anisotropy in the indian ocean upper mantle from rayleigh and love waveform inversion a tomographic method is applied to love and rayleigh wave seismograms in order to address the problem of upper mantle elastic anisotropy in the indian ocean the first step in our approach is a waveform inversion for each path of the study a 1 d depth dependent model compatible with the waveforms of the fundamental mode and several higher modes of love and rayleigh waves is obtained then the models related to the different paths are inverted in order to retrieve 3 d velocity heterogeneities and anisotropy in this paper both the radial anisotropy and the azimuthal anisotropy of s waves are investigated in detail we find a significant radial anisotropy in the uppermost 300 km of the mantle with an overall amplitude smaller than what is found in global studies azimuthal s wave anisotropy is also present the directions of fast s velocities show rather simple patterns from 100 to 300 km depth and strong correlations with the direction of absolute plate motion apm are found in the uppermost 100 km the pattern of fast velocity directions is complex and does not correlate simply with plate motions from 100 to 200 km depth a correlation with apm is found in most oceanic regions the main lack of correlation is located in the vicinity of la reunion and mauritius islands in this region the anisotropy exhibits a complex pattern between 100 and 150 km possibly due to a disturbing effect of the hotspot at 200 km depth and deeper azimuthal anisotropy vanishes progressively beneath most of the oceanic regions no significant azimuthal anisotropy is found beyond 300 km depth the depth extent of both azimuthal and radial anisotropy the amplitude of radial anisotropy and the pattern of azimuthal anisotropy support the idea of a preferential orientation of olivine crystals in a low viscosity zone beneath the lithosphere this region would be located between 100 and 200 km in oceanic areas and possibly deeper between 200 and 300 km in continental areas not well resolved in this study

seismic anisotropy tracing plate dynamics in the mantle elastic anisotropy is present where the speed of a seismic wave depends on its direction in earth9s mantle elastic anisotropy is induced by minerals that are preferentially oriented in a directional flow or deformation earthquakes generate two seismic wave types compressional p and shear s waves whose coupling in anisotropic rocks leads to scattering birefringence and waves with hybrid polarizations this varied behavior is helping geophysicists explore rock textures within earth9s mantle and crust map present day upper mantle convection and study the formation of lithospheric plates and the accretion of continents in earth history

interferometric daylight seismic imaging summary claerbout s daylight imaging concept is generalized to a theory of interferometric seismic imaging ii interferometric seismic imaging is defined to be any algorithm that inverts correlated seismic data for the reflectivity or source distribution as examples we show that ii can image reflectivity distributions by migrating ghost reflections in passive seismic data and generalizes the receiver function imaging method used by seismologists interferometric seismic imaging can also migrate free surface multiples in common depth point cdp data and image source distributions from passive seismic data both synthetic and field data examples are used to illustrate the different possibilities of ii the key advantage of ii is that it can image source locations or reflectivity distributions from passive seismic data where the source position or wavelet is unknown in some cases it can mitigate defocusing errors as a result of statics or an incorrect migration velocity the main drawback with ii is that severe migration artefacts can be created by partial focusing of virtual multiples

elimination of the overburden response from multicomponent source and receiver seismic data with source designature and decomposition into pp ps sp and ss wave responses this paper shows that betti s reciprocity theorem gives an integral equation procedure to eliminate from the physical multicomponent source multicomponent receiver seismic measurements the effect of the physical source radiation pattern and the response of the physical overburden that is the medium above the receiver plane the physical multicomponent sources are assumed to be orthogonally aligned anywhere above the multicomponent receiver depth level other than the position of the sources no source characteristics are required the method denoted the betti designature elastic demultiple has the following additional characteristics it preserves primary amplitudes while eliminating all waves scattered from the overburden it requires no knowledge of the medium below the receiver level it requires no knowledge of the medium above the receiver level it requires information only of the local density and elastic wave propagation velocities at the receiver level to decompose the physical seismic meas

shear velocity structure radial anisotropy and dynamics of the tibetan crust geophysical and geological data suggest that tibetan middle crust is a partially molten mechanically weak layer but it is debated whether this low viscosity layer is present beneath the entire plateau what its properties are how it deforms and what role it has played in the plateau s evolution broad band seismic surface waves yield resolution in the entire depth range of the tibetan crust and can be used to constrain its shear wave velocity structure indicative of crustal composition temperature and partial melting and radial anisotropy indicative of the patterns of deformation we measured love and rayleigh wave phase velocity curves in broad period ranges up to 7 200 s for a few tens of pairs and groups of stations across tibet combining in each case hundreds of interstation measurements made with cross correlation and waveform inversion methods shear velocity profiles were then determined by extensive series of non linear inversions of the data designed to constrain the depth dependent ranges of isotropic average shear speeds and radial anisotropy shear wave speeds within the tibetan middle crust are anomalously low and also show strong lateral variations across the plateau the lowest mid crustal shear speeds are found in the north and west of the plateau 3 1 3 2 km s 1 within a pronounced low velocity zone in southeastern tibet crustal shear wave speeds increase gradually towards southeast whereas in the north the change across the kunlun fault is relatively sharp the lateral variations of shear speeds within the crust are indicative of those in temperature a mid crustal temperature of 800 c reported previously can account for the low shear velocities across lhasa in the north the temperature is higher and exceeds the solidus resulting in partial melting that we estimate at 3 6 per cent strong radial anisotropy is required by the data in western central tibet 5 per cent but not in northeastern tibet the amplitude of radial anisotropy in the crust does not correlate with isotropic average shear speed and by inference with crustal rock viscosity or with surface elevation instead radial anisotropy is related to the deformation pattern and is the strongest in regions experiencing extension crustal flattening as noted previously the growth of tibet by the addition of indian crustal rocks into its crust from the south is reflected in the higher crustal seismic velocities and thus lower temperatures in the southern compared to northern parts of the plateau more recently added rocks having had less time to undergo radioactive heating within the thickened tibetan crust gravity driven flattening the basic cause of extension and normal faulting in the southern western and central tibet is evidenced by pervasive radial anisotropy in the middle crust beneath the regions undergoing extension the overall eastward flow of the crust is directed by the boundaries and motions of the lithospheric blocks that surround tibet

finite difference modeling experiments for seismic interferometry in passive seismic interferometry new reflection data can be retrieved by crosscorrelating recorded noise data the quality of the retrieved reflection data is among others dependent on the duration and number of passive sources present during the recording time the source distribution and the source strength to investigate these relations we set up several numerical modeling studies to carry out the modeling in a feasible time we design a finite difference algorithm for the simulation of long duration passive seismic measurements of band limited noise signatures in the subsurface novel features of the algorithm include the modeling of thousands of randomly placed sources during one modeling run the modeling experiments explore the dependency relation between the retrieved reflections and source signature length source positions number of sources and source amplitude variations from these experiments we observed that the positions of the passive sources and the length of the source signals are of direct influence on the quality of the retrieved reflections random amplitude variations among source signals do not seem to have a big impact on the retrieved reflections

dynamics of the india eurasia collision zone we present simple new dynamic calculations of a vertically averaged deviatoric stress field over a depth average of 100 km for asia from geodetic geologic topographic and seismic data a first estimate of the minimum absolute magnitudes and directions of vertically averaged deviatoric stress is obtained by solving force balance equations for deviatoric stresses associated with gravitational potential energy differences within the lithosphere plus a first order contribution of deviatoric stresses associated with stress boundary conditions this initial estimate of the vertically averaged deviatoric stress field is obtained independent of assumptions about the rheology of the lithosphere absolute magnitudes of vertically averaged deviatoric stresses vary between 5 and 40 mpa assuming bulk viscous behavior for the lithosphere the magnitudes of deviatoric stresses together with the magnitudes of strain rates inferred from quaternary fault slip rate and gps data yield vertically averaged effective viscosities for tibet of 0 5 5 1022 pa s compared with 1 2 5 1023 pa s in more rigid areas elsewhere in the region a forward modeling method that solves force balance equations using velocity boundary conditions allows us to refine our estimates of the vertically averaged effective viscosity distribution and deviatoric stress field the total vertically averaged deviatoric stress and effective viscosity field are consistent with a weak lower crust in tibet they are consistent with some eastward motion of tibet and south china lithosphere relative to eurasia and they confirm that gravitational potential energy differences have a profound effect on the spatially varying style and magnitude of strain rate around the tibetan plateau our results for the vertically averaged deviatoric stress argue for a large portion of the strength of the lithosphere to reside within the seismogenic upper crust to get deviatoric stress magnitudes there to be as high as 100 300 mpa in accord with laboratory and theoretical friction experiments indicating that stress drops in earthquakes are small fractions of the total deviatoric stress

seismic anisotropy and mantle creep in young orogens summary seismic anisotropy provides evidence for the physical state and tectonic evolution of the lithosphere we discuss the origin of anisotropy at various depths and relate it to tectonic stress geotherms and rheology the anisotropy of the uppermost mantle is controlled by the orthorhombic mineral olivine and may result from ductile deformation dynamic recrystallization or annealing anisotropy beneath young orogens has been measured for the seismic phase pn that propagates in the uppermost mantle this anisotropy is interpreted as being caused by deformation during the most recent thermotectonic event and thus provides information on the process of mountain building whereas tectonic stress and many structural features in the upper crust are usually orientated perpendicular to the structural axis of mountain belts pn anisotropy is aligned parallel to the structural axis we interpret this to indicate mountainparallel ductile i e creeping deformation in the uppermost mantle that is a consequence of mountain perpendicular compressive stresses the preferred orientation of the fast axes of some anisotropic minerals such as olivine is known to be in the creep direction a consequence of the anisotropy of strength and viscosity of orientated minerals in order to explain the anisotropy of the mantle beneath young orogens we extend the concept of crustal escape or extrusion tectonics to the uppermost mantle we present rheological model calculations to support this hypothesis mountain perpendicular horizontal stress determined in the upper crust and mountain parallel seismic anisotropy in the uppermost mantle require a zone of ductile decoupling in the middle or lower crust of young mountain belts examples for stress and mountain parallel pn anisotropy are given for tibet the alpine chains and young mountain ranges in the americas finally we suggest a simple model for initiating mountain parallel creep

anoxic dissolution processes of biotite implications for fe behavior during archean weathering abstract iron rich biotite fe mg 5 dissolution experiments were carried out in a batch system under o 2 deficient conditions p o 2 5 atm referred to as anoxic conditions at 1 atm of p co 2 ph 4 6 and 100 c for 7 120 days for a better understanding of anoxic weathering processes and fe behavior during weathering before 2 2 ga for comparison oxic fe rich biotite dissolution experiments were conducted under present atmospheric conditions at ph 4 7 and 100 c for 7 80 days referred to as oxic conditions by using the same starting biotite as that for the anoxic experiments the concentrations of fe in solution after the dissolution experiments were larger by one to more than two orders of magnitude under anoxic conditions than under oxic conditions high resolution scanning and transmission electron microscopy revealed that fe ii rich vermiculite or smectite was precipitated as a secondary mineral at the edge of biotite under anoxic conditions in contrast to the formation of fe iii and al hydr oxides under oxic conditions the results suggest that part of fe ii is released to water as anoxic weathering proceeds explaining the decrease of fe content in pre 2 2 ga paleosols relative to their parent rocks the fe mg molar ratio of the secondary vermiculite or smectite was more than 7 while the starting fe rich biotite had a value of about 5 the fe mg molar ratio was less than 2 5 in solution the results of the anoxic experiments suggest that fe ii rich vermiculite or smectite could be the precursor of the chlorite preserved in pre 2 2 ga paleosols this is further corroborated by the increase in fe mg molar ratios in chlorite with decreasing depth in some precambrian paleosols

azimuthal anisotropy of rayleigh waves in east asia 1 we present a surface wave study aimed to resolve the azimuth dependent propagation velocities of rayleigh waves 10 184s in east asia the resultant anisotropy patterns demonstrate that east asia consists of a number of tectonic domains each displays a characteristic and self consistent deformation mode the depth variation of the anisotropy pattern in each domain helps to understand the boundary actions on the lithosphere and the depth varying deformation styles owing to rheology change

siderite cannot be used as co2 sensor for archaean atmospheres abstract it was proposed to utilize siderite feco3 in mid to late archaean superior type banded as a proxy to constrain the co2 partial pressure of archaean atmospheres implicit in this proposition is that siderite was a primary carbonate mineral that crystallized directly from fe2 enriched archaean seawater in equilibrium with atmospheric co2 to our knowledge that proposition has not been demonstrated to be valid we test with water gas exchange experiments under controlled co2 partial pressures if siderite can be stabilized as a primary mineral in fe2 bearing seawater reduced seawater proxies enriched in fe2 and mn2 are equilibrated with reduced n2 ch4 co2 h2 gas phases with variable co2 the solid phases stabilized in fe2 enriched water compositions are amorphous ferrous iron hydroxy carbonates crystalline siderite feco3 is not found to be a stable phase the phases precipitating from mn2 enriched water include crystalline rhodochrosite mnco3 and possibly amorphous mn enriched phases based on these results we advise against using siderite in banded iron formations as a co2 sensor for the archaean atmosphere

the s velocity structure of the east asian mantle from inversion of shear and surface waveforms summary the structure of the mantle beneath east asia down to 800 km depth is investigated using full waveforms of seismic shear and surface waves epicentral distances are limited to less than 40 in contrast with previous waveform inversions we avoid ray theoretical or path integral approaches instead we use 1 exact 3 d waveform sensitivity kernels that correctly reflect off path sensitivity and the existence of fresnel zones we apply 2 an accurate 3 d forward modelling technique based on a coupled mode multiple forward scattering approach allowing us 3 to iterate the inversion procedure through several 3 d models and 4 to evaluate the true misfit between the data and the synthetics for the 3 d model average lateral resolution of the model in regions with good path coverage is 400 km throughout the upper mantle in the depth range from 100 to 250 km the lateral resolution even approaches 200 km since wave front healing is taken into account the amplitudes of velocity perturbations are larger than in other tomographic models moreover the waveform sensitivity kernels provide an intrinsic physical smoothing of the model stabilizing the inversion finally owing to the use of exact sensitivities better resolution can be achieved with a given set of seismograms major features of the model are high velocity subducting slabs along the west pacific subduction zones stagnating at the 660 km discontinuity a strong low velocity zone between 80 and 250 km depth in the west pacific backarc regions a plume like low velocity feature beneath the southern tip of the baikal rift zone extending into the transition zone a low velocity region under the tien shan with connection into the transition zone and thick crust under tibet reaching its maximum thickness of approximately 80 km close to the 35th parallel the lithospheric mantle underneath southern tibet is very fast indicating underthrusting of the indian lithosphere in contrast the upper mantle beneath northern tibet exhibits average to slow values from the moho down to the 660 km discontinuity very high s velocities are again observed beneath the tarim basin the high velocity mantle lithospheres under southern tibet and the tarim join beneath the karakorum range at the western tip of the tibet plateau

adaptive decomposition of multicomponent ocean bottom seismic data into downgoing and upgoing p and s waves with wavefield decomposition the recorded wavefield at a certain depth level can be separated into upgoing and downgoing wavefields as well as into p and s waves the medium parameters at the considered depth level e g just below the ocean bottom need to be known in order to be able to do a decomposition in general these parameters are unknown and in addition measurement related issues such as geophone coupling and crosstalk between the different components need to be dealt with in order to apply decomposition to field data an adaptive five stage decomposition scheme was developed in which these issues are addressed in this study the adaptive decomposition scheme is tested on a data example with a relatively shallow water depth 120 m consisting recordings from of a full line of ocean bottom receivers although some of the individual stages in the decomposition scheme are more difficult to apply because of stronger interference between events compared to data acquired over deeper water

observations of regional phase propagation across the tibetan plateau we present observations of regional phase velocity and propagation characteristics using data recorded during a 1 year deployment of broadband digital seismic stations across the central tibetan plateau along the qinghai tibet highway from golmud to lhasa previous seismological studies within this region have had to rely on earthquakes recorded almost exclusively at stations outside of the plateau we have the opportunity to study numerous source receiver paths confined entirely within the tibetan plateau our analysis concentrates on travel time amplitude and frequency content measurements of the pg pn and sn phases pn can be clearly picked for all observed paths and propagates at an average velocity of 8 16 0 07 km s within the tibetan plateau sn however shows dramatic variations in propagation efficiency across the tibetan plateau that is strongly dependent on frequency we observe that sn rapidly decreases in frequency and amplitude as it passes through the northern portion of the plateau we show that in general sn propagation efficiency decreases with increasing frequency content we use 122 events from outside of the plateau and 61 from within to refine the boundaries of a region of inefficient high frequency sn propagation specifically we show that a larger portion of the northern tibetan plateau attenuates sn energy than was previously suggested in the southern plateau where high frequency sn is observed we computed an average velocity of 4 59 0 18 km s we also observed that the pn velocity within this region of inefficient high frequency sn propagation is nearly 4 slower than the pn velocity computed for paths restricted to the southern plateau and that the crust is about 10 km thinner than in the south the coincident locations of inefficient sn propagation and slow pn velocity is commonly observed in regions of active tectonics our results add constraints to the velocity structure of the lithosphere beneath the tibetan plateau and require first order lateral variations in the uppermost mantle structure despite the relatively uniform topography of the plateau

depth variant azimuthal anisotropy in tibet revealed by surface wave tomography azimuthal anisotropy derived from multimode rayleigh wave tomography in china exhibits depth dependent variations in tibet which can be explained as induced by the cenozoic india eurasian collision in west tibet the e w fast polarization direction at depths 100 km is consistent with the accumulated shear strain in the tibetan lithosphere whereas the n s fast direction at greater depths is aligned with indian plate motion in northeast tibet depth consistent nw se directions imply coupled deformation throughout the whole lithosphere possibly also involving the underlying asthenosphere significant anisotropy at depths of 225 km in southeast tibet reflects sublithospheric deformation induced by northward and eastward lithospheric subduction beneath the himalaya and burma respectively the multilayer anisotropic surface wave model can explain some features of sks splitting measurements in tibet with differences probably attributable to the limited back azimuthal coverage of most sks studies in tibet and the limited horizontal resolution of the surface wave results

elastic interferometry for ocean bottom cable data theory and examples elastic imaging from ocean bottom cable obc data can be challenging because it requires the prior estimation of both compressional wave p wave and shearwave s wave velocity fields seismic interferometry is an attractive technique for processing obc data because it performs model independent redatuming retrieving pseudo sources at positions of the receivers the purpose of this study is to investigate multicomponent applications of interferometry for processing obc data this translates into using interferometry to retrieve pseudo source data on the sea bed not only for multiple suppression but for obtaining p converted p to s wave ps wave and possibly pure mode s waves we discuss scattering based elastic interferometry with synthetic and field obc datasets conventional and scattering based interferometry integrands computed from a synthetic are compared to show that the latter yields little anti causal response a four component 4c pseudo source response retrieves pure mode s reflections as well at p and ps reflections pseudo source responses observed in obc data are related to p wave conversions at the seabed rather than to true horizontal or vertical point forces from a gulf of mexico obc data set diagonal components from a ninecomponent pseudo source response demonstrate that the p wave to s wave velocity ratio v p v s at the sea bed is an important factor in the conversion of p to s for obtaining the pure mode s wave reflections

evaluation of elastodynamic interferometric redatuming a synthetic study on salt dome flank imaging summary the recent introduction of novel redatuming techniques based on the principles of timereversed acoustics and source receiver reciprocity has allowed seismic measurements collected on one datum to be extrapolated to another datum under favourable conditions this process can approximately remove the complicated overburden structures without any velocity information we introduce an elastic redatuming scheme for walkaway vertical seismic profiles wvsps based on these principles using p and s wave potentials which are derived from the spatial derivatives of the measured wavefields additional field acquisition is utilized to directly measure these derivatives we test our elastic redatuming scheme on data simulated with an elastic finite difference algorithm the elastodynamic green s functions between multicomponent receivers in a borehole are then extracted from data recorded using multicomponent p and s wave sources at the surface the wavefields are redatumed as four parts two related to the p wave potential and another two related to the s wave potential these parts are migrated separately and form four independent images of the salt dome flank providing a more complete elastic description of the subsurface

increasing the sensitivity of controlled source electromagnetics with synthetic aperture controlled source electromagnetics csem has been used as a derisking tool in the hydrocarbon exploration industry we apply the concept of synthetic aperture to the lowfrequency electromagnetic field in csem synthetic aperture sources have been used in radar imaging for many years using the synthetic aperture concept big synthetic sources can be constructed by adding the response to small sources building blocks in different ways and consequently big sources with different radiation patterns can be created we show that the detectability of hydrocarbons is significantly enhanced by applying synthetic aperture to csem data more challenging targets such as deep reservoirs 4km belowsea floor can be detected the synthetic aperture technique also increases the sensitivity of the field to subsurface targets in the towing streamer acquisition we also show that a pseudovertical source orthogonally distributed dipole pairs can be constructed synthetically and that the detection capability of this pseudovertical source is increased by applying field steering the synthetic aperture concept opens a new line of research in csem with the freedom to design suitable synthetic aperture sources for a given purpose

seismic imaging of the downwelling indian lithosphere beneath central tibet a tomographic image of the upper mantle beneath central tibet from indepth data has revealed a subvertical high velocity zone from 100 to 400 kilometers depth located approximately south of the bangong nujiang suture we interpret this zone to be downwelling indian mantle lithosphere this additional lithosphere would account for the total amount of shortening in the himalayas and tibet a consequence of this downwelling would be a deficit of asthenosphere which should be balanced by an upwelling counterflow and thus could explain the presence of warm mantle beneath north central tibet

rayleigh wave tomography of china and adjacent regions 1 this paper presents a tomographic study on the s wave velocity structure of china and adjacent regions group velocity dispersions of fundamental rayleigh waves along more than 4000 paths were determined with frequency time analysis the study region was divided into a 1 1 grid and velocities in between grid nodes were calculated by bilinear interpolation the occam s inversion scheme was adopted to invert for group velocity distributions this method is robust and allows us to use a fine grid in model parameterization and thus helps to restore a more realistic velocity pattern checkerboard tests were carried out and the lateral resolution was estimated to be 4 6 in china and its eastern continental shelves the resulting group velocity maps from 10 to 184 s showed good correlation with known geological and tectonic features the pure path dispersion curves at each node were inverted for shear wave velocity structures the three dimensional velocity model indicates thick lithospheres in the yangtze and tarim platforms and hot upper mantles in baikal and western mongolia coastal area and continental shelves of eastern china and indochina and south china sea regions the tibetan plateau has a very thick crust with a low velocity zone in its middle beneath the crust a north dipping high velocity zone mimicking a subducting plate reaches to 200 km in depth and reaches to the kunlun mountains northward in northern tibet a low velocity zone immediately below the moho extends eastward then turns southward along the eastern edge of the plateau until it connects to the vast low velocity area in indochina and the south china sea

estimating changes in seismic wave velocity from a pneumatic source in an operational mine rock mass characteristics such as applied stress pore pressure and fracture density are coupled to the seismic wave propagation velocity therefore measuring small relative changes of seismic wave velocities in operational mines has the potential for tracking changes in these important rock mass properties to this end we have conducted an experiment with the aim of measuring seismic body wave velocities in situ at williams mine hemlo canada using a pneumatic source and multiple receivers we determine that the signal to noise ratio is improved by deconvolving the source signal from the signal recorded at the remotely grouted geophones we evaluate the estimated relative changes in traveltimes and assume that these are due to changes in the body wave velocity caused by mining related activities we are able to successfully detect the source response at remote receivers in an operational mine at a distance of more than 430 m

iron formation carbonates paleoceanographic proxy or recorder of microbial diagenesis the chemical and isotopic compositions of carbonates are commonly used as proxies for ancient seawater or paleoenvironments iron formation if carbonates have been used as evidence for an anoxic fe ii rich archean and paleoproterozoic ocean and high atmospheric co 2 contents it has been proposed however that microbial fe cycling dominates the chemical and isotopic compositions of if carbonates suggesting less direct applicability as an oceanic proxy here were use an isotope tracer that is not affected by biological processes or isotopic fractionation the radiogenic 87 rb87 sr system to test the applicability of if carbonates as a paleoenvironmental proxy we focus on the 2 5 ga campbellrand platform transvaal basin south africa that records a shift from ca mg carbonates to if carbonates during a marine transgression when coupled with previously determined fe c and o isotope compositions it becomes clear that the if carbonates studied here do not refl ect seawater compositions but instead record extensive microbial fe cycling in the soft sediment prior to lithifi cation these results question the use of if carbonates to infer seawater compositions and paleoenvironmental conditions including estimates for atmospheric co 2 contents

detection of widespread fluids in the tibetan crust by magnetotelluric studies magnetotelluric exploration has shown that the middle and lower crust is anomalously conductive across most of the north to south width of the tibetan plateau the integrated conductivity conductance of the tibetan crust ranges from 3000 to greater than 20 000 siemens in contrast stable continental regions typically exhibit conductances from 20 to 1000 siemens averaging 100 siemens such pervasively high conductance suggests that partial melt and or aqueous fluids are widespread within the tibetan crust in southern tibet the high conductivity layer is at a depth of 15 to 20 kilometers and is probably due to partial melt and aqueous fluids in the crust in northern tibet the conductive layer is at 30 to 40 kilometers and is due to partial melting zones of fluid may represent weaker areas that could accommodate deformation and lower crustal flow

revisiting hcn formation in earth s early atmosphere abstract using a new photochemical model the hcn chemistry in earth s early atmosphere is revisited we find that hcn production in a ch 4 rich early atmosphere could have been efficient similar to the results of a previous study zahnle 1986 for an assumed ch 4 mixing ratio of 1000 ppmv hcn surface deposition increases from 2 10 9 cm 2 s 1 at fco 2 3 to more than 1 10 10 cm 2 s 1 30 tg yr at fco 2 0 3 and 1 these conditions may well have applied throughout much of the archean eon 3 8 2 5 ga prior to the origin of life and the advent of methanogens hcn production rates would likely have been at 1 10 7 cm 2 s 1 or lower thereby providing a modest source of hcn for prebiotic synthesis

anisotropic rayleigh wave tomography of northeast china using ambient seismic noise abstract the ambient noise data recorded by 249 seismic stations in the permanent and temporary networks in northeast china are used to invert for the isotropic phase velocity and azimuthal anisotropy of rayleigh waves in the period band 5 50 s the inversion results reflect the structure from the shallow crust to upper mantle up to approximately 120 km depth beneath the songliao basin both the fast direction in shallow crust and strike of a low velocity anomaly in the middle crust are nne ssw which is coincident with the main tectonic trend of the paleo pacific tectonic domain this indicates that the rifting of the songliao basin is influenced by the subduction of paleo pacific plate the upper mantle of songliao block except the central area of songliao basin to the west of mudanjiang fault and the east of the north south gravity lineament is characterized by high velocity and weak anisotropy up to approximately 120 km depth we infer that there is delamination of lithospheric mantle beneath the songliao block obvious n s ne sw and e w trending fast directions are found in the lithospheric mantles of the east west and south sides of songliao block respectively which coincide with the strikes of the paleozoic tectonic in these areas this suggests that the frozen in anisotropic fabric in the lithospheric mantle can be used to indicate the historical deformation of the lithosphere in the northern margin of the north china craton the spatial variations of phase velocity and azimuthal anisotropy are more dramatic than those in northeast china blocks which indicates that the lithosphere of the north china craton has experienced more complicated tectonic evolution than that of the northeast china blocks

iron isotopes constrain biologic and abiologic processes in banded iron formation genesis abstract the voluminous 2 5 ga banded iron formations bifs from the hamersley basin australia and transvaal craton south africa record an extensive period of fe redox cycling the major fe bearing minerals in the hamersley transvaal bifs magnetite and siderite did not form in fe isotope equilibrium but instead reflect distinct formation pathways the near zero average 56fe values for magnetite record a strong inheritance from fe3 oxide hydroxide precursors that formed in the upper water column through complete or near complete oxidation transformation of the fe3 oxide hydroxide precursors to magnetite occurred through several diagenetic processes that produced a range of 56fe values 1 addition of marine hydrothermal fe 2 aq 2 complete reduction by bacterial dissimilatory iron reduction dir and 3 interaction with excess fe 2 aq that had low 56fe values and was produced by dir most siderite has slightly negative 56fe values of 0 5 that indicate equilibrium with late archean seawater although some very negative 56fe values may record dir support for an important role of dir in siderite formation in bifs comes from previously published c isotope data on siderite which may be explained as a mixture of c from bacterial and seawater sources several factors likely contributed to the important role that dir played in bif formation including high rates of ferric oxide hydroxide formation in the upper water column delivery of organic carbon produced by photosynthesis and low clastic input we infer that dir driven fe redox cycling was much more important at this time than in modern marine systems the low pyrite contents of magnetite and siderite facies bifs suggests that bacterial sulfate reduction was minor at least in the environments of bif formation and the absence of sulfide was important in preserving magnetite and siderite in the bifs minerals that are poorly preserved in the modern marine record the paucity of negative 56fe values in older early archean and younger early proterozoic bifs suggests that the extensive 2 5 ga hamersley transvaal bifs may record a period of maximum expansion of dir in earth s history

low hesperian pco2 constrained from in situ mineralogical analysis at gale crater mars carbon dioxide is an essential atmospheric component in martian climate models that attempt to reconcile a faint young sun with planetwide evidence of liquid water in the noachian and early hesperian in this study we use mineral and contextual sedimentary environmental data measured by the mars science laboratory msl rover curiosity to estimate the atmospheric partial pressure of co2 pco2 coinciding with a long lived lake system in gale crater at 3 5 ga a reaction transport model that simulates mineralogy observed within the sheepbed member at yellowknife bay ykb by coupling mineral equilibria with carbonate precipitation kinetics and rates of sedimentation indicates atmospheric pco2 levels in the 10s mbar range at such low pco2 levels existing climate models are unable to warm hesperian mars anywhere near the freezing point of water and other gases are required to raise atmospheric pressure to prevent lake waters from being lost to the atmosphere thus either lacustrine features of gale formed in a cold environment by a mechanism yet to be determined or the climate models still lack an essential component that would serve to elevate surface temperatures at least locally on hesperian mars our results also impose restrictions on the potential role of atmospheric co2 in inferred warmer conditions and valley network formation of the late noachian

the biogeochemistry of ferruginous lakes and past ferruginous oceans abstract anoxic and iron rich ferruginous conditions prevailed in the ocean under the low oxygen atmosphere that occurred through most of the archean eon while euxinic conditions i e anoxic and hydrogen sulfide rich waters became more common in the proterozoic ferruginous conditions persisted in deep waters ferruginous ocean regions would have been a major biosphere and earth surface reservoir through which elements passed through as part of their global biogeochemical cycles understanding key biological events such as the rise of oxygen in the atmosphere or even the transitions from ferruginous to euxinic or oxic conditions requires understanding the biogeochemical processes occurring within ferruginous oceans and their indicators in the rock record important analogs for transitions between ferruginous and oxic or euxinic conditions are paleoferruginous lakes their sediments commonly host siderite and ca carbonates which are important precambrian records of the carbon cycling lakes that were ferruginous in the past or euxinic lakes with cryptic iron cycling may also help understand transitions between ferruginous and euxinic conditions in shallow and mid depth oceanic waters during the proterozoic modern ferruginous meromictic lakes which host diverse anaerobic microbial communities are increasingly utilized as biogeochemical analogues for ancient ferruginous oceans such lakes are believed to be rare but regional and geological factors indicate they may be more common than previously thought while physical mixing processes in lakes and oceans are notably different many chemical and biological processes are similar the diversity of sizes stratifications and water chemistries in ferruginous lakes thus can be leveraged to explore biogeochemical controls in a range of marine systems near shore off shore silled basins or those dominated by terrestrial or hydrothermal element sources ferruginous systems both extant and extinct lacustrine and marine host a continuum of biogeochemical processes that highlight the important role of iron in the evolution of earth s surface environment

data driven adaptive decomposition of multicomponent seabed recordings dual sensor hydrophone and three component geophone data recorded on the seafloor allow the elastic wavefield to be decomposed into its upgoing and downgoing p and s wave components most decomposition algorithms require accurate knowledge of the elastic properties of the seafloor in the vicinity of the receivers and properly calibrated sensors in order for the data to be a faithful vector representation of the ground motion we present a multistep adaptive decomposition scheme that provides the necessary information directly from the data by imposing constraints on intermediate decomposition results the proposed scheme requires no a priori information and only a minimal amount of user defined input thus allowing multicomponent data to be decomposed in an automated data driven fashion the performance of the technique is illustrated using seabed data acquired in the north sea with prototype single sensors multicomponent geophones individually sampled realistic seafloor properties and sensor calibration operators are obtained and elastic decomposition of the calibrated data generally yields good results dominant water layer reverberations are successfully attenuated and primary reflections are substantially enhanced in the computed upgoing p wave potential just below the seafloor in contrast the result for the upgoing s wave potential is somewhat less convincing although the energy of water layer multiples is substantially reduced notable amounts of undesired multiple energy remain in this section after decomposition particularly at high offsets these imperfections may point to inaccuracies in the parametrization of the seafloor or remaining inaccuracies in the vector fidelity of the horizontal geophone recordings nevertheless the results obtained with the extended data driven decomposition scheme are at least comparable to previously published results

interferometric ground roll removal attenuation of scattered surface waves in single sensor data land seismic data are contaminated by surface waves or ground roll these surface waves are a form of source generated noise and can be strongly scattered by near surface heterogeneities the resulting scattered ground roll can be particularly difficult to separate from the desired reflection data especially when this scattered ground roll propagates in the crossline direction we have used seismic interferometry to estimate scattered surface waves recorded during an exploration seismic survey between pairs of receiver locations where sources and receivers coincide these interreceiver surface wave estimates were adaptively subtracted from the data this predictive subtraction process can successfully attenuate scattered surface waves while preserving the valuable reflected arrivals forming a new method of scattered ground roll attenuation we refer to this as interferometric ground roll removal

seismic and electromagnetic controlled source interferometry in dissipative media abstract seismic interferometry deals with the generation of new seismic responses by crosscorrelating existing ones one of the main assumptions underlying most interferometry methods is that the medium is lossless we develop an interferometry by deconvolution approach which circumvents this assumption the proposed method applies not only to seismic waves but to any type of diffusion and or wave field in a dissipative medium this opens the way to applying interferometry to controlled source electromagnetic csem data interferometry by deconvolution replaces the overburden by a homogeneous half space thereby solving the shallow sea problem for csem applications we demonstrate this at the hand of numerically modeled csem data

azimuthal anisotropy of lg attenuation in eastern tibetan plateau 1 the q of seismic phase lg is measured to approach the crustal q which can be used to infer crustal rheology a reverse two station event method is used to measure the interstationqlg which theoretically eliminates effects from the source and site response strong azimuthal anisotropy of 1 qlg in northern tibet is observed which approximately correlates with the seismic velocity anisotropy observed using both surface waves and teleseismic s waves both the intrinsic and scattering attenuation contribute the isotropic q whereas the anisotropic q may be associated with the fractures and faults in the crust strong 1 qlgis observed in the northwestern songpan ganzi fold belt where the high qdirections correspond with the orientations of major strike slip faults intrinsic and scatteringq values are estimated for different tectonic terranes in eastern tibetan plateau the intrinsic qvalues are approximately 500 in the northern tibetan plateau which suggests a hot 700 c crust consistent with low seismic velocity measurements our result suggests that the crust of northwestern songpan ganzi fold belt may be hotter than that of the qiangtang terrane which may be related to the strain heating along the major strike slip fault zones that dominate the northernmost tibet

micro scale tracing of fe and si isotope signatures in banded iron formation using femtosecond laser ablation we have detected micrometre scale differences in fe and si stable isotope ratios between coexisting minerals and between layers of banded iron formation bif using an uv femtosecond laser ablation system connected to a mc icp ms in the magnetite carbonate chert bif from the archean old wanderer formation in the shurugwi greenstone belt zimbabwe magnetite shows neither intra nor inter layer trends giving overall uniform d 56 fe values of 0 9 but exhibits intra crystal zonation bulk iron carbonates are also relatively uniform at near zero values however their individual d 56 fe value is highly composition dependent both siderite and ankerite and mixtures between both are present and d 56 fe end member values are 0 4 for siderite and 0 7 for ankerite the data suggest either an early diagenetic origin of magnetite and iron carbonates by the reaction of organic matter with ferric oxyhydroxides catalysed by fe iii reducing bacteria or more likely an abiotic reaction of organic carbon and fe iii during low grade metamorphism si isotope composition of the old wanderer bif also shows significant variations with d 30 si values that range between 1 0 and 2 6 for bulk layers these isotope compositions suggest rapid precipitation of the silicate phases from hydrothermal rich waters interestingly fe and si isotope compositions of bulk layers are covariant and are interpreted as largely primary signatures moreover the changes of fe and si isotope signatures between bulk layers directly reflect the upwelling dynamics of hydrothermal rich water which govern the rates of fe and si precipitation and therefore also the development of layering during periods of low hydrothermal activity precipitation of only small amounts of ferric oxyhydroxide was followed by complete reduction with organic carbon during diagenesis resulting in carbonate chert layers during periods of intensive hydrothermal activity precipitation rates of ferric oxyhydroxide were high and subsequent diagenesis triggered only partial reduction forming magnetite carbonate chert layers we are confident that our micro analytical technique is able to detect both the solute flux history into the sedimentary bif precursor and the bif s diagenetic history from the comparison between coexisting minerals and their predicted fractionation factors

remote sensing of hydrocarbon layers by seabed logging sbl results from a cruise offshore angola detecting and assessing hydrocarbon reservoirs without the need to drill test wells is of major importance to the petroleum industry seismic methods have traditionally been used in this context but the results can be ambiguous another approach is to use electromagnetic sounding methods that exploit the resistivity differences between a reservoir containing highly resistive hydrocarbons and one saturated with conductive saline fluids modeling presented by eidesmo et al 2002 demonstrates that by using seabed logging sbl a special application of frequency domain controlled source electromagnetic csem sounding the existence or otherwise of hydrocarbon bearing layers can be determined and their lateral extent and boundaries can be quantified such information provides valuable complementary constraints on reservoir geometry and characteristics obtained by seismic surveying

detecting the oldest geodynamo and attendant shielding from the solar wind implications for habitability abstract the onset and nature of the earliest geomagnetic field is important for understanding the evolution of the core atmosphere and life on earth a record of the early geodynamo is preserved in ancient silicate crystals containing minute magnetic inclusions these data indicate the presence of a geodynamo during the paleoarchean between 3 4 and 3 45 billion years ago while the magnetic field sheltered earth s atmosphere from erosion at this time standoff of the solar wind was greatly reduced and similar to that during modern extreme solar storms these conditions suggest that intense radiation from the young sun may have modified the atmosphere of the young earth by promoting loss of volatiles including water such effects would have been more pronounced if the field were absent or very weak prior to 3 45 billion years ago as suggested by some models of lower mantle evolution the frontier is thus trying to obtain geomagnetic field records that are 3 45 billion years old as well as constraining solar wind pressure for these times in this review we suggest pathways for constraining these parameters and the attendant history of earth s deep interior hydrosphere and atmosphere in particular we discuss new estimates for solar wind pressure for the first 700 million years of earth history the competing effects of magnetic shielding versus solar ion collection and bounds on the detection level of a geodynamo imposed by the presence of external fields we also discuss the prospects for constraining hadean paleoarchean magnetic field strength using paleointensity analyses of zircons

mineralogical constraints on precambrian pco2 arising from m t rosing d k bird n h sleep c j bjerrum 744 747 2010 10 1038 nature08955 rosing and others1 recently proposed a new model for the early earth s atmospheric composition they suggest on the basis of mineral assemblages in sedimentary rocks that atmospheric co2 and ch4 concentrations have been consistently overestimated this proposal is intriguing given that high concentrations of these gases are traditionally considered to be the solution to the faint young sun paradox of why the earth did not freeze over in our sun s youth when solar luminosity was greatly decreased rosing and others1 instead invoke a decreased planetary albedo during the earth s early history new insights into this problem which has long captivated those interested in the earth s early climate and biological history are welcome however we believe that the arguments presented by rosing and others1 for roughly modern values of the partial pressure of co2 are not robust

rayleigh wave phase velocities in the pacific with implications for azimuthal anisotropy and lateral heterogeneities summary the lateral distribution of fundamental mode rayleigh wave phase velocities in the pacific has been calculated in order to determine the variation in velocity as a function of the age of the oceanic plate the importance of azimuthal anisotropy and the presence of secondary lateral heterogeneities the data set used in this study consists of phase velocity measurements in the period range 20 125 s for 178 paths traversing the pacific both the pure path and spherical harmonic inversion techniques are used in this investigation with an emphasis placed on determining the effectiveness and resolving power of these methods in calculating the velocity distribution in an oceanic regime the first order velocity age relation which dominantly characterizes surface wave dispersion in an oceanic environment can be adequately modelled by either the pure path or spherical harmonic techniques azimuthal anisotropy is the dominant second order effect and is best developed in regions less than 80 myr in age in this age zone the faster direction of wave propagation is either in the direction of fossil seafloor spreading or in the direction of present day absolute plate motion in the western pacific there is a correlation with paleo relative motion fossil seafloor spreading for periods less than 50 s lateral velocity heterogeneities which are not related to the age of the oceanic plate are found by pure path inversions and are correlated with regions associated with shallow residual depth anomalies this effect slow velocities with respect to the velocity age model monotonically decreases as a function of period and is indicative of a shallow origin age independent anomalies were also modelled by the application of the pure path and spherical harmonic methods in a sequential inversion

a review of shear wave splitting in the crack critical crust summary over the last 15 years it has become established that crack induced stress aligned shear wave splitting with azimuthal anisotropy is an inherent characteristic of almost all rocks in the crust this means that most in situ rocks are pervaded by fluid saturated microcracks and consequently are highly compliant the evolution of such stress aligned fluid saturated grain boundary cracks and pore throats in response to changing conditions can be calculated in some cases with great accuracy using anisotropic poro elasticity ape ape is tightly constrained with no free parameters yet dynamic modelling with ape currently matches a wide range of phenomena concerning anisotropy stress shear waves and cracks in particular ape has allowed the anisotropic response of a reservoir to injection to be calculated predicted with hindsight and the time and magnitude of an earthquake to be correctly stress forecast the reason for this calculability and predictability is that the microcracks in the crust are so closely spaced that they form critical systems this crack critical crust leads to a new style of geophysics that has profound implications for almost all aspects of pre fracturing deformation of the crust and for solid earth geophysics and geology we review past present and speculate about the future of shear wave splitting in the crack critical crust shear wave splitting is seen to be a dynamic measure of the deformation of the rock mass there is some good news and some bad news for conventional geophysics many accepted phenomena are no longer valid at high spatial and temporal resolution a major effect is that the detailed crack geometry changes with time and varies from place to place in response to very small previously negligible changes however at least in some circumstances the behaviour of the rock in the highly complex inhomogeneous earth may be calculated and the response predicted opening the way to possible control by feedback the need is to devise ways to exploit these new opportunities in the crack critical crust recent observations from the smsites project at husavik in northern iceland gathered while this review was being written display the extraordinarily sensitivity of in situ rock to small changes at great distances the effects are far too large to occur in a conventional elastic brittle crust and their presence confirms the highly compliant nature of the crack critical crust

oxidative sulfide dissolution on the early earth recent evidence suggests that the biological production of oxygen in earth s surface oceans may have preceded the initial accumulation of large amounts of oxygen in the atmosphere by 100 million years or more however the potential effects of early oxygen production on surface ocean chemistry have remained little explored and questions persist regarding the locus of oxidation of crustal material i e subaerial and or submarine settings here we revisit the notion of spatially restricted oxygen oases in the archean surface ocean by employing a simple steady state box model of the surface ocean in a coastal upwelling system using pyrite as an example we then explore the possibility that oxygenic photosynthesis in such a system could support the widespread oxidation of crustal sulfide minerals without concomitant accumulation of oxygen in the atmosphere we find that it is possible to establish strong air sea gas exchange disequilibrium with respect to o 2 however in marine settings there is an apparent timescale mismatch between the kinetics of oxidative dissolution and the rate at which sulfide minerals delivered physically to shallow marine sediments will be buried below the zone of oxygen penetration estimated dissolution timescales compare somewhat more favorably with typical timescales of soil development and physical weathering transport in the subaerial realm despite the much lower dissolved oxygen concentrations inferred at gas exchange equilibrium with atmospheric po 2 relative to those attainable in productive regions of the surface ocean however broad scale subaerial weathering of sulfides would likely have required either local o 2 production at the site of weathering or transient increase in atmospheric po 2 above the most plausible levels characteristic of a pervasively reducing atmosphere

the lithosphere of north china craton from surface wave tomography abstract fundamental mode rayleigh wave group velocity dispersion data were used in a tomographic study to reconstruct the crust and upper mantle shear velocity structure in east asia this article expounds the features of lithosphere structure in different sub regions of north china and discusses their implications for the tectonic evolution and present tectonics of north china craton ncc the study revealed that strong earthquakes took place in the low velocity zones and along the boundaries between high and low velocities in the middle and lower crust the active nw trending zhangjiakou bohai seismic belt corresponds to a prominent low velocity zone in the crust and upper mantle lid in addition to the well known structural difference between western and eastern ncc we are able to resolve clearly the lithospheric features of sub regions in eastern ncc the southern north china basin and north yellow sea are characterized by a high velocity crust and upper mantle lid down to 70 80 km depth while the northern north china basin has a low velocity crust and the velocity below moho to about 130 km is rather uniform and low there is also a general trend of velocity decrease in the upper mantle from west to east the velocity images bear the imprints of subduction and collision that took place in the periphery of ncc since mesozoic and shed some light on the mechanism of mesozoic lithospheric thinning of eastern ncc

experimental quantification of the seismoelectric transfer function and its dependence on conductivity and saturation in loose sand under certain circumstances seismic propagation within porous media may be associated to a conversion of mechanical into electromagnetical energy known as a seismoelectromagnetic phenomemon the propagation of fast compressional p waves is more specifically associated to manifestations of a seismoelectric field linked to fluid flow within the pores the analysis of seismoelectric phenomena which requires combining the theory of electrokinetics to biot s theory of poroelasticity provides us with a transfer function noted e u that links the coseismic seismoelectric field e to the seismic acceleration u in order to measure the transfer function we have developed an experimental set up enabling seismoelectric laboratory observation in unconsolidated quartz sand within the kilohertz range the investigation focused on the impact of fluid conductivity and water saturation over the coseismic seismoelectric field during the experiment special attention was given to the accuracy of electric field measurements we concluded that in order to obtain a reliable estimate of the electric field amplitude the dipole from which the potential differences are measured should be of much smaller length than the wavelength of the propagating seismic field time lapse monitoring of the seismic velocities and seismoelectric transfer functions were performed during imbibition and drainage experiments in all cases the quantitative analysis of the seismoelectric transfer function e u was in good agreement with theoretical predictions while investigating saturation variations from the residual water saturation to full saturation we showed that the e u ratio undergoes a switch in polarity at a particular saturation s also implying a sign change of the filtration traducing a reversal of the relative fluid displacement with respect to the frame this sign change at critical saturation s stresses a particular behaviour of the poroelastic medium the dropping of the coseismic electric field to zero traduces the absence of relative pore fluid displacements representative of a biot dynamically compatible medium we concluded from our experimental study in loose sand that measurements of the coseismic seismoelectric coupling may provide information on fluid distribution within the pores and that the reversal of the seismoelectric field may be used as an indicator of the dynamically compatible state of the medium

integration of electromagnetic and seismic data to assess residual gas risk in the toe thrust belt of deepwater niger delta during the last couple of years drilling activities in the deepwater thrust belt play of nigeria figure 1 have shown mixed results with some wells finding pay but others coming across low saturation gas and or brine sands in this frontier exploration context shell nigeria e p co snepco has been particularly successful in a number of toe thrust prospects where the key risk is trap integrity related to both earlier thrust faults and later extensional faults

seismic anisotropy beneath the eastern margin of the tibetan plateau from sks splitting observations abstract the eastern margin of the tibetan plateau is one of the areas of the entire region with the most complex deformation seismic anisotropy reveals the imprints of past and present deformation in the lithosphere and asthenosphere which could help elucidate the deformation mechanism this study investigated the characteristics of seismic anisotropy beneath 512 seismic stations based on sks phases from 200 earthquake the measured splitting delay time varied from 0 2 1 4 s average 0 7 s in the central sichuan basin scb the average delay time is only 0 35 s consistent with the characteristics of the relatively thick and rigid lithosphere of the stable scb however the northeastern scb has relatively stronger anisotropy with a fast polarization direction fpd with ne sw orientation which is markedly different from the weak anisotropy with nw se orientation in the central scb we speculate that owing to e w compressive stress caused by eastward escape of material from the tibetan plateau the lithosphere in this area has undergone considerable deformation along the right lateral maximum shear direction in the chuandian diamond block the fpd of several adjacent stations was approximately e w i e almost perpendicular to the fpd in the northern and southern regions which might be related to partial melting in the lithosphere and to compressive stress the fpd in the west qinling orogenic belt is approximately parallel to the strike of the surface faults and the average delay time is approximately 1 3 s as the lithosphere is insufficient to produce such strong anisotropy in this region we infer that the lithosphere and asthenosphere both contribute to the seismic anisotropy delay time in the southern songpan ganzi block is much smaller than in the north which might be attributable to low velocity mantle upwelling and complex tectonic stress in this area

geoscience fossil raindrops and ancient air an analysis of fossil imprints of ancient raindrops suggests that the density of the atmosphere 2 7 billion years ago was much the same as that today this result casts fresh light on a long standing palaeoclimate paradox see letter p 359 about 2 7 billion years ago towards the end of the archaean period a rain shower left its mark on ash deposits from a volcanic eruption in what is now the south african veldt as the ash hardened to form tuff rock the crater like imprints of the individual raindrops were fossilized an analysis of these imprints aided by comparison with similar prints formed during the 2010 eyjafjallajokull eruption in iceland suggests that air density during the archaean was no more than twice modern levels at that time the sun was dimmer than it is today but the climate was warm most theories to explain this faint young sun paradox have assumed that the atmosphere was denser in the archaean than it is now and that the greenhouse effect was stronger but this latest work seems to rule out higher carbon dioxide levels nitrogen pressure broadening remains unlikely but possible

determination of the fe ii aq magnetite equilibrium iron isotope fractionation factor using the three isotope method and a multi direction approach to equilibrium abstract magnetite is ubiquitous in the earth s crust and its presence in modern marine sediments has been taken as an indicator of biogeochemical fe cycling magnetite is also the most abundant fe oxide in banded iron formations bifs that have not been subjected to ore forming alteration magnetite is therefore an important target of stable fe isotope studies and yet interpretations are currently difficult because of large uncertainties in the equilibrium stable fe isotope fractionation factors for magnetite relative to fluids and other minerals in this study we utilized the three isotope method 57fe 56fe 54fe to explore isotopic exchange via an enriched 57fe tracer and natural mass dependent fractionation using 56fe 54fe variations during reaction of aqueous fe ii fe ii aq with magnetite importantly we employed a multi direction approach to equilibrium by reacting four 57fe enriched fe ii solutions that had distinct 56fe 54fe ratios which identifies changes in the instantaneous fe isotope fractionation factor and hence identifies kinetic isotope effects we find that isotopic exchange can be described by two 56fe 54fe fractionations where an initial rapid exchange 66 isotopic mixing within 1 day involved a relatively small fe ii aq magnetite 56fe 54fe fractionation followed by slower exchange 25 isotopic mixing over 50 days that was associated with a larger fe ii aq magnetite 56fe 54fe fractionation this later fractionation is interpreted to approach isotopic equilibrium between fe ii aq and the total magnetite all four fe ii solutions extrapolate to the same final equilibrium 56fe 54fe fractionation for fe ii aq magnetite of 1 56 0 20 2 at 22 c additional experiments that synthesized magnetite via conversion of ferrihydrite by reaction with aqueous fe ii yield final 56fe 54fe fractionations that are identical to those of the exchange experiments our experimental results agree well with calculated fractionation factors using the reduced partition function ratios for fe h 2 o 6 2 from rustad et al 2010 and stoichiometric magnetite from mineev et al 2007 and these relations may be combined with the experimental constraints to determine the temperature dependence of the fe ii aq magnetite fractionation factor 10 3 ln f e i i aq magnetite 0 145 0 002 10 6 t 2 0 10 0 02 where t is in k part of the reason for large discrepancies in calculated fe isotope fractionation factors for magnetite likely lies in the stoichiometry of the mineral in specific studies given the significant effect of octahedral versus tetrahedral fe isotope fractionation that has been calculated when our results are applied to bif genesis our experimentally determined fe ii aq magnetite fractionation factor indicates that magnetite siderite mineral pairs in 2 5 ga bifs did not form in fe isotope equilibrium with each other or with ancient seawater iron oxides in such bifs are therefore more likely to have formed through processes that were isolated from equilibrium with the oceans indicating that such bif minerals may not be suitable proxies for ancient paleoenvironments

complex multilayered azimuthal anisotropy beneath tibet evidence for co existing channel flow and pure shear crustal thickening of the two debated end member models for the late cenozoic thickening of tibetan crust one invokes channel flow rapid viscous flow of the mid lower crust driven by topography induced pressure gradients and transporting crustal rocks eastward and the other pure shear faulting and folding in the upper crust with viscous shortening in the mid lower crust deep crustal deformation implied by each model is different and would produce different anisotropic rock fabric observations of seismic anisotropy can thus offer a discriminant we use broadband phase velocity curves each a robust average of tens to hundreds of measurements to determine azimuthal anisotropy in the entire lithosphere asthenosphere depth range and constrain its amplitude inversions of the differential dispersion from path pairs region average inversions and phase velocity tomography yield mutually consistent results defining two highly anisotropic layers with different fast propagation directions within each the middle crust and the asthenosphere in the asthenosphere beneath central and eastern tibet anisotropy is 2 4 per cent and has a nne ssw fast propagation azimuth indicating flow probably driven by the nne ward shallow angle subduction of india the distribution and complexity of published shear wave splitting measurements can be accounted for by the different anisotropy in the mid lower crust and asthenosphere the estimated splitting times that would be accumulated in the crust alone are 0 25 0 8 s in the upper mantle 0 5 1 2 s depending on location in the middle crust 20 45 km depth beneath southern and central tibet azimuthal anisotropy is 3 5 and 4 6 per cent respectively and its e w fast propagation directions are parallel to the current extension at the surface the rate of the extension is relatively low however whereas the large radial anisotropy observed in the middle crust requires strong alignment of mica crystals implying large finite strain and consistent with high rate horizontal flow together radial and azimuthal anisotropy suggest eastward mid crustal channel flow in central tibet along the regional topography gradient in ne high tibet mid crustal azimuthal anisotropy is 4 8 per cent and has wnw ese and nw se fast propagation directions parallel to the net extension at the surface these fast directions are inconsistent with channel flow following the sw ne regional topography gradient instead they suggest similar net deformation in the decoupled shallow and deep crust in the brittle upper crust it is accommodated by strike slip faulting in the ductile mid lower crust by shear oriented at 45 to the faults although mid crustal flow beneath ne tibet may transport some material towards the plateau periphery at a low region average rate the dominant mid crust deformation pattern is shear parallel to the plateau boundary this implies that channel flow from central tibet is not the main cause of the on going crustal thickening farther northeast

multimode rayleigh wave inversion for heterogeneity and azimuthal anisotropy of the australian upper mantle summary we present an azimuthally anisotropic 3 d shear wave speed model of the australian upper mantle obtained from the dispersion of fundamental and higher modes of rayleigh waves we compare two tomographic techniques to map path average earth models into a 3 d model for heterogeneity and azimuthal anisotropy method i uses a rectangular surface cell parametrization and depth basis functions that represent independently constrained estimates of radial earth structure it performs an iterative inversion with norm damping and gradient regularization method ii uses a direct inversion of individual depth layers constrained by bayesian assumptions about the model covariance we recall that bayesian inversions and discrete regularization approaches are theoretically equivalent and with a synthetic example we show that they can give similar results the model we present here uses the discrete regularized inversion of independent path constraints of method i on an equal area grid with the exception of westernmost australia we can retrieve structure on length scales of about 250 km laterally and 50 km in the radial direction to within 0 8 per cent for the velocity 20 per cent for the anisotropic magnitude and 20 for its direction on length scales of 1000 km and longer down to about 200 km there is a good correlation between velocity heterogeneity and geologic age at shorter length scales and at depths below 200 km however this relationship breaks down the observed magnitude and direction of maximum anisotropy do not in general appear to be correlated to surface geology the pattern of anisotropy appears to be rather complex in the upper 150 km whereas a smoother pattern of fast axes is obtained at larger depth if some of the deeper directions of anisotropy are aligned with the approximately n s direction of absolute plate motion this correspondence is not everywhere obvious despite the fast 7 cm yr 1 northward motion of the australian plate more research is needed to interpret our observations in terms of continental deformation predictions of sks splitting times and directions an integrated measure of anisotropy are poorly matched by observations of shear wave birefringence

crust and uppermost mantle beneath the north china craton northeastern china and the sea of japan from ambient noise tomography 1 a 3 d shear velocity model of the crust and uppermost mantle to a depth of 100 km is presented beneath the north china craton ncc northeastern china the korean peninsula and the sea of japan ambient noise rayleigh wave tomography is applied to data from more than 300 broadband seismic stations from chinese provincial networks cearray the japanese f net and the iris global seismic network continuous data from 2007 to 2009 are used to produce group and phase velocity maps from 8 s to 45 s periods the model is motivated to constrain the distributed intraplate volcanism crustal extension cratonic rejuvenation and lithospheric thinning that are hypothesized for the study region numerous robust features are observed that impose new constraints on the geometry of these processes but discussion concentrates only on four 1 the north south gravity lineament follows the 40 km contour in crustal thickness and crustal thickness is anticorrelated with water depth beneath the sea of japan consistent with crustal isostasy for a crust with laterally variable composition 2 the lithosphere is thin 70 km beneath the songliao bohai graben but seismically fast 3 even thinner more attenuated lithosphere bounds three sides of the eastern ncc in a horseshoe shape identifying a region of particularly intense tectonothermal modification where lithospheric rejuvenation may have reached nearly to the base of the crust 4 low velocity anomalies reach upward in a y shape in the mantle beneath the eastern and western borders of the sea of japan extending well into continental east asia in the west and are separated by a 60 km thick lithosphere beneath the central sea of japan this anomaly may reflect relatively shallow slab dehydration in the east and in the west may reflect deeper dehydration and convective circulation in the mantle wedge overlying the stagnant slab

faint young sun problem more severe due to ice albedo feedback and higher rotation rate of the early earth 1 during the archean 3 8 2 5 billion years ago the sun was up to 25 less luminous than today yet there is strong evidence that the earth s ocean surface was not completely frozen the most obvious solutions to this faint young sun problem demand high concentrations of greenhouse gases such as carbon dioxide here we present the first comprehensive 3 dimensional simulations of the archean climate that include processes as the sea ice albedo feedback and the higher rotation rate of the earth these effects lead to co2 partial pressures required to prevent the earth from freezing that are significantly higher than previously thought for the early archean we find a critical co2partial pressure of 0 4 bar in contrast to 0 06 bar estimated in previous studies with 1 dimensional radiative convective models our results suggest that currently favored greenhouse solutions could be in conflict with constraints emerging for the middle and late archean

decomposition of electromagnetic fields into upgoing and downgoing components this paper gives a unified treatment of electromagnetic em field decomposition into upgoing and downgoing components forconductiveandnonconductivemedia wheretheelectromagneticdataaremeasuredonaplaneinwhichtheelectricpermittivity magneticpermeability andelectricalconductivityareknown constants with respect to space and time above and below the plane of measurement the medium can be arbitrarily inhomogeneousandanisotropic in particular the proposed decomposition theory applies to marine em low frequency data acquired for hydrocarbon mapping where the upgoing components of the recordedfield guided and refracted from the reservoir that are of interest for the interpretation the direct source field the refracted airwave induced by the source the reflected field from the sea surface and most magnetotelluric noise traveling downward just below the seabed are field components that are considered to be noise in electromagneticmeasurements the viability and validity of the decomposition method is demonstrated using modeled and real marine em data also termed seabed logging sbl data the synthetic data are simulated in a model that is fairly representative of the geologic area wheretherealsblwerecollected theresultsfromthesynthetic data study therefore are used to assist in the interpretation of the realdatafromanareawith320 mwaterdepthaboveaknowngas province offshore norway the effect of the airwave is seen clearly in measured data after field decomposition just below the seabed the upgoing component of the recorded electric field has almost linear phase indicating that most of the effect of the airwavecomponenthasbeenremoved

the free surface assumption for marine data driven demultiple methods in the past integral formulations for marine data driven demultiple methods have been derived from reciprocity theorems two fundamental assumptions in these derivations were that the sea surface is flat and has a known reflection coefficient often taken to be minus one in this paper we show that for dual sensor data these assumptions can be relaxed the sea surface has to obey the same conditions as any other reflecting boundary in the subsurface it must be constant in time but shape and reflection strength can vary in space for both surface related multiple elimination and multiple attenuation by multi dimensional deconvolution we derive integral equations that depend only on the measured pressure and particle velocity fields finally we show there is an intimate connection between the integral equations for the methods

monitoring changes in velocity and q using non physical arrivals in seismic interferometry application of seismic interferometry to records from receivers at the earth s surface from sources in wells retrieves the reflection response measured at the receivers as if from virtual sources located also at the surface when the wavefields experience intrinsic losses during propagation non physical arrivals ghosts would appear in the retrieved result these ghosts appear due to waves that reflect inside a subsurface layer thus a ghost contains information about the seismic properties of the specific layer we show how such ghosts can be used to monitor layer specific changes in the velocity and intrinsic losses in the subsurface we show how to identify the ghosts using numerical modelling results from a vertical well and how to estimate the layer specific velocity and quality factor changes using numerical modelling results from a horizontal well as well as ultrasonic s wave laboratory data

on the relation between seismic interferometry and the simultaneous source method summary in seismic interferometry the response to a virtual source is created from responses to sequential transient or simultaneous noise sources in the simultaneous source method overlapping responses to sources with small time delays are recorded seismic interferometry and the simultaneous source method are related in this paper we make this relation explicit by discussing deblending as a form of seismic interferometry by multidimensional deconvolution

mobility of nutrients and trace metals during weathering in the late archean abstract the evolution of the geosphere and biosphere depends on the availability of bio essential nutrients and trace metals consequently the chemical and isotopic variability of trace elements in the sedimentary record have been widely used to infer the existence of early life and fluctuations in the near surface environment on the early earth particularly fluctuations in the redox state of the atmosphere in this study we applied late archean weathering models hao et al 2017 developed to estimate the behavior of major elements and the composition of late archean world average river water to explore the behavior of nutrient and trace metals and their potential for riverine transport we focused on p mn cr and cu during the weathering of olivine basalt in our standard late archean weathering model pco2 g 10 1 5 bars ph2 g 10 5 0 bars crustal apatite was totally dissolved by the acidic rainwater during weathering our model quantitatively links the pco2 g of the atmosphere to phosphate levels transported by rivers the development of late archean river water ph 6 4 resulted in riverine phosphate of at least 1 7 molar much higher than at the present day at the end of the early proterozoic snowball earth event when pco2 g could be 0 01 0 10 bars river water may have transported up to 70 molar phosphate depending on the availability of apatite thereby stimulating high levels of oxygenic photosynthesis in the marine environment crustal levels of mn in olivine dissolved completely during weathering except at large extents of weathering where mn was stored as a component of a secondary carbonate mineral the corresponding mn content of river water about 1 2 molar is higher than in modern river water whiffs of 10 5 mole o2 gas or hno3 kg 1 h2o resulted in the formation of pyrolusite mno2 and abundant hematite and simultaneous dramatic decreases in the concentration of mn ii in the river water chromite dissolution resulted in negligible dissolved cr in archean river water however amorphous cr oh 3 representing easily weatherable cr bearing minerals dissolved totally during the weathering simulations resulting in concentrations of cr iii in the river water of up to 0 14 molar higher than at the present day late archean weathering of accessory chalcopyrite produced chalcocite and bornite and extremely low concentrations of cu 10 15 molar because of the low solubilities of the copper sulfides however pulses of either o2 g or hno3 produced native copper chalcocite and bornite much more hematite and river water containing levels of dissolved cu comparable to the present day copper mineralogy predicted by weathering models might provide a new correlation with evidence from studies of copper mineral evolution overall our results implied that the redox state of the atmosphere the ph of surface waters and the availability of easily weatherable minerals are all important factors controlling the dissolution of trace elements in river water interpretation of the sedimentary signatures of trace elements should consider not only the redox state but also the ph and availability of accessory minerals

green s functions extraction and surface wave tomography from microseisms in southern california we use crosscorrelations of seismic noise data from 151 stations in southern california to extract the group velocities of surface waves between the station pairs for the purpose of determining the surface wave velocity structure we developed an automated procedure for estimating the green s functions and subsequent tomographic inversion from the 11 325 station pairs based on the characteristics of the noise field we eliminate specific events by a procedure that does not introduce any spurious spectral distortion in the band of interest 0 05 0 2 hz further we only used the emerging arrival structure above a threshold signal to noise ratio the result is that mostly station pairs with their axes oriented toward the sea are used consistent with the noise having a microseism origin finally it is the time derivative of the correlation function that is actually related to the green s function the emergence of the time domain green s function is proportional to the square root of the recording time and inversely proportional to the square root of the distance between stations the tomographic inversion yields a surface wave velocity map that compares favorably with more conventional and elaborate experimental procedures