the ivrea zone and lower crustal magnetization abstract the ivrea zone in northern italy is an arcuate belt of rocks whose geological and geophysical properties indicate that they were once deep in the crust it is one of several presumed lower crustal cross sections whose magnetic properties have been well studied in a systematic way the others are the lofoten vesteralen terrane in norway the pikwitonei cross lake subprovince in manitoba the kapuskasing uplift in ontario and to a lesser extent the lewisian terrane in scotland rocks from the ivrea zone have highly variable magnetic properties consistently strong magnetization is associated with serpentinized ultramafics and the main metagabbro sequence many amphibolites and a few metapelites are also strongly magnetic natural remanent magnetization generally with a strong present field overprint has intensities comparable to induced magnetization in the more highly magnetic rocks although the strongly magnetic units have magnetizations high enough about 4 a m to account for the magnetic anomaly associated with the zone they would be required to make up a considerably larger proportion of the subsurface body than they represent in the surface outcrop a common observation in the other crustal cross sections as well like the other cross sections magnetization in the ivrea zone is carried by magnetite with some pyrrhotite in less magnetic metapelitic samples the ivrea zone contains more ultramafic rocks than most of the other sections but shows no direct correlation between magnetization and composition degree of serpentinization has a far greater influence on the magnetization of the ultramafic rocks

a geomagnetic induction anomaly from ims data near hudson bay and its relation to crustal electrical conductivity in central north america summary canadian geomagnetic data collected during the international magnetospheric study are used to investigate the terrestrial electrical conductivity structure of north eastern manitoba and part of the north west territories the computed transfer functions resolve a major conductor trending east west between the communities of gillam and back in manitoba regional trends in the surface geology suggest that this conductor may be linked with the north american central plains electrical conductor twodimensional modelling of the data suggests that the conductor dips to the north from a small depth beneath gillam and may extend to the lowermost cnst

global and local influences on the chemical composition of snowfall at dye 3 greenland the record between 10 ka b p and 40 ka b p abstract wisconsin age ice from dye 3 greenland shows a number of 18o level changes which indicate the occurrence of rapid climate transitions in order to study the effect of climate change on geochemical fluxes we have selected several of these transitions for chemical analysis at each transition we have measured chloride nitrate and sulfate concentrations with 5 cm depth resolution all three anionic species show significant variations which correlate with the measured 18o shifts in general periods of high 18o warm periods have lower anion concentrations than adjacent periods of low 18o cold periods however the relative concentration shifts are not the same for all species indicating that the concentration variations cannot be caused only by changes in snow accumulation rate acting on a constant anion flux in addition to these rapid concentration changes over the last 40 ka slower secular variations in baseline concentrations also occurred baseline chloride and sulfate concentrations reached maxima near time of maximum ice volume baseline nitrate on the other hand remained relatively constant until near the end of the glaciation when its concentration rose if a constant sulfate flux is assumed the measured sulfate concentrations imply a dependence of snow accumulation rate on 18o for the wisconsin which is similar to that presently observed in greenland the sulfate concentrations would then suggest that wisconsin snowfall rates were at times as much as eight times lower than today

rotating horizontal convection horizontal convection hc is the generic name for the flow resulting from a buoyancy variation imposed along a horizontal boundary of a fluid we study the effects of rotation on three dimensional hc numerically in two stages first when baroclinic instability is suppressed and second when it ensues and baroclinic eddies are formed we concentrate on changes to the thickness of the near surface boundary layer the stratification at depth the overturning circulation and the flow energetics during each of these stages our results show that for moderate flux rayleigh numbers o 1 0 11 rapid rotation greatly alters the steady state solution of hc when the flow is constrained to be uniform in the transverse direction rapidly rotating solutions do not support a boundary layer exhibit weaker overturning circulation and greater stratification at all depths in this case diffusion is the dominant mechanism for lateral buoyancy flux and the consequent buildup of available potential energy leads to baroclinically unstable solutions when these rapidly rotating flows are perturbed baroclinic instability develops and baroclinic eddies dominate both the lateral and vertical buoyancy fluxes the resulting statistically steady solution supports a boundary layer larger values of deep stratification and multiple overturning cells compared with non rotating hc a transformed eulerian mean approach shows that the residual circulation is dominated by the quasi geostrophic eddy streamfunction and that the eddy buoyancy flux has a non negligible interior diabatic component the kinetic and available potential energies are greater than in the non rotating case and the mixing efficiency drops from sim 0 7 to sim 0 17 the eddies play an important role in the formation of the thermal boundary layer and together with the negatively buoyant plume help establish deep stratification these baroclinically active solutions have characteristics of geostrophic turbulence

inhomogeneous structure of the senegal lithosphere from deep magnetotelluric soundings data from 23 magnetotelluric sites were used to determine electrical conductivities within the crust and upper mantle in senegal west africa along a profile about 600 km long data have been obtained in a variety of tectonic zones the west african craton stable since 1850 250 m y the west african mobile belt with ages in the range of 350 650 m y and the senegal sedimentary basin an earth model is derived from two dimensional modeling the model shows several heterogeneities extending deep in the lithosphere and perhaps more deeply in the asthenosphere to a depth of about 460 km at the stations in the western part of the basin the average resistivity is anomalously low 20 30 ohm m down to about 10 km the basin anomaly in the upper crust is assumed to mark an initial rift zone it may reveal a pattern of crustal weakness in this area beneath this zone the resisitivity is in the range of 1000 3000 ohm m down to at least 300 km in the eastern basin the resistivity is decreasing in the depth range of 20 30 km and the top of the ultimate mantle conductor is at a depth of 300 km at the sites in the marginal part of the west african craton conductive layers can be recognized in the depth range of 30 40 and 80 100 km the resistivity drops to 10 ohm m at a depth of about 460 km the model in this region is compared with available geological and geophysical information and an attempt is made to explain some of the observed discontinuities by plate tectonics it is suggested that the major discontinuity which separates the basin and the mobile belt and extends to about 460 km depth could be interpreted as marking a precambrian subduction slab dipping eastward in the west african craton a low resistivity has been determined for a depth range of 130 150 km the upper 130 km were found to have the rather high resistivities of 1000 3000 ohm m a sharp rise in conductivity occurs at a depth of about 460 km beneath the craton in contrast a surprising feature in the kedougou region in the craton is that the major portion of the lower crust is conductive the existence of a birrimian geosynclinal pair in eastern senegal consisting of a eugeosynclinal marine andesitic volcanic trough particularly volcanics with ophiolite affinity and a miogeosynclinal sedimentary sequence could characterize a modern plate tectonic phase dominated by widespread ocean opening and continental collision the conductive material in the lower crust might be explained by the buried ocean crust

an audiofrequency magnetotelluric traverse across the mana pools basin northern zimbabwe summary audiofrequency magnetotelluric soundings in the frequency range 0 016 128 hz have been conducted to investigate the structure of the mana pools basin in northern zimbabwe a karoo sedimentary basin in the zambezi valley data from nine tensorial soundings along a 40 km n s traverse from the river zambezi to the zimbabwe craton and a further sounding on the escarpment have been processed giving the sounding curves required for preliminary modelling studies a variety of l d inversion algorithms have subsequently been applied to these processed data sets to yield interpretive models an approximate 2 d geoelectrical section has been derived by collating the 1 d models a marked and rapid contrast in subsurface resistivity distribution between the escarpment and the basin is apparent within the basin the results show the presence of a top sequence of variable thickness up to 10 km overlying a more resistive 0 1 s m 1 unit that may represent the geoelectrical basement these top sequences which have tentatively been interpreted as sedimentary formations with different stratigraphic units consist of highly conducting 0 3 s m 1 to 1 s m 1 materials overlain by a slightly more resistive typically 0 2 s m 1 unit in some cases with another good conductor adjacent to the surface the data also suggest a rapid thinning of the top sequences close to the escarpment in contrast to the more gradual decrease of the sediments in previous geophysical models for the area on the escarpment the resistivity is higher at all depths the mana pools basin thus appears to be a continuation of the good conductor running from namibia into western zimbabwe identified by de beer 1982a and provides a link to another beneath the lower zambezi basin to the east of mana pools discovered magnetotellurically by losecke knodel muller 1988 comparisons between the geoelectrical model and other available results for the region are presented

telluric currents and magnetic anomalies current densities of 2 5 10 4 amp m can be generated by constant groundwater migration in artesian aquifers these dc currents are sufficient to generate magnetic anomalies exceeding 200 nt in both the menindee trough australia and the karoo basin south africa telluric currents associated with ionospheric activity can be detected by variations in magnetic induction but constant offsets associated with streaming potentials and geochemical activity sp anomalies are obscured by noise consequently some regional magnetic anomalies may be wrongly attributed to variations in magnetic susceptiblity with residuals explained by remanence

synchronizing ice cores from the renland and agassiz ice caps to the greenland ice core chronology 1 four ice cores from the agassiz ice cap in the canadian high arctic and one ice core from the renland ice cap in eastern greenland have been synchronized to the greenland ice core chronology 2005 gicc05 which is based on annual layer counts in the dye 3 grip and ngrip ice cores volcanic reference horizons seen in electrical conductivity measurements ecm have been used to carry out the synchronization throughout the holocene the agassiz ice cores have been matched to the ngrip ice core ecm signal while the renland core has been matched to the grip ice core ecm signal thus tying the cores to gicc05 furthermore it has been possible to synchronize the renland ice core to ngrip gicc05 in the glacial period back to 60 000 years b2k years before a d 2000 on the basis of a matching of transitions between stadials and interstadials this work brings the total number of ice core records that have been rigorously tied to the gicc05 timescale up to nine renland annual layer thicknesses are increasing with depth during the period from 7 to 8 5 ka b2k a highly unusual observation only matched by a similar thickness increase in the glacial section of the renland core some 60 ka ago annual layer thicknesses in the agassiz ice cores point to a well developed raymond bump in the agassiz ice cap

a magnetotelluric study of the damara belt in namibia 2 mt phases over 90 reveal the internal structure of the waterberg fault omaruru lineament abstract magnetotelluric mt observations at some sites in the vicinity of the waterberg fault omaruru lineament wf ol a major tectono stratigraphic zone boundary in the central zone of the damara belt show evidence for strong three dimensional 3d effects we observe very high skew values phases over 90 and a strong correlation of parallel components of the electric and magnetic fields at long periods because of the dense site spacing and good spatial coverage we can positively attribute these effects to local geology and are able to resolve structural detail within the wf ol mapping latorraca s electric characteristic vectors in form of ellipses proved particularly useful in identifying key elements of the conductivity structure for subsequent modelling 3d and 2d anisotropic modelling can reproduce most of the observed 3d effects the conductivity anomalies revealed in the area are i a conductive ring structure in the shallow crust along the northern part of the profile ii an anisotropic region in the upper crust with high conductivity parallel to the wf ol iii anisotropy in the lower crust with a different but undetermined strike direction and iv a shallow elongated conductor sub normal to the wf ol modelling studies further suggest that the anisotropic fault zone is approximately 10 km wide and may reach down to a depth of 14 km or more

co2 record between 40 and 8 kyr b p from the greenland ice core project ice core co2 ice core records show an increase in the atmospheric concentration of 80 100 parts per million by volume ppmv from the last glacial maximum lgm to the early holocene we present co2 measurements performed on an ice core from central greenland drilled during the greenland ice core project grip this co2 profile from grip confirms the most prominent co2 increase from the lgm with a mean concentration of 200 ppmv to the early holocene with concentrations between 290 and 310 ppmv some structures of the new co2 record are similar to those previously obtained from the dye 3 ice core greenland which indicated a dilemma between greenland and antarctic co2 records oeschger et al 1988 both greenland cores show high co2 values for rather mild climatic periods during the last glaciation whereas co2 records from antarctica do not show such high co2 variations during the glaciation and furthermore the co2 values in the early holocene are about 20 30 ppmv higher in the grip record than in antarctic records there is some evidence that the difference could be due to chemical reactions between impurities in the ice leading to an increase of the co2 concentration under certain conditions if in situ processes can change the co2 concentration in the air bubbles the question arises about how reliably do co2 records from ice cores reflect the atmospheric composition at the time of ice formation the discrepancies between the co2 profiles from greenland and antarctica can be explained by in situ production of excess co2 due to interactions between carbonate and acidic species since the carbonate concentration in antarctic ice is much lower than in greenland ice co2 records from antarctica are much less affected by such in situ produced co2

electrical conductivity measurements from the gisp2 and grip greenland ice cores the direct current electrical conductivity of glacial ice depends on its acidity1 3 and can also indicate changes in climate as ice formed in cold dusty periods has a high concentration of alkaline dust1 4 5 which significantly reduces the conductivity6 7 compared to warmer less dusty periods here we present electrical conductivity records for the greenland ice sheet project 2 gisp2 and greenland ice core project grip ice cores drilled 28 km apart to enable direct comparison of the results the upper parts of both records are consistent with previous evidence from other greenland cores4 8 12 for a stable greenland climate during the holocene and a series of warm events punctuating the last glacial period however there is a significant discrepancy between the two records in the bottom 10 of the cores calling into question recent reports of climate variability in the last interglacial4 8 and the penultimate glaciation8 at this stage it is too early to say what exactly is causing the discrepancy although ice flow may have introduced some discontinuities into the records further work will be necessary to establish how much climatic information it will eventually be possible to extract from the lower parts of the two cores

field impressed anisotropies of magnetic susceptibility and remanence in minerals the application of alternating af or direct df magnetic fields to samples containing dispersed particles of magnetite titanomagnetite or maghemite has been found to alter significantly the measured low field susceptibility anisotropy even though the particle orientation is unchanged the acquisition of isothermal remanent magnetization irm is also strongly dependent on the previous af treatment given to the samples a method of quantifying the changes brought about by application of the fields is described the samples studied were intrinsically isotropic or only weakly anisotropic and the computed impressed susceptibility or irm ellipsoids produced by the applied fields were ellipsoids of revolution with the unique axis aligned with the applied field axis the size and shape of the field impressed ellipsoids were strongly dependent upon particle size these effects might thus form the basis of a rapid nondestructive means of estimating particle size in rocks the results also have important implications for magnetic fabric analysis and the static af demagnetization of rocks in particular to ensure that the intrinsic magnetic fabric is measured without any field impressed component of anisotropy samples should not be subjected to static af demagnetization prior to the determination of either low field susceptibility anisotropy or irm anisotropy samples should also not carry a significant remanence such as an irm acquired in a field greater than about 5 mt prior to low field susceptibility anisotropy analysis

a new greenland deep ice core the polar ice sheets are rich sources of information on past atmospheric conditions including paleoclimates a new deep ice core has been drilled in south greenland comparison of the oxygen isotopic profile with that from camp century and with a deep sea foraminifera record indicates that the new core reaches back to about 90 000 years before present in a continuous sequence the details in the wisconsin part of the ice core records seem to be climatically significant and the general trends reveal all of the relevant emiliani stages recorded in deep sea cores the redated camp century record suggests a dramatic termination of the eem sangamon interglacial

anisotropic magnetic susceptibility in the continental lower crust and its implications for the shape of magnetic anomalies magnetic anisotropy measurements on samples from the lower continental crust were made to test the hypothesis that anisotropy may cause deflection of the peak to trough axes of magnetic anomalies caused by mid to lower crustal sources average anisotropy p for these samples is 1 5 but can be as high as 3 4 felsic granulite facies rocks show the highest anisotropy magnetic sources with p equal to the maximum determined in this study can cause azimuthal rotations of magnetic anomalies by up to about 25 but p lower than or equal to 1 5 causes no significant rotation comparison of the model results to the abnormal shape of some magnetic anomalies in southern italy indicate that these anomalies cannot be related easily to a strong and coherent ams of the source rocks the most probable explanation of such shapes is the existence of a strong remanent magnetization in subsequently rotated source bodies

revised tectonic implications for the magnetic anomalies of the western weddell sea abstract ten years after the usac u s a rgentina c hile project which was the most comprehensive aeromagnetic effort in the antarctic peninsula and surrounding ocean basins questions remain regarding the kinematics of the early opening history of the weddell sea key elements in this complex issue are a better resolution of the magnetic sequence in the western part of the weddell sea and merging the usac data set with the other magnetic data sets in the region for this purpose we reprocessed the usac data set using a continuation between arbitrary surfaces and equivalent magnetic sources the equivalent sources are located at a smooth crustal surface derived from the existing bathymetry topography and depths estimated by magnetic inversions the most critical area processed was the transition between the high altitude survey over the antarctic peninsula and the low altitude survey over the weddell sea that required downward continuation to equalize the distance to the magnetic source this procedure was performed with eigenvalue analysis to stabilize the equivalent magnetic source inversion the enhancement of the mesozoic sequence permits refining the interpretation of the seafloor spreading anomalies in particular the change in shape and wavelength of an elongated positive in the central weddell sea suggests that it was formed during the cretaceous normal polarity interval the older lineations in the southwestern weddell sea are tentatively attributed to susceptibility contrasts modeled as fracture zones numerical experimentation to adjust synthetic isochrons to seafloor spreading lineations and flow lines to fracture zones yields stage poles for the opening of the weddell sea since 160 ma to anomaly 34 time the corresponding reconstructions look reasonable within the known constraints for the motions of the antarctic and south america plates however closure is not attained between 160 and 118 ma if independent published east antarctica africa south america rotations are considered the lack of closure may be overcome by considering relative motion between the antarctic peninsula and east antarctica until 118 ma time an important component of convergence

the role of remanent magnetization in the southern italian crust from aeromagnetic anomalies induced magnetization in the intermediate to deep crust is insufficient to justify regional magnetic anomalies this is an unresolved problem that cannot be completely explained by induced and viscous magnetization in this context the contribution of remanent magnetization in the southern italian region has been investigated from a study of long wavelength magnetic anomalies by means of a shape analysis several anomalies were found which indicate a total magnetization vector direction different from the induced one from the good correspondence between these deduced declinations with palaeomagnetic and palaeotectonic data we infer a remarkable manifestation of remanent magnetization in the intermediate crust

a magnetotelluric traverse across the mauritanides orogenic belt west africa summary magnetotelluric measurements with reference magnetometers for study of the anomalous geomagnetic variation field were made in the period range 10 10000 s at 10 sites crossing the main geological zones of eastern senegal west africa the results indicate that the electromagnetic fields are controlled by the dominant structural trends of the mauritanides orogenic belt apparent resistivities reflect the major structural units identified from surface geology data in the principal directions for most of the stations exhibit anisotropic behaviour and at certain sites the magnetotelluric responses are affected by surface distortion effects the study of the geomagnetic variation field indicates the presence of a concentration of telluric current flow beneath the craton a lack of obviously conductive geological structures at the surface suggests that this flow of telluric current may be attributed to increased electrical conductivity in the crust the quantitative interpretation of the magnetotelluric results is given in terms of 1 d and 2 d resistivity models it is shown that the data require the presence of a highly anomalous crustal structure 40 rm dipping to the east from a small depth beneath the outcropping margin of the west african craton in the mauritanides the crust is resistive 3000 rm t o a depth of 30 km and overlies a conducting layer of 10 km thick in the depth range 250 300 km resistivities decrease to less than 50 rm this may coincide with the top of the asthenosphere the geoelectric model is discussed in relation to the general geological background of the region considered

rotating horizontal convection and the potential vorticity constraint horizontal convection with a focus on the essential physics it can disclose for the oceanic overturning circulation is studied in an annular gap where differential heating is applied along the bottom based on the three dimensional distribution of both velocity and temperature fields we propose that in addition to differential buoyancy forcing and diapycnal mixing potential vorticity pv can also be a limiting factor for the overturning circulation by imposing a constraint on the meridional flow with rotation flow has to be along pv contours excepting that effects of diffusion or friction exist to break the pv constraint in the axisymmetric rotating experiment the flow along isotherms is to zero order azimuthal due to the fact that pv varies only along the radial direction consequently the radial flow appears at the next order with much smaller amplitude leading to a much weaker overturning circulation compared with that of the non rotating experiment in the presence of a full radial barrier the annular gap can be viewed as a four sided domain with its northern boundary along the inner cylinder and the meridional flow shows remarkable inhomogeneity in space in the absence of any background pv gradient the flow field equilibrates itself to a state where the pv contours above the bottom boundary layer are largely radial particularly in a sub region near the western boundary allowing a relatively strong radial geostrophic flow and greatly strengthening the overturning circulation from the case without the barrier

potential for stratigraphic folding near ice sheet centers lack of agreement between the deep portions of the greenland icecore project grip and greenland ice sheet project ii gisp2 ice cores from central greenland suggests that folds may disrupt annual layering even near ice divides we use a simple kinematic flow model to delineate regions where slope disturbances wrinkles introduced into the layering could overturn into recumbent folds and where they would flatten leaving the stratigraphic record intact wrinkles are likely to originate from flow disturbances caused internally by inhomogeneities and anisotropy in the ice rheological properties rather than from residual surface structures sastrugi or from open folds associated with transient flow over bed topography if wrinkles are preferentially created in anisotropic ice under divides where the resolved shear stress in the easy glide direction can be weak and variable then the deep intact climate record at dye 3 may result from its greater distance from the divide alternatively the larger simple shear at dye 3 may rapidly overturn wrinkles so that they are not recognizable as folds the ice core record from siple dome may be intact over a greater fraction of its depth compared to the central greenland records if its flat bedrock precludes fluctuations in the stress orientation near the divide

the magnetic properties of metaperidotitic rocks as a function of metamorphic grade implications for crustal magnetic anomalies seventeen samples from the malenco serpentinite in the swiss alps representing systematic prograde metamorphic conditions from prehnite pumpellyite to upper amphibolite are moderately to strongly magnetic average susceptibility of 6 61 10 2 si units due predominantly to the presence of a magnetite rich spinel some greenschist facies samples contain the metal alloy awaruite ni3fe these contain in addition much greater amounts of magnetite small amounts of pyrrhotite in about half the samples complete the contributions to the total magnetization susceptibilities generally decrease with increasing metamorphic grade because of the production of increasing amounts of chromerich spinel which dilutes the magnetite component these results indicate that significant magnetic anomalies should be generated in suture zones containing metamorphosed serpentinite and in areas where serpentinite is being actively subducted

the effects of geothermal heating on the ocean overturning circulation 1 we examine the response of an overturning circulation driven by differential thermal forcing along the top horizontal boundary to a small additional heat flux applied at the bottom horizontal boundary the system forms a simple thermally driven flow that provides insight into the ocean s meridional overturning circulation we conclude that the additional destabilising geothermal heat flux tends to promote a more vigorous full depth overturning having approximately 10 greater volume flux than with no bottom heating no significant change is observed in the vertical density structure in contrast the addition of a stabilising heat flux at the base leads to a shallow partial depth circulation the key diagnostic for the significance of the geothermal flux appears to be the ratio of the buoyancy flux supplied at the bottom to the residual buoyancy flux driving the downwelling plume through the base of the thermocline

a magnetotelluric study of the damara belt in namibia 1 regional scale conductivity anomalies abstract the namibian margin is dominated by the late proterozoic to early cambrian fold belts of the damara orogen which wrap around and separate the congo and kalahari cratons this mosaic of relatively soft fold belts and hard cratons apparently controlled the path for the opening of the south atlantic in the early cretaceous the continents split along the coast parallel fold belts of the damara orogen while the inland fold belt damara belt was effected by extension and widespread igneous intrusion but never developed to the rift stage this paper is concerned with the interpretation of magnetotelluric mt data along a 200 km nw se profile across the damara belt in nw namibia the regional two dimensional electrical resistivity model and the induction vector data exhibit three distinctive zones i a generally very resistive upper crust which is typical for the granites and metasediments of the damara belt ii two subvertical conductors in upper to mid crustal levels which correlate with major tectonic zone boundaries and iii a highly conductive middle to lower crust in the southern part of the profile the geometry of the conductive structures could reflect a regional shear system in which upper crustal listric faults pass into a detachment zone in the middle crust we interpret the high electrical conductivity in terms of graphite or other forms of mineralization enrichment along the shear planes this zone of crustal weakness may have originated in pre damara times and had probably experienced several episodes of crustal reactivation before the intrusion of basaltic dike swarms during the cretaceous rifting and magmatism associated with the opening of the south atlantic

laboratory simulation of the geothermal heating effects on ocean overturning circulation motivated by a desire to understand the geothermal heating effects on ocean circulation a large scale circulation generated and sustained by thermal forcing at the surface subject to a small amount of heating from the bottom boundary is investigated through laboratory experiments motivated by understanding the geothermal heating effects on ocean circulation despite its idealization our experiments demonstrate that the leading order effect of geothermal heating is to significantly enhance the abyssal overturning in agreement with the findings in ocean circulation models our experiments also demonstrate that geothermal heating cannot influence the poleward heat transport due to the strong stratification in the thermocline our study further reveals that the ratio of geothermal flux induced turbulent dissipation to the dissipation due to other energies is the key parameter determining the dynamical importance of geothermal heating this quantity explains why the impact of geothermal heating is sensitive to the deep stratification the diapycnal mixing and the amount of geothermal flux moreover it is found that this dissipation ratio may be used to understand results from different studies in a consistent way this article is protected by copyright all rights reserved

grimm the gfz reference internal magnetic model based on vector satellite and observatory data summary in this paper the new gfz reference internal magnetic model grimm is presented the model has been derived from nearly 6 yr of champ satellite data and 5 yr of observatory hourly means at high latitudes full vector satellite data are used at all local times which allows a separation between on one hand the fields generated by ionosphere and field aligned currents and on the other hand the fields generated in the earth s core and lithosphere this selection technique leads to a data set without gaps during the polar summers resulting in a core field model that has an unprecedented time resolution the modelled static core field secular variation and lithospheric field are all in good agreement with previously published magnetic field models order five b splines are used to model the variation in time of the core field the energy in the secular acceleration has therefore a smooth behaviour in time and increases continuously from 2003 5 mapping the field acceleration from 2001 5 to 2005 5 reveals its rapid and complex evolution over this time period at the earth s surface due to the applied regularization technique the acceleration energy in spherical harmonics 6 11 is significantly larger than for other models and we show that such a spectrum is acceptable

simple models for the beattie magnetic anomaly in south africa abstract the origin of the approximately 1000 km long beattie magnetic anomaly bma in south africa remains unclear and contentious key issues include the width depth and magnetization of its source in this study we use uniformly magnetized spheres prisms and cylinders to provide the simplest possible models which predict the 1 km altitude aeromagnetic measurements along a profile across the bma the source parameters are adjusted by forward modeling in case of a sphere an inversion technique is applied to refine the parameters our results suggest that two similarly magnetized and adjacent sources with a vertical offset can explain the observed magnetic anomaly the best fitting model corresponds to two highly magnetized 5 a m 1 sheet like prisms extending from 9 to 12 km depth and from 13 to 18 km depth respectively and with a total width reaching 80 km other less preferred models show thicker and deeper magnetized volumes associated magnetizations seem to be mostly induced although a weak remanent component is required to improve the fit we also compare our results with the interpretation of independent magnetotelluric and seismic experiments along the same profile it suggests that the geological sources for the bma are mostly located in the middle crust and may be displaced by a shear zone or a fault contrary to previous models suggesting a serpentinized sliver of paleo oceanic crust within the natal namaqua mobile belt we propose that granulite facies mid crustal rocks within this belt may cause the bma

age difference between polar ice and the air trapped in its bubbles air entrapped in bubbles formed in cold ice has essentially the same composition as that of the atmosphere at the time of bubble formation the analysis of dated ice samples therefore enables the history of atmospheric composition to be investigated 1 3 the age of the entrapped air is however not the same as that of the surrounding ice because air bubbles only become isolated from the atmosphere during the transition from firn to ice typically the age of the ice at this transition is between 100 and 3 000 yr depending mainly on firn temperature and snow accumulation rate the mean age difference between ice and enclosed air as well as the age distribution width for a given sample are especially important for the investigation of the anthropogenic increase of co2 and trace gases in the atmosphere over the last centuries and for the comparison of climatic parameters recorded in the ice with parameters recorded in the bubbles for siple station antarctica this age difference and age distribution width were deduced from the bubble volume measured as a function of depth the values are 95 yr and 22 yr respectively

the role of the geothermal heat flux in driving the abyssal ocean circulation 1 the results presented in this paper demonstrate that the geothermal heat flux ghf from the solid earth into the ocean plays a non negligible role in determining both abyssal stratification and circulation strength based upon an ocean data set we show that the map of upward heat flux at the ocean floor is consistent within a factor of 2 with the ocean floor age dependent map of ghf the observed buoyancy flux above the ocean floor is consistent with previous suggestions that the ghf acts to erode the abyssal stratification and thereby enhances the strength of the abyssal circulation idealized numerical simulations are performed using a zonally averaged single basin model which enables us to address the ghf impact as a function of the depth dependence of diapycnal diffusivity we show that ignoring this vertical variation leads to an under prediction of the influence of the ghf on the abyssal circulation independent of the diffusivity profile introduction of the ghf in the model leads to steepening of the southern ocean isopycnals and to strengthening of the eddy induced circulation and the antarctic bottom water cell the enhanced circulation ventilates the ghf derived heating to shallow depths primarily in the southern ocean

magnetic properties of exposed deep crustal rocks from the superior province of manitoba abstract a traverse across precambrian rocks of the superior province in central manitoba provided 56 samples for magnetic study from the pikwitonei granulite domain and the adjacent cross lake subprovince these rocks represent a cross section of the continental crust presently exposed at the surface thirty one samples are dominantly paramagnetic and three contain pyrrhotite twenty two samples from silicic plutonic rocks which make up about 80 of the section by volume contain nearly pure magnetite and have susceptibilities that are almost constant between room temperature and 540 c averaging about 1 10 2 si units the mafic supracrustal rocks with much higher iron content are about an order of magnitude less magnetic because they contain little or no magnetite susceptibility is generally independent of metamorphic grade or apparent depth in the crust the discrepancy between induced magnetizations of measured lower crustal rocks and the source requirements of long wavelength magnetic anomalies may be due to a high temperature viscous magnetization in the deep crust

microseismic activity and basement controls on an active intraplate strike slip fault ceres tulbagh south africa earthquakes far from plate boundaries are poorly understood because of few well studied examples and uncertainty of what controls the location of these events in 1969 a damaging local magnitude m l 6 3 strike slip earthquake with no surface expression occurred in the ceres tulbagh region in the south african stable continental interior here we present a microseismic study of the ceres tulbagh area conducted over three months in 2012 in which 172 events recorded on at least three stations follow a gutenberg richter relationship for 1 5 m l 0 5 the events delineate a 5 km wide subvertical zone that is microseismically active to a depth of 15 km this fault zone is subparallel to the 1969 aftershock zone and at low angle to the regionally inferred greatest horizontal stress we argue that the microseismically active zone is guided by inherited structures in the basement geology this and similar structures may represent significant earthquake hazard in plate interiors online material tables of station and event parameters and p and s wave arrival times

magnetotelluric measurements across the beattie magnetic anomaly and the southern cape conductive belt south africa 1 the beattie magnetic anomaly bma and the southern cape conductive belt sccb two of earth s largest continental geophysical anomalies extend across the southern african continent in an east west direction to resolve structural details of the sccb a high resolution magnetotelluric study was conducted in march 2004 along a 150 km long n s profile across the karoo basin in south africa a two dimensional conductivity model at a scale of the entire crust exhibits three distinct zones of high conductivity 1 beneath the surface trace of the axis of the bma we observe a high conductivity anomaly at 5 to 10 km depth however it remains enigmatic whether both geophysical anomalies have the same source 2 a shallow regionally continuous subhorizontal band of high conductivity can be linked to a 50 to 70 m thick pyritic carbonaceous marker horizon intersected in deep boreholes 3 several highly conductive synformal features in the mid crust are newly imaged near the northern extremity of the profile our data generally support the existence of a deeper conductivity belt but allow for a much better definition of crustal conductivity in this region such laterally confined zones of crustal conductivity can only be resolved with a dense site spacing and broadband recordings

petrology of felsic granulites metapelites metabasics ultramafics and metacarbonates from southern calabria italy prograde metamorphism uplift and cooling of a former lower crust

crustal structure beneath southern africa and its implications for the formation and evolution of the kaapvaal and zimbabwe cratons the formation of archean crust appears to in volve processes unique to early earth history initial results from receiver function analysis of crustal structure beneath 81 broadband stations deployed across southern africa re vealsignicantdierences inthenatureof thecrustandthe crust mantle boundary between archean and post archean geologic terranes with the notable exception of the colli sional limpopo belt where the crust is thick and the moho complex the crust beneath undisturbed archean craton is typically thin 35 40 km unlayered and characterized byastrongvelocitycontrastacrossarelativelysharpmoho thiscrustalstructurecontrastsmarkedlywiththatbeneath post archeanterranesandbeneatharcheanregionsaected bylarge scaleproterozoicevents thebushveldcomplexand the okwa magondi belts where the crust tends to be rel atively thick 45 50 km and the moho is complex

the ice flow velocity profile for dye 3 greenland uniaxial compression tests were made on 72 deep ice core specimens taken from 14 depth intervals between 247 m and 2021 m on the dye 3 greenland ice core test results give stress exponent values ranging from 2 7 to 3 2 which are in close agreement to the 3 0 measured on laboratory manufactured polycrystalline ice specimens these test results are compared with similar measurements made in a field laboratory immediately after the ice core was recovered the data sets showed no significant differences over the four year time span the horizontal velocity profile is calculated for dye 3 using a strain rate enhancement factor relationship and using stress exponent and activation energy values measured for artificial laboratory ice the calculated profile is in good agreement with the results from direct borehole tilting measurements this is the first time such a close correlation has been affected between laboratory generated experimental data and field measurements in the analysis of a horizontal velocity profile for a large ice sheet

the δ18o record along the greenland ice core project deep ice core and the problem of possible eemian climatic instability over 70 000 samples from the 3029 m long greenland ice core project grip ice core drilled on the top of the greenland ice sheet summit have been analyzed for 8o a highly detailed and continuous 8o profile has thus been obtained and is discussed in terms of past temperatures in greenland we also discuss a three core stacked annual 8o profile for the past 917 years the short term 50 years variability of the annual 8o signal is found to be 1 in the holocene and estimates for the coldest parts of the last glacial are 3 or higher these data also provide insights into possible disturbances of the stratigraphic layering in the core which seems to be sound down to the onset of the eemian spectral analysis of highly detailed sequences of the profile helps determine the smoothing of the 8o signal which for the holocene ice is found to be considerably stronger than expected we suggest this is due to a process involving diffusion of water molecules along crystal boundaries in the recrystallizing ice matrix deconvolution techniques were employed for restoring with great confidence the highly attenuated annual 8o signal in the holocene we confirm earlier findings of dramatic temperature changes in greenland during the last glacial cycle abrupt and strong climatic shifts are also found within the eem sangamon interglaciation which is normally recorded as a period of warm and stable climate in lower latitudes the stratigraphic continuity of the eemian layers is consequently discussed in section 3 of this paper in terms of all pertinent data which we are not able to reconcile

earth s lithospheric magnetic field determined to spherical harmonic degree 90 from champ satellite measurements summary the champ magnetic field mission is providing highly reliable measurements from which the global lithospheric magnetic field can be determined in unprecedented resolution and accuracy using almost 5 yr of data we derive our fourth generation lithospheric field model termed mf4 which is expanded to spherical harmonic degree and order 90 after subtracting from the full magnetic field observations predicted fields from an internal field model up to degree 15 an external field model up to degree two and the predicted magnetic field signatures for the eight dominant ocean tidal constituents we fit and remove remaining external fields and polar electrojet signatures in a track by track scheme from a subset of least disturbed tracks we estimate the mf4 model by least squares damping ill determined coefficients by regularization the resulting mf4 model provides a good representation of the lithospheric field down to an altitude of about 50 km at lower latitudes with reduced accuracy in the polar regions crustal features come out significantly sharper than in previous models in particular bands of magnetic anomalies along subduction zones become visible by satellite for the first time

folding in the greenland ice sheet the deformation of layering into folds is modeled for a linear viscous medium moving over a decollement folds are generated by flow variations caused by relief on the decollement variations in friction or both the model is applied to folds forming now in the greenland ice sheet near dye 3 for which more complete data are available than for analogous solid earth situations and for which the decollement is at or near the bed the folds wavelength 4 8 km are detected by radio reflection sounding measured surface deformation and deformation rate are used with the radar results to test the theory calculated fold amplitude is only 20 less than that measured which indicates that the theory is substantially correct inversion of the data to calculate basal drag and velocity variations is not helpful for near dye 3 because many different basal boundary conditions can lead to the observed deformations

seismic waves velocities and anisotropy in serpentinized peridotites from xigaze ophiolite abundance of serpentine in slow spreading ridge the effect of serpentinization on seismic wave velocity and anisotropy has been analysed in 6 peridotite samples of the xigaze ophiolite having harzburgitic composition and a degree of serpentinization ranging from 3 to 70 we found i p and s wave velocities are linearly correlated with serpentine content ii anisotropy of p and s waves decreases with increasing serpentinization while in fresh peridotites anisotropy is of the order of 6 for 70 of serpentinization it drops to 1 for p waves and to 2 for s waves comparison of published laboratory velocity data in oceanic lithologies like gabbros and dolerites with serpentinized peridotite data shows the difficulty of distinguishing between gabbros and peridotites having a percentage of serpentine between 10 and 40 by seismic waves velocities only this comparison suggests that seismic velocity data are consistent with the existence of partialy serpentinized peridotites up to 40 of serpentinization in the oceanic crust formed at slow spreading ridges in accordance with geological observation

on the special rheological properties of ancient microparticle laden northern hemisphere ice as derived from bore hole and core measurements on observe des differences dans le fluage des glaces a forte concentration en microparticules wisconsin recent qui sont plus douces que celles de la glace contemporaine ou holocene on analyse les parametres suivants concentration ou microparticules 18 o taille des cristaux axe c ca na conductivite et densite dans des carottes prelevees a agassiz iles ellesmere sur l ile devon et au groenland

rotation of the italian peninsula from aeromagnetic evidence abstract palaeomagnetic data have been used extensively in order to study the geodynamical history of a region similar information can be obtained by analysing the shape of aeromagnetic anomalies and the direction of the total magnetization vector of the anomaly sources we can suppose that the direction of the earth s magnetic induction vector is almost constant with time along a n s direction if a significant directional difference between total magnetization and induction vector is observed we can argue that the anomaly sources were affected by conspicuous rotational movements in the central southern tyrrhenian area the anomaly shape is generally normal i e n s except for several anomalies which have a maximum minimum direction parallel to the italian coast that is nw se rather than n s this feature and the location of these anomalies on the border of the bathyal plain seems to be in agreement with the anticlockwise rotation movements suggested in some geodynamical models of the region

nanoscale haematite ilmenite lamellae in massive ilmenite rock an example of lamellar magnetism with implications for planetary magnetic anomalies summary massive nearly pure haemo ilmenite layers from historic ore deposits in rogaland norway contain very few silicates or other oxides and typically produce remanence dominated magnetic anomalies these rocks are ideal for evaluating the magnetic properties of fine exsolution intergrowths and the larger titanohaematite lamellae in the host ilmenite grains a typical bulk composition ilm 84 exsolved at high temperature to produce host ilmenite ilm 94 and micron sized haematite lamellae ilm 23 as measured by electron microprobe emp subsequent undercooling of the ilmenite and the micron scale haematite lamellae led to metastable nucleation of nanoscale lamellae down to unit cell scale leaving depleted hosts between lamellae with compositions of ilm 98 and ilm 15 13 as measured by tem edx samples have high coercivities and average nrm values of 25 a m 1 which typically show 2 per cent saturation in the nrm state the amount of magnetization in these samples is too high to be solely accounted for by a spin canted af moment in the haematite based on monte carlo simulations of haematite ilmenite interfaces at the atomic scale and on measured rock magnetic properties we predict that the magnetization is carried by a ferrimagnetic substructure produced at the contacts of the very fine scale titanohaematite and ilmenite exsolution lamellae

paleo pole positions from martian magnetic anomaly data magnetic component anomaly maps were made from five mapping cycles of the mars global surveyor s magnetometer data our goal was to find and isolate positive and negative anomaly pairs which would indicate magnetization of a single source body from these anomalies we could compute the direction of the magnetizing vector and subsequently the location of the magnetic pole existing at the time of magnetization we found nine suitable anomaly pairs and from these we computed paleo poles that were nearly equally divided between north south and mid latitudes these results suggest that during the existence of the martian main magnetic field it experienced several reversals and excursions

a magnetovariational study of a geothermal anomaly summary during 1980 september and october an array of 31 threecomponent magnetometers was operated over a known geothermal anomaly in the rocky mountains foothills of alberta the purpose of the array was to investigate the possibility of the existence of a geomagnetic induction anomaly coincident with the geothermal anomaly examples of magnetograms and fourier transform anomaly maps are presented and show that although there is some indication of an anomaly this is to a great extent masked by the presence of large external source field gradients a detailed transfer function analysis of the data is presented both single station transfer functions and a solution of the induction matrix are calculated the latter involves the representation of the normal fields by least squares fitted planes and a justification of the validity of this method is given contour plots of the response of the anomalous vertical field to horizontal hypothetical events and pseudosections of iza hni for traverses across the array are shown these indicate the existence beneath the geothermal anomaly of a body of anomalously high electrical conductivity by treating the observed values of the in phase part of iza ha i along a traverse across the anomaly as due to vertical dipoles it is shown that the top of the conductive body probably lies at 10 20km depth the pseudosections suggest that the body extends at least to lower crustal depths the correlation of the conductivity and geothermal anomalies supports the inference that both are due to the presence of partial melt in the crust

magnetotelluric soundings and the geological structure and tectonics of the senegalo mauritanian basin in northern senegal west africa the results of the magnetotelluric soundings provide new insights on the geological structure and tectonics of the onshore senegalo mauritanian basin in northern senegal west africa electrical resistivities can be correlated with the major geological units in the upper 1000 m resistivity variations indicate a transition from freshwater saturated sediments near the surface to saline water saturated sediments above the basement the resistivity of the sediments is dominated by their porosity and the resistivity of the contained fluids the resistivity of the cretaceous sediments is unexpectedly low especially in the western part of the basin the top of the metamorphic granitic basement cannot be located on the west portion of the profile because of the feeble resistivity contrast between the impervious compacted aptian jurassic limestones and the metamorphic crystalline rocks the magnetotelluric estimates of the depth of the basement range from 600 m in the east to 4000 m in the west based on the magnetotelluric interpretation and other independent geophysical and geological information a cross section through the senegalo mauritanian basin along the magnetotelluric traverse is presented a zone of high electrical conductivity is observed in the crust at depths of 4 6 km on the eastern margin of the deep basin this conductive structure may be the extension of one further south that was revealed by an earlier investigation the entire conductive zone which runs north south between longitudes 15 and 16 w may be a major fracture zone related to the opening of the atlantic ocean however its origin remains to be established

are changes in atmospheric cleansing responsible for observed variations of impurity concentrations in ice cores are changes in atmospheric cleansing responsible for observed variations of impurity concentrations in ice cores

the southern cape conductive belt south africa its composition origin and tectonic significance abstract magnetometer array studies have led to the discovery and mapping of the southern cape conductive belt sccb crossing the southern tip of africa from west to southeast coasts the sccb lies just south of the namaqua natal belt of cratonic rocks remobilized about 1000 m y b p it is shown that it coincides with a zone of weakness which has been exploited by three major geosynclinal accumulations over some 600 m y relationships between the sccb and the basement geochronology geology and tectonics are considered in detail these relationships support the view that the conductive belt was formed by an accumulation of marine sediments and oceanic lithosphere at the top of a proterozoic subduction which stopped about 1000 to 800 m y b p associated with this subduction we propose a proterozoic range of andean mountains whose roots are now exposed in the namaqua natal belt later subduction further south near the present south coast is proposed to account for the intrusion between the south coast and the sccb of the cape granites in the time interval 600 500 m y b p there is some evidence for a third yet more distant subduction episode off permian gondwanaland after outlining this tectonic history the paper turns to a closer examination of the hypothesis that the southern cape conductive belt consists of partly serpentinized basalt accumulated at the top of a proterozoic subduction a large static magnetic anomaly which correlates with the sccb over most of its length is well fitted by a model which strongly supports this hypothesis bouguer gravity anomalies along western and central profiles likewise support the hypothesis a discussion follows of the process of formation of the proposed block of serpentinized marine rocks beginning with serpentinization of the crust near oceanic ridges by reaction of warm porous newly extruded basalt with seawater convecting through it the serpentinized basalt is stable at crustal temperatures and pressures and so is transported in the seafloor until it reaches a subduction where it accumulates because of its low density examples of such accumulations are cited finally it is shown that any iron in the olivine and pyroxene in the original basalt precipitates when these silicates are hydrated to serpentine as magnetite which is the dominant mineral conferring high electrical conductivity and high magnetic susceptibility on serpentinites in particular the beattie static magnetic anomaly requires in our model calculation a very high susceptibility readily attainable in basalt at 15 20 serpentinization the authors know of no other rock able to provide this high susceptibility a similar percentage of serpentinization provides the density required to model the gravity anomalies

creep buckling of ice shelves and the formation of pressure rollers pour mieux comprendre le phenomene des vagues de pression on etudie la stabilite des shelfs aux perturbations des champs moyens de contrainte et de vitesse de deformation l analyse des perturbations est basee sur la loi de fluage de glen on montre que si les vitesses de deformation sont suffisamment compressives on peut s attendre a voir apparaitre des ondulations les extensions laterales ont un effet stabilisateur et les vagues de pression ont plus de chance de se former lorsqu elles existent

geoelectrical structure of the northern part of the senegal basin from joint interpretation of magnetotelluric and geomagnetic data magnetotelluric mt and differential geomagnetic sounding dgs surveys were made in the senegal basin west africa to investigate the conductivity structure of the crust and upper mantle magnetic and electric variations were recorded at 11 sites along a east west profile 220 km in length the period range of the measurements was 10 10 000 s for the western stations the mt response displays low skew with moderate anisotropy the dgs data indicate the presence of an anomalous geomagnetic variation field in the deep basin and on the eastern margin of our study area a telluric current concentration in the thick conductive sediments of the deep basin might cause this anomaly both mt and dgs data are then compared with two dimensional model results to explain the observed features along the profile results reveal a strong anomalous crustal structure in the central part of the profile involving a conductive zone 2 ohm m at depths in the range 4 23 km within the deep basin the results of the two dimensional modeling are not conclusive acceptable fits between observed and calculated data have been obtained with alternate models a continuous conductive zone 0 7 2 ohm m may exist beneath the uppermost layers at a depth of approximatively 1 km but the observed data can be explained equally well by poorly conducting material intruded into the highly conductive sedimentary cover this ambiguity is inherent in the two dimensional models themselves both models show that the sediments increase westward to reach a thickness greater than 4000 m when approaching the coast it is suggested that the flow of current observed in this study is largely controlled by strong lateral variation in the resistivity of the basin and or by variation of the sedimentary thickness the models also indicate the existence of two decreased resistivity layers in the eastern part of the profile one at the base of the crust and the second at 80 km with a resistivity of 20 ohm m no such layers were detected at these depths below the deep basin at some depth greater than 150 km there is a general trend toward lower resistivities from 2000 to about 2 ohm m across the entire profile this may indicate the presence of a small degree of partial melt

evidence for general instability of past climate from a 250 kyr ice core record recent results1 2 from two ice cores drilled in central greenland have revealed large abrupt climate changes of at least regional extent during the late stages of the last glaciation suggesting that climate in the north atlantic region is able to reorganize itself rapidly perhaps even within a few decades here we present a detailed stable isotope record for the full length of the greenland ice core project summit ice core extending over the past 250 kyr according to a calculated timescale we find that climate instability was not confined to the last glaciation but appears also to have been marked during the last interglacial as explored more fully in a companion paper3 and during the previous saale holstein glacial cycle this is in contrast with the extreme stability of the holocene suggesting that recent climate stability may be the exception rather than the rule the last interglacial seems to have lasted longer than is implied by the deep sea specmap record4 in agreement with other land based observations5 6 we suggest that climate instability in the early part of the last interglacial may have delayed the melting of the saalean ice sheets in america and eurasia perhaps accounting for this discrepancy

the first greenland ice core record of methanesulfonate and sulfate over a full glacial cycle methanesulfonate msa in ice cores has attracted attention as a possible tracer of past oceanic emissions of dimethylsulfide dms after sulfate msa is the second most prevalent aerosol oxidation product of dms but in contrast to sulfate dms oxidation is the only known source of msa the hypothesis by charlson et al 1987 of a climate feedback mechanism with sulfur emissions from marine phytoplankton influencing the cloud albedo adds to the interest in establishing long records of msa and non seasalt sulfate spanning large climatic changes records of msa and non seasalt sulfate covering time periods from a few years to thousands of year have been extracted from antarctic ice cores ivey et al 1986 saigne and legrand 1987 legrand and feniet saigne 1991 mulvaney et al 1992 but only the record from the vostok ice core legrand et al 1991 covers a full glacial cycle the concentrations of msa and non seasalt sulfate in antarctica have been found to increase under glacial conditions here we present the first northern hemisphere record of msa and the first continuous record of non seasalt sulfate both extracted from the renland ice core east greenland the records are extending from the holocene to the eem interglacial 130 000 years b p the contrast to the southern hemisphere records is striking with a decreasing concentration of msa with the advance of glaciation but an increasing concentration of non seasalt sulfate a strong linear relationship is found in the renland ice core between the ratio of msa to non seasalt sulfate and the temperature with higher ratios associated with warmer climatic stages while the opposite relationship to temperature is found in the vostok ice core a more complicated picture is emerging of the use of msa in ice cores as a quantitative tracer which suggests that previous interpretations can have been overly simplistic

effect of fine scale microstructures in titanohematite on the acquisition and stability of natural remanent magnetization in granulite facies metamorphic rocks southwest sweden implications for crustal magnetism mid proterozoic granulites in sw sweden having opaque minerals hematiteilmenite with minor magnetite and occurring in an area with negative aeromagnetic anomalies have strong and stable reversed natural remanent magnetization 9 2 a m with 100 remaining after demagnetization to 100 mt samples were characterized by optical microscopy electron microprobe emp transmission electron microscopy tem and rock magnetic measurements earliest oxide equilibrium was between grains of titanohematite and ferri ilmenite at 650 600 c initial contacts were modified by many exsolution cycles hematite and ilmenite ilm hosts and lamellae by emp are ilm 24 25 ilm 88 93 like titanohematite and ilmenite above 520 c on burton s diagram 1991 finer hosts and lamellae by tem are ilm16 3 and ilm 88 4 like coexisting antiferromagnetically ordered af hematite and ilmenite below 520 c on burton s diagram this may be the first example of analytical identification in one sample of former hematite now finely exsolved and af hematite tem microstructures consist of gently curving semicoherent ilmenite lamellae within hematite flanked by precipitate free zones and abundant ilmenite disks down to unit cell scale 1 2 nm strain contrast of disks suggests full coherence with the host and probable formation at the reaction titanohematite af hematite ilmenite at 520 c magnetic properties are a consequence of chemical and magnetic evolution of hematite and ilmenite with bulk compositions ilmenite richer than ilm 28 that apparently exsolved without becoming magnetized down to 520 c where hematite broke down to af hematite plus ilmenite producing abundant af hematite below its neel temperature intensity of magnetization is greater than possible with hematite alone and tem work suggests that ultrafine ilmenite disks in af hematite are associated with a ferrimagnetic moment due to local imbalance of up and down spins at coherent interfaces

rock magnetic investigation of possible sources of the bangui magnetic anomaly the bangui magnetic anomaly bma is the largest lithospheric magnetic field anomaly on earth at low latitudes previous studies investigated its geological source using constraints from satellite and ground magnetic field measurements as well as from surface magnetic susceptibility measurements on rocks from the panafrican mobile belt zone pmbz here we combine magnetic field data modelling and rock magnetic property measurements susceptibility and natural remanent magnetization nrm on many samples from this pmbz and the surrounding formations it reveals that nrm is a significant component of the total magnetization mt of the bma source which reaches 4 3 a m with maximum thicknesses of 38 and 54 km beneath the western and eastern parts of the bma only the isolated and relatively thin banded iron formations and some migmatites show such mt values thus we suggest that the thick bma source may be composed either by overlapped slices of such metamorphic rocks or by an iron rich mafic source or by a combination of these two geological structures

on the driving processes of the atlantic meridional overturning circulation because of its relevance for the global climate the atlantic meridional overturning circulation amoc has been a major research focus for many years yet the question of which physical mechanisms ultimately drive the amoc in the sense of providing its energy supply remains a matter of controversy here we review both observational data and model results concerning the two main candidates vertical mixing processes in the ocean s interior and wind induced ekman upwelling in the southern ocean in distinction to the energy source we also discuss the role of surface heat and freshwater fluxes which influence the volume transport of the meridional overturning circulation and shape its spatial circulation pattern without actually supplying energy to the overturning itself in steady state we conclude that both wind driven upwelling and vertical mixing are likely contributing to driving the observed circulation to quantify their respective contributions future research needs to address some open questions which we outline

grain scale processes folding and stratigraphic disturbance in the gisp2 ice core flow disturbances have been shown to alter stratigraphic order in the lower part of the ice sheet in central greenland vertical thin sections of the greenland ice sheet project 2 ice core show that in the lower 30 the expected c axis vertical fabric is interrupted by planes of grains stripes with c axes oriented approximately in the dip directions of the planes stripe parallel shear produces small scale folds the stripes can be explained qualitatively by a simple nucleation and growth model based on the strong anisotropy of ice crystal deformation nucleation probably is sensitive to spatially variable rates of polygonization producing spatially variable stripe densities stripes are modeled to affect the ice viscosity so variations in stripe density may contribute to viscosity contrasts that might produce larger deformational features and loss of stratigraphic order

the eem stable isotope record along the grip ice core and its interpretation a 3029 m long deep ice core extending nearly to bedrock has been drilled at the very top of the greenland ice sheet summit by the greenland ice core project grip an international european joint effort organized by the european science foundation the ice core reaches back to 250 000 yr b p according to dating based partly on stratigraphic methods and partly on ice flow modeling a continuous and detailed stable isotope 18o profile along the entire core depicts dramatic temperature changes in greenland through the last two glacial cycles including abrupt climatic shifts during the eem sangamon interglaciation which is elsewhere recorded as a warm and stable period the stratigraphic continuity of the eemian layers has therefore been scrutinized new ice core studies comprising cloudy band observations deconvolution and frequency analyses lead to the conclusion that the climate instability suggested during the eem interglaciation in greenland is likely to be real though no conclusive evidence is available whereas latitudinal displacements of the north atlantic ocean current are considered the immediate cause of the glacial climate instability longitudinal displacements may be the immediate cause of the eemian instability if so the eemian climate changes will be much subdued outside the arctic region and will probably only be recognizable in sedimentary sequences of high sensitivity and temporal resolution

extending comprehensive models of the earth s magnetic field with orsted and champ data summary a new model of the quiet time near earth magnetic field has been derived using a comprehensive approach which includes not only pogo and magsat satellite data but also data from the orsted and champ satellites the resulting model shows great improvement over its predecessors in terms of completeness of sources time span and noise reduction in parameters with its well separated fields and extended time domain of 1960 to mid 2002 the model is able to detect the known sequence of geomagnetic jerks within this frame and gives evidence for an event of interest around 1997 because all sources are coestimated in a comprehensive approach intriguing north south features typically filtered out with other methods are being discovered in the lithospheric representation of the model such as the s atlantic spreading ridge and andean subduction zone lineations in addition this lithospheric field exhibits significantly less noise than previous models as a result of improved data selection the f region currents through which the satellites pass are now treated as lying within meridional planes as opposed to being purely radial results are consistent with those found previously for magsat but an analysis at orsted altitude shows exciting evidence that the meridional currents associated with the equatorial electrojet likely close beneath the satellite besides the model a new analysis technique has been developed to infer the portion of a model parameter state resolved by a particular data subset this has proven very useful in diagnosing the cause of peculiar artefacts in the magsat vector data which seem to suggest the presence of a small misalignment bias in the vector magnetometer

geothermal heating and its influence on the meridional overturning circulation the effect of geothermal heating on the meridional overturning circulation is examined using an idealized coarse resolution ocean general circulation model this heating is parameterized as a spatially uniform heat flux of 50 mw m 2 through the flat ocean floor in contrast with previous studies that have considered an isolated hotspot or a series of plumes along the mid atlantic ridge the equilibrated response is largely advective a deep perturbation of the meridional overturning cell on the order of several sv is produced connecting with an upper level circulation at high latitudes allowing the additional heat to be released to the atmosphere rising motion in the perturbation deep cell is concentrated near the equator the upward penetration of this cell is limited by the thermocline analogous to the role of the stratosphere in limiting the upward penetration of convective plumes in the atmosphere the magnitude of the advective response is inversely proportional to the deep stratification with a weaker background meridional overturning circulation and a less stratified abyss the overturning maximum of the perturbation deep cell is increased this advective response also cools the low latitude thermocline the qualitative behavior is similar in both a single hemisphere and double hemisphere configuration the anomalous circulation driven by geothermal fluxes is more substantial than previously thought we are able to understand the structure and strength of the response in the idealized geometry and further extend these ideas to explain the results of adcroft et al 2001 where the impact of geothermal heating was examined using a global configuration

electromagnetic geophysics and global tectonics lateral variations of electrical conductivity in the lithosphere and below it can be informative concerning both contemporary and ancient global tectonics the methods of investigation include classical four electrode resistivity depth probing which is of special value in continental shields magnetotellurics long used on land and recently in the pacific to show that in electrical terms the lithosphere is thicker near an oceanic ridge than had been expected and magnetometer arrays which are useful primarily in mapping highly conductive structures examples of the use of all three methods will be reviewed with emphasis on the last in conjunction with other geophysical geological and geochemical data magnetometer arrays have been useful in mapping partial melt under the western united states in discovering an extension of the southern end of the african rift system to the atlantic coast in detecting a major fracture zone in central north america which may be a proterozoic interplate collision feature and in discovering a conductive belt under southernmost africa which may result from accumulation of serpentinized marine crustal rocks at the top of a subduction under proterozoic gondwanaland inherent difficulties of quantitative modeling limit the utility of magnetometer arrays largely to discovery and mapping of two and three dimensional conductive structures this remains valuable however particularly when other physical properties can be used to limit the range of possible interpretations of the conductive structures recent array studies now in progress in western canada have defined two new conductivity anomalies one of these may represent a local partial melt concentration under the rocky mountains trench the other a precambrian rift valley in the lower crust previously suggested by others on the basis of seismological gravity and static magnetic data both are being investigated by more concentrated arrays

characterization of sulfide deposits by airborne magnetic and gamma ray responses in eastern finland abstract integrated interpretation of aerogeophysical data permits geophysical characterization of sulfide deposits in eastern finland the structural control and the hydrothermal alteration of the rocks can be discerned from airborne magnetic and radiometric data the cu co zn deposits associated with serpentinite dolomite calc silicate rock siliceous rock black shale settings of the outokumpu type are locally characterized by reduced intensity of magnetization and electrical conductivity and reduced eth and k radiation enhanced eu eth anomalies are commonly spatially related to major structural lineaments such as the boundaries of structurally more coherent bedrock blocks or intersections of faults and shear zones electromagnetic surveys can be used to distinguish black shales and serpentinites in the region the sulfide deposits in eastern finland can be divided into two groups on the basis of airborne geophysical characteristics 1 intense magnetization and high electrical conductivity and 2 moderate magnetization and electrical conductivity in the sulfide deposits of group 1 ore forming constituents mainly precipitated during primary sedimentation for example the black shale hosted ni cu zn ore at talvivaara group 2 is related to sulfide deposits where major ore forming processes have affected the associated black shale formation after the sedimentation phase outokumpu type cu co zn deposits in cases where the magnetization and electrical conductivity of black shales are especially weak ore formation processes probably continued during late metamorphic or post metamorphic deformation the theoretical basis for the observed differences in magnetic response between different deposits has to do with the mineralogy of the black shales pyrite changes into ferrimagnetic monoclinic pyrrhotite during metamorphism above greenschist facies especially along tectonic structures if no further alteration occurs pyrrhotite bearing black shale formations appear as highly magnetic and electrically conductive zones on geophysical maps wherever intense tectonic deformation emplacement of quartz sulfide veins hydrothermal alteration or contact metamorphism has affected the black shale formation after sedimentation both magnetic and electromagnetic responses may be weakened because of diminished amounts of the pyrrhotite bearing and graphite rich material in addition the presence of antiferromagnetic pyrrhotite of hexagonal crystal structure may decrease the magnetic response the classification presented here applies specifically to eastern finland but may be widely applicable to terranes with abundant black shale formations that have undergone regional metamorphism of upper greenschist facies or higher association of sulfide ores with black shales is a common feature in the geological record our integrated geophysical method offers a quick way of examining large areas and it should prove useful in the search for sulfide deposits in other parts of the world

characterization of double diffusive convection steps and heat budget in the deep arctic ocean 1 in this paper we explore the hydrographic structure and heat budget in the deep canada basin by using data measured with mclane moored profilers mmp bottom pressure recorders bpr and conductivity temperature depth ctd profilers upward from the bottom a homogeneous bottom layer and its overlaying double diffusive convection ddc steps are well identified at mooring a 75 n 150 w we find that the deep water is in weak diapycnal mixing because the effective diffusivity of the bottom layer is 1 8 10 5 m2s 1 while that of the other steps is 10 6 m2s 1 the vertical heat flux through the ddc steps is evaluated by using different methods we find that the heat flux 0 1 11 mwm 2 is much smaller than geothermal heating 50 mwm 2 this suggests that the stack of ddc steps acts as a thermal barrier in the deep basin moreover the temporal distributions of temperature and salinity differences across the interface are exponential whereas those of heat flux and effective diffusivity are found to be approximately lognormal both are the result of strong intermittency between 2003 and 2011 temperature fluctuations close to the sea floor were distributed asymmetrically and skewed toward positive values which provide a direct observation that geothermal heating was transferred into the ocean both bpr and ctd data suggest that geothermal heating and not the warming of the upper ocean is the dominant mechanism responsible for the warming of deep water as the ddc steps prevent vertical heat transfer geothermal heating is unlikely to have a significant effect on the middle and upper arctic ocean

an integrated geophysical study of the beattie magnetic anomaly south africa abstract the source of the beattie magnetic anomaly bma still remains unclear with several competing hypotheses here we add a piece to the puzzle by investigating available potential field data over the anomaly filtered magnetic data show the bma as part of a group of linear magnetic anomalies as the linear anomaly north of the bma is associated with exposed supracrustals migmatites and shear zones within the natal thrust terranes we assume a similar source for the bma this source geometry constrained by seismic and mt data fits potential field data over the bma and other magnetic linear anomalies in the south central and south western karoo in these models the bodies deepen from 5 km towards the south with horizontal extents of 20 60 km and vertical extents of 10 15 km densities range from 2800 to 2940 kg m 3 and magnetic susceptibilities from 10 to 100 10 3 si these magnetic susceptibilities are higher than field values from supracrustal rocks 10 60 10 3 si but could be due to the fact that no remanent magnetisation was included in the model the lithologies associated with the different linear anomalies vary as is evident from varying anomaly amplitudes the strong signal of the bma is linked to high magnetic susceptibility granulite facies supracrustals 10 50 10 3 si as seen in the antarctic where the mobile belt continued during gondwana times