paleomagnetic evidence for dextral strike slip motion in the pyrenees during alpine convergence mauleon basin france abstract the paleomagnetic study of the aptian albian black marls from the inverted early cretaceous mauleon basin in the northern pyrenees reveals two components of magnetization that both yield negative fold tests the carriers of each component show different unblocking temperatures indicating that ferrimagnetic iron sulphides are the main carriers of the intermediate temperature component itc unblocks around 360 400 c and magnetite is the main carrier of the high temperature component htc unblocks around 460 500 c both components show a clockwise deviation in declination with respect to the reference field directions itc di and htc di both in in situ coordinates the reference direction for stable europe for the cenozoic is d i 001 50 d 5 i 3 however the reversal test for both of the magnetization components in situ is positive the structural evolution of the basin involves transtensional deformation during the main stage of basin formation and inversion during compression with migration of hydrocarbons and development of thrusts and folds with axial planar cleavage secondary magnetizations were acquired at the latest stages of the evolution of the inverted basin post folding as the result of a chemical mechanism and they were blocked during two different and likely close spaced in time polarity chrons the blocking of the two different magnetizations with different carriers implies chemical changes in the environment from anoxic sulphate reduction to oxic inferred clockwise rotation of the paleomagnetic vectors can be interpreted in terms of dextral shear across the north pyrenean zone during the tertiary probably eocene according to the plate tectonic scenario this distributed dextral shear was probably favored by discrete shear along cleavage planes as shown by geologic markers of consistent dextral strike slip motion in non competent units with strong cleavage development there is no evidence of such dextral movement in the south pyrenean zone thus indicating a strong partitioning of deformation during the pyrenean orogeny

the mass balance of circum arctic glaciers and recent climate change the sum of winter accumulation and summer losses of mass from glaciers and ice sheets net surface mass balance varies with changing climate in the arctic glaciers and ice caps excluding the greenland ice sheet cover about 275 000 km2of both the widely glacierized archipelagos of the canadian norwegian and russian high arctic and the area north of about 60 n in alaska iceland and scandinavia since the 1940s surface mass balance time series of varying length have been acquired from more than 40 arctic ice caps and glaciers most arctic glaciers have experienced predominantly negative net surface mass balance over the past few decades there is no uniform recent trend in mass balance for the entire arctic although some regional trends occur examples are the increasingly negative mass balances for northern alaska due to higher summer temperatures and increasingly positive mass balances for maritime scandinavia and iceland due to increased winter precipitation the negative mass balance of most arctic glaciers may be a response to a step like warming in the early twentieth century at the termination of the cold little ice age arctic ice masses outside greenland are at present contributing about 0 13 mm yr 1to global sea level rise

magnetic polarity stratigraphy of the tertiary igneous rocks of skye scotland summary palaeomagnetic results are given for 115 sites mainly from the central igneous intrusive complex and also radiometric dates by the 40ar 39ar step heating method these are combined with published dates and palaeomagnetic results for the lavas and regional dyke swarms with the aim of establishing the magnetic polarity stratigraphy igneous activity spanned at least the approximate interval 61 54 ma and a polarity sequence of at last r n r n r but could span seven polarity intervals r n r n r n r depending on the age of the loch ainort epigranite however these polarities cannot be identified with specific chrons of either the harland et al 1982 or berggren et al 1985 polarity time scales the skye igneous activity therefore spans more polarity intervals than in mull ardnamurchan or rhum and this is consistent with the radiometric ages interpretations of the results for the coire uaigneich and related granites and for the loch ainort epigranite present problems and partial solutions are offered

wetting front advance and freezing of meltwater within a snow cover 1 observations in the canadian arctic in a naturally stratified snow cover the movement of meltwater into dry snow is complicated by the interaction of the wetting front with stratigraphic horizons field observations showed that when the wetting front reached premelt stratigraphic horizons water ponded at the interface and then flow fingers developed and penetrated the lower stratum the flux in these fingers which was increased to about twice that of the surface flux was used to feed water to the impeding horizons where it froze to form ice layers these ice layers were the major source of latent heat released within the snow cover and they were responsible for the warming of the snow and the underlying soil these continuous ice layers grew only at stratigraphic boundaries because of this ice layer growth the wetting front advance was retarded and the arrival of meltwater at the snow cover base was significantly delayed owing to a cold substrate the strong heat flux from the snow into the soil delays the warming of the snow cover and limits runoff after the snow is isothermal at 0 c by the refreezing of soil infiltration and the development of a basal ice layer

palaeomagnetic study of the late mesoproterozoic torridon group nw scotland age magnetostratigraphy tectonic setting and partial magnetic overprinting by caledonian orogeny abstract the late mesoproterozoic torridonian supergroup is a predominantly fluviatile 7 km thick succession of red sediments deposited onto palaeoproterozoic lewisian metamorphic terrane of the hebridean craton in nw scotland although a subject of investigation since the earliest days of palaeomagnetism the age and constitution of the remanent magnetisation in the upper torridonian torridon group have remained unclear in this study we undertake detailed statigraphically controlled sampling through the succession and conduct progressive thermal demagnetisation to resolve component structures rock magnetic analysis shows that magnetite although volumetrically small is a significant component of the ferromagnetism in these sediments in addition to specularite and pigmentary hematite unlike the lower torridonian stoer group which was fully lithified prior to late grenville deformation 1100 ma the torridon group was not influenced by tectonism until early palaeozoic times and records a complex history of partial and sometimes total later overprinting the a remanence is extensively overprinted by a b remanence with e to s directed intermediate to steep positive directions and subordinate reversed representatives and less commonly by a c remanence with ne direction b is resolved mainly in the magnetite spectrum and incorporates a swathe of directions coincident with mid ordovician palaeofields predicted from baltica and adjoining caledonian terranes it is therefore interpreted as a remanence residing in small amounts of authigenic magnetite acquired during burial of this margin of the hebridean craton by nappe stacking during the grampian orogeny 470 430 ma following final closure of the iapetus ocean due to the merging of the b population with the a direction a has not been defined as an antiparallel axis by former studies and can be inseparable by cleaning however defining a to include components with i d i 115 31 95 2 9 for a and d i 293 25 95 3 5 for a the poles at 238 e 1 s dp dm 2 0 3 8 and 235 e 1 n dp dm 1 8 3 3 are close to the laurentian apwp at 1039 1025 ma on the pre drift reconstruction and constrain sedimentation within the 1060 to 994 977 ma range of previous evidence only minor polar movement is detected through the entire thickness of the torridon group and suggests a sedimentation rate of the order of 0 5 m 1000 years for this clastic succession a magnetic stratigraphy in a suggested by early work is confirmed in at least 13 polarity zones and identifies a rapid reversal rate preserved in specular hematite fixed by post depositional mechanisms postdating slumping and sand volcano formation in wet sediment the palaeomagnetic age of the a remanence links torridonian sedimentation to rifting and subsidence accompanying break up of the laurentian and fennoscandian shields at 1050 950 ma following the grenville orogeny rifting between the two shields was followed by rotation and rewelding into a neoproterozoic configuration these events correlate with transition in torridon group palaeoenvironments from enclosed basin to open basin with river systems feeding onto a passive margin to marine transgression and finally return to enclosed basin conditions diagenesis continued in post palaeozoic times and a d remanence with n s shallow axis is sporadically preserved in authigenic hematite and interpreted as mainly a variscan acquisition

from surge type to non surge type glacier behaviour midre lovenbreen svalbard since the beginning of the 1900s the polythermal valley glacier midre lovenbreen area 5 km 2 in northwestern svalbard has retreated approximately 1km the glacier has not surged for at least 110 years and the present smooth longitudinal profile and negative mass balance suggest that it is not currently building up to a surge based on limited evidence it has often been assumed that midre lovenbreen surged around 1880 the ongoing retreat is probably due to quiescent phase stagnation amplified by 20th century climate change as is the case for several other glaciers in svalbard recently small eskers have been found in the glacier forefield based on their geomorphological appearance these are interpreted as concertina eskers and they add to the dynamic history of the glacier midre lovenbreen is interpreted as an example of a change from surge type to non surge type glacier behaviour

the north atlantic igneous province reconstructed and its relation to the plume generation zone the antrim lava group revisited summary large igneous provinces lips have recently been suggested to originate at the edges of low velocity zones on the core mantle boundary plume generation zones if true lips can potentially be used to constrain paleolongitude in plate tectonic reconstructions to validate the hypothesis it is essential to study lips of which the paleolongitude can be constrained by other methods such as hotspot reference frames an ideal candidate to this end is the early cenozoic north atlantic igneous province naip despite being the largest volcanic unit of the british tertiary igneous province btip part of the naip the age and paleoposition of the antrim lava group alg in northern ireland which is key to the naip as a whole was hitherto poorly constrained in this paper we therefore present an integrated high resolution paleomagnetic and geochronological study the alg is divided into three formations the lower basalt formation lbf interbasaltic formation ibf and the upper basalt formation ubf the ibf is mostly lateritic and encloses the tardree rhyolite we offer new age constraints from all three formations using the 40 ar 39 ar method and propose that 62 6 0 3 61 3 0 3 and 59 6 0 3 ma 1 internal uncertainties are sound estimates of the age of emplacement of the lbf tardree rhyolite ibf and ubf respectively this constrains the nominal duration of emplacement of the alg to 3 0 6 ma 1 this reevaluation of the magnetic signature in the alg revealed reverse polarity remanence in all three formations and an overall paleomagnetic north pole at latitude 78 9 n longitude 167 e a95 6 3 age 61 ma in the european reference system this appears consistent with paleomagnetic poles from the rest of the naip both in europe and greenland as well as predictions from modern apparent polar wander paths the new radiometric ages span magnetochron c26r c27n and c27r the normal polarity chron c27n most probably occurred during the ibf hiatus explaining why no normal polarity remanence was detected in the paleomagnetic investigation emplacement of the lbf falls in magnetochron c27r making this one of the oldest lava sequence in the naip older than the c27n lava pile in western greenland the 60 ma position of the naip in a paleomagnetic reference frame puts it close to the northern edge of the african large low shear wave velocity anomaly at the core mantle boundary and therefore in the line with the plume generation zone hypothesis however the back projected icelandic hotspot normally considered to have formed the naip is located 1500 km north of the latitude at which the naip erupted the northward motion of the north atlantic lithosphere since the late cretaceous challenging the existing correlation of the naip to the icelandic hotspot normally used to explain the observed pre and syn breakup north atlantic magmatism 63 55 ma and either an additional plume located further south in the north atlantic may be invoked to create the naip or the icelandic hotspot must have

modelling the impact of superimposed ice on the mass balance of an arctic glacier under scenarios of future climate change a surface energy mass balance model with an explicit calculation of meltwater refreezing and superimposed ice formation is applied to midre lovenbreen spitsbergen svalbard the model is run with meteorological measurements to represent the present climate and run with scenarios taken from global climate model predictions based on the is92a emissions scenario to represent future climates model results indicate that superimposed ice accounts for on average 37 of the total net accumulation under present conditions the model is found to be highly sensitive to changes in the mean annual air temperature and much less sensitive to changes in the total annual precipitation a 0 58c decade 1 temperature increase is predicted to cause an average mass balance change of 0 43 m a 1 while a 2 decade 1 increase in precipitation will result in only a 0 02 m a 1 change in mass balance an increase in temperature results in a significant decrease in the size of the accumulation area at midre lovenbreen and hence a similar decrease in the net volume of superimposed ice the model predicts however that the relative importance of superimposed ice will increase to account for 50 of the total accumulation by 2050 the results show that the refreezing of meltwater and in particular the formation of superimposed ice make an important positive contribution to the mass balance of midre lovenbreen under present conditions and will play a vital future role in slowing down the response of glacier mass balance to climate change

retention of greenland runoff by refreezing implications for projected future sea level change melting produced at the surface of subfreezing permeable firn does not freely percolate but instead refreezes near its point of origin raising the firn temperature and decreasing the pore space if sufficient meltwater is introduced the firn will warm to 0 c and subsequent water will fill the remaining pore space without freezing only after the residual water content of the firn is exceeded will water flow through the firn and possibly escape as runoff this process prevents summer melt on high arctic glaciers and ice caps from escaping in its entirety and it must be accounted for in modeling glacier runoff on the basis of surface energy balance a model is presented here which describes in a simple way the transient process of infiltration refreezing and runoff in a future warming climate the model is applied to greenland for which predictions of runoff induced sea level rise that do not consider the refreezing process could be as much as 5 0 cm too high over 150 years

reassembling the paleogene eocene north atlantic igneous province new paleomagnetic constraints from the isle of mull scotland abstract the paleomagnetic data sets from the british tertiary igneous province btip have recently been criticized as being unreliable and discordant with data from elsewhere in the north atlantic igneous province naip riisager et al earth planet sci lett 201 2002 261 276 riisager et al earth planet sci lett 214 2003 409 425 we offer new paleomagnetic data for the extensive lava flow sequence on the isle of mull scotland and can confirm the paleomagnetic pole positions emanating from important earlier studies our new north paleomagnetic pole position for eurasia at 59 0 2 ma has latitude 73 3 n longitude 166 2 e dp dm 5 2 7 0 a re evaluation and an inter comparison of the paleomagnetic database emanating from the naip were carried out to test for sub province consistency we find a general agreement between the eurasian part of naip btip and faeroes and east greenland data however a compilation of west greenland data displays a large and unexplained dispersion we speculate on if this is related to different sense of block rotation of the tertiary west greenland constituents combining all data from the naip constituents give a pole position at 75 0 n 169 9 e n 25 k 84 3 a95 3 2 in eurasian reference frame

palaeomagnetism and radiometric dating of the british tertiary igneous province muck and eigg summary the lavas dykes and other intrusive bodies of muck and eigg have been sampled for palaeomagnetic purposes all the bodies are reversely magnetized except for four dykes of muck which are normally magnetized the lavas show variations of direction of magnetism believed to be a record of secular variation the radiometric dates support the geological evidence that the lavas of eigg and muck predate parts of the rhum central igneous complex the lavas of rhum and canna and some of those of skye the combined evidence shows that most of the activity took place during an r n r n sequence between 63 and 58 ma the younger r n sequence is the same as the first part of the r n r sequence found in mull but the sgurr pitchstone of eigg and parts of the eastern redhills of skye belong to later reversed intervals and correspond to chron 26r and chron 26 the igneous activity occurred over a 10 myr period from about 63 ma and can probably be associated with chrons 27r to 23r of the harland et al time scale

the british tertiary igneous province palaeomagnetism of the dyke swarm along the sleat coast of skye summary a palaeomagnetic investigation has been made of a swarm of more than 400 dykes along the south coast of skye scotland by the sound of sleat seven red lamprophyre dykes have palaeomagnetic directions inconsistent with tertiary age and not inconsistent with their previously held caledonian age the remaining 409 dykes have palaeomagnetic directions that are consistent with a lower tertiary age we present evidence suggesting that the tertiary dykes might have been emplaced during a short time over which the geomagnetic polarity occupied as few as three polarity intervals nrn or rnr certain intermediate directions of magnetization have also been found and are presented here

multiphase formation of superimposed ice during a mass balance year at a maritime high arctic glacier meteorological and snow temperature data from midtre lovenbreen spitsbergen svalbard indicate two distinct annual phases of rapid snowpack warming and superimposed ice formation in 1998 99 short periods of positive air temperatures in early winter lasting up to 36 hours and often coinciding with rainfall caused rapid glacier wide melting percolating water froze to form superimposed ice on the lower half of the glacier and wetted refrozen snow and ice lenses at higher altitudes the second period of superimposed ice formation commenced in may june 1999 and continued for 5 weeks at low altitudes and throughout the summer at high altitudes these observations at midtre lovenbreen are typical of spitsbergen glaciers and reflect the unique climatology of the region they contrast with those from glaciers in more continental climatic settings where superimposed ice formation is confined to a single period during summer there are significant implications for glacier mass balance with superimposed ice locally comprising up to 20 of winter balances and accounting for 16 25 of the annual accumulation since projected climatic warming is greatest during the winter months in arctic regions superimposed ice may become an increasingly important component of the winter and potentially the net balance of spitsbergen glaciers

determination of timing and location of water movement and ice layer formation by temperature measurements in sub freezing snow measurements of temperature in snow along a vertical profile during the onset of spring melting are used to calculate spatial and temporal temperature gradients and terms of the conduction equation with an internal energy production term are calculated heat transfer information is combined with stratigraphic observations made during melting and allow detailed determination of the timing and location of heterogeneous water movement and of refreezing internal energy production is interpreted as latent heat of refreezing of percolated meltwater times and locations of flow and refreezing of meltwater are calculated and compared to stratigraphic observations of layering and changes in density and liquid water content sequences of melt piping layering and refreezing seen in calculations and in stratigraphy demonstrate retarded flow at fine to coarse transitions flow along such transitions and refreezing at the transitions to form ice layers downslope flow is also observed in the absence of an impermeable horizon to redirect flow from the vertical

palaeomagnetic and anisotropy of magnetic susceptibility data bearing on the emplacement of the western granite isle of rum nw scotland the western granite is the largest of several granitic bodies around the margin of the rum central igneous complex we report palaeomagnetic and anisotropy of magnetic susceptibility ams data that bear on the emplacement and deformation of the western granite the collection includes samples from 27 sites throughout the western granite five sites in adjacent feldspathic peridotite and two sites in intermediate to mafic hybrid contact aureole rocks palaeomagnetic data from 22 of the 27 sites in the granite provide an in situ group mean d 213 2 i 69 5 95 5 5 that is discordant to an early paleocene reverse polarity expected field about 184 66 95 4 3 the discrepancy is eliminated by removing an inferred 15 of northwest side down tilting about a best fit horizontal tilt axis trending 040 data from the younger peridotite and hybrid rocks of the rum layered suite provide an in situ group mean of d 182 6 i 64 8 95 4 0 which is statistically indistinguishable from an early paleocene expected field and imply no post emplacement tilting of these rocks since remanence acquisition the inferred tilt recorded in the western granite which did not affect the younger layered suite suggests that emplacement of the ultrabasic rocks resulted in roof uplift and associated tilt of the western granite to make space for mafic magma emplacement magnetic fabric magnitude and susceptibility parameters yield two subtle groupings in the western granite ams data set group 1 data defined by rocks from exposures to the east and south have comparatively high bulk susceptibilities kmean 29 51 10 3 in si system stronger anisotropies pj 1 031 and oblate susceptibility ellipsoids group 2 data from rocks in the west part of the pluton have lower values of kmean 15 89 10 3 si and pj 1 014 and triaxial susceptibility ellipsoids magnetic lineations argue for emplacement of the granite as a tabular sheet from the south southeast toward the north and west moderate to steeply outward dipping magnetic foliations together with deflection of the country rock bedding in the north are consistent with doming accompanying magma emplacement

quantitative assessment for the rockfall hazard in a post earthquake high rock slope using terrestrial laser scanning abstract geometric information and discontinuity characterization of rock masses are the key aspects of the analysis of the evolution and failure mechanisms of rockfalls with the advantageous use of terrestrial laser scanning tls the accurate three dimensional 3d spatial information of rock slopes can be obtained without contact by conducting a fuzzy k means algorithm in this study the automatic identification of discontinuity sets is achieved and dominant occurrences of rock mass discontinuities in each local region can be acquired in great detail this automatic identification method permits the user to visually identify the discontinuity sets and acquire their spacial distribution features e g occurrences spacings trace lengths and their geometric compounding relationships at the same time based on the shortest distance sd algorithm and the surface to plane volume calculation algorithm the distributions shapes volumes and scars of the rockfalls can be accurately detected over the monitored time interval these methods are able to provide adequate investigations and quantitative assessment for the rockfall failure mechanisms and evolutions of the hongshiyan post failure rock slope after the 2014 ludian earthquake the topography of the landslide surface mechanical properties scales and the structural features of the discontinuities have a significant effect on the failure mechanisms distributions and volumes of rockfalls the main failure mechanisms of the rockfall investigated in this rock slope can be divided into plane failure wedge failure and toppling failure the plane and wedge failures mainly occurred in the landslide surface while the toppling failures are mainly observed along the edge of the post failure slope which is obviously responded by preferred discontinuities the reverse dip stratified structure characteristics and the excavation disturbances during the recovering and slope treatment stage induce numerous progressive and continuous failures of rockfalls the results are beneficial for the design and optimization of rockfall treatments

climate sensitivity of glaciers in southern norway application of an energy balance model to nigardsbreen hellstugubreen and alfotbreen three glaciers in southern norway with very different massbalance characteristics are studied with an energy balance model of the ice snow surface the model simulates the observed mass balance profiles in a satisfactory way and can thus be used with some confidence in a study of climate sensitivity calculated changes in equilibrium line altitude for a 1 k temperature increase are 110 108 and 135 in for nigardsbreen hellstugubreen and alfotbreen respectively the corresponding changes in mass balance averaged over the entire glacier area are 0 88 0 715 and 1 11 in year water equivalent runs with an ice flow model for nigardsbreen to which calculated mass balance profiles are imposed predict that the front will advance by 3 km for a 1 k cooling and will retreat by as much as 6 5 km for a 1 k warming the response to a 10 increase in precipitation would be a 2 km advance of the snout whereas a 4 km retreat is predicted for a 10 decrease this large sensitivity as compared to many other glaciers is to a large extent due to the geometry of nigardsbreen

modeling the refreezing of meltwater as superimposed ice on a high arctic glacier a comparison of approaches 1 the refreezing of meltwater is known to be a critical factor when assessing the mass balance state of glaciers in the high arctic several different models of this process are widely used in numerical glacier simulations but as yet no attempt has been made to compare these approaches or to assess their relative performance against field measurements such models are likely to include implicit calibrations which may prove to be invalid under conditions different from those for which they were constructed we test a selection of six refreezing algorithms taken from the literature against field measurements of net mass balance and superimposed ice formation made at a high arctic glacier running these models for 1971 2002 using daily meteorological data and forward until 2050 using a range of future scenarios for the climate of svalbard we are able to make the following conclusions concerning mass balance modeling 1 the choice of meltwater refreezing model has a significant impact on the calculated net mass balance of a glacier 2 three of the models tested could with suitable tuning simultaneously reproduce both superimposed ice quantities and net mass balance measurements within their respective bounds of uncertainty while the other three could not 3 in situations where the model can be tuned to field measurements indicative of mass balance future predictions of mass balance are largely independent of the specific meltwater refreezing model used

the thermal conductivity of seasonal snow twenty seven studies on the thermal conductivity of snow k eff have been published since 1886 combined they comprise 354 values of k eff and have been used to derive over 13 regression equation and predicting k eff vs density due to large and largely undocumented differences in measurement methods and accuracy sample temperature and snow type it is not possible to know what part of the variability in this data set is the result of snow microstructure we present a new data set containing 488 measurements for which the temperature type and measurement accuracy are known a quadratic equation where is in g cm 3 and k eff is in w m 1 k 1 can be fit to the new data r 2 0 79 a logarithmic expression can also be used the first regression is better when estimating values beyond the limits of the data the second when estimating values for low density snow within the data set snow types resulting from kinetic growth show density independent behavior rounded grain and wind blown snow show strong density dependence the new data set has a higher mean value of density but a lower mean value of thermal conductivity than the old set this shift is attributed to differences in snow types and sample temperatures in the sets using both data sets we show that there are well defined limits to the geometric configurations that natural seasonal snow can take

a palaeomagnetic study of recumbently folded and thermally metamorphosed torridon group sediments eishort anticline skye scotland two geologically significant palaeomagnetic components have been isolated in samples of torridon group sandstones from the kishorn nappe of nw scotland the samples were collected from the eishort anticline a large recumbent fold of caledonian age the softer component which is demagnetized by low temperatures and weak magnetic fields when in situ displays a steep upward direction typical of reversed tertiary magnetizations both polarities of the harder component which is demagnetized by high temperatures only are present and when dip corrected display either northwesterly upward or southeasterly downward directions characteristic of torridonian remanences it is suggested that the tertiary component is of chemical origin and the result of alteration of magnetite titanomagnetite grains the geographical distribution of these grains is irregular and as a result the distribution of the tertiary overprint is irregular the torridonian palaeomagnetic components from both limbs of the fold can be restored by rotation about the fold axis to internally consistent directions which lie close to previously published data from the caledonian foreland no correction is necessary for finite strain which suggests that the eishort anticline formed by a flexural slip mechanism however an anticlockwise rotation of 26 is required to reconcile the torridonian directions from the kishorn nappe with those of the caledonian foreland which suggests that during emplacement the thrust sheet rotated on the underlying fault surface

variations of near surface firn density in the lower accumulation area of the greenland ice sheet pâkitsoq west greenland firn density variations have been studied in the lower accumulation area of the greenland ice sheet 1440 1620 m a s l near p kitsoq west greenland the main control on density in the near surface firn layer of 5 10 m thickness is the formation of ice layers by the refreezing of meltwater that reaches depths of 2 4 m below the surface the density variations are described by the ratio of annual surface melt m to the annual accumulation c the ratio m c is about 0 6 at the run off limit at about 1400 m a s l in the study area where refreezing of meltwater transforms snow into impermeable ice the mean density of near surface firn decreases with elevation reflecting a decrease in melt with elevation there is a surprising decrease in firn density at depths of more than about 4 m below the 1991 summer surface which reflects lower melt rates and or higher accumulation in the early 1980s and late 1970s when this firn was passing through the surface layer the formation of such low density firn may have partially contributed to the 1978 85 thickening of the ice sheet observed by satellite radar altimetry near surface firn density is therefore very sensitive to climate change and might be an attractive target for climate monitoring

internal accumulation in firn and its significance for the mass balance of storglaciaren sweden the discussion on global change has led to increased interest in glacier mass balance since glaciers can be used as climatic indicators to meet the need for high quality mass balance data requires critical examination of traditional mass balance methods and their possible errors one issue regarding mass balance measurements that has received little attention is internal accumulation our study shows that internal accumulation in the firn layer of storglaciaren sweden significantly affects the mass balance of the glacier this occurs because the winter cold wave penetrates below the previous year s summer surface and into underlying firn we estimated internal accumulation from measurements of temperature and water content in firn the depth of the 0 c isotherm correlated with snow depth and air temperature so that low snow depth and low air temperature separately cause a deeper 0 c isotherm we determined irreducible gravimetric water content in firn to 2 3 which corresponds to an irreducible water saturation of 6 8 our value for firn is relatively high compared with that for snow probably due to trapped water in isolated firn pores refreezing of percolating meltwater in spring accounted for 30 of annual internal accumulation the remaining 70 was due to refreezing of retained capillary water in firn pores during winter disregarding internal accumulation would lead to underestimation of annual net mass balance by 0 04 0 06 m w e corresponding to 3 5 of annual accumulation of the entire glacier in an average year hence internal accumulation potentially becomes a source for systematic error if not accounted in mass balance measurements

plate motions and the geomagnetic field ii jurassic to tertiary summary the analysis of the time averaged geomagnetic field is extended back to 200ma palaeomagnetic poles from the major plates have been carefully selected from recent compilations of reliable results for each region these were returned with their corresponding sampling sites to their locations at the estimated dates of magnetization in a fixed hotspots framework the corrected results were then grouped into 20ma windows at intervals of 10ma representing the past 100ma and 40ma windows at 30ma intervals for the more sparse 100 200ma data global means and fisher statistics were calculated for each window having included the axial quadrupole in the calculation the value of this coefficient which gave the maximum value for the fisher precision parameter tightest grouping of poles was taken as representative of each interval the results indicate that a small axial quadrupole of the same sign as the axial dipole may have persisted throughout the cenozoic this is equivalent to a northward offset axial dipole field far sided effect during the late cretaceous this component appears to have changed sign with respect to the dipole negative values seem to have obtained throughout the cretaceous long normal polarity interval corresponding to a southward offset dipole near sided effect the data distribution is inadequate for the resolution of the quadrupole at earlier times and zero values cannot be discounted little relative motion is implied between the hotspots and the geomagnetic axis for the past 90 ma the global mean polar path curving around the predicted fixed hotspots pole at a distance of typically 5 latitude with little sign of rapid tertiary polar wander as implied by studies of pacific data alone between 100 and 200 ma however there is a clear difference between the two reference frames amounting to 17 19 in the jurassic this may reflect motion of the mantle relative to the geomagnetic axis but may also include errors due to inaccurate determination of hotspot tracks and inter hotspot motion

palaeomagnetic dating of sub torridon group weathering profiles nw scotland verification of neoproterozoic palaeosols a palaeoplain that cuts across lewisian gneiss in the cape wrath area northwest scotland is weathered to depths of 1 6 m beneath fluvial red conglomerates and sandstones of the applecross formation of the early neoproterozoic torridon group weathered lewisian rocks are enriched in k 2 o and fe 2 o 3 and contain hematite as an alteration product of biotite and magnetite palaeomagnetic analysis of the weathering profiles has been carried out to constrain the age of alteration thermal step demagnetization of 87 oriented core specimens of biotite gneiss and amphibolite spanning four profiles at poll a mhurain and sheigra reveal a stable high temperature component with a mean direction of remanence of d 149 1 i 57 2 95 5 0 corrected for dip of the overlying applecross strata that gives a pole at 9 8 s 199 2 e dp 5 3 dm 7 3 this direction differs significantly from that for little weathered lewisian at those two localities and also from directions for the late mesoproterozoic stoer group the remanence direction for the weathered rocks is however near the dip corrected high temperature remanence direction d 122 3 i 50 5 95 10 8 obtained by us from the thermal step demagnetization of 39 sandstone specimens of the applecross formation in the cape wrath area furthermore the direction for the weathered rocks agrees closely with other dip corrected results for the torridon group such as the detrital remanent magnetization drm mean direction d 148 3 i 59 9 95 13 8 for the applecross formation on rum and that for the main outcrop of the aultbea formation d 139 7 i 52 6 95 14 5 the concordance of palaeomagnetic results for the sediments and weathered rocks implies that the weathering profiles in the cape wrath area acquired their remanence penecontemporaneously with deposition of the applecross and aultbea formations the palaeomagnetic results therefore argue strongly that the weathering profiles are palaeosols of applecross age c 980 ma that formed at a palaeolatitude of 38 and constitute the oldest known palaeosols in the british isles views that the weathering is substantially older or younger than applecross deposition cannot be sustained our findings carry implications for applecross palaeoclimate and source and strengthen the conclusion that atmospheric oxygen was a substantial fraction of its present abundance by early neoproterozoic time

changes in geometry and subglacial drainage of midre lovenbreen svalbard determined from digital elevation models digital elevation models dems of the bed and surface of the polythermal midre lovenbreen svalbard are used to identify changes in glacier geometry between 1977 and 1995 the calculated mean annual mass balance 0 61 m water equivalent w e a 1 is more negative than that derived from field measurements 0 35 m w e a 1 although the error associated with this value 0 7 m a 1 suggests that the difference may be accounted for by errors however similar discrepancies between dem based and field based measurements of mass balance have been reported elsewhere in svalbard although errors may be responsible patterns of surface elevation change may also be explained in terms of patterns of ablation accumulation and dynamics the theoretical structure of the subglacial drainage system is modelled using different assumptions about subglacial water pressure in 1977 and 1995 these reconstructions are compared with the observed positions of proglacial outlet streams decreasing subglacial water pressure results in a decrease in the influence of surface morphology and an increase in the role of the bed topography on drainage routing which generally leads to more dispersed drainage long term changes in the position of proglacial outlet streams occur as a result of changes in glacier geometry but short term changes may also occur in response to early season water pressures controlled by meteorological and hydrological conditions copyright 2003 john wiley sons ltd