westward traveling low frequency hemispheric waves in the atmosphere the resonant response of a shallow water atmosphere is examined by applying forcing for each hemisphere separately it is shown that westward traveling waves tend to concentrate in hemispheres of their forcing for real damping and this tendency is greater for low frequency waves these waves appear from the global rossby modes and have resonant peaks only in one hemisphere since they are largely confined to one hemisphere and are thus qualitatively more similar to observations then are normal modes the results were essentially unchanged when the basic experiments were repated many times each with a different randomly generated forcing structure

retrograding wintertime low frequency disturbances over the north pacific ocean abstract retrograding westward moving features over the middle and high latitudes of the northern hemisphere during winter are examined in observations and data from a gcm simulation using complex principal component analysis the first eigenvector of the low frequency middle troposphere geopotential height over the north pacific has a zonal scale of 120 160 deg longitude and a dipole like meridional structure with maximum amplitude over the gulf of alaska and the bering sea these disturbances propagate westward and go through a life cycle of growth and decay over a period of about three weeks the temporal evolution of the energy and its conversions during the life cycle of the simulated pacific disturbances are composited using the time dependent coefficient of the principal component analysis as an index for their occurrence the composite energy cycle indicates that the kinetic and available potential energy of these disturbances grow and decay together both baroclinic and barotropic conversion

mountain torques and atmospheric oscillations theoretical work and general circulation model gcm experiments suggest that the midlatitude jet stream s interaction with large scale topography can drive intrasea sonal oscillations in large scale atmospheric circulation pat terns in support of this theory we present new observa tional evidence that mountain induced torques play a key role in 15 30 day oscillations of the northern hemisphere circulation s dominant patterns the affected patterns in clude the arctic oscillation ao and the pacific north american pna pattern positive torques both accelerate and anticipate the midlatitude westerly winds at these peri odicities moreover torque anomalies anticipate the onsets of weather regimes over the pacific as well as the break ups of hemispheric scale regimes

observed and gcm simulated westward propagating planetary scale fluctuations with approximately three week periods the structural characteristics and vorticity dynamics of westward traveling patterns wtp in the troposphere are examined using the national centers for environmental prediction national center for atmospheric research ncep ncar reanalyses based on observations for the 1973 95 period as well as the output from a 100 yr integration of a general circulation model gcm with a rhomboidal truncation at 30 wavenumbers and 14 vertical levels an identical set of diagnostic tools including progressive retrogressive variance analysis crossspectra and complex empirical orthogonal functions eofs are applied to the reanalysis and gcm datasets for 300 mb height these diagnoses all indicate that the wtp are most prominent during the cold season in the high latitude zone extending westward from northwestern canada to northeastern siberia with a typical period of 22 days outstanding episodes are identified on the basis of the temporal coefficients of the leading complex eof composite charts of the anomalous 300 mb height sea level pressure and 850 mb temperature fields at various phases of these events are constructed the typical circulation changes accompanying the passage of the wtp are similar to those associated with well known regional weather phenomena such as amplified pressure ridges over alaska cold air outbreaks over western north america and east asia and heavy snowfall over the pacific northwest the occurrence of the wtp over the north pacific is also characterized by notable changes in the spatial distribution and intensity of synoptic scale activity the contributions of relative vorticity advection planetary vorticity advection the b effect and horizontal divergence to the vorticity tendency in various phases of the composite wave at 300 500 and 850 mb are investigated in the mid and upper troposphere the vorticity dynamics of the wtp is similar to that of free external rossby waves with the b effect which leads to westward propagation being the dominant term whereas the eastward advection of relative vorticity is less important due to the weak mean zonal flow in the alaska siberia sector most of the essential characteristics of the observed wtp deduced from the ncep ncar reanalyses are well reproduced by the gcm the realism with which this phenomenon can be simulated in a model environment offers considerable promise for using the gcm as a tool for studying the impact of wtp on intraseasonal atmospheric variability in extended model experiments and for assessing the dependence of the locality and activity level of the wtp on various states of the ambient circulation

genetic modelling for banded iron formation of the hamersley group pilbara craton western australia abstract the banded iron formation bif of the hamersley group pilbara craton western australia particularly from the well studied dales gorge member is unique in its lateral stratigraphic and petrological continuity throughout an area exceeding 60 000 km 2 enabling reasonable estimates for the annual input of components to the depository in the model of this paper varying supply of materials for the medley of mesoband types particularly of iron and silica in the oxide bif can be accommodated by the interaction of two major oceanic supply systems 1 surface currents and 2 convective upwelling from mid oceanic ridge mor or hot spot activity both modified by varied input of pyrochastic material 1 the surface currents were saturated in silica and carried minimal iron due to photic precipitation but were periodically recharged by storm mixing precipitation from them gave rise to the banded chert rich horizons including the varves whose regular and finely laminated iron silica distribution resulted from seasonal meteorological influences 2 precipitation from convection driven upwelling of high iron solution from mor or hot spot activity periodically overwhelmed the delicate seasonal patterns of 1 to produce the iron dominated mesobands a wide range of intermediate mesoband types resulted where the deep water supply was modified by varied mor activity or by partial blocking of upwelling waters by surface currents such as by the present el nino during these periods of oxide dominated bif silica was deposited from saturated solution mainly by evaporative concentration and iron by oxidation due to photolysis and photosynthetically produced oxygen superimposed on these supply differences was the varying effect of fine aluminous ash from dominantly northern distal volcanic sources changing the meteorological and depositional conditions occasional input of extremely fi ash during bif precipitation produced mesoband cm scale variations involving increased carbonate silicate precipitation sustained volcanic periods resulted in s macroband deposition chert carbonate silicate bif with shale gradually returning to the dominant hematite magnetite chert bif as the volcanic input waned during volcanic periods the normally high capacity of sunlight to precipitate ferric iron directly by photolytic oxidation of ferrous iron and by photosynthetic production of oxygen was modified by turbidity in the atmosphere aerosols and dust and in the water colloids from reactive ash s surface precipitated ferric hydroxyoxide redissolved in the presence of decaying organic matter in the subphotic zone augmenting the iron content of the zone precursor ferrous carbonates and silicates were precipitated when the iron concentration of this sub photic zone exceeded their respective solubilities during volcanism the increased availability of nutrients particularly phosphorus to surface waters increased the organic contribution despite lower light values leading to an almost total absence of ferric iron oxides in the s macrobands i e no magnetite or hematite cooling of warm silica saturated sea water during these periods of olcanic winter increased the ratio of precipitation of silica to iron which however was still controlled by seasonal conditions intermediate concentrations of organic matter insufficient to totally convert the ferric compounds either during precipitation or diagenesis resulted in overgrowths of magnetite on hematite and eventually in the substantial conversion of hematite to magnetite where higher temperatures were achieved during low grade regional metamorphism changes in sea level to explain facies changes in bif are not required in this model but are not excluded the preferred conditions are for a very low oxygen to anoxic atmosphere a much higher level of mor activity than at present the presence of photosynthetic plankton the absence of si silica secreting organisms and a deep sea water temperature higher than 20 c however none of these conditions is essential to the model a narrow carbonate bank is postulated for part of the fortescue river valley area during marra mamba iron formation times basal hamersley group with bif precipitation on either side the reef is postulated to have grown northward becoming a major shallow water carbonate platform on the pilbara continent during upper marra mamba iron formation and wittenoom dolomite times but ceased to play an important role in subsequent periods

geographical dependence observed in blocking high influence on the stratospheric variability through enhancement and suppression of upward planetary wave propagation abstractprevious studies have suggested the importance of blocking high bh development for the occurrence of stratospheric sudden warming ssw while there is a recent study that failed to identify their statistical linkage through composite analysis applied to high amplitude anticyclonic anomaly events observed around every grid point over the extratropical northern hemisphere the present study reveals a distinct geographical dependence of bh influence on the upward propagation of planetary waves pws into the stratosphere tropospheric bhs that develop over the euro atlantic sector tend to enhance upward pw propagation leading to the warming in the polar stratosphere and in some cases to major ssw events in contrast the upward pw propagation tends to be suppressed by bhs developing over the western pacific and the far east resulting in the polar stratospheric cooling this dependence is found to arise mainly from the sensitivity of the interference between the climatological pws and upward p

extratropical low frequency variability as a low dimensional problem ii stationarity and stability of large scale equilibria stationarity and stability properties of large scale persistent anomalies in the northern hemisphere are addressed the low order model developed in part i is used for this purpose it was obtained as the projection of a three level quasi geostrophic system on the ten leading empirical orthogonal functions three global quasi stationary states are identified which represent the arctic high and the positive and negative phases of the main teleconnections pacific north american and north atlantic oscillation the quasi stationary solutions have only a partial correspondence to the weather regimes found in part i stability analysis shows a growing mode that describes an oscillation between the two phases of the teleconnections the negative teleconnection state is also shown to be much more stable than the other two possible decay mechanisms are also discussed copyright 2002 royal meteorological society

study of block onset using sensitivity perturbations in climatological flows this work describes the dynamics of adjoint sensitivity perturbations that excite block onsets over the pacific and atlantic oceans appropriate functions are derived for the blocking indices for these two regions and the model basic flow is constructed from northern hemisphere climatological data the concepts of sensitivity analysis are extended to forced problems this tool is used to investigate block onset due to atmospheric forcing such as that resulting from tropical sea surface temperature anomalies these linear studies are carried out in a hemispherical primitive equations u coordinate two layer model results show that wind sensitivity perturbations less than 10 m s 21 and sensitivity forcing of vorticity sources of the order of 1 5 3 10210 s22 are sufficient to excite block onset both for the pacific and atlantic blocking sensitivity perturbations and forcing perturbations when expressed in terms of vertical vorticity display a rossby wave train structure mainly found on the southward flanks of the pacific and atlantic jets that is near the philippines and the caribbean regions from inferences based on the flow evolution of these sensitivity perturbations and with the help of potential vorticity analyses on the two constant potential temperature surfaces in this model a dynamical framework that may explain pacific and atlantic block onsets is proposed the nonuniform potential vorticity distribution in the jets in particular the concentration of these gradients on potential vorticity waveguides and the lagrangian advection of potential vorticity by the eddies making up the stationary rossby wave train and their energy propagation and convergence all conspire to play a key role in the growth of the synoptic scale eddies supported by baroclinic as well as barotropic processes it is proposed that the structural modification of the eddies in the wave train leads to the planetary structures that become associated with block onset more specifically the wave train in the pacific evolves into a blocking dipole while the atlantic block is found at the leading edge of the rossby wave train across the atlantic furthermore this study shows that at the initial time the pacific block displays a clear baroclinic structure while the wave train associated with the atlantic block has a much more barotropic structure the significance of these results and their potential applications to predictions of blocking are discussed

midlatitude westward propagating disturbances preceding intraseasonal oscillations of convection over the subtropical western north pacific during summer 1 we start with an analysis of the abruptly enhanced convection over the subtropical western north pacific wnp around late july the so called convection jump and indicate that intraseasonal oscillations isos play a dominant role in this feature then we analyze the extratropical circulation anomalies in association with the convection isos over the subtropical wnp which are more common than the convection jump and thus provide a greater sample size it is found that prior to convection iso peaks a well defined wave train of alternating cyclonic and anticyclonic perturbations appears and propagates westward in the upper troposphere over the midlatitude north pacific and bends southward to the subtropical wnp the present results imply that midlatitude circulation anomalies in the north pacific may affect intraseasonal variability of climate in the subtropical wnp and east asia during summer

a synvolcanic origin for magnetite rich orebodies hosted by bif in the weld range district western australia abstract iron ore deposits hosted by archean banded iron formation bif in the weld range greenstone belt are representative of most of the documented iron ore deposits in the yilgarn craton they include near surface supergene goethite hematite orebodies that overlie and partly modify deeper occurrences of hypogene magnetite and specular hematite ores the cenozoic goethite hematite rich orebodies in these deposits are unequivocally the product of meteoric fluid alteration affecting bif in the near surface supergene environment however the deeper and likely older magnetite and specular hematite rich orebodies have a more contentious origin this study is the first to present a fluid alteration model for hypogene iron mineralization in the yilgarn craton that uses fluid inclusion and mineral chemistry data to constrain the physical chemical characteristics and source of hypogene fluids responsible for mineralization high grade 57 wt fe magnetite rich iron ore at the beebyn deposit defines a discontinuous series of 440 c and co2 rich paired o and c stable isotope data for stage 1 ferroan dolomite suggest that these fluids had a magmatic source while stage 1 magnetite chemistry e g enrichments in mg mn ca and na indicates chemical exchange took place between the fluids and mafic igneous rocks prior to crystallization of magnetite the presence of monophase carbonic fluid inclusions in stage 1 ferroan dolomite suggest that phase separation of a bicarbonate rich aqueous fluid took place in deeper parts of the hydrothermal system which led to the separation of the resultant volatile rich and brine phases during transport of the stage 1 fluid to shallower crustal levels cooling of the hydrothermal system during the stage 2 fluid event involved i an early brine 275 327 c 36 40 equiv wt nacl with cl br and c and o isotopes values that overlap the ranges for magmatic fluids with minor involvement of archean seawater and extensive chemical exchange with country rocks followed by ii pulses of moderate and lower temperature stage 2 brines 125 260 c 2 24 equiv wt cacl2 with cl br o and c isotope na br and ca ca na signatures that suggest cooling of magmatic derived fluids that mixed with archean seawater and reacted with mafic igneous country rocks in areas more distal to fluid pathways the last stage of formation of magnetite rich ore at the beebyn deposit involved the flow of stage 3 fluids through the existing fault network that controlled earlier fluids stage 3 fluids are lower temperature 98 175 c low to high salinity brines with cl br values that overlap reported ranges for 3 2 ga vent fluids and seawater thus they are likely the product of heated archean seawater that was chemically modified through interaction with mafic country rocks magnetite talc veins at the madoonga deposit have fluid halogen ratios for fluid inclusions hosted by magnetite that are compatible with a range of possible sources including low grade metamorphic fluids geothermal brines or oil field formation waters although their common spatial association with semi massive sulfides suggests their likely precipitation from brines derived from heated archean seawater more locally developed specular hematite quartz veins that cut folded magnetite rich ores at the beebyn and madoonga deposits are the product of iron redistribution in bif by heated meteoric fluids or seawater with precipitation of specular hematite as a consequence of oxidation and cooling of the fluid

prospectivity of western australian iron ore from geophysical data using a reject option classifier abstract there has recently been a rapid growth in the amount and quality of digital geological and geophysical data for the majority of the australian continent coupled with an increase in computational power and the rising importance of computational methods there are new possibilities for a large scale low expenditure digital exploration of mineral deposits here we use a multivariate analysis of geophysical datasets to develop a methodology that utilises machine learning algorithms to build and train two class classifiers for provincial scale greenfield mineral exploration we use iron ore in western australia as a case study and our selected classifier a mixture of a gaussian classifier with reject option successfully identifies 88 of iron ore locations and 92 of non iron ore locations parameter optimisation allows the user to choose the suite of variables or parameters such as classifier and degree of dimensionality reduction that provide the best classification result we use randomised hold out to ensure the generalisation of our classifier and test it against known ground truth information to demonstrate its ability to detect iron ore and non iron ore locations our classification strategy is based on the heterogeneous nature of the data where a well defined target iron ore class is to be separated from a poorly defined non target class we apply a classifier with reject option to known data to create a discriminant function that best separates sampled data while simultaneously protecting against new unseen data by closing the domain in feature space occupied by the target class this shows a substantial 4 improvement in classification performance our predictive confidence maps successfully identify known areas of iron ore deposits through the yilgarn craton an area that is not heavily sampled in training as well as suggesting areas for further exploration throughout the yilgarn craton these areas tend to be more concentrated in the north and west of the yilgarn craton such as around the twin peaks mine 27 s 116 e and a series of lineaments running east west at 25 s within the pilbara craton potential areas for further expansion occur throughout the marble bar vicinity between the existing spinifex ridge and abydos mines 21 s 119 121 e as well as small isolated areas north of the hamersley group at 21 5 s 118 e we also test the usefulness of radiometric data for province scale iron ore exploration while our selected classifier makes no use of the radiometric data we demonstrate that there is no performance penalty from including redundant data and features suggesting that where possible all potentially pertinent data should be included within a data driven analysis this methodology lends itself to large scale reconnaissance mineral explorations and through varying the datasets used and the commodity being targeted predictive confidence maps for a wide range of minerals can be produced

the isotopic composition of radiogenic helium and its use to study groundwater movement in confined aquifers abstract the 3 he 4 he ratio for radiogenic he is shown to be primarily determined by the abundance of li and neutron absorbing elements in the host formations and is almost independent of the radioelement abundance the 3 he 4 he ratios have been computed for various li contents in rock matrices with average abundances of other elements the 3 he 4 he ratio is specific for each formation and may therefore be used to study crustal diffusion of he it is shown however that both the 3he and 4he contents of continental basements are likely to exceed those of the mantle so that diffusion of mantle he through continental crust is not likely to occur changes in the 3 he 4 he ratio for dissolved he have potential for studying groundwater movement and mixing processes atmospheric he which was dissolved at recharge is progressively diluted by radiogenic he with an isotopic ratio characteristic of the aquifer as the groundwater migrates down dip mixing proportions for formation he and atmospheric he in aquifers for which there is admixture of fracture borne water and interstitially stored water can also be estimated from isotopic measurements these processes are discussed in relation to two aquifers in eastern england

the role of deformation in the formation of banded iron formation hosted high grade iron ore deposits hamersley province australia abstract the hamersley province western australia hosts some of the world s largest iron ore deposits but despite decades of research their genesis is still extensively debated many iron ore deposits are hosted in complexly deformed archean to paleoproterozoic banded iron formations comprising thin chert and iron oxide bands interlayered with silicate rich shales and carbonates current iron ore genesis models have identified a strong structural control on ore formation linked to extensive hypogene and supergene fluid circulation along fault structures these fluid pathways facilitate the removal of vast amounts of gangue minerals leading to enrichment of the iron oxide residue to iron ore however the evolution of the associated structures has not yet been considered as a key element in ore genesis here we show through multiscale structural analyses that deformation not only forms suitable fluid channels but that folding and shearing also result in significant synkinematic removal of gangue minerals our multidisciplinary investigation of the structural evolution of the mount tom price deposit combines microtectonic field geology and 3d implicit modelling techniques to establish a link between deformation structures at various scales microscale shear bands and outcrop scale asymmetric parasitic folds share striking similarities in their evolution and their controlling mechanisms both features record substantial non coaxial deformation accompanied by volume changes due to stress induced silica remobilisation the closely spaced layering of rheologically different lithologies within hamersley province strata plays a crucial role in complex multilayer deformation which resulted in extensive strain partitioning our study suggests that deformation was of major significance in the upgrading of banded iron formation to iron ore and was active from the early stages of banded iron formation during diagenesis deformation structures also established a micro to deposit scale lateral and vertical fluid network which enabled infiltration by hypogene and supergene fluids during or after deformation these new insights have important implications for iron ore genesis models structural applications in the mine environment and for understanding complex multilayer deformation with volume loss

weather regimes low frequency oscillations and principal patterns of variability a perspective of extratropical low frequency variability abstract the dynamical basis of extratropical low frequency variability lfv is investigated using a quasigeostrophic model on a sphere with realistic northern hemisphere topography the model is driven by newtonian relaxation to an axisymmetric radiative equilibrium temperature two versions of the model are used one with two vertical levels and horizontal t15 resolution and the other with five levels and t21 resolution in previous investigations by the authors the former model has been found to possess multiple attractors in a stable range of the model parameters and to wander irregularly among attractor ruins for unstable parameter sets a similar behavior is found for the higher resolution model as well three aspects of lfv are considered in this paper the first two are intermittent appearances of quasi stationary weather regimes and low frequency oscillations the third is the dominance of a few principal patterns of variability that show red noise like temporal behavior in the real atmospher

origin and age of saline waters in busko spa southern poland determined by isotope noble gas and hydrochemical methods evidence of interglacial and pre quaternary warm climate recharges abstract in the busko and solec spas saline sulphide and sulphate waters are exploited at different depths the shallow water system in busko is related to an interglacial or interstadial period shown by the lack of tritium and 14 c and its 18 o and d values equal to that of modern water the noble gas temperatures ngt are somewhat lower than the present air temperatures but the 4 he and 40 ar ages of about 90 480 ka confirm a pleistocene recharge the 18 o and d values of the deep system are much heavier than those of local modern waters which may mean the recharge of a warm pre quaternary climate the 4 he 40 ar and 21 ne contents and the ngt values confirm the deep system to be recharged at the pre quaternary stages of the present hydrologic cycle i e after the sea transgression in the badenian high so 4 2 contents indicate that the cl and so 4 2 in both systems originated from simultaneous leaching of nacl and gypsum other components can be explained by cation exchange between water and badenian clay minerals in marls e g ca 2 for mg 2 and na for k and by the decay of marine organic matter supplying br i and b

dating ultra deep mine waters with noble gases and 36cl witwatersrand basin south africa abstract concentrations and isotopic ratios of dissolved noble gases 36cl d and 18o in water samples from the ultra deep gold mines 0 718 to 3 3 km below the surface in the witwatersrand basin south africa were investigated to quantify the dynamics of these ultra deep crustal fluids the mining activity has a significant impact on the concentrations of dissolved gases as the associated pressure release causes the degassing of the fissure water the observed under saturation of the atmospheric noble gases in the fissure water samples 70 98 normalized to asw at 20 c and 1013 mbar is reproduced by a model that considers diffusive degassing and solubility equilibration with a gas phase at sampling temperature corrections for degassing result in 4he concentrations as high as 1 55 10 1cm3stp4he g 1 40ar 36ar ranging between 806 and 10331 and 134xe 132xe and 136xe 132xe ratios above 0 46 and 0 44 respectively corrected 134 136 xe 132xe and 134 136 xe 4he ratios are consistent with their production ratios whereas the nucleogenic 4he 40ar and 134 136 xe 40ar ratios generally indicate that these gases are produced in an environment with an average u th k content 2 3 times above that of crustal average in two scenarios one considering only accumulation of in situ produced noble gases the other additionally crustal flux components the model ages for 14 individual water samples range from 13 to 168 ma and from 1 to 23 ma respectively the low 36cl ratios of 4 37 10 15 and comparatively high 36cl concentrations of 8 350 10 15 atoms 36cl l 1 reflect subsurface production in secular equilibrium indicating an age in excess of 1 5 ma or 5 times the half life of 36cl in combination the results suggest residence times of the fluids in fissures in this region up to 3 3 km depth are of the order of 1 100 ma we cannot exclude the possibility of mixing and that small quantities of younger water have been mixed with the very old bulk

microplaty hematite ore in the yilgarn province of western australia the geology and genesis of the wiluna west iron ore deposits abstract the wiluna west small 130 mt high grade bedded hematite ore deposits consisting of anhedral hematite mesobands interbedded with porous layers of acicular hematite show similar textural and mineralogical properties to the premium high grade low phosphorous direct shipping ore from pilbara sites such as mt tom price mt whaleback etc in the hamersley province and goldsworthy shay gap and yarrie on the northern margin of the pilbara craton both margins of the pilbara craton and the northern margin of the yilgarn craton were subjected to sub aerial erosion in the paleoproterozoic era followed by marine transgressions but unlike the hamersley basin the jfgb was covered by comparatively thin epeirogenic sediments and not subjected to proterozoic deformation or burial metamorphism the joyner s find greenstone belt jfgb in the yilgarn region of western australia was exhumed by middle to late cenozoic erosion of a cover of unmetamorphosed and relatively undeformed paleoproterozoic epeirogenic sedimentary rocks that preserved the jfgb unaltered for nearly 2 ga thus providing a unique snapshot of the early proterozoic environment acicular hematite pseudomorphous after acicular iron silicate is only found in iron ore and bif that was exposed to subaerial deep weathering in early paleoproterozoic times pre 2 2 ga and in the overlying unconformable paleoproterozoic conglomerate derived from these rocks and is absent from unweathered rocks lascelles 2002 high grade ore and bif weathered during later subaerial erosion cycles contain anhedral hematite and acicular pseudomorphous goethite the acicular hematite was formed from goethite pseudomorphs of silicate minerals by dehydration in the vadose zone under extreme aridity during early paleoproterozoic subaerial weathering the principal high grade hematite deposits at wiluna west are interpreted as bedded ore bodies that formed from bif by loss of chert bands during diagenesis and have been locally enriched to massive hematite by the introduction of hydrothermal specular hematite no trace of chert bands are present in the deep saprolitic hematite and hematite goethite ore in direct contrast to shallow supergene ore in which the trace of chert bands is clearly defined by goethite replacement voids and detrital fill abundant hydrothermal microplaty hematite at wiluna west is readily distinguished by its crystallinity the genesis of the premium ore from the pilbara region has been much discussed in the literature and the discovery at wiluna west provides a unique opportunity to compare the features that are common to both districts and to test genetic models

atmospheric forcing of fram strait sea ice export a closer look fram strait is the primary region of sea ice export from the arctic and therefore plays an important role in regulating the amount of sea ice and freshwater within the arctic we investigate the variability of fram strait sea ice motion and the role of atmospheric circulation forcing using daily data during the period 1979 2006 the most prominent atmospheric driver of anomalous sea ice motion across fram strait is an east west dipole pattern of sea level pressure slp anomalies with centers of action located over the barents sea and greenland this pattern also observed in synoptic studies is associated with anomalous meridional winds across fram strait and is thus physically consistent with forcing changes in sea ice motion the association between the slp dipole pattern and fram strait ice motion is maximized at 0 lag persists year round and is strongest on time scales of 10 60 days the slp dipole pattern is the second empirical orthogonal function eof of daily slp anomalies in both winter and summer when the analysis is repeated with monthly data only the barents center of the slp dipole remains significantly correlated with fram strait sea ice motion however after removing the leading eof of monthly slp variability e g the north atlantic oscillation the full east west dipole pattern is recovered no significant slp forcing of fram strait ice motion is found in summer using monthly data even when the leading eof is removed our results highlight the importance of high frequency atmospheric variability in forcing fram strait sea ice motion

helium accumulation in groundwater ii a model for the accumulation of the crustal 4he degassing flux abstract a model of 4 he accumulation in a confined aquifer is presented which includes in situ production and a crustal degassing flux of 4 he into the bottom of a confined aquifer the model is consistent with the measurements of 4 he concentration in the great artesian basin australia torgersen and clarke 1985 and also with the measurements of 4 he concentration in the auob sandstone of namibia heaton 1981 the measured value of the crustal degassing flux of 4 he from these two areas is comparable with the 4 he degassing flux calculated from the atmospheric budget of 4 he wasserburg et al 1963 ozima and podosek 1983 a literature review suggests that a crustal degassing 4 he source may account for the 4 he accumulation in many other groundwater systems it is concluded from the literature review the detailed modelling of 4 he accumulation in gab and the auob sandstone and the atmospheric budget that crustal degassing of 4 he may be a common and widespread phenomena

simultaneous atlantic pacific blocking and the northern annular mode a synoptic situation termed high latitude blocking hlb is shown to occur frequently in both the atlantic and pacific sectors and to result in flow anomalies very similar to those associated with the negative phase of the northern annular mode nam in the respective sector there is a weak but significant link between the occurrence of hlb in the two sectors with atlantic hlb tending to lead pacific hlb by 1 3 days this link arises from rare events in which both sectors are almost simultaneously affected by a large scale wave breaking event which distorts the polar trough over northern canada in several cases the tropospheric wave breaking occurs in tandem with a large scale disturbance of the stratospheric polar vortex there is therefore a physical link between the atlantic and pacific sectors but analysis suggests that this does not contribute to determining the pattern of the nam as conventionally defined from monthly mean data however an alternative version of the nam derived directly from daily data does appear to reflect this physical link these conflicting results highlight the sensitivity of the nam to the period over which data are averaged copyright c 2008 royal meteorological society

geochemistry and pb pb geochronology of the neoarchean nkout west metamorphosed banded iron formation southern cameroon the nkout west iron deposit of the northern edge of the congo craton in southern cameroon forms part of the northwest plunging isoclinal mega fold composed of metamorphosed banded iron formations bifs which have been deformed and metamorphosed resulting in recrystallized magnetite quartz and silicates here we present whole rock geochemistry together with pb pb isotopic compositions and dating of metamorphosed bif with the aim to constrain the genesis and elucidate the metamorphic resetting age based on the mineral composition two types of bif magnetite bif garnet bif have been distinguished the magnetite bif shows high sio2 and fe2o3 and low al2o3 tio2 hfse and ree contents similar to detritus free chemical sediments in contrast their high concentrations in garnet bif indicate clay rich contaminated input the co zn ratios are consistent with trace metals of hydrothermal sources 0 14 for magnetite bif or with minor hydrogenous component 0 27 for garnet bif the paas normalized ree y patterns of the studied metamorphosed bifs show lree depletion relative to hree and positive eu anomalies the magnetite bif exhibits positive y anomaly y y paas 1 3 which is the signature inherited from sea water but its y ho 39 93 ratio suggests a mixture of hydrothermal solution with sea water the garnet bif shows positive eu negative y anomalies and chondritic y ho ratio 27 49 which is consistent with hydrothermal solution with significant clastic contamination whole rock pb concentration is roughly on average of 2 0 6 0 ppm suggesting a hydrothermal pb source the pb pb dating yielded an age of 2679 ma interpreted as the neoarchean metamorphic event in the ntem complex

cross frequency coupling skewness and blocking in the northern hemisphere winter circulation abstract variability in daily wintertime december february djf 500 hpa heights on low l 6 day 1 frequencies is examined using 40 yr ecmwf re analysis era 40 data leading eofs of l correspond to planetary scale teleconnection patterns those of m to retrograding eastward dispersing long waves oriented along great circle routes and those of h to baroclinic waves in the climatological mean storm tracks in the atlantic sector eof 1 of m appears to be embedded in eof 1 of l cross frequency coupling between l and m exhibits distinctive patterns in the atlantic sector the negative polarity of the north atlantic oscillation nao with above normal heights over greenland is associated with enhanced m variability over greenland an analogous relationship is observed in the pacific sector between an nao like pattern and the variance of m over alaska cross frequency coupling between l and h in both sectors is indicative of a reinforcement

upper regime parallel lamination as the result of turbulent sediment transport and low amplitude bed forms a series of laboratory experiments has been carried out in which parallel laminated deposits were produced from an upper regime plane bed the laminae had thicknesses of a few mm and could be traced continuously over distances up to the length and width of the depositional area 0 3 m by 1 5m fluctuations in bed elevation were measured both during deposition and at equilibrium much of the bed fluctuation occurs at time scales that are too long to be due directly to turbulence as most theories for lamina formation would require we suggest instead that extremely low amplitude bed forms are present even on apparently flat beds and that the migration of these bed forms produces laterally continuous lamination all the lamination produced in the laboratory experiments was normally graded using high speed photography it was observed that the normal grading results from rapid deposition of a layer of loosely packed coarse sand several grain diameters thick followed by the slow sieving out of a well packed surface layer of finer sand the initial deposition is the result of small scale turbulent fluctuations in boundary shear stress the sieving out that follows results in a smooth surface whose low friction coefficient temporarily inhibits further deposition we term this process glazing the alignment of small scale turbulent scour and fill structures along the paths traced by migrating bedform troughs produces laterally continuous parallel lamination

geographical dependence of upper level blocking formation associated with intraseasonal amplification of the siberian high abstract intraseasonal amplification events of the surface siberian high in winter are generally associated with blocking ridge formation in the upper troposphere composite analysis applied to the 20 strongest intraseasonal events of upper level anticyclonic anomalies at every grid point over siberia reveals that the blocking formation differs fundamentally between the east and west of the climatological upper level trough over the far east to the west what can be called wave train atlantic origin type is common where a blocking ridge develops from anomalies as a component of a quasi stationary rossby wave train propagating across the eurasian continent under modest feedback forcing from transient eddies to the east of the trough what can be called pacific origin type dominates where a blocking ridge forms in association with westward development of anticyclonic anomalies from the north pacific under stronger feedback forcing from the pacific storm track regardless of a particular type of bl

the bif hosted high grade iron ore deposits in the archean koolyanobbing greenstone belt western australia structural control on synorogenic and weathering related magnetite hematite and goethite rich iron ore the koolyanobbing banded iron formation bif hosted iron ore deposits total premining resources 150 million metric tons mt indicated reserves 32 mt are located in the mesoarchean lower succession bif of the koolyanobbing greenstone belt younami terrane yilgarn craton in western australia in the koolyanobbing greenstone belt a deformation sequence that broadly correlates with the proposed deformation history of most greenstones belts within the southern cross domain includes d 1 structures mainly small scale f 1a and f 1b folds formed in a north south to northwest southeast compressional regime a ductile to brittle deformation sequence d 2 to d 4 generated during east west compression and a late stage brittle segmentation of bif and reactivation of faults attributed to d 5 the formation of the seven known medium 45 58 wt fe to high grade 58 68 wt fe magnetite martite specularite and goethite bearing orebodies can be subdivided into four archean stages and one weathering related upgrade from the permian and or mesozoic to recent times the archean ore forming stages comprise 1 early fe mg ca metasomatism causing local ferroan carbonate and ferroan talc alteration of the metamorphosed quartz magnetite bif protolith 2 sequential syn d 2a coaxial to syn d 4 transpressional tight folding driven removal of carbonate quartz and minor ferroan talc by solution and mechanical transfer producing residual enrichment of medium to high grade magnetite ore 3 magnetite mineralization in syn d 2b and syn d 4 breccias and fractures forming medium grade ore zones or overprint magnetite in bif and first stage magnetite ore and 4 mineralization of hydrothermal specularite and locally associated ferroan dolomite quartz alteration and local oxidation of magnetite in and near brittle d 4 faults fractures and reactivated f 1 and f 2a fold cores modern weathering related leaching of carbonate and minor quartz pseudomorphic goethite replacement of existing iron oxides and gangue and coeval or subsequent to oxidation in the vadose zone formed goethite martite ore with local relics of specularite or magnetite and or kenomagnetite the intensity and localization of this supergene modification is in most deposits at koolyanobbing controlled by existing hypogene magnetite specularite rich medium to high grade ore zones and or carbonate altered bif at depth the existence of high grade ore below the weathering horizons suggests the possibility of further concealed magnetite and or specularite rich orebodies within the deposits and region

geochemical evolution of the banded iron formation hosted high grade iron ore system in the koolyanobbing greenstone belt western australia the banded iron formation bif hosted iron ore deposits in the lower greenstone succession of the koolyanobbing greenstone belt 50 km north of southern cross in western australia are a 200 mt high grade fe 58 pre mining resource and represents one of the most important iron ore districts in the yilgarn craton four hypogene alteration ore forming stages and one supergene upgrading event took place 1 during ore stage 1 lree depleted transition metal enriched mg fe ca carbonates replaced quartz in bifs the deposit scale alteration was most likely induced by devolatilization of sea floor altered ca si depleted mafic rocks in the vicinity of the bif during early regional syn d 1 very low to low grade metamorphism and was most strongly developed on reactivated bif basalt contacts 2 ore stage 2 involved the formation of patchy magnetite ore by a syn d 2 to d 4 dissolution of early carbonate enrichment of fe 2o 3total in magnetite iron ore was by a factor of 2 to 2 4 and compatible trace elements in magnetite such as ga v and al were immobile a subdeposit scale ferroan talc footprint proximal to magnetite iron ore in the largest deposit k deposit was associated with ore stage 2 and resulted from dissolution of magnesite due to reaction with silica in the bif under greenschist facies conditions and potentially high fluid rock ratio 3 magnetite growth during ore stage 3 forming granular magnetite martite ore is related to a subsequent hydrothermal event occurring locally throughout the belt especially in d 2b faults 4 ore stage 4 was associated with fe ca p l ree y enriched hydrothermal fluids possibly from a magmatic source such as the postmetamorphic lake seabrook granite that crops out about 10 km west of the koolyanobbing deposits and at the southern margin of the greenstone belt these ca enriched fluids interacted with distal metamorphosed mafic rock and influenced the bif ore system in a small number of deposits they were channelled through regional d 4 faults and caused specularitedolomite quartz alteration resulting in fe grades of up to 68 5 supergene upgrade ore stage 5 by further gangue leaching in the weathering zone was most effective in carbonate altered bifs and magnetite ore this process together with supergene martitization and goethite replacement of magnetite led to the formation of high grade locally at k deposit high p goethite martite ore at koolyanobbing the two geochemically distinct stages of archean carbonate alteration clearly controlled the formation of hypogene magnetite specularite martite rich ore and recent supergene modification including the further upgrade of fe ore 2012 by economic geology

genesis of high grade hematite orebodies of the hamersley province western australia a reply sir we are very thankful for the constructive comments provided by kneeshaw and kepert unfortunately the competitive ownership structure of the iron ore mining industry in the hamersley province limits our personal knowledge of the mount whaleback deposit to occasional mine visits and to the available literature published and unpublished we do however have access to the data on the smaller giles mini and lido deposits 50 and 10 km west of mount whaleback respectively and to extensive exploration mapping in the ophthalmia ranges to the north direct proof for rotation of the orebody as it is preserved at paraburdoo in the approximately 50 dip of the overlying proterozoic hematite conglomerates of the mount mcgrath formation is not preserved at mount whaleback however we believe that there is sufficient information available in the literature to think that a rotational model for a large part of the deposit is perfectly plausible first and above all as stated in taylor et al 2001 f2 folds are more upright with axial planes dipping about 50 s in the southern and western part of the deposit away from the whaleback fault e g as shown on section no 315120e fig 4 in powell et al 1999 interestingly the ore controlling faults are also more upright in this part of the deposit which suggests an intimate link between the dip of these fault planes and dip of the axial planes of f2 folds secondly

geochemistry and sedimentology of a facies transition from limestone to iron formation deposition in the early proterozoic transvaal supergroup south africa this study deals with the geochemistry and sedimentology of a facies transition from interbedded carbonate shale to banded iron formation in the campbellrand carbonate sequence to the overlying kuruman iron formation of the transvaal supergroup in south africa which is approximately 2 3 ga old four major lithologies are 1 limestone and dolomite 2 shale and interbedded shale carbonate 3 siderite rich banded iron formation and 4 iron oxide rich banded iron formation these rocks are unaltered and essentially unmetamorphosed with a maximum metamorphic diagenetic overprint ranging in temperature from 110 degrees to 170 degrees c and with pressures of not more than 2 kbars the oldest rocks are limestones and lesser dolomite with abundant cryptalgalaminae and intraclastic textures interbedded with the limestones and dolomites are carbonaceous shales some of which are unusually ferruginous or pyrite rich these carbonates and shales are overlain by meso and microbanded siderite chert iron formation which grades upward into magnetite chert and carbonate rich iron formation the averages for major and trace elements and rare earth element ree contents of the limestones dolomites and shales are distinct from those of the two types of iron formation the two most chemically diverse rock types are the shales enriched in alkalis and most trace elements and the two iron formation types siderite and magnetite rich which are almost totally depleted in all elements except si fe mg and ca and co 2 the high al 2 o 3 contents of the shales avg 9 55 wt correlate well with their high organic carbon contents avg 3 91 wt the limestones and dolomites have average al 2 o 3 and organic carbon contents of about 3 0 and 0 7 wt percent respectively the two iron formation types have al 2 o 3 average values of 0 099 wt percent siderite rich and 0 066 wt percent magnetite rich and corresponding averages for organic carbon of 0 080 wt percent siderite rich and 0 012 wt percent magnetite rich the ree are generally concentrated in the shales by a factor of about 10 over the iron formations the limestones and dolomites have intermediate values and the iron formations are most depleted the iron formation ree patterns in a ratio with north american shale composite nasc have pronounced positive eu anomalies and slight negative ce anomalies both of these anomalies are absent in the shales but the limestones and dolomites show slight positive eu anomalies the siderite rich iron formations consist of chert siderite ankerite or ferroan dolomite with traces of pyrite and stilpnomelane the siderite in these commonly microbanded assemblages is very fine grained along well defined bedding planes it is concluded from petrographic study that siderite is a primary precipitate the ankerites and ferroan dolomites in these siderite rich occurrences are commonly euhedral rhombohedral much coarser grained and appear to be of later diagenetic origin the magnetite rich iron formations consist of chert magnetite ankerite or ferroan dolomite or siderite or hematite or stilpnomelane or minnesotaite in these oxide rich iron formations the ankerites and ferroan dolomites are similarly much coarser grained than the finely banded siderite on the basis of the geochemical data and a reconstruction of the depositional basin for the carbonate shale to iron formation transition we conclude that the limestone dolomite shale lithologies originated in a water column quite distinct from that in which the iron formations were precipitated we propose a model with a stratified water column in which the surface waters during a regressive stage in the depositional basin were the site of much organic carbon productivity and the locus of cryptalgal limestones and intraclastic limestone deposition with at somewhat greater depth below the chemocline deposition of pyritic carbonaceous shale our model depicts the deeper waters during a transgressive stage of the basin with the kaapvaal craton more deeply submerged as the site for iron formation deposition these deeper waters were depleted in organic carbon and enriched in dissolved ferrous iron relative to the shallower water mass with continued availability of oxygen along the chemocline separating the two water masses the ultimate source of the iron and probably the sio 2 in the iron formations appears to be a very dilute hydrothermal input in the deep ocean waters as concluded from mixing calculations of ree values for modern atlantic ocean water and hydrothermal solutions from the east pacific rise

low frequency variability in the midlatitude atmosphere induced by an oceanic thermal front abstract this study examines the flow induced in a highly idealized atmospheric model by an east west oriented oceanic thermal front the model has a linear marine boundary layer coupled to a quasigeostrophic equivalent barotropic free atmosphere the vertical velocity at the top of the boundary layer drives the flow in the free atmosphere and produces an eastward jet parallel to the oceanic front s isotherms a large gyre develops on either side of this jet cyclonic to the north and anticyclonic to the south of it as the jet intensifies during spinup from rest it becomes unstable the most unstable wave has a length of about 500 km it evolves into a meander and eddies detach from the eastern edge of each gyre the dependence of the atmospheric dynamics on the strength t of the oceanic front is studied the gulf stream and kuroshio fronts correspond roughly in the scaling used here to t 7 c for weak fronts t 4 c the circulation is steady and exhibits two large antisymmetric gyres sepa

s c mylonites two types of foliations are commonly developed in mylonites and mylonitic rocks a s surfaces related to the accumulation of finite strain and b c surfaces related to displacement discontinuities or zones of relatively high shear strain there are two types of s c mylonites type i s c mylonites described by berthe et al typically occur in deformed granitoids they involve narrow zones of intense shear strain which cut across mylonitic foliation type ii s c mylonites described here have widespread occurrence in quartz mica rocks involved in zones of intense non coaxial laminar flow the c surfaces are defined by trails of mica fish formed as the result of microscopic displacement discontinuities or zones of very high shear strain the s surfaces are defined by oblique foliations in the adjacent quartz aggregates formed as the result of dynamic recrystallization which periodically resets the finite strain clock these oblique foliations are characterized by grain elongations alignments of segments of the grain boundary enveloping surfaces and by trails of grains with similar c axis orientations examples of this aspect of foliation development in mylonitic rocks are so widespread that we suggest the creation of a broad class of s c tectonites and a deviation from the general tradition of purely geometric analysis of foliation and time relationships kinematic indicators such as those discussed here allow the recognition of kilometre scale zones of intense non coaxial laminar flow in crustal rocks and unambiguous determination of the sense of shear

multiscale dynamical processes underlying the wintertime atlantic blockings abstractthe wintertime atmospheric blocking over the atlantic is investigated using a newly developed methodology namely localized multiscale energy and vorticity analysis ms eva and the theory of canonical energy transfer through a multiscale window transform mwt the atmospheric fields from the era 40 data are reconstructed on three scale ranges or scale windows basic flow window blocking window and synoptic window the blocking event is obtained by compositing the wintertime blocking episodes and a clear westward retrograding signal is identified on the blocking window likewise the local multiscale energetics following the signal are composited it is found that a life cycle of the blocking scale kinetic energy ke may be divided into three phases onset phase amplification phase and decay phase different phases have different mechanisms in play in general pressure work and the canonical transfer from the synoptic eddies initiate the generation of the blocking while the latter contri

the origin of terrestrial pisoliths and pisolitic iron ore deposits raindrops and sheetwash in a semi arid environment abstract ooliths evidently form by chemical precipitation in limnic paralic fluvial and marine environments pisoliths however appear to be restricted to terrestrial environments typically composed of iron aluminium and manganese sesquioxides with minor admixtures of quartz and kaolinite they are widely distributed in tropical to subtropical regions overlying deeply weathered soil profiles although iron aluminium and manganese rich end members are important sources of these metals their genesis is still enigmatic their formation has never been observed or produced experimentally and current models for their origin are little more than guesses a new model is presented based on a unique personal observation in which pisoliths are formed by the action of charged raindrops during thunderstorms impacting on dry deeply weathered powdery soils the pisoliths are transported across pediments by sheetwash accumulating as thick deposits in the valley floors pisolites are characteristically unfossiliferous and typically clearly pedogenic the absence of fine depositional layering fossil seeds leaves and pollen in pisolites is explained by bioturbation and the action of soil organisms during extended pedogenesis while the major coarse bedding features derive from erosional and depositional events in the evolution of the pediment pisolitic iron ores aka channel iron deposits cid are a special case of transported pisolitic ferricrust that form an important resource of medium grade iron ore 57 60 wt fe in the pilbara region of western australia apart from minor deposits in the northern yilgarn province of western australia they have not been found elsewhere they differ from normal transported ferricrust and terrestrial pisolites not only in the exceptionally high iron and low alumina and silica content but also in containing abundant fossilised wood particles

high grade iron ore at windarling yilgarn craton a product of syn orogenic deformation hypogene hydrothermal alteration and supergene modification in an archean bif basalt lithostratigraphy banded iron formation bif hosted iron ore deposits in the windarling range are located in the lower greenstone succession of the marda diemals greenstone belt southern cross domain yilgarn craton and constitute a total hematite martite goethite ore resource of minimum 52 mt at 60 wt fe 0 07 p banded iron formation is interlayered with high mg basalts at windarling and precipitated during episodes of volcanic quiescence trace element content and the rare earth element ree ratios y ho 42 to 45 sm yb 1 5 together with positive la and gd anomalies in least altered hematite magnetite metachert bif indicate the precipitation from archean seawater that was fertilised by hydrothermal vent fluids with a basaltic hree y signature hypogene iron ore in sub greenschist facies metamorphosed bif formed during three distinct stages ore stage 1 was a syn to post metamorphic syn d1 fe ca mg ni co p ree metasomatism that produced local ni ree rich fe dolomite magnetite alteration in bif hydrothermal alteration was induced by hot fluid flow controlled by brittle ductile reactivation of bif basalt margins and crosscutting d1 faults the ni co rich content of dolomite and a shift in ree ratios in carbonate altered bif towards archean mafic rock signature y ho to 31 to 40 sm yb to 1 to 2 and gd gd to 1 2 to 1 4 suggest that high mg basalts in the windarling range were the primary source of introduced metals during ore stage 2 a syn deformational and likely acidic and oxidised fluid flow along bif basalt margins and within d1 faults leached carbonate and precipitated lepidoblastic and anhedral granoblastic hematite high grade magnetite hematite ore is formed during this stage ore stage 3 hydrothermal specular hematite spch fe dolomite quartz alteration was controlled by a late orogenic brittle compressional transpressional stage d4 the regional scale shear zone related d3 is not preserved in windarling this minor event remobilised iron oxides carbonate and quartz to form veins and breccia but did not generate significant volumes of iron ore ore stage 4 involved mesozoic to recent supergene oxidation and hydration in a weathering environment reaching down to depths of 100 to maximum 200 m below surface supergene ore formation involved goethite replacement of dolomite and quartz as well as martitisation important ground preparation for supergene modification and upgrade were mainly the formation of steep d1 to d4 structures steep bif basalt margins and particularly the syn d1 to syn d2 carbonate alteration of bif that is most susceptible to supergene dissolution the windarling deposits are structurally controlled supergene modified hydrothermal iron ore systems that share comparable physical chemical and ore forming characteristics to other iron ore deposits in the yilgarn craton e g koolyanobbing beebyn in the weld range mt gibson however the remarkable variety in pre syn and post deformational ore textures relative to d1 and d2 has not been described elsewhere in the yilgarn and are similar to the ore deposits in high strain zones such as of brazil quadrilatero ferrifero or iron quadrangle and nigeria the overall similarity of alteration stages i e the sequence of hydrothermal carbonate introduction and hypogene leaching with other greenstone belt hosted iron ore deposits supports the interpretation that syn orogenic bif alteration and upgrade was crucial in the formation of hypogene supergene iron ore deposits in the yilgarn craton and possibly in other archean paleoproterozoic greenstone belt settings worldwide

weather regime transitions and the interannual variability of the north atlantic oscillation part ii dynamical processes inthisstudy attentionisfocused on identifyingthedynamicalprocessesthatcontributetothenegativenorth atlantic oscillation nao 2 to positive nao nao 1 and nao 1 to nao 2 transitions that occur during 1978 90 p1 and 1991 2008 p2 by constructing atlantic ridge ar and scandinavian blocking sbl indices thecompositeanalysis demonstratesthatinastrongerar sbl winter nao 2 nao 1 eventcanmore easily transition into an nao 1 nao 2 event composites of 300 hpa geopotential height anomalies for the nao 2 tonao 1 andnao 1 tonao 2 transitioneventsduringp1andp2arecalculated itisshownforp2 p1 that the nao 1 to sbl to nao 2 nao 2 to ar to nao 1 transition results from the retrograde drift of an enhanced high latitude large scale positive negative anomaly over northern europe during the decay of the previous nao 1 nao 2 event this finding cannot be detected for nao events without transition moreover itisfoundthattheamplificationofretrogradingwavenumber1ismoreimportantforthenao 2 to nao 1 transitionduringp1 but themarkedreintensificationandretrogrademovementofbothwavenumbers1 and 2 after the nao 1 event decays is crucial for the nao 1 to nao 2 transition during p2 it is further shown that destructive constructive interference between wavenumbers 1 and 2 over the north atlantic during p1 p2 is responsible for the subsequent weak nao 1 strong nao 2 anomaly associated with the nao 2 to nao 1 nao 1 to nao 2 transition also the weakening strengthening of the vertically integrated zonal wind upstream atlantic storm track is found to play an important role in the nao regime transition

hydrothermal origin for the 2 billion year old mount tom price giant iron ore deposit hamersley province western australia giant iron ore deposits such as those in the hamersley province of northwestern australia may contain more than a billion tonnes of almost pure iron oxides and are the world s major source of iron it is generally accepted that these deposits result from supergene oxidation of host banded iron formation bif accompanied by leaching of silicate and carbonate minerals new textural evidence however shows that formation of iron ore at one of those deposits mount tom price involved initial high temperature crystallisation of magnetite siderite iron silicate assemblages this was followed by development of hematite and ferroan dolomite bearing assemblages with subsequent oxidation of magnetite leaching of carbonates and silicates and crystallisation of further hematite preliminary fluid inclusion studies indicate both low and high salinity aqueous fluids as well as complex salt rich inclusions with the range of fluid types most likely reflecting interaction of hydrothermal brines with descending meteoric fluids initial hematite crystallisation occurred at about 250 c and high fluid pressures and continued as temperatures decreased although the largely hydrothermal origin for mineralisation at mount tom price is in conflict with previously proposed supergene models it remains consistent with interpretations that the biosphere contained significant oxygen at the time of mineralisation

iron deposits hosted by banded iron formations in the yilgarn craton products of sequential iron enrichment by magmatic marine and meteoric fluids abstract mesoarchean algoma type bif in the yilgarn craton are locally enriched by secondary processes to iron concentrations approaching 72 wt fe while their near surface goethite hematite ores are unequivocally the product of supergene fluid alteration their deeper magnetite and crystalline hematite martite ores have a contentious origin this study shows that magnetite ores formed after diagenesis and peak greenschist facies metamorphism of bif coinciding with regional deformation events the ores formed by a multi stage process involving initial replacement of primary quartz bands in bif by hypogene carbonate minerals followed by the dissolution of the carbonate and concentration of magnetite at the beebyn deposit the earliest fluids were hot 440 c co2 vapor rich and were likely derived from felsic magmatic fluids in contrast the fluids responsible for carbonate alteration at the matthew ridge koolyanobbing and windarling deposits were cooler 366 to 125 c and aqueous rich with salinities from 0 3 to 43 9 wt nacl equiv their cl br and c o isotopic ratios suggest they are the product of mixing between magmatic fluids and heated seawater in all deposits later cooling 178 to 98 c of these fluids led to dissolution of carbonate minerals and concentration of magnetite dated monazite and xenotime from magnetite ores demonstrates that fluid alteration of bif began in the mesoarchean and continued episodically through to the paleoproterozoic later specular hematite rich veins that cut and modify magnetite ores crystallized from aqueous rich fluids that are more oxidized with salinities from 0 to 28 7 wt nacl equiv and crystallization temperatures of 243 to 85 c their cl br and c o isotopic fluid ratios indicate that they formed from mixing between heated seawater and meteoric waters the bif hosted iron deposits in the yilgarn craton show remarkable similarities with the well studied iron deposits in the carajas mineral province and hamersley basin they all show the same transition through time from magnetite to hematite rich ores subsequent episodic tectonically driven fluid alteration events led to further iron enrichment in these deposits the transition in ore types most likely corresponds to a common guiding process namely the uplift of crustal rocks containing bif resulting in the progressive shallowing of fluid alteration pathways and the greater influence of cooler more oxidized hydrothermal fluids unlike iron deposits hosted by algoma type bif in the yilgarn and carajas iron deposits hosted by superior type bif in the hamersley basin display a greater influence of oxidized marine and meteoric waters without involvement of magmatic fluids

the genesis of the hope downs iron ore deposit hamersley province western australia the banded iron formation bif hosted hope downs high grade hematite ore deposits are situated within the marra mamba iron formation with subsidiary deposits in the brockman iron formation of the archean to proterozoic hamersley group of western australia the main orebody extends to 260 m below the surface and is unusually rich in martite pseudomorphous hematite after magnetite and poor in limonite and goethite compared to other ore deposits of the marra mamba iron formation the high grade hematite ore is mainly within the newman member but also occurs in parts of the nammuldi member together with low grade limonitic ore that becomes high grade after calcining karst erosion of the overlying wittenoom formation has produced steep sided buried valleys adjacent to the in situ orebodies that contain thick deposits 160 m of goethitic and sideritic sediments including remnants of robe pisolite formation bedded siderite hematite gravels red ochreous detrital material and enriched scree deposits that are additional sources of ore the ore consists of low phosphorous martite limonite goethite derived from chert free bif by supergene weathering no evidence of the complete carbonate replacement of chert has been found at hope downs nor were any traces of preexisting chert bands seen in the ore despite the abundance of chert bands in bif elsewhere a variety of textures and composition shown by cherty bif adjacent to the orebodies is described from which the origin of the chert free bif is inferred including sedimentary structures consistent with density current deposition a model is presented for the origin of the host iron formation and the ore deposits in which density currents transported reworked iron silicates and hydroxides in colloidal suspension onto an unstable sea floor the amorphous silica produced during diagenesis of al poor iron silicates formed the characteristic chert bands of bif but some of the hydrous amorphous silica was lost prior to lithification to form chert free bif weathering of the chert free bif produced the high grade hematite ore that is exposed today

the dynamics of euro atlantic blocking onsets the mechanisms leading to euro atlantic blocking are investigated using 31 winters of 500 hpa geopotential height from nmc and ecmwf daily analyses and a 30 000 day perpetual winter run from a simple 3 level quasi geostropic model a composite analysis of the atmospheric flow during days preceding blocking events is carried out it indicates that euro atlantic blocking events have two simultaneous main precursors a high latitude retrograding wave number 1 pattern at planetary scale and an enhanced baroclinic wavetrain travelling across the north atlantic at synoptic scale these phenomena are identified both in observed and in simulated data the nonlinear interactions between small scale large scale and the mean flow during the establishment of the blocking anomaly are examined by a budget analysis of the quasi geostrophic potential vorticity equation these diagnostics demonstrate that wave wave interactions contribute to about 50 of the local amplification of the blocking anomaly while wave mean flow interactions contribute to propagation of the large and small scale anomalies and their phase locking finally the question is addressed as to whether the two precursors are necessary and sufficient conditions for blocking onsets by performing various sets of initial value experiments performed with the quasi geostrophic model these experiments show that the two precursors occur simultaneously but only the synoptic scale baroclinic wavetrain is necessary to trigger euro atlantic blocking

book review helium isotopes in nature developments in geochemistry 3 by b a mamyrin and i n tolstikhin elsevier amsterdam 1984 xiv 273 pp price u s 53 75 u s a and canada dfl 140 00 rest of world hardback

4he diffusion in 40ar retentive minerals abstract 4he vacuum diffusion experiments were carried out by stepwise degassing minerals normally used in k ar chronometry such as hornblende nepheline and others the study aims at 4he diffusion parameters for uppermost crustal temperatures and thus at he retentivities of the minerals the results indicate long time near quantitative 4he retention in hornblende pyroxene langbeinite and perhaps nepheline for 0 5 mm grains at least at temperatures below about 70 c sanidine and muscovite lose 4he over geologic periods at lower temperatures only the minerals of the first group are possible u th he chronometers or geothermometers provided that other dating problems such as alpha loss and u migration can be settled closure temperatures for rapidly cooling 2 mm hornblende crystals are found to be in the range of 190 to 170 c tentative u he indices indicate that diffusion parameters of the same order of magnitude as determined for the he retentive minerals may be used to select minerals to be further examined for their u th he datability e g ore minerals

mountain torques and northern hemisphere low frequency variability part i hemispheric aspects the ncep ncar reanalysis dataset for 1958 97 is used to analyze intraseasonal variations in mountain torques and the large scale atmospheric circulation patterns associated with them spectral analysis of the atmospheric angular momentum aam budget shows that the dominant variations of mountain torque have periodicities near 30 days and shorter while the dominant aam variations occur in the 40 60 day band this difference is due to the 40 60 day aam variations being primarily related to equatorial processes while mountain torque variations are associated mostly with extratropical processes the northern hemisphere nh mountain torque has enhanced power and significant spectral peaks in the 20 30 day band the signal in this band accounts for 33 of the nh mountain torque variance once the seasonal cycle has been removed lag composites of the nh 700 hpa geopotential heights based on the 20 30 day mountain torque signal show the latter to be associated with coherent large scale patterns that resemble lowfrequency oscillations identified in this band by previous authors the composite patterns that are in phase quadrature with the 20 30 day nh mountain torque have a pronounced zonally symmetric component these patterns are associated with substantial aam variations arguably driven by the nh mountain torque in this band principal component pc analysis of the nh 700 hpa heights shows that in the 20 30 day band the mountain torque is also in phase quadrature with the two leading pcs the first corresponds to changes in the midlatitude jet intensity near the subtropics while the second corresponds to the arctic oscillation the relationships with aam of the latter essentially occurs through the mass term mountain torques are furthermore nearly in phase with dominant patterns of low frequency variability that exhibit substantial pressure gradients across the rockies and the tibetan plateau

dating very old pore waters in impermeable rocks by noble gas isotopes the sup 4 he sup 40 ar and sup 136 xe content dissolved in the pore water of sedimentary rock samples was measured on samples from borehole cores near the repository for nuclear waste in morsleben germany due to the very low permeabilities of the rock formations conventional groundwater sampling was almost impossible hence the authors developed a new sampling method for noble gases in the pore water of freshly drilled rock cores this method provides vertical noble gas profiles in high depth resolution even in impermeable rocks by application of the new technique quantitative age information of groundwater and pore water have been derived the authors find palaeowaters from the last glaciation depleted in delta d and delta sup 18 o with a sup 4 he age of about 55 kyr the high saline pore solutions below are at least 6 mio years old this has been concluded from the profiles of radiogenic sup 4 he and sup 40 ar close to diffusion in steady state and from xenon isotopes produced by spontaneous fission of sup 238 u in the rocks a sup 4 he flux of 2 center dot 10 sup minus 7 cc stp cm sup 2 yr is derived from the profile which is due to local sup 4 he production within the investigatedmore sediments less

the relation between iron formation and low temperature hydrothermal alteration in an archean volcanic environment abstract the uppermost section of the hunter mine group hmg 2728 ma a bimodal volcanic complex in the abitibi greenstone belt contains both oxide and carbonate facies banded iron formation bif this paper explores the relationship between volcanic activity and the development of the two types of iron formation the oxide facies represented by chert jasper magnetite iron formation is widespread and recurs at several stratigraphic levels field evidence suggests it deposited directly on the sea floor during periods of volcanic quiescence and is most probably derived from fluids seeping from volcaniclastic sediments and the compaction of felsic shard rich tuffs seeping occurred along synsedimentary and synvolcanic faults and then along bedding planes carbonate siderite facies iron formation in contrast formed locally below a silica cap rock by in situ low temperature hydrothermal replacement of chert tuff beds and even of the oxide formation thus the two types of iron formation although spatially and stratigraphically juxtaposed are not the result of the same process and only the carbonate facies appears to be directly related to a low temperature hydrothermal volcanic process the contrasting genesis of oxide and carbonate iron formations indicates that current simplistic models relating banded iron formation to volcanic massive sulphide deposits need to be re evaluated the notion that oxide iron formation represents a distal manifestation of volcano generated hydrothermal activity may be the exception and not the rule because as indicated by this study bif occurs in the central part of a volcanic edifice rather than on the flanks

stages of evolution of a toarcian compacted claystone around galleries excavated between 1 and 124 years ago by the study of noble gases dissolved in pore water at the tournemire underground research laboratory france abstract the purpose of this work is to study the diffusion properties in an argillaceous formation and how these properties change after the excavation of galleries over a long period the tournemire underground research laboratory url located in aveyron france offers the unique opportunity to investigate different stages of the evolution of the so called excavation damaged zone edz over a period of about 124 a four horizontal boreholes parallel to the bedding and one vertical borehole were drilled in order to study diffusion transport in the edz in both directions in galleries excavated 124 13 6 and 1 a ago respectively noble gases dissolved in the pore water of 47 core samples were extracted in order to establish precise depth profiles mainly produced in situ by decay of u and th radiogenic he accumulates in the pore water of rocks and escapes toward the gallery allowing a characterization of the diffusive transport in the edz and its evolution with time for the oldest gallery 1882 1888 core mapping reveals an edz well developed with fractures up to a depth of 1 8 m in this zone all radiogenic he has been lost and at the same time heavy noble gases are trapped in the rock with an enrichment of xe relative to kr and of kr relative to ar from 1 8 m he concentration increases with depth to reach a plateau at about 3 5 m corresponding to a concentration normally found in the unaltered zone considering diffusive transport of the radiogenic he to the edz model calculations allowed proposing transport properties in the unfractured and undisturbed zone uz with a pore diffusion coefficient dp of 1 10 10 m 2 s deduced from the best fit curve to experimental data and a mean porosity of 0 1 obtained form petrophysical measurements the most recent gallery 2008 did not show any evidence of edz at depths greater than 7 5 cm a fracture located around 0 75 m possibly of tectonic origin induced large advection and diffusion losses of he on both sides in the next 50 cm excesses of ar kr and xe are found suggesting diffusion of air from the gallery associated with he losses the 6 a old gallery is clearly influenced by fractures of tectonic origin increasing the edz thickness of 0 7 m up to a damaged zone of 1 8 m based on the he concentration profile the 13 a old gallery 1996 has a net edz of 1 2 m larger than the 0 7 m deduced from the structural analysis by core mapping followed by a disturbed zone of up to 3 5 m that could represent an intermediate stage before the development of fractures in the edz the vertical borehole drilled in the same gallery shows a less extensive damaged zone 1 4 m and allows the determination of a dp of 3 10 11 m 2 s taking into account the anisotropy relative to the stratification plane the rare gas analysis has therefore enabled confirmation and better description of the net time dependent evolution of the edz around drifts in the argillaceous tournemire site no obvious edz for the 1 a gallery 2008 1 2 m width for the 13 a old gallery 1996 and 2 m width for the century old tunnel the occurrence of a disturbed and unfractured zone dz around the 13 a old gallery undetected by structural studies and geophysical methods clearly demonstrates the power of radiogenic he in assessing the transport properties around drifts excavated at different ages in compacted clay rocks

low frequency variability in a simplified atmospheric global circulation model storm track induced spatial resonance the interaction between two storm tracks is analysed by using idealized experiments with a simplified global circulation model a zonal distance of 180 between the storm tracks drives a retrograde travelling zonal wave number two wave with an amplitude modulation of about 50 days the stream function tendency equation demonstrates that the amplitude modulation is excited in the storm track regions by the high frequency periods between two and eight days eddy forcing spatial resonance of a retrograde propagating zonal wave number two rossby wave and high frequency eddy forcing are proposed as the amplifying mechanisms while friction and self interaction between the low frequency eddies ensure damping sensitivity experiments indicate that the pronounced modulation of 50 days vanishes with decreasing zonal distance between the storm tracks thereby enhancing the intensity and duration of blocking events

diffusion as the main process for mass transport in very low water content argillites 2 fluid flow and mass transport modeling 1 on the basis of chloride concentrations of pore water in the tournemire massif part 1 a conceptual model for mass transport in argillites by diffusion is proposed from this conceptual model and current knowledge of the geological history of the massif one dimensional numerical simulations are formulated for chloride transport in tournemire massif over the past 53 ma good agreement between experimental data and calculated values for both diffusion coefficients and concentrations of chloride confirms that diffusion is the main process for mass transport in the massif this model is also tested using deuterium contents of pore water applying variable concentrations to meteoric water circulating in system boundary layers based on the thermal dependency of its isotopic composition these simulations reveal the likely important role of lithologic heterogeneities such as fractures in the horizontal distribution of tracer concentrations

an updated analysis of northern hemisphere submonthly retrograde waves abstractthis study performs an updated analysis of northern hemisphere retrograde disturbances that were first identified by classical observational studies as one of the dominating coherent struct

storm track organization and variability in a simplified atmospheric global circulation model a storm track is simulated in a coarse resolution multilevel primitive equation model with linear surface friction and heating terms a restoration temperature distribution consisting of a dipole embedded in a zonally symmetric profile forces the model to simulate the surface heating three simulations each with a different dipole orientation are performed to test the sensitivity of storm track organization to the external forcing field the climatological time mean circulation and the transient disturbances of the reference simulation agree well with observations of northern hemisphere wintertime storm tracks local energetics show that baroclinic instability is responsible for the enhanced eddy kinetic energy downstream of the jet and the downstream end of the storm track results from the barotropic conversions of eddy kinetic energy to the time mean flow low frequency fluctuations with a period of about 50 days associated with a retrograding large scale wave pattern are identified by a complex empirical orthogonal function analysis of the vertically averaged stream function a composite life cycle of the low frequency variability reveals the growth and decay of a blocking anticyclone downstream of the storm track a cyclogenesis initiated by an eastward propagating wave train is observed a few days before the amplitude of the low frequency anomaly attains its peak the net forcing of the high frequency eddies contributes to the growth and decay of the blocking anticyclone the results suggest that the occurrence of the blocking like event is part of the storm track dynamics

the formation of magnetite in the early archean oceans abstract banded iron formations bifs are iron and silica rich chemical sedimentary rocks that were deposited throughout much of the precambrian the biological oxidation of dissolved fe ii led to the precipitation of a ferric oxyhydroxide phase such as ferrihydrite in the marine photic zone upon burial ferrihydrite was either transformed into hematite through dehydration or it was reduced to magnetite via biological or abiological fe iii reduction coupled to the oxidation of buried microbial biomass however it has always been intriguing as to why the oldest bifs are characteristically magnetite rich while bifs formed after the neoarchean are dominated by hematite here we propose that some magnetite in early archean bif could have precipitated directly from seawater through the reaction of settling ferrihydrite and hot fe ii rich hydrothermal fluids that existed in the deeper waters we conducted experiments that showed the reaction of fe ii with biogenic ferric iron mats under strict anoxic conditions lead to the formation of a metastable green rust phase that within hours transformed into magnetite our model further posits that with the progressive cooling and oxidation of the earth s oceans the above reaction shuts off and magnetite was subsequently restricted to reactions associated with diagenesis and metamorphism

noble gas composition of deep brines from the palo duro basin texas deep groundwaters in sedimentary basins can be sufficiently old 10 m y to have accumulated both radiogenic 4he and 40ar differences in the ratio of u th k for aquifer lithologies lead to variations for 4he40ar in the groundwater these variations can be used to resolve chronologically significant noble gas components application of a binary model to brine samples from the palo duro basin texas yields u th 4he residence times of 100 to 300 m y concentrations of two non radiogenic noble gases 20ne and 36ar provide information on the salinities of the water that recharged these aquifers

helium and argon isotopes in rocks minerals and related ground waters a case study in northern switzerland abstract to address the problems related to the transfer of helium and argon isotopes from rocks into related aquifers the concentrations of u th li and k and helium and argon isotopes were measured in a sedimentary sequence from tertiary to permo carboniferous and in crystalline rocks in northern switzerland in addition to whole rock samples mineral separates have also been investigated the observed concentrations of 3 he 4 he and 40 ar in rocks minerals and groundwaters are compared to calculated values which would result from in situ radiogenic production in a closed system this comparison shows that the rocks and minerals have almost completely lost radiogenic in situ produced helium loss of 3 he and 4 he has been controlled by different retention capacities of the rock forming and accessory minerals and fillings therefore measured 3 he 4 he ratios in these rocks can differ from the calculated production ratios the deviation is generally less than a factor of 3 however in certain chemical sediments e g anhydrites elevated ratios of 3 he 4 he up to 15 10 8 which is 20 times the production ratio were observed it is postulated that 3 h the precursor of 3 he might be chemically bound in such sediments during the 12 3 years half life of 3 h and that the beta decay energy of 3 h is too small to liberate 3 he the calculated closed system concentrations of helium in groundwater exceed the measured ones by up to three orders of magnitude implying that the water rock system has not been a closed system since the time of sedimentation and that movable waters have removed helium to a discharge area and to the atmosphere stagnant old waters however could contain high concentrations of helium and supply it to younger movable waters aquitards with low permeability permian shales appear to be a more important source of helium isotopes than water bearing rocks permian sandstones high abundances of all parent elements in the shales almost complete loss of radiogenic helium and the enhanced production ratio of 3 he 4 he 7 2 10 8 in these rocks similar to the ratio observed in the neighbour permian aquifer 9 4 10 8 suggest the shale as a major source of helium isotopes for this aquifer and diffusion as a principal process for helium transfer from the stagnant porewaters in shales into the movable waters in sandstones some sedimentary rocks e g permian sandstones and shales contain more helium than could have been produced since the time of sedimentation while permian sandstones mainly have retained this excess rare gas component they have released radiogenic helium the difference in the retention of these two components most probably demonstrates that migration of radiogenic helium from damage tracks along crystal imperfections to grain boundaries is an important process controlling its loss the abundances of excess 3 he and 4 he in rocks cannot be predicted by in situ production calculations studies of rocks and minerals are required the average ratio of 40 ar 36 ar 385 in groundwaters collected from permian sediments from the weiach borehole is much higher than that in the aquifers from the crystalline basement below the sediments radiogenic 40 ar in permian sediments is mainly liberated by diagenetic water rock interactions the low contribution of radiogenic 40 ar in deep aquifers of the crystalline basement and the fact that chlorine concentrations in groundwaters from the basement are also much lower than in the overlaying permian aquifer eliminates a hypothetical upwards fluid flux from this study we envisage intrabasin sources for helium and argon isotopes in groundwaters with mainly lateral migration of waters through sedimentary layers at different rates and diffusion of helium from slower velocity older waters in more stagnant zones shales into flow passes with highervelocity younger waters in sections with higher permeability sandstones an external source for noble gases is not required in such a model

the mount gibson banded iron formation hosted magnetite deposit two distinct processes for the origin of high grade iron ore the mount gibson banded iron formation lies within the windanning formation of the luke creek group which is found in almost all greenstone belts throughout the murchison province the banded iron formation bif typically consists of alternating bands of magnetite and microcrystalline quartz chert with rare carbonaceous and iron silicate rich shale partings and layers and rare thin fine grained tuff bands owing to repetition by isoclinal folding and attenuation by faulting the true thickness of the mount gibson bif is unknown but appears to be on the order of 100 m although the mount gibson bif is typical of many algoma type iron formations situated within a greenstone belt on an archean craton it is also similar to hamersley type bif in petrology areal extent and ore genesis high grade hematite deposits formed within bif were thought to have formed by the supergene leaching of chert from typical cherty bif recent evidence suggests that at least some of these deposits are formed by hypogene replacement of chert by carbonates with subsequent supergene leaching of the carbonate and accessory minerals and oxidation of magnetite to hematite magnetite carbonate bif in which there is clear evidence of hydrothermal replacement of chert by carbonate forms distinctive magnetite goethite ore with magnetite locally persisting to the surface mount gibson shows clear evidence of the formation of high grade ore by this process but also contains high grade hematite ore and chert free bif that show no evidence of the hypogene replacement of chert high grade hematite occurrences up to 1 km in strike length are found within the weathered zone overlying the magnetite bif at mount gibson and continue into unweathered chert free bif at depth that show no evidence of hydrothermal carbonate or supergene enrichment the cherty bif shows sharp contacts against chert free bif and high grade ore even when strongly weathered this suggests that deep saprolitic in situ high grade ore may be produced by different processes including hydrothermal replacement of chert mesobands by carbonates with subsequent supergene leaching of the carbonate and by the oxidation of chert free bif in which chert bands either never developed or were apparently removed during diagenesis neither model requires supergene selective leaching of quartz chert during deep weathering

using excess 4he to quantify variability in aquitard leakage summary fluid flux through aquitards controls the rate of recharge discharge cross formational fluid flow and contaminant transport in subsurface systems in this paper concentrations of 4 he are used to investigate the spatial distribution of vertical fluid flux through the regionally extensive great artesian basin aquitard system in northern south australia two vertical profiles of 4 he concentration in aquitard pore water augmented with regional sampling of aquifers above and below the aquitard were used to estimate fluid flux at multiple locations over a large spatial area 4 he concentrations in the shallow aquifer above the great artesian basin range from atmospheric equilibrium to 1000 times enriched over atmosphere fluid flux through the aquitard was estimated by fitting observed helium concentrations at each sampling site with a 1 d model of helium transport through the aquitard estimated fluid fluxes through the aquitard vary over three orders of magnitude across the study area in areas of competent aquitard fluid fluxes are less than 0 003 mm yr and mass transport of helium is dominated by molecular diffusion preferential discharge zones are clearly identifiable with fluid fluxes up to 3 mm yr our results show that fluid flux through a regionally extensive aquitard can be highly variable at large spatial scales and that 4 he concentrations in aquifers bounding the aquitard system provide a convenient and sensitive method for investigating aquitard flux at the regional scale