interannual enso and interdecadal enso like variability in the southern hemisphere tropospheric circulation abstract recent work has identified variability in the pacific ocean sst with a structure qualitatively similar to enso but at lower frequencies than enso zhang et al have documented the atmospheric circulation anomalies in the tropics and northern hemisphere that are associated with decadal enso like variability and compared these anomalies to those associated with the interannual enso cycle here the authors extend the study of zhang et al to the southern hemisphere using the national centers for environmental prediction national center for atmospheric research reanalysis data for 1958 96 consistent with previous studies the southern hemisphere circulation anomalies associated with enso display a teleconnection pattern from the central tropical pacific into the far southeastern pacific ocean comparatively larger circulation anomalies are found in the southern hemisphere associated with the decadal enso like variability though aloft the structure of the anomalies emphasizes the polar vortex wit

key trends in the resource sustainability of platinum group elements abstract platinum group elements pges are increasingly used in a variety of environmentally related technologies such as chemical process catalysts catalytic converters for vehicle exhaust control hydrogen fuel cells electronic components and a variety of specialty medical uses amongst others almost all of which have strong expected growth to meet environmental and technological challenges this century economic geologists have been arguing on the case of abundant geologic resources of pges for some time while others still raise concerns about long term supply yet there remains no detailed analysis of formally reported mineral resources and key trends in the pges sector this paper presents such a detailed review of the pges sector including detailed mine production statistics and mineral resources by principal ore types providing an authoritative case study on the resource sustainability for a group of elements which are uniquely concentrated in a select few regions of the earth the methodology compiled data sets and trends provide strong assurance on the contribution that pges can make to the key sustainability and technology challenges of the 21st century such as energy and pollution control

decadal changes in the south pacific western boundary current system revealed in observations and ocean state estimates observations and ocean state estimates are used to investigate the nature and mechanism of decadal variability in the east australian current eac system and south pacific subtropical gyre a 62 year record on the tasmanian continental shelf shows decadal variations of temperature and salinity as well as a long term trend which has been related to wind driven variations in the poleward extension of the eac repeat expendable bathythermograph lines spanning the last 15 years suggest that low frequency variations in the transport of the eac extension and tasman front are anticorrelated but the time series are too short to draw firm conclusions here we use two ocean state estimates spanning the past 50 years to diagnose the physical mechanisms and spatial structure of the decadal variability of the south pacific subtropical gyre the observations and state estimates paint a consistent picture of the decadal variability of the gyre and eac system strengthening of the basin wide wind stress curl drives a southward expansion of the subtropical gyre as the gyre shifts south the eac extension pathway is favored at the expense of the tasman front resulting in the observed anticorrelation of the these two major currents the results suggest that the subtropical gyre and western boundary current respond to decadal variability in basin scale wind stress curl consistent with island rule dynamics that strong decadal variability of the south pacific gyre complicates efforts to infer trends from short term records and that wind stress curl changes over the south pacific basin drive changes in the eac system that are likely to have implications for marine ecosystems and regional climate

precious and base metal geochemistry and mineralogy of the grasvally norite pyroxenite anorthosite gnpa member northern bushveld complex south africa implications for a multistage emplacement the grasvally norite pyroxenite anorthosite gnpa member within the northern limb of the bushveld complex is a mineralized layered package of mafic cumulates developed to the south of the town of mokopane at a similar stratigraphic position to the platreef the concentration of platinum group elements pge in base metal sulfides bms has been determined by laser ablation inductively coupled plasma mass spectrometry these data coupled with whole rock pge concentrations and a detailed account of the platinum group mineralogy pgm provide an insight into the distribution of pge and chalcophile elements within the gnpa member during both primary magmatic and secondary hydrothermal alteration processes within the most unaltered sulfides containing pyrrhotite pentlandite and chalcopyrite only the majority of ipge rh and some pd occur in solid solution within pyrrhotite and pentlandite with an associated pt as and pd bi te dominated pgm assemblage these observations in conjunction with the presence of good correlations between all bulk pge and base metals throughout the gnpa member indicate the presence and subsequent fractionation of a single pge rich sulfide liquid which has not been significantly altered in places the primary sulfides have been replaced to varying degrees by a low temperature assemblage of pyrite millerite and chalcopyrite these sulfides are associated with a pgm assemblage characterized by the presence of pd antimonides and pd arsenides which are indicative of hydrothermal assemblages the presence of appreciable quantities of ipge pd and rh within pyrite and to a lesser extent millerite suggests these phases directly inherited pge contents from the pyrrhotite and pentlandite that they replaced the replacement of both the sulfides and pgm occurred in situ thus preserving the originally strong spatial association between pgm and bms but altering the mineralogy precious metal geochemistry indicates that fluid redistribution of pge is minimal with only pd au and cu being partially remobilized and decoupled from bms this is also indicated by the lower concentrations of pd evident in both pyrite and millerite compared with the pentlandite being replaced the observations that the gnpa member was mineralized prior to intrusion of the main zone and that there was no local footwall control over the development of sulfide mineralization are inconsistent with genetic models involving the in situ development of a sulfide liquid through either depletion of an overlying magma column or in situ contamination of crustal s we therefore believe that our observations are more compatible with a multistage emplacement model where preformed pge rich sulfides were emplaced into the gnpa member such a model explains the development and distribution of a single sulfide liquid throughout the entire 400 800 m thick succession it is therefore envisaged that the gnpa member formed in a similar manner to its nearest analogue the platreef notable differences however in pge tenors indicate that the ore forming process may have differed slightly within the staging chambers that supplied the platreef and gnpa member

antarctic ozone depletion causes an intensification of the southern ocean super gyre circulation 1 recent climate trends over the southern hemisphere sh summer feature a strengthening of the circumpolar westerly and a weakening of the midlatitude westerly extending from the stratosphere to earth s surface much of the change is attributable to antarctic ozone depletion however the consequential ocean circulation changes are unknown here i demonstrate that the observed surface wind changes have forced a southward shift and spin up of the super gyre which links the subtropical south pacific indian and atlantic ocean circulation advecting more warm water southward the circulation change includes a strengthening of the east australian current eac flow passing through the tasman sea the southward shift may be responsible for the observed unusually large warming in the sh midlatitude ocean and may contribute to the reported range extension to the south of many marine species in the south west pacific

decadal sea level variability in the south pacific in a global eddy resolving ocean model hindcast sea level variability and related oceanic changes in the south pacific from 1970 to 2003 are investigated using a hindcast simulation of an eddy resolving ocean general circulation model ogcm for the earth simulator ofes along with sea level data from tide gauges since 1970 and a satellite altimeter since 1992 the first empirical orthogonal function mode of sea level anomalies slas of ofes exhibits broad positive slas over the central and western south pacific the corresponding principal component indicates roughly stable high low and high slas separated by a rapid sea level fall in the late 1970s and sea level rise in the late 1990s consistent with tide gauge and satellite observations these decadal changes are accompanied by circulation changes of the subtropical gyre at 1000 m depth and changes of upper ocean zonal current and eddy activity around the tasman front in general agreement with previous related studies it is found that sea level variations in the tasman sea can be explained by propagation of long baroclinic rossby waves forced by wind stress curl anomalies if the impact of new zealand is taken into account the corresponding atmospheric variations are associated with decadal variability of el nino southern oscillation enso thus decadal sea level variability in the western and central south pacific in the past three and half decades and decadal enso variability are likely to be connected the sea level rise in the 1990s which attracted much attention in relation to the global warming is likely associated with the decadal cooling in the tropical pacific

the upper critical zone of the eastern bushveld complex precursor of the merensky reef the upper critical zone in the central sector of the eastern bushveld complex consists predominantly of norites although bronzitites anorthosites chromitites and other chromitic rocks appear at intervals in the sequence gabbronorites appear near the middle of the zone the rock sequence is divided into 14 principal units plus the series from the base of the merensky reef to the top of the bastard reef mineralogical compositions of successive units have been determined from modal analyses and compositions of bronzite and plagioclase in the various rock units have been determined by microprobe analysis the results indicate that although parts of the zone show normal fractionation the zone as a whole is not the product of progressive fractionation of a magma crystal settling appears to have played a relatively minor role in evolution of the zone successive pulses of parent magma into the chamber may account for some of the mineralogical and chemical variations but features of certain thick norite units suggest stratification of the magma in the chamber the study suggests that the merensky reef registers a sudden event in the bushveld magma chamber an event that altered the chemistry of the system and triggered precipitation of sulfides and platinoid minerals the alternative hypothesis that the reef is the culmination of a long period of magma fractionation is not supported by the evidence presented chromite occurs in a variety of associations with plagioclase and bronzite the associations are consistent with phase relations in the system mg 2 sio 4 iron oxide caal 2 si 2 o 8 sio 2 at elevated pressure

major influences on the evolution of the 2 67 2 1 ga transvaal basin kaapvaal craton abstract the transvaal is one of three structural basins of the transvaal supergroup preserved on the kaapvaal craton the evolution of the neoarchaean palaeoproterozoic transvaal basin is ascribed predominantly to magmatism palaeoclimate and eustasy with plate tectonics playing a sporadic role the supergroup comprises basal protobasinal rocks followed by the black reef formation chuniespoort group and uppermost pretoria group immature siliciclastic and bimodal volcanic rocks of the protobasinal unit reflect a wide zone of rifting related to the c 2 7 ga ventersdorp supergroup mantle plume individual protobasinal successions were laid down in separate fault bounded basins controlled at least partially by greenstone belt orientations in the kaapvaal basement post magmatic post rifting thermal subsidence accommodated black reef fluvial sheet sandstones and the subsequent thick carbonate bif epeiric platform succession of the chuniespoort group subordinate mechanical subsidence accompanied this long lived thermal relaxation intense weathering due to neoarchaean atmosphere composition greatly reduced clastic sedimentation and the greenhouse palaeoclimate further encouraged carbonate sedimentation globally enhanced sea levels due to enhanced mid ocean ridge growth consequent upon either global magmatic events or supercontinent break up also played a pivotal role in chuniespoort epeiric basin evolution pretoria group sedimentation is ascribed to two cycles of rifting and subsequent thermal subsidence the first cycle appears to reflect plate tectonically induced rifting with an epeiric sea drowning the rift basin despite reduced sea levels due to the first major global glaciation the second pretoria cycle is most likely related to a major continental flood basalt event with thermal subsidence allowing a second and probably larger epeiric sea to advance onto the northern kaapvaal craton evaluation of the inferred relatively minor importance of plate tectonics in the evolution of the essentially intracratonic transvaal basin depends to a large degree on the age assigned to the limpopo orogeny during which the zimbabwe and kaapvaal cratons collided recent evidence for a c 2 0 ga collision supports the concept that catastrophic global magmatic events gradually gave way to plate tectonics during transvaal basin evolution

geochronological evidence for an extension of the northern lobe of the bushveld complex limpopo province south africa abstract we have conducted the first geochronological study of the ultramafic mafic succession the waterberg project located in the southern marginal zone of the limpopo belt north of the northern lobe of the bushveld complex using u pb dating on zircons extracted from both mineralized reefs as well as on detrital zircons of the waterberg group unconformably overlying the mafics this contribution is the first in a series of publications describing the well mineralized succession and will therefore give an overview of the local geology u pb baddeleyite ages for the main mafic sill intruding the area supplement the data the succession is extremely well mineralized and therefore of interest to the wider geological community the intrusion ages for the lobe are 2059 3 and 2053 5 ma and within error coeval with the intrusion ages of the bushveld complex detrital zircon age probability density distributions indicate a maximum age of deposition of c 2045 ma for the waterberg group and show abundant peaks of archean age paleoproterozoic zircons are interpreted to represent zircon from the detritus of the now eroded roof rocks of the lobe while the archean zircons are most likely sourced from the limpopo complex to the northeast the major dolerite sill intruding the project area has been confirmed to belong the umkondo igneous province with a baddeleyite age of 1125 5 ma the combination of these new age dates local geology and existing literature is used to argue that i the ultramafic mafic succession is related to the bushveld complex ii a significant unconformity is present between igneous rocks and the overlying sediments and iii the area was exposed to a level of paleoproterozoic tectonism not previously observed in the bushveld complex

spatiotemporal variability of the south pacific convergence zone fresh pool eastern front from coral derived surface salinity data abstractdirect observations indicate a south eastward expansion of the south pacific convergence zone spcz fresh pool and a freshening trend since the 1970s understanding decadal and longer term variability of the spcz fresh pool and of the salinity front located at its southeastern margin has been limited by the scarcity of instrumental sea surface salinity sss measurements in this study we use coral 18o as a proxy for sss to extend the salinity record back to the 1880s from three different locations across the sss front fiji tonga and rarotonga ftr high percentages of observed sss variance are explained by multi coral 18o mean composite at each site at the interannual time scale the salinity front displacement over the last 200 years follows the el nino southern oscillation index enso the different el nino flavours are observable in the amplitude of the salinity front interannual displacement however we do not observe any significant changes in either the frequency or the amplitud

the southwest pacific ocean circulation and climate experiment spice the southwest pacific ocean circulation and climate experiment spice is an international research program under the auspices of clivar the key objectives are to understand the southwest pacific ocean circulation and the south pacific convergence zone spcz dynamics as well as their influence on regional and basin scale climate patterns south pacific thermocline waters are transported in the westward flowing south equatorial current sec toward australia and papua new guinea on its way the sec encounters the numerous islands and straits of the southwest pacific and forms boundary currents and jets that eventually redistribute water to the equator and high latitudes the transit in the coral solomon and tasman seas is of great importance to the climate system because changes in either the temperature or the amount of water arriving at the equator have the capability to modulate the el nino southern oscillation while the southward transports influence the climate and biodiversity in the tasman sea after 7 years of substantial in situ oceanic observational and modeling efforts our understanding of the region has much improved we have a refined description of the spcz behavior boundary currents pathways and water mass transformation including the previously undocumented solomon sea the transports are large and vary substantially in a counter intuitive way with asymmetries and gating effects that depend on time scales this paper provides a review of recent advancements and discusses our current knowledge gaps and important emerging research directions

decadal variability of east australian current transport inferred from repeated high density xbt transects a ctd survey and satellite altimetry a time series of the net geostrophic transport through the tasman sea representing the flow of the east australian current eac extension is determined from a full depth ctd section 15 years of high density xbt transects and satellite altimetry data a section between sydney and wellington px34 has been occupied four times per year since 1991 with high resolution xbt sampling two methods to infer baroclinic transport from proxy data along the section are presented the first uses shallow xbt transects to derive geostrophic transport relative to a deep 2000 m reference level in the second approach synts the subsurface temperature and salinity structure are inferred from satellite surface height and temperature fields using a model developed from historical in situ observations the baroclinic transport is then computed in the usual manner the methods are validated using both a full depth ctd occupation of the px34 section and further transects crossing the eac in the northern tasman sea there is close agreement between the 49 xbt and synts px34 transport estimates obtained between 1992 and 2006 the time series of transport through the sydney wellington section shows a range of temporal signals from eddyscale seasonal interannnual to decadal in particular we note that the net eac flow ranges from 5 sv in 1995 to a maximum of 16 sv in 2000 2001 this decadal variation confirms the eac response to a spin up of the south pacific circulation forced by changes in the basin wide winds and matches the changes in oceanic properties observed in the tasman sea

platinum group elements in the boulder bed western bushveld complex south africa concentrations of platinum group elements in samples from the boulder bed at five localities in the western bushveld complex range between 50 ppb and 70 ppm boulders thus have much more variable and sometimes highly enriched pge contents relative to the other lithologies in the immediate foot wall sequence of the merensky reef the pge enrichment can largely be modelled as a result of primary magmatic processes including collection of pge by segregating sulphide melt and fractionation of mss other features of the boulder bed such as the selvages of pure anorthosite and the chromitite stringers surrounding some of the boulders bear evidence of recrystallisation a model is proposed by which the boulder bed formed as a result of a combination of early and late magmatic processes the pges were collected by magmatic sulphide melt which accumulated in a pyroxenite layer the host rock to the pyroxenite was a thick package of norites which recrystallised in response to upward migrating magmatic fluids the fluids caused partial hydration melting of the norites adjacent to the pyroxenite producing anorthosite the boulders represent the broken up remnants of the pyroxenite layer the selvages of chromite and pure anorthosite around some of the boulders remain poorly understood but may represent the latest recrystallisation event in response to localised late magmatic fluid overpressure upon cooling

petrology and mineralisation of the southern platreef northern limb of the bushveld complex south africa the platreef the putative local analogue of the merensky reef forms the floor to the mafic succession in the northern limb of the bushveld complex we define the platreef as the lithologically variable unit dominated by pyroxenite which is irregularly mineralised with pge cu and ni between the transvaal metasedimentary footwall or archaean basement and the overlying main zone gabbronorite we define the mineralisation around calcsilicate xenoliths within the main zone in the far north of the limb as a platreef style mineralisation the platreef ss has a strike extent of 30 km whereas platreef style mineralisation occurs over a strike length of 110 km the platreef varies from 400 m thick in the s to 30 modal in some intersections these are dominated by pyrrhotite with lesser pentlandite and chalcopyrite minor pyrite and traces of a wide compositional range of sulphides in the southern sector mineralised zones have cu grades of 0 1 0 25 and ni 0 15 0 36 massive sulphides are localised commonly but not exclusively towards the contact with footwall metasedimentary rocks magmatic sulphides are disseminated or net textured ranging from a few microns to 2 cm grains of pyrrhotite and pentlandite with chalcopyrite and minor pyrite much of the sulphide is associated with intergranular plagioclase or quartz feldspar symplectites along the margins of rounded cumulus orthopyroxenes the pges in the southern sector occur as tellurides bismuthides arsenides antimonides bismuthoantimonides and complex bismuthotellurides pgm are rarely included in the sulphides but occur as micron sized satellite grains around interstitial sulphides and within alteration assemblages in serpentinised zones the pt pd ratio 1 and pge grade may be decoupled from s and base metal abundance

relationships between low frequency variability in the southern hemisphere and sea surface temperature anomalies abstract long term trends and interannual variations of circulation anomalies in the southern hemisphere are examined using the national centers for environmental prediction national center for atmospheric research reanalysis from 1949 to 1998 the changes in planetary circulation regimes are linked to global sea surface temperature anomalies sstas empirical orthogonal function eof analysis was performed on 500 hpa height anomalies the leading mode eof1 shows a strong zonal symmetry with a phase reversal between height anomalies in high and midlatitudes apart from zonal symmetry a zonal wavenumber 3 is evident with three centers located in three southern oceans in the low frequency band with fluctuations longer than 60 months eof1 is associated with the second rotated eof mode of sstas with positive loadings over three southern oceans and negative loadings in the north pacific and the north atlantic the next two modes are the pacific south american psa patterns they depict wave 3 patterns in

flows in the tasman front south of norfolk island the tasman front is a narrow band of eastward flowing subtropical water crossing the tasman sea from australia to north cape new zealand it is the link between the two subtropical western boundary currents of the south pacific the east australian current eac off eastern australia and the east auckland current eauc off northeastern new zealand here we report the first direct measurements of flow in the tasman front from a moored array deployed across gaps in the submarine ridges south of norfolk island and hydrographic and adcp measurements during the deployment and recovery voyages the mean flow through the array over july 2003 to august 2004 was found to be eastward only in the upper 800 m with a transport of 6 sv below 800 m a weak westward mean flow 1 5 sv was measured associated with antarctic intermediate water aaiw using sea surface height to account for additional transport south of the moored array results in a total mean eastward transport between norfolk island and north cape new zealand of 8 sv varying between 4 and 18 sv the measurements show that the tasman front is much shallower than either the eac or eauc both of which extend below 2000 m depth has less transport than either the eac or eauc and has instances of flow reversal thus the tasman front is a weaker connection between the eac and eauc than the paradigm of a contiguous south pacific western boundary current system would suggest

the tasman front a study using satellite infrared imagery abstract monthly charts of the approximately zonal tasman front between the australian coast near 152 e and 164 e and between latitudes 31 to 37 s have been constructed from infrared satellite imagery for 29 months from march 1982 to april 1985 from these it was found that one to three meanders but usually two are present between 152 and 164 e and that the second meander pinches off at times to form a warm core ring in the same manner as is known to occur for the first meander both second meander and ring tend to propagate westwards the mean zonal widths of the first and second meander are similar and are 3 6 to 3 7 longitude 330 345 km the average and the most likely modal positions of the tasman front were determined and the two positions are most different when the front meanders northward the zonal length scale of frontal perturbations increases eastward from 140 near 155 e to 300 km near 161 e and perturbations propagate westward at approximately 0 1 m s 1

geochemistry and mineralogy of the platreef and critical zone of the northern lobe of the bushveld complex south africa implications for bushveld stratigraphy and the development of pge mineralisation the northern lobe of the bushveld complex is currently a highly active area for platinum group element pge exploration this lobe hosts the platreef a 10 300 m thick package of pge rich pyroxenites and gabbros that crops out along the base of the lobe to the north of mokopane formerly potgietersrus and is amenable to large scale open pit mining along some portions of its strike an early account of the geology of the deposit was produced by percy wagner where he suggested that the platreef was an equivalent pge rich layer to the merensky reef that had already been traced throughout the eastern and western lobes of the bushveld complex wagner s opinion remains widely held and is central to current orthodoxy on the stratigraphy of the northern lobe this correlates the platreef and an associated cumulate sequence that includes a chromitite layer known as the grasvally norite pyroxenite anorthosite gnpa member directly with the sequence between the ug2 chromitite and the merensky reef as it is developed in the upper critical zone of the eastern and western bushveld implicit in this view of the magmatic stratigraphy is that similar critical zone magma was present in all three lobes prior to the development of the merensky reef and the platreef however when this assumed correlation is examined in detail it is obvious that there are significant differences in lithologies mineral textures and chemistries mg of orthopyroxene and olivine and the geochemistry of both rare earth elements ree and pge between the two sequences this suggests that the prevailing interpretation of the stratigraphy of the northern lobe is not correct the critical zone of the northern lobe cannot be correlated with the critical zone in the rest of the complex and the simplest explanation is that the gnpa platreef sequence formed from a separate magma or mixture of magmas chilled margins of the gnpa member match the estimated initial composition of tholeiitic main zone type magma rather than a critical zone magma composition where the gnpa member is developed over the ultramafic lower zone hybrid rocks preserve evidence for mixing between new tholeiitic magma and existing ultramafic liquid this style of interaction and the resulting rock sequences are unique to the northern lobe the gnpa member contains at least seven sulphide rich horizons with elevated pge concentrations some of these are hosted by pyroxenites with similar mineralogy crystallisation sequences and pd rich pge signatures to the platreef chill zones are preserved in the lowest main zone rocks above the gnpa member and the platreef and this suggests that both units were terminated by a new influx of main zone magma this opens the possibility that the platreef and gnpa member merge laterally into one another and that both formed in a series of mixing quenching events involving tholeiitic and ultramafic magmas prior to the main influx of tholeiitic magma that formed the main zone

the influence of subglacial hydrology on the flow of kamb ice stream west antarctica 1 ice streams on the siple coast west antarctica have a complex history of flow because their basal motion is governed by time varying basal conditions although the mechanical interaction between ice and till is well established very little is known about the potential effect of regionally scaled water transport in a basal water system which has only recently become apparent to investigate the combined effect of hydrological and mechanical processes we developed the hydrology ice and till model in which ice flow is coupled to a coulomb plastic till layer and a basal water system consisting of discrete conduits when the model is applied to kamb ice stream kis results confirm that it is capable of oscillating between fast and stagnant modes of flow we show that when subglacial conduits are disregarded or do not extend to the grounding line the oscillatory behavior of the ice stream is governed by the basal thermal regime when conduits extend to the grounding line the modelled ice stream oscillation period is increased peak speeds are reduced and oscillations may ultimately cease if the volume of water supplied is sufficiently high three different hydrological states characterize the behavioral patterns of ice flow and these states are distinguished by conditions at the grounding line modelled ice stream velocities were found to oscillate with fast and slow periods typically lasting a few hundred years although varying according to hydrological activity our results indicate that kis could reactivate this century given its hydrological setting and 170 years of stagnation

the merensky reef at winnaarshoek eastern bushveld complex a primary magmatic hypothesis based on a wide reef facies the merensky reef on the winnaarshoek property in the central sector of the eastern limb of the bushveld complex belongs to what may be described as a wide reef facies in which platinum group element pge mineralization is largely contained within a relatively thick succession of medium grained nonpegmatoidal feldspathic orthopyroxenite bounded top and bottom by thin chromite stringers the wide reef merensky reef facies at winnaarshoek presents the opportunity to study subtle processes which could potentially be obscured or obliterated in the thinner reef facies found in many areas of the western limb especially in places where the reef zone contains a component of pegmatoidal orthopyroxenite at winnaarshoek a pegmatoidal feldspathic orthopyroxenite is developed below the lower chromite stringer and a second pegmatoidal orthopyroxenite is intermittently developed several centimeters above the upper chromite stringer in contrast to most occurrences in the western limb however both these pegmatoidal orthopyroxenites are only sporadically mineralized if at all in addition rather than being at the base of a feldspathic orthopyroxenite unit as is the case throughout most of the western limb the merensky reef at winnaarshoek occurs toward the top of a sequence of feldspathic orthopyroxenite despite the gross relative uniformity of the merensky reef on a property wide scale at winnaarshoek we demonstrate using underground channel samples that individual intersections spaced a matter of meters apart commonly display remarkable variations in grade distribution our study is based primarily on whole rock and mineral chemical profiles from drill cores through three separate and distinctive subfacies of essentially normal i e nonpothole reef one of the drill core intersections represents typical reef in which pge mineralization forms an upper reef extending from the upper chromitite stringer approximately 1 m into the underlying feldspathic pyroxenite and a much thinner lower reef centered on the lower chromitite stringer the other two drill core intersections represent extremes in terms of the vertical distribution of the pge one having mineralization confined to the immediate vicinity of the two chromitite stringers while the other is richly mineralized throughout the reef zone peaks in pge concentrations in all three intersections are coincident with the most primitive compositions of orthopyroxene plagioclase and chromite the dominant rock forming minerals whereas the cumulate layers below major pge peaks are the most evolved as indicated by mineral compositions and elevated whole rock incompatible element concentrations furthermore the lowest grade merensky reef intersection has the most chemically evolved footwall whereas the richest intersection has a relatively primitive footwall we consider the origin of the merensky reef in the context of its position as the culmination of a sequence of several pge mineralized chromite bearing layers in the upper critical zone and suggest that the regular upward variations in the chemistry of the chromitite from one layer to the next and the overall cyclicity are indicative of complex magma replenishment and mixing processes in the chamber we apply a model of lateral streaming of magma with chromite crystallization and consequent sulfide and platinum group mineral pgm precipitation resulting from magma mixing specifically the merensky reef cannot be properly interpreted outside the context of the sequence of successive events that evolved throughout the development of the upper critical zone genetic models also need to account for lateral facies variations in the reef

distribution of mafic sills in the transvaal supergroup northeastern south africa systematic measured profiles on a regional scale are used to document the distribution of noritic and amphibolitic sills 787 in total in the largely sedimentary latest archaean early proterozoic succession of the transvaal supergroup in northeastern south africa an aggregate thickness of over 2 2 km of sills intruded this 12 km thick succession the proportion of sill to sill plus sedimentary rock increases upward average 16 towards the overlying mafic phase of the bushveld complex the average thickness of sills is 34 m but increases in the same sense the emplacement level of the bushveld complex transgresses across some 5 km of the stratigraphy of the upper formations of the transvaal supergroup a node of maximum intensity of noritic sill intrusion is developed in these uppermost strata adjacent to the location where the transgressive relationship is most pronounced the structural styles of the transvaal supergroup strata influence intensity of sill intrusion other factors being equal maximum intrusion intensity is associated with areas of shallow homoclinal dip supplementary material a spreadsheet including all 440 formation traverses including grid references formation thicknesses and number and thicknesses of every sill therein is available at http www geolsoc org uk sup18807

the mineralogy and mineral associations of platinum group elements and precious metals in the aurora cu ni au pge deposit northern limb bushveld complex abstract aurora is a platinum group element pge prospect hosted in the northern limb of the bushveld complex south africa it is one of only three deposits discovered in the northern limb so far to be hosted in the melanocratic leucocratic gabbroic cumulates of the main zone of the rustenberg layered suite aurora moorddrift and waterberg t zone deposits rather than in predominantly ultramafic rocks e g platreef the host cumulates at aurora have been divided into three principal units and they intrude the dolomites of the lower transvaal supergroup base metal sulphide bms mineralisation with pge is present in the leucogabbronorites and gabbronorites of unit 2 and in coarse grained gabbronorite veins which intrude the peridotites of unit 1 these veins contain up to 50 interstitial pyrrhotite pentlandite chalcopyrite pyrite unit 2 contains 1 3 pentlandite pyrrhotite chalcopyrite assemblages and 1 5 chalcopyrite pyrite pyrrhotite associated with hydrothermal alteration the pge content of aurora however is predominantly controlled by the presence of platinum group minerals pgm not bms la icp ms analysis of sulphides shows the bms in aurora have lower pge concentrations than other bushveld magmatic sulphides with pentlandite carrying much lower concentrations of pd average 23 ppm than the platreef or the merensky reef sem eds analysis of 26 sections characterised 995 platinum group minerals pgm and precious metal bearing minerals pmm with a total area of 27850 m2 and an average size of 28 2 m2 of the pgm and pmm identified in aurora 85 by area are pd te bi minerals with 6 pd te minerals 4 electrum and 3 ag te minerals along with minor pd bi pd as pt te bi pt as and pt s minerals that collectively comprise 2 of total area only 25 of the pgm and pmm in aurora are bms hosted with the rest hosted in silicates of the total pgm and pmm area 22 are hosted in alteration silicates quartz chlorite or actinolite in an alteration halo around sulphides unusually 52 of the pgm and pmm are spatially removed from bms instead hosted in alteration silicates and within cracks in primary silicates away from any bms this indicates a multi stage ore genesis model with hydrothermal remobilisation of pge important for ore formation the style and host rocks for mineralisation in the aurora deposit are fundamentally different from other deposits in the northern limb of the bushveld hosted in ultramafic rocks such as the platreef gnpa member deposits and the f zone of the waterberg deposit all of which contain a greater diversity of pgm and bms with higher precious metal contents the mineralisation most similar to aurora is the t zone of the waterberg deposit located to the north of aurora which been suggested to be an along strike equivalent of the aurora main zone mineralisation however despite strong similarities in pgm mineralogy and s isotope signatures there are significant differences in bms mineralisation and host lithology meaning it is unlikely they are directly linked stratigraphically at present it seems more likely that aurora and the waterberg t zone reflect similar fluid influenced processes operating in different parts of the main zone perhaps at different times and in different structural basins rather than a continuous mineralised zone along strike

decadal variability in the large scale sea surface height field of the south pacific ocean observations and causes large scale sea surface height ssh changes in the extraequatorial south pacific ocean are investigated using satellite altimetry data of the past 12 yr the decadal ssh signals in the 1990s were dominated by an increasing trend in the 30 50 s band and a decreasing trend in the central south pacific ocean poleward of 50 s in recent years since 2002 there has been a reversal in both of these trends spatially varying low frequency ssh signals are also found in the tropical region of 10 25 s where the decadal ssh trend is negative in the eastern basin but positive in the western basin to clarify the causes for these observed spatially varying ssh signals a 11u2 layer reduced gravity model that includes the wind driven baroclinic rossby wave dynamics and the responses forced by ssh changes along the south american coast was adopted the model hindcasts the spatially varying decadal trends in the midlatitude and the eastern tropical regions well accumulation of the wind forced ssh anomalies along rossby wave characteristics is found to be important for both previously reported long term trends and their reversals in recent years the boundary forcing associated with the time varying ssh signals along the south american coast is crucial for understanding the observed ssh signals of all time scales in the eastern tropical south pacific basin but it has little impact upon the midlatitude interior ssh signals

evidence for magma mixing processes within the critical and lower zones of the northwestern bushveld complex south africa abstract during deposition of the lower lz lower critical lcz and upper critical ucz zones the course of crystallization was dominated by repeated emplacement of primitive mafic magma evidence for this is provided by textural chemical and isotopic data emplacement of fresh magma into resident residual liquids produced reversal in fractionation trends i e progressive increase in mg mg fe 2 ratios and enrichment of cr in orthopyroxenes through intervals of 350 m in the lz 100 200 m in the lcz and 10 80 m within the lower part of the ucz reversals culminated with the deposition of olivine cumulates upon pyroxenite within the lz and lcz but a change in the style of reversals led ultimately to ultramafic cumulates with or without olivine resting directly upon anorthositic cumulates at the top of the ucz merensky and bastard units these incursions of fresh mafic liquid led to deposition of ultramafic rocks at the base of units constituting the ucz whereas the cumulus feldspar bearing rocks at the tops of such units crystallized from hybrid liquids resulting from the mixing of the new and the resident residual liquids there is a general decoupling of trends of cryptic variation of the feldspar and orthopyroxene populations in the ucz wide variations of initial sr isotope ratios exist throughout the entire section within individual units and within individual members of units isotopic disequilibrium also exists between feldspar and pyroxene fractions of some samples and between individual feldspar grains of different textural habit the first appearance of cumulus feldspar bearing cumulates of the ucz is presaged by the appearance of small partially resorbed plagioclase inclusions within the orthopyroxene grains of pyroxenites at the top of the lcz this texture is conspicuous throughout the lowermost 400 m of the ucz but is still sporadically developed within the bastard unit at the top of the ucz anorthositic rocks of the lower part of the ucz are held to have been derived from residual liquids evolved during crystallization of earlier cumulates but anorthosites with a main zone isotopic signature cap the merensky and bastard units

textural and compositional features of chromite in the lower and critical zones of the bushveld complex south of potgietersrus chromite is developed as an accessory mineral in a 1 700 m thick sequence of lower and lower critical zone cumulates in the area south of potgietersrus and as a major constituent in a number of chromitite layers within this sequence the lower zone consists of a 1 500 m thick sequence of ultramafic rocks characterized by varying proportions of olivine orthopyroxene and chromite which allows the pile of cumulates to be subdivided into 37 cyclic units the critical zone on the other hand is only 350 m thick of which only the lower 150 m contains chromite bearing lithologies electron microprobe analyses of chromite and coexisting silicates indicate a strong positive correlation between the mg mg fe super 2 and cr fe super 2 fe super 3 ratios of chromite and the mg mg fe super 2 ratio of the mafic silicates the composition of the chromite is also strongly influenced by the mineralogical composition of the host rock and probably reflects conditions within the parental magma during crystallization of these rocks the analytical data have also shown the tio 2 content of the disseminated chromite to be the most informative indicator of the degree of differentiation of the magma a detailed investigation of the chromite across the contact of a thin chromitite layer has shown that the chromitite layers crystallized in response to an influx of magma and that chromite crystallization could have been enhanced by an increase in the f sub o 2 and a rise in the cr 2 o 3 content of the liquid thereby causing an increase in the cr 2 o 3 content of the chromite within the chromitite layers postcumulus modification of the chromitite layers is extensive and primary textural features have been obliterated essentially by sintering sintering took place in the presence of a reactive mg rich liquid which resulted in an increase in the mg mg fe super 2 ratio of the chromite this process resulted in a densification of the chromitite layers from an estimated initial porosity of 30 to 45 percent to less than 10 percent

the geologic setting of the platiniferous ug 2 chromitite layer on the farm maandagshoek eastern bushveld complex the platiniferous ug 2 chromitite layer forms part of the upper chromitite group of the critical zone on the farm maandagshoek 254kt in the eastern bushveld complex the upper chromitite group is made up of four cyclic units namely ug 1 ug 2 ug 3 and ug 3a each of which consists of a basal chromitite layer with or without olivine overlain successively by melanorite and norite and terminated with anorthosite the ug 2 chromitite layer and its associated basal pegmatoid is found close to the base of the ug 2 cyclic unit the cyclic units are separated by sharp contacts and have a remarkable persistence along strike and downdip whole rock cu total ni pt and pd show an upward enrichment from the base of the ug 2 unit reaching a maximum in the main chromitite layer they are depleted in the hanging wall sequence of melanorite bulk analyses of the ug 2 chromitite and its enclosing lithologies for pt pd ru rh ir au s sulfide ni cu and co show that the platinum group elements and base metals are concentrated 1 close to the top and base of the main chromitite layer 2 in the leader layer and 3 in minor chromitite lenses and layers both in the footwall pegmatoid and the hanging wall melanorite the pt pt pd and cu ni ratios are low in the footwall pegmatoid and base of the main chromitite layer values of these ratios however increase upward with pt pt pd decreasing at the upper contact of the ug 2 layer the calculated concentration of platinum group metals in the sulfide phase in the ug 2 chromitite layer ranges from 3 000 to just under 12 000 ppm the above determination is primarily controlled by the concentration of s in the layer with the average precious metal distribution remaining relatively constant the calculated d super sulfide liquid sub silicate liquid with the parental liquid values in parentheses are pt 52 575 40 ppb pd 132 940 16 ppb rh 22 080 8 ppb ir 730 000 0 4 ppb and au 2 190 21 ppb evidence of an influx of magma into the eastern lobe of the bushveld complex at a level between the upper and middle chromitite groups is indicated by discontinuous and disturbed layers in borehole drilling on maandagshoek it is suggested that this magma batch was slightly more enriched in the platinum group elements in order to produce the increased value of these elements in the ug 2 chromitite layer it is proposed that preexisting sulfides in the ug 2 chromitite layer collected platinum group elements from upward migrating highly fractionated intercumulus fluids under highly reducing conditions during the early stages of formation of the layer the relatively large volume of sulfide initially present in the layer was later reduced by volatiles enriched in o 2 passing through the layer resorbing the sulfides and blowing off so 2 the partial resorption of the sulfides accounts for the presence of minor pt fe alloys and the abnormally high partition coefficients of the platinum group elements in the sulfide liquid of the ug 2 chromitite layer

diapirism initiated by the bushveld complex south africa the bushveld complex induced the development of a unique type of diapir during its intrusion these diapirs are preserved as large domal structures exposed on the margin and within the northeastern mafic layered sequence they were produced by gravitational loading and heating of argillaceous and arenaceous floor rocks gravity induced deformation is restricted to rocks above a detachment level that corresponds approximately with the 550 c peak metamorphic isotherm and with lithological contacts of contrasting competency strongly foliated and boudinaged contact metamorphosed sedimentary rocks between domes were deformed by lateral extension along flow lines directed toward dome culminations at the highest structural levels domes have bulbous geometries and the enclosing layered igneous rocks are deformed into series of outward verging folds that define a broad rim syncline deformation within the domal cores is represented by constriction that produced radial curtain type folds that have steeply plunging lineations and concentrically oriented folds in the outer shell diapirism is closely linked to magma emplacement mechanisms floor folds in the country rocks were initiated in the interfinger areas of a fingered intrusion with the coalescence of intrusion fingers into a single sheet interfinger folds matured into large diapiric domes that rose to the upper levels of the magma chamber strain rates estimated from strain analyses dome geometry and model cooling calculations are on the order of 10 14 s 1 corresponding to diapiric uplift rates of 0 6 cm yr

precise age of the platiniferous merensky reef bushveld complex south africa by the u pb zircon chemical abrasion id tims technique the bushveld complex south africa is the world s largest layered intrusion and host to the majority of the known resources of platinum group elements chromium and vanadium despite intensive study determining the precise crystallization age of the bushveld complex has proven to be difficult in this study zircon and rutile were identified in thin section and separated from a sample of pegmatitic feldspathic orthopyroxenite from the pge rich merensky reef west mine townlands shaft rustenburg section low u 21 105 ppm zircon occurs with interstitial biotite and is locally in direct contact with sulfide rutile occurs mainly as inclusions within or overgrowths on chromite especially in the chromitite stringers that bound the reef and also as acicular grains with biotite a u pb crystallization age of 2054 4 1 3 ma was determined from single crystal chemical abrasion id tims u pb dating of zircon and multigrain fractions of rutile yielded a u pb cooling age of 2055 0 3 9 ma integration of the merensky reef crystallization age with existing u pb age determinations for intrusions that pre and postdate emplacement and crystallization of the layered rocks of the bushveld complex indicate that magmatism occurred at ca 2054 ma and is contemporaneous with related magmatic activity across the northern kaapvaal craton of south africa and botswana

the response of the southern annular mode the east australian current and the southern mid latitude ocean circulation to global warming 1 climate models predict an upward trend of the southern annular mode sam in response to increasing atmospheric co2 concentration however the consequential impact of this change on oceanic circulation has not been explored here we analyse the outputs of a series of global warming experiments from the csiro mark 3 climate model we show that although for the zonal wind stress change the maximum is located at approximately 60 s in terms of the change in surface wind stress curl the maximum is situated at approximately 48 s this change in the wind stress curl causes a spin up of the entire southern midlatitude ocean circulation including a southward strengthening of the subtropical gyres particularly the east australia current eac the intensified eac generates a warming rate in the tasman sea that is the greatest in the southern hemisphere sh with significant implications for sea level rise the pan southern ocean scale suggests a broad impact on the marine ecosystem of the entire southern midlatitude ocean

cyclicity in the main and upper zones of the bushveld complex south africa crystallization from a zoned magma sheet the major element composition of plagioclase pyroxene olivine and magnetite and whole rock sr sr data are presented for the uppermost 2 1 km of the layered mafic rocks upper main zone and upper zone at bierkraal in the western bushveld complex initial sr sr ratios are near constant 0 7073 0 0001 for 24 samples and imply crystallization from a homogeneous magma sheet without major magma recharge or assimilation the 2125 m thick section investigated in drill core comprises 26 magnetitite and six nelsonite magnetite ilmenite apatite layers and changes up section from gabbronorite an72 plagioclase mg 74 clinopyroxene to magnetite ilmenite apatite fayalite ferrodiorite an43 mg 5 clinopyroxene fo1 olivine the overall fractionation trend is however interrupted by reversals characterized by higher an of plagioclase higher mg of pyroxene and olivine and higher v2o5 of magnetite in the upper half of the succession there is also the intermittent presence of cumulus olivine and apatite these reversals in normal fractionation trends define the bases of at least nine major cycles we have calculated a plausible composition for the magma from which this entire succession formed forward fractional crystallization modeling of this composition predicts an initial increase in total iron near constant sio2 and an increasing density of the residual magma before magnetite crystallizes after magnetite begins to crystallize the residual magma shows a nearconstant total iron an increase in sio2 and decrease in density we explain the observed cyclicity by bottom crystallization initially magma stratification developed during crystallization of the basal gabbronorites once magnetite began to crystallize periodic density inversion led to mixing with the overlying magma layer producing mineralogical breaks between fractionation cycles the magnetitite and nelsonite layers mainly occur within fractionation cycles not at their bases in at least two cases crystallization of thick magnetitite layers may have lowered the density of the basal layer of melt dramatically and triggered the proposed density inversion resulting in close but not perfect coincidence of mineralogical breaks and packages of magnetitite layers

pge tenor and metal ratios within and below the merensky reef bushveld complex implications for its genesis the merensky reef is the consequence of several major influxes of magma into the bushveld magma chamber that gave rise to at least two cyclic units including the pre merensky and merensky cyclic units analysis of the s ni cu platinum group element pge and au contents of 1140 samples from 24 profiles through the rocks forming these units and two profiles extending downward from the reef to close to the ug 2 horizon of the western bushveld complex provide important petrogenetic constraints on the origin of the reef particular attention is paid to variations in the calculated pge tenors and cu pd ratios of sulfides both vertically and laterally all of the profiles show a decrease in pt tenor and concomitant increase in cu pd ratio upward across the pyroxenite of the merensky unit and laterally away from certain centres that are interpreted as feeder zones the data also document that more mafic horizons within the merensky to ug 2 interval of the southwestern bushveld are characterized by much lower cu pd 100 400 than the ambient ratio 4000 this is interpreted as demonstrating that these horizons are the consequence of incursions of magma whose composition has been modified in a staging chamber by reaction with sulfide resident there the favoured interpretation for the merensky reef is that the bushveld chamber s was occupied by a resident magma crystallizing orthopyroxene and plagioclase pre merensky and subsequently merensky magmas entered the chamber s intermittently from localized feeder structures mixing or mingling with the resident magma and giving rise to the more mafic cumulates at specific horizons including the merensky reef the mixing or mingling caused sulfide immiscibility in these cases as the new magma pulses flowed laterally away from the entry zones they deposited sulfide becoming depleted in pge on coming to rest the magma continued to segregate sulfide and crystallize pyroxene so that the sulfides became progressively depleted in pge and acquired a higher cu pd upward through the pyroxenite this model requires that the merensky magmas could dissolve up to two orders of magnitude more pge than current measurements on dry diopside anorthite melts have indicated it is suggested that very pge rich magmas can develop as a result of sulfide unsaturated magma reacting with early formed sulfide in a staging chamber dissolving fes and enriching the remaining sulfide in pge later magmas equilibrating with this enriched sulfide will themselves become highly enriched in pge

a new occurrence of merensky reef on the flanks of the zaaikloof dome northeastern bushveld complex relationship between diapirism and magma replenishment exploration for platinum group elements has resulted in the discovery of an occurrence of merensky reef in the eastern limb of the bushveld complex south africa the new discovery was made on the dwarsrand property in the poorly known boundary area between the intensively explored western and central sectors of the eastern limb this boundary area is largely composed of domes and basins that may in part be aligned along a regional lineament the wonderkop fault transvaal supergroup metasediments in the core parts of the domes form prominent hills whereas the bushveld complex in the intervening basins is restricted to areas of poor outcrop the formation of domes is attributed to diapirism triggered by gravitational loading and heating of the transvaal supergroup in response to intrusion of the bushveld complex merensky reef was located beneath cover rocks in an area where the critical and main zones are uplifted and attenuated against the southern flank of the zaaikloof dome drill holes although collared in upper zone intersected the critical and main zones at relatively shallow depths as a result of the uplift the critical and main zones including the merensky reef are steeply dipping the base of the upper zone forms an intrusive unconformity and the layering in this zone has a much shallower dip economically significant grades were obtained on the dwarsrand property but exploration was terminated because of poor strike continuity the new findings suggest that uplift and attenuation of the bushveld complex on the flanks of domes in the northeastern part of the complex are both more extensive and more severe than previously realized a complex history of intrusion and uplift is implied some domes even penetrate into the overlying bushveld granite the occurrence of markedly angular unconformities may be contrasted with the layercake sequences typically associated with the bushveld complex diapirism in response to crustal loading and heating by ultramafic sequences occurred in the early stages of magma emplacement episodic intrusion and uplift are consistent with multiple replenishment models in which each of the many dozens of cycles that comprise the critical and main zones corresponds to a new magma pulse with the oldest layers being dragged to higher structural levels in response to the uplift periodic reactivation of doming is indicated by the lowermost layers within the critical and main zones abutting against the southern flank of the zaaikloof dome at progressively higher levels the transgressive nature of the upper zone and bushveld granite relative to the critical and main zones may indicate that the upper zone formed by new intrusions of magma rather than the generally accepted model of differentiation within the chamber the discovery of hidden merensky reef at relatively shallow depths beneath transgressive upper zone and granite could provide the impetus for further exploration possibilities within the bushveld complex particularly on the flanks of regionally extensive domal structures

bushveld symplectic and sieve textured chromite is a result of coupled dissolution reprecipitation a comparison with xenocrystic chromite reactions in arc basalt textures of bushveld chromite from thin seams and accessory disseminations in the platreef and the northernmost waterberg project area were compared with textures of xenocrystic chromite from mantle xenoliths found in neogene basalt in the kurile island arc the sieve textured to symplectic rims around the resorbed chromite in the kurile samples resulted from the reaction between chromite and chromite undersaturated basaltic melt with the inclusions in chromite being entrapped during episodes of chromite primary growth chemical dissolution and reprecipitation or secondary growth the relics of the lattice oriented etch tunnels suggest that the dissolution preferentially developed along the crystallographic planes and defects the bushveld chromites exhibiting similar textures are interpreted as reaction textured chromites by analogy with the kurile samples the bushveld sieve textured fish hook to symplectic and amoeboidal to atoll like chromites are believed to have been formed due to coupled dissolution reprecipitation of the earlier cumulus or xenocrystic chromite during interaction with chromite undersaturated evolved melt the electron backscattered diffraction data confirm the same single crystal crystallographic orientation of all domains of the reaction textured chromites as well as their clustered semi dissolved relics therefore bushveld inclusion rich chromite might have captured different populations of melt inclusions during its discontinuous out of equilibrium growth with fast episodic resorption and regeneration the occurrence of reaction textured chromites indicates a zone of interaction between dynamic magmatic influxes where chemical equilibrium was not achieved whereas a complete re equilibration between chromite and the stagnant and sequestered interstitial liquid was attained during the formation of the massive chromitites

closing the time varying mass and heat budgets for large ocean areas the tasman box abstract the role of oceanic advection in seasonal to interannual balances of mass and heat is studied using a 12 yr time series of quarterly eddy resolving expendable bathythermograph xbt surveys around the perimeter of a region the authors call the tasman box in the southwestern pacific the region contains the south pacific s subtropical western boundary current system and associated strong mesoscale variability mean geostrophic transport in the warm upper ocean temperature greater than 12 c is about 3 8 sv 1 sv 106 m3 s 1 southward into the box across the brisbane australia fiji northern edge net outflows are 3 3 sv eastward across the auckland new zealand fiji edge and 2 7 sv southward across sydney australia wellington new zealand mean ekman convergence of 2 2 sv closes the mass budget net water mass conversions in the upper ocean consist of inflow of waters averaging about 26 c and 35 4 psu balanced by outflow at about 18 c and 35 7 psu and reflect the net evaporation and heat los

recent freshening of the east australian current and its eddies the east australian current eac has a relatively weak mean flow and an energetic eddy field that dominates the circulation the properties of the mean flow have been studied in detail but the changes in the eddy field have received little attention we analyze argo temperature and salinity profiles for 2005 2012 to construct a picture of the time mean and time varying properties of eac eddies we find that eddies and the surrounding waters of the western tasman sea are freshening at a rate of 0 017 0 025 practical salinity unit yr over the top 100 m with no significant temperature change consistent with the observations fields from an eddy resolving ocean model show freshening with no temperature trend moreover the model results indicate that observed changes are significant in the context of the variability over the last 20 years and may be part of a multiyear perhaps decadal cycle we attribute the freshening of the region to increased precipitation off eastern australia

intensification and variability of the confluence of subtropical and subantarctic boundary currents east of new zealand the confluence of subtropical and subantarctic boundary currents east of new zealand creates strong fronts the fronts have clear signatures in sea surface height ssh and sea surface temperature sst which make the confluence a good region to investigate the variability of the boundary currents of the south pacific analysis of the 20 year time series of the ssh is used to investigate the location and strength of fronts measured as the gradient in ssh ssh and the eddy kinetic energy eke and their relationship to local and large scale wind forcing the intensity of the ssh and the eke have increased at a rate of 0 02 cm km 1 and 32 cm2 s 2 decade 1 respectively there is a significant correlation r 0 7 p 0 01 between the ssh and eke signals reflecting baroclinic instabilities inherent in the fronts differences between northward and southward wind driven transports across the confluence from the island rule are also increasing at 7 5 sv decade 1 along with an upward trend in the sst differences across the region time series of the southern annular mode sam and southern oscillation index soi and local winds were compared to the frontal strength although the positive trend in the soi coincides with increasing subtropical inflows there is little correspondence of the indices and local winds with short term variability while these results indicate a connection between the intensification of the confluence and south pacific winds there is little change in frontal location north of bollons seamount which suggests bathymetry influences the location of the confluence

the chronology of magma influxes to the eastern compartment of the bushveld complex as exemplified by its marginal border groups the floor contact of the eastern compartment of the bushveld complex has been mapped in detail for 130 km field relations petrographical and geochemical data have enabled 3 important marginal border groups and a possible parent to the lower zone to be demarcated these represent the 4 main magma influxes which formed the malic phase of the bushveld complex and are in chronological order 1 a contaminated magnesian basalt b1 which gave rise to a saucer shaped laminated marginal zone at the base of the complex with diverse related sills 2 a harrisitic peridotite representing quenched lower zone magma 3 a border group to the more evolved probably basaltic critical zone magma which was intruded in two phases an early quenched norite facies n being followed by a chilled bulk critical zone facies b2 which forms a skin to the whole critical zone finally 4 a border group to the main and upper zone b3 which represents an early magnesian pulse of this last immense volume of tholeiitic liquid there is also evidence for minor influxes of magma within the critical zone and main zone

the role of contamination in the evolution of the platreef of the bushveld complex the platreef is a thick greater than 100 m coarse grained feldspathic orthopyroxenite which transgresses northward across metasediments of the transvaal sequence and onto a granitic floor it is overlain by main zone gabbronorites vertical profiles for rb and sr isotope ratios through the platreef and overlying main zone show differences depending upon footwall type against a granitic floor the platreef has initial 87 sr 86 sr ratios ranging from 0 7107 to 0 7226 calculated at 2 05 b y suggesting heterogeneous contamination by a melt derived from the granite overlying a dolomitic footwall the platreef shows lower initial ratios 0 7054 to 0 7147 but high rb contents again suggest addition of a granitic component the extent of contamination is least at the base the role of dolomite assimilation is difficult to quantify the lowermost rocks of the main zone show high rb contents due to contamination but their initial sr isotope ratios have not been changed significantly due to the high proportion of cumulus plagioclase orthopyroxene has a much lower initial ratio than its whole rock indicating that the main cumulus phase formed prior to contamination and that the granitic liquid reacted with the interstitial basic magma to produce the contaminated intercumulus phases the tenor of platinum group elements in the platreef precludes the formation of immiscible sulfide liquid from an interstitial liquid rather it must be an early formed phase hence the separation of sulfide liquid may predate the main siliceous contamination process

the merensky cyclic unit and its impact on footwall cumulates below normal and regional pothole reef types in the western bushveld complex the merensky reef of the bushveld complex occurs in its highest stratigraphic position as a heterogeneous pegmatitic feldspathic melanorite bounded by two narrow chromitite stringers at the base of the merensky cyclic unit mcu in the swartklip facies of the rustenburg layered suite the occurrence of widespread thermal and mechanical erosion termed potholing has led to the subdivision of the merensky reef into normal reef and regional pothole reef sub facies the transition between the two sub facies occurs where the mcu transgresses the lower chromitite stringer of the normal merensky reef and cuts down into the underlying cumulate lithologies in the regional pothole reef at the northam platinum mine several economic reef types are identified where the merensky reef becomes conformable to cumulate layering in particular to the footwall marker np2 reef type and the upper pseudoreef p2 reef type the normal merensky reef as well as the p2 and np2 reefs contains economic platinum group element pge grades and includes the lower portion of the mcu melanorite and the merensky chromitite whole rock geochemistry indicates that this package is compositionally identical in normal p2 and np2 reefs suggesting that the base of the mcu is a relatively homogeneous drape over both normal and regional pothole reef regions however the lower sections of the three reefs are variables depending on the depth of transgression of the mcu in the normal and p2 reef types transgression by the mcu was arrested within harzburgites melanorites and norites resulting in coarse pegmatitic textures in the immediate footwall units for the np2 reef transgression by the mcu was arrested within leucocratic rocks and resulted in the formation of troctolites below the merensky chromitite these troctolites are characterised by a coupled relationship between olivine and sulphides and by changes in major element chemistry and pge contents relative to equivalent units in the footwall of the normal reef along with micro textural relationships these features suggest that troctolization of leucocratic cumulates in the np2 reef beneath the merensky chromitite was a result of a reactive infiltration of a chromite saturated melt and an immiscible sulphide liquid from the overlying mcu rather than a significant fluid flux from below in all reef types the concentration of s defines symmetrical peaks centred on the merensky chromitite and chromitites from pre existing cyclic units in normal and p2 reefs whereas pge concentrations define asymmetrical peaks with higher pge contents in reconstituted footwall rocks relative to the mcu melanorite this signature is attributable to a magmatic model of pge collection followed by deposition towards the base of the mcu and within reconstituted footwall rocks the continuity of the asymmetrical magmatic pge signature between the normal reef and regional pothole reef sub facies indicates that pge mineralization inherent to the merensky magma occurred as a drape over a variably eroded and subsequent texturally and geochemically reworked or reconstituted footwall

decadal spinup of the south pacific subtropical gyre abstract an increase in the circulation of the south pacific ocean subtropical gyre extending from the sea surface to middepth is observed over 12 years datasets used to quantify the decadal gyre spinup include satellite altimetric height the world ocean circulation experiment woce hydrographic and float survey of the south pacific a repeated hydrographic transect along 170 w and profiling float data from the global argo array the signal in sea surface height is a 12 cm increase between 1993 and 2004 on large spatial scale centered at about 40 s 170 w the subsurface datasets show that this signal is predominantly due to density variations in the water column that is to deepening of isopycnal surfaces extending to depths of at least 1800 m the maximum increase in dynamic height is collocated with the deep center of the subtropical gyre and the signal represents an increase in the total counterclockwise geostrophic circulation of the gyre by at least 20 at 1000 m a comparison of woce and

mesoscale structure of the mean east australian current system and its relationship with topography abstract we document the mean circulation of the east australian current eac system which is the major western boundary current in the southwest pacific ocean a new high resolution climatology of the region csiro atlas of regional seas cars is used to produce regional fields of steric height and sections of geostrophic velocity the realism of these fields has been enhanced by the high spatial resolution allowance for bathymetric control and the influence of land barriers the increased detail in the maps and sections reveal the major influence that the complex regional topography has on the circulation the results also provide the appropriate resolution for comparison with recent model studies of the region we have been able to both resolve well known features with greater detail and identify previously unrecognized aspects of the circulation important results include 1 the zonal inflow north of 30 s breaks into a series of individual jets after flowing through the island chains as suggested by recent model results 2 the interaction of the zonal inflow with the local topography as it undergoes bifurcation is documented 3 a detailed picture of the evolution of the eac at the western boundary is presented including surface currents and transports 4 separation and recirculation of the eac within the tasman abyssal basin are shown to occur as a double cell structure constrained by bathymetry 5 the eac outflow is resolved as a series of eastward and northeastward currents 6 the path of the tasman front is defined as it interacts with the tasman ridge systems 7 a quasi permanent eddy is identified for the first time adjacent to the norfolk ridge northwest of new zealand

the geological setting of the platinoid and base metal sulfide mineralization in the platreef of the bushveld complex in drenthe north of potgietersrus on the farm drenthe the platreef consists of a complex assemblage of norites melanorites pyroxenites serpentinites and xenoliths of dolomite it is a zone of up to 250 m thick between the basal contact of the complex and the overlying main zone norites the composition of the orthopyroxene shows a rapid increase from en 60 to en 70 in the lower 30 m of the sequence and a sharp drop from en 70 to en 60 at the top of the platreef this compositional break at the upper contact is due to a change from primary bronzite in the platreef to inverted pigeonitc in the main zone high temperature metamorphic assemblages are preserved in the larger xenoliths whereas their outer margins and the smaller xenoliths contain retrograde metamorphic mineral assemblages the igneous rocks display irregularly distributed zones of alteration to bronzite bastitites and serpentinites the alteration process was probably dominated by degassing of the xenoliths the distribution and composition of the major sulfide minerals indicate a change from a sulfur rich to a metal rich environment during the crystallization of the platreef pyrite monoclinic pyrrhotite pentlandite and chalcopyrite dominate the lower part of the platreef and hexagonal pyrrhotite pentlandite and chalcopyrite or cubanite prevail in the upper portion whole rock analyses show an upward decrease in the ni cu ratio from 1 95 to 1 70 large variations occur in the distribution and grade of ni cu and platinum group elements although highest values are associated with a zone of serpentinization the ni and cu values are unreliable indicators in predicting pt and pd concentrations in zones of major metal enrichment ni and cu correlate strongly while pt and pd have more significant correlations with ni than cu although the minerals of the platinum group elements are primarily associated with chalcopyrite the pt pd ratio decreases upward in the sequence the mineralization is considered to have been caused by the breakup of dolomite xenoliths that added co 2 h sub 2 o and s to the magma thus simultaneously lowering the solubility of sulfur in the magma and adding sulfur from an external source which resulted in the precipitation of an immiscible sulfide liquid

interaction of floor rocks with the platreef on overysel potgietersrus northern transvaal the platreef on the basal contact of the rustenburg layered suite has been studied on the farm overysel 30 km north of potgietersrus the major element trace element and sr isotope geochemistry of the platreef feldspathic pyroxenite and the adjacent floor rocks is presented the immediate floor rocks consist of a suite of highly metamorphosed banded tonalitic gneisses with leucotonalitic veins to the south the floor rocks are metamorphosed sediments of the transvaal sequence these were intruded by granites which show signs of metamorphism close to the layered suite one facies of the granite yielded an age of 2 504 or 0 109 b y with an initial 87 sr 86 sr ratio of 0 7035 or 6 the cumulate textured pyroxenites of the platreef are geochemically different from other units of the rustenburg layered suite in having a high and variable initial 87 sr 86 sr ratio ranging from 0 7104 to 0 7227 with an average of 0 7160 calculated at 2 05 b y whereas overlying gabbro has a ratio of 0 7079 other ratios such as rb sr and ba zr are anomalously high in the platreef ranging from 0 05 to 0 26 and 16 to 44 respectively these data and the presence of relatively sodic plagioclase biotite and quartz suggest substantial assimilation of siliceous material the sedimentary transvaal sequence forming the footwall to the platreef to the south of overysel and consisting of dolomites and banded iron formation because of their low 87 sr 86 sr ratios and low k 2 o rb and sio 2 cannot produce the observed anomalies the banded tonalitic gneisses and associated veins are an unusual suite of rocks pyroxene geothermometry indicates that they have been metamorphosed to or 900 degrees c rb sr ratios range from 0 02 to 0 21 and 0 to 1 8 percent k 2 o 87 sr 86 sr ratios at 2 05 b y range from 0 7024 to 0 7258 total assimilation of these rocks cannot have produced the anomalous features of the platreef however their low k 2 o contents and high 87 sr 86 sr ratio in relation to their present low rb sr suggest that they have undergone a partial melting or loss of fluid component as a result of contact metamorphism however there are isotopic grounds for rejecting this as the sole source of contamination the granite close to the platreef has a high rb sr ratio initial 87 sr 86 sr ratios of 0 73 to 0 79 at 2 05 b y ago and 3 9 percent k 2 o total assimilation of these rocks could produce the trace element characteristics of the platreef but only if over 60 percent assimilation has occurred major element considerations preclude this a partial melt or fluid phase generated from these granites could have been the contaminant a composition calculated to produce the observed trace element and isotopic features of the platreef with less than 25 percent contamination has 150 ppm sr 87 sr 86 sr of 0 8 9 000 ppm ba 600 ppm rb and 10 ppm zr

mean circulation and variability of the tropical atlantic during 1952 2001 in the gecco assimilation fields abstract the shallow subtropical tropical cells stc of the atlantic ocean have been studied from the output fields of a 50 yr run of the german partner of the estimating the circulation and climate of the ocean gecco consortium assimilation model comparison of gecco with time mean observational estimates of density and meridional currents at 10 s and 10 n which represent the boundaries between the tropics and subtropics in gecco shows good agreement in transports of major currents the variability of the gecco wind stress in the interior at 10 s and 10 n remains consistent with the ncep forcing although temporary changes can be large on pentadal and longer time scales an stc loop response is found between the poleward ekman divergence and stc layer convergence at 10 s and 10 n via the equatorial undercurrent euc at 23 w where the divergence leads the euc and the convergence suggesting a pulling mechanism via equatorial upwelling the divergence is also associated with changes in the easter

non sequential injection of pge rich ultramafic sills in the platreef unit at akanani northern limb of the bushveld complex evidence from sr and nd isotopic systematics

the genesis of sulfide mineralization in a portion of the potgietersrus limb of the bushveld complex in the potgietersrus limb of the bushveld complex south africa sulfide mineralization is associated with the intrusive contact between bushveld pyroxenitic gabbros and floor rocks consisting of malmani dolomite the overlying penge banded ironstone and sedimentary rocks of the pretoria series whole rock values of up to 0 5 and 1 8 weight percent copper and nickel respectively were obtained in borehole intersections through the contact zone and values of up to 28 ppm platinum group metals have been reported from the area petrographic and isotope geochemical studies were undertaken on a representative suite of rocks it was shown that the pyroxenitic gabbro in contact with the floor rocks was equivalent in composition to the merensky reef xenoliths of malmani dolomite incorporated into the bushveld rocks reacted with the magma and were metamorphosed to calc silicate and carbonate silicate hornfelses increased sulfide concentrations are found in association with the xenoliths these rocks together with the bushveld pyroxenitic gabbros and recrystallized argillaceous sediments of the pretoria series and the pre bushveld noritic sills constitute a 120 m thick mineralized zone known as the platreef the results of carbon and oxygen isotope determinations on unaltered malmani dolomite are consistent with values expected for an evaporitic environment of deposition during the precambrian anhydrite normally associated with evaporitic sequences is likely to have been preserved in the malmani dolomite when it was incorporated as xenoliths into the bushveld magma sulfur isotope studies on sulfides from the platreef zone give a range of delta 34 s values of from 6 8 to 9 2 per mil as the delta 34 s values for primary magmatic bushveld sulfides fall in the range 0 6 to 3 5 per mil the isotope geochemistry clearly indicates a contribution of sedimentary derived sulfur to the original bushveld magma sulfide mineralization in the platreef is considered therefore to have been derived from the combined influence of a sulfur rich merensky reef type magma and the incorporation of additional sulfur from the malmani dolomite either through direct diffusion or in association with a carbonate melt between the banded ironstone and the platreef zone there is a 50 m thick zone of coarsegrained iron and sulfide rich pegmatoidal pyroxenite pyrrhotite from these rocks gave delta 34 s values of 2 7 and 8 7 per mil which are within the possible range of magmatic sulfur these rocks may be related to the intrusive pipes found elsewhere in the bushveld complex

the bushveld complex south africa formation of platinum palladium chrome and vanadium rich layers via hydrodynamic sorting of a mobilized cumulate slurry in a large relatively slowly cooling subsiding magma chamber platinum group element pge deposits in the bushveld complex and other layered intrusions form when large incompletely solidified magma chambers undergo central subsidence in response to crustal loading resulting in slumping of semi consolidated cumulate slurries to the centres of the intrusions and hydrodynamic unmixing of the slurries to form dense layers enriched in sulfides oxides olivine and pyroxene and less dense layers enriched in plagioclase the most economic pge cr and v reefs form in large multiple replenished intrusions because these cool relatively slowly and their central portions subside prior to termination of magmatism and complete cumulate solidification the depth of emplacement has to be relatively shallow as otherwise ductile crust would not be able to flex and collapse in smaller intrusions cooling rates are faster subsidence is less pronounced and where it occurs the cumulate may be largely solidified resulting in insignificant mush mobility and mineral sorting layering is thus less pronounced and less regular and continuous and the grades of the reefs are lower but the reefs can be relatively thicker an additional factor controlling the pge cr and v prospectivity of intrusions is their location within cratons intra cratonic environments offer more stable emplacement conditions that are more amenable to the formation of large layered igneous bodies furthermore intrusions sited within cratons are more readily preserved because cratons are underlain by thick buoyant keels of harzburgite that prevent plate tectonic recycling and destruction of crust

the ug2 merensky reef interval of the bushveld complex northwest of pretoria northwest of pretoria the ug2 merensky reef interval overlies a critical zone lower zone sequence that contains numerous large blocks of floor material nevertheless individual layers can be correlated with equivalent units at crocodile river mine the rustenburg impala union and amandelbult sections concentrations of platinum group elements in two borehole intersections of the ug2 chromitite are 4 ppm over 1 2 m and 2 4 ppm over 2 2 m therefore bulk pge levels appear to be only moderately lower than those at western platinum mine this renders models explaining pge enrichment by upward percolating melt or fluids problematic the merensky reef although containing sulphides is only weakly mineralized with pge 0 6 ppm the ug2 pyroxenite is separated from the ug2 chromitite by a 15 m noritic layer the introduction of feldspathic cumulates between two units that elsewhere directly overly each other may be explained by the more evolved composition of resident magma in those parts of the chamber distally located with regard to a major feeder zone at union section it also suggests that the ug2 unit is a multiple rather than a single cyclic unit

litho and chemostratigraphy of the flatreef pge deposit northern bushveld complex the flatreef is a world class platinum group element pge deposit recently discovered down dip from existing mining and exploration operations on the northern limb of the bushveld complex current indicated resources stand at 42 moz pge 346 mt with 3 8 g t pt pd rh au 0 32 ni and 0 16 cu which in the case of pt is equivalent to 10 years global annual production making it one of the largest pge deposits on earth the grade and thickness of the flatreef mineralised interval is highly unusual with some drill core intersections containing up to 4 5 g t pt pd rh au over 90 m in drill core here we document the down dip and along strike litho and chemostratigraphy of the flatreef and its footwall and hanging wall rocks based on a diamond drill core database totalling 720 km at the base of the sequence intersected in the drill cores are up to 700 m thick sills of ultramafic rocks dunite harzburgite pyroxenite emplaced into pelitic dolomitic and locally quartzitic and evaporitic rocks belonging to the duitschland formation of the transvaal supergroup next is an approximately 100 200 m sequence of low grade sulphide mineralised layered mafic ultramafic rocks containing abundant sedimentary xenoliths and in places several chromite seams or stringers this is overlain by a 100 m thick sequence of well mineralised mafic ultramafic rocks the flatreef sensu strictu overlain by a laterally persistent mottled anorthosite layer at the base of 1 km of homogenous main zone gabbronorite based on stratigraphic lithological and compositional analogies to the layered rocks in the eastern and western bushveld complex we correlate the flatreef and its chromite bearing footwall rocks with the upper critical zone notably the interval between the ug2 chromitite and the bastard reef as found elsewhere in the bushveld complex this includes recognition of a merensky reef correlative the ultramafic rocks below the main chromitite seam ug2 correlative in the flatreef footwall are correlated with the lower critical and lower zones however compared to the western and eastern bushveld limbs the studied sequence is strongly enriched in sulphide and pge many of the rocks show elevated cao k2o rb and zr contents and lateral continuity of layers between drill cores is less pronounced than elsewhere in the bushveld whereas ultramafic units are locally considerably thickened these compositional and lithological traits are interpreted to result from a range of processes which include contamination with calcsilicate and hornfels intrusion of granitic magmas and the influence of multiple structural events such as pre to syn emplacement regional scale open folding and growth faults evidence for the existence of potholes also exists in the shallow up dip portions of the project area the entire magmatic sequence below the main zone becomes increasingly contaminated to the extent that individual units are somewhat more difficult to correlate between drill cores this package represents the platreef as exposed in outcrop and shallow bore holes across much of the northern limb of the bushveld complex the new data presented here thus indicate that the platreef is a relatively more contaminated up dip extension of parts of the critical and lower zones

revisiting the circulation of the east australian current its path separation and eddy field abstract the traditional view of the east australian current eac as depicted in many schematics is of a continuous boundary current that flows along the shelf off eastern australia between approximately 18 s and about 32 5 s where it separates from the coast and continues either towards new zealand along the tasman front or towards tasmania as the eac extension additionally it is widely recognised that eddies are prevalent in the eac region particularly south of the eac separation we revisit this long standing paradigm and suggest that the eac is perhaps better viewed as a continuous meandering stream flowing adjacent to the coast that feeds a field of mesoscale eddies observations show that eac eddies are prevalent over a broad region of the western tasman sea as far north as 25 s typically with maximum intensity between 30 and 35 s at any instant in time the eac jet is usually evident as a poleward stream adjacent to the continental shelf edge other commonly accepted components of the eac system including the tasman front and eac extension are rarely seen as distinct identifiable features rather these features are evident only in time mean fields when the eddy variability is filtered out it is also common for the eac to be spatially discontinuous due to the presence of eddies often with multiple short streams that sometimes separate and re attach to the coast recognition of the eac as an eddy dominated current system has seen many recent studies focus on various aspects of eddies in the eac system providing new insights into mesoscale ocean dynamics recent studies of individual eddies have shown that the circulation within eddies including tilting and vertical motion is more complex than previously understood a summary of these studies along with a review of the eac system particularly its path separation and eddy field is presented here

zonation of merensky style platinum group element mineralization in turfspruit thick reef facies northern limb of the bushveld complex the stratigraphy whole rock composition and mineralogy from a thick undisturbed section of the platreef at depth on turfspruit in the northern limb of the bushveld complex have been examined in detail this section has much more in common with the critical zone than with the platreef structure updip although the high grade platinum group element pge reefs located close to the boundary with the main zone are much thicker than any known merensky reef facies a key feature that marks the conversion of the platreefstyle mineralization into the merensky style is the regular appearance of chromite seams which become more persistent with decreasing contamination westward and downdip the similar zonal distribution of platinum group minerals pgm in two consecutive thick pge reefs correlates with changes in silicate compositions and vertical zonation of pge concentrations in whole rocks and base metal sulfides this supports orthomagmatic in situ crystallization of ore minerals during mixing of consecutive magmatic influxes the typical merensky style ore assemblages of predominant pt sulfides and alloys are developed within 1 m thick intervals which correspond to the highest temperature mixing zones between two magmas whereas the rest of the reefs host pt pd bismuthotellurides and arsenides pge content and tenor as well as cu pd in major base metal sulfides in the turfspruit reefs are within the range of the typical high grade merensky values suggesting the same enrichment process