crustal structure of the northern nova scotia rifted continental margin eastern canada 1 the nova scotia continental margin off eastern canada marks a transition from a volcanic to a nonvolcanic style of rifting the northern nonvolcanic segment of the margin was studied by a 490 km long refraction seismic line with dense air gun shots coincident with previous deep reflection profiles a p wave velocity model was developed from forward and inverse modeling of the wide angle data from 19 ocean bottom seismometers and coincident normal incidence reflection profiles the continental crust has a maximum thickness of 36 km and is divided into three layers with velocities of 5 7 6 9 km s crustal thinning down to 3 km occurs in a 180 km wide zone and the sediment cover in this area is up to 15 km thick farther seaward a 150 km wide transition zone is observed with a 5 km thick lower layer 7 2 7 6 km s interpreted as partially serpentinized mantle at the landward end this layer is overlain by highly altered continental crust 5 4 km s extending up to the seaward limit of the jurassic salt province farther seaward the upper layer is interpreted as exhumed and highly serpentinized mantle 5 1 km s separated from the lower layer by subhorizontal reflectivity which probably represents a serpentinization front oceanic crustal thickness is 4 km with layer 2 velocities of 4 6 5 0 km s layer 3 velocities of 6 4 6 55 km s are lower than typical lower oceanic crust velocities but consistent with a low magma supply and increased tectonism as observed on the reflection profile this reduced magma production might be related to the proximity of the newfoundland transform margin

density of basalt core from hilo drill hole hawaii abstract density measurements of 1600 samples of core from 889 to 3097 m depth below sea level in the hawaii scientific drilling program hole near hilo hawaii show marked differences between the basaltic rock types and help define stratigraphy in the hole water saturated densities of subaerial lava flows occurring above 1079 m depth have the broadest range because of the large density variation within a single lava flow water saturated densities commonly range from 2 0 to 3 0 with an average of 2 55 0 24 g cc dikes and sills range from 2 8 to 3 1 g cc densities of hyaloclastite commonly range from 2 3 to 2 7 with an overall average of about 2 5 g cc the low density of most hyaloclastite is due primarily to palagonitization of abundant glass and presence of secondary minerals in the interstices between fragments four principal zones of pillow lava separated by hyaloclastite occur in the drill core the shallowest 1983 2136 m is paradoxically the densest averaging 3 01 0 10 g cc the second 2234 2470 m is decidedly the lightest averaging 2 67 0 13 g cc the third 2640 2790 m and fourth 2918 bottom at 3097 m are high averaging 2 89 0 17 and 2 97 0 08 g cc respectively the first pillow zone includes degassed pillows i e lava erupted on land that flowed into the sea these pillows are poor in vesicles because the subaerial one atmosphere vesicles were compressed when the flow descended to deeper water and higher pressure the second low density non degassed pillow zone is the most vesicle rich apparently because it was erupted subaqueously at a shallow depth the higher densities of the third and fourth zones result from a low vesicularity of only a few percent and an olivine content averaging more than 5 for the third zone and about 10 for the fourth zone the uppermost hyaloclastite extending about 400 m below the bottom of the subaerial basalt is poorly cemented and absorbs up to 6 wt of water when immersed progressing downward the hyaloclastite absorbs less water and becomes better cemented this change is apparently due to palagonitization of glass and addition of secondary minerals in the deeper older hyaloclastite a process favored by the increase of temperature with depth the cementation is largely complete at 1800 m depth where the temperature attains about 20 c the zone of freshest uncemented hyaloclastite represents the weakest rock in the drill hole and is a likely level for tectonic or landslide disruption

geophysical studies of the structure of the appalachian orogen in the atlantic borderlands of canada results of 6000 km of crustal seismic profiling are presented with gravity and aeromagnetic maps for the appalachian orogen in eastern canada wide angle seismic surveys show that the central mobile belt of the orogen has a thinner crust than its margins high velocity lower crust attributed to underplating is found below the former laurentian continental margin in newfoundland and below the magdalen basin potential field data are used to trace the surface zones of the orogen from the northeast newfoundland shelf to cape breton but extrapolation to new brunswick and quebec is unclear because of late paleozoic basin development the central mobile belt of the orogen is only a few tens of kilometres wide in southwest newfoundland and cape breton but broadens substantially to around 200 km elsewhere reflection images show a strong deep crustal fabric that runs along the orogen with a margin that crosses into the avalon zone in southern newfoundland but coincides with the avalon gander zone boundary el

global marine gravity from retracked geosat and ers 1 altimetry ridge segmentation versus spreading rate 1 three approaches are used to reduce the error in the satellite derived marine gravity anomalies first we have retracked the raw waveforms from the ers 1 and geosat gm missions resulting in improvements in range precision of 40 and 27 respectively second we have used the recently published egm2008 global gravity model as a reference field to provide a seamless gravity transition from land to ocean third we have used a biharmonic spline interpolation method to construct residual vertical deflection grids comparisons between shipboard gravity and the global gravity grid show errors ranging from 2 0 mgal in the gulf of mexico to 4 0 mgal in areas with rugged seafloor topography the largest errors of up to 20 mgal occur on the crests of narrow large seamounts the global spreading ridges are well resolved and show variations in ridge axis morphology and segmentation with spreading rate for rates less than about 60 mm a the typical ridge segment is 50 80 km long while it increases dramatically at higher rates 100 1000 km this transition spreading rate of 60 mm a also marks the transition from axial valley to axial high we speculate that a single mechanism controls both transitions candidates include both lithospheric and asthenospheric processes

evidence for winter spring denitrification of the stratosphere in the nitrate record of antarctic firn cores a firn core from within the polar vortex in the weddell sea sector of antarctica shows nitrate peaks that occur in the spring or early summer they are far less prominent in two other cores from sites that are further north although other data are needed circumstantial evidence suggests that the peak may be due to sedimentation of polar stratospheric clouds from the stratosphere during winter and spring no change in the form of the peak is seen over the last three decades it may be possible to observe past stratospheric conditions over longer time scales from ice cores but other sources may obliterate the signal in ice from the glacial periods 34 refs 5 figs

the structure evolution and symmetry of the magma poor rifted margins of the north and central atlantic a synthesis abstract magma poor rifted margins consistently show extreme crustal thinning accompanied by normal faulting the serpentinization of the mantle beneath crust thinned to less than 8 2 km and the unroofing of a broad zone of mantle within the continent ocean transition accompanied by the development of detachment and other large offset faults these observations are the logical result of the progressive extension of cool lithosphere away from thermal anomalies such as plumes although the paucity of magmatism may be explained by depth dependent extension of the lithosphere and by pre depleted sub crustal lithosphere rifting above initially cool sub lithospheric mantle also may explain the subsidence deficit observed at some margins synrift subsidence is buffered by the simultaneous influx of warmer oceanic asthenosphere as it extends thins and cools ductile creeping layers in the mid and deep crust become progressively more brittle resulting in increased coupling between the upper and lower crust and eventually the embrittlement of the entire crust faults cutting from the surface across the moho bringing water into the mantle and causing its serpentinization increased coupling and the development of serpentine detachments predict the development of late stage asymmetry once the entire crust is brittle such detachments are imaged on some margins and inferred on others analysis of the crustal structure across conjugate margins shows that these are approximately symmetric until this late stage when they become markedly asymmetric similar analyses show that depth dependent stretching of the crust is insufficient to explain the discrepancy between the amount of the visible extension along faults and the amount of crustal thinning instead this extension discrepancy may be related to the complex evolution of brittle deformation through multiple phases and styles of faulting related to the changes in the rheological character and strength of the lithosphere as it is thinned complex polyphase faulting continues after complete crustal separation resulting in the exhumation of broad expanses of peridotitic basement the top of which is everywhere marked by an exhumed slip surface similar to the corrugated surface observed at mid ocean ridges the similarity in processes between mantle unroofing and seafloor spreading makes the distinction between the cot and true oceanic crust difficult and possibly moot

from continental extension to seafloor spreading crustal structure of the goban spur rifted margin southwest of the uk we present a 169 km wide angle velocity model across the goban spur rifted margin southwest of the uk a 120 km wide intermediate region is identified between the first clear seafloor spreading magnetic anomaly anomaly 34r and thinned continental crust where velocities increase from 4 5 km s1 to 6 8 km s1 in the top 4 km beneath acoustic basement at depth it can be divided into a region where a 1 5 km thick high velocity layer 7 2 7 6 km s1 exists and a region where this layer is absent but velocities of 7 km s1 are present wide angle pmp arrivals are observed across the whole of the intermediate region but a normal incidence reflection moho is not present based on these velocities and combined with seismic reflection gravity and magnetic modelling along this transect we interpret the intermediate region to consist of a 70 km wide zone of exhumed mantle with high poisson s ratio at top basement 0 34 an extremely low topographic expression and a high velocity deep layer empirical relationships between velocity and degree of serpentinization suggest that serpentinite content decreases with depth from 100 per cent at top basement to 25 per cent from 5 7 km into basement the observed magnetic anomaly is best fit by a thin magnetized layer 1 km of magnetization 2 3 a m1 the intense magnetization may be due to magnetite formation during a prolonged interaction of serpentinite with sea water between this exhumed mantle and anomaly 34r is a 50 km wide region raised 400 m above the adjacent exhumed mantle consisting of a series of basement ridges poisson s ratio is not well defined in this region which may be composed of anomalous oceanic crust or exhumed mantle

site information and initial results from deep ice drilling on law dome antarctica the aim of deep ice drilling on law dome antarctica has been to exploit the special characteristics of law dome summit i e low temperature and high accumulation near an ice divide to obtain a high resolution ice core for climatic environmental studies of the holocene and the last glacial maximum lgm drilling was completed in february 1993 when basal ice containing small fragments of rock was reached at a depth of 1196 m accurate ice dating obtained by counting annual layers revealed by fine detail 18 o peroxide and electrical conductivity measurements is continuous down to 399 m corresponding to a date of ad 1304 sulphate concentration measurements made around depths where conductivity tracing indicates volcanic fallout allow confirmation of the dating for agung in 1963 and tambora in 1815 or estimates of the eruption date from the ice dating for the kuwae vanuatu eruption 1457 the lower part of the core is dated by extrapolating the layer counting using a simple model of the ice flow at the lgm ice fabric measurements show a large decrease 250 to 14 mm 2 in crystal size and a narrow maximum in c axis verticality the main zone of strong single pole fabrics however is located higher up in a broad zone around 900 m oxygen isotope 18 o measurements show holocene ice down to 1113 m the lgm at 1133 m and warm 18 o about the same as holocene ice near the base of the ice sheet the lgm holocene 18 o shift of 7 0 only 1 larger than for vostok indicates that law dome remained an independent ice cap and was not overridden by the inland ice sheet in the glacial

crustal structure and magmatism of north atlantic continental margins enormous variations exist along rifted margins of the north atlantic in the magmatic activity that accompanied continental breakup the north atlantic margins exhibit a full range of magmatic behaviour one end of the spectrum commonly termed volcanic margins is found in the northern north atlantic off greenland norway and northwest britain where huge volumes of igneous rock were added to the continental crust as it rifted and the adjacent oceanic crust is considerably thicker than normal the other end of the spectrum so called non volcanic margins occurs further south off france and spain where only minor volcanism accompanied rifting and the oceanic crust immediately adjacent to the continental margin is thinner than normal oceanic crust results from seismic profiles are used to determine the volume and distribution of igneous crust accumulated as the continents broke up and seafloor spreading commenced the volume of melt produced during rifting is controlled mainly by the temperature of the underlying asthenospheric mantle and by the amount and rate of decompression as it rises beneath stretching and thinning lithosphere large quantities of igneous rock were produced as the continents broke up to form the north atlantic where the newly initiated iceland plume brought abnormally hot mantle beneath the rift much smaller quantities of melt were produced further south where mantle temperatures were normal during rifting the distribution of igneous rocks across the continental margin and the adjacent oceanic crust is controlled by the rate at which melt is produced and by the ease with which it can intrude laterally in the crust where the rate of stretching is very slow and the rifting is spread over many millions or even tens of millions of years as is common during the initial stages of rifting then the rising asthenospheric mantle has time to cool by conduction heat is lost from the asthenospheric mantle both vertically into the overlying water layer and laterally into the colder wedges of continental lithosphere on either side of the rifted region as a consequence of the reduced mantle temperatures considerably less partial melting occurs in the slowly rising asthenospheric mantle than in mantle which rises rapidly beneath rifts this explains the abnormally thin oceanic crust found adjacent to the non volcanic rifted continental margins in the north atlantic as the new ocean widens the spreading centre becomes isolated from the continental margins on either side by newly formed warm oceanic lithosphere and the rate of rifting generally increases the oceanic crust then attains its equilibrium thickness as conductive heat loss from the rising asthenospheric mantle reduces and all the melt formed by decompression of the mantle solidifies close to the spreading axis basaltic melts can intrude continental crust far more easily than oceanic crust basaltic dykes may extend many hundreds of kilometres away from the magma source beneath continental rifts this redistributes melt laterally towards the continent in both volcanic and to a lesser extent in non volcanic margins and may be a contributory factor in leaving unusually thin crust in the first formed ocean immediately adjacent to some rifted margins it also allows melt formed under volcanic rifts to migrate large distances laterally as is seen in the rockall area northwest of britain

mechanisms of low flux intraplate volcanic fields basin and range north america and northwest pacific ocean we compare two intraplate pliocene pleistocene volcanic fields in different tectonic settings the central basin and range and the northwest pacific ocean both fields are characterized by widely scattered small volume alkali basaltic volcanoes within the fields each volcano apparently originates from a separate volatile enriched parental melt from the upper mantle there is no evidence at either field for locally anomalous heat flow or ongoing introduction of new fluids into the upper mantle such as might occur above a subducting slab we conclude that the volcanic fields reflect deformation driven collection of already existing partial melts in a heterogeneous upper mantle deformation driven melt collection may be an important mechanism for other diffuse intraplate volcanic fields and this is consistent with a tectonically controlled low flux end member for intraplate fields where magmatism is a passive response to regional deformation differences in the degree of fractionation and contamination between the two fields are inferred to be related to flexure induced vertical variations in the orientation of principal stresses in the northwest pacific ocean which cause stalling of ascending dikes in the lithosphere

three dimensional gravity and magnetic modelling of the irish sector of the ne atlantic margin abstract a new 3d lithospheric model has been constructed using high resolution gravity data from the irish national seabed survey the sedimentary component of the model incorporated density variations due to laterally varying overcompaction associated with cenozoic denudation after optimisation based on gravity inversion regional crustal thickness variations were defined which are in reasonable agreement with the results of wide angle seismic experiments high crustal extension factors 5 characterise the deeper parts of the rockall and porcupine basins and in places the model indicates extreme stretching 10 beneath these basins this could be because of instability in the gravity inversion although other recent investigations have independently suggested similarly high extension factors in contrast the hatton basin is characterised by an apparent extension factor of about 2 the modelling resolves a pattern of ne to nne trending local mesozoic basins on the margins of the rockall trough helping to delineate structures that were previously only sparsely sampled by seismic surveys it appears possible that rifts with similar trends underlie the volcanic rocks which obscure the deeper parts of the hatton basin the linear trends of the basins to the south and east of ireland are interpreted to have been inherited from a basement fabric that was initially established during the late precambrian assembly of this basement and subsequently subjected to caledonian and variscan reactivation magnetic modelling indicates that the variations in the thickness of the crystalline crust predicted by the gravity models can explain the regional magnetic anomaly patterns over the rockall and porcupine basins but that significant additional magnetic material probably igneous rocks of both palaeogene and cretaceous ages is required to explain the anomalies in the hatton basin region the magnetic signature of the rockall basin is distinctly different to that over the basement of similar apparent thickness formed during mid cretaceous c34n opening of the ocean basin to the south this is an impediment to hypotheses that invoke mid cretaceous sea floor spreading rather than intracontinental rifting to explain the development of the basin the exception is in the extreme south of the basin where the volcanism associated with the barra volcanic ridges combined with indications of relatively strong lithosphere could be evidence of incipient ocean opening

new evidence for the origin of the porcupine median volcanic ridge early cretaceous volcanism in the porcupine basin atlantic margin of ireland 1 two dimensional pre stack depth migrated seismic reflection data gravity and velocity models are used to assess the nature and origin of a prominent buried ridge the porcupine median volcanic ridge pmvr within the porcupine basin offshore ireland the debate on the origin of the pmvr during the past 30 years has followed the evolution of the concept of continental margin genesis in this paper the origin of the ridge is evaluated on the basis of the internal geometry and velocity structure revealed by the seismic data implication of the presence of these type of ridges in hyper extensional rifted margins is discussed and compared with other margins the analysis indicates that the ridge is an extrusive volcanic ridge probably tholeiitic in composition constructed by stacked hyaloclastite deltas and topped by carbonate platforms the results invalidate previously proposed models involving highly rotated fault blocks and the serpentinite mud volcanism the extension magnitude analysis suggests a highly stretched setting where limited mantle serpentinization may have occurred but the architecture and velocity of the pmvr demonstrates that it is made of lower velocity materials than serpentinite during the opening of the north atlantic the pmvr represents the northern time equivalent magmatic event expressed along the newfoundland iberia galicia recorded by the j anomaly that originate from cretaceous volcanic deposits

crustal structure of the central nova scotia margin off eastern canada summary the central nova scotia margin off eastern canada is located at a transition from a volcanic margin in the south to a non volcanic margin in the north in order to study this transition a wide angle refraction seismic line with dense airgun shots was acquired across the central nova scotia margin the 500 km long transect is coincident with previous deep reflection profiles across the lahave platform and extending into the sohm abyssal plain a p wave velocity model was developed from forward and inverse modelling of the wide angle data from 21 ocean bottom seismometers and coincident normal incidence reflection profiles the velocity model shows that the continental crust is divided into three layers with velocities of 5 5 6 9 km s 1 the maximum thickness is 36 km a minor amount 5 km of thinning occurs beneath the outer shelf while the major thinning to a thickness of 8 km occurs over the slope region the seaward limit of the continental crust consists of 5 km thick highly faulted basement there is no evidence for magmatic underplating beneath the continental crust on the contrary a 4 km thick layer of partially serpentinized mantle 7 6 7 95 km s 1 begins beneath the highly faulted continental crust and extends 200 km seawards forming the lower part of the oceancontinent transition zone the upper part of the transition zone consists of the highly faulted continental crust and 4 to 5 km thick initial oceanic crust the continent ocean boundary is moved 50 km farther seawards compared to an earlier interpretation based only on reflection seismic data the oceanic crust in the transition zone consists of layer 2 and a high velocity lower crustal layer layer 2 is 1 3 km thick with velocities of 5 6 6 0 km s 1 the high velocity lower crustal layer is 1 2 km thick with velocities of 7 25 7 4 km s 1 suggesting a composite layer of serpentinized peridotite and gabbroic layer 3 oceanic crust with normal thickness of 5 7 km and more typical layer 3 with velocities of 6 95 7 3 km s 1 is observed at the seaward end of the profile

calibrating the ice core paleothermometer using seasonality high resolution oxygen isotope measurements on the dome summit south dss ice core from law dome antarctica provide a seasonal profile that is sufficiently stable and undistorted by biases in accumulation to permit comparison with measured temperature seasonality this comparison yields an isotope temperature relation with a gradient d dt of 0 44 0 02 c and provides a new method for exploring the isotope temperature relationship at high accumulation sites if applied to the observed isotope record from the dss core which extends through the last glacial and beyond this calibration suggests that at its coldest the last glaciation was 13 c colder than present at this site after correcting for elevation change this finding compares with a temperature difference of 8 c computed by using the local spatially derived calibration

beryllium 10 transport to antarctica results from seasonally resolved observations and modeling 1 cosmogenic 10be measured in polar ice cores has important application in the reconstruction of past solar activity however the processes controlling its atmospheric transport and deposition to the ice sheets are not fully understood here we use the seasonal changes in 10be concentrations in a 10 year monthly resolved ice core record from the law dome site east antarctica in conjunction with echam5 ham general circulation model gcm simulations of 10be and 7be deposition as tools to examine this problem maximum 10be concentrations are observed in the ice core during the austral late summer to early autumn summer autumn while minimum concentrations are observed during the austral winter the gcm simulations corroborated by earlier observations of 10be 7be ratios in antarctica from the georg von neumayer air sampling station suggest that the 10be concentration maximum is linked to direct input of stratospheric 10be from the antarctic stratosphere to the lower levels of the antarctic troposphere during the austral summer autumn this result contrasts with the modeled transport of 10be to greenland where the seasonal maximum in stratospheric input is seen in the late winter to spring synchronous with the timing of the seasonal maximum in midlatitude stratosphere to troposphere exchange our results suggest that a different combination of processes is responsible for the transport of 10be to the antarctic and greenland ice sheets

high time resolution profiles of soluble ions in the last glacial period of a dome fuji antarctica deep ice core we measured the depth profiles of soluble ions in the dome fuji antarctica ice core to search for possible paleoclimate indications of seasonal climate variations in the last glacial period a 523 mm long core section between 587 65 and 588 18 m depth was selected for this pilot study and the high resolution chemical analysis was done on 2 mm thick samples our results indicate that anion cation trapping in ice affects the profiles of the soluble ions and na in the core may preserve its seasonal signal correlation coefficients and the equivalent balances of the soluble ions suggest the following selective coexistences 1 cl na 2 no3 1 2 ca2 k and 3 so4 2 1 2 ca2 h mg2 these coexistences are probably due to 1 a sea salt source of na and cl 2 reaction of no3 with dust for ca2 and no3 and 3 anion cation trapping in ice for so4 2 and ca2 mg2 respectively

results from a detailed aeromagnetic survey across the northeast newfoundland margin part i spreading anomalies and relationship between magnetic anomalies and the ocean continent boundary abstract a detailed aeromagnetic survey carried out across the northeast newfoundland margin clearly shows the presence of sea floor spreading anomalies 25 to 34 correlation of these anomalies with synthetic profiles shows an increase in the rate of spreading soon after anomaly 27 time three fracture zones can be identified by dislocations in the magnetic anomalies their positions are confirmed on the depth to basement map of this region an eastward extension of the southernmost fracture zone at latitude 49 n matches well with the faraday fracture zone across the mid atlantic ridge and with a basement ridge known as pastouret ridge mapped off goban spur by combining the present survey data with the previously collected shipborne measurements we have also traced the westward continuation of the charlie gibbs fracture zone under the newfoundland shelf a large amplitude magnetic anomaly lies along the margin and separates two zones with different magnetic characteristics long wavelength small amplitude anomalies on the landward side and quasi lineated anomalies on the seaward side seismic data compilations show that this large anomaly coincides with the ocean continent boundary at most places north of flemish cap modelling of the magnetic anomalies indicate that the large amplitude anomaly is caused by the juxtaposition of highly magnetized oceanic crust against weakly magnetized continental crust this situation is similar to that observed across the goban spur margin which is a conjugate of the flemish cap margin the presence of highly magnetized oceanic crust landward of anomaly 34 and within the cretaceous magnetic quiet zone is attested to by the presence of similar large amplitude anomalies south of the flemish cap and goban spur regions but these do not mark the ocean continent transition

rift related magmatism on magma poor margins structural and potential field analyses of the mesozoic notre dame bay intrusions newfoundland canada and their link to north atlantic opening abstract the magma poor newfoundland margin formed following lithospheric stretching rifting and breakup resulting in the separation of the grand banks from iberia and northern newfoundland from ireland mesozoic igneous rocks contemporaneous with rifting have been documented on and offshore newfoundland with later offshore magmatism occurring post breakup the notre dame bay magmatic province ndbmp is a jurassic cretaceous potentially rift related occurrence of igneous rocks onshore newfoundland that includes the gabbroic budgell harbour stock bhs and marginal lamprophyre dykes in addition to the nearby potentially related dildo pond intrusion dpi and leading tickles stock lts although the bhs is penetrated by wells it is poorly exposed at surface and the deeper structure remained unknown here results of field based mapping combined with inversion of full tensor gradiometry ftg and aeromagnetic data covering the bhs are presented although the ftg data are of a higher resolution than the magnetic data comparable results were obtained from these independent datasets we found that the dykes form clusters at the ends of density and susceptibility anomalies interpreted to be lobe like magmatic conduits furthermore potential field analysis has revealed the irregular geometry and southward dipping nature of the bhs in addition to the presence of multiple near surface anomalies that may correspond to dyke clusters or other igneous bodies structural analysis indicates that emplacement of the dykes was controlled by pre existing geological structures and that they have probably been deformed post intrusion potentially via the reactivation of pre rift faults the potential field investigations and observations of dyke margin lineations from field based structural analysis indicate that multiple dyke sources and bodies similar to the bhs may also exist finally this study demonstrates that magma poor margins such as the newfoundland margin may host significant volumes of extension related magmatic rocks challenging the concept of a distinction between magma rich and magma poor rifting

features of meteorological events preserved in a high resolution law dome east antarctica snow pit snow pitandshallow firn corerecordsofoxygenisotoperatios d 18 o and traceionspeciesweregeneratedatahigh accumulationsiteonlaw dome eastantarctica concordancebetweenaccumulationeventsidentifiedin recordsupto 7 7kmapartconfirms that the observed glaciochemical variations are the result of regional rather than local sur face effects this allows calibration of the snow pit records with measured meteorological parameters netaccumulationperiodsthatcomprisethesnow pitrecordareidentifiedusing hourly snow accumulation measurements from a co located automatic weather station aws particular focus isgiventothreenetaccumulationperiods preservedduringaustral summer 1999 2000 that correspond to the top 0 5m of the snow pit local meteorological conditions recorded during the summer accumulation periods by the aws are combined with regional and synoptic scale meteorology derived from casey station 110km away and advancedvery high resolution radiometer satellite imagery to identify potential source regions forchemical signals preserved in summer snowat law dome

results from a detailed aeromagnetic survey across the northeast newfoundland margin part ii early opening of the north atlantic between the british isles and newfoundland abstract poles of rotation for the north atlantic have been derived from the results of a new aeromagnetic survey northeast of newfoundland reconstruction of the north atlantic at anomaly 34 time shows a band of large amplitude magnetic anomalies which parallels anomaly 34 on both sides of the atlantic from flemish cap and goban spur to the azores gibraltar fracture zone a group of similar anomalies has also been identified in the bay of biscay north of goban spur and flemish cap these anomalies follow the ocean continent boundary poles of rotation derived for this anomaly show that it forms an isochron 100 110 m y during the long cretaceous normal polarity interval the cause of this anomaly is not definite but it may represent an increase in the magnetization of the crust during a limited time within the cretaceous magnetic quiet zone by a process such as replacement of thermoremanent magnetization by chemical remanent magnetization as proposed by raymond and labrecque the north atlantic has also been reconstructed at the time of the initial opening in the region between flemish cap and the charlie gibbs fracture zone using inferred ocean continent boundaries on the west and east sides it has been shown that the entire region could not have saparated at one time but that spreading between the british isles and newfoundland had to progress from south to north consequently when active sea floor spreading was taking place between goban spur and flemish cap about 110 m y the region to the north was still being stretched the calculated amount of stretching as derived from the reconstructions about 25 agrees well with the extension of the lithosphere obtained from modelling the subsidence history of this region and with the results of deep seismic studies active spreading in the north started about 100 m y ago

structure across the northeastern margin of flemish cap offshore newfoundland from erable multichannel seismic reflection profiles evidence for a transtensional rifting environment we present the results from processing and interpreting nine multichannel seismic reflection lines collected during the 1992 erable experiment over the northeastern margin of flemish cap offshore newfoundland these lines combined into five cross sections provide increased seismic coverage over this lightly probed section of the margin and reveal tectonically significant along strike variations in the degree and compartmentalization of crustal thinning similar to the southeastern margins of flemish cap and the grand banks a transitional zone of exhumed serpentinized mantle is interpreted between thinned continental and oceanic crust the 25 km wide transitional zone bears similarities to the 120 km wide transitional zone interpreted as exhumed serpentinized mantle on the conjugate irish atlantic margin but the significant width difference is suggestive of an asymmetric conjugate pair a 40 50 km wide zone of inferred strike slip shearing is interpreted and observed to extend along most of the northeastern margin of flemish cap individual shear zones szs may represent extensions of szs and normal faults within the orphan basin providing further evidence for the rotation and displacement of flemish cap out of orphan basin the asymmetry between the flemish cap and irish conjugate pairs is likely due in large part to the rotation and displacement of flemish cap which resulted in the flemish cap margin displaying features of both a strike slip margin and an extensional margin

snow chemistry across antarctica un updated compilation of published and new data of major ion ca cl k mg na no3 so4 and methylsulfonate ms concentrations in snow from 520 antarctic sites is provided by the national itase international trans antarctic scientific expedition programmes of australia brazil china germany italy japan korea new zealand norway united kingdom the united states and the national antarctic programme of finland the comparison shows that snow chemistry concentrations vary by up to four orders of magnitude across antarctica and exhibit distinct geographical patterns the antarctic wide comparison of glaciochemical records provides a unique opportunity to improve our understanding of the fundamental factors that ultimately control the chemistry of snow or ice samples this paper aims to initiate data compilation and administration in order to provide a framework for facilitation of antarctic wide snow chemistry discussions across all itase nations and other contributing groups the data are made available through the itase web page http www2 umaine edu itase content syngroups snowchem html and will be updated with new data as they are provided in addition recommendations for future research efforts are summarized

high precision dating of volcanic events a d 1301 1995 using ice cores from law dome antarctica a record of volcanic activity over the period a d 1301 1995 has been extracted from three law dome ice cores east antarctica the record dating is unambiguous at the annual level from a d 1807 to 1995 and has an uncertainty of 1 year at a d 1301 signals from 20 eruptions are preserved in the record including those of two unknown eruptions with acid deposition beginning in a d 1810 8 and a d 1685 8 the beginning of the ice core signal from the a d 1815 tambora eruption is observed in the austral summer of a d 1816 1817 the mean observed stratospheric transport and deposition time to law dome from the eruption site is 1 5 years 0 6 years from 11 well dated eruptions the largest eruption observed in the law dome record has its maximum in a d 1460 with volcanic sulfate deposition beginning in the austral winter of a d 1459 this event is also observed in other ice core records and is attributed to the volcano kuwae with an eruption date in the range a d 1455 9 1459 9 if all sources of error are considered this is at least three years later than the date previously ascribed by dendrochronological and historical studies

crustal structure beneath orphan basin and implications for nonvolcanic continental rifting a wide angle seismic profile was acquired across the northeast newfoundland margin seaward to orphan knoll the profile is complemented by existing multichannel reflection data two online deep wells and other deep seismic reflection and refraction lines modeling of data from 15 ocean bottom seismometers provides the seismic velocity structure for a significant portion of the 400 km wide stretched continental crust on this margin prerift metasedimentary rocks have a topographic relief of up to 5 km with a velocity of 5 2 km s increasing to 5 9 km s unaltered crystalline crust below the metasedimentary section has a velocity range of 6 1 6 5 km s most of the lower crust landward of orphan knoll has a typical continental lower crustal velocity 6 8 7 0 km s and the lower crust shallows significantly beneath orphan knoll our model shows no evidence for the 5 km thick 7 35 km s layer modeled from data of an earlier experiment and interpreted as magmatic underplating but instead indicates that thinned continental crust extends seaward for 360 km without underplating implying nonvolcanic rifting for the ne newfoundland margin we suggest that continental stretching persisted from 180 ma to chron 34n time 101 ma when the final breakup between canada and western europe took place leaving a 400 km wide zone of thinned continental crust underneath the shelf and deep water area of the orphan basin on the landward side of the basin gravity modeling indicates a zone of very thin 6 8 km crust possibly a failed rift center formed as a result of the northward progression of nonvolcanic rifting between canada and europe a possible sub moho reflector suggests generally uniform stretching of the crust and upper mantle and the absence of volcanics and underplating is in contrast to the observations on related rift basins of the grand banks

lead ba and bi in antarctic law dome ice corresponding to the 1815 ad tambora eruption an assessment of emission sources using pb isotopes lead ba and bi concentrations and pb isotopic compositions have been measured in antarctic law dome 66 8 s 112 4 e ice dated from 1814 ad to 1819 ad by thermal ionisation mass spectrometry to investigate the possible deposition of heavy metals from the 1815 ad eruption of tambora volcano 8 5 s 117 4 e in indonesia although volcanic s emissions from tambora observed as so42 are present in the antarctic ice core record there are grounds to question the origin of the pb and bi also deposited at law dome from late 1817 as the pb isotope data suggest this pb originated from mount erebus 77 5 s 167 2 e on ross island antarctica it is shown that at least 97 of any pb and bi emitted from tambora was removed from the atmosphere within the 1 6 year period required to transport aerosols from indonesia to antarctica consequently increased pb and bi concentrations observed in law dome ice about 1818 ad are attributed to either increased heavy metal emissions from mount erebus or increased fluxes of heavy metals to the antarctic ice sheet resulting from climate and meteorological modifications following the tambora eruption elevated ba concentrations observed from mid 1816 to mid 1818 indicate increased atmospheric loading of rock and soil dust also occurred at the time

covariation of sea ice and methanesulphonic acid in wilhelm ii land east antarctica sea ice plays an important role in ocean atmosphere heat exchange global albedo and the marine ecosystem knowledge of variation in sea ice extent is essential in order to understand past climates and to model possible future climate scenarios this paper presents results from a short firn core spanning 15 years collected from near mount brown wilhelm ii land east antarctica variations of methanesulphonic acid msa at mount brown were positively correlated with sea ice extent from the coastal region surrounding mount brown 60 1208 e and from around the entire antarctic coast 0 3608 e previous results from law dome identified this msa sea ice relationship and proposed it as an antarctic sea ice proxy curran and others 2003 with the strongest results found for the local law dome region our data provide supporting evidence for the law dome proxy at another site in east antarctica but a deeper mount brown ice core is required to confirm the sea ice decline suggested by curran and others 2003 results also indicate that this deeper record may also provide a more circum antarctic sea ice proxy

evolution of nonvolcanic rifted margins new results from the conjugate margins of the labrador sea the crust across each margin of the labrador sea a conjugate rift margin pair consists of three well defined zones thinned continental crust 70 80 km wide transitional crust and oceanic crust the transition zone is characterized by a low velocity upper crust 4 5 km s underlain by a 6 4 7 7 km s layer we propose that the lower layer is serpentinized upper mantle it is less likely to be gabbroic igneous lower crust the low velocity upper crust may be either an oceanic basaltic layer or very thin 2 km continental crust results from this first combined seismic reflection and refraction study of a conjugate margin pair support very slow initial continental rifting 1 mm yr on average resulting in an asymmetric crustal thinning profile with breakup occurring against the west greenland shelf this asymmetry may be an important feature of rifting at nonvolcanic margins and may be related to strain hardening of the cooling and thinning lithosphere or to small scale convection in the lower lithosphere and asthenosphere at the sides of the rift the thin crust in the transition zone implies that the amount of melt generated in the upwelling mantle is negligible consistent with slow spreading or rifting the transition from this slow formational period to normal oceanic accretion processes is marked by a sharp decrease in basement depth signaling an increase to normal oceanic crustal thickness

spatial and seasonal variations of the snow chemistry at the central filchner ronne ice shelf antarctica the chemical stratigraphy of the surface firn of the central filchner ronne ice shelf was determined in conjunction with stable isotopes from shallow firn cores and snow pit samples collected at 1 1 widely distributed sites and covering a time period of at least 20 years the chemical analysis included ecm profiling and the determination of chloride non sea salt nss sulphate methanesulphonate msa nitrate and partly sodium and bromide throughout the investigated area winter time nss sulphate levels are found to be substantially negative indicating that the sulphate to sodium ratio in airborne sea salt particles is depleted by a factor of 5 approximately in relation to the bulk sea water ratio while winter firn layers appear to be marked by episodic events of large sea salt inputs pronounced annual cycles with maxima in summer firn layers are commonly observed for the ecm signal and for nss sulphate nitrate and msa at all sites for msa however this phase relation is almost reversed for depths greater than 3 4m the mean impurity levels consistently are strongly depleted with increasing distance from the ice edge by about 30 100 km for sea salt 25 100 km for msa and only 10 100 km for nss sulphate however no substantial trend is observed for nitrate it is concluded therefore that the sea salt and the biogenic sulphur compounds deposited on the filchner ronne ice shelf mainly originate from the adjacent weddell sea further important implications of the continental effects are a an atmospheric residence time of nss sulphate apparently exceeding that of msa probably due to the supplementary sulphate production on the ice shelf from biogenic so 2 and b a substantial limitation of the potential of deep ice cores already drilled on the filchner ronne ice shelf in extracting reliable net temporal changes of sea salt and biogenic sulphur species

kinematic evolution of the southern north atlantic implications for the formation of hyperextended rift systems we focus on the southern north atlantic rifted margins to investigate the partitioning and propagation of deformation in hyperextended rift systems using plate kinematic modeling the kinematic evolution of this area is well determined by oceanic magnetic anomalies after the cretaceous normal polarity superchron however the rift and early seafloor spreading evolution 200 83 ma remains highly disputed due to contentious interpretations of the j magnetic anomaly on the iberia newfoundland conjugate margins recent studies highlight that the j anomaly is probably polygenic related to polyphased magmatic events and therefore does not correspond to an isochron we present a new palinspastic restoration without using the j magnetic anomaly as the chron m0 we combine 3 d gravity inversion results with local structural stratigraphic and geochronological constraints on the rift deformation history the restoration of the southern north atlantic itself is not the primary aim of the study but rather is used as a method to investigate the spatiotemporal evolution of hyperextended rift systems we include continental microblocks that enable the partitioning of the deformation between different rift segments which is of particular importance for the evolution of the iberia eurasia plate boundary our modeling highlights the following 1 the segmentation of the iberia newfoundland rift system during continental crust thinning 2 the northward v shape propagation of mantle exhumation and seafloor spreading 3 the complex partitioning of deformation along the iberia eurasia plate boundary and 4 a three plate propagation model which implies transtension

ice core evidence for antarctic sea ice decline since the 1950s the instrumental record of antarctic sea ice in recent decades does not reveal a clear signature of warming despite observational evidence from coastal antarctica here we report a significant correlation p

a volcanic province near the western termination of the charlie gibbs fracture zone at the rifted margin offshore northeast newfoundland a mid cretaceous to late cretaceous volcanic province named here the charlie gibbs volcanic province is described near the western termination of the charlie gibbs fracture zone against the rifted continental margin northeast of newfoundland we used seismic data to map 14 volcanic seamounts now buried below younger sediments they rise 0 7 to 2 s two way time twt above the surrounding basement level and are about 8 30 km wide some are conical while others are more flat topped underlying igneous units resembling flows and sills are also observed based on magnetic modeling of the large positive magnetic anomalies associated with the seamounts the total thickness of igneous rocks can locally reach about 8 km this magmatism occurred in the vicinity of the charlie gibbs fracture zone and extends about 150 km to the north along the rifted continental margin the volcanic province also forms the northern boundary of the jurassic early cretaceous orphan basin along a major transform margin there truncation of rift related structures which extend to deep crustal levels is observed at the transform along trends similar to those of prerift appalachian terrane boundaries on the adjacent shelf this suggests the existence of a preexisting weak zone in the continental lithosphere within which a complex strike slip fault system developed and may have controlled the location of final continental breakup between the rockall and north american plates in the late cretaceous

magmatic breakup as an explanation for magnetic anomalies at magma poor rifted margins during continental breakup the onset of seafloor spreading is thought to be marked by the first occurrence of a magnetic anomaly analysis of seismic and magnetic data from the iberia newfoundland continental rift system suggests that the first magnetic anomaly observed here instead represents a magmatic event that pre dates seafloor spreading

the peridotite ridge province in the southern iberia abyssal plain seismic constraints revisited the west iberia margin has become the type example of a magma starved margin in which extension during breakup led to the exhumation of mantle rocks apparently over a broad area much of our understanding of this process is based on geophysical and drilling data from the southern iberia abyssal plain where the seaward part of the region of exhumed mantle is comprised of a series of margin parallel basement ridges here we analyse wide angle and normal incidence seismic data from a series of intersecting profiles across this peridotite ridge province and develop a three dimensional picture of its structure using these seismic data we infer a location and age magnetic anomaly m1 125 127 ma of first formed oceanic crust in this region that are consistent with ocean drilling program data and data from the conjugate margin we infer that the peridotite ridge province is about 70 km wide with strong serpentinization 75 at the top of basement reducing to much lower degrees 25 around 2 km below we map the geometry of some prominent reflectors on the landward margin of the peridotite ridge province that likely represent detachment faults and show that one of these has a domal structure similar to that of detachment faults at slow to ultra slow spreading mid ocean ridges

major cretaceous volcanic province in southern rockall trough abstract a group of three large curvilinear ridges called the barra volcanic ridge system has been mapped in the acoustic basement of southern rockall trough typically each ridge is about 2 km high and 20 km wide at its base a crudely layered acoustic character moderate density but high strength of magnetisation point to a volcanic sedimentary ash composition for the ridges seismic continuity with the acoustic basement of the rest of rockall trough suggests that the trough basement is of similar composition an age for the ridges of lower cretaceous is indicated by well ties and consideration of regional geology volumetrically the ridges are on the scale of hot spot features such as the wyville thomson ridge

a reduced crustal magnetization zone near the first observed active hydrothermal vent field on the southwest indian ridge 1 inversion of near bottom magnetic data reveals a well defined low crustal magnetization zone lmz near a local topographic high 37 47 s 49 39 e on the ultraslow spreading southwest indian ridge swir the magnetic data were collected by the autonomous underwater vehicle abe on board r v dayangyihao in february march 2007 the first active hydrothermal vent field observed on the swir is located in area a within and adjacent to the lmz at the local topographic high implying that this lmz may be the result of hydrothermal alteration of magnetic minerals the maximum reduction in crustal magnetization is 3 a m the spatial extent of the lmz is estimated to be at least 6 7 104 m2 which is larger than that of the lmzs at the tag vent field on the mid atlantic ridge mar as well as the relict field bastille dante grotto and new field vent sites on the juan de fuca ridge jdf the calculated magnetic moment i e the product of the spatial extent and amplitude of crustal magnetization reduction is at least 3 107 am2 for the lmz on the swir while that for the tag field on the mar is 8 107 am2 and that for the four individual vent fields on the jdf range from 5 107 to 3 107 am2 together these results indicate that crustal demagnetization is a common feature of basalt hosted hydrothermal vent fields at mid ocean ridges of all spreading rates furthermore the crustal demagnetization of the area a on the ultraslow spreading swir is comparable in strength to that of the tag area on the slow spreading mar

a seasonal deuterium excess signal at law dome coastal eastern antarctica a southern ocean signature the snow isotopic composition 18o and d of two shallow cores from the high accumulation summit region of law dome east antarctica was measured at monthly resolution over the 1980 1992 period while the 18o or d signals clearly reflect the local temperature cycle the deuterium excess d d 8 18o is shifted with respect to 18o cycle by a 4 months lag interpretation of this phase lag is investigated using both an atmospheric general circulation model agcm which includes the water isotopic cycles and a simple isotopic model which better describes the microphysical processes within the cloud using this dual approach we show that the seasonality of 18o and d at law dome summit results from a combination of the southern ocean temperature cycle shifted by 2 3 months with respect to the local insolation and seasonal moisture origin changes due to a strong contribution of the local ocean when ice free both approaches are consistent with a dominant temperate to subtropical moisture origin we thus demonstrate from our present day subseasonal study that the record of d in the dome summit south dss deep ice core represents a potential tool for identifying changes in southern ocean temperatures and or sea ice cover at the scale of the past thousand years

characterization of sills associated with the u reflection on the newfoundland margin evidence for widespread early post rift magmatism on a magma poor rifted margin author posting the authors 2010 this article is posted here by permission of john wiley sons for personal use not for redistribution the definitive version was published in geophysical journal international 182 2010 113 136 doi 10 1111 j 1365 246x 2010 04635 x

models of the development of the west iberia rifted continental margin at 40 30 n deduced from surface and deep tow magnetic anomalies the ocean continent transition oct on nonvolcanic rifted continental margins is important in that it contains evidence concerning the breakup of the continents and the onset of seafloor spreading the nature of the oct off western lberia has recently been attracting attention following seismic and other geophysical studies there and drilling of acoustic basement by leg 149 of the ocean drilling program here we concentrate on the interpretation of a new digital magnetic anomaly chart for the iberia abyssal plain between 39 5 o and 42 2 o n the most striking anomaly is a trough which exists immediately east of and locally parallel to anomaly j and is closely coincident with a basement peridotite ridge we conclude from modeling surface magnetic anomalies and a deep tow magnetometer profile that seafloor spreading began about 129 9 m y ago barremian at a rate of 10 0 mm yr consistent with drilling and seismostratigraphic results from the southern lberia abyssal plain a second feature is the existence of linear low amplitude isochron parallel magnetic anomalies east of the peridotite ridge trough and a clear change in the trend of these anomalies east of about 11 o 15 w it is possible to model these anomalies by a series of 7 to 30 km wide weakly magnetized crustal blocks of about the same thickness as the oceanic crust the west to east anticlockwise change in trend at 11 o 15 w suggests either a major change in stress regime and or in the type of magnetic anomaly source we speculate that the region between the peridotite ridge and 11 o 15 w represents a transitional crust whose formation was dominated by the same stress regime which accompanied eventual seafloor spreading we explain the change in trend at 11 o 15 w by a model featuring a rift which propagated through continental crust from south to north we consider three scenarios for the formation of the transitional zone crust first continental crust is surrounded and impregnated by intrusive and extrusive material formed from passive upwelling of magma created by the limited adiabatic decompression partial melting which occurs as the two continental lithospheric plates first begin to separate second such crust is surrounded by unroofed upper mantle material or third the zone is the product of ultraslow seafloor spreading present evidence tends to support the first scenario in this scenario the top of the asthenosphere eventually becomes shallow enough to enable magma to penetrate the thinnest continental crust fill the space caused by additional extension and form the transitional crust already described over a broad zone at least several tens of kilometers wide at breakup upper mantle rocks become exposed on the seafloor and undergo extensive serpentinization in a final stage we envisage focusing of the igneous activity leading to the onset of seafloor spreading initially producing thin oceanic crust because of a restricted melt supply the width of continuous tectonically extended continental crust in the southern iberia abyssal plain is less than was previously supposed

seismic volcanostratigraphy of large volume basaltic extrusive complexes on rifted margins large volume extrusive basaltic constructions have distinct morphologies and seismic properties depending on the eruption and emplacement environments the presence and amount of water is of main importance while local rift basin configuration erosion and resedimentation determine the overall geometry of the volcanic constructions we have developed the concept of seismic volcanostratigraphy a subset of seismic stratigraphy to analyze volcanic deposits imaged on seismic reflection data the method places special focus on identification and mapping of seismic facies units and the volcanological interpretation of these units interpretation of seismic reflection data along the atlantic and western australia rifted margins reveals six characteristic volcanic seismic facies units named 1 landward flows 2 lava delta 3 inner flows 4 inner seaward dipping reflectors inner sdr 5 outer high and 6 outer sdr these units are interpreted in terms of a five stage tectonomagmatic volcanic margin evolution model comprising 1 explosive volcanism in a wet sediment broad basin setting 2 subaerial effusive volcanism forming gilbert type lava deltas along paleoshorelines 3 subaerial effusive volcanism infilling a fairly narrow rift basin 4 shallow marine explosive volcanism as the injection axis is submerged below sea level and finally 5 deep marine sheet flow or pillow basalt volcanism further erosion and resedimentation processes are particularly important during the shallow marine stages seismic volcanostratigraphy provides important constraints on rifted margin development in particular on the prevolcanic basin configuration relative timing of tectonomagmatic events total amount of volcanic rocks location of paleoshorelines and margin subsidence history these parameters give key boundary conditions for understanding the processes forming volcanic margins and other large volume basaltic provinces

crustal structure of the flemish cap continental margin eastern canada an analysis of a seismic refraction profile summary the crustal structure of the ne flemish cap margin was determined along a 460 km long refraction wide angle reflection seismic transect flame line to define the thickness structure and composition of the crust and uppermost mantle along the line a p wave velocity model was developed from forward and inverse modelling of dense airgun shots recorded by 19 ocean bottom seismometers a coincident multichannel seismic profile was used to guide the modelling as reflections could be identified down to moho the model displays a sediment cover of up to 3 6 km thick subdivided into three layers with velocities of 1 8 1 9 km s 1 2 8 3 1 km s 1 and 4 7 4 8 km s 1 for the western part of the flame line over flemish cap the p wave velocity model displays an up to 32 km thick three layer continental crust the continental crust has velocities of 5 8 6 1 km s 1 6 3 6 45 km s 1 and 6 65 6 85 km s 1 and thicknesses of about 5 km 7 km and 20 km in the upper middle and lower layers respectively the thick continental crust thins to a two layer 6 km thick crust upper layer is 5 55 6 0 km s 1 and the layer below is 6 65 6 8 km s 1 over a distance of 45 km s wave velocities are determined in the upper layer of the thick continental crust over flemish cap and the transition zone by assigning poisson s ratios in the p wave velocity model comparison of calculated to observed arrival times gives a poisson s ratio of 0 27 in the upper layer and 0 28 in the layer below which suggests that the composition of the crust is primarily continental in both the thick crust and the thin crust of the transition zone the thin continental crust is stretched over a width of 80 km and is underlain by a layer with velocities of 7 5 7 9 km s 1 we interpret this layer as partially serpentinized mantle which is consistent with observationsfromthenewfoundlandmargintothesouth theserpentinizedmantleterminates 30 km seaward of the thick continental crust at the seaward most end of the thin continental crust a prominent ridge feature is observed the seismic refraction and multichannel seismic data results indicate a mixed character between serpentinized mantle with volcanic extrusions or continental crust the reflection seismic data show a high relief basement from the ridge feature and seaward the flame line crosses magnetic anomaly 34 and extends another 50 km seaward well into oceanic crust the ridge is flanked seaward by a two layer oceanic crust the upper layer layer 2 has velocities of 4 8 5 0 km s 1 for the landward most 35 km of oceanic crust and 4 8 6 2 km s 1 for the seaward 60 km the average thickness of layer 2 is 2 km the lower layer layer 3 has velocities of 6 7 7 2 km s 1 and a thickness of 3 5 km the velocity model is consistent with a sharp onset of seafloor spreading seaward of the ridge feature

crustal structure of atlantic fracture zones ii the vema fracture zone and transverse ridge summary the crustal structure beneath the vema fracture zone and its flanking transverse ridge was determined from seismic refraction profiles along the fracture zone valley and across the ridge relatively normal oceanic crust but with an upwarped seismic moho was found under the transverse ridge we suggest that the transverse ridge represents a portion of tectonically uplifted crust without a major root or zone of serpentinite diapirism beneath it a region of anomalous crust associated with the fracture zone itself extends about 20 km to either side of the central fault gradually decreasing in thickness as the fracture zone is approached there is evidence to suggest that the thinnest crust is found beneath the edges of the 20 km wide fracture zone valley under the fracture zone valley the crust is generally thinner than normal oceanic crust and is also highly anomalous in its velocity structure seismic layer 3 is absent and the seismic velocities are lower than normal the absence of layer 3 indicates that normal magmatic accretionary processes are considerably modified in the vicinity of the transform fault the low velocities are probably caused by the accumulation of rubble and talus and by the extensive faulting and fracturing associated with the transform fault this same fracturing allows water to penetrate through the crust and the apparently somewhat thicker crust beneath the central part of the fracture zone valley may be explained by the resultant serpentinization having depressed the seismic moho below its original depth

the southern margin of the rockall plateau stratigraphy tertiary volcanism and plate tectonic evolution the geological evolution of the southwest rockall plateau in the area of edoras bank has been clarified using seismic reflection gravity and magnetic data four principal reflectors are observed within the tertiary sedimentary sequence i late miocene ii latest early miocene iii late eocene iv earliest eocene a period of pronounced sediment drift accumulation marks the interval i ii reflector iii the top of a sediment wedge prograding southward from edoras bank marks a change from terrigenous to pelagic sedimentation that is probably related to subsidence of the rockall plateau following the separation of greenland from eurasia in the earliest eocene reflector iv marks the top of a wedge shaped seismically transparent layer that also thins southward away from edoras bank on the basis of its seismic attributes and magnetic signature this layer is interpreted as a volcanic sheet formed as part of the north atlantic tertiary volcanic province during rifting of greenland from eurasia the recognition of voluminous volcanic rocks south of edoras bank extends the known area of the tertiary volcanic province several hundred kilometres to the south gravity anomaly modelling and continental reconstructions suggest that the region south of edoras bank is underlain by thinned continental crust a four stage geological evolution for this region is indicated i initial rifting associated with the separation of labrador from greenland in the late cretaceous is characterized by enhanced crustal thinning and subsidence in the region of a rift triple junction ii passive subsidence and accumulation of late cretaceous and earliest tertiary sediments followed the initiation of seafloor spreading in the labrador sea iii blanketing of the area by palaeocene volcanic rocks masked pre existing magnetic lineations providing an explanation for some of the problems in earlier interpretations based mainly on magnetic data iv post volcanic sedimentation continuing to the present day

determining continuous basins across conjugate margins the east orphan porcupine and galicia interior basins of the southern north atlantic ocean abstract the east orphan porcupine and galicia interior basins are several of the most promising basins for hydrocarbon exploration along the rifted continental margins of the southern north atlantic ocean despite having formed at similar geological times the basins exhibit fundamental differences in symmetry crustal thickness sedimentary cover thickness and amount of extension interpretation of seismic reflection and well data was integrated with published 3d grids of depth to basement and moho proxy depth to interpret and restore select seismic reflection lines publicly available kinematic evolution models were also integrated to evaluate and compare the restored seismic reflection lines in a more global regional context interpretation of five seismo stratigraphic units and three tectonostratigraphic megasequences along the east orphan porcupine and galicia interior basins reveals similar seismic character for each unit and comparable tectonic history the structural restoration of the selected lines indicates that evolution sedimentary cover thickness faulting style crustal structure and kinematic evolution of the east orphan and porcupine basins differ significantly a variable and asymmetrical crustal structure is found in the east orphan basin contrasting with the elongated and symmetric porcupine and galicia interior basins rift domain maps of the three basins reveal that they are each underlain by hyperextended crust with possible exhumed mantle in the centre of the porcupine basin based on a holistic analysis of the results obtained the linkage between the east orphan and the porcupine basins seems implausible but rather a contemporaneous relationship is interpreted moreover a potential connection between the galicia interior basin and the porcupine basin during the early to late jurassic is proposed such scenarios imply an oblique and synchronous rifting evolution around the bay of biscay triple junction of the southern north atlantic

seasonal variability in the input of lead barium and indium to law dome antarctica lead pb isotopic compositions and concentrations and barium ba and indium in concentrations have been determined at monthly resolution in five law dome coastal eastern antarctica ice core sections dated from 1757 ad to 1898 ad natural background pb concentrations in 1757 ad average 0 2 pg g 1 and can be attributed to mineral dust and volcanic emissions with 206pb 207pb ratios reaching up to 1 266 0 002 from 1887 ad to 1898 ad pb concentrations reached 5 pg g 1 and 206pb 207pb ratios decreased to 1 058 0 001 as a result of additional inputs of pb from anthropogenic sources seasonal variability in the late 1880s has been investigated by decoupling volcanic pb from the total measured pb concentrations revealing spring and autumn maxima and consistent winter minima in anthropogenic pb and mineral dust ba concentrations we link this variability to the annual cycle in the position and strength of the antarctic circumpolar trough and the southern ocean westerly winds to the north of the trough region during the autumn and spring seasons these systems increase in strength transporting more impurity laden air from the southern hemisphere continental regions to eastern antarctica and law dome as this pb is isotopically identical to that emitted from south eastern australia broken hill port pirie this implies a relatively direct air trajectory pathway from southern australia to law dome eastern antarctica

comparison of lithosphere structure across the orphan basin flemish cap and irish atlantic conjugate continental margins from constrained 3d gravity inversions regionally constrained 3d gravity inversion results on the orphan basin flemish cap and the irish atlantic conjugate continental margins are compared to investigate crustal structure early rifting history and geological evolution of this part of the north atlantic the full crustal density anomaly distributions provide some of the first depth images of how rifted structures compare along and across these conjugate margins broad similarities in crustal structure are identified with some noticeable differences linked to rifting and crustal stretching processes extreme crustal thinning stretching factors 3 5 is indicated beneath much of the southern porcupine basin the western half of west orphan basin the eastern half of jeanne d arc basin the southeastern half of east orphan basin and in pockets beneath rockall basin this appears to have resulted in the serpentinization and possible exhumation of mantle lithosphere on the irish atlantic and flemish cap margins but not beneath orphan basin a simple evolution model is proposed for the early stages of rifting between the margins it is suggested that ancient orogenic sutures played an important role in controlling the northward migration of rifting and the rotation and displacement of flemish cap out of orphan basin supplementary material enlarged maps from this paper are available at www geolsoc org uk sup18527

a 700 year record of southern hemisphere extratropical climate variability annually dated ice cores from west and east antarctica provide proxies for past changes in atmospheric circulation over antarctica and portions of the southern ocean temperature in coastal west and east antarctica and the frequency of south polar penetration of el nino events during the period ad 1700 1850 atmospheric circulation over the antarctic and at least portions of the southern hemisphere underwent a mode switch departing from the out of phase alternation of multi decadal long phases of eof1 and eof2 modes of the 850 hpa field over the southern hemisphere as defined in the recent record by thompson and wallace 2000 thompson and solomon 2002 that characterizes the remainder of the 700 year long record from ad 1700 to 1850 lower tropospheric circulation was replaced by in phase behavior of the amundsen sea low component of eof2 and the east antarctic high component of eof1 during the first phase of the mode switch both west and east antarctic temperatures declined potentially in response to the increased extent of sea ice surrounding both regions at the end of the mode switch west antarctic coastal temperatures rose and east antarctic coastal temperatures fell respectively to their second highest and lowest of the record polar penetration of el nino events increased during the mode switch the onset of the ad 1700 1850 mode switch coincides with the extreme state of the maunder minimum in solar variability late 20th century west antarctic coastal temperatures are the highest in the record period and east antarctic coastal temperatures close to the lowest since ad 1700 extratropical regions of the southern hemisphere have experienced significant climate variability coincident with changes in both solar variability and greenhouse gases

seismic volcanostratigraphy of the norwegian margin constraints on tectonomagmatic break up processes voluminous volcanism characterized early tertiary continental break up on the mid norwegian continental margin the distribution of the associated extrusive rocks derived from seismic volcanostratigraphy and potential field data interpretation allows us to divide the more voring and lofoten vesteralen margins into five segments the central more margin and the northern voring margin show combinations of volcanic seismic facies units that are characteristic for typical rifted volcanic margins the lofoten vesteralen margin the southern voring margin and the area near the jan mayen fracture zone show volcanic seismic facies units that are related to small volume submarine volcanism the distribution of subaerial and submarine deposits indicates variations of subsidence along the margin vertical movements on the mid norwegian margin were primarily controlled by the amount of magmatic crustal thickening because both the amount of dynamic uplift by the icelandic mantle plume and the amount of subsidence due to crustal stretching were fairly constant along the margin thus subaerial deposits indicate a large amount of magmatic crustal thickening and an associated reduction in isostatic subsidence whereas submarine deposits indicate little magmatic thickening and earlier subsidence from the distribution of volcanic seismic facies units we infer two main reasons for the different amounts of crustal thickening 1 a general northward decrease of magmatism due to increasing distance from the hot spot and 2 subdued volcanism near the jan mayen fracture zone as a result of lateral lithospheric heat transport and cooling of the magmatic source region furthermore we interpret small lateral variations in the distribution of volcanic seismic facies units such as two sets of inner seaward dipping reflectors on the central voring margin as indications of crustal fragmentation

a 200 year sulfate record from 16 antarctic ice cores and associations with southern ocean sea ice extent chemistry data from 16 50 115m deep sub annually dated ice cores are used to investigate spatial and temporal concentration variability of sea salt ss so4 2 and excess xs so4 2 over west antarctica and the south pole for the last 200 years low elevation ice core sites in western west antarctica contain higher concentrations of so4 2 as a result of cyclogenesis over the ross ice shelf and proximity to the ross sea polynya linear correlation analysis of 15 west antarctic ice core so4 2 time series demonstrates that at several sites concentrations of ssso4 2 are higher when sea ice extent sie is greater and the inverse for xsso4 concentrations of xsso4 2 from the south pole site east antarctica are associated with sie from the weddell region and west antarctic xsso4 2 concentrations are associated with sie from the bellingshausen amundsen ross region the only notable rise of the last 200 years in xsso4 around 1940 is not related to sie fluctuations and is most likely a result of increased xsso4 2 production in the mid low latitudes and or an increase in transport efficiency from the mid low latitudes to central west antarctica these high resolution records show that the source types and source areas of ssso4 2 and xsso4 2 delivered to eastern and western west antarctica and the south pole differ from site to site but can best be resolved using records from spatial ice core arrays such as the international trans antarctic scientific expedition itase

a serpentinite hosted ecosystem the lost city hydrothermal field the serpentinite hosted lost city hydrothermal field is a remarkable submarine ecosystem in which geological chemical and biological processes are intimately interlinked reactions between seawater and upper mantle peridotite produce methane and hydrogen rich fluids with temperatures ranging from 40 to 90 c at ph 9 to 11 and carbonate chimneys 30 to 60 meters tall a low diversity of microorganisms related to methane cycling archaea thrive in the warm porous interiors of the edifices macrofaunal communities show a degree of species diversity at least as high as that of black smoker vent sites along the mid atlantic ridge but they lack the high biomasses of chemosynthetic organisms that are typical of volcanically driven systems

volcano growth mechanisms and the role of sub volcanic intrusions insights from 2d seismic reflection data abstract temporal and spatial changes in volcano morphology and internal architecture can determine eruption style and location however the relationship between the external and internal characteristics of volcanoes and sub volcanic intrusions is often difficult to observe at outcrop or interpret uniquely from geophysical and geodetic data we use high quality 2d seismic reflection data from the ceduna sub basin offshore southern australia to quantitatively analyse 56 pristinely preserved eocene age volcanogenic mounds and a genetically related network of sub volcanic sills and laccoliths detailed seismic mapping has allowed the 3d geometry of each mound to be reconstructed and distinct seismic facies within them to be recognised forty six continental basaltic shield volcanoes have been identified that have average flank dips of 3 parallel seismic reflections within the shield volcanoes are interpreted to represent interbedded volcanic and clastic material suggesting that a series of temporally separate eruptions emanated from a central vent the shield volcanoes typically overlie the lateral tips of sills and we suggest that the intermittent eruption phases correspond to the incremental emplacement of discrete magma pulses within the laterally extensive sill complex eight volcanogenic hydrothermal vents which are also associated with the lateral tips of sills were also recognised and these appear to have formed from the seepage of intrusion related hydrothermal fluids onto the seafloor via emplacement induced fractures this work highlights that deformation patterns preceding volcanic eruptions may i be offset from the eruption site ii attributed to intrusions with complex morphologies and or iii reflect magma movement along pre existing fracture systems these complexities should therefore be considered in eruption forecasting models that link pre eruption ground deformation to subterranean magma emplacement depth and volume more generally our study highlights the key role that seismic reflection data can play in understanding the geometry distribution and evolution of ancient and modern volcanic systems

characterizing braided rivers in two nested watersheds in the source region of the yangtze river on the qinghai tibet plateau abstract large and complex braided channels 1 km wide in the source region of the yangtze river sryr on the qinghai tibet plateau china are characterized by a unique alpine environment with very high elevations and glacier sources yet little is known about the morphological complexity and changes of large braided channels owing to the scarcity of hydrological and topographical data in this study we provided the first insight into these issues using compiled long term several decades climatic and suspended sediment data and landsat images of braided channels in two nested headwater watersheds i e tuotuo and tongtian river watersheds in the sryr analysis of annual and monthly mean temperature precipitation and water discharges illustrated the nature of the hydrological regime and their response to glacier melting in the two watersheds examination of annual mean suspended sediment loads and concentrations c led to two sediment rating curves and generally similar c values between the two watersheds among a suite of morphological indices for braided rivers braiding intensity may be quantified using four different ones after comparing the indices we showed that they had similar ability of characterizing braiding patterns and adopted branch count index bit3 for further analysis we found that bit3 was linearly correlated with active valley width wr during both flood and dry seasons for the 17 selected braided reaches distributed within the two nested watersheds in addition braiding density defined as the ratio of bit3 to wr remained approximately unchanged irrespective of elevations and locations of these reaches indicating clearly that morphological structures of braided channels are spatially uniform in the sryr along the main channels of the two nested watersheds over the entire study period bit3 always reached the maximum as rw defined as the ratio of water body to exposed active riverbed area was around 0 43 furthermore rw was always positively correlated with the associated water discharge q variable degrees of braiding adjustment in response to changes of hydrological regime were represented by hysteresis loops between bit3 and q these results revealed relatively homogeneous morphologic properties of braided channels across the two nested watersheds and provided an important benchmark for future morphodynamic research on braided rivers in the sryr

how continents break up insights from papua new guinea the woodlark basin in the western pacific forms a continuous system of active continental rifting evolving to well developed seafloor spreading thin sediment cover in the basin and a dominantly nonvolcanic rift phase permit basement fabric and structures to be imaged by swath mapping and seismic reflection data in the continental and oceanic parts of the basin magnetic isochrons indicate a single euler pole of opening for most of the basin history and allow us to infer the opening kinematics along the rifted margins in agreement with rigid plate tectonic models continental rifting initiated geologically synchronously at 6 ma along the length of the protomargins within a deforming plate boundary zone strain localization and seafloor spreading however developed in a time transgressive fashion from east to west within this zone of deformation spreading centers formed within the rheologically weaker protocontinental margins surrounded by stronger oceanic lithosphere in the solomon and coral seas the transition to spreading occurred after a rather uniform degree of continental extension 200 40 km both early and late stage rifting involved high and low angle normal faults we identify distinct styles in the transition from rifting to spreading which we refer to as nucleation propagation and stalling these breakup styles impart varyingly concordant to discordant relationships between the adjacent oceanic and continental rift structures continental transform margins which are or were juxtaposed against the ends of spreading centers show no evidence for thermal uplift or igneous underplating the initial spreading segments achieved much of their length at nucleation within rift basins separated along strike by accommodation zones with subsequent lengthening by spreading propagation into rifting continental crust this early propagation and the subsequent development of transform faults between initially nontransform spreading segment offsets produced rift and spreading segmentation boundaries that are not simply correlated the spreading centers nucleated approximately orthogonal in strike to the opening direction but as the protomargins were oblique to this direction nucleation jumps occurred in order to maintain the new spreading centers within the protomargins thus stepwise spreading nucleation in order to remain within a rheologically weak zone rather than rupturing of the lithosphere by stress concentration at the tip of a propagating ridge axis is the dominant form of the rifting to spreading transition in the woodlark basin