impact of 3 d earth structure on fennoscandian glacial isostatic adjustment implications for space geodetic estimates of present day crustal deformations 1 the importance of including lateral earth structure in the analysis of fennoscandian glacial isostatic adjustment gia is investigated using a finite volume numerical formulation comparing output from radially varying 1 d earth models and models which account for the presence of plate boundaries lateral variations in lithospheric thickness and viscosity heterogeneities in the upper and lower mantle we find that perturbations to present day rates of surface deformation due to the inclusion of 3 d earth structure significantly exceed current observational uncertainties predicted residuals between 1 d and 3 d earth models may be improved with the use of a 1 d model which approximates the local depth dependent mean of the 3 d model however the remaining misfit is still large enough to significantlybiasinferencesofearthstructureandicehistory weconcludethatlateralvariationsatbothglobalandregional scales must be accounted for when interpreting gps observations from fennoscandia citation whitehouse p k latychev g a milne j x mitrovica and r kendall 2006 impact of 3 d earth structure on fennoscandian glacial isostatic adjustment implications for space geodetic estimates of presentday crustal deformations geophys res lett 33 l13502

bryozoans as carbonate sediment producers on the cool water lacepede shelf southern australia abstract modern sediments on the lacepede shelf and adjacent slope are typical mid latitude cool water palimpsest deposits dominated by bryozoan mollusc and quartz particles bryozoan bioclasts form autochthonous and to a lesser extent allochthonous accumulations across the grain size spectrum from mud to boulder size the bryozoan skeletons are grouped on the basis of architecture and style of disarticulation fragmentation into nine morphotypes erect rigid forms comprise 1 fenestrate 2 foliose 3 flat robust branching and 4 delicate branching morphotypes large adeona sp skeletons are recognised as a separate type of fenestrate form because of their distinctive shape mineralogy and restricted environment erect flexible forms are either 5 articulated branching or 6 articulated zooidal other important morphotypes are 7 encrusting 8 nodular arborescent and 9 vagrant differences in skeletal architecture result in distinct particle types encrusting morphotypes on soft organic substrates and the wide variety of erect rigid forms are broken into fragments erect flexible forms disintegrate upon death into definitive sand and silt size grains encrusting morphotypes on hard substrates nodular arborescent and vagrant forms remain whole and when large accumulate in place morphotypes are mineralogically distinct and so bryozoan sediment composition is environmentally controlled shallow water

the formation of polar surface water the ice export and the exchanges through the fram strait abstract the polar surface water is formed from two parent water masses inflowing atlantic water and fresh from river run off and net precipitation two possible modes for this formation are considered 1 a entrainment driven by mechanical stirring takes place over the entire polar ocean and brings atlantic water into the surface layer 2 b the interaction between the atlantic water and the fresh water supply occurs only on the s shelves while the mixing in the interior is inhibited by a thick advective layer of intermediate density formed on the shelves the exchanges between the polar ocean and the north atlantic and the resulting stratification are determined for the two cases the derived transports are much smaller than those often encountered in the literature which mode dominates depends upon the fresh water discharge from the analysis it is found that a larger fresh water contribution favours mixing on the shelves while the entrainment in the interior becomes more important as the run off diminishes the present day situation is probably one where the shelf processes dominate however a modest decrease in the river run of the ocean it is found that a complete removal of the river run off may not affect the extent of the ice cover too seriously so that comparatively high ice export can be maintained this is because the higher salinity of the polar surface water gradually increases the fresh water buffer present in the pacific inflow

glacial rebound and plate spreading results from the first countrywide gps observations in iceland iceland is one of the few places on earth where a divergent plate boundary can be observed on land direct observations of crustal deformation for the whole country are available for the first t

crustal accretion at high temperature spreading centres rheological control of crustal thickness abstract new determinations of lateral crustal thickness variations at anomalous oceanic spreading centres such as iceland have shown that the crust may be thinner at the ridge axis above the plume thickening towards the sides bjarnason and schmeling 2009 to understand this behaviour crustal accretion models have been carried out solving the conservation equations of mass momentum and energy with melting melt extraction and feedback of extracted material as newly formed crust for an extending lithosphere system underlain by a hot mantle plume the dynamics of rifting are thermally and rheologically controlled by the feedback due to accreted new crust four accretional modes with characteristic crustal thickness variations are identified depending on the width of the volcanic emplacement zone the accretional heating rate which can be associated with the thickness of the surface layer in which magmatic emplacement takes place and the spreading rate mode 1 zero crustal thickness at the spreading axis develops for cool accretion and a wide emplacement zone mode 2 strongly or moderately crustal thickening away from the axis develops in case of warm deep reaching accretion and wide emplacement zones mode 3 nearly constant crustal thickness develops in case of warm deep reaching accretion but narrow emplacement zones dynamic topography of mode 3 shows only a weak or no regional minimum at all near the axis modes 2 or 3 may be identified with the situation in iceland mode 4 a stagnating central crustal block evolves for cool accretion and narrow emplacement this mode disappears for increasing spreading rates no accretional mode with maximum crustal thickness above the plume at the rift axis has been found the absence of mode 1 accretion with zero crust at ridge axis on earth may be an indication that in general crustal accretion is not cold and shallow the model is also applied to other hotspot ridge settings azores galapagos and suggests modes 2 3 accretion

an analysis of anisotropic mantle viscosity and its possible effects on post glacial rebound abstract this paper presents a preliminary analysis of the possibility of using post glacial rebound observations to place constraints on the anisotropy of upper mantle viscosity we extend existing analyses by considering a visco elastic spherical self gravitating earth we show that three independent viscosity parameters are required to describe a maxwell solid with transversely isotropic viscosity and isotropic elasticity we develop a theoretical formalism for including the effects of transversely isotropic visco elasticity in post glacial rebound models using this formalism to describe anisotropy in the upper mantle we model the free air gravity anomaly relative sea level changes and present day horizontal motion caused by the rebound to assess their dependence on the upper mantle viscosity parameters we find that as long as those parameters are no larger than the lower mantle isotropic viscosity the results for the gravity and sea level changes are notably sensitive to only one of the three viscosity parameters this suggests at least for the case where the lower mantle viscosity is significantly larger than the upper mantle viscosity that post glacial rebound studies of gravity and sea level will be hard pressed to usefully constrain viscous anisotropy we do find that the horizontal motion is relatively sensitive to transverse isotropy and so might be used in principle to constrain the anisotropy in practice though it is likely that the strong dependence of horizontal motion on details of the ice model coupled with the uncertainties in existing ice models would make such attempts impractical

holocene uplift and rapid fluvial erosion of iceland a record of post glacial landscape evolution abstract in actively deforming regions fluvial systems are strongly regulated by uplift river geometries record histories of vertical motions that can be used to examine the driving forces generating topographic relief iceland s rapidly evolving landscapes provide an opportunity to disentangle histories of uplift generated by postglacial rebound volcanism dynamic support and plate spreading broad knickzones observed along iceland s large rivers and its powerful waterfalls and deep canyons hint that regional processes have generated significant relief we combine high resolution drone photogrammetry and cosmogenic 3he dating of fluvial terraces to measure the erosional history of one of iceland s largest knickzones jokulsarglufur in the northeast part of the island progressive younging of terraces indicates knickpoint propagation rates of up to 70 cm a 1 during the last 8 ka knickpoint velocities appear to be controlled partly by toppling of basalt columns these rates were used to calibrate a model that inverts iceland s drainage networks for uplift rate histories calculated uplift and isostatic calculations indicate that rifting sub plate support and isostatic adjustment resulted in tens to hundreds of meters of regional holocene uplift our results suggest regional uplift and fluvial erosion can rapidly generate hundreds of meters of relief in post glacial landscapes

modelling postglacial sea levels with power law rheology and a realistic ice model in the absence of ambient tectonic stress previous studies of postglacial rebound with power law rheology and simple ice models over laurentia have found that sea level data in the transition zone e g boston cannot be easily explained unless non linear rheology is restricted to lie within a thin layer between the lithosphere and the upper mantle the reason is the existence of a viscously stationary zone which is characteristic of non linear mantles in this study the simple ice model is replaced by a realistic ice history adapted from the ice 3g model which takes into account important effects such as retreating ice margins and eustatic ocean loading a 3 d finite element model is used to calculate the deformation and the relative sea levels in eastern canada and along the us east coast the purpose is to investigate whether the realistic load model allows sea level data near the centre of rebound and along the atlantic east coast of canada and the usa to be explained by non linear rheology simultaneously the assumptions made are that 1 postglacial rebound sees the steady state creep and not the transient creep 2 either ambient tectonic stresses do not interact with rebound stress or the ambient stress level is low 3 lateral heterogeneity of rock properties and anisotropic flow can be neglected results indicate that the retreating ice margin does make it easier for non linear rheology in the mantle to explain the sea level data in the transition zone around boston however reconciling all the sea level data near the centre of rebound and along the atlantic east coast simultaneously is still difficult for a non linear rheology so far two types of models with non linear rheology have been found to be able to explain the sea level data inside and outside laurentia simultaneously they are 1 earth models with a thin 270 km non linear zone overlying a 1021 pa s lower mantle 2 earth models with a linear upper mantle with an average viscosity of 1021 pa s overlying a non linear lower mantle with a 3 10 35 pa 3 s 1 and n 3

regional perturbations in a global background model of glacial isostasy abstract in schotman and vermeersen 2005 earth planet sci lett 236 the effect of crustal and asthenospheric low viscosity zones lvzs on geoid heights was shown as predicted by models of glacial isostatic adjustment gia the governing equations were solved analytically in the spectral domain which makes the method used accurate and fast however it does not allow for large lateral variations in earth stratification as the properties of shallow lvzs can be expected to vary laterally we have developed a finite element model based on abaqus global spherical 3d finite element models are currently not capable of providing high resolution predictions which we expect due to the shallowness of the lvzs we therefore use a regional flat 3d model and compute geoid heights from the predicted displacements at density boundaries by solving laplace s equation in the fourier transformed domain the finite element model is not self gravitating but we compare the results with a self gravitating spectral model under the assumption that the lack of self gravitation is partly compensated by the lack of sphericity and that long wavelength differences largely cancel out when using perturbations which are the difference between a model with and without an lvz we show that geoid height perturbations due to an lvz can be computed accurately though the accuracy deteriorates somewhat with the depth of the lvz moreover we show that horizontal rates of displacement though not accurate for total displacements are accurate for perturbations in the near field we show the effect of lateral variations in the properties of the lvz and in lithospheric thickness and compute geoid height perturbations for northern europe based on a simple laterally heterogeneous model the model is forced with a realistic ice load history and a eustatic ocean load history the errors introduced by using a eustatic ocean load history instead of realistic oceans are generally smaller than 10 but might be critical for perturbations due to crustal lvzs

two circulation regimes of the wind driven arctic ocean the major goal of this paper is to demonstrate the existence in the arctic ocean of two regimes of wind forced circulation we simulated the vertically averaged currents sea level heights and ice drift in the arctic ocean from 1946 to 1993 using a two dimensional wind forced barotropic model that includes frictional coupling between the ocean and ice the model has a spatial resolution of 55 5 km and is driven by winds river runoff and an imposed but realistic sea level slope between the pacific and the atlantic oceans there is a good agreement between velocities from observed buoy motions and velocities of modeled ice drift even though the model lacks ocean baroclinicity and ice thermodynamics the results indicate that wind driven motion in the central arctic alternates between anticyclonic and cyclonic circulation with each regime persisting for 5 7 years based upon our analysis of the modeled sea level and ice motion anticyclonic wind driven motion in the central arctic appeared during 1946 1952 1958 1963 1972 1979 and 1984 1988 and cyclonic motion appeared during 1953 1957 1964 1971 1980 1983 and 1989 1993 shifts from one regime to another are forced by changes in the location and intensity of the icelandic low and the siberian high the two regimes may help explain the significant basin scale changes in the arctic s temperature and salinity structure observed recently the great salinity anomaly and the variability of ice conditions in the arctic ocean

sea ice thickness distribution in fram strait hm nuclear submarine sovereign has obtained the first synoptic measurements of the ice thickness distribution in fram strait the main outflow from the arctic ocean the ice nearest the ice edge is of quite different character from ice in the interior of the strait with a greater proportion of first year ice and less pressure ridging an estimate of the ice flux through fram strait can be made from the distribution of mean ice drafts

glacial isostatic adjustment on 3 d earth models a finite volume formulation summary we describe and present results from a finite volume fv parallel computer code for forward modelling the maxwell viscoelastic response of a 3 d self gravitating elastically compressible earth to an arbitrary surface load we implement a conservative control volume discretization of the governing equations using a tetrahedral grid in cartesian geometry and a low order linear interpolation the basic starting grid honours all major radial discontinuities in the preliminary reference earth model prem and the models are permitted arbitrary spatial variations in viscosity and elastic parameters these variations may be either continuous or discontinuous at a set of grid nodes forming a 3 d surface within the regional or global modelling domain in the second part of the paper we adopt the fv methodology and a spherically symmetric earth model to generate a suite of predictions sampling a broad class of glacial isostatic adjustment gia data types 3 d crustal motions long wavelength gravity anomalies these calculations based on either a simple disc load history or a global late pleistocene ice load reconstruction ice 3g are benchmarked against predictions generated using the traditional normal mode approach to gia the detailed comparison provides a guide for future analyses e g what grid resolution is required to obtain a specific accuracy and it indicates that discrepancies in predictions of 3 d crustal velocities less than 0 1 mm yr 1 are generally obtainable for global grids with 3 106 nodes however grids of higher resolution are required to predict large amplitude 1 cm yr 1 radial velocities in zones of peak post glacial uplift e g james bay to the same level of absolute accuracy we conclude the paper with a first application of the new formulation to a 3 d problem specifically we consider the impact of mantle viscosity heterogeneity on predictions of present day 3 d crustal motions in north america in these tests the lateral viscosity variation is constructed with suitable scaling from tomographic images of seismic s wave heterogeneity and it is characterized by approximately 2 orders of magnitude peak to peak lateral variations within the lower mantle and 1 order of magnitude variations in the bulk of the upper mantle below the asthenosphere we find that the introduction of 3 d viscosity structure has a profound impact on horizontal velocities indeed the magnitude of the perturbation of order 1 mm yr 1 is as large as the prediction generated from the underlying 1 d radial reference model and it far exceeds observational uncertainties currently being obtained from space geodetic surveying the relative impact of lateral viscosity variations on predicted radial motions is significantly smaller

north pole ice thickness and association with ice motion history 1977 1992 ice drafts measured by u s navy submarine sonar near the north pole from 1977 to 1992 were used together with arctic ocean drifting buoy data from 1979 to 1992 to investigate north pole ice thickness changes during this 15 year period a primary objective was to determine dynamical forcing mechanisms and associated time scales relevant to ice thickness variation north pole ice thickness showed much interannual variability and no systematic decrease of ice thickness was observed changes in ice thickness were closely associated with long term directional deviation of ice motion from geostrophic wind forcing and with ice convergence and divergence at the north pole during the 1 2 weeks prior to submarine measurement the geographic origin of ice within the arctic ocean was not associated with its eventual thickness at the north pole

application of bryozoan zoarial growth form studies in facies analysis of non tropical carbonate deposits in new zealand abstract the fragmental remains of bryozoan colonies are the dominant skeletal contributor in many modern and ancient occurrences of non tropical shelf carbonate deposits the taxonomic complexity of the bryozoa the difficulties of systematic classification except by specialists the wide range of particle sizes and of preservation state in the carbonate deposits and the often well cemented nature of host limestones all limit the sedimentological interpretation of the commonly diverse bryozoan component however following the pioneer work of stach in the 1930 s the potential exists for sedimentologists to obtain useful paleo environmental information about the deposits by recording simply the nature and relative abundances of the various growth shapes of the bryozoan material known as their zoarial colonial growth forms here we propose a simple descriptive terminology covering both cheilostome and cyclostome bryozoans that involves four main growth forms encrusting erect rigid erect flexible and free living and a selection of subcategories based on shape identification of these habitat related growth form types is aided by reference to line drawings specimen photographs and photomicrographs of thin section slices the scheme is illustrated by examining briefly some possible environmental controls on the spatial and or time variations in the nature and distribution of colonial growth forms in several examples of modern and tertiary bryozoan dominated carbonate deposits in new zealand we anticipate that the routine description and quantification of bryozoan colonial growth types in non tropical carbonates generally will facilitate the recognition and interpretation of contrasting sub facies within the global foramol bryomol group of carbonate deposits

numerical models of mantle lithosphere weakening erosion and delamination induced by melt extraction and emplacement continental rifting caused by extension and heating from below affects the lithosphere or cratons in various ways volcanism and melt intrusions often occur along with thinning weakening and even breaking lithosphere although mechanical necking models of the lithosphere are often applied the aspects of melting and the implications due to melt transport and emplacement at shallower depths are not well understood a two phase flow approach employing melt extraction and shallow emplacement associated with thermal weakening is developed and compared with observations the results of this comparison indicate the importance of partial melts and an asthenospheric magma source for increasing the rising rate of the lithosphere asthenosphere boundary during extension thermo mechanical physics of visco plastic flow is approximated using the finite difference method with eulerian formulation in 2d the conservation of mass momentum and energy equations are solved for a multi component crust mantle and two phase melt matrix system rheology is temperature and stress dependent in consideration of depletion and enrichment melting and solidification are controlled by a simplified linear binary solid solution model melt is extracted and emplaced in predefined depth regions emplacement zones in the lithospheric mantle and crust the compaction boussinesq approximation was applied its validity was tested against the full compaction formulation and found fully satisfactory for the case of sublithospheric melting models a simple model guided by the geodynamic situation of the rwenzori region typically results in updoming asthenosphere with melt assisted erosion of the lithosphere s base even with a conservative approach for a temperature anomaly melting alone doubles the lithospheric erosion rate in comparison with a model without melting with melt extraction and intrusion lithospheric erosion and upwelling of the lithosphere asthenosphere boundary speeds up by a factor 3 4 in an extreme case delamination may occur if weakening fully decouples a hanging mantle block models with an emplacement zone of up to approximately 70 km agree well with observations especially for a concept based on seismological and petrological data

iceland rising solid earth response to ice retreat inferred from satellite radar interferometry and visocelastic modeling a broad uplift occurs in iceland in response to the retreat of ice caps which began circa 1890 until now this deformation signal has been measured primarily using gps at points some distance away from the ice caps here for the first time we use satellite radar interferometry interferometric synthetic aperture radar to constrain uplift of the ground all the way up to the edge of the largest ice cap vatnajokull this allows for improved constraints on the earth rheology both the thickness of the uppermost earth layer that responds only in an elastic manner and the viscosity below it the interferometric synthetic aperture radar velocities indicate a maximum displacement rate of 24 4 and 31 4 mm yr at the edge of vatnajokull during 1995 2002 and 2004 2009 respectively the fastest rates occur at outlet glaciers of low elevation where ice retreat is high we compare the observations with glacial isostatic adjustment models that include the deglaciation history of the icelandic ice caps since 1890 and two earth layers using a bayesian approach we derived probability density functions for the average earth model parameters for three satellite tracks based on our assumptions the three best fit models give elastic thicknesses in the range of 15 40 km and viscosities ranging from 4 10 1018 pa s

ridging and strength in modeling the thickness distribution of arctic sea ice a theory describing evolution of the ice thickness distribution the probability density of ice thickness was proposed by thomdike et al 1975 and has been used in several sea ice models the advantage of this theory over the widely used two level formulation is that it treats ridging explicitly as a redistribution of ice thickness and ice strength as a function of energy losses incurred by ridge formation however the parameterization of these processes remains rather speculative and largely untested and so our purpose here is to explore these parameterizations using a numerical model based on this theory the model uses a 160 km resolution grid of the arctic and 7 years of observed atmospheric forcing data 1979 1985 monthly oceanic heat flux and current fields are obtained from a 40 km resolution coupled ice ocean model run separately with the same forcing by requiring the computed monthly mean ice drift to have the same magnitude as observed buoy drift we estimate the primary strength parameter the ratio of total to potential energy change during ridging this ratio depends on the value of other parameters however the standard case has a ratio of 17 which is within the range estimated by hopkins 1994 in simulations of individual ridging events the effects of ridge redistribution and shear ridging parameters are illustrated by a series of sensitivity studies and comparisons between observed and modeled ice thickness distributions and ridge statistics in addition these comparisons highlight the following shortcomings of the thickness distribution theory as it is presently implemented first the process of first year to multiyear ridge consolidation is ignored and second the observed preferential melt of thick ridged ice is not reproduced

monitoring ice thickness in fram strait time series of ice thickness in fram strait are evaluated together with ice drift for estimation of the ice flux from the arctic ocean ice draft was measured from moored sonars every 4 min suggesting a nominal sampling interval of 30 m on the basis of these data the annual cycle of many statistics can be resolved the modal ice thickness shows a maximum of 3 15 m in may and a minimum in september of 2 43 m indicating an annual variation range of 0 72 m this compares with model results comprehensive drillings and most of the submarine observations during the last 2 3 decades indicating no significant long term ice thickness change this is also concurrent with the lack of significant temperature changes in the arctic ocean during the same period the mean ice thickness in the central core of the ice stream shows an annual amplitude of 1 m with a minimum in september of 2 25 m and a maximum in april may of 3 25 m the mean thickness of ice only open water excluded is 3 27 m and compares with 3 29 m which is the mean thickness of ice only from 17 submarine tracks in the arctic ocean 1960 1982 the mean annual ice export which is equal to the mean annual net production of ice in the arctic ocean is 2850 km 3 yr 1 for the period 1990 1996 this efflux shows significant variations from year to year with a minimum in 1990 1991 of 2050 km 3 yr 1 and a maximum in 1994 1995 of 4700 km 3 yr 1 this significant variation of nearly 130 is mainly caused by an increased atmospheric forcing and to a lesser degree by an increase in the annual mean ice thickness of 20 over the same period

the experimental deformation of dunite deformation experiments have been carried out on two dunites anita bay of 100 m grain size and aheim of 900 m grain size at strain rates from 10 3 to 10 6 s 1 and temperatures from 1000 c to 1300 c in a gas medium deformation apparatus at 300 mpa confining pressure most of the tests were under wet conditions defined by the presence of small amounts of water from hydrous minerals initially present constant strain rate and relaxation experiments covering ranges of flow stress down to about 70 mpa and 7 mpa respectively show that there is a change in flow law in going below about 100 mpa differential stress and that the coarser grained rock is stronger than the finer grained one power law parameters above the transition are n 4 48 0 31 and q 498 38 kj mol 1 for aheim dunite and n 3 35 0 17 and q 444 24 kj mol 1 for anita bay dunite while below the transition relaxation tests on anita bay dunite give n 2 44 0 18 and q 386 27 kj mol 1 it is concluded that there is a weakening effect of water that this effect is mainly in the grain boundaries and that grain boundary sliding is probably a significant deformation mechanism at the lower stresses under wet conditions

sea ice topography of the arctic ocean in the region 70 degrees w to 25 degrees e in october 1976 a cooperative experiment was made to survey the sea ice topography in the european sector of the arctic ocean h m submarine sovereign acquired 4000 km of ice draft data by using an upward looking sonar while a canadian forces aircraft flew along the submarine s track and acquired 2200 km of ice elevation data by using a laser profilometer the two types of profile were processed in corresponding 100 km section lengths and the following statistical analyses and comparisons were made i probability density functions o f ice draft and elevation each distribution shows a peak for young ice and for undeformed multi year ice at large ice thicknesses the distributions take the analytical form of a negative exponential the mean drafts enable two distinct geographical ice regimes to be identified there is an offshore zone of very heavy pressure ridging extending up to 400 km from the coasts of ellesmere island and of north greenland with mean ice draft in the range 5 0 to 7 5 m while out in the central arctic ocean the mean ice draft is lower 3 9 5 1 m and the characteristics of the ice cover remain homogeneous over a length scale of 1000 km the transition between the two regimes is abrupt taking place in less than 25 km data from the same part of the central arctic taken in march 1971 showed a mean ice draft 0 3 m lower while data from the central beaufort sea showed a mean draft more than 0 8 m lower ii level ice distributions ice with a local gradient of less than 1 in 40 was defined as level ice and used as an indicator of the quantity and thickness distribution of undeformed i e thermodynamically grown ice in the arctic ocean the distribution has a mode at 3 0 3 1 m draft and level ice percentages are in the range 30 40 bottom side and 70 80 top side in the offshore zone and 45 55 bottom and 85 95 top in the central arctic thus about half of the arctic ice cover consists of deformed ice iii pressure ridge spacings the spacings of ridge keels fit a negative exponential distribution characteristic of randomness except at close spacings where there is a deficit of keels explained as a geometrical effect and at very large spacings where there is an excess due to the contribution of polynyas the distribution of sail spacings exhibits these two effects but also differs from a random distribution at moderate sail separations iv ridge elevations and drafts keel drafts fit a law of form p h d oc exp ah 2 d h except for an excess of keels at drafts beyond 20 m there is a positive correlation between mean keel draft and keel frequency there are 3 5 4 5 keels per kilometre with draft exceeding 9 m in the offshore zone and 2 3 in the central arctic sail elevations fit a law of form p h d h x exp a h d h with a positive correlation between mean sail elevation and sail frequency the sail elevation and ice elevation distributions can be related by assuming that all thick ice is contained in pressure ridges of triangular cross section v keel sail comparison for the 21 corresponding 100 km sections there are positive correlations between mean sail height and mean keel draft and between keel frequency and sail frequency from these it is possible to convert a sail distribution computed from a laser profile into a keel distribution enabling sea ice bulk characteristics to be derived from airborne surveys alone vi leads and polynyas a lead was defined as a continuous sequence of depth points greater than 5 m long and not exceeding 1 m draft the number density n d of leads per kilometre of width d m fits the power law n d 15 d 2 exceptionally wide leads were concentrated in the offshore zone and in the marginal ice zone close to the open water of the greenland sea

the under ice thickness distribution of the arctic basin as recorded in 1958 and 1970 the uss nautilus ssn 571 was the first vessel to cross the arctic basin via the north pole in early august 1958 during this expedition almost continuous acoustic under ice thickness profiles were recorded in august 1970 uss queenfish ssn 651 retraced nautilus route yielding the only duplicate transect of under ice topography across the arctic basin comparisons of the statistical analysis of the under ice draft measurements obtained through use of wide beam and narrow beam and narrow beam only acoustic profilers by nautilus and queenfish respectively along the coincidental route are presented geographic areas found to have distinct under ice characteristics and ice composition are identified the under ice statistics of both cruises are considered in relation to representative field observations and modeling results of other researchers principal findings are 1 nautilus recorded generally more severe ice conditions within the canada basin than did queenfish 12 years later overall mean drafts were 3 08 and 2 39 m respectively 2 the under ice topography becomes progressively more severe when proceeding from the canadian to the eurasian side of the arctic basin 3 the canada basin may contain the most moderate under ice topography and the greatest number of open water and refrozen polynyas and leads within the central arctic basin 4 the makarov and amundsen basins and the arctic mid ocean ridge may contain some of the most severe under ice topography within the arctic basin and 5 the present study indicates an overall arctic basin mean of 3 4 open water new ice less than 30 cm in summer

geodetic gps measurements in south iceland strain accumulation and partitioning in a propagating ridge system 1 gps observations in south iceland between 1994 and 2003 are compared with two dimensional elastic half space and viscoelastic coupling models for two parallel rift zones representing the western and eastern volcanic zones wvz evz gps data from the hreppar block between the wvz and evz fit a rigid block model within uncertainties spreading rates across the wvz increase from 2 6 0 9 mm yr in the northeast to 7 0 0 4 mm yr in the southwest conversely spreading rates in the evz decrease from 19 0 2 0 mm yr in the northeast to 11 0 0 8 mm yr in the southwest the direction of ridge propagation summed extension rates across the two rift zones are approximately constant and equal to the total plate rate 18 20 mm yr consistent with a simple propagating ridge model whereby the wvz is deactivating in the direction of evz propagation the coupling model confirms results from the simple elastic half space model including relatively shallow locking depths 5 km beneath the rift zones and allows for an estimate of mean viscosity 1019 1020 pa s beneath the elastic layer the location of maximum surface velocity gradient in the evz presumably the locus of subsurface magma accumulation and future rifting does not coincide with the 1783 1784 lakagigar fissure eruption but is 20 km to the west on the bar abunga veidivotn fissure swarm this had a small volume eruption in 1862 1864 but a major eruption in 1477 a d

using commercial finite element packages for the study of earth deformations sea levels and the state of stress summary modifications to commercial finite element fe packages must be applied before they can be used for geophysical studies involving long wavelength deformation or viscoelasticity this paper provides in detail how and why the commercial codes have to be modified when incompressibility is assumed both the non self gravitating flat earth and self gravitating spherical earth will be considered the latter involves an iterative procedure which converges within 5 iterations this is demonstrated both analytically and numerically in addition implementation of the gravitationally self consistent sea level equation on a self gravitating spherical earth is also described good agreement between numerical results obtained with this coupled finiteelement method and the conventional spectral method is also demonstrated in all cases the interpretation of the outputs of fe models are particularly important in modelling the state of stress

influence of melt on the creep behavior of olivine basalt aggregates under hydrous conditions the influence of melt on the creep behavior of olivine basalt aggregates under hydrous conditions has been investigated by performing a series of high temperature triaxial compression experiments samples with melt fractions of 0 02 0 12 were deformed under water saturated conditions at temperatures between 1373 and 1473 k and a confining pressure of 300 mpa in a gas medium apparatus at constant differential stress and temperature the rate of deformation increased rapidly but systematically with increasing melt fraction in the diffusion creep regime at a given differential stress samples with melt fractions of 0 02 and 0 12 deformed a factor of 2 and 20 respectively faster than a melt free sample in the dislocation creep regime a sample with a melt fraction of 0 12 deformed a factor of 40 faster than a melt free sample for partially molten olivine basalt aggregates deformed under hydrous conditions the dependence of creep rate on melt fraction can be expressed in the form 0 exp where 26 for diffusion creep and 31 for dislocation creep the results of this study combined with reasonable estimates for the spatial variation in the concentrations of water and melt as well as for the geotherm and the activation volume for creep provide constraints on the viscosity structure of earth s upper mantle as an example we present a viscosity profile for the mantle wedge above a subducting plate demonstrating that the viscosity in that region can vary by 3 orders of magnitude over a depth of 60 km due to the combined effects of water and melt weakening

continuous gps measurements of postglacial adjustment in fennoscandia 1 geodetic results 1 data collected under the auspices of the bifrost gps project yield a geographically dense suite of estimates of present day three dimensional 3 d crustal deformation rates in fennoscandia johansson et al 2002 a preliminary forward analysis of these estimates milne et al 2001 has indicated that models of ongoing glacial isostatic adjustment gia in response to the final deglaciation event of the current ice age are able to provide an excellent fit to the observed 3 d velocity field in this study we revisit our previous gia analysis by considering a more extensive suite of forward calculations and by performing the first formal joint inversion of the bifrost rate estimates to establish insight into the physics of the gia response in the region we begin by decomposing a forward prediction into the three contributions associated with the ice ocean and rotational forcings from this analysis we demonstrate that recent advances in postglacial sea level theory in particular the inclusion of rotational effects and improvements in the treatment of the ocean load in the vicinity of an evolving continental margin involve peak signals that are larger than the observational uncertainties in the bifrost network the forward analysis is completed by presenting predictions for a pair of fennoscandian ice histories and an extensive suite of viscoelastic earth models the former indicates that the bifrost data set provides a powerful discriminant of such histories the latter yields bounds on the assumed constant upper and lower mantle viscosity nu um nu lm specifically we derive a 95 confidence interval of 5 x 10 20 less than or equal to nu um less than or equal to 10 21 pa s and 5 x 10 21 less than or equal to nu lm less than or equal to 5 x 10 22 pa s with some preference for elastic lithospheric thickness in excess of 100 km the main goal of the bayesian inverse analysis is to estimate the radial resolving power of the bifrost gps data as a function of depth in the mantle assuming a reasonably accurate ice history we demonstrate that this resolving power varies from similar to 200 km near the base of the upper mantle to similar to 700 km in the top portion of the lower mantle we conclude that the bifrost data are able to resolve structure on radial length scale significantly smaller than a single upper mantle layer however these data provide little constraint on viscosity in the bottom half of the mantle finally elements of both the forward and inverse analyses indicate that radial and horizontal velocity estimates provide distinct constraints on mantle viscosity

glacial isostatic adjustment constrains dehydration stiffening beneath iceland abstract during melting in the upper mantle the preferred partitioning of water into the melt will effectively dehydrate the solid residue linear extrapolation of laboratory experiments suggests that dehydration can produce a sharp viscosity contrast increase of a factor 500 across the dry solidus in this study we show that the suggested magnitude of dehydration stiffening in a plume ridge setting is incompatible with the present glacial isostatic adjustment gia in iceland using gps observations of current gia in iceland we find that the data are best fit by a viscosity contrast over the dry solidus in the range 0 5 3 a viscosity contrast higher than 10 requires a mantle viscosity below the dry solidus lower than 4 8 10 18 pa s depending on the thickness of the dehydrated layer a viscosity contrast of 100 or more demands a mantle viscosity of 10 18 pa s or less however we show here that a non linear extrapolation of the laboratory data predicts a viscosity contrast as low as a factor 3 29 assuming conditions of constant strain rate to constant viscous dissipation rate this is compatible with our gia results and suggests that the plume ridge interaction beneath iceland is governed by a non linear rheology and controlled by a combination of kinematic and dynamic boundary conditions rather than buoyant forces alone

glacial isostatic adjustment and the viscosity structure underlying the vatnajokull ice cap iceland we examine the dependence of glacial isostatic adjustment gia due to changes in the vatnajokull ice cap iceland on the underlying viscosity structure iceland offers a unique case study for gia research with a thinner elastic lithosphere underlain by a low viscosity zone or asthenosphere as opposed to regions such as fennoscandia or north america described by a thicker lithosphere while not necessarily featuring an asthenosphere

the role of sea ice and other fresh water in the arctic circulation salinity stratification is critical to the vertical circulation of the high latitude ocean we here examine the control of the vertical circulation in the northern seas and the potential for altering it by considering the budgets and storage of fresh water in the arctic ocean and in the convective regions to the south we find that the present day greenland and iceland seas and probably also the labrador sea are rather delicately poised with respect to their ability to sustain convection small variations in the fresh water supplied to the convective gyres from the arctic ocean via the east greenland current can alter or stop the convection in what may be a modern analog to the halocline catastrophes proposed for the distant past the north atlantic salinity anomaly of the 1960s and 1970s is a recent example it must have had its origin in an increased fresh water discharge from the arctic ocean similarly the freshening and cooling of the deep north atlantic in recent years is a likely manifestation of the increased transfer of fresh water from the arctic ocean into the convective gyres finally we note that because of the temperature dependence of compressibility a slight salinity stratification in the convective gyres is required to efficiently ventilate the deep ocean

carbonate mineralogy of free living bryozoans bryozoa otionellidae otago shelf southern new zealand on the open temperate shelf off otago peninsula eastern south island new zealand in water depths ranging from 90 to 140 m free living motile bryozoans of the genus otionellina survive in shifting quartzofeldspathic sands unlike most bryozoans these free living colonies characteristically precipitate skeletons of aragonite here we report on skeletal carbonate mineralogy of 104 specimens of otionellina from four different species across the shelf which are also in different stages of development all were dominantly composed of aragonite with only five specimens containing 0 15 wt calcite but with an overall mean of 1 wt with low to intermediate mgco3 content in the calcite 5 wt mgco3 mineralogy in otionellina varies but is not related to species colony size or environmental factors such as water temperature salinity water depth and substratum otionellinids may lack the environmentally influenced biomineralisation pathways found in other carbonate producing taxa it appears that most free living or vagrant bryozoans are aragonitic although some rare examples contain up to 5 calcite there may be no free living genera which are always entirely aragonite while preservation of aragonitic skeletons is by no means certain the presence of free living bryozoans indicates a sandy outer shelf environment and a diagenetic history that allows for preservation of aragonite

skeletal carbonate mineralogy of new zealand bryozoans abstract on many cool water carbonate shelves both modern and ancient bryozoans are the dominant sediment contributor because skeletal carbonate mineralogy can be related to both taxonomic and environmental controls and has important implications for understanding the geochemistry and diagenesis in carbonate deposits it is highly relevant to develop a comprehensive database for bryozoan skeletal mineralogy this x ray diffraction study of modern new zealand bryozoans more than doubles the published data available on bryozoan mineralogy and allows for statistical analysis of variability in mineralogy particularly the hitherto unknown background variation within individuals and within populations such a database allows evaluation of differences among populations taxa and growth forms the 412 analyses of 49 bryozoan species fall into four mineralogical categories a 61 are single calcite which ranges widely from low to high wt mgco3 varieties b 6 in the genera cellaria and macropora comprise dual calcites a dominant one low in mg x 1 8 wt mgco3 and the other much higher x 9 0 wt mgco3 c 27 are mainly calcite with some aragonite and d 6 are mainly aragonite with some calcite the bimineralic calcite aragonite skeletons can arise from different processes including biologically mediated changes in precipitation and diagenetic alteration following death there is small but significant variation in mg content in calcite both within colonies mg increasing with age and within populations for a total of within species variance of 0 526 we found however more variation among species in both calcite aragonite ratio and mg content than that within species whereas some genera and families are reasonably consistent mineralogically e g low mg calcite cyclostomes such as telopora buski others are highly variable e g chaperia acanthina among the higher taxa cyclostomes tend to be entirely or mainly calcite with a low to moderate range of mgco3 x 2 1 wt whereas cheilostomes are far more variable with anascans x 4 1 wt having a lower mgco3 content than ascophorans x 5 6 wt all dual calcite species are anascans in the calloporoidea the proportions of aragonite and of mg in calcite are greater in evolutionarily younger taxa at least at the higher levels in relation to growth forms erect rigid varieties are calcitic except for several robust branching genera such as adeonellopsis encrusting forms particularly unilaminar sheets are highly variable and contain most of the bimineralic calcite aragonite species erect flexible forms are either single or dual calcite and extremely variable in mg content free living or vagrant forms are usually aragonitic in relation to latitude and temperature variation few species show any consistent or significant mineralogical trends although schizoporella unicornis deposits increasing amounts of aragonite at lower warmer latitudes mecynoecia purpurascens includes more but still low mg in its calcite skeleton with increasing latitude a reversal of the mg temperature relationship noted in several other skeletal carbonate producers

reconstructing the origin and trajectory of drifting arctic sea ice recent studies have indicated that drifting arctic sea ice plays an important role in the redistribution of sediments and contaminants here we present a method to reconstruct the back ward trajectory of sea ice from its sampling location in the eurasian arctic to its possible site of ori gin on the shelf based on historical drift data from the international arctic buoy program this method is verified by showing that origins derived from the backward trajectories are generally con sistent with other indicators such as comparison of the predicted backward trajectories with known buoy drifts and matching the clay mineralogy of sediments sampled from the sea ice with that of the seafloor in the predicted shelf source regions the trajectories are then used to identify regions where sediment laden ice is exported to the transpolar drift stream from the new siberian islands and the central kara plateau calculation of forward trajectories shows that the kara sea is a major contributor of ice to the barents sea and the southern limb of the transpolar drift stream

postseismic deformation following the june 2000 earthquake sequence in the south iceland seismic zone 1 we observe postseismic deformation on two spatiotemporal scales following mw 6 5 earthquakes in the south iceland seismic zone on 17 and 21 june 2000 we see a rapidly decaying deformation transient lasting no more than 2 months and extending about 5 km away from the two main shock ruptures this local month scale transient is captured by several radar interferograms and is also observed at a few campaign gps sites located near the faults a slower transient with a characteristic timescale of about a year is detected only by gps measurements the month scale deformation pattern has been explained by poroelastic rebound due to postearthquake pore pressure changes in contrast the year scale deformation can be explained by either afterslip at 8 14 km depth or viscoelastic relaxation of the lower crust and upper mantle in response to the coseismic stress changes the optimal viscoelastic models have lower crustal viscosities of 0 5 1 1019 pa s and upper mantle viscosity of 3 1018 pa s because of the limitations of our gps campaign data we consider both afterslip and viscoelastic relaxation as plausible mechanisms explaining the deformation field both types of postseismic deformation models suggest that the areas of large coseismic stress increase east of the 17 june and west of the 21 june ruptures continue to be loaded by the postseismic deformation

sea ice thickness distribution in the greenland sea and eurasian basin may 1987 upward sonar data from a 3400 km track in the greenland sea and the eurasian basin of the arctic ocean obtained by a british submarine in may 1987 have been statistically analyzed mean ice thicknesses in the greenland sea show significant melting with downstream distance at an average rate of 0 38 m per degree of latitude equivalent to about 2140 km3 yr 1 of freshwater input between 80 n and 75 n in the region north of fram strait between 82 n and 80 n there is remarkable consistency between mean ice drafts in 1987 and in three previous cruises 1976 1979 1985 while between greenland and the north pole there was a significant thinning in 1987 relative to 1976 in fram strait the mean ice draft shows evidence of thinning within the region of influence of a semipermanent eddy ice draft distributions show that the probability density function of deformed ice follows a negative exponential distribution and that the change of shape of the distribution with downstream distance in the greenland sea is due to preferential melting of thicker ice pressure ridge draft distributions also fit a negative exponential while their spacings fit a lognormal distribution lead width distribution follows a power law with an exponent of 1 45 for narrow leads and 2 50 for wider leads if leads are defined as having a 1 m maximum draft 1 67 and 2 76 for a 0 5 m maximum draft lead spacings follow a negative exponential distribution for moderate spacings 400 1500 m but with an excess of lead pairs at small and very large spacings

controls variation and a record of climate change in detailed stable isotope record in a single bryozoan skeleton abstract the long lived about 20 yr bryozoan adeonellopsis sp from doubtful sound new zealand precipitates aragonite in isotopic equilibrium with seawater exerting no metabolic or kinetic effects oxygen isotope ratios 18 o in 61 subsamples along three branches of a single unaltered colony range from 0 09 to 0 68 pdb mean 0 36 pdb carbon isotope ratios 13 c range from 0 84 to 2 18 pdb mean 1 69 pdb typical of cool water carbonates 18 o derived water temperatures range from 14 2 to 17 5 c adeonellopsis has a minimum temperature growth threshold of 14 c recording only a partial record of environmental variation by correlating seawater temperatures derived from 18 o with the southern oscillation index however we were able to detect major events such as the 1983 el nino interannual climatic variation can be recorded in skeletal carbonate isotopes the range of within colony isotopic variability found in this study 0 77 in 18 o and 1 34 in 13 c means that among colony variation must be treated cautiously temperate bryozoan isotopes have been tested in less than 2 of described extant species this highly variable phylum is not yet fully understood

can observations of postglacial rebound tell whether the rheology of the mantle is linear or nonlinear it has been argued that since a power law mantle may appear to the postglacial rebound process as linear if the rebound stress is less than the ambient tectonic stress therefore observations of rebound cannot be used to determine whether the flow law in the mantle is linear or nonlinear the purpose of this paper is to show that this may not be completely true if the rebound process sees the steady state creep it will be demonstrated that for sites near the centre of rebound the relative sea level rsl observations cannot distinguish a newtonian mantle from a power law mantle with large ambient stress however for sites outside the ice margin the rsl signature for a power law mantle is clearly distinguishable from that due to deep flow in a newtonian mantle this characteristic of power law medium is independent of the ambient stress level and is unlikely to change with more complicated rebound models if these results remain valid in future work then rsl data near the ice margin can be used to argue against a steady state power law mantle

secular oscillations in the carbonate mineralogy of reef building and sediment producing organisms driven by tectonically forced shifts in seawater chemistry the primary mineralogy of oolites and early marine carbonate cements led sandberg nature 305 1983 19 22 to divide the phanerozoic eon into three intervals of aragonite seas and two intervals of calcite seas hardie geology 24 1996 279 283 has shown that these oscillations together with synchronous oscillations in the mineralogy of marine potash evaporites can be explained by secular shifts in the mg ca ratio of seawater driven by changes in spreading rates along mid ocean ridges the hardie model also predicts that high mg calcite should precipitate along with aragonite as it does in today s aragonite sea we have uncovered oscillations in the carbonate mineralogy of hypercalcifying organisms ones that have produced massive skeletons large reefs or voluminous bodies of sediment that correspond to sandberg s aragonite seas and calcite seas and that are predicted by the hardie model particular groups of corals sponges and algae appear to have been dominant reef builders only when favored by an appropriate mg ca ratio in seawater in early and middle paleozoic calcite seas calcite i reefs were dominated by calcitic tabulate heliolitid and rugose corals and calcitic stromatoporoids in contrast during the period of late paleozoic early mesozoic aragonite seas aragonite ii aragonitic groups of sponges scleractinian corals and phylloid algae as well as high mg calcitic red algae were principal reef builders during late cretaceous time at the acme of calcite ii massive rudists displaced aragonitic hermatypic corals in today s aragonite sea aragonite iii scleractinian corals are again dominant reef builders along with high mg calcitic coralline algae major sediment producing algae exhibit temporal patterns similar to those of reef builders calcitic receptaculitids flourished during calcite i whereas aragonitic dasycladaceans did not become dominant rock formers until aragonite ii during calcite ii calcitic nannoplankton formed massive coccolith chalks in warm shallow seas of the late cretaceous after the mg ca ratio of seawater had reached a very low value and calcium concentration a very high value as the mg ca ratio of seawater rose and calcium concentration fell during the cenozoic era individual coccoliths on average became less massive and encrusted cells less thickly by pliocene time during aragonite iii the prominent genus discoaster secreted only narrow rayed coccoliths that covered less than 25 of the cell surface also during aragonite iii the aragonitic green alga halimeda emerged as the dominant skeletal sediment producer in reef tracts the influence of seawater chemistry on skeletal secretion appears to have been especially strong for morphologically simple taxa that exert relatively weak control over their own calcification such groups include algae sponges corals and bryozoans morphological simplicity also permits these groups to adopt vegetative or colonial modes of growth that confer success in competition for space on reefs this linkage in addition to the basic chemical demands of hypercalcification has given the mg ca ratio of seawater strong control over the success of individual reef building taxa more generally this ratio appears to have strongly influenced evolutionary changes in the skeletal mineralogy of sponges and cheilostome bryozoans throughout their history we conclude that throughout phanerozoic time a chain of causation has extended from mid ocean ridge processes via seawater chemistry to the mineralogical and biological composition of reef communities and bioclastic carbonate deposits

climate driven vertical acceleration of icelandic crust measured by continuous gps geodesy earth s present day response to enhanced glacial melting resulting from climate change can be measured using global positioning system gps technology we present data from 62 continuously operating gps instruments in iceland statistically significant upward velocity and accelerations are recorded at 27 gps stations predominantly located in the central highlands region of iceland where present day thinning of the iceland ice caps results in velocities of more than 30 mm yr and uplift accelerations of 1 2 mm yr2 we use our acceleration estimates to back calculate to a time of zero velocity which coincides with the initiation of ice loss in iceland from ice mass balance calculations and arctic warming trends we show through a simple inversion a direct relationship between ice mass balance measurements and vertical position and show that accelerated unloading is required to reproduce uplift observations for a simple elastic layer over viscoelastic half space model

carbonate minerals oxygen and carbon isotopes in modern temperate bryozoa eastern tasmania australia abstract x ray analysis of cool temperate bryozoa from eastern shelf carbonates indicates the occurrence of a spectrum of low mg calcite to high mg calcite with variable amounts of aragonite bryozoa contain variable amounts of caco3 cements in bryozoa 18o values increase and 13c values decrease with increasing aragonite content probably due to the occurrence of vaterite aragonite contents decrease with decreasing water temperatures as established by oxygen isotope thermometry tasmanian bryozoa may be enriched in 18o up to 0 7 due to aragonite and high mg calcite contents 13c values of bryozoa composed of pure calcite are similar to those of associated brachiopods 18o and 13c values of bryozoa are unaffected by kinetic and metabolic effects the isotopic field of temperate bryozoa differs from tropical carbonates by having heavier 18o 1 3 0 5 and lighter 13c 1 8 0 8 values than bulk tropical carbonates the bryozoa are in equilibrium with upwelling deep seawater and surface subantarctic water and therefore intersect seafloor diagenesis and upwelling water trend lines mol mgco3 content ranges from 2 to 11 in calcite and corresponds to 5 14 c water temperatures the 18o values of bryozoa give a range of temperatures from 7 to 13 c slightly less than the 4 15 c obtained from 18o of tasmanian brachiopods this discrepancy is probably due to either minor biochemical fractionation or to bryozoa being in equilibrium with warmer waters

the underside topography of sea ice over the continental shelf of the beaufort sea in the winter of 1990 a spatial section of draft across 348 km of sea ice was obtained by moored subsea sonars in the beaufort sea in the winter of 1990 the ice field was a mixture of first year and multiyear types with mean draft of the order of 3 m level ice of 0 to 4 m draft occupied approximately 50 of the section within this fraction draft values of 0 and 1 5 m were most common while at 2 to 3 m draft a noticeable contribution was made by rafted first year ice deformed ice was observed to a maximum draft of nearly 27 m keels within the section were identified using both the rayleigh criterion and a new criterion based on level ice both criteria defined exponentially distributed populations but with different e folding scales the observations revealed an ice cover whose statistical properties are consistent with other data from the periphery of the polar pack in the beaufort sea but which are much more severe than those of ice found in the seasonal ice zone only a few tens of kilometres to the south the cross sectional shape of individual keels was highly variable but on average the distribution of ice within a keel was an exponential function of draft the topography of both first year and old ice subsections of the data was fractal in character with a dimension of about 1 5 for both ice types the average mass of sea ice per unit area was estimated as 2540 2970 kg m 2 with 68 73 of this mass contributed by deformed ice the large uncertainties in these estimates arise because the porosity of keels and the burden of snow or melt water upon the ice are not known

physical properties of summer sea ice in the fram strait the physical properties of sea ice in the fram strait region of the greenland sea were examined during june and july 1984 in conjunction with the marginal ice zone experiment field program most of the ice sampled within fram strait during this period was multiyear thicknesses and other properties indicated that none of the multiyear ice was older than 4 to 5 years snow cover on the multiyear ice averaged 29 cm while that on first year ice averaged only 8 cm deep this difference may be related to enhanced sublimation of the snow on the thinner first year ice the salinity profiles of first year ice clearly show the effects of ongoing brine drainage in that profiles from cores drilled later in the experiment are substantially less saline than earlier cores thin section examinations of crystal structure indicate that about 75 of the ice consisted of congelation ice with typically columnar type crystal structure the remaining 25 consisted of granular ice with only a few occurrences of snow ice the granular ice consisted primarily of frazil found in small amounts at the top of floes but mainly observed in multiyear ridges the horizontally oriented crystal c axes showed various degrees of alignment ranging from no alignment to strong alignments in which the alignment direction changed with depth implying a change in floe orientation with respect to the ocean current at the ice water interface during ice growth evidence of crystal retexturing was observed in the upper meter of nearly every multiyear core this retexturing consisting of grain boundary smoothing and nearly complete obliteration of the ice platelet brine layer substructure is attributed to summer warming

on the interaction between small and large scale convection and postglacial rebound flow in a power law mantle abstract some consequences arising from the superposition of flows of two different kinds or scales in a non newtonian mantle are discussed and applied to the cases mantle convection plus postglacial rebound flow as well as small plus large scale mantle convection if the two flow types have similar magnitude the apparent rheology of both flows becomes anisotropic and the apparent viscosity for one flow depends on the geometry of the other if one flow has a magnitude significantly larger than the other the apparent viscosity for the weak flow is linear but develops direction dependent variations about a factorn n being the power exponent of the rheology for the rebound flow lateral variations of the apparent viscosity about at least 3 are predicted and changes in the flow geometry and relaxation time are possible on the other hand rebound flow may weaken the apparent viscosity for convection secondary convection under moving plates may be influenced by the apparent anisotropic rheology other mechanisms leading to viscous anisotropy during shearing may increase this effect a linear stability analysis for the onset of convection with anisotropic linear rheology shows that the critical rayleigh number decreases and the aspect ratio of the movement cells increases for decreasing horizontal shear viscosity normal viscosity held constant applied to the mantle this model weakens the preference of convection rolls along the direction of plate motion under slowly moving plates rolls perpendicular to the plate motion seem to have a slight preference these results could be useful for resolving the question of newtonian versus non newtonian or isotropic versus anisotropic mantle rheology

carbonate minerals major and minor elements and oxygen and carbon isotopes and their variation with water depth in cool temperate carbonates western tasmania australia cool temperature bryomol carbonates predominate on shallow shelves around western tasmania many samples are bored and encrusted due to the occurrence of submarine hardgrounds and to slow rates of sedimentation intragranular caco3 cementation is common tasmanian whole carbonates are mainly mixtures of high mg calcite hmc and low mg calcite lmc with minor amounts of aragonite hmc and aragonite contents decrease while lmc content increases with increasing water due to a decrease in water temperature mg concentrations in calcite correspond to temperatures of 10 c sr and na concentrations in bryozoa decrease due to calcite cementation whereas their concentrations increase with aragonite content mn and fe concentrations are higher than in warm shallow marine counterparts and increase with increasing aragonite percentage sr and na concentrations in calcite slightly increase with increasing lmc percentage due to decreasing water temperatures mn and fe values in calcite are independent of water temperature hmc percentage and hmc lmc ratios decrease while lmc percentage increases with increasingly heavier 18o values due to decreasing temperatures lmc is a marine mineral and not a product of meteoric diagenesis unusually high concentrations of sr na mn and fe in aragonite are due either to the tendency of aragonite formed in cold seas to incorporate more of these elements or to aragonite formation by transformation of metastable cold water caco3 minerals such as vaterite or hydrous calcite sr contents decrease with increasing water depth due to a decrease in aragonite and increase in low mg calcite na values increase with increasing water depth due to increasing salinity mn and fe concentrations decrease with increasing water depth due to decreasing continental terrigenous input depth related variations in mg sr na mn and fe are due to marine diagenesis and are unrelated to meteoric diagenesis 18o and 13c values are positively correlated and this isotopic field is bisected by the diagenesis trend line for upwelling water that passes through the point 1 18o 0 13c the 18o values become heavier and mg values decrease with increasing water depth due to decreasing temperatures from about 17 to 6 c for normal seawater and from 13 to 2 c for upwelling water these temperatures are lower than measured summer values because of the strong seasonal effect of surface waters influx of subantarctic water in winter and mixing of cold upwelling waters in the bottom water as carbonates in upwelling regions are in equilibrium with upwelling water having 18o values of 1 in paleotemperature determinations seawater values that are lower by 1 than those postulated for different geological periods should be considered for those carbonates suspected of being non tropical in origin on the basis of geological and geochemical evidence originally calcitic limestonesare abundant in ancient sequences and their temperatures of formation and diagenesis can be better understood by using the data base provided by this study

secular changes in colony forms and bryozoan carbonate sediments through geological history ever since their first radiation in the ordovician bryozoans have contributed significantly to carbonate sedimentation most of the numerous colony forms developed by bryozoans have evolved repeatedly in different taxonomic groups and vary in their sediment producing potential there are nine basic bryozoan colony forms encrusting dome shaped palmate foliose fenestrate robust branching delicate branching articulated and free living the proportion of these morphotypes in bryozoan faunas period by period is shown to change significantly through the phanerozoic notable patterns include i steady increase in the number and proportion of encrusting species through time interrupted by a transient drop in the late palaeozoic ii post triassic decrease in robust branching colonies iii rise in the proportion of fenestrate colonies through the palaeozoic followed by their absence in the triassic and jurassic rarity in the cretaceous and reappearance in smaller proportions in the cenozoic and iv scarcity of articulated colonies and absence of free living colonies until the cretaceous most palaeozoic bryozoan sediments come from two architecturally distinct groups of colonies i domal delicate branching robust branching and palmate and ii fenestrate the former generate coarse particles both as sediment and components of stromatoporoid coral reefs in the early and mid palaeozoic whereas the delicate lacy fans of the latter create both prolific coarse sediment and form the cores of late palaeozoic deep water sub photic biogenic mounds nearly all post palaeozoic bryozoan sediments comprise cyclostomes and cheilostomes with many of the same growth forms but with the addition of free living colonies and significant numbers of articulated colonies the latter produced sand and mud sized bryozoan sediment via disarticulation for the first time in contrast to the palaeozoic post palaeozoic bryozoans generated sediment varying more widely across the grain size spectrum from mud to sand to gravel this article highlights the need to consider evolutionary changes in carbonate producing organisms when interpreting facies changes through time

post glacial rebound and asthenosphere viscosity in iceland during the weichselian glaciation iceland was covered with an ice cap which caused downward flexure of the earth s surface the post glacial rebound in iceland was very rapid being completed in about 1 000 years the length of this time interval constrains the maximum value of asthenosphere viscosity in iceland to be 1 times 10 sup 19 pa s or less further clarification of the ice retreat and uplift history may reveal lower viscosity current changes in the mass balance of icelandic glaciers must lead to measurable elevation changes considering this low viscosity expected current elevation changes around the vatnajoekull ice cap are of the order of 1 cm per year due to mass balance change in this century

glacio isostatic deformation around the vatnajokull ice cap iceland induced by recent climate warming gps observations and finite element modeling 1 glaciers in iceland began retreating around 1890 and since then the vatnajokull ice cap has lost over 400 km 3 of ice the associated unloading of the crust induces a glacio isostatic respo

surface wave tomography of the upper mantle beneath the reykjanes ridge with implications for ridge hot spot interaction 1 we modeled fundamental mode love and rayleigh waves to study the seismic properties of the upper mantle beneath the reykjanes ridge these waves were generated by regional earthquakes occurring in the north atlantic to the south of iceland and were recorded by stations located on iceland over 12 000 measurements of the phase group arrival time and amplitude of narrow pass filtered waveforms over the period range of 9 5 100 s were used to solve for mantle shear wave velocity structure and anisotropy in a vertical plane oriented normal to the ridge axis the velocity structure contains a broad and deep low velocity zone in the upper mantle beneath the ridge a joint analysis of the seismic structure with gravity data reveals that the low velocities are consistent with elevated temperatures 75 our study shows that plume material spreads broadly outward beneath the reykjanes ridge from iceland and is not confined to a narrow lithospheric channel at distances 200 km from the ridge shear wave anisotropy indicates a predominant horizontal alignment of the fast axes of anisotropic crystals mainly above 50 km depth which can be interpreted as past horizontal ridge perpendicular flow within 200 km of the ridge the anisotropy indicates a general vertical alignment of the fast axes or an alignment such that the fast axes point along the ridge the transition to this type of anisotropy coincides with the appearance of increased hot spot ridge interaction 20 myr ago indicating that plume flow has largely disrupted mantle flow beneath the ridge since that time

skeletal mineralogy of bryozoans taxonomic and temporal patterns skeletal carbonate mineralogy of 1183 specimens of marine bryozoans from the literature was examined for phylogenetic patterns in order to elucidate the effects of bryozoan mineralogy on geochemical and paleoenvironmental analysis colonies are composed of calcite 66 of specimens aragonite 17 of specimens or various mixtures of the two 17 specimens phylum mean 72 9 wt calcite n 1051 when calcite is present it ranges from 0 0 to 13 7 wt mgco3 mean 5 0 wt mgco3 n 873 most 61 calcitic specimens are formed of intermediate mg calcite 4 to 8 wt mgco3 others 28 of low mg calcite 0 to 4 wt mgco3 and few of high mg calcite 8 wt mgco3 the phylum occupies at least 63 of the theoretical mineralogical space available to biomineralisation most of this variation occurs in the class gymnolaemata order cheilostomata suborder neocheilostomata fossil and recent stenolaemate taxa are generally low to intermediate mg calcite mean 99 7 wt calcite 2 6 wt mgco3 17 of available biomineral space variability among families is related in a general way to first appearance datum families younger than 100 ma display greater mineralogical complexity than older ones the cheilostome infraorder flustrina includes unusual free living aragonitic families dual calcite skeletons mainly low mg calcite but with secondary high mg calcite and some genera with considerable mineralogical variability families e g membraniporidae and phidoloporidae and species e g schizoporella unicornis with the highest degree of variability have potential for environmental correlations with mineralogy paleoenvironmental interpretation and possibly molecular investigation for potential cryptic species stenolaemate families genera and species with low variability on the other hand are well suited for geochemical work such as stable isotope analysis variability in the skeletal mineralogy of bryozoans suggests that they may be useful in geochemical phylogenetic and paleoenvironmental studies with careful choice of study material

postrifting anelastic deformation around the spreading plate boundary north iceland 1 modeling of the 1987 1992 deformation field using a viscoelastic earth structure a third global positioning system gps survey of a regional network surrounding the krafla volcanic system north iceland was conducted in 1992 following a major crustal spreading episode which began in this system in 1975 differencing the 1992 results with those from 1987 and 1990 reveals a regional deformation field with a maximum rift normal expansion rate of 4 5 cm year near the rift decreasing to 3 cm year at large distances the time averaged spreading rate in north iceland 1 8 cm year cannot account for this deformation the vertical deformation field reveals regional uplift throughout the network area at its maximum closest to the rift and decreasing with distance three different models are applied to study the postdike injection ground deformation 1 stress redistribution in an elastic layer over a viscoelastic half space 2 stress redistribution in an elastic viscous layered medium and 3 continued opening at depth on the dike plane in an elastic half space using model 1 the effects of historical episodes in the region are subtracted from the observed displacement fields and the remaining motion is modeled as relaxation following the recent krafla rifting episode the best fit model involves a half space viscosity of 1 1 x 10 18 pa s a relaxation time of 1 7 years and an elastic layer thickness for northeast iceland of 10 km the vertical field indicates that the krafla dike complex rifted the entire elastic layer using model 2 the motion 1987 1990 and 1990 1992 can be simulated adequately given the survey errors but the 1987 1992 deformation is poorly fitted suggesting that a more realistic geophysical model is required using model 3 a range of dikes will fit the deformation field