effect of iron content on electrical conductivity of ringwoodite with implications for electrical structure in the transition zone abstract electrical conductivity of ringwoodite with various iron contents fe fe mg 0 09 0 2 and 0 3 was measured at pressure 20 gpa and temperature up to 1900 k conditions of the lower part of the mantle transition zone in a kawai type multi anvil apparatus the conductivity increased with increasing total iron content all electrical conductivity data were fitted to the formula of electrical conductivity 0 xfe exp h kt where 0 is the pre exponential term xfe is the mole fraction of iron content in the mg site h is the activation enthalpy k is the boltzmann constant and t is absolute temperature the activation enthalpy becomes higher at a certain temperature at high temperatures the activation enthalpy decreased from 1 44 to 0 92 ev with increasing total fe content at low temperatures less than 1000 k the activation enthalpy also decreases from 1 15 to 0 74 ev with total fe content dependence of the activation enthalpy on fe content suggests that the dominant mechanism of charge transport is fe2 fe3 hopping small polaron recent electrical conductivity depth profiles of the transition zone beneath the pacific ocean obtained from the electromagnetic induction study shows that the conductivity values between 520 and 660 km depths may be explained by ringwoodite with fe fe mg 0 10 on the other hand assuming a normal geotherm conductivity values beneath the continent or stable craton are considerably lower than those of ringwoodite with fe fe mg 0 10 taking into consideration results from the global seismic tomographic studies relatively low conductivity in these regions can be explained by the existence of a cooler region compared with the surrounding mantle rather than the presence of iron poor ringwoodite or a combination of both

geomagnetic variation anomalies and deflection of telluric currents summary we show that most of the abnormal variations of the transient magnetic field have deflection and canalization of telluric currents as main sources in order to do so we develop a formalism which is valid in a limited frequency range which satisfies the following hypotheses a the thickness of the laterally inhomogeneous upper layer is small with respect to the penetration depth of the transient field b self induction of the heterogeneities of conductivity can be neglected theoretical results and observations performed at mid latitudes are in good agreement the main result we establish is that deviation and canalization of the current responsible for the transient abnormal field do not affect their temporal variations the study of temporal variations of the abnormal field thus provides us with information about the structure of the crust in the region where the deviated and canalized currents are induced

on the heterogeneous electrical conductivity structure of the earth s mantle with implications for transition zone water content 1 we have investigated the lateral variations in mantle electrical conductivity structure using electromagnetic sounding data for this purpose we used very long time series up to 51 years of geomagnetic observatory data at six locations encompassing different geological settings to compute response functions that cover the broadest possible frequency range 3 9 to 95 2 days furstenfeldbruck fur europe hermanus her south africa langzhou lzh china alice springs asp australia tucson tuc united states north america and honolulu hon united states north pacific we inverted the response functions beneath each observatory for a local radial conductivity profile using a stochastic sampling algorithm specifically we found significant lateral variations in conductivity throughout the mantle with resolution limited to the depth range 500 1200 km at 600 km depth conductivity varies between 0 1 and 0 4 s m and increases to 1 3 2 0 s m at 800 km depth beneath all stations except her 0 5 s m at 900 km depth conductivity increases further to 1 4 2 4 s m with her hon and asp being most conductive this trend persists to a depth of 1200 km comparison with conductivity profiles constructed from laboratory measurements of mantle mineral conductivities and models of earth s mantle composition and thermal state reveal that significant thermochemical variations are at the origin of the observed heterogeneities in mantle conductivity found here because of the somewhat large error bounds on sampled conductivity profiles and the reduced sensitivity of the electromagnetic sounding data above 500 km depth constraints on transition zone water content are less conclusive although h2o contents 0 5 wt in the midtransition zone appear less likely

continental scale deep electrical resistivity structure beneath china abstract electrical resistivity structure in the deep earth can provide important information and constraints on regional geodynamics especially in regions with complex tectonics and geodynamical processes in this study we image the electrical resistivity structure in the depth range of 350 1200 km beneath china by inverting the frequency dependent ratios of geomagnetic field components at a relatively dense network of geomagnetic observatories very long time series of hourly mean values of the three component geomagnetic field from 45 geomagnetic observatories are collected and processed to derive robust ratios of the geomagnetic components which are then converted to the apparent resistivities and impedance phases in the period range of 5 3 113 8 days and used in 1 d occam inversion with smoothing constraint and the method to obtain the electrical resistivity profiles and delta function models at individual observatories 1 d electrical resistivity models suggest that low electrical resistivity structure exists in the mantle transition zone beneath eastern china high water content in the mantle minerals due to the subduction of seismically fast pacific slab is proposed as the main reason for the low electrical resistivity beneath eastern china our results provide further evidence that the western front of the subducting pacific slab in the mantle transition zone roughly coincides with the abrupt change in the surface topography in eastern china

shear at the base of the oceanic mixed layer generated by wind shear alignment observations are used to evaluate a simple theoretical model for the generation of near inertial shear spikes at the base of the open ocean mixed layer when the upper ocean displays a two layer structure the model predicts that large changes in shear squared can be produced by the alignment of the wind and shear vectors a climatology of stratification and shear variance in drake passage is presented which shows that these assumptions are most applicable to summer fall and spring but are not highly applicable to winter temperature salinity and velocity data from a high spatial resolution cruise in drake passage show that the model does not predict all large changes in shear variance the model is most effective at predicting changes in shear squared when it arises owing to near inertial wind driven currents without requiring a rotating resonant wind stress the model is also more effective where there is a uniform mixed layer above a strongly stratified transition layer rotary spectral and statistical analysis of an additional 242 drake passage transects from 1999 to 2011 confirmed the presence of this shear spiking mechanism particularly in summer spring and fall when stratification is stronger

conductivity and fluids in the oceanic upper mantle abstract three deep electromagnetic soundings in the north pacific ocean were re examined our inversion of existing data showed the presence of conductors below the three sites in very good agreement with previous results we showed that h 2 o rich fluids as well as partial melting account for the conductivities observed this led us to determine temperatures ranging firstly from 1050 to 1150 c when only h 2 o rich fluids were present and secondly from 1300 to 1350 c for partial melting only we suggest that the upper mantle below the two oldest sites 30 and 72 ma is characterized by about 1 of partial melting in the presence of about 1 of h 2 o rich fluids below the third site that nearest to the ridge zone 1 ma the degree of partial melting might reach 10 the presence of 1 of h 2 o rich fluids would reduce the effect of the asthenospheric outflows on the lithospheric magmatism this agrees with the principle of thermodynamic decoupling of the asthenosphere and lithosphere the weakening effect of fluids particularly water on upper mantle rheology is also discussed we suggest that the asthenosphere and lithosphere might be decoupled mechanically which if true would affect small scale convection below the oceanic lithosphere

deuterium hydrogen exchange in olivine implications for point defects and electrical conductivity 1 knowledge about hydrogen self diffusion dh is critical for determining mantle hydrogen distribution and understanding point defects also chemical diffusion of hydrogen in olivine such as redox exchange with polarons dredox depends on dh in this study deuterium 2h was exchanged into hydrogen 1h saturated single crystals of san carlos olivine between 750 and 900 c at 2 gpa we measured and fit the resulting 2h profiles to obtain dh 100 10 4 9 1 4 e 140 30kj mol rt m2 s which is 1 log unit lower than dredox 100 with similar activation enthalpy ha by comparing these two diffusion coefficients we estimate the small polaron diffusion coefficient additionally we estimate dh in the 010 and 001 orientations demonstrating that dh is highly anisotropic in olivine these dhvalues were used with the nernst einstein relation to estimate the electrical conductivity by hydrogen in olivine h 101 1 e 130kj mol rt s m for 10 2 wt h2o that is lower in magnitude than previous measurements our results suggest that hydrogen alone cannot account for high electrical conductivity anomalies observed at asthenospheric depths 10 2 to 10 1 s m the maximum anisotropic variation of dh and hin olivine is 2 log units between 750 and 900 c and increases when extrapolated to higher temperature 3 3 at 1400 c anisotropy observed in the mantle may indicate substantial amounts of hydrogen in olivine with lattice preferred orientation

stabilities and equations of state of dense hydrous magnesium silicates abstract the thermodynamic stability of high pressure phases in the ternary system mgo sio2 h2o is reviewed with special emphasis on hydrated phases including the alphabet phases on the basis of recent experimental data a stability diagram for these phases along a geotherm appropriate for a subducting slab is constructed this suggests that at realistic water contents the breakdown of antigorite will produce olivine plus phase a in the upper mantle but that the ability of wadsleyite to accommodate wt levels of water means that phases e and d are unlikely to occur in the upper portion of the transition zone for the lower part of the transition zone in which ringwoodite becomes stable the coexisting hydrous phase would be superhydrous phase b provided ringwoodite is not able to accommodate significant amounts of water at the top of the lower mantle phase d is stable but only at temperatures below 1300 c as the maximum solubility of h2o in magnesium silicate perovskite is also quite low slabs may dehydrate somewhat as they enter the lower mantle the available thermodynamic data for these phases is also reviewed from the most recent measurements of equations of state it is concluded that the elasticity of the phases in the mgo sio2 h2o ternary system is primarily dependent upon the density and not upon the water content it is therefore concluded that the presence of even significant amounts of hydrated phases in subducting slabs could not be unequivocally identified from seismological observations

the energy flux from the wind to near inertial motions in the surface mixed layer abstract time series of wind stress computed from long term meteorological buoy data off north america are used to examine the forcing of surface inertial currents by the wind a simple damped slab model of the mixed layer is used to compute h the average flux of energy from the wind to mixed layer inertial currents in a mixed layer of fixed depth h the forcing of mixed layer inertial motions is highly intermittent most of the forcing occurs during the winter months with a few dozen events accounting for typically over half the total energy flux major forcing events are usually associated with translating cold fronts or small lows with scales of about 100 km the larger synoptic scale features have little energy at the inertial frequency and thus result in only weak forcing of inertial currents a strong seasonal signal exists in the inertial forcing at ows p 50 n 145 w 50 m is largest from october to february and is a factor of 12 above the june and july values if seasonally varying

surface mixed layer deepening through wind shear alignment in a seasonally stratified shallow sea inertial oscillations are a ubiquitous feature of the surface ocean here we combine new observations with a numerical model to investigate the role of inertial oscillations in driving deepening of the surface mixed layer in a seasonally stratified sea observations of temperature and current structure from a mooring in the western irish sea reveal episodes of strong currents 0 3 m s 1 lasting several days resulting in enhanced shear across the thermocline while the episodes of strong currents are coincident with windy periods the variance in the shear is not directly related to the wind stress the shear varies on a subinertial time scale with the formation of shear maxima lasting several hours occurring at the local inertial period of 14 85 h these shear maxima coincide with the orientation of the surface current being at an angle of approximately 90 to the right of the wind direction observations of the water column structure during windy periods reveal deepening of the surface mixed layer in a series of steps which coincide with a period of enhanced shear during the periods of enhanced shear gradient richardson number estimates indicate ri 1 4 at the base of the surface mixed layer implying the deepening as a result of shear instability a one dimensional vertical exchange model successfully reproduces the magnitude and phase of the shear spikes as well as the step like deepening the observations and model results therefore identify the role of wind shear alignment as a key entrainment mechanism driving surface mixed layer deepening in a shallow seasonally stratified sea

construction and destruction of the north china craton with implications for metallogeny magnetotelluric evidence from the hengshan wutai fuping region within trans north china orogen abstract the hengshan wutai fuping region within the trans north china orogen is an important corridor that preserves the records of precambrian evolutionary history and phanerozoic magmatic and metallogenic events in the north china craton here we present 2 d and 3 d inversion models of magnetotelluric data collected from the hengshan wutai fuping region and surrounding regions our data reveal nearly horizontal high conductivity layer at lower crustal and upper mantle depths of 35 45 km beneath the hengshan and wutai mountains providing direct evidence for crust mantle decoupling and magmatic underplating in the mesozoic yixingzhai gold deposits and boqiang cu mo au deposits two nearly upright conductors cut through the crust and connect the lower crustal high conductivity layer suggesting frozen channels of fluids and magmas a west dipping slab shaped conductor within the high resistivity lithosphere beneath the wutai complex appears to be the deep extension of the exposed longquanguan ductile shear zone these resistivity anomalies can provide new constraints on the subduction collision events during precambrian tectonic evolution of the trans north china orogen as well as on the phanerozoic tectonic magmatic and metallogenic events in the hengshan wutai fuping region the west dipping slab shaped conductor may indicate a westward oceanic subduction between the wutai and fuping complexes with subsequent collision associated with the formation of the trans north china orogen in the paleoproterozoic we interpret the crustal and upper mantle conductors to be part of an interconnected fluid melt transport channel which probably played an important role for the mesozoic tectonic magmatic and metallogenic events as well as the cenozoic basaltic volcanism in the hengshan wutai fuping region

remote sensing of hydrogen in earth s mantle hydrogen1 in earth s interior is known to play a key role in a number of processes consequently inferring the distribution of hydrogen is a critical step in our study of dynamics and evolution of earth usually hydrogen distribution is inferred from two types of samples at the surface first a magma may contain hydrogen water and under some conditions the water content of the magma can be quenched upon cooling in these cases measurements of the water content of the magma provide us with some constraints on the hydrogen water content of the source region if we know the partitioning of hydrogen between magmas and the source rocks and the degree of melting second hydrogen content of some xenoliths transported by magma can be measured they provide a direct clue as to the hydrogen content in a region from which a xenolith has been carried however these direct petrologic approaches have two major problems firstly the sampling is limited by the distribution of volcanoes and even if there are volcanoes that carry rocks from earth s interior the depth extent that volcanoes sample rocks is limited usually 200 km secondly there is no guarantee that the hydrogen content that one measures on these samples actually represents the hydrogen content in a region where these samples came from for example hydrogen is known to diffuse very easily so hydrogen dissolved in minerals could diffuse out during the transport of a rock or conversely a piece of rock may acquire extra hydrogen during its ascent also hydrogen atoms dissolved in minerals may precipitate in the mineral to form fluid inclusions or micro scale hydrous minerals in summary the direct method to infer the distribution of hydrogen from rock samples has major limitations and an alternative approach i e remote sensing hydrogen content from geophysical

an estimate of average lower mantle conductivity by wavelet analysis of geomagnetic jerks it has recently been proposed that geomagnetic jerks observed at the earth s surface could be viewed as singularities in the time behavior of the geomagnetic field with a regularity of about 1 5 when wavelet analyzed such a signal should have suffered some distortion when diffusing from the core mantle boundary cmb through the conducting mantle assuming that the upper mantle is an insulator and given the electromagnetic time constant of the mantle we compute the distortion that a pure singularity introduced at the cmb suffers as it traverses the mantle we compute this distortion through its effects on the so called ridge functions extracted from the wavelet transform of the signal this distortion is very similar to the small but significant one that we observe in real data we therefore speculate that jerks must have been pure singularities at the base of the mantle and infer an average estimate for the mantle electromagnetic time constant from the way the signal is distorted by fitting the synthetic ridge functions to the experimental ones assuming for example a thickness of 2000 km for a uniform lower conducting mantle we find an electrical conductivity smaller than 10 s m 1 this value is in reasonable agreement with values derived from high pressure experiments for a silicate mantle

contribution at satellite altitude of electromagnetically induced anomalies arising from a three dimensional heterogeneously conducting earth using sq as an inducing source field summary the induced magnetic signature at satellite altitude associated with crustal and mantle electrical heterogeneities is examined using five different earth models the three dimensional induction problem is solved in these models using the solar quiet daily variation as an inducing source field every model has an upper shell representing the land ocean electrical conductivity distribution the different models have either a homogeneous mantle or a heterogeneous mantle containing either subduction zones a ridge system or conductive blocks electromagnetic anomalies presented in the form of geographical maps are calculated for wavelengths related to spherical harmonics of degrees in the ranges 1 12 and 13 27 respectively magnetic anomalies are calculated at an altitude of 400 km and for three different local times 6 12 and 18 h

model of the geoelectrical structure of the mid and lower mantle in the europe asia region summary the conductivity structure of the earth s mantle was estimated using the induction method down to 2100 km depth for the europe asia region for this purpose the responses obtained at seven geomagnetic observatories irt kiv mos nvs hlp wit and ngk were analysed together with reliable published results for 11 yr variations 1 d spherical modelling has shown that beneath the mid mantle conductive layer 600 800 km the conductivity increases slowly from about 1 s m 1 at 1000 km depth to 10 s m 1 at 1900 km while further down 1900 2100 km this increase is faster published models of the lower mantle conductivity obtained using the secular 30 60 yr variations were also considered in order to estimate the conductivity at depths down to the core the new regional model of the lower mantle conductivity does not contradict most modern geoelectrical sounding results this model supports the idea that the mantle base situated below 2100 km depth has a very high conductivity

momentum balance of the wind driven and meridional overturning circulation abstractwhen a force is applied to the ocean fluid parcels are accelerated both locally by the applied force and nonlocally by the pressure gradient forces established to maintain continuity and satisfy the kinematic boundary condition the net acceleration can be represented through a rotational force in the rotational component of the momentum equation this approach elucidates the correspondence between momentum and vorticity descriptions of the large scale ocean circulation if two terms balance pointwise in the rotational momentum equation then the equivalent two terms balance pointwise in the vorticity equation the utility of the approach is illustrated for three classical problems barotropic rossby waves wind driven circulation in a homogeneous basin and the meridional overturning circulation in an interhemispheric basin in the hydrostatic limit it is shown that the rotational forces further decompose into depth integrated forces that drive the wind driven gyres and overturning forces

p wave tomography of the mantle under the alpine mediterranean area 1 we study the upper mantle p wave velocity structure below the euro mediterranean area down to 1000 km depth by seismic travel time tomography we invert summary residuals constructed with both regional and teleseismic first arrival data reported by the international seismological centre isc 1964 1995 introducing some alternative strategies in the travel time tomographic approach and a new scheme to correct teleseismic data for global mantle structure our high resolution model pm0 5 is parameterized with three dimensional 3 d linear splines on a grid of nodes with 0 5 spacing in both horizontal directions and 50 km vertical spacing we obtain about 26 root mean square rms reduction of residuals by inversion in addition to roughly 31 reduction after summary rays formation and selection sensitivity analyses are performed through several test inversions to explore the resolution characteristics of the model at different spatial scales the distribution of large scale fast anomalies suggests that two different stages of a convection process presently coexist in very close regions the mantle dynamics of western central europe is dominated by blockage of subducted slabs at the 660 km discontinuity and ponding of seismically fast material in the transition zone contrarily in the eastern mediterranean fast velocity material sinks into the lower mantle suggesting that the flow of the cold downwelling here is not blocked by the 660 km discontinuity on a smaller scale the existence of tears in the subducted slab lithospheric detachment all along both margins of the adriatic plate as proposed by some authors is not supported by our tomographic images

evolution of oceanic near surface stratification in response to an autumn storm abstractunderstanding the processes that control the evolution of the ocean surface boundary layer osbl is a prerequisite for obtaining accurate simulations of air sea fluxes of heat and trace ga

near inertial kinetic energy budget of the mixed layer and shear evolution in the transition layer in the arabian sea during the monsoons we present the horizontal kinetic energy ke balance of near inertial currents in the mixed layer and explain shear evolution in the transition layer using observations from a mooring at 15 26 n in the arabian sea during the southwest monsoon the highly sheared and stratified transition layer at the mixed layer base varies between 5 m and 35 m and correlates negatively with the wind stress results from the mixed layer near inertial ke nike balance suggest that wind energy at times can energize the transition layer and at other times is fully utilized within the mixed layer a simple two layer model is utilized to study the shear evolution in the transition layer and shown to match well with observations the shear production in this model arises from alignment of wind stress and shear although the winds are unidirectional during the monsoon the shear in the transition layer is predominantly near inertial the near inertial shear bursts in the observations show the same phasing and magnitude at near inertial frequencies as the wind shear alignment term

conductivity discontinuities in the upper mantle beneath a stable craton we present evidence for approximate collocation of seismic and electrical transitions in the upper mantle more than two years of very long period magnetotelluric mt data were recorded at a lakebottom observatory in the central canadian shield after processing to contend with non stationary source effects and removal of galvanic distortion the underlying structure is 1d for periods of one hour to four days the response was extended to periods of 100 days by appending geomagnetic depth sounding data to the mt curves minimum structure linearised inversion nonlinear extremal inversion and a new genetic algorithm for nonlinear hypothesis testing reveal discrete jumps in conductivity at depths near the major upper mantle seismic discontinuities the jumps occur over limited depth ranges

seismic discontinuities and subduction zones abstract the seismic discontinuities at 410 and 660 km depth in the mantle are generally believed to be due to phase transformations in its major component olivine an increasing observational base of short period regional seismic network data supports this view here we analyse cross sections of discontinuity topography in the northwest pacific subduction zones to infer the thermal and chemical state of the mantle in a subduction environment most of the data is from the southern part of the izu bonin subduction zone penetration of the lower mantle by the steeply dipping subducting slab is clearly indicated by depression of the 660 km discontinuity elevation of the 410 km discontinuity in this region by up to 60 km implies a thermal anomaly of approximately 900 1000 c relative to the ambient mantle observation of the elevated 410 in the seismically active part of the slab indicates that the olivine wadsleyite phase transformation occurs under essentially equilibrium conditions there which argues against the transformational faulting hypothesis as the mechanism for deep earthquakes upgoing p wave reflections from the 410 km discontinuity indicate that it is sharp in the mantle far from subducting slabs reflection coefficient variability in and near slabs indicates either a chemical effect or the consequences of topographic focusing

water in the mantle results from electrical conductivity beneath the french alps 1 a deep magnetotelluric sounding in the french alps provided a vertical electrical conductivity profile between 200 1000 km two prominent features are observed first the conductivity in the depth range 400 800 km is smaller than the conductivity of a pyrolite mantle obtained from laboratory results for a normal geotherm second the data do not require the conductivity to change throughout the transition zone 410 660 km in this part of the mantle a temperature of 350 450 c less than normal explains the magnetotelluric conductivity profile at 200 400 km our model favors a cold mantle with 1000 1500 ppm of water dissolved in olivine if correct this model suggests that the subducted slab is dehydrated before reaching the transition zone

global 3 d imaging of mantle conductivity based on inversion of observatory c responses ii data analysis and results summary the global 3 d electrical conductivity distribution in the mantle in the depth range between 400 and 1600 km is imaged by inverting c responses estimated on a global net of geomagnetic observatories very long time series up to 51 years 1957 2007 of hourly means of three components of the geomagnetic field from 281 geomagnetic observatories are collected and analysed special attention is given to data processing in order to obtain unbiased c responses with trustworthy estimates of experimental errors in the period range from 2 9 to 104 2 d after careful inspection of the obtained c responses the data from 119 observatories are chosen for the further analysis squared coherency is used as a main quality indicator to detect and then to exclude from consideration observatories with a large noise to signal ratio during this analysis we found that along with the c responses from high latitude observatories geomagnetic latitudes higher than 58 the c responses from all low latitude observatories geomagnetic latitudes below 11 also have very low squared coherencies and thus cannot be used for global induction studies we found that the c responses from the selected 119 mid latitude observatories show a huge variability both in real and imaginary parts and we investigated to what extent the ocean effect can explain such a scatter by performing the systematic model calculations we conclude that 1 the variability due to the ocean effect is substantial especially at shorter periods and it is seen for periods up to 40 d or so 2 the imaginary part of the c responses is to a larger extent influenced by the oceans 3 two types of anomalous c response behaviour associated with the ocean effect can be distinguished 4 to accurately reproduce the ocean effect a lateral resolution of 1 1 of the conductance distribution is needed and 5 the ocean effect alone does not explain the whole variability of the observed c responses we also detected that part of the variability in the real part of the c responses is due to the auroral effect in addition we discovered that the auroral effect in the c responses reveals strong longitudinal variability at least in the northern hemisphere europe appears to be the region with smallest degree of distortion compared with north america and northern asia we found that the imaginary part of the c responses is weakly affected by the auroral source thus confirming the fact that in the considered period range the electromagnetic em induction from the auroral electrojet is small assuming weak dependence of the auroral signals on the earth s conductivity at considered periods and longitudinal variability of the auroral effect we developed a scheme to correct the experimental c responses for this effect with these developments and findings in mind we performed a number of regularized 3 d inversions of our experimental data in order to detect robust features in the recovered 3 d conductivity images although differing in details all our 3 d inversions reveal a substantial level of lateral heterogeneity in the mantle at the depths between 410 and 1600 km conductivity values vary laterally by more than one order of magnitude between resistive and conductive regions the maximum lateral variations of the conductivity have been detected in the layer at depths between 670 and 900 km by comparing our global 3 d results with the results of independent global and semi global 3 d conductivity studies we conclude that 3 d conductivity mantle models produced so far are preliminary as different groups obtain disparate results thus complicating quantitative comparison with seismic tomography or and geodynamic models in spite of this our 3 d em study and most other 3 d em studies reveal at least two robust features reduced conductivity beneath southern europe and northern africa and enhanced conductivity in northeastern china

divalent cation diffusion in mg2sio4 spinel ringwoodite β phase wadsleyite and olivine implications for the electrical conductivity of the mantle mg fe and mg ni interdiffusion coefficients have been measured in single crystal olivine alpha mg ni fe sub 2 sio sub 4 polycrystalline beta phase beta mg ni fe sub 2 sio sub 4 and silicate spinel gamma mg ni fe sub 2 sio sub 4 at 1473 k between 1 and 14 gpa under controlled thermodynamic conditions in olivine d sub mg ni sup alpha ranges from 10 sup 17 m sup 2 s sup 1 at 1 gpa to 4x10 sup 18 m sup 2 s sup 1 at 9 gpa and d sub mg fe sup alpha ranges from 3x10 sup 15 m sup 2 s sup 1 at 1 gpa to 10 sup 15 m sup 2 s sup 1 at 4 gpa at 9 gpa the cation diffusion rates in gamma spinel are approx 3 orders of magnitude higher than those in olivine between 9 and 14 gpa d sub mg ni sup gamma decreases from 8x10 sup 15 to 9x10 sup 16 m sup 2 s sup 1 and d sub mg fe sup gamma decreases from 2x10 sup 13 to 5x10 sup 14 m sup 2 s sup 1 between 10 and 14 gpa the diffusion coefficients for both fe and ni are similar in magnitude and pressure dependence between beta phase and gamma spinel the activation volumes for mg ni diffusion are 3 2 and 6 7 cm3 mol 1 in the alpha and gamma beta phases respectively and for mg fe diffusion are 5 4more and 6 1 cm3 mol 1 in the alpha and gamma beta phases respectively our results demonstrate that extreme differences in transport properties are expected to occur across the 400 km discontinuity a model for electrical conductivity in the upper mantle involving two mechanisms for electrical conduction with one mechanism being associated with a hydrogen or defect related conduction process and the other involving a straightforward application of the nernst einstein equation to our cationic diffusion data reproduces geomagnetic sounding results for the upper mantle and shallow transition zone this modeling yields an order of magnitude increase in electrical conductivity across the 400 km discontinuity with little or no change in conductivity occurring near 520 km depth our modeling suggests that divalent cation diffusion is the dominant mechanism of charge transport between 400 and 670 km depth and that the high pressure olivine polymorphs dominate the electrical behavior of the deep upper mantle comparison between field based deep mantle conductivity profiles derived from magnetotelluric studies and extrapolation of our model suggests that a discontinuity in electrical transport properties exists at the base of the transition zone as well similarly given the dramatic differences in chemical diffusion between different polymorphs of olivine it is possible that other transport property mediated processes such as viscous flow could dramatically change across the 400 and 670 km discontinuities such changes are less likely to occur at 520 km depth c 2000 american geophysical union less

electromagnetic detection of a 410 km deep melt layer in the southwestern united states a deep seated melt or fluid layer on top of the 410 km deep seismic discontinuity in earth s upper mantle as proposed in the transition zone water filter hypothesis may have significant bearing on mantle dynamics and chemical differentiation the geophysical detection of such a layer has however proved difficult magnetotelluric and geomagnetic depth sounding are geophysical methods sensitive to mantle melt here we use these methods to search for a distinct structure near 410 km depth we calculate one dimensional forward models of the response of electrical conductivity depth profiles based on mineral physics studies of the effect of incorporating hydrogen in upper mantle and transition zone minerals these models indicate that a melt layer at 410 km depth is consistent with regional magnetotelluric and geomagnetic depth sounding data from the southwestern united states tucson the 410 km deep melt layer in this model has a conductance of 3 0 x 10 4 s and an estimated thickness of 5 30 km this is the only regional data set that we have examined for which such a melt layer structure was found consistent with regional seismic studies we infer that the hypothesized transition zone water filter occurs regionally but that such a layer is unlikely to be a global feature

generation of bulk shear spikes in shallow stratified tidal seas abstract recent finescale observations of shear and stratification in temperate shelf sea thermoclines show that they are of marginal stability suggesting that episodes of enhanced shear could potentially lead to shear instability and diapcynal mixing the bulk shear between the upper and lower boundary layers in seasonally stratified shelf seas shows remarkable variability on tidal inertial and synoptic time scales that has yet to be explained in this paper observations from the seasonally stratified northern north sea are presented for a time when the water column has a distinct two layer structure bulk shear estimates based on adcp measurements show a bulk shear vector that rotates in a clockwise direction at the local inertial period with episodes of bulk shear spikes that have an approximately twice daily period and occur in bursts that last for several days to explain this observation a simple two layer model based on layer averaging of the one dimensional momentum equation is developed

observed upper ocean response to typhoon megi 2010 in the northern south china sea typhoon megi passed between two subsurface moorings in the northern south china sea in october 2010 and the upper ocean thermal and dynamical response with strong internal tides present was examined in detail the entire observed water column 60 360 m was cooled due to strong ekman pumped upwelling up to 50 m in the thermocline by megi with maximum cooling of 4 2 c occurring in thermocline a relatively weak maximum amplitude of 0 4 m s 1 and quickly damped e folding time scale of 2 inertial periods near inertial oscillation nio was observed in the mixed layer power spectrum and wavelet analyses both indicated an energy peak appearing at exactly the sum frequency fd1 with maximum amplitude up to 0 2 m s 1 of nio f and diurnal tide d1 indicating enhanced nonlinear wave wave interaction between f and d1 during and after typhoon numerical experiments suggested that energy transfer from nio to fd1 via nonlinear interaction between f and d1 may have limited the growth and accelerated the damping of mixed layer nio generated by megi the occurrence of fd1 had a high correlation with nio the vertical nonlinear momentum term associated with the vertical shear of nio and vertical velocity of d1 or vertical shear of d1 and vertical velocity of nio was more than 10 times larger than the horizontal terms and was responsible for forcing fd1 after megi surface layer diurnal energy was enhanced by up to 100 attributed to the combined effect of the increased surface layer stratification and additional megi forced diurnal current

hydrogen diffusivity in wadsleyite and water distribution in the mantle transition zone abstract the kinetics of the hydrogen diffusivity in synthesized polycrystalline wadsleyite was measured by ir spectroscopy in order to determine the diffusion coefficient of hydrogen in wadsleyite the major constituent mineral in the mantle transition zone with the composition of mg0 89fe0 11 2sio4 the hydration experiments were conducted at 15 16 gpa and temperature range from 900 to 1200 c with mg oh 2 as the water source by kawai type multi anvil apparatus the diffusion rate obtained here is considered to be an effective diffusion coefficient with the grain size of 9 m involving contributions both from the lattice diffusion and the grain boundary diffusion the temperature dependence of diffusion of hydrogen in polycrystalline wadsleyite was determined to be dh 9 6 10 6 exp 123 32 kj mol 1 rt at 15 gpa hydrogen diffusion rate in wadsleyite is roughly consistent with the average diffusivity of hydrogen in olivine in recent years several authors have suggested a possibility of the hydrous transition zone and the distribution and transport properties of water are still debated on the basis of water dependence of the electrical conductivity the water content in the mantle transition zone was estimated from the observed conductivity by using the nernst einstein relation the results obtained here show that the distribution of water should be quite heterogeneous throughout the mantle transition zone

observing geomagnetic induction in magnetic satellite measurements and associated implications for mantle conductivity 1 currents induced in earth by temporal variations in the external magnetic field have long been used to probe mantle electrical conductivity but almost exclusively from sparsely distributed land observatories satellite borne magnetometers such as flown on magsat orsted and champ offer the prospect of improved spatial coverage the approach we have taken is to isolate induction by harmonic dst disturbance storm time excitation of the magnetospheric ring current in satellite magnetic measurements this is done by removing the magnetic contributions of the main core magnetic field the crustal magnetic field and ionospheric fields cause of the daily variation using sabaka et al s 2000 2002 cmp3 comprehensive model the dst signal is then clearly evident in the midlatitude satellite passes lower than 50 degrees geomagnetic latitude at higher latitudes auroral and field aligned currents contaminate the data we fit the internal and external components of the dst signal for each equatorial pass exploiting the fact that the geometry for the internal and external components is different for the azimuthal and radial vector components the resulting timeseries of internal and external field variations shows that the dst signals for the dawn passes are half those of the dusk passes the sum of equatorial external and internal components of the field averaged over dawn and dusk passes provides an excellent estimate for the dst index and may in fact be superior when used as a proxy for the purposes of removing induced and magnetospheric fields from satellite magnetic data we call this estimate satellite dst cross spectral analysis of the internal and external timeseries shows both greater power and higher coherence in the dusk data we processed the transfer function between internal and external dusk timeseries to provide globally averaged frequency dependent impedances that agree well with independently derived estimates we estimate earth s radial electrical conductivity structure from these impedances using standard regularized inversion techniques a near surface conductor is required of thickness less than 10 km with a conductivity thickness product almost exactly that of an average earth ocean inversions suggest that an increase in conductivity at 440 km depth predicted by recent laboratory measurements on high pressure phases of olivine is not favored by the data although as in previous studies the 670 km discontinuity between the upper and lower mantle is associated with a two orders of magnitude jump in conductivity a new feature in our inversions is a further increase in lower mantle conductivity at a depth of 1300 km a global map of the internal induced component of the magnetic field provides a qualitative estimate of three dimensional 3 d variations in earth electrical conductivity demonstrating graphically that the satellite data are responsive to lateral variations in electrical conductivity caused by the continents and oceans

a geophysical perspective on mantle water content and melting inverting electromagnetic sounding data using laboratory based electrical conductivity profiles abstract this paper applies electromagnetic sounding methods for earth s mantle to constrain its thermal state chemical composition and water content we consider long period inductive response functions in the form of c responses from four stations distributed across the earth europe north america asia and australia covering a period range from 3 9 to 95 2 days and sensitivity to 1200 km depth we invert c responses directly for thermo chemical state using a self consistent thermodynamic method that computes phase equilibria as functions of pressure temperature and composition in the na 2 o cao feo mgo al 2 o 3 sio 2 model system computed mineral modes are combined with recent laboratory based electrical conductivity models from independent experimental research groups yoshino 2010 and karato 2011 to compute bulk conductivity structure beneath each of the four stations from which c responses are estimated to reliably allocate water between the various mineral phases we include laboratory measured water partition coefficients for major upper mantle and transition zone minerals this scheme is interfaced with a sampling based algorithm to solve the resulting non linear inverse problem this approach has two advantages 1 it anchors temperatures composition electrical conductivities and discontinuities that are in laboratory based forward models and 2 at the same time it permits the use of geophysical inverse methods to optimize conductivity profiles to match geophysical data the results show lateral variations in upper mantle temperatures beneath the four stations that appear to persist throughout the upper mantle and parts of the transition zone calculated mantle temperatures at 410 and 660 km depth lie in the range 1250 1650 c and 1500 1750 c respectively and generally agree with the experimentally determined temperatures at which the measured phase reactions olivine spinel and spinel ferropericlase perovskite occur the retrieved conductivity structures beneath the various stations tend to follow trends observed for temperature with the strongest lateral variations in the uppermost mantle for depths 300 km conductivities appear to depend less on the particular conductivity database conductivities at 410 km and at 660 km depth are found to agree overall with purely geophysically derived global and semi global one dimensional conductivity models both electrical conductivity databases point to 2 o in the upper mantle for transition zone minerals results from the laboratory database of yoshino 2010 suggest that a much higher water content up to 2 wt h 2 o is required than in the other database karato 2011 which favors a relatively dry transition zone 2 o incorporating laboratory measurements of hydrous silicate melting relations and available conductivity data allows us to consider the possibility of hydration melting and a high conductivity melt layer above the 410 km discontinuity the latter appears to be 1 regionally localized and 2 principally a feature from the yoshino 2010 database further there is evidence of lateral heterogeneity the mantle beneath southwestern north america and central china appears wetter than that beneath central europe or australia

a semi global reference model for electrical conductivity in the mid mantle beneath the north pacific region 1 one dimensional electrical conductivity structure in the mid mantle of the one fourth of the earth beneath the north pacific ocean was obtained by a semi global electromagnetic induction study electromagnetic response functions estimated from electric field variations measured by submarine cables and geomagnetic field variations obtained by magnetic observatories and long term observations sites were inverted into radially symmetric conductivity distribution by taking the distribution of land and ocean at the surface into account as a most preferred model a smooth conductivity depth profile was obtained with two abrupt increases that possibly correspond to the seismic discontinuities at 410 and 660 km

the heterogeneous electrical conductivity structure of the lower mantle abstract we developed a new three dimensional electromagnetic time domain technique to invert 32 years of magnetic monthly mean values 1958 1990 from 120 geomagnetic observatories to image the mid lower mantle a preliminary global analysis of the data provided a source field model and an initial one dimensional 1 d conductivity profile for the inversion the 1 d analysis showed that monthly mean magnetic data could resolve the lower mantle conductivity structure in the depth range 900 1400 km we run a series of synthetic data inversion to test the approach the inversion of the true data with different initial conditions and different fractions of the data set was found fairly stable regions of low and high conductivity values were identified the most resistive structures are beneath the australian region near japan north and central america while highly conductive structures are identified beneath eastern africa and eurasia some of these significant variations in the lower mantle conductivity are well correlated with structures in seismic tomography model the uncorrelated features suggest that temperature and compositional effects known as predominant on both conductivity and velocity could be associated with other processes affecting mainly the conductivity probably in relation with minor phases

observations of a diapycnal shortcut to adiabatic upwelling of antarctic circumpolar deep water in the southern ocean small scale turbulence causes diapycnal mixing which influences important water mass transformations in turn impacting large scale ocean transports such as the meridional overturning circulation moc a key controller of earth s climate we present direct observations of mixing over the antarctic continental slope between water masses that are part of the southern ocean moc a 12 h time series of microstructure turbulence measurements hydrography and velocity observations off elephant island north of the antarctic peninsula reveals two concurrent bursts of elevated dissipation of o 10 6 w kg 1 resulting in heat fluxes 10 times higher than basin integrated drake passage estimates this occurs across the boundary between adjacent adiabatic upwelling and downwelling overturning cells ray tracing to nearby topography shows mixing between 300 and 400 m is consistent with the breaking of locally generated internal tidal waves since similar conditions extend to much of the antarctic continental slope where these water masses outcrop diapycnal mixing may contribute significantly to upwelling

constraints on thermal state and composition of the earth s lower mantle from electromagnetic impedances and seismic data despite the tight constraints put by seismology on the elastic properties of the earth s lower mantle its mineralogical composition and thermal state remain poorly known because the interpretation of seismic measurements suffers from the trade off between temperature iron content and mineralogical composition in order to overcome this difficulty we complement seismic data with electromagnetic induction data the latter data are mostly sensitive to temperature and iron content while densities and acoustic speeds mostly constrain the mineralogy a 0 5 log unit increase in electrical conductivity can be caused either by a 400 k increase of the temperature or by an increase of iron content from 10 to 12 5 acoustic velocity is only marginally sensitive to temperature but it increases by 0 8 km s 1 on average as the perovskite fraction increases from 50 to 100 olsen s 1999 apparent resistivities in the period range 15 days 11 years and preliminary reference earth model prem densities and acoustic speeds are jointly inverted in the depth range 800 km 2600 km by using a monte carlo markov chain method given the uncertainties on these data estimates of perovskite fraction are well constrained over the whole depth range but information on temperature and iron content is only obtained for depths less than 2000 km corresponding to the penetration depth of the long period electromagnetic field all parameter values are determined with an uncertainty better than 15 20 at the 1 confidence level the temperature in the uppermost lower mantle i e down to 1300 km depth is close to a value of 2200 k and increases along a superadiabatic gradient of 0 4 k km 1 between 1300 and 2000 km depth extrapolation of this gradient at greater depth leads to a temperature close to 2800 k at 2600 km depth the iron content of the lower mantle is found to be almost constant and equal to 10 11 whatever the depth while a significant linear decrease of the perovskite content is observed throughout the whole depth range from 80 at 800 km depth down to 65 at 2600 km depth

the electrical conductivity of the mantle beneath europe derived from c responses from 3 to 720 hr summary the c response which connects the magnetic vertical component and the horizontal gradient of the horizontal components of electromagnetic fluctuations forms the basis for estimating the conductivity depth profile of the earth this paper describes new estimates of the c response obtained from observatory hourly mean values the z y method is applied which means that the vertical component is used locally whereas the horizontal components are used globally by expansion into terms of spherical harmonics a special evort is made to obtain unbiased estimates of c when applied to 90 months of hourly mean values from about 90 observatories the method yields consistent results for european observatories in the entire period range from 3 to 720 hr and for two diverent source mechanisms s and d st a good description of the source structure for individual time segments is essential this was achieved by a separate spherical harmonic analysis of the data for each month for d st or each day for s and by estimating a large number 120 of expansion coeycients the results are interpreted by means of 1 d conductivity models which show that the upper mantle has remarkably little structure with a monotonic decrease of resistivity from 100 v m near z 200 km to 0 7 v m below z 1000 km

intermittent intense turbulent mixing under ice in the laptev sea continental shelf vertical mixing in the bottom boundary layer and pycnocline of the laptev sea is evaluated from a rapidly sampled 12 h time series of microstructure temperature conductivity and shear observations collected under 100 sea ice during october 2008 the bottom boundary turbulent kinetic energy dissipation was observed to be enhanced 10 4 w m 3 beyond background levels 10 6 w m 3 extending up to 10 m above the seabed when simulated tidal currents were directed on slope upward heat fluxes into the halocline class waters along the laptev sea seabed peaked at 4 8 w m 2 averaging out to 2 w m 2 over the 12 h sampling period in the laptev sea pycnocline an isolated 2 h episode of intense dissipation 10 3 w m 3 and vertical diffusivities was observed that was not due to a localized wind event observations from an acoustic doppler current meter moored in the central laptev sea near the m2 critical latitude are consistent with a previous model in which mixing episodes are driven by an enhancement of the pycnocline shear resulting from the alignment of the rotating pycnocline shear vector with the under ice stress vector upward cross pycnocline heat fluxes from the arctic halocline peaked at 54 w m 2 resulting in a 12 h average of 12 w m 2 these results highlight the intermittent nature of arctic shelf sea mixing processes and how these processes can impact the transformation of arctic ocean water masses the observations also clearly demonstrate that absence or presence of sea ice profoundly affects the availability of near inertial kinetic energy to drive vertical mixing on the arctic shelves

spatial and temporal patterns of small scale mixing in drake passage temperature and salinity profiles obtained with expendable ctd probes throughout drake passage between february 2002 and july 2005 are analyzed to estimate turbulent diapycnal eddy diffusivities to a depth of 1000 m diffusivity values are inferred from density temperature inversions and internal wave vertical strain both methods reveal the same pattern of spatial variability across drake passage diffusivity estimates from inversions exceed those from vertical strain by a factor of 3 over most of drake passage the polar front pf separates two dynamically different regions strong thermohaline intrusions characterize profiles obtained north of the pf south of the pf stratification is determined largely by salinity and temperature is typically unstably stratified between 100 and 600 m depth in the upper 400 m turbulent diapycnal diffusivities are o 10 3 m2 s 1 north of the pf but decrease to o 10 4 m2 s 1 or smaller south of the pf below 400 m diffusivities typically exceed 10 4 m 2 s 1 diffusivities decay weakly with depth north of the pf whereas diffusivities increase with depth and peak near the local temperature maximum south of the pf the meridional pattern in near surface mixing corresponds to local maxima and minima of both wind stress and wind stress variance near surface diffusivities are also found to be larger during winter months north of the pf wind driven near inertial waves strong mesoscale eddy activity and double diffusive convection are suggested as possible factors contributing to observed mixing patterns

the effect of water on mantle phase transitions water has played an important role in the earth s evolution the incorporation of water as hydroxyl into solid mineral phases or as coexisting hydrous fluids and melts affects the chemical and physical properties of crust and mantle constituents i e it weakens rocks and minerals reduces viscosity and strength of the materials and depresses dramatically the melting temperature of silicate minerals e g karato 1990 inoue 1994 hirth and kohlstedt 1996 chen et al 1998 kubo et al 1998 mei and kohlstedt 2000 many recent studies have suggested the possible existence of water in the earth s mantle especially in the transition zone e g smyth and frost 2002 ohtani et al 2004 litasov et al 2005a hae et al 2006 where wadsleyite and ringwoodite can accommodate up to 3 wt of h2o in their structures e g kohlstedt et al 1996 low velocity zones observed seismologically at the top of the 410 km discontinuity may indicate the existence of trapped high density melt e g revenaugh and sipkin 1994 song et al 2004 matsukage et al 2005 sakamaki et al 2006 which is likely to be hydrous as it is not possible to melt the base of the upper mantle without water at these conditions electrical conductivity anomalies in the upper mantle and transition zone that are related to subduction zones have also been interpreted as an effect of water in the mantle e g fukao et al 2004 tarits at el 2004 hae et al 2006 koyama et al 2006 studies of the kinetics of the hydrous olivine wadsleyite transformation ohtani et al 2004 hosoya et al 2005 seem to be consistent with seismological observations e g koper et al 1998 that indicate the absence of a metastable olivine wedge in subducting

mixing in the transition layer during two storm events abstract upper ocean dynamics analyzed from mooring array observations are contrasted between two storms of comparable magnitude particular emphasis is put on the role of the transition layer the strongly stratified layer between the well mixed upper layer and the deeper more weakly stratified region the midlatitude autumn storms occurred within 20 days of each other and were measured at five moorings in the first storm the mixed layer follows a classical slab layer response with a steady deepening during the course of the storm and little mixing of the thermocline beneath in the second storm rather than deepening the mixed layer shoals while intense near inertial waves are resonantly excited within the mixed layer these create a large shear throughout the transition layer generating turbulence that broadens the transition layer details of the space time structure of the frequencies in both short waves and near inertial waves are presented small scale waves are excited within the transition

electrical conductivity of wadsleyite at high temperatures and high pressures abstract the electrical conductivity of wadsleyite aggregates has been determined under the broad range of thermodynamic conditions using the impedance spectroscopy for a frequency range of 10 2 to 10 6 hz two branches are observed in the complex impedance one at high frequency range showing a half circle originated at z real part of impedance z imaginary part of impedance 0 in the z z plot and another branch in the low frequency range the results from high frequency semi circles correspond to the electric properties of a sample whereas the results from a low frequency branch correspond to the electrode effects from the analysis of the results from the semi circles we have identified two distinct mechanisms of electrical conduction having different activation enthalpies and different sensitivity to oxygen fugacity and water content one mechanism dominating at water poor condition has a high activation enthalpy 147 kj mol and the conductivity increases with oxygen fugacity we suggest that electrical conduction in this regime is due to charge transfer involving ferric iron polaron conduction under water rich conditions electrical conductivity increases with water content but decreases with oxygen fugacity and the activation enthalpy is smaller 88 kj mol we infer that electrical conduction in this regime is due to protons the activation enthalpy in this regime is insensitive to water content and the conductivity is proportional to water content c w as c w r with r 0 72 the value of r is smaller than one suggests that minority defects such as h m or h are responsible for electrical conduction our results show that a completely dry transition zone is incompatible with most of the geophysical observations on the mantle transition zone and some water 0 1 0 3 wt in the pacific is required to explain the observed electrical conductivity

global mantle tomography progress status in the past 10 years abstract we review the present status of global mantle tomography and discuss two main classes of models that have been developed in the past 10 years p velocity models based on large datasets of travel times from the international seismological centre bulletins often referred to as high resolution models and s velocity models based on a combination of surface wave and hand picked body wave travel times or waveforms referred to as long wavelength models we discuss their respective strengths and weaknesses as well as progress in the resolution of other physical parameters such as anisotropy anelasticity density and bulk sound velocity using tomographic approaches we present the view that future improvements in global seismic tomography require the utilization of the rich information contained in complete broadband seismic waveforms this is presently within our reach owing to theoretical progress as well as the increase in computational power in recent years

experimentally based water budgets for dehydrating slabs and consequences for arc magma generation phase diagrams of hydrous mid ocean ridge mor basalts to 330 km depth and of hydrous peridotites to 250 km depth are compiled for conditions characteristic for subduction zones a synthesis of our experimentally determined phase relations of chlorite lawsonite epidote zoisite amphibole paragonite chloritoid talc and phengite in basalts and of phase relations from the literature of serpentine talc chlorite amphibole and phase a in ultramafics permits calculation of h2o contents in hydrous phase assemblages that occur in natural compositions this yields the information necessary to calculate water budgets for descending slabs starting from low grade blueschist conditions 10 20 km depth with h2o contents between 5 and 6 wt for hydrated oceanic crust complete dehydration is achieved between 70 and 300 km depth as a function of individual slab geotherms hydrous phases which decompose at depth below volcanic arcs are lawsonite zoisite chloritoid and talc phengite in mafic compositions and chlorite and serpentine in peridotite approximately 15 35 of the initially subducted h2o are released below volcanic arcs the contribution of amphibole dehydration to the water budget is small 5 20 and occurs at relatively shallow depth 65 90 km in any predicted thermal structure dehydration is a combination of a stepwise and a continuous process through many different reactions which occur simultaneously in the different portions of the descending slab such a dehydration characteristic is incompatible with single phase dehydration models which focus fluid flow through a unique major dehydration event in order to explain volcanic fronts as a consequence of continuously progressing dehydration water ascending from the slab will be generally available to depth of ca 150 200 km the fluid rising from the subducting lithosphere will cause partial melting in the hot portion of the mantle wedge we propose that the volcanic front simply forms above the mantle wedge isotherm where the extent of melting is sufficient to allow for the mechanical extraction of parental arc magmas thermal models show that such an isotherm ca 1300oc locates below volcanic fronts slab surface depths below such an isotherm are compatible with the observed depths of the slab surface below volcanic fronts 1998 elsevier science b v all rights reserved

resonant wind driven mixing in the ocean boundary layer abstract the role of resonant wind forcing in the ocean boundary layer was examined using an ocean large eddy simulation les model the model simulates turbulent flow in a box measuring 100 300 m on a side whose top coincides with the ocean surface horizontal boundary conditions are periodic and time dependent wind forcing is applied at the surface two wind forcing scenarios were studied one with resonant winds that is winds that rotated at exactly the inertial frequency at 45 n and a second with off resonance winds from a constant direction the evolution of momentum and temperature for both cases showed that resonant wind forcing produces much stronger surface currents and vertical mixing in comparison to the off resonance case surface wave effects were also examined and found to be of secondary importance relative to the wind forcing the main goal was to quantify the main processes via which kinetic energy input by the wind is converted to potential energy in the form of changes in the

very long period magnetotellurics at tucson observatory implications for mantle conductivity in a companion paper egbert et al this issue we describe the estimation of very long period 0 16 t 91 days magnetotelluric mt impedances from 11 years of data collected at the tucson geomagnetic observatory here we discuss the implications of these data for mantle conductivity using minimum norm flattest and smoothest inversions we find simple one dimensional models of electrical conductivity in the depth range 0 1500 km we use forward modeling a linearized resolution analysis and constrained one dimensional inverions to delineate the range of models which are consistent with the estimated impedances although the mt data have limited resolution large scale vertical averages of mantle conductivity are well constrained we reach the following conclusions concerning mantle conductivity beneath tucson 1 the upper 200 km has a conductance of order 104 siemens s this anomalously high conductance may be concentrated in an aesthenospheric high conductivity layer but the geometry of the conductive zone is not constrained 2 typical conductivities in the transition zone 400 700 km are 0 1 0 3 s m 1 a step increase to reach this value at or near the 400 km olivine spinel phase transition is consistent with but not required by the data an upper mantle which is resistive throughout 0 05 s m 1 or less is not allowed by the data 3 resolvable large scale averages of conductivity increase from 0 2 s m 1 to 1 0 s m 1 between 600 and 900 km depth a range of models including those with step increases and step decreases at the 670 km seismic discontinuity are consistent with the data 4 between 900 1500 km conductivity increases slowly average conductivities in this region are of the order of 1 s m 1 to within a factor of 2 or 3 while limited zones of highly resistive mantle are consistent with the data a lower mantle which is resistive throughout is not models in which conductivity is always above 5 s m 1 below 1000 km can also be ruled out in conjunction with improved laboratory estimates of electrical conductivities of mantle minerals at high temperatures and pressures these constraints can provide important clues to the composition and physical state of the mantle

variability and mixing of the filchner overflow plume on the continental slope weddell sea abstracta large fraction of antarctic bottom water is produced in the weddell sea through mixing between the cold and dense shelf water masses and the warm and saline off shelf water we present o

a joint interpretation of electromagnetic and seismic tomography models suggests the mantle transition zone below europe is dry abstract the three dimensional 3d electrical conductivity distribution in the mantle transition zone mtz below europe is resolved by directly inverting geomagnetic response functions from 12 selected european observatories comparison of the conductivity image with the results of global seismic tomography reveals a good correlation between low conductivity and high velocity anomalies at depths of 400 700 km beneath the mediterranean subduction zone in central europe interpretation of the 3d conductivity model combined with seismic tomography indicates that the european mtz is drier than the mtz beneath the western pacific subduction zone even though the subducted slab material is stagnating similarly in both regions

stagnant slab a review a stagnant slab is a subducted slab of oceanic lithosphere subhorizontally deflected above across or below the 660 km discontinuity this phenomenon has now been widely recognized beneath subduction zones around the circum pacific and in the mediterranean collaboration of seismic and electromagnetic observations mineral physics measurements and geodynamic modeling has begun to provide a consistent picture of stagnant slab

vertical structure and transport of the antarctic circumpolar current in drake passage from direct velocity observations 1 the structure of the antarctic circumpolar current acc in drake passage is examined using 4 5 years of shipboard acoustic doppler current profiler adcp velocity data the extended 1000 m depth range available from the 38 khz adcp allows us to investigate the vertical structure of the current the mean observed current varies slowly with depth while eddy kinetic energy and shear variance exhibit strong depth dependence objectively mapped streamlines are self similar with depth consistent with an equivalent barotropic structure vertical wavenumber spectra of observed currents and current shear reveal intermediate wavenumber anisotropy and rotation indicative of downward energy propagation above 500 m and upward propagation below 500 m the mean observed transport of the acc in the upper 1000 m is estimated at 95 2 sv or 71 of the canonical total transport of 134 sv mean current speeds in the acc jets remain quite strong at 1000 m 10 20 cm s 1 vertical structure functions to describe the current and extrapolate below 1000 m are explored with the aid of full depth profiles from lowered adcp and a 3 year mean from the southern ocean state estimate sose a number of functions including an exponential are nearly equally good fits to the observations explaining 75 of the variance fits to an exponentially decaying function can be extrapolated to give an estimate of 154 38 sv for the full depth transport

optimal estimation of hydrographic station data and derived fields abstract optimal estimation is applied to contouring and analysis of hydrographic sections measured fields such as temperature and salinity and derived fields such as geostrophic velocity are decomposed into large scale and small scale components first a heavily sampled basin scale field is estimated and subtracted from the data a small scale field barely resolved by the hydrography is then found from the residuals the power and utility of the technique are illustrated by means of examples using a short section from the straits of florida and a transatlantic section along 24 n

time variations of the earth s magnetic field from daily to secular variations in time of the earth s natural magnetic field have a frightfully wide spectrum ranging over more than 20 orders of magnitude they extend from well over 103 hz where they merge into the radioelectric noise spectrum to probably more than 100 m y in rough summary the higher frequencies stem from external sources in the ionosphere and above into the magnetosphere whereas the longer periods arise from internal sources primarily fluid motions in the core at depths in excess of 2900 km methods for separating external and internal signals have been available since the time of gauss 1 838 but the accuracy the space distribution and the time duration of available data are often not sufficient to unambiguously solve the problem for all types of variations although periods shorter than 1 yr should clearly be assigned to external sources the extent to which the spectra of the external and internal parts overlap is the subject of recent debate and progress currie 1966 1 968 proposed that there was little overlap and a cutoff of 3 7 yr a long debated l l yr contribution due to the solar cycle was clearly demonstrated by currie 1973a courtillot le mouei 1976a b and alldredge 1976 and the cutoff period was thus rejected to at least 20 yr the situation seemed to be confused again when courtillot et al 1978a provided evidence for sharp impulses of 1 yr duration i n the internal so called secular variation the debate that followed over the reality of brief internal events is one of the topics addressed in this review we first provide a short historical account of successive discoveries

upper mantle conductivity structure of the back arc region beneath northeastern china the upper mantle electrical conductivity structure of the pacific back arc beneath northeastern china was investigated through long period magnetotelluric mt and geomagnetic depth sounding gds experiments mt and gds responses were obtained up to periods of 105 106 seconds and were inverted to one dimensional 1 d models with minimum and smooth structure constraints respectively the resulting conductivity model was compared with past models and the mantle transition zone beneath northeastern china is more conductive than those of other tectonic settings by almost one order of magnitude in the depth range of 400 600 km this feature may correspond to the presence of a stagnant slab beneath this region in the mantle at a depth of less than 400 km the conductivity profile has a similar feature to that in the thermal and extensional tectonic region in the southwestern united states

ocean nutrient pathways associated with the passage of a storm storms that affect ocean surface layer dynamics and primary production are a frequent occurrence in the open north atlantic ocean in this study we use an interdisciplinary dataset collected in the region to quantify nutrient supply by two pathways associated with a storm event entrainment of nutrients during a period of high wind forcing and subsequent shear spiking at the pycnocline due to interactions of storm generated inertial currents with wind the post storm increase in surface layer nitrate by 20 mmol m 2 was predominantly driven by the first pathway nutrient intrusion during the storm alignment of post storm inertial currents and surface wind stress caused shear instabilities at the ocean pycnocline forming the second pathway for nutrient transport into the euphotic zone during the alignment period pulses of high turbulent nitrate flux through the pycnocline up to 1 mmol m 2 day 1 approximately 25 times higher than the background flux were detected however the impact of the post storm supply was an order of magnitude lower than during the storm due to the short duration of the pulses cumulatively the storm passage was equivalent to 2 5 5 of the nitrate supplied by winter convection and had a significant effect compared to previously reported sub mesoscale dynamics in the region as storms occur frequently they can form an important component in local nutrient budgets

southern ocean mixed layer depth from argo float profiles 1 argo float profiles of temperature salinity and pressure are used to derive the mixed layer depth mld in the southern ocean mld is determined from individual profiles using both potential density and potential temperature criteria and a monthly climatology is derived from individual mlds using an objective mapping method quantitative data are available in the auxiliary material the spatial structures of mlds are similar in each month with deep mixed layers within and just north of the antarctic circumpolar current acc in the pacific and indian oceans the deepest mixed layers are found from june to october and are located just north of the acc where antarctic intermediate water aaiw and subantarctic mode water samw are formed examination of individual mlds indicates that deep mixed layers mld 400 m from both the density and temperature criteria are concentrated in a narrow surface density band which is within the density range of samw the surface salinity for these deep mixed layers associated with the samw formation are slightly fresher compared to historical estimates differences in air sea heat exchanges wind stress and wind stress curl in the pacific and indian oceans suggest that the mode water formation in each ocean basin may be preconditioned by different processes wind mixing and ekman transport of cold water from the south may assist the samw formation in the indian ocean in the eastern pacific the formation of mode water is potentially preconditioned by the relative strong cooling and weak stratification from upwelling

on the electrical conductivity of the mid mantle ii delineation of heterogeneity by application of extremal inverse solutions summary long period equivalent scalar magnetotelluric impedance functions from 22 magnetic observatories distributed over the earth s surface are inverted to test the hypothesis that there exists significant lateral heterogeneity in the electrical conductivity structure of the mid mantle limited spaces of extremal models are examined and strict bounds are placed on the range of permissible models joint d inversion of combined response functions from observatory pairs proves the requirement for global heterogeneity while exploration of h layered earth model space suggests systematic regional heterogeneity indicating that enhanced upper mantle conductivity is found within the pacific basin and baikal rift zone and is absent elsewhere the results of these extremal inverse solutions are compared with the backus gilbert like schmucker inversion and a heuristic inversion

the 3d electromagnetic response of the earth to ring current and auroral oval excitation summary we report on a new compilation of functions representing the geoelectromagnetic response of the earth s mantle the c response functions and newly defined d response functions the latter of which constitutes a sensitive indicator of lateral heterogeneities in conductivity were estimated for periods from 5 to 106 7 days the spatial coverage and statistical quality of these response function estimates were improved over previous compilations in part by making use of a newly assembled global catalogue of magnetic observatory hourly mean values and by applying a statistically robust form of empirical orthogonal function eof analysis the eof results suggest that for periods longer than 5 days the geomagnetic field variation can be represented predominantly by a single eigenmode we have identified that the primarysource fields of the dominant mode consist of two current systems in the same direction an equatorial ring current in the magnetosphere and two conjugate auroral oval ring currents in the ionosphere the influence of the auroral current system is seen at surprisingly low latitudes 40 and 50 for the vertical and northern components respectively which results in non negligible bias on the conventional c response at high latitudes a preliminary correction of the auroral current effect has been made on the c and d responses obtained for data from 55 and 52 observatories respectively at latitudes from 60 to 60 we also present a set of approximate equivalent internal mantle electrical current distributions that prove a useful indicator of the presence of deep earth heterogeneities these new sets of electromagnetic information will increase the resolving power of 3d distribution of electrical conductivity in the earth s mantle

fluid flow near reservoir lakes inferred from the spatial and temporal analysis of the electric potential electric self potential sp variations have been monitored continuously from 1995 to 1998 at 14 points on a ridge separating the roselend and la gittaz reservoir lakes in the french alps the lakes have level variations of at least 50 m over yearly cycles seasonal variations of sp associated with lake level variations are observed on five points of the array for three points located on the banks of the lakes a positive correlation to the lake level variations is observed with a maximal amplitude of about 180 mv corresponding to an average response of 2 4 mv per meter of water for two points located on the bottom of each lake the correlation is negative with a maximal magnitude of about 50 mv corresponding to an average response of 1 1 mv per meter of water two independent temporary electrical arrays located on the banks of each lake confirm these measurements and allow a better spatial characterization of the sources associated with the observed sp variations in particular near the roselend lake the electrical response to lake level variations is increasing for decreasing altitude the measured sp variations are proposed to result from the electrokinetic coupling associated with a vertical groundwater flow connecting a constant pore pressure source to the bottom of the lakes numerical modeling indicates that the spatial variation of the response and the nonlinear response observed at one point can be explained by leakage currents in the conductive lake water the values of the streaming potential coefficient spc measured in the laboratory with crushed rock samples from the site range from 14 to 50 mv 0 1 mpa for an electrolyte resistivity of 40 m and are compatible to first order with the magnitude of the observed seasonal sp variations a detailed quantitative electrokinetic modeling is currently limited mainly by the poor knowledge on the contribution of electrical leakage currents and the local variability of the spc this experiment indicates that spatial and temporal variations of the electric potential are promising tools to characterize and monitor shallow groundwater flow and provide practical data for the investigation of groundwater flow associated with volcanic or tectonic activity

electrical conductivity of majorite garnet and its implications for electrical structure in the mantle transition zone abstract electrical conductivities of majorite garnet with compositions of pyrolite minus olivine pyrolite majorite and mid ocean ridge basalt morb majorite were measured under physical conditions of the mantle transition zone 18 and 23 gpa and temperatures up to 2000 k in a kawai type multi anvil apparatus the samples with morb composition are mainly composed of majorite which has higher fe and al contents and contain a small amount of stishovite the conductivity of the morb majorite is more than twice higher than those of the pyrolite majorite at the same temperature the activation energies of these majorites are both 1 4 ev at temperature of 1000 1600 k suggesting that the dominant mechanism of charge transportation is fe 2 fe 3 hopping small polaron conduction at higher temperatures 1600 k corresponding to temperature conditions of the transition zone conduction mechanism of the pyrolite majorite would change from small polaron to ionic conduction the pyrolite majorite has only slightly higher and lower conductivity than dry wadsleyite and ringwoodite respectively and will not largely change the conductivity depth profile predicted for the dry mantle transition zone the laboratory based conductivity profile of the mantle transition zone with pyrolitic composition can explain well the current semi global conductivity depth profile obtained from electromagnetic study beneath pacific on the other hand the garnetite originating from the oceanic crust has remarkably higher conductivity than the surrounding mantle because the conductivity of morb majorite is significantly higher than those of wadsleyite and ringwoodite conductivity values of morb majorite agree with those of the stagnant slab beneath the northeastern china

electrical conductivity model of al bearing bridgmanite with implications for the electrical structure of the earth s lower mantle electrical conductivity measurements of bridgmanite with various al contents and a constant mg of 90 were performed at temperatures ranging from room temperature up to 2000 k at pressures of 26 28 gpa in a kawai type multianvil apparatus by impedance spectroscopy analysis the incorporation of al into bridgmanite raises its electrical conductivity significantly but it is a small conductivity variation with respect to the quantity of al synchrotron mossbauer spectroscopy of recovered samples showed significant amounts of ferric iron in aluminous bridgmanite the mobility of the charge carriers in bridgmanite was calculated based on the conductivity and the fe3 fe ratio the relationship between the logarithm of the electrical conductivity and the reciprocal temperature is consistent with fe2 fe3 electron hopping small polarons as the dominant conduction mechanism at low temperatures 1600 k by taking various conduction mechanisms into account we develop an electrical conductivity model for aluminous bridgmanite as a function of the al and fe contents the small polaron conduction model indicates that the electrical conductivity of aluminous bridgmanite has a maximum at around 0 06 al atoms per formula unit and any further increase in the al content in bridgmanite reduces the conductivity in contrast the ionic conduction model indicates that the electrical conductivity simply increases with increasing al content the resulting conductivity of al bearing bridgmanite first increases up to 0 06 al atoms per formula unit and then remains constant or increases with increasing al content at higher temperatures the increase in conductivity observed in the uppermost part of the lower mantle by electromagnetic studies can be explained by the gradual decomposition of majorite garnet the deeper lower mantle conductivity would be controlled by small polaron conduction because of the large positive activation volume required for ionic conduction

electrical conductivity anisotropy of dry and hydrous olivine at 8 gpa the effects of dissolved h2o on the electrical conductivity and its anisotropy in olivine fo90 at 8 gpa were investigated by complex impedance spectroscopy at nominally anhydrous conditions conduction along 1 0 0 and 0 0 1 is slightly higher than along 0 1 0 in contrast to observations made at lower pressures in earlier studies increasing h2o content increases conductivities but activation energies are lower and h2o concentration dependent the use of polarized ftir spectroscopy to determine h2o concentrations reveals a weaker than expected effect that water has on olivine conductivity and distinguishes our results from earlier studies based on analyses using non polarized infrared spectroscopy we show that at h2o concentrations of a few hundred wt ppm or less that the dominant conduction mechanism at mantle temperatures continues via small polarons such as that observed for anhydrous olivine our results also suggest that at depths greater than 200 km the presence of h2o may not be necessary to explain regions in the upper mantle where both electrical and seismic anisotropy are observed this can be explained by differences in the pressure dependence of the activation energy for conduction along each of the three crystallographic axes however while electrical anisotropy of anhydrous olivine remains weak at 8 gpa it is nevertheless enhanced by elevated concentrations several hundred wt ppm of dissolved h2o at these conditions dominated by proton hopping conductivity along 0 1 0 is highest approximately an order of magnitude greater than along 1 0 0 additionally at 1000 wt ppm and 1500 c an isotropic conductivity derived from the data is about 1 order of magnitude higher than that for nominally anhydrous olivine thus in regions of the mantle characterized by anomalously high conductivities and both electrical and seismic anisotropy significant amounts of dissolved hydrogen can be expected

observations of the transition layer abstract the transition layer is the poorly understood interface between the stratified weakly turbulent interior and the strongly turbulent surface mixed layer the transition layer displays elevated thermohaline variance compared to the interior and maxima in current shear vertical stratification and potential vorticity a database of 91 916 km or 25 426 vertical profiles of temperature and salinity from seasoar a towed vehicle is used to define the transition layer thickness acoustic doppler current measurements are also used when available statistics of the transition layer thickness are compared for 232 straight seasoar sections which range in length from 65 to 1129 km with typical horizontal resolution of 4 km and vertical resolution of 8 m transition layer thicknesses are calculated in three groups from 1 vertical displacements of the mixed layer base and of interior isopycnals into the mixed layer 2 the depths below the mixed layer depth of peaks in shear stratification and potenti