exchange of neodymium and its isotopes between seawater and small and large particles in the sargasso sea the concentrations and isotopic ratios of nd were measured in small 1 53 m and large 53 m in situ filtered particles sediment trap material sediment and aerosol samples collected during oligotrophic and productive conditions in the sargasso sea atmospheric input is the main source of exogeneous nd into the sargasso sea based on ndal ratios in trapped material we suggest that up to 50 of atmospheric nd could dissolve at the air sea interface the concentration and isotopic composition of nd vary with both depth and the particle size neodymium concentrations of small and large particles increase with depth from 2 9 12 g g and 5 8 11 8 g g respectively at 40 m the nd signature of large particles is significantly more negative nd 0 13 1 0 2 than that of the small ones nd 0 11 7 0 3 and the particles filtered deeper below 237 m the nd 0 values for small particles match the seawater whereas the nd 0 values of large particles remain less radiogenic these differences are related to the different behaviors of the particulate fractions small particles apparently exchange nd with seawater and then sink rapidly to depth as biologically formed aggregates large particles these aggregates isolate the small particles slowing the exchange with the water to produce different signatures between large particles and seawater since these exchange processes modify the particulate nd signature the shallow radiogenic nd 0 signature observed in the surface and subsurface waters in the nw atlantic is not due to external contributions and is likely of advective origin at depth below 800 m the magnitude of the aggregation disaggregation process varies significantly with season the fraction of small particles repackaged into large aggregates can vary from 0 8 to 80 100 in productive and oligotrophic conditions respectively this seasonal variation may correspond to sinking particles of different zooplanktonic origin vertical fluxes of nd into and out of the sargasso sea as well as exchange fluxes in the water column are examined about 50 of the particulate flux from the surface is remineralized at depth a net scavenging flux of 0 8 10 4 g m2 y is required this value corresponds to a scavenging nd residence time of 1900 y considerably longer than previous estimates of global oceanic residence time of nd on the order of 400 y

field relationship of high grade neo to mesoarchaean rocks of south east greenland tectonometamorphic and magmatic evolution abstract south east greenland forms part of the north atlantic craton and is characterized by migmatitic orthogneisses narrow bands of mafic granulite ultramafic and possible meta sedimentary rocks and alkaline carbonatitic intrusive rocks mafic granulite meta sedimentary and ultramafic rocks form the basement for the emplacement of granitic intrusions at ca 2865 ma that lasted episodically until ca 2790 ma and continuously during 2750 2700 ma the area is structurally complex with evidence of at least seven deformation events including reclined and mushroom like fold interference patterns an older 2790 ma foliation formed in granitic rocks and the basement during the timmiarmiut orogeny dt deformation associated with the ca 2790 2700 ma skjoldungen orogeny folded this early foliation and is associated with a penetrative foliation that is refolded progressively in a northeast southwest oriented stress field the orientation of the stress field progressively rotated into a northnorthwest southsoutheast orientation during the last stages of the orogeny the orogeny is also characterized by syn deformational anatexis at granulite facies at approximately 800 c and 5 8 kbar ca 2790 2740 ma which decreased to the amphibolite facies at ca 2730 ma the late to post tectonic granite and alkaline rocks assigned to the skjoldungen alkaline province intruded the central northern part around 2710 ma this was followed by north south extensional deformation during the singertat stage forming discrete shear zones at greenschist facies grades which is coeval with the emplacement of pegmatite ijolite and carbonatite emplacement during ca 2680 2650 ma similar lithology and tectonic processes in the tasiusarsuaq terrane of southern west greenland and the lewisian complex in scotland suggest a possibly large archaean terrane at that time which taking the present size at least covered around 500 600 km in an east west direction and approximately 200 km in a north south direction

new constraints on the sources and behavior of neodymium and hafnium in seawater from pacific ocean ferromanganese crusts the behavior of dissolved hf in the marine environment is not well understood due to the lack of direct seawater measurements of hf isotopes and the limited number of hf isotope time series obtained from ferromanganese crusts in order to place better constraints on input sources and develop further applications a combined nd hf isotope time series study of five pacific ferromanganese crusts was carried out the samples cover the past 38 myr and their locations range from sites at the margin of the ocean to remote areas sites from previously unstudied north and south pacific areas and water depths corresponding to deep and bottom waters for most of the samples a broad coupling of nd and hf isotopes is observed in the equatorial pacific end and ehf both decrease with water depth similarly end and ehf both increase from the south to the north pacific these data indicate that the hf isotopic composition is in general terms a suitable tracer for ocean circulation since inflow and progressive admixture of bottom water is clearly identifiable the time series data indicate that inputs and outputs have been balanced throughout much of the late cenozoic a simple box model can constrain the relative importance of potential input sources to the north pacific assuming steady state the model implies significant contributions of radiogenic nd and hf from young circum pacific arcs and a subordinate role of dust inputs from the asian continent for the dissolved nd and hf budget of the north pacific some changes in ocean circulation that are clearly recognizable in nd isotopes do not appear to be reflected by hf isotopic compositions at two locations within the pacific ocean a decoupling of nd and hf isotopes is found indicating limited potential for hf isotopes as a stand alone oceanographic tracer and providing evidence of additional local processes that govern the hf isotopic composition of deep water masses in the case of the southwest pacific there is evidence that decoupling may have been the result of changes in weathering style related to the buildup of antarctic glaciation

reduced north atlantic deep water flux to the glacial southern ocean inferred from neodymium isotope ratios the global circulation of the oceans and the atmosphere transports heat around the earth broecker and denton1 suggested that changes in the global ocean circulation might have triggered or enhanced the glacial interglacial cycles but proxy data for past circulation taken from sediment cores in the south atlantic ocean have yielded conflicting interpretations of ocean circulation in glacial times 13c variations in benthic foraminifera2 3 4 5 6 support the idea of a glacial weakening or shutdown of north atlantic deep water production whereas other proxies such as cd ca ba ca and 231pa 230th ratios show little change from the last glacial maximum to the holocene epoch7 8 9 here we report neodymium isotope ratios from the dispersed fe mn oxide component of two southeast atlantic sediment cores both cores show variations that tend towards north atlantic signatures during the warm marine isotope stages 1 and 3 whereas for the full glacial stages 2 and 4 they are closer to pacific ocean signatures we conclude that the export of north atlantic deep water to the southern ocean has resembled present day conditions during the warm climate intervals but was reduced during the cold stages an increase in biological productivity may explain the various proxy data during the times of reduced north atlantic deep water export

hydrothermal flake graphite mineralisation in paleoproterozoic rocks of south east greenland flake graphite mineralisation is hosted in the kuummiut terrane of the paleoproterozoic nagssugtoqidian orogen south east greenland eclogite facies peak metamorphic assemblages record temperatures of 640 830 c and pressures of 22 25 kbar and are retrogressed in the high pressure amphibolite facies during ca 1870 1820 ma graphite occurs as lenses along cleavage planes in breccia and as garnet quartz graphite veins in various metamorphic host rocks in the tasiilaq area at auppaluttoq kangikajik and nuuk ilinnera graphite contents reach 30 vol in 0 2 4 20 m wide semi massive mineralisation auppaluttoq kangikajik supergene alteration formed 1 to 2 m thick and up to a 2 5 2 5 km wide loose limonitic gravel containing graphite flakes in places the flake size ranges from 1 to 6 mm in diameter with an average of 3 mm liberation efficiency is at minimum 60 hydrothermal fluids at 600 c transporting carbon as co2 and ch4 formed the mineralisation commonly hosted by shear zones which acted as pathways for the mineralising fluids the hydrothermal alteration assemblage is quartz biotite grunerite edenite pargasite k feldspar titanite the 13c values of graphite varying from 30 to 18 pdb indicate that the carbon was derived from organic matter most likely from metasedimentary sources devolatilisation of marble may have contributed a minor amount of carbon by fluid mixing precipitation of graphite involved retrograde hydration reactions depleting the fluid in h2o and causing graphite saturation although the high grade mineralisation is small it represents an excellent example of hydrothermal mineralisation in an eclogite facies terrane during retrograde exhumation

the assessment of ree patterns and 143nd 144nd ratios in fish remains abstract the ree content and isotopic composition of sr and nd have been determined in fish teeth ranging in age from the trias to the present and from various localities mostly around the atlantic these measurements have been carried out on selachian and teleost remains from the same locality in togo and show no appreciable difference which suggests with the help of a mass balance calculation of the ce anomaly that diagenetic effects are not responsible for the ree enrichment of biogenic phosphates one group of fossil teeth has about 3 times the ree abundances of shale and a shale normalized pattern with a minimum at sm it is thought to reflect deposition in the open sea environment a second group has ree concentration about 10 times higher than the first group with either a regular light ree enrichment or more frequently a maximum in the middle ree both being probably indicative of deposition in estuarine or near shore conditions the shape of the ree spectra and the size of the ce anomaly can be used semi quantitatively to determine the depth of deposition the results presented here on late cretaceous eocene fish teeth samples from morocco reflect an increasing influx of deep waters with a lowla yb ratio and strong negative ce anomaly which agrees well with the evolution of sediment chemistry and microfauna associations in contrast nd is typical of the water mass in which the fish debris decayed examples of nearly isolated basins identified with nd isotopes include the south atlantic prior to the lutetian nd 13 5 the miocene persian gulf nd 3 1 and bolivia during the late cretaceous nd 12 8 togo and guinea bissau results suggest that in the south atlantic the meridional oceanic circulation had not started before 45 ma ago combination of ree and nd data suggests that the assignment of jurassic cretaceous samples measured so far to open sea water masses is still ambiguous

the hafnium isotope composition of pacific ocean water the first hf isotope data for seawater are reported for a series of stations in the northwestern pacific and define a range from epsilon hf 3 5 1 4 to 8 6 1 6 most samples have values within error of the average of epsilon hf 5 9 but significant variations are found in intermediate waters at a depth of 600 m as well as in deep waters the nd and hf isotope compositions of the deep waters fall within the range of values found for surfaces of hydrogenetic ferromanganese crusts in the region confirming that hf in the fe mn crusts has been derived from the overlying water column which thus provide an archive of past seawater compositions although the seawater samples are generally close to the global epsilon nd epsilon hf correlation obtained from ferromanganese crusts there are significant deviations from this correlation indicating that there is some additional decoupling between nd and hf isotope signals most likely caused by local water mass mixing and differences in residence times this is not resolved in the crust samples which integrate seawater signals over 10 4 years the combined use of these two isotope systems in seawater therefore provides an additional dimension for tracing water masses in the oceans studies of the distribution of oceanic hf isotope compositions that have been confined to deep water and boundary waters as recorded in seafloor ferromanganese crusts can now be extended and aimed at characterising the entire present day water column average hf concentrations measured in this study are somewhat lower than previously reported suggesting a shorter residence time for hf in the global oceans although the uncertainty in the extent of hf removal from the water column during estuarine mixing as well as a lack of data on hydrothermal and dust inputs remains a limit on how well the residence time can be defined c 2008 elsevier ltd all rights reserved

preface to the special issue of precambrian research on the understanding of gneiss complexes in honour of clark r l friend gneiss complexes are the world s most horrendously complexrocks to understand see cover of this issue yet they form a largeamount of the continents albeit they are normally hidden fromview by sedimentary sequences the last half century has seenenormous advances in understanding these rocks which form notonly a large amount of the continental crust but also hold much of earth s oldest rock record the archaean disciplines medicine and health sciences social and behavioral sciences publication details nutman a p 2014 preface to the special issue of precambrian research on the understanding of gneiss complexes in honour of clark r l friend precambrian research 255 part 3 771 773 this journal article is available at research online http ro uow edu au smhpapers 2672 preface to the special issue of precambrian research on the understanding of gneiss complexes in honour of clark r l friend

the isotopic composition of neodymium in the north pacific abstract we have determined the isotopic composition of neodymium in two water column profiles in the western north pacific and in bottom waters from three other locations in the north pacific the vertical profiles reveal that the water column is stratified with respect to isotopic composition mid depths associated with pacific deep water are uniform having nd 0 3 shallow waters are variable but deep waters are less radiogenic having md 0 4 4 to 5 in the western basin the more negative bottom waters in the western north pacific provide evidence for the advective transport of rare earth elements from antarctic sources to this region bottom waters in the eastern north pacific are more radiogenic nd 0 3 3 to 3 8 indicating very little transport of ree from antarctic sources to this basin overall the isotopic composition of nd in pacific bottom waters appears to be conservative when compared to the silica distribution in spite of evidence for nd removal the findings are generally compatible with other parameters such as temperature salinity and nutrient distributions the presence of nd isotopic signatures associated with antarctic sources in these waters provides evidence for a small component of rare earth elements of north atlantic origin in the bottom waters of the north pacific this is the first tracer evidence to corroborate the suggestion of reid and lynn 1971 that some of the temperature and salinity properties of the deep waters of the north pacific originated in the norwegian and greenland seas

u pb and lu hf isotopes of the archean orthogneiss complex on eastern hall peninsula southern baffin island nunavut identification of exotic paleo to mesoarchean crust beneath eastern hall peninsula abstract eastern hall peninsula on southeastern baffin island lies at the junction of a complex paleoproterozoic collision between the rae craton meta incognita microcontinent and the north atlantic craton from ca 1 88 to 1 80 ga several different interpretations of crustal correlations and the location of intervening sutures have been proposed based on reconnaissance scale geologic investigation therefore in this study complex zircon grains from archean orthogneiss units on eastern hall peninsula were analyzed in situ to elucidate the detailed magmatic history of the region and assess crustal provenance magmatic zircons yielded u pb crystallization ages between ca 2976 and 2720 ma and metamorphic zircons record tectonothermal disturbances between ca 2740 and 2700 ma a period coinciding with metamorphism documented in adjacent crustal blocks e g west greenland and northern labrador magmatic rocks older than ca 2740 ma generally have positive ehf t signatures between 0 and 7 2 and depleted mantle model ages of ca 3 1 3 0 ga indicating the time that protolith melt was extracted from the mantle the oldest granodioritic crust crystallized at ca 2976 ma and was then reworked periodically at ca 2 93 2 84 2 81 and 2 77 2 69 ma zircons from two orthogneiss samples with u pb crystallization ages younger than ca 2740 ma yielded negative ehf t values ranging from 4 to 12 and mean depleted mantle model ages of ca 3 4 and 3 3 ga respectively indicating derivation from an older potentially exotic crustal source yet to be identified in outcrop on hall peninsula synthesizing regional u pb data we propose a new regional correlation model that retains the essentials of previous models and incorporates new data from eastern hall peninsula and other recent studies this new tectonic correlation model groups eastern hall peninsula southern cumberland peninsula and the aasiaat domain into a core zone that shared a geologic history prior to 1 90 ga and potentially prior to 2 75 ga

the palaeoproterozoic anatomy of the lewisian complex nw scotland evidence for two laxfordian tectonothermal cycles a new structural examination of palaeoproterozoic high p granulites on south harris nw scotland when integrated with previous geochronological structural and metamorphic studies on key areas of the lewisian complex suggests the existence of two distinct tectonothermal cycles within the palaeoproterozoic laxfordian event which on south harris are separated by a 100 myr hiatus in deformation the older cycle from c 1 91 to 1 85 ga records the development of an active continental margin on the archaean gneisses that dominate the complex and the subsequent onset of continent continent collision this represents the continuation of the nagssugtoqidian orogen of greenland evidence for this is concentrated in allochthonous slivers of the former active continental margin displaced during the younger cycle the younger cycle around 1 75 1 65 ga began with thrust related crustal thickening that initiated regionally extensive amphibolite facies metamorphism and ductile deformation which dominates the preserved laxfordian deformation history this may be the peripheral expression of the accretion of the malin block to the sw of the lewisian and represents the lateral continuation of the labradorian ketilidian orogen of north america supplementary material additional figures are available at http www geolsoc org uk sup18863

2 7 ga metamorphic peridotites from southeast greenland record the oxygen isotope composition of archean seawater abstract the 18 o value of seawater is presently buffered by high temperature hydrothermal alteration and low temperature weathering of lithospheric rocks it is much debated whether the 18 o of seawater has been buffered to this steady state value throughout earth s history or alternatively whether it gradually increased towards the present value since the archean a third possibility is that the 18 o of seawater was buffered at a higher value before continents emerged and has been buffered at its current value 18 o 1 since the emergence of continents in this contribution we reconstruct the 18 o of archean seawater from triple oxygen isotope 17 o 18 o variations in 2 7 ga ultramafic rocks that reacted with fluids that in turn had been derived from seawater in the archean the samples that were studied are peridotites from the ivnartivaq complex an ultramafic lens in the archean kuummiut terrane rae craton southeast greenland that have unusually low 18 o values for peridotites in olivine 1 7 18 o 4 6 relative to vsmow bulk rock trace element concentrations mineral compositions and u pb dating of zircon grains in the peridotites indicate that these samples are metamorphic peridotites that formed by the dehydration of serpentinites 2 7 ga ago the serpentinite protoliths in turn had formed by alteration of ultramafic cumulate rocks at high temperatures 250 450 c by fluids that were derived from seawater triple oxygen isotope variations in olivine from the peridotites indicate that the fluids could not have been derived from seawater with a significantly higher or lower 18 o value than that of seawater in the phanerozoic but could have been derived from seawater with a value of 18 o 1 i e the modern ice free seawater composition we conclude that the 18 o of seawater had reached its current steady state value by 2 7 ga ago

timing of magmatism and metamorphism in the gruinard bay area of the lewisian gneiss complex comparison with the assynt terrane and implications for terrane accretion reply u pb zircon data from selected gneiss samples provide estimates of the timing of igneous protolith emplacement and granulite facies metamorphism in the gruinard bay badcall bay and lochinver areas of the mainland lewisian gneiss complex of north west scotland zircons from the gneisses contain complex zoning patterns making them amenable to shrimp ion microprobe analysis with inherited zircon distinguishable from magmatic growth and metamorphic recrystallisation from gruinard bay a tonalitic granulite yielded a protolith age of 2 825 8 ma for oscillatory zoned zircon cores with recrystallised low u rims preserving an age of 2 733 12 ma an inherited 2 905 15 ma age component was recognised in a sample of trondhjemitic gneiss the protolith of which is estimated to have been emplaced at 2 858 11 ma low u rims yielding an age of 2 729 20 ma are also present which together with the rim age from the granulite are considered to date granulite facies metamorphism in the gruinard region for comparison with the gruinard bay gneisses a trondhjemitic partial melt and trondhjemite sheet were analysed from the granulite facies rocks at badcall bay in the assynt terrane age data do not show definitive relationships with zoning patterns in the zircons and show a spread along concordia typical of previous ion microprobe age data from the scourie gneisses both samples do however show a significant isotopic disturbance at 2 480 ma the most recent estimate for the badcallian metamorphism protolith ages for components of the assynt terrane range between ca 2 900 and 3 030 ma and appear to be older than the 2 860 2 825 ma gneiss protoliths from the gruinard bay area the timing of granulite facies metamorphism also appears to be different between the two areas suggesting that the two granulite regions of the mainland may not be related as previously thought instead they are interpreted in terms of a terrane accretion model whereby the two areas evolved as separate blocks prior to their juxtaposition during the palaeoproterozoic the boundary between the two terranes is thought to lie to the south of lochinver corresponding with the strathan line because of the observed 2 480 ma age signature to the north and 2 730 ma signature to the south by defining a new mainland terrane the gruinard terrane the number of mainland blocks which includes the assynt central rhiconich northern and southern terranes has been expanded

study on vertical profiles of rare earth elements by using an ocean general circulation model 1 observed vertical profiles of rare earth elements rees in the north pacific ocean show a systematic change from lighter to heavier rees their source and sink consist of external fluxes coming from the surface and the bottom of the ocean and internal recycling within the ocean in order to evaluate the role of each source and sink in controlling the vertical profiles of rees we conduct numerical simulations of rees by using an ocean general circulation model these simulations demonstrate that the shape of their vertical profiles is not directly related to the external sources from river runoff atmospheric dust deposition and coastal regions but is sensitive to the internal recycling processes within the ocean nutrient like treatment where rees are removed at the surface and released in the deep ocean is suitable for simulating the vertical profiles of lighter rees but not for heavier rees when the irreversible scavenging is considered as a sink term and particle dissolution is taken into consideration as a source term the vertical profiles of heavier rees can be reproduced but those of lighter rees cannot the observed difference in the vertical profiles among rees can be reproduced only when the reversible scavenging process is incorporated into the model only the reversible scavenging process accounts for the observed vertical profiles of different rees in comprehensive manner

the nature and significance of the faroe shetland terrane linking archaean basement blocks across the north atlantic abstract core samples of the continental basement rocks that underlie the eastern faroe shetland basin fsb and its inshore margins west of shetland reveal a suite of predominantly granodioritic to granitic orthogneisses including ttg together with lesser volumes of foliated granitoids and subordinate dioritic to mafic gneisses and amphibolites a small area of lithologically similar gneisses also crops out onshore at north roe uyea west of the caledonian front in shetland in the core samples coarse grained gneissose textures and mineralogies consistent with upper amphibolite facies metamorphism are overprinted by a weak but ubiquitous static greenschist facies retrogression later structures include widespread epidote quartz veining and local developments of mylonite cataclasite pseudotachylite and phyllonite regions associated with the rona ridge oil fields e g clair lancaster also preserve extensive brittle fracturing and associated low temperature hydrothermal mineralization quartz adularia calcite pyrite chalcopyrite with significant fracture hosted hydrocarbons new and published u pb zircon analyses from the gneisses offshore give a relatively narrow range of neoarchaean protolith ages ca 2 7 2 8 ga spread over a geographic area of 60 000 km2 west of shetland detrital zircon data from overlying triassic cretaceous sedimentary sequences in the fsb suggest an equivalent limited range of neoarchaean source materials hf isotopic data indicate involvement of palaeo to mesoarchaean crustal sources overall our findings suggest that a major phase of neoarchaean crustal formation and associated high grade metamorphism dominates basement rocks in the region they are similar in age and lithology to the protoliths of the nearest onshore lewisian complex of nw scotland rhiconich terrane however they lack geochronological or textural evidence for the widespread proterozoic laxfordian events ca 1 7 1 8 ga which are widespread in scotland this suggests that the precambrian rocks west of shetland the faroe shetland terrane can be correlated with the archaean rocks of the central greenland rae craton and that a northern limit of proterozoic reworking lies just offshore from the north coast of scotland

controls on magmatism in an island arc environment study of lavas and sub arc xenoliths from the tabar lihir tanga feni island chain papua new guinea the tabar lihir tanga feni tltf islands of papua new guinea mainly comprise high k calc alkaline and silica undersaturated alkaline rocks that have geochemical features typical for subduction related magmatism numerous sedimentary mafic and ultramafic xenoliths recovered from tubaf seamount located on the flank of lihir island provide a unique opportunity to study the elemental and isotopic composition of the crust and mantle wedge beneath the arc and to evaluate their relationships to the arc magmatism in the region the sedimentary and mafic xenoliths show that the crust under the islands is composed of sedimentary sequences and oceanic crust with pacific affinity a majority of the ultramafic xenoliths contain features indicating wide spread metasomatism in the mantle wedge under the tltf arc leaching experiments reveal that the metasomatized ultramafic xenoliths contain discrete labile phases that can account for up to 50 or more of elements such as cu zn rb u pb and light ree rare earth elements most likely introduced in the xenoliths via hydrous fluids released from a subducted slab the leaching experiments demonstrated that the light ree enrichment pattern can be more or less removed from the metasomatized xenoliths and the residual phases exhibit ree patterns that range from flat to light ree depleted sr nd isotopic data for the ultramafic residues show a coupled behavior of increasing 87sr 86sr with decreasing 143nd 144nd ratios the labile phases in the ultramafic xenoliths represented by the leachates show decoupling between sr and nd with distinctly more radiogenic 87sr 86sr than the residues both leachates and residues exhibit very wide range in their pb isotopic compositions indicating the involvement of three components in the mantle wedge under the tltf islands two of the components can be identified as pacific oceanic mantle and pacific sediments some of the ultramafic samples and clinopyroxene separates however exhibit relatively low 206pb 204pb at elevated 207pb 204pb suggesting that the third component is either indian ocean type mantle or australian subcontinental lithospheric mantle geochemical data from the ultramafic xenoliths indicate that although the mantle wedge in the area was extensively metasomatized it did not significantly contribute to the isotopic and incompatible trace element compositions of tltf lavas compared to the mantle samples the tltf lavas have very restricted pb isotopic compositions that lie within the pacific morb range indicating that magma compositions were dominated by melts released from a stalled subducted slab with pacific morb affinity interaction of slab melts with depleted peridotitic component in the mantle wedge followed by crystal fractionation most likely generated the geochemical characteristics of the lavas in the area

shrimp u pb zircon geochronology of the late archaean ruinnaesset syenite skjoldungen alkaline province southeast greenland abstract a sample of the ruinnaesset syenite of the southeast greenland skjoldungen alkaline province contains a homogeneous population of zircons up to 1 mm diameter that are devoid of new overgrowths and old inherited cores u pb isotopic analyses of these zircons form a single population with weighted mean 207 pb 206 pb and 206 pb 238 u ages of 2699 4 ma and 2690 20 ma 2 respectively these dates are interpreted as giving the time of crystallisation of the syenite

a critical discussion of the subduction collision model for the neoproterozoic aracuai west congo orogen abstract the neoproterozoic aracuai west congo orogen in brazil and congo represents a branch of the brasiliano pan african orogenic system that is considered to terminate northward into a confined cratonic environment defined by the horseshoe shaped pre atlantic sao francisco congo craton the prevailing interpretation is that this orogen formed as a result of 50 m y of subduction leading to a classical continent continent collision this subduction collision model hinges on interpretation of 630 580 ma granitoids in the core of the orogen as being arc related and of locally exposed mafic and ultramafic metamorphic rocks as being ophiolitic we show that when tested beyond geochemical signature the model has fundamental problems that cannot be accounted for in particular there is an insurmountable oceanic space problem in this confined setting that is overlooked in most of the current literature there are also problems with subduction initiation lack of unambiguous evidence for oceanic crust no trace of any high p metamorphism and the abrupt termination of an ocean with no realistic way to transfer the large amount of oceanic opening displacement and subsequent convergence required by the model we conclude that the prevailing subduction collision model cannot possibly work and argue that the existing data are more consistent with hot intracontinental orogeny we stress the importance of building tectonic models on more than one type of data making realistic restorations and palinspastic reconstructions and taking into account modern geotectonic knowledge

the use of foraminifera as a record of the past neodymium isotope composition of seawater 1 we present new isotopic data for sedimentary planktonic foraminifera as well as for potential water column and sedimentary sources of neodymium nd which confirm that the isotopic composition of the foraminifera is the same as surface seawater and very different from deep water and sedimentary nd the faithfulness with which sedimentary foraminifera record the isotopic signature of surface seawater nd is difficult to explain given their variable and high nd ca ratios ratios that are often sedimentary foraminifera ratios that are often much higher than is plausible for direct incorporation within the calcite structure we present further data that demonstrate a similarly large range in nd ca ratios in plankton tow foraminifera a range that may be controlled by redox conditions in the water column cleaning experiments reveal in common with earlier work that large amounts of nd are released by cleaning with both hydrazine and diethylene triamine penta acetic acid but that the nd released at each step is of surface origin while further detailed studies are required to verify the exact location of the surface isotopic signature and the key controls on foraminiferal nd isotope systematics these new data place the use of planktonic foraminifera as recorders of surface water nd isotope ratios and thus of variations in the past supply of nd to the oceans from the continents via weathering and erosion on a reasonably sure footing

a reappraisal of the lewisian gneiss complex geochronological evidence for its tectonic assembly from disparate terranes in the proterozoic new u pb single zircon geochronology undertaken on tonalitic gneisses granite sheets migmatites and metasediments from the lewisian gneiss complex on the mainland and the northern part of the outer hebrides nw scotland have been used to test the correlation of so called laxfordian events across the complex from the outer hebrides to the mainland and the current model for the evolution of the complex as a whole the study has revealed that the granite sheets originated in two quite different melting events those on the mainland at loch laxford are ca 1 855 ma old whereas those on harris and lewis with which they are presently correlated are ca 1 675 ma old grey gneisses associated with granites on the south side of loch laxford are confirmed to belong to the northern region a migmatitic grey gneiss on harris has given a protolith age of ca 3 125 ma the currently oldest recognised in the complex detrital zircons in the leverburgh and langavat belts range in age from 2 780 to 1 880 ma and unequivocally demonstrate deposition in the palaeoproterozoic the granulite facies metamorphism in this block is dated from zircon overgrowths at ca 1 880 ma the laxford shear zone which separates the northern and central regions is interpreted to have evolved post 1 860 ma during amphibolite facies metamorphism accompanying deformation which took place at ca 1 740 ma in both regions on harris the langavat finsbay shear zone developed after 1 675 ma when a ca 1 880 ma granulite facies proterozoic arc was juxtaposed against amphibolite facies archaean rocks to the north therefore the shear zones which bound tectonic blocks in the lewisian complex evolved at different times and can be interpreted as terrane boundaries the new data confirm that the lewisian complex was not constructed from one contiguous piece of archaean crust reworked in the proterozoic but was progressively assembled from several discrete terranes during the proterozoic accordingly the former regional divisions of the lewisian complex are here renamed as follows on the mainland the northern region is called the rhiconich terrane and the central region the assynt terrane on the outer hebrides the archaean gneisses of lewis and the northern part of harris comprise the tarbert terrane whereas the newly accreted proterozoic blocks are called the roineabhal terrane in harris and the niss terrane in the north on lewis wider correlations show that the geology of the outer hebrides has more in common with east greenland than mainland scotland on the eastern side of the minch fault

paleocurrent reconstruction of the deep pacific inflow during the middle miocene reflections of east antarctic ice sheet growth today the deep western boundary current dwbc east of new zealand is the most important route for deep water entering the pacific ocean large scale changes in deep water circulation patterns are thought to have been associated with the development of the east antarctic ice sheet eais close to the main source of bottom water for the dwbc here we reconstruct the changing speed of the southwest pacific dwbc during the middle miocene from 15 5 12 5 ma a period of significant global ice accumulation associated with eais growth sortable silt mean grain sizes from ocean drilling program site 1123 reveal variability in the speed of the pacific inflow on the timescale of the 41 kyr orbital obliquity cycle similar orbital period flow changes have recently been demonstrated for the pleistocene epoch collectively these observations suggest that a strong coupling between changes in the speed of the deep pacific inflow and high latitude climate forcing may have been a persistent feature of the global thermohaline circulation system for at least the past 15 myr furthermore long term changes in flow speed suggest an intensification of the dwbc under an inferred increase in southern component water production this occurred at the same time as decreasing tethyan outflow and major eais growth between 15 5 and 13 5 ma these results provide evidence that a major component of the deep thermohaline circulation was associated with the middle miocene growth of the eais and support the view that this time interval represents an important step in the development of the neogene icehouse climate

limits on conservative behavior of nd isotopes in seawater assessed from analysis of fish teeth from pacific core tops abstract neodymium nd isotopes have become an accepted water mass tracer for paleoceanographic reconstructions however high rates of radiogenic nd input from volcanic islands arcs in the western equatorial pacific wep and the subarctic north pacific sanp can significantly alter the isotopic composition of ambient seawaters leading to questions about the conservative nature of nd isotopes in the pacific here we present the first comprehensive study of nd isotopes preserved in core top fossil fish teeth debris from 31 sites in the pacific to define the vertical and horizontal distributions of seawater nd isotopes and to test the reliability of nd isotopes as a water mass tracer in the pacific we found that nd isotopic values of northward flowing lower circumpolar deep water lcdw are significantly modified in the western central equatorial pacific and the sanp as a result of local radiogenic inputs from young circum pacific island arcs consequently modified lcdw with 5 end is created in the central equatorial pacific while recirculated north pacific deep water npdw acquires a radiogenic nd isotope value 2 end in the sanp nd isotopic ratios of fish teeth debris in the central north pacific plotted against salinity or phosphate concentration from the overlying bottom water produce a mixing envelope between modified lcdw and npdw indicating that these nd isotopic values represent the southern and northern sourced end member values for the central north pacific and that mixing of these water masses defines vertical profiles of nd isotopes in this region in the eastern equatorial pacific eep three water masses modified lcdw npdwsanp and npdwwestern pacific combine to produce nd isotopic values that reflect this conservative mixture therefore nd isotopes behave as a conservative water mass tracer in the central north pacific and the eep but high rates of lithogenic inputs from young circum pacific island arcs compromise the conservative behavior of nd isotopes in the wep and sanp

modeling the distribution of nd isotopes in the oceans using an ocean general circulation model nd isotopic compositions of authigenic phases in marine archives are increasingly used as a tracer of past changes in ocean circulation such applications assume that water masses are tagged with distinct nd isotope compositions in source regions and that away from these regions the nd isotope ratios observed in deep waters mainly reflect water mass mixing however the degree to which the simple assumption of quasi conservative mixing reflects the complicated reality of general circulation in the oceans is not well understood here we present numerical simulations of the nd isotope distribution in seawater using an offline ocean general circulation model ogcm treating the nd isotopic composition of seawater as a conservative tracer nd isotope data from modern surface waters are used to generate an interpolated map of nd isotope compositions for the entire surface ocean this map is treated as a fixed surface boundary condition that is propagated into the interior ocean by the advective diffusive transport of the ocean model until a steady state is reached this treatment of nd isotopes was chosen because it mimics the behavior of well known and well understood conservative tracers such as potential temperature and salinity the model results for all non surface grid points are then compared to published data this simple approach produces nd isotope estimates for north atlantic deep water that are consistent with observations but produces values lower than observed in the deep pacific and southern oceans indicating the need for a source of radiogenic nd directly to the deep pacific ocean sensitivity tests show this to be a robust result by fixing the return flow of the pacific ocean to the southern ocean with nd isotope ratios that are consistent with modern observations the model results are brought into close agreement with a majority of the available data with the addition of a deep pacific source of radiogenic nd more than half of the simulated values from the interior ocean fall within one e unit of the corresponding observations our results thus indicate that the deep pacific ocean requires a source of radiogenic nd in addition to inputs to the surface ocean and that when the deep waters of the north atlantic and north pacific are set with realistic values a majority of intermediate areas are consistent with quasi conservative mixing of these two end members

preservation of seawater sr and nd isotopes in fossil fish teeth bad news and good news abstract we analyzed 87sr 86sr ratios in foraminifera pore fluids and fish teeth for samples ranging in age from eocene to pleistocene from four ocean drilling program sites distributed around the globe site 1090 in the cape basin of the southern ocean site 757 on the ninetyeast ridge in the indian ocean site 807 on the ontong java plateau in the western equatorial pacific and site 689 on the maud rise in the southern ocean sr isotopic ratios for dated foraminifera consistently plot on the global seawater sr isotope curve for sites 1090 757 and 807 sr isotopic values of the pore fluids are generally less radiogenic than contemporaneous seawater values as are values for fossil fish teeth in contrast pore fluid 87sr 86sr values at site 689 are more radiogenic than contemporaneous seawater and the corresponding fish teeth also record more radiogenic values thus sr isotopic values preserved in fossil fish teeth are consistently altered in the direction of the pore fluid values furthermore there is a correlation between the magnitude of the offset between the pore fluids and the seawater curve and the associated offset between the fish teeth and the seawater curve these data suggest that the hydroxyfluorapatite of the fossil fish teeth continues to recrystallize and exchange sr with its surroundings during burial and diagenesis therefore sr chemostratigraphy can be used to determine rough ages for fossil fish teeth in these cores but cannot be used to fine tune age models in contrast to the sr isotopic system our nd concentration data combined with published isotopic and rare earth element data suggest that fish teeth acquire nd during early diagenesis while they are still in direct contact with seawater the concentrations of nd acquired at this stage are extremely high relative to the concentrations in surrounding pore fluids as a result nd isotopes are not altered during burial and later diagenesis therefore fossil fish teeth from a variety of marine environments preserve a reliable and robust record of deep seawater nd isotopic compositions from the time of deposition

timing and climatic consequences of the opening of drake passage age estimates for the opening of drake passage range from 49 to 17 million years ago ma complicating interpretations of the relationship between ocean circulation and global cooling secular variations of neodymium isotope ratios at agulhas ridge southern ocean atlantic sector suggest an influx of shallow pacific seawater approximately 41 ma the timing of this connection and the subsequent deepening of the passage coincide with increased biological productivity and abrupt climate reversals circulation productivity linkages are proposed as a mechanism for declining atmospheric carbon dioxide these results also indicate that drake passage opened before the tasmanian gateway implying the late eocene establishment of a complete circum antarctic pathway

structure of the palaeoproterozoic nagssugtoqidian orogen south east greenland model for the tectonic evolution abstract the palaeoproterozoic nagssugtoqidian orogen extends over 250 km along the east coast of greenland around the settlement of tasiilaq the orogen includes archaean rocks from the adjoining rae craton to the north and the north atlantic craton to the south and palaeoproterozoic rocks the rae craton consists of orthogneiss and amphibolite included in the schweizerland and kuummiut terranes and is tectonically overlain in the kuummiut terrane by ca 2100 2200 ma units that include marble meta pelite and psammite and amphibolite assigned to the helheim and kuummiut units the kuummiut terrane was probably subducted underneath the se trending ca 1885 ma ammassalik intrusive complex which has a high temperature metamorphic halo and is characterised by a change from sinistral faulting to pure shear deformation the southern isortoq terrane consists of medium pressure amphibolite facies bimodal meta volcanic and the rocks of the kuummiut terrane were tectonically imbricated in ene verging structures during ca 1870 ma high pressure metamorphism this was followed by ne sw convergence and close to orthogonal extrusion in the weakened crust which is characterised by partial melting during decompression the rocks of the isortoq terrane were imbricated in a se vergent thrust and ramp system either during oblique subduction of the kuummiut terrane or an earlier tectonic stage elsewhere ne sw compression as in the northern terranes formed sw vergent thrust systems and folds this was most likely caused by a change in the regional stress field during collision between ca 1870 and 1820 ma in the north the schweizerland terrane was juxtaposed to the kuummiut terrane in southeasterly direction causing refolding of earlier structures in the lower amphibolite facies this hinteland type of deformation was possibly related to tectonism in western greenland the latest recognised deformation event was during ca 1740 1680 ma associated with ne sw extension which is interpreted as orogenic collapse the complex structural evolution of the orogen was caused by oblique convergence during wsw directed subduction the convergence of irregularly shaped cratons and the change of the regional stress field from ene wsw to nw se during progressive collisional tectonics between the two archaean cratons

worldwide distribution of continental rock lithology implications for the atmospheric soil co2 uptake by continental weathering and alkalinity river transport to the oceans 1 the silicate rock weathering followed by the formation of carbonate rocks in the ocean transfers co2 from the atmosphere to the lithosphere this co2 uptake plays a major role in the regulation of atmospheric co2 concentrations at the geologic timescale and is mainly controlled by the chemical properties of rocks this leads us to develop the first world lithological map with a grid resolution of 1 1 this paper analyzes the spatial distribution of the six main rock types by latitude continents and ocean drainage basins and for 49 large river basins coupling our digital map with the gem co2 model we have also calculated the amount of atmospheric soil co2 consumed by rock weathering and alkalinity river transport to the ocean among all silicate rocks shales and basalts appear to have a significant influence on the amount of co2 uptake by chemical weathering

neogene evolution of atlantic thermohaline circulation perspective from walvis ridge southeastern atlantic ocean 1 we present new nd isotope records from walvis ridge ocean drilling program odp sites 1262 1264 southeastern atlantic spanning the past 24 ma to investigate the neogene evolution of atlantic thermohaline circulation the new data indicate that deepwater nd t values from odp site 1262 decrease from 11 0 at 10 6 ma to 12 5 by 7 3 ma this decrease parallels the nd isotope trends contained in fe mn crust records from the northwestern atlantic however the shift at odp site 1262 4755 m water depth occurred up to 6 ma earlier than the corresponding changes in crust records from the atlantic and earlier than any atlantic site shallower than 2700 m paleowater depth recent interpretations of the rapid decrease in fe mn crust nd nd t ssvalues invoke changes in weathering inputs to the labrador sea region rather than a fundamental change in deepwater convection in the labrador sea however the new evidence for significant depth stratification of the nd isotope signal in the southeastern atlantic between 10 6 and 7 3 ma suggests that the onset of deepwater convection in the labrador sea may have played a role in the deepwater decrease in nd isotopic composition climatic conditions during the middle to late miocene likely favored an increase in the importance of glacially induced mechanical weathering while at the same time promoting deep convection in the labrador sea

new insights on the orosirian carbon cycle early cyanobacteria and the assembly of laurentia from the paleoproterozoic belcher group abstract the orosirian period 2050 1800 ma of the paleoproterozoic era represents an interval between the great oxidation event and lomagundi jatuli excursion and the apparent environmental stability of the late paleoproterozoic to mid mesoproterozoic 1800 1300 ma here we present stratigraphic data carbon isotope chemostratigraphy element abundances and u pb zircon depositional ages from the belcher group in subarctic canada providing an unparallelled global reference section for the orosirian period u pb dates of 2018 5 1 0 ma and 2015 4 1 8 ma for two tuffs in the kasegalik formation lowest belcher group provide the first robust age constraints for earth s earliest unambiguous cyanobacterial fossil eoentophysalis belcherensis a tuffaceous shale 4 km stratigraphically higher at the contact of the flaherty and omarolluk formations 1854 2 1 6 ma provides an age for foreland basin development associated with the collision of the superior and hearne cratons carbonate carbon isotope data show smooth fluctuations ranging from 2 3 to 3 6 reaching a maximum near ca 1 88 ga this trend toward heavier carbon isotope values is interpreted as the result of global orogenesis during nuna assembly that elevated nutrient supply and sedimentation rates leading to an increase in the proportion of organic carbon burial relative to carbonate carbon burial