sea ice motion in response to geostrophic winds it is an old rule of thumb that sea ice moves with a speed of about 2 of the surface wind and about 45 to the right of the wind a similar relationship between the ice velocity and the geostrophic wind is examined here it is found that only about half of the long term several month average ice motion is directly related to the geostrophic wind the other half being due to the mean ocean circulation on shorter time scales and in all seasons more than 70 of the variance of the ice velocity in the central arctic ocean is explained by the geostrophic wind within about 400 km of the coasts the geostrophic wind is less successful in explaining the ice motion the spatial variations in ice velocity are also partly explained by the geostrophic wind about half of the variance in the large scale ice vorticity and shear are accounted for on the other hand none of the large scale ice divergence can be explained by the wind the long term average ocean current is estimated by subtracting the share of the ice motion caused by the wind from the total ice motion

utilizing the aspect sea ice thickness data set to evaluate a global coupled sea ice ocean model 1 simulated sea ice thickness in the orca2 lim coupled sea ice ocean model is compared with thicknesses from the antarctic sea ice processes and climate aspect database we find a qualitative agreement of the large scale patterns of ice thickness distribution regional averages for the various sectors of the southern ocean yield a very good correspondence between observations and model data exceptions are the eastern bellingshausen and northwestern weddell seas a poor representation of the antarctic peninsula in the atmospheric forcing data and the related overestimation of westerly winds in this region lead to a spurious accumulation of sea ice on the western side of the peninsula and to an underestimation of sea ice coverage on the eastern side since the spatial scale of observations is not comparable to the size of a model grid cell there is little agreement between individual observations and the corresponding model ice thicknesses a model analysis of the seasonal and interannual variability indicates that the aspect data underestimate the climatological ice thickness in the central and southern weddell sea and the eastern ross sea by up to 1 m because of a winter bias in the observations an overestimation of similar magnitude is expected in the bellingshausen sea ice thickness data in most of the indo pacific sector appear to be representative for the long term climatology a model estimate of the bias is used to compute a revised distribution of climatological sea ice thickness

crustal lineaments distribution of lower crustal intrusives and structural evolution of the voring margin ne atlantic new insight from wide angle seismic models abstract five lineaments on the volcanic voring margin ne atlantic have been identified in crustal scale models derived from ocean bottom seismograph obs data it is suggested that the voring basin can be divided in four compartments bounded by the jan mayen fracture zone lineament a new lineament defined from this study the gleipne lineament the surt lineament and the bivrost lineament the nw se trending jan mayen gleipne and bivrost lineaments probably represent old zones of weakness controlling the onset of the early eocene seafloor spreading whereas the surt and new lineaments rotated ca 30 symmetrically from the azimuth of the gleipne lineament may represent adjustment features related to the early cretaceous early tertiary rifting the longest landward extent of a lower crustal high velocity body assumed to represent intrusions related to the last phase of rifting is found between the new lineament and the gleipne lineament where the body extends across the helland hansen arch northeastwards in the voring basin the landward limit of the body steps gradually seawards closely related to the interpreted lineaments northeast of the gleipne lineament the body terminates close to the fles fault complex north of the surt lineament it extends across the nyk high and northeast of the bivrost lineament the intrusions terminate around the voring escarpment evidence for an interplay between active and passive rifting components is found on regional and local scales on the margin the active component is evident through the decrease in magmatism with increased distance from the icelandic plume and the passive component is documented through the fact that all found crustal lineaments to a certain degree acted as barriers to magma emplacement the increased thickness of the continental crust on the seaward side of the voring escarpment the upwarping of moho and thinning of the lower crustal high velocity layer in the western part of the voring basin as well as a strong shallowing of the moho observed in parts of the area between the jan mayen fracture zone lineament and the new lineament can be explained by lithospheric delamination models

influence of tides on sea ice in the weddell sea investigations with a high resolution dynamic thermodynamic sea ice model 1 the effects of tides on the seasonal fluctuations and regional differences of the sea ice cover in the weddell sea are investigated using a dynamic thermodynamic sea ice model operating at high temporal and spatial resolution tidal currents add a highly variable stress force at the ice water interface they are derived from an eight constituent barotropic ocean only tidal model thermodynamics are calculated for seven ice classes plus open water dynamics consider an averaged ice class and open water only results from runs of the sea ice model with and without the additional tidal stress demonstrate that tidal currents alter the local and to lesser extent basin wide evolution of sea ice the tidal currents reduce the expansion of the ice cover mostly cause a speed up of the retreat and lead to a smaller minimum ice extent the most significant local differences between the two model simulations are found in tidally active regions over the continental shelf breaks of the western weddell sea in front of filchner ronne ice shelf and offshore dronning maud land

estimation of sea ice thickness in ross and weddell seas from ssm i brightness temperatures in polar regions ocean atmosphere interactions are strongly influenced by sea ice and its thickness since satellite passive microwave observations became available in the 1970s significant progress has been made in the study of snow depth and sea ice concentration and extent in these regions estimating sea ice thickness sit instead turned out to be considerably more difficult we present a new empirical algorithm to estimate sit in the ross and weddell seas from special sensor microwave imager brightness temperatures this algorithm combines brightness temperature polarization difference and ratio values to obtain sit for seasonal ice up to a thickness of about 90 cm during freezing conditions a series of filters accounts for open water new ice and snow on sea ice our sit estimates are consistent with colocated visual ship based sit observations made according to the antarctic sea ice processes and climate project showing linear correlation values between 0 73 and 0 96 and root mean square error values between 14 and 24 cm the seasonal development of the region average sit derived with our approach agrees with the corresponding values derived from u s national ice center ice charts comparison with colocated polynya distribution maps suggests that the algorithm could be optimized for its performance with regard to sit values around 50 cm and that a closer investigation of the snow impact on the sit retrieval is required

atmospheric and oceanic forcing of weddell sea ice motion the data from sea ice buoys which were deployed during the winter weddell sea project 1986 the winter weddell gyre studies 1989 and 1992 the ice station weddell in 1992 the antarctic zone flux experiment in 1994 and several ship cruises in austral summers are uniformly reanalyzed by the same objective methods geostrophic winds are derived after matching of the buoy pressure data with the surface pressure fields of the european centre for medium range weather forecasts the ratio between ice drift and geostrophic wind speeds is reduced when winds and currents oppose each other when the atmospheric surface layer is stably stratified and when the ice is under pressure near coasts over the continental shelves the spatial inhomogeneity of tidal and inertial motion effectively controls the variability of divergence for periods below 36 hours far from coasts speed ratios which presumably reflect internal stress variations in the ice cover are independent of drift divergence on the spatial scale of 100 km to study basin scale ice dynamics all ice drift data are related to the geostrophic winds based on the complex linear model thorndike and colony 1982 for daily averaged data the composite patterns of mean ice motion geostrophic winds and geostrophic surface currents document cyclonic basin wide circulations geostrophic ocean currents are generally small in the weddell sea significant features are the coastal current near the southeastern coasts and the bands of larger velocities of 6 cm s 1 following the northward and eastward orientation of the continental shelf breaks in the western and northwestern weddell sea in the southwestern weddell sea the mean ice drift speed is reduced to less than 0 5 of the geostrophic wind speed and increases rather continuously to 1 5 in the northern central and eastern weddell sea the linear model accounts for less than 50 of the total variance of drift speeds in the southwestern weddell sea and up to 80 in the northern and eastern weddell sea

igneous intrusions in the faroe shetland basin and their implications for hydrocarbon exploration new insights from well and seismic data igneous sills and dykes that intrude pervasively into prospective sedimentary basins are a common occurrence in volcanic margins impacting the petroleum system and causing geological and technical drilling challenges during hydrocarbon exploration the faroe shetland basin fsb ne atlantic margin has been the focus of exploration for over 45 years with many wells penetrating igneous intrusions utilising 29 fsb wells with 251 intrusions and 3d seismic data this study presents new insights into the impacts that igneous intrusions have on hydrocarbon exploration examination of cores reveals additional igneous material in individual wells compared to estimates using seismic or petrophysical data alone leading to potential underestimation of the volume of the igneous component in a basin furthermore analysis of petrophysical data shows that within the fsb there are silicic intrusions such as diorite and rhyolite in addition to the commonly encountered mafic intrusions these silicic intrusions are difficult to recognise in seismic and petrophysical data due to their low density and compressional velocity and have historically been misidentified on seismic reflection data as exploration targets drilling data acquired through intrusions provide valuable insight into the problems exploration wells can encounter often unexpectedly many of which can be detrimental to safe drilling practice and result in prolonged non productive time

the sea ice thickness distribution in the northwestern weddell sea we present new data on distribution of snow and sea ice thicknesses in the northwestern weddell sea the data were obtained through direct measurements along 19 profiles each approximately 100 m long on 17 different floes located between 54 46 w and 59 64 s the overall probability density functions pdfs for ice thicknesses reflect the complex mixture of first second and multi year ice to be expected in the outflowing branch of the weddell gyre further differentiation of the data reveals four distinct thickness classes which reflect differences in the formation and subsequent histories of the ice encountered these classes i iv represent strongly deformed first year ice less deformed first and second year ice and deformed second or multi year ice respectively each of the classes is characterized by a specific set of quantities related to ice texture and surface snow characteristics and by distinct pdfs for snow and ice thicknesses in addition geometric surface and bottom roughness characteristics differ significantly for each of the floe classes

application of allochthonous organic carbon and phosphorus forms in the interpretation of past environmental conditions organic matter in sediments for instance carbon nitrogen and phosphorus can be used to reconstruct the paleoecological and pollution history of lakes and their catchment basins in this paper the contents of allochthonous organic carbon allochthonous oc and autochthonous organic carbon autochthonous oc in sediment cores taken from wuliangsuhai lake and daihai lake in northern china are quantified by using a binary model and phosphorus forms in the sediment cores from the two lakes are extracted by sequential extraction techniques the results indicate that the palaeoenvironment and paleoclimate of daihai lake and its catchment basin in the recent 250 years can be well reconstructed based on the content of allochthonous oc the climate was relatively humid and warm in the period of 1865 2005 while relatively dry and cold in the period of 1765 1865 the sedimentary information of allochthonous oc in the 22 42 cm portion of the sediment cores in daihai lake corresponds to the final cold fluctuation of the little ice age that occurred since the middle holocene the difference of phosphorus forms in the sediment cores between the two lakes indicates that phosphorus input to the lakes and the correlation between phosphorus forms and distribution and the changes of environment are influenced by the eutrophication mechanisms and environmental conditions of the two lakes

miocene igneous activity in the northern subbasin offshore senegal nw africa abstract more than 70 enigmatic discordant seismic anomalies of limited lateral extent associated with eye shaped structures and folds have been imaged on 2d seismic data from the offshore northern subbasin of the senegal basin nw africa in this paper we define their distribution characterise their morphology and compare them to similar features imaged by seismic data along the ne atlantic margin that have been demonstrated to be igneous sills hydrothermal vents and intrusion related forced folds we propose that the cretaceous and paleogene strata in the northernmost part of the offshore northern subbasin were heavily intruded by igneous sills during the miocene the proposed timing of intrusion is coincident with that previously suggested for the development of the cayar seamount located 100 km nnw of dakar and onshore volcanism in the cap vert area near dakar and in the cape verde islands this stage of miocene igneous activity is interpreted to be closely associated with the coeval collision between africa and europe the interpreted igneous sills give rise to an unexpected soft seismic response the preferred explanation for the soft appearance of the sills is that the majority of sills are thin

formation of segmented normal faults a 3 d perspective the interpretation of fault kinematics from geometric data is an essential step in developing an understanding of the growth of fault systems constraints on fault geometry are however often restricted to 2 d maps or cross sections in this article we consider the extent to which kinematic interpretations of faulting benefit from a 3 d rather than 2 d geometrical perspective concentrating on relay zones and segmented normal fault arrays we suggest that very different interpretations of their evolution arise from the recognition that the propagation directions of faults and fault segments will rarely be contained within the inspection plane of 2 d data a 3 d perspective favours an interpretation in which the segments of a fault array are kinematically interrelated from their initiation individual segments in such systems may link into a single fault surface out of the plane of inspection or may be unconnected in 3 d we argue that this interpretation which conflicts with the often suggested model of incidental overlap of originally isolated faults should be the preferred model for the generation and growth of segmented normal fault arrays

amundsen sea ice production and transport 1 drift and variability of sea ice in the amundsen sea are investigated with ice buoys deployed in march 2000 and a coupled ice ocean model the bremerhaven regional ice ocean simulations brios model results are compared with in situ ocean atmosphere and sea ice measurements satellite observations and 8 19 months of buoy drift data we identify a zone of coastal westward drift and a band of faster eastward drift separated by a broad transition region characterized by variable ice motions the model represents drift events at scales approaching its resolution but is limited at smaller scales and by deficiencies in the national centers for environmental prediction forcing two thirds of the modeled ice production in the southern amundsen moves westward near the coast its transport modulated by meridional wind strength damping sea ice formation in the eastern ross sea half of the ice exported from the ross moves eastward into the northern amundsen sea a net sea ice sink that also receives more than one third of the ice generated to its south a low rate of exchange occurs with the bellingshausen sea which must have a more independent ice regime snow ice formation resulting from high precipitation accounts for one quarter of the ice volume in the amundsen sea aiding the formation of thick ice in a region with generally divergent ice drift freshwater extraction by sea ice formation is roughly balanced by precipitation and ice shelf melting but a positive trend in the surface flux is consistent with an amundsen source for reported freshening in the ross sea

sill morphology and comparison of brittle and non brittle emplacement mechanisms magmatic sheet intrusions contribute significantly to the upper crustal magma transport network the emplacement mechanism of the magmatic sheets controls the final geometry of the intrusions and the characteristics of host rock deformation previous observations have highlighted the preponderance of brittle structures associated with shallow level sheet intrusions however recent studies have suggested that non brittle host rock behaviour also occurs particularly related to the formation of magma fingers during shallow level sill intrusion here we examine both brittle and non brittle intrusion mechanisms and expand upon them with field observations from a series of widespread and variable magmatic systems non brittle emplacement appears primarily associated with viscous flow of the host rock during intrusion and is therefore intimately linked to the contemporaneous host rock rheology as well as magma dynamics purely brittle and non brittle emplacement processes are found to be end members with many intrusions containing evidence of both behaviours deriving the host rock characteristics is therefore important for discerning potential diagnostic intrusion indicators and intrusion geometries both within the field and in modelling incorporation of variable host material behaviours in numerical and analogue modelling tuned using direct field observations may consequently further our understanding of the controls on shallow level intrusion

simulations of ice ocean dynamics in the weddell sea 2 interannual variability 1985 1993 investigations of sea ice ocean interaction on the continental shelf in the southwestern weddell sea reveal a strong correlation between fluctuations of atmospheric forcing and the variability of sea ice formation anomalies of meridional wind stress in the inner weddell sea are consistent with the phase of the antarctic circumpolar wave acw positive anomalies of northward wind stress cause an increase of sea ice export in the same and of sea ice formation in the following year leading to an increased production of high salinity shelf water driven by a varying zonal density distribution over the continental shelf the circulation in the filchner ronne ice shelf cavity fluctuates between two modes each of which features a characteristic distribution of basal freezing and melting regions thus signals of interannual atmospheric variability propagate into the deep ocean and the sub ice shelf cavities

magma transport through the crust via interconnected sill complexes three dimensional seismic data from the volcanic margin offshore norway have been used to map the complex three dimensional geometry of two major sill complexes these data show that the complexes consist of bowl shaped or inclined sheetlike discordant sills that intersect with one another over a vertical distance of 8 12 km and transect basement and sedimentary rocks of the middle to upper crust using a simple graphical technique we demonstrate the interconnectedness of the sill complex and hence the maximum time for sill emplacement we show that sills are interconnected by junctions occurring systematically at the lowest points of the overlying sills and hence that the sill complex could have acted as a single throughgoing magmatic plumbing system transporting mafic melts from mid lower crustal depths to near surface based on this imaging of the magmatic plumbing system we conclude that sills can form viable and efficient conduits for crustal magma transport in volcanic margins and may play a role in feeding large extrusive complexes

displacement geometry in the volume containing a single normal fault fault displacements measured in coal mines and from seismic data are used to develop a model describing the near field displacements associated with an ideal single normal fault displacement on a fault surface ranges from a maximum at the center of the fault to zero at the edge or tip line the tip line is elliptical with the shorter axis of the ellipse parallel to the displacement direction contours of equal displacement form concentric ellipses centered on the point of maximum displacement displacement gradients vary with fault size and with mechanical properties of host rock fault radius to maximum displacement ratios range from 5 to 500 plotting of displacement contour diagrams and knowledge of displacement gradients are useful in interpreting seismic reflectio data both for quality control of interpretations and for quantitative extrapolation of limited data displacements associated with faulting decrease systematically with increasing distance along the normal to the fault surface this decrease is seen as reverse drag in both hanging wall and footwall hanging wall rollover and tilting of the reflectors cannot be used to distinguish listric from planar normal faults even where fault block rotation can be demonstrated neither listric fault geometry nor a flat detachment surface is geometrically necessary because faulting is accommodated by ductile deformation rigid fault bounded blocks cannot exist except in some special circumstances related to a free surface the displacements within the rock volume affected by a single fault are not simply related to regional extension apparent horizontal extension by faulting varies from one layer to another and a significant proportion of the extension in a basin may be due to ductile deformation

sill related seafloor domes in the zhongjiannan basin western south china sea abstract magmatism can exert significant impact on sedimentary basins such as the zhongjiannan basin zjnb western south china sea we have evaluated multibeam bathymetric and multichannel seismic reflection data acquired by the guangzhou marine geological survey in recent years in order to investigate the distribution the characteristics and the subsurface structures related to seafloor domes found in the northeastern zjnb our data revealed forty two domes at water depths between 2312 m and 2870 m which are clustered around volcanic mounds large seamounts and along the edge of the central depression in the study area these domes are generally circular to elongate or irregular in plan view with large basal areas and they also have gentler flanks dips of 1 46 7 73 with vertical reliefs ranging from tens to hundreds of meters in seismic sections majority of the domes are underlain by variably shaped and complex magmatic sills which provide a cause effect relationship between domes formation and igneous intrusions these intrusions heat surrounding organic rich sediments release hydrocarbons fluidize sediment pore waters and form gas rich fluids which fill in sediment and uplift overlying strata immediately above the sills to form forced folds which are manifested as seafloor domes these sill folds dome structures have important implications for understanding geomorphologic features caused by sills emplaced at depth

oligocene to lower pliocene deposits of the norwegian continental shelf norwegian sea svalbard denmark and their relation to the uplift of fennoscandia a synthesis abstract this study provides the results of the first integrated study of oligocene pliocene basins around norway within the study area three main depocentres have been identified where sandy sediments accumulated throughout the oligocene to early pliocene period the depocentre in the norwegian danish basin received sediments from the southern scandes mountains with a general progradation from north to south during the studied period the depocentre in the basinal areas of the uk and norwegian sectors of the north sea north of 58 n received sediments from the scotland shetland area because of the sedimentary infilling there was a gradual shallowing of the northern north sea basin in the oligocene and miocene a smaller depocentre is identified offshore northern nordland between ranafjorden approximately 66 n and vesteralen approximately 68 n where the northern scandes mountains were the source of the oligocene to early pliocene sediments in other local depocentres along the west coast of norway sandy sedimentation occurred in only parts of the period shifts in local depocentres are indicative of changes in the paleogeography in the source areas in the barents sea and south to approximately 68 n the oligocene to early pliocene section is eroded except for distal fine grained and biogenic deposits along the western margin and on the oceanic crust this margin was undergoing deformation in a strike slip regime until the eocene oligocene transition the early oligocene sediments dated in the vestbakken volcanic province and the forlandssundet basin represent the termination of this strike slip regime the change in the plate tectonic regime at the eocene oligocene transition affected mainly the northern part of the study area and was followed by a quiet tectonic period until the middle miocene when large compressional dome and basin structures were formed in the norwegian sea the middle miocene event is correlated with a relative fall in sea level in the main depocentres in the north sea formation of a large delta in the viking graben frigg area and uplift of the north and south scandes domes in the norwegian danish basin the sorgenfrei tornquist zone was reactivated in the early miocene possibly causing a shift in the deltaic progradation towards the east a late pliocene relative rise in sea level resulted in low sedimentation rates in the main depositional areas until the onset of glaciations at about 2 7 ma when the scandes mountains were strongly eroded and became a major source of sediments for the norwegian shelf whilst the frigg delta prograded farther to the northeast

sea ice thickness in the weddell sea antarctica a comparison of model and upward looking sonar data sea ice thickness is a key parameter for estimates of salt fluxes to the ocean and the contribution to global thermohaline circulation observations of sea ice thickness in the southern ocean are sparse and difficult to collect an exception to this data gap is time series data from upward looking sonars uls which sample the drifting sea ice continuously in this study we use uls data from ten different locations over periods ranging from 9 to 25 months to compare with model data although these data are limited in space and time they provide a qualitative indication of the ability of global climate models gcms to adequately represent southern ocean sea ice we compare the uls data to output from four different gcms bccr bcm2 0 echam5 mpi om ukmo hadcm3 and ncar ccsm3 which were used for the fourth assessment report of the intergovernmental panel on climate change they simulate the ice thickness reasonably well but in most cases average model ice thickness is less than thicknesses derived from uls data the seasonal cycle produced by the models correlates well with the uls except for locations near maud rise where in summer the uls find a low concentration of thick ice floes this overly thin ice will have implications for both the salt flux to the central weddell sea during the growth season and the freshwater flux during the melt season using satellite derived ice drift data to calculate transports in the weddell sea we find that the underestimation of ice thickness results in underestimated salt fluxes

on the structure and the transport of the eastern weddell gyre abstract the circulation pattern and volume transports in the eastern weddell gyre are estimated on the basis of hydrographic data collected by r v polarstern between 1989 and 1996 in the northeastern edge of the weddell gyre eastward flowing water masses from the antarctic circumpolar current and the weddell sea converge due to the strong effect of topographic constraints on ocean currents in the weakly stratified waters of high latitudes the wedge like structure of the southwest indian ridge can cause the convergence the increased shear leads to instabilities of the current at the eastern end of the ridge which produce an intense mesoscale eddy field between 15 and 30 e in the eddies water from the weddell cold regime and the antarctic circumpolar current waters mix and form the water masses of the weddell warm regime these waters are advected southward and flow towards the westward southern rim current which is driven by the antarctic eastwind band hence there is not a continous flow from the northern to the southern rim but a decay of the mean flow in the northeast and a reformation in the south volume transports across the greenwich meridian estimated on the basis of a combined ctd adcp data set result in an eastward flow of 61 sv in the northern rim current and a westward return flow of 66 sv in the southern part of the gyre the transport is about twice as high as previous estimates between kapp norvegia and the northern tip of the antarctic pensinsula indicating a significant gyre circulation north of 70 s

the three dimensional geometry and growth of forced folds above saucer shaped igneous sills forced folds formed at the seabed immediately overlying shallow 1 km saucer shaped sills along the ne atlantic margin during the early paleogene examples of this sill fold relationship are exceptionally well imaged by high resolution 3d seismic datasets from the ne rockall basin the forced folds are domal in shape 2 4 km in diameter exhibit a structural relief of up to 350 m and comprise sediment volumes of ca 1 km3 a comparison of the thickness distribution across and volume of a saucer shaped sill with a high intrusion diameter to depth ratio and the structural relief and volume of its associated forced fold shows a remarkable equivalence this has the important implication that the structural relief on intrusion related forced folds can be used as an estimate of the thickness of the underlying sill the analysed forced folds are interpreted to have formed through three continual growth stages that are directly linked to the mechanical emplacement of the underlying saucer shaped sills their growth was associated with an increase in faulting of the overlying strata and influenced coeval or subsequent development of polygonal fault systems within the overburden these structures represent a new type of four way dip closed hydrocarbon trap

zircon dating ties ne atlantic sill emplacement to initial eocene global warming abstract the earth experienced rapid greenhouse gas induced global warming during the palaeocene eocene thermal maximum petm the source of the gas is however debated we have for the first time determined the ages of magmatic sills in the voring basin offshore norway zircon u pb ages of 55 6 0 3 and 56 3 0 4 ma demonstrate that sill emplacement was synchronous with the petm within small errors this discovery strengthens the hypothesis that global warming was triggered by rapid release of greenhouse gases generated by heating of organic rich sediments around intrusions in the ne atlantic rather than from dissociation of gas hydrates supplementary material u pb data are available at http www geolsoc org uk sup18392

flow of bottom water in the northwestern weddell sea the weddell sea is known to feed recently formed deep and bottom water into the antarctic circumpolar water belt from whence it spreads into the basins of the world ocean the rates are still a matter of debate to quantify the flow of bottom water in the northwestern weddell sea data obtained during five cruises with r v polarstern between october 1989 and may 1998 were used during the cruises in the weddell sea five hydrographic surveys were carried out to measure water mass properties and moored instruments were deployed over a time period of 8 5 years to obtain quasi continuous time series the average flow in the bottom water plume in the northwestern weddell sea deduced from the combined conductivity temperature depth and moored observations is 1 3 0 4 sv intensive fluctuations of a wide range of timescales including annual and interannual variations are superimposed the variations are partly induced by fluctuations in the formation rates and partly by current velocity fluctuations related to the large scale circulation taking into account entrainment of modified warm deep water and weddell sea deep water during the descent of the plume along the slope between 0 5 sv and 1 3 sv of surface ventilated water is supplied to the deep sea this is significantly less than the widely accepted ventilation rates of the deep sea if there are no other significant sources of newly ventilated water in the weddell sea either the dominant role of weddell sea bottom water in the southern ocean or the global ventilation rates have to be reconsidered

18o concentrations in sea ice of the weddell sea antarctica we present data on ice texture salinity and delta o 18 obtained from identical sections of ice cores during the winter weddell sea project 1986 on rv polarstern from july through august 1986 in the longitude range between 5 degrees w and 7 degrees e we find no uniquely definable relationship between delta o 18 values and ice texture in a particular section however most of the snow ice as well as some sections of frazil ice ar found to have negative delta o 18 concentrations this is due to varying degrees of admixtures of meteoric ice snow and sea water during formation of snow ice in contrast to common assumptions our results seem to indicate that a snow cover contributes positively to sea ice growth rather than slowing down the overall growth rate based on a simple model we have estimated the contributions of meteoric ice mean of 3 3 and the combined meteoric ice sea water fraction a minimum of 7 6 to the total ice thickness for the majority of the sampled floes although this is only a moderate contribution to the overall mass balance in the absence of congelation growth it nevertheless enhances ice growth in general this hypothesis is dependently supported by our snow and ice thickness data wadhams and others 1987 which demonstrate that the depression of the snow ice interface below the water line i e a negative freeboard and the formation of snow ice is a common occurrence in the weddell sea therefore we hypothesize that the major part of the observed apparent increase in ice thickness between our inbound and outbound tracks of wwsp 86 may not be derived from regular thermodynamically driven congelation growth but rather from the snow ice component in floes of the weddell sea

three dimensional 3 d seismic imaging of conduits and radial faults associated with hydrothermal vent complexes voring basin offshore norway abstract here we document a suite of radial faults associated with hydrothermal vent complexes in the voring basin offshore norway these complexes have pyramid shaped cylindrical and conical shaped conduits with a dome or eye shaped morphology at their summit intruding on paleogene sedimentary rocks hydrothermal vents are intimate with the tips of magmatic sills that were emplaced at depths ranging between 1800 and 5800 ms two way travel time twtt at shallower depths of 1800 to 3000 ms twtt and intermediate depths of 3000 to 5000 ms twwt magmatic sills regularly intersect the lower parts of the vent conduits which are characterized here as pipes an important parameter that is used to characterize the morphology of a hydrothermal vent conduit is the width of the conduit which is defined as the longest axis marking the extent of the vents conduit within the surrounding host rock strata our findings reveal that radial faults are commonly associated with the summits of hydrothermal vents implying the existence of local stress fields around the vents where the maximum compressive stress is radial and minimum stress is circumferential which overrides the regional stress field and indicate variable stress regimes as opposed to tectonic faults importantly circumferential stretching due to catastrophic plumbing of hydrothermal fluids differential compaction and intensive fracturing enabled the polygonal faults to realign in a radial pattern resulting in the formation of radial faults at the vent summit as a corollary of this work we hypothesize that pyramid shaped hydrothermal conduits are possibly markers of protracted sill emplacement in sedimentary basins

weddell sea ice drift kinematics and wind forcing ice drift in the weddell sea was studied on the basis of positional and meteorological data from argos buoys drifting in 1990 1992 and surface pressure analyses from the european centre for medium range weather forecasts ecmwf the drift kinematics showed differences between the eastern and western parts of the weddell sea close to the antarctic peninsula the ice drifted as an almost nonrotating uniform field at a low speed having reduced small scale motions with little meandering compared to regions further to the east inertial motion was detected from the ice drift in areas east of 35 w and in the region of the antarctic circumpolar current on timescales of days wind was the primary forcing factor for the drift a linear model between the wind and ice drift explained 40 80 of the drift velocity variance the degree of explanation was higher in the central weddell sea around 40 w and lower closer to the antarctic peninsula the geostrophic wind was found to provide almost as good a basis for the general drift estimation as the surface wind observed by the buoys although strong cyclones were not well detected by the ecmwf analyses the data suggest a dependency upon atmospheric stability such that stable stratification reduces the wind forcing on the drift for 60 80 of the time the direction of the drift deviated less than 45 from the geostrophic wind and for 45 70 of the time less than 45 from the ocean current ice transport through a transect crossing the weddell sea from the antarctic peninsula tip to kapp norwegia was estimated on the basis of the geostrophic winds the drift s observed response to the wind and literature based information on ice concentration and thickness the estimated annual mean net export in 1992 1994 varied from 8000 to 22 000 m3 s most of the net export took place in winter and spring export prevailing west of 35 w and import east of it

formation and maintenance of a polynya in the weddell sea a dynamic thermodynamic sea ice mixed layer model for the weddell sea is complemented by a simple diagnostic model to account for local sea ice atmosphere interaction to consider the atmospheric influence on the oceanic mixed layer the pycnocline upwelling velocity is calculated using the theory of ekman pumping in several experiments formation and conservation of a polynya in the weddell sea are investigated intrusion of heat into the lower atmosphere above the polynya area is assumed to cause a thermal perturbation and a cyclonic thermal wind field superposed with daily ecmwf surface winds this modified atmospheric forcing field intensifies oceanic upwelling and induces divergent ice drift simulation results indicate that in case of a weak atmospheric cross polynya flow the formation of a thermal wind field can significantly extend the lifetime of a large polynya the repeated occurrence of the weddell polynya in the years 1974 76 thus appears to be an effect of feedback mechanisms between sea ice atmosphere and oceanic mixed layer

thermodynamic sea ice growth in the central weddell sea observed in upward looking sonar data upward looking sonar uls data were used to analyze thermodynamic sea ice growth the study was carried out for an ocean region in the central weddell sea for which data of sea ice thickness variability and of the oceanic heat flux through the ice are rare in the study area the contribution of sea ice deformation to vertical ice growth is relatively small in some years this provides the opportunity to simulate thermodynamic sea ice growth considering the influence of a snow cover and of the oceanic heat flux to this end a modified version of stefan s law was used the resulting ice thickness variations were then compared with the uls measurements for the investigated cases the best consistency between data and model results was obtained assuming a snow layer of less than 5 cm thickness and average oceanic heat fluxes between 6 and 14 w m 2 it is demonstrated that in conjunction with ice drift data and analytical models for thermal sea ice growth uls ice thickness measurements are useful for studying the seasonal cycle of growth and decay and for inferring the magnitude of the average oceanic heat flux under sea ice

melting of ice shelves and the mass balance of antarctica we calculate the present ice budget for antarctica from measurements of accumulation minus iceberg calving run off and in situ melting beneath the floating ice shelves the resulting negative mass balance of 469 gt year 1 differs substantially from other recent estimates but some components are subject to high temporal variability and budget uncertainties of 20 50 annual accumulation from an earlier review is adjusted to include the antarctic peninsula for a total of 2144 gt year 1 an iceberg production rate of 2016 gt year 1 is obtained from the volume of large icebergs calculated from satellite images since 1978 and from the results of an international iceberg census project ice shelf melting of 544 gt year 1 is derived from physical and geochemical observations of meltwater outflow glaciological field studies and modeling of the sub ice ocean circulation the highest melt rates occur near ice fronts and deep within sub ice cavities run off from the ice sheet surface and from beneath the grounded ice is taken to be 53 gt year 1 less than half of the negative mass balance need come from the grounded ice to account for the unattributed 0 45 mm year 1 in the ipcc best estimate of the recent global sea level rise

response of the weddell sea pack ice to wind forcing sea ice drift in the weddell sea was studied on the basis of data from seven buoys deployed on ice floes in january february 1996 six of the buoys formed an array with initial mutual distances of 50 km which by october 1996 were stretched to 400 km in an east west direction the differential kinematical parameters i e divergence shearing rate and vorticity were estimated from the buoy array positions by applying a time dependent method the large scale drift of the array was divergent until august 1996 but after the array came under the influence of the antarctic circumpolar current the deformation was mainly shearing meteorological data from european centre for medium range weather forecasts analyses and special sensor microwave imager derived ice concentrations were interpolated at the buoy sites the drift velocity was highly wind dependent it was also wind dependent in winter when the momentum balance was mainly between the internal ice stress and the air drag the drift divergence and shearing rate were related to the wind forcing while the array vorticity correlated better with the air pressure itself we estimated the air ice drag coefficient to be 1 8 10 3 at a height of 10 m over the ice and the geostrophic drag coefficient to be 6 1 10 4 the stability effects on the coefficients were mostly felt in conditions of light winds the average ice water drag coefficient based on 5 day periods was 2 1 10 3 the ice transport through a transect crossing the weddell sea was computed on the basis of the geostrophic winds observed wind drift dependence ssm i derived ice concentrations and the literature on ice draft statistics the resulting annual mean net export in 1996 was 1600 km3 yr 1

sea ice dynamics in the weddell sea simulated with an optimized model the advection of sea ice and associated freshwater salt fluxes in the weddell sea in 1986 and 1987 are investigated with a large scale dynamic thermodynamic sea ice model the model is validated and optimized by comparison of simulated sea ice trajectories with observed drift paths of six buoys deployed on the weddell sea ice the skill of the model is quantified by an error function that measures the deviations of simulated trajectories from observed 30 day sea ice drift a large number of sensitivity studies show how simulated sea ice transports and associated freshwater salt fluxes respond to variations in physical parameterizations the model reproduces the observed ice drift well provided ice dynamics parameters are set to appropriate values optimized values for the drag coefficients and for the ice strength parameter are determined by applying the error function to various sensitivity studies with different parameters the optimized model yields a mean northward sea ice volume export out of the southern weddell sea of 1693 km3 in 1986 and 2339 km3 in 1987 this shows the important role of sea ice transport for the freshwater budget of the weddell sea and gives an indication of its high interannual variability

ice drift in the weddell sea in 1990 1991 as tracked by a satellite buoy ice drift in the weddell sea has been studied by using a satellite buoy deployed on an ice floe the buoy survived for a 20 month period indicating a drift trajectory of 10 000 km and yielding 13 months of marine meteorological data the drift of the ice floe was studied with respect to the winds measured by the buoy in the central weddell sea the mean drift speed of the ice floe was 0 15 m s which was about 3 of the wind speed more specifically the drift ratio was 3 4 in the marginal ice zone and 2 4 in the inner pack ice field on average the drift was directed 36 left of the wind direction but the turning angle was larger during the austral summer and smaller during the winter on time scales of days the drift was primarily wind dependent except for cases during winter periods of high ice concentration and internal ice resistance for time scales of several months purely wind based simulations of the drift resulted in a discrepancy between the observed and simulated trajectories but the inclusion of a slow 0 02 m s residual current made the simulations significantly better the geostrophic wind based on european centre for medium range weather forecasts pressure analyses was estimated for a 1 month period and the ice floe was found to drift almost parallel to the geostrophic wind with a speed of 2 of the geostrophic wind speed inertial type motion superimposed on the wind induced drift was found to be a characteristic feature in the marginal ice zone during the austral summer but it could not be found from the drift in winter when kinetic energy was transferred to larger scales of motion and dissipated into the ice field

the influence of normal fault geometry on igneous sill emplacement and morphology magma flow within the subsurface is heavily influenced by the pre existing structure of the upper crust during continental rifting normal faults modify the geometry of igneous networks by providing preferential pathways for the intrusion of magma however the way in which magma intrudes into fault planes is poorly understood here we quantitatively document the relationship between fault architecture and intrusion distribution and geometry using three dimensional seismic reflection data from the exmouth sub basin offshore northwest australia inclined segments of saucer shaped sills intrude several faults along convex into the hangingwall fault plane corrugations we suggest that stress field perturbations associated with the fault plane corrugations provide suitable conditions for fault reactivation as magma conduits pre existing faults also modify sill geometries through the offset of stratigraphic horizons that may be preferentially intruded potentially resulting in the formation of a new sill or the development of minor intrusive steps this work emphasizes the importance of the pre existing structural template in controlling the growth and final geometry of intrusive networks

mid cenozoic post breakup deformation in the passive margins bordering the norwegian greenland sea abstract since the initial breakup of the norwegian greenland sea in early eocene time chron 24b 53 7 ma the conjugate passive margins have been subject to compression extension magmatism and broad uplifts and subsidence mid to late cenozoic compressional doming basin inversion and reverse faulting occurred predominantly offshore within the rheologically weak deep cretaceous depocenters on the norwegian margin the domes seem to have formed primarily in middle eocene to early oligocene and in early miocene while doming in the faroes rockall region developed in late paleocene to early eocene oligocene and mid to late miocene we suggest that the main driving force was plume enhanced ridge push a testable working hypothesis is that discrete phases of compression were associated with periods of high plume flux in addition the oligocene chron 13 35 ma change in plate motion appears to have generated local shear adjustments along pre existing basement weaknesses forming compressional structures in the overlying sedimentary section this change in plate motion was first manifest by significant extension between the sw barents sea and the conjugate nne greenland margin and led to spreading along the knipovitch ridge mid cenozoic extension and magmatism also preceded the development of the kolbeinsey ridge the associated separation of the jan mayen microcontinent from east greenland must have been strongly influenced by the position of the iceland plume center in addition to the change in extension direction with respect to initial breakup of the norwegian greenland sea in early eocene time the mid cenozoic extension and magmatism is a post breakup phenomena widely spaced areas of neogene uplift occurred along all bordering margins these enigmatic uplifted areas acted as nucleation sites for growth of ice caps and initiated large glacial deposits that built out on the adjacent shelves

the ice thickness distribution across the atlantic sector of the antarctic ocean in midwinter during the first leg of the winter weddell sea project antarktis v 2 cruise of f s polarstern the entire width of the antarctic sea ice zone was traversed in the vicinity of 0 longitude in the period july 18 to september 10 1986 ice thicknesses were measured by direct drilling and by helicopter profiling using an exstar 100 mhz impulse radar system in addition aerial photography of the ice cover was done from 100 to 2000 m altitude using a 70 mm aerial camera mounted in the helicopter the results of the point measurements drilling are reported in this paper together with an indication of how the radar and photography data will be used to extend them so as to yield area averaged ice thickness distributions it was found that the main ice type across the entire width of the ice cover was consolidated pancake ice occurring in vast floes this formed out of a 250 km wide band at the advancing ice edge which comprised a concentrated field of individual pancakes in a matrix of frazil ice preferred thicknesses of undeformed floes were 40 60 cm of ice covered with 5 15 cm of snow the individual pancakes attained almost all of this thickness before consolidation subsequent congelation growth was slow estimated at 0 4 cm d 1 the floes contained much small scale roughness on the upper and lower surfaces due to rafting of pancakes at the time of consolidation but pressure ridging was modest except in the far south a few very thick 8 11 m multiyear floes were observed embedded in the pack at latitudes beyond 66 s

sea ice thickness in the southwestern sea of okhotsk revealed by a moored ice profiling sonar 1 using a moored ice profiling sonar along with a moored acoustic doppler current profiler a total spatial section of draft across 3334 km of sea ice was obtained in the southwestern sea of okhotsk near hokkaido in the winters of 1999 2001 using this draft data set the average draft and keel statistics are discussed in this sea for the first time the mean draft was 0 60 m which corresponds to the thickness of 0 71 m over the three winters with the range of 0 49 0 72 m for each winter the classification of level and deformed ice reveals a small range of the monthly mean level ice draft 0 18 0 27 m and the dominance of the deformed ice in terms of volume 80 the mean draft varied with the areal ratio of the deformed ice fairly well these results suggest that dynamic processes such as ridging and rafting are important for the evolution of draft in the region of observation the observed draft probability density distribution and keel statistics show that the thick ice ratio and keel frequency are lower than the similar data in polar regions and closer to those observed in davis strait west of greenland along with the ice concentration and speed derived from the satellite data the southward ice transport to the southwestern sea of okhotsk is estimated on the basis of the observed sea ice thickness the estimated ice transport ranged from 15 to 70 km3 in each winter the heat and freshwater transport associated with the ice transport ranged from 3 9 1017 to 1 8 1018 j and from 12 to 57 km3 respectively

sea ice transports in the weddell sea time series of sea ice draft in the weddell sea are evaluated together with hydrographic observations satellite passive microwave data and ice drift for estimation of the freshwater fluxes into and out of the weddell sea ice draft is measured with moored upward looking sonars since 1990 along two transects across the weddell gyre one transect extending from the tip of the antarctic peninsula to kapp norvegia was sampled between 1990 and 1994 and covers the flow into and out of the southern weddell sea the other transect sampled since 1996 and extending from the antarctic continent northward along the greenwich meridian covers the exchange of water masses between the eastern and the western weddell sea in order to relate results obtained during the different time periods empirical relationships are established between the length of the sea ice season derived from the satellite passive microwave data and defined as the number of days per year with the sea ice concentration exceeding 15 and 1 the annual mean ice draft and 2 the annual mean ice volume transport by using these empirical relationships estimates of annual mean ice drafts and ice volume transports are derived at all mooring sites for the period february 1979 through february 1999 wind and current force a westward ice transport in the coastal areas of the eastern weddell sea and a northward ice transport in the west during the 2 year period 1991 1992 the mean ice volume export from the weddell sea is 50 19 103 m3 s 1 this freshwater export is representative for a longer term 20 year mean and exceeds the average amount of freshwater gained by precipitation and ice shelf melt by about 19 103 m3 s 1 yielding an upper bound for the formation rate of newly ventilated bottom water in the weddell sea of 2 6 sv

impacts of fault sill interactions on sill emplacement in the voring basin norwegian north sea abstract pre existing faults may facilitate subsurface magma transport and sill trangression in sedimentary basins although widely recognized interaction between faults and intrusion networks in sedimentary basins remains poorly imaged by seismic reflection data to understand how sills interact with pre existing faults in sedimentary basins we use high resolution 3 d seismic reflection data from the naglfar dome voring basin to examine the seismic expression of sills interpret fault geometries and displacement patterns and characterize sill fault interactions the sills are expressed as tuned reflection packages meaning they are below

seismic interpretation of sill complexes in sedimentary basins implications for the sub sill imaging problem application of 3d seismic reflection data to igneous systems in sedimentary basins has led to a revolution in the understanding of mafic sill complexes however there is considerable uncertainty on how geometries and architecture of sill complexes within the subsurface actually relates to geometries in seismic reflection data to provide constraints on how sill complexes in seismic data should be interpreted we present synthetic seismograms generated from a seismic scale 22 0 25 km outcrop in east greenland constrained by abundant field data this study highlights how overlying igneous rocks adversely affect imaging of underlying intrusions and rocks by decreasing seismic amplitude frequency and making steeply dipping features near impossible to image furthermore seismic modelling in this study shows that because of the high impedance contrast between siliciclastic host rock and dolerites very thin 1 5 m intrusions should in principle be imaged in reflection seismic data at 3 km depth however comparison with actual seismic data with well data shows significant amounts of unimaged sill intrusions and this is likely due to limited seismic resolution overburden complexity inadequate velocity models and interference between reflections from closely spaced sills and sill splays significant improvements to sill imaging and interpretation could be made by better predicting occurrence and geometry of sill intrusions and including these in velocity models supplementary material a scaled version of the input panel fig 5d used for seismic modelling a1 and very high resolution versions of subfigures in figure 11 a2 3 are available at https doi org 10 6084 m9 figshare 5607160

geometry and growth of sill complexes insights using 3d seismic from the north rockall trough doleritic sill complexes which are an important component of volcanic continental margins can be imaged using 3d seismic reflection data this allows unprecedented access to the complete 3d geometry of the bodies and an opportunity to test classic sill emplacement models the doleritic sills associated with basaltic volcanism in the north rockall trough occur in two forms radially symmetrical sill complexes consist of a saucer like inner sill at the base with an arcuate inclined sheet connecting it to a gently inclined commonly ragged outer rim bilaterally symmetrical sill complexes are sourced by magma diverted from a magma conduit feeding an overlying volcano with an elongate concave upwards trough like geometry bilaterally symmetrical sills climb away from the magma source from which they originate both sill complex types can appear as isolated bodies but commonly occur in close proximity and consequently merge producing hybrid sill complexes radial sill complexes consist of a series of radiating primary flow units with dimensions up to 3 km each primary flow unit rises from the inner saucer and is fed by primary magma tube primary flow units contain secondary flow units with dimensions up to 2 km each being fed by a secondary magma tube branching from the primary magma tube secondary flow units in turn are composed of 100 m scale tertiary flow units a similar branching hierarchy of flow units can also be seen in bilaterally symmetrical sill complexes with their internal architecture resembling an enlarged version of a primary flow unit from a radial sill complex this branching flow pattern as well as the interaction between flow units of varying orders provides new insights into the origin of the structures commonly seen within sill complexes and the hybrid sill bodies produced by their merger the data demonstrate that each radially symmetrical sill complex is independently fed from a source located beneath the centre of the inner saucer grows by climbing from the centre outwards and that peripheral dyking from the upper surface is a common feature these features suggest a laccolith emplacement style involving peripheral fracturing and dyking during inner saucer growth and thickening the branching hierarchy of flow units within bilaterally symmetrical sill complexes is broadly similar to that of primary flow units within a radially symmetrical sill complex suggesting that the general features of the laccolith emplacement model also apply

modeling the water masses of the atlantic ocean at the last glacial maximum 1 we produced gridded monthly sea surface boundary conditions for the atlantic ocean at the last glacial maximum lgm based on the sea surface temperature reconstruction of the glamap project we used an ocean general circulation model ogcm subject to these sea surface boundary conditions and a corresponding wind stress field from an atmospheric general circulation model to study the differences in the distribution of the main water masses between the lgm and the present our global ogcm is characterized by high vertical resolution low vertical diffusion and isopycnal mixing and hence allows for a realistic representation of the hydrology and circulation of the modern atlantic ocean according to a series of lgm experiments with an increasing sea surface salinity anomaly in the weddell sea the ventilated thermocline was colder than today by 2 3 c in the north atlantic ocean and in the experiment with the largest anomaly 1 0 beyond the global anomaly by 4 5 c in the south atlantic ocean its depth was reduced by 50 m on average most notably in the tropics in the north atlantic ocean the outcrop locations of the thermocline isopycnal surfaces migrated southward by 5 10 and the ventilation increased in the south atlantic ocean the mixed layer and thermocline water masses were dominated by cold water originating from drake passage and the import of warm water from the indian ocean was reduced to about 4 sv or 40 of its modern value antarctic intermediate water was colder by 3 4 c and could be traced as far as 10 n the meridional overturning rates of north atlantic deep water nadw and antarctic bottom water aabw in the atlantic ocean were similar to those of the present day experiment 9 10 sv and 4 sv respectively however nadw cooled by 2 5 c and aabw by 1 c aabw was near the freezing point of seawater at the surface and the saltiest water mass in the atlantic ocean even saltier than nadw we show that the differences between the lgm and the present day experiments can be traced back to the changes in the subpolar and interhemispheric sea surface density gradients

controls on transgressive sill growth igneous sills represent an important contribution to upper crustal magma transport and storage this study focuses on an exemplary 20 50 m thick transgressive sill in the faroe islands on the european atlantic passive margin which is hosted in layered lavas 1 20 m thick and basaltic volcaniclastic units 1 30 m thick preserved steps in the sill and offset intrusive segments are consistent with initial propagation via segmented fractures followed by inflation to create a through going sheet although steps correspond to the position of some host rock interfaces and volcaniclastic horizons most interfaces are bypassed transgressive sill contacts are subparallel to thrust faults that record ene wsw shortening which are observed within the surrounding country rock and within the sill remnant sill segments are elongate along a nnw sse axis parallel to the derived intermediate stress axis for thrust faults the overall transgressive geometry is consistent with regional horizontal shortening with steps indicating transitions between transgressive and lateral sill propagation are controlled locally by layering this work emphasizes the importance of scale of observation in considering the controls on sill emplacement and in particular that layering is not the primary control on geometry

igneous sills record far field and near field stress interactions during volcano construction isle of mull scotland abstract sill emplacement is typically associated with horizontally mechanically layered host rocks in a near hydrostatic far field stress state where contrasting mechanical properties across the layers promote transitions from dykes or inclined sheets to sills we used detailed field observations from the loch scridain sill complex isle of mull uk and mechanical models to show that layering is not always the dominant control on sill emplacement the studied sills have consistently shallow dips 1 25 and cut vertically bedded and foliated metamorphic basement rocks and horizontally bedded cover sedimentary rocks and lavas horizontal and shallowly dipping fractures in the host rock were intruded with vertical opening in all cases whilst steeply dipping discontinuities within the sequence i e vertical fractures and foliation in the basement and vertical polygonal joints in the lavas were not intruded during sill emplacement mechanical models of slip tendency dilation tendency and fracture susceptibility for local and overall sill geometry data support a radial horizontal compression during sill emplacement our models show that dykes and sills across mull were emplaced during nw se horizontal shortening related to a far field tectonic stress state the dykes generally accommodated phases of ne sw horizontal tectonic extension whereas the sills record the superposition of the far field stress with a near field stress state imposed by emplacement of the mull central volcano we show that through detailed geometric characterisation coupled with mechanical modelling sills may be used as an indication of fluctuations in the paleostress state

a quick response of the equatorial ocean to antarctic sea ice salinity anomalies the potential mechanisms for a quick response of the equatorial ocean to processes occurring in the high latitude southern ocean have been investigated in the southern ocean at the drake passage latitudes there is an zonally unbounded channel the antarctic circumpolar current belt accb it is demonstrated that in a multiconnected domain such as the accb signals generated by anomalies in the southern ocean sea ice cover salinity distribution can propagate in a wave like manner in the form of fast moving barotropic rossby waves a disturbance induced near the drake passage is demonstrated to generate a rossby wave that propagates across the pacific in only a few days this signal is reflected at the western boundary of the pacific and generates an equatorward propagating coastally trapped kelvin wave the resulting temperature anomaly moves northward along the western coastline up to the vicinity of the equator and increases in amplitude over time the anomaly in the western edge of equatorial pacific then begins to propagate eastward along the equator as a trapped equatorial wave after about 2 3 months this wave reaches the eastern coast reflects and generates the coastal trapped wave which propagates north and south along the coastline in both hemispheres this process is suggested as one possible direct mechanism by which the extratropical ocean can induce anomalies in the equatorial ocean

the morphology of intrusion related vent structures and their implications for constraining the timing of intrusive events along the ne atlantic margin intrusion related vent structures found within palaeocene sediments and near the palaeocene eocene transition in basins along the ne atlantic margin have been studied in detail using 3d seismic data the vents formed at the palaeo seabed as a result of sill intrusion and the vent fill ranges in composition from entirely remobilized sedimentary to dominantly magmatic the vents are commonly developed above sill edges and crests in sills to which they are linked by pipe and diatreme like structures the vents also develop above the upper tips of deep seated faults and at fault intersections the vents can be used to constrain the approximate timing of igneous sill intrusion and duration of hydrothermal activity it is estimated that using the seismically constrained stratigraphic context of these vents to establish the timing of sill emplacement is associated with an uncertainty of 100 ka and up to as much as 800 ka interpretation of the vents indicates that sill intrusion took place during a number of discrete phases within the palaeocene and earliest eocene these structures and their timing of formation have important implications for climate change studies basin scale processes including fluid flow and diagenesis as well as hydrocarbon exploration

wind forcing and ice motion in the weddell sea region the wind forcing on the ocean surface and temporal and spatial characteristics of ice motion are analyzed from data of automatic buoy stations drifting for 14 months on ice floes in the weddell sea generally fluctuations with periods from 2 to 5 days which are caused by synoptic scale atmospheric wind fields primarily contribute to the variance of ice drift peaks in the spectra of ice motion are also found at 12 hours due to inertial motion and weak tidal effects which are nearly coincident in this geographical region inertial motion is only missing in winter when the ice concentration is high in the western branch of the weddell gyre semidiurnal peaks are stronger over the continental shelf than over the deep ocean diurnal peaks are weaker than semidiurnal peaks they are found over the continental shelf and are missing over the deep ocean at the same time the tidal motion reflects non wind related coupling between water and ice motion for periods of 1 2 to 1 day the integral length scales of ice motion are between 550 and 680 km longitudinal correlation and between 360 and 540 km lateral correlation respectively when the ice concentration exceeds eight tenths in the marginal ice zone the lateral length scale of ice motion is reduced to 270 km the integral length scales of air pressure fields exceed the length scales of ice motion by a factor of at least 1 5 longitudinal and lateral length scales of ice motion in the weddell sea are slightly smaller than those published for the beaufort sea where the ice moves within a basin of similar diameter of approximately 1500 km the smaller scales in the weddell sea presumably are due to differences of the forcing fields about 70 to 95 of the variance of 12 hourly averaged drift velocities can be linearly related to the wind velocity except when the mean wind speed drops below 3 5 m s the correlation with geostrophic winds is close to that with locally measured winds the twelve hourly averaged ice drift amounts to about 3 5 of the actual surface wind velocity at a height of 3 m or to about 1 6 of the geostrophic wind velocity derived from surface pressure analyses the drift geostrophic wind ratio is 30 smaller in central winter with ice concentrations above nine tenths in the central weddell sea than during summer periods when the buoys are closer to the coast in the periphery of the gyre and in the marginal ice zone the drift local wind ratio scatters more than the drift geostrophic wind ratio the ice drift in winter for ice concentrations above eight tenths on an average is deflected by 20 degrees to the left of the surface wind direction and is parallel to the isobars in summer under reduced ice concentrations it deviates by 40 degrees to the left of the surface wind and by 10 to 15 degrees to the left of the geostrophic wind for periods of 1 month the ice drift is also linearly well correlated to geostrophic wind velocity winds in the weddell sea usually do not average out over longer time scales ocean currents are too weak to dominate mean ice motion

sea ice transport driving southern ocean salinity and its recent trends the southern ocean has freshened over the past few decades but the underlying mechanisms involved have remained unclear alex haumann and colleagues bring together multiple lines of evidence and despite large uncertainties in the underlying datasets they find that northward transport of sea ice formed around the antarctic coast can explain the bulk of the observed freshening

persistent iceberg groundings in the western weddell sea antarctica abstract we use the time average of a series of satellite synthetic aperture radar sar images from 2005 to 2007 to identify grounded icebergs in the western weddell sea satellite laser altimetry along repeated ground tracks confirms that regions of high mean backscatter are associated with stationary tabular icebergs the altimeter data also provide iceberg freeboard from which we infer maximum ice draft assuming the lightly grounded limit and a two layer density model consistent with a source at filchner ronne ice shelf the maximum iceberg draft agrees very well with the ocean depth at a point where the bathymetry is well constrained by ship soundings however for an extensive region near 71 s south of larsen c ice shelf the maximum grounding depth for several icebergs reveals the ocean to be locally more than 200 m shallower than in bathymetry products this previously uncharted bank is expected to have a profound effect on the northward flow of the western weddell gyre and the cross slope transport of offshore warm deep water towards the eastern antarctic peninsula ice shelves passive microwave data also show that sea ice concentration is reduced in the vicinity of this group of grounded icebergs indicating an iceberg influence on ocean atmosphere heat and freshwater exchanges

the contribution of the weddell gyre to the lower limb of the global overturning circulation the horizontal and vertical circulation of the weddell gyre is diagnosed using a box inverse model constructed with recent hydrographic sections and including mobile sea ice and eddy transports the gyre is found to convey 42 8 sv 1 sv 106 m3 s 1 across the central weddell sea and to intensify to 54 15 sv further offshore this circulation injects 36 13 tw of heat from the antarctic circumpolar current to the gyre and exports 51 23 msv of freshwater including 13 1 msv as sea ice to the mid latitude southern ocean the gyre s overturning circulation has an asymmetric double cell structure in which 13 4 sv of circumpolar deep water cdw and relatively light antarctic bottom water aabw are transformed into upper ocean water masses by mid gyre upwelling at a rate of 2 2 sv and into denser aabw by downwelling focussed at the western boundary 8 2 sv the gyre circulation exhibits a substantial throughflow component by which cdw and aabw enter the gyre from the indian sector undergo ventilation and densification within the gyre and are exported to the south atlantic across the gyre s northern rim the relatively modest net production of aabw in the weddell gyre 6 2 sv suggests that the gyre s prominence in the closure of the lower limb of global oceanic overturning stems largely from the recycling and equatorward export of indian sourced aabw

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

weddell sea iceberg drift five years of observations since 1999 52 icebergs have been tagged with gps buoys in the weddell seato enable monitoring of their position the chosen icebergs were of small tomedium size with a few icebergs larger than 10 km associatedwith the calving of icebergs a38 and a43 from the ronne ice shelf the majority of icebergs were tagged off neumayer station 8e 70s it was found that smaller bergs with edges shorter than 200 m had the shortestlife cycle 0 5 yr iceberg and thus freshwater export out of theweddell sea was found to be highly variable in one year the majority of buoysdeployed remained in the weddell sea constituting about 40 of the ncep p efreshwater input whereas in other years all of the tagged icebergs were exported the observed drifts of icebergs and sea ice showed a remarkably coherent motion the analysis of an iceberg sea ice buoy array in the western weddell seaand an iceberg array in the eastern weddell sea showed a coherent sea iceiceberg drift in sea ice concentrations above 86 dynamic kinematic parameter dkp during the course of coherent movement were low and deviations from the meancourse associated with the passage of low pressure system the length scale ofcoherent movement was estimated to be less than 250km about half the value found forthe arctic ocean

an experimental investigation of sill formation and propagation in layered elastic media a series of experiments are described where dyed water a magma analogue was intruded into solid gelatine a crustal analogue to investigate the formation of sills we considered a layered gelatine system with contrasting adjacent layers by varying the density and rigidity of the gelatine we found that experimental sills form when the upper layer is more rigid than the lower layer with intrusion occurring in a plane directly below the interface experimental dykes were observed to propagate to the surface when the young s modulus ratio of upper to lower gelatine layers was less than one experimental dyke arrest occurred when the upper layer was more rigid and the interface was strong two varieties of experimental sill formed when the upper layer was more rigid than the lower layer and the interface was sufficiently weak the form of the intrusion depends on the balance of driving pressures and the young s modulus ratio of contrasting adjacent layers when the rigidity ratio is high and there is a large driving pressure the experimental feeder dyke completely converts to propagate as a sill however when the rigidity ratio and driving pressure are both close to one a dyke sill hybrid forms under these conditions the experimental sill formation is accompanied by contemporaneous dyke intrusion into the overlying more rigid layer during sill propagation deformation structures such as faults and en echelon fractures are formed into the lower layer experimental sill propagation dynamics are controlled by viscous dissipation along the length of the sill causing acceleration with increasing length our study suggests that rigidity contrasts may play a major role in the location of sills and development of igneous complexes in ancient cratonic areas the moho is a suitable site for the preferential formation of sills with higher rigidity continental crust overlying weaker mantle mantle plumes impacting ancient continents provide a situation in which large sills can form to fractionate prior to eruption of flood basalts the boundary between the upper and lower crust conrad discontinuity may provide a preferential focus for the emplacement of sheets of silicic magma at continental arcs where the lower crust is weakened by prolonged heating and possible hydration

thick and deformed antarctic sea ice mapped with autonomous underwater vehicles satellites have documented trends in antarctic sea ice extent and its variability for decades but estimating sea ice thickness in the antarctic from remote sensing data remains challenging in situ observations needed for validation of remote sensing data and sea ice models are limited most have been restricted to a few point measurements on selected ice floes or to visual shipboard estimates here we present three dimensional 3d floe scale maps of sea ice draft for ten floes compiled from two springtime expeditions by an autonomous underwater vehicle to the near coastal regions of the weddell bellingshausen and wilkes land sectors of antarctica mean drafts range from 1 4 to 5 5 m with maxima up to 16 m we also find that on average 76 of the ice volume is deformed ice our surveys indicate that the floes are much thicker and more deformed than reported by most drilling and ship based measurements of antarctic sea ice we suggest that thick ice in the near coastal and interior pack may be under represented in existing in situ assessments of antarctic sea ice and hence on average antarctic sea ice may be thicker than previously thought

tectonic development of the mid norway continental margin abstract the area reviewed covers the mid norway continental margin between latitudes 62 n and 68 n main structural elements as defined at the base cretaceous level are the trondelag platform underlying the inner shelf the more and voring basins located beneath the outer shelf and slope and the more platform and the outer voring plateau forming a base of slope trend of highs sediments contained in the mid norway basin range in age from late palaeozoic to cenozoic the basement was consolidated during the caledonian orogenic cycle devonian and early carboniferous wrench movements along the axis of the arctic north atlantic caledonides are thought to have preceded the namurian onset of crustal extension rifting processes were intermittently active for some 270 my until crustal separation between greenland and fennoscandia was achieved during the early eocene during the evolution of the norwegian greenland sea rift system a stepwise concentration of tectonic activities to its axial zone the area of subsequent continental separation is observed during the late palaeozoic to mid jurassic a broad zone was affected by tensional faulting during the late jurassic and cretaceous the trondelag platform was little affected by faulting whilst major rift systems in the more and voring basins subsided rapidly and their shoulders became concomitantly upwarped during the latest cretaceous and early palaeogene terminal rifting phase only the western more and voring basins were affected by intrusive and extrusive igneous activity following the early eocene crustal separation and the onset of sea floor spreading in the norwegian greenland sea the voring segment of the mid norway marginal basin subsided less rapidly than the more segment during the early and mid tertiary minor compressional deformations affected the voring basin and to a lesser degree the more basin tensional forces dominated the late palaeozoic to early cenozoic evolution of the mid norway basin and effected strain mainly in the area where the crust was weakened by the previous lateral displacements the lithosphere thinned progressively and the effects of the passively upwelling hot asthenospheric material became more pronounced massive dyke invasion of the thinned crust preceded its rupture

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

salinity profiles of antarctic sea ice field data and model results in order to describe and compare sea ice salinity distributions which are of interest in the study of physical and biological ice characteristics as well as ice ocean interaction we have computed composite profiles and fitted third degree polynomials to salinity data of 129 cores from the weddell sea four characteristic profile shapes have been recognized and described through the polynomial coefficients c s i and type profiles the field data have been compared with ideal salinity profiles generated by a simulation scheme based on thermodynamic growth under climatological conditions representative of the weddell sea composite salinity profiles agree well with simulations irrespective of the growth mechanism i e frazil or congelation suggesting that ice properties which depend on salinity or porosity may evolve independent of ice growth conditions analysis of salinity and 18o data in conjunction with the simulations demonstrates that apart from meteorological conditions controlling growth velocity and temperature distribution within the ice flooding and upward brine expulsion are important in raising top salinities as frequently observed low top salinities are typical of ice that has undergone retexturing and differential desalination salinity decreases towards the bottom of a floe are a result of high oceanic heat fluxes and snow cover effects as a consequence of these processes the bulk salinity of weddell sea ice cannot be described by a simple linear age thickness relationship as characteristic of arctic first year ice