strain heating as a mechanism for partial melting and ultrahigh temperature metamorphism in convergent orogens implications of temperature dependent thermal diffusivity and rheology 1 we explore the importance of strain heating in ductile shear zones for production of leucogranites and high temperature metamorphism in collisional orogens with temperature dependent thermal diffusivity d and rock rheology new measurements on metamorphic rocks from the bohemian massif show that d is 1 3 mm2 s at 25 c and decreases exponentially to as low as 0 4 mm2 s at 600 c this temperature dependence causes model lithospheric geotherms to be straighter compared to geotherms calculated with a constant d using power law parameters for rheology we show that deformation of quartzite at strain rate of 3 10 13 s 1 produces 100 w m3 at 550 c and 7 w m3 at 800 c deformation of stronger clinopyroxenite at this strain rate produces 8 w m3 even at 1050 c when strain heating is introduced by a 3 km thick ductile shear zone with quartz rheology at the depth of 35 km in a lithosphere that has 70 km thick crust and 60 km thick lithospheric mantle the schist solidus is reached by 8 myr in the vicinity of the shear zone deforming at the strain rate of 3 10 13 s 1 for clinopyroxenite rheology ultrahigh temperature uht metamorphic conditions are reached in 40 myr after initiation of strain heating two dimensional models that replicate the exhumation of the greater himalaya crystalline rocks above the main central thrust produce extensive partial melting and an inverted metamorphic field gradient occurrences of leucogranites within crustal shear zone systems may be evidence of the coupling between deformation and heat production in the crust

potential links between surging ice sheets circulation changes and the dansgaard oeschger cycles in the irminger sea 60 18 kyr surface and deepwater paleoclimate records in irminger sea core so82 5 59on 31 ow and icelandic sea core ps2644 68on 22ow exhibit large fluctuations in thermohaline circulation thc from 60 to 18 calendar kyr b p with a dominant periodicity of 1460 years from 46 to 22 calendar kyr b p matching the dansgaard oeschger d o cycles in the greenland ice sheet project 2 gisp2 temperature record grootes and tuiver 1997 during interstadials summer sea surface temperatures sstsu in the irminger sea averaged to 8oc and sea surface salinities sss averaged to 36 5 recording a strong irminger current and atlantic thc during stadials sstsu dropped to 2o 4oc in phase with sss drops by 1 2 they reveal major meltwater injections along with the east greenland current which turned off the north atlantic deepwater convection and hence the heat advection to the north in harmony with various ocean circulation and ice models on the basis of the ird composition icebergs came from iceland east greenland and perhaps svalbard and other northern ice sheets however the southward drifting icebergs were initially jammed in the denmark strait reaching the irminger sea only with a lag of 155 195 years we also conclude that the abrupt stadial terminations the d o warming events were tied to iceberg melt via abundant seasonal sea ice and brine water formation in the meltwater covered northwestern north atlantic in the 1 1460 year frequency band benthic 15 80 brine water spikes led the temperature maxima above greenland and in the irminger sea by as little as 95 years thus abundant brine formation which was induced by seasonal freezing of large parts of the northwestern atlantic may have finally entrained a current of warm surface water from the subtropics and thereby triggered the sudden reactivation of the thc in summary the internal dynamics of the east greenland ice sheet may have formed the ultimate pacemaker of d o cycles

pacific ocean and cenozoic evolution of climate 1 the pacific ocean has played a major role in climate evolution throughout the cenozoic 65 0 ma it is a fundamental component of global heat transport and circulation the dominant locus of primary productivity and consequently the largest reservoir for carbon exchange between the oceans and the atmosphere a satisfactory understanding of the cenozoic evolutionary history of the pacific and its impact on global climate is currently data limited nevertheless the large dynamic range of cenozoic conditions sets the stage to greatly expand our understanding of global climate and biogeochemical cycles past earth experiments are particularly useful to understand interactions between climate and geosystems under different greenhouse gas loads we highlight in this review four important problems in which the pacific played a major role the effect of changing geographic boundary conditions on ocean circulation interactions between the carbon cycle and climate the pacific ocean s influence on north american climate and its water cycle and the gradual evolution of climate systems

contrasting archaean crustal records in western part of the napier complex east antarctica new constraints from shrimp geochronology abstract u pb zircon geochronology of 6 samples collected from fyfe hills 2 felsic orthogneisses and 1 quartzo feldspathic gneiss and mt cronus 1 quartzo feldspathic gneiss and 2 quartzites in the western part of the napier complex east antarctica revealed an archaean proterozoic history ranging from 3128 to 1824 ma the absence of ancient evidence 3800 3600 ma indicates that these ages were not always dominant in these two areas the 3128 2580 ma protolith ages are similar to those of other areas e g mt riiser larsen tonagh island while the magmatic protolith age of 2741 ma obtained for the fyfe hills orthogneiss is a new component of the archaean magmatic protolith in the napier complex metamorphic overgrowth or recrystallized zircons yielded systematic disconformable ages for both fyfe hills 2518 ma and mt cronus 2490 ma the presence of 2580 ma inherited zircons provides important constraints that place the ultrahigh temperature metamorphic peak later than 2580 ma the post metamorphic ages suggest local fluid infiltration or local deformation event s

experimental zircon melt and zircon garnet trace element partitioning and implications for the geochronology of crustal rocks garnet is the most commonly used mineral in thermobarometry whereas zircon is the most robust chronometer to date high grade metamorphic rocks to provide a basis for correlation of zircon and garnet growth we determined experimentally the trace element partitioningbetweenzircon ahydrousgraniticmeltandgarnetat20kbarand800 1000 cforp y rareearthelements ree zr hf th andu inrespecttomelt zirconpreferentiallyincorporatesallinvestigatedelementsapartfromreewithatomicnumberlowerthansm at this pressure and in the chosen composition the distribution coefficient between zircon and melt d zrn melt for ree increases with increasingatomicnumberofthereeandwithdecreasingtemperature dyb is 20at1000 c butmorethananorderofmagnitude higher at800 c the solubilityofzrinhydrousgraniticmelts buffered by zirconisabouta factoroftwo lower at20kbar than what has been previously established for mid crustal pressures large garnet produced in the experiments allowed determination of garnet melt trace element partitioning d grt melt at temperatures of 800 1000 c conditions relevant for partial melting of crustal rocks there is a systematic increase indree with decreasing temperature zircon contains significantly more heavy ree than garnet at temperatures of 800 850 c zircon garnetpartitioncoefficientsofheavy reedecreasewithincreasingtemperaturefromdlu of12at800 cto 1 4at1000 c middle reepartitioningisclosetounityforthewholeinvestigatedtemperaturerange differentempiricallydetermined zircon garnet partition coefficients from granulites and ultra high temperature granulites can potentially be explained by the experimentallydeterminedchangeofpartitioningasafunctionoftemperature thesedatacanassistinestablishingequilibriumbetween garnetandzirconzonesinnaturalrocks andintheconstructionofdetailedpressure temperature timepathsinhigh grademetamorphic rocks

the geochronology structure and metamorphism of early archaean rocks at fyfe hills enderby land antarctica abstract the fyfe hills of enderby land antarctica are composed of archaean cratonic rocks of the napier complex initial crustal formation possibly of plutonic volcanic and sedimentary origin was in the early archaean but probably significantly later than the 4000 ma ages previously reported at 3100 ma the rocks underwent granulite facies metamorphism and an intense regional strain d1 which rotated into parallelism pre existing discordances and produced a recumbent gneissic pile approximate sr isotopic homogenisation over at least a metre and new zircon growth and or major resetting apparently occurred at this time subsequent ductile deformation d2 also characterised by tight to isoclinal folding produced characteristic microstructure only within f2 folds but no axial plane foliation was developed peak granulite facies metamorphic conditions close to 900 c and 9 11 kbars were reached during d1 d2 considerable pb loss occurred in zircon during asymmetric open to tight folding d3 and upper amphibolite to granulite facies metamorphism 650 c 7 kbars at 2500 ma rb sr equilibration on the scale of centimetres occurred at that time after which the napier complex was essentially cratonised a subsequent semiductile deformation did not affect the whole napier complex but produced upright shear zones in the fyfe hills area which lies close to the boundary between the napier complex and the younger rayner complex to the south

the exhaust valve of the north atlantic during glacial periods climate records are marked by large amplitude oscillations believed to be a result of north atlantic na freshwater anomalies which weakened the thermohaline circulation thc and introduced instabilities such oscillations are absent from the present interglacial period with the aid of a semiglobal analytical model it is proposed that the bering strait bs acts like an exhaust valve for the above na freshwater anomalies specifically it is suggested that large instabilities in the thc are only possible during glacial periods because during these periods the bs is closed during interglacial periods when the bs the exhaust valve is open low salinity anomalies are quickly flushed out of the north atlantic by the strong southern ocean winds

the sensitivity of the climate response to the magnitude and location of freshwater forcing last glacial maximum experiments abstract proxy records indicate that the locations and magnitudes of freshwater forcing to the atlantic ocean basin as iceberg discharges into the high latitude north atlantic laurentide meltwater input to the gulf of mexico or meltwater diversion to the north atlantic via the st lawrence river and other eastern outlets may have influenced the north atlantic thermohaline circulation and global climate we have performed last glacial maximum lgm simulations with the ncar community climate system model ccsm3 in which the magnitude of the freshwater forcing has been varied from 0 1 to 1 sv and inserted either into the subpolar north atlantic ocean or the gulf of mexico in these glacial freshening experiments the less dense freshwater provides a lid on the ocean water below suppressing ocean convection and interaction with the atmosphere above and reducing the atlantic meridional overturning circulation amoc this is the case whether the freshwater is added directly to the area of convection south of greenland or transported there by the subtropical and subpolar gyres when added to the gulf of mexico the amoc reduction is less for the smaller freshwater forcings but is not linear with the size of the freshwater perturbation the recovery of the amoc from a slow state is 200 years for the 0 1 sv experiment and 500 years for the 1 sv experiment for glacial climates with large northern hemisphere ice sheets and reduced greenhouse gases the cold subpolar north atlantic is primed to respond rapidly and dramatically to freshwater that is either directly dumped into this region or after being advected from the gulf of mexico greenland temperatures cool by 6 8 c in all the experiments with little sensitivity to the magnitude location or duration of the freshwater forcing but exhibiting large seasonality sea ice is important for explaining the responses the northern hemisphere high latitudes are slow to recover antarctica and the southern ocean show a bipolar response with warming and reduced sea ice this warming continues after the cessation of the freshwater forcing and shows a dependence on the duration of the freshwater forcing equatorward of the expanded sea ice the simulated temperature and salinity anomalies are sensitive to the amount of colder and fresher waters that are advected out of the subpolar north atlantic in the tropical atlantic the recovery of the intertropical convergence zone itcz from its more southerly position during the freshwater forcing is much more rapid than the recovery of the amoc and is more related to the recovery of low latitude surface temperatures than greenland temperature or sea ice these results have implications for using proxy records as indirect measures of the amoc

isotopic resetting of u pb zircon and rb sr and sm nd whole rock systems in enderby land antarctica implications for the interpretation of isotopic data from polymetamorphic and multiply deformed terrains abstract extensive isotopic resetting occurred in precambrian basement rocks of enderby land antarctica three examples are highlighted to show the folly of reliance on a single isotopic system and the dangers of isotopic dating without adequate geological constraints in the first example the rb sr whole rock age of a proterozoic granite is 200 ma younger than its emplacement age derived from u pb zircon analyses the opposite relationship is observed in an archaean granite whose rb sr age from structural considerations appears to date emplacement but is some 500 ma older than its u pb zircon age which is inconsistent with the deduced time of emplacement the third example documents isotopic resetting of the sm nd whole rock system during transitional amphibolite to granulite facies tectonism however in both the rb sr and sm nd whole rock systems it is the development of penetrative fabric rather than intensity of metamorphic grade which facilitates isotopic resetting the scale of lithological homogeneity is probably a contributing factor

prolonged 100 ma ultrahigh temperature metamorphism in the napier complex east antarctica a petrochronological investigation of earth s hottest crust

rapid changes of glacial climate simulated in a coupled climate model abrupt changes in climate termed dansgaard oeschger and heinrich events have punctuated the last glacial period approximately 100 10 kyr ago but not the holocene the past 10 kyr here we use an intermediate complexity climate model to investigate the stability of glacial climate and we find that only one mode of atlantic ocean circulation is stable a cold mode with deep water formation in the atlantic ocean south of iceland however a warm circulation mode similar to the present day atlantic ocean is only marginally unstable and temporary transitions to this warm mode can easily be triggered this leads to abrupt warm events in the model which share many characteristics of the observed dansgaard oeschger events for a large freshwater input such as a large release of icebergs the model s deep water formation is temporarily switched off causing no strong cooling in greenland but warming in antarctica as is observed for heinrich events our stability analysis provides an explanation why glacial climate is much more variable than holocene climate

can we use ice sheet reconstructions to constrain meltwater for deglacial simulations 1 freshwater pulses from melting ice sheets are thought to be important for driving deglacial climate variability this study investigates challenges in simulating and understanding deglacial climate evolution within this framework with emphasis on uncertainties in the ocean overturning sensitivity to meltwater inputs the response of an intermediate complexity model to a single northern hemisphere meltwater pulse is familiar a weakening of the ocean overturning circulation in conjunction with an expansion of sea ice cover and a meridional temperature seesaw nonlinear processes are vital in shaping this response and are found to have a decisive influence when more complex scenarios with a history of pulses are involved a meltwater history for the last deglaciation 21 9 ka was computed from the ice 5g ice sheet reconstruction and the meltwater was routed into the ocean through idealized ice sheet drainages forced with this meltwater history model configurations with altered freshwater sensitivity produce a range of outcomes for the deglaciation from those in which there is a complete collapse of the overturning circulation to those in which the overturning circulation weakens slightly the different outcomes are interpreted in terms of the changing hysteresis behavior of the overturning circulation i e non stationary freshwater sensitivity as the background climate warms through the course of the deglaciation the study illustrates that current uncertainties in model sensitivity are limiting in efforts to forward model deglacial climate variability furthermore ice sheet reconstructions are shown to provide poor constraints on meltwater forcing for simulating the deglaciation

a diagnostic indicator of the stability of the atlantic meridional overturning circulation in ccsm3 a diagnostic indicator dmov is proposed in this paper to monitor the stability of the atlantic meridional overturning circulation amoc the dmov is a diagnostic for a basinwide salt advection feedback and defined as the difference between the freshwater transport induced by the amoc across the southern border of the atlantic ocean and the overturning liquid freshwater transport from the arctic ocean to the north atlantic as validated in the community climate system model version 3 ccsm3 for an amoc in the conveyor state a positive dmov freshwater convergence in the atlantic basin indicates a monostable amoc and a negative dmov freshwater divergence indicates a bistable amoc based on dmov the authors investigate the amoc stability in the last glacial maximum lgm and analyze the modulation of the amoc stability by an open closed bering strait moreover the authors estimate that the real amoc is likely to be bistable in the present day since some observations suggest a negative dmov freshwater divergence is currently in the atlantic basin however this estimation is very sensitive to the choice of the observational data

coupled wind forced controls of the bering chukchi shelf circulation and the bering strait throughflow ekman transport continental shelf waves and variations of the pacific arctic sea surface height gradient abstract we develop a conceptual model of the closely co dependent bering shelf bering strait and chukchi shelf circulation fields by evaluating the effects of wind stress over the north pacific and western arctic using atmospheric reanalyses current meter observations satellite based sea surface height ssh measurements hydrographic profiles and numerical model integrations this conceptual model suggests bering strait transport anomalies are primarily set by the longitudinal location of the aleutian low which drives oppositely signed anomalies at synoptic and annual time scales synoptic time scale variations in shelf currents result from local wind forcing and remotely generated continental shelf waves whereas annual variations are driven by basin scale adjustments to wind stress that alter the magnitude of the along strait meridional pressure gradient in particular we show that storms centered over the bering sea excite continental shelf waves on the eastern bering shelf that carry northward velocity anomalies northward through bering strait and along the chukchi coast the integrated effect of these storms tends to decrease the northward bering strait transport at annual to decadal time scales by imposing cyclonic wind stress curl over the aleutian basin and the western subarctic gyre ekman suction then increases the water column density through isopycnal uplift thereby decreasing the dynamic height sea surface height and along strait pressure gradient storms displaced eastward over the gulf of alaska generate an opposite set of bering shelf and aleutian basin responses while ekman pumping controls canada basin dynamic heights proshutinsky et al 2002 we do not find evidence for a strong relation between beaufort gyre sea surface height variations and the annually averaged bering strait throughflow over the western chukchi and east siberian seas easterly winds promote coastal divergence which also increases the along strait pressure head as well as generates shelf waves that impinge upon bering strait from the northwest

how long lived is ultrahigh temperature uht metamorphism constraints from zircon and monazite geochronology in the eastern ghats orogenic belt india abstract along the coast of peninsular india the eastern ghats expose a deep crustal section through a composite proterozoic orogenic belt to quantify the late mesoproterozoic early neoproterozoic pressure p temperature t time t evolution of the eastern ghats province new shrimp u pb zircon and monazite age data from multiple localities are reported and integrated with the results of phase equilibria modelling samples of residual granulite migmatite and enderbite yield a spread of weighted mean 207pb 206pb zircon and monazite ages between ca 970 and ca 930 ma based on ranges of spot ages from several samples the late prograde to peak ultrahigh temperature uht metamorphism counter clockwise evolution ccw to t 950 c at p 8 kbar and initial cooling is interpreted to have occurred between ca 1130 and ca 970 ma regionally extensive enderbite and charnockite magmas were emplaced into the hot suprasolidus crust around the time of peak metamorphism for the residual granulites and migmatites the retrograde p t t path is characterized by close to isobaric cooling to the variable but elevated solidi for different samples weighted mean ages between ca 970 and ca 930 ma in several samples are interpreted to record the timing of crystallization of melt trapped by the percolation threshold in each of these samples two additional weighted mean ages of ca 980 ma from korhonen f j saw a k clark c brown m bhattacharya s 2011 new constraints on uht metamorphism in the eastern ghats province through the application of phase equilibria modelling and in situ geochronology gondwana research 20 764 781 extend this range back in time by 10 my the variability in the calculated weighted mean ages across the region is interpreted to be due mainly to differences in the temperature of the elevated solidus from sample to sample suggesting a slow cooling rate of 1 c my during the retrograde stage of this long lived uht metamorphism these results demonstrate that the eastern ghats province sustained uht conditions t 900 c for 50 my and perhaps for as long as 200 ma from ca 1130 to 930 ma during a single ccw tectono metamorphic event

salinity changes in the north west pacific ocean during the late pliocene early quaternary from 2 73 ma to 2 52 ma recent research has increasingly advocated a role for the north pacific ocean in modulating global climatic changes over both the last glacial cycle and further back into the geological record here a diatom 18o record is presented from ocean drilling program site 882 over the pliocene quaternary boundary from 2 73 ma to 2 52 ma mis g6 mis 99 large changes in 18odiatom of c 4 from 2 73 ma onwards are documented to occur on a timeframe broadly coinciding with glacial interglacial cycles these changes are primarily attributed to large scale inputs of meltwater from glacials surrounding the north pacific basin and the bering sea despite these inputs and associated change in surface water salinity on the basis of existing opal and uk37 temperature data and new modelled water column densities no evidence exists to suggests a removal of the halocline stratification or a resumption of the high productivity system similar to that which prevailed prior to 2 73 ma the permanence of the halocline suggests that the region played a key role in driving global climatic changes over the early glacial interglacial cycles that followed the onset of major northern hemisphere glaciation by inhibiting deep water upwelling and ventilation of co2 to the atmosphere

thermal history of uht metamorphism in the napier complex east antarctica insights from zircon monazite and garnet ages abstract high grade gneisses from mt riiser larsen east antarctica have been dated by whole rock mineral sm nd and shrimp zircon and monazite u pb to help define the thermal history of ultrahigh temperature uht metamorphism in the napier complex both the monazite and youngest zircon yield a range of apparent ages 2 51 2 47 ga consistent with crystallization during an extended period of metamorphism some zircon also preserves an isotopic record of earlier events placing an upper limit of a few million years on the duration of peak metamorphic conditions the similarity of the monazite and zircon u pb ages implies rapid initial postpeak cooling to below the blocking temperature of these minerals 900 c consistently lower sm nd whole rock mineral isochron ages 2 38 ga indicate that cooling slowed before the temperature reached 650 c the history of the uht metamorphism is interpreted to be 1 protracted high temperature 800 c conditions 2 51 2 47 ga 2 peak conditions up to 1100 c

uht metamorphism peaking above 1100 c with slow cooling insights from pelitic granulites in the jining complex north china craton

the hydraulics of an evolving upwelling jet flowing around a cape upwelling jets flow alongshore in approximate geostrophic balance with the onshore pressure gradient induced by coastal upwelling observations of such jets have shown that they often move offshore downstream of capes leaving a pool of upwelled water inshore comparisons are made between this behavior and the hydraulic transition of a potential vorticity conserving coastal current as it passes a topographic anomaly at which it is exactly critical to long coastal trapped waves an analytic 1 5 layer model of coastal hydraulics with constant potential vorticity in each layer predicts flow fields i e jet separation in critical situations that resemble observations when scales approximate cape blanco on the oregon coast separation occurs at a jet transport of around 0 76 3 106 m3 s21 similar to observed transports time dependent semigeostrophic calculations suggest that during an upwelling season the jet would evolve from a weak flow which was subcritical everywhere and symmetric about the cape to an exactly critical state that made a transition from subcritical to supercritical structure at the head of the cape the predicted flow field at critical transition consists of a narrow upwelling jet upstream of the cape that moves offshore and broadens at the cape this critical state would be accompanied by a downstream jump back to subcritical conditions further upwelling favorable winds would lead to transient waves that propagated upstream and downstream modifying the upstream and downstream conditions and restoring criticality thus the head of the cape exerts hydraulic control on the flow and prevents the jet transport from increasing above its critical level inherent in the hydraulic approach is the assumption that alongshore scales are large for realistic alongshore scales solutions modified by coastline curvature suggest that the convexity of the head of a cape slightly inhibits the transition to a strongly upwelled downstream state by increasing the required critical transport in the presence of topographic features with finite alongshore scale the hydraulic approach can be used to construct a flow field although this flow field has an inherent error arising from the implicit assumptions regarding scales estimation of this error for topography representing cape blanco suggests that in places the cape is rather abrupt for hydraulic theory to be valid

instability of glacial climate in a model of the ocean atmosphere cryosphere system in contrast to the relatively stable climate of the past 10 000 years during glacial times the north atlantic region experienced large amplitude transitions between cold stadial and warm interstadial states in this modeling study we demonstrate that hydrological interactions between the atlantic thermohaline circulation thc and adjacent continental ice sheets can trigger abrupt warming events and also limit the lifetime of the interstadial circulation mode these interactions have the potential to destabilize the thc which is already more sensitive for glacial conditions than for the present day climate thus providing an explanation for the increased variability of glacial climate

the dominant role of the east siberian sea in driving the oceanic flow through the bering strait conclusions from grace ocean mass satellite data and in situ mooring observations between 2002 and 2016 it is typically stated that the pacific to arctic oceanic flow through the bering strait important for arctic heat freshwater and nutrient budgets is driven by local wind and a poorly defined far field pressure head forcing related to sea surface height differences between the pacific and the arctic using monthly arctic wide ocean bottom pressure satellite data and in situ mooring data from the bering strait from 2002 to 2016 we discover the spatial structure of this pressure head forcing finding that the bering strait throughflow variability is dominantly driven from the arctic specifically by sea level change in the east siberian sea ess in turn related to westward winds along the arctic coasts in the comparatively calm summer this explains approximately two thirds of the bering strait variability in winter local wind variability dominates the total flow but the pressure head term while still correlated with the ess dominated sea level pattern is now more strongly related to bering sea shelf sea level variability

thermal regimes in the chukchi sea from 1941 to 2008 the summer june october temperature observations in the surface 0 m and subsurface 20 mbottom chukchi sea layers collected from 1941 to 2008 have been analyzed using the self consistent data recovery procedure based on correlation analysis and iterative empirical orthogonal function eof decomposition the analysis of the surface and subsurface eofs identified cold normal and warm thermal states with variability of 2 3 years and also 4 7 years we found that the chukchi sea water temperature has gradually increased since 1941 warming in the surface layer since 1941 has been minimal in the bering strait 0 012 1 cy r 1 total 0 8 1c and maximal in long strait 0 030 0 036 1 cy r 1 total 2 0 2 4 1c in the subsurface layer temperatures have increased about half as much minimal 0 0030 0 0075 1 cy r 1 total 0 2 0 5 1c in long strait and rather uniform 0 010 0 015 1 cy r 1 total 0 7 1 0 1c for the remaining chukchi sea analysis of the satellite sea surface height anomaly data shows that during the warm periods there is a stronger flow through the bering strait and intensification of the northwestward currents in the central chukchi sea extensive correlation analysis shows that the thermal state of the chukchi sea agrees well with the flow of pacific water through the bering strait and by an increase of the global atmospheric temperature in addition typical circulation during warm and cold periods was reconstructed using four dimensional variational 4dvar data assimilation into the ocean model and estimates of volume and heat fluxes in the chukchi sea during cold and warm periods were derived which are consistent with eof and correlation analyses

bering strait throughflow and the thermohaline circulation received 25 august 2005 revised 10 november 2005 accepted 14 november 2005 published 21 december 2005 1 in this study we employ a global coupled climate model to show that the bering strait is important to the variations of the thermohaline circulation three experiments are analyzed a present day control run and two freshwater hosing runs results show that as the thermohaline circulation weakens due to freshwater forcing in the northern north atlantic the bering strait throughflow weakens and even reverses its direction and the export of the arctic freshwater into the northern north atlantic is reduced or the arctic even takes freshwater from the latter thus helping the thermohaline circulation to recover relatively quickly after the end of the freshwater hosing citation hu a and g a meehl 2005 bering strait throughflow and the thermohaline circulation geophys res lett 32 l24610 doi 10 1029 2005gl024424

thermobarometric modelling of zircon and monazite growth in melt bearing systems examples using model metapelitic and metapsammitic granulites u pb age data collected from zircon and monazite are used to draw fundamental inferences about tectonic processes in the earth despite the emphasis placed on zircon and monazite ages the understanding of how to relate the timing of growth of zircon and monazite to an evolving rock system remains in its infancy in addition few studies have presented large datasets of geochronological data from zircon and monazite occurring in the same metamorphic rock sample such information is crucial for understanding the growth of zircon relative to monazite in a systematic and predictive manner as per this study the data that exist support the generally held conception that zircon ages tend to be older than monazite ages within the same rock here experimental data for zircon and monazite saturation in melt bearing rocks are integrated with phase diagram calculations the calculations constrain the dissolution and growth behaviour of zircon and monazite with respect to evolving pressure temperature and silicate mineral assemblages in high grade melt bearing metasedimentary rocks several key results emerge from this modelling first that in aluminous metapelitic rocks i e garnet cordierite sillimanite assemblages zircon ages are older than monazite ages in the same rock second that the growth rate of accessory minerals is nonlinear and much higher at and near saturation than at lower temperatures and third that the difference in zircon and monazite ages from the same rock may be ascribed to differences in the temperature s at which zircon and monazite grow rather than differences in closure temperature systematics using our methodology the cooling rate of granulites from the reynolds range central australia have been constrained at 4 c myr 1 this study serves as a first pass template on which further research in applying the technique to a field study can be based

dynamics of a quasi stationary jet along the subarctic front in the north pacific ocean the western isoguchi jet an ideal two layer model abstractthe dynamics of a quasi stationary jet along the subarctic front in the north pacific ocean the western isoguchi jet were investigated using an idealized two layer model the experiments suggested that a seafloor topography which is 500 m high produces a jet along its eastern flank the formation mechanism of the jet can be explained via baroclinic rossby wave characteristics baroclinic rossby waves propagate along characteristic curves which are significantly distorted by anticyclonic barotropic flow on the seafloor topography a baroclinic surface jet is formed where a characteristic curve originating in the subtropical gyre and one originating in the subpolar gyre meet because the pycnocline depth varies discontinuously at this location the barotropic flow on the seafloor topography is induced by eddies

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

the effect of arctic freshwater pathways on north atlantic convection and the atlantic meridional overturning circulation abstractthis study examines the relative roles of the arctic freshwater exported via different pathways on deep convection in the north atlantic and the atlantic meridional overturning circulation amoc deep water feeding the lower branch of the amoc is formed in several north atlantic marginal seas including the labrador sea irminger sea and the nordic seas where deep convection can potentially be inhibited by surface freshwater exported from the arctic the sensitivity of the amoc and north atlantic to two major freshwater pathways on either side of greenland is studied using numerical experiments freshwater export is rerouted in global coupled climate models by blocking and expanding the channels along the two routes the sensitivity experiments are performed in two sets of models cm2g and cm2m with different control simulation climatology for comparison freshwater via the route east of greenland is found to have a larger direct impact on labrador sea convection in response to the changes of

interpreting granulite facies events through rare earth element partitioning arrays the use of rare earth element ree partition coefficients is an increasingly common tool in metamorphic studies linking the growth or modification of accessory mineral geochronometers to the bulk silicate mineral assemblage the most commonly used mineral pair for the study of high grade metamorphic rocks is zircon and garnet the link from u pb ages provided by zircon to the p t information recorded by garnet can be interpreted in relation to experimental data the simplistic approach of taking the average ree abundances for zircon and garnet and comparing them directly to experimentally derived partition coefficients is imperfect in that it cannot represent the complexity of a natural rock system this study describes a method that uses all the zircon analyses from a sample and compares them to different garnet compositions in the same rock using the most important ree values it is possible to define zircon garnet equilibrium using an array rather than an average the array plot describes partitioning between zircon and garnet using dyb and dyb dgd as the defining features of the relationship this approach provides far more sensitivity to mineral reactions and diffusional processes enabling a more detailed interpretation of metamorphic history of the sample this article is protected by copyright all rights reserved

u pb isotopic behaviour of zircon during upper amphibolite facies fluid infiltration in the napier complex east antarctica understanding the factors that contribute to u pb discordance in zircon is essential for interpreting isotopic data and for assessing the validity of concordia intercept ages modification caused by interaction with metamorphic fluids is often cited as a primary means by which significant or even complete isotopic resetting of u pb systematics in zircon might be achieved under subsolidus conditions we present a field example from the napier complex east antarctica in which a palaeoproterozoic 2450 70 ma zircon population interacted locally with an early palaeozoic 498 1 7 ma aqueous fluid at upper amphibolite facies conditions conventional ion microprobe analysis of sectioned and polished grain surfaces indicates that fluid interaction resulted in minor disturbance of u and pb in zircons both normal and reverse discordance with limited displacement along a chord with a lower intercept that coincides with the timing of fluid infiltration in contrast ion probe drilling or depth profiling into unpolished natural zircon crystal surfaces revealed extensive disturbance of u pb systematics to depths of 0 2 m with near surface ages consistent with the timing of fluid influx at 498 ma although zircon underwent some radiogenic pb redistribution during fluid interaction infiltrating fluids resulted in minimal grain scale isotopic modification of zircon based on ion probe depth profiling results we propose that limited normal discordance observed in the conventional ion microprobe zircon analyses in this case is controlled by an analytical mixture of reset and or recrystallised zircon along penetrative micro fracture networks with that of adjacent unaffected zircon we also suggest that the observed reverse discordance is genuine resulting from localised intra grain net accumulations of radiogenic pb we conclude that the isotopic response of zircon in this case is controlled by the interaction of an aqueous metamorphic fluid of low to moderate salinity resulting in sub micrometre depth scale isotopic modification at natural crystal faces and along penetrative micro fracture networks and that grain scale isotopic modification was negligible therefore we urge caution when considering regional chronological interpretations that appeal to significant zircon isotopic resetting caused exclusively by metamorphic fluid interaction at upper amphibolite facies conditions

influence of bering strait flow and north atlantic circulation on glacial sea level changes sea level fluctuations of about 20 30m occurred throughout the last glacial period these fluctuations seem to have been derived primarily from changes in the volume of northern hemisphere ice sheets 1 3 and cannot be attributed solely to ice melt caused by varying solar radiation 4 here we use a fully coupled climate model to show that the transport of relatively fresh pacific water into the north atlantic ocean was limited when lower sea level restricted or closed the bering strait resulting in saltier north atlantic surface waters this invigorated deep convection in the north atlantic ocean strengthening meridional overturning circulation and northward heat transport in our model which consequently promoted melting of ice sheets in north america and europe our simulations show that the associated sea level rise led to a reopening of the bering strait the flux of relatively fresh water into the north atlantic ocean muted meridional overturning circulation and led to cooling and ice sheet advance in the northern hemisphere we conclude that the repetition of this cycle could produce the sea level changes that have been observed throughout the last glacial cycle

a model based study of ice and freshwater transport variability along both sides of greenland we investigate some aspects of the variability of the arctic freshwater content during the 1965 2002 period using the drakkar eddy admitting global ocean sea ice model 12 km resolution in the arctic a comparison with recent mooring sections shows that the model realistically represents the major advective exchanges with the arctic basin through bering fram and davis straits and the barents sea this allows the separate contributions of the inflows and outflows across each section to be quantified in the model the arctic freshwater content variability is explained by the sea ice flux at fram and the combined variations of ocean freshwater inflow at bering and outflow at fram and davis at all routes except trough fram strait the freshwater transport variability is mainly accounted for by the liquid component with small contributions from the sea ice flux the ocean freshwater transport variability through both davis and fram is controlled by the variability of the export branch baffin island current and east greenland current respectively the variability of the inflow branches playing a minor role we examine the respective role of velocity and salinity fluctuations in the variability of the ocean freshwater transport fram and davis straits offer a striking contrast in this regard freshwater transport variations across davis strait are completely determined by the variations of the total volume flux 0 91 correlation on the other hand the freshwater transport through fram strait depends both on variations of volume transport and salinity as a result there is no significant correlation between the variability of freshwater flux at fram and davis although the volume transports on each side of greenland are strongly anti correlated 0 84 contrary to davis strait the salinity of water carried by the east greenland current through fram strait varies strongly due to the ice ocean flux north of greenland

effect of global ocean temperature change on deep ocean ventilation 1 a growing number of paleoceanographic observations suggest that the ocean s deep ventilation is stronger in warm climates than in cold climates here we use a general ocean circulation model to test the hypothesis that this relation is due to the reduced sensitivity of seawater density to temperature at low mean temperature that is at lower temperatures the surface cooling is not as effective at densifying fresh polar waters and initiating convection in order to isolate this factor from other climate related feedbacks we change the model ocean temperature only where it is used to calculate the density to which we refer below as dynamic temperature change we find that a dynamically cold ocean is globally less ventilated than a dynamically warm ocean with dynamic cooling convection decreases markedly in regions that have strong haloclines i e the southern ocean and the north pacific while overturning increases in the north atlantic where the positive salinity buoyancy is smallest among the polar regions we propose that this opposite behavior of the north atlantic to the southern ocean and north pacific is the result of an energy constrained overturning

impact of freshwater release in the north atlantic under different climate conditions in an oagcm abstract the response of climate to freshwater input in the north atlantic na has raised a lot of concern about the issue of climate stability since the discovery of abrupt coolings during the last glacial period such coolings have usually been related to a weakening of the atlantic meridional overturning circulation amoc probably associated with massive iceberg surges or meltwater pulses additionally the recent increase in greenhouse gases in the atmosphere has also raised the possibility of a melting of the greenland ice sheet which may impact the future amoc and thereby the climate in this study the extent to which the mean climate influences the freshwater release linked to ice sheet melting in the na and the associated climatic response is explored for this purpose the simulations of several climatic states last interglacial last glacial maximum mid holocene preindustrial and future 2 co2 are considered and the climatic response to a freshwater input computed interactively acc

why is the north america cordillera high hot backarcs thermal isostasy and mountain belts global mountain belts are commonly concluded to be a consequence of crustal thickening resulting from continental collision with high elevations supported by crustal roots however accumulating seismic structure data indicate that many mountain belts have no crustal root most of the north american cordillera has a 30 35 km crust in contrast to 40 45 km for the lower elevation craton and other stable areas it has been shown previously that most such mountain belts are in present or recent backarcs that are uniformly hot from thermal constraints we predict a uniform 1600 m elevation support of the cordillera by thermal expansion compared to stable areas over most of the cordillera relative to stable areas the elevations after correction for variable crustal thickness and density are in excellent agreement when subduction and shallow backarc convection stop the lithosphere may cool and the elevations of mountain belts subside over 300 m y

metastable growth of corundum adjacent to quartz in a spinel bearing quartzite from the archaean napier complex antarctica in a granulite facies spinel bearing quartzite corundum orthopyroxene and sapphirine and rarely cordierite and sillimanite form partial rims separating spinel from quartz textures indicate the reactions spinel quartz orthopyroxene corundum and spinel quartz orthopyroxene sapphirine thus corundum and sapphirine are produced by reactions involving quartz the low al content of the orthopyroxene 0 5 2 8 wt and low values for mg fe distribution coefficient for spinel sapphirine and spinel orthopyroxene reflect low temperature conditions during formation of the reaction products absence of zoning in spinel and a constant mg fe distribution coefficient for spinel sapphirine and spinel orthopyroxene over a compositional range indicate mg fe equilibration it is suggested that stable reactions such as spinel quartz cordierite or spinel quartz garnet sillimanite were over stepped and that metastable reactions give rise to the anomalous juxtaposition of corundum quartz

the collapse of the bering strait ice dam and the abrupt temperature rise in the beginning of the holocene this paper focuses on the abrupt increase in the oceanic and atmospheric temperature in the northern hemisphere at the beginning of the holocene approximately 11 000 yr before the present de boer and nof hypothesized that at that time the bering strait bs opened up abruptly because of the breakup of an ice dam by rising sea levels it is proposed further here that this sudden opening caused an abrupt increase in the mean temperature of the northern hemisphere an analytical coupled ocean atmosphere model is applied to the north atlantic in an attempt to quantify the temperature change resulting from the opening of the bs heat salt and mass are all conserved within a box in the north atlantic a convection condition allows water to enter the deep layer and the island rule relates the wind field to the mass fluxes the conventional approach that the meridional overturning cell moc was not operating during the younger dryas because of an overwhelming freshwater flux is adopted here opening the bs in the early holocene allowed these freshwater anomalies to be flushed out into the pacific reviving convection and the transport of heat northward restarting convection with an open bs increases mean oceanic and atmospheric temperature by 3 and 23 c respectively these values are comparable to those found in both the centre europeen de recherche et d enseignement des geosciences de l environnement cerege alkenone and greenland ice sheet project 2 gisp 2 ice core records of course restarting convection increases the temperature even with a closed bs but in the closed bs case the oceanic increase is slightly higher 4 c instead of 3 c whereas the atmospheric is much lower 17 5 c instead of 23 c this is because by requiring a continuous sea level around the americas an open bs allows the wind field to limit the amount of southern ocean water that enters the south atlantic this controlled volume flux 8 sv is considerably smaller than that allowed into the atlantic in the no wind control closed bs case 17 sv

feldspar thermometry in ultrahigh temperature metamorphic rocks evidence of crustal metamorphism attaining 1100 c in the archean napier complex east antarctica ultrahigh metamorphic temperatures attained in the mid to lower crust have been assessed by examining the mineral chemistry of ternary feldspars with relatively coarse exsolution lamellae from the archean napier complex east antarctica chemical compositions of re integrated perthitic mesoperthitic and antiperthitic feldspars are calculated from the modal proportions and the chemical analyses of host and lamellar domains formed through exsolution based on ternary feldspar solvus models re integrated compositions of feldspars from a variety of rock types yield the minimum equilibrium temperatures ranging from 1000 to 1110 c 0 8 gpa these data confirm the suggestion that the regional thermal conditions of the napier complex reached or exceeded 1100 c as feldspar is one of the common constituents of the crustal rocks this approach could be applicable to a wide variety of rocks in which feldspar represents exsolution textures

the sources of energy for crustal melting and the geochemistry of heat producing elements crustal melting to produce granite magmas requires a tremendous amount of energy in principle there are three main mechanisms of heating that can extensively melt a fertile crust radiogenic heat caused by the decay of 40k 230th 235u and 238u increased subcrustal heat flux caused by the upwelling of deeper therefore hotter mantle materials advection of heat caused by the emplacement and crystallization of hot mantle magmas two dimensional finite elements modeling reveals that a fertile crust thickened to 65 70 km would produce copious granite magmatism after 30 40 m y if its average heat production is a 1 2 w m 3 but it would scarcely melt if a 0 65 w m 3 increasing the subcrustal heat flow from the normal value of qm 0 025 w m 2 to qm 0 04 w m 2 may also lead to extensive crustal melting especially if the crust does not thin to less than 30 35 km very high qm 0 06 w m 2 affecting the continental crust is unlikely but the combination of moderately high qm 0 04 w m 2 and a thick fertile crust with a 1 2 w m 3 such as often happens in the volcanic and back arc areas of subduction zones is ideal to produce copious granite magmatism lastly the emplacement of hot mantle magmas in a fertile crust can produce crustal melts in just a few thousand years but the volume of these is equal to or less than the volume of the intruding magma a clue for understanding the relative importance of each of these three mechanisms comes from the radiogenic heat production of granite rocks calculated from the concentration of 40k 230th 235u and 238u at the time of their formation this parameter estimated on more than 3400 granites samples of different ages and provenance reveals a strongly asymmetric distribution peaking around 2 4 w m 3 a value much higher than the average continental crust about 1 1 2 w m 3 and certainly much higher than the average lower continental crust about 0 4 0 8 w m 3 only those granite rock types that are clearly connected with mantle heat sources such as the archean ttg post archean subduction related trondhjemites and recent adakites have a heat production equal to or smaller than the lower continental crust since the bulk melt solid partition coefficient of the heat producing elements hpe k th and u is k 1 the elevated hpe contents of granites indicates that most of them have been derived from hpe rich sources we conclude that radiogenic heating is often essential and always advantageous for generating large volumes of granite magmas and that granite magmatism is the main cause of the accumulation of hpe in the upper crust

response of thermohaline circulation to freshwater forcing under present day and lgm conditions responses of the thermohaline circulation thc to freshwater forcing hosing in the subpolar north atlantic ocean under present day and the last glacial maximum lgm conditions are investigated using the national center for atmospheric research community climate system model versions 2 and 3 three sets of simulations are analyzed with each set including a control run and a freshwater hosing run the first two sets are under present day conditions with an open and closed bering strait the third one is under lgm conditions which has a closed bering strait results show that the thc nearly collapses in all three hosing runs when the freshwater forcing is turned on the full recovery of the thc however is at least a century earlier in the open bering strait run than the closed bering strait and lgm runs this is because the excessive freshwater is diverged almost equally toward north and south from the subpolar north atlantic when the bering strait is open a significant portion of the freshwater flowing northward into the arctic exits into the north pacific via a reversed bering strait throughflow which accelerates the thc recovery when the bering strait is closed this arctic to pacific transport is absent and freshwater can only be removed through the southern end of the north atlantic together with the surface freshwater excess due to precipitation evaporation river runoff and melting ice in the closed bering strait experiments after the hosing the removal of the excessive freshwater takes longer and this slows the recovery of the thc although the background conditions are quite different between the present day closed bering strait run and the lgm run the thc responds to the freshwater forcing added in the north atlantic in a very similar manner

arctic perennial ice cover over the last 14 million years knowledge of the long term history of the perennial ice is an important issue that has eluded study because the cenozoic core material needed has been unavailable until the recent arctic coring expedition acex detrital fe oxide mineral grains analyzed by microprobe from the last 14 ma 164 m of the acex composite core on the lomonosov ridge were matched to circum arctic sources with the same mineral and 12 element composition these precise source determinations and estimates of drift rates were used to determine that these sand grains could not be rafted to the acex core site in less than a year thus the perennial ice cover has existed since 14 ma except for the unlikely rapid return to seasonal ice between the average sampling interval of about 0 17 ma both north america and russia contributed significant fe grains to the acex core during the last 14 ma

the timing and duration of high temperature to ultrahigh temperature metamorphism constrained by zircon u pb hf and trace element signatures in the khondalite belt north china craton understanding the timing and duration of metamorphism is fundamental to correctly interpreting the geodynamic evolution of orogenic events metamorphic constraints are still ambiguous in the khondalite belt north china craton so paleoproterozoic orogenic processes remain unclear here we investigate a suite of high grade rocks from the dongpo locality in the daqingshan terrane by integrating zircon trace elements and hf isotopic signatures with u pb ages to track zircon formation events and confidently constrain the timing and duration of metamorphism the application of the ti in zircon thermometer confirms the occurrence of regional ht uht metamorphism linking zircon hree patterns and u pb ages in combination with hree partitioning between zircon and garnet reveals zircon formation during partial melting to thermal peak stage and during the post peak cooling stage local reaction control by garnet breakdown has affected some zircons which show a more radiogenic 176hf 177hf ratio 0 28164 0 28174 relative to unaffected grains 0 28128 0 28156 metamorphic zircon u pb ages combined with results from previous studies constrain partial melting to uht conditions with decompression heating at 1 94 1 90 ga post peak near isobaric cooling at 1 90 1 86 ga and the thermal peak very close to ca 1 90 ga local garnet breakdown occurred during another decompression heating stage to ht uht conditions at 1 86 1 84 ga and further cooling occurred until ca 1 80 ga our work reveals relatively short lived partial melting less than 40 million years in comparison to long lived cooling stages nearly 100 million years in the eastern part of the khondalite belt two pulses of decompression heating to ht uht conditions in the deep crust could result from repeated asthenospheric upwelling driven by lithospheric extension after subduction collision our study sheds new light on zircon behaviour during ht uht metamorphism and elucidates the underlying driver of uht metamorphism in this part of the khondalite belt

deceleration of a finite width stratified current over a sloping bottom frictional spindown or buoyancy shutdown abstract the deceleration of an unforced two dimensional finite width current over a sloping bottom in a stratified fluid is studied to quantify the relative importance of frictional spindown and buoyancy shutdown when both act simultaneously frictional spindown decelerates the current through ekman suction and pumping at the current edges which transmit stresses into the interior fluid buoyancy shutdown is the process by which lateral advection of density in the bottom boundary layer generates thermal wind shears that reduce the bottom stress thereby halting deceleration a theoretical model of a downwelling current suggests that buoyancy shutdown always reduces the deceleration timescale from that for frictional spindown alone and produces a nonzero steady along isobath current overlying an arrested bottom mixed layer the model is most sensitive to the burger number s n f where n is the buoyancy frequency the bottom slope and f the coriolis parameter larger s produces a stronger steady cu

first application of titanium in zircon thermometry to ultrahigh temperature metamorphism titanium in zircon thermometry has been applied to evaluate the temperatures of metamorphism for two ultrahigh temperature uht granulite localities the neoproterozoic anapolis itaucu complex in central brazil and lower crustal xenoliths from the neo archean kaapvaal craton in south africa uht metamorphism in the anapolis itaucu complex occurred ca 640 ma ti in zircon thermometry on metamorphic zircon yields temperatures of 965 811 c a population of late zircon that formed from ilmenite and rutile breakdown records temperatures of 780 c maximum temperatures are less than those inferred from quantitative phase equilibria modeling based on textural setting inclusions in peak phases and as matrix grains these zircons likely record growth during the pro grade and or postpeak evolution in contrast the kaapvaal xenoliths record two phases of zircon growth during uht metamorphism at 2720 2715 ma followed by growth during isobaric cooling in kyanite bearing leucosomes ca 2690 ma temperatures recorded by the two phases of growth are 1024 878 c and 936 839 c respectively suggesting that zircon growth occurred during uht conditions and continued throughout the postpeak evolution over a 200 c interval these examples demonstrate that the ti in zircon thermometer is a powerful new tool with the potential to determine crystallization temperatures of zircon during granulite facies metamorphism throughout the prograde to postpeak evolution of such terranes

zircon the metamorphic mineral a mineral that forms under conditions as variable as diagenesis to deep subduction melt crystallization to low temperature alteration and that retains information on time temperature trace element and isotopic signatures is bound to be a useful petrogenetic tool the variety of conditions under which zircon forms and reacts during metamorphism is a great asset but also a challenge as interpretation of any geochemical data obtained from zircon must be placed in pressure temperature deformation fluid context under which condition and by which process zircon forms in metamorphic rocks remains a crucial question to answer for the correct interpretation of its precious geochemical information in the last 20 years there has been a dramatic evolution in the use of zircon in metamorphic petrology with the advent of in situ dating techniques zircon became relevant as a mineral for age determinations in high grade metamorphic rocks since then there has been incredible progress in our understanding of metamorphic zircon with the documentation of growth and alteration textures its capacity to protect mineral inclusions zircon thermometry trace element patterns and their relation to main mineral assemblages solubility of zircon in melt and fluids and isotopic systematics in single domains that go beyond u pb age determinations metamorphic zircon is no longer an impediment to precise geochronology of protolith rocks but has become a truly indispensable mineral in reconstructing pressure temperature time fluid paths over a wide range of settings an obvious consequence of its wide use is the rapid increase of literature on metamorphic zircon and any attempt to summarize it can only be partial in this chapter reference to published works are intended as examples and not as a compilation this chapter approaches zircon as a metamorphic mineral reporting on its petrography and texture deformation structure and mineral chemistry including trace element and isotopic systematics linking this information together highlights the potential of zircon as a key mineral in petrochronology

significance of post peak metamorphic reaction microstructures in the ultrahigh temperature eastern ghats province india ultrahigh temperature uht granulites in the eastern ghats province egp have a complex p t t history we review the p t histories of uht metamorphism in the egp and use that as a framework for investigating the p t t history of mg al rich granulites from anakapalle with the express purpose of trying to reconcile the down pressure dominated p t path with other uht localities in the egp mafic granulite that is host to mg al rich metasedimentary granulites at anakapalle has a protolith age of c 1580 ma mg al rich metasedimentary granulites within the mafic granulite at anakapalle were metamorphosed at uht conditions during tectonism at 960 875 ma meaning that the uht metamorphism was not the result of contact metamorphism from emplacement of the host mafic rock reworking occurred during the pan african c 600 500 ma event and is interpreted to have produced hydrous assemblages that overprint the post peak high temperature retrograde assemblages in contrast to rocks elsewhere in the egp that developed post peak cordierite the metasedimentary granulites at anakapalle developed post peak generation 2 reaction products that are cordierite absent and nominally anhydrous therefore rocks at anakapalle offer the unique opportunity to quantify the pressure drop that occurred during so called m2 that affected the egp we argue that m2 is either a continuation of m1 and that the overall p t path shape is a complex counter clockwise loop or that m1 is an up temperature counter clockwise deviation superimposed on the m2 path therefore rather than the rocks at anakapalle having a metamorphic history that is apparently anomalous from the rest of the egp we interpret that other previously studied localities in the egp record a different part of the same p t path history as anakapalle but do not preserve a significant record of pressure decrease this is due either to the inability of refractory rocks to extensively react to produce a rich mineralogical record of pressure decrease or because the earlier high p part of the rocks history was erased by the m1 loop irrespective of the specific scenario models for the tectonic evolution of the egp must take the substantial pressure decrease during m2 into account as it is probable the p t record at anakapalle is a reflection of tectonics affecting the entire province this article is protected by copyright all rights reserved

plio pleistocene trends in ice rafted debris on the lomonosov ridge although more than 700 sediment cores exist from the arctic ocean the plio pleistocene evolution of the basin and its marginal seas remains virtually unknown this is largely due the shallow penetration of most of these records and difficulties associated with deriving chronologies for the recovered material the integrated ocean drilling program s iodp expedition 302 arctic coring expedition acex recovered 197 m of neogene quaternary sediment from the circumpolar regions of the lomonosov ridge as detailed analyses of this material emerge research is beginning to formulate a long term picture of paleoceanographic changes in the central arctic ocean this paper reviews the acex plio pleistocene age model identifies uncertainties and addresses ways in which these may be eliminated within the established stratigraphic framework a notable reduction in the abundance of ice rafted debris ird occurs in the early part of the pleistocene and persists until marine isotope stage 6 mis 6 therefore while global oceanographic proxies indicate the gradual growth of terrestrial ice sheets during this time ird delivery to the central arctic ocean remained comparatively low and stable within the resolution of existing data the pleistocene reduction in ird is synchronous with predicted changes in both the inflow of north atlantic and pacific waters which in modern times are known to exert a strong influence on sea ice stability

geochronology and geological evolution of metamorphic rocks in the field islands area east antarctica detailed geochronological structural and petrological studies reveal that the geological evolution of the field islands area east antarctica was substantially similar to that of the adjacent archaean napier complex though with notable differences in late and post archaean times these differences reflect the area s proximity to the proterozoic rayner complex and consequent vulnerability to tectonic process involved in the formation of the latter distinctive structural features of the field islands are 1 consistent development of a discordant pervasive s3 axial plane foliation 2 re orientation of s3 axial planes to approximate to the subsequent e w tectonic trend of the nearby rayner complex 3 selective retrogression by a post d3 static thermal overprint and 4 relatively common development of retrogressive e w trending mylonitic shear zones peak metamorphic conditions in excess of 800 c at 900 100 m pa 9 kbar were attained at one locality following but probably close to the time of d2 folding d3 took place in late archaean times when metamorphic temperatures were about 650 c and pressures were about 600 mpa 6 kbar later temperatures of 600 50 c and pressures of 700 mpa 7kbar were attained in an amphibolite facies event presumably associated with the widespread granulite to amphibolite facies metamorphism and intense deformation involved in the formation of the rayner complex at about 1100 ma the area was subsequently subjected to near isothermal uplift rb sr isotopic data indicate that the pervasive d3 fabric developed at about 2400 2500 ma and this age can be further refined to 2456 8 5 ma by concordant zircon analyses from a syn d3 pegmatite all zircons were affected by only minor 7 10 pb loss and or new zircon growth during the rayner event at about 1100ma thus the 450 850 g gu concentrations of these zircons were too low to cause sufficient lattice damage over the 1350 ma from 2450 ma for excessive pb to be lost during the 1100 ma event the emplacement of pegmatite at 522 10 ma substantially changed the rb sr systematics of the only analysed rock that developed a penetrative fabric during the 1100 ma event monazite in this pegmatite contains an inherited pb component which probably resides in small opaque inclusions a good correlation is found between rb sr total rock ages and rock fabric u pb zircon intercepts with concordia also mostly correspond to known events however in one example a near perfect alignment of zircon analyses probably developed by mixing of unrelated components produced concordia intercepts that appear to have no direct geochronological significance

experimental determination of ree partition coefficients between zircon garnet and melt a key to understanding high t crustal processes the partitioning of rare earth elements ree between zircon garnet and silicate melt was determined using synthetic compositions designed to represent partial melts formed in the lower crust during anatexis the experiments performed using internally heated gas pressure vessels at 7 kbar and 900 1000 c represent equilibrium partitioning of the middle to heavy ree between zircon and garnet during high grade metamorphism in the mid to lower crust the dree zircon garnet values show a clear partitioning signature close to unity from gd to lu because the light ree have low concentrations in both minerals values are calculated from strain modelling of the middle to heavy ree experimental data these results show that zircon is favoured over garnet by up to two orders of magnitude the resulting general concave up shape to the partitioning pattern across the ree reflects the preferential incorporation of middle ree into garnet with dgd zircon garnet ranging from 0 7 to 1 1 dho zircon garnet from 0 4 to 0 7 and dlu zircon garnet from 0 6 to 1 3 there is no significant temperature dependence in the zircon garnet ree partitioning at 7 kbar and 900 1000 c suggesting that these values can be applied to the interpretation of zircon garnet equilibrium and timing relationships in the ultrahigh t metamorphism of low ca pelitic and aluminous granulites

osumilite in the sapphirine quartz terrane of enderby land antarctica implications for osumilite petrogenesis in the granulite facies

rotating stratified barotropic flow over topography mechanisms of the cold belt formation off the soya warm current along the northeastern coast of hokkaido abstractthe soya warm current swc flows through a shallow strait between the japan sea and the sea of okhotsk the swc has a jet structure downstream of the strait along the northern coast of hokkaido with a maximum speed exceeding 1 m s 1 at its axis in summer and fall a surface cold belt with a subsurface doming structure forms offshore of the swc axis mechanisms of the cold belt formation are discussed from a point of view of resonant interaction between a barotropic stratified flow and a shallow sill and subsequent baroclinic adjustment along the swc when a stratified current rides a slope upstream the thermocline displaces upward greatly and outcrops owing to resonant generation of internal kelvin waves if the upper layer is thinner than the lower layer the control section where a froude number is unity occurs upstream from the sill crest when the ambient inflow has a barotropic flow component these upwelling features closely resemble those along the southwestern coast of sakhalin islan

peel back controlled lithospheric convergence explains the secular transitions in archean metamorphism and magmatism abstract the preserved archive of continental crust suggests that various secular geologic and geochemical transitions took place during the mesoarchean and neoarchean 3 2 2 5 ga these transitions which are imprinted in the metamorphic and magmatic rock record include the emergence of paired metamorphism an increasing depth of tonalite trondhjemite granodiorite ttg formation the widespread appearance of granites and changes in the composition of mafic rock previous studies have argued that these secular transitions reflect secular cooling of the mantle and herald a gradual transition from pre plate tectonic to plate tectonic regimes however their tectonic driver remains elusive in part due to a lack of detailed understanding of lithospheric dynamics prevalent during this transitional period when the mantle was warmer compared to the present day here we demonstrate that lithospheric convergence driven by peel back peeling process under warmer mantle conditions termed as peel back convergence may explain the late archean secular transitions this tectonic phenomena features large scale peeling a form of delamination but not dripping of the mantle lithosphere with or without lower crustal rocks during convergence we simulated numerical models of peel back convergence under archean crust mantle conditions and predicted the pressure temperature p t conditions of crustal metamorphism and melting the evolution of peel back convergent setting features juxtaposition of a colder compressional regime with thickened crust that forms at the site of peeling and a warmer extensional regime with thinned crust forming behind it the metamorphic and magmatic p t conditions prevalent in these two tectonothermal sites replicate the features of the late archean rock record including the appearance of coeval high t p and intermediate t p metamorphic rocks greater abundance of high t p relative to intermediate t p metamorphic rocks dominant formation of ttgs at higher pressures where garnet rutile is stable in the residue reworking of pre existing felsic rocks sediments to produce potassic granites and interaction of crustal melts and mantle to form hybrid granitoids thus peel back convergence can explain the thermobaric bimodality of late archean rocks within an asymmetric tectono thermal framework however unlike modern convergent plate boundaries the asymmetry is controlled by lithospheric peeling

hot orogens and supercontinent amalgamation a gondwanan example from southern india abstract the southern granulite terrane in southern india preserves evidence for regional scale high to ultrahigh temperature metamorphism related to the amalgamation of the supercontinent gondwana here we present accessory mineral zircon and monazite geochronological and geochemical datasets linked to the petrological evolution of the rocks as determined by phase equilibria modelling the results constrain the duration of high to ultrahigh temperature 900 c metamorphism in the madurai block to be c 40 ma with peak conditions achieved c 60 ma after the formation of an orogenic plateau related to the collision of the microcontinent azania with east africa at c 610 ma a 1d numerical model demonstrates that the attainment of temperatures 900 c requires that the crust be moderately enriched in heat producing elements and that the duration of the orogenic event is sufficiently long to allow conductive heating through radioactive decay both of these conditions are met by the available data for the madurai block our results constrain the length of time it takes for the crust to evolve from collision to peak p t i e the prograde heating phase then back to the solidus during retrogression this evolution illustrates that not all metamorphic ages date sutures

subtropical western boundary currents over slopes detaching from coasts with inshore pool regions an indication to the kuroshio nearshore path abstractthe dynamics of subtropical western boundary currents over slopes detaching from coasts with inshore pool regions where the water of the subtropical gyre does not enter and the velocity is small are investigated this study is intended to understand the dynamics of the nearshore path of the kuroshio which has a distinct boundary between the boundary current and the coastal water numerical experiments under idealized conditions are made the results show flow patterns with pool regions similar to the kuroshio under simple conditions a deep countercurrent is present on the lower bottom slope which represents observed deep currents this is part of a deep cyclonic recirculation north of the jet which extends to the lower bottom slope despite steep topography this extension can be explained by the geostrophic contours in this region the upper boundary current feels the bottom slope and the westward intensification is blocked in the other region where the bottom layer velocity is very small

the flow system in the japan sea caused by a sea level difference through shallow straits we investigate how the current system through the japan sea is driven and what determines the volume transport we suppose that a part of the difference in geopotential anomaly between the subtropical and subpolar gyre is converted into a barotropic sea level difference across the three shallow straits which connect the japan sea with the pacific and that this difference is the primary driving force of the current system then we examine the flow under the condition that there is a constant sea level difference between two oceans connected through a shallow strait we found that the strait acts as a source of arrested shelf waves or steady coastal flows for the timescale beyond the inertial period in the northern hemisphere steady flows are established along the shelves with the coast to the right left in the ocean of low high sea level we apply this notion to the current system through the japan sea the tsushima nearshore branch the tsugaru coastal mode and the soya current can all be interpreted as a coastally trapped flow whose source is the upstream strait further a series of northeastward flows along the south and east china seas shelves should be interpreted as coastally trapped flows whose source is the downstream tsushima strait numerical model experiments incorporating the realistic topography also simulate the observed flow fields the volume flux through each strait being limited geostrophically relations between the sea level difference and volume transport can be represented by simple formulas

radiogenic heat production in the continental crust abstract the thermal structure and evolution of continents depend strongly on the amount and distribution of radioactive heat sources in the crust determining the contribution of crustal rocks beneath a superficial layer is a major challenge because heat production depends weakly on major element composition and physical properties such as seismic wavespeed and density enriched granitic intrusives that lie at the current erosion level have a large impact on the surface heat flux but little influence on temperatures in the deep crust many lower crustal rocks that are poor in heat producing elements are restites from ancient orogenic events implying that enrichment of the upper crust was achieved at the expense of deeper crustal levels for the same total heat production concentrating heat sources in an upper layer acts to reduce temperatures in the lower crust thereby allowing stabilization of the crust the present day structure of the crust is a consequence of orogeny and should not be adopted for thermal models of the orogenic event itself this review summarizes information extracted from large data sets on heat flow and heat production and provides estimates of crustal stratification and heat production in several geological provinces analysis of global and regional data sets reveals the absence of a positive correlation between surface heat flow and crustal thickness showing that the average crustal heat production is not constant differences of heat flow between geological provinces are due in large part to changes of crustal structure and bulk composition collating values of the bulk crustal heat production in a few age intervals reveals a clear trend of decrease with increasing age this trend can be accounted for by radioactive decay indicating that thermal conditions at the time of crustal stabilization have not changed significantly for the average crustal thickness of 40 km moho temperatures are near solidus values at the time of stabilization suggesting an intrinsic thermal control on crustal thickness and heat production distribution crustal thickening by more than about 10 km above this mean value induces changes of gravitational potential energy that exceed the strength of the lithosphere for several provinces where strong constraints on heat production are available it is shown that prior to intracrustal fractionation only modest amounts of thickening were needed to generate the conditions of ultra high temperature metamorphism the tell tale signature of crustal heat production is anatectic and metamorphic events that lag the cessation of orogenic activity by several tens of million years crustal heat production is decreasing with time because it is due to the radioactive decay of uranium thorium and potassium its rundown is responsible for the secular cooling of lithospheric roots at a typical rate of about 100 k gy 1 implying complex thermal interactions with a convecting mantle that is not cooling at the same rate

northward flow in the bering and chukchi seas the ocean circulation on the bering and chukchi sea shelves is investigated using a barotropic numerical model in the presence of the winter seasonal wind stress from the northeast observed northward transport through bering strait of 0 6 sv for the 8 month winter season 1981 1982 can be driven by a sea level difference of 0 4 m between the pacific and arctic oceans this sea level difference between basins is in agreement with hydrographic estimates in the absence of wind stress a 0 4 m sea level difference drives a northward transport through bering strait of 1 1 sv maximum transport through bering strait is geostrophically limited not frictionally or inertially limited major qualitative circulation features of the region are evident in the simulations westward intensification of the northward transport across the bering sea shelf is confirmed with the major transport occurring in the relatively deep gulf of anadyr in the absence of wind anadyr strait contributes 72 of the northward transport through bering strait with northeasterly winds there is a net clockwise circulation around saint lawrence island within the 20 m isobath norton and kotzebue sounds are little influenced by the general circulation northward transport in the chukchi sea bifurcates in the vicinity of point hope the predominant branch is directed toward the northwest in a broad canyon feature south of herald shoal and the balance becomes the alaskan coastal current between cape lisburne and point barrow

an integrated microtextural and chemical approach to zircon geochronology refining the archaean history of the napier complex east antarctica integrated textural and chemical characterisation of zircon is used to refine the u pb geochronology of the archaean ultra high temperature napier complex east antarctica scanning electron microscope characterisation of zircon and the rare earth element compositions of zircon garnet and orthopyroxene are integrated to place zircon growth in an assemblage context thereby providing tighter constraints on the timing of magmatic and metamorphic events data indicate that magmatism occurred in the central and northern napier complex at ca 2 990 ma a regional relatively low pressure metamorphic event occurred at ca 2 850 2 840 ma mineral ree data from garnet bearing orthogneiss indicate that ca 2 490 2 485 ma u pb zircon ages provide an absolute minimum age for the ultrahigh temperature uht foliation preserved in this rock internal zircon zoning relationships and estimated zircon garnet dree values from paragneiss suggest that an absolute minimum age of ultra high temperature metamorphism is ca 2 510 ma but that it is more likely to be older than ca 2 545 ma we suggest that the high proportion of published zircon u pb data with ages between ca 2 490 2 450 ma reflects late post peak zircon growth and does not date the timing of peak uht metamorphism

the fate of zircon during uht uhp metamorphism isotopic u pb δ18o hf and trace element constraints garnet clinopyroxene ultra high pressure uhp rocks from the northern bohemian massif contain zircon with micro diamond inclusions trace element concentrations oxygen and hafnium isotopic composition and u pb age of distinct textural domains in zircon characterize their growth conditions and temporal evolution diamond bearing zircon mantle domains with relicts of oscillatory zoning have uniform th u ratios 0 1 0 2 high ti contents 110 190 ppm corresponding to temperatures of at least 1100 c and some two of 17 mantle analyses preserve steep heavy rare earth element hree patterns with ybn gdn 10 11 with a weak negative eu anomaly these signatures are consistent with crystallization from a melt under uhp ultra high temperature uht conditions some of the bright cathodoluminscence cl rims preserve th u and ti values characteristic of the zircon mantles but others show elevated th u ratios of 0 3 0 4 and lower ti contents 20 40 ppm only 13 ppm in a rare low cl outer rim as they feature flat hree patterns and negative eu anomalies and commonly make embayments and truncate the mantle zoning we suggest that they have formed through recrystallization in the solid state during exhumation of the rock when both garnet and plagioclase were stable the three zircon domains that is cores mantles and rims yield u pb concordia ages of 340 9 1 5 340 3 1 5 and 341 2 3 4 ma respectively when linked to the previously reconstructed p t path of the rock the error limits of the zircon mantle and rim ages constrain the exhumation of the rocks from depth of 140 km uhp to 80 km hp to a minimum rate of 1 5 cm yr 1 the zircon cores are heterogeneous in terms of th u ratio below 0 1 but also above 0 2 and ree characteristics and their ehf values scatter between 15 7 and 4 8 with similar values for individual domains within a single zircon grain suggesting a very localized control on hafnium isotope composition on a grain scale the non equilibrated ehf values as well as a large range of the hf depleted mantle model ages possibly reflect the presence of a heterogeneous population of old zircon consequently the uniform and young 238u 206pb ages may represent near complete resetting of the u pb geochronometer during the uhp uht event at c 340 ma through dissolution reprecipitation process in contrast to hf the oxygen isotope composition of zircon is homogeneous ranging between 7 8 and 9 6 vsmow reflecting a source containing upper crustal material and homogenization at uhp uht conditions our study documents that continental crust was subducted to mantle depths at c 340 ma during the variscan orogeny and was subsequently very rapidly exhumed implying that the sequence of events was faster than can be resolved by the secondary ion mass spectrometry technique

foundering triggered by the collision of india and asia captured in xenoliths xenoliths that erupted in the se pamir of tajikistan from 1000 to 1050 c and 90 km depth illuminate what happens when crust founders into the mantle 40ar 39ar dating of minerals from the xenoliths and volcanic host rocks of the shoshonitic dunkeldik pipe and dike field indicates eruption at 11 2 0 2 ma u pb and trace element laser ablation split stream inductively coupled plasma mass spectrometry of zircon shows that the igneous and metasedimentary xenoliths were likely derived from the crustal section into which they were intruded the jurassic cretaceous andean style magmatic arc and its proterozoic mesozoic host rocks along the southern margin of asia recrystallization of these zircons was extensive yielding a range of dates down to 11 ma the zircons show distinct changes in eu anomaly lu gd ratio and ti concentrations compatible with garnet growth and minimal heating at 22 20 ma and then 200 300 c of heating 25 km of burial and alkali carbonate melt injection at 14 11 ma these changes are interpreted to coincide with i heat input due to indian slab breakoff at 22 20 ma and ii rapid thickening and foundering of the pamir lithosphere at 14 11 ma prior to and synchronous with collision between deep indian and asian lithospheres beneath the pamir

anatexis during high pressure crustal metamorphism evidence from garnet whole rock ree relationships and zircon rutile ti zr thermometry in leucogranulites from the bohemian massif in situ mineral garnet zircon trace element and garnet rock ree distribution data obtained on leucogranulites from the bohemian massif are consistent with an origin of these now strongly deformed rocks as migmatitic leucogranites formed through dehydration melting of muscovite bearing protoliths at high pressures during the variscan orogeny partial melting at p t conditions of 900 9408c and 1 6 1 8 gpa consumed mica and feldspar to produce peritectic kyanite and high ca 20 24 mol grossular high eu garnet along with leucogranitic melt dree grt rock values calculated from this garnet and its host rock are consistent with garnet melt equilibrium at these p t conditions ternary feldspar and rutile 2000 3200 ppm zr crystallized from the leucogranite melts in the migmatites at minimum temperatures of 4880 9208c these formed along with apatite prior to and accompanying the crystallization of garnet rims and the majority of the garnet that occurs in the leucogranulites this garnet is typified by strong depletion in eu and dree grt rock values consistent with garnet melt equilibrium only after the fractionation of apatite 50 3 wt and ternary feldspar 5 15 wt zircons in one leucogranulite are dominated by oscillatory zoned grains that yield pre variscan u pb ages 495 and 433 ma and crystallized from ti in zircon thermometry at 700 8108c in their original host magmas these xenocrystic grains are potentially preserved because of the short timespan of the high p t variscan event at 340 ma and the moderate amounts of leucogranitic partial melt present in the rocks estimated as at least 30 wt melt from zr mass balance the production of new zircon associated with the variscan event in this leucogranulite is limited to weakly zoned zircon rims characterized by flat heavy ree patterns that approach ree equilibrium with garnet rims this post peak zircon was formed at minimum p t conditions of 8408c and 0 8 1 0 gpa along the decompression cooling path defined by previous work for the bohemian massif granulites the garnet whole rock ree relationships observation of eu depletion between early and later garnet growth phases and zr ti thermometric estimates indicating temperatures of48408c for this evolution cannot be explained by the alternative interpretation of the leucogranulites as reworked and recrystallized pre variscan low p granitoids but support a model in which the bohemian leucogranulites were high p variscan age migmatites containing significant amounts of melt

th u ratios in metamorphic zircon this is the peer reviewed version of the following article yakymchuk c kirkland c l clark c 2018 th u ratios in metamorphic zircon journal of metamorphic geology which has been published in final form at https doi org 10 1111 jmg 12307 this article may be used for non commercial purposes in accordance with wiley terms and conditions for self archiving

chemical th u total pb dating by electron microprobe analysis of monazite xenotime and zircon from the archean napier complex east antarctica evidence for ultra high temperature metamorphism at 2400 ma abstract the chemical th u total pb isochron method chime of dating was carried out on monazite xenotime zircon and polycrase in six samples of ultra high temperature uht granulites and one of pegmatite from five localities in the napier complex enderby land east antarctica despite the chronological heterogeneity in many grains the chime ages overall show strong evidence for a major event near 2420 ma analyzed monazite and xenotime grains except monazite from mt riiser larsen have a narrow range of chime ages 2412 42 ma from beaver island and 2429 36 2421 18 and 2431 22 ma from reference peak these ages are indistinguishable from chime ages on zircon 2436 17 and 2420 20 ma and on central older domains of monazite 2404 54 ma from mt riiser larsen and on a zircon rim 2430 15 ma from mt cronus we conclude that the ca 2420 ma ages relate to the uht metamorphism but we do not exclude the possibility that lead loss has lowered the ages by some 50 ma this interpretation is consistent with a preponderance of ages obtained by other methods on the napier complex particularly the ca 2475 25 ma age for a generation of pegmatites coeval with the uht event and containing the uht mineral assemblage beryllian sapphirine khmaralite quartz events prior to 2500 ma and after 2420 ma have also left their imprint on the chime ages for example a 3646 ma chime age suggests that a zircon core is inherited whereas ca 2200 and 2000 ma chime ages in zoned monazite could be due to further lead loss during post uht activity the much younger chime age of 1094 67 ma for monazite from a pegmatite at zircon point casey bay confirms 1000 1100 ma lead loss ages reported for this region and could result from fluid activity in an outlier of rayner activity near the boundary between the rayner and napier complexes approximately 2500 ma ages have been reported from granulite facies but not uht rocks in the madras and nilgiri blocks in south india these blocks and the napier complex could have constituted a single structural unit by late archean time

lu hf systematics of the ultra high temperature napier metamorphic complex in antarctica evidence for the early archean differentiation of earth s mantle abstract the napier complex of the east antarctic craton comprises some of the oldest rocks on earth 3 8 billion years old overprinted by an ultra high temperature uht metamorphic event near the archean proterozoic boundary garnet orthopyroxene sapphirine osumilite rutile and a whole rock representing a fully equilibrated assemblage from this uht granulite belt have yielded a lu hf isochron age of 2403 43 ma the first ever determined on a uht mineral assemblage preservation of the uht mineral assemblage in the rock analyzed without any significant retrogression suggests rapid cooling with closure likely to have occurred for the lu hf system at post peak uht conditions near a temperature of 800 c this mineral whole rock isochron yields an initial 176 hf 177 hf ratio corresponding to an e hf value of 14 1 acquired during uht metamorphism such a low value demonstrates that overall uht granulites evolved in a low lu hf environment probably formed when the rocks were first extracted from a highly depleted mantle zircon e hf values we have measured see through the uht metamorphism and show that the source materials for the magmas that formed the napier complex were extremely depleted 5 6 e hf at 3 85 ga relative to the chondritic uniform reservoir chur these results also suggest significant depletion of the early archean mantle in agreement with the early differentiation of the earth that the latest core formation models require

arctic ocean glacial history while there are numerous hypotheses concerning glacial interglacial environmental and climatic regime shifts in the arctic ocean a holistic view on the northern hemisphere s late quaternary ice sh

four zircon ages from one rock the history of a 3930 ma old granulite from mount sones enderby land antarctica ion microprobe u th pb analyses of zircons from a granulite grade orthogneiss from mount sones enderby land antarctica record the ages of four principal events in the history of the gneiss three of which already have been recognized through previous isotopic dating of other samples the structure of the zircons indicates at least four different stages of growth the several zircon ages were obtained by grouping the analyses according to the stage they represented in the observed stratigraphic succession of growth and thereby defining separate u pb discordance patterns for each stage the stratigraphically oldest zircon rare discrete cores is indistinguishable in age from the most common euhedrally zoned zircon both crystallized when the tonalitic precursor of the orthogneiss was emplaced into the crust 3927 10 ma ago making the orthogneiss currently the oldest known terrestrial rock the outer parts of most grains and some whole grains recrystallized at 2948 31 17 ma during or immediately after possibly 100 ma of high granulite grade metamorphism the recrystallized zircon was isotopically disturbed by tectonism associated with reactivation of the southern margin of the napier complex at 1000 ma in the intervening time at 2479 23 ma the cores and zoned zircon suffered a major isotopic disturbance involving movement of radiogenic pb which left most of the crystals with radiogenic pb deficiencies but produced local radiogenic pb excesses in others a new generation of zircon characterized by very high th u and low u grew at that time that event deformation and possibly a minor rise in temperature produced widespread perturbations of other isotopic systems throughout the napier complex

on the driving processes of the atlantic meridional overturning circulation because of its relevance for the global climate the atlantic meridional overturning circulation amoc has been a major research focus for many years yet the question of which physical mechanisms ultimately drive the amoc in the sense of providing its energy supply remains a matter of controversy here we review both observational data and model results concerning the two main candidates vertical mixing processes in the ocean s interior and wind induced ekman upwelling in the southern ocean in distinction to the energy source we also discuss the role of surface heat and freshwater fluxes which influence the volume transport of the meridional overturning circulation and shape its spatial circulation pattern without actually supplying energy to the overturning itself in steady state we conclude that both wind driven upwelling and vertical mixing are likely contributing to driving the observed circulation to quantify their respective contributions future research needs to address some open questions which we outline

hot orogens tectonic switching and creation of continental crust many granulite terrains were too hot to have formed during continental collision rather along with many high grade metamorphic terrains that typify continental crust most formed in accretionary orogens during tectonic switching when prolonged lithospheric extension was interrupted by intermittent transient contraction based on modern and ancient examples tectonic switching occurs when slab retreat induces upper plate extension causing arc splitting formation of microcontinent slivers and backarc basins then intermittent arrival of buoyant oceanic plateaus induces transient flat subduction or slab flip and crustal thickening during slab retreat basaltic magmas produced from decompressed asthenosphere advect into the extending orogen causing granulite facies metamorphism and granite generation but subsequent thickening during flat subduction cools the region thickening is focused in the thermally softened backarc region and if sediment filled a hot short lived 10 m y narrow 50 100 km orogenic belt forms such thickening is often misleadingly ascribed to arc or microcontinent collision once slab retreat mode is reestablished lithospheric extension recommences and a new arc backarc system forms generally outboard arrival of another plateau will reverse the procedure and another short lived hot orogen will form within the orogenic system cycles of tectonic switching efficiently produce continental crust

the origin of retrograde shear zones in the napier complex implications for the tectonic evolution of enderby land antarctica abstract retrograde shear zones rszs in granulites at fyfe hills in the napier complex east antarctica formed during a late proterozoic or early palaeozoic reactivation event these rszs preserve a structural and metamorphic record of excavation from depth equivalents of 6 8 kbar 20 26 km to depth equivalents of 3 5 kbar 10 17 km excavation was essentially isothermal suggesting that the rszs formed in an allochthonous terrain during crustal thickening by low angle thrusting rehydration of the rszs was accompanied in part by large ion lithophile element enrichment and reduction in k rb features which are attributed to 1 the retrograde fluid having originated in continental crust that had not previously undergone granulite facies metamorphism and 2 the fluid having been introduced into the rszs by the intrusion of granitic dykes and pegmatites the saturation of these rsz pegmatites in components characteristically depleted in deep crustal and mantle sources such as boron and beryllium supports a low grade source for the retrograde fluid furthermore a rehydration mechanism involving intrusion of pegmatites suggests that the fyfe hills granulites are underlain by a significantly lower grade hydrous terrain

u pb geochronology from tonagh island east antarctica implications for the timing of ultra high temperature metamorphism of the napier complex abstract ion microprobe u pb zircon geochronology of an orthopyroxene bearing felsic orthogneiss from central tonagh island enderby land east antarctica provides insight into the chronological metamorphic evolution of the archaean napier complex the details of which have been the source of debate for over two decades the orthogneiss crystallised at 2626 28 ma predating peak ultra high temperature uht metamorphism and development of an intense regional s1 gneissosity two subsequent episodes of zircon growth resetting can be identified a minor period of zircon growth occurred at 2546 13 ma the regional significance and geological nature of which is unclear this was followed by an episode of abundant zircon growth as mantles on 2626 ma cores and as anhedral grains partly characterised by high th u 1 2 at 2450 2480 ma this age coincides with both lower and upper concordia intercept ages from other u pb zircon studies and several rb sr and sm nd whole rock isochron ages from the napier complex we conclude that uht metamorphism occurred at 2450 2480 ma and find no compelling evidence that uht occurred much earlier as has been postulated the zircon u pb data from this study also indicates a lower intercept age of 500 ma which coincides with the emplacement of early palaeozoic pegmatite swarms and synchronous infiltration of aqueous fluids into the southwestern regions of the napier complex

stable isotope and lithologic evidence of late glacial and holocene oceanography of the northwestern pacific and its marginal seas abstract stable isotopes geochemical lithological and micropaleontological results from cores from the far northwest fnw pacific and the okhotsk and bering seas are used to reconstruct the regional environment for the last glaciation the deglacial transition and the holocene 18 o records of planktonic foraminifera of the region show two light shifts during deglacial time provoked by the freshening of the surface water and climate warming these north pacific terminal events t1anp and t1bnp with ages of 12 500 and 9300 yr b p respectively occur almost simultaneously with two episodes of accelerated glacier melting around the north atlantic along with the isotopic shifts the caco 3 content in regional sediments increased abruptly 1a and 1b carbonate peaks probably due to changes of productivity and pore water chemistry of surface sediments organic matter and opal concentration increased during the transition between t1anp and t1bnp events in the sediments of the fnw pacific and the southern part of the bering sea and opal content increased in the holocene in the bering and okhotsk seas 13 c records of cores from the okhotsk and bering seas and the fnw pacific do not contradict the hypothesis of increased intermediate water formation in the region during glaciation during deglaciation accumulation of the coarse terrigenous component decreased in sediments of the bering sea and the fnw pacific before the t1anp event probably as a result of rising sea level and opening of the bering strait

parameter sensitivity of primitive equation ocean general circulation models abstract experiments with a low resolution primitive equation ocean general circulation model with idealized basin geometry and surface forcing have been carried out in order to identify the processes controlling the climatically important aspects of the circulation emphasis was placed on the sensitivity of the model solutions to the magnitude of the vertical diffusivity scaling arguments suggest and the numerical experiments confirm that the solutions are most sensitive to the magnitudes of the wind stress curl and the vertical diffusivity for small vertical diffusivity wind forcing dominates the solution the vertical scale of the thermocline is set by the strength of the ekman pumping and there is a multiple gyre circulation in the upper layers for large vertical diffusivity diabatic surface forcing dominates the solution vertical diffusion controls the vertical scale of the thermocline and there is a single large anticyclonic gyre in the upper layers both the meridionally and zonally inte

interconnectivity between volume transports through arctic straits arctic heat and freshwater budgets are highly sensitive to volume transports through the arctic subarctic straits here we study the interconnectivity of volume transports through arctic straits in three models two coupled global climate models one with a third degree horizontal ocean resolution higem1 1 and one with a twelfth degree horizontal ocean resolution hadgem3 and one ocean only model with an idealized polar basin tenth degree horizontal resolution the two global climate models indicate that there is a strong anti correlation between the bering strait throughflow and the transport through the nordic seas a second strong anti correlation between the transport through the canadian artic archipelago caa and the nordic seas transport and a third strong anti correlation is found between the fram strait and the barents sea throughflows we find that part of the strait correlations is due to the strait transports being coincidentally driven by large scale atmospheric forcing patterns however there is also a role for fast wave adjustments of some straits flows to perturbations in other straits since atmospheric forcing of individual strait flows alone cannot lead to near mass balance fortuitously every year idealized experiments with an ocean model nemo3 6 that investigate such causal strait relations suggest that perturbations in the bering strait are compensated preferentially in the fram strait due to the narrowness of the western arctic shelf and the deeper depth of the fram strait