chromium isotopes in carbonates a tracer for climate change and for reconstructing the redox state of ancient seawater abstract strontium and carbon isotopes of marine carbonates are routinely applied for chemostratigraphic cross correlations of time equivalent sedimentary sequences and for calibration of the compositional evolution of seawater throughout earth s history mainly for the purpose of reconstructing ancient climatic changes we here present results of a new isotopic tracer system stable chromium isotopes applied to a late ediacaran marine carbonate sequence exposed in the calera de recalde syncline arroyo del soldado group uruguay the aim was to compare cr isotope signatures directly to 13 c 87 sr 86 sr and 143 nd 144 nd fluctuations in a well defined stratigraphic profile comprising sediments that were deposited during cold warm periods accompanied by sea level changes in response to glaciation deglaciation at higher latitudes the studied section is characterized by a pronounced negative down to 3 3 13 c excursion in carbonates paralleled by a decrease of 87 sr 86 sr values chromium isotope signatures over this section also show a correlated decrease in 53 cr 53 cr 53 cr 52 cr sample 53 cr 52 cr srm979 1 1000 values from 0 29 to 0 17 which mirrors a decrease in positively fractioned seawater signatures to slightly negative values characteristic of high temperature magmatic sources linear correlations between 53 cr and end 570 ma 87 sr 86 sr and cr concentrations can be explained by mixing between two major input sources of cr nd and sr into the shallow seawater 1 a source characterized by negative 53 cr values of 0 2 low 87 sr 86 sr values of 0 707 and elevated 147 sm 144 nd values of 0 13 recognized as a subaqueous hydrothermal dominated input source and 2 a source characterized by positively fractionated 53 cr values of 0 2 higher 87 sr 86 sr values of 0 708 and lower 147 sm 144 nd values of 0 11 a source which is strongly affected by continentally derived input chromium isotopes provide a powerful tool for reconstructing the redox state of ancient seawater since positive values indicate that at least locally neoproterozoic shallow ocean waters were sufficiently oxidized to fractionate chromium and or that oxygen levels of the atmosphere were sufficient to transform cr iii into the more mobile hexavalent cr vi formed during weathering processes on land the fact that 87 sr 86 sr values despite 13 c fluctuations remain low indicative of a strong hydrothermal input into the basin at his time implies that co 2 limitation was the cause of negative 13 c and 53 cr excursions in otherwise nutrient rich late neoproterozoic basins and that glaciation is only one more consequence of a tectonically driven biologically mediated system in such a scenario glaciation acts as an amplifier of 53 cr signals these signals in marine carbonates are a sensitive tracer for redox processes in the ocean and or on land and have the potential to contribute significantly in combination with the other commonly used isotopic tracers to the reconstruction of climatic changes particularly those that are associated with major glaciation periods in earth s history

possible role of warm sst bias in the simulation of boreal summer monsoon in sintex f2 coupled model reasonably realistic climatology of atmospheric and oceanic parameters over the asian monsoon region is a pre requisite for models used for monsoon studies the biases in representing these features lead to problems in representing the strength and variability of indian summer monsoon ism this study attempts to unravel the ability of a state of the art coupled model sintex f2 in simulating these characteristics of ism the coupled model reproduces the precipitation and circulation climatology reasonably well however the mean ism is weaker than observed as evident from various monsoon indices a wavenumber frequency spectrum analysis reveals that the model intraseasonal oscillations are also weaker than observed one possible reason for the weaker than observed ism arises from the warm bias over the tropical oceans especially over the equatorial western indian ocean inherent in the model this warm bias is not only confined to the surface layers but also extends through most of the troposphere as a result of this warm bias the coupled model has too weak meridional tropospheric temperature gradient to drive a realistic monsoon circulation this in turn leads to a weakening of the moisture gradient as well as the vertical shear of easterlies required for sustained northward propagation of rain band resulting in weak monsoon circulation it is also noted that the recently documented interaction between the interannual and intraseasonal variabilities of ism through very long breaks vlbs is poor in the model this seems to be related to the inability of the model in simulating the eastward propagating madden julian oscillation during vlbs

ediacaran δ13c chemostratigraphy of south china abstract we report new 13 c chemostratigraphic data of the ediacaran system in south china using litho and biostratigraphic markers as independent calibrators we combine new and previously published 13 c data to construct a composite 13 c profile for the ediacaran system in south china the composite curve shows in chronostratigraphic order 1 a negative 13 c excursion en1 in the doushantuo cap carbonate that overlies the ghaub age nantuo diamictite 2 a pronounced positive 13 c excursion ep1 in the lower doushantuo formation where doushantuo type acanthomorphic acritarchs red algae and animal embryos first appear 3 a mild negative 13 c excursion en2 in the middle doushantuo formation 4 a positive 13 c excursion ep2 in the upper doushantuo formation where doushantuo type acanthomorphic acritarchs micrometazoans and stem group florideophyte red algae become abundant and diverse 5 a pronounced negative 13 c excursion en3 near the uppermost doushantuo formation 6 a strongly positive 13 c excursion up to 6 3 pdb ep3 in the lower dengying formation 7 a stable 13 c plateau ca 2 5 pdb ei in the middle and upper dengying formation where abundant vendotaenid algae macroscopic ediacaran fossils biomineralized animal tubes and mostly horizontal animal traces occur and 8 a 10 negative 13 c excursion en4 immediately below the ediacaran cambrian boundary the general pattern of this composite 13 c curve is robust despite a considerable amount of variation among the 13 c profiles of different sections this composite 13 c curve provides a first order chemostratigraphic framework for the subdivision and correlation of the ediacaran system in south china

dispersion and attenuation of mantle waves through waveform inversion summary we propose a new approach to the determination of elastic and anelastic parameters of the earth s structure from seismic data instead of measuring such functionals of the medium response as the phase delay or spectral ratios we perturb parameters of the model to satisfy the observed waveforms the advantage of our waveform inversion technique wit is that it uses the properties of the earth as a smoothing filter also because the earth s structure is the common denominator the method allows simultaneous interpretation of different functionals of this structure for example the waveforms of rayleigh and love waves for the same source receiver pair following extensive testing of the method on synthetic and actual data we subject results of our analysis of 37 recordings for various sources and stations to pure path decomposition the period range of analysis extends from 160 to 630s we distinguish four types of regions stable continents areas tectonically active within the last 400myr ocean floors younger than 38 myr and old ocean floors older than 38 myr the results indicate significantly different responses of the oceanic and continental areas at long periods stable continental and tectonic regions have nearly the same dispersion for periods greater than 300 35os and then diverge rapidly with stable continents showing higher velocities the young and old oceans on the other hand become distinct at periods as long as 500 s the old oceans are faster in terms of shear velocity models the data are consistent with a difference of about 4 per cent between the young and old oceans in a depth range from 400 to 670 km while the continental regions are similar and close to the global average at shallower depths our structures are conceptually similar to those inferred from previous pure path analyses and short period surface wave studies while we cannot yet establish statistically significant differences in q for the four regions such differences are obtained if one distinguishes only between continents and oceans the q for the latter is 10 20 per cent lower in the period range of the analysis

attenuation tomography of the earth s mantle a review of current status resolving the lateral variations of attenuation in the deep mantle by tomographic methods holds potential for constraining its thermal structure and dynamics it is a challenging subject which has been addressed by only a few studies until now we here review the main motivations behind pursuing this challenge the difficult issues involved in separating effects of anelastic attenuation from scattering and focusing due to propagation in 3 d elastic structure and finally discuss the current status of global attenuation tomography

insights on upper mantle melting rheology and anelastic behavior from seismic shear wave tomography in seismic tomography we map the wave speed structure inside the earth but we ultimately seek to interpret those images in terms of physical parameters this is challenging because many parameters can trade off with each other to produce a given wave speed the problem is compounded by the convention of mapping seismic structures as perturbations relative to a 1 d reference model rather than absolute wave speeds using a full waveform tomography model of europe as a case study we quantify the extent to which thermochemical and dynamic properties can be constrained using only s wave speed expressed in absolute values the wave speed distributions of this tomography model are compared with 4 million thermochemical models whose seismic properties are computed via thermodynamic modeling these models sample the full range of realistic mantle compositions including variable water and melt contents and mineral intrinsic anelasticity is taken into account intrinsic anelasticity causes waves to travel more slowly at higher temperatures leading to seismic attenuation but the sensitivity of the wave speed reduction to temperature is in turn controlled by the wave frequency global studies of surface waves indicate an anticorrelation between s wave speed and attenuation we therefore only retain thermochemical models satisfying this anticorrelation our study indicates that the frequency dependence of anelasticity depends on temperature or rheology with 0 1 being most appropriate in cold or lithospheric mantle and 0 3 in warmer regions i e the asthenosphere additionally the slowest regions require specific compositions and or a velocity weakening mechanism such as partial melting elastically accommodated grain boundary sliding or water

the potential influence of radiative heat transfer on the formation of megaplumes in the lower mantle abstract recent seismic tomographic models have revealed broad low velocity anomalies in the lower mantle beneath africa and the central pacific which suggest a break in the symmetry between hot and cold regions in lower mantle dynamics we have considered the possible impact from radiative heat transfer which can be described by a nonlinear temperature dependent coefficient in the thermal conductivity in 2d numerical simulations results for rayleigh numbers up to 106 show a strong stabilizing influence from radiative heat transfer on mantle upwellings and the production of extremely hot thermal anomalies in the interior this nonlinearity is responsible for producing a strong attractor in the mantle convective system which greatly simplifies its time dependent dynamics the possible link of the simplified lower mantle time dependent dynamics with polar wander is discussed the main point here is that slow time dependence of the huge anomalies in the lower mantle can be the main mechanism controlling long term rotational dynamics

early cambrian ocean anoxia in south china the cause of the most marked changes in the evolution of life which define the first order stratigraphic boundary between the precambrian and the phanerozoic eon remains enigmatic and a highly topical subject of debate a global ocean anoxic event triggered by large scale hydrogen sulphide h 2 s release to surface waters has been suggested by wille et al on the basis of two data sets from south china and oman to explain the fundamental biological changes across the precambrian cambrian pc c boundary here we report a new precise shrimp u pb zircon age of 532 3 0 7 million years myr ago fig 1 for a volcanic ash bed in the critical unit that reflects the ocean anoxic event the lowermost black shale sequence of the niutitang formation in the guizhou province south china this age is significantly younger than the precise pc c boundary age of 542 0 0 3 myr ago approximately 10 myr younger than the extinction of the ediacaran fauna and thus challenging the view of a major ocean anoxic event having been responsible for the major changes in the direction of evolution at the pc c boundary

post depositional origin of highly 13c depleted carbonate in the doushantuo cap dolostone in south china insights from petrography and stable carbon isotopes abstract extremely negative carbon isotope values from the doushantuo cap dolostone succession in the yangtze gorges south china were interpreted as direct evidence of the hypothesis of methane hydrate destabilization during the marinoan deglaciation however this suggestion remains uncertain due to the obscurity of the paragenetic sequence of carbonate minerals with diverse carbon isotopic compositions here we conducted macroscopic and microscopic petrographic and carbon isotopic investigations of the cap dolostone succession at the jiulongwan section in the yangtze gorges our results show that extreme 13c values down to 44 exclusively occur in calcite whereas 13c values for the host dolomite range mostly between 4 and 2 and clearly reveal that the host dolomite were locally dissolved and or replaced by the calcite bearing highly 13c depleted signals this finding suggests that methane oxidation activity as indicated by the strongly depleted 13c signals did not occur until after the cap dolostone deposition thus the presence of the extremely negative 13c values could not be taken as evidence for the proposed methane hypothesis nevertheless this conclusion is not contradictory to the scenario that massive releases of methane from permafrost hydrates during deglaciation much prior to methane oxidation documented here could drive substantial negative 13c shifts well exhibited by the doushantuo cap dolostone and others around the world

palaeoceanographic redox environments for the lower cambrian hetang formation in south china evidence from pyrite framboids redox sensitive trace elements and sponge biota occurrence abstract the lower cambrian hetang formation a black shale sequence contains a stone coal a flammable organic rich mudstone unit abundant pyrite framboids occur in this unit with the average mean sizes falling in a narrow range from 4 6 to 5 4 m and the maximum diameters being around 10 m the size distribution pattern of the pyrite framboids indicates a euxinic depositional environment the redox sensitive trace metals of the stone coal samples reveal a large enrichment of mo 10 180 times v 4 40 times u 10 60 times and ni 2 20 times compared to the average upper continental crust value consistent with an anoxic environment for their deposition the redox sensitive element ratios th u v v ni v cr indicate that the depositional environment for the lower part of the stone coal unit was the most anoxic and euxinic in contrast a sponge biota including eleven species of demosponges and hexactinellids and two undetermined forms indicate an oxic or dysoxic environment to reconcile these two facts we propose that although an anoxic euxinic environment predominated during the deposition of the lower cambrian hetang formation black shales occasional currents may also have brought free oxygen to the bottom water column to allow the growth of the sponges on the sea floor

rare earth element and srnd isotope geochemistry of phosphate nodules from the lower cambrian niutitang formation nw hunan province south china abstract phosphate nodules are widely distributed in the early cambrian black shale sequence in yangtze platform south china in this study we carried out x ray diffraction major element rare earth element and sr nd isotopic studies on phosphate nodules from two pc c sections in hunan province south china two different types of phosphate nodules can be recognized based on ree and nd isotopic characteristics type 1 with seawater like ree patterns higher end t values and type 2 with hat shaped ree patterns and more negative end t values we suggest that the difference in ree patterns and nd isotopic compositions between the two types reflects changes in pore water geochemistry from seawater dominated towards diagenetic fluid dominated conditions during early diagenesis positive eu anomalies are displayed in most samples and are likely to record not only the anoxic environments but also the involvement of submarine exhalative hydrothermal fluids which prevailed in south china during early cambrian

the behaviour of manganese in atlantic carbonate sediments abstract manganese porewater and solid phase data are considered from two different atlantic carbonate sediments both with good geochemical characterisation the first case is a continuously accumulated alternating pelagic marl carbonate ooze where there is a correlation in post oxic conditions between mn ii porewater levels and an acetate leachable mn fraction of the sediment this leachable mn appears to have an association with calcite and in this example the leached residue mn content approximates to that of shale in the second case a very recent compositionally homogeneous calcareous turbidite is underlain by mn oxyhydroxide at the former sediment surface which is now acting as a source of mn ii by reduction sorption of mn by the turbidite sediment and shale like leached residue mn levels in anoxic conditions are again observed as mn ii diffuses towards present oxic conditions although no unique equilibrium is achieved similar older buried turbidites have concentration decreases in solid phase mn content preserved from bottom to top as relict evidence that this process has occurred in these turbidites 54 64 of the total mn is leachable by the acetate method and therefore presumed to be sorbed by calcite the oldest example has persisted for 125 kyr it is suggested that the slow breakdown of metastable mn oxyhydroxide and sorption of mn ii to calcite surfaces are important controls on the behaviour of mn in pelagic carbonates in mildly reducing conditions where accumulation fluctuations occur these processes act together with accumulation to limit overall mn migration and to prevent the large mn enrichments seen near the sediment surface in more reducing environments

recent progress in studies of the elastic properties of earth materials abstract technological advances now allow the elastic moduli and acoustic wave velocities of minerals to be measured or calculated under extreme conditions of pressure and temperature p and t that closely mimic the p t conditions throughout much of earth s interior in this special issue the papers on the elasticity of earth materials report new results that provide excellent examples of these advances here we briefly discuss the scientific context for the elasticity papers in this special issue and some of the future challenges that are critical for advancing our understanding of the compositional state of the deep earth through studies of the elastic properties of materials

degree 2 pattern of attenuation structure in the upper mantle from apparent complex frequency measurements of fundamental spheroidal modes the authors present preliminary results of an analysis of global lateral variations in elastic and anelastic structure of the upper mantle using the sompi method they have analyzed ida records to measure apparent complex frequencies of fundamental spheroidal modes sub 0 s sub 21 sub 0 s sub 34 which are primarily sensitive to the structure of the transition zone the apparent complex frequencies have then been inverted into local frequency and attenuation maps using the new inversion method the authors have obtained consistent degree 2 patterns in the local attenuation maps as well as in the local frequency maps the high attenuation anomalies correlate with regions of slow p wave velocity in the lower mantle and with regions of high hotspot density this may represent a continuation of lower mantle upwelling into the transition zone

generation of long wavelength heterogeneity in the mantle by the dynamic interaction between plates and convection lateral variations in seismic velocity through its dependence on temperature can easily be generated at the gravest harmonics including degrees one and two by the dynamic interaction between plates and convection models of thermal convection with a single non subducting plate have been formulated in a cylindrical geometry plates of width one to four times times the thickness of the convecting region strongly modulate the flow by being pushed over cold downwellings and inhibiting cooling of the fluid beneath during rapid motion off of hot regions a large scale pattern of shear is developed causing small uprising limbs to be swept into the largest upwellings both insulation and plume plume collisions pump energy into the lower wavenumber harmonics

structure of axisymmetric mantle plumes results of high resolution numerical calculations of axisymmetric thermal plumes in an infinite prandtl number fluid with thermally activated viscosity are used to infer the structure of axisymmetric subsolidus thermal plumes in the earth s lower mantle we calculate the velocity and temperature distribution for axisymmetric convection above a heated disk in an incompressible fluid cylinder 2400 km in height and 1200 km in diameter initially at a uniform temperature t the lower boundary representing the plume source region near the core mantle boundary is stress free and maintained at temperature tc t the upper boundary is permeable allowing the plume to escape the cylinder we determine the sensitivity of plume structure to viscosity variations c ranging from 102 to more than 104 and temperature differences t tc t in the range 400 850 k starting plumes consist of a large leading diapir and a trailing conduit connected to the thermal boundary layer above the heated surface the leading diapir expands during ascent to about 400 km diameter at 2200 km height in steady state the plume consists of a narrow high velocity conduit imbedded within a broader thermal halo the width of both these structures is proportional to height above the heated boundary and inversely proportional to t and the viscosity ratio heat transport in the plume is nearly independent of viscosity variations a simplified boundary layer model yields a heat transfer law nu ra where nu is the plume nusselt number and ra is the rayleigh number based on t and the radius of the heated circular region feeding the plume calculated steady state plume heat transports fit this law to within a few percent we present several calculations of plumes with heat transport in the range 100 400 gw similar to the advective heat transport by the hawaiian hotspot at the top of the lower mantle plumes with large viscosity variations have high velocity conduits 50 60 km in diameter and 125 km diameter thermal halos a centerline temperature anomaly of about 500 k and a centerline velocity of about 75 cm yr 1 plumes with weak viscosity variations are approximately twice as broad and have centerline temperature anomalies less than 160 k at the same height and much smaller velocity large viscosity variations and a minimum heat transport are both

the seismic velocity and attenuation structure beneath the tatun volcanic area taiwan abstract we have investigated the structure beneath the tatun volcanic zone north of taipei metropolitan area of taiwan area of five million people we used the data collected from a seismic network deployed for 5 years over the volcanic zone this plus another data from taiwan regional networks allow us to carry out tomographic inversions for v p v p v s and q p structures beneath the tatun volcanic zone based on our results and other geological tectonic and seismic findings we reconstruct the structural evolution of the crust in the tatun volcanic zone and discuss the implication to the surrounding faults fractured zones and discuss potential future volcanic activities from the tomographic results there appear to exist a tube shaped highly fractured ancient magma passage with high seismic velocities that parallel to the chinshan fault and magma passage extends to the southeast at the depth about 20 km this structure suggests plutonic intrusion passage beneath the tatun volcano group that may have been associated with the earlier subduction of the philippine sea plate melting of the subducted plate at depth has generated the magma intrusion that has brought about the tatun volcanic activities the high seismicity today also implies a highly fractured crust due to the hydrothermal activities and induced crustal stress the hydrothermal fluid rich upper crust as indicated by the low v p v s ratio may have important bearing on the potential hazards associated with the two active faults cutting through both the taipei basin as well as the tatun volcanic groups

rare earth element geochemistry of late devonian reefal carbonates canning basin western australia confirmation of a seawater ree proxy in ancient limestones rare earth element and yttrium ree y concentrations were determined in 49 late devonian reefal carbonates from the lennard shelf canning basin western australia shale normalized sn ree y patterns of the late devonian samples display features consistent with the geochemistry of well oxygenated shallow seawater a variety of different ancient limestone components including microbialites some skeletal carbonates stromatoporoids and cements record seawater like ree y signatures contamination associated with phosphate fe oxides and shale was tested quantitatively and can be discounted as the source of the ree y patterns co occurring carbonate components that presumably precipitated from the same seawater have different relative ree concentrations but consistent ree y patterns clean devonian early marine cements n 3 display ree y signatures most like that of modern open ocean seawater and the highest y ho ratios e g 59 and greatest light ree lree depletion average nd sn yb sn 0 413 sd 0 076 however synsedimentary cements have the lowest ree concentrations e g 405 ppb non contaminated devonian microbialite samples containing a mixture of the calcimicrobe renalcis and micritic thrombolite aggregates in early marine cement n 11 have the highest relative ree concentrations of tested carbonates average total ree 11 3 ppm stromatoporoid skeletons unlike modern corals algae and molluscs also contain well developed seawater like ree patterns samples from an estuarine fringing reef have very different ree y patterns with lree enrichment nd sn yb sn 1 possibly reflecting inclusion of estuarine colloidal material that contained preferentially scavenged lree from a nearby riverine input source hence devonian limestones provide a proxy for marine ree geochemistry and allow the differentiation of co occurring water masses on the ancient lennard shelf although appropriate partition coefficients for quantification of devonian seawater ree concentrations from out data are unknown hypothetical devonian canning basin seawater ree patterns were obtained with coefficients derived from modern natural proxies and experimental values resulting devonian seawater patterns are slightly enriched in lree compared to most modem seawaters and suggest higher overall ree concentrations but are very similar to seawaters from regions with high terrigenous inputs our results suggest that most limestones should record important aspects of the ree geochemistry of the waters in which they precipitated provided they are relatively free of terrigenous contamination and major diagenetic alteration from fluids with high non seawater like ree contents hence we expect that many other ancient limestones will serve as seawater ree proxies and thereby provide information on paleoceanography paleogeography and geochemical evolution of the oceans copyright c 2004 elsevier ltd

diagenetic constraints on the use of cerium anomalies as palaeoseawater redox proxies an isotopic and ree study of cambrian phosphorites seawater rare earth element ree distributions show heavy ree enrichment and cerium depletion but these characteristics are only rarely found in ancient marine authigenic minerals such as carbonate fluorapatite and calcite this enigma may be due to changing ree distributions in seawater through time or to post depositional redistribution of ree differentiating between these two factors is however far from straightforward as their relative importance is likely to vary and evidence is often ambiguous in this article we present ree as well as strontium and sulphur isotope data for basal cambrian phosphorites of south china which confirm the emerging rule of thumb that granular phosphorites seldom reveal extreme departures from the seawater norm and tend to undergo therefore less diagenetic alteration of ree spectra than skeletal apatites and other phosphate types this study reveals stratigraphic trends in ce anomalies ce ce that could be taken to imply the preservation of a palaeoenvironmental signal however further investigation of our data shows that such trends in phosphorites can also be caused by post depositional alteration which tends to 1 increase ree contents 2 reverse ce depletion 3 alter eu anomalies and 4 concentrate preferentially the middle ree mree further alteration which may be due to weathering has led to hree depletion and positive la and y anomalies tetrad effect this is the first study where ceanom and phosphate bound sulphate s isotope values could be compared for one particular stratigraphic section which provides us with an additional tool for the interpretation of diagenetic redox conditions least altered phosphorites from meishucun that yield a strongly negative ceanom constrain early cambrian seawater 34s to about 33 we conclude that phosphorite ceanom primarily records variations in the local conditions of diagenesis while outlining some special cases in which diagenetic redox conditions may broadly reflect local bottom water redox conditions

paleobathymetric influence on the development of the late ediacaran yangtze platform hubei hunan and guizhou provinces china abstract a compiled facies map based on 48 sections positioned in three chinese provinces hunan guizhou and hubei and field data emphasize the changes in depositional environments on the shelf of the ediacaran yangtze platform the facies interpretation suggests three depositional environments 1 a deep water basin with a slope facies on its margins maoping and wuhe sections hubei province 2 intertidal or shoals environments subjected to wave energy yangjiaping and zhongling sections hunan province wengan section guizhou province 3 a shallow water subtidal shelf environment subjected to currents xikou section guizhou province during the doushantuo formation time period 635 ma 551 ma condon d zhu m y bowring s yang a jin y g 2005 u pb ages from the neoproterozoic doushantuo formation china science science 308 95 98 the shelf showed an irregular bathymetry interpreted as inherited from the rifting history of the yangtze platform following the break up of rodinia approx 700 ma the tilted blocks influenced the platform development and the facies distribution locally more than the doushantuo formation time period 84 ma has been necessary to compensate the variations of bathymetry inherited of the tilted blocks geometry

a burial diagenesis origin for the ediacaran shuram wonoka carbon isotope anomaly marked negative 13c excursions in ediacaran age carbonate sediments have been identified in several sections globally but are not recognized in all sections of similar age the presence of 13ccarb values as low as 12 has been interpreted as recording fundamentally different processes in the global carbon cycle than those recognized today the 13ccarb anomalies are strongly correlated with 18ocarb values but are not represented in 13corg records while no primary depositional processes have been identified that can produce the correlated 18o 13c arrays simulations show that fluid rock interaction with high pco2 fluids is capable of producing such arrays at geologically reasonable pco2 and water rock ratios variations in the mg ca ratio and sulfate concentration of the altering fluid determine the extent of dolomite vs calcite and anhydrite in the resulting mineral assemblage incorporation of an initially aragonitic mineralogy demonstrates that high sr low mn sr and modest alteration of 87sr 86sr in ancient carbonates are all compatible with a burial diagenesis mechanism for generation of the 13c anomalies and do not necessarily imply preservation of primary values the profound ediacaran negative 13c anomalies can be adequately explained by well understood diagenetic processes conflated with the difficulty of correlating precambrian sections independently of chemostratigraphy they are not a record of primary seawater variations and need not have independent stratigraphic significance

chemical diagenesis of a multicomponent carbonate system 2 stable isotopes abstract this study examines the oxygen and carbon isotopic distribution in the brachiopods crinoids rugose corals and matrix cement from the mississippian burlington limestone iowa and missouri and the silurian read bay formation arctic canada the isotopic data can be reconciled with textural and trace element observations only if the 18o content of ancient oceans differed from that of the present oceans this postulated secular variation in the 18o content of seawater requires a separate evaluation of the mississippian and silurian carbonate components examination of the 18o content of the least altered low mg calcite fossil components of the two formations suggests that the 18o of mississippian seawater was on average about 1 5 lighter and that of silurian seawater was about 5 5 lighter than the 18o of present day ocean water the average diagenetic shift in 18o for the read bay formation which stabilized in a partly closed diagenetic phreatic meteoric system is about 1 for the original low mg calcite and about 2 for the original high mg calcite and aragonite components conversely for the burlington limestone which stabilized in an open diagenetic phreatic meteoric system the average diagenetic shift in 18o is about 3 for the original low mg calcite and about 4 f r the original high mg calcite components the 13c values of the originally low mg calcite or high mg calcite with less than 7 mole mgco3 brachiopods and rugose corals the originally high mg calcite crinoids and the originally aragonite lime mud of the burlington limestone and read bay formation are bimodally distributed this bimodal 13c distribution is independent of geologic age inferred original mineralogy of the component degree of diagenetic alteration and the type of diagenetic meteoric system with components from both the read bay formation and burlington limestone contributing to both 13c groups the mode of the light 13c group is 0 25 and for the heavy 13c group it is 3 25 pdb both 13c groups also show a decrease in 13c of about 1 with increasing diagenetic alteration stabilization

global lateral variations of shear wave attenuation in the upper mantle we analyze several thousand high quality globally recorded ss s differential waveforms to constrain the lateral variation of shear wave attenuation q in the upper mantle we use a multitaper frequency domain technique to measure attenuation parameterized by a t operator and implement a robust estimation technique to compute t and its variance the differential waveform technique minimizes the effect of factors such as finite source duration and structural complexity near the source and receiver so the differential ss s waveforms are mainly sensitive to the shear attenuation in the upper mantle under the ss bounce point we use seismograms recorded at ranges of 45 to 100 and compute the ss s differential t from the broadening of the ss waveform relative to the hilbert transform of the s waveform a careful choice of fitting windows allows us to reduce the biasing effects of interfering phases which can affect t by up to 0 5 s the t residuals with respect to preliminary reference earth model prem vary by 1 5 s with an average of 0 24 s our study suggests an average q value of 112 most of the lateral variations of q are within 30 of this value in the top 400 km of the mantle slightly lower than the prem value of 128 there is a qualitative correlation of t residual with tectonic region with distinctly higher attenuation observed under young oceans compared to platforms and shields also the lateral variations of the residuals are similar in trend to those observed in studies of the attenuation of scs multiples at long wavelengths the qp map shows a modest correlation with shear wave attenuation maps computed from surface wave analyses and with the patterns of lateral variations of shear velocities at certain upper mantle depths predicted by the model s16b30 the correlation with the velocity model is highest at 300 500 km depth indicating that there may be a contribution to long wavelength attenuation from relatively deep regions formal inversion for an upper mantle q model shows that while lateral resolution is quite good depth resolution is poor as might be expected better depth resolution must await combined body wave and surface wave inversions

neodymium isotopes in planktonic foraminifera a record of the response of continental weathering and ocean circulation rates to climate change the neodymium nd isotope composition of ancient seawater is a potentially useful tracer of changes in continental inputs and ocean circulation on timescales of a few ka here we present the first nd isotope record for seawater using sedimentary foraminifera cleaned using standard oxidative reductive techniques the data along with mn ca ratios suggest that cleaned foraminifera provide a reliable record of nd in seawater and hold out the prospect of using nd in foraminifera to examine changes in seawater that accompany glacial interglacial climatic cycles the principal potential problem to be overcome with the use of forams as records of trace elements in ancient seawater is their diagenetic fe mn coatings these contain large amounts of nd and other trace elements but can be cleaned off using highly reducing reagents mn ca ratios for the majority of the cleaned sedimentary foraminifera analysed here lie within the range 10 100 mol mol that has yielded success in studies of transition elements in forams mass balance modelling suggests that for residual mn ca ratios 100 mol mol nd added to the foram in the coating will never shift the measured nd isotope composition significantly away from the seawater value acquired by the foram test in the water column additionally nd concentrations measured in cleaned sedimentary foraminifera are comparable with those for a modern sample that has never encountered diagenetic fluids finally core top planktonic foraminifera for two sites have nd isotope compositions that are identical to local surface seawater the data we present here for labrador sea forams over the past 2 5 m y are interpreted in terms of changes in the seawater isotopic composition the data show a pronounced shift from end values of 12 to 19 in the period 2 5 1 5 ma this change is interpreted to result from the initiation of northern hemisphere glaciation and the increased derivation of labrador sea nd via ice rafting from archaean terranes in central canada in combination with stable isotope and foraminiferal relative species abundance data the new nd data are consistent with the surface hydrography of the labrador sea being dominated by a fluctuating balance between cold polar waters containing unradiogenic nd and warm subtropical waters containing more radiogenic nd the major change in labrador sea nd that is observed in the past 2 5 ma can on its own account for the change in the nd isotope composition of north atlantic deep water over the same time period

new perspectives on mantle dynamics from high resolution seismic tomographic model p1200 recently a high resolution tomographic model the p1200 based on p wave travel times was developed which allowed for detailed imaging of the top 1200 km of the mantle this model was used in diverse ways to study mantle viscosity structure and geodynamical processes in the spatial domain there are lateral variations in the transition zone suggesting interaction between the lower mantle plumes and the region from 600 km to 1000 km some examples shown here include the continental region underneath manchuria ukraine and south africa where horizontal structures lie above or below the 660 km discontinuity the blockage of upwelling is observed under central africa and the interaction between the upwelling and the transition zone under the slow icelandic region appears to be complex an expansion of the aspherical seismic velocities has been taken out to spherical harmonics of degree 60 for degrees exceeding around 10 the spectra at various depths decay with a power law like dependence on the degree with the logarithmic slopes in the asymptotic portion of the spectra containing values between 2 and 2 6 these spectral results may suggest the time dependent nature of mantle convection details of the viscosity structure in the top 1200 km of the mantle have been inferred both from global and regional geoid data and from the high resolution tomographic model we have considered only the intermediate degrees l 12 25 in the nonlinear inversion with a genetic algorithm approach several families of acceptable viscosity profiles are found for both oceanic and global data the families of solutions for the two data sets have different characteristics most of the solutions asociated with the global geoid data show the presence of asthenosphere below the lithosphere in other families a low viscosity zone between 400 and 600 km depth is found to lie atop a viscosity jump other families evidence a viscosity decrease across the 660 km discontinuity solutions from oceanic geoid show basically two low viscosity zones one lying right below the lithosphere the other right under 660 km depth all of these results bespeak clearly the plausible existence of strong vertical viscosity stratification in the top 1000 km of the mantle the presence of the second asthenosphere may have important dynamical ramifications on issues pertaining to layered mantle convection numerical modelling of mantle convection with two phase transitions and a realistic temperature and pressure dependent viscosity demonstrates that a low viscosity region under the endothermic phase transition can indeed be generated self consistently in time dependent situations involving a partially layered configuration in an axisymmetric spherical shell model

a global tomographic model of shear attenuation in the upper mantle we present a global three dimensional model of shear attenuation in the upper mantle based on the measurement of amplitudes of low frequency 100 300s rayleigh waves observed at stations of the geoscope and iris networks attenuation coefficients are measured on r1 and r2 paths using a method which minimizes the effects of focussing due to propagation in a three dimensional elastic earth through a series of tests which in particular involve the computation of synthetic models of attenuation and focussing we demonstrate that long wavelength lateral variations in attenuation in the first 400 500 km of the mantle can indeed be resolved the model is obtained in a two step procedure the first step consists in the computation of maps of rayleigh wave attenuation at different periods using an inversion method without a priori parametrisation which involves the introduction of a correlation length chosen here at 3000 km to optimize the trade off between resolution and variance in the model in the second step after corrections for shallow structure an inversion with depth is performed assuming lateral heterogeneity is confined to depths between 80 and 650 km the resulting model presents lateral variations in qp that are correlated with tectonic features in particular ridges and shields in the first 250 km of the upper mantle below that depth the pattern shifts and becomes correlated with the hotspot distribution particularly so if the buoyancy strength of hotspots is taken into account two major low velocity zones appear to be located in the central pacific and beneath northern africa in the depth range 300 500 km this pattern seems to continue at greater depth but resolution becomes insufficient below 500 km to draw definitive conclusions the smooth lateral variations retrieved are on the order of 50 down to 400 km we propose an interpretation in terms of plume lithosphere ridge interaction in the upper mantle arguing for deflection of the bulk of hot upwelling material from plumes towards ridges which may be occurring between 200 and 300 km depth

multiproxy constraints on alteration and primary compositions of ediacaran deep water carbonate rocks yangtze platform south china the occurrence of shallow and deep water sedimentary facies has established the yangtze platform in south china as a key site for the study of neoproterozoic ocean oxidation and ediacaran animal evolution following the marinoan glaciation the yanwutan section in hunan province is one of the few coherent sections on the yangtze platform where ediacaran deep water carbonate sediments predominantly dolostones are preserved together with organic carbon rich shales here we present new major and trace element abundance data as well as sr o and c isotope compositions of leachates from carbonates of the doushantuo formation we evaluate the role of diagenetic modification of the carbonate rocks and constrain the redox evolution of ediacaran seawater in space and time sr 87 sr 86 systematically varies with delta o 18 carb sr and ba abundances indicating variable but mostly strong modification of fluid mobile elements by continental basin fluids in contrast ree y patterns have preserved seawater like compositions cap dolostones unit i on top of the nantuo diamictites differ from cap dolostones at shallow water sections on the yangtze platform in that they show no ce anomalies and little alteration near the top sr 87 sr 86 0 7078 delta o 18 4 0 delta c 13 carb 1 1 suggesting that delta c 13 carb and delta o 18 of cap dolostones at many other sections were compromised by hydrothermal alteration the overlying organic carbon poor micritic dolostone unit ii shows negative ce anomalies that disappear towards the top of the unit no ce anomalies occur in subsequent organic carbon rich muddy dolostone units units iii to iv these observations enrichments in toc that correlate with variations in redox sensitive metals in the carbonates negative delta c 13 carb in units ii to iv and the decoupling of delta c 13 carb from delta c 13 org argue for the existence of mostly anoxic deep water at the yangtze passive continental margin during the ediacaran the negative ce anomalies at the base of unit ii with negative delta c 13 carb may reflect fluctuations towards suboxic or oxic conditions or an allochthonous origin of this unit however trace metal enrichments in carbonates of the same unit argue for reducing conditions in pore water whereas the carbonates may have preserved the ree y signatures inherited from suboxic to oxic seawater the trace element and negative delta c 13 carb values in units ii to iv are consistent with a stratified basin model with a large partially remineralised organic matter reservoir in anoxic bottom and pore waters c 2015 elsevier ltd all rights reserved

co variation of mg and c isotopes in late precambrian carbonates of the siberian platform a new tool for tracing the change in weathering regime abstract this study reports significant mg isotopic variations in 32 samples of late precambrian dolomites from various parts of the siberian platform exhibiting a range of 13 c from 10 to 6 v pdb in the ediacaran dolomites from the n and ne part of the patom paleobasin a systematic increase of 26 mg from 2 3 0 2 to 0 8 0 3 dsm3 along with a 13 c decrease from 6 to 10 26 mg 0 072 13 c 1 83 with a correlation coefficient of r 2 0 73 n 20 are observed within the stratigraphic sequence of ediacaran and ediacaran lower cambrian transition successions all positive c isotope excursions co vary with negative mg isotope shifts and vice versa other samples from the siberian platform lay on this regression with r 2 0 66 n 28 except for 4 samples probably diagenetic dolostones that exhibit a positive 26 mg 13 c correlation the enrichment of sedimentary dolomite in heavy carbon and light mg isotopes may be interpreted as a result of a complex interplay of contrasting sedimentation conditions sea regression versus sea transgression regime yielding different weathering regimes and relative contributions of carbonate versus silicate rocks the enrichment of both oceanic water and carbonate sediments by 26 mg could occur during transgression and would coincide with sedimentation of low 13 c carbonates whereas sedimentation of the depleted in 26 mg and enriched in 13 c carbonates could happen under regression regime when exposed carbonate platforms and coastal planes are subjected to intense weathering

inversion of surface wave spectra for source parameters of large earthquakes using aspherical earth models abstract amplitude and phase spectra of rayleigh and love waves of periods of 150 to 300 s from eight large earthquakes which occurred between 1990 and 1993 are corrected for wave propagation and then inverted for the source duration rupture directivity and moment tensor of the earthquake source for various propagation anomalies predicted by models of the earth s aspherical elastic and anelastic structure the application of corrections for the focusing and take off azimuth anomaly in addition to conventional corrections for the phase anomaly improves inversions of surface waves rayleigh waves provide better constraints than love waves in the determination of source parameters the data consist of either only rayleigh waves or both love and rayleigh waves whenever the full moment tensor is involved for several earthquakes there is clear discrepancy between source parameters determined from different data sets particularly from rayleigh or love waves the moment tensor solution tends to underpredict observed amplitude spectra for data sets with large amplitude anomalies especially for love waves this study shows that 1 the 1990 sudan earthquake is anomalous for its unusually large non double couple component of the moment tensor 2 the 1990 iran earthquake has a strike slip mechanism with a subvertical nw striking nodal plane which is the fault plane inferred from surface ruptures 3 the 1991 costa rica earthquake may have a seismic moment larger than previously determined from long period surface waves 4 the 1992 nicaragua earthquake involves a predominant rupture parallel to the trench and a large centroid offset up dip to the trench axis and the latter process may be responsible for large tsunami generation 5 the 1993 hokkaido earthquake has a shallow dipping and west plunging nodal plane which is consistent with the p wave first motions but differs from the centroid moment tensor solutions

shear wave attenuation tomography of the lithosphere asthenosphere system beneath the mediterranean region abstract 3 d tomographic images of the shear wave attenuation structure of the lithosphere asthenosphere system beneath the mediterranean region are derived from the inversion of anelastic attenuation coefficients obtained for the fundamental mode of nearly 3100 rayleigh wavetrains recorded at very broad stations placed in the mediterranean region the two station method is used to compute 124 interstation path averaged attenuation coefficients within the 15 120 s period range an adaptation to attenuation coefficients of the yanovskaya ditmar formulation specifically devoted to path averaged surface wave dispersion data and body waves permits to obtain the local attenuation coefficients and their uncertainties for all the period range at every node of a grid covering the studied region maps of stretching azimuths and mean radii of the resolution ellipses are also obtained for every period all of these parameters are strongly related to the spatial path coverage data quality and data coherency a local attenuation dispersion curve is defined then in each grid node and the stochastic inversion of these curves yields 1 d models of inverse shear wave intrinsic quality factor q 1 the shear wave velocity models obtained for every grid node in an elastic tomography developed in a previous work for the same domain are taken as reference models for the inversion process the kriging contouring algorithm is used to represent the resulting 3 d q 1 structure by means of horizontal and vertical cross sections down to 160 km depth the main patterns of this anelastic structure are mostly in agreement with the structural and tectonic features obtained in the reference shear velocity tomography the local attenuation maps manifest notable lateral changes for periods of 20 and 40 s whereas more moderate spatial variation is observed for longer periods the 3 d anelastic models show that at shallow levels the attenuation in the western mediterranean is usually slightly higher than in the central eastern part the area from corsica sardinia islands to italy presents high q 1 values of 40 0 70 0 10 3 down to 75 km depth whereas q 1 does not exceed 20 0 10 3 in the rest of the mediterranean basin except for the south eastern part where slightly higher values are obtained 20 0 30 0 10 3 those high q 1 values suggest the presence of asthenospheric material at shallow levels which would be related to distensive tectonic processes responsible for the recent opening of extensional basins as the tyrrhenian sea and the ligurian provencal basin in the eastern mediterranean the vertical cross sections at different latitudes and longitudes reveal at deep levels some structural patterns consistent with signs of subduction which are linked to the convergence between the african and eurasian plates

ocean redox structure across the late neoproterozoic oxygenation event a nitrogen isotope perspective abstract the end of the neoproterozoic era 1000 to 541 ma is widely believed to have seen the transition from a dominantly anoxic to an oxygenated deep ocean this purported redox transition appears to be closely linked temporally with metazoan radiation and extraordinary perturbations to the global carbon cycle however the geochemical record of this transition is not straightforward and individual data sets have been variably interpreted to indicate full oxygenation by the early ediacaran period 635 to 541 ma and deep ocean anoxia persevering as late as the early cambrian because any change in marine redox structure would have profoundly impacted nitrogen nutrient cycling in the global ocean the n isotope signature of sedimentary rocks 15 n sed should reflect the neoproterozoic deep ocean redox transition we present new n isotope data from amazonia northwest canada northeast svalbard and south china that span the cryogenian glaciations 750 to 580 ma these and previously published data reveal a n isotope distribution that closely resembles modern marine sediments with a mode in 15n close to 4 and range from 4 to 11 no apparent change is seen between the cryogenian and ediacarian data from earlier proterozoic samples show a similar distribution but shifted slightly towards more negative 15 n values and with a wider range the most parsimonious explanation for the similarity of these n isotope distribution is that as in the modern ocean nitrate and hence o2 was stable in most of the middle late neoproterozoic ocean and possibly much of proterozoic eon however nitrate would likely have been depleted in partially restricted basins and oxygen minimum zones omzs which may have been more widespread than in the modern ocean

anelastic tomography a new perspective on upper mantle thermal structure abstract anelastic tomography provides new insights into the thermal structure of the earth s upper mantle lateral variations in q are largest in the upper 300 km but remain significant down to at least 500 600 km they correlate with the distribution of plates in the upper 300 km in agreement with cooling plate models and with the distribution of hotspots at greater depths two major upwellings are revealed under hawaii and northeastern africa which may originate at the bottom of the transition zone or deeper low q structure beneath the main ridge systems on the other hand does not appear to extend vertically below about 300 km

mantle phase transitions and layered chaotic convection we report preliminary results from a sequence of very high resolution simulations of the mantle convection process based upon an axisymmetric anelastically compressible spherical model with both olivine spinel and spinel post spinel phase transitions our analyses strongly suggest that at earth like rayleigh numbers near 107 the circulation may be quite strongly layered although a very long timescale transient exists that is characterized by brief excursions into the whole mantle state we suggest the use of a new diagnostic tool that may be employed to determine whether or not the layered state is characteristic of the present day earth

seismic wave dispersion and attenuation in aheim dunite an experimental study summary a machine has been developed which provides for the study of both shear modulus dispersion and internal friction in geological materials through the observation of forced torsional oscillations of low frequency 10 1000 mhz and strain amplitude 1000 c the eventual goal is an understanding of the dispersion and attenuation of seismic shear waves in the earth s upper mantle measurements have been made on cylindrical specimens of an olivine rich rock from aheim norway which contains in addition to olivine about 10 per cent pyroxene and 5 10 per cent of hydrous silicate phases dominantly clinochlore serpentine and talc in order to separate aspects of the mechanical behaviour associated with the olivine aggregate from those attributable to the hydrous phases and or their dehydration products the specimens were either previously fired under controlled oxygen fugacity at 1200 c for 24 hours in order to effect complete dehydration within the olivine stability field or simply oven dried at 110 c linearity of the mechanical behaviour has been demonstrated by the amplitude independence of the results for the strain amplitude range 10 8 10 6 and by the quantitative consistency between the observed modulus dispersion and that calculated from the measured internal friction through the kramers kronig relations of linear theory both the pressure dependence at room temperature and the temperature dependence at 300 mpa of the shear modulus and internal friction have been investigated much of the variation of the shear modulus is attributed to changes of crack porosity associated with changes in pressure and temperature and with in situ dehydration temporal evolution of q 1 towards a lower asymptotic value over periods of hours of exposure to given conditions of high pressure and temperature is tentatively attributed to the gradual diminution of enhanced anelastic relaxation associated with regions of decaying stress concentration at asperities on cracks and or grain boundaries at the highest pressures and temperatures achieved in this study 300 mpa and 1000 c marked dispersion of the shear modulus amounting to 5 per cent between periods of 3 and 100 s and concomitant strong internal friction varying with oscillation period to approximately as 0 011to1 6 are observed the energy dissipation appears to be concentrated within rather than at the boundaries between the olivine grains these results provide the clearest indication yet that solid state probably intragranular anelastic relaxation in ultramafic rocks gives rise to losses comparable with those observed seismologically in the earth s upper mantle although the mechanistic basis for the observed anelasticity remains to be established

spatial changes of seismic attenuation and multiscale geological heterogeneity in the baikal rift and surroundings from analysis of coda waves abstract the baikal rift system is undergoing an active tectonic deformation expressed by a high level of seismic activity this deformation leads to physical and mechanical changes of crustal properties which can be investigated by the seismic quality factor and its frequency dependence using a single backscattering model a seismic quality factor qc a frequency parameter n and an attenuation coefficient have been estimated by analyzing coda waves of 274 local earthquakes of the baikal rift system for nineteen lapse time windows w from 10 to 100 s every 5 s and for six central frequencies 0 3 0 75 1 5 3 6 and 12 hz the average qc value increases with the frequency and lapse time window from 46 52 at 0 75 hz to 502 109 at 12 hz for w 10 s and from 114 49 at 0 3 hz to 1865 679 at 12 hz for w 100 s the values of qc f and were estimated for the whole baikal rift system and for separate tectonic blocks the stable siberian platform main rift basins spurs and uplifts along the rift system the q0 value qc factor at the frequency f 1 hz varies within 72 109 and the frequency parameter n ranges from 0 87 to 1 22 whereas q0 is 134 and n is 0 48 for the stable siberian platform vertical variations of attenuation reveal that sharp changes of and n are confined to the velocity discontinuities the comparison of lateral variations of seismic wave attenuation and geological and geophysical characteristics of the baikal rift system shows that attenuation is correlated with both seismic activity and heat flow and in a lesser degree with the surface fault density and the age of the crust seismic wave attenuation found across the main shear zones of the south western baikal rift main sayan strike slip fault zone and tunka obruchev and primorsky normal faults is increased by more than 25 60 compared to the neighboring areas

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

3 d upper mantle shear velocity and attenuation from fundamental mode free oscillation data summary we present the results of global measurements of spheroidal and toroidal fundamental mode eigenfrequencies as well as fundamental spheroidal mode attenuation made from a data set consisting primarily of records from the geoscope network data processing includes variable filtering and account is taken of higher order effects along the dispersion branch local frequency and attenuation maps are first obtained as a function of angular order by a regionalization scheme based on the introduction of a correlation length in a second step after correction for shallow layer effects shear velocity maps at different depths in the mantle are obtained by simultaneous inversion of spheroidal and toroidal local frequency maps the results confirm the correlation of shear velocity with large scale tectonic features in the first 200 km of the mantle below that depth the maps are dominated by a degree 2 pattern whose power is peaked around a depth of 300 km shallower than inferred in the first 3 d global studies of the upper mantle and correlated with a degree 2 in attenuation thus confirming the results of our earlier study of degree 2 in the upper mantle we also report estimates of spherically averaged eigenfrequencies for fundamental spheroidal and toroidal mode branches

integrated ediacaran sinian chronostratigraphy of south china abstract the ediacaran period was a critical time in the history of life and earth understanding the complex interactions between geological and biological events during this interval requires high resolution chronostratigraphy thanks to its uniquely favorable palaeogeographic setting south china is an ideal region for constructing a framework for global chronostratigraphic correlation of ediacaran strata here continuous ediacaran successions contain both siliciclastic and carbonate rocks that range from shallow to deep marine and contain rich fossil assemblages showing a variety of preservation styles as part of the present study 62 reference sections distributed throughout the yangtze platform were investigated analyses of the sedimentary facies and sequence stratigraphy of more than 40 sections revealed four major sequences in the shallow marine facies three major sequence boundaries occur respectively in the middle and upper parts of the doushantuo formation and in the middle part of the dengying formation subdivision and correlation of the ediacaran successions of the yangtze platform based on sequence stratigraphy is congruent with the carbon isotope chemostratigraphy of 12 reference sections 1075 samples the sequence boundary sb2 in the middle part of the doushantuo formation is suggested here as the lower xiadongian upper yangtzean series boundary of the ediacaran which coincides with several globally recognizable events namely 1 the end of the neoproterozoic glaciations 2 the first appearance of animals 3 the sudden increase in the diversity of acanthomorphic acritarchs and 4 a negative 13 c excursion the ediacaran system of south china may be subdivided into five stages furthermore the criteria for defining these stages may be applicable throughout the world also the composite carbon isotope profile of the ediacaran of south china can be used as a global reference for chronostratigraphic subdivision and correlation in addition to the globally recognizable negative 13 c excursion in the cap carbonate at the base of the ediacaran the negative 13 c excursion in the uppermost doushantuo wonoka shuram interval represents another globally correlatable major excursion the positive 13 c excursion immediately following the uppermost doushantuo excursion and ending at 548 ma is also useful for global correlation current ediacaran chronostratigraphy and global correlations indicate that the upper doushantuo wonoka shuram negative 13 c excursion ranges from ca 560 ma to 551 ma and therefore coincides with the evolutionary renovation of life feedback between evolutionary innovation and geochemical cycling was the key mechanism for this unique and most strongly negative 13 c excursion in earth history our chronostratigraphic correlations also imply that the earliest known metazoan biotas namely the weng an and characteristic ediacaran biotas do not constitute two temporally separate evolutionary lineages as proposed previously but represent two parallel evolutionary pathways preceding the cambrian explosion of animal life

on the anelastic contribution to the temperature dependence of lower mantle seismic velocities abstract there is little agreement regarding the importance of anelastic contributions to the temperature dependence of seismic wave velocities some studies consider the anelastic contribution to be negligible whereas others argue that it may be as important as the purely elastic contribution in order to evaluate these claims we review physically plausible models for mantle relaxation processes and construct a model of activation enthalpy based on solid state physics as well as on recent experimental results we then deduce self consistent 1 d q profiles and evaluate the anelastic contribution to the temperature dependence of v s and v p physically plausible values of activation enthalpies yield anelastic contributions that increase with depth and account for at least 20 of the velocity temperature derivatives at 2600 km we show that anelasticity may significantly moderate temperature variations in hot slow velocity zones this suggests that inferred lateral changes in temperature as well as arguments regarding the presence of chemical heterogeneity and or temperature induced phase transformations are likely to be significantly affected by anelastic effects in the mantle we also compute values of the dimensionless parameters r s p r s and r s we find that anelasticity can either increase or decrease r s p but the magnitude of the correction of the order of 15 is smaller than the uncertainty in the elastic value anelasticity cannot change the sign of r s and r s but it can decrease their values by as much as 50 however given current uncertainties in seismic and mineral data the effects of anelasticity on the dimensionless parameters do not bring any quantitative constraints on lateral variations of temperature chemical composition and or mineral phase

integrated chemostratigraphy of the doushantuo formation at the northern xiaofenghe section yangtze gorges south china and its implication for ediacaran stratigraphic correlation and ocean redox models abstract the ediacaran doushantuo formation in the yangtze gorges of south china plays an important role in our understanding of biological evolution global correlation and ocean redox conditions because of the availability of high resolution paleontological and geochemical data and numerous radiometric dates however integrated study has been focused on the jiulongwan section that was largely deposited below wave base in a restricted shelf lagoon jiang et al 2011 zhu et al 2011 studies of shallower water successions are lacking and this presents a challenge to test ediacaran stratigraphic correlation and ocean redox models to fill this knowledge gap we conducted a high resolution integrated study of the doushantuo formation at the northern xiaofenghe nxf section approximately 35 km to the northeast and paleogeographically updip of the jiulongwan section with the exception of the basal 20 m nxf sediments were deposited above normal wave base integrated biostratigraphic and chemostratigraphic data indicate that the 140 m thick nxf section correlates with the lower doushantuo formation member i and much of member ii i e the lower ca 70 m of the formation at jiulongwan geochemical data from nxf and other doushantuo sections indicate that euxinic conditions may have been limited to a shelf lagoon represented by the jiulongwan section that was restricted between the proximal inner shelf and a distal shelf margin shoal complex at least during early doushantuo time following the deposition of the doushantuo cap dolostone further integrated studies are necessary to test whether euxinic conditions existed in open marine shelves in south china and elsewhere during the ediacaran period

lateral variations in upper mantle thermal structure inferred from three dimensional seismic inversion models global shear wave velocity variations in the upper mantle woodhouse and dziewonski 1984 are employed as a basis for inferring lateral heterogeneities in temperature or composition at a depth of 150 km using experimentally determined elastic moduli for the olivine solid solution and hashin shtrikman bound theory vs and its partial derivatives with respect to temperature or composition iron content are calculated for the depth of interest based on these results the observed seismic vs variations are interpreted as temperature and compositional deviations relative to a spherically symmetric reference model if it is assumed that the vs variations primarily represent lateral temperature changes then the present results can be used to construct a global surface heat flow map from the geotherm families of pollack and chapman 1977 our inferred heat flow patterns calculated independently of actual heat flow data show clear agreement with the surface heat flow maps proposed by chapman and pollack 1975 and chapman 1985 this result is consistent with and supports thermal structure as the primary governing factor in the generation of upper mantle lateral velocity variations compositional changes appear to correlate with temperature but affect velocity structure to a lesser extent

marine biogeochemical cycling during the early cambrian constrained by a nitrogen and organic carbon isotope study of the xiaotan section south china the precambrian cambrian boundary is claimed as one of the most crucial biological breakthroughs on our planet when changes in chemical and physical conditions together with key biological innovations helped to trigger a biodiversity explosion the yangtze platform south china mainly characterized by continuous and unaltered successions of this age is ideally suited to high resolution palaeomarine investigations of this event in this study 15n and 13corg records from xiaotan section yunnan sw china were investigated in order to provide insight into variations in primary productivity ecological developments and marine environment the ediacaran early cambrian xiaotan section is characterized by relatively high sedimentation rates and variable lithologies carbonates cherts phosphorites sandstones and siltstones that alternate through nearly 600 meters thickness organic carbon isotope values vary between 36 and 21 tightly following and helping to complete published 13ccarb trends the base of the cambrian is characterized in this section by a significant drop from 25 to 35 mirroring the 13ccarb fall observed at this and other sections worldwide higher in the section in the dahai member values increase to 20 again demonstrating communication between doc and dic pools during the early cambrian on 105 106 year time scales from the base of the overlying shiyantou formation 13corg values remain around 30 until the end of the section testifying to an interval of more modest change in dic 13c that would be consistent with biostratigraphic correlations nitrogen isotope values vary independently from carbon isotope trends exhibiting several major 15n cycles with values fluctuating between 9 and 1 robust trends in 15n within the dahai member testify to changes in the ratio between nitrogen fixation and denitrification possibly reflecting fluctuations in the water column chemocline as 13c values reached their global acme up section nitrogen isotope values show dampened cyclicity with values remaining between 0 and 5 indicating establishment of an equilibrium state in marine biogeochemistry and nutrient cycles the overall variability in 15n closely resembles that of recent marine sediments while the observed cyclicity is defined by several samples in all cases these observations argue for good preservation of original isotopic signatures which bodes well for future n isotope studies of the precambrian cambrian boundary

tomographic study of upper mantle atttenuation in the pacific ocean the quality factor q has been measured in the pacific ocean along many paths using long period seismograms of the geoscope and ida networks the love and rayleigh phase velocity and qr 1 have been regionalised in the period range 60s to 200s and then simultaneously inverted at depth to obtain 3d images of s wave velocity and attenuation q 1 in the depth range 60 300km up to 100s three very attenuating zones appear one under the east pacific rise e p r another one around hawaii a third one east of kermadec by inversion those three anomalies are found stable down to 160km below 200km lateral variations are less intense and differently distributed the maximum attenuation seems to concentrate along a northeast trend in the north central pacific

dynamically supported geoid highs over hotspots observation and theory hotspots are associated with long wavelength geoid highs an association that is even stronger when the geoid highs associated with subduction zones are removed we quantify these associations by expanding the hotspot distribution in spherical harmonics and calculating correlation coefficients as a function of harmonic degree the hotspot distribution spectrum is nearly white except for peaks at degrees 2 and 6 it is correlated positively with the slab residual geoid for degrees 2 6 with low seismic velocity in the lower mantle at degree 2 and with low seismic velocity in the upper mantle at degree 6 we test fluid mechanical models for hotspots including lithospheric delamination and hot plumes by calculating their predicted dynamic geoid responses and comparing them to the observations these models include the effects of temperature dependent rheology our preferred hotspot model based on observations of the geoid and seismic tomography has plumes preferentially occurring in regions of large scale background temperature highs in a mantle which has a substantial viscosity increase with depth although other models are possible alternatively strong plumes in the lower mantle may neck down and attenuate in the upper mantle due to changes in plume viscosity the major low density anomalies causing the geoid highs associated with plumes appear to be in the lower mantle we hypothesize that cold lower mantle underlying the complementary geoid lows is due to deeply subducted oceanic lithosphere and that hotspot plumes tend to be excluded from these cold regions of the mantle

source parameters and three dimensional attenuation structure from the inversion of microearthquake pulse width data qp imaging and inferences on the thermal state of the campi flegrei caldera southern italy the three dimensional p wave attenuation structure of the campi flegrei caldera and the estimate of source parameters for 87 local microearthquakes is obtained by the nonlinear inversion of pulse width and rise time measurements by using the method described by zollo and de lorenzo this issue source radii represent the better resolved parameters with values ranging from 70 m to 230 m the dip and strike angles defining fault orientations are usually affected by larger uncertainties and are well constrained only for 11 events the dip fault is usually confined in the range 30 60 with an average uncertainty of 12 the fault strikes mainly range between 60 and 60 and seem to define preferential directions oriented radially from the symmetry axis of the ground deformation stress drop estimates indicate rather low values 0 01 1 mpa which suggest low strength properties of the incoherent and brittle materials filling the caldera primarily yellow tuffs the three dimensional qp images obtained from the inversion of p pulse durations show two significant low qp anomalies between 0 and 1 km of depth in the north eastern sector and at 2 3 km of depth in the central eastern sector of the caldera the high degree of spatial correlation of the low qp zone and low vs as inferred by aster and meyer 1988 at 0 1 km in depth and other geophysical and geochemical observations suggest that this anomaly can be related to the presence of densely fractured porous and fluid filled rocks in the ne sector of the caldera the deeper low qp anomaly is interpreted as being related to a dominant thermal effect we used the surface and deep borehole temperature measurements available in the area to obtain a local calibration curve to convert qp in temperature at campi flegrei the retrieved t qp map shows a high thermal deep disturbance 450 500 c at depths between 2 and 3 km in the eastern sector of the caldera where the most recent eruptive activity is concentrated the present day temperature field retrieved by qp images has been interpreted by using a three dimensional thermal conduction model assuming an extended heat source initial temperature of 800 c located underneath the attenuation anomalous region the results indicate that the qp inferred temperature field can be related to the heat conduction effect of one or more molten bodies whose top should be at about 4 km depth consistent with recent seismic estimates of the magma chamber top at campi flegrei ferrucci et al 1992 this study suggests that the present thermal state and rock rheology of the inner caldera could be controlled by the cooling of molten bodies that originally intruded at depths of 1 4 1 6 km during one or more recent time of 10 kyr eruptive events

a dislocation model of seismic wave attenuation and micro creep in the earth harold jeffreys and the rheology of the solid earth a microphysical model of seismic wave attenuation is developed to provide a physical basis to interpret temperature and frequency dependence of seismic wave attenuation the model is based on the dynamics of dislocation motion in minerals with a high peierls stress it is proposed that most of seismic wave attenuation occurs through the migration of geometrical kinks micro glide and or nucleation migration of an isolated pair of kinks bordoni peak whereas the long term plastic deformation involves the continuing nucleation and migration of kinks macro glide kink migration is much easier than kink nucleation and this provides a natural explanation for the vast difference in dislocation mobility between seismic and geological time scales the frequency and temperature dependences of attenuation depend on the geometry and dynamics of dislocation motion both of which affect the distribution of relaxation times the distribution of relaxation times is largely controlled by the distribution in distance between pinning points of dislocations l and the observed frequency dependence of q q w is shown to require a distribution function of p l l m with m 4 2 the activation energy of q 1 in minerals with a high peierls stress corresponds to that for kink nucleation and is similar to that of long term creep the observed large lateral variation in q 1 strongly suggests that the q 1 in the mantle is frequency dependent micro deformation with high dislocation mobility will temporarily cease when all the geometrical kinks are exhausted for a typical dislocation density of 108 m 2 transient creep with small viscosity related to seismic wave attenuation will persist up to the strain of 10 6 thus even a small strain 10 6 10 4 process such as post glacial rebound is only marginally affected by this type of anelastic relaxation at longer time scales continuing nucleation of kinks becomes important and enables indefinitely large strain steady state creep causing viscous behavior

the upper mantle degree 2 constraints and inferences from global mantle wave attenuation measurements we present the results of an analysis of global lateral variations in anelasticity of the upper mantle as measured from very long period rayleigh waves observed on the geoscope network four consecutive wave trains are used on each record to eliminate uncertainty on the amplitude at the source and to take into account in a linear manner focussing effects due to lateral variations in elastic parameters local estimates of attenuation are obtained using an inversion method based on the introduction of a correlation length as an alternative to spherical harmonics expansion comparison of phase velocity and attenuation maps obtained at various periods shows a strong correlation of high q and high velocity regions respectively low q and low velocity both for great circle and minor arc data at periods around 200 s the detailed analysis of the degree 2 pattern well resolved in the attenuation maps around this period shows that it originates in the depth range 250 500 km and indicates that of the upper mantle degree 2 pattern observed in shear velocity is likely to be of thermal origin possibly related to the topmost part of the large scale convective system corrections for dispersion due to anelastic attenuation in degree 2 lead to significant decrease of corresponding lateral variations in shear velocity and a better agreement in phase and amplitude of the upper mantle degree 2 pattern with that observed in the geoid this study shows that it is important to consider correcting presently available upper mantle tomographic models for the effects of intrinsic attenuation at long periods before performing any comparison with short period data or other geophysical parameters

high temperature experiments on the elastic and anelastic behaviour of magmatic rocks measurements of the complex shear modulus have been performed on various ultrabasic and basic rocks dunite peridotite gabbro and basalts and a synthetic forsterite at temperatures between 600 and 1500 c depending on the sample material and in the frequency range from 0 003 to 30 hz under forced torsional oscillations the maximum shear stress was 0 3 mpa and the strains were of the order 5 10 6 to 5 10 5 measurements of the electrical resistivity as a function of temperature have been carried out together with the mechanical experiments to indicate the beginning of partial melting the absolute values of the shear modulus below tm are comparable with the results of other authors the onset of melting shows up in a rapid decrease of both shear modulus and electrical resistivity at high temperatures t 0 8 tm the absorption factor q 1 follows a power law q 1 exp a rt with 0 15 0 30 and a between 100 and 200 kj mol 1 towards lower temperatures this high temperature background is superimposed by a flat absorption peak which shifts from high to low frequencies with falling temperature this peak can often be described by a lognormal distribution of simple relaxation processes with a standard deviation of about three and a mean relaxation time exp a rt with a 400 600 kj mol 1 above tm a third contribution to q 1 is observed that can be regarded as independent of frequency between 1 and 30 hz this last component which contributes 50 to the total absorption is attributed to the melt phase in the samples an explanation for the high temperature background and the peak is proposed which is based on the relaxation of pinned dislocation lines by some coupling mechanism via the reorganisation of the stress field with increasing deformation this process may be responsible for both features at very high temperatures and low frequencies a small influence of steady state flow on q 1 is observed and below t 0 6 tm thermal cracking or thermoelastic losses may contribute to q 1 the influence of lithostatic pressure on q 1 is estimated and the extrapolation of the experimental data to the p t conditions of the uppermost mantle are in good agreement with published earth models an attempt is made to establish a general q t relationship from the experimental data

boron and calcium isotope composition in neoproterozoic carbonate rocks from namibia evidence for extreme environmental change the level and evolution of atmospheric carbon dioxide throughout earth s history are key issues for palaeoclimate reconstructions especially during times of extreme climate change such as those that marked the neoproterozoic the carbon isotope ratios of marine carbonates are crucial in the correlation and identification of neoproterozoic glacial deposits and they are also used as a record for biogeochemical cycling and potential proxy for atmospheric pco2 likewise the boron and calcium isotope compositions of marine carbonates are potential proxies for palaeo seawater ph and the ratio of calcium fluxes into and out of seawater respectively and together they may be used to estimate atmospheric carbon dioxide here we use b and ca isotopes to estimate palaeoenvironmental conditions in the aftermath of a major neoproterozoic glaciation in namibia the validity of the b and ca isotope variation in the ancient marine carbonates is evaluated using the oxygen isotope composition of the carbonates and its correlation to the carbon isotope variation a negative 2 7 to 6 2 11b excursion occurs in the postglacial carbonates and is interpreted to reflect a temporary decrease in seawater ph associated variations in 44ca values ranging between 0 35 and 1 14 are linearly coupled with the carbon isotope ratios and imply enhanced postglacial weathering rates the reconstructed seawater ph and weathering profiles indicates that high atmospheric co2 concentrations were likely during the melt back of neoproterozoic glaciations and precipitation of cap carbonates however the b isotope trend suggests that these concentrations rapidly ameliorated and they do not co vary with 13c thus models attempting to link long lived negative 13c excursions to elevated pco2 need to be reconsidered

carbon isotope chemostratigraphy and sedimentary facies evolution of the ediacaran doushantuo formation in western hubei south china abstract the doushantuo formation of south china consisting of intercalated siliciclastic and carbonate rocks which exhibit considerable variation in thickness and sedimentary facies is one of the key sedimentary units in the world for understanding ediacaran geobiological processes however stratigraphic correlation and sedimentary facies evolution of the doushantuo formation remain controversial sedimentary facies analysis of the well known sections of the yangtze gorges area and new sections in northern yichang western hubei indicate that the doushantuo succession in the yangtze gorges area was deposited in an offshore intra shelf basin within the yangtze platform in contrast the doushantuo formation in the northern yichang area which is characterized by one or a few phosphate rich horizons represents deposition on the subtidal or intertidal shelf and so is similar to the succession in the weng an area central guizhou province the carbon isotope profiles of the different sections exhibit significant variation which is interpreted to result from facies differences sequence and high resolution carbon isotope stratigraphy have clarified problematic correlations and the temporal relationship between fossil assemblages of the doushantuo formation on the yangtze platform the results confirm that a prominent negative 13ccarb excursion baince nadir down to 5 in the middle portion of the second sequence of the doushantuo formation is a widely traceable event baince together with three other 13ccarb excursions namely cance at the base wance near the first sequence boundary near the middle and dounce at the top of the doushantuo formation constitute a complete carbon isotope profile for the doushantuo formation that reflects contemporaneous seawater and so serves as a reference chemostratigraphic profile for global stratigraphic correlation and for evaluating the chemical evolution of seawater during the ediacaran period

secular changes of water chemistry in shallow water ediacaran ocean evidence from carbonates at xiaofenghe three gorges area yangtze platform south china ediacaran carbonates from the shallow water section near xiaofenghe three gorges area hubei province on the yangtze platform south china have been studied to understand post marinoan changes in seawater chemistry major and trace element abundances and sr nd isotopic compositions were obtained on acetic acid leachates of carbonate sediments from both the doushantuo formation members d1 d4 and the overlying dengying formation c and o isotopic compositions of these samples were analysed using the phosphoric acid method with the exception of the d1 cap carbonates 87sr 86sr 0 7088 0 713 most samples show low mn sr and 87sr 86sr 0 7078 in d2 d4 and 0 709 in the dengying formation similar to seawater values inferred from ediacaran carbonates elsewhere these data together with the absence of a correlation between 87sr 86sr and 18ocarb indicate minor diagenetic fluid overprinting the initial nd values of the carbonate leachates show a limited variation range from 4 3 to 7 5 with the highest values in d1 these data overlap with published data for doushantuo sediments in south china and possibly indicate an isotopically homogenous nd source in the source area during most of the ediacaran d1 samples of cap carbonates display negative 13ccarb values of 2 0 to 3 6 vpdb and 18ocarb values ranging from 6 5 to 7 3 vpdb whereas the overlying d2 strata show positive 13ccarb values around 6 carbonate rocks from d3 and d4 show a large variation of 13ccarb from 1 4 to 9 4 with some samples having unusually high 13ccarb values coupled to low toc contents most carbonate leachates at the base of the doushantuo display superchondritic y ho and variable hree enrichment pr yb 1 similar to modern seawater this suggests a similar seawater chemistry as under modern open ocean conditions at the beginning of the ediacaran whereas lower y ho in d3 correlate with flat ree y patterns indicating a change to variable freshwater seawater mixing and suppression of hree lree fractionation as in modern estuaries carbonate rock samples from d4 are rich in clay minerals up to 15 of possible authigenic origin such as nontronite and saponite formation of these clay minerals may only be possibly under extreme alkaline and saline conditions the d4 carbonate leachates show the highest y ho and lowest ce ce values and have unusually high 13ccarb values which may also support the conditions of extreme shallowing and perhaps temporarily restricted basins that led to a rise in salinity due to high evaporation rates ce anomalies in carbonate leachates show strong variation throughout the profile a secular increase of seawater oxygenation through the ediacaran is not obvious however the most pronounced ce anomalies can be found in the upper doushantuo ce ce as low as 0 64

trace and rare earth element geochemistry of black shale and kerogen in the early cambrian niutitang formation in guizhou province south china constraints for redox environments and origin of metal enrichments abstract early cambrian black shales of south china not only host important sponge arthropod and other soft bodied fossils that have helped to trace early metazoan diversification but also show extreme enrichments of a number of trace metals in particular ni mo and v in this study we use a new approach by analyzing rare earth elements in kerogen extracted from the black shales together with a number of redox sensitive trace element compositions and total organic carbon toc concentrations in an early cambrian black shale sequence in zunyi guizhou province south china to place better constraints on the oceanic redox conditions and the origin of the extreme metal enrichment our data show significant negative ce anomalies ce ce as low as 0 4 occurring in kerogen which indicate an oxygenated surface environment of primary productivity in consistent with the concept that the organic matter is mainly derived from organisms in the euphotic zone mass balance calculation suggests that the kerogen associated ree can dominate the measured black shale ree budget while similarity between our measured ree patterns and those of similarly aged phosphorites indicates that the ree content of ancient phosphorites may have also derived initially from organic matter the redox sensitive trace elements such as u v mo and their ratios of u al v al and mo al in black shales show different correlation patterns with toc contents the upper black shales show a good metal toc correlation but such a correlation is absent in the lower part the lower black shales exhibit much higher metal enrichments compared to black sea sulphidic euxinic sediment this is taken to indicate the presence of sulphidic bottom waters during the deposition of the lower black shales including the ni mo ore layer in contrast anoxic non sulphidic conditions occurred during the deposition of the upper black shales taking all these geochemical data together we suggest that the early cambrian south china seaway was strongly stratified and stagnant and that euxinic bottom water conditions may have led to enrichment of the redox sensitive metals such as u v and mo in the lower black shales and in one case the occurrence of a polymetallic ni mo sulphide ore bed bearing an extraordinarily extreme metal enrichment which according to the mo toc and ni toc ratios and much other geochemical evidence may have been additionally influenced by hydrothermal input of metals within the rift basin as suggested by a number of previous studies

oceanic topography and heatflow indications for a silent discharge of cold rock into the convecting earth the 2 5 km mean relative elevation of mid ocean ridges is arguably consistent with a steady state thickness of 100 km for the oceanic lithosphere here we explore some consequences of this particular value based on seismic models of the uppermost oceanic mantle we infer that no significant heat is added to normal asthenosphere following accretion at spreading centers therefore we adopt the view that kinematic thinning moderates the conductive thickening to yield the steady state thickness of 100 km we devise a method for estimating mean rates of thinning directly from topography age data with no assumptions on the detailed dynamics of delamination we find that delamination delivers a significant fraction 20 of the global downwelling discharge this large non subducting discharge of cold lithospheric material provides an explanation for the most robust features of upper mantle tomography and should be considered in studies of mantle cooling and mixing

neoproterozoic crustal growth and reworking of the northwestern yangtze block constraints from the xixiang dioritic intrusion south china abstract neoproterozoic magmatism along the northwestern margin of the yangtze block south china produced voluminous adakitic and normal arc felsic intrusions associated with many mafic ultramafic bodies dioritic plutons are rare but are important for understanding the crustal growth and secular evolution of the region the xixiang dioritic intrusion at hannan on the northwestern margin of the yangtze block provides a good opportunity to investigate the petrogenesis of intermediate plutons and to document the neoproterozoic continental crustal evolution of the region the pluton has a zircon u pb age of 764 9 ma rocks from the pluton have relatively low sio 2 59 3 63 0 wt and k 2 o na 2 o 5 8 6 2 wt and high mgo 2 3 3 1 wt and al 2 o 3 15 8 17 1 wt they have chondrite normalized ree patterns enriched in lree la ybn 7 2 14 7 with negative eu anomalies their primitive mantle normalized trace element patterns are characterized by enrichment of lile and depletion of hfse nb and ta with positive pb and negative ti anomalies they have a narrow range of initial 87 sr 86 sr ratios 0 7033 to 0 7035 with positive end values 1 94 to 3 16 their zircon ehf values range from 5 5 to 9 8 with an average of 7 32 these isotopic data are similar to those of the neoproterozoic mafic ultramafic intrusions in the hannan region suggesting that the diorites were generated by partial melting of newly emplaced basaltic rocks the diorites have mg values 50 3 54 4 higher than those of experimental melts of mafic igneous rocks in addition they contain fe rich clinopyroxene suggesting that partial melts of the newly formed mafic lower crust interacted with the mafic residue based on its arc related geochemical affinity and emplacement preceding voluminous adakitic magmatism the xixiang intrusion is proposed to have been generated in a neoproterozoic arc setting during crustal growth and reworking that lasted from 950 ma to