representation of the carbon cycle in box models and gcms 1 solubility pump 1 bacastow 1996 broecker et al 1999 and archer et al 2000 have called attention recently to the fact that box models and general circulation models gcms represent the thermal partitioning of co2 between the warm surface ocean and cold deep ocean in different ways they attribute these differences to mixing and circulation effects in gcms that are not resolved in box models the message that emerges from these studies is that box models have overstated the importance of the ocean s polar regions in the carbon cycle a reduced role for the polar regions has major implications for the mechanisms put forth to explain glacial interglacial changes in atmospheric co2 in parts 1 and 2 of this paper a new analysis of the ocean s carbon pumps is carried out to examine these findings this paper part 1 shows that unresolved mixing and circulation effects in box models are not the main reason for box model gcm differences the main factor is very different kinds of restrictions on gas exchange in polar areas polar outcrops in gcms are much smaller than in box models and they are assumed to be ice covered in an unrealistic way this finding does not support a reduced role for the ocean s polar regions in the cycling of organic carbon the subject taken up in part 2

deep crustal structure and magmatic processes the inception of seafloor spreading in the norwegian greenland sea summary an extensive investigation into the deep crustal structure of the conjugate norwegian and e greenland margins utilizing two ship expanded spread and wide aperture cdp profiling has led to the identification of a unique suite of crustal structures developed during the onset of seafloor spreading on both margins a region of low velocity crust was found proximal to marginal escarpments immediately seaward of the low velocity crustal block seaward dipping reflectors occur within thick crust that is interpreted to be wholly igneous a product of voluminous melt production enhanced by convective partial melting processes this earliest formed oceanic crust is actually several kilometres thicker than the low velocity crust or the adjacent continental crust normal oceanic crustal thicknesses are not produced until about 5 my after the initiation of spreading when convective partial melting has abated and the magma budget has reduced the extrusive unit reduces to normal thicknesses long before the intrusive and plutonic section indicating that the thickening or thinning of oceanic crust is accomplished primarily by variations in the latter

crustal structure of the sheared southwestern barents sea continental margin abstract eighteen expanding spread profiles from the southwestern barents sea continental margin were studied with a variety of seismic interpretation techniques and in conjunction with seismic reflection data the velocity depth profiles from the slope intercept herglotz wiechert and sum methods were compared and contrasted to the velocity depth curve calculated from the iterative forward modelling of the amplitudes the velocity depth profiles derived from the sum and forward modelling of the amplitudes were used to produce synthetics in both the x t and p domains to choose the technique that most accurately represented the data the results showed the oceanic crust at the senja fracture zone to be different from standard oceanic crust with a higher than average velocity for layer 2 of 5 7 6 4 km s 1 and slightly smaller crystalline crustal thickness the continent ocean transition at crustal depths was shown to be narrow in the range of 10 20 km our results were consistent with two different crustal provinces east of the senja fracture zone the tertiary marginal basin and the cretaceous basin region the velocity depth curves enabled us in a gross sense to suggest the depths to deep sedimentary interfaces and the top of the crystalline crust that were normally not observed in the reflection profiles depths down to 20 km were inferred for the base of the sedimentary section in the southwestern barents sea the crust thinned gently towards the continent ocean boundary at the senja fracture zone whereas the crustal thickness changed abruptly below the hornsund fault zone at the margin farther north we associated the change in crustal structure with cenozoic in the thick crust beneath the svalbard platform in the north the depths to mantle determined on the shelf were consistent with that observed on a deep multichannel seismic line that crosses the area

a sea ice climate switch mechanism for the 100 kyr glacial cycles a box model of the ocean atmosphere sea ice land ice climate system is used to study a novel mechanism for self sustained oscillations of the climate system on a time scale of 100 000 years without external forcing the oscillation in land ice volume has the familiar sawtooth shape of climate proxy records the most novel aspect of the climate oscillations analyzed here is the crucial role played by the sea ice the sea ice acts as a switch of the climate system switching it from a growing land glaciers mode to a retreating land glaciers mode and shaping the oscillation s sawtooth structure a simple explanation of the 100 kyr timescale is formulated on the basis of the mechanism seen in the model finally rapid sea ice changes such as those seen in our model and their drastic effects on the climate system may provide an explanation to some of the rapid climate changes observed to be a part of the variability at all timescales in the paleorecord

the role of mesozoic rifting in the opening of the ne atlantic evidence from deep seismic profiling across the faroe shetland trough the fast deep seismic reflection profile traverses the whole width of the faroe shetland trough the principal target of the profile was the structure of the crust beneath the faroe basalts in this region bright reflections are seen from between 7 and 9 km depth beneath the basalts dipping westwards in the opposite direction to the dip of the basalts and reflections within the basalts these sub basalt reflections are regarded as originating from near top basement the moho has not been imaged beneath the basalts possibly because of the absence of any impedance contrast at the base of the crust the profile shows that the basement of the faroe shetland trough thins to c 10 km beneath the centre of the trough thinning of the crystalline basement is probably the result of more than one phase of extension the most recent of which occurred in the mid to late cretaceous extension appears to have been concentrated on a series of east dipping normal faults cutting through the basement these faults may have originated during a precambrian rifting event it is suggested that opening of the ne atlantic occurred to the west of the faroe islands as mesozoic rifting in the faroe shetland trough had strengthened the lithosphere in this region

the structure of the faeroe shetland trough from integrated deep seismic and potential field modelling in the faeroe shetland trough thick basaltic lava units make it difficult to image underlying structure with conventional seismic profiles we have therefore used potential field modelling to estimate crustal structure along the fast deep seismic line and other regional transects we introduce the additional constraints of crustal isostasy to gravity models and the observations of the polarity of magnetic remanence within the lava pile to magnetic models the crustal stretching factor ranges up to 3 along the fast traverse and our model also suggests that a sedimentary section with a thickness of up to 3 4 km is present beneath a c 2 km basalt layer on the faeroese shelf

differentiating bubble free layers from melt layers in ice cores using noble gases melt layers are clear indicators of extreme summer warmth on polar ice caps the visual identification of refrozen meltwater as clear bubble free layers cannot be used to study some past warm periods because in deeper ice bubbles are lost to clathrate formation we present here a reliable method to detect melt events based on the analysis of kr ar and xe ar ratios in ice cores and apply it to the detection of melt in clathrate ice from the eemian at neem greenland additionally melt layers in ice cores can compromise the integrity of the gas record by dissolving soluble gases or by altering gas transport in the firn which affects the gas chronology we find that the easily visible 1 mm thick bubble free layers in the wais divide ice core do not contain sufficient melt to alter the gas composition in the core and do not cause artifacts or discontinuities in the gas chronology the presence of these layers during winter and the absence of anomalies in soluble gases suggests that these layers can be formed by processes other than refreezing of meltwater consequently the absence of bubbles in thin crusts is not in itself proof of a melt event

crustal structure beneath the faroe islands from teleseismic receiver functions we use teleseismic receiver function analysis to constrain the crustal structure beneath the faroe islands on the northwest european volcanic continental margin a cluster of 45 broad band seismometers on the glyvursnes peninsula streymoy island recorded 10 teleseismic events ranging in magnitude from 6 1 to 8 0 during july december 2003 receiver functions show a clear ps peak for the first time in this region the depth of the converting boundary is estimated as 29 32 km using forward and inverse modelling and thickness versus vp vs ratio stacking techniques modelling experiments suggest that this estimate may represent the conversion from a region of high velocity lower crust rather than the crust mantle transition at the moho the best fit modelling results were achieved with a gradational high velocity region at least 6 km thick in the lower crust this is interpreted as due to the emplacement of sills into pre existing continental crust rather than the simple underplating of a block of high velocity igneous material at its base

imaging and regional distribution of basalt flows in the faeroe shetland basin we demonstrate that the use of long offset seismic data allows wide angle reflections and diving waves to be recorded and that these can be used in conjunction with prestack depth migrations to constrain and to image the base of the basalt flows and the underlying structure in the faeroe shetland basin crustal velocity models are built first by inverting the traveltimes of the recorded reflections and diving waves using ray tracing methods finer details of the velocity structure can then be refined by analysis of the amplitudes and waveforms of the arrivals we show that prestack depth migration of selected wide angle arrivals of known origin such as the base basalt reflection using the crustal velocity model allows us to build a composite image of the structure down to the pre rift basement this has the advantage that the wide angle first arriving energy must be primary and not from one of the many multiples or mode converted phases that plague near offset seismic data this allows us to tag these primary arrivals with confidence and then to identify the same arrivals on higher resolution prestack migrations that include data from all offsets examples are drawn from the faeroe shetland basin with a series of regional maps of the entire area showing the basalt depths and the thickness of the basalt flows and underlying sediment down to the top of the pre rift basement the maps show how the basalts thin to the southeast away from their presumed source west of the present faeroe islands and also show the extent to which the structure of the pre rift basement controls the considerable variations in sediment thickness between the basement and the cap formed by the overlying basalt flows

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

the impact of magnetic viscosity on time domain electromagnetic data from iron oxide minerals embedded in rocks at opemiska quebec canada abstractthe magnetic viscosity mv effects observed at time scales between 0 01 and 10 ms at opemiska are associated with magnetic grains of variable size in rocks recent observations made during a ground time domain electromagnetic tdem survey at opemiska are consistent with four aspects of the spatial and amplitude characteristics of a mv response 1 the bz t decay rate is roughly proportional to 1 t1 where 0 4 0 4 2 the anomalies are mainly visible on the z component when the em receiver sensor is located inside or just outside the transmitter loop 3 there is no obvious x or y component response and 4 the sites where mv effects are seen in the tdem data are coincident with an airborne magnetic anomaly previous studies have demonstrated that mv could be caused by 1 fine grained particles of maghemite or magnetite in the overburden regolith or soil that were formed through lateritic weathering processes 2 volcanic glass shards from tuff containing approximately 1 by weig

contrasts of seismic velocity density and strength across the moho abstract we provide an overview of contrast of elastomechanical parameters across the moho basically contrasts in seismic wave velocities density and yield strength these can be regarded as dynamic and quasi static endmembers of elastomechanical parameters of the crust mantle transition with respect to practical applications they are closely related because seismic investigations play the role of exploring the earth structure and physical state forming the boundary conditions for determining density and rheological models we address both average global moho models and petrophysical and structural factors that cause deviations from the global averages such as variations in moho depth temperature and rock composition fabric anisotropy and macro scale heterogeneity besides principle considerations these factors are put in a regional context in order to demonstrate how they are related to past and present tectonic processes seismic velocity and density contrasts are found between 0 and 25 whereas strength or viscosity contrasts may be orders of magnitude higher but may vanish as well or show a discontinuity of higher order only especially oceanic and old cratonic crust and near moho mantle may appear as a rheological unit although they are seismically well distinguishable in terms of seismic velocities the moho may vanish mainly under the influence of serpentinization or eclogitization originally defined as a first order discontinuity in seismic velocity the moho has turned out to be an interface across which other seismic properties such as seismic velocity gradients anisotropy and heterogeneity scale parameters can change strongly too however knowledge of these parameters as well as their relation to the rheology of the crust mantle transition is still restricted to local or regional examples so no global conclusions can be drawn

older crust underlies iceland summary the oldest rocks outcropping in northwest iceland are 16 myr old and in east iceland 13 myr the full plate spreading rate in this region during the cenozoic has been 2c m a 1 and thus these rocks are expected to be separated by 290 km they are however 500 km apart the conclusion is inescapable that an expanse of older crust 210 km wide underlies iceland submerged beneath younger lavas this conclusion is independent of any considerations regarding spreading ridge migrations jumps the simultaneous existence of multiple active ridges three dimensionality or subsidence of the lava pile such complexities bear on the distribution and age of the older crust but not on its existence or its width if it is entirely oceanic its maximum age is most likely 26 37 ma it is at least 150 km in north south extent but may taper and extend beneath south iceland part of it might be continental a southerly extension of the jan mayen microcontinent this older crust contributes significantly to crustal thickness beneath iceland and the 40 km local thickness measured seismically is thus probably an overestimate of present day steady state crustal production at iceland

different mechanisms of magnetisation recorded in experimental fires archaeomagnetic implications abstract we present here the archaeomagnetic and rock magnetic results obtained from the experimental recreation of prehistoric fires on a clayish soil substratum by using a system of thermocouples that allows the control of temperatures and heat penetration with depth the magneto mineralogical transformations as well as the ability to acquire magnetic remanences are studied a detailed set of rock magnetic analyses carried out both on surface and depth with a millimetre scale resolution shows the creation of post burn magnetite over the entire surface of the hearth thus promoting a distinguishable magnetic enhancement on the basis of a detailed control of temperatures interesting differences between areas centre vs periphery were observed and two different mechanisms of magnetisation have been recorded the central area which exceeded 600 c acquired a thermal remanent magnetisation trm and the periphery which was heated up to 300 c recorded a thermo chemical tcrm remanent magnetisation the most noticeable mineralogical changes are focused to the first centimetre of the central area where single domain magnetite has been created the implications of these results for archaeomagnetic and palaeointensity studies as well as their utility for the detection of fire in archaeological sites are discussed

the composition and structure of volcanic rifted continental margins in the north atlantic further insight from shear waves abstract imaging challenges caused by highly attenuative flood basalt sequences have resulted in the understanding of volcanic rifted continental margins lagging behind that of non volcanic rifted and convergent margins massive volcanism occurred during break up at 70 of the passive margins bordering the atlantic ocean the causes and dynamics of which are still debated this paper shows results from traveltime tomography of compressional and converted shear wave arrivals recorded on 170 four component ocean bottom seismometers along two north atlantic continental margin profiles this traveltime tomography was performed using two different approaches the first a flexible layer based parameterisation enables the quality control of traveltime picks and investigation of the crustal structure the second with a regularised grid based parameterisation requires correction of converted shear wave traveltimes to effective symmetric raypaths and allows exploration of the model space via monte carlo analyses the velocity models indicate high lower crustal velocities and sharp transitions in both velocity and vp vs ratios across the continent ocean transition the velocities are consistent with established mixing trends between felsic continental crust and high magnesium mafic rock on both margins interpretation of the high quality seismic reflection profile on the faroes margin confirms that this mixing is through crustal intrusion converted shear wave data also provide constraints on the sub basalt lithology on the faroes margin which is interpreted as a pre break up mesozoic to paleocene sedimentary system intruded by sills

regional distribution of volcanism within the north atlantic igneous province an overview of the distribution of volcanic facies units was compiled over the north atlantic region the new maps establish the pattern of volcanism associated with breakup and the initiation of seafloor spreading over the main part of the north atlantic igneous province naip the maps include new analysis of the faroe shetlands region that allows for a consistent volcanic facies map to be constructed over the entire eastern margin of the north atlantic for the first time a key result is that the various conjugate margin segments show a number of asymmetric patterns that are interpreted to result in part from pre existing crustal and lithospheric structures the compilation further shows that while the lateral extent of volcanism extends equally far to the south of the iceland hot spot as it does to the north the volume of material emplaced to the south is nearly double of that to the north this suggests that a possible southward deflection of the iceland mantle plume is a long lived phenomenon originating during or shortly after impact of the plume

representation of the carbon cycle in box models and gcms 2 organic pump 1 box models of the ocean atmosphere co 2 system rely on mechanisms at polar outcrops to alter the strength of the ocean s organic carbon pump gcm based carbon system models are reportedly less sensitive to the same processes here we separate the carbon pumps in a three box model and the gcm based princeton ocean biogeochemistry model to show how the organic pumps operate in the two kinds of models the organic pumps are found to be quite different in two respects deep water in the three box model is relatively well equilibrated with respect to the pco 2 of the atmosphere while deep water in the gcm tends to be poorly equilibrated this makes the organic pump inherently stronger in the gcm than in the three box model the second difference has to do with the role of polar nutrient utilization the organic pump in the gcm is shown to have natural upper and lower limits that are set by the initial po 4 concentrations in the deep water formed in the north atlantic and southern ocean the strength of the organic pump can swing between these limits in response to changes in deep water formation that alter the mix of northern and southern deep water thus unlike the situation in the three box model the organic pump in the gcm can become weaker or stronger without changes in polar nutrient utilization

depth imaging of basalt flows in the faeroe shetland basin summary we use a grid of long offset seismic profiles to map the thickness of subsurface basalt flows that extend southeastward across the faeroe shelf offsets of up to 38 000 m were achieved using two seismic acquisition vessels towing multichannel streamers and shooting alternately wide angle reflections and refractions from the basalt and underlying strata allow us to build a velocity model down to the basement lewisian crust the velocity models give a first order picture of the geology of low velocity sediments of presumed early palaeocene and mesozoic age beneath the basalt flows they also provide a control for pre stack depth migration of the entire seismic data set thus allowing us to produce good depth images of the structure pre stack depth migration of selected wide angle arrivals such as those from the base of the basalt and the underlying basement produce strong reflection images that allow these interfaces to be identified unambiguously on the higher resolution images produced from the entire data set in particular we can distinguish between the primary and multiple reflections we thus use the very long offset arrivals to tag the velocities of the geological interfaces from which they are returned and hence to improve both the imaging and the interpretation of the basalt flows and the underlying structure

crustal structure of edoras bank continental margin and mantle thermal anomalies beneath the north atlantic we discuss the structure of the continent ocean transition of the edoras bank area of the ne atlantic margin and the influence of the iceland mantle plume during continental breakup edoras bank lies close to the present day limit of influence of the iceland plume sequences of seaward dipping reflectors imaged on multichannel seismic profiles are well developed over much of the margin they form aerially extensive dipping sheets with individual reflectors imaged on margin parallel profiles as laterally continuous and subhorizontal features we interpret them as basalt flows extruded from linear fissure vents close to sea level at the time of continental breakup a crustal velocity model for the margin was developed by travel time modeling of wide angle refractions and reflections recorded on digital ocean bottom hydrophones and the structure confirmed by gravity modeling oceanic crust formed immediately after breakup is 10 km thick and exhibits classic layer 2 and layer 3 seismic velocities the crustal thickness increases to the se reaching 17 km beneath the continental fragment of edoras bank the continent ocean transition is dominated in the lower crust by a lens up to 8 km thick of high velocity material 7 2 7 5 km s 1 which exhibits a very low velocity gradient it extends over a 60 km wide region beneath the seaward dipping reflectors the high velocities and densities of the lower crust are probably caused by igneous rock with a high mgo content that underplated the margin or heavily intruded pre existing crustal material during continental breakup both the addition of large quantities of melt to the margin and the subsidence curves inferred from postrift sediments sampled by deep sea drilling project dsdp boreholes on the margin indicate the presence of abnormally hot mantle beneath the area at the time of continental breakup comparison of the structure and subsidence history of the edoras bank margin with other areas of the nw european margin allows us to deduce the spatial extent and temporal variation of the thermal anomalies caused by the iceland mantle plume during and following continental breakup we find that at the time of breakup a thin layer of anomalously hot asthenospheric mantle 150 c hotter than normal lay beneath the 2000 km long rift extending from the lofoten margin off norway in the north to the edoras bank margin in the south over a period of 5 10 m y following the onset of rifting the mantle thermal anomaly decayed rapidly to only 50 c or so above normal as the thin layer of abnormally hot mantle decompressed beneath the rift and produced a large pulse of magmatism

transform marginal plateaus abstract numerous submarine plateaus form highstanding bathymetric highs at continent to ocean transitions due to their proximity to continents they have been frequently labelled marginal plateaus although this term has not been clearly defined or associated with a specific geology or geodynamic process until now these elevations have been interpreted as submerged thinned continental fragments detached from continents basaltic buildups formed by hotspots volcanic margins or oceanic plateaus many of these plateaus formed at transform margins connecting oceanic basins of contrasted ages we propose for the first time to define and review a class of marginal plateaus related to a specific tectonic setting transform marginal plateaus tmps based on a compilation of 20 tmps around the world we show that most of them have a polyphased history and have undergone at least one major volcanic phase our review highlights in particular a hitherto unrecognized close link between hotspots volcanic activity and transform margins we also propose that due to their polyphased history tmps may contain several successive basins and overlooked long lived sedimentary archives we finally highlight that because these tmps were transform plate boundaries perpendicular or oblique to surrounding rifts many of them were close to last contact points during final continental breakup and may have formed land bridges or bathymetric highs between continents therefore we discuss broader scientific issues such as the interest of tmps in recording and studying the onset and variations of oceanic currents or past biodiversity growth bio connectivity and lineage evolution

a review of the ne atlantic conjugate margins based on seismic refraction data the ne atlantic region evolved through several rift episodes leading to break up in the eocene that was associated with voluminous magmatism along the conjugate margins of east greenland and nw europe existing seismic refraction data provide good constraints on the overall tectonic development of the margins despite data gaps at the ne greenland shear margin and the southern jan mayen microcontinent the maximum thickness of the initial oceanic crust is 40 km at the greenland iceland faroe ridge but decreases with increasing distance to the iceland plume high velocity lower crust interpreted as magmatic underplating or sill intrusions is observed along most margins but disappears north of the east greenland ridge and the lofoten margin with the exception of the vestbakken volcanic province at the sw barents sea margin south of the narrow lofoten margin the european side is characterized by wide margins the opposite trend is seen in greenland with a wide margin in the ne and narrow margins elsewhere the thin crust beneath the basins is generally underlain by rocks with velocities of 7 km s 1 interpreted as serpentinized mantle in the porcupine and southern rockall basins while off norway alternative interpretations such as eclogite bodies and underplating are also discussed

deep seismic and geochemical constraints on the nature of rift induced magmatism during breakup of the north atlantic abstract the deep crustal structure of the norwegian and east greenland conjugate passive margins has been investigated with two ship multichannel reflection and refraction techniques in the region from the iceland faeroe ridge to the greenland fracture zone the data demonstrate the presence of thick 15 20 km igneous crust that was emplaced along the conjugate margins during the initiation of seafloor spreading seaward dipping units comprise the uppermost 3 6 km of the thick crust the thickness of igneous crust diminishes seaward with oceanic layer 2 approaching normal thickness nearer to the margin than does oceanic layer 3 wide angle reflection techniques used in conjunction with expanded spread profiling reveal deep moho reflections beneath 20 km thick oceanic crust that continue seaward into the ocean basin margins that have experienced unusually prolific magmatic activity during breakup have been termed volcanic to distinguish them from non volcanic margins that have evolved into accreting plate boundaries without excessive volcanism the origin of volcanic margins cannot be determined from seismically derived crustal thicknesses alone however thermal and chemical properties of the mantle source may be revealed in the compositions of magmatic products emplaced within volcanic passive margins a combined investigation of crustal structure and associated basalt composition could better resolve processes that result in volcanic passive margins on the voring plateau of the norwegian margin seismic lines coincide with dsdp leg 38 drill sites whole rock major element abundances in the freshest basalts are compatible with moderate extents of partial melting of mantle much like that believed to underlie modern mid ocean ridge systems and would be expected to result in the emplacement of oceanic crust about 9 2 km thick this is about the thickness of oceanic crust located seaward of the dipping basaltic units off the hatton more and voring plateau margins and 10 km south of iceland on the reykjanes ridge these results together with regional heat flow depth gravity and geoid anomalies suggest that the mantle beneath this region has experienced a moderate long lived temperature increase compared to mantle beneath normal ocean basins the much greater thicknesses of igneous crust adjacent to the margins do not appear to reflect an unusually hot flux associated with their construction since this would lead to much greater extents of partial melting than are inferred from the basalt geochemistry evidently the difference between volcanic and non volcanic margins is not simply proximity to a hot spot instead we suggest that the magmatism is closely related to the continental rifting phase tectonism resulting in an abrupt transition from continental to oceanic lithosphere may induce small scale convection that results in the circulation of a greater amount of mantle material into the pressure temperature field where partial melting begins than would occur through passive upwelling alone initial crustal thicknesses are enhanced as a result of the convective partial melting crustal thickness is expected to diminish as this secondary convection slows but the degree of partial melting of the source should not change dramatically

constraining ventilation during deepwater formation using deep ocean measurements of the dissolved gas ratios 40ar 36ar n2 ar and kr ar equilibrium saturation and for d 40 ar a value of 1 188 0 055 relative to air these data are used to constrain high latitude ventilation processes in the framework of three box and seven box ocean models for the three box model tracer data we constrain the appropriate surface area of the high latitude region in both models to be 3 6 2 5 1 7 of ocean surface area and the bubble air injection rate to be 22 7 8 8 7 3 mol air m 2 yr 1 results for the seven box model were similar with a high latitude area of 3 3 2 2 1 3 our results provide geochemical support for suggestions that the effective area of high latitude ventilation is much smaller than the region of elevated preformed nutrients and demonstrate that noble gases strongly constrain the ocean solubility pump reducinghigh latitude surfaceareaweakens theco2solubility pumpintheboxmodels and limits communication between the atmosphere and deep ocean these tracers should be usefulconstraintsonhigh latitudeventilationandthestrengthofthesolubilitypumpinmore complex ocean general circulation models

deep crustal reflectors under reydarfjordur eastern iceland crustal accretion above the iceland mantle plume summary deep seismic reflection profiling on the faroe iceland ridge experiment fire imaged a sequence of dipping events in the upper to crust beneath the tertiary lava pile in reydarfjordur eastern iceland these events dip towards the spreading centre at which the crust was formed and continue down to 6 s two way time corresponding to depths down to 15 18 km similar crustal reflections have been observed previously within the crust in other areas of iceland although the reflective events located by previous surveys have not been interpreted rigorously in this study we calculate synthetic seismograms from modelled sequences of lava flows to examine the reflective character of a lava le the synthesized lava pile sections are based on field measurements made in the reydarfjordur area the 1 km vertical exposures on the sides of the fjord are composed of lava flows within stratigraphic units which have a range of compositions and some intercalated tuffaceous clastic horizons we compare the synthetic seismograms with our seismic reflection data to test the interpretation that the deep dipping reflections are caused by a gently dipping sequence of subaerially erupted lavas we conclude that the upper and mid crustal reflections that we observe can be interpreted as lava flow sequences similar to those seen at the surface with the main reflections resulting from packets of flows of similar thicknesses the reflectors that we have imaged beneath iceland are similar to those defining the seaward dipping reflectors on the volcanic continental margins and we therefore suggest that the seaward dipping reflections commonly result from packets of lavas with similar distributions of thicknesses the occurrence of lavas down to 15 18 km beneath reydarfjordur can be explained partly by multiple episodes of accretion of the upper crust although the volcanic processes forming the crust in iceland generally lead to advection of extrusive material to mid crustal levels the production of an average extrusive layer totalling 15 km thickness is within reasonable estimates of the current volcanic productivity the advection of frozen lava flows to mid crustal levels contributes to the maintenance of the subsolidus crust that is observed on iceland and the zeolite minerals within the extrusive sequence may be responsible for the crustal low resistivity zone that has been observed using magnetotelluric techniques

early cenozoic crust at the norwegian continental margin and the conjugate jan mayen ridge multichannel seismic profiles at the voring plateau margin off norway and the northern jan mayen ridge have provided a framework for the early tertiary evolutionary history of these areas which were adjacent prior to the opening of the norwegian sea we propose that their paleogene evolution is quite similar and that the two acoustic basement reflectors ee and jo were formed in the early eocene by extrusion of flow basalts from the time of opening to about anomaly 23 time oceanic seafloor was generated by icelandic type spreading during that time the central jan mayen fracture was the main transform the seaward dipping reflector sequences are flow basalts generated by the icelandic spreading the existence of less well developed dipping sequences seaward of the primary ones probably reflects that these regions submerged later because of higher initial elevation during the early eocene subaerial spreading episode which is related to the north atlantic volcanic province basalt flows also covered the adjacent thinned and heavily intruded continental crust it is proposed that the continent ocean boundary is located just landward of anomaly 24b a short distance seaward of the voring plateau escarpment reflector k is only recognized landward of this boundary in a region of transitional crust the escarpment experienced syndepositional faulting and the most elevated parts of the voring marginal high did not subside below shallow water depths before oligocene miocene time we suggest that the voring margin and the jan mayen ridge represent volcanic passive margins the evolution of this margin type may relate to initial uplift due to rifting in previously thinned crust

excess helium and neon in the southeast pacific tracers for glacial meltwater 1 we analyze helium he and neon ne isotope data sets from the southeast pacific sector of the southern ocean collected in 1992 and 1994 and describe a new method to estimate glacial meltwater fluxes independent of previous approaches the waters sampled during these cruises reveal large atmospheric he and ne excess concentrations in the upper 500 m that are derived predominantly from the dissolution of air contained in glacial ice during melting underneath and at the front of floating ice shelves at elevated hydrostatic pressure on the stations sampled the meltwater fractions are largest near the northern and southern ice shelf fronts of george vi ice shelf 1 5 2 5 consistent with prior inferences from 18o the inventories of 4he and ne with local glacial meltwater origin south of 70 s between 210 e and 295 e are 2 5 0 7 1011 cm3stp 4he and 7 9 2 7 1011 cm3stp ne respectively the associated meltwater volumes are 430 120 gt for 4he and 400 140 gt for ne for an estimated seawater residence time of 2 years on the continental shelf the amounts of glacial meltwater translate into bulk melting rates of 215 170 gt yr 1 for 4he and 200 170 gt yr 1 for ne the values derived from 4he are expected to be larger than those obtained from ne because of the terrigenic he component which accounts for 10 15 of the he excess in this region our regional melting rate of 200 215 gt yr 1 is comparable to a prior estimate but subject to uncertainties resulting from the station distribution the air sea gas exchange and the estimate of the net transport through the study region

the structure of the rockall trough from seismic refraction and wide angle reflection measurements there are few detailed descriptions of the deep crustal structure of failed cratonic rifts and passive margins despite their import ance in constraining the variety of structural models invoked to account for their common structural symmetry or gross half graben form1 3 such deep crustal structure profiles also have considerable value in defining the continent ocean boundary con straining plate reconstructions and determining the overall setting of sedimentary basin development the rockall trough in the north atlantic ocean is a sinuous elongate bathymetric depression 200 km wide and more than 2 000 m deep lying between rockall plateau and the british isles and the structure nature and age of its underlying crust are disputed in the absence of deep crustal structure profiles the presence of oceanic crust has been assumed because the trough has to be accommodated in reconstructions of the north atlantic although stretched continental crust is equally plausible the nature of the crust is important for the precise reconstruction of the late jurassic early cretaceous plate history and setting of sedimentary basins of the north atlantic4 8 the nature of the crust beneath the trough as well as the structure thickness and composition of the sedimentary cover is also relevant to oil exploration here we report the first observations of deep crustal structure across the rockall trough and its margins from seismic refraction and wide angle reflection profiles our results show that the northern rockall trough is underlain by highly stretched continental crust and not by oceanic crust as previously postulated

in situ evaluation of air sea gas exchange parameterizations using novel conservative and volatile tracers measurements of air sea gas exchange rates are reported from two deliberate tracer experiments in the southern north sea during february 1992 and 1993 a conservative tracer spores of the bacterium bacillus globigii var niger was used for the first time in an in situ air sea gas exchange experiment this nonvolatile tracer is used to correct for dispersive dilution of the volatile tracers and allows three estimations of the transfer velocity for the same time period the first estimation of the power dependence of gas transfer on molecular diffusivity in the marine environment is reported this allows the impact of bubbles on estimates of the transfer velocity derived from changes in the helium sulphur hexafluoride ratio to be assessed data from earlier dual tracer experiments are reinterpreted and findings suggest that results from all dual tracer experiments are mutually consistent the complete data set is used to test published parameterizations of gas transfer with wind speed a gas ex change relationship that shows a dependence on wind speed intermediate between those ofliss and merlivat 1986 and wanninkhof 1992 is found to be optimal the dual tracer data are shown to be reasonably consistent with global estimates of gas exchange based on the uptake of natural and bomb derived radiocarbon the degree of scatter in the data when plotted against wind speed suggests that parameters not scaling with wind speed are also influencing gas exchange rates

the seismic structure of thinned continental crust in the northern bay of biscay summary the stretching and thinning of the continental crust which occurs during the formation of passive continental margins may cause important changes in the velocity structure of such crust further crust attenuated to a few kilometres thickness can be found underlying oceanic water depths this paper poses the question of whether thinned continental crust can be distinguished seismically from normal oceanic crust of about the same thickness a single seismic refraction line shot over thinned continental crust as part of the north biscay margin transect in 1979 was studied in detail tau p inversion suggested that there are differences between oceanic and continental crust in the lower crustal structure this was confirmed when synthetic seismograms were calculated the thinned continental crust p 7 0 exhibits a two gradient structure in the non sedimentary crust with velocities between 5 9 and 7 4 km s l an upper 0 8 s l layer overlies a 0 4 s l layer no layer comparable to oceanic layer 3 was detected the uppermost mantle also contains a low velocity zone

moho and basement depth in the ne atlantic ocean based on seismic refraction data and receiver functions seismic refraction data and results from receiver functions were used to compile the depth to the basement and moho in the ne atlantic ocean for interpolation between the unevenly spaced data points the kriging technique was used free air gravity data were used as constraints in the kriging process for the basement that way structures with little or no seismic coverage are still presented on the basement map in particular the basins off east greenland the rift basins off nw europe are mapped as a continuous zone with basement depths of between 5 and 15 km maximum basement depths off ne greenland are 8 km but these are probably underestimated plate reconstructions for chron c24 c 54 ma suggest that the poorly known ammassalik basin off se greenland may correlate with the northern termination of the hatton basin at the conjugate margin the most prominent feature on the moho map is the greenland iceland faroe ridge with moho depths 28 km crustal thickness is compiled from the moho and basement depths the oceanic crust displays an increased thickness close to the volcanic margins affected by the iceland plume

volcanism on the rockall continental margin continental break up in the northern north atlantic was accompanied by massive volcanism this produced sequences of basalt flows erupted close to sea level which are imaged as seaward dipping reflectors on seismic reflection profiles they comprise the upper 4 5 km of transitional crust in both the edoras bank and hatton bank areas of the rockall continental margin detailed surveys show that the reflectors are convex upward on dip lines but are sub horizontal on strike lines they are probably produced by aerially extensive basalt flows extruded from linear fissure vents aligned parallel to the rift axis along the rockall margin two sequences of seaward dipping reflectors occur separated by an 9outer high9 near the foot of the continental margin the landward set of basalt flows are emplaced above stretched and intruded continental crust while the seaward set span the region from highly distended continental crust to fully oceanic crust the outer high is capped by basalt flows but exhibits little coherent internal reflectivity we discuss possible origins of the outer high as a residual block of thinned and intruded continental crust or as a central volcano cut by fissure vents similar to those found in the neovolcanic zones of iceland

estimating the recharge properties of the deep ocean using noble gases and helium isotopes the distribution of noble gases and helium isotopes in the dense shelf waters of antarctica reflect the boundary conditions near the ocean surface air sea exchange sea ice formation and subsurface ice melt we use a non linear least squares solution to determine the value of the recharge temperature and salinity as well as the excess air injection and glacial meltwater content throughout the water column and in the precursor to antarctic bottom water the noble gas derived recharge temperature and salinity in the weddell gyre are 1 95 c and 34 95 psu near 5500 m these cold salty recharge values are a result of surface cooling as well as brine rejection during sea ice formation in antarctic polynyas in comparison the global value for deep water recharge temperature is 0 44 c at 5500 m which is 1 5 c warmer than the southern hemisphere deep water recharge temperature reflecting the contribution from the north atlantic the contrast between northern and southern hemisphere recharge properties highlight the impact of sea ice formation on setting the gas properties in southern sourced deep water below 1000 m glacial meltwater averages 3 5 by volume and represents greater than 50 of the excess neon and argon found in the water column these results indicate glacial melt has a non negligible impact on the atmospheric gas content of antarctic bottom water

the afar depression transition between continental rifting and sea floor spreading abstract detailed geophysical and geological studies were undertaken in the 1970 s in the afar depression ethiopia in order to study the processes of continental break up occurring in this unique setting the afar is located between the ethiopian and somali plateaus and the red sea we have re evaluated the available seismic refraction and gravity data using modern interpretation techniques and incorporated new information derived from magnetic gravity seismicity and seismic refraction data the results of the re evaluation indicate that the afar depression is a transition zone between the continental rifts of kenya and the present ongoing sea floor spreading of the red sea and the gulf of aden in the depression itself the crust is greatly attenuated and is underlain by a low velocity 7 4 7 5 km s high temperature upper mantle material however it is still a continental crust which thins to the northeast and east towards the red sea and the gulf of aden it is not a newly formed oceanic crust as has been suggested in the past the trends of the areas of crustal thinning are indicated by the gravity anomalies the trend of the ethiopian rift reaches south afar where the trend of crustal thinning changes and becomes parallel to the red sea trend it is offset en echelon from the spreading area of the red sea which is marked by an alignment of gravity maxima the gulf of aden trend continues west towards the ethiopian rift the gravity trends coincide with areas of maximum crustal thinning and may thus suggest a possible triple junction in development

structure of the lower crust beneath the carolina trough u s atlantic continental margin data from three large offset seismic profiles provide information on the crustal structure beneath the carolina trough the profiles obtained by the u s geological survey the naval oceanographic research development agency and the scripps institution of oceanography in 1985 were oriented parallel to the trough and were located 1 seaward of the east coast magnetic anomaly ecma which is generally thought to represent the boundary between oceanic and continental crust 2 along the axis of the trough between the ecma and the hinge zone which is thought to reflect the landward limit of highly stretched and altered transitional crust and 3 along the carolina platform landward of the basement hinge zone on crust thought to have been thinned only slightly during rifting these data constrain the velocity structure of the lower crust and provide evidence for a thick lens of high velocity 7 1 km s lower crustal material that extends beneath the carolina trough and the adjacent ocean basin this lens reaches a maximum thickness of about 13 km beneath the deepest part of the trough thins to about 5 km seaward of the ecma and is either very thin or absent landward of the hinge zone it is interpreted to represent material that was underplated beneath and or intruded into the crust during the late stage of continental rifting and that led to an anomalously thick plutonic layer during the early seafloor spreading phase these data thus support the recent conclusions of white et al 1987b and mutter et al 1988 that the initiation of seafloor spreading is attended in many if not most cases by the generation of an anomalously large volume of melt

north atlantic volcanic margins dimensions and production rates early tertiary lithospheric breakup between eurasia and greenland was accompanied by a transient 3 m y igneous event emplacing both the onshore flood basalts of the north atlantic volcanic province navp and huge extrusive complexes along the continent ocean transition on the rifted continental margins seismic data show that volcanic margins extend 2600 km along the early eocene plate boundary in places underlain by high velocity 7 2 7 7 km s lower crustal bodies quantitative calculations of navp dimensions considered minimum estimates reveal an areal extent of 1 3 106 km2 and a volume of flood basalts of 1 8 106 km3 yielding a mean eruption rate of 0 6 km3 yr or 2 4 km3 yr if two thirds of the basalts were emplaced within 0 5 m y the total crustal volume is 6 6 106 km3 resulting in a mean crustal accretion rate of 2 2 km3 yr thus navp ranks among the world s larger igneous provinces if the volcanic margins are considered the velocity structure of the expanded crust seaward of the continent ocean boundary differs from standard oceanic and continental crustal models based on seismic velocities this volcanic margin crust can be divided into three units of which the upper unit corresponds to basaltic extrusives the regionally consistent velocity structure and geometry of the crustal units suggest that the expanded crust including the high velocity lower crust which extends some distance landward of the continent ocean boundary was emplaced during and subsequent to breakup the volcanic margin crust was formed by excess melting within a wide zone of asthenospheric upwelling probably reflecting the interaction of a mantle plume and a lithosphere already extending

integrated wide angle and near vertical subbasalt study using large aperture seismic data from the faeroe shetland region acquiring large aperture seismic data 38 km maximum offset along a profile crossing the faeroe shetland basin in the north atlantic enables us to use wide angle reflections and refractions in addition to conventional streamer data 0 6 km for subbasalt imaging the wideangle results are complemented and confirmed by images obtained from the conventional near vertical offset range traveltime tomography applied to the wide angle data shows a low velocity layer 3 5 4 5 km s underneath southeastward thinning lava flows suggesting a 2 5 3 0 km thick sedimentary layer the velocity model obtained from traveltime tomography is used to migrate wide angle reflections from large offsets that arrive ahead of the water wave cone the migrated image shows base basalt and sub basalt reflections that are locally coincident with the tomographic boundaries application of a new multiple suppression technique and controlled stacking of the conventional streamer data produces seismic sections consistent with the wide angle results prestack depth migration of the near vertical offsets shows a continuous base basalt reflection and a clearly defined termination of the basalt flows

model sensitivity in the effect of antarctic sea ice and stratification on atmospheric pco2 1 several recent papers have demonstrated a decrease in atmospheric pco2 resulting from barriers to communication between the deep sea and the atmosphere in the southern ocean stephens and keeling 2000 decreased pco2 by increasing antarctic sea ice in a seven box model of the world ocean and toggweiler 1999 showed a similar response to southern ocean stratification in box models the pco2 of the atmosphere is controlled by the region of the surface ocean that fills the deep sea archer et al 2000a by severing the southern ocean link between the deep sea and the atmosphere atmospheric pco2 in these models is controlled elsewhere and typically declines although the models range widely in their responses continuum models such as three dimensional 3 d and 2 d general circulation models control pco2 in a more distributed way and do not exhibit box model sensitivity to high latitude sea ice or presumably stratification there is still uncertainty about the high latitude sensitivity of the real ocean until these model sensitivities can be resolved glacial pco2 hypotheses and interpretations based on southern ocean barrier mechanisms see above mentioned references plus elderfield and rickaby 2000 francois et al 1998 gildor and tziperman 2001 sigman and boyle 2000 and watson et al 2000 are walking on thin ice

crust generated above the iceland mantle plume from continental rift to oceanic spreading center since the north atlantic continental breakup in the early tertiary the process of rifting above a mantle plume has produced large thicknesses of igneous crust we report results of an integrated offshore onshore seismic study of the crust and upper mantle along a transect of the aseismic faroe iceland ridge between the continental fragment beneath the faroe islands and the present day spreading center in northeast iceland normal incidence seismic data provide an image of the uppermost crust which is complemented by a velocity model from streamer refraction analyses these data together image four sedimentary basins up to 400 m deep along the crest of the ridge the streamer refraction velocity model in turn forms the uppermost section of a full crustal velocity model derived from travel time modeling of air gun and explosive data the compressional to shear wave velocity ratio in the crust of the faroe iceland ridge is 1 77 0 02 similar to that of eastern iceland the moho beneath the faroe iceland ridge lies at a depth of 25 30 km crust of 25 30 km thickness requires an upper mantle potential temperature elevated 200 250 c above normal if formed by passive adiabatic decompression melting of the mantle or a somewhat less elevated temperature if as is likely there is a component of active convection in the underlying mantle plume core

the stratigraphy and structure of the faroese continental margin this paper presents a summary of the stratigraphy and structure of the faroese region as the faroese area is mostly covered by volcanic material the nature of the pre volcanic geology remains largely unproven seismic refraction data provide some indications of the distribution of crystalline basement which probably comprises archaean rocks with the overlying cover composed predominantly of upper mesozoic cretaceous and cenozoic strata the cenozoic succession is dominated by the syn break up faroe islands basalt group which crops out on the faroe islands where it is up to 6 6 km thick and shelf areas post break up sediments are preserved in the adjacent deep water basins including the faroe shetland basin seismic interpretation of the post volcanic strata shows that almost every sub basin in the faroe shetland basin has been affected by structural inversion particularly during the miocene these effects are also observed on the faroe platform the munkagrunnur ridge and the fugloy ridge where interpretation of low gravity anomalies suggests a large scale fold pattern the structure of the iceland faroe ridge which borders the nw part of the faroe area remains ambiguous the generally thick crust together with the absence of well defined seawards dipping reflectors may indicate that much of it is underlain by continental material

ambient noise tomography reveals basalt and sub basalt velocity structure beneath the faroe islands north atlantic abstract ambient noise tomography is applied to seismic data recorded by a portable array of seismographs deployed throughout the faroe islands in an effort to illuminate basalt sequences of the north atlantic igneous province as well as underlying sedimentary layers and precambrian basement rayleigh wave empirical green s functions between all station pairs are extracted from the data via cross correlation of long term recordings with phase weighted stacking implemented to boost signal to noise ratio dispersion analysis is applied to extract inter station group travel times in the period range 0 5 15 s followed by inversion for period dependent group velocity maps subsequent inversion for 3 d shear wave velocity reveals the presence of significant lateral heterogeneity up to 25 in the crust main features of the final model include i a near surface low velocity layer interpreted to be the malinstindur formation which comprises subaerial compound lava flows with a weathered upper surface ii a sharp velocity increase at the base of the malinstindur formation which may mark a transition to the underlying beinisvor formation a thick laterally extensive layer of subaerial basalt sheet lobes iii a low velocity layer at 2 5 7 0 km depth beneath the beinisvor formation which is consistent with hyaloclastites of the lopra formation iv an upper basement layer between depths of 5 9 km and characterized by s wave velocities of approximately 3 2 km s consistent with low grade metamorphosed sedimentary rocks v a high velocity basement with s wave velocities in excess of 3 6 km s this likely reflects the presence of a crystalline mid lower crust of archaean continental origin compared to previous interpretations of the geological structure beneath the faroe islands our new results point to a more structurally complex and laterally heterogeneous crust and provide constraints which may help to understand how continental fragments are rifted from the margins of newly forming ocean basins

morphologies and emplacement mechanisms of the lava flows of the faroe islands basalt group faroe islands ne atlantic ocean the palaeogene faroe islands basalt group fibg comprises three eruptive sequences or formations all emplaced into a subaerial environment during the development of the extensive continental flood basalt province that stretches from east greenland through the faroe islands and into the faroe shetland basin the beinisvor formation having a tabular classic facies architecture is composed of a sequence of simple flows each comprising a single sheet lobe the beinisvor formation is overlain by the distinctly contrasting malinstindur formation that has a compound braided facies architecture the enni formation occurs at the top of the sequence and consists of a mixture of simple and compound flows with tabular classic and compound braided facies architectures respectively surface and internal characteristics of the sheet lobes of the beinisvor and enni formations indicate emplacement through inflation which is more obvious for the tube fed compound flows of the malinstindur and enni formations the difference between the simple and compound flow sequences of the fibg is most likely linked to the manner in which the lava was supplied during the eruption and the eruptive style of the volcanic system the sheet lobes were erupted over laterally extensive areas from fissure systems which had a continuous supply of lava which contrasts with the tube fed compound flows which were erupted in a gradual piecemeal manner from point sourced low shield volcanoes with limited areal extents

early cenozoic saucer shaped sills of the faroe islands an example of intrusive styles in basaltic lava piles abstract the early cenozoic igneous activity of the north atlantic igneous province that generated widespread sill complexes in sedimentary basins at the nw european margins also generated various intrusive systems in the contemporaneous basaltic lava pile of the faroe islands the faroe island basalt group comprises seven formations with a total thickness of about 6 5 km of which four major formations are built up of tholeiitic lava flows each being several hundred metres thick and three thinner formations are mostly built up of volcaniclastic lithologies each being a few metres to a few tens of metres thick the largest sills are exposed as partly saucer shaped bodies in the three uppermost formations where inner gently dipping basal sill sections gradually give way to more steeply inclined discordant outer rims that commonly cut several hundred metres into overlying lava flows numerous subvertical and moderately inclined dykes ranging in thickness from c 0 5 to c 4 0 m intersect the areas affected by sill intrusion but only inclined dykes or sheets have been positively identified as sill feeders locally controlled rotations of least principal stress axes 3 during initial sill intrusion or propagation may have been an important contributing factor in determining the overall geometry of the investigated intrusions

ridge plume interaction in the north atlantic and its influence on continental breakup and seafloor spreading development of the rifted continental margins and subsequent seafloor spreading in the north atlantic was dominated by interaction between the iceland mantle plume and the continental and oceanic rifts there is evidence that at breakup time a thin sheet of particularly hot asthenospheric mantle propagated beneath the lithosphere across a 2500 km diameter region this event caused transient uplift massive volcanism and intrusive magmatism and a rapid transition from continental stretching to seafloor spreading subsequently the initial plume instability developed to an axisymmetric shape with the 100 km diameter central core of the iceland plume generating 30 40 km thick crust along the greenland iceland faeroes ridge the surrounding 2000 km diameter region received the lateral outflow from the plume causing regional elevation and the generation of thicker and shallower than normal oceanic crust we document both long term 10 20 ma and short term 3 5 ma fluctuations in the temperature and or flow rate of the mantle plume by their prominent effects on the oceanic crust formed south of iceland lateral ridge jumps in the locus of rifting are frequent above the regions of hottest asthenospheric mantle occurring in both the early history of seafloor spreading when the mantle was particularly hot and throughout the generation of the greenland iceland faeroes ridge the northern north atlantic is a classic example of continental breakup above a thermal anomaly in the mantle created by a mantle plume the most obvious consequence of this was the generation of huge volumes of basaltic magmas by partial melting of the mantle at the time of continental breakup white mckenzie 1989 most of this magma bled up to the crust where the majority perhaps 60 80 was frozen in the lower crust while the rest was extruded as lava flows and pyroclastic deposits often after undergoing fractionation in crustal magma chambers if the extrusion rates are sufficiently high lava flows may form extensive sub horizontal flood basalts extending across the continental hinterland on rifted continental margins the extrusive lavas are typically expressed as seaward dipping reflector sdr sequences that are readily identifiable on seismic reflection profiles all these features have been found along more than 2 500 km of the rifted continental margins in the northern north atlantic on both the european and the greenland sides furthermore the mantle plume that gave rise to the thermal anomaly in the mantle is still going strong lying beneath iceland at the present day the northern north atlantic is therefore an excellent natural laboratory in which to investigate the interplay between rifting and magmatism because the history of rifting is well recorded from the time of continental breakup in the early tertiary to the seafloor spreading at the present day magnetic anomaly field reversals during the tertiary were frequent and south of iceland there are few large fracture zones in the oceanic crust so the seafloor spreading magnetic anomalies contain a clear record of the rifting history throughout the ocean basin there have also been several well constrained wide angle seismic surveys of the continental margins on both sides of the atlantic e g barton white 1997b smallwood et al 1999 korenaga et al 2000 often with coincident drilling penetrating the volcanic rocks so both the structure of the rifted margins and the timing of the igneous activity are well known in this paper we examine the interaction between the mantle plume and rifting first of the continental lithosphere to form the present continental margins and subsequently of the oceanic lithosphere to form the mid ocean ridge and the land mass of iceland smallwood white rift plume interaction 3

regional uplift gas hydrate dissociation and the origins of the paleocene eocene thermal maximum much of the north atlantic region experienced hundreds of metres of transient uplift during the latest paleocene regional uplift should cause dissociation of gas hydrates stored in marine sedimentary strata and the subsequent release of carbon to the atmosphere should enhance warming through the greenhouse effect we have developed models of the spatial distribution of uplift to match observational constraints from sedimentary basins fringing the north atlantic in order to provide quantitative estimates of the carbon released upon uplift these models are coupled to a simple parameterisation of carbon storage in sub seafloor gas hydrates results of modelling indicate that large quantities of carbon can be released by uplift with some models generating over 2000 gt of carbon release the possibility that latest paleocene regional uplift occurred over the same time interval as onset of the paleocene eocene thermal maximum petm around 30 kyr cannot be ruled out using current stratigraphical observations or mantle convection models in this case the uplift mechanism alone can account for the magnitude and timing of the carbon isotope excursion associated with the petm alternatively if the duration of latest paleocene uplift was closer to the upper bound of around 1 myr or if other paleocene uplift events were equally significant then gas hydrate dissociation upon uplift could have contributed to longer term changes in ocean temperature and carbon isotope composition through the late paleocene these temperature changes in combination with development of the north atlantic land bridge during the latest paleocene culmination of uplift may be able to force rapid changes in ocean circulation and cause phenomena such as the petm

densities and porosities in the oceanic crust and their variations with depth and age we have used drilling and downhole logging results from deep sea drilling project holes 395a and 418a measured properties of core samples from the seafloor and from ophiolites and seismic velocity structure models of the oceanic crust to constrain empirical statistical relationships between in situ densities porosities and seismic velocities and to estimate densities and porosities in the oceanic crust with the following principal results our review of drilling results and studies of ophiolites suggests that the velocity structure of the oceanic crust is more closely related to changes in porosity and alteration or metamorphic grade than to its igneous structure the transition from layer 2a 3 7 0 4 km s 1 to 2b 5 2 0 4 km s 1 occurs within the extrusive pile the top of layer 2c 6 2 0 2 km s 1 is associated with the top of the sheeted dike complex and with the first appearance of greenschist facies alteration products ophiolite studies have shown that the top of layer 3 occurs within the sheeted dikes where the metamorphic grade of alteration products increases from greenschist to amphibolite facies a comparison of reprocessed and corrected downhole logs from holes 395a and 418a indicates that in situ densities and fractional porosities differ by 0 2 mg m 3 and 0 07 respectively these differences are consistent with prevailing views of the progressive alteration of the upper oceanic crust statistical relationships between in situ density porosity and slowness are highly linear and indicate that the density slowness relation estimated from laboratory samples pb 3 81 0 02 6 0 0 1 s applies to in situ properties as well we obtained a similar empirical relationship for porosities from the hole 418a log data f 0 35 0 03 2 37 0 15 s r2 0 84 s e 0 015 applying these empirical models to seismic velocity structure models yields the following crustal densities and porosities layer 2 2 62 2 69 mg m 3 0 10 0 12 and layer 3 2 92 2 97 mg m 3 the estimated average density of the oceanic crust is 2 86 0 03 mg m 3 densities and porosities estimated from a seismic crustal structure model for old atlantic crust agree very well with the logging data from hole 418a suggesting that in situ densities and porosities can be estimated from seismic velocities high density and porosity gradients are characteristic of the upper oceanic crust increasing densities reflect both a progressive decrease of porosity and increasing grain density with depth based on the seismic properties of layer 2a we estimate that the layer thins at an approximate rate 20 to 45 m m y 1 while the velocity increases by 0 040 km s 1 m y 1 the corresponding rates of change of density and porosity are 0 016 0 009 mg m 3 m y 1 and 0 005 0 003 m y 1 respectively

definitive superparamagnetic source identification through spatial temporal and amplitude analysis of airborne electromagnetic data the aim of this paper is to add confidence to existing methods using decay shape analysis to detect superparamagnetic responses in airborne electromagnetic data while expensive to acquire vertical spatial gradient measurements of the electromagnetic signals can discriminate near surface superparamagnetic sources this research investigated the use of horizontal spatial gradients and amplitude information as further indicators of superparamagnetic high horizontal gradients were shown both theoretically and in field data to help discriminate superparamagnetic from deep mineral targets further superparamagnetic responses have characteristically small amplitudes inconsistent with realistic mineral exploration targets at shallow depths

a method for precise measurement of argon 40 36 and krypton argon ratios in trapped air in polar ice with applications to past firn thickness and abrupt climate change in greenland and at siple dome antarctica we describe a method for measuring the 40ar 36ar ratio and the 84kr 36ar ratio in air from bubbles trapped in ice cores these ratios can provide constraints on the past thickness of the firn layer at the ice core site and on the magnitude of past rapid temperature variations when combined with measured 15n 14n both variables contribute to paleoclimatic studies and ultimately to the understanding of the controls on earth s climate the overall precision of the 40ar 36ar method 1 standard error of the mean is 0 012 for a sample analyzed in duplicate corresponding to 0 6 m in reconstructed firn thickness we use conventional dynamic isotope ratio mass spectrometry with minor modifications and special gas handling techniques designed to avoid fractionation about 100 g of ice is used for a duplicate pair of analyses an example of the technique applied to the gisp2 ice core yields an estimate of 11 3k of abrupt warming at the end of the last glacial period 15 000 years ago the krypton argon ratio can provide a diagnostic of argon leakage out of the bubbles which may happen naturally during bubble close off or artifactually if samples are warmed near the freezing point during core retrieval or storage argon leakage may fractionate the remaining 40ar 36ar ratio by 0 007 per change in 84kr 36ar introducing a possible bias in reconstructed firn thickness of about 2 m if thermal diffusion is not accounted for or 6 m if thermal diffusion effects are quantified with measured 15n 14n reproducibility of 84kr 36ar measured in air is about 0 2 1 standard error of the mean but is about 1 for ice core samples ice core samples are systematically enriched in 84kr 36ar relative to atmosphere by 5 probably reflecting preferential size dependent exclusion of the smaller argon atom during bubble entrapment recent results from the siple dome ice core reveal two climate events during the last deglaciation including an 18 m reduction in firn thickness associated with an abrupt warming at sometime between 18 and 22 kyr bp and a partial or total removal of the firn during an ablation event at 15 3 kyr bp

a geophysical study of the hatton bank volcanic margin a summary of the results from a combined seismic gravity and magnetic experiment abstract this paper summarizes the results of a geophysical study of the hatton bank volcanic margin in the north atlantic normal incidence and synthetic aperture profiles saps show seaward dipping reflectors interpreted as subaerial basalt flows on the continental slope and in the adjacent ocean basin velocity depth profiles derived from expanding spread profiles esps shot along strike show elevated velocities in the lower crust on the continental slope the high velocities are interpreted as evidence for igneous underplating or intrusion gravity modelling shows high densities in the lower crust supporting a model of igneous accretion an ocean bottom seismometer obs and multichannel refraction line shot perpendicularly to strike through the midpoints of the esps further constrained the velocity structure across the margin the velocity model was refined as each dataset was processed clearly showing that multiple datasets were required in order to constrain the velocity structure tightly in particular esps were found to produce misleading results in places where the assumption that the sub surface structure is one dimensional inherent in the esp technique was not valid the final velocity model is used to infer the thickness of igneous material accreted across the hatton margin which is estimated to increase westwards from zero at the top of the bank to 18 km at the bottom of the continental slope and then thin rapidly to 11 km in the adjacent iceland basin as the oceanic crust thins its seismic velocity decreases suggesting a decrease in percentage partial melting these observations strongly support a model in which volcanic margins are produced by elevated asthenospheric temperatures

crustal structure beneath the faroe islands and the faroe iceland ridge abstract we have mapped the transition from the continental faroe block the faroe islands and surrounding shelf to the thickened oceanic crust of the faroe iceland ridge in the north atlantic using the results of a detailed sea to land seismic profile with wide angle to normal incidence recordings of explosive and airgun shots fired at sea along the faroe iceland ridge interpretation of all available seismic and gravity data indicates that this aseismic ridge is composed of 30 3 km thick oceanic crust with a gradual transition to ancient continental crust from 100 to 40 km northwest of the faroe islands close to the shelf edge this confirms that the crust beneath the faroe islands which may be up to 46 km thick comprises continental material in agreement with previous seismic and geochemical results results suggest that the upper 5 2 0 7 km of the faroe crust consists of tertiary basalts generated during continental breakup overlying the continental crust beneath the lower crust where seismic constraint is poor may exhibit high seismic velocities 7 1 7 6 km s 1 which we attribute to underplating or intrusion by mafic melts during continental breakup in the early tertiary

lispb revisited a new look under the caledonides of northern britain summary the lithospheric seismic profile in britain lispb shot in 1974 included a 700 km long north south profile of the crust and upper mantle of northern britain the wide angle seismic data from lispb was of exceptional quality and coverage making a reinterpretation using current modelling programs worthwhile this reinterpretation shows that the original crustal model published by bamford et al 1978 satisfies the observed first arrival traveltimes but that further detail of crustal velocity structure may be obtained by model shows strong variations in crustal velocity structure that appear to be correlated with geological and tectonic structures mapped at the surface the new lithospheric model is compatible with the observed gravity field when converted to appropriate densities and the variations in loading along the base of the density model show that the lithosphere is close to local isostatic equilibrium profiles of the variation of velocity and density with depth from the new model are compared with measurements of lewisian rocks made at confining pressures up to 8 kbar allowing the seismic model to be interpreted geologically a synthetic normal incidence profile of the new model is superimposed on the nearby nec deep seismic reflection profile and the features imaged by the two techniques are compared

palaeocene recent plate boundaries in the ne atlantic and the formation of the jan mayen microcontinent abstract breakup and sea floor spreading between greenland and eurasia established a series of new plate boundaries in the north atlantic region since the late palaeocene a conventional kinematic model from pre breakup to the present day assumes that eurasia and greenland moved apart as a two plate system however new regional geophysical datasets and quantitative kinematic parameters indicate that this system underwent several adjustments since its inception and suggest that additional short lived plate boundaries existed in the ne atlantic among the consequences of numerous plate boundary relocations is the formation of a highly extended or even fragmented jan mayen microcontinent and subsequent deformation of its margins and surrounding regions the major oligocene plate boundary reorganization and microcontinent formation might have been precluded by various ridge propagations and or short lived triple junctions ne and possibly sw of the jan mayen microcontinent from the inception of sea floor spreading 54 ma to c18 40 ma our model implies a series of failed ridges offshore the faeroe islands a northern propagation of the aegir ridge ne of the jan mayen microcontinent and a series of triple junctions and or propagators in the southern greenland basin

a 3d regional crustal model of the ne atlantic based on seismic and gravity data abstract we present a 3d regional crustal model for the north atlantic which is based on the integration of seismic constraints and gravity data the model addresses the crustal thickness geometry and includes information on sedimentary thickness the presence of high velocity zones in the lower crust and information on the crustal density distribution in the continental and oceanic domains using an iterative forward and inverse modelling approach we adhere to the seismic constraints within their uncertainty but manage to enhance the crustal geometry in areas where seismic data are sparse or absent a number of basins are resolved with more detail recently released seismic reflection data beneath the ne greenland shelf allowed for a major improvement of the crustal thickness estimates estimated moho depths beneath the basins there vary between 15 and 25 km which is compatible with the conjugate norwegian margin a major lower crustal seismic velocity anomaly in the vicinity of the greenland iceland faroe ridge complex is supported by density modelling we discuss the validity and uncertainties of our model assumptions and discuss the correlation with the main structural elements of the north atlantic

convective partial melting 1 a model for the formation of thick basaltic sequences during the initiation of spreading stretching and thinning of continental lithosphere during rifting and initiation of seafloor spreading cause upwelling of mantle rocks into a pressure temperature environment where partial melting occurs the amount of melt liberated is however insufficient to account for the thickness of igneous crust emplaced near the continent ocean boundary at some passive margins these igneous structures are known from seismic reflection and deep refraction studies on numerous passive margins worldwide seismic studies also suggest that the transition from continental to oceanic crust on these volcanic margins is relatively abrupt their evolution apparently does not include a prolonged extensional phase preceding the inception of spreading to account for these observations we propose a model for the generation of partial melt in which the lateral temperature contrasts evoked by rifting and asthenospheric upwelling drive convection in the upper mantle deep hot mantle material is transported upward by the convection and hence much larger volumes flow through the region where partial melting occurs than during passive upwelling numerical calculations show that convective partial melting can provide substantial additions to melting due to passive upwelling alone using this model we propose an evolutionary scheme for volcanic passive margins emphasize the contrast with nonvolcanic margins and describe conditions which might lead to the development of both types

quantifying transient mantle convective uplift an example from the faroe shetland basin the icelandic mantle plume generated maximum uplift in early paleogene times the faroe shetland basin which fringes the north atlantic margin of europe was close to the center of early paleogene icelandic plume activity three dimensional seismic reflection data from the faroe shetland basin reveal sedimentary geometries that allow a phase of transient uplift to be accurately reconstructed and quantified close to the paleocene eocene boundary circa 56 ma rapid uplift resulted in fluvial incision into marine sediments this unconformity was buried by nonmarine sediments recording the decay of transient uplift relief on the unconformity was 550 m constraining the minimum amount of surface uplift some 60 m of this uplift can be attributed to the isostatic response to erosional unloading tectonic uplift of over 490 m peaked and decayed within 3 ma rates of water loaded tectonic subsidence following peak uplift are several times greater than maximum expected postrift subsidence rates the amplitude and duration of this transient effect is best explained by a mantle convective phenomenon we suggest that a region of hot plume material flowed laterally beneath the lithosphere producing transient uplift which decayed when plume material was advected farther away our analysis suggests that under certain circumstances stratigraphic records can yield valuable quantitative information about aspects of mantle convective circulation

mechanisms controlling the global oceanic distribution of the inert gases argon nitrogen and neon 1 dissolved inert gas measurements in the ocean yield important information about processes that occur during water mass formation we present argon nitrogen and neon data from the subtropical and subpolar north pacific and the subtropical north atlantic all three gases were supersaturated at the surface in the deep ocean ar and n2 were undersaturated while ne remained supersaturated all the data fell within the range predicted by a quasi steady state mixed layer model that allows only temperature change and diffusive and bubble mediated gas exchange this result suggests that these three processes are the first order controls on the global oceanic distribution of inert gas saturations

continent ocean transition on the voring plateau ne atlantic derived from densely sampled ocean bottom seismometer data 1 a 180 km long seismic wide angle profile was acquired across the voring plateau ne atlantic using a tuned air gun array and three component ocean bottom seismometers deployed with 5 km spacing the seismic p wave data have been modeled by ray tracing inversion and the model has been constrained by s wave and gravity modeling the data and modeling have allowed us to depict the crustal structure and nature of the continent ocean transition cot in a classical volcanic margin case the p wave velocity near the top of the main crustal layer is estimated to 6 0 km s landward of the 25 km wide cot the seaward increase to 6 5 km s in the cot is conformable with heavily intruded continental crust within this zone farther seaward the velocity increase to 6 9 km s in the same layer suggests the presence of oceanic crust the abundant magmatism landward of and within the cot is primarily observed as extrusives forming wedges of seaward dipping reflectors and mafic lower crustal intrusions underplating the maximum thickness of the oceanic crust is measured to 23 5 km at the seaward termination of the cot

magmatism at rifted continental margins the intense volcanism and uplift observed on many rifted continental margins forming basaltic seaward dipping reflector sequences is accompanied by the emplacement of a thick igneous section at depth partial melting by decompression of passively upwelling asthenosphere that is hotter than normal because it is near a hotspot explains both the thickness of the new igneous crust and the initial elevation of the rifted margins

a refraction seismic transect from the faroe islands to the hatton rockall basin 1 the crustal structure of the faroe rockall plateau was studied by a 790 km long refraction seismic transect consisting of two intersecting lines the air gun shot spacing was 200 m and the signals were recorded by 77 ocean bottom seismometers a p wave velocity model was developed from forward and inverse modeling of the wide angle seismic data and incorporation of coincident multichannel reflection seismic data continental crust with velocities ranging from 5 6 to 6 8 km s can be traced from the faroe islands across the banks to the sw of the faroes and into the hatton rockall basin the thickness of the subvolcanic crust is up to 25 km on the banks but is as little as 8 km in the channels between the banks the thinning in the channels may be related to nw trending shear zones extending from major lineaments in ne rockall trough basalt layers are found along the entire transect with a total thickness of up to 4 km two layers with velocities of 4 9 5 2 and 5 3 5 6 km s are thought to represent paleogene flood basalts that can be correlated from the faroe islands to george bligh bank close to george bligh bank an 80 km wide and up to 9 km thick body with velocities of 6 5 km s is interpreted as intrusion a 5 km thick high velocity lower crustal layer 7 3 km s extends from the area of the intrusion into the northern hatton rockall basin at the northern flank of lousy bank the transition zone to oceanic crust was encountered

a hot spot model for early tertiary volcanism in the n atlantic s u m m a r y the initial stages of rifting of the n atlantic were accompanied by massive extrusion of volcanic rocks and the accretion to the lower crust of even greater volumes of igneous rock emplacement of the bulk of the igneous rocks along the 2000 km long continental rift occurred in as little as 2 3 my the pattern and timing of magmatic activity can be explained by partial melting of anomalously hot asthenosphere as it welled up to fill the space left by the stretched and thinned continental lithosphere asthenosphere temperatures were elevated regionally by 100 150 over a 2000 km diameter region by the mushroom shaped head of hot material fed by a mantle plume which at present is centred beneath iceland

regional anomalies of sediment thickness basement depth and isostatic crustal thickness in the north atlantic ocean abstract we calculate the anomalous basement topography for the north atlantic ocean from 30 to 70 n latitude and from 0 to 70 w longitude at a resolution of roughly 6 6 km using grids of total sediment thickness and observed and predicted sea floor bathymetry to correct for the effects of isostatic sediment loading and lithospheric age plotting this residual topography for various plate reconstructions during opening of the north atlantic we delineate consistent patterns of basement highs related to variations in hotspot related volcanism in addition to iceland and the azores we recognize three centers of excess volcanism at the mid atlantic ridge the milne seamounts and azores biscay rise 75 40 ma the southeast newfoundland ridge and madeira tore rise 130 110 ma and the east and west thulean rises 60 50 ma the duration of volcanic activity ranges from 8 to 10 m y thulean rises to 60 m y iceland and thus it appears that both long and short lived hotspots coexist even in relatively close proximity in contrast during the period 110 60 ma we observe little excess volcanism during either continental breakup or seafloor spreading we estimate isostatic crustal thickness from the anomalous basement depths after first removing dynamic effects created by mantle flow maximum thicknesses of volcanic features from 30 km beneath the greenland iceland faeroe ridge to 15 km beneath the azores biscay rise are broadly consistent with seismic data and predictions of decompression melting widths of volcanic features indicate that thickening primarily occurs within 100 200 km of hotspots except along continental margins that rifted at the time of the hotspot activity i e east greenland and the hatton rockall bank we observe conjugate structures south of greenland and edoras bank where excess volcanism appears to have extended beyond the margin proper and into oceanic crust similar conjugate features appear in the labrador sea south of davis strait finally we identify anomalous oceanic regions adjacent to some continental margins where unusually low values of predicted crustal thickness suggest either additional variations in plate properties or non isostatic effects within the mantle

a new look at the rockall region offshore ireland abstract a 700 km wide angle reflection refraction profile carried out in the central north atlantic west of ireland crossed the erris trough rockall trough and rockall bank and terminated in the western hatton rockall basin the results reveal the presence of a number of sedimentary basins separated by basement highs the rockall trough with a sedimentary pile up to 5 km thick is underlain by thinned continental crust 8 10 km thick some major fault block structures are identified especially on the eastern margin of the rockall trough and in the adjacent erris trough the hatton rockall basin is underlain by westward thinning continental crust 22 10 km thick sedimentary strata are up to 5 km thick the strata in the rockall trough and hatton rockall basin probably range in age from late palaeozoic to cenozoic however the basins have different sedimentation histories and differ in structural style the geometry of the crust and sediments suggests that the rockall trough originated by pure shear crustal stretching associated with rift deposits and cenozoic thermal sag strata in contrast the development of the erris trough located on unthinned continental crust was facilitated by shallow brittle extension with little deep crustal attenuation a two layered crust occurs throughout the region the lower crustal velocity in the hatton rockall basin is higher than that in the rockall trough the velocity structure shows no indication of crustal underplating by upper mantle material in the region

the hatton bank continental margin ii deep structure from two ship expanding spread seismic profiles summary the continent ocean transition adjacent to hatton bank was studied using a dense grid of single ship and two ship multichannel seismic profiles the interpretation of the explosive expanding spread profiles esps which were shot as part of this survey are discussed here in detail extensive seaward dipping reflectors are developed in the upper crust across the entire margin these seaward dipping reflectors continue northwards on the faeroes and voring margins where they have been shown to be caused by basaltic lavas as well as on the conjugate margin of east greenland the dipping reflectors are an important feature of the rifting history of the margin and show that extensive volcanism was associated with the extension the esps show clear seismic arrivals out to ranges of 100 km wide angle moho reflections can be seen on all the lines as well as good mid and lower crustal arrivals the determination of seismic velocity structure was constrained by ray tracing and by amplitude modelling using reflectivity synthetic seismograms the results from the esps show that there is a thick region of lower crustal material beneath the margin with an unusually high crustal velocity of 7 3 7 4 km s this lower crustal material reaches a maximum thickness of 14 km beneath the central part of the margin and is terminated at depth by the moho the lower crustal lens of high velocity material is interpreted as underplated or intruded igneous rocks associated with the large volumes of extrusive basaltic lavas now seen as dipping reflectors on the margin

exploration in the shetland faeroe basin using densely spaced arrays of ocean bottom seismometers recent exploration activity in the peripheral regions of the shetland faeroe basin offshore northwest scotland has led to the discovery of some of the largest oil reserves on the united kingdom uk continental shelf we present results from two ocean bottom seismometer profiles acquired by mobil north sea ltd across the center of the shetland faeroe basin these data provide a powerful tool for delineating long wavelength velocity variations and thus have potential for reducing the nonuniqueness associated with conventional seismic exploration methods analysis of the first arrival traveltime data using both forward and inverse ray based techniques produces a well constrained velocity depth model of the basin fill we estimate that the uncertainty in the velocity structure is 5 from a series of trial and error perturbations applied to the final models the velocity structure of the faeroe basin has three principal layers 1 a near surface layer with velocities in the range 1 6 to 2 2 km s 2 a 3

crustal accretion at the reykjanes ridge 61 62 n we report results of a seismic gravity and magnetic survey of the reykjanes ridge spreading center at 61 62 n about 600 km from the center of the iceland mantle plume anomalously shallow water on the ridge crest enabled us to record seismic refractions on a 2 4 km hydrophone streamer the velocity within layer 2a is 2 4 0 3 km s 1 and its mean thickness is 400 100 m the velocity at the base of layer 2a is 3 3 0 3 km s 1 on the ridge axis increasing with crustal age to 4 0 km s 1 at 1 5 ma and 4 5 km s 1 at 5 ma assuming that seismic layer 2a on the ridge axis is also the extrusive layer i e the magnetic source layer we have successfully modeled the variations in amplitude of the magnetic field the best magnetic model includes enhanced magnetization within layer 2a at the sites of recent volcanic activity as independently recognized in side scan sonar data we also present a full crustal seismic model based on wide angle seismic recordings on digital ocean bottom hydrophones and disposable sonobuoys the seismic model is complemented by gravity modeling which further suggests that the ridge crest is in isostatic equilibrium the zero age crust is 10 0 km thick while crust of age 5 ma is 7 8 km thick these crustal thicknesses are greater than those of normal oceanic crust which we attribute to the presence of anomalously hot asthenospheric mantle beneath the spreading center we suggest that the variation in thickness between 0 ma and 5 ma crust is caused by temporal variation in the plume fed asthenospheric temperature beneath the reykjanes ridge

a comparison between airguns and explosives as wide angle seismic sources the relative merits of a 48 gun 9324 cu in 153 litre airgun array and a 200 kg explosive source are considered for the purposes of long range 0 400 km refraction seismic work with particular reference to traveltime modelling theoretical source calculations indicate that in the frequency range 2 5 12 0 hz the airgun source will produce an rms pressure 8 of that produced by the explosive source and an initial burst pressure 17 of that produced by the explosive source observed data support these calculations at short ranges and illustrate the greater attenuation of the airgun signal with range due to its lack of very low frequency 5 hz content at short offsets the airgun array provides a preferable seismic source to the explosives due to densely spaced shots and a consistent waveform resulting in excellent trace to trace coherence with increasing offsets it may be necessary to stack the airgun data to enhance its signal to noise ratio here we use a 4 fold stack large explosive shots although more powerful produce a less consistent waveform and are more widely spaced due to operational constraints the offset at which airguns provide a preferable source is dependent on the ambient noise this practical comparison of real sources demonstrates that even without advanced processing a well tuned airgun array may provide a preferable source to explosives at offsets up to 160 km under favourable experimental conditions

precise determination of ar kr and xe isotopic fractionation due to diffusion and dissolution in fresh water abstract dissolved noble gases are ideal conservative tracers of physical processes in the earth system due to their chemical and biological inertness although bulk concentrations of dissolved ar kr and xe are commonly measured to constrain physical models of atmosphere ocean and terrestrial hydrosphere processes stable isotope ratios of these gases e g 136xe 129xe are seldom used because of low signal to noise ratios here we present the first results from a new method of dissolved gas sampling extraction and analysis that permits measurement of stable ar kr and xe isotope ratios at or below 5 per meg amu 1 precision 1 two orders of magnitude below conventional kr and xe isotopic measurements this gain in precision was achieved by quantitative extraction and subsequent purification of dissolved noble gases from 2 l water samples via helium sparging and viscous dual inlet isotope ratio mass spectrometry we have determined the solubility fractionation factors sol for stable ar kr and xe isotope ratios between 2 and 20 c via laboratory equilibration experiments we have also conducted temperature controlled air water gas exchange experiments to estimate the kinetic fractionation factors kin of these isotope ratios we find that both sol and kin normalized by isotopic mass difference m decrease in magnitude with atomic number but are proportional to m for isotope ratios of the same element with the new ability for high precision isotopic measurements we suggest that dissolved kr and xe isotope ratios in groundwater represent a promising novel geochemical tool with important applications for groundwater modeling water resource management and paleoclimate

mantle thermal structure and active upwelling during continental breakup in the north atlantic seismic reflection and refraction data acquired on four transects spanning the southeast greenland rifted margin and greenland iceland ridge gir provide new constraints on mantle thermal structure and melting processes during continental breakup in the north atlantic maximum igneous crustal thickness varies along the margin from s 30 km in the near hotspot zone 6 500 km from the hotspot track to v18 km in the distal zone 500 1100 km magmatic productivity on summed conjugate margins of the north atlantic decreases through time from 1800 300 to 600 50 km 3 km ma in the near hotspot zone and from 700 200 to 300 50 km 3 km ma in the distal zone comparison of our data with the british faeroe margins shows that both symmetric and asymmetric conjugate volcanic rifted margins exist joint consideration of crustal thickness and mean crustal seismic velocity suggests that along margin changes in magmatism are principally controlled by variations in active upwelling rather than mantle temperature the thermal anomaly vt at breakup was modest v100 125 c varied little along the margin and transient data along the gir indicate that the potential temperature anomaly 125 50 c and upwelling ratio v4 times passive of the iceland hotspot have remained roughly constant since 56 ma our results are consistent with a plume impact model in which 1 a plume of radius v300 km and vt of v125 c impacted the margin around 61 ma and delivered warm material to distal portions of the margin 2 at breakup 56 ma the lower half of the plume head continued to feed actively upwelling mantle into the proximal portion of the margin and 3 by 45 ma both the remaining plume head and the distal warm layer were exhausted with excess magmatism thereafter largely confined to a narrow 6 200 km radius zone immediately above the iceland plume stem alternatively the warm upper mantle layer that fed excess magmatism in the distal portion of the margin may have been a pre existing thermal anomaly unrelated to the plume fl 2001 elsevier science b v all rights reserved

eucrust 07 a new reference model for the european crust 1 we present a new digital model eucrust 07 for the crust of western and central europe and surroundings 35n 71n 25w 35e available results of seismic reflection refraction and receiver functions studies are assembled in an integrated model at a uniform grid 15 0 15 0 the model consists of three layers sediments and two layers of the crystalline crust besides depth to the boundaries we provide average p wave velocities in the upper and lower parts of the crystalline crust the new model demonstrates large differences in the moho depth compared to previous compilations over 10 km in some specific areas e g the baltic shield furthermore the velocity structure of the crust is much more heterogeneous than in previous maps eucrust 07 offers a starting point for numerical modeling of deeper structures by allowing correction for crustal effects beforehand and to resolve trade off with mantle heterogeneities citation tesauro m m k kaban and s a p l cloetingh 2008 eucrust 07 a new reference model for the european crust geophys res lett 35 l05313 doi 10 1029 2007gl032244

estimation and geologic interpretation of regolith chargeability and superparamagnetic susceptibility in airborne electromagnetic datainterpretation of chargeability and spm in aem

an integrated gravity model for europe s crust and upper mantle we present an integrated gravity model of the european lithosphere based on an analysis of a number of new data sets leading to a much higher resolution than provided by previous models first of all a recent crustal model eucrust 07 is used to quantify the crustal contribution to the observed gravity field and to identify the effect of mantle heterogeneity the new gravity field model is based on a combination of satellite champ and grace and terrestrial data we also use these data sets to estimate residual mantle gravity anomalies and residual topography reflecting the effect of mantle density variations induced by temperature and compositional heterogeneity the separation of these effects is vital for a proper assessment of mantle structure and evolution in addition we utilize a new tomographic model for p and s velocity anomalies beneath europe which is a priori corrected for crustal structure using eucrust 07 the seismic velocity anomalies were subsequently converted into temperature anomalies using a mineral physics approach we estimate the effect of temperature variations on the gravity field and subtract it from the total mantle field the residual fields point to an important role of compositional density anomalies in the upper mantle a number of key features of the compositional density distribution so far invisible in seismic tomography data are detected for the first time

the onset of walvis ridge plume influence at the continental margin highlights crustal structure of walvis ridge reveals high seismic velocities in the lower crust intruding the african continent this modified crust is localized to approx 100 100 km within the continent no indication for a large plume head observed the opening of the south atlantic is a classical example for a plume related continental breakup flood basalts are present on both conjugate margins as well as aseismic ridges connecting them with the current plume location at tristan da cunha to determine the effect of the proposed plume head on the continental crust we acquired wide angle seismic data at the junction of the walvis ridge with the african continent and modelled the p wave velocity structure in a forward approach the profile extends 430 km along the ridge and continues onshore to a length of 720 km crustal velocities beneath the walvis ridge vary between 5 5 km s and 7 0 km s a typical range for oceanic crust the crustal thickness of 22 km however is approximately three times larger than of normal oceanic crust the continent ocean transition is characterized by 30 km thick crust with strong lateral velocity variations in the upper crust and a high velocity lower crust hvlc where velocities reach up to 7 5 km s the hvlc is 100 to 130 km wider at the walvis ridge than it is farther south and impinges onto the continental crust of the kaoko fold belt such high seismic velocities indicate mg rich igneous material intruded into the continental crust during the initial rifting stage however the remaining continental crust seems unaffected by intrusions and the root of the 40 km thick crust of the kaoko belt is not thermally abraded we conclude that the plume head did not modify the continental crust on a large scale but caused rather local effects thus it seems unlikely that a plume drove or initiated the breakup process we further propose that the plume already existed underneath the continent prior to the breakup and ponded melt erupted at emerging rift structures providing the magma for continental flood basalts

the five stable noble gases are sensitive unambiguous tracers of glacial meltwater the five inert noble gases he ne ar kr and xe exhibit a unique dissolved gas saturation pattern resulting from the formation and addition of glacial meltwater to seawater he and ne become oversaturated and ar kr and xe become undersaturated to varying percentages for example addition of 10 glacial meltwater to seawater results in a saturation anomaly of he 12 8 ne 8 9 ar 0 5 kr 2 2 and xe 3 3 this pattern in noble gas saturation reflects a unique meltwater signature that is distinct from the other major physical processes that modify the gas concentration and saturation namely seasonal changes in temperature at the ocean surface and bubble mediated gas exchange we use optimum multiparameter analysis to illustrate how all five noble gases can help distinguish glacial meltwater from wind driven bubble injection making them a potentially valuable suite of tracers for glacial melt and its concentration in the deep waters of the world ocean