behavior of gold in the lateritic equatorial environment weathering and surface dispersion of residual gold particles at dondo mobi gabon abstract a supergene gold dispersion halo has developed at dondo mobi southern gabon from the weathering of an auriferous lisvenite under equatorial humid rain forest conditions morphological and chemical studies were performed on gold particles extracted from both fresh and weathered rocks from different parts of the dispersion halo the results demonstrate that the gold particles are residual and are subjected to increasing weathering upward through the profile from the protore to the surface in the central part of the halo and at the surface both laterally upslope and downslope weathering continues downward into the formerly barren saprolites which were derived from proterozoic schist and archean gneiss rim of the halo thus the au content progressively decreases as does the ag content of gold particles with distance from the center of the ore body gold particles subjected to increased weathering are smaller with more rounded shapes and surfaces pitted by dissolution the dispersion pattern is controlled by au dissolution and translocation of residual gold particles these processes involve an autochthonous evolution in the central part of the halo and a para autochthonous evolution at the rim of the halo at dondo mobi chloride and humic substances are the only ligands present in significant concentrations that are capable of complexing au thermodynamic calculations demonstrate that under such acidic conditions ph ranges from 4 to 6 gold stability increases with increasing silver content the dominant au complexes formed have been calculated to be an aurous hydroxichloride complex and an organometallic complex both of which are stable at surficial conditions thus both chemical and translocation processes control the mobility of au and gold particles in equatorial lateritic weathering profiles

an upper dalradian microfossil reassessed an acritarch from the highest dalradian macduff slate of northeast scotland is confirmed as veryhachium cf lairdii its presence implies that the macduff slate is ordovician this conclusion is compatible with the view that there is stratigraphical and structural continuity between the dalradian and lower palaeozoic rocks in the vicinity of the highland border e g the lower cambrian keltie water grit formation but runs counter to other lines of evidence notably biotite cooling ages which suggest that the dalradian had undergone metamorphism by the late cambrian further investigations are needed to reproduce and confirm the biostrati graphical evidence for the age of the macduff slate

eddies and the annular modes of climate variability the dominant modes of month to month varia tion in the troposphere are realistically simulated in a gen eral circulation model whose lower boundary is prescribed with realistic topography and seasonally varying climato logical sea surface temperatures in both hemispheres these internal modes describe a nearly zonally symmetric north south shifting of the zonal jets as anomalous westerlies vac illate between high 50 60 a nd low 30 40 latitudes the eddy structures evolve with the jets and the corre sponding eddy momentum forcings support the shifts in jet position stationary synoptic wave variations mainly ac count for the total eddy forcings in the northern south ern hemisphere pronounced eddy variations occur over the north atlantic sector

significance of the early fabric in the contact metamorphic aureole of the 590 ma ben vuirich granite perthshire scotland an early fabric is preserved in pseudomorphs of contact metamorphic cordierite and andalusite found in hornfelsed dalradian rocks at the nw margin of the ben vuirich granite as previous work had suggested that this fabric may be of regional tectonic d1 origin and taken as evidence that a pre 590 ma neoproterozoic orogenic event had affected the dalradian supergroup the textures found in the hornfelses and their porphryoblasts are illustrated and described in detail three types of hornfels are recorded unspotted cordierite and andalusite bearing cordierite was altered to mica garnet and kyanite and andalusite pseudomorphed by kyanite during the subsequent d2 regional metamorphism the early fabric is finely spaced and associated with a weak shape fabric except in the higher grade hornfelses where it is more intensely developed and formed during the deformation of irregularly shaped cross cutting granitic veinlets it is concluded that the hornfels fabric is probably bedding modified by intrusion related deformation rather than a d1 fabric parallel to bedding the implication of this finding is that intrusion of the ben vuirich granite occurred during the vendian c 650 ma rifting phase at about the time that the tayvallich lavas were being deposited and before any significant deformation of the dalradian rocks had taken place there is no unambiguous evidence to show that the early fabric in the hornfels at ben vuirich formed during a precambrian regional scale deformation of the dalradian block

testing for the presence of a terrane boundary within neoproterozoic dalradian to cambrian siliceous turbidites at callander perthshire scotland the southern highland group dalradian and keltie water grit formation which includes the lower cambrian leny limestone form an inverted 1 4 km thick largely arenaceous sequence at callander the grits have the same detrital mineralogy throughout mainly quartz plagioclase an 1 3 muscovite and biotite chlorite formed from detrital biotite during low grade regional metamorphism t 40 ar 39 ar laser fusion dating of detrital muscovites yields an age spectrum with a peak at 1600 1800 ma in the dalradian rocks similar old ages occur in the keltie water grits but are diluted by ages of 507 886 ma we interpret these new data as showing that the rocks were most likely deposited as a single sequence possibly with a disconformity in neoproterozoic to early cambrian times before the onset of grampian orogenesis in the early palaeozoic no major structural or stratigraphical breaks have been identified and there is no direct evidence for the presence of two separate terranes

geochemical behavior of au in terrigenous and pelagic marine sediments abstract a total of 224 marine sediments of terrigenous and pelagic origin were collected from the obama bay suruga bay south of kii strait off northeast japan and the japan trench terrigenous sediments and the central pacific pelagic sediments these samples were analyzed for au by atomic absorption spectrometry using a graphite furnace atomizer after solvent extraction separation the average au content of the terrigenous 2 4 1 2 ppb n 85 and the pelagic 1 4 1 3 ppb n 139 sediments were similar to crustal abundance a significant correlation r 0 82 was found for au and cu in terrigenous sediments while no such trend was found in the pelagic sediments sediments collected around the au mineralized regions are known to exhibit high au cu ratios therefore the au cu ratio should serve as a useful indicator for the geochemical exploration of au deposits with respect to the content of au and organic carbon in the pelagic sediments surface samples had higher values whereas deeper sediments were depleted there was no such vertical concentration trend for au and organic carbon in the terrigenous sediments the vertical profile of au in the pelagic sediments is thought to be a result of upward migration of au organometallic complexes during early diagenesis since the most probable form of au in the terrigenous sediments is elemental significant migration during diagenesis would not occur explaining the lack of a vertical trend for au

reconstruction of historical sea surface temperatures using empirical orthogonal functions abstract studies of climate variability often rely on high quality sea surface temperature sst anomalies although the high resolution national centers for environmental prediction formerly the national meteorological center optimum interpolation oi sst analysis is satisfactory for these studies the oi resolution cannot be maintained before november 1931 due to the lack of satellite data longer periods of ssts have come from traditional analyses of in situ ship and buoy sst observations alone a new interpolation method is developed using spatial patterns from empirical orthogonal functions e0fs that is a principal component analysis to improve analyses of sst anomalies from 1950 to 1981 the method uses the more accurate oi analyses from 1982 to 1993 to produce the spatial eofs the dominant eof modes which correspond to the largest variance are used as basis functions and are fit in a least squares sense to the in situ data to determine the time dependence of each mode a complete field

some comparative marine chemistries of rhenium gold silver and molybdenum abstract four metals re au ag and mo whose dissolved forms in seawater can potentially be reduced to insoluble states have been measured in a variety of solids depositing under anoxic conditions hydrothermal sulfides coastal sediments and phosphorites for comparative purposes these elements have also been determined in ferromanganese minerals and pelagic sediments which have accumulated under oxidizing conditions rhenium appears to be a unique sentinel for identifying reducing depositional environments where enrichments of greater than three orders of magnitude above crustal concentrations are found molybdenum follows re in these sediments but mo is enriched also in those that accumulate under oxidizing conditions the reported mo crustal concentrations appear low in comparison to our measured sedimentary values gold and ag are concentrated in hydrothermal deposits and sulfides appear to be involved in the precipitation of these elements coastal sediments adjacent to a domestic outfall of los angeles california contain extraordinary high accumulations of anthropogenic au ag and pt the study seeks an understanding of the mobilities and sinks of these metals in the marine environment during the major weathering cycle further anthropogenic contributions to their sedimentary concentrations are identified

new evidence that the lower cambrian leny limestone at callander perthshire belongs to the dalradian supergroup and a reassessment of the exotic status of the highland border complex

the transient atmospheric circulation response to north atlantic sst and sea ice anomalies abstract the objective of this study is to investigate the transient evolution of the wintertime atmospheric circulation response to imposed patterns of sst and sea ice extent anomalies in the north atlantic sector using a large ensemble of experiments with the ncar community climate model version 3 ccm3 the initial adjustment of the atmospheric circulation is characterized by an out of phase relationship between geopotential height anomalies in the lower and upper troposphere localized to the vicinity of the forcing this initial baroclinic response reaches a maximum amplitude in 5 10 days and persists for 2 3 weeks diagnostic results with a linear primitive equation model indicate that this initial response is forced by diabatic heating anomalies in the lower troposphere associated with surface heat flux anomalies generated by the imposed thermal forcing following the initial baroclinic stage of adjustment the response becomes progressively more barotropic and increases in both spatial extent an

vendian and riphean rifting in nw scotland in the neoproterozoic nw scotland was located in a tract of extending continental lithosphere that evolved into the eastern passive margin of laurentia when iapetus opened in the vendian on the nw caledonian foreland major rift events are inferred in the riphean and vendian corresponding to the intra torridonian and sub cambrian unconformities respectively in this context current views that the moine and dalradian rocks of the scottish highlands were subjected to repeated orogeny during the same period become difficult to sustain we propose a tectonostratigraphic history for the two areas in which neoproterozoic orogeny plays no part

the antarctic dipole and its predictability this study investigates the nature of interan nualvariability ofantarcticseaiceanditsrelationshipwith the tropical climate we nd that the dominant interan nual variance structure in the sea ice edge and surface air temperature elds is organized as a quasi stationary wave which we call the antarctic dipole adp it is charac terized by an out of phase relationship between the ice and temperature anomalies in the central eastern pacic and atlantic sectors of the antarctic the dipole consists of a strong standing mode and a weaker propagating motion within each basin s ice eld it has the same wavelength as the antarctic circumpolar wave acw and dominates the acw variance the dipole is clearly associated with tropical enso events it can be predicted with moderate skill using linear regression involving surface temperature two to four months ahead the prediction performs better in extreme warm cold years and best in la ni na years

major northwest directed caledonian thrusting and folding in precambrian rocks northwest mayo ireland structural mapping of the north west mayo inlier has investigated the kinematics of ductile fault and shear zones within the dalradian and pre dalradian rocks the area is characterized by an intense east to southeast trending elongation direction pre caledonian deformation is preserved as west northwest trending sinistral mylonite zones these are cross cut by grenville pegmatites and by undated mafic dykes subsequent deformation has been influenced by the majorbasement structures during the caledonian deformation the pre existing sinistral structures were reactivated coaxially as steep sinistral shear zones deforming the mafic dykes major northwesterly verging thrusts are widespread in flat lying zones which occur over much of the inlier these are interpreted as d2 in age the s2 fabric is the dominant fabric and contains an east to northeast trending mineral lineation the southeastern part of the inlier is characterized by very large scale wavelengths of kilometres reclined d2 folding this is interpreted as a crustal scale ramp zone in the vicinity of this ramp there is evidence of earlier f1 folds which are possibly the product of ramprelated deformation all of these structures are interpreted as the product of northwest verging thrusting with crustal scale frontal and lateral ramps

neonereites uniserialis from c 600 ma year old rocks in western scotland and the emergence of animals a chain of pellets neonereites uniserialis usually attributed to a coelomate or pseudocoelomate metazoan is described from the middle dalradian bonahaven formation of western scotland in sandstones not far above the port askaig tillite this trail lies more than 8 km below u pb zircon dated tayvallich lavas 595 4 ma and predates early deformation plus intrusion by granites at ben vuirich dated at 597 11 and 590 2 ma u pb zircon chronology for early metazoan evolution suggests that this is the earliest putative evidence for coelomates or pseudocoelomate discovered so far with a minimum interglacial age of c 600 ma bp it is from about this time that a fall takes place in the maximum 13 c of carbonates suggestive of a decline in the long term rate of carbon burial one explanation for this decline is the increasing impact of metazoan bioturbation through the late neoproterozoic to cambrian interval

high precision u pb monazite geochronology of the c 806 ma grampian shear zone and the implications for the evolution of the central highlands of scotland two pegmatites and mylonitic rocks from the grampian shear zone yield u pb monazite ages of 806 3 ma 808 9 11 ma and 804 12 13 respectively these confirm that shearing and pegmatite crystallization was essentially synchronous lower intercepts of c 440 ma indicate disturbance during ordovician reworking these new data question the concept of a single latest proterozoic early palaeozoic orogeny in the grampian terrane and the significance of the great glen fault

twentieth century simulation of the southern hemisphere climate in coupled models part ii sea ice conditions and variability we examine the representation of the mean state and interannual variability of antarctic sea ice in six simulations of the twentieth century from coupled models participating in the intergovernmental panel on climate change fourth assessment report the simulations exhibit a largely seasonal southern hemisphere ice cover as observed there is a considerable scatter in the monthly simulated climatological ice extent among different models but no consistent bias when compared to observations the scatter in maximum winter ice extent among different models is correlated to the strength of the climatological zonal winds suggesting that wind forced ice transport is responsible for much of this scatter observations show that the leading mode of southern hemisphere ice variability exhibits a dipole structure with anomalies of one sign in the atlantic sector associated with anomalies of the opposite sign in the pacific sector the observed ice anomalies also exhibit eastward propagation with the antarctic circumpolar current as part of the documented antarctic circumpolar wave phenomenon many of the models do simulate dipole like behavior in sea ice anomalies as the leading mode of ice variability but there is a large discrepancy in the eastward propagation of these anomalies among the different models consistent with observations the simulated antarctic dipole like variations in the ice cover are led by sea level pressure anomalies in the amundsen bellingshausen sea these are associated to different degrees in different models with both the southern annular mode and the el nino southern oscillation enso there are indications that the magnitude of the influence of enso on the southern hemisphere ice cover is related to the strength of enso events simulated by the different models

timing of barrovian metamorphism and older granite emplacement in relation to dalradian deformation detailed studies of the relationship between structure and metamorphism in the southern scottish highlands and new rb sr isotope data for the rough craig granite have resulted in a revision of the tectonothemal history of the dalradian two main events are recognized the first dl probably at c 590 ma produced a regiondy inverted stratigraphic succession on the underlib of the tay nappe the second event possibly between 520 and 470 ma encompassed regional d2 and d3 deformation phases and was synchronous with peak barrovian metamorphic conditions

variability of sea ice in the northern weddell sea during the 20th century the record of winter fast ice in the south orkney islands northern weddell sea antarctica is over a century long and provides the longest observational record of sea ice variability in the southern hemisphere here we present analyses of the series of fast ice formation and breakout dates from 1903 to 2008 we show that over the satellite era post 1979 the timing of both final autumn formation and complete spring breakout of fast ice is representative of the regional sea ice concentrations sic in the northern weddell sea and associated with atmospheric conditions in the amundsen sea region to the west of the antarctic peninsula variation in the fast ice breakout date is influenced by the intensity of the westerly north westerly winds associated with the southern annular mode sam in contrast the date of ice formation displays correlations with regional oceanic and sea ice conditions over the previous 18 months which indicate a preconditioning during the previous summer and winter and exhibits variability associated with variation in tropical pacific sea surface temperature i e the el nino sothern oscillation enso a reduction in fast ice duration at the south orkney islands around the 1950s was associated with both later formation and earlier breakout however there were marked changes in variability with periodicities of 3 5 year 7 9 year and 20 year in each of the series and in their relationships with enso and sam indicating the need for caution in interpreting changes in ice conditions based on shorter term satellite series

the effects of north atlantic sst and sea ice anomalies on the winter circulation in ccm3 part ii direct and indirect components of the response the wintertime atmospheric circulation responses to observed patterns of north atlantic sea surface temperature and sea ice cover trends in recent decades are studied by means of experiments with an atmospheric general circulation model here the relationship between the forced responses and the dominant pattern of internally generated atmospheric variability is focused on the total response is partioned into a portion that projects onto the leading mode of internal variability the indirect response and a portion that is the residual from that projection the direct response this empirical decomposition yields physically meaningful patterns whose distinctive horizontal and vertical structures imply different governing mechanisms the indirect response which dominates the total geopotential height response is hemispheric in scale with resemblance to the north atlantic oscillation or northern hemisphere annular mode and equivalent barotropic in the vertical from the surface to the tropopause in contrast the direct response is localized to the vicinity of the surface thermal anomaly sst or sea ice and exhibits a baroclinic structure in the vertical with a surface trough and upper level ridge in the case of a positive heating anomaly consistent with theoretical models of the linear baroclinic response to extratropical thermal forcing both components of the response scale linearly with respect to the amplitude of the forcing but nonlinearly with respect to the polarity of the forcing the deeper vertical penetration of anomalous heating compared to cooling is suggested to play a role in the nonlinearity of the response to sst forcing

short paper the depositional age of the dalradian supergroup u pb and sm nd isotopic studies of the tayvallich volcanics scotland zircons from a keratophyre associated with the tayvallich volcanics in the dalradian rocks of the sw scottish highlands have been dated by u pb methods yielding an age of 595 4 ma this age indicates that most or all of the dalradian is precambrian in age and that dalradian sedimentation may have lasted for about 200 million years the age also constrains the time interval between cessation of dalradian sedimentation and subsequent crustal thickening and regional metamorphism during the grampian orogeny sm nd isotopic data for the tayvallich volcanins and related metadolerite sills yield initial ond values of 2 to 4 which are thought to reffect the effects of melting of lithospheric mantle

mechanisms forcing an antarctic dipole in simulated sea ice and surface ocean conditions abstract the mechanisms forcing variability in southern ocean sea ice and sea surface temperature from 600 years of a control climate coupled model integration are discussed as in the observations the leading mode of simulated variability exhibits a dipole pattern with positive anomalies in the pacific sector associated with negative anomalies in the atlantic it is found that in the pacific ocean circulation changes associated with variable wind forcing modify the ocean heat flux convergence and sea ice transport resulting in sea surface temperature and sea ice anomalies the pacific ice and ocean anomalies persist over a number of years due to reductions in ocean shortwave absorption reinforcing the initial anomalies in the atlantic sector no single process dominates in forcing the anomalies instead there are contributions from changing ocean and sea ice circulation and surface heat fluxes while the absorbed solar radiation in the atlantic is modified by the changing surface albedo the anomalies

current controversies in the caledonides this set of papers arose from a discussion meeting held in the department of geology applied geology university of glasgow on 9 10 september 1997 the aim of the meeting was to review the progress which had been made in resolving some of the more controversial aspects of the pre silurian caledonide geology of the british isles and scandinavia it was also held to mark the retirements of two well known caledonide geologists professors bernard leake and donald ramsay there were 34 talks and poster contributions and the meeting was attended by over 100 people we would like to thank our colleague tim dempster for helping to convene and organize the meeting it was fitting that the meeting should be held in the gregory building which was named after the glasgow professor who not only pioneered work on rift tectonics and regional uplift in africa but also wrote a perceptive book on dalradian geology gregory 1931 in this work gregory pointed out that his observations were made over a period of 25 years being made on recreational weekends and mainly in winter when the ground is less obscured by vegetation and the highland mountains most attractive an approach motivated by a spirit of enquiry and very productive but one which would be out of step with current research strategies we would like to dedicate this set of papers to mark piasecki who died on 13 june 1999 mark was the first person to draw attention to major problems concerning the relationship

interpretation of recent antarctic sea ice variability 1 trends in the satellite derived antarctic sea ice concentrations 1979 2002 show pronounced increase decrease in the central pacific sector bellingshausen western weddell sector by 4 10 per decade confidence levels for these regional trends exceed 95 positive polarities of the antarctic oscillation aao lead to more less ice in the eastern ross amundsen sector bellingshausen northern weddell sector which are qualitatively opposite to the impacts of positive polarities of the el nino southern oscillation enso the mechanisms responsible for the covariability between the ice and the a aao and b enso are demonstrated over the last 24 years a positive aao trend and a slightly negative enso trend produce a spatial pattern of ice changes similar to the regional ice trends however the magnitude of the ice changes associated with the aao and enso is much smaller than the regional ice trends more local or less understood large scale processes should be investigated for the explanations

unravelling dates through the ages geochronology of the scottish metamorphic complexes the paper by giletti et al 1961 is seen as a major landmark in the evolution of dating techniques in polymetamorphic terrains we consider certain critical issues from each of the main complexes of the scottish highlands studied by giletti et al to illustrate how subsequent developments in geochronological methodology have influenced our understanding of metamorphic belts lewisian examples focus on the formation of archaean crust and the age of the main high grade metamorphism and the scourie dyke swarm the antiquity of moinian sedimentation its relationship to the torridonian sandstones and the timing of precambrian metamorphism have been controversial issues the timing and nature of caledonian orogenesis most clearly expressed in the dalradian complex have been the focal points for the refinement of radiometric investigation these complexes have been subject to successive developments in methodology with ever tighter constraints from rb sr and k ar mineral dating through rb sr and pb pb whole rock studies u pb dating of bulk zircon fractions and sm nd whole rock and mineral investigation up to the latest technologies of single grain zircon and ion microprobe analysis the rocks have released their secrets reluctantly and many of the questions posed in 1961 have still not been definitively answered however the hope of unambiguous solution leads towards greater efforts ever more reliable data and a clearer evolutionary picture

a linkage observed between austral autumn antarctic oscillation and preceding southern ocean sst anomalies abstractin this study the authors apply a lagged maximum covariance analysis mca to capture the cross seasonal coupled patterns between the southern ocean sea surface temperature sosst and extratropical 500 hpa geopotential height anomalies in the southern hemisphere from which nino 3 4 signals and their linear trends are removed to a certain extent statistically significant results show that the dominant feature of ocean atmosphere interaction is likely the effect of atmosphere on sosst anomalies with a peak occurring when the atmosphere leads the sosst by 1 month however the most eye capturing phenomenon is that the austral autumn atmospheric signal characterized by a negatively polarized antarctic oscillation aao is significantly related to the gradual evolution of preceding sosst anomalies suggesting that the sosst anomalies tend to exert an effect on the southern hemisphere atmospheric circulation a regression analysis based on sosst anomaly centers confirms these features it is also

influence of low arctic sea ice minima on anomalously cold eurasian winters 1 influence of low arctic sea ice minima in early autumn on the wintertime climate over eurasia is investigated observational evidence shows that significant cold anomalies over the far east in early winter and zonally elongated cold anomalies from europe to far east in late winter are associated with the decrease of the arctic sea ice cover in the preceding summer to autumn seasons results from numerical experiments using an atmospheric general circulation model support these notions the remote response in early winter is regarded as a stationary rossby wave generated thermally through an anomalous turbulent heat fluxes as a result of anomalous ice cover over the barents kara seas in late autumn which tends to induce an amplification of the siberian high causing colder conditions over the far east the late winter cold anomalies over eurasia are also reproduced in our experiment which is associated with the negative phase of the north atlantic oscillation

u pb and rb sr geochronology of magmatism and metamorphism in the dalradian of connemara western ireland new geochronological data which clarify the timing of syn orogenic magmatism and regional metamorphism in the connemara dalradian are presented u pb zircon data on four intermediate to acid foliated magmatic rocks show important inherited components but the most concordant fractions demonstrate that major magmatism continued until 465 ma whereas the earliest basic magmatism has been dated previously at 490 ma a fine grained fabric cutting granite contains discordant zircons which also appear to be 465 ma old arc magmatism in connemara therefore spanned a period of at least 25 ma recent u pb data on titanite from central connemara which gave a peak metamorphic age of 478 ma are supplemented by u pb data on titanite and monazite from metamorphic veins in the east of connemara which indicate that low p high t regional metamorphism continued there to 465 ma i e at least 10 ma later than in the central region dated previously new rb sr data on muscovites from coarse grained segregations in different structural settings range from 475 to 435 ma in part this range probably also reflects differences in age from west to east with three ages close to 455 ma from the eastern area which is also the site of the lowest pressure metamorphism thermal modelling indicates that at any one locality the duration of metamorphism was probably as little as 1 2 ma the new dates emphasize the complexity in the spatial and temporal distribution of high level regional metamorphism caused by magmatic activity the relatively simple overall distribution of mineral appearance isograds revealed by regional mapping masks the complexity of a prolonged but punctuated metamorphic history related to multiple intrusions primarily in the southern part of connemara the later stages of magmatic activity followed progressive uplift and erosion after the onset of magmatism and were localized in the eastern part of the region

observed impact of atlantic sst anomalies on the north atlantic oscillation the large scale patterns of covariability between monthly sea surface temperature sst and 500 mb height anomalies z500 in the atlantic sector are investigated as a function of time lag in the ncep ncar reanalysis 1958 97 in agreement with previous studies the dominant signal is the atmospheric forcing of sst anomalies but statistically significant covariances are also found when sst leads z500 by several months in winter a panatlantic sst pattern precedes the north atlantic oscillation nao by up to 6 months such long lead time covariance is interpreted in the framework of the stochastic climate model reflecting the forcing of the nao by persistent atlantic sst anomalies a separate analysis of midlatitudes 208 708n and tropical 208s 208n sst anomalies reveals that the bulk of the nao signal comes from the midlatitudes a dipolar anomaly with warm sst southeast of newfoundland and cold sst to the northeast and southeast precedes a positive phase of the nao and it should provide a prediction of up to 15 of its monthly variance several months in advance since the forcing sst pattern projects significantly onto the tripole pattern generated by the nao these results indicate a positive feedback between the sst tripole and the nao with a strength of up to 25 mk 21 at 500 mb or 2 3 mb k21 at sea level additionally a warming of the tropical atlantic 208s 208n roughly symmetric about the equator induces a negative nao phase in early winter this tropical forcing of the nao is nearly uncorrelated with and weaker than that resulting from the midlatitudes and is associated with shorter lead times and reduced predictive skill

observations on the behavior of gold during supergene oxidation at summitville colorado u s a and implications for electrum stability in the weathering environment abstract the textural relations morphology and composition of gold grains in both oxidized and unoxidized ore from the summitville colorado au cu ag deposit have been investigated in this study gold from oxidized summitville ore is finer grained than that from unoxidized ore and occurs exclusively in intimate intergrowths with goethite of presumed supergene origin these oxide gold grains show no evidence of silver depletion and are enriched in silver relative to primary gold in a few cases this trend can be explained using the stability relations of silver in electrum at 25 c at this temperature electrum with up to 20 wt ag may be relatively stable in the weathering environment provided that chloride contents are sufficiently low in the coexisting aqueous phase this is in accord with the maximum silver values observed in gold grains in the oxidized zone of the summitville deposit and with the negligible chloride content of present mine drainage the absence of chloride that is implied by these electrum compositions effectively rules out gold chloride complexes as an explanation for the local remobilization of gold in the oxidized zone at summitville it strongly suggests that gold chloride complexes are not necessary to transport gold at least on a local scale in the acidic environment that characterizes many weathering sulfide ore bodies local gold mobilization in this environment is believed to reflect the formation of transient gold thiosulfate complexes

observed influence of north pacific sst anomalies on the atmospheric circulation abstract a lagged maximum covariance analysis mca of monthly anomaly data from the ncep ncar reanalysis shows significant relations between the large scale atmospheric circulation in two seasons and prior north pacific sea surface temperature sst anomalies independent from the teleconnections associated with the enso phenomenon regression analysis based on the sst anomaly centers of action confirms these findings in late summer a hemispheric atmospheric signal that is primarily equivalent barotropic except over the western subtropical pacific is significantly correlated with an sst anomaly mode up to at least 5 months earlier although the relation is most significant in the upper troposphere significant temperature anomalies are found in the lower troposphere over north america the north atlantic europe and asia the sst anomaly is largest in the kuroshio extension region and along the subtropical frontal zone resembling the main mode of north pacific sst anomaly variability in late winter

a high precision u pb age for the ben vuirich granite implications for the evolution of the scottish dalradian supergroup high precision high accuracy u pb analyses of zircons from the ben vuirich granite in the scottish dalradian succession were performed on carefully picked high integrity abraded fractions of specific morphologies zircon needles which grew during crystallization give an age of 590 2 ma stubby prisms show inheritance with an upper concordia intercept of 1448 7 ma the age of intrusion at 590 ma contrasts strongly with a previous u pb zircon age of 514 6 7 ma determined on bulk fraction unabraded grains which it is argued must have suffered significant pb loss as the ben vuirich granite was emplaced between the regional d2 and d3 deformation events in the dalradian block the new data imply that all dalradian sedimentation was precambrian supposed lower cambrian fauna1 links between the dalradian succession and laurentia are thus discounted and the possibility that the dalradian supergroup has non laurentian affinities must be considered the early d1 d2 nappe forming grampian structures are also precambrian and may be associated with terrane docking along a tectonically active ocean margin later d3 structures and associated peak barrovian metamorphism suggest a subsequent thickening event possibly c 520 ma ago

role of sea ice in forcing the winter climate of antarctica in a global climate model 1 two different experiments have been conducted using the hadley centre s atmosphere only global climate model to investigate the role of sea ice in forcing the antarctic climate during the winter months the first experiment carried out by the met office hadley centre involved forcing the model with observed sea ice concentration from the hadisst1 data set for the period 1870 1999 and showed that the observed temperature at faraday vernadsky station on the west coast of the antarctic peninsula was well reproduced correlation coefficient of 0 872 for the period 1973 1999 while at stations around the rest of the continent the observed temperature was not so highly correlated with the modeled temperature the second experiment used two different yearly repeating sea ice concentrations one being the average for the period 1975 1979 and the second the average for 1995 1999 the later period has generally less sea ice this second experiment showed that the climate response to the sea ice forcing in the amundsen bellingshausen sea was confined to the layers closest to the surface while the response around the rest of antarctica where the katabatic wind flowing off the continent plays a role in the process is over a much deeper layer of the atmosphere it is also suggested that the katabatic flow may play a role in making the sea ice around east antarctica insensitive to external climate forcing

wave maintained annular modes of climate variability abstract the leading modes of month to month variability in the northern and southern hemispheres are examined by comparing a 100 yr run of the geophysical fluid dynamics laboratory gcm with the ncep ncar reanalyses of observations the model simulation is a control experiment in which the ssts are fixed to the climatological annual cycle without any interannual variability the leading modes contain a strong zonally symmetric or annular component that describes an expansion and contraction of the polar vortex as the midlatitude jet shifts equatorward and poleward this fluctuation is strongest during the winter months the structure and amplitude of the simulated modes are very similar to those derived from observations indicating that these modes arise from the internal dynamics of the atmosphere dynamical diagnosis of both observations and model simulation indicates that variations in the zonally symmetric flow associated with the annular modes are forced by eddy fluxes in the free troposphere while

polymetamorphism in the dalradian of the central scottish highlands systematic variations in both timing and conditions of metamorphism are observed in the dalradian rocks of central perthshire scotland early d2 d3 and syn d3 relatively low pressure metamorphism 550 c 7 kb occurs in the flat belt rocks of the tay nappe to the south and later post d3 higher pressure conditions 550 c 9 kb dominate the steep belt to the north the latter appear to be superimposed on earlier low pressure assemblages chloritoid biotite with the pressure increase probably relating to formation of the tummel steep belt possibly near 460 ma this polymetamorphism in addition to the generally high pressures of barrovian metamorphism throughout perthshire may account for the poor development of the metamorphic zonal pattern in this region compared to the eastern dalradian

evidence for the lower palaeozoic age of the tay nappe the timing and nature of grampian events in the scottish highland sector of the laurentian margin stratigraphical and structural evidence for latest proterozoic phanerozoic deposition of the dalradian supergroup in the scottish highlands sector of the laurentian margin is incompatible with geochronological data suggesting c 750 ma knoydartian and c 600 ma grampian orogenesis the age of the 590 2 ma ben vuirich granite is critical to the dating of grampian orogenesis fabric evidence indicates that the granite predates d2 while its relationship to d1 is unclear in this paper the d1 d2 overprint during the evolution of the tay nappe a regional grampian structure is examined documentation of the distribution style and orientation of d1 and d2 structures from well exposed sections near the highland border has allowed the development of a kinematic model for the d1 d2 deformation in this model early crustal shortening resulted in d1 upright structures that were subsequently overturned in a d2 se directed shear regime associated with underthrusting of a southern landmass the model is best explained in terms of a single continental collision event with no evidence of a time gap between d1 and d2 this analysis is consistent with pre regional d1 intrusion of the ben vuirich granite and an early palaeozoic grampian orogeny

coupled ocean atmosphere feedback in the southern annular mode abstract previous studies have demonstrated that while the southern annular mode sam is an intrinsic feature of the atmosphere it projects strongly onto the ocean and sea ice properties and circulation this study investigates the extent of back interaction whereby these oceanic sam anomalies feed back to the atmosphere a comparison between atmosphere only and full coupled climate models demonstrates that air sea interactions in the coupled system act to increase the persistence of the sam in the atmosphere to identify the nature of feedback from the ocean to the atmosphere ensemble experiments are carried out in both atmosphere only and full coupled models whereby a continuous sam like sea surface temperature sst anomaly is imposed both coupled and uncoupled experiments show a direct thermal response that affects the lower tropospheric temperature and surface meridional winds an indirect upper troposphere wide response is also seen whose characteristics are sensitive to the coupling for the

generation of the tay nappe scotland by large scale se directed shearing recent mapping in the glen shee area central scotland has revealed an array of several large scale f1 folds within the 9flat belt9 of the tay nappe strata within the 9flat belt9 forming part of the lower limb of the tay nappe are thus not entirely inverted and the traditional view that the nappe in this area comprises a single se facing recumbent anticline is rejected it is demonstrated that the sense of shear of d2 was top to the se resulting in nw verging f2 folds by strong d2 strain partitioning strain estimates indicate adisplacement of 10 50 km during d2 shearing in the proposed model originally upright f1 folds were modified by a major d2 shearing event that produced a shallow nw dipping s2 foliation in the 9flat belt9 the upper limit of d2 deformation forms the boundary between the strongly deformed and inverted 9lower limb9 and the right way up 9upper limb9 which is undeformed by d2 and is now mostly removed by erosion the d2 shear event was responsible for the gross stratigraphic inversion in the flat belt the tay nappe is considered here as formed by a crustal scale shear zone in this model there is no necessity to invoke a root zone

slow oceanic teleconnections linking the antarctic circumpolar wave with the tropical el nino southern oscillation a case study for the period 1982 1994 shows that a major source for the antarctic circumpolar wave is in the western subtropical south pacific where interannual anomalies in sea surface temperature sst and precipitable water prwat form once established these interannual anomalies in tandem with anomalies in sea level pressure slp move south toward the southern ocean the system then migrates east around the globe through a combination of oceanic advection with the antarctic circumpolar current and ocean atmosphere coupling the coincidence of interannual anomalies in sst slp and prwat indicates the extratropical ocean and atmosphere are tightly linked on these timescales large portions of the interannual sst anomalies branch advectively northward into the south atlantic and indian oceans ultimately reaching the tropics in each basin some 6 8 years after appearing in the low latitude pacific this constitutes a slow oceanic teleconnection that is unique in climate dynamics made possible by the continuity of earth s oceans via the southern ocean in the tropical indian ocean these interannual anomalies move east and arrive at the indo pacific transition in advance of the trans pacific propagation of the respective el nino southern oscillation enso phases the interannual sst and prwat anomalies that appear in the subtropical south pacific are directly linked with the enso cycle on the equator through anomalous vertical convection and a regional overturning cell in the troposphere the same cell that initiates fast planetary waves in the atmosphere that carry enso signals around the southern hemisphere on much shorter timescales

synchronous variability in the southern hemisphere atmosphere sea ice and ocean resulting from the annular mode zonally symmetric fluctuations of the midlatitude westerly winds characterize the primary mode of atmospheric variability in the southern hemisphere during all seasons this is true not only in observations but also in an unforced 15 000 yr integration of a coarse resolution r15 coupled ocean atmosphere model here it is documented how this mode of atmospheric variability known as the southern annular mode sam generates ocean circulation and sea ice variations in the model integration on interannual to centennial timescales that are tightly in phase with the sam the positive phase of the sam is associated with an intensification of the surface westerlies over the circumpolar ocean around 608s and a weakening of the surface westerlies farther north this induces ekman drift to the north at all longitudes of the circumpolar ocean and ekman drift to the south at around 308s through mass continuity the ekman drift generates anomalous upwelling along the margins of the antarctic continent and downwelling around 458s the anomalous flow diverging from the antarctic continent also increases the vertical tilt of the isopycnals in the southern ocean so that a more intense circumpolar current is also closely associated with positive sam in addition the anomalous divergent flow advects sea ice farther north resulting in an increase in sea ice coverage finally positive sam drives increases in poleward heat transport at about 308s while decreases occur in the circumpolar region ocean and sea ice anomalies of the opposite sign occur when the sam is negative the ocean and sea ice fluctuations associated with the sam constitute a significant fraction of simulated ocean variability poleward of 308s year round the robustness of the mechanisms relating the sam to oceanic variability suggests that the sam is likely an important source of large scale variability in the real southern hemisphere ocean

gold in seawater abstract a suite of seawater samples has been analyzed for au by inductively coupled plasma quadrupole mass spectroscopy icpms the recently developed method involves preconcentration by anion exchange followed by flow injection sample introduction into a vg plasmaquad icpms profiles in the atlantic gulf stream northeast pacific and mediterranean are presented in addition the first measurements of au in hydrothermal fluids are reported the measured concentrations of au in the atlantic and northeast pacific 50 150 fmol l are within a factor of 2 3 of recently reported values for pacific waters 1 and nearly three orders of magnitude less than reported in the literature prior to 1988 indicating contamination problems with the earlier data in general au concentrations for atlantic and pacific samples reported here are the same 50 fmol l however differences of up to a factor of 2 between the oceans cannot be ruled out due to large uncertainties associated with the pacific data mediterranean deep waters contain higher concentrations of au 100 150 fmol l this is attributed to proximal aeolian dust and or riverine sources gold is enriched in hydrothermal fluids 10 100 s of pmol l with respect to ambient seawater although the fluxes remain poorly constrained preliminary budget considerations indicate that rivers and hydrothermal venting are the most significant sources of au to the oceans

coherent wave zonal mean flow interactions in the troposphere abstract spatial structure and temporal evolution of synoptic time scale variations of the tropospheric zonal mean flow are studied by extensive cross correlation analyses and the degree to which observations agree with two dimensional adiabatic theory is determined observational data from seven years of operational daily global analyses are studied along with data from the ncar general circulation model observed zonal mean zonal wind and temperature tendencies are compared with analyzed adiabatic forcing terms although significant correlations are found throughout the extratropics there are significant equation residuals in both operational analyses and model data model momentum residuals result from calculational inaccuracies interpolation to pressure surfaces and spectral aliasing of nonlinear terms and biases introduced by once daily sampling diabatic terms are also important for the daily thermodynamic balance coherent wave zonal mean flow interactions are revealed via cross correlation a

geochemistry of the amazon 1 precipitation chemistry and the marine contribution to the dissolved load at the time of peak discharge analyses of precipitation and surface water are used to estimate the fluxes of marine cyclic salts through that part of the amazon river system draining past obidos 80 of the basin at the time of peak discharge in june amazon precipitation chemistry can be devided into two principal components marine and terrestrial the marine component determined from analyses of marine rain consists of na k mg ca and cl in approximately sea salt proportions with s doubly enriched the excess sulfur is probably derived from gas phase inputs the terrestrial component makes an important contribution of k ca s and n much of which can be related to biological emissions the emission of reduced sulfur in the marine and terrestrial environment and nitrogen in the terrestrial environment is responsible for a natural acid rain in the amazon region with a ph from 4 7 to 5 7 this is about one tenth the acidity of polluted urban rain the chloride content of lowland rivers which drain regions lacking significant geologic sources of chloride shows a systematic decrease in chloride with increasing distance from the ocean this trend is used to define the cyclic salt background for amazonian surface waters cyclic salts in general make only a minor contribution relative to terrestrial inputs to the chemistry of amazon basin rivers even those draining intensely weathered terrains an estimated 17 6 cl 6 9 na 1 3 mg 3 6 s 0 4 k and 0 1 ca of the dissolved load at obidos during peak discharge is cyclic

observed evidence of an impact of the antarctic sea ice dipole on the antarctic oscillation abstracta lagged maximum covariance analysis mca is applied to investigate the linear covariability between monthly sea ice concentration sic and 500 mb geopotential height z500 in the southern hemisphere sh the dominant signal is the atmospheric forcing of sic anomalies throughout the year but statistically significant covariances are also found between austral springtime z500 and prior sic anomalies up to four months earlier the mca pattern is characterized by an antarctic dipole adp like pattern in sic and a positively polarized antarctic oscillation aao in z500 such long lead time covariance suggests the forcing of the aao by persistent adp like sic anomalies the leading time of sic anomalies provides an implication for skillful predictability of springtime atmospheric variability

terrane amalgamation in the clew bay region west of ireland the caledonides of the west of ireland provide a well exposed and well mapped example of an oblique collision zone the east northeast trending deer park and achill beg fault system is a crustal scale ductile sinistral strike slip duplex of late ordovician age imbricating late precambrian granulite facies lower crustal rocks near eclogite facies supracrustal rocks up to amphibolite facies dalradian metasedimentary rocks and greenschist facies cambro ordovician rocks this fault system is correlated with a pre devonian component of the highland boundary fault system in southern scotland in the clew bay area the high pressure low temperature facies metamorphic rocks in tectonic contact with greenschist facies cambro ordovician rocks are together interpreted as an accretionary prism complex related to northwestward directed subduction both of these are allocthonous terrains with respect to the dalradian terrane to the north north west mayo to the south the cambro ordovician rocks docked with a probable dalradian block containing ultramafic intrusives deer park complex during the late ordovician the deer park complex and south mayo trough linked earlier during the arenig silurian and lower middle devonian redbed successions sit unconformably on the metamorphic rocks deposition and deformation of these cover rocks was controlled by oblique strike slip movements on the leek fault whose strike swings from west northwest to north northeast following earlier basement trends as it is traced eastwards from clew bay the leek fault system may be correlated with the leannan fault of northwest donegal a splay of the great glen fault system of central scotland east of clew bay this sinistral shear generated local dilation on the more northerly trending bend of the leek fault lower and middle old red sandstone redbeds were developed here the west northwest trend of the leek fault in clew bay acted as a compressional bend during these sinistral movements and transpressional southwest directed thrusting developed in silurian rocks post middle old red sandstone pre late tournaisian dextral displacement on the leek fault reversed this pattern with transtension in clew bay allowing intrusion of small carbonated peridotite bodies into silurian rocks and easterly directed thrusting of middle old red sandstone rocks east of the bay on the transpressional north south bend a tectonic model for the region is presented here this model involves a northwestward directed subduction system 150 to 750 km of arenig sinistral strike slip movement and eastwards insertion of the connemara block with formation of the ordovician south mayo trough as a pull apart basin subsequently a further 130 to 650 km eastward displacement of rocks took place south of the deer park fault in later ordovician times the magnitudes of these estimates are directly proportional to an assumed maximum wavelength of 1500 km for promontories on the original laurentian margin and using the current juxtaposition of terranes a minimum wavelength of 300 km is inferred

a pre d2 age for the 590 ma ben vuirich granite in the dalradian of scotland regional mapping demonstrates that s2 is the main fabric in the central highland dalradian and that it is this fabric not s3 as previously thought which overprints the 590 ma ben vuirich granite the granite appears to have been emplaced following a tectonic event d1 in the dalradian rocks for a finely spaced penetrative fabric seen in newly discovered hornfelses from the granite aureole is overgrown by contact metamorphic cordierite and andalusite porphyroblasts now both altered which predate the d2 metamorphism the ben vuirich granite cuts the neoproterozoic dalradian sequence of the central highlands of scotland fig 1 inset and was first recognized as an older foliated granite by barrow et al 1905 later workers bradbury et al 1976 bradbury 1978 concluded that the granite was intruded between d2 and d3 in the dalradian rocks harris et al 1976 and for many years an u pb zircon age of 514 7 ma from the intrusion pankhurst pidgeon 1976 provided a valuable datum point in the evolution of the scottish caledonides however dalradian chronology has recently undergone a radical revision following the determination of a precise u pb age of 590 2 ma for the emplacement of the ben vuirich granite rogers et al 1989 in accepting the interpretation of bradbury et al 1976 that the regional penetrative fabric which affects the country rocks and is continuous with that in the granite is of d3 age rogers et al were also guided by the conclusions of harte et al 1984 and dempster harte

caledonian transpressional terrane accretion along the laurentian margin in co mayo ireland the highland border fair head clew bay baie verte brompton suture clew bay suture on achill island western ireland is an e w trending zone of vertical ductile dextral transpression bounding the southern edge of the dalradian terrane the achill dalradian a sequence of coarse grained psammite overlain by conglomerate quartzite schist and limestone disposed in recumbent curvilinear folds represents laurentian continental margin deposits which were translated westward during deformation parallel to the suture trace studies of fabric development and albite porphyroblast growth indicate that e w dextral transpression and west directed recumbent folding were synchronous and are interpreted as separate expressions of the same nw directed terrane impingement event this process of terrane docking was marked by a c 4kbar phase of decompression within the suture four separate terrane slivers are recognized the presence of pyrope garnet and crossite amphibole within two imply the clew bay zone experienced sufficient displacement to exhume rocks from deep lithospheric levels in addition the extensive retrogression and destruction suffered by these remnant high pressure mineral assemblages during the albite generating transpressional phase implies that ductile deformation was enhanced by the introduction of fluid into and along the suture zone during impingement

was scotland a vendian rrr junction in early palaeozoic time northern scotland occupied a 120 promontory between the appalachian and greenland iapetus facing margins of laurentia it is proposed that this originated at a triple rift rrr jundion between laurentia and two other continents possibly baltica and west gondwana the third arm is marked by trans caledonoid extensional lineaments that controlled vendian sedimentation and morb type volcanicity in the grampians the model accounts for the huge scale of the se prograding moine dalradian clastic wedge as compared with neoproterozoic sequences elsewhere on the e laurentian margin where an extensional regime persisted throughout the grenville caledonian interval evidence currently cited for precambrian orogeny in the highlands needs to be reappraised from that viewpoint major stretching events are inferred at c 750 and 600 ma anorogenic older granite magmatism was associated with the latter which culminated in the opening of iapetus

the arctic oscillation signature in the wintertime geopotential height and temperature fields the leading empirical orthogonal function of the wintertime sea level pressure field is more strongly coupled to surface air temperature fluctuations over the eurasian continent than the north atlantic oscillation nao it resembles the nao in many respects but its primary center of action covers more of the arctic giving it a more zonally symmetric appearance coupled to strong fluctuations at the 50 hpa level on the intraseasonal interannual and interdecadal time scales this arctic oscillation ao can be interpreted as the surface signature of modulations in the strength of the polar vortex aloft it is proposed that the zonally asymmetric surface air temperature and mid tropospheric circulation anomalies observed in association with the ao may be secondary baroclinic features induced by the land sea contrasts the same modal structure is mirrored in the pronounced trends in winter and springtime surface air temperature sea level pressure and 50 hpa height over the past 30 years parts of eurasia have warmed by as much as several k sea level pressure over parts of the arctic has fallen by 4 hpa and the core of the lower stratospheric polar vortex has cooled by several k these trends can be interpreted as the development of a systematic bias in one of the atmosphere s dominant naturally occurring modes of variability

eddy zonal flow feedback in the northern hemisphere winter abstract the variability of the zonal mean zonal wind in the northern hemisphere winter december march is studied using eof analysis and momentum budget diagnostics of ncep ncar reanalysis data 1976 2001 the leading eof of the zonal mean zonal wind is well separated from the remaining eofs and represents the north south movement of the midlatitude westerlies analysis of the momentum budget shows that a positive feedback between the zonal mean wind anomalies and the eddy momentum fluxes selects the leading eof of midlatitude variability like the southern hemisphere the baroclinic eddies reinforce the zonal wind anomalies while external rossby waves damp the wind anomalies in the northern hemisphere the quasi stationary eddies also reinforce the zonal wind anomalies but the baroclinic eddies are most important for the positive eddy zonal flow feedback the observations support the following feedback mechanisms 1 above normal baroclinic wave activity is generated in the region of enhanced wester