristics are compatible with a comagmatic origin for the metagabbro and metavolcanic rocks that were fed through and deposited on an older rift related supracrustal sequence of the dwyer group the synvolcanic metagabbro dikes have extended the strike length of the volcanic section by at least 100 the mafic rocks of the chan formation are geochemically similar to mid ocean ridge basalt possibly with a minor subduction zone component preliminary end values for metagabbroic rocks are consistent with the derivation of magmas predominantly from a normal depleted mantle source the chan formation is interpreted to have formed in a marginal basin like setting adjacent to

the 1991 1996 natmap slave province project introduction the 11 papers contributed to this issue on the geology of the slave province are summarized and where appropriate links between data from the various contributions are emphasized in the discussion we provide a synthesis of the present knowledge base on pre yellowknife supergroup basement rocks throughout the slave province all available age data on mesoarchean and paleoarchean basement rocks are compiled and in a first attempt to seek regional correlations 10 events of crustal growth and or reworking are recognized spanning well over 1 billion years of earth s history the spatial distributions of these events across the craton are discussed finally we pose a series of important questions that remain and that may help guide future research

deglaciation of pope glacier implies widespread early holocene ice sheet thinning in the amundsen sea sector of antarctica abstract the amundsen sea sector of the antarctic ice sheet presently dominates the contribution from antarctica to sea level rise several large ice streams that currently drain the sector have experienced rapid flow acceleration grounding line retreat and thinning during the past few decades however little is known of their longer term millennial scale retreat history despite the reliance of several ice sheet and glacial isostatic adjustment models on such data for improving sea level prediction from this critical region this study investigates the timing and extent of surface lowering of one of those ice streams pope glacier since the last glacial maximum lgm using glacial geological evidence for former ice cover we present a new deglacial chronology for the glacier derived from surface exposure dating of glacially deposited cobbles and ice scoured bedrock from mount murphy and its surrounding peaks cosmogenic 10be exposure ages from 44 erratic cobbles and 5 bedrock samples and in situ 14c exposure ages from one erratic and 8 bedrock samples are predominantly in the range 5 5 16 ka although 10be inheritance from prior exposure is prevalent in some erratics and probably all bedrock samples none of the ages pre date the lgm from these results we infer that the surface of pope glacier lowered by 560 m during the early to mid holocene 9 6 ka at an average rate of 0 13 0 09 0 04 m yr 1 the lowering coincided with a period of enhanced upwelling of warm circumpolar deep water onto the continental shelf in the region a reduction in buttressing facilitated by such upwelling by an ice shelf that is thought to have spanned the embayment until 10 6 cal kyr bp could have triggered simultaneous early holocene thinning of pope glacier and glaciers elsewhere in the amundsen sea embayment

deglacial temperature history of west antarctica the most recent glacial to interglacial transition constitutes a remarkable natural experiment for learning how earth s climate responds to various forcings including a rise in atmospheric co2 this transition has left a direct thermal remnant in the polar ice sheets where the exceptional purity and continual accumulation of ice permit analyses not possible in other settings for antarctica the deglacial warming has previously been constrained only by the water isotopic composition in ice cores without an absolute thermometric assessment of the isotopes sensitivity to temperature to overcome this limitation we measured temperatures in a deep borehole and analyzed them together with ice core data to reconstruct the surface temperature history of west antarctica the deglacial warming was formula see text c approximately two to three times the global average in agreement with theoretical expectations for antarctic amplification of planetary temperature changes consistent with evidence from glacier retreat in southern hemisphere mountain ranges the antarctic warming was mostly completed by 15 kybp several millennia earlier than in the northern hemisphere these results constrain the role of variable oceanic heat transport between hemispheres during deglaciation and quantitatively bound the direct influence of global climate forcings on antarctic temperature although climate models perform well on average in this context some recent syntheses of deglacial climate history have underestimated antarctic warming and the models with lowest sensitivity can be discounted

petrogenesis of mantle derived lile enriched archean monzodiorites and trachyandesites sanukitoids in southwestern superior province reply in southwestern superior province diorite monzodiorite and trachyandesite sanukitoids occurring within syn to post tectonic intrusive complexes and within greenstone belts have the following chemical characteristics 55 60 wt sio2 mgo 6 wt mg 0 60 ni and cr both 100 ppm na2o k2o 6 wt sr and ba both 600 1800 ppm and rare earth element ree patterns that are strongly light rare earth element lree enriched cen 80 250 ybn 4 10 and show no eu anomalies sanukitoids and their granodioritic derivatives constitute at least 5 of the exposed crust in the study area the sanukitoids cannot be derived by melting fractionation or crustal contamination of basalts or lamprophyres that are coeval with the sanukitoids crustal contamination of komatiites fails to explain the high large ion lithophile element lile contents of the sanukitoids rather we suggest that the sanukitoids were derived by hydrous melting of lile enriched mantle peridotite at pressures between 10 and

stacked uppermost mantle layers within the slave craton of nw canada as defined by anisotropic seismic discontinuities 1 a 20 station seismic array in nw canada recorded 336 teleseismic events with distribution in back azimuth and epicentral distance sufficient to characterize uppermost mantle discontinuities between depths typical of the mohorovicic and lehman discontinuities following wavefield decomposition groups of seismograms were source normalized through simultaneous deconvolution to estimate the near receiver impulse response and thus detect major discontinuities beneath each seismic station stations within the lac de gras kimberlite field display an unusually strong negative impulse on the radial component within the nw quadrant and two moderate impulses on the transverse component forward modeling of these impulses suggests a mantle layer dipping at 22 to the southeast with a mildly anisotropic 2 upper discontinuity at 120 135 km depth and another mildly anisotropic 2 discontinuity at about 170 km depth superimposed on these layers is another stronger anisotropic 4 layer between 110 and 180 km depths that dips to the west stations outside of the lac de gras field but within the southeastern slave craton display more numerous but weaker impulses the most prominent of these occurs at about 150 km depth on the transverse component and has opposite polarity to that observed farther north the prominent negative impulse observed on the radial component is interpreted to arise from structural preferred orientation in the form of a stockwork of wehrlite dykes beneath the lac de gras field interpretation of the other layers in the context of known surface geology as well as xenolith petrology and garnet geochemistry of diamondiferous kimberlites favors previous suggestions that they represent 4000 2900 ma depleted harzburgite and eclogite layers underthrust from the northwest at 2600 or 1880 ma the layer beneath the se slave craton has a similar but distinct tectonic history of nw verging underthrusting associated with the 2635 2615 ma defeat suite of plutonism taken together these interpretations indicate that the slave craton was assembled from at least four lithospheric blocks prior to its cratonization about 2580 ma each block is 90 120 km thick and all four blocks abut across a steep suture boundary beneath mackay lake in the central slave craton significant amounts of carbon and eclogites could have become incorporated into the central slave mantle within the diamond stability field during the proposed underthrusting of lithosphere to explain their relatively common occurrence in kimberlite eruptions within the slave craton

sub ice shelf sediments record history of twentieth century retreat of pine island glacier many glaciers and ice shelves in the west antarctic ice sheet are retreating or thinning rapidly but the triggering mechanism has been unclear now the retreat of pine island glacier is found to have begun in the 1940s following warming el nino events in the pacific ocean showing that glacial retreat can continue long after an initial push from the climate many glaciers and ice shelves in the amundsen sea sector of the antarctic ice sheet are retreating or thinning rapidly but to date the initial triggering mechanism has been unclear james smith and colleagues now show that pine island glacier s exceptional retreat began in the 1940s following a pulse of warmth associated with el nino events in the tropical pacific thereby confirming that glacial retreat can persist well after the initial push from climate the west antarctic ice sheet is one of the largest potential sources of rising sea levels1 over the past 40 years glaciers flowing into the amundsen sea sector of the ice sheet have thinned at an accelerating rate2 and several numerical models suggest that unstable and irreversible retreat of the grounding line which marks the boundary between grounded ice and floating ice shelf is underway3 understanding this recent retreat requires a detailed knowledge of grounding line history4 but the locations of the grounding line before the advent of satellite monitoring in the 1990s are poorly dated in particular a history of grounding line retreat is required to understand the relative roles of contemporaneous ocean forced change and of ongoing glacier response to an earlier perturbation in driving ice sheet loss here we show that the present thinning and retreat of pine island glacier in west antarctica is part of a climatically forced trend that was triggered in the 1940s our conclusions arise from analysis of sediment cores recovered beneath the floating pine island glacier ice shelf and constrain the date at which the grounding line retreated from a prominent seafloor ridge we find that incursion of marine water beyond the crest of this ridge forming an ocean cavity beneath the ice shelf occurred in 1945 12 years final ungrounding of the ice shelf from the ridge occurred in 1970 4 years the initial opening of this ocean cavity followed a period of strong warming of west antarctica associated with el nino activity thus our results suggest that even when climate forcing weakened ice sheet retreat continued

the central slave basement complex part i its structural topology and autochthonous cover new field and geochronological data are used to define the distribution of mesoarchean basement rocks in the south central slave province this distribution reflects a single contiguous basement terrane that we propose to call the central slave basement complex it shows a structural topology that is internally consistent and compatible with known regional folding and faulting events a sample of a proposed basement gneiss below the courageous lake greenstone belt central slave province has been dated by u pb methods and yields an age of 3325 8 ma consistent with the new basement distribution this sample also contains 2723 3 ma metamorphic zircon and ca 2680 ma titanite the central slave basement complex is overlain by a thin discontinuous but distinctive cover sequence that includes minor volcanic rocks clastic sedimentary rocks and banded iron formation all previously known and some new occurrences of this distinctive cover sequence occur in the immediate stratigraphic hanging wall of the